EXTRACTION OF GOLD. 23REDUCTION WITH AQUEOUS REDUCING AGENTS.- Selenium Dioxide and Pure Selenium from Crude Selenium.
From a dilute aqueous solution of selenious acid, the element selenium is pre-
cipitated by soluble reducing agents, such as sulphurous acid or hydrazine salts:
SeO 2 + 2 H2SO3 = 2 H 2 SO* + Se.A suitable solution of selenious acid may be obtained by oxidizing the raw
material with nitric acid; seleme acid is not formed under these conditions
(difference from sulphur).
Place 20 g. of crude selenium in an evaporating dish, and
after covering the dish with an inverted funnel, oxidize by the
gradual addition of 50 g. of concentrated nitric acid. Evap-
orate the solution upon the water bath; and after drying the
residue at 110° to 130°, sublime it from wide test-tubes, or from
an evaporating dish, into a large inverted beaker. The subli-
mate is usually somewhat reddish in color, due to the presence
of a small amount of elementary selenium; therefore treat it
once more with nitric acid, evaporate the solution to dryness,
and take up the residue in water. Reduce the selenious acid,
at the room temperature, by passing into the solution sulphur
dioxide gas which has been washed with water. A red precipi-
tate of selenium is slowly formed; the precipitation takes place
more rapidly, however, in the presence of a little hydrochloric
acid. The red, amorphous product can be dried in vacuo over
Sulphuric acid without change, but by warming it in contact
with the solution from which it has been precipitated, it goes
over into gray selenium.
1
Selenium imparts a blue tinge to the Bunsen flame. A porce-
lain dish held in this flame becomes coated with a brick-red
(reduction) spot which is surrounded by a white (oxidation) ring.
At the same time a characteristic, radish-like odor is noticed,
which can be obtained even more distinctly by heating selenium
on charcoal before the blowpipe.
- Extraction of Gold.
Gold can be obtained mechanically by "panning" the stamped, or ground
ore, or, more advantageously, by treating the powdered ore with mercury
which dissolves out the gold. Ores with a small gold content, and those
(^1) Vessels stained with selenium can be cleaned with potassium cyanide
solution, whereby readily soluble KCNSe is formed.