This element, which fills an intermediate place between tellurium and sulphur, occurs mainly in connection with the latter body, being found in association with native sulphur itself, and with the sulphides of iron, copper, lead, and molybdenum, as well as with tellurium ores and pitchblende (uranium ore). The commercial sources of selenium are three-fold:-(a) the native double copper-lead selenide found in the Hartz; (6) the deposits formed in sulphuric acid chambers and flues where seleni-ferous brimstone or pyrites is burned; (c) the flue-dust of roasting furnaces in works where lead desilverization is carried on. Following are the processes of extraction employed.
(a) The pulverized copper-lead selenide is treated with hydrochloric acid to dissolve the earthy carbonates; the residue is washed, dried, and ignited for some time with an equal quantity of black flux; by this means, the selenium is converted into potassium selenide, which is dissolved away by boiling water from the oxides simultaneously formed; the solution undergoes oxidation on exposure to the air, and a grey deposit of selenium results; this is washed, dried, and distilled.
(6, 1) The seleniferous deposit is mixed with potassium nitrate and carbonate, and deflagrated in a led-hot crucible; the residue, containing potassium selenate, is heated with hydrochloric acid, and the whole is evaporated down to small bulk; a reducing action is thus set up, and selenious acid is liberated; on saturating the solution with sulphurous acid, and heating to boiling, red amorphous flakes of selenium are thrown down.
(6, 2) The deposit is treated with aqua-regia; the excess of this acid is expelled by heat after addition of sulphuric acid; the cooled residue is washed with water, the solution is neutralized with sodium carbonate, and the mass is evaporated to dryness; the residue is roasted at a gentle heat with an equal quantity of sal ammoniac till it assumes a reddish-brown colour; the mass is then treated with water, which leaves the selenium undissolved.
(6, 3) The deposit is washed, and digested with a moderately strong solution of potassium cyanide at 176° to 212° F. (80° to 100° C.) till the red colour just changes to pure grey; the residue is lixiviated with boiling water till the liquid ceases to assume a reddish opalescent appearance on addition of hydrochloric acid; after filtering the solution, the selenium is precipitated by hydrochloric acid; it contains as impurities small quantities of copper, iron, and perhaps mercury, from all which it may be freed by converting it into selenious oxide, subliming this compound, and precipitating the selenium by sulphurous acid.
(6, 4) In order to recover that portion of the selenium which is carried into the sulphuric acid in the chambers, this latter (" chamber acid ") is allowed to run down a Glover tower without the addition of any nitrated acid; it is thus brought into contact with the reducing gases from the chambers (the SO2), which precipitate the selenium encountered as a red amorphous powder; the reddish turbid acid thus obtained is allowed to settle, and decanted, and the deposit of selenium is washed, and dried at 212° F. (100° C.)
(c) The flue dust, containing 9 Per cent. of uncombined selenium, is levigated, the heavier parts being washed out with water containing hydrochloric acid, and then with pure water; the mass is then fused with crude sodic or potassic carbonate, pulverized and lixiviated; on leaving the brown-red liquid exposed to the air, the selenium deposits; this is separated by filtration, and distilled.
Selenium is insoluble in hot and cold water, very slightly soluble in concentrated sulphuric acid, and is oxidized and dissolved by nitric acid and aqua-regia. In the variety of its modifications of form, it bears a striking resemblance to sulphur. The crystalline form conducts electricity more readily than the vitreous, its capacity in that respect increasing with the temperature.