Calcination, 3.38

Calcination is used to produce value-added kaolin products. Commonly, there are two families of calcined kaolin. One is metakaolin, which is produced by heating 

Surface treatment is another value-added step that can improve the performance of kaolin. Since the filler is naturally very hydrophilic due to its hydroxyl groups, a treatment can be applied to render its surface hydrophobic or organophilic. These surface-modified kaolins are useful especially in plastics and rubber industries, where they improve adhesion and dispersion and hence act more effectively as functional fillers. Silanes, titanates, and fatty adds as discussed in Chapters 4-6, respectively, may be used to modify the surface charaderistics of either hydrous or calcined kaolins, promoting dea lomeration, often lower viscosities, and improved mechanical and eledrical properties. 

Typical chemical compositions of hydrous and calcined kaolins are shown in Table 13.1 [8]. The major effeds of the removal of water of hydration are an increase in refradive index, a moderate increase in the otherwise low Mohs hardness, a decrease or increase in spedfic gravity depending on the extent of caldnation, and a decrease in dieledric constant (Table 13.2) [8]. The calcined materials, due to their higher void structure, are capable of considerably higher oil absorption, as indicated in Table 13.3, which also includes comparisons of additional properties of various 

Major suppliers of kaolin with their corresponding capacities in million tons per armum (mtpa) are Imerys with 5.0 mtpa, Huber Engineered Materials (business sold in 2008 to IMin Partners, Fort Worth, TX) with 2.0 mtpa, Engelhard (business 

Metal oxide Hydrous grade Calcined grade 

During the calcination with or without gases, there are decompositions of hydroxides, carbonates (gel, xerogel) transforming into oxides which are chemical reactions, transforming one compound to other compounds. During thermal treatments, there are following cases  

It shows that the specific surface area decreases with increasing temperature of calcination. 

On supported catalysts, the effect of calcination is significant on particle sizes, dispersions, and crystallite sizes. Measurements of particle sizes or metal dispersions after calcination and reduction of supported catalysts indicate different situations and there are three cases that are illustrated below. 

1 Effect of calcination and digestion over reduction and metallic surface area 

The first case illustrates how the calcination temperature and digestion time affect metallic area and degree of reduction of the alumina-supported NiO for different Ni contents. 

Phase transformation can be followed by thermogravimetry, measuring the mass loss and the temperature difference with increasing temperature, as shown in Fig. 8.6. 

The effects of calcination are significant, modifying textural and morphological properties, affecting the surface area, pore volumes, and structures. There are also differences between bulk and supported materials. In the first case, one observes from Fig. 8.7 how these parameters change with increasing calcination temperature. 

In the manufacturing of ceramic materials, three thermal treatments are important calcination, sintering, and glazing. 

Calcination was discussed in Section 12.2, Raw Materials. It was required for some ceramics to be obtained in the desired compound. For example, in the preparation of alumina, it was mentioned that aluminum hydroxide was calcined to get the alumina. Calcination is the process of heating in the absence of any atmosphere. This heating decomposes the compound into a solid and a gas or vapor. In the case of aluminum hydroxide, it decomposes into aluminum oxide and water vapor. 

In sintering, interparticle bonding is allowed to take place at a temperature below the melting point of the material. In order to facilitate this bonding, certain additives are added. These are called sintering aids. These sintering aids may get melted in the process. The particles may be crystalline or amorphous. 

y is the surface energy, and r is the radius of the spherical shape. This radius of curvature is positive or negative depending on whether the 

Coalescence of two particles in contact during sintering (a) two spherical particles in contact (b) formation of grain boundary. 

This is essentially the reverse of the extraction process, except that the product s nature is carefully controlled by the plant conditions, including seeding or selective nucleation, precipitation temperature, and the cooUng rate. The gjbbsite crystals are then classified into size fractions and fed into a rotary or fluidized-bed calcination kiln (step E). Undersized particles are fed back into the precipitation stage. 

The gibbsite crystals are calcined (step E) to form high-purity alumina (a-Al203) as the precursor material for a variety of alumina ceramics according to  

The calcination process must be carefully controlled since it dictates the properties of the final product A secondary process stream containing dried gibbsite can be separated to produce aluminates, zeolites, filler materials for toothpaste, fire retardants, and others (steps F-H). As will be shown below, the stepwise removal of water and OH groups, respectively, leads to a plethora of transitional alumina polytypic structures that eventually will determine the performance of the end products. 

The worldwide production of alumina was estimated to be 62.4 million tons in 2006, an increase from the 1995 value of 38 million tons (Evans, 1996), with Australia (17.7 milUon tons), China (8.6 million tons), Brazil (5.3 million tons) and USA (5.2 million tons) as the leading producers (Source USGS Mineral Resources Program). While 90% of the alumina is used to extract metallic aluminum by electrolysis, approximately 10% of the total is non-metallurgical alumina used for advanced ceramic allocations (see Section 7.1.4.3 below). 

Removal of soluble cadmium and production of a fine powder. 

