DRY PROCESSES. The extraction of silver in the dry way is effected by converting the metal into a silver-lead alloy and then submitting this to an oxidizing melting in the cupellation fur nace. The production of the silver-lead alloy depends upon the power which lead possesses of extracting silver from its ores or from various products containing it, the lead readily alloying with the silver, and the process is carried out either by a simple melting or by a combination of roasting and melting. The sil•er-lead alloy obtained is called 'work lead.' if the amount of silver in the work lead is not great enough to make direct cupellation profitable, then an intermediate process of concentration is intro duced. Therefore. the dry process in its most extruded form comprises: (1) the production of work lead; (2) the concentration of the silver in the work lead; and (3) the extraction of the silver from the concentrate or enriched work lead. In the production of work lead we have to distinguish between its production from ores and its production from metallurgical products; fur ther we have to distinguish between its produc tion from rich ores, from medium ores, and from poor ores, for each of which the process differs, and its production from matte, speiss, alloys, and other metallurgical products, each of which likewise requires a different process.
These different processes are all variations of two general processes. One of these consists essentially in introducing the ore, matte, or other product into a bath of molten lead in a reverberatory furnace; the other consists in smelting the ore, matte, or other product with materials rich in lead in a blast furnace. The result in either case is the production of a silver lead alloy, or work lead, more or less rich in silver. It the silver content is less than about 0.12 per cent. it is generally assumed that it cannot be economically treated by cupellation until the work lead is enriched by concentration. The two processes of concentration employed are the Pattinson process and the zinc process. In the Pattinson process the work lead, by slow cooling from the molten state, is separated into crystals poor in silver and a fluid portion rich in silver. If the richer liquid portion be sepa rated, it again can be divided into a poorer solid portion and a still richer liquid alloy. and this operation can be repeated until the enriched lead contains 2.5 per cent. of silver, when the maxi
mum is reached. The Pattinson process is con ducted in large pots of cast iron or cast steel. The crystals are separated from the mother liquor either by leveling them out from the pot or else by tapping of the mother liquor and leaving the crystals behind, and the formation and separation of the crystals is effected either by stirring the cooling mass or by blowing steam through it.
In the zinc process the silver is separated from the work lead in the form of a silver zinc-lead alloy; the lead poor in silver remains behind. The process is based upon the fact that if argen tiferous lead be melted. pieces of zinc forming altogether from to 2 per cent. of the weight of the lead thrown on its surface, the tempera ture of the bath raised to the melting point of the zinc, and the whole thoroughly stirred and al lowed to cool, a crust or scum forms upon the surface as the temperature is lowered. This scum is a solidified mixture of alloys of lead. zinc, and silver, lighter than the molten lead and containing all the silver originally present in the lead, and it can easily he separated from the rest of the metal forming the bath. After separation the excess of lead present is removed by Equation, a process based upon the fact that the alloy has a higher melting point than lead itself. The scum is placed in pots or reverbera tory furnaces and heated until the excess of lead melts and separates from the solid alloys. The latter, known as rich scum, is next heated for the separation of the zinc by the processes of distillation, oxidation, or treatment with fluxes, so that only silver and lead remain. The final process of eupelling the a rgentifer one lead consists of an oxidizing melting of the work lead in a reverberatory furnace. This process may be performed in stages or con tinuously. The work lead is charged into the furnace with a quantity of litharge and the mass is slowly melted by an increasing heat. As the melting progresses successive scums are formed on the molten surface which contain litharge mixed with the oxides of lead and of the other impurities, and which are drawn off from time to time. The final product remaining is silver with about 10 per cent. of impurities. This is refined by a similar oxidizing process.