Economic Exploitation of Rice Straw
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Transcript of Economic Exploitation of Rice Straw
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Acknowledgement
First of all and above all great thanks to ALLAH blessings on me cannot be
counted.
I am greatly and deeply indebted to Prof. Dr. Raafat M. Issa and Prof. Dr.
Morsy M. Abou-Sekkina, Professors of Physical and Inorganic Chemistry,
Department of Chemistry, Faculty of Science, Tanta University, for suggesting the
subject of the present thesis, their kind supervision, unlimited help, appreciable
encouragement and continuous scientific discussions during carrying out the work
and reviewing entirely the thesis from the initial to the final stages.
I would like also to express my sincere gratitude to Prof. Dr. Alam El-deenMohamed Bastawisy Professor of Chemical Engineering, Faculty of Engineering,
Tanta University for his constructive supervision, great help, valuable instruction,
fruitful criticism, continuous support to continue this work.
My great and sincerest thanks to Dr. Abdalla Mohamed Khedr, Assistant
Professor of Analytical Chemistry, Chemistry Department, Faculty of Science, Tanta
University for his kindly instructions during the experimental work, valuable and
scientific discussions during carrying out this work and reviewing critically the
thesis.
The author
Wael Abd-Allah El-Helece
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Dedication
To my family with my deep and sincere appreciation for their great efforts
during my life and my studies
To my wife, daughters, sun and my brother with my great thanks for hishelp
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CONTENTS
CONTENTS page
LIST OF TABLES 9
LIST OF FIGURES... 12
KEY OF SAMPLES 16
ABBREVIATIONS... 17
AIM OF THE WORK... 18
CHAPTER I
INTRODUCTION ...... 19
1. Annual plants.... 19
2. Rice straw.... 26
2.1. On -farm non-burn alternatives .... 28
2.1.1. Straw decomposition... 28
2.1.2. Chopping and incorporating straw into soil.... 28
2.1.3. Rolling..... 29
2.2. Baling and transportation / disposal... 29
2.3. Off-farm non-burn alternatives.. 30
2.3.1. Energy conversion... 30
2.3.1.1. Anaerobic digestion. 30
2.3.1.2. Direct combustion.... 31
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2.3.1.3. Ethanol production... 31
2.4. Pulp products.. 33
2.4.1. Paper / cardboard.... 33
2.4.2. Fiberboard... 33
2.5. Construction products.... 34
2.5.1. Wood replacement materials... 34
2.5.2. Bricks and cement boards... 34
2.5.3. Panel construction... 35
2.5.4. Straw bale structure construction.... 35
2.6. Composting.... 35
2.7. Mushroom production.... 36
2.8. Erosion control... 36
2.9. Livestock feed.... 37
2.10. Additional available resources..... 37
2.11. Chemical composition of wood and straw... 40
3. Pulping processes .... 42
3.1. Mechanical pulping .. 46
3.2. Chemical pulping .. 46
3.2.1. Soda process .. 46
3.2.2. Kraft process .. 47
3.2.3. Sulfite process ... 48
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3.2.4. IRSP (impregnation rapid steam pulping process) process. 48
a- Impregnation .... 48
b- Rapid steam pulping .... 49
4. Bleaching... 50
4.1. Chlorination (C). 51
4.2. Alkaline extraction (E)... 51
4.3. Hypochlorite bleaching (H)... 51
4.4. Chlorine dioxide bleaching (D). 51
4.5. Oxygen bleaching (O).... 52
4.6. Ozone bleaching (Z)... 52
4.7. Peroxide bleaching (P)... 52
5. Cellulose derivatives. 53
6. Mercerization.... 54
7. Black liquor usage.... 55
CHAPTER II
EXPERIMENTAL..... 57
1- Raw material used... 57
2. Equipments 57
3. Analysis of raw material... 57
3.1 Moisture content..... 58
3.2 Water soluble matter... 58
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3.3 Determination of ash and silica in rice straw and pulp.. 58
3.4 Alpha cellulose estimation ..... 59
4. Cooking in sodium hydroxide solutions (pulping)... 59
Cooking conditions... 60
4.1 Effect of sodium hydroxide concentration .... 60
4.2 Effect of time at optimum alkalinity... 60
4.3 Effect of weight / volume ratio..... 60
4.4 Effect of cooking temperature on the pulp yield ... 61
4.5. Effect of the nature of rice straw on the pulp yield.. 61
5. Bleaching.... 61
6. Permanganate number. 62
7. Determination of the contents of black liquor.... 63
7.1. Isolation of alkali lignin..... 63
7.2. Separation of silica from sodium silicate solution obtained.. 64
8. Xanthation....... 64
CHAPTER III
RESULTS AND DISCUSSION.... 65
1. Analysis of raw materials..... 65
1.1. Moisture content... 65
1.2. Water soluble matters... 65
1.3. Ash content... 66
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2. Effect of cooking conditions .... 67
2.1. Effect of sodium hydroxide concentration ...... 67
2.2. Effect of time at optimum alkalinity...... 68
2.3. Effect of weight / volume ratio (w/v).... 69
2.4. Effect of cooking temperature on the pulp yield .. 70
2.5. Effect of the nature of rice straw on the pulp yield..... 71
2.6. Alpha cellulose estimation... 72
3. Infrared spectrophotometric study of cellulose obtained from high yield
soda rice straw pulps
73
3.1. Infrared spectrophotometric determination of lignin in soda yield ricestraw pulp ...
80
3.2. TGA study of cellulose samples ... 94
3.3. Differential thermal analysis (DTA) of cellulose sample.. 98
3.4. Kappa number .. 99
4. Bleaching ..... 102
5. Determination of the content of black liquor ... 106
5.1. Extraction of lignin and other organic matters ... 106
5.2. Studying the alkali lignin contained in black liquor . 110
5.3. Infrared spectrophotometric determination of lignin in samples
separated from black liquor remained.
113
5.4. TGA Study of lignin sample. 118
5.5. Differential thermal analysis (DTA) of lignin sample .. 119
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6. Relationship between bleaching and brightness of the produced pulpaccording to kappa number ......................................................
120
7. Silica precipitated from sodium silicate solution obtained.. 121
7.1. TGA studies on silicate samples... 125
7.2. Differential thermal analysis (DTA) of silicate sample ... 126
8. Xanthation .. 127
8.1. IR spectral analysis for xanthated cellulose samples prepared... 127
8.2. TGA studies for xanthated cellulose samples.. 129
8.3. Differential thermal analysis (DTA) of xanthated cellulose samples. 130
9. SEM microscopic investigations ... 131
9.1. Super molecular structure of cellulose samples .. 132
9.2. Super molecular structure of bleached cellulose samples . 135
9.3. Super molecular structure of xanthated cellulose sample . 136
CHAPTER IV
SUMMARY ........................................................................................ 139
CHAPTER V
REFERENCES ... 142
ARABIC SUMMARY ...... 153
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List of tables
Table
no.
Title Pageno.
1 Chemical composition of natural fibers 21
2 Average annual yields of some raw materials 24
3 Worldwide availability of annual plant fiber 25
4 The technological, economic, and commercial
feasibility of each non-burn alternative
27
5 Average chemical composition of rice straw 39
6 Pulping processes for annual plants 44
7 Pulping processes and yields 45
8 The approximate composition of dry rice straw 66
9 Rice straw composition according to Houston 67
10 The effect of NaOH concentration on the yield of the
pulp
68
11 The effect of time of pulping 69
12 The effect of weight / volume ratio 70
13 The effect of cooking temperature on the pulp yield 70
14 The effect of rice straw nature on the pulp yield 71
15 -cellulose content of rice straw 72
16 -cellulose content of pre-treated rice straw (pulp) 73
17 Assignments of bands in the IR spectrum of rice straw
pulp
75
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18 a The lignin content of the pulp of rice straw samples 1,
2, 3 and 4 related to the relative intensity according to
Saad et. al. method
83
18 b The lignin content of the pulp of rice straw samples 1,
2, 3 and 4 related to the relative intensity according tothe background correction method
84
18 c The lignin content of the pulp of rice straw samples 1,
2, 3 and 4 related to the relative intensity according to
the base line technique method85
18 d The lignin content of the pulp of rice straw samples 1,
2, 3 and 4 related to the relative intensity according to
the Pislot method
86
19 a The lignin content of the pulp of rice straw samples 5,
6, 7 and 8 related to the relative intensity according to
Saad et. al. method
87
19 b The lignin content of the pulp of rice straw samples 5,
6, 7 and 8 related to the relative intensity according to
the background correction method
88
19 c The lignin content of the pulp of rice straw samples 5,
6, 7 and 8 related to the relative intensity according to
the base line technique method
89
19 d The lignin content of the pulp of rice straw samples 5,
6, 7 and 8 related to the relative intensity according to
the Pislot method
90
20 a The relation between sodium hydroxide solution andthe lignin content determined from the methods from
IR charts when pulping for 1 hr
92
20 b The relation between sodium hydroxide solution and the
lignin content determined from different methods from
93
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IR charts when pulping for 2 hrs
21 TGA analysis for cellulose samples 96
22 The kappa number for samples 1, 2, 3 and 4 100
23 The kappa number for samples 5, 6, 7 and 8 101
24 IR analysis of RSA samples 108
25 IR analysis of lignin from rice straw samples 111
26 a Lignin content of samples 15, 16, 17 and 18 related to
the relative intensity according to Saad et al. method
113
26 b Lignin content of samples 15, 16, 17 and 18 related to
the relative intensity according to the background
correction method
114
26 c Lignin content of samples 15, 16, 17 and 18 related to
the relative intensity according to the base line
technique method
115
26 d Lignin content of samples 15, 16, 17 and 18 related to
the relative intensity according to the pislot method
116
27 Lignin content of samples 15, 16, 17 and 18 related tothe relative intensity determined by different methodsfrom IR charts
117
28 The relation between conditions of preparation ofsamples and the brightness (opacity) according toKappa number
120
29 IR analysis of samples 19, 20 and 21 and calciumsilicate hydrate
121
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LIST OF FIGURES
Fig.no.