Not all manufacturers produce cadmium oxide, as some prefer to dissolve cadmium metal directly in acid. However cadmium oxide is used in many other applications, including nickel cadmium batteries and stabilizers for polymers. 

The oxide is then packed and sold, or metered into water to form a slurry suitable for dissolving in acid. Sulfuric acid (50%) is preferred, as the heat of solution is not too violent. In this way, a strong stock solution of about 300 g/L Cd can be produced. The solution is treated as necessary to remove other undesirable metal contaminants, filtered and diluted down to the required strength prior to precipitation. 

Clean air from the filter bag housing is typically passed to a scavenger unit to ensure that discharged gases are within permitted limits for cadmium emissions. 

While cadmium will dissolve in sulfuric, hydrochloric and nitric acids, a mixture of sulfuric and nitric acids is preferred, to obtain a controllable reaction rate to avoid producing noxious fumes. 

One of the now-classic examples of process Raman spectroscopy is monitoring the efficiency of converting the anatase form of titanium dioxide to the rutile form by calcining [67-69]. The forms have dramatically different spectra, enabling simple univariate band ratio models to be used. The uncertainty in the mean 

Hydrothermal treatment can however be used constructively to modify the properties of an adsorbent. Perhaps the best example is the formation of ultrastable Y by hydrothermal treatment of sodium ammonium Y zeolite. The change is accompanied by a contraction in the unit cell parameter and an increase in the Si/Al ratio due to elimination of aluminum from the lattice. The resulting product shows greater thermal stability than Y zeolites of similar composition which have not been subjected to the hydrothermal treatment. However, with X zeolite the usual result is a loss of crystallinity with attendant deterioration of the adsorptive properties while with A zeolite a more subtle effect referred to as pore closure occurs. Hydrothermally treated 4A zeolite behaves as if the window aperture is somewhat smaller than in normal 4A sieve. This effect can be useful since a pore-closed 4A does not admit chlorinated hydrocarbons and is therefore useful for drying freon refrigerants. If a wider pore sieve is used for this purpose premature breakdown and loss of capacity may occur due to formation of HF and/or HCl by hydrolysis. The precise mechanism of pore closure has not yet been established and it remains uncertain whether it involves a true modification of the ciystal structure or merely a rearrangement of the surface layers. 

A brief list of some important separations which are carried out with zeolite adsorbents is given in Table 1.3. A much more comprehensive list has been given by Collins and reprinted by Breck. The large majority of these applications are purification processes in which the zeolite is used as a selective adsorbent to remove an undesirable impurity such as water, HjS, or radioactive Kr. Important examples of true separation processes include the linear/branched chain hydrocarbon separation, carried out with zeolite A and the separation of xylene isomers which utilizes various cationic forms of the X and Y zeolites. Some of these processes are considered in greater detail in Chapters 10-12. 

When reactive hydrocarbons such as olefins are present, slow formation of polymeric species may occur within the zeolite crystals. On thermal regeneration these species are converted to coke, leading to a decline in the useful capacity of the adsorbent and in some cases also a decline in intracrystalline diffusivity. In the drying of cracked gas this problem may be avoided by using 

Framework Form Unit Ceil Window (A) Application 

X Ca Ca4oN a6[(AI02)s6(S 2) 1 oel 12-ring 8.0 Removal of mercaptans from 

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Petroleum coke is in reality a hydrocarbon whose C/H ratio is very high it is usually higher than 20 and can attain 1000 after calcination. It is not, therefore, elementary carbon. 

It is possible to modify the quality of the coke by calcination at high temperatures (1200-1400°C) this has the effect of reducing the volatile material and to increase the density. 

Petroleum coke is an excellent fuel, and that is its main use, especially for the coke from fluid coking". There are some other markets that have to do with calcined coke electrodes for aluminum production or for all other electrolytic cells, carbons for electro-mechanical equipment, graphite, and pigments. 

Sulfated ash in lubricating oils in greases NF T 60-143 ISO 3987 ASTM D 874 NF T 60-144 ASTM D 128 Weight of residue after treatment of the ash by sulfuric acid and calcination As above... 

Figure Bl.25.9. Positive SIMS spectra of a Zr02/Si02 catalyst, (a) after preparation from Zr(OC2Ftj), (b) after drying at 40 °C and (c) after calcination in air at 400 °C (from [17]).

Eschka s mixture (sulfur in coal) mix 2 parts of porous calcined MgO with 1 part of anhydrous Na2C03 not a solution but a dry mixture. 

A final example appears in Fig. 3.26(c) and (d) where the experimental substance was a magnesium oxide prepared by hydrolysis of magnesium methylate followed by calcination at 500°C. Curve (c) gives a comparison plot of adsorption on a compact against the adsorption on the... 

ALUMDIUMCOMPOUNDS - ALUMINIUMOXIDE(ALUMINA) - CALCINED,TABULAR, AND ALUMINATE CETffiNTS] (Vol 2) -in automobile polishes pOLISITES] (Vol 19)... 

Calcia [1305-78-8] Calci-Bind Calcimar Calcimycin [52665-69-7] Calcination Calcined gypsum Calcined limestone... 

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