Firstly—That separation by filtration from impurities and so forth left by the oxide of lead used, and insoluble in the acetate of ammonium, is made possible, and thereby a pigment of exceeding whiteness and purity can be obtained, besides high basicity, with corresponding body and covering power.
Secondly—That the rapidity of the conversion of oxide of lead into basic acetate of lead is immensely increased—it becomes in fact almost instantaneous, much time being saved.
Thirdly—That the conversion is effected quickly and in a perfectly closed vessel; no chance of the escape of ammonia occurs, and, in the carbonating stage of the process, the free ammonia present in the liquid assists in securing the formation of basic carbonate of lead, and the maintenance of this basicity throughout the conversion.
This free ammonia is converted by the carbonic acid into the less volatile but still alkaline carbonate, and, later on, this ammonium carbonate reacts upon the basic acetate of lead, converting it into basic carbonate of lead, acetate of ammonium, still less volatile, being simultaneously produced, ready for use over again.
It must be understood then, that as the carbonic acid passes into and through the ammoniacal mixture in the carbonator, it continually precipitates or forms basic carbonate of lead, in presence of more or less of the volatile alkali, which however continually diminishes in quantity as the conversion proceeds.
Were no ammonia present, but only tribasic acetate of lead, as in the case of the earlier methods of precipitating white lead, unless a very slow current of carbonic acid gas were passed through, some of the first formed basic carbonate of lead would be in danger of being over-carbonated and losing its basicity, being converted into mono-carbonate of lead. This danger is much lessened, and consequently a much greater possibility of rapidity of carbonating is conferred, in the case of the process as above described, and by virtue of the ammonia which is present.
Nevertheless the carbonic acid gas must not be passed, even into the ammoniacal liquid containing the tribasic acetate of lead, with such rapidity that distinct alkalinity to the usual tests ceases to be maintained, and the process must be terminated whilst the liquid is still alkaline. If a little lead salt on the one hand, and a little ammonia on the other, be left in the mother liquors ultimately obtained on filtering from the white lead, they will be returned and circulated.
[Fig. 27] shows an elevation (partly in section) of the apparatus employed.
a is an iron vessel made of boiler plate, and lined internally with lead. It may be here added that all the apparatus is thus lined, or is constructed of material invulnerable to the action of lead salts, ammonia, or acetate of ammonia.
The vessel a, which is termed the digestor, should be furnished with, a thermometer, pressure gauge, safety valve, charging hole b, and suitably arranged sampling pipe with cock.