TitlePageno.
1 The steam explosion effect of a fibril 50
2 Important cellulose derivatives 54
3 IR. absorption spectra of samples 1, 2, 3 and 4 78
4 IR. absorption spectra of samples 5, 6, 7 and 8 79
5 The relation between the lignin content of the pulp of rice
straw samples 1, 2, 3 and 4 to the relative intensity
according to Saad et. al. method
83
6 The relation between the lignin content of the pulp of rice
straw samples 1, 2, 3 and 4 to the relative intensity
according to the background correction method
84
7 The relation between the lignin content of the pulp of rice
straw samples 1, 2, 3 and 4 to the relative intensity
according to base line technique method
85
8 The relation between The lignin content of the pulp of rice
straw samples 1, 2, 3 and 4 to the relative intensity
according to Pislot method
86
9 The relation between the lignin content of the pulp of rice
straw samples 5, 6, 7 and 8 to the relative intensity
according to Saad et. al. method
87
10 The relation between the lignin content of the pulp of rice
straw samples 5, 6, 7 and 8 to the relative intensity
according to the background correction method
88
11 The relation between the lignin content of the pulp of rice
straw samples 5, 6, 7 and 8 to the relative intensity
89
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according to the base line technique method
12 The relation between the lignin content of the pulp of rice
straw samples 5, 6, 7 and 8 to the relative intensity
according to the Pislot method
90
13 The relation between sodium hydroxide solution and the
lignin content determined from different methods from IR
charts when pulping for 1 hr
92
14 The relation between sodium hydroxide solution and the
lignin content determined from different methods from IR
charts when pulping for 2 hrs
93
15 TGA thermograph of sample 3 94
16 TGA thermograph of sample 7 95
17 TGA thermograph of sample 8 95
18 TGA thermograph of sample 10 96
19 DTA thermograph of sample 8 98
20 The relation between sodium hydroxide solution
concentration and Kappa number (boiling for 1hr)
100
21 The relation between sodium hydroxide solution
concentration and Kappa number (boiling for 2 hrs)
101
22 Oxidation of residual lignin during bleaching sequences 103
23 IR. absorption spectra of samples 9, 10 and 11 105
24 IR. absorption spectra of samples 12, 13 and 14 109
25 A typical IR spectrum for lignin extracted 110
26 IR. absorption spectra of samples 15, 16, 17 and 18 112
27 The relation between the lignin content of samples 15, 16, 113
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17 and 18 to the relative intensity according to Saad et al.
method
28 The relation between the lignin content of samples 15, 16,
17 and 18 related to the relative intensity according to the
background correction method
114
29 The relation between the lignin content of samples 15, 16,
17 and 18 related to the relative intensity according to the
base line technique method
115
30 The relation between the lignin content of samples 15, 16,
17 and 18 to the relative intensity according to the pislot
method
116
31 The relation between lignin content of samples 15, 16, 17
and 18 to the relative intensity determined by different
methods from IR charts
117
32 TGA thermograph of lignin sample 15 118
33 DTA thermograph of lignin sample 15 119
34 The relation between conditions of treatment and the
brightness of the produced pulp according to Kappa number
120
35 IR. absorption spectra of samples 19, 20 and 21 122
36 XRD patterns of samples 19, 20 and 21 124
37 TGA thermograph of sample 21 125
38 DTA thermograph of sample 21 126
39 IR. absorption spectra of samples 8, 22, 23 and 24 128
40 TGA thermograph of sample 22 129
41 TGA thermograph of sample 24 130
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42 DTA thermograph of 24 131
43 SEM of rice straw sample 132
44 SEM of sample no.1 133
45 SEM of sample no. 4 133
46 SEM of sample no. 5 134
47 SEM of sample no. 8 134
48 SEM of sample no. 11 137
49 SEM of sample no. 24 138
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ABBREVIATIONS
AGU Anhydrous glucose unit
APMP Alkaline peroxide mechanical pulping
AQ Anthraquinone
ASAM Alkaline sulfite anthraquinone methanol
ASTM American society for testing and materials
ATR Attenuated total reflection
CMC Carboxymethyl cellulose
CTMP Chemi-thermomechanical process
DMAc Dimethylacetamide
DMSO Dimethylsulfoxide
DP Degree of polymerization
DS Degree of substitution
ECF Elementary chorine free
FAO Food and agriculture organization
IR Infra-red spectroscopy
HPLC High performance liquid chromatography
IDE Impregnation de-polymerization extractionIRSP Impregnation rapid steam pulping
ISO International standards organization
MC Methylcellulose
NMR Nuclear magnetic resonance
SEC Size exclusion chromatography
SEM Scanning electron microscopy
TAPPI Technical association of the pulp and paper industry (USA)
TCF Totally chlorine free
THF Tetrahydrofuran
TGA Thermo gravimetric analysis
XRD X-ray diffraction
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AIM OF THE WORK
The objective of this dissertation is to study rice straw as an alternative source
for the production of cellulose pulp and some other useful materials such as xanthated
cellulose. The conditions of preparation (pulping, bleaching and xanthation) and
factors affecting the main properties of the produced cellulose pulp and xanthated
cellulose (the effect of alkali concentration, time of pulping and weight / volume ratio
when boiling) are intended. The resulting intermediate products as well as the final
products, were characterized and compared at each experimental step.
The IR absorption spectral analysis were undertaken according to differentmethods (Saad et. al., base line technique, back ground and Pirlot methods)
Experiments were done according to the following six steps;
1. Characterization of rice straw from El-Delta region, Egypt.
2. Mechano-Chemical pulping and bisulphate pulping methods.
3. Bleaching with bisulphate and H2O2.
4. Improving accessibility and reactivity of bleached pulps.
5. Xanthation of both unbleached and bleached pulps.
6. Characterization of synthesized xanthates.
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INTRODUCTION
1. Annual plants
Crop residues and other agricultural by-products once categorized as wastes have
become major components of livestock feed in many countries. The rapid increase in
their use is due to several factors, such as increasing demand for food, greater
pressure for agricultural land use, raising cost of better-quality feed, pollution
problems due to waste disposal, and the realization of the wasting of enormous
quantities of potential sources of carbohydrates [1, 2].
Agricultural by-products have many uses, rice straw is used in paper industry,
small quantity of straw used for the feeding and bedding of cattle and buffaloes, most
of the straw produced in Egypt is either ploughed in the land or burned directly on the
field. Cereal straws are often used for thatching houses in Asian countries. Straw is
also a good packing material. Many farmers use straw and stubble as a mulch.
The chemical compositions of annual fibers vary greatly, not only according to
their species, plantation location, and growth environment but also to their harvest
times. Studies have reported many varieties in fiber sources, fiber ages, anddetermination methods. Data on the chemical composition of several common plant
fibers are shown in table 1 [3]. Generally, about 40 to 50 % of the weight of annual
plants is cellulose (which is the main component of these plants), except for cotton,
which has much higher cellulose content. About 10 to 30 % of the weight of annual
plants is lignin and 20 to 30 % is hemicellulose. The ash content varies greatly.
Annual plants have much higher ash content than woods [4].The chemical compositions in table 1 show that all annual plants have similar
chemical properties, such as lower lignin contents, higher pentosan or hemicellulose
contents and higher ash contents than woods.
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Annual plants and agricultural residues, received more attention in recent years
for produce of pulp, paper, paperboard, and cellulose derivatives. In fact, non-wood
materials had been used to produce cellulosic products since the invention of
papermaking by a Chinese, Cai Lun [5].
Wood is not available in sufficient quantities in many countries, alternative new
non-wood raw materials need to be investigated and exploited for the potential
substitution of wood. Therefore, the cellulose industry included the investigation of
such new resources as overproduced crops, agricultural waste, unconventional plants
and common wild plants to decide whether it is feasible to use them to produce paper,
paperboard and cellulose derivatives, such as tailor-designed methylcellulose as an
additive for cement, food and drug [6, 7].
