Associated with the pyrites in small quantities are various substances, some of which, becoming volatilised when the ore is burnt, enter the chambers with the mixed gases, and thus find their entrance into the acid, whilst others remain behind in the iron residue of impure ferric oxide, left on the hearth of the furnace after roasting. The former of these foreign bodies, which are found in most commercial acids, are described below under the section “Purification.” Amongst the solid non-volatile matters, the extraction of which
from the burnt iron has been found in many works to yield a profit, are zinc, copper, silver and thallium.
At Wolcrum, in Germany, the zinc which exists in the residue in the form of sulphate is extracted by lixiviation, and then treated with common salt, the reaction giving rise to the production of sulphate of soda and chloride of zinc. The soda obtained is sufficient to pay for the working of the operation, whilst a good profit is made by the sale of the large quantities of chloride of zinc which are thus yielded.[213]
[213] Ibid.
The copper, which in some residues is met with to the amount of 4 per cent., also pays for extraction, and is sold to the smelter. It is first converted into chloride, and then precipitated by iron. The silver is recovered by Claudet’s process, which consists in precipitating it from a saline solution in which it is in the state of a soluble chloride, by iodide of potassium.
In the Widnes Copper Works the silver so extracted yields an annual profit of £3000.[214]
[214] Ibid.
Thallium is found in the fine dust caused by the combustion of the pyrites, which dust deposits in the flues between the furnace and the chambers. The metal is extracted from the dust by treating this latter with dilute sulphuric acid. The resulting sulphate is converted into chloride, and again reconverted
alternately into sulphate and chloride several times, the sulphate last obtained being reduced by metallic zinc.[215]
[215] Ibid.