Fig. 27.—MacIvor’s White Lead Plant.
Agitation of the contents of the vessel is effected by means of the circulating pump c, which, drawing off supernatant liquor along with air and ammonia vapour from the upper part of the digestor a, by the pipe d, forces it through the pipe e to the heater f, consisting of a coil suitably heated by steam in a steam vessel g, or by similar means, and then is forced through a pipe h, leading to the bottom of the digestor a, where it terminates in a cone-spreader i, provided with a regulating valve j. By means of this spreader, the liquid is forced through the heavy and dense mixture containing the oxide of lead, which it vigorously agitates and keeps more or less in circulation as indicated.
The charges of litharge and acetate of ammonium will vary according to the strength of the latter. For a 5 per cent. strength of acetate of ammonium solution, however, it will be best to calculate the proportions as follows, viz. about 1200 gallons of 5 per cent. acetate of ammonium liquor for 1 ton of litharge, which should be finely ground.
The temperature of the liquor may vary between 140° F. and 212° F.; but within these limits, the lowest temperature consistent with the sufficiently rapid conversion and solution of the litharge is preferable. The reason of this is simply that the less the heat employed, the less is the tension of the ammonia, and the chances of the loss of ammonia are thus minimised; in addition to this, less delay is involved and less refrigeration or cooling is needed before carbonating.
The object of the violent agitation of the litharge amongst the heated acetate of ammonium in the digestor a, is that caking of the former may be prevented, and a most rapid conversion of the oxide of lead into basic acetate of lead, with minimum expenditure of heat, be secured.
The effect of the heat in the closed space is greatly aided by that of the pressure due to the tension of the ammonia and aqueous vapour at the increasing temperatures.
When the litharge is converted into tribasic acetate of lead, and brought into solution, the liquor and sludge of insoluble matters is preferably pumped through pipe k, by means of the filter-press pump l, and forced through the filter-press m, which removes and retains the insoluble matter, allowing the clear liquor to pass to the cooler n, and through this to the carbonator o, similar in construction to the digestor a.
Here the cooled liquid is circulated by means of pump p (in a similar manner to that adopted in the digestor process already described), carbonic acid being simultaneously pumped in through the pipe q, which is perforated as shown, or introduced in any other way.