refractories WORLDFORUM 2 (2010) [2] 63

1 Introduction

Fused cast alumina-zirconia-silica (AZS) ma-terials are the preferred refractories for thefurnace area in contact with the glass meltwhen melting soda-lime-silica glass. Theyare also used as sidewall material in thecombustion chamber as well as crown mate-rial especially with oxygen fired furnaces.Their microstructure (mineral phases, grainsize) is of great interest since it heavily influ-ences their properties such as chemical re-

trodes. After melting at temperatures above2000 °C and bubbling with oxygen at theend of the melting campaign the casting iscarried out. The cast blocks are slowlycooled. All these production steps have aneffect on the developing microstructure.The choice of raw materials depends onavailability, purity and price. Especially theexpensive zirconium based raw materials areadapted for the demands on the refractorywhen used. Sodium is used, among otherthings, to lower the melting temperature sothat less energy is necessary and the corro-sion of the arc furnace is diminished. Thebubbling oxidizes the melt and decreasesthe danger of bubble generation when therefractory materials get in contact with glassmelts.The formation of the microstructure duringproduction of the fused cast AZS blocks canbe illustrated by phase diagrams and withthe help of an ice block.Fig. 1 shows a phase diagram of ZrO2 –Al2O3 – SiO2 [1, 2] that can be used to cal-culate the concentration of the main miner-al phases found in fused cast AZS blocks. Thediagram includes three single components

sistance to surrounding media (glass melt,flue gases, condensation products, etc.).If unexpected incidents occur during a glassmelting campaign, such as heavy corrosionor increasing appearance of glass defects,test specimens may be taken to determinethe cause. Analysis of chemical and minera-logical composition as well as other investi-gations may lead to wrong interpretations ifthe microstructure and mineral phases of theblocks during and after production are notwell known. Moreover, the microstructure ofthese materials is changing during use,which is one reason for the investigationsthat are presented here. The paper willtherefore deal with the microstructure offused cast AZS blocks after production, dur-ing use and after cooling the furnace andblock, respectively.

2 Microstructure after production

The production process of fused cast AZS re-fractories can shortly be described as fol-lows: The raw materials commonly used con-tain zirconia, silica, alumina, sodium oxideand other additives. They are mixed andmelted in an arc furnace with graphite elec-

Fused Cast AZS Refractory Materials — Structure and Structural Changes during Application

B. Fleischmann

The microstructure of alumina-zirconia-silica (AZS) materials ischanging during use, which isone reason for the investigationsthe results of which are present-ed here. The paper will thereforedeal with the microstructure offused cast AZS blocks after pro-duction, during use and aftercooling the furnace and block,respectively. The presence of dif-ferent mineral phases found insamples removed from an oper-ating glass furnace to answerquestions respecting glass pro-duction problems is explained.

Dipl.-Ing. Bernhard Fleischmann

Hüttentechnische Vereinigung der

Deutschen Glasindustrie e.V.

63071 Offenbach on Main

Germany

E-mail: fleischmann@hvg-dgg,de

Keywords: alumina-zirconia-silica (AZS) re-

fractories, microstructure, soda-lime-silica

glass, glass melting, glass furnaces

Fig. 1 Equilibrium phase diagram of Al2O3 – SiO2 – ZrO2 [2]

64 refractories WORLDFORUM 2 (2010) [2]

(ZrO2, Al2O3 and SiO2) and two intermediatecompounds (mullite 3 Al2O3·2 SiO2 and zir-con ZrO2·SiO2). The first question to answer is which phasecrystallizes out of the melt in each area. It iscorundum in the alumina rich corner, badde-leyite in the zirconia rich corner, cristobalitein the small area in the silica corner and mul-lite in the field between cristobalite andcorundum according to [1]. The secondquestion is what happens to zircon. As weare dealing with a five and more componentsystem with impurities in reality rather thana three component one, the decompositiontemperature of zircon is far below 1775 °C(down to 1450 °C depending on reaction

partner and its concentration [3]). That iswhy baddeleyite cristallizes in the field of theZrO2 corner.As can be seen in Fig. 2, where the startingpoint S represents the chemical composition41 mass-% ZrO2, 46 mass-% Al2O3 and12 mass-% SiO2 (the rest is sodium oxide),the crystallisation path follows first the redand then the blue line to the eutectic point.Using the phase diagram to estimate theshare of the mineral phases and the compo-sition of the residual melt the following cal-culation results:• Chemical composition of the melt at start-

ing point: 41 mass-% ZrO2, 46 mass-%Al2O3 and 12 mass-% SiO2

• Crystallisation of baddeleyite along thered line: Mineral phase and chemistry atthe end of the red line: 13,8 mass-% bad-deleyite in 86,2 mass-% residual meltwith 32,4 mass-% ZrO2, 53,3 mass-%Al2O3 and 14,2 mass-% SiO2