Annual plants are considered as potential resources because of overproduction of
agricultural crops [8, 9], their higher yield of cellulose than wood [10, 11], lower
lignin contents and consumption of less pulping chemicals and energy [12]. Cellulose
can be obtained from annual plants by a mild pulping process, which consumes less
energy and chemicals in a shorter cooking time [13]. The investment on producing
processes reduces at the same time. Annual plants can be planted, cultivated, and
harvested every year.
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Table 1. Chemical composition of natural fibers
Fiber source Cellulose (%) -Cellulose (%) Lignin (%) Pentosans (%) Ash (%) Silica (%)
Leaf fibers
Abaca 56-63 7-9 15-17 1-3
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These special characters were the dominant direct importance for their
development. However, the annual plants have some specific problems as raw
materials of cellulosic products. Harvesting is limited to only a few weeks of a year.
Annual plants are planted and scattered in many small fields that thus cause the
difficulty of transportation and collection. A sufficient store capacity is needed to set
up to ensure an all-year supply. Most annual plants are attacked easily by
microorganisms. To minimize degradation, these plants should be stored as dry as
possible.
Transportation of wood is more expensive and difficult than annual plant, under
the consideration of the economical objective, the environment influence, the
sufficient supply, and the higher yield of cellulose, annual plants are now gradually
substituting woods as alternative resources of cellulosic products [14].
Currently, about 55 % of the feedstock for the production of pulps is virgin
wood, 9 % is non-woody sources, and 16 % is recycled paper [15]. The main woods
for cellulosic products are from rapid growth species such as eucalyptus and pine.
Agricultural crops (especially straw and bagasse) and natural plants can be alternative
sources to forest woods if they can be found in sufficient supply, the most important
annual plants for the pulp industry are agricultural residues (bagasse and cereal straw)
and naturally cultivated bamboo and reeds. Other important annual plants, such as
miscanthus, flax, kenaf, sisal, jute, hemp, and cotton, are valuable raw materials for
the production of special pulps, special papers, and cellulose derivatives [16].
Annual plants have several advantages over wood resources. Firstly, they grow
to maturity much more quickly than wood species. Hemp can be harvested within
three to four months. Other annual plants such as straw, flax, abaca etc. can be
harvested yearly. This brings quicker profits for the farmers and obtains a higher
cellulose yield. Secondly, crop residual fibers such as bagasse, straw, flax, jute, and
wild plants can be used, so profits are higher profit thanks to these low-value
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lignocellulosic sources. Thirdly, annual plant sources are usually chipped to smaller
sizes (about 4 cm in length) than wood chips in the pulp digester. Annual plant stalks
have more porous fiber structures and weaker inter-fiber lignin deposits. This
requires less cooking energy and less time. Bagasse and straw, for example, cooked
within 10 to 15 minutes, save a lot of energy in a short time. Finally, fewer cooking
and bleaching chemicals are used for annual plants than for wood chips.
Annual plants generally have lower lignin contents, higher pentosan contents,
higher hemicellulose contents, and higher ash contents (especially silica) than woods,
while the cellulose contents are almost equivalent. So far these plants have only been
used in the manufacture of textiles and paper products that constantly compete with
synthetic and wood fibers. The feasibility of using annual plant fibers in other
applications has not been widely researched or developed [17]. The most widely
available annual plants are the straw of cereals, the stems of corn and sugar cane,
which are listed in tables 2 and 3.
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Table 2.Average annual yields of some raw materials [18]
Plant Fiber yield
(Tones /year/ha)
Pulp yield
(Tones/year/ha)
Scandinavian softwood 1.5 0.7
Fast growing softwood 8.6 4
Temperate softwood 3.4 1.7
Fast growing hardwood 15 7.4
Wheat straw 4 1.9
Rice straw 3 1.2
Bagasse 9 4.2
Bamboo 4 1.6
Kenaf 15 6.5
Hemp 15 6.7
Miscanthus 12 5.7
Canary grass 8 4.0
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Table 3. Worldwide availability of annual plant fibers
Fiber type Potential availability for pulping (MillionBDMT)*
Agricultural residues
Wheat straw 600.0Other cereal straws 290.0
-Barley straw 195.0
-Oat straw 55.0
-Rye straw 40.0
-Rice straw 360.0
Grass seed straw 3.0
Seed flax straw 2.0
Corn stalks 750.0
Sorghum stalks 252.0
Cotton fibers 89.0
-Cotton staple fiber 18.3
-Cotton linters (first & second 2.7
-Cotton stalks 68.0
Sugar cane bagasse 102.2
Non woody crop fibersStem fibers 13.9
-Jute, kenaf, hemp, etc.
Leaf fibers 0.6
-Sisal, henequen, maguey, abaca
Natural growing plants
Reeds (Estimate) 30.0
Bamboo (Estimate) 30.0
Papyrus (Estimate) 5.0
Esparto grass (Estimate) 0.5
Sabai grass 0.2
*: Fibers available for delivery to pulp mills. Bone Dry Metric Ton (BDMT).
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2. Rice strawRice straw, unlike other crop residues, is difficult to dispose off and has limited
economical alternative uses. Traditionally, burning was the preferred method of
ridding the field of the waste straw. However, recent clean-air legislation phase out
rice straw burning. Farmers now are faced with the challenge of finding new ways to
eliminate the straw in time to plant the subsequent year's crop.
As burning is phased out, alternative disposal methods for straw residues must
be found. Although limited burning is still possible, other alternative methods of in-
field disposal being used more frequently include various combinations of chopping,
rolling discing and alternate flooding and draining [19, 20].
Other alternative uses for rice straw have been investigated by growers and
industry members and through rice industry organizations such as the California Rice
Industry Association (CRIA). Some alternative uses that have been looked at include
straw bale construction, industrial building materials, packing materials, animal feed
and bedding, erosion control and a variety of there uses. The CRIA has produced a
booklet entitled "Rice Straw Information" that provides more complete information
on rice straw, its uses and surrounding issues [21-25].
Disadvantages of rice straw burning are primarily related to air quality
I- Generation of air pollutants, including; particulates, carbon monoxide (CO),
hydrocarbons, nitrogen oxides (NOx) and sulfur dioxide (SO2).
II- Production of poly-nuclear aromatic hydrocarbons in both gas and particulate
forms, many of which are carcinogenic.
III- Release of airborne silica fibers (small particles of straw ash with possible
carcinogenic health effects).
The amount of pollutants emitted by rice straw burning depends on the moisture
content of the straw, the manner in which the field is burned (heading fire, backing
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fire, strip-light fire), and the emission factor (the pollution emitted per weight unit of
the fuel being burned). The farmer is responsible for monitoring the moisture content
of the straw and proceeding with the burn only if the straw passes the so- called
"crackle" test (indicating low moisture and emission factor) in the field. It is also up
to the farmer to select the method of burning that best suits the environmental
circumstances on the day that the burn is scheduled.
Two central questions for the rice industry are: what are the alternatives to
burning, and how will they affect the competitiveness of California rice according to
the CRIA, it costs the average rice farmer approximately $ 2 per acre to burn rice
straw, and between $ 25 and $ 70 per acre to either plow it under or remove it [26].
Table 4. The technological, economic, and commercial feasibility of each non-burn
alternative
On-farm non-burn alternatives Off-farm non-burn alternatives
Crop rotationStraw decompositionBaling
Energy conversionPulp products manufacturingConstruction products manufacturing
Straw bale structureConstructionCompostingMushroom productionErosion controlLivestock feed
The pressing need for economic alternatives to burning has spurred substantial
public and private investment in research into solutions, as well as implementation ofpilot-scale, and even large-scale programs. The California Rice Research Board has
invested over 15 million $ of its income from rice farmer's contributions since 1969
to fund research projects concerned with the use of rice straw [27].
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2.1. On-farm non-burn alternatives
Straw decomposition is a management practice developed as an alternative to
burning. It is implemented by either of the following methods; chopping the straw
and incorporating it into the soil by tillage and rolling the standing straw into the soil
using a mechanical crimping/rolling device.
A variety of field implements can be used to chop straw. Harvester-mounted
choppers shred the straw into long pieces; flail choppers pulled behind a tractor
produce a range of sizes of shredded pieces. Self-propelled forage choppers leave the
straw in pieces less than 2 inches long. Smaller pieces are easier to incorporate with
field tillage equipment. The use of choppers on rice straw is effective, but the high
silica content of the straw causes a great deal of wear on chopper blades relative to
other crops.
Straw incorporation is usually accomplished by chisel or disk tillage. The
number and type of field operations required to achieve a good straw/soil mixture
depends on soil type. Clayey rice growing soils are difficult to till, making
incorporation more difficult.
Even though micro-organisms are abundant in rice soils, certain other
environmental conditions are required to accomplish the decomposition process [28].
