Fig. 17.
Theoretically, the quantity of acetic acid with which the process is commenced is sufficient to form an unlimited quantity of white lead, since all the acetic acid brought into the decomposing tank in the form of basic acetate is returned to the dissolving tub as neutral acetate. In practice, however, matters are somewhat different. Small quantities of acetic acid are lost in the wash waters; each time the solution of lead acetate is pumped back into the dissolving tub, a small quantity of acetic acid must be added to make up the loss.
Fig. 18.
The method pursued by Ozouf, in France, is a considerable improvement on Thénard’s process. Pure carbonic acid is used for the precipitation, and white lead of similar composition to that produced by the Dutch process is obtained, since the introduction of the carbonic acid can be regulated according to the volume and strength of the lead solution, and thus white lead of any desired composition can be produced. The most elaborate precautions for the health of the work-people are taken.
The preparation of pure carbonic acid gas is based upon the absorption of this gas from a mixture of gases by a solution of sodium carbonate, and its evolution on heating the solution. The apparatus is shown in Figs. [18] and [19]. The products of combustion obtained from the stove, A, are drawn by the air pump, E, through the pipe, C, into the cooler, B, which is regularly fed with cold water by D. The gases compressed in the receiver, E´, deposit moisture there, and then proceed through 3 horizontal cylinders, F, of sheet-iron, provided with agitators, in which the carbonic acid is absorbed by a cold solution of sodium carbonate of 9° B. The unabsorbed gases escape into the atmosphere through G ([Fig. 19]). The sodium bicarbonate solution is received in the wooden tank, H, after passing through the 3 cylinders, F. The pump, I, of the alternating pump, I I´, lifts the sodium bicarbonate solution out of H and sends it through the pipe, K, into the tubular cylinder, J, which stands upon a cylinder of larger diameter, J´, communicating with it only by the vertical tubes. The bicarbonate solution rises between the tubes in J, passes through the pipe, L, drops in a fine spray through the rose forming its mouth, and by means of the vertical tubes passes into J´ and thence into M, where it is heated by means of a steam coil to 100° C. Carbonic acid is then evolved, and the residual solution of neutral sodium carbonate, after cooling in the vessel, R, by means of the cold coil, is drawn off by the pump, I´, again to enter the cylinders, F, by means of the pipe, K. The carbonic acid evolved in M, together with steam, enters J´ through N, and in rising in the tubes of the cylinder, J, is cooled by the falling spray of bicarbonate solution. The cooling is completed in the coil, O, surrounded by water; the vessel, P, separates the condensed water and passes the gas on into the holder, Q. The pipe, S, connecting P with the suction pipe of the pump, J´, serves to restore to the solution of sodium carbonate the water it has lost, thus maintaining the proper concentration. The cost of 1 cubic metre of carbonic acid is 10 centimes, of 1 kilogramme 5 centimes.
Fig. 19.
For the production of white lead, the carbonic acid, by means of the pipe, U (Figs. [19] and [20]), enters the cylinder, T, provided with an agitator and containing a solution of basic lead acetate. By means of the pump, V, the lead solution is fed into the cylinder, T, through W. The absorption of the gas proceeds rapidly; the progress of the operation is followed by the observation of a pointer moving over a scale; as the gas holder sinks the pointer moves upwards. After the precipitation of white lead, the contents of T are emptied into the tub, b, in which rotate rakes attached to a vertical axis of iron plated with copper. When the white lead has settled, the supernatant solution of neutral lead acetate is drawn off through the pipe, c, by means of the pump, d, and conducted into the water-tight vessel, X, containing a stirrer on the vertical axis, W, made of coppered steel. Here litharge is added, and the resulting solution of basic lead acetate is conveyed to the cylinders, T, by means of the pump, V, as already described. The white lead in the tub, b, by putting the stirrer in motion, is washed once with water which has been previously purified by a little lead acetate. It then goes into another tub provided with stirrers, where it is several times washed, sodium carbonate being added to the last wash water until a sample of the white lead is not coloured by a drop of potassium iodide solution. In this way the wash water is obtained free from lead, and the product is said to be of better quality. This, however, is not in accordance with the fact that good Dutch white lead generally contains some lead acetate. The two-cylinder pump, h, which is in connection with the gas holder, forces gas over the surface of the liquid in T in order to drive it into tubs which are not in the position shown for b in the illustration, and into which there is no direct flow. The washed white lead is brought into bags which are pressed in a hydraulic press, dried, ground, sieved and packed in casks. These troublesome and often dangerous operations have been modified by Ozouf in the following manner. The pulp white lead runs from the tub, b, into the hopper, g, where it is kept mixed by a small stirrer, and from which it passes on to the cylinder, f, heated by gas from the inside. In its rotation the cylinder carries along the white lead and dries it, it is then removed by a knife below the hopper, and falls on to an inclined plane. The hopper and cylinder are in a room provided with a good draught.