Impure Metal Oxides

The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 

IR spectroscopy of adsorbed probe molecules is mostly performed with either the transmission/absorption technique or with the DRIFT technique. In the tiansmis-sion/absorption technique, self-supporting pressed disks ofthe pure oxide powders 

The advantages of the DR technique over the transmission/absorption technique in the fleld of the surface chemistry of oxides are  

2) applicability to powders that scatter too much for the transmission/absorption technique, assuming the surface area is sufficiently high to detect surface vibrahons with a sufficiently high signal-to-noise ratio  

3) slightly lower sensitivity to bulk conduction phenomena, because of a higher surface-to-bulk sensitivity ratio. 

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In the United States, germanium is obtained as a by-product of zinc production from zinc blende ores. The ore is concentrated by the flotation process. Concentrated ore is then roasted, converting zinc and the impurity metals to their oxides. Heating the crude oxides with sodium chloride and coal converts germanium and other impurity metal oxides into their volatile chlorides. The chloride vapors are condensed and germanium chloride, GeCh, is separated from the condensate by fractional distillation. 

When a reactive metal impurity is to be removed from a more noble metal, the impure metal is refined by controlled-potential electrolysis in which the impure metal oxidizes as the anode and the pure metal is reduced as the cathode Au, Ag, and Cu are... 

High Purity Aluminum Trifluoride. High purity anhydrous aluminum triduoride that is free from oxide impurities can be prepared by reaction of gaseous anhydrous HF and AlCl at 100°C, gradually raising the temperature to 400°C. It can also be prepared by the action of elemental fluorine on metal/metal oxide and subsequent sublimation (12) or the decomposition of ammonium duoroaluminate at 700°C. 

Also present are 0.3 wt % mixed metal oxides (R2O2) and 5 ppm of arsenic. Impurities in fluorspar may affect yield, plant operabiUty, or product quaUty. 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

The niter and fresh caustic soda, required to maintain the fluidity of the salt bath in the reactor chamber, are added gradually. When the color of the saturated salts turns from a dark gray to white, the impurity metals are at their highest state of oxidation, and the lead content of the spent salts is very low. In a modification, the arsenic and tin are selectively removed as sodium arsenate and sodium stannate, followed by the removal of antimony as sodium antimonate. 

Impurities. Impurities usually found in manganese ore may be classified into metal oxides, eg, iron, 2inc, and copper gangue volatile matter such as water, carbon dioxide, and organic matter and other nonmetaUics. 

Refining Processes. AH the reduction processes yield an impure metal containing some of the minor elements present in the concentrate, eg, cadmium in 2inc, or some elements introduced during the smelting process, eg, carbon in pig iron. These impurities must be removed from the cmde metal in order to meet specifications for use. Refining operations may be classified according to the kind of phases involved in the process, ie, separation of a vapor from a Hquid or soHd, separation of a soHd from a Hquid, or transfer between two Hquid phases. In addition, they may be characterized by whether or not they involve oxidation—reduction reactions. 

Mullite Refractories. MuUite refractories are classified under ASTM C467. This brick must have an Al O content between 56 and 79% and contain less than 5% impurities. Impurities are considered metal oxides other than those of aluminum and siUcon. The hot-load subsidence is 5% max is 1593°C. 

The transformations are aided by or may requke the presence of impurities or added mineralizers such as alkaH metal oxides. Indeed, it has been suggested that tridymite cannot be formed at all in the absence of impurities, and some texts assert that pure Si02 occurs in only two forms quartz and cristobaHte... 

The devitrification rate is extremely sensitive to both surface and bulk impurities, especially alkah. Increased alkah levels tend to increase the devitrification rate and lower the temperature at which the maximum rate occurs. For example, a bulk level of 0.32 wt % soda increases the maximum devitrification rate 20—30 times and lowers the temperature of maximum devitrification to approximately 1400°C (101). The impurity effect is present even at trace levels (<50 ppm) and can be enhanced with the addition of alumina. The devitrification rate varies inversely with the ratio of alumina-to-alkah metal oxide. The effect is a consequence of the fact that these impurities lower glass viscosity (102). 

For capillary columns fused siHca is the material of choice for the column container. It has virtually no impurities (<1 ppm metal oxides) and tends to be quite inert. In addition, fused siHca is relatively easily processed and manufacture of columns from this material is reproducible. In trace analysis, inertness of tubing is an important consideration to prevent all of the tiny amounts of sample from becoming lost through interaction with the wall during an analysis. 

The Kroll process for tire reduction of tire halides of refractory metals by magnesium is exemplified by the reduction of zirconium tetrachloride to produce an impure metal which is subsequently refined with the van Arkel process to produce metal of nuclear reactor grade. After the chlorination of the impure oxide in the presence of carbon... 

In 1794 the Finnish chemist J. Gadolin, while examining a mineral that had recently been discovered in a quarry at Ytterby, near Stockholm, isolated what he thought was a new oxide (or earth ) which A. G. Ekeberg in 1797 named yttria. In fact it was a mixture of a number of metal oxides from which yttrium oxide was separated by C. G. Mosander in 1843. This is actually part of the fascinating story of the rare earths to which we shall return in Chapter 30. The first sample of yttrium metal, albeit very impure, was obtained by F. Wohler in 1828 by the reduction of the trichloride by potassium. 

Pseudomorphism has less desirable consequences, and usually means are sought to suppress it. If the substrate has been scratched, ground or abrasively polished, or if it has been cold rolled or cold formed, the surface is left in a peculiar state. Cold working reduces the surface grain size, and produces deformed, shattered and partly reoriented metal. It may produce microcrevices between the deformed grains, and, with some processes, non-metallic impurities and oxides are embedded in the surface. The disturbed state of the substrate is copied by a pseudomorphic electrodeposit with several consequences (Fig. 12.7). One is aesthetic it has often been noted that almost invisible abrasion of the substrate develops as more prominent... 

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