Preparation of Mullite Ceramics from Bayer Electrofilter Fines and Low Kaolinite Clay

In this paper are investigated microstructural, phase, physical and mechanical properties of mullite ceramics obtained from clay and Bayer electrofilter fines. Clay and alumina were mixed in such ratio that provides the amount of Al O 2 3 and SiO2 corresponding composition of mullite. One mixture was manually mixed and other was milled three hours in ceramic mill. After isostatic molding and heat treating at 1580°C and 1635°C XRD analysis revealed approximately 60% of mullite and 40% of corundum in first mixture and in second mixture 70% of mullite and 30% of corundum. Two types of mullite were observed in SEM micrographs beside corundum grains and glassy phase. The results of investigation indicate that preparation of mullite ceramics from these raw materials is possible, if clay and alumina are well homogenized and milled before molding.


Introduction
Mullite is an aluminosilicate compound that is used for conventional and advanced ceramic applications due to low density, low thermal conductivity, low thermal expansion, low dielectric constant, excellent mechanical properties at high temperature, low creep rate and good chemical stability [1,2,3].These properties make mullite increasingly important in electronic, optical and high temperature structural applications, while traditional uses of mullite include refractories in the metallurgical industries.Mullite is the only stab.intermediate phase in Al 2 O 3 -SiO 2 system at atmospheric pressure.It is a solid solution phase of alumina and silica with stoichiometric ranging from relatively silica -rich 3Al 2 O 3 •2SiO 2 (3:2 mullite) to alumina rich 2Al 2 O 3 •SiO 2 (2:1 mullite) [4].Mullite has been synthesized in many ways to prepare dense sintered bodies, and these methods involve very high cost starting materials and powder processing and are not cost effective for large scale production, but use of kaolinitic clay and alumina as starting materials are cheaper alternatives [3,5,6].Applying syntheses based on natural minerals it is possible to produce a high quality mullite ceramic by conventional processing -sintering methods [7].Clay has been widely used, not only for reasons of economy, but also due to its potential for the formation of needle like mullite grains [3] which are preferable from the viewpoint of high strength and toughness.Mullite synthesis process occurs at high temperatures by diffusion of particles [1], i.e. below 1300°C the major reaction is the kaolinite series reaction which converts into primary mullite and the cristobalite phase, amorphous silica and the reactive alumina remains unreacted.It acts as an inert material below 1300°C [8].At temperatures higher than 1400°C, secondary mullite is formed by dissolution of alumina to silica rich glass and precipitation of secondary mullite [3,8].The rate of secondary mullite formation is very slow at temperatures below 1555°C, but extremely fast at temperatures above 1600°C due to strong effect of the eutectic liquid formation at 1595°C [5].Temperature and the level forming secondary mullite depend on the nature and dispersity of starting materials and on the amount and composition of glassy phase [9,10,11].Secondary mullite crystals are smaller because they are nucleated from the transient liquid phase and primary mullite crystals are larger and with a higher aspect ratio.Primary mullite is arisen from kaolinite and has grown in the firing [4,5,6,12,13].There are many papers concerning mullite preparing from kaolin and alumina [2,5,6,8,[12][13][14][15], but there is a little investigation on using cheaper raw materials [21].The objective of this research is to examine the possibility of preparing mullite ceramics from domestic raw materials and to determine the effect of simultaneous milling on properties of material obtained by sintering of clay and Bayer electrofilter fines.

Materials and experimental procedure
Raw materials used in this investigation are clay "Klokoti" [20] and electrofilter fines, by-products of the Bayer process for the production of alumina from bauxite.The deposit of clay "Klokoti" is situated in the Central Bosnia and Bayer electrofilter fines are from alumina factory "Alumina" d. o. o.Zvornik, B&H.The chemical and mineralogical composition and particle size distribution were investigated.Chemical composition was determined by atomic absorption spectrometry.Particle size analysis was carried out using laser analyzer Malvern Mastersizer 2000.XRD was performed on a Shimadzu diffractometer XRD-6000 with Cu Kα radiation, with accelerating voltages of 40 kV and current 30 mA, in the range of angles 2-80° 2θ with a step 0.02° 2θ and a dwell time of 0.6 seconds.According to chemical composition of clay and Bayer electrofilter fines, in order to get a mixture which has approximate composition of mullite (72 % Al 2 O 3 and 28 % SiO 2 ), two mixtures were made: -Specimens which are in this paper marked as A contain 39 % of clay sieved through mesh openings 0.5 mm and 61 % of Bayer electrofilter fines, mixed manually, -Specimens marked as B contain 38.5 % of the clay and 61.5 % of Bayer electrofilter fines, milled together in ceramic mill for 3 hours.
Both types of specimens, without any additions, were isostatic pressed at 100 MPa into cylindrical cartridge case and treated at 1580°C and 1635°C in the tunnel furnace within the cycle of 36 hours, with two hours keeping on the highest temperature.
After thermal treatment volume shrinkage and loss of mass were calculated.The determination of true density was according to ISO 5018 -1983 (E).The bulk density, water absorption, true, apparent and closed porosity have been determined according to BAS EN 623 -2.Compressive strength was determined on universal machine for static testing.Refractoriness (softening point) was determined in criptol furnace according to EN 993-13:1995.The experimental diffraction data were collected on the X-ray powder Ital Structure APD2000 in Bragg-Brentano geometry, using the X-ray tube with a Cu anode (CuKα radiation, λ = 1.5418Å) and Ni-filter.The conditions for collecting diffraction data were: U = 40 kV, I = 30 mA, interval 5-70º 2θ, step 0.02º 2θ, dwell time 1 s.Program PowderCell (W.Kraus, G. Nolze, PowderCell for Windows V.2.4, the Federal Institute for Materials Research and Testing, Berlin, Germany, 2000) was used to determine semi quantitative phase analysis.Scanning electron microscopy was performed on a scanning electron microscope MIRA 3 TESCAN.