Temperature, moisture, and available oxygen are all essential factors affecting
decomposition. Straw breakdown occurs between 40 and 86 oF and is more rapid at
the upper end of this temperature range. Soil moisture also influences the rate of
straw decomposition. Straw can decay with or without air, but the pathways and
byproducts produced under each condition are quite different. For straw to
decompose most rapidly, an optimal mixture of air and water in soil pores is helpful.
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For the clay soils on which rice is grown, this moisture content is about 30 %, with an
air content of about 20 %, both by volume. Decomposition rates decrease as soil
moisture levels become extremely dry or wet.
Rice straw rolling is a decomposition method that was developed to reduce
straw incorporation costs while leaving unharvested rice grain accessible to foraging
wildlife. The roller design was developed with funding from the Dow Chemical
Company, Dow Elanco, and the National Fish and Wildlife Foundation [29]. Rollers
take a variety of forms (e.g., rolling cages, fluted drums). Each design flattens most
straw to the soil, while pressing some slightly into the soil.
A typical protocol involves draining the field and harvesting the grain,
reflooding to a depth of 2 to 6 inches, and then using the roller to crush straw and
stubble into the soil. The stirring action creates a good mixture of soil, water, and
straw, bringing the crop residue into contact with the soil micro-organisms that begin
the decomposition process. This approach has the demonstrated advantage to
waterfowl of preserving residual rice seed as a carbohydrate source and of creating
winter habitat that fosters the growth and development of dietetically important
invertebrate species [30].
Baling can involve several operations, the most important of which is cutting the
straw below the water line, which is the principal infection point for stem rot. Baling
and removing rice straw from the field can be as effective as burning in controlling
stem rot if the straw is cut below the waterline and completely removed from the
field. The usefulness of baling is restricted by the limited markets available for rice
straw, rice straw products and the high cost of bale transport. Purchase, removal, and
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transport of straw bales from the field currently range from $ 4 to $ 6 per bale,
depending on the transporter and destination. Alternative uses for rice straw, such as
bio-energy production and building materials, are currently being evaluated for
feasibility [31].
2.3. Off-farm non-burn alternatives
This is an attractive alternative because rice straw has relatively high energy
content (up to 8,000 Btu per pound). Bio-energy production plants often cannot
afford to transport feedstock more than 15 to 20 miles, which precludes this option
for the majority of rice producers. Potential methods of energy conversion include:
a- Anaerobic digestion to produce methane gas
b- Direct combustion to produce electric power
c- Ethanol production
This is a fermentation process in which organic waste is converted to methane
and carbon dioxide gases in three stages:
1. Pretreatment to break down complex organic compounds into soluble
components.
2. Oxidation to produce low-molecular-weight organic acids.
3. Fermentation to produce methane gas.
The economic feasibility of agriculturally produced methane is highly dependent
on the demand for methane and on the cost of competitive materials, e.g., natural gas.
Taking into account high transportation costs, anaerobic digestion can only compete
with natural gas in remote and isolated areas where it is feasible to generate methane
on-farm [32].
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Direct combustion alternatives include burning rice straw in biomass power
plants for power generation, and the use of rice straw in logs or pellets for home
heating.
At power plants, the alkalinity of rice straw (associated with the potassium and
chloride content) creates costly, and seemingly insurmountable, slugging problems in
furnaces. When rice straw is burnt, a large volume of ash is generated because of
the high silica content of the straw. The high silica content also compromises the
straw's energy conversion efficiency. Ash disposal is a significant logistical and
permitting challenge. The large volumes produced, as well as the potential presence
of crystalline silica, can cause its classification as a hazardous waste, potentially
making ash disposal time-consuming and costly.
The process for converting rice straw to ethanol includes the following steps:
a- Pulverizing the straw.
b- Blending to produce a liquid slurry mix.
c- Hydrolyzing cellulose molecules in the slurry mixture to produce simple
sugars.
d- Fermenting the sugar-rich liquid.
f- Distilling fermentation products to ethanol.
Two methods of hydrolysis are available, acid and enzymatic. They differ in the
means by which the cellulose is broken down into fermentable sugars. Acid
hydrolysis occurs in a single step in which the cellulose is exposed to a strong acid to
produce the sugar liquor. Enzymatic hydrolysis is a two-step process in which the
straw is pretreated to separate the cellulose and hemicellulose components. A dilute
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acid breaks down the hemicellulose to simple sugars, and then enzymes produced
from genetically engineered fungi or bacteria break down cellulose. The two-step
process is potentially more efficient because it can produce higher overall yield of
ethanol from a given amount of rice straw.
The Sutter ethanol partners project proposes to construct a cogeneration facility
north of Sacramento, California. The facility will burn natural gas to produce steam
for power generation. Residual steam will be used to break down rice straw into sugar
components that will then be converted to ethanol and other byproducts. The facility
would convert 132,000 tons of rice straw into 10 million gallons of ethanol annually.
If successful, the plant will consume up to 15 percent of the Valley's rice straw while
producing clean- burning ethanol to power internal combustion engines and for other
uses. The Sutter project would eliminate the need to burn rice straw on 50,000 acres
near Sacramento while making cleaner-burning fuel available for automobiles [33].
The City of Gridley (California USA) plans to construct an experimental plant to
convert rice straw into ethanol and power. The facility would dispose of
approximately 20 percent of California's rice straw, harvested annually from 80,000
acres of nearby rice fields. The facility would produce about 20 million gallons of
ethanol annually, which would in turn be used to generate surplus power in an
amount equal to half of Gridley's annual demand. The project is part of a feasibility
study funded by Congress since 1994. It is being conducted at the University of
California, Davis, and at the U. S. Department of Energy pilot plant in Golden,
Colorado, that went into operation in mid-1995. The CRIA is currently preparing to
deliver 85 tons of straw to the National Renewable Energy Labs in Golden, Colorado,
for this study. Construction of the Gridley facility is planned to begin in 1998,
pending favorable feasibility study results [34, 35].
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The pulping process changes raw, cellulose-rich materials (e.g., rice straw) to a
form that can be used in the production of paper, fiberboard, and many molded
products. Rice straw contains the highest level of silica. High-silica content makes
handling difficult because it is abrasive and rigid. Also, disposal of residual high-
silica black liquor sludge is difficult. These factors result in increased manufacturing
costs and economic disincentives for the use of rice straw when other less demanding
materials are available [36].
In the early 1980s, the Rice Research Board and Louisiana Pacific joined in a
study on the technological and economic feasibility of producing corrugated paper
from rice straw. The study indicated that the market was inadequate to support a
production facility on the West Coast. A plant producing corrugated paper from rice
straw was established in California, but the plant was closed in 1989 [37].
Manufacturing fiberboard from rice straw requires the use of chemical binders
which are different from the binders used in the manufacture of fiberboard from
wood chips. These binders represent a fairly new technology which is still under
development. Difficulties encountered during experimental production of medium-
density fiberboard from a 50/50 mixture of rice straw and hardwood chips are
reported in the "Economic Uses for Rice Straw" from the Report to California Rice
Farmers, 1969-84.
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2.5. Construction products
Construction of products, for which rice straw is a raw material candidate,
include, wood replacement materials, bricks and cement boards, panels, and straw
bale structure.
Agronomic systems is the name of a process for manufacturing a wood
replacement material using 70 percent biomass and 30 percent recycled plastic. The
material, marketed under the name Bio Comp, is waterproof, resistant to rot and
insects, withstands the sun, and can be shaped and nailed. This biological composite
uses a patented steam process to explosively break apart the biomass, releasing the
starch, sugars, resin, and other raw materials of the fiber. The process works on any
long-cell biomass, including wheat, rye, corn, rice, and barley straws.
Fiber reinforced composite building materials have been used for centuries in
the form of adobe bricks and other products. When straw is combined with cement,
the alkalinity of cement can have adverse effects on the long-term durability of the
fibers. Although alkalinity can be controlled with additives, the resulting product is
heavy and difficult to handle, cut, and fasten.
When rice straw is combined with clay, the resulting product insulates well, but
is not waterproof. If the straw/clay mixture is kiln fired, the composite end-product
loses biomass during the firing process, resulting in a lighter weight product and
further improves insulation properties.
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Manufacture of straw panels or boards, was pioneered in Sweden in the 1930s
and first commercialized in Germany in the 1940s. A number of areas of the world
including Belgium, Australia, China, and the Philippines produce and use straw
panels in construction today.
2.5.4. Straw bale structure construction
Straw bale construction was first used in the Midwestern U.S. in the 1800s and
its use is currently being revived. A 1,600- square-foot house requires approximately
500 bales. At 80 pounds per bale, this corresponds to about 20 tons or the rice straw
from 6 acres. The bale wall structure is typically sealed with plaster or stucco and
will have walls approximately 18 to 24 inches thick with an insulation R value
around 50. The National Research Council of Canada demonstrated the plastered
surface will withstand 1850F for 2 hours before cracking. The straw is sufficiently
dense that it does not readily support combustion.
