The properties of zinc are such that the metal is not recoverable like iron, copper, and lead, which we have been' studying. Zinc melts at 419.4° C. and boils at 940° C. Its reduction temperature is about 1050° C. Zinc is reduced to metal above its boiling point. Zinc vapor also decomposes carbon dioxide and at once reverts to zinc oxide. No blast furnace yet has been induced to produce the metal; it can be obtained only by reduction in small retorts with a large excess of carbon.
There are two chief types of furnace for roasting zinc ores. In the first type the flames which heat the ore play directly over the charge; the products of combustion and the sulphur dioxide from the roasting mix and pass up the flue together. All zinc furnaces are built with mechanical arrangements for stirring. In this first sort of furnace the ore and the flames enter at opposite ends; the ore gradually is worked to the fire end of the furnace under constant stirring. If 12 per cent is the amount of sulphur left in a mechanically roasted copper ore and 4 per cent is the amount in a roast-sintered lead charge, the permissible sulphur in a roasted zinc ore is more like 1 per cent.
The second type of furnace lacks the innumerable variations found in the first class of furnaces.
They must keep the gases of combustion separate from the sulphurous gases, and the sulphurous gases are to be kept as concentrated as possible and used for sulphuric-acid manufacture. This second type of furnace is muffle built; there will be seven superimposed hearths with the three lower ones muffled so as to be heated with extraneous fuel. These furnaces have to be stirred mechanically, as do all zinc-roasting furnaces. The use of so many hearths, the muffling of the first three hearths, and the ducts for pre-heating the air and leading it into the furnace and for leading the gases out make a structure difficult to describe and even harder to draw.
Fig. 46, Block of Zinc Retorts in Action Courtesy of "Mining and Scientific Press".
The retorts for holding the charge during zinc smelting are about 5 feet long, 10 inches in diameter, 1½ inch thick, and closed at one end. Several hundred such retorts will be placed nearly level in a long double furnace which is heated by gas, either natural or producer. It is now customary to have three or four rows of these retorts one above the other and as many as a thousand of them in one block. The retorts tip slightly downward so that they can be filled and cleaned easily.
The best practice pre-heats the gas and air in regenerators, and the gas is burned by letting in air at intervals in just the right amount to furnish a uniform combustion the length of the furnace. Fig. 46 shows the external appearance of a large furnace in operation. The flame is seen playing from the end of each condenser as the monoxide escapes into the air. These furnaces will be charged only once a day, the smelting cycle completing itself , in about 24 hours. Too high a temperature of course is not desirable because of its effect on the retorts; otherwise it is highly desirable to have a large excess temperature in the furnace to replace the heat absorbed by the reaction and to keep the reaction going lively.
The metal is graded and sold on specification, and refining is accomplished by redistillation; this latter is not done very often, as may be assumed.