This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
Zinc is obtained from the sulphide, or "blende"; and to a smaller extent from the carbonate and oxide. There are several complex zinc ores, and some of these, as the zinc and lead sulphides, are abundant, but so far no method has been found for the profitable extraction of the metals.
The essential steps in extracting zinc are:
(1) Changing the ore to an oxide, by calcining the less stable ores and roasting the sulphides. Calcining is done in kilns, and roasting is done in reverberatory furnaces.
(2) Reducing the oxide. This is done by mixing the pulverized oxide with carbon and heating in clay retorts. The high heat necessary to accomplish the reduction (1900° F.) not only liberates the zinc, but vaporizes it, and the vapor is condensed in suitably placed iron tubes which drain the liquid zinc into ladles from which it is poured into ingots.
(3) Refining the ingots. They are remelted in quantities of several tons in a specially constructed reverberatory furnace. The molten metal is allowed to remain quiet for a few days, when the floating impurities are skimmed off. Molten lead and iron, which are almost always present, sink to the lowest part of the furnace basin and are quietly tapped out, the lead running out first, followed by the iron, which carries some zinc also. The remaining zinc is run out and cast into slabs or ingots for marketing. These are known as "spelter," and should contain not over 1% of lead and only traces of iron.
A recent refining process for zinc consists of passing the vapor from the reducing retorts through a filter of coke kept hot enough to prevent condensation. This is said to remove iron and lead better than by the method of re-melting.