Clays , thermal

Illite and montmorillonite are similar in structure and differ slightly from kaolinite in this regard. The first two are composed of two silicon-oxygen layers per octahedral layer containing iron, magnesium and aluminum and in kaolinite the ratio of tetrahedral and octahedral layers is 1. In clays thermal modification occurs at lower temperature than silica because the bonds formed between the Al, Fe, and Mg atoms and oxygen are weaker than the Si-0 bonds. 

Al-pillared clays thermally stable up to 750 C have been prepared from easily available starting materials a natural clay, namely the saponite, and a commercial aqueous solution of Chlorhydrol. 

Potassium Chloride. The principal ore encountered in the U.S. and Canadian mines is sylvinite [12174-64-0] a mechanical mixture of KCl and NaCl. Three beneficiation methods used for producing fertilizer grades of KCl ate thermal dissolution, heavy media separation, and flotation (qv). The choice of method depends on factors such as grade and type of ore, local energy sources, amount of clay present, and local fuel and water availabiUty and costs. 

Two undesirable aspects of FCC naphtha quaUty are that it may contain unacceptably high amounts of foul smelling mercaptans, and that its thermal stabiUty may be too low. Mercaptans are usually found in the light FCC naphtha and may be removed or converted to sulfides and disulfides by a sweetening process such as Merox, developed by UOP. Thermal stabiUty is improved in sweetening processes through removal of cresyUc and naphthenic acids. It may be further improved by clay treating and by addition of oxidation inhibitors such as phenylene diamine. 

Moleculady mixed composites of montmorillonite clay and polyimide which have a higher resistance to gas permeation and a lower coefficient of thermal expansion than ordinary polyimides have been produced (60). These polyimide hybrids were synthesized using montmorillonite intercalated with the ammonium salt of dodecylamine. When polymerized in the presence of dimethyl acetamide and polyamic acid, the resulting dispersion was cast onto glass plates and cured. The cured films were as transparent as polyimide. 

A fourth alkalinity control additive is magnesium oxide [1309A8A], which is used in clay-free polymer-base fluids (47). Magnesium oxide provides an alkaline environment and, as it is only slightly soluble, also has a buffering effect. It enhances the thermal stabHity of polymer solutions by preventing a pH decrease to neutral or slightly acidic conditions at elevated temperatures. It is mainly appHed in completion or workover operations where clay-free acid-soluble fluids are desired. 

Mixed-layer clays, particularly lUite—smectite, are very common minerals and illustrate the transitional nature of the 2 1 layered siHcates. The transition from smectite to iUite occurs when smectite, in the presence of potassium from another mineral such as potassium feldspar, or from thermal fluids, is heated and/or buried. With increasing temperature smectite plus potassium is converted to iUite (37,39). 

The hydrated alumina minerals usually occur in ooUtic stmctures (small spherical to eUipsoidal bodies the size of BB shot, about 2 mm in diameter) and also in larger and smaller stmctures. They impart harshness and resist fusion or fuse with difficulty in sodium carbonate, and may be suspected if the raw clay analyzes at more than 40% AI2O2. Optical properties are radically different from those of common clay minerals, and x-ray diffraction patterns and differential thermal analysis curves are distinctive. 

R. C. Mackenzie, ed.. The Differential Thermal Investigation of Clays, Mineralogical Society, London, 1957. 

MiscelDneous. Other important properties are resistance to thermal shock, attack by slag, and, in the case of refractories (qv), thermal expansion. For whiteware, translucency, acceptance of glazes, etc, may be extremely important. These properties depend on the clay mineral composition, the method of manufacture and impurity content. 

Other Polymerization Methods. Although none has achieved commercial success, there are a number of experimental alternatives to clay-catalyzed or thermal oligomeriza tion of dimer acids. These iaclude the use of peroxides (69), hydrogen fluoride (70), a sulfonic acid ion-exchange resia (71), and corona discharge (72) (see Initiators). 

Thermally efficient calcination of lime dolomite and clay can be carried out in a multicompartmeut fluidized bed (Fig. 17-27). Fuels are burned in a fluidized bed of the product to produce the required heat. Bunker C oil, natural gas, and coal are used in commercial units. Temperature control is accurate enough to permit production of hme of very high availability with close control of slaking characteristics. Also, half calcination or dolomite is an accepted practice. The requirement of large crystal size for the hmestoue limits apphcatiou. SmaU-sized crystals in the hmestoue result in low yields due to high dust losses. 

Tne insulating firebrick is a class of brick that consists of a highly porous fire clay or kaolin. Such bricks are light in weight (about one-half to one-sixth of the weight of fireclay), low in thermal conductivity, and yet sufficiently resistant to temperature to be used successbilly on... 

Bricks of silicon carbide, either recrystaUized or clay-bonded, have a high thermal conductivity and find use in muffle walls and as a slag-resisting material. 

Thermogravimetric analysis has also been used in conjunction with other techniques, such as differential thermal analysis (DTA), gas chromatography, and mass spectrometry, for the study and characterisation of complex materials such as clays, soils and polymers.35... 

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