Alumina Membranes

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Academic year 2011-‐2012

Erasmus Mundus Master in Membrane Engineering Semester S1

Report on the six-‐month project

A review of Alumina: Most abundant and productive material of the

mother nature.

KAYAALP Umay PAPOUTSOGLOU Dimitra

January 2012

Supervisor:

AYRAL Andre; [email protected]‐montp2.fr

BACCHIN Patrice ; [email protected]‐tlse.fr

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TABLE OF CONTENT

1. INTRODUCTION ........................................................................................................................... 3 2. ALUMINA AS A MATERIAL ....................................................................................................... 4 2.1. NOMENCLATURE ..................................................................................................................... 4 2.2. STRUCTURE AND MINERALOGICAL PROPERTIES ........................................................ 8 2.2.1. STRUCTURE OF ALUMINA PHASES ..................................................................................................... 8 2.2.2. PSEUDOMORPHOSIS .............................................................................................................................. 9 2.2.3. SURFACE AREA OF ALUMINA ............................................................................................................. 12 2.2.4. POROSITY .............................................................................................................................................. 13 2.2.5. SORPTIVE CAPACITY ........................................................................................................................... 15

2.3. MECHANICAL PROPERTIES OF ALUMINA ..................................................................... 16 2.4. THERMAL PROPERTIES OF ALUMINA ............................................................................ 20 2.5. CHEMICAL PROPERTIES OF ALUMINA ........................................................................... 22 2.5.1. WET CHEMICAL REACTIONS OF SINTERED ALUMINA .................................................................. 22 2.5.2 REACTION OF CHEMICAL ELEMENTS WITH ALUMINA ................................................................... 23

1.8. COLLOIDAL PROPERTIES OF ALUMINA ......................................................................... 24 3. ALUMINA MEMBRANES .......................................................................................................... 26 3.1. INTRODUCTION ..................................................................................................................... 26 3.1 PREPERATION OF ALUMINA MEMBRANES .................................................................... 29 3.2.1. MACROPOROUS ALUMINA MEMBRANE PREPARATION ................................................................ 29 3.2.2.MESOPOROUS ALUMINA MEMBRANES ............................................................................................. 31 3.2.3 MICROPOROUS ALUMINA MEMBRANES ........................................................................................... 35

4.DESIGN OF THE MEMBRANE MODULES ............................................................................. 38 4.1 DIFFERENT TYPES OF MODULES ...................................................................................... 38 4.1.1 ALUMINA MEMBRANE MODULES ....................................................................................................... 39 4.1.2. COMMERCIALIZED MODULES OF MEMBRANE ALUMINA ............................................................... 42

4.2. SEPARATION CHARACTERISTICS FOR ALUMINA MEMBRANES ............................. 45 5. APPLICATIONS ........................................................................................................................... 49 5.1. CERAMIC MEMBRANES ........................................................................................................ 49 5.2. ALUMINA MEMBRANES APPLICATIONS ........................................................................ 52 5.2.1. LIQUID PHASE SEPARATION APPLICATIONS ............................................................................... 52 5.2.2. GAS PHASE SEPARATION ................................................................................................................. 63

6.SUMMARY AND CONCLUSIONS .............................................................................................. 74 APPENDIX A : MEMBRANE MATERIAL SHEET ..................................................................... 75 APPENDIX B: CHEMICAL INTEREST OF ALUMINA .............................................................. 77 REFERANCES ................................................................................................................................... 85

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1. INTRODUCTION

Ceramists are not close agreement as to the substances included in the term of

‘’ceramics’’, nor do they seem to have devised as simple, consistent definition of

the term that is entirely satisfactory. Kingerly defined it as ‘’the art and science of

making and using solid articles which have as their essential component, and are

composed in large of inorganic nonmetallic materials.’’ L. Mitchell defined

ceramics as ‘’all high-‐temperature chemistry and physics of nonmetallic materials,

and the techniques of forming products at high temperatures.’’ The first definition

allows inclusion of materials having melting points below room temperature, as,

for example ice; while the second does not exclude certain organic substances

that may been produced at high temperatures, such as carbon disulfide. Although

materials of all kinds, including organic substances, are involved in the

preparation of ceramics, it is believed that these definitions are too broad to

cover the ceramic applications of alumina.

The investigation of alumina as a ceramic material was undertaken to provide

information under the following specifications: a review dealing with alumina

both from a theoretical and a practical point of view, and including information

on the nomenclature, properties of alumina, alumina as a membrane material

and finally industrial alumina membrane applications.

The following information has been gathered:

• occurrence in nature

• crystal or mineralogical characteristics

• mechanical, thermal, chemical and colloidal properties.

• alumina membranes fabrication, modules and industrial applications.

A general review tried to be gathered to understand deeply about alumina

material and also alumina membranes.

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2. ALUMINA AS A MATERIAL

2.1. NOMENCLATURE

De Morveau suggested the word ‘alumine’ in 1786 as the proper name for the

basic earth of alum. This was Anglicized to ‘Alumina’ in England while Germany

‘tornerde’ is still used, meaning clay earth. The term ‘alumina’ is presently used

rather indefinitely in ceramic literature to denote;

1. aluminous material of all types taken collectively

2. the anhydrous and hydrous aluminum oxides taken indiscriminately

3. the calcined or substantially water free alimunium oxides without

distinguishing the phases present

4. corundum or alpha alumina, specifically. It is often used interchangeably

with the molecular formula Al2O3.

The true meaning is sometimes hard to determine from the context.

More than 25 alumina solid phases is defined in recent years. But it is doubtful if

all of them really exist or not. These phases includes, amorphous hydrous and

anhydrous oxides, crystalline hydroxides and oxides, and aluminas containing

small amounts of oxides of alkalies or alkaline earths, designated as beta

aluminas.

The phases found in the nature, and few of artificial types, have common or

mineralogical names. Most of them also are defined by greek letter formulas.

Corundum, emery, sapphire, and ruby are more or less pure forms found in the

nature and known for antiquity as abrasives and gem stones. All consist of the

the phase designated alpha alumina (! – Al2O3). Figure 1 shows the dehydration

sequence of alumina hydrates in air.

Diaspore

Another native mineral, described by Haüy in 1801, was named ‘diaspore’ by

him, from the Greek for ‘scatter’ because it flew apart upon heating.

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Figure 1: Dehydration Sequence Of Alumina Hydrates In Air

Gibbsite

Vaquelin in 1802 gave its formula as Al2O3 ∙ 3H2O. Dewey named a well-‐

crystallized mineral ‘gibbsite’ for G. Gibbs an American mineralogist. It

corresponded with the formula Al(OH)3 or Al2O3 ∙ 3H2O. It is the principal phase

of the ‘trihydrate’ bauxites.

Bauxites

Berthier examined a mineral from Les Baux in southern France, containing about

%52 Al2O3 and 20% bound water, from which it was supposed that the mineral

was Al2O3 ∙ 2H2O. The mineral was named ‘bauxite’ by St. Clair Deville.

Bohemite

After X-‐ Ray diffraction became generally used for analysis of chemical

components a new pattern of bauxite is discovered. This bauxite has %15 bound

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water Al2O3 ∙ H2O. This component named ‘bohemite’ for both natural and

artificial products. These phases are the izomers of diaspore.

In USA, the word ‘bauxite’ has come to mean any highly aluminous ore composed

mainly of one or more of phases, gibbsite, boehimite and diaspore. In reality no

phase corresponding to Al2O3 ∙ 2H2O has been found.

In 1925, Haber devised a system of nomenclarature for trivalent alumina phases

the known. The ‘alpha’ series included diaspore and corundum (alpha alumina);

the ‘gamma’ series included hydrargillite (gibbsite), bauxite (bohemite), and

gamma alumina. The classification was obviously based on the end product of

calcination, but this is somewhat arbitrary because gamma alumina also

transforms to alpha alumina.

Bayerite

A new phase is identified by Böhm in aluminum hydroxide precipitates that had

aged moist for several months at room temperature. The water content was

about that of gibbsite, but the X-‐Ray pattern was different, indicating an isomer

of gibbsite. This phase called ‘Bayerite’ by Fricke in 1928 on the erroneous

supposition that it was normal product of the Bayern process. L. Milligan had

already shown in 1922 that the Bayer product is predominantly gibbsite.

Bayerite was claimed to have been found in nature by Gedeon, and more recently

(1963) by Gross and Heller.

The failure of Haber classification system to distinguish between bayerite and

gibbsite prompted the devising of the Alcoa system of nomenclature (Frary). In

this system, the choice of Greek letters was initially based on the relative

abundance of phase in nature. Gibbsite was called alpha alumina trihydrate;

bohemite, alpha alumina monohydrate; bayerite beta alumina trihydrate; and

diaspore, beta alumina monohydrate. Gamma alumina and beta alumina had the

same significance as in the Haber system.

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Table 1: Nomenclature of Crystalline Aluminas (WALTER , Alumina as a Ceramic Material, 1970, page 5)

Mineralogical Name

Phase of Form Name

Symposium (1) Alcoa (2) Haber (3) British (4) French (5) Other

Hydroxides

Gibbsite (6) Hydrargillite Al(OH)3

! − !"!!! ∙3!!!

! − !"(!")!

! − !"!!! ∙3!!!

Bayerite (7) Al(OH)3 ! − !"!!! ∙3!!!

!− !"(!")!

! − !"!!! ∙3!!!

Nordstrandite (1) Randomite (8) Bayerite II

(8)

Al(OH)3

Bauxite (9) Al(OH)2

Bohemite (10) AlOOH ! − !"!!! ∙!!!

! − !"(!")!

! − !"!!! ∙ !!!

Diaspore AlOOH ! − !"!!! ∙!!!

! − !"(!")!

Tohdite (11) AlOOH 5 !"!!! ∙ !!! ! − !"!!! (12)

Aluminas Rho

Chi Chi Chi + Gamma Chi + Gamma

Eta Eta Gamma Eta Gamma Gamma Gamma Delta Gamma Xi1, Xi2 (13)

Kappa Kappa Ioata (17) Kappa + Theta Kappa +

Delta

Corundum, Sapphire Alpha Alpha Alpha Alpha Alpha

AlO Al2O M2O ∙ 11 Al2O3 (14) M2O ∙ 6 Al2O3 (15) MO ∙ 6 Al2O3

Zeta Alumina Li2O ∙ 5 Al2O3 (1) Ginsberg, Huttig, Strunk-‐Lichtenberg (2) Edwards, Frary, Stumpf, et al. (3) Haber, Weiser, and Milligan (4) Rooksby, Day, and Hill (5) Thibon, Tertian, and Papee (6) Dewey (7)Fricke (8)Teter, Gring, and Keith (9)Böhm

(10)de Lapparent (11)Yamaguchi (12)Steinheil (13)Cowley (14)Rankin and Merven (15)Scholder (16)Barlett (17)P.A. Foster

8

Standardization of the nomenclature for aluminas is very desirable particularly

to avoid the confusion in the hydrous phases. Gingsberg reported the

conclusions of a symposium held in 1957, in which an attempt was made to

devise a universal standard nomenclature. Some features of the proposed system

are improvements, for example the substitution of ‘hydroxide’ instead of

‘hydrate’, namely, aluminum trihydroxide for alumina trihydrate; aluminum

oxide hydroxide for alumina trihydrate; aluminum oxide hydroxide for alumina

monohydrate. Also, it was agree to use the Alcoa nomenclature for the transition

aluminas, but to designate some of them as ‘forms’ rather than ‘phases’, to imply

the present uncertainty about them. The confusion in naming the hydroxide

phases has not been resolved, however.

2.2. STRUCTURE AND MINERALOGICAL PROPERTIES

The remarkable range of properties of the hydrous and non hydrous crystalline

properties of alumina has been interesting for researchers, and its structure has

been induced much scientific curiosity.

Examples of these structural peculiarities are the factors determining the

phenomena of transition phases, and the exceptional strength and hardness of

corundum. Besides the ideal crystal structures, which are a rarity in actual

ceramic systems, defect crystal structure and microstructure are significant.

Gross structure beyond the crystal lattice beyond the crystal lattice is also of

ceramic interest.

2.2.1. Structure of Alumina Phases

Crystal structure is the main factor controls the properties of aluminas. In

general, the phases of most significance in alumina are those produced by

pseudomorphic dehydration.

The crystal structures of alumina phases are shown in Table 2. Mineralogical

properties of the various phases are shown in Table 3.

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2.2.2. Pseudomorphosis Achenbach (1931), Damerell (1932), and Teritan (1950) have shown that the

dehydration of gibbsite crystals is pseudomorphic, that is, external shape of the

crystals is retained and there is an orientation relationship of the crystal axes of

the new phases to those of the original. The crystals lose transparency and

smoothness upon heating, and fine-‐grained fibers develop parallel to the

hexagonal surface. Void space resulting from the loss of water from the gibbsite

and increasing density of the transition phases is distributed in microporosity of

very high surface area in the porous skeleton (Weitbrecht and Fricke).

Pseudomorphosis is of considerable importance because of its effect on surface

area of the intermediate phase structures, and on crystal size and size

distribution of the fully calcined aluminas for ceramic forming processes.

With the basis of alumina structure, all the transition aluminas have oxygen ions

in approximately cubic close packing. The differences in their patterns represent

changes in intensities of reflections resulting from differences in distribution of

the aluminum ions. The initial cationic disorder of the low-‐temperature phases

depends upon the source of the alumina. The transitions become more ordered

with increasing heat treatment.

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Table 2: Crystal Structure of the Aluminas (WALTER , Alumina as a Ceramic Material, 1970, page 30)

Phase Formula Crystal System Space

Group

Mole-‐cules

Unit Cell Parameters Angle Ref. Angstroms

a b c Hydrated Aluminas Gibbsite ! − !"!!! ∙ 3!!! Monoclinic !!!ℎ 4 8.641 5.07 9.72 1

Bayerite ! − !"!!! ∙3!!! Monoclinic !!!ℎ

2 4.716 8.679 5.06 3 Nordstrandite !"!!! ∙ 3!!! Monoclinic 8 8.63 5.01 19.12 4 Bohemite ! − !"!!! ∙ !!! Orthorhombic !!!"ℎ 2 2.868 12.227 3.7 5,12 Diaspore ! − !"!!! ∙ !!! Orthorhombic !!!"ℎ 2 4.396 9.426 2.844 5 Transition Aluminas

Chi Cubic 10 7.95 9 Eta Cubic (Spinel) !!! 10 7.9 9,8 Gamma Tetragonal 7.95 7.95 7.79 6,7 Delta Tetragonal 32 7.967 7.967 23.47 10,11 Iota Orthorhombic 4 7.73 7.78 2.92 9 Theta Monoclinic !!!ℎ 4 5.63 2.95 11.86 13 Kappa Orthorhombic 32 8.49 12.73 13.96 9 Corundum ! − !"!!! Rhombohedral !!!! 2 4.758 12.991 2 Al2O Cubic 4.98 20 !"# ∙ !"!!! Cubic (Spinel) !!! 7.915 22 Beta Aluminas (21) !"!! ∙ 11!"!!! Hexagonal !!!ℎ 1 5.58 22.45 14 !!! ∙ 11!"!!! Hexagonal !!!ℎ 1 5.58 22.67 14 !"# ∙ 11!"!!! Hexagonal !!!ℎ 1 5.56 22.55 16 !"# ∙ 6!"!!! Hexagonal !!!ℎ 2 5.54 21.83 15 !"# ∙ 6!"!!! Hexagonal !!!ℎ 2 5.56 21.95 15 !"# ∙ 6!"# Hexagonal !!!ℎ 2 5.58 22.67 17 Zeta Alumina !"!! ∙ 11!"!!! Cubic !!! 2 7.9 18,19

(1) Megav (2) Swanson, Coo Isaacs, & Evans (3) Unmack (4) Lippens (5) Swanson & Fuyat (6) Saalfeld (7) Brindey & Nakahira (8) Verwey

(9) Stumpf (10) Teritan & Papee (11) Rooymans (12) Reichertz & Yost (13) Kohn (14) Beevers & Brouhult (15) Laderqvist (16) Bragg (17) Adelsköld

(18) Kordes (19) Braun (20) Hoch & Johnson (21) Scholder & Mansmann (22) Filonenko, Larov, Andreeva & Prevszer

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Table 3 : Mineralogical Properties of the Alumina (WALTER , Alumina as a Ceramic Material, 1970, page 31)

Phase Index of Refraction nd Cleav-‐

erge Mohs Hard

Micro Hard

kg/mm2

Dens. Measured

g/ml Ref.