2.6. Composting
Commercial compost companies use a mixture of several agricultural materials
to produce a desired end product. Candidate materials include straw, manure, fruit
and vegetable waste, leaves, grass clippings, and any other widely available
biodegradable materials. The mix of ingredients is piled into long windrows
(typically 5 feet high, 10 feet wide, and several hundred feet long). Compost turning
equipment mixes and aerates the piles.
The carbon to nitrogen (C: N) ratio of a compost mix is critical. The goal of the
composter is to acquire and mix the ingredients to a 30 C: 1 N ratio and then compost
them down to a finished product of significantly smaller mass and a C: N ratio of
approximately 10:1. Commercial compost-makers use straw in their production mix
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and storage) have not been well established and appear to be the major constraint in
establishing a solid niche for rice straw in erosion-control or fire-rehabilitation
markets.
2.9. Livestock feed
Rice straw is poorly digested by cattle. Cattle use 42 to 48 percent of ingested
rice straw as compared to 65 to 70 percent utilization of alfalfa hay [39]. Part of the
reason for this may be that rice straw is high in fiber, low in protein, and does not
supply enough nitrogen for the efficient metabolism and growth of rumen microbes
necessary to carry out the initial breakdown of the straw. Also, silica has no nutritive
value and may interfere with the digestion process. It can be used, however, by adult
or pregnant animals because their requirements for energy and protein are small
compared to their capacity to consume feed. Diets composed of 75 to 85 percent rice
straw were adequate to support pregnant cows and result in calves with a birth weight
comparable to those kept under conventional management practices on dry range or
on irrigated pasture for an equivalent period of time.
2.10. Additional available resources
a- Energy
Centre of Biomass Technology:This is a Danish biomass information network
comprising four technological institutes dedicated to generating power from biomass.
CBT collects and disseminates technical and economic know-how and experiences
associated with the establishment and operation of straw and wood chip-fired
combustion plants.
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b- Construction
Straw bale construction has been used for more than 100 years in America.
Recently, many new efficient homes have been built with straw bales.
Rice straw, the major agricultural by-product of Egypt, is high in lignin and
silica. Both of these components play an important role in reducing the digestibility
of straw. The crude protein content of rice straw is generally between 3 and 5 per cent
of the dry matter. Any crop residue with less than 8 per cent crude protein is
considered inadequate as a livestock feed because it is unlikely that such residues,
without supplementation, could sustain nitrogen balance in an animal. A further
deficiency in most fibrous material, especially in rice straw, is the low content of
calcium, phosphorus, and trace elements
table 5 [40].
The composition of residues varies with variety, location, and the cultural
practices employed in growing the crop from which they are obtained. If the full
potential of agricultural residues available in vast quantities throughout Asia is to be
realized, it is necessary that some types of treatment before feeding them to livestock
should be considered.
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Table 5. Average chemical composition of rice straw
Component Percentage (%)
Digestible energy 1.9 (kcal/kg)
Crude protein 4.5
Crude fiber 35.0
Ether extract 1.5
Lignin 4,5
Cellulose (%) 34.0
Nitrogen-free extract 42.0
Total digestible nutrients 43.0
Ash 16.5
Silica 14.0
Calcium 0.19
Phosphorus 0.10
Potassium 1.2
Magnesium 0.11
Sulphur 0.10
Cobalt 0.05 (mg/kg)
Copper 5.0 (mg/kg)
Manganese 4.0 (mg/kg)
A number of physical, biological, and chemical methods of treatment have been
described. Their aim was to increase digestibility and voluntary consumption, thereby
increasing the intake of digestible energy (DE). The treated material is often enriched
with nitrogen and mineral supplements in order to make it more completenutritionally. Some of these methods will be described, the emphasis being placed on
chemical methods of treatment.
Fibrous raw materials for pulp and paper production are generally divided into
three main categories, wood fibers which constitute about
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75 % of all fibrous raw materials supply of paper mills, reclaimed waste paper about
20 % and remaining 5 % are non wood fibers [41]. Rice straw could be very
important as a source for paper production especially in Egypt. Cereal straw, in
particular wheat straw, is a major source of pulp for paper production in china and
other Asian countries [42].
The high silica content of rice straw (9-14 %) however prohibits the economic
use of rice straw for this purpose. The silica will cause problems in the recovery of
chemicals used in the pulping process. For rice straw, there is currently no
commercially available solution for this problem. Other problems with the use of rice
straw for pulp are the higher water retention capacity of straw, the lower yield per ton
of raw material compared to wood, straw yields 45 % of pulp whereas wood yields
55 %, and the low bulk density of straw [43].
2.11. Chemical composition of wood and straw
The main components of wood and straw are cellulose, lignin, hemicelluloses
and soluble substances (extractives). The major polysaccharides component of wood
is cellulose, which has high molecular weight and is a highly crystalline material. The
term hemicelluloses refers to a mixture of low molecular weight polysaccharide
which is closely associated in plant tissues with cellulose. Lignin comprises 20-35 %
of wood substance and consists of the total non-carbohydrate fraction of extractive
free wood. Lignin compounds are essentially substituted phenyl propane three
dimensional polymers which are held together by ether or carbon bonds.
Some international applied solutions
1- The NACO (North American papermaking Cooperation) International system,
devised in Foggia, Italy in 1982, claims that its recovery system can handle high
levels of silica. The Arisa group in Australia intends to build a second NACO
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process for straw, but plans haven't materialized yet. It is not clear why other
paper manufacturers don't seem interested in this process [44].
2- Al Wong of Arbokem proposes to run a pulp mill in such a way that the effluent
can be used as a liquid fertilizer. According to Mr. Wong [45], growers would be
interested in exchanging their straw for his fertilizer. The concept has been setup
in a pilot plant in Alberta but hasn't matured yet. Some trials were carried out
with a blend of California rice straws and Oregon Rye Grass with mixed results.
3- Granit SA of Switzerland claims a new technology to solve the recovery problem.
They're working on a pilot plant in Thonon, France, to prove the concept, which
seems very promising. This technology is not mature either, and needs a couple
of more years to develop [46].
4- Weyerhaeuser spent a great deal of time and money in the 90's figuring out a
decent straw supply system, and figuring out the staketech steam explosion
system. Weyco built a complete pilot plant in Springfield, Oregon and made it
work. Nevertheless Weyco decided not to pursue straw pulping, and as of yet it is
unclear what the exact reason for discontinuation was. The project reports are not
public, but if you contact Bill Fuller at the Weyco research facility in Tacoma one
may be able to find more knowledge [47].
5- A pulp and paper manufacturing group called ABC pulp and paper in India claims
two patents and a pilot process proof of treating silica rich black liquor effluent
from a straw pulping process. The process is defunct and has not been proved on
industrial scale [48].
6- The Finnish company Conox claims similar results.
7- Universal Pulping of Eugene, Oregon claims a low-temperature, low pressure, low
emission process particularly suitable for non-wood pulping. The process,
patented by Eric Prior, was evaluated by the pulping labs at NCSU, WSU and the
University of Washington with very promising results. Although promising, the
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technology is not "off-the-shelf" yet, it has to be scaled up and debugged first. UP
is working on a pilot project to establish the technology [49].
Only the NACO process in Italy has been demonstrated on an industrial scale
for a significant period of time. All other technologies are in the R&D stage and do
not have a guaranteed performance [50].
The ultimate solution
Government support for straw utilization in papermaking is the most likely
way to get straw pulping accepted in Egypt.
Straw competes with wood as a raw fiber material. Only when we run low on
wood fiber will straw become an economically viable alternative. It is impossible to
anticipate a rise in pulping wood fiber cost in the near future to the extent where an
investment in straw based pulping can be justified. Fiber supply experts talk about a
variety of market effects:
a- Will paper use continue to increase?
b- Will the new Economy and the electronic office slow down the use of paper?
c- Will the recycling rate increase?
d- Will tree farms be able to supply sufficient fiber?
e- Will fiber imports from tropical countries and overseas tree farms continue?
f- Will consumer awareness shape the demand for paper that uses particular types of
fiber?
3. Pulping processes
There are many pulping processes, see table 7, these include mechanical pulping,
semi-mechanical pulping, chemical pulping, and bio-pulping. The pulping processes
suitable for annual plants are listed in table 6 the most common commercial method
for annual plant pulping is the soda method [51]. There are also several new
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physiochemical methods table 6 with good potential for producing high quality pulp
from annual plants. The kraft and neutral sulfite processes are less used. The acid
sulfite process is not used because it produces brittle pulps with high ash contents and
inadequate strengths. For higher yield pulping, the chemi-refiner mechanical pulping
process is used. Mechanical pulps are suitable for newspaper but not for cellulose
derivatives, which need celluloses of high purity to ensure high quality [52]. The
pulping processes concentrate not only on optimizing pulp quality but also on
improving pulp yields, reducing energy consumption, reducing chemical
consumption (and improving the recovery processes of the chemicals), reducing
pollution and developing sulfur-free pulping processes and chlorine-free bleaching
sequences .
