Page:The New International Encyclopædia 1st ed. v. 18.djvu/221

* SILVER. 177 SILVER. with the silver, and the process is carried out either by a simple melting or by a combination of roasting and melting. The silver-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 moat extended 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 difl'ers, and its production from matte, speiss, alloys, and other metallurgical products, each of which likewise requires a difl'erent 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 off the mother liquor and leaving the crystals behind, and the formation and separation of the crystals is ell'ected 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 jjoor in silver remains behind. The process is based upon the fact that if argen- tiferous lead be melted, pieces of zinc forming altogether from 1% to 2 per cent, of the w-eight 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 be separated from the rest of the metal forming the bath. After separation the excess of lead present is removed by liquation, 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 (inal process of cupelling the argentifer- ous 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 (luantity of litharge and the mass is slowly melted hy 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 ofl' from time to time. The final proiluct remaining is silver with about 10 per cent, of impurities. This is refined by a similar oxidizing process. Wet Processes. Of the various combined wet and drj' processes for extracting silver, the amalgamation process is the first which demands consideration. In the amalgamation process, the silver in ores or metallurgical products is con- verted into a mercury alloy, or amalgam, which is subsequently distilled, the silver being left behind and the mercury condensed and used over again. The various amalgamation processes may be grouped into three classes: (1) Amalgama- tion with mercury alone; (2) amalgamation with mercury and certain reagents without roast- ing ; and ( 3 ) amalgamation with mercury and reagents after a ehlorodizing roasting. ( 1 ) Amalgamation with mercury alone, usu- ally called direct amalgamation, is practiced only with ores consisting chiefly of native silver. It was formerly extensively used in Peru, Chile, and Mexico, and is yet used to some extent in those countries where suitable ore is available. The process consists in rubbing the crushed ores with mercuiy, the crusliing either going on at the same time or having been done previously, and is of comparatively limited application. (2) Amalgamation with reagents and without roasting is employed when the silver exists in sulphur, arsenic, and antimony compounds, and includes what are known as the Gazo, Krohnke, Patio, and Washoe processes. Of these the Patio anil Washoe processes are the most important and they only will be described further. The Patio process is extensively used in Mexico, and to a less extent in South American countries. In carrying it out the first operation is to crush and grind the ore. The coarse crushing is usu- ally performed in edge-nmner mills, stamps, rolls, or rock-crushers (see Grinding AND Crush- ing M.vciiiNERV), while the fine grinding is done in special mills called arrastras. Described brief- ly, the arrastra is a circular pit. the sides and bottom of which are paved with hard stone such as quartz or porphyry. In the centre of the pit floor is a pyramidal stone with a hole in its top into which pivots a vertical post supported at its upper end by a horizontal beam. This post car- ries two or four horizontal arms, to each of which are attached by chains or thongs one or more rectangular blocks of porphyry weighing from 6 to 12 ewt. These blocks are attached in such a way that their front edges are about two inches above the floor while their rear edges drag on the floor. By revolving the vertical shaft these stone blocks are dragged round and round the pit, grind-