!

Hydrated Aluminas Gibbsite 1.568 1.568 1.587 (100) 2.5 -‐3.5 2.42 1,2 Bayerite 1.583* 2.53 3,2 Bohemite 1.649 1.659 1.665 (010) 3.5 -‐ 4 3.01 4,5,14 Diaspore 1.702 1.722 1.75 (010) 6.5 -‐ 7 3.44 1,14

* Transition Aluminas

Chi 3.0 19 Eta 1.59 -‐1.65 2.5-‐3.6 6

Gamma 3.2 20 Delta 3.2

Iota 1.604 3.71 7 Theta 1.66-‐1.67 3.56 6 Kappa 1.67-‐1.69 3.3 6

Corundum Al2O 1.7604 1.7686

none 9.0 2150 3.96-‐3.98 21,15

AlO Al2O3

1.77 -‐ 1.80

2070 3.84 13

Beta Aluminas

Sodium Beta 1.635 -‐1.650 1.676

8,10,18

Potassium Beta

1.642 1.675

17,18

1.640 1.668 Magnesium

Beta 1.629 1.665-‐1.680

16,8 Calcium Beta 1.752 1.759

1.754 1.763

3.731 11,12 Barium Beta 1.694 1.702

3.69 10

Lithium Zeta

1.735

3.61 9 *Average Estimate (1) Dana (2) Roth (3) Montoro (4) Ervin (5) Bonshtedt Kupletskkaya (6) Thibon (7) Foster, P. A.

(8) Rankin & Mervin (9) Kordes (10) Toropov (11) Filonenko (12) Wisnyi (13) Filonenko, Larov, Andreeva & Pevzner (14) Fricke & Severin

(15) Coble (16) Bragg, Gottfried, West (17) Kato & Yamauchi (18) Beevers & Brohult (19) Stumpf (20) Gingsberg (21) Biltz & Lemke

! !

! !

!

!!!

!

!

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2.2.3. Surface Area of Alumina

Alumina is widely used as a catalyst or catalyst support in many heterogeneous

catalytic processes owing to its high surface area, superior chemical activity and

low cost. In order to prepare the thermal-‐stable alumina with high surface area

and large pore volume, two ways have been adopted. One is that some additives

including silica, phosphoric oxide, barium oxide, cerium oxide and lanthanum

oxide have been added to alumina. But the presence of these additives will

modify the original properties. The other way is using some new methods and

techniques, such as sol-‐gel method and supercritical drying techniques.

Table 4 lists the specific surface areas, pore volumes, and pore diameters

measured for samples of Al2O3. The results show that the alumina samples

prepared using β-‐cyclodextrin template had the higher surface areas (124-‐484

m2/g), larger pore volumes (0.7-‐1.27 mL/g) and more thermal stability than

samples prepared without using β-‐cyclodextrin. The sample A-‐773 exhibits the

highest SBET (484 m2/g) among the alumina samples calcined at different

temperatures. When the temperature exceeds 773 OK, the SBET decreases rapidly,

but the pore volume changes a little. After calcination at 1273 OK, the A-‐1273

maintains surface area of 124.2 m2/g and pore volume of 0.70 mL/g. However,

B-‐773 and B-‐1273 have the surface areas of 348 and 98.3 m2/g, respectively.

The pore volume of B-‐1273 is only 0.54 mL/g.

Table 4: Specific surface areas, pore volumes, and pore diameters measured for samples of Al2O3

Sample SBET (m2/g) Pore Volume (mL/g) Pore Diameter (A)

A-‐773 484.34 0.98 60.85 A-‐923 285.27 1.27 24.41 A-‐1073 220.86 0.87 20.49 A-‐1273 124.22 0.7 72.23 B-‐773 348.01 1.08 24.54 B-‐923 228.28 1.14 28.15 B-‐1073 201.2 0.74 24.67 B-‐1273 98.3 0.54 72.16

Harris and Sing study on gels formed by hydrolysis of aluminum isopropoxide.

1100 m2/g of surface area is detected. Storage of these unstable gels in the

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presence of water vapor caused loos in surface area to about 500 m2/g. The

adsorption isotherms of nitrogen, determined at -‐196 OC on the outgassed

products were of the reversible S-‐type, characteristic of physical adsorption on

nonporous solids. Gels that have been dehydrated at room temperature

approximately to the formula Al2 . 3H2O showed no X-‐ray evidence of crystalline

structure.

2.2.4. Porosity Porosity, and special case, permeability significantly affect the properties of

alumina ceramics, and in a wide range of magnitude. Porosity is generated in

sintered alumina structures for various reasons, some of which are to improve

permeability to gases and liquids for porous diaphragms and diffuser plates, to

increase the thermal insulation of refractories, and to improve the fuel

combustion in radiant heaters. Volatile or combustible burn-‐outs (sawdust,

naphthalene) have been used to generate pores. Gas generators include:

hydrogen peroxide and aluminum powder with acids or alkalies.

Calcining mixtures of ground and unground Bayer alumina at high temperatures

can develop gross porosity beyond 50% by volume. Uniformly distributed

porosity is attained by ‘bisque’ firing fine-‐ground alumina in the undersintered

range 1000 to 1400 OC.

Barrett, Joyner and Hallenda applied adsorption -‐ desorption on sintered

alumina to have further information about pore shape and pore distribution. In

literature five general types of hysteresis loops are defined, from which fifteen

capillary shapes could be deduced. The adsorption isotherms of the activated

forms of alumina fit the three main types, A, B and E, all of which have steep

desorption curves. Type A has a steep sorption branch, type B a gradual sorption

branch, with a broad hysteresis range, and type E a gradual sorption branch with

a narrow hysteresis range. The pore shapes are mainly open and closed tubular

capillaries, ink bottle shapes, and slit shapes. In figure 2 the pore size

distribution curves were derived from the N2 physisorption isotherms according

to the B-‐J-‐H method, Figure 2 a and b show, respectively, the pore size

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distributions for the commercial Al2O3; and the ceramic foams obtained from the

same aluminas.

Figure 2: a) N2 physisorption isotherm of commercial -‐alumina; (b) Pore size distribution of commercial -‐alumina; (c) N2 physisorption isotherm of commercial -‐alumina ceramic foam; (d) Pore size distribution of commercial ?-‐alumina ceramic foam.

Hayes, Budworth, and Roberts; investigated the permeability of sintered

aluminum tubes (total porosity 4 to 9 %, purity 99.3 to 99.8% Al2O3). These

tubes at first were impermeable to oxygen, nitrogen and argon at temperatures

below 1500 OC. At 1500 to 1750 OC, the specimens showed appreciable

permeation to oxygen, presumably by a surface diffusion process. The diffusion

coefficient was about 100 !"2/sec. very slight or no permeation was found for

nitrogen, and none for argon. After continued exposure for 100 hours at 1700 OC,

permeation by normal channel-‐flow developed suddenly and swamped the

earlier phenomena. The permeation of nitrogen through hot-‐pressed sintered

alumina (4 to 14% total porosity) was predominantly by Knudsen flow.

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2.2.5. Sorptive Capacity

The strong desiccating action of activated alumina has been known at least since

1879. The properties that make the activated aluminas particularly suitable for

desiccant use are: the ability to develop high surface area during formation of

dehydration; a high degree of chemical inertness; resistance to softening,

swelling, and disintegration when immersed in water or other liquids; high

resistance to shock and abrasion; and the ability to return to the original highly

adsorptive from by a suitable thermal regenerative treatment.

Alumina is also use for special type of adsorption called chromatography, in

which the identification and separation of adsorbed ions are usually based on a

visual, spatial order of adsorption. Typical properties of desiccant, commercial

alumina samples are shown in Table 5.

A partial list of gasses and liquids that can be dried by activated alumina (Alcoa

brochure, June 1, 1967) includes the following.

Gasses

Acetylene, air, ammonia, argon, carbon dioxide, chlorine, cracked gas, ethane,

ethylene, freon, furnace gas, helium, hydrogen, hydrogen chloride, hydrogen

sulfide, methane, natural gas, nitrogen, oxygen, propane, propylene, and sulfur

dioxide.

Liquids

Benzene, butadiene, butane, butene, butyl acetate, carbon tetrachloride, chloro

benzene, cyclohexane, ethyl acetate, freon, gasoline, heptane, n hexane, jet fuel,

kerosene, lubricating oils, naphtha, nitrobenzene, pentane, pipe-‐line products,

propane, propylene, styrene, toluene, transformer oils, vegetable oils and xylene.

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Table 5: Typical Properties of Desiccant, Chromatographic And Catalytic Aluminas (WALTER , Alumina as a Ceramic Material, 1970, page 39)

F-‐1 H-‐151 F-‐20 T-‐71 F-‐110 F-‐7 Typical properties Al2O3 % 92.00 90.00 92.00 99.5 + 92-‐94 84.00 Na2O % 0.90 1.60 0.90 0.01 0.08 0.90 Fe2O3 % 0.08 0.13 0.08 0.06 0.03 0.08 SiO2 % 0.09 2.20 0.09 0.04 0.01 0.09 Loss on ignition (1100 OC) % 6.50 6.00 6.20 0.00 6.0-‐8.0 12.10 SO3 % 0.09 CaO % 0.06 Nickel Formate %

2.50

Form Granular Ball Granular Granular Ball Granular Surface area m2/g 210.00 390.00 210.00 0.50 180-‐280 Bulk density, loose kg /m3 832.9620 816.9435 929.0730 1217.4060 800.9250 832.9620 Bulk density, packed, kg/m3 881.0175 848.9805 1089.2580 1361.5725 881.0175 881.0175 Spesific Gravity 3.30 3.1-‐3.3 3.30 Static sorption at 60% RH 14-‐16 22 -‐ 25 Crushing Strenght 55.00 75.00 Pore volume ml/gm 0.15-‐0.20 0.38 pH 9.00 Sieve Analysis On 80 mesh % 2 max Through 270 mesh % 5 max

2.3. MECHANICAL PROPERTIES OF ALUMINA Alumina has remarkable mechanical properties in comparison with conventional

porcelains and other single oxide ceramics. None of the likely refractory single

oxide contenders approaches pure sintered alumina in bending and tensile

strength, and is exceed only by stabilized ZrO2 in compressive strength. Many if

the advantageous strength characteristics are retained to lesser extent by the

high and low alumina porcelains.

The interest in mechanical properties stems from several applications such as

possible substitution of alumina ceramics for refractory metal parts in air-‐bone

equipment, or fabrication forms in which high mechanical strength, membrane,

hardness or thermal shock resistance is important.

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The mechanical tests of particular significance include: flexural, compressive,

tensile torsional, and impact strengths; modulus of elasticity and rigidity;

Poisson’s ratio and bulk modulus; fatigue, creep, internal friction, thermal shock

resistance, and flaw detection; and hardness.

Structural applications of aluminum oxide in the high temperature field require

knowledge of the effect of temperature on mechanical properties. Data on

mechanical properties of alumina is collected in Table 6 and 7. The data in the

table include information taken from ceramic literature as well as average values

for commercial production, taken from the standards of alumina ceramic

manufacturers association and the literature of several studies.

Table 6 is belonging to typical properties and specifications of commercial

grades of 3 kinds of alumina. Hydrated aluminas, prepared in a modern Bayer

plant, is indicated the specimen of C. And specimen A belongs to calcined

aluminas, which specimen T indicates tabular alumina.

Data on the mechanical properties of alumina is collected in Table 7. The data in

the table include information taken from the ceramic literature, as well as

average values for commercial production, taken from the standarts of the

Alumina Ceramic Manufacturers Association.

Table 6: Typical Properties and Spesifications of Hydrated Aluminas-‐Series C-‐30 (WALTER , Alumina as a Ceramic Material, 1970, page 21)

Typical Properties C A T Al2O3 % 65 98.9 99.5+

SiO2 % 0.01 0.02 0.06

Fe2O3 % 0.003 0.03 0.06

Na2O % 0.16 0.45 0.20

Moisture (110 OC) % 0.04 1.0 Specific Gravity 2.42 3.6-‐3.8 3.65 -‐3.8 Sieve Analysis on 100 mesh % 0-‐1 4-‐15 on 200 mesh % 5-‐10 50-‐75 on 325 mesh % 30-‐55 88-‐98

through 270 mesh % 45-‐70 2-‐12

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Table 7: Mechanical Properties of Alpha Alumina Oxide (WALTER , Alumina as a Ceramic Material, 1970, page 45)

Bending Strength (Modulus of Rupture) Temp OC MPa

(1)

Sapphire Flame-‐fused,

oriented 0O between optic axis and bar axis a

25 680 600 180

1000 300

Sapphire Flame-‐fused, oriented 45 O between optic axis and

bar axis a ,b

25 480 600 313

1000 567

Ruby Flame-‐fused, oriented 45 O

between optic axis and bar axis a ,b

25 333 600 220

1000 567 Polycrystalline Alumina

S25OC=142500 e-‐11.83PG-‐0.60+3.33P

(18) S1200OC=73000e

-‐11.33PG-‐0.60+3.33P Polycrystalline Alumina (99.9% Al2O3 98% theoretical density, hot pressed) (2)

Crystal Size (microns) 1 – 2 10 – 15 40 – 50 Temperature OC

25 OC 447 320 233 400 OC 347 247 227 1000 OC 327 247 207 1350 OC 247 107 93

Commercial Grades of Polycrystalline Alumina Nominal % Al2O3 %99.9 (5) %99 (3) %94 (3) %85 (3)

25 OC 413 347 307 307 980 OC

153 113 80

Compressive Strength (MPa) Sapphire Polycrystalline Temperature OC 100 % Al2O3 (7) 99% Al2O3 (3) 94 % Al2O3 (3) 85 % Al2O3 (3)

25 OC 295 3733 2000 2000 1600 25 OC 3300 2840

400 OC

1420 800 OC

1233

1000 OC

853 1200 OC

473

1400 OC

237 1600 OC

47

Hardness on the Mohs Scale 9

19

Table 7 continued

Tensile Strength MPa

Temperature OC

Single Crystal Filaments Polycrystalline Orientation 45 O to optic axis

(9) Uncoated (10) Coated (10) (e) 94 % Al2O3 (3) 85 % Al2O3 (3)

30 OC 473 467 1400 251 173 117 300 OC 350 243 800 OC 350 227 1050 OC 225 1100 OC 587 209 1200 OC 123 63 57 1400 OC 28

Modulus of Elasticity (E) X 108 MPa E (polycrystalline)=59.49 X 106 e -‐3.95P, where P = fractional pore volume (8) Single Crystal (12) Polycrystalline

Temperature OC Sapphire Ruby

(0.75%Cr2O3)

(11) 94 % Al2O3 (3) 85 % Al2O3 (3)

25 OC 35.07 36.07 39.53 26.80 21.27 500 OC 32.07 33.00 38.18

1000 OC 29.00 30.07 36.59 1200 OC 27.93 29.00 35.77 Elastic constants Elastic Compliances

( X108 MPa) ( X108 MPa) (13)

48.03 2.35 48.16 2.17 14.27 6.94 15.82 -‐0.72 10.72 -‐0.36 -‐2.27 0.49

Modulus of Ridity(G) X 108 MPa Polycrsytallined Alumina

Hot-‐Pressed Cold-‐Pressed 94 % Al2O3 (3) 85 % Al2O3 (3)

Temperature OC Sapphire (13) Zero Porosity (14) Zero Porosity (14) Density 3.62 (3) Density 3.42 (3)

25 OC 15.53 (Reuss) 15.51 15.93 11.33 8.67

16.05 (Vogit)

a Loading rate 94.7 MPa/minute; b minimum creep at 45O; c zero porosity; d less than 5% porosity; e rupture time less than one minute. (1) Wacthman &Maxwell (1959) (2) Springs Mitchell & Vasilos (1964) (3) Coors Prorcelain Data Sheet 0.001, August 1964 (4) Diamonite Products Manufacturing Company (1963) (5) Frenchtown Porcelain Co., Bull. 5462 (7) Ryshewitch (1941) (9) Wachtman & Maxwell (1954) (8) Knudsen

(10) Berezhkova &Rozhanskii (11) Crandall, Chung, & Gray (1961) (12) Wachtman &Lam (1959) (13) Wahtman, Tefft, Lam & Stinchfield (1960) (14) Lang (1960) (15) Ryshewitch (1951) (16) Spriggs & Brisette (17) Kingery & Pappis (18) Passmore, Spriggs & Vasilos (1965)

20

2.4. THERMAL PROPERTIES OF ALUMINA Chemical and thermal stability, relatively good strength, thermal and electrical

insulation characteristics combined with availability in abundance have made

alumina attractive for engineering applications.