• Crystallisation of baddeleyite and corun-dum along the blue line. Chemistry at theend of the blue line: 18,7 mass-% badde-leyite and 5,0 mass-% corundum in 76,2 mass-% residual melt with 30,1 mass-% ZrO2, 53,4 mass-% Al2O3

and 16,2 mass-% SiO2

• At the eutectic point baddeleyite, corun-dum and mullite should crystallise at aconstant temperature of 1726 °C regard-ing the phase diagram. But in reality only

baddeleyite and corundum crystallise anda vitreous phase freezes due to the sodiumcontent; looking at the phase diagram ofSiO2 – Al2O3 – Na2O this will be unter-standable

• After crystallisation at the eutectic:40,2 mass-% ZrO2, 41,3 mass-% Al2O3

are present as crystalline mineral phasebesides 18,5 mass-% vitreous phase with4,4 mass-% ZrO2, 25,4 mass-% Al2O3,64,8 mass-% SiO2 and 5,4 mass-% Na2O.

While calculating the composition of the re-fractory blocks there has to be borne in mindthat alumina and zirconia do have a com-mon solubility in glass melts of about 30 mass-% depending on the temperature.The microstructure of these blocks shouldconsist of some free baddeleyite and corun-dum embedded into an intercrystalline link-age of both minerals due to the crystallisa-tion in the eutectic point (model: spongewith frozen water in the pores (Fig. 3)) witha vitreous phase between the grains. All thegrains are about the same size.This calculation so far as well as the descrip-tion of the microstructure is valid only forequilibrium conditions, i.e. cooling as slowlyas necessary so that the whole block is atthe same temperature! When cooling realblocks the cooling rate should be as high aspossible to save time and money. The result-

Fig. 2 Phase diagram of Al2O3 – SiO2 – ZrO2 with crystallization path of an AZS refractory material of the chemical composition 41 mass-% ZrO2, 46 mass-% Al2O3

and 12 mass-% SiO2

Fig. 3 Model for visualizing the intercrystalline linkage of two minerals

Fig. 4 Sketch of the onion like structureof a real AZS block that was not in equilibrium during cooling procedure

refractories WORLDFORUM 2 (2010) [2] 65

ing composition and microstructure showthe following changes:• Increased portion of vitreous phase that

shows decreased part of sodium oxide(same content of sodium oxide dissolvedin more melt)

• Decreased portion of free baddeleyite andcorundum and increased part of intercrys-talline linked phased

• No homogeneous cooling of the block re-sulting in an onion like structure with dif-ferent grain sizes (fast cooling → smallgrain size; areas of slow cooling rate →large crystals and grain size) as sketchedin Fig. 4

• Inhomogeneous chemical composition ofthe block (Fig. 5).

Fig. 6 helps to understand the origin ofpores inside the fused cast AZS blocksthough water behaves different to AZSmelts. When freezing water in a pot ice de-velops first on the top and at the sides as the

3 Changes in Microstructure during use3.1 In contact with glass melt

Figs. 7 and 8 show two remains of AZS sol-dier blocks that were in contact with glassmelt. Fig. 7 shows a cut through a soldierblock of a float glass furnace near the metal

bottom is protected against the cold by thesupport of the pot. Having a lower densitythan water the ice is expanding and be-tween the residual water and the ice a cavi-ty appears. Thus the long pores develop ascan be seen in Fig. 6a and sketched inFig. 6b. When producing a fused cast AZS block it isquite opposite but the same structure andeffects result. The crystalline phase has ahigher density and therefore needs less vol-ume than the melt. As the outside of the re-fractory block solidifies and crystallises veryfast the residual melt in the interior adjoinsa cavity (see Fig. 4). The crystals that growout of the melt during the long coolingprocess need less space than is availablefrom the first solidified outer shape. Poresare formed between them, thus, the fusedcast AZS blocks have an open porosity ofabout 1 vol.-%. Non-destructive investiga-tions with ultrasound confirmed this.

