Zinc Whites.—Originally and properly the term zinc white was reserved for the white pigment consisting of zinc oxide; but latterly many kinds of white pigment have been introduced containing a large proportion of sulphide of zinc, sometimes associated with more or less oxide, and sometimes without any oxide, and these are also by many people called zinc whites, to which name they are perhaps as well entitled as the original zinc oxide. It will therefore be convenient to arrange them all under the same general heading of zinc whites.
(1) Oxide.—Under the influence of a white heat metallic zinc is volatilised, and if the vapour is thus brought into contact with oxygen, either in the pure state or as air, combustion takes place, and the oxygen unites with the metal to form zinc oxide. On this very simple principle is based the manufacture of zinc oxide white.
The operation is conducted in plant similar to that shown in [Fig. 28], which consists essentially of two departments, that in which the zinc is volatilised and that in which the oxidised vapour is deposited for collection.
The volatilising process takes place in a series of oblong fire-clay retorts a, varying somewhat in form but always with a contracted and rising neck. Ordinary dimensions are about 2 feet long and 9 inches in diameter each way, with walls about 1½ inches thick. These are heated to whiteness and then charged with ingots of metallic zinc.
The retorts are arranged in double rows in reverberatory furnaces b, two furnaces being arranged back to back so as to economise heat. The furnaces are fired at the side, and the heat is conveyed around the retorts by means of the flues c, the products of combustion of the fuel finally escaping by the chimney stack d.
Fig. 28.—Apparatus for Making Zinc Oxide.
As the vaporised zinc is emitted at the mouths of the retorts a in a partially ascending current, it immediately encounters a plentiful supply of air, and thereupon takes fire (undergoes combustion or oxidation). In this condition it enters the lower and funnel-shaped end of the sheet-iron flue e, by which it is conveyed into the series of settling compartments f.
While the bulk of the zinc oxide thus formed passes into the settling chambers, a portion of it is too heavy to do so; its specific gravity is such that the force of the draught is not sufficient to carry it up. This portion falls at once into a receptacle placed beneath the mouth of the flue e.