Selenium production

It is generally unacceptable to emit sulfur dioxide, thus the scmbber effluent must be treated for sulfur dioxide removal. If the plant aheady possesses faciUties for the production of sulfuric acid, this rather concentrated sulfur dioxide stream can be easily fed into the wet gas cleaning circuit and disposed of in the sulfuric acid plant. The quantity is so small that it does not put any additional burden on the sulfuric acid plant. Because no tellurium is carried over with the selenium dioxide during roasting, it is possible to produce a selenium product which can be purified to commercial grade (99.5-99.7%). 

Japan, Canada, and the United States accounted for 70% of the 1995 estimated world production of 2000 t (Table 5). At least 100 t of selenium was also available to Western markets from the former Soviet bloc. Selenium production is expected to rise in South America, particularly Chile, as the copper industry continues rapid expansion. A considerable amount of unrefined selenium is also shipped to Chile and the Philippines for conversion to final commercial product by either hydrometaHurgical or distillation processes. 

Combination reactions occur with several nonmetals. With sulfur, it forms sulfur monobromide, S2Br2. With the addition of selenium, products are selenium monobromide, Se2Br2, and selenium tetrabromide, SeBr4. It yields unstable tellurium monobromide, Te2Br2, and a stable tetrabromide, TeBr4,with tellurium. 

Cooling and examination of the mixture shows that about 50 per cent, of the original quantity of selenium fosdibenzoylmethane has undergone transformation, and that the dimeric selenium product represented on the right of the equation consists of isoselenium dibenzoylmethane together with a small proportion of selenium dibenzoylmethane. iso Selenium dibenzoylmethane,... 

Another 25% of the selenium production in 1996 was used in inorganic pigments (principally as cadmium sulfoselenide used in plastics, paints, enamels, inks, mbber, and ceramics), and 10-15% of selenium is used in a broad spectrum of applications including accelerators and vulcanizing agents in mbber production, in stainless steel, and as selenides of refractory metals for use in lubricants. Some of these products may end up as disposed waste. Medical and pharmaceutical uses such as in topical preparations for treatment of dandmff, for... 

Worn-out and damaged photocopier drums, other industrial scrap and residues from chemical processes are of increasing importance as raw material. About 15% of selenium production come from these secondary sources. 

The western world figures for selenium production from 1964 to 1973 average 1042 tons [33]. The 1976 figure was only slightly higher at 1087 tons. The US produced 184 tons and imported the remaining portion of the 453 tons used from Canada (42%), Japan (27%) and others. 

Diselenium dichloride acts as a solvent for selenium. Similarly disulphur dichloride is a solvent for sulphur and also many other covalent compounds, such as iodine. S Clj attacks rubber in such a way that sulphur atoms are introduced into the polymer chains of the rubber, so hardening it. This product is known as vulcanised rubber. The structure of these dichlorides is given below ... 

The following oxidation of camphor to camphor-quinone illustrates the oxidising action of selenium dioxide, and readily gives a crystalline product. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

We also developed a number of other useful new fluorinating reagents. They ineluded a convenient in situ form of sulfur tetrafluoride in pyridinium polyhydrogen fluoride, selenium tetrafluoride, and ey-anurie fluoride. We introdueed uranium hexafluoride (UFg), depleted from the U-235 isotope, which is an abundant by-product of enrichment plants, as an effective fluorinating agent. 

Medicated Dandruff Shampoos. Dandmff is a scalp condition characterized by the production of excessive cellular material (18). A number of shampoos have been marketed which are designed to control and alleviate this condition, and many additives have been included in shampoo compositions to classify them as treatment products for dandmff. These additives include antimicrobial additives, eg, quaternary ammonium salts keratolytic agents, eg, saUcychc acid and sulfur heavy metals, eg, cadmium sulfide coal tar resorcinol and many others. More recent (ca 1993) systems use selenium sulfide [7488-56-4] or zinc pyrithione [13463-41 -7] as active antidandmff shampoo additives. Both of these additives are classified as dmgs, but can be found in over-the-counter products. A stronger version, incorporating the use of higher levels of selenium sulfide in a shampoo, is available but requires a prescription for purchase. 

Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

Pigment color is determined by the ratio of Cd, and Zn or Hg if present, to S and Se in the product and can be changed all the way from primrose to maroon. Mercury substitution for cadmium yields (Cd, Hg)S pigments with ted and maroon shades similar to those obtained with selenium substitution. The Cadmium Association provides the simple diagram given in Figure 5 showing the color and composition correlation. 

Selenium and precious metals can be removed selectively from the chlorination Hquor by reduction with sulfur dioxide. However, conditions of acidity, temperature, and a rate of reduction must be carefliUy controlled to avoid the formation of selenium monochloride, which reacts with elemental selenium already generated to form a tar-like substance. This tar gradually hardens to form an intractable mass which must be chipped from the reactor. Under proper conditions of precipitation, a selenium/precious metals product substantially free of other impurities can be obtained. Selenium can be recovered in a pure state by vacuum distillation, leaving behind a precious metals residue. 

Purifying Selenium and Tellurium. Selenium recovery processes generally yield a metal product which contains some tellurium, and, correspondingly, recovered tellurium generally contains some small amount of selenium (37,41). 

Also reported as commercial products are selenium amino acid chelate, selenium ascorbate, selenium aspartate, and selenium citrate triturations, as well as selenium dibromide, selenium dichloride, selenium lysinate, and selenium sulfide bentonite. 

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