After a time (about three or four hours), this oxidation begins to slacken, when he pumps up a second time the acid solution from the lower vat on to the granulated lead, and allows them to stand in contact for one hour; he then again draws off the liquid into the lower tank, and again exposes the metallic lead to atmospheric oxidation, allowing three or four hours for the latter operation; and if the solution of lead has not already attained the specific gravity of 1·040, at 60° F., he again repasses the liquor over the metallic lead partially oxidised, until it has attained that specific gravity, when he places the dissolved lead with fresh granulated lead and recommences the manufacture in the same way.
This operation succeeds much better on the large scale than on a laboratory scale. In vats containing upwards of one ton of lead, the result is all that can be desired, and can be obtained by passing the liquor from twice to three times over the metallic lead partially oxidised. A little practice enables the operator so to control the process that he can obtain the solution of the desired specific gravity with perfect ease.
This plan of manufacturing tribasic acetate of lead possesses the advantage of producing that substance wholly or nearly wholly free from the impurities contained in metallic lead, such as copper and silver, which are not taken up, or soluble, in the presence of metallic lead. In consequence of the circumstance that foreign matter is left almost untouched, it is practicable to make white lead of a fine quality from old lead such as lead piping, roofing, and worn out lead generally, which can thus be utilised to greater advantage than in any other way.
Having obtained this solution of basic acetate of lead of the specific gravity of 1·040 at 60° F., Condy proceeds as follows:—To the solution produced by each 60 lb. of acid and 750 lb. of water previously pumped up into another vat or tank, he adds bicarbonate of soda in the proportion of 30 lb. for each 60 lb. by weight of acid originally employed, and agitates the mixture. This will generally precipitate all the white lead, but it is necessary to test the filtrate to ascertain the exact point when all the lead is thrown down. Sufficient bicarbonate of soda should be added to do this completely, and it would be better to use bicarbonate of soda in excess rather than leave any lead unprecipitated, as by this means greater certainty is obtained in securing on the large and practical scale a white lead capable of standing the effect of light and grinding in oil without changing. The white lead after precipitation can be washed, pressed, and dried in the usual way.
The following variation may be made from the method described of making tribasic acetate of lead, thus:—To each 60 lb. of acid and 750 lb. of water may be added sufficient of the tribasic solution to make neutral acetate of lead, with which to recommence the manufacture of tribasic acetate of lead by the process described. Vague reference has been made in works on technical chemistry to the possibility of using metallic lead in the manufacture of sugar of lead, but such references have been practically worthless, as they contain no information of a practical nature even for the manufacture of neutral acetate of lead, and no process at all has ever been described for the manufacture of basic acetate of lead from metallic lead acted on by acetic acid or acetate of lead.
Gardner’s Process.—The conditions observed and fulfilled in the arrangements adopted by Prof. E. V. Gardner, are founded upon a study of the nature, properties and behaviour of the substances concerned, under certain methods of treatment.
There are several oxides of lead which may be formed under special conditions.—(1) Pb2O, and the same oxide combined with water Pb2H2O2; (2) PbO, and the same oxide combined with water PbH2O2. In the hydrated form these oxides combine readily with carbonic acid, but they do not combine readily with carbonic acid when dehydrated. The hydrates are most readily formed at about 120°-130° F., and are decomposed after they are formed if heated to 212° F.
These oxides and their hydrates combine with acetic acid to form acetate and sub-acetate of lead, and with nitric acid to form nitrate and sub-nitrate of lead.
Lead, submitted to the action of air, watery vapour, and acetic or nitric acid, or a mixture of these acids, with air or oxygen, with proper precautions, forms sub-acetate or sub-nitrate of lead, and this sub-acetate or sub-nitrate of lead readily absorbs carbonic acid and forms carbonate and sub-carbonate of lead.
Sufficient acetic or nitric acid, or a mixture of these and air or oxygen, and watery vapour, must be constantly supplied to form the subsalts of lead, to carry on the operation of converting blue lead into white lead; but an excess or an insufficiency of these agents will in either case prevent the formation of sub-acetate or sub-nitrate, and consequently of the sub-carbonates.