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Table 6.Pulping processes for annual plants
Pulping process Chemical treatment Mechanical treatment Frequency
Chemical pulping
Soda (+AQ NaOH (+AQ) None Commonly used
Kraft (+AQ) Na2S + Na2OH (+AQ) None Commonly used
Sulfite NaHSO3 and/or SO2/Na2CO3 None Commonly used
Phosphate Na3PO4 None Potentially used
Milox Formic acid None Potentially used
Impregnation- NaOH, sodium carbonates None Potentially used
Depolymerization- ethanol- water blend
Extraction (IDE)
Alcell Ethanol-water blend None Potentially used
Processes other than chemical pulping
Thermo-mechanical Steam None Potentially used
Biopulping White rot fungi Disc refiner Potentially used
Ceriporiopsis subvermispora Disc refiner Potentially used
Alkaline peroxide NaOH, H2O2 Disk refiner Potentially used
mechanical pulping (APMP)
Chemi-thermomechanical Steam + NaHSO3 + Disc refiner Potentially used
(CTMP) NaOH
Cold caustic soda mechanical NaOH Disc refiner Potentially used
IRSP NaOH (+AQ) + Steam None Potentially used
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Table 7. Pulping processes and yields*
Pulping process Chemical treatmentMechanicaltreatment
Plant**Yield
%
Mechanical pulpingStone groundwood None Grindstone S 9399Steamedgroundwood Steam Grndstone S 8090Refiner mechanical None Disc refiner A, S 9398Thermomechanical Steam
Disc refiner(Pressure)
A, S 9198Asplund Steam Disc refiner A, S 8090Biopulping
White rot fungi Ceriporiopsis
subvermisporaDiscrefinerDisc refiner
A, H, SA, H, S
Chemi -mechanical and Chemi-thermomechanical pulpingChemigroundwood
Neutral sulfite or Na2S +NaOH
GrindstoneH, SH, S
H, S
80928090
8590Chemi refinermechanical pulp
NaOHorNaHSO3or Alkalinesulfite orAcidic sulfite
Disk refiner A, H, S 8090
Chemithermomechanicalpulp
Steam+Na2SO3+NaOHDisc refiner(pressure)
A, H, S 6597
Semi-mechanical pulpingNeutral sulfite Na2SO3+Na2CO3or NaHCO3 Disk refiner A, H 6590Cold soda NaOH Disk refiner A, H 6590Alkaline sulfite Na2CO3, Na2S, NaOH Disk refiner A, H, S 65-90Sulfate Na
2S + NaOH Disk refiner A, H 6590
Soda NaOH Disk refiner A, H 6590Green liquor Na2CO3 + Na2S Disk refiner A, H 6590Non-sulfur Na2CO3+ NaOH Disk refiner A, H 6590
Chemical ProcessesKraft (High yield) Na2S + NaOH Disk refiner A, H, S 5565Sulfite (High yield)
Acidic sulfite (Ca, Na, Mg)Or Bisulfite (Na, Mg)
Disk refiner A, H 5570
Kraft (+AQ) Na2S+NaOH (+AQ) Mild to none A, H, S 4555Kraft (Polysulfide) (Na2S+NaOH)x None A, H, S 4560Soda NaOH None A, H 4055Soda- AQ NaOH + AQ Mild to none A, H 4555Soda- oxygen NaOH, O2 Disk refiner A, H 4560
Acidic sulfiteAcidic sulfite (Ca, Na, Mg,
NH3)Mild to none A, S 4555
Bisulfite Bisulfite (Na, Mg, NH3) Mild to none A, H, S 4560Neutral sulfite Neutral sulfite Mild to none A, H, S 4560
**: A: annual plants; H: hardwood; S: softwood.
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3.1. Mechanical pulping
Mechanical pulps are obtained by disintegrating and physically separating the
fibers. These pulps have very intense yellow color and are often used for newspapers
or paperboards. Because of the large quantity of residue lignin in the pulps, the color
of these pulps easily turns yellow, but this can be overcome by subsequent chemical
bleaching. Softwood is the most common raw material of mechanical pulps, which
are relatively white. Annual plants are the easiest materials to use with mechanical
pulping because of their porous stalks. Mechanical pulping does not use chemicals to
eliminate lignin and hemicellulose, so yield is often high (90-98 %) as shown in table
7.
3.2. Chemical pulping
With chemical pulping, delignification is carried out with the help of acidic or
alkaline reagents in reactors. The lignin and hemicellulose are partially eliminated so
yields are between 40 and 60 %. On the other hand, the fibers are whiter and better
separated. Chemical pulping is divided into sulfite pulping and alkaline pulping
depending on the pH and nature of the pulping reagents. Sulfite pulping is a stronger
process because the separation of cellulose is better and their pulps can be used to
produce chemicals and papers of particularly good quality.
3.2.1. Soda process
This is the oldest and simplest pulping process. The soda process is a
common way to produce annual pulp. With this process, the cooking chemical is
mainly sodium hydroxide. Soda process leaves more insoluble carbohydrates in the
pulp and obtains a better yield than the kraft method. The strength and lignin content
of pulps produced with the soda and kraft processes are similar. Easily bleachable
short fibers that are abundant in pentosan are produced. This process often uses easily
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pulped species such as cereal straws, flax, abaca etc. Stevens et al. [53], proposed a
soda pulping process in which a catalyst, anthraquinone (AQ), is added. This catalyst
has two fundamental effects: the alkaline delignification process is accelerated and
the carbohydrates are stabilized. Soda-AQ pulping improves the yields under the
same operation conditions as conventional soda pulping. The use of this catalyst
(AQ) is only limited to 0.1 % of the dry biomass. Since annual plants are impregnated
easily, and have a low reactive lignin content table 6, the amount of pulping
chemicals needed for annual plants is lower than for woods. With soda pulping, 10
15 % NaOH, which depends on the raw material, is normally used at a pulping
temperature of 160170 C. Yields range from 40 to 55 % and are influenced greatly
by the species and quality of the raw material.
3.2.2. Kraft process
Kraft pulping is the most important pulping method. At present, more than half
of the worldwide production of pulps is manufactured using this method [54]. Yields
vary between 40 and 60 %. Kraft pulping requires shorter cooking times and is not
very selective. The pulping chemicals used are mainly NaOH and Na2S [55, 56]. The
raw material is treated with a highly alkaline solution of NaOH, which is known to
cleave lignin but also eliminates some of the hemicellulose. The undesirable
breakdown of hemicellulose is largely avoided by adding Na2S to the solution, which
avoids a very high concentration of NaOH in the pulping liquor. Kraft pulping
usually operates in batch reactors with a temperature between 160 and 180 C and a
cooking time between 4 and 6 hours. Continuous kraft pulping operates at a
temperature between 190 and 200 C and a cooking time between 15 and 30 min [57].
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3.2.3. Sulfite process
The main pulping chemicals are NaHSO3and/or Na2SO3[58, 59]. The reactors
for this process can be continuous or batch and operate at temperatures between 125
and 180 C depending on the final product (paper, cardboard, etc.). This process has a
yield of between 40 and 60 %. In the pulping process, sulphonates form and are
hydrated and the swelling of fibers helps delignification. The strongly ionized
sulphonic acids increase the acidity of the pulping medium, which results in
condensation reactions between phenolic moieties in lignin. This forms insoluble
resin-like polymers. These side reactions include degradation of the hemicelluloses
and celluloses. However, these carbohydrates are less degraded, which causes a
higher degree of polymerization and therefore a lower resistance of the pulps than in
the kraft process. Sulphite pulps are easier to bleach and are used to produce paper
with specific properties, such as toilet and tissue paper, which must be soft,
absorbent, and strong.
3.2.4. IRSP (impregnation rapid steam pulping process) process
Montan et al. [60], developed the IRSP process using wheat straw, which is
also tested by other annual plants and woods such as pine, miscanthus, sugar cane,
cardoon, and eucalyptus [61]. This process differs from the steam explosion pulping
in the nature of the impregnation, which generally uses concentrated NaOH solutions,
moderate pressures, and short impregnation times of 12 hours. This process consists
of two steps:
a- Impregnation
The aim of impregnation is to obtain a uniform distribution of pulping
chemicals in chips. Uniform distribution leads to more uniform pulp, better quality,
fewer rejects, and shorter cooking times [62]. The reactive pulping chemicals are
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mass-transferred into the stalk voids by penetration (which is governed by the
pressure gradient) and by diffusion (which is controlled by the concentration gradient
of the penetrating chemicals) [63].