Thermal properties of alumina are listed below on the Table 8.

Table 8: Thermal properties of alumina. (WALTER , Alumina as a Ceramic Material, 1970, page 64)

Melting Point !Al2O3 2051.0 ± 9.7 ℃ (1) Boiling Point !Al2O3 3530 ℃ (3800 ± 200 OK) (2) Vapor Pressure T OK Atm

(2)

2309 8.7 X 10 -‐6 2325 1.03 X 10 -‐6 2370 1.66 X 10 -‐6 2393 1.68 X 10 -‐6 2399 2.15 X 10 -‐6 2459 3.78 X 10 -‐6 2478 5.81 X 10 -‐6 2487 9.1 X 10 -‐6 2545 2 X 10 -‐6 2565 1.29 X 10 -‐6 2605 1.91 X 10 -‐6

Heat of Formation at 298.16 OK (kcal/mole)

Entrophy at 298.16 OK (kcal/mole)

! Al2O3∙ 3H2O -‐612.8 33.51 (5,3) ! Al2O3∙ 3H2O -‐609.4

(4)

Amorphous -‐304.2

(3) ! Al2O3∙ H2O -‐417.8 23.15 (4) ! Al2O3∙ H2O

8.43 (3)

! Al2O3 -‐400.4 12.16 (7,6) AlO -‐138 48.967 (2,14) Al2O -‐248 59.75 (2,14) Enthalpy (E+PV) in kcal/mole ! Al2O3 (∆!!"#.!" °!

! ) 0.03550922 ! − 4.0884 10!! !! − 11.23206 !"!!" ! + 19.63341 (a) (9)

0.035549846 ! − 3.9085 10!! !! − 11.2306 !"!!" ! + 17.23778 (b) (16)

0.03031602 ! + 8.3979 10!! !! − 2.81406 ! 10!/ ! − 12.87764 (c) (15)

Specific Heat (cal/g K) ! Al2O3∙ 3H2O 0.2694 + 6.43 X 10!! t (d) (8)

0.2855 at 25 ℃ (9)

0.348264 – 8.019 X 10-‐6 T – 47.8423/T (d,e) (9)

21

Table 8: continued

! Al2O3 OK cal/g OK OK cal/g OK 400 0.22545 900 0.28789

(9) 500 0.24857 1000 0.2924 600 0.26372 1100 0.29595 700 0.27431 1200 0.29877 800 0.28205

1318 0.2783 1787 0.4196 (10)

1510 0.3364 1660 0.3814 2575 0.468 (11)

Thermal Expansion, Linear (X 10 -‐6 /OC) ! Al2O3 Temperature Range Single Crystal (12) Polycrystalline

OC Orientation O0 C-‐axis 90O (12) -‐273.16 to 0 1.95 1.65 1.89 -‐73 to 0 4.39 3.75 4.1 0 to 127 6.26 5.51 6.03 327 7.31 6.52 6.93 527 7.96 7.15 7.5 927 8.65 7.8 8.08 1127 8.84 7.96 8.25 1327 8.98 8.12 8.39 1527 9.08 8.2 8.49 1727 9.18 8.3 8.58

Thermal Conductivity (cal/sec cm O C)

Temperature OC cal/sec cm O C -‐263 3

(13)

-‐253 9 -‐233 14 -‐223 12 25 0.086 100 0.069 300 0.038 500 0.025 700 0.018 900 0.015 1100 0.014 1300 0.014 1500 0.013 1700 0.014 1900 0.015

22

(a) Range 400 OK to 1200 OK; (b) Range 678 OK to 1330 OK; (c) Range 1290 OK 1673 OK; (d) T= OKelvin; (e) Range of equation 400

to 1200 OK.

(1) Schneider, Nat. Bur. Stds. Private communication, Nov , 1968 (2) Brewer & Searcy (3) Rossini, Wagman, et. Al. N. B. S. Circ.

500, 1952 (4) Russel et. Al. (1955) (5) Barany & Kelley (6) Kerr, Johnson, & Hallett (7) Mah (8) Roth, Wirths, & Berendt (1942)

(9)Furukawa et. Al. (10) Shomate &Naylor

(11) Sheindlin(1964) (12) Wachtman, Scuderi, & Cleek (13)Coors Porcelain Co. (AD 995)

(14) Panyushkin &Maltsev (15) Banashek, Sokolov, Rubinchik & Fomin (16) Sokolov, Banashek, & Rubinc

2.5. CHEMICAL PROPERTIES OF ALUMINA

Chemical reactions of alumina of general ceramic interest include the resistance

to attack of sintered alumina by various reagents, particularly at high

temperatures. High temperature chemistry includes those chemical phenomena,

which occur above 1000 OC. Such temperatures are attained by combustion, by

electrical heating, or by chemical explosions and nuclear reactions (Margrave,

1962). Some example studies are listed below due to understand further

chemical behavior of alumina.

2.5.1. Wet Chemical Reactions of Sintered Alumina In aqueous solution, aluminum oxide exhibits an amphoteric behavior. It can be

expressed by the following equilibra:

!!!! + 3 !!! ↔ !"(!")! ↔ !"!!! + !!!!

Impermeable alumina has been marked resistance to wet chemical corrosion. As

a rule the lower phases of alumina and the hydrous aluminas show increasing

chemical reactivity as they decrease in density. Early experiments on the

resistance of sintered alumina to attract were intended to demonstrate its

suitability as a container, crucibles, etc., for thermal reactions (Winzer, 1932).

Concentrated H2SO4, HCl, HNO3, H3PO4, and 20% NaOH dissolved no more than

0.02% of a 30 X 35 mm crucible within six hours, as indicated by loss in weight of

the crucible. This is not necessarily indicative of chemical inertness, as for

example; phosphoric acid readily reacts even with coarsely crystalline tabular

alumina to form slowly soluble phosphate bonds at temperatures below the

23

boiling point of the acid. Dawihl and Klingler (1967) state that sintered alumina

containing 3% silicates is far more resistant to corrosion by HCl, HNO3, and

H2SO4 in concentrations from 10 to 95% acid and up to 100 OC than titanium,

cast silicon and Cr – Ni steels.

Finely divided alumina is rapidly dissolved by HF, hot concentrated H2SO4,

mixtures of these acids, ammonium fluoride, molten alkali bisulfates or

pyrosulfates, and by concentrated HCl, especially when under pressure. All these

reagents have been used to dissolve alumina in analytical procedures. Sintered

alumina dissolves in concentrated H2SO4 faster than some high alumina

porcelains containing clay binders (85% Al2O3). Karpacheva and Rozen found

that the densest sintered alumina reacts with heavy water, H218O, the following

reaction rates, as percent reaction, within 80 minutes, were observed.

Temp °C 200 400 600 900 Rate (% 80 min) 20 30 50 97

Hot water solutions of the free alkali hydroxides and carbonates cause

perceptible reaction, the rate being correspondingly faster and higher

temperatures and under pressure.

2.5.2 Reaction of Chemical Elements with Alumina

Some reactions of interest between the chemical elements and alumina are

shown in the Table 9. Reaction of molten alumina with some refractory metals

and allots are included. These are presented in alphabetical order, in general, for

convenience of reference. Detailed information is given with references in

Appendix A related to element interactions with alumina. For references see

Appendix A.

24

Table 9: Interest Between The Chemical Elements And Alumina

Aggressively attraction Molten Lithium

No Attraction

Antimony

Attraction

Potassium Arsenic Carbon Beryllium Florine Chlorine Copper Cobalt

Limited attraction

BaO Gallium SRO Lead

Bishmut Mercury Cerium Nitrogen

Chromium Phosphorus Iron Silver

Manganse Sulfur Molybdenum Selenium

Nickel Tellurium Niobium Tin Palladium Uranium Platinum

Tantalum Titanium

Zirconium Tungusten Sodium

1.8. COLLOIDAL PROPERTIES OF ALUMINA Surface of pure alumina is defined alkaline. By electrophoretic methods Fricke

and Keefer (1949) determined the isoelectric point of zero point charge (zpc) of

gamma alumina to be at 9.0 pH, that of amorphous Al(OH)3 at pH 9.4, that

gibbsite at pH 9.20, and that of bohemite at pH 9.40 to 9.45. The potential

determining ions were considered to be H+ and OH-‐, which enter into

electrochemical reaction at the surface in the case of aluminum oxide. The

essential part of the surface reaction schematically is as follows:

OHH (+)+H2O

H3O+

! "## OH OH $

! "## O($)+H2O

Positive Surface Uncharged surface Negative Surface

Isoelectric point of hydrated alumina is listed in Table 10:

25

Table 10 : isoelectric point of hydrated alumina (O’Conor 1956)

Aging Conditions Precipitation Fresh Preparation Rapid Aging by Heating Suspension Aged 4 Months Excess Alkali 5.08 6.79 5.78 Equivalent Alkali 6.63 7.28 7.06 Deficent Alkali 7.29 7.43 7.32

Zeta potential of natural corundum reported positive in water 17-‐20 OC, but it

changes to negative on heating to 1000 OC. (O’Conor 1956)

The interaction between the liquid and the surface can be estimated by contact

angle (q) measurements. The details of the interactions between the surface and

the solvents were explored by analysis of the effect the properties of the liquid

on varying the equilibrium contact angle with water (!!") with the different

anodic alumina surfaces. These values were determined from the intersection of

the fits for advancing and receding angles versus contact angle hysteresis with

ordinate at ! q=0, one finds the equilibrium angle !!" . For different samples

results vary between 62.6 -‐ 73.3. Results obtained Wihelmy Method. (Redon et.

al., 2005)

26

3. ALUMINA MEMBRANES

3.1. INTRODUCTION In recent years there has been a growing interest in utilizing inorganic

membranes to address a variety of separation problems in many industries

Various inorganic membranes made from metals, inorganic polymers and

ceramics have been proposed for liquid and gas separation applications.

Membrane technologies play an increasingly important role in pollution

prevention, resource recovery and waste treatment activities. Due in large part

to cost considerations, polymeric membranes dominate these applications,

however, the use of polymeric membranes in separations involving aggressive

materials such as many organic solvents, acids, bases and oxidants is often

limited by the tolerance of the polymeric material to extreme conditions. The

interest in utilizing such membranes in separations has increased since the

advent of consistent-‐quality, commercially available ceramic membranes with

narrow pore size distributions.

Ceramic membranes are noted for their excellent mechanical strength and

tolerance to solvents, as well as pH, oxidation, and temperature extremes. For

example, they can be used at significantly higher temperatures, have better

structural stability without the problems of swelling or compaction, generally

can withstand more harsh chemical environments, are not subjected to

microbiological attack, and can be backflushed, steam sterilized or autoclaved.

An ideal ceramic membrane must be highly selective, permeable and durable.

For aqueous applications, or aqueous/organic separations it is desirable for the

ceramic to be hydrophilic to maximize flow and minimize fouling. The membrane

selectivity is primarily dependent upon the pore size distribution; the narrower

the pores size distribution, the more selective the membrane. Mechanical

integrity is, enhanced in such applications by slip-‐casting a relatively thin

selective membrane onto a thicker, more permeable support yielding an

asymmetric membrane.

27

The former forms of dense (or nonporous) membranes of palladium or its alloys,

silver and zirconia have shown to be permeable only to certain gases (e.g.,

hydrogen and oxygen). These materials are used in sensors, electrodes and

coatings. Their industrial use as a separation tool, however, is limited primarily

by low permeability compared to microporous metal or ceramic membranes.

Currently, microporous stainless steel, silver and ceramic membranes such as

alumina, zirconia and glass are available commercially. In Table 11 the selected

commercial alumina membranes are listed. Data collected from web based

information. Among the ceramic membranes, alumina is gaining acceptance for

liquid phase separations. Like zirconia membranes, alumina membranes for

liquid phase separations came to commercial fruition from uranium isotope

enrichment work. (Hsieh, Bhave, and Fleming, 1988)

Table 11 : Selected commercial Alumina Membranes

Manufacturer Trade Name Application Membrane Material

Support Material

Membrane pore

diameter

Membrane support

configuration

Alcoa / SCT Membralox UF ! − !"!! ! Al2O3 40-‐100 ! Tube and multichannel element

MF ! Al2O3 ! Al2O3 0.2-‐5 µ m

Norton Ceraflo MF ! Al2O3 ! Al2O3 0.2-‐1µ m Tube

NGK MF Al2O3 Al2O3 0.2-‐5µ m Tube and Plate

Alcan/Anotec Anopore UF Al2O3 Al2O3 250 ! Disk

MF Al2O3 Al2O3 0.2µ m

Alumina membranes are constantly growing area. In the Figure 3, it can be

seen that, the publication numbers are highly increasing parallel with the

membrane research especially during recent years.

This data is collected from sciencedirect.com.

28

Figure 3 : Number of publications on membranes and alumina membranes based on years.

The main reasons why alumina is still trending research topic can be explained

by following reasons. During the last several decades, many investigations have

been focused on porous anodic alumina (PAA) due to its benefits of tunable

nanopore diameter and long-‐range ordered feature of the porous nanochannels

in macroscopic domain. The pore diameter of the PAA nanochannel can precisely

be controlled from a few nanometers to several hundreds of nanometers by

applying regarding electrolyte, voltage (or current), and reaction temperature

during the electrochemical anodization reaction of aluminum substrate.

