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CERAMIC RAW MATERIALS. In studying ceramics processing it is necessary to be familiar with the types of raw materials available.
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In studying ceramics processing it is necessary to be familiar with the types of raw materials available. • Clay minerals, which provide plasticity when mixed with water; feldspar, which acts as a nonplastic filler on forming and a fluxing liquid on firing; and silica, which is a filler that resists fusion, have been the back bone of the traditional ceramic porcelains.
Other silicate minerals are used in white wares such as ceramic tile, thermal shock-resistant cordierite products,and steatite electrical porcelains. • Silica, aluminosilicates, tabular aluminium oxide, magnesium oxide, calcium oxide, and mixtures of these minerals have long been used for structural refractories. • Alumina, magnesia, and aluminosilicates are now used in some advanced structural ceramics.
Silicon carbide and silicon nitride are used for refractory, abrasive, electrical, and structural ceramics. • Finely ground alumina, titanates, and ferrites are the backbone of the electronic ceramics industry. • Stabilized zirconias are used for advanced structural and electrical products and zircon, zirconia, and other oxides doped with transition and rare-earth metal oxides are widely used as ceramic pigments.
These materials are commonly prepared by calcining particle mixtures, but some are now produced using special chemical techniques. • In Chapter 3, the more common ceramic materials produced in large tonnage and widely used in ceramics are considered. • Special materials of exceptional purity and homogeneity which are being developed for research and some very advanced products are discussed in Chapter 4.
In this chapter we briefly consider the nature of the starting materials, traditionally called raw materials, that can be purchased from a vendor and received at a manufacturing site. • These materials can vary widely in nominal chemical and mineral composition, purity, physical and chemical structure, particle size, and price.
Categories of raw materials include • (1) nonuniform crude material from natural deposits, • (2) refined industrial minerals that have been beneficiated to remove mineral impurities to significantly increase the mineral purity and physical consistency. • (3) high-tonnage industrial inorganic chemicals that have undergone extensive chemical processing and refinement to significantly upgrade the chemical purity and improve the physical characteristics.
The choice of a raw material for a particular product will depend on material cost, market factors, vendor services, technical processing considerations, and the ultimate performance requirements and market price of the finished product. • Accordingly, a higher-quality and more expensive material may be acceptable for microelectronics, coatings, fibers, and some high-performance products.
But the average cost of raw materials for building materials and traditional ceramics such as tile and porcelain must be relatively low. • Cost-benefit considerations may suggest substitutions of materials of lower cost that do not impair the quality, or alternatively, a more expensive material, which may be more economically processed and/or which will increase the quality and per formance of the product.
Many early ceramics industries were based near a natural deposit containing a combination of crude minerals that could be conveniently processed into usable products. • Construction materials such as brick and tile and some pottery items are historical examples, and many are still identified by the regional name.
Some crude materials are of sufficient purity to be used in heavy refractories: • Crude bauxite, a nonplastic ore containing hydrous alumina minerals, clay minerals, and mineral impurities such as quartz and ferric oxides, is used in producing some refractories. • Today, however, most ceramics are produced from more refined minerals.
Industrial minerals are used in large tonnages for producing construction materials, refractories, whitewares, and some electrical ceramics. • They are used extensively as additives in glazes, glass, and raw materials for industrial chemicals. • Common examples are listed in Table 3.1.
Clays are produced by the weathering of aluminosilicate rocks and sedimentation. • Clay minerals are layer-type hydrous aluminosilicates which can be dispersed into fine particles (Fig. 3.1).
Kaolin is a relatively pure, white firing clay composed principally of the mineral kaolinite Al2Si2O5(OH)4 but containing other clay minerals, as indicated in Table 3.2, and a minor amount of impurity minerals such as quartz Si02, ilmenite FeTiO3, rutile TiO2, and hematite Fe203.
Ball clay is a sedimentary clay of fine particle size containing complex organic matter ranging down to a submicron size. • Bentonite is a high proportion of the clay mineral monllonite. • Clays are used in whiteware formulations and aluminosilicate refractories to produce plasticity in forming and resistance to deformation when partial fusion occurs during firing. • Crushed and milled quartz SiO2, derived from relatively pure deposits of sandstone granular silicate mineral used extensively in whitewares refractory and glaze compositions (Fig 3-2)
The beneficiation of industrial minerals begins with crushing and grinding to a small enough size to liberate undesired mineral phases. • Further beneficiation may include settling and flotation to segregate minerals by density or size the separation of magnetic minerals using powerful electromagnets/ • Mending of different processing runs for consistency, and perhaps particle size classification.
Solids may be concentrated by filtration or centrifugation, and a portion of the soluble impurities are eliminated with the liquid. • A typical flow diagram for refining kaolin is shown in Fig. 3.3.
Concentrated solids are usually dried using a rotary or belt dryer or by spray drying. • Some materials are calcined, and a hard aggregate is formed Dried cake or calcined materials may be pulverized or ground and then sized or air elutriated before bagging or loading in hopper cars. • Many fine materials are loaded and unloaded using pneumatic fluidization and are stored at the plant site m large silos.
Important industrial ceramic chemicals include tabular and calcined aluminas, magnesium oxide, silicon carbide, silicon nitride, alkaline earth titanates soft and hard femtes, stabilized zirconia, and inorganic pigments. • Extensive chemical beneficiation reduces the content of accessary minerals and may increase the chemical purity up to about 99.5. • For many materials, the scale of operation is extremely large, which aids in lowering the unit processing costs and selling price.