NaOH and anthraquinone (AQ) are used as pulping chemicals under mild
pressure. Chemicals penetrate and diffuse into the capillaries and stalk voids. The
stalk fibers swell until maximum absorption is reached. Water, NaOH, AQ, and
alkaline soluble chemicals transfer between the fiber and the bulk solution until an
equilibrium stage is reached. Delignification, the softening of fibers and defibration
occur during the swelling and penetration stages. Some lignin that reacts with NaOH
degrades and dissolves in the alkaline solution. The initial white color of the alkaline
solution becomes darker and blacker [64].
b- Rapid steam pulping
Explosion pulping was invented by Mason [65]. Vit and Kokta [66] developed
the process to produce pulps that are suitable for papermaking, using techniques such
as the chemical impregnation of chips, short-duration saturated steam cooking and
sudden pressure release. Steam explosion pulping can be divided into two stages:
rapid steam cooking and steam explosion. In the rapid steam cooking stage, typical
cooking time is several minutes and typical cooking temperature is above 180oC. The
short cooking time prevents side reactions, it improves the selectivity and the yield of
pulps. Water has a plasticizing action on the glass transition temperature of lignin and
hemicellulose, and their softening temperature is reduced to about 100 oC. Steam
cooking at temperatures above their glass transition temperature leads to an additional
permanent fiber softening because of internal structural changes. The increasing
numbers of voids helps and improves the effect of the subsequent steam explosion
pulping.
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Fig. 1. The steam explosion effect of a fibril
During steam cooking, interior capillaries and fibril voids are gradually filled
with high-pressure liquid. When the cooking pressure is suddenly released, the high-
pressure liquid evaporates, which subjects the fibers to high impact forces. The fibers
are lacerated figure 1.
There are some other techniques as Pulping with organic solvents,
ASAM (alkaline sulfite-anthraquinone-methanol) process, Organocell
process, Alcell process, Acetocell process, Milox process and The IDE
(impregnationdepolymerizationextraction) process are collected in table 7.
4. Bleaching
Pulp bleaching is carried out in a sequence of several stages to eliminate as
much residual lignin as possible. Usually lignins are physically dissolved in alkaline
solution or chemically modified to form soluble chemicals in aqueous/alkaline
solutions [67, 68].
This process often uses two types of reagents (oxidants and alkali) though
reductants are sometimes used [69]. The oxidants are used to degrade and whiten the
lignins. The alkali is also used to dissolve the lignin. The alkali extraction can be used
to eliminate hemicellulose if the objective is to obtain dissolving pulps. The
following bleaching stages are often used in the contemporary bleaching industry
[70].
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4.1. Chlorination (C)
Chlorine is a common, effective, selective bleaching agent that reacts quickly
with lignin to form water-soluble degraded chemicals, which can be extracted with
alkaline solution. Chlorination is carried out at about 300oC and usually lasts for 30
min for sulfite pulp and up to 60 min for kraft pulp at a consistency of about 3 %.
Shortly after this chlorination bleaching, the next bleaching process is alkaline
extraction.
4.2. Alkaline extraction (E)
Alkali solution can dissolve some degraded lignins, degraded hemicelluloses
and some depolymerized celluloses of low molecular weights. Alkaline extraction is
often carried out using 11.5 % NaOH (based on o.d. pulp) for sulfite pulps and 3 %
for kraft pulps, which often lasts for 6090 min at 4060 C at a consistency of about
10 %. If dissolving pulps of high as 90 C and the alkali charge as high as 5 % NaOH
based on o.d. (oven dried) pulp.
4.3. Hypochlorite bleaching (H)
The oxidation reagents attack the free phenolic hydroxyl groups or the phenolic
ethers of the phenyl propane side chain of lignins. Usually, 12 % hypochlorite
based on o.d. pulp is used at 3050 C at a consistency of 10 % and the bleaching
lasts for 24 h.
4.4. Chlorine dioxide bleaching (D)
Chlorine dioxide is an extremely effective and selective bleaching
agent. The chlorine dioxide attacks phenolic OH groups of lignins. Phenoxy
radicals formed in this way undergo further reactions either to provide
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quinoid structures or to form muconic acid derivatives after a ring cleavage.
Chlorine bleaching uses 0.5 and 1.5 % active chlorine based on o.d. pulp,
which is carried out at 7080 C for 34 h at a consistency of 10-12 %.
4.5. Oxygen bleaching (O)
The bleaching agent is gaseous oxygen. The process must be carried out under
mild pressure for a sufficient amount of oxygen to be available in the bleaching
liquor. As a biradical, oxygen can remove an electron from the phenolate ions, which
are present in the alkaline medium. The formed phenoxy radical undergoes further
degradation reactions. Hydroperoxides are produced, which are further degraded by
intermolecular nucleophilic attack of the peroxide anions. The oxygen bleaching, in
which 24 % alkali and 12.5 % oxygen are used, lasts 3090 min.
4.6. Ozone bleaching (Z)
The most important reaction of ozone with lignin is the cleavage of the bonds
between the lignin units. Ozone can attack both the aryl and the alkyl moieties. The
attack on the aromatic rings leads to ring cleavage. Double bonds in the aliphatic side
chain, where carbonyl and peroxide structures are formed, are also attacked.
4.7. Peroxide bleaching (P)
The bleaching of mechanical pulp destroys the chromophoric groups by
cleaving conjugated double bonds. At 7080 C, the highly nucleophilic per
hydroxyl ion formed can further degrade quinoid lignin structures, which are
produced by the electrophilic bleaching chemicals. The peroxide bleaching
uses 12 % based on o. d. pulp at a consistency of 10 % at 7080 C.
Traditionally, the bleaching reagents are chosen for their economy and
selectivities as well as their capacity for bleaching efficiency and quality. Currently,
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due to the strict environmental restrictions on the emission of organic chlorides and
dioxins in effluents, the bleaching sequences increasingly use Elementary Chlorine
Free (ECF) or Totally Chlorine Free (TCF) processes [71]. TCF bleaching is the
current trend for contemporary delignification. TCF bleaching produces no
organochlorines, which are hazardous substances such as dioxin, an endocrine
disrupter, and human carcinogen.
5.Cellulose derivativesCellulose cannot dissolve in water. Introducing hydrophilic groups along the
chain of cellulose cleaves hydrogen bonds and renders its derivatives soluble in
conventional solvents, widening its applications to, for example, functional celluloseethers and esters [72-76].
Commercial cellulose derivatives are either ethers or esters that are soluble in
water or organic solvents. The three free hydroxyl groups in the AGUs react with
various functional substitution groups. The resultant substituents therefore disturb the
inter- and intra-molecular hydrogen bonds in cellulose, reduce the hydrophilic
character of the numerous hydroxyl groups, and increase the hydrophobicity.Introducing ester and ether groups separates the cellulose chains so completely that
the fiber structure is either altered or destroyed. The solubility of a cellulose
derivative in a solvent or in water depends on the type of substituents, the degree of
substitution and the molecular weight. These cellulose derivatives are grouped
according to the processes and chemical substituents. The most important commercial
cellulose derivatives are shown in figure 2 [77-82].
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Fig. 2. Important cellulose derivatives
6. Mercerization
In modern mercerization processes, 3070 % NaOH solution is sprayed onto
dry cellulose powder in fast-turning, dry-mixing aggregates. The cellulose powder
can also be impregnated with an inert organic solvent, which is used to produce
carboxymethyl cellulose (CMC). The cellulose can be mercerized in an organic
solvent in normal stirred vessels, which use powder NaOH after the slurry of
cellulose is formed.
The alkali cellulose for subsequent etherification must contain at least 0.8 mol
of NaOH per mole of anhydroglucose, which is a basic requirement to produce
Cellulose derivatives
esters ethers
Soluble in organic
solvents
Soluble in waterinorganic organic
Cellulose
nitrate
xanthate
Acetate
propionate
Acetate
ro ionate
Acetate
buthirate
Ethyl cellulose
Benz cellulose
Methyl cellulose
Carboxymethyl
cellulose
Hydroxymethylcellulose
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uniformly substituted ethers. Cellulose ethers of lower viscosities are usually adjusted
in the alkalization step, which is referred to as the ageing process. In the ageing
process, carefully designed conditions must be adjusted according to the products
final application. The optimum parameters which often control the process are ageing
time, temperature, NaOH concentration and the presence of catalytic amounts of iron,
cobalt, or manganese salts, which catalyze the oxidative de-polymerization.
7. Black liquor usage
Lignins are probably the most complex and the least well characterized group
of substances in nature. It comprises 20-35 % of wood substance; lignin forms the
adhesive reinforcing component and binds together the cellulosic fiber structure.
Lignin is obtained during pulping of lignocellulosic material. Lignin is essentially a
substituted phenyl propane unit held together by ether and carbon bonds.
Characteristics of lignin are highly affected by the pulping process and kind of
lignocellulosic material. So, during pulping process, lignin is demethylated and
degrades which causes the increase in phenolic hydroxyl groups [83]. The lignin oforganosolv pulping process has many physical and chemical properties which
distinguish it from produced lignin from kraft and sulfite process [84]. Increasing
temperature, time and pH of the pulping process increase hydroxyl groups in the
separated lignin [85].