Results and discussion
The results of chemical analysis of unsieved clay "Klokoti" and clay sieved through mesh openings 0.5 mm and Bayer electofilter fines are given in tab. 1.It shows that the percentage of impurity in clay is high, especially the content of K 2 O, that indicates that clay has illite and muscovite.Low loss of ignition indicates low content of kaolinite [9].Sieving was slightly changed the chemical composition of clay.
Tab.I Chemical composition of clay "Klokoti" and Bayer electrofilter fines.
Chemical The particle size distribution of the clay "Klokoti" and Bayer electrofilter fines is shown in figs. 1 and 2, respectively.The mean particle size (d 50 ) of the clay "Klokoti" is 18.102 µm.Bayer electrofilter fines have finer particle size distribution with mean particle size of 5 µm (fig.2).These grain distributions are coarser in comparing with other investigations [2,5,6,8,[12][13][14][15]. The XRD patterns of the clay and Bayer electrofilter fines are shown in fig. 3. The main phases in clay are kaolinite, muscovite and quartz.Muscovite and illite have very similar XRD patterns [10], so that is very difficult to differentiate them in this way.Bayer electrofilter fines consist mainly of α-Al 2 O 3 and some gibbsite.Namely, grain size has a significant effect on mullitization, because activity of components present in the system largely depends on the particle size [7,11,14].Specimens treated at higher temperature have only about 3.5% greater amount of mullite than specimens treated at lower temperature, because the temperature difference is only 55°C.On SEM micrographs of specimen A treated at 1580°C, shown in fig.8, it can be observed three types of grains: coarse grains of alumina, high aspect ratio mullite crystals and fine equiaxed mullite crystals raised from glassy phase.On higher temperature primary mullite crystals become larger (fig.9) especially in specimen A. The primary mullite is formed at temperature 1000 -1200°C but with a further increase in temperature mullite crystals just become larger [3,15,16].Temperature increase also leads to increased pore size as can be seen in fig. 10 [15,17].Pore growth is caused by glassy phase wetting and providing a capillary attraction at the surrounding particles.This leading the initial small pores to coalescence and grow [15].The physical properties of specimens A and B are given in tab.III.According to the results shown in tab.III it can be concluded that volume shrinkage of specimens B is greater than volume shrinkage of specimens A because of finer particles in specimens B [18,19].The volume shrinkage of both specimens is greater at the lower than at higher temperature.Creation of secondary mullite is accompanied by an increase of volume and as a result there is a loosening of structure in specimens at higher temperature [16].

Type of specimens
Density of specimens B is also greater than density of specimens A because of higher volume shrinkage, but for both specimens it is lower at higher temperature due to increase in volume when creating a secondary mullite which leads to a loosening of structure.True porosity has the opposite behavior in comparison to density.Specimens B have low apparent porosity and water absorption, but higher closed porosity.Increase in closed porosity may be due to gas production from decomposing of thermally unstab.materials in glassy phase at high sintering temperature [15].Because of low apparent porosity these specimens have higher compressive strength.Specimens with the highest apparent porosity (specimens A treated at 1635°C) have the lowest compressive strength.
All specimens have refractoriness above 1780°C, but specimen A treated at 1635°C was the most deformed.
Heat treating of specimen A at 1635°C, despite higher content of mullite, gives specimen with lower density, lower compressive strength and refractoriness and higher porosity and water absorption than specimens A treated at 1580°C.It seems that oversintering phenomenon occurs at 1635°C.Oversintering is stage of thermal treating which can lead to expansion of material and reducing the density [16].

Conclusion
Unlike most studies which dealt with the pure synthetic aluminas and clays with high kaolinite content, this paper present investigation of mullite producing from low-cost raw materials: non-treated natural fireclay and electrofilter fines, by-products of the Bayer process.
In addition to kaolinite, clay used for sample preparation contains quartz, illite, muscovite and other impurities.These impurities enhanced liquid phase forming upon heating which results in substantial reducing the amount of mullite in the final product.XRD analysis of mullite showed the presence of relatively large quantities of corundum although the starting mixtures were designed to mullite (3:2) composition.Granulation of the starting material is rather rough, so their reactivity is lower.Therefore, larger particles of alumina did not have enough time to dissolve in the glassy phase and to react by creating mullite.
SEM analysis showed the presence of two type of mullite: high aspect ratio mullite corresponding to primary mullite formed from clay and smaller equiaxed secondary mullite formed in reaction of alumina and released SiO from clay.The formation of secondary 2 mullite is accompanied by an increase in volume and decrease in density.Increasing the temperature leads to an increase in grain size of mullite and increasing porosity.
Simultaneous grinding of the starting materials provides the material with more uniform structure, increased amounts of mullite, increased strength and density, and reduced porosity.
Generally, it can be concluded that preparation of mullite ceramics from low cost raw materials is possible, but clay and alumina have to be milled and homogenized before molding.

Fig. 8 .
Fig. 8. Micrograph of fracture surface of specimen A treated at 1580°C.