Inertness, refractories

The most extensive group of nitrides are the metallic nitrides of general formulae MN, M2N, and M4N in which N atoms occupy some or all of the interstices in cubic or hep metal lattices (examples are in Table 11.1, p. 413). These compounds are usually opaque, very hard, chemically inert, refractory materials with metallic lustre and conductivity and sometimes having variable composition. Similarities with borides (p. 145) and carbides (p. 297) are notable. Typical mps (°C) are ... 

Among the more important catalysts are metals, which may be promoted by other metals, or by oxides and oxides, which are usually rendered more effective by mixing with other oxides. It is usual to distinguish between supported catalysts, generally metals in a finely divided condition on the surface of silicate minerals, and promoted catalysts, where an oxide, or occasionally some other compound, is mixed with the metal the mixture being sometimes also supported on an inert refractory support. The distinction is not, however, absolutely sharp. 

Oxides with layered stmcture or those whose structures contain large tunnels or cavities may display abnormal ion movement or serve as templates for heterogeneous catalysis (see Ionic Conductors Intercalation Chemistry Oxide Catalysts in Solid-state Chemistry andZeolites). Many oxides are stabilized by the formation of structures that are highly defective nature and have similar properties to those listed above (see Defects in Solids). The strong bonds, which result in three-dimensional cross-linked structures, give rise to inert, refractory materials that have a variety of uses (see Section 5.3.6 and Ceramics). 

In a well-crystallized condition the iron (III) spinels are chemically inert refractory substances. They lose oxygen before melting at very high temperatures, 1500 to 1800°. They all dissolve readily in boiling hydrochloric acid except nickel iron(III) oxide, which is very inert toward common reagents. In order to determine the nickel or iron content, nickel iron(III) oxide is first reduced to metal by heating it under hydrogen at 500°. 

To this point, moderate and high temperature lining details for inert atmospheres have been discussed. When impurities are added to the atmospheres being contained in the form of products of combustion or feed stocks, or when the atmospheres are oxidizing or reducing rather than inert, refractory lining performance can be radically altered. 

Inert refractory supports have long been used as economizers to extend the surface of active catalysts 1,2 thus asbestos has been used to support platinum to catalyze the oxidation of the S02. The original purpose of supports seems to have been to extend the surface of expensive catalysts. Atoms that are buried in the bulk phase, when a catalyst acts as its own support, are brought to the active surface when distributed over another, cheaper, support. 

Adams (1987) reported the reactions of PCBs and other halogen-containing organics with sulfur in an inert atmosphere at 500-1500°C (932-2732°F), converting into nontoxic, nonflammable, inert, refractory byproducts. 

Decomposition of Zircon. Zircon sand is inert and refractory. Therefore the first extractive step is to convert the zirconium and hafnium portions into active forms amenable to the subsequent processing scheme. For the production of hafnium, this is done in the United States by carbochlorination as shown in Figure 1. In the Ukraine, fluorosiUcate fusion is used. Caustic fusion is the usual starting procedure for the production of aqueous zirconium chemicals, which usually does not involve hafnium separation. Other methods of decomposing zircon such as plasma dissociation or lime fusions are used for production of some grades of zirconium oxide. 

At elevated temperatures, CaH2 reacts with halogens, sulfur, phosphoms, alcohols, and ammonia. At high temperatures, it reacts with refractory metal oxides and haUdes. Calcium hydride is substantially inert to organic compounds that do not contain acidic hydrogens. 

Once initiated, zirconium and carbon powders react exothermically in a vacuum or inert atmosphere to form zirconium carbide. With the greater availabiHty of relatively pure metal powders, this technique is coming into common use for the production of several refractory carbides. Zirconium carbide is not a fixed stoichiometric compound, but a defect compound with a single-phase composition ranging from ZrCQ to ZrCQ at 2400°C. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

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