Fig. 10.
[Fig. 10] represents the construction of Lefèbre’s white lead mill, which is designed to give the greatest protection possible to the workmen. At A, the white lead masses are enclosed in a hopper lined with bronze, with internal angular projections; by means of M, which is grooved in a similar manner, the larger lumps are broken up and enter the mill itself by means of the hopper under A. The mill consists of bed stone, H, and runner, K. The grinding surfaces are grooved to facilitate the delivery of the ground white lead. As the illustration shows, the stones are completely covered by M, so that the escape of dust is almost completely prevented. The ground white lead is conducted by the tubes, O O, to the drums, O′ O′, from which it is packed.
In order to obtain hard white lead, the material which has been ground dry under the edge-runners is mixed with water to a soft paste, or if water has been added whilst the lead was under the edge stones, more is now added to make the paste thin enough to be ready for the mills, where it is ground, being fed in regularly by means of a copper spoon. The paste issuing from the mill is collected in earthenware pans or plaster of Paris moulds, in which it is dried.
These pans have usually the shape of a truncated cone, which was the form in which white lead was formerly brought into trade from Holland. In drying, water is lost, and the mass of white lead shrinks, so that the lump can be removed from the mould after a few days on turning it upside down. The drying is accomplished either in the air or in artificially heated stoves. The heating must at first be gradual, or the mass of white lead would shrink so rapidly that the cone would be full of cracks, and then easily fall to pieces. When once the drying has reached a certain stage, the temperature of the stove may be raised to 50° C. (122° F.) without danger of breaking the lumps. When quite dry, the surface of the white lead, which is now rough, must be smoothed by scraping, when it is ready for the market.
Soft White Lead.—The hard white lead prepared as just described consists of very heavy and very hard lumps of the purest white. When soft white lead is required, the admixed lead acetate must be removed by washing. This is accomplished by adding a larger quantity of water, either when grinding under the edge-runners or in the mills, so that a thin pulp is formed; this is run into a receiver, in which is a stirrer. The white lead is not completely prevented from sinking by the stirrer; a soft mud is deposited at the bottom of the vessel, which can only be stirred up with difficulty on account of its high density. When this vessel is full, the stirrer is stopped and the milky liquid allowed to settle, which happens in a short time on account of the high specific gravity of white lead. The clear liquid above the white lead is drawn off into a tank lined with cement. It is advisable to arrange the stirrer so that it may be placed at any height in the vessel; if this is the case, by gradually lowering the stirrer whilst in motion, the white lead lying at the bottom can be mixed up with fresh water. These operations are repeated until all the lead acetate is removed.
Soda solution is added to the wash waters in order to recover the lead dissolved in them as lead acetate; lead carbonate is precipitated and settles at the bottom of the tank. This precipitation may also be effected by putting lumps of limestone in the tank. The paste remaining in the washing tubs, which now contains only pure white lead and water, is filled into bags of closely woven material and the water pressed out by a gradually increasing pressure, until a stiff pulp remains behind. This is then completely dried, either in the air or in drying stoves.
Soft white lead forms either irregular lumps or a soft, heavy powder. The lower qualities of white lead contain a smaller or larger quantity of finely ground barytes; the higher the proportion of barytes the smaller is the covering power of the mixture. A very simple method for detecting barytes in white lead will be given later.
(b) German Process.
The German or Austrian method of making white lead is also known as the chamber process, since the formation takes place in closed chambers, constructed of wood or masonry. In the older processes, cast lead sheets were bent double and hung on cross bars in a wooden box with a water-tight bottom, with the precaution that the plates did not touch. A number of these boxes, generally 90, were arranged in a hot room, each box being about 1·6 metre long, 0·4 metre wide and 0·3 metre high. On the bottom of each box was poured a mixture of vinegar or dilute acetic acid and wine refuse, and the box was covered with a well-fitting lid. The temperature of the room was gradually raised week by week; during the first week remaining at 25° C., during the second at 38° C., the third at 45° C. At the commencement of the fourth week the temperature was raised to 50° C., at which it was kept for a fortnight. At this high temperature a considerable quantity of acetic acid is evaporated, causing the formation of lead acetate, which is converted, by the carbonic acid evolved from the wine refuse, into basic lead carbonate. When the proper temperatures have been maintained, on opening the boxes almost all the lead is found changed into white lead, which is knocked off, and the residual lead used in casting new plates. It is easy to conceive that the boxes may be well replaced by brick chambers, in which a large number of lead plates are brought and into which acetic acid vapours and carbonic acid are introduced after the room has been closed. Chambers are used which can contain 12,000 to 12,500 kilogrammes of lead.
These chambers, in which the lead plates are hung upon wooden supports, have an opening immediately above the bottom, which is connected with a retort in which vinegar, containing 4 to 5 per cent. of acetic acid, is boiled. After about 12 hours, by the simultaneous action of the vapours of acetic acid and the oxygen of the air, lead acetate is formed. Carbonic acid is now led into the chambers. The carbonic acid is obtained by burning charcoal in a cylindrical furnace; it is cooled by being passed through a long iron tube before it enters the chamber. For 12,500 kilogrammes of lead there are required every day about 482 litres of dilute acetic acid, obtained by mixing strong acetic acid with water until the mixture contains 4·5 per cent. of the acid, and 18 kilogrammes of charcoal, from which the carbonic acid is obtained. The time required is 5 or 6 weeks, and the residue of unaltered lead varies from 10 to 35 per cent.