Moreover, reaction time provides a tunability in the thickness of the porous

nanochannels from a hundred of nanometers to a hundred of micrometers. Such

easy control ability of the pore diameter and the thickness makes the PAA one of

the interesting materials, which are frequently being applied in nanoscience. So

In the first figure publications

on alumina membranes is

evaluated.

Second figure evaluates the

trend of membrane

publications during years.

It can be concluded that;

trends of alumina membranes

Second figure evaluates the

trend of membrane

publications during years.

It can be concluded that;

trends of alumina membranes

are rising correlated with

general trends with

membranes. Alumina

membranes are promising

and developing area in

membrane society.

29

far, the studies to utilize the PAA have been performed in a wide range of

research fields such as nanomaterial design, molecular sieving, photonic and

optical device, and catalysis.

3.1 PREPERATION OF ALUMINA MEMBRANES 3.2.1. Macroporous Alumina Membrane Preparation

Porous ceramic membranes with pore size ranging from 0.1 to 50 mm and

porosity above about 40% are used for filtration (e.g., hot gas filtration),

diffusion (e.g., waste water treatment), dispersion rolls, inkpads for

fingerprinting and numerous other applications. In particular, filtration is

important for the petrochemical, mining and chemical industries.

The anodizing of pure aluminum in various acidic electrolytes has attracted

considerable attention in recent years since it opens up the possibility of

preparing films with well-‐controlled, uniform pores from tens to several

hundreds of nanometers in diameter. The basic cell structure containing

cylindrical pores has long been known and methods for preparing regular pore

arrangements in a hexagonal pattern with interpore distances of between 50 nm

and 500 nm have been reported. The preparation of regular pore arrays typically

involves electrolytic polishing and multiple anodising steps or even mechanical

pre-‐texturing. As-‐prepared porous anodic alumina (PAA) membranes are

amorphous to X-‐ray diffraction (XRD). Their chemical composition is not

stoichiometric Al2O3 but incorporates a considerable quantity of anion impurities

and hydroxyl groups incorporated from the electrolyte into the alumina

structure or bound to the alumina surface. (Kirchner, et. al)

PAA has proved to be useful for fabricating many materials, especially as a

template for the synthesis of metallic or semiconductor nanometer -‐scaled wires

and particles. After chemical functionalizing, PAA membranes can be utilized for

catalytic or optical purposes. Further, the porous film itself may be employed for

filtration, gas separation or as a photonic crystal. Because it is a ceramic oxide,

30

PAA has considerable potential in high-‐temperature applications, although

severe problems can occur when certain types of PAA membranes are heated.

Macroporous alumina membranes also can be made from particles or

discontinuous fibers by the use of a binder or by sintering. The sintering of

ceramic particles is perhaps the simplest approach to forming a porous ceramic

filter, however, the sintering of bulk ceramics is a very energy expensive process

due to the high temperatures required. The pore size is controlled by the starting

particle size, sintering time and temperature. This method is generally used to

produce alumina microfiltration filters, which contain larger pores and supports

for ultrafiltration membranes, which contain smaller pores. (Avci et. al. )

Binders are most commonly in the form of fine particle dispersions (colloid). The

silica colloid is an example. After application by wet forming (as in paper

making), drying and appropriate heat treatment is needed. The binder

technology is central to the technology of membrane fabrication. A binder is

necessary to hold the ceramic particles or fibers together to form a membrane. It

must be used in a sufficient quantity in order for the membrane to have

acceptable mechanical strength. However, it must not block the pores in the

membrane, as is the case if it is used excessively. Thus, an effective binder should

be able to bind the particles or fibers together and result in a mechanically

strong membrane, even when it is used in a very small proportion. In addition,

the binder must be able to withstand high temperatures, as encountered in hot

gas filtration.

Silica and vitreous glass are widely used binders in the refractory and ceramic

industry. It is often used in the form of an aqueous dispersion. The silica colloid is

particularly attractive due to the high temperature resistance of silica compared to

vitreous glass and the good binder dispersion enabled by the small particle size of the

silica in the colloid. The average size of the silica particles in the colloid can range

from under 10 nm to over 80 nm. The silica binder is easy to use, but it tends to fill

the open or continuous porosity of the filter membrane.

31

Phosphate has been utilized as a binder in the refractory industry for many years.

Pirogov et al. used a phosphoric acid (H3PO4) binder in a mullite-corundum body in

their study to determine the optimum content of graphite and SiC additives. Birchall

et al. studied the mechanical properties of an unsintered SiC compact bonded by

aluminum phosphate (AlPO4) glass. Toy and Whittemore evaluated the reactivities of

several calcined aluminas with phosphoric acid and demonstrated that a glassy AlPO4

phase and aluminum metaphosphate (Al(PO3)3) are effective bonding phases.

3.2.2.Mesoporous Alumina Membranes

Of the present technologies, sol–gel is the best method for making ceramic

ultrafiltration membranes. However, the pore size is generally limited to the

sizes of the ceramic precursor particles prior to sintering. For sol–gels, the

particle size distribution is difficult to control, and they must be used

immediately after preparation to avoid aggregation or precipitation. (Johns et. al)

Commercially available membranes are currently ! -‐Al2O3 membranes are

currently thermally stabilized at 650 OC, with a mean pore size equal to 5 nm.

Such membranes can be used for Nanofiltration of aqueous solutions containing

inorganic salts or amino acids. They can also be used for gas permeation

applications. However these membranes present a rather low chemical stability

in aqueous media at very high pH values. More over, their structural evolution

has to be taken into account for high temperature applications. (Ayral)

Sol-‐gel process can be applied generally;

• Solution must be prepared An example of Boehmite (! -‐AlOOH) sol from

an inorganic precursor was prepared as in shown figure 4.

• Pretreatment alumina supports (synthesized or provided). Heating, or

solution applications.

• Membrane solution contacts with support.

• Penetration of the solution into the voids of the support occurs by

capillary action.

• The support was then shaken to remove any excess solution, and dried at

room temperature.

32

• The coated support was heated up to 600-‐1000 OC step by step and held

remained in temperature range for stabilization.

• Multiple coatings were obtained by treating a previously coated filter.

It was reported the unsupported membrane was obtained by dying the sol layer:

!"#$% ! − !"##$ !"# !"#$%&

!ℎ!" ! − !"##$ !"# !"#$% !"#$%&

!ℎ!" ! − !"##$ !"# !"#$% !"#$%&

!ℎ!" !"#$! ! − !"##$ !"#$%"& !"#$% !!"#℃! !!"#$%!"&

! − !!!!! !"#!$$%&'() !"!#$%&"

the sol layer was formed by dipping the layer was dried at room temperature,

the drying step removes excess water, cracks formed easily during drying step.

Fig.4. Preparation procedure of boehmite sol

In order to obtain the Al2O3 membrane without crack, a method, rapid gelation

processing for preparation of gel layer can be used. The sols of ! − !"##$ are

atomized firstly then sprayed onto a substrate. The gel layer was obtained from

sols directly. No crack was observed.

33

Figure 5 is the schematic diagram of the rapid gelation processing. The sols are

atomizing by a high-‐velocity gas, and sprayed into the substrate (glass or

ceramics plate coated with cellulose acetate film. After drying, the resulting gel is

placed in acetone to dissolve the cellulose acetate and to obtain the unsupported

gel film). The 0.1-‐ 200 µm of thickness of the membrane can be obtained

without cracking at conventional drying rate. When the thickness of membrane

is higher than 200 ~ m, cracking can be prevented at controlled drying rate.

The transparent boehmite gel membranes obtained by rapid gelation were fired

at a temperature (>350°C) to form ! -‐alumina membrane.

Figure 5: Schematic drawing of the rapid gelation processing, 1 -‐ nozzle, 2 -‐ atomizing sol and 3 -‐substrate.

Mesoporous γ-‐alumina membranes are formed by dip-‐coating a porous substrate

in a Boehmite (γ-‐AlOOH) precursor sol, will be treated by heat and sintering

steps. The quality and properties of the membrane depend on the dispersion

rheology and quality of the Boehmite sol and the dip-‐coating process as such. A

high quality Boehmite sol is prepared by hydrolysis and condensation of an

aluminum alkoxide: ATSB ([C4H9O]3Al), followed by one or more purification

µ

34

steps. Five steps in total are necessary for a single layer mesoporous γ-‐alumina

membrane preparation:

1. Boehmite sol preparation

2. Boehmite sol purification

3. Dip-‐coating sol preparation

4. Dip-‐coating procedure

5. Heat treatment/sintering

In the Figure 6 an example of dip coating process is shown.

Figure 6: Filter ring assembly for mesh filtering: top filter ring (left), bottom filter ring (middle) and assembly diagram with nylon mesh (right). The thickness of the top filter clamp ring can be increased to increase the filtration volume. Currently it can hold ~10 ml.

In laboratory scale experiments to the synthesis of mesoporous alumina

membranes using slip-‐casting technique has been tried. Catalytic membranes

with precise active layer width and location were prepared by sequential slip

casting of Pt/Al and alumina sols. Different distributions of catalyst within the

membrane can be easily obtained by varying the thickness of the slip-‐cast layers

as well as their arrangement. The layer widths are controlled by the slip-‐casting

parameters, including slip-‐casting time, alumina content and particle size. The

catalyst loading and dispersion of the active layer can be controlled precisely.

The effects of sintering temperature (600-‐1200°C) on the membrane's pore

35

structure, morphology, phase structure, surface area and gas permeability are

observed. ( Yeung et. al; 1996)

3.2.3 Microporous Alumina Membranes

In the slipcasting method, a porous support is usually made first by conventional

ceramic processing techniques to provide rigid structure with relatively large

pore size for slip deposition. Since particle size directly related with pore size,

the slip used, as the membrane precursor needs to contain well-‐dispersed

particles of uniform size. Depending on the desired pore size of the membrane,

the membrane precursor particles may be prepared by precipitation,

classification, sol-‐gel method, etc. In the sol-‐gel techniques, ultrafine particles of

a few nm in diameter can be prepared by polycondensation or redox reactions of

aluminum salts or hydrolysis and condensation of aluminum alkoxides.

After treatment with a peptizing agent such as an acid and optionally with a

viscosity modifier, the slip is deposited on the porous support by the dipping or

slipcasting procedure. This procedure for filtration based on capillary pressure

drop created by the contact of the slip with the support. (Leenaars, Burggraaf

1985) This pressure drop forces the dispersion medium (e.g water to flow into

the dry pores of the support while the slip particles are retained and

concentrated at the surface forming a thin membrane. The membrane precursor

is then dried and calcined to provide the required pore size for specific

applications and the needed bonding between the membrane and the support.

These steps must be done with a great sensibility to avoid cracks, which can

occur due to shrinkage upon drying and/or calcining. (Hsieh, Bhave, and

Fleming, 1988)

The slipcasting method is commonly used for making commercial alumina

membranes. Method can be replied to organize intermediate layers between

thin, permeable, selective membrane and the thick, porous support, which

provides the needed mechanical strength. T Thus, alumina membranes, like

many other porous inorganic membranes, are composite in nature. The cross-‐

36

section of a multi-‐layered alumina membrane composite can be exemplified by

the scanning electron micrograph of Figure 7. In general, the selective membrane

layer has a thickness of 2-‐10 !m. This thickness is a trade-‐off between high flux

and mechanical stability. The intermediate layers are generally 10-‐20, !m thick.

The bulk support constitutes the majority of the pore volume and thickness. If

the membrane precursor particles are so fine that they penetrate the relatively

large pores in the support, the permeability of the membrane/sup port

composite will decrease due to partial or complete blockage of the pores in the

support. To prevent fine particles from penetrating into the support, one or more

intermediate layers of graded particle size are used.

Figure 7: Cross-‐sectional scanning electron micrograph of a three-‐layered alumina membrane/support composite (pore diameter of 0.2, 0.8 and 10 μm, respectively, in each layer)

The ability to consistently produce high quality alumina membranes on a

commercial scale has been the key to wider acceptance of ceramic membranes as

a separation tool. The surface morphology of a high quality alumina membrane,

shown in Figure 8, displays uniform particle size.

37

Figure 8: Top-‐view scanning electron micrograph of an alumina membrane (pore diameter of 0.2 !m)

Typically, membranes and supports have a porosity of 30-‐60% by volume. This

range provides a good compromise between permeability and strength.

Membrane science has a leading role in innovative processes and is considered

one of the main strategic axes of research activities in all developed countries.

Advanced technology programs in the USA or Japan involve them, and with an

annual growth rate of 10 to 20 %, and a total world market above 10 billion

Euros around 2010, membranes are likely to become more and more important

in the future. Besides flat membranes as motivated by literature, anisotropic

membranes consist of an extremely thin surface layer supported on a much

thicker, porous substructure.

The surface layer and its substructure may be formed in a single operation or

separately. Multi layer membranes and their production will be motivated in

next chapter.

38

4.DESIGN OF THE MEMBRANE MODULES

The separation science and technology using all the kinds of different

membranes is highly influenced by the membrane module configuration as far as

the efficiency of a membrane process depends on the design of its module. The

cost reduction of membrane module has led to the commercialization of

membrane process some decades before depends on the sort of application and

the different materials used. In practice, the commercial available membrane

modules are assembled in units consisting usually of several membranes,

sometimes from many thousands of them.

4.1 DIFFERENT TYPES OF MODULES

Currently, four main types of membranes modules are in the market: (a) planar,

(b) spiral, (c) tubular (d) hollow fibers modules and (e) honeycomb. The

modules are closely related to the geometry of the membranes and therefore of

the materials. Each commercial module is consisted of a number of membranes,

placed in different ways. The structure of membrane can be (a) symmetric, (b)

anisotropic and (c) composite and according to the sort of use, also two main

categories can be distinguished: (a) vibrating membrane and (b) submersible

membranes. Besides, in terms of geometry, two types of different geometry can

exist: (a) planar and (b) tubular [Remigy, 2007]. From these basic configurations,

many other secondary modes can also occur such as vibrating hollow fiber

modules and many others.

In Table 12, main parameters of membrane modules are gathered. The Table 1 is

presented in “Filtration Membranaire” published by CNRS in 2007 and it is only

representative for actual membrane technology trends. All the information

concerning membrane technology needs frequent updating because membrane

engineering is a state-‐of-‐the-‐art research and market field.

39

Table 12 Comparison of performances for different modules and membranes [Remigy, 2007]

Different perfomance of membranes modules

Geometry Planar Spiral Tubular Hollow

fibers

Cost of inventissement

(U$$/m2, 2000)

50 to 200 5 to 100 50 to 200 5 to 20

Energy cost Moyen Medium Important Low

Hydraulic diameter 1 to 5 0,8 to 1,2 12 to 20 0,1 to 1

Comptability 100 to 400 300 to 1000 10 to 300 1000 to

15000

Membranes replacement Membrane by

membrane

Entire

module

Tube by

tube

Entire

module

Pretreatment Moyen Medium Low Medium

(entire

filtration

internal/exte

rnal)

Medium to

low

(external/int

ernal)

Cleaning Good Difficult to

medium

Excellent Medium

Material All the materials Polymers All the

materials

Polymer

4.1.1 Alumina membrane modules

Alumina membranes characterized as inorganic, ceramic membranes are usually

fabricated in tubular or less often planar module, composite or anisotropic, in

some cases immerged, monolith, disk mode. On the other side, vibrant or

immerged modules in hollow fibers and planar or spiral geometry are commonly

polymer (organic) membranes modes. The most recent trend appeared in

alumina membrane market is honeycomb module which combines some of the

most important advantages of previous modes.

