Page:The American Cyclopædia (1879) Volume XV.djvu/59

Rh behind the spongy copper. This process has almost everywhere given way to humid methods. (See, , and .)—The method of amalgamation, invented in Mexico in 1557 by Bartolomé de Medina, led to the enormous production of silver there and in South America during the next 200 years, and has remained substantially in extensive use ever since. The Mexican, known as the patio process, is suited to ores which contain native silver or silver chloride (bromide, iodide) and sulphide, and are measurably free from other sulphides and from arsenides and antimoniurets. The ore is first crushed and then ground fine in arrastras. If gold is present, 50 or 60 per cent. of it may be saved by introducing silver or copper amalgam into the arrastra. Ores containing pyrites, antimony, or arsenic are incompletely roasted, to break up the combination of silver with these elements. The presence of silver sulphide does hot necessitate roasting as a preliminary for patio amalgamation. The fine paste from the arrastra is spread on the patio floor (of stone, calked boards, or asphaltum) in round heaps (tortas) about 0.3 metre high and 10 to 16 metres in diameter, containing each from 5,000 to 100,000 kilos; average, about 60 tons. The paste having stiffened by the evaporation of its water, from 2½ to 10 per cent. of impure salt is added, according to the contents of silver in the ore. This is intermixed with shovels and subsequently by the treading of mules or men, and occasionally by means of kneading machines, with travelling wheels, set up in the torta. After one or two days the magistral is added; this is copper vitriol and salt, or rich oxidized copper ores mixed with pyrites which has been roasted with salt, or simply copper pyrites which has been so roasted. The quantity of magistral required varies according to the season, the temperature, and the quantity of the ore; it usually ranges from ½ to 1 per cent. Its function is to cause certain reactions with the salt and the sulphide of silver and promote the formation of amalgam. Too much of it causes too high a temperature in the mass, particularly in winter; hence cold weather and poor ores require the smallest amount. After another treading, quicksilver is sprinkled over the torta by squeezing through a leather or canvas bag. The quantity used is six to seven times the weight of silver in the ore, sometimes much more. It is rarely added all at once; the usual practice is to give fresh quicksilver every alternate day, treading the mass for six to eight hours on each intervening day. The termination of amalgamation is observed by panning samples (see ) from the torta, and examining the amount and condition of the quicksilver and amalgam. The period required for the whole operation down to this point varies from 5 to 30 days; average, about 19 days. Various theories have been proposed concerning the chemical reactions of the patio. Too low a temperature stops the reactions, and may

be remedied by more frequent treading or by additional magistral. The amalgam is collected in settlers, which are circular vats of wood or masonry, about 9 ft. in diameter and 8 ft. in depth, in which the mass, thinned with water, is stirred and allowed to deposit its heavy amalgam, while the lighter portion is drawn off. The amalgam, being concentrated still further, is at last collected in a leather or canvas bag, where it is freed by squeezing from free mercury, which passes through, carrying a little silver with it, while the mass remains in a coherent, plastic condition. The former is used again on the patio; the latter is moulded into 30 lb. blocks, piled on an iron plate, covered with a large iron bell, and heated by means of a charcoal fire around the bell. The mercury is vaporized, and (the joint at the edge of the bell being carefully luted) passes down through a pipe in the iron plate into a cistern of water. The bell furnace is less economical of fuel and mercury than muffle or retort furnaces; it loses 0.8 per cent, of mercury. The silver, found in solid masses when the bell is raised, is cast into ingots of 80 or 90 lbs. By the patio process the usual product of silver is 50 to 66 per cent. of that contained in the ore; the most docile ores, under favorable circumstances, have yielded 90 per cent. The loss of quicksilver is given by Kerl as 3 to 5 per cent. of the quantity used; earlier accounts make it considerably greater. This loss is due to the formation in the torta of soluble mercury dichloride (calomel), which is afterward washed away.—The cazo process, used in Mexico and Chili, is a hot amalgamation in kettles. The ore (in Mexico chloride, in Chili sulphide) is placed, in the form of a watery pulp, in a vat with copper bottom and wooden or stone sides. Here it is heated and stirred with salt and quicksilver, copper vitriol being added in the treatment of sulphides. The process is rapid and effects a tolerably complete extraction of silver, but involves great loss of quicksilver (2 to 2.5 times the weight of silver) when applied to sulphide ore. Silver ores free from sulphides of other metals are amalgamated at Guanajuato, Mexico, in arrastras, by simple grinding and mixing with quicksilver and water.—Pan amalgamation, called the Washoe process, consists in rubbing together in pans (usually of cast iron) the watery mixture of crushed ore (pulp) with quicksilver, with or without the addition of other chemicals. The simplest form of it may be thus described: The ore suitable for this process (usually containing silver sulphide or chloride and native silver, with little antimony, arsenic, base sulphides, in a gangue of quartz) is first crushed in a stamp mill, similar in most respects to that employed for gold-bearing quartz. (See .) The screens which regulate the size of the crushed particles are of wire cloth with 40 to 60 meshes to the inch, or of Russia sheet iron, perforated with holes to  in. in diameter. The pulp reduced to this fineness