Selenium precipitation

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and teUurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 W with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxylamine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and deterrnination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

Hydrochloric acid is next added, and, after ten minutes, warm water in considerable quantity. If necessary a little bisulphite may be added to remove any excess of permanganate. After cooling and allowing all suspended matter to settle thoroughly, the clear liquid is syphoned off and the selenium precipitated by blowing sulphur dioxide through for four hours. The selenium is then removed, washed and dried. 

Purification may also be effected by oxidation to selenious acid, e.g. by heating with dilute nitric acid. On evaporation the solid selenium dioxide may be obtained, and this can be purified by repeated sublimation in a current of dust-free dry air.3 It may then be redissolved in water, the solution acidified with hydrochloric acid, and the selenium precipitated by passing in sulphur dioxide.4 For further purification the element can be sublimed in a current of carbon dioxide, and after heating for some time at 100° C. to convert it into the crystalline condition, it may be heated with carbon disulphide to extract any traces of residual sulphur. 

MAN/GAL] Manceau, A., Gallup, D. L., Removal of selenocyanate in water by precipitation characterization of copper-selenium precipitate by X-ray diffraction, infrared, and X-ray absorption spectroscopy, Environ. Sci. Technol., 31, (1997), 968-976. Cited on page 297. 

Inorganic anions such as iodide, iodate, nitrate, and chlorate have been determined on the basis of redox reactions. Iodides have been determined by their reaction with chromium(vi) in an acidic medium. The unreacted Cr is then extracted into MIBK from a 3 M HCl solution. The iodide concentration is quantitatively related to the increase of the Cr atomic signal in aqueous solution or the decrease of the Cr signal in the organic phase. Selenium(iv) is reduced by iodide to elemental selenium. The atomic absorption signal of selenium is then measured in the selenium precipitate. 

Then cool the reaction-mixture, filter it at the pump, leaving a black residue of selenium, and wash out the flask twice with 2x5 ml. of acetic acid, passing the washings also through the filter. Dilute the united filtrates with water, and make the solution alkaline with 10% aqueous sodium hydroxide, which precipitates the camphorquinone. Cool, filter off the yellow camphorquinone at the pump, wash with water and drain thoroughly. 

Wet Process. The sodium arsenate and stannate slag are treated by a leach and precipitation process to produce calcium arsenate, calcium stannate, and a sodium hydroxide solution for recycle. The sodium antimonate filtercake containing selenium, tellurium, and indium is treated in a special metals refinery to recover indium and tellurium. 

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. 

A widely used procedure for determining trace amounts of selenium involves separating selenium from solution by reduction to elemental selenium using tellurium (as a carrier) and hypophosphorous acid as reductant. The precipitated selenium, together with the carrier, are collected by filtration and the filtered soflds examined directly in the wavelength-dispersive x-ray fluorescence spectrometer (70). Numerous spectrophotometric and other methods have been pubHshed for the deterruination of trace amounts of selenium (71—88). 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

Many analytical methods depend on the conversion of the tellurium in the sample to teUurous acid, H2Te02. Should teUurous acid precipitate on dilution, it can be redissolved with hydrochloric acid. Although tellurium is not as readily volatile as selenium, precautions should be taken to prevent the volatilisation of tellurium when halogen or hydrohaUde media are used during sample decomposition. 

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. 

Zinc smelters use x-ray fluorescence spectrometry to analyze for zinc and many other metals in concentrates, calcines, residues, and trace elements precipitated from solution, such as arsenic, antimony, selenium, tellurium, and tin. X-ray analysis is also used for quaUtative and semiquantitative analysis. Electrolytic smelters rely heavily on AAS and polarography for solutions, residues, and environmental samples. 

The color obtained is a function of both the composition and the particle size of the precipitated crystals. A redder color results from both increased selenium to sulfur ratio and from larger crystals, caused by a more severe heat treatment. Hence, it is possible to make, from the same glass, a series of color filter types, by controlled reheating. 

By-Product Recovery. The anode slime contains gold, silver, platinum, palladium, selenium, and teUurium. The sulfur, selenium, and teUurium in the slimes combine with copper and sUver to give precipitates (30). Some arsenic, antimony, and bismuth can also enter the slime, depending on the concentrations in the electrolyte. Other elements that may precipitate in the electrolytic ceUs are lead and tin, which form lead sulfate and Sn(0H)2S04. 

The fumes from the roaster are passed through a train of water-spray scmbbers and an electrostatic precipitator. In the scmbber, selenium dioxide [7446-08-4] reacts with sulfur dioxide (eq. 37) to produce elemental selenium, which is purified to provide a commercial product. 

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