Lead chloride is solublo in boiling saturated solution of sodium chloride (brine) to the extent of about 53} parts (=40 parts metallic lead) to 1000 parts by measure of boiling brine; whereas, on cooling down to ordinary temperatures, most of it crystallizes out until no more than about 10.8 of lead chloride (=8 of metallic lead) to 1000 of brine remains in solution. Silver is soluble in the same liquid to the extent of about 1 of silver chloride (= 0.75 part of metallic silver) to 1000 of brine at about 50° to 53 1/2° F. (10° to 12° 0.) and about 1.2 of silver chloride1 to 1000 of brine at about 77° to 7.3° F. (25o to 26° C), and about double that quantity at a boiling temperature. The presence of lead chloride in the brine would appear not to diminish this rate of solubility of silver as chloride. The solution of silver chloride in brine appears to be peculiarly liable to that condition chemically termed super - saturation, so much so, that it will often be found that hot brine saturated with silver chloride will remain clear on cooling, until some determining cause, Such as the addition of an atom of solid silver chloride, intervenes, when immediately the whole liquid will turn milky from the separation of silver chloride.

It thus becomes evident that when lead and silver chlorides are present together, they are dissolved, and if care be taken to have brine enough present, all the silver is retained in solution on cooling, while most of the lead becomes deposited as lead chloride.

If the cooled brine, still containing, as it generally does, some ferric chloride, be now decanted from the precipitated lead chloride, and a strip of zinc be immersed in it, the lead chloride still remaining in solution and the whole of the silver chloride are reduced, together with formation of spongy lead and silver on the one hand, and zinc chloride which passes Into solution on the other. It suffices, however, and is more convenient in practice, to only precipitate part of the lead from the brine, and by leaving this light spongy lead floating about, and agitating the brine with it, this lead gradually picks up all the silver from the brine, till it becomes very argentiferous. Successive charges of argentiferous brine may be run into a tub properly disposed for the purpose, with bits of spongy lead floating about, in it, and the silver be picked up from each such successive charge, the brine being thus desilverized. This brine, as it now contains but a fraction of the lead it had taken from the ore, and no silver, may be pumped up, neutralized if necessary, reheated, and used afresh.

Thus the same brine may be employed over and over again to take more lead and silver out of successive charges of ore, depositing lead chloride in one tub, and all the silver with but a little lead in a third, while, by mere reheating, the same lot of brine becomes repeatedly regenerated in its solvent powers.

The above - mentioned employment of spongy lead is by far the most suitable and convenient in these circumstances, and offers special facilities for the subsequent separation and refining of the silver, being for this reason superior to iron, copper, or any of the metals whose employment has been previously proposed as a means of collecting silver from its solutions. The fact of the silver remaining in the brine together with what lead is still retained in solu - tion after fooling, especially facilitates this mode of its recovery, while it admits of the utilization of the same brine over and over again as a carrier and depositer of the lead on the one hand, and of silver on the other; arid, as shown already, by so proportioning the quantity of the brine as to make it sufficient to retain the whole of the silver in solution, this metal can be entirely carried over into a separate vessel, where it may be thrown down completely, the brine being thus freed from silver at each successive operation.

The process of desilverizing usually employed - or rather, what should be termed concentrating the silver in lead previous tocupellation - may be dispensed with when this process is employed, fur the silver is thus at once obtainable with lead in sufficientrly concentrated form for immediate cupellation. Heating the brine during the desilverizing materially accelerates the operation, a lubful being thus able to be completely desilverized in a few hours. As the brine me often contains traces of copper gathered from the ore, it is usually convenient to so appor - tion the lead thrown down as to have enough of that metal present to ensure proper cupellation, and further,in order to avoid, loss, it is well not to obtain the silver in too concentrated a form. Lead with 3 to 4 Per cent. of silver will be huh enough; the quantity of lead pre - cipitated being of course proportionate to the time of immersion of the spelter, the quantity of the spelter employed, and the temperature of the brine. Argentiferous precipitates are easily, however, obtainable with 28 or 30 Per cent. of silrer from ores containing but a few oz. of silver per ton.

Instead of spelter, zinc - dust may be used for this precipitation; or, instead of employing spongy lead at all for the collection of the silver, the same end may he obtained by stirring into the argentiferous brine some precipitated lead or copper sulphide. This will also gather the silver 3 from the brine, though not perhaps quite so efficaciously or so rapidly as the spongy lead. This process is now being worked on a grand scale in lancashire.

Softening Lead

Crude metallic lead is always more or less "hard," that quality being mainly dependent upon the proportion of antimony, and to a less extent upon that of the other foreign metals which it contains. These may occur in the following extreme proportions: antimony, 1/2 to 3/ 1/4 per cent.; copper, 1/2 to 1/3 per cent.; iron,1/23 to 1/4 per cent.; silver, in almost any proportion. When the last - named metal is largely present, a particular procss turned "desilverization" is employed; "but in ordinary cases, the softening is accomplished by exposing the metal in a reverberatory furnace with a cast-iron bed to the action of dull - red heat with free access of air. This oxidizes the foreign metals, some of whose oxides pass off as fume, and the remainder combine with the lead oxide formed, and with it are skimmed off as dross, which is subsequently reduced, together with litharge made in the cupellation of silver, in a reverberatory or blast furnace, in which it is mixed with carbonaceous matter.