Fig. 5 Inhomogeneous distribution of zirconia in a real AZS block [4]

Fig. 6 Appearance of pores when freezingwater; a) picture of a half frozen blockand b) sketch of the process

Fig. 7 Investigated remain of a soldier block cut at the metal line area

a

b

Fig. 8 Side view of a piece of a soldierblock from a container glass furnace cutfrom top to bottom

66 refractories WORLDFORUM 2 (2010) [2]

line with about 9 cm residual thickness. Theblock was placed near the hot spot. The fur-nace was operated more than 12 years. Theblock had a nominal zirconium oxide contentof 41 mass-%.Previous investigations with a light micro-scope and an electron microscope were car-ried out to depict the corrosion process asfar as understood with the help of the re-sults [5]. Watching the chemical and miner-alogical changes somewhere in the middleof an AZS fused cast block the followingprocess steps roughly describe the course ofevents:During production three phases grow out ofthe melt – corundum, baddeleyite and a vit-reous phase. The baddeleyite and corundumexist as free primary crystal or both are en-tangled due to the eutectic transformationby inter-crystalline linkage. At the beginningof operation time, the temperature slowly in-creases in the middle of the block and thediffusion of alkaline and alkaline earth ele-ments into the vitreous phase of the blockcan be observed. The vitreous phase with in-creased alkaline content starts to dissolve

corundum whereas the baddeleyite remainscrystalline and may start to grow since thecommon solubility of zirconia and alumina[6] is exceeded. During the furnace cam-paign the corrosion at the side with glasscontact continues, i.e., the block loosesweight and the temperature in the middle ofthe block rises. The content of alkaline andearth alkaline elements increases and morecorundum is dissolved. When the tempera-ture is up to more than about 1300 °C andthe content of the vitreous phase with alu-mina and zirconia reaches saturation crys-tallisation of secondary zirconia and recrys-tallisation of corundum as well as formationof mullite needles can be observed in the vit-reous phase. When the corrosion, i.e. theboundary layer, arrives at the middle of theblock free baddeleyite in a vitreous phase,containing a lot of components that origi-nate from the glass melt can be seen. Thefree baddeleyite will be dissolved or may befound as stone in the glass product whencorrosion continues. When cooling the blockthe vitreous phase may contain feldsparcrystals that grow out of the vitreous phase

during slow cooling but are not present inthe refractory at high temperatures.Fig. 8 shows a soldier block from a contain-er glass furnace. The block was placed nearthe barrier. The furnace was operated morethan 11 years. The block had a nominal zir-conia content of 31 – 33 mass-%. The resid-ual thickness at the smallest area is 14 mm.Fig. 9 shows electron microprobe picturesthat represent the residual thickness at thesmallest area of 14 mm. Different phasescan be distinguished: the light grey grainsare corundum, the small white grains arebaddeleyite and the dark grey areas are vit-reous phase. The small arc at the right side isthe residue of the glass melt and boundarylayer, respectively. Fig. 10 shows the chemi-cal composition determined following theyellow line in Fig. 9. The chemistry was de-termined over areas with a diameter ofabout 1,4 mm each. One can see the diffu-sion of sodium from the glass melt into theblock (right side) and the diffusion of sodiumfrom the backside (left side). The soldierblocks were already backed up with AZSplates. Glass, that leaves the furnace to gobetween soldier block and plate, places sodi-um at the disposal. Diffusion of alkaline canoccur as the temperature between originalblock and plate is high enough.The increased alumina content at position4,7 to 6,2 m and the corresponding de-creased zirconia content at the same placeare probably due to production rather thancorrosion effects during operation. In Fig. 9at the left side of the right picture the corre-sponding area is visible with a lot of corun-dum but less baddeleyite.

