Refractory substance

Mendeleev predicted that eka-silicon is a refractory substance as predicted by Mendeleev. In fact it melts at the relatively low temperature of 950°C.16... 

The section Analysis starts with elemental composition of the compound. Thus the composition of any compound can be determined from its elemental analysis, particularly the metal content. For practically all metal salts, atomic absorption and emission spectrophotometric methods are favored in this text for measuring metal content. Also, some other instrumental techniques such as x-ray fluorescence, x-ray diffraction, and neutron activation analyses are suggested. Many refractory substances and also a number of salts can be characterized nondestructively by x-ray methods. Anions can be measured in aqueous solutions by ion chromatography, ion-selective electrodes, titration, and colorimetric reactions. Water of crystallization can be measured by simple gravimetry or thermogravimetric analysis. 

Robert Hare, 1781-1858. Professor of chemistry at the University of Pennsylvania. At the age of twenty years he invented the oxy-hydrogen, or compound, blowpipe, with which he fused and volatilized platinum and other refractory substances. He was most ingenious in devising chemical apparatus. 

The fusibility of a substance is not solely Influenced by tho elements which enter into its composition, but also by tho manner in which these elements ate arranged and oombined together. The chief constituent of clay—alumina—is a base which, In combination with silica, forms one of the most refractory substances, and this property is possessed by the clays in proportion as they ore unmixed. with other bases, as alkalies, oxide of iron, lime, arid magnesia, in the order here given. In tho purer clays, which for ajl ordinary purposes may be considered fire-proof, the refractory quality is augmented in proportion to the quantity of silica they contain. 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

The diseases attributed to asbestos are a result of the fiber morphology and stability of the fibers rather than any specific chemical reactions between the asbestos and a host organism. It is probable that any refractory substance of similar morphology should stimulate similar diseases. 

The various reagents may be created in several ways. They may be obtained by photolysis, pyrolysis or discharge of a suitable precursor. Refractory substances, such as metals and metal salts, can be produced in the gas phase by direct vaporisation or, more recently, by laser... 

The nitrides and carbides of titanium and zirconium and the carbide of hafnium are extremely hard substances, resembling metals both in appearance and in electrical conductivity. Their formulae approach AxBh but some departure from stoichiometry is possible. Each of these refractory substances has the sodium chloride structure, described alternately (p. 190) as cubic close-packed arrays of metal atoms with the small nonmetal atoms in the octahedral holes. Note, however, that the parent metals themselves do not have cubic close-packed structures. Thus, the older view of such nitrides and carbides as lattices of the parent metals that are expanded to accommodate nitrogen or carbon atoms in the holes (interstices) is not admissible. The nature of the bonding in such refractory nitrides and carbides appears to be linked to the nature of bonding in metals in general, an important and interesting topic, but best pursued in more advanced works. 

Enhancing -> passivation of iron and steel and dissolving refractory substances such as titanium slag, copper-yttrium oxide, and metal fluorides. 

This black, refractory substance is made by reduction of V2O5 with hydrogen or carbon monoxide. It has the corundum structure but is difficult to obtain pure, since it has a marked tendency to become oxygen deficient without change in structure. Compositions as low in oxygen as VOi35 are reported to retain the corundum structure. 

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°. 

The sulfides cerium monosuifide, CeS, and thorium monosulfide, ThS, and related sulfides have been found valuable as refractory substances. The melting point of cerium monosulfide is 2450° C. 

AWLICATION OF PREVIOUSLY PRECIPITATED ACTIVE ALUMINIUM HYDROXIDE (AAH) FOR REMOVAL OF REFRACTORY SUBSTANCES FROM WASTEWATER... 

Pieviously ptecipitated amorphous aluminium hydroxide with many surface groups and developed specific surface area, can be used for the removal of refractory substances from wastewaters. 

During this study the removal efficiencies for refractory substances were compared for conventional coagulation process and for adsorptive removai on previously precipitated AAH. The results confirm the suitability of AAH for the removal contaminants from wastewater. 

Further research is planned to find out either operating conditions able to degrade biologically these refractory substances or physicochemical treatment to cope with biological methods to solve completely the problem of olive oil waste water discharge. 

Thermal expansion. The thermal expansion coefficient of ceramics is a resultant of the expansion of the individual phases, crystalline and glassy. Their coefficients may differ considerably and, furthermore, are also temperature dependent, again to varying degrees. Figure 211 shows, for the sake of illustration, this dependence for several refractory substances which do not undergo any phase changes. 

A refractory substance is a material that is resistant to heat and attack by strong chemical agents. 

Acidic flux A salt that exhibits acidic properties in the molten state used to convert refractory substances into water-soluble products. 

Note that these standard substances are adequate to effectively remove the excess oxygen. Sulfuric acid is included as a refractory substance. It has not been observed as a gas-phase species, but might be observed in infrared spectra of clouds. It is likely that most of the chemistry forming these species occurs in hot, dense stellar envelopes during their mass loss. 

The principal direction of the published photochemical research to date has been to elucidate pathways which might be responsible for the degradation of these refractory substances in nature. This basic research has not been extensively applied either to on-or off site treatment of hazardous substances. In this chapter we extend the existing PCB photochemistry literature by considering this potential. The development and fabrication of a prototype surface photoreactor, and its preliminary evaluation at a PCB-contaminated site are described. Photochemical treatments may be applicable to many other classes of hazardous chemicals as well including the polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) about which this monograph is chiefly concerned. 

The actinide element halides run the entire gamut of volatility from very high melting and refractory substances such as the trifluorides to the exceedingly volatile hexafluorides. Table VIII lists the actinide element... 

Hafnium is a lustrous, silvery, ductile metal with a high melting point. The mechanical properties of the metal are markedly affected by traces of impurities such as O, N, and C which have an embrittling effect on the metal, making it difficult to fabricate. The alloy tantalum hafnium carbide (Ta4HfCs), with a melting point of 4215 °C, is one of the most refractory substances known. 

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