The French process for making white lead is based upon the reaction which occurs when carbonic acid is passed into a solution of basic lead acetate; basic lead carbonate is precipitated and neutral lead acetate remains dissolved; the latter is again converted into basic acetate, from which carbonic acid again separates white lead and so on. This process, at present used on an enormous scale, is due to the French chemist Thénard, who first put it into operation on the large scale at Clichy, near Paris. The method is also known as the Clichy process.
The operations of this process are divided into the production of the basic acetate and the treatment of its solution with pure carbonic acid, whereby the basic carbonate is precipitated.
1. Preparation of the Solution of Basic Lead Acetate.—The preparation of this compound has been already described; the following is supplementary to what was previously given. If litharge be used, it is dissolved in wooden tubs heated by steam. The acetic acid is brought nearly to boiling by open steam, and the finely ground litharge gradually added. In consequence of its high specific gravity, the litharge would quickly sink to the bottom, so that it is advisable to keep the liquid in motion by means of a stirrer, and to allow the litharge to fall in in a thin stream. The introduction of the latter is continued until the specific gravity of the solution indicates that the liquid contains three equivalents of lead oxide to one equivalent of acetic acid.
In working with metallic lead, this must be used in a finely divided form; it is cast, as in the Dutch process, into thin sheets or gratings, or into flat wires or ribbons. These ribbons are easily made by melting the lead in a pan provided with a delivery pipe with stop cock; beneath the latter is brought a vessel, which can be moved backwards and forwards upon a tram line, filled with water. When the melted lead is allowed to flow into this vessel, which is being moved backwards and forwards, the metal forms long, thin ribbons, which possess a large surface. A wooden tub is almost completely filled with these lead ribbons, which are then covered with acetic acid. After a short time the acid is run off, when, by the action of the air, so energetic an oxidation of the lead takes place that the contents of the tub become heated, and steam and acetic acid vapours begin to rise. When this is seen, the original acetic acid is pumped back into the tub, and left there for some hours in contact with the lead in order that it may dissolve the lead oxide. When the solution has reached a specific gravity of 1·1326 to 1·1415, it is drawn off from the undissolved lead, which again, in a short time, in consequence of the rapid oxidation, becomes warm, and is treated with fresh acetic acid.
The lead ribbons become finally so thin that they fall together by their own weight, and form tight masses, upon which air and acetic acid can no longer act. These residues are then removed from the solution vessel and new ribbons introduced. The residues have a velvety appearance; when they are mixed up with water they make it dark, and from the turbid liquid a fine, velvet-black powder soon separates, which consists of finely divided silver. The liquid still remains turbid owing to the suspension of fine particles of carbon. The lead ores, especially galena, often contain notable quantities of silver; silver lead is generally desilverised before use; usually, however, small quantities of silver remain in the lead. When the lead is dissolved in acetic acid, this silver settles as a soft powder at the bottom of the vessels in which the lead residues from the solution tubs are washed.
2. Preparation of the Carbonic Acid and Precipitation of the White Lead.—The carbonic acid required for precipitating the basic carbonate is obtained either by heating limestone in a small furnace, from which the gas is drawn by a pump, or directly by burning charcoal. In the former case very pure carbonic acid is obtained, and, as a by-product, valuable quicklime; in the latter case precautions must be taken to produce pure carbonic acid.
Fig. 16.
The furnace designed by Kindler for preparing carbonic acid ([Fig. 16]) consists of a conical furnace, burning coal or coke upon the hearth, a. The passage, c, is divided by a vertical wall, in order to avoid obstruction from the piling up of fuel. The space, K, is filled with limestone, through which the carbonic acid passes; the tanks, e, filled with water, cool the limestone, and the gas, which then passes through the water in the washing vessel, D, is drawn off by a pump.
In the old process at Clichy the apparatus depicted in [Fig. 17] was used. The basic lead acetate was made in the wooden tub, A, provided with the stirrer, B C. The solution was run off from this vessel by means of the cock, b, into the settling tank, E, in which the mechanical impurities separated from the solution. The clear liquid ran into the decomposing vessel, a large shallow covered tank, holding 9,000 to 10,000 litres. In this tank opened 800 copper tubes, given off from the large pipe, S. The small furnace, D, in which limestone was burnt with coke, produced the carbonic acid; from the pipe at the top of the furnace the carbonic acid was brought into the Archimedean screw, h K, was washed with water, and pumped into the solution of lead acetate. The introduction of the carbonic acid was continued from 10 to 12 hours, after which the apparatus was left at rest until the liquid in the decomposing tank had become quite clear, through deposition of the white lead. The clear solution, now containing neutral lead acetate, was run off into the receiver, m, from which the pump, P, carried it back into the dissolving tub, A, where it was treated with fresh quantities of litharge. The solution of neutral lead acetate drawn off from the white lead had approximately a specific gravity of 1·0901. The white lead at the bottom of the decomposing tank was a tolerably thick paste. It was transferred to the tank, O, and washed several times with water. The first wash waters, which contained small quantities of lead acetate, were returned to the dissolving tub. The resulting white lead formed a very soft powder; it was at once placed in the drying pans. The white lead prepared by this process is a precipitate containing no coarse lumps, so that grinding is unnecessary.