Separation tellurium

A widely used procedure for determining trace amounts of tellurium involves separating tellurium in (1 1) hydrochloric acid solution by reduction to elemental tellurium using arsenic as a carrier and hypophosphorous acid as reductant. The arsenic, reduced from an addition of arsenite to the solution, acts as a carrier for the tellurium. The precipitated tellurium, together with the carrier, is collected by filtration and the filter examined directly in the wavelength-dispersive x-ray fluorescence spectrometer. 

The reactions between Grignard reagents and tellurium dihalides frequently reported in the older literature (Vol. IX, p. 1051) produced mixtures of diaryl tellurium products and other organic tellurium compounds that were difficult to separate. Tellurium dihalides are rather unstable compounds with a strong tendency to disproportionate to tellurium and tellurium tetrahalides. The Grignard reagents, which had to be used in excess of the stoichiometri-cally required amounts, then reacted with the tellurium (p. 172) and the tellurium tetrahalide and probably not with the tellurium dihalide. 

A frequently used method for separating tellurium from most elements consists in its reduction in acid medium to the element [1], usually by SnCh, SO2, or hypophosphite. Se, As, Hg, Au, Ag, Pd, and Pt are precipitated together with Te. Selenium and arsenic are suitable collectors for traces of tellurium [ 1 ]. 

Pemoval of Other Impurities. After softening, the impurities that may stiU remain in the lead are silver, gold, copper, tellurium, platinum metals, and bismuth. Whereas concentrations may be tolerable for some lead appHcations, the market values encourage separation and recovery. The Parkes process is used for removing noble metals and any residual copper, and the KroU-Betterton process for debismuthizing. 

The roasted pellets or extmdes are ground and leached in water. The hexavalent selenium dissolves as sodium selenate [13410-01 -0] Na2Se04. Sodium teUurate, being highly insoluble in the now very strongly alkaline solution, remains in the residue. The separation between selenium and tellurium is readily achieved, provided all tellurium is oxidized to the hexavalent state. 

Complete separation of selenium from tellurium caimot be achieved owing to classical Raoult s law considerations and what appears to be the... 

Most commercial tellurium is recovered from electrolytic copper refinery slimes (8—16). The tellurium content of slimes can range from a trace up to 10% (see Seleniumand selenium compounds). Most of the original processes developed for the recovery of metals of value from slimes resulted in tellurium being the last and least important metal produced. In recent years, many refineries have changed their slimes treatment processes for faster recovery of precious metals (17,18). The new processes have in common the need to remove the copper in slimes by autoclave leaching to low levels (<1%). In addition, this autoclave pretreatment dissolves a large amount of the tellurium, and the separation of the tellurium and copper from the solution which then follows places tellurium recovery at the beginning of the slimes treatment process. 

Production and Economic Aspects. Thallium is obtained commercially as a by-product in the roasting of zinc, copper, and lead ores. The thallium is collected in the flue dust in the form of oxide or sulfate with other by-product metals, eg, cadmium, indium, germanium, selenium, and tellurium. The thallium content of the flue dust is low and further enrichment steps are required. If the thallium compounds present are soluble, ie, as oxides or sulfates, direct leaching with water or dilute acid separates them from the other insoluble metals. Otherwise, the thallium compound is solubilized with oxidizing roasts, by sulfatization, or by treatment with alkaU. The thallium precipitates from these solutions as thaUium(I) chloride [7791 -12-0]. Electrolysis of the thaUium(I) sulfate [7446-18-6] solution affords thallium metal in high purity (5,6). The sulfate solution must be acidified with sulfuric acid to avoid cathodic separation of zinc and anodic deposition of thaUium(III) oxide [1314-32-5]. The metal deposited on the cathode is removed, kneaded into lumps, and dried. It is then compressed into blocks, melted under hydrogen, and cast into sticks. 

Molybdenite [1309-56 ] M0S2, normally floats with the copper sulfides. Therefore, the copper concentrate from the cleaner cells frequently has to be separated from molybdenite in a separate flotation circuit before the copper concentrate goes to the smelter. Gold, silver, selenium, and tellurium are collected with the copper concentrate. 

A small amount of tellurium (95 mg.) is formed during the preparation. The amount of tellurium increases slowly as the heating is prolonged. In a separate experiment pure bis(4-methoxyphenyl)-... 

Separation of Se and Te can also be achieved by neutralizing the alkaline selenite and tellurite leach with H2SO4 this precipitates the tellurium as a hydrous dioxide and leaves the more acidic selenous acid, H2Se03, in solution from which 99.5% pure Se can be precipitated by S02 ... 

Determination of copper as copper(I) thiocyanate Discussion. This is an excellent method, since most thiocyanates of other metals are soluble. Separation may thus be effected from bismuth, cadmium, arsenic, antimony, tin, iron, nickel, cobalt, manganese, and zinc. The addition of 2-3 g of tartaric acid is desirable for the prevention of hydrolysis when bismuth, antimony, or tin is present. Excessive amounts of ammonium salts or of the thiocyanate precipitant should be absent, as should also oxidising agents the solution should only be slightly acidic, since the solubility of the precipitate increases with decreasing pH. Lead, mercury, the precious metals, selenium, and tellurium interfere and contaminate the precipitate. 

Discussion. This gravimetric determination depends upon the separation and weighing as elementary selenium or tellurium (or as tellurium dioxide). Alkali selenites and selenious acid are reduced in hydrochloric acid solution with sulphur dioxide, hydroxylammonium chloride, hydrazinium sulphate or hydrazine hydrate. Alkali selenates and selenic acid are not reduced by sulphur dioxide alone, but are readily reduced by a saturated solution of sulphur dioxide in concentrated hydrochloric acid. In working with selenium it must be remembered that appreciable amounts of the element may be lost on warming strong hydrochloric acid solutions of its compounds if dilute acid solutions (concentration <6M) are heated at temperatures below 100 °C the loss is negligible. 

With the exception of the reactions of trifluoromethyl radicals with sulfur vapor, which is really a separate class of reactions, if the power supplied to the load coil surrounding the reactor (see Fig. 2) was maintained at, or near, the minimum amount needed to support the discharge, in only two cases were compounds found that clearly resulted from reactions other than replacement of halogen by trifluoromethyl. The reaction of tellurium tetrabromide (or the chloride) gave, in addition to the products just reported, very small proportions of such species as BrCF2TeCF2Br and (C2F5)2Te, which were isolated in yields of... 

Selenium (IV) adsorbed as selenotrisulfate was then eluted from the column with either 0.1 M penicillamine or 0.1M cysteine. The eluate was then subjected to an acid digestion procedure to reduce selenium to the tetravalent state with diaminonaphthalene for fluorometric determination. Approximate agreement with the tellurium coprecipitation method was obtained. The application of both methods to the analysis of estuarine waters permitted the separate determination of both selenium (IV) and selenium (VI), since the tellurium coprecipitation methods did not differentiate between the two species. 

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