40

Planar module: In general, membranes of planar module are less met in current

market trends and represent either older membrane systems either pilot or

patent scale applications. They are based on conventional filter press design and

membrane feed spacers and product spacers are layered together between two

end plates. The most common uses of plate and frame modules concerns

electrochemical applications as ion exchange, electrodialysis, pervaporation

systems, electrosensors and membrane electrode assembly. These applications

concern all the different kinds of membranes as metal or polymer membranes.

Figure 9 a planar aluminium module is given as a patent for an improved proton

exchange membrane. The membrane could be used in membrane module unit as

electrochemical cells having internal passages parallel to the membrane surface.

The passages in the membrane extend from one edge to another and allow fluid

flow through the membrane and give access directly to the membrane. The

invention, awarded by NASA in 1997, is related to applications in as electrode

assemblies for fuel cells, electrolyzes, electrochemical hydrogen and oxygen

pumps, and related devices [Gonzalez-‐Martin et al., 1997].

Figure 9 Membrane electrode assembly containing internal passages [Gonzalez-‐Martin et al., 1997]

Tubular module: Alumina membranes most of the times have tubular mode

which is composed of a number of tubular membranes with an internal relatively

41

small diameter. They may be unique tubes assembled in a module using joints or

monolithic composed of several tubes (Figure 10 and 11). The selective part is

located inside the tube or tubes. Feed is made at one end of the module and the

fluid flows inside the tubes or, the retentate springs at the other end. The

permeate pass through the body of the tube or out of the monolith to the

external part.

Figure 10 Schematic side-‐view of membrane module consisting of multi-‐channel elements

Figure 11 Cross-‐section of a monolithic multi-‐channel membrane element [Hsieh et al., 1998]

In general, the main advantages and disadvantages of tubular noticed in the

literature are because of ceramic material properties, the most common

substance for which they are made of.

(a) Advantages:

42

• No pre-‐treatment, accepts slurries in large particles (about one-‐tenth the

diameter of the tube)

• Easy to clean, use of mechanical cleaning

(b) Disadvantages:

• Very low compactness of the order of 10 to 300 m2/m3

• Relatively high investment

• Energy costs can be significant, due to a high rate of circulation

• Fragility in the case of ceramic membranes

As an example for diameter difference, in laboratory scale of Stoitsas et al.

(2005) research work, alumina and silica membrane specimens for gas

separation are of tubular geometry with a length 340∙103m, an internal diameter

of 8·103m and an external diameter of 14·103m when the membrane surface per

specimen is 8.5·103m2. In this case, the ceramic membrane is an asymmetric 4-‐

layer system. The first layer operates as a support, the third as a microfiltration

layer (pore sizes of 100 or 200nm depending on the firing temperature) and the

fourth as an ultrafiltration layer with a pore size of 3–5nm. The second layer

(pore size of 500nm) serves to bridge the gap between the macroporous support

and the microfiltration layer.

Honeycomb: Honeycomb pore size membrane is part of the new generation

membranes for microfiltration and ultrafiltration applications. It is available for

ceramic membranes either of alumina or carbon supports. The term honeycomb

concerns the microstructure of porous in porous layer and because of this

geometry, the membrane is suitable for special applications.

4.1.2. Commercialized modules of membrane alumina

Tubular mode

The figure below (Figure 12) is from Veolia Water Solutions & Technologies

(VWS) manual for new ceramic membrane modules.

43

Figure 12 Large diameter monolith membrane element with permeate conduits

The industry utilizing the CeraMem® technology provides a variety of inorganic

microfiltration (MF) and ultrafiltration (UF) membranes. In CeraMem®

technology, typical multi-‐channel ceramic membrane has multiple parallel

passageways that run from a feed inlet end face to an opposing outlet end face.

The surfaces of the passageways are coated with permselective membrane. A

feed stream is introduced under pressure at the inlet end face, flows through the

passageways over the membrane, and is withdrawn at the downstream end face

as retentate. Permeate flux passes through the membrane flows into the porous

monolith material. Under an applied pressure, the combined permeate from all

the passageways flows through the porous monolith support to the periphery of

the monolith and is removed at the monolith exterior surface

[veoliawaterst.com/ceramem].

Honeycomb module

The Anopore™ inorganic membrane (Anodisc™) in honeycomb module is a novel

material with precise, no deformable honeycomb pore structure (Figure 13) with

no lateral crossovers between individual pores that filters at precisely the stated

cut-‐off, allowing no larger sized particles to pass through the membrane.

44

Figure 13 AnoporeTM alumina membrane with honeycomb pore size distribution [whatman.com]

The AnoporeTM inorganic membrane is composed of a high purity alumina matrix

that is manufactured electrochemically [whatman.com]. This kind of membrane

shows a high thermal permeability and selectivity and it can be applied for a

wide range of special applications as pharmaceutical or laboratory ultrafiltration

process.

Generally, commercial alumina membranes have an asymmetric structure with

membrane deposition on the inside surface of a tube (or channel). The diameter

of module depends on the kind of application. Large scale units can be used eg. in

wastewater treatment plants and small scale units can be used eg. in gas

separation. A typical industrial installation will have several of these modules

arranged in series and/or parallel configuration [Sondhi et al., 2007].

45

4.2. SEPARATION CHARACTERISTICS FOR ALUMINA MEMBRANES

For future looking through membrane uses it is important to summarize the

most crucial separation characteristics as “crossflow mode”, “flux “ or “rejection”

properties.

Crossflow

Crossflow filtration is a pressure-‐driven separation process in which a stream

flows parallel to the filter surface. For microfiltration applications such a parallel

flow creates shear forces and/or turbulence to sweep away particulates and

prevent blinding of the filter surface. The feed flows past the inside surface of the

tube (or channel) and the permeate (filtrate) flows through the porous support

to the shell side of the module. This is generally true for all crossflow membrane

modules such as tubular, hollow fiber, spiral wound, or plate and frame modules.

Inorganic membranes used in commercial large-‐volume applications are,

however, available only in single tubes or multi-‐channel elements as described

earlier. Such membrane-‐based separation devices have to satisfy certain

performance cri-‐membranes used in commercial large-‐volume applications are,

however, available only in single tubes or multi-‐channel elements as described

earlier. Such membrane-‐based separation devices have to satisfy certain

performance criteria to justify a cost-‐effective process as flux or filtration rate,

long term flux stability and separation or rejection properties of the membrane.

Flux

This is one of the most important performance criteria for cost effective

membrane technology, particularly for large-‐scale separations. In membrane

separation processes the (permeate) flux may be influenced by such factors as

crossflow velocity, transmembrane pressure differential, temperature and feed

characteristics.

46

Crossflow velocity

This is the average rate at which the process fluid flows parallel to and across the

membrane surface. Due to continuous removal of filtrate (permeate) through the

membrane, the crossflow velocity is somewhat higher at the inlet than at the

outlet of the membrane element. For many particulate filtration applications (e

.g., MF) it is generally recommended to be in the velocity range of 1-‐5 m/sec

depending on process stream properties such as viscosity, particulate loading,

etc ., and on constraints imposed by pressure drop limitation In MF with an

alumina membrane, for instance, a crossflow velocity in the range 3-‐5 m/sec is

recommended. An increase in crossflow velocity generally results in an increase

in flux, since at higher shear rates the removal of particulates at the membrane

surface is more effective. A higher crossflow velocity, however, results in a

substantially higher tube side pressure drop which may pose a problem to the

membrane material. For most polymeric hollow fibers, the allowable

transmembrane pressure difference is limited to about 30 psig, since the typical

fiber burst pressure is only about 50-‐60 psig. In contrast, alumina membranes do

not suffer from this limitation and usually have a pressure rating of 250 psig or

higher. In Figure 14, a simplified circuit of crossflow membrane is designed, with

feed and retentate and pressure application points illustrated.

Figure 14: Simplified schematic of crossflow membrane filtration [Hsieh et al., 1998]

47

Feed

It has been found that the flux behavior for pure water (e.g., filtered tap or

deionized water) through a single tube is similar to that through a multi-‐channel

alumina membrane element (such as Membralox®). However, for scale-‐up

studies, alumina membranes with tube bundles or multi-‐ channel monolithic

elements should be used over single tubes to obtain hydrodynamic and fluid

management data.

Flux stability

Critical factor determining process and economic viability of separation

applications with membranes is also flux stability. In MF and OF applications,

flux decay can be a serious problem. Flux decay is usually a direct result of an

increase in the hydraulic resistance of the membrane due to fouling, eg. excessive

accumulation of debris/particulates at the membrane surface or in the pores.

The phenomenon of membrane fouling is only partially understood. In general,

fouling is more severe with symmetric membranes. The majority of alumina

membranes have an asymmetric pore structure with a pore size variation across

the entire thickness of membrane/support composite. Such asymmetric

membranes are thus expected to be less susceptible to internal pore fouling.

Many liquid streams contain extraneous matter in the form of small particles,

macromolecules, etc. During the filtration operation, these substances are often

concentrated near the membrane surface and can precipitate or agglomerate on

the surface as well as in the pores. Flux decay was observed in the filtration of

tap water with a single-‐tube alumina membrane when the backflushing

technique for cleaning was not employed. This may be due to membrane fouling

by trace quantities of particulate and colloidal matter, since prefiltered water

showed a higher flux in comparison to unfiltered tap water. A stable flux for

extended operation can be obtained if flux decay due to fouling can be controlled.

48

Rejection characteristics

One of the important parameters determining membrane rejection

characteristics is the mean pore diameter of MF and OF membranes having

narrow pore size distributions. In general, the smaller the pore diameter (i.e., the

tighter the membrane), the higher will be the rejection coefficient to

particulates/ solutes of greater nominal size. However, as pore size decreases,

permeability to liquid also decreases rapidly.

49

5. APPLICATIONS

5.1. CERAMIC MEMBRANES There are a variety of industrially important liquid or gas phase systems where

the general membrane technology has been successfully applied for separation

purposes. Such membrane processes as MF, UF, and reverse osmosis (RO) using

polymeric membranes have been in commercial practice for over two decades.

However, in recent years ceramic membranes are also being considered for

crossflow MF and OF applications and also gas separation application [Laitinen,

2004].

Ceramic membranes are usually composed by metal oxides as aluminum,

titanium or zirconium oxide as they are or mixed. They are formed from

different layers of particular length and the support is consisted of porous α-‐

alumina in tubes unit. The active layer is consisted from alumina (Al2O3, α or γ),

from zirconium (ZrO2) or titanium oxide (TiO2). The main advantages or ceramic

membranes can be listed as:

• sufficient resistance at high temperature ( to 300 ˚C)

• chemical resistance

• pH range from 0 to 14

• compatibility of organic solvents and ionizing radiation

• support of sterilization

Organic membranes are hardly resistant to solvents eg. polymeric membranes

including PTFE used in wastewater treatment process and on the other side

inorganic membranes are more readily used for such kind of application [Remigy

et al., 2007]. In applications of industrial wastewater treatment as solvent

recycling from polluted solution, conventional reclamation technologies, such as

distillation and standard filtration, suffer significant limitations in terms of

technical viability, cost, and user friendliness when ceramic membrane

technology performs important advantages, as listed in Table 13 [Ciora et al.,

2003].

50

Table 13 Advantages of ceramic membrane technology for the recovery of spent high flash solvents [Ciora et al., 2003]

M&P Ceramic Membrane Technology

Advantage Comments

1. Excellent solvent

resistance

Can be used to treat entire range of high flash solvents

2. Excellent recoverd

product quality

Finished product quality similar to virgin material

3. Low temperature

operation

No thermal degradation of solvent

4. Good product recovery

ratios

>90% solvent recovery can be aschieved

5. No additional waste

disposal problem

Waste volume necessary for disposal is <10% of original volume

6. Low tech Technology is easily implemented. No special training required.

Minimal maintenance etc.

7. Implemented on small

scale

Most high flash solvent waste is highly segmented with numerous

small-‐scale generators of waste solvent

At the same moment, considerable disadvantages often limit ceramic

membranes in pilot or small scale applications and render polymeric membranes

as the most widely applied membranes. Ceramic membranes are related to high

cost of preparation, fragility and high cost of installation and as also limited

variety of modules (only tubular and more rarely planar). In addition, some

ceramic active layers are sensitive to basic attacks when they are deemed to be

generally very resistant to alkaline pH [Remigy et al., 2007].

Inorganic membranes can be classified according to the principal component

they consist of apart from other layers or additives to carbon, metal and

composite membranes [Remigy et al., 2007].

Analytically, the most important applications of these categories are gathered

below:

51

• Carbon

If the membranes are absolutely consisted of carbon they are used for gas

separation. For the filtration field, carbon membranes comprise a carbon porous

layer on which a metal oxide layer is deposited (zirconium or alumina).

• Metals

A wide variety of metals can be used for the manufacture of filtration

membranes. Two metals, however, stand out: steel (including stainless steel) and

aluminum. The stainless steel membranes are composed of sintered particles

while those of aluminum are formed of a film of anodized aluminum. The metal

membranes are exclusively microfiltration membranes or high UF with a pore

size of not less than 20 nm.

• Combination of different materials

In Table 13, the most important advantages and drawbacks are revised for the

different kinds of materials. Note that the comparisons in terms of thermal and

chemical resistance ignore the other materials in the manufacture of modules

(seals, adhesives).

52

5.2. ALUMINA MEMBRANES APPLICATIONS In the recent years of membrane technology development, alumina ceramic or

composite membranes are of great interest for a wide range of applications and

new structures compositions. They offer a largely independent control of pore

size, pore topology, pore orientation, molecular functionalization and pore wall

composition.

A developing research field focuses on nanosize of porous alumina membranes

creating a potential for molecular separators, materials for the inclusion of

conducting or semiconducting nanostructures. The use of porous ceramic

membranes often offers a solution for aggressive environments as corrosive

solutions or high temperature process [Levanen et al., 2004].

The following sections will give more information on applications or possible

application areas of alumina ceramic membranes divided in two main categories:

1. Liquid phase separation

2. Gas phase separation

Remarkable is that in the analysis of applications below, alumina is one the

components of membranes. In many cases composite membranes are studied

with many different layers and at least one alumina coat.

5.2.1. Liquid phase separation applications Most of the commercial liquid phase applications for ceramic membranes are

either for microfiltration or ultrafiltration. The first large-‐scale commercial

success of ceramic membranes has been in the food and beverage industries.

However, significant applications are found also in other areas, such as

biotechnology, pharmaceutical, petrochemical, and other process industries as

well as in environmental control. In recent years also ceramic nanofiltration

membranes have been developed. Microfiltration is mostly applied in cases

53

where liquid streams contain particulates, and ultrafiltration is used when

smaller molecules are removed.

Some of the important liquid phase applications where alumina membranes are

employed are: treatment of industrial and municipal water, sterilization of

liquids in the pharmaceutical industry, clarification and sterilization of

beverages, e .g. wine, beer and fruit juices, cell harvesting and sterilization,

cheese-‐making by ultrafiltration and wastewater treatment. A number of studies

on crossflow filtration can be found in the recent literature [Laitinen et al., 2004].

Although many of them deal specifically with polymeric membranes, the

principles of crossflow filtration, in general, can also be applied to membranes

made of inorganic materials such as alumina. In all these application areas the

ceramic membranes have to compete with the polymeric membranes which are

becoming more and more stable and which have an advantage of much lower

prices. The use of ceramic membranes can only be justified in cases, where they

give much better performance, or in cases, where there are no suitable polymeric

membranes available [Hsieh et al., 1998].

