MacIvor’s Processes.—The name of MacIvor is a familiar one in improvements in manufacturing chemistry, and not the least in connection with the subject of pigments, sometimes in conjunction with other inventors.
In 1889 MacIvor and others introduced some modifications in the process of producing white lead or carbonate of lead, by the treatment of oxide of lead (litharge or massicot) with a solution of acetate of ammonia, whereby the oxide of lead is transformed into hydrate and acetate, which are subsequently converted into carbonate of lead by the injection of carbonic acid. The hydrate and acetate of lead, in presence of free ammonia formed in the reaction, are quickly decomposed by
Fig. 25.—Lewis’s White Lead Plant.
the carbonic acid, yielding the final product, namely carbonate of lead, and re-forming the acetate of ammonia.
The rapidity with which the conversion of oxide of lead into hydrate can be effected depends upon the strength of the acetate of ammonia solution employed, that is to say, the weaker the solution the slower will be the conversion. It has been found that, for commercial purposes, the solution of acetate of ammonia may be used with advantage of a strength which need not be more than 25 per cent. nor less than 5 per cent. A strength of 25 per cent, operates in a comparatively short space of time; but a strength as low as ½ per cent. will effect the hydration if there be a sufficient quantity of the weak solution, the lead oxide being in a fine state of division, and time of no object. This ½ per cent. strength, however, or any strength below 5 per cent., is not recommended for commercial purposes, having regard to the time required for completing the operation.
The conversion of the oxide of lead into the hydrate and acetate of lead is effected in the cold (heat may be used, but for commercial operation it is not recommended). The conversion is facilitated by employing a mechanical arrangement, similar in many respects to that adopted in a previous specification, but modified by connecting the digesting vat with a cistern or vat containing acetic acid, whereby any free ammonia carried over during the operation may be absorbed. Also by introducing carbonic acid to the mass of hydroxide and acetate of lead formed by the oxide of lead and acetate of ammonia, either by a series of concentric rings perforated with small holes in the bottom of the vat itself, or by passing carbonic acid down a hollow shaft to which are attached stirrers, and through perforated tubes attached to the blades of the stirrers, or by any other means that may ensure the thorough saturation of the hydroxide and acetate of lead so as to form carbonate of lead.
The solution of acetate of ammonia may be recovered from the white lead, and be repeatedly used, for the conversion of further quantities of oxide of lead into hydrate and acetate of lead, which hydrate and acetate are converted into carbonate of lead by the injection of carbonic acid. Theoretically, a given weight of acetate of ammonia in solution, used in conjunction with carbonic acid, should be capable of converting an unlimited quantity of oxide of lead into carbonate of lead; but during the manufacture of white lead by this process, it may be reckoned that there will be a loss of ammonia acetate varying with the strength of the solution employed, but it should not exceed 10 per cent. on each charge. This loss arises in the washing of the final product, and through the escape of some ammonia during the process.