3.2 In contact with flue gases –evaporation products and dustparticles

Fig. 11 shows the front part of a block thatwas installed above the glass level and thathad contact with the flue gases, evaporationproducts and dust particles from the batch.The block was placed slightly above theglass melt surface. The results of X-ray dif-fraction (Fig. 12) may suggest the followingcorrosion process.Alkaline evaporated from the surface of theglass melt (the block was placed near thehot spot of the furnace) and/or dust from thebatch is disposed on the surface of the re-fractory. Due to the high temperatures espe-cially the alkaline metals are incorporated

Fig. 9 Joined pictures of the AZS block at the smallest part (see Fig. 8) determined byelectron microprobe

Fig. 10 Concentration of different oxides in mass-% at the smallest area of soldierblock (see Fig. 8)

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68 refractories WORLDFORUM 2 (2010) [2]

into the vitreous phase of the refractory andlower its viscosity. Alumina is dissolved andthe vitreous phase with its high content ofalkaline and earth alkaline leaks from theblock at the hot front side (viscosity of a meltwith 25 mass-% Al2O3, 10 mass-% Na2O, 5 mass-% ZrO2, 5 mass-% CaO and 55 mass-% SiO2 between 1500 and 1600 °C ≈ 2 – 3 Pa · s). A skeleton of bad-deleyite (estimated: > 90 mass-%), somecorundum and a small amount of a silicaphase remain (right side of the block inFig. 9) with pores and fr ee space betweenthem. When cooling this block slowly during

may be found in AZS blocks after productionif the chemistry is slightly different. Mullitemay develop during the corrosion process ifthe vitreous phase is saturated with aluminathat is dissolved in the alkaline enriched vit-reous phase. Moreover, mullite may growwhen the block is cooled after furnace shutdown. Especially when the cooling is carriedout very carefully to reuse parts of the fur-nace, the saturation limit decreases withtemperature and there is enough time fornucleation and crystal growth. Since parts ofthe block vanish with corrosion and thechemistry changes due to diffusion process-es, the presence of a mineral phase has to bereassessed in order not to misinterpret it.

Acknowledgements

The author wishes to thank Dr Gerda Boy-manns for preparing the transparent cutsand for carrying out the optical mi-croscopy, Mrs K. Hitzler for the investiga-tions with the scanning electron micro-scope and the microprobe (EDX) on thesample from a float glass furnace as wellas Prof. F. Gebhardt for his kind support.He also would like to thank Dr H. Wuth-now for the electron microscope and X-raydiffraction investigations of the blockskindly displaced at our disposal by ArdaghGlass GmbH.

References

[1] Ondik, H.M.; McMurdie, H.F.: Phase diagrams

for zirconium + zirconia systems. The Ameri-

can Ceramic Society, Westerville (Ohio), 1998,

Zr 366 – Zr 370

[2] Levin, M.E. et al,: Phase diagrams for cera-

mists. The American Ceramic Society, Colum-

bus (Ohio), 1964, Fig. 772

[3] Telle, R.: Studies on the thermal stability of

ZrSiO4 and its corrosion resistance to alkaline

and alkaline-earth oxides. 48. Int. Colloquium

on Refractories, Aachen 2005

[4] Hreglich, S.: Metodologie di controllo sui ma-

teriali refrattari da vetro. Vetro Informazione

49 (1989) [1] 8–13

[5] Fleischmann, B.: A travel through time in a

soldier block – the microstructure of fused

cast AZS materials before and during use for

glass melting. Proc. UNITECR 2007 in Dres-

den

[6] Manfredo, L.J.; McNally, R.N.: Solubility of re-

fractory oxides in soda-lime glass. J. Am. Ce-

ram. Soc. 67 (1984) [8] C-155 – C-158

furnace shut down the silica phase starts tocrystallise and feldspar modifications (X-raydiffraction indicates fine grains of them)grow out of it.If the temperature at the surface of the re-fractory that is in contact with the combus-tion chamber is lower, the leakage of the vit-reous phase by gravity will not occur. Thesame is the case if the alkaline concentrationis smaller (alkaline free or poor glass, lessevaporation, etc.).

4 Summary

Post mortem investigations of fused cast AZSblocks produced in Europe or North America,and the illustration of the crystallisationprocess during production with the aid of

Fig. 11 Front part of an AZS block placed above themetal line and in contact with flue gases and evaporation products

Fig. 12 Results of X-ray diffraction measurement on the front part of an AZS block in the sidewallof a container glass furnace

equilibrium phase diagrams allow to picturethe corrosion process during operation. Thepresence of different mineral phases foundin samples removed from an operating glassfurnace to answer questions respectingglass production problems is explained.Therefore, an assessment of the microstruc-ture and its changes is possible regardingcorrosion procedure. But the productionprocess and the cooling of the blocks has tobe considered as the microstructure de-pends, among other things, on raw materialsand their concentration, on cooling rate,block size and shape. Mullite as an example

refractories WORLDFORUM 2 (2010) [1] 69