In the following part, the applications of alumina membranes in gas phase

separation process are gathered in Table 14.

i. Food processing

Generally, the major advantages of ceramic membranes in the area of food and

beverages are their resistance to alkaline cleaning solutions, their stability in

steam sterilization, better capability to withstand higher operating pressures,

and a consistent pore size. In crossflow microfiltration applications, the major

advantage of the ceramic membranes has been the possibility to use uniform

transmembrane pressure, which for example in dairy industry has enabled high

fluxes and low fouling. The market share of inorganic membranes in the food and

dairy industry was about 10% at 1990s and tomorrow this percentage is

54

Table 14 Liquid phase separation process of alumina membranes and composite alumina membranes

Porous alumina membrane appications

Liquid phase separation applications

Food processing

Clarification of natural fruit juices fruit juice industry (orange, apple, grape etc.)

Filtration of sugar cane juice sugar concenration

Alcoholic beverages breweries, wineries

Environmental applications

Petrochemical industry wastewater treatment petrochemical industry, oil recovery

Pulp and paper industry wastwater treatment water reuse in production line

Municipal wastewater treatment municipal wastewater treatment plants, water

reuse

Activated sludge treatment depollution of wastewater sludge, compost

Nuclear industry wastewater treatment

Fluoride removal municipal wastewater, chemical industry

wastewater

Arsenic removal from water resources depollution of water sources and groundwater

Arsenic and fluoride removal depollution of water sources and groundwater

Chromium and arsenic removal metal industry wastewater, valuable metals

recovery

Biotechnology and Pharmaceutical

applications

Fermentation broths recovery of valuable antibiotics

Fungal cells separation diafiltrationm polysaccharide production

Penicillin recovery

Lysozyme ultrafiltration recovery of a water-‐soluble

supposed to be doubled [Bhave et al., 1991]. The main applications can be

roughly divided into (a) concentration of soluble molecules and suspended

solids and (b) clarification by removing suspended solids. As already mentioned,

the main usage of inorganic membranes is in the dairy industry, in which the key

to success has been the invention of transmembrane pressure mode. Some

studies have also been made concerning the usage of inorganic membranes for

the concentration of other vegetable and animal proteins as well as for the

processing of starch and sugar.

55

(a) Clarification of natural fruit juices

Clarification or fruit juice such as apple, cranberry and grape is one of the most

successful and widely practiced industrial applications of ceramic membranes. Ceramic

membrane filtration provides a particularly attractive alternative, replacing such

conventional treatments as gelatin addition, holding/decanting, diatomaceous earth,

cake filtration, and polishing, with a single unit operation. Membranes produce superior

clarity juice and deliver higher yields compared with conventional clarification

processes.

(b) Filtration of sugar cane juice

In the filtration of sugar cane juice, ceramic membranes as carbon/carbon or

alumina/alumina [Jiratananon et al., 1997] can be used in several different stages in the

raw and refined sugar production. One interesting opportunity is in the

microfiltration/ultrafiltration of clarified juice (7–14 Brix) and/or pre-‐evaporated juice

(20-‐25 Brix) as a pretreatment prior to ion exchange or chromatographic separations.

Pretreated and filtered juice is softened, evaporated and purified using ion exchange

and chromatographic processes leading to a better quality refined sugar. Typical

operating conditions include feed temperatures of 90–100oC, a high crossflow velocity

(4–7 m/s) and transmembrane pressures up to 5bar. The need to purchase, use and

dispose of filter aids is eliminated. In some applications, the cost of equipment necessary

to dewater filter-‐aid sludge may be comparable to the cost of the membrane system.

[Sondhi et al., 2005]

(c) Alcoholic beverages

In the area of alcoholic beverages, alumina membranes are used for the

clarification and sterilization of the final products. Composite multilayer ceramic

membranes membranes applications exist for the filtration of wine and beer

[Gillot et al., 1986] as well as for sake and vinegar [Hagasewa et al., 1991]. By

membrane filtration the amount of waste generated and chemicals used during

the production can be considerably decreased. The use of membrane filtration

instead of a conventional process is advantageous also since membrane filtration

produces a sterile product and further sterilization is not necessary. For

Membralox® membranes these applications have existed since 1988 [Gillot et

al., 1990]. In breweries as well as in wineries membrane filtration can be used to

56

replace clarification, stabilization and sterilization steps. The alumina membrane

Membralox® is for example used to further concentrate tank bottoms or

centrifuge concentrates [Gillot et al., 1990].

ii. Environmental applications

The increasing concern for the environment and tighter legislation on emissions are

increasing widely the applications of ceramic membranes. In many chemical process

applications there is a growing need to treat not only the waste streams, to meet the

increasingly stringent environmental regulations, but also to recover and reuse the

chemicals. The nature of these process streams can vary and in some cases the process

may require aggressive operating and/or cleaning conditions. Examples of such

applications include the filtration of chemical solvents, dye and pigment wastewater

from dye processing and colouring plants, and highly variable wastewater containing

detergents, polymers and organic solvents [Sondhi et al., 2007]. Environmental

applications can be roughly divided into wastewaters from various sources, as well as

oily wastes and sludges. In the following part the process industry wastewaters from the

chemical, textile, and pulp and paper industry are discussed as well as wastewaters

from the nuclear industry and sludge from municipal wastewater dehydration.

(a) Petrochemical industry wastewater

The wastewater applications in the petrochemical industry include the treatment of

acidic and alkaline process wastewaters from a vinyl chloride monomer manufacturing

facility. Lahier and Goodboy (1993) have shown that the high content of heavy metals

from acidic wastewater can be removed by using two-‐stage membrane filtration with

alumina microporous membranes. As a pretreatment the pH of the wastewater was

adjusted from 0.7 to 12. In the first filtration stage, the precipitated metal hydroxides

were separated from the water and in the second filtration stage the filtered metals

were concentrated to 17-‐20 w.w%. The water recoveries from the filtrations were 80%

and 99.5%. In Lahier and Goodboy research work, alumina microfiltration membranes

were evaluated for treatment of 3 aqueous streams containing heavy metals, oils, and

solids at petrochemical manufacturing facilities. They have also shown that an alkaline waste stream containing dichloroethane, water, and calcium/iron hydroxides can be

treated with ceramic membranes.

57

(b) Pulp and paper industry

In pulp and paper industry the main interest in studying the possible applications of

ceramic membranes has been in bleach plant effluent treatments. The wastewater sources in paper industry comes from disk filtration, flotation and close loop water

circulation as well as the clarification of coating colour basin. In Laitinen et al. (1998) as

well as in many 90s references [Petterson et al., 1988; Gaddis et al., 1995], alumina

membranes are used for wastewater ultrafiltration for suspended solids (SS), COD, color

and colloidal material removal. Laitinen tried flat sheet α-‐ and γ-‐alumina membranes

and modified α-‐alumina membranes and concluded in quite low COD removal is

achieved (under 22%), iron and odor from color wastewater treated sufficiently and SS

and colloids was almost totally remover. Fouling of membrane seems to be the most

considerable problem for large scale application of the method. Great advantage of

wastewater treatment of pulp industry without using chemical compounds is recycling

feasibility, In some cases [Laitinen et al., 2001] existing results have showed that

ultrafiltration permeate was satisfactory for continuous recycling and the pulp

produced with recycling had better strength properties than when recycling was not

used. Hence, toxic discharges were reduced, waste management became more effective,

and the need for maintenance was reduced.

(c) Municipal wastewater

Ultrafiltration/microfiltration is suitable for tertiary treatment of municipal wastewater

after standard treatment steps. The final product can be reused as the level of pollutants

and microorganisms falls short of environmental legislation limits. In Vera et al. (1997)

a study on the filtration of the secondary clarified suspension is studied using alumina

microfiltration membranes of 0.14μm pore size. The membrane system proved to serve

as a total barrier for suspended solids, total coliform, fecal coliform and fecal

streptococci. This fact together with the reductions of turbidity, COD and phosphorus

removal from secondary treatment made the microfiltered water perfectly adapted to

irrigation. A critical flux of 100 l/(m²h) was achieved at 1bar pressure and 3 m/s

crossflow velocity. Other possible application studied is ceramic membrane

microfiltration of hotel wastewater in order to reuse it in secondary purposes, such as

toilet flushing [Laitinen, 2004]. Nevertheless, Because of the high cost of membrane

installation and performance, the wide scale application seems to be not yet necessary

in this kind of municipal wastewater and activated sludge treatment.

58

(d) Activated sludge

Xing et al. (2001) has studied the use of a bioreactor combined with γ-‐alumina and

zirconia ceramic ultrafiltration membranes for the reclamation of urban wastewater.

According to results, the treated water could be reused directly for municipal purposes,

such as toilet flushing and car washing, and after softening treatment for industrial

purposes such as cooling supply and process water. Removal of ammoniac nitrogen and

suspended solids was product of membrane separation however COD removal efficiency

was attributed more than 90% by bioreactor and no by ceramic membranes.

(e) Nuclear industry

In the nuclear industry, composite alumina ceramic membranes have been studied for

the treatment of radioactive waste streams. AEA Technology in England has done

excessive pilot plant studies with ceramic membranes and the results have shown that,

for a low-‐level radioactive general site waste, good alpha and beta/gamma removal can

be achieved. [Laitinen,2004]. Cadmium adsorption from silica-‐alumina membrane is

simulated by Pacheso et al. (2006) from nuclear and chemical wastewater treatment.

Sol-‐gel structured nanoparticles of silica and alumina are used for multilayer composite

alumina membranes fabrication.

(f) Fluoride removal

Nanofiltration (NF) is an attractive technique for reducing fluoride anions (F-‐)

concentrations to acceptable levels in drinking water, however commercial

membranes show minimal chloride of fluoride selectivity. In laboratory scale,

multilayer organic membranes (polystyrene sulfonate) with porous alumina

supports exhibited Cl, F and Br high selectivity, three times more than the

commercial available membranes [Pagana et al., 2007]. Moreover,

chloride/fluoride selectivity is essentially constant over Cl-‐/F-‐ feed ratios from 1

to 60, so these separations will be viable over a range of conditions [Seong et.al,

2007].

59

(g) Arsenic removal from water resources

Alumina ceramic membranes can be used for As(V) from water resources – as

contaminated water. For such kind of application, γ-‐Al2O3/α-‐Al2O3 can be used.

The concentration of arsenic ions decreased from 1ppm, which was originally to

power in 5ppb. Therefore, the concentration of ions As (V) in output was lower

than the maximum permissible limit set for drinking water [Pagana et al.,

2009].In Laitinen’s doctorate thesis, published in 2004 from Finland Technical

University, from all the membrane separation process studied applied in a wide

range of wastewaters, alumina membranes assumed to be suitable for arsenic

removal from bore well water if flocculation was used as a pretreatment as well

as for the treatment of the stone cutting wastewater. The fluxes achieved in

short-‐term experiments for the bore well water were promising and the

optimization of operation conditions should be considered [Laitinen, 2004].

(h) Arsenic and fluoride removal

In a similar recent study, Wu et al. (2011) highly ordered mesoporous aluminas

and calcium-‐doped aluminas were synthesized. Their fluoride adsorption

characteristics, including adsorption isotherms, adsorption kinetics, the

effect of pH and co-‐existing anions were investigated. These materials

exhibited strong affinity to fluoride ions and extremely high defluoridation

capacities. The highest defluoridation capacity value reached 450 mg/g. These

materials also showed superb arsenic removal ability: 1 g of mesoporous

alumina was able to treat 200 kg of arsenic contaminated water with a pH

value of 7, reducing the concentration of arsenate from 100 ppb to 1 ppb.

60

(i) Chromium and arsenic removal

In laboratory scale, separation processes have been developed for ion Cr (III)

removal from water using γ-‐alumina membranes [Pagana et al., 2000; Sklari et

al., 2005]. In Pagana et al. (2000) is described how two pilotic system have been

developed with concentration of chromium ions. In the conclusion of this study,

the concentration of ions Cr (III) in output is assumed to be significantly lower

than the maximum permissible limit set for drinking water (greek standars). By

the same research group, a combined process of removing ions As (V) and Cr(III)

from simulated wastewater using nanostructruded alumina membranes

provided also sufficient results. The concentrations of arsenic and chromium

ions decreased from 1ppm and 0.5ppm, which was originally to power in 5ppb

and 3ppb, respectively. Therefore, the concentrations of ions As (V) and Cr (III)

in output was lower than the maximum permissible limits set for drinking water.

In the figure below (Figure 15) the pilot system is illustrated [Pagana et al.,

2008].

Figure 15 Flow diagram of the Cr (III) removal process [Pagana et al., 2008]

In this work, asymmetric multilayer porous ceramic membranes are developed.

Composite γ-‐Al2O3 membranes made by sol–gel method are prepared for

chromium and aersinc ions removal from water solutions. As(V) removal is

achieve d by a two stage adsorption – ultrafiltration processes in series.

Moreover, Cr(III ) removal is achieve d by an adsorption–ult rafiltrat ion parallel

61

process (Figure 15) using membrane ultrafiltration in a pilot system (see

characteristics inTable 15 ).

Table 15: Basic characteristics of membrane ultrafiltration process applied in Pagana et al., 2007 pilot system for chromium removal (Pagana et al., 2007)

Chromium removal: basic characteristics of the membrane ultrafiltration process

Feed volume 100%

Final permeate volume 100%

Average permeate flux 60ml min-‐1

Pressure difference 3.105 Nm-‐2

Membrane surface A (A=2πrl) 0.05m2

In general, the adsorption-‐ultrafiltration ion process using ceramic membranes

may offer a low cost effective alternative arsenic and chromium purification

technology basically in terms of membrane stability, applied pressure and

product flux with the additional advantage of being suitable for small local and

decentralized units for point-‐of-‐use. However, further experiments should be

carried out in order to provide more accurate an d widely accepted conclusions

[Pagana et al., 2007].

iii. Biotechnology and Pharmaceutical applications

In general framework, in biotechnology and pharmaceutical applications, the

biocompatibility of the membranes is important [Shackleton et al., 1987]. In such

kind of applications, the main interest has been on the filtration of fermentation

broths. In biotechnology, ceramic membranes have been used for primary

extraction, purification, and concentration of biomass, antibiotics, vitamins,

amino acids, organic acids, enzymes, biopolymers, and biopesticides [Cueille et

al., 1990]. There are also applications in the treatment of vaccines, recombinant

proteins, cell cultures, and monoclonal antibodies as well as in continuous

fermentation.

The alumina membranes have several features that indicate the practical

benefit in analytical and diagnostic separations. They are transparent,

allowing to view the retained materials from either side of the membrane.

62

The capillary pore structure allows no lateral diffusion of liquids along the

membrane. Nanoporous alumina membranes are usually prepared

electrochemically in sulfuric or phosphoric acid bath. After preparation they

are used for microfiltration of biological media, such as human red blood cells

and bovine serum albumin [Laitinen, 2004].

(a) Fermentation broths

Alumina membranes are used for fermentation broth clarification at numerous

installations worldwide, successfully competing with other technologies such as

polymeric membranes, vacuum filtration and centrifugation. These systems are

often used for the recovery of valuable antibiotics from dilute solutions. Typical

operating conditions include feed temperatures of 30–60°C, high crossflow

velocity (5–8 m/s) and trans-‐membrane pressures up to 6bar [Laitinen, 2004].

