The white lead obtained by this process is quite usable, but the process has the considerable disadvantage that there is no control over the quantities of materials used. In order to produce a certain quantity of white lead of a certain composition, definite quantities of lead, oxygen, acetic acid and carbonic acid are necessary. If, then, the apparatus can be so arranged that the quantities of material employed can be accurately measured, a great advance will have been made, for the operation will be no longer conducted at random, but under definite unchangeable conditions. The quantities of carbonic acid and acetic acid can be calculated beforehand. The volumes of the gases required may be measured without difficulty by meters of the type used for measuring coal gas. Upon this principle are based a number of more modern methods.
Fig. 11.
According to the process of Major, the vapours of water and acetic acid are introduced at the same time into the chambers filled with lead plates, in order to produce basic lead acetate. This part of the process requires about 12 hours. Then carbonic acid is introduced; it is produced by burning charcoal in an iron cylinder, through which air is forced. By means of this carbonic acid, which is at a comparatively high temperature, about 60° C., the lead acetate is quickly changed into white lead. A portion of the lead acetate remains undecomposed; in order to remove this, at the close of the operation ammonia is injected, which decomposes the lead salt. The ammonia salts now present are finally driven out by means of superheated steam. Major’s apparatus is depicted in [Fig. 11]. A and B are chambers with horizontal gratings upon which the lead lies; C is the furnace for burning the charcoal, provided with a fan. The products of combustion from C pass under the boiler D, their passage being regulated by the valve, a; they then heat the boiler E, containing acetic acid, and enter the chambers, A and B, through the flue, b, or are directed by the valve in b into the chimney, d. The gases, after passing through the chamber, find an exit at e. The steam pipe, f, conducts steam into the boiler E, through g, and also into the chambers, A and B, thus heating them and providing the necessary moisture; h is the funnel for filling E; the pipe, k, carries the acetic acid vapours into the chambers. After the thin lead plates have been brought into the chambers they are closed and the temperature raised to 49° to 60° C., steam and acetic acid vapours are led in for 10 to 20 hours in order to form the basic acetate, then the furnace gases are introduced at a temperature of 60° C. The white lead obtained in this manner can be finished in the ordinary way by washing and grinding, but it is better to remove the lead acetate by introducing ammonia, and then hot air or superheated steam, as previously stated.
The process is complete in 2 to 4 weeks. Gartner, working according to this process with 150 kilogrammes of lead in a chamber 1·26 metre long, 0·78 metre high and 0·78 metre wide, obtained good white lead in 28 days.
The apparatus designed by H. Kirberg for the manufacture of white lead is illustrated in [Figs. 12, 13] and [14]. The lead plates are hung upon the laths, a, in the chambers; the supports of the laths, b, go through the slits, c, in the supports, d, and project through the walls at e. The carriers, b, hang from bolts by brass wire. By striking the end of the laths, e, they are made to swing so that the white lead loosely adhering to the lead plates is shaken off and fresh surfaces of metal are exposed. The openings in the walls through which the ends of the laths project are closed by indiarubber. In order to prevent the production of dust when the chambers are emptied, water is introduced in a fine spray upon the lead plates from the copper tubes, v, placed under the roof of the chamber. The water washes off the remains of the white lead from the plates.
Fig. 14.
In a similar manner, the white lead process may be carried out in a shorter time when the gases enter the chamber under increased pressure; but this is attended with difficulties, since continuous supervision of the apparatus and of the tightness of the chamber is necessary.