CERAMIC RAW MATERIALS

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CERAMIC RAW MATERIALS
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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.

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• 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.

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• 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.

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• 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.

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CHAPTER 3COMMON RAW MATERIALS
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• 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.

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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.

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• 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.

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• 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.

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3.1 CRUDE MATERIALS
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• 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.

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• 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.

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3.2 INDUSTRIAL MINERALS
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• 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.

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• 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).

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• 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.

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• 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)

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• 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.

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• 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.

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• 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.

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3.3 INDUSTRIAL INORGANIC CHEMICALS
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• 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.

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• 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).

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• During the digestion, most of the hydrated
  alumina goes into solution as sodium aluminate
• ImpurityAl(OH)3(solid)NaOH(sol)
• ?NaAl(OH)4-(sol)Impurity_
• and insoluble compounds of iron, silicon, and
  titanium are removed by settling and filtration.

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• 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 1100-1200C is crushed and
  ground to obtain a range of sizes (Fig. 3.5).

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• Tabular aluminas are obtained by calcining to a
  higher temperature, about 1650C.
• 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)

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• 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 1750C and the zirconium separated
  by leaching with sulfuric acid
• ZrO2 SiO2 2H2SO4?Zr(SO4)2SiO22H2O

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• 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.
• SiO23C?SiC2CO(gas)

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• 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

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• Titania TiO2 is produced by the sulfate or
  chloritle process.
• In the sulfate process ilmenite FeTiO3 is
  treat with sulfuric acid at 150-180c to from
  the soluble titanyl sulfate TiOSO4
• FeTiO32H2SO45H2O? FeSO4.7H2O TiOSO4

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• After removing undissolved solids and then the
  iron sulfate precipitate the titanyl sulfate is
  hydrolyzed at 90C 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
  900-1000C and the chloride TiCl4 formed is
  subsequently oxidized to Ti02.

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• As indicated by the nominal solid-state reaction
  for the formation of barium titanate at a
  temperature above 1250C
• 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.

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• 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.

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• 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

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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.

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• 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.
  

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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.