Fig. 15.
The apparatus of W. Thompson ([Fig. 15]) consists of a chamber, A, constructed with a false roof, B, to carry off the condensed vapours, and provided at both sides with doors, through which the waggons, carrying the lead, are introduced and removed. The pipes, n, which convey air and carbonic acid, are provided with branches, e, which reach to the sides of the chamber, and are joined to the perforated tubes, D. The pipes, n, are provided with reservoirs, b, which prevent the too rapid entry of the air or carbonic acid, and at the same time give a sufficient heating surface. Upon the waggon, E, are the frames, v, which support the lead plates. The plates may be solid or gratings; they are 3 to 12 millimetres thick, and are arranged at intervals of about 25 millimetres in the frames. The troughs, F, for the reception of the acetic acid are filled from the reservoir, Z. The steam pipe, C, effects the evaporation of the acid; the steam pipe, d, heats the chamber at the commencement of the process, and also when C is out of use. When the chamber is filled with lead, the troughs, F, are filled with acetic acid of 5 per cent. strength. The chambers are closed, and steam is led through C and d until the temperature in the chamber reaches 25° to 50° C., and the evaporation of the acid begins. Then air is forced in under a small pressure through n, e, D during 3 to 4 days, after which follows a mixture of equal parts of carbonic acid and air, continued until the formation of white lead is complete. When plates 3 millimetres thick are used, and the temperature is gradually raised from 25° to 50° C., about 12 days are required; but if the plates are 12 millimetres thick, 28 days are necessary. For the proper carrying out of this process it is important that the temperature of the chamber should be very gradually raised from 25° to 50° C.
In the process of P. Rey molten lead is poured in a thin stream into water, and the “granulated” metal then placed in vessels, in a layer 30 centimetres thick, upon a grating 5 centimetres above the bottom. In the bottom are narrow tubes which, reaching above the lead, serve to admit air. Acetic acid is allowed to flow over the lead from vessel to vessel. In order to obtain the proper solution, a layer of lead, 2 metres thick, is necessary, so that 6 to 7 vessels are arranged, one above the other. If lead acetate solution be used, the layer of lead need only be 1·2 metre thick. The solution of basic lead acetate is then treated with carbonic acid according to the ordinary process.
In addition to the modifications of the German white lead process which have been described, many others have been proposed. The principle of many of these methods is, that finely divided lead is much more rapidly converted into white lead than lead in the form of plates; finely divided lead exposes an enormously greater surface to the action of the acetic acid and other materials than do lead plates.
In Rostaing’s method the lead is changed into very small pellets, by allowing the melted metal to flow on to a rapidly rotating iron disc; the molten lead, in consequence of the centrifugal force due to the rapid rotation, assumes the form of very small drops, which are thrown off the disc and cooled in a vessel of cold water.
Torassa recommends that lead, obtained in pellets by pouring into cold water, should be brought into a rapidly rotating vessel, whereby the greater part will be converted into fine dust (?). This lead dust is said to be then converted into white lead by simple exposure to air, lead oxide being first formed, and from it basic lead carbonate. This process is not workable on a large scale. The rapid oxidation of finely divided lead by air has been applied by several inventors to the manufacture of white lead. The processes of Woods, M’Cannel and Grüneberg are founded upon this transformation. In essentials they are as follows: lead is finely divided in iron or earthenware cylinders, and either carbonic acid and air, or a mixture of these with acetic acid vapour, are introduced by means of the axle of the cylinders.
If a special apparatus is not provided, as it should be, on the large scale, to separate the unaltered lead from the white lead, the lumps coming from the chambers must be subjected to a process of levigation, in which the lead, being the heaviest body, will be first deposited; the last portions deposited consist of the purest white lead, the lower layers of which will be tinged more or less grey by an admixture of finely divided lead and lead peroxide.
White lead manufactured by the German process occasionally exhibits perceptible reddish or greyish tinges, the cause of which lies in the defective execution of the process. The red tinge denotes the presence of free lead oxide, caused by the use of an insufficient quantity of acetic acid. A grey tint is due to the presence of metallic lead or an excess of lead carbonate.