[Fig. 26] is a vertical section of the apparatus employed.
a is a vat, which may be made of wood or other material capable of resisting the chemicals employed; it may conveniently be 6 feet in diameter and 4 feet deep, and it is provided with a closely fitting cover, b is a cistern situate at a higher level, and intended to contain a solution of acetate of ammonia, c is a pipe by which the solution can be drawn down from the cistern b into the vat a; a cock is provided upon this pipe, as the drawing indicates. There is a man-hole d in the cover of the vat, and the vat contains an agitator e, with a vertical shaft which can be turned by gearing f as shown, or the agitator may be driven by any suitable motor. The shaft of the agitator e is hollow, and pipes g, which stand immediately behind the stirring tines, are connected with the hollow shaft, to deliver the carbonic acid gas into the vat; or this may be effected by means of coils of pipe laid at the bottom of the vat, and pierced with small holes. The pipes g are open at their lower ends. h is a cock by which the liquor can be drawn off from the vat into the receiver i. A pump k is provided upon the cover of the receiver, by which the liquor may be returned into the vat a. l are outlets by which the white lead is discharged from the
Fig. 26.—MacIvor’s White Lead Plant.
vat into the washing cisterns m and n. o is a man-hole, which may be opened to facilitate the emptying of the vat. p is a plug which is removed to let the white lead run out of the vat. r is an ammonia catch box, charged with acetic acid.
All the metal-work of the apparatus with which the acetate solution comes into contact should be of such a character as to resist corrosion, or should be coated with a material capable of withstanding attack by the chemicals employed in or formed during the process.
The operation is by preference conducted in the following manner, but the details admit of variation. The charge of monoxide of lead for an apparatus of the dimensions indicated may weigh about 1120 lb.
The monoxide should be in fine powder, and may be either moist or dry. Having received this charge, which is introduced by the upper man-hole d, the vat a is closed, and a solution of acetate of ammonia is let down upon the charge from the cistern b, or pumped out of the receiver i. The vat a should be charged with the solution of acetate of ammonia in the proportion of three parts of said solution to one part of lead monoxide by weight. It is convenient to employ a solution containing 5 per cent. of acetate of ammonia, and the quantities above stated are suited to a solution of this strength; but the strength of the acetate solution may be varied within wide limits, as hereafter explained.
The charge of monoxide of lead and acetate of ammonia in the vat should be kept constantly stirred by the agitator until it becomes whitish in colour, when it will be found that the monoxide of lead has become converted into hydrate and acetate. The workman will know that this change is complete when the reddish or yellowish appearance of the monoxide of lead disappears, and the mass in the vat becomes whitish in colour.