The product is pumped from the condensers to a settling vat. Here it settles for an hour, the deposit being pumped into a second vat and washed with dilute sulphuric acid, in order to remove impurities. The resulting product is washed several times with water, and is then passed through a filter press. From the press it drops into bogies, which carry the pulp to the drying house, where it is dried by hot air. The process would thus appear an extremely simple and practical one. Several chemical authorities of repute, as well as manufacturing firms and others who have used the new white lead, have expressed themselves strongly as to its merits. One point immensely in favour of sulphate of lead as opposed to carbonate, is that the former is almost entirely innocuous, whereas the poisonous properties of the other are well known. It is claimed that the sulphate is not acted upon by coal gas or by the atmosphere of towns, which is always more or less impregnated with gases such as sulphuretted hydrogen, whose reactions with various metals have only too good reason to be known.
According to the patent specification bearing the names of French and Hannay, they employ lead ores, lead fume, or lead slags containing sulphur; and when these materials do not contain sufficient sulphur to form a sulphate with all the lead which sublimes in the process, they add to them pyrites or other sulphur-yielding substance to make up the deficiency. They heat the materials mixed with a suitable proportion of coke in an air-blast cupola furnace, which is by preference of an improved and special construction shown in [Figs. 18 to 20], and hereinafter described; and they thereby produce sulphite of lead as a sublimate, provided that there are no chlorides such as common salt present in the charge, in which case sulphate and chloride of lead will be formed.
The sublimate is carried forward with the current of gases through flues to a fume condenser, which is by preference of the kind known as Wilson’s and French’s. As the gases and sublimate pass through the flues, hydrochloric acid is mixed
French and Hannay’s White-lead Furnace.
with them, being by preference formed in a chamber in connection with the flue, by introducing a solution of chloride of sodium in spray, and by providing a sufficient excess of sulphurous acid beyond that required for forming the sulphite of lead. Air is also present, and a well known reaction takes place, yielding hydrochloric acid and sulphate of soda, the operation taking place at a part of the flue near enough to the furnace to be always at a red heat. The hydrochloric acid thus mixed with the gases and sublimate causes the formation of chloro-sulphite of lead, or other combinations of lead, sulphur, oxygen, and chlorine of variable constitution, depending on the proportions of the several constituents, but in most cases the product is a body which forms a white pigment of extremely good quality.