(b) Fungal cells separation

Haarstrick et al. [1990] have studied microfiltration using alumina microporous

membrane in the separation of fungal cells and the purification of the produced

polysaccharide. They concluded that crossflow microfiltration could be an

alternative to conventional separation including purification processes and

suggested that a diafiltration mode should be used in order to avoid the

problems arising from the high viscosity of the broths.

(c) Penicillin recovery

An alumina microfiltration membrane has been used in laboratory scale

[Adikane et al., 2001] to optimize penicillin G recovery from fermentation broth.

Reduction over 40% has been achieved in processing time by optimizing the

crossflow velocity. Moreover, it is showed that the operating fluxes could be

regenerated and a 98% recovery of penicillin G could be obtained over 12 cycles.

63

(d) Lysozyme ultrafiltration

Group of Baudry et al. (2001) has studied the use of modified ceramic

membranes by alumina and zirconium oxide used for lysozyme and lactoferrin

ultrafiltration. It is concluded that as other studies had prouved, adsorbed

protein on the membrane surface was able to highlight UF mechanism. The

enhanced selectivity was observed when the protein to be retained had

additional interactions with the membrane surface. Conrad et al. (1998) studied

microfiltration for the recovery of a water-‐soluble, chiral compound of

therapeutic interest from a bioconversion broth of whole cells and soybean oil. It

is assumed that that it was possible to use microfiltration to harvest an aqueous

product stream from the oil-‐water bioconversion broth.

5.2.2. Gas phase separation

The use of membranes in gas separations has grown at a very rapid pace in

recent times. One particularly interesting application of gas separation with

membranes is the removal of dilute heavy organics from light gas streams such

as the removal of solvents from the exhaust of different process industries

[Javaid et al., 2005]. Gas mixtures can be separated by either dense or porous

membranes. Microporous inorganic membranes have often been modified using

in many cases different types of alumina and studied as possible materials for

achieving solubility based separation.

The dense ceramic membranes are mostly impermeable to all other gases, giving

extremely high selectivity towards oxygen or hydrogen. Oxygen permeation

through a dense ceramic membrane is due to a large number of oxygen vacancies

that are generated by doping and the electron holes produced by the defect

reaction exist in the solid electrolyte. Under a gradient of oxygen partial pressure

imposed on the membrane at a high temperature, the oxide ions are transported

along with holes from the high partial pressure side to the low partial pressure

side. Similarly, when hydrogen is exposed to a mixed proton conducting

membrane, it may be transferred through the membrane under a hydrogen

partial pressure gradient. Again, apart from the membrane bulk diffusion, the

64

surface reactions are also important and need to be taken into consideration for

the hydrogen permeation. In microporous ceramic membranes, the gas

permeation behaviour may be dominated by Knudsen diffusion, surface

diffusion, multilayer diffusion, capillary condensation or molecular sieving (i.e.

configurational diffusion) and is strongly dependent on the pore size and pore

size distribution of the membrane, operating temperature and pressure, and the

nature of the membrane and the permeating molecules [Laitinen, 2004]. Gas

separation using porous ceramic membranes is one of the important research

topics [Wiley, 2007].

Recently, most of the development membrane separation activities have been

focused on gas separations, particularly as ionic conductors for oxygen transport

and as molecular sieve membranes for hydrogen separations. Their use in

environmental applications has been very limited due to cost considerations,

although they offer several unique advantages in this area, such as chemical and

thermal stability and rugged structural stability [Sondhi et al., 2007].

Table 16 : Gas separation by porous alumina membranes [Kaldis et al., 2004]

Pore diameter (Å) Operating temperature

(oC)

Applied pressure

differenece (kPa)

Gas mixture

~100 25-‐75 3.03 -‐ 16.2 He/N2

~100 25-‐75 3.03 -‐ 16.2 He/C2H6

~100 25-‐75 3.03 -‐ 16.2 He/Kr

1020 800 343 H2/H2S

200-‐400 10 152-‐252 H2/H2S

~10 31-‐77 4.04-‐39.4 H2O/air

~10 65-‐70 H2O/CH3OH

~10 78-‐84 H2O/C2H5OH

~10 82-‐88 H2O/(CH3)2CHOH

~30 65-‐70 H2O/CH3OH

~30 78-‐89 H2O/C2H5OH

~30 82-‐88 H2O/(CH3)2CHOH

100-‐200 196-‐197 23.2-‐106 H2/He

100-‐200 196-‐197 23.2-‐106 H2/CO

65

100-‐200 196-‐197 23.2-‐106 He/N2

100-‐200 196-‐197 23.2-‐106 H2/CO2

100-‐200 196-‐197 23.2-‐106 H2/CO

100-‐200 196-‐197 23.2-‐106 He/N2

100-‐200 196-‐197 23.2-‐106 H2/CO2

100-‐200 196-‐197 23.2-‐106 H2/C2H6

100-‐200 196-‐197 23.2-‐106 H2/C3H8

100-‐200 20 H2/He

100-‐200 20 He/N2

100-‐200 20 H2/CO2

300-‐400 65 13.1-‐92.9 235U/238U

Sustainable membranes in high temperatures are strongly needed for energy

production application as biomass burning (organic waste gasification, coal

gasification etc.). Ceramic membranes are appropriate for such kind of

applications because of their high thermal stability and corrosion resistance. In

installations as coal gasification plants, gas separation is needed for emission

treatment after combustion or other processes. Usually, carbon dioxide is the

substance which must be isolated in order to fit environmental requirements.

Membrane technology is one of the promising tools in these applications and

many research works has been realized in recent years.

In the following part, the applications of alumina membranes in gas phase

separation process are gathered in Table 17.

Table 17 : Gas phase separation process of alumina membranes and composite alumina membranes

Porous alumina membrane appications

Gas phase separation applications Industrial/Scientific applications

Carbon dioxide capture

CO2/N2 separation Nox control, gas phase streams from energy plants

H2/CO2 separation gazification, combustion emissions

Hydrocarbons separation

Acetone recovery paint, ink, oil industry

Propane separation natural gas processing, petroleum refining

Catalytic reactors

VOCs oxidation air pollution, VOCs recovery

Methane to ethane reaction gas treatment

66

i. Carbon dioxide capture

There is growing consensus among the scientific community that the rising

atmospheric levels of CO2 as a result of human activities (e.g., fossil fuel burning,

cement production). Membrane processes appear to be an attractive option to

carry out gas separations in terms of their lower environmental impact and

energy costs compared to more conventional separation technologies (e.g.,

distillation, absorption, adsorption, crystallisation). On the guidance of a recent

study recently published by Lito et al. (2011), different adsorption models have

been screened to account for CO2 and N2 adsorption in MFI-‐alumina samples [C.-‐

H. Nicolas et al., 2011].

(a) CO2/N2 separation

In Sang H. Hyun et al., (1995) top layers of γ-‐Al2O3 composite membranes have

been modified for improving the separation factor of CO2 to N2. The separation

factor through the TiO2 supported γ-‐Al2O3membrane was found to be fairly

enhanced by silane coupling, but in case of the α-‐Al2O3 supported membrane was

not. The CO2/N2 separation factor through the modified γ-‐Al2O3 / TiO2 composite

membrane is 1.7 at 90°C. The separation factor is proportional to the CO2

concentration in the gas mixture, and the modified membrane is stable up to

100°C. The main mechanism of the CO2 transport through the modified γ-‐Al2O3

layer is known to be a surface diffusion.

(b) H2/CO2 separation

Coal gasification is called any process of converting coal into gas for many different uses as illuminating, heating etc. In the figure above (

Figure ) an integrated gasification combined cycle is shown (IGCC). In IGCC (a)

air separation and gasification, (b) gas cleaning, (c) gas conditioning and (d) gas

separation are combined for total air treatment after coal gasification are aligned

(Figure 16). Using conventional technology, it is possible to build the IGCC plant

67

above, technologies such as cryogenics, solvent extraction, adsorbents in

pressure or temperature swing adsorption, and low temperature polymeric

membranes. Due to compliance with sorption mechanisms, these technologies

operate best at low temperatures (<50oC). The problem here is that gas

separation follows downstream from gas conditioning. The gasification of coal

predominantly produces syngas (CO and H2) with some remaining

hydrocarbons, CO2 and water. Hence, the syngas requires further processing.

Figure 16: A conventional air treatment process with carbon capture [Diniz et al., 2007]

The syngas requires further processing through the WGS reaction (see equation

below), in order to maximise H2 production.

CO + H2O à CO2 + H2 ΔH =-‐41. 2 kJ.mol-‐1

The WGS reaction is exothermic and the conversion is limited by thermodynamic

equilibrium as the conversion to H2 and CO2 decreases with increasing

temperature. The reaction is therefore carried out in two stages, in high (350-‐

400oC) and low temperature (250-‐300oC) shift reactors with interstage cooling.

In addition, further cooling to <50oC is required to reduce the temperature to the

temperature acceptable by conventional gas separation technologies.

Cooling large stream of hot gases is capital intensive, and incurs a loss of power

production; furthermore, these processes are likely to deliver CO2 at reduced

pressures, which will have to be re-‐compressed to >100atm for transportation

and storage. Hence, conventional processes will attract large energy penalties.

In order to reduce efficiency losses, an alternative is to separate gases at higher

temperatures. In this case, inorganic membranes derived from ceramics, silica,

68

metal and by further doping or alloying showed preferential H2 selectivity over

CO2 at high temperatures (>200oC) [see e.g. recent work of Joe Da Costa in

Australia]. These technologies can also operate in membrane reactor

arrangements for the water gas shift reaction, which allow for shifting the

reactions to higher conversions due to the extraction of hydrogen from the

reaction chamber. The advantage here is two:

1. CO2 is kept at high pressure thus reducing requirements for CO2 compression

downstream.

2. As H2 is selectivity taken, the syngas stream is reduced by up 30-‐35%

depending on the recovery rate.

Although CO2 will have to be cooled down prior to compression, the volumes are

reduced thus requiring a lower cooling duty. Therefore, inorganic membranes

and their incorporation in MRs are foreseen to be the technology of choice for

advanced IGCC plants as depicted in Figure 17.

Figure 17: An Advanced scheme for IGCC with carbon capture [Diniz et al., 2007]

The scheme shown above (Figure 18) for gas separation and conditioning is

being developed in Australia by the University of Queensland as the research

provider under the R&D program directed by the Centre for Low Emission

Technology (www.clet.net). The use of a higher temperature membrane allows

further simplification of the gas cooling, conditioning and separation step, but

this scheme requires a high temperature membrane tolerant to the syngas.

69

Figure 18: Schematic of a multi tube membrane module for H2 and CO2 separation [Diriz et al., 2007]

In Diriz et al. (2007) for high temperature gas separation membrane, commercial

D-‐alumina tubes coated with a top γ-‐alumina layer, dip-‐coated with cobalt and

selective silica layer. The best membrane performance delivered H2 purity in

excess of 98% at 300o C. The positive energy of activation for H2 permeation

coupled with the negative energy of activation for CO2 permeation of metal

doped silica membranes provide a favourable fundamental property for

engineering design of gas separation at higher temperatures (up to 500o C) such

as those required in a coal gasification process. The process integration provided

by Diriz et al. is potentially beneficial for the next generation of high temperature

processing unit operations in low emissions coal gasification.

70

ii. Hydrocarbons separation

(a) Acetone recovery

Huang et al. (1997) modified alumina membranes for recovery of acetone from

nitrogen by reducing the pore size to enhance multilayer diffusion and capillary

condensation transport mechanisms [Huang et al. 1997]. Depending on the

temperature and the feed composition, the separation factor varied from being

less than 10 to as high as 200 is dominant. The modified membranes showed

higher acetone permeability and higher separation factors as compared to

polymeric membranes. However, the performance was strongly influenced by

temperature and feed composition.

(b) Propane separation

Hydrocarbons as propane or butane are usually by-‐products of natural gas

processing and petroleum refining and they are commonly used as a fuel for

engines, heating appliances etc. In Randon et al. (1995) alumina membrane are

modified and gas permeation experiments were conducted using nitrogen and

propane. The modified membrane exhibited significantly high propane

permeance. The modification had made the membrane hydrophobic and

improved the membrane’s solubility-‐based separation characteristics. Other

attempt for alumina membrane modification using different alkyl

trichlorosilanes showed a significant increase in propane/nitrogen selectivity

accompanied, by a loss in permeance, after modification. In comparison to PDMS,

one of the best-‐known polymers for solubility-‐based separations, the hybrid

membranes exhibited equal or greater propane/nitrogen selectivity although at

lower propane permeance [Javaid et al., 2005]. McCarley and Way (2001), in a

similar study, modified 5nm alumina membranes with C18 trichlorosilane. They

conducted both single gas and mixed gas permeation experiments at fixed

transmembrane pressures. The treated membrane showed a significant increase

in ideal selectivity for heavier gases (n-‐butane) over lighter gases (nitrogen and

methane).

71

iii. Catalytic reactors

The application of porous ceramic membranes as catalytic reactors starts in the 1980s.

The driving force for this change was the possibility of integrating reaction and

separation, which had already been achieved in the field of biochemical reaction

engineering using polymeric membranes. These, however, were not applicable at the

temperatures used in most of the processes of interest in the chemical process industry

[Lafarga et al., 1998]. Since the materials used in the manufacture of ceramic

membranes are also commonly used as conventional catalyst supports (as alumina),

there has been a strong interest in the development of membrane reactors by

researchers with previous experience on heterogeneous catalysis, who adapted many of

the preparation and characterization techniques used in this field. The most important

and valuable fact is that membrane itself is the catalyst [Coronas et al., 1999] and it can

be used for a wide range of applications. Catalytic membranes are used to improve

contact efficiency with the objective of attaining higher conversions by decreasing mass

transfer resistances. Membrane reactor with catalytic surface aims to improve the

contact in gas–liquid–solid systems by providing a well-‐defined contact region: the

liquid-‐filled pores of the catalytic zone of the membrane in close proximity to the gas

interphase. This does not require a permselective membrane and avoids the problem of

catalyst recovery that appears in slurry reactors.

Poorly permselective inert porous membranes such as mesoporous alumina,

mesoporous glass etc. can reveal attractive for a number of dehydrogenation

reactions. Infiltrated MFI/alumina composite membranes have been used as O2

distributors but also as barriers for dehydrogenation reactions in MRs [Julbe et

al., 2001].

(a) VOCs removal

Volatile Organic Compounds (VOCs) are among the most common air pollutants

emitted from chemical, petrochemical, and allied industries. VOCs are one of the

main sources of photochemical reaction in the atmosphere leading to various

environmental hazards and on the other hand, these VOCs have good commercial

value. Hence, growing environmental awareness has put up stringent regulations

to control the VOCs emissions. In such circumstances, it becomes mandatory for

72

each VOCs emitting industry or facility to opt for proper VOCs control measures.

There are many techniques available to control VOCs emission (destruction

based and recovery based) with many advantages and limitations [Khan et al.,

2000]. One of these methods for removal and no so often for VOCs recovery is

gas separation with ceramic membranes -‐ zeolithe and alumina membranes in

most of the cases. By the University of Zaragoza in Spain team and other research

groups this concept has already been demonstrated eg. using hydrogenation

reactions over Pt/Al2O3 catalysts in membrane module.