Alumina Al2O3 is the most widely used inorganic chemical for ceramics (Table 1.2) and is produced worldwide in tonnage quantities for the aluminum and ceramics industries using the Bayer process. • The principal operations in the Bayer process are the physical beneficiation of the bauxite, digestion (inthe presence of caustic soda NaOH at an elevated temperature and pressure), clarification, precipitation, and calcinations, followed by crushing, milling, and sizing (see Fig. 3.4).
During the digestion, most of the hydrated alumina goes into solution as sodium aluminate: • Impurity+Al(OH)3(solid)+NaOH(sol) • →Na++Al(OH)4-(sol)+Impurity_ • and insoluble compounds of iron, silicon, and titanium are removed by settling and filtration.
After cooling, the filtered sodium aluminate solution is seeded with very fine gibbsite Al(OH)3, and at the lower temperature the aluminum hydroxide reforms as the stable phase. • The agitation time and temperature are carefully controlled to obtain a consistent gibbsite precipitate. • The gibbsite is continuously classified, washed to reduce the sodium content, and then calcined. • Material calcined at 1100-1200°C is crushed and ground to obtain a range of sizes (Fig. 3.5).
Tabular aluminas are obtained by calcining to a higher temperature, about 1650°C. • Magnesium oxide MgO of greater than 98% purity is prepared by precipitating magnesium hydroxide in a basic mixture of treated dolomite and natural brines or seawater containing MgCl4 and MgSO4, followed by washing, filtration, drying, and calcination. • Zirconia ZrO2 of 99% purity is obtained by the caustic fusion of zircon ZrSi04: • ZrSi04 + 4NaOH → Na2ZrO3 + Na2SiO3 + 2H2O (3.2)
Chemical dissolving of the silicate in water simultaneously hydrolyzes the sodium zirconate to hydrated zirconia. • Zirconia is also produced by hot chlorination of zircon in the presence of carbon, and the hydrolysis of the zirconium tetrachloride product to form ZrOCl2 . • The ZrOCl2 can be calcined directly or reacted with a base in water to form hydrous zirconia. • Zircon may also be dissociated to ZrO2 + SiO2 by heating above 1750°C and the zirconium separated by leaching with sulfuric acid: • ZrO2 + SiO2 +2H2SO4→Zr(SO4)2+SiO2+2H2O
Silicon carbide SiC is produced in large tonnages using the Acheson process by reacting a batch consisting principally of high-purity sand and low-sulfur coke at 2200-2500 °C in an electric arc furnace. • SiO2+3C→SiC+2CO(gas)
The crystalline product is crushed, washed in acid and alkali, and then dried after iron has been removed magnetically. • Granular material is used in refractories and bonded abrasives. • Milled material chemically treated to remove impurities introduced in milling is used industrially for structural ceramics introduced in milling is used industrially for structural ceramics
Titania TiO2 is produced by the sulfate or chloritle process. • In the sulfate process ilmenite FeTiO3 is treat with sulfuric acid at 150-180°c to from the soluble titanyl sulfate TiOSO4 • FeTiO3+2H2SO4+5H2O→ FeSO4.7H2O +TiOSO4
After removing undissolved solids and then the iron sulfate precipitate the titanyl sulfate is hydrolyzed at 90°C to precipitate the hydroxide • TiO(OH)2 • TiOSO4 + 2H20 → TiO(OH)2 + H2 (3.11) • The titanyl hydroxide is calcined at about 1000o-C to produce titania TiO2. • In the chloride process, a high-grade titania ore is chlorinate in the presence of carbon at 900-1000°C and the chloride TiCl4 formed is subsequently oxidized to Ti02.
As indicated by the nominal solid-state reaction for the formation of barium titanate at a temperature above 1250°C • BaCO3 + Ti02 → BaTi03 + CO2 (3.14) • the partial pressure of C02 in the pores of the product influences the reaction kinetics. • Also the none quilibrium phase Ba2TiO4 initially forms between BaTiO3 and unreacted BaC03 and is undesirable in the calcined product; • this phase is minimized by dispersing agglomerates of titania and mixing thoroughly to maximize the particle contacts and reduce the diffusion path between BaCO3 and Ti02.
Calcination in a furnace, which provides a more uniform temperature in the material and mixing of material with air, as shown in Fig 3 10 may produce a more uniform product.
In calcining ferrites and pigments, the oxygen pressure of the air must be controlled to obtain the requisite oxidation states of the transition metal ions. • The calcining temperatures and atmosphere must also be controlled to prevent the loss of nonrefractory chemical dopants
SUMMARY • Few ceramics are produced today using crude raw materials. • Industrial minerals are refined physically to reduce the concentration of undesirable mineral impurities and to produce a particular particle size distribution; water-soluble impurities are removed by washing.
Industrial inorganic chemicals used to produce the majority of technical ceramics are chemically processed on a large scale to improve both the chemical and the mineral purity; • the calcined product containing hard aggregates is commonly milled to disperse the aggregates and obtain a product of controlled size distribution.
SUMMARY • Mixed-oxide industrial chemicals are commonly produced by calcining a mixture of these industrial chemicals. • The completeness of the reaction and uniformity of the product depend on the particle size and mixedness of the reactants and the time, temperature and atmosphere and their uniformity during calcination. • Different lots of processed materials are blended to maintain a higher level of uniformity.
