Selenium Production

Sulphuric acid production sludge as well as copper-electrolyte slimes are the main source of selenium recovery. After flotation concentration the separation of molten selenium is the next step. Selenium is extracted from residues after via leaching by Na₂S or Na₂SO₃ with H₂SO₄. Oxidizing roasting with water capturing of the sublimated product and bottoming by NaHSO₃ in HCl or slime sintering with Na₂CO₃ is also applied. After that leaching, HCl acidifying SeO₂ residue is filtered out and bottomed by FeSO₄ for Se(VI) reducing and SO₂.

Sulphide cycle processing based on reversible reaction between selenium and Na₂SO₃ is one of alternative chemical methods of selenium preparation. Other methods include selenium oxidizing (burning in O₂), sublimation refining of the yielded SeO₂, vapours filtering, ion exchange etc., with following SeO₂ reduction. High purity selenium is obtained refining H₂Se, SeOCl₂ and so on.

Refined the selenium is remelted, filtered through fiberglass fiber fabric or activated coal, exposed to vacuum distillation and or put to rectification in inert atmosphere.

Selenium Extraction From Pyrites

In the oxidation of the pyrites (or other sulphur mineral) for the formation of sulphur dioxide in the manufacture of sulphuric acid, foreign elements like arsenic and selenium also undergo oxidation and pass off as vapours with the sulphur dioxide. The selenium dioxide produced in this manner then suffers more or less complete reduction by the sulphur dioxide, when finely divided selenium separates, mainly in the lead chambers, as a red, amorphous powder, accompanied possibly by some of the greyish-black form; a portion of the dioxide is also found in the Glover tower acid. The amount of selenium in the chamber "mud" depends, of course, on the nature of the pyrites; relatively large quantities of compounds of arsenic,- zinc, tin, lead, iron, copper or mercury are always present, arising almost entirely from impurities in the pyrites.

In the smelting of copper ores some selenium is found in the flue dust; the amount may be as much as 12 per cent., whilst some passes into the anode slimes of the electrolytic refineries.

The method of extraction and purification of the selenium from such mixtures has to be adapted to suit each individual case and the process is therefore frequently lengthy. Generally speaking, most of the methods involve oxidation of the selenium in the mixture to selenious acid, and precipitation of the element from the latter compound by treatment with sulphur dioxide. With powerful oxidising agents, selenic acid is produced, and this compound must be reduced to selenious acid, generally by boiling with hydrochloric acid, before passing sulphur dioxide.

The process first employed on a large scale consisted in steeping coke or pumice in the washed sludge and after drying, igniting in a current of air at such a temperature that the selenium burned to the dioxide without any distilling over unburnt. The sublimate of selenium dioxide was dissolved in water and the element precipitated by treatment with sulphur dioxide. The method, however, was too wasteful, not more than 70 per cent, of the selenium being recoverable. Various oxidising agents have therefore been employed, for example, nitric acid, aqua regia, a mixture of alkali carbonate and nitrate (with fusion), potassium or sodium chlorate, potassium permanganate, and more recently sodium dichromate, manganese dioxide and concentrated sulphuric acid, and sulphur trioxide in the form of oleum.

When chlorate or permanganate is used the sludge is first treated with water and dilute sulphuric acid, the oxidising agent then being added slowly and in small quantities (to avoid risk of explosion) until the suspended matter is wholly greyish-white

5Se + 6H₂SO₄ + 4KMnO₄ = 2K₂SO₄ + 4MnSO₄ + 5H₂SeO₃ + H₂O.

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.

The use of sodium chlorate possesses certain advantages; it is cheaper than permanganate and a better yield of selenium is obtained, 90 to 95 per cent, being usual; with permanganate the yield is 80 to 85 per cent., some of the selenium remaining in the solution as selenic acid, which is not directly reduced by the sulphur dioxide.

The use of oleum is still more satisfactory, and although the yield of selenium (85 per cent.) is not so high as in the chlorate process, oleum is the cheapest oxidising agent, and no selenic acid is produced. The sludge is warmed with oleum containing 20 per cent. SO₃, oxidation proceeding according to the equation

Se + 2SO₃ = SeO₂ + 2SO₂.

The reaction is reversible under the conditions obtaining, but the oxidation may be completed in the presence of excess of sulphur trioxide, the expelled sulphur dioxide being led away and a little oleum containing 80 per cent. S03 added. After cooling, very dilute sulphuric acid (about 1 per cent.) is added, the solution allowed to settle for 24 hours, and finally filtered. The selenium is then precipitated with sulphur dioxide in the usual manner.

The sludge which separates from the oxidation process contains sulphates of lead and calcium, and forms a source of the former metal, which is often an important by-product from the selenium extraction.

A further method of extraction of the selenium from the " mud " is based on the solubility of the element in solutions of potassium cyanide, forming selenocyanide. The solution deposits selenium when acidified, and any sulphur is retained in solution as thiocyanic acid. Treatment with a solution of an alkali hydroxide, or fusion with sodium carbonate (the latter more especially for the extraction of selenium from the dust of the flues between the pyrites burners and the Glover tower), has also been applied, the mass being extracted with water in the latter case. The resulting aqueous solutions deposit selenium on atmospheric oxidation, whilst any extracted sulphur passes mainly to thiosulphate.

Chlorine also converts the selenium and sulphur of dried "chamber mud" into the volatile chlorides, from which the selenium can be liberated by decomposition with water followed by reduction with an alkali sulphite.

The presence of much selenium in the Glover tower acid imparts a red colour to the liquid. The element can be separated by dilution followed by treatment with sulphur dioxide.

A method applied to dried anode slimes consists in heating first with concentrated sulphuric acid to convert metals present to sulphates, and then with an alkali hydrogen sulphate, when selenium distils over above 700° C.

Selenium Extraction From the Metallic Selenides

With these substances, the chief of which is zorgite, or lead-copper selenide, the usual primary process is again one of oxidation. If a powerful oxidising agent such as aqua regia is used, selenic acid is formed. After removal of excess of the oxidising acid by evaporation, and filtering off the separated lead chloride, the selenic acid is reduced to selenious acid by boiling with hydrochloric acid, and the selenium then precipitated by sulphur dioxide.

The most usual impurities in the crude selenium are lead and calcium sulphates, tellurium and sulphur. The most satisfactory method of purification is distillation, the temperature being gradually raised to about 900° C. Water and any free sulphuric acid escape first, whilst other impurities are left in the still. The latter should be made of cast iron containing 5 to 6 per cent, of silicon, though fused quartz may be employed. The receiver is kept at about 360° C. so that the selenium is collected in the molten condition and can be cast into ingots. Some of the element remains in the residue and cannot be distilled even at 1000° C.; this is particularly the case when calcium sulphate is the main impurity, and other methods of extraction have then to be used.

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. It may then be redissolved in water, the solution acidified with hydrochloric acid, and the selenium precipitated by passing in sulphur dioxide. 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.

The purified selenium of commerce is usually about 99.5 per cent, pure; it may contain a trace of iron and sulphur but is usually free from tellurium.