Spectroscopic methods e.g. ultra-violet (UV), infrared (IR) and
(H NMR) can be used to give information about the structure of lignin[86, 87]. It was found that the methoxyl contents as well as oxidation level of soda
lignin is higher than that in case of kraft lignin [88]. Also, phenolic and carbonyl
groups of organosolv are higher than that in case of kraft lignin, while as hydroxyl
and methoxyl groups in kraft lignin are higher than organosolv lignin [89]. Alecell
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lignin has a lower molecular weight and lower methoxyl groups than kraft and soda
lignin, it might be sensitive to highly depolymerizing bleaching methods e.g.
hypochlorite bleaching [90, 91].
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EXPERIMENTAL
1. Raw material used
The raw material used in this work was Egyptian rice straw. It was directly
collected from different regions from the Nile delta.
2. Equipments
IR absorption spectra were recorded as KBr discs within the 4000-200 cm-1
range on a Perkin Elmer 1430 infrared spectrophotometer. The thermogravimetricanalysis (TGA) was carried out on a Shimadzu TG 50 thermogravimetric analyzer
from room temperature up to 1000oC using 10
oC/min heating rate under nitrogen as
atmosphere. The differential thermal analysis (DTA) was performed on 990 Du-Pont
differential thermal analyzer of 1200 C cell using Al2O3 as a reference. X-ray
powder diffraction diagrams were measured on Debye-Scherrer PW 1050 (Cux-K;
Ni-filter) from Philips. The surface of rice straw and pulp from rice straw samplesimaged with the (SEM) scanning electron microscopy type, JEOL JEM-850 operating
at 35 kV employed in the Central laboratory, National Research Centre, Cairo, Egypt.
Samples were investigated as it is without any change in their physical form.
3. Analysis of raw material
Many methods for the analysis of the plant materials and pulp have been
developed and tested for their reliability. The standard methods mostly in current
usage nowadays are the American Tappi standard, the German (Deutsche Einheiten)
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methods, and the Swedish methods. In this work the Tappi standards [92] were used
in most of the chemical analysis of the raw material and pulp.
3.1. Moisture content
An amount of air dry sample (w) was dried in an oven at 105 oC till constant
weight (w1) and the moisture content was calculated using equation (1).
100w
)w-(wcontentMoisture 1 = (1)
3.2. Water soluble matter
An amount of air dry sample was washed in distilled water in different ratios
with boiling for different time periods to determine the water soluble matters in the
samples used.
3.3. Determination of ash and silica in rice straw and pulp
The contents of ash and silica were determined by the Chinese standard
methods for non-wood raw materials [93]. The test specimen was transferred to a
crucible, carbonized gently over a Bunsen burner, then ignited in a muffle furnace at
575 +25 C, and the residue was weighed as ash. Because of the high silica content
of rice straw, a solution in ethanol of magnesium acetate, which contained 4.054 g of
Mg(AcO)2.4H2O in one liter of 95 % aqueous ethanol, was added to the test
specimen that originated from rice straw to prevent incomplete carbonization caused
by fusion of ash and silica. The ash obtained as described above was treated with
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concentrated HCl. The acid-insoluble residue was filtered, washed with hot water
until no chlorides were detectable, ignited, and finally weighed as silicon dioxide.
3.4. -cellulose estimation [94]
The term alpha cellulose is meant to describe the part of cellulose which does
not dissolve in 17.5 % NaOH (w/w). About 3 grams of rice straw were placed in the
porcelain beaker with 250 ml capacity, then 25 ml of sodium hydroxide (17.5 % w/w)
were added and left to swell for 4 minutes. The pulp was pressed with a glass rod for
3 minutes. After pressing, another 25 ml of sodium hydroxide were added and the
contents were mixed thoroughly till one gets a homogeneous past. The beaker was
then covered and left for 35 minutes at 25 oC. Then 100 ml of distilled water were
added and quickly filtered under suction using a sintered glass funnel (1G2 of 5 cm
diameter and 4.5 cm length). After washing with distilled water till neutrality 100 ml
of 10 % acetic acid were added drop wise for washing followed by distilled water.
The alpha cellulose was estimated gravimetrically after drying in an air oven at 105-
106 oC then weighing, the produced alpha cellulose was also ignited to calculate the
ash in the -cellulose samples.
4. Cooking in sodium hydroxide solutions (pulping)
The raw material, rice straw was cut into small pieces of 2-3 cm length before
pulping. Pulping was carried out in a porcelain beaker heated on electrical hot plate
under atmospheric pressure. All cooks were made with oven dried raw material from
rice straw samples. After pulping, the pulp was defibrated then filtered and washed
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with water till neutrality. The pulp was screened and analyzed chemically as in case
of the raw material.
Cooking conditions
In these tests, samples from rice straw were treated with NaOH solutions of
different concentrations for different time periods at different temperatures.
4.1. Effect of sodium hydroxide concentration
Sodium hydroxide acts as a solublizing agent for both silica and lignin found in
raw materials. 10 gm samples were boiled with 100 ml NaOH solution of certain
concentrations (4 %, 6 %, 8 %, 10 % and 12 %) for 2 hours.
4.2. Effect of time at optimum alkalinity
10 g samples were leached with 100 ml of 10 % NaOH solutions and were boiled
for different time periods (1, 2, 3 and 4 hours).
4.3. Effect of weight / volume ratio
10 g samples from rice straw were treated with 10 % NaOH solutions whileboiling for 2 hrs. in different weight per volume ratios 1/5, 1/10, 1/20 and 1/50 (w/v).
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4.3. Effect of cooking temperature on the yield of pulp
10 g samples from rice straw were treated with 100 ml 10 % NaOH solutions for
2 hrs. at different temperatures (40, 60, 80 and 100 oC).
4.4. Effect of the nature of rice straw on the pulp yield
10 g samples from rice straw were treated with 100 ml of 10 % NaOH solutions
for 2 hrs. at boiling point, length of samples used were the whole plant (80-120 cm
length), 20 cm, 10 cm, 5 cm and mechanically devided plant (less than 1cm).
5. Bleaching
The pulps produced by NaOH pulping are pale to intense yellow in color and
requires bleaching to reach acceptable brightness. Chlorine (C) and sodiumhypochlorite (H) with intermediate caustic extraction (E) and hydrogen peroxide (P)
with CEHEP and HEP sequences were used to evaluate bleaching capability of rice
straw pulps.
There are many sequences for bleaching rice straw pulp as examples; S-O-D1-
E/P-D2-P and O-D1-E/P-D2-P where:
S (saponification): 412 % NaOH on pulp, 5 % consistency, 60 C and 2hrs.
O-stage: 410 % NaOH on pulp, 12 % consistency, O2 pressure of 5kg/cm
2, 100 C and 1 hr.
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D-stage: 0.6 % (D1) or 0.4 % (D2) ClO2on pulp, 12 % consistency, 70 Cand 3 h.
E/P-stage: 2 % NaOH on pulp, 0.5 % H2O2on pulp, 12 % consistency, 70C and 2 h.
P-stage: 0.5 % H2O2 on pulp, 1.5 % NaOH on pulp, 12 % consistency, 70C and 2 h.
The brightness of bleached pulp depends on the bleaching sequences and
conditions. This is used on the industrial range and stated here to show that it is a
very expensive and complicated condition. So in this study the pulp were treated
with sodium bisulphate solution (4 % and 8 %), and with sodium hydroxide
solution (4 %) then with hydrogen peroxide solution (4 %) and the resulting
samples were subjected to IR spectrophotometric analysis to compare these results
with that obtained from previous work [95].
6. Permanganate number [96]
It is a method of expressing bleachability of pulp. It is determined by the
number of mls of 0.1 N KMnO4 consumed by one gram of moisture free pulp under
certain specific conditions of time, temperature and acidity.
Required volume of 0.1 N KMnO4(20 40 ml depending upon the rawness of
the pulp) were paced in one beaker, an equal amount of 4N H2SO4 is placed into
another beaker and enough water combined with H2SO4, so that, in the final reactionmixture of the permanganate solution will be 1/300 N. When the reagent is ready, the
pulp specimen is added to the reaction beaker, followed by addition of sulfuric acid
and then by addition of permanganate. After exactly 5 minutes at 25 oC an excess of
KI is added to stop the reaction. The residual KMnO4 in the mixture released an
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equivalent weight of iodine from the iodide salt solution. The liberated iodine is then
titrated against stander sodium thiosulphate solution [95]. The volume of
permanganate consumed by the pulp is then calculated. The permanganate number is
obtained by dividing the number of mls of 0.1 N KMnO4consumed by the moisture
free weight of the test specimen.
W
V-25numbertePermengana = (2)
where V is the number of milliliters of 0.1 N Na2S2O3consumed in the titration,
W is the weight of moisture free pulp and 25 is the number of mls of 0.1 N KMnO4.
7. Determination of the contents of black liquor
The recovery of the contents of the black liquors of rice straw is very difficult
because of the high viscosity of the liquor, the low caloric value and, in particular,
the high silica content, which is much higher in straw, especially rice straw, than inwood. As a result, such black liquors are discharged without any treatment and cause
serious water pollution. Novel and non-polluting pulping