In Figure 19, it is illustrated a flow-‐through membrane in which the permeation

of a premixed feed stream takes place. This was the approach employed in works

by Saracco et al. (1999), who used catalytically modified fly ash filters for alcohol

dehydration and for the reduction of nitrogen oxides with NH3. Pina et al. (1999)

used Pt/Al2O3 and perovskite-‐containing membranes operating in the Knudsen

regime for the purification (by catalytic combustion) of air streams containing

volatile organic compounds (VOCs) in low concentrations. According to this

study, since in the Knudsen diffusion regime the probability of collisions

between the molecules and the wall of the pores is maximised, this type of

membrane would be expected to give high contact efficiency in the reaction of

diluted streams, such as those commonly encountered in VOC removal. The

results of Pina et al. (1999) showed that the membrane could perform very

efficiently in the combustion of VOCs at low temperatures, although at the

expense of a significant pressure drop.

Figure 19: Applications of membrane reactors [Coronas et al., 1999]

73

The industrial application of this type of reactor surely requires optimization of

the membrane structure aimed to reducing the pressure drop. Otherwise, its use

would be restricted to applications involving reaction simultaneous with gas

filtration, where the pressure drop is already present.

(b) Methane to ethane reaction

In Lafarga D. and Santamaria J. projects published in international literature

[Lafarga et al., 1994; Sebastian et al., 2009] a new reactor concept has been

developed and tested for the conversion of methane into ethane, ethylene and

higher hydrocarbons via oxidative coupling. The basic idea consists of providing

a low oxygen concentration in the reactor in order to increase hydrocarbon

selectivity. To this end, oxygen is supplied to the reacting mixture by permeation

through a porous wall as a modified alumina membrane, which is capable to

withstand the temperatures and pressures involved, and at the same time give

adquate values of oxygen flux [Lafarga et al., 1994].

Concluding, despite the plethora of studies appeared in the open and patent

literature focusing on the development of ceramic materials for gas mixture

separations [Koutsonikolas et al., 2010] no many industrial processes have been

commercialized. Although alumina membrane is the most used in large scale,

polymer membranes are still dominating in commercial field of separation

process. The lack of selectivity combined with their high unit cost most often

ascribed to the support, as well as their lack of hydrothermal stability in some

cases, act as key economical and technological barriers for the industrial

implementation.

74

6.SUMMARY AND CONCLUSIONS The term alumina includes large number of chemical compounds: amorphous,

crystalline trihydroxides, monohydroxides, transition aluminas and stable

anhydrous alumina.

Alumina takes attention with good thermal, mechanical and chemical stabilitiy,

as a material for industrial applications. With wider applications of membrane,

alumina is trending topic. Alumina pore size distributions can be controlled

easily modifications on operation conditions from a hundred of nanometers to a

hundred of micrometers.

Besides traditional pathways like sintering and anodic oxidation, alternative

methods -‐ sol-‐gel methods, dip coating, rapid gelation-‐ are introducing with

alumina membrane synthesis. The optimum range that will provide a good

compromise between permeability and strength is being improved also with

additives.

According to literature overview, alumina membranes are applied especially in

the areas where the advantages of ceramic membranes can be exploited. In

biotechnology and pharmaceutical industry their resistance to microbial attack

and biological degradation is really very important and their good thermal and

chemical stability are often the factors that have made them beneficial in

wastewater treatment and gas separation applications. However, they suffer

from poor steam stability, fouling and cracking and because of this alumina is

often mixed with additives such as metals or oxides to improve such kind of

drawbacks [Laitinen, 2004]. Modifications can also be used to increase

selectivity by surface adsorption or molecular sieving effects and a wide range of

research field in ceramic membrane is oriented to this direction.

75

APPENDIX A : Membrane Material Sheet MEMBRANE MATERIAL NAME Alumina 1. INTRINSIC PROPERTIES Nature High Low 1. 1. General physicochemical properties Bulk density at RT kg/m3 1217.406 800.925 1. 2. Mechanical properties Tensile Strength (MPa) 173 a 117 b Bending Strenght Mpa 413 c 307 d Modulus of Elasticity (E) X 108 MPa 26.8 a 21.27 b Compressive Strenght Mpa 3733 e 1600 b Modulus of Ridity(G) X 108 MPa 11.3 a 8.67 b Hardness on the mohs scale 9 1. 3. Thermal properties Melting point OC 2051.0 ± 9.7 Boiling Point OC 3530 ± 200 1. 4. Surface properties Zero point of charge 9.45 f 9 g Water contact angle 73.3 62.6 SBET (m2/g) 484.34 98.3 Pore Diameter (A) 72.23 20.49 Pore Volume (mL/g) 1.27 0.54

1. 5. Chemical properties Operating pH range

Compatibility with solvent 1 phosphoric acid -‐ HF well

Compatibility with solvent 2 HCl-‐ HNO3 weak a polycrystalline alumina 94% e polycristalline alumina 100% b polycrystalline alumina 85% f gamma alumina c polycristalline alumina 99.9% g bohemite d polycristalline alumina 85.5%

76

2. CHARACTERISTICS OF COMMERCIAL MEMBRANES BASED ON THIS MATERIAL Membrane name Memrbalox Membrane manufacturer Pall Membrane shape capillary tube, multichannel, monolithic Membrane structure symmetric Support nature ultrapure µ-‐alumina, zirconia, titania Membrane type porous Average pore size 5μm Molecular weight cut-‐off 1-‐5kg/mol Water permeance at RT 300 l/(m²h bar) Membrane name Anopore Membrane manufacturer Myriad Membrane shape disc Membrane structure symmetric Support nature polypropylene Membrane type porous Average pore size 0.02-‐0.1μm Molecular weight cut-‐off 0,1μm Water permeance at RT 1 m³/m² Membrane name Céram inside Membrane manufacturer TAMI Membrane shape Disc Membrane structure Porous, plate, tubular, multichannel Support nature Alumina/titania/zirconia Membrane type dense, porous Average pore size 0,14-‐1,40μm Molecular weight cut-‐off 1-‐300kg/mol Water permeance at RT 800 -‐1200 l/(m²h bar)

77

APPENDIX B: Chemical Interest Of Alumina

Alkali Metals

Ryshkewitch (1960) stated that molten lithium attracts alumina aggressively,

potassium to lesser degree, and sodium the least. Reed claimed that the

corrosion resistance of sintered alumina to both liquid and vaporized sodium is

good at 900 OC. A commercial sintered alumina article disintegrated within 168

hours at 940 OC, however. A synthetic sapphire cylinder (1/4 in. diameter by 1

in. length lost about 1% in weight in 168 hours ay 900 OC but remained clear.

Kelman, Wilkinson, and Yaggee found that sodium potassium did not attract

below 500 OC, but caused corrosion at 600 OC. Kolosova et al. found a weight loss

of only 0.01 to 0.06% in 35 hours at 400 OC for sintered alumina immersed in

molten alkali (79%K, 21%Na).

The behavior of vaporized alkali metals on oxides and other dielectric materials

has been of interest for thermoelectric converters. Wagner and Coriell (1959)

tested Al2O3, BN, ZrO2, MgO, HfO2, ThO2, CaO, and NbC to exposure to cesium

vapor at temperatures as high as 1475 OC. Only fused alumina remained

unaffected by the test. It was concluded that cesium probably reacts with

impurities in the contacting material, but the effect can ben observed only when

the surface areas available for reaction are relatively large, as is the case with

sintered specimens. Higgings attributed pitting of single crystals by cesium

vapor at 600 OC to silicon. And barium impurities; neutron irradiation increased

the attack. C. E. Addams (1959) observed that rubidium effectively condensed at

high temperatures only on oxides with which it could form stable complex

compounds. Cowan and Stoddard (1964) stated that glasses could not be used in

thermionic converter seals because of alkali metal (cesium) corrosion.

Alkaline Earth Metals

78

Jaeger and Krasemann (1952) observed no reaction of calcium, barium, or

strontium to their boiling points (about 1150 OC). This is likely, since BaO and

SRO can be reduced to the metals (thermite reaction) by metallic aluminum at

1100 OC in a vacuum (Gvelisiani and Pazukhin). Magnesium also shows no

attract to its boiling point (1100 OC).

Aluminum

Aluminum reacts with alumina at 1100 OC to form Al2O, and at 1600 OC to form

AlO (Hoch and Johnston).

Antimony, Arsenic

Jaeger and Krasemann claimed no attract of sintered alumina.

Beryllium

Beryllium (melting point 1500 OC) shows no attract of sintered alumina. A slight

darkening of the Alumina, caused by formation an interfacial layer of

chrysoberyl, BeO ∙ Al2O3 (melting point 1870 OC), and Be ∙ 3Al2O3 (melting point

1910 OC) are identified compounds (Lang, Fillmore, and Maxwell, 1952;

Galakhov, 1957).

Bishmut

The corrosion resistance is good to 1400 OC (Reed). The reaction product is

Bi2O3∙2Al2O3 (Levin and Roth, 1964).

Carbon

Carbon reduces alumina to normal carbide Al4C3 (Prescott and Hinckle) at above

1700 OC (Jaeger and Kresemann), initiating at as low as 1310 OC (Komarek et al.),

79

but requiring about 2400 OC for complete conversion (Kohlmeyer and

Lundquist). Two oxicarbides also exist at about 2030 OC, Al4O4C (Foster, Long

and Hunter; Cox and Pidgeon). The presence of Fe2O3, SiO2, TiO2, and V2O5 as

impurities in sintered alumina might induce deterioration at lower temperatures

(Stroup), as low as 1380 OC (Kroll and Schlechten). The reduction to metallic

aluminum occurs at about 2000 OC (Miller, Foster, and Baker). Graphite does not

wet molten alumina, but severely pitted in contact with it in water-‐free, inert

atmosphere or vacuum (Barlett and Hall).

Cerium

Ceric oxide, CeO, forms no compounds with alumina (Wartenberg and Eckhardt).

Cerous oxide, Ce2O3, forms Ce2O3 ∙ 11 Al2O3 and Ce2O3 ∙ Al2O3, both of which

decompose in air to form Al2O3 and CeO2 at temperatures above 800 OC (Leonov

and Keler).

Chlorine

Chlorine does not attack, except in the presence of carbon (Singer and

Thurnauer).

Chromium

Chromium wets alumina at 1650 OC in a reducing atmosphere (Blackburn,

Shelvin and Lowers).

Cobalt

Cobalt neither wets nor reacts with sintered alumina to above its melting point

(1480 OC) in a reducing atmosphere (Sieverts and Moritz; Tumanov et. al.).

Copper

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Copper reacts with the transition aluminas at 800 OC in air to form CuAl2O4,

which is stable to 1000 OC. It then converts to CuAlO2, which is stable in air to

about 1260 OC (Hahn and de Lorent, 1955; Misra and Chaklader, 1963).

Fluorine

Winzer claimed attack of sintered alumina at 1700 OC by dry fluorine.

Gallium

Sintered alumina is inert to gallium to 1000 OC (Kelman et al.). Wartenberg and

Reusch (1932) found solid solutions above 810 OC with Ga2O3.

Hydrogen

Jaeger and Krasemann observed no reduction of alpha alumina in hydrogen up to

the melting point, only a surface darkening L. J. Trostel, Jr. (1965) noted that

hydrogen attacks alumina refractories below 1600 OC, however in the presence

of water vapor.

Iron

Iron can be melted in sintered alumina under reducing conditions, but wets

about 1600 OC (Blckburn et. al.). The spinel FeO ∙ Al2O3, dissolves to about 6%

alumina at 1750 OC (McIntosh, Rait, and Hay; Fischer and Hoffman).

Muan and Gee (1956), and Muan (1958) found limited solubility for Fe2O3 in

corundum. Richards and White; Atlas and Sumida; and Turnoc and Lindsley

investigated the spinel reactions; Fe2O3 ∙ Al2O3 has a structure similar to kappa

alumina, and requires above 1320 OC prepare.

Lead

81

No reaction occurs with alumina at the melting point of lead (327 OC). In lead-‐

bismuth eutectic alloy (44.5% Pb), Gangler found 0.000 mils/year loss at 1090 OC. Lead aluminate is unstable above 970 OC (Geller and Bunting).

Lithium (See alkali metals)

Manganese

Manganese does not attack sintered alumina to above the melting point (1260 OC) in a reducing atmosphere. Although it is more active than iron, cobalt or

nickel it can be distilled in sintered alumina to give a spectroscopicially pure

product (Sieverts and Moritz).

Mercury

Kelmann et al., and Hahn, Frank, et. al. found no reaction with alumina at 300 OC.

Molybdenum

Alumina is not reduced by molybdenum even above the melting point of

alumina. Discoloration may occur at 2100 OC in a dry, inert atmosphere (He).

Nickel

No attack occurs in dry inert atmosphere Nickel can be melted in sintered

alumina in hydrogen atmosphere (Economos and Kingery). Wetting occurs at

1800 OC.

Niobium

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Mass spectrometric and thermogravimetric analysis at 1800 to 2200 OC indicates

the principal reaction is

!!!!! + 3 !" → 2 !" ! + 3!"# ! .

Secondary reactions under neutral conditions are:

!!!!! + !" → !!!! ! + !"!! !

!!!!! + 2 !" + 1 2!! → !!!! ! + !"!! + !"# (!)

(Grosmann, 1966).

Nitrogen

Nitrogen does not attack (Jaeger and Krasemann).

Palladium, Platinum

Both metals can be handled in the molten conditions in sintered alumina. (Jaeger

and Krasemann)

Phosphorus

No attack was observed in moderate temperatures (Jaeger).

Silver

No compounds can be prepared with silver and alumina (Hahn, Frank et. al.).

Sulfur, Selenium, Tellurium

These elements do not attack alumina.

Tantalum

Tantalum reacts only slightly with molten alumina in water free inert

atmosphere, H2, N2, CO, or in vacuum (Bartlett and Hall). Alloy 90 Ta-‐10W shows

83

slight reaction. Tantalum carbide and 4 TaC ∙ ZrC react only slightly (Bartlett and

Hall).

Tin

No reaction occurred in molten tin at 1000 OC.

Titanium, Zirconium

No reaction occurred in an inert atmosphere below 1800 OC, at which

temperature black discoloration of the grains and corrosion occurred (Economs

and Kingery). Titanium nitride wets molten alumina with negligible corrosion in

water free inert atmosphere (Barlett and Hall).

Titanium + Aluminum-‐Hardened Nickel-‐Base Alloy

Decker, Rowe and Freeman found that trance amounts of zirconium or boron

picked up from zirconia or magnesia crucibled reduced cracking of the hardened

nickel-‐base alloys during hot-‐working and increased their rupture strength and

ductility. It was desirable to compensate for this effect when making heats in

sintered alumina crucibles.

Tungusten

Jager and Kresemann observed no reaction between tungusten and alumina.

Wallace et. al. (1961) investigated the reactions in a Knudsen cell-‐oven operable

at 2500 OC in conjunction with a Nier-‐type mass spectrometer. Jacodine (1961)

observed a growth of nodules or hillocks of alumina in the investigation of

heater-‐cathode breakdown of alumina-‐coated heaters operated at 1200 OC and

180 volts dc. Tungusten wets molten alumina with negligible corrosion in water-‐

free inert atmosphere (Barlett and Hall).

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Uranium

Jaeger and Krasemann found no reaction between uranium and sintered alumina

to 1200 OC. Dykstra (1960) found no solid solution between Al2O3 and uranium

oxide.

Vanadium

Burdese obtained reaction between V2O5 and gamma alumina at 500 OC to form

Al2O3 ∙ V2O5.

Zirconium Boride (ZrB2) and Zirconium Carbide (ZrC)

Molten alumina wets and reacts moderately in water free, inert atmosphere

(Barlett and Hall).

85

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