Page:The American Cyclopædia (1879) Volume XV.djvu/60

52 is ground and amalgamated in pans, of which there are numerous forms. The charge for a pan is 800 to 1,500 lbs.; the very large pans treating tailings which have been already ground fine, can take 3,000 to 4,500 lbs. To maintain a proper temperature, steam is introduced into the pulp or into a steam chamber under the bottom, and a wooden cover is usually kept on the pan. The pulp is generally ground for one or two hours; then the quicksilver is sprinkled in (usually 60 to 70 lbs. to a charge of 1,200 or 1,500 lbs.), and, the mullers being raised to avoid too much grinding, which would “flour” the mercury, stirring is continued for two or three hours longer, after which the pulp is diluted and drawn off into a settler. The modification of the Washoe process invented by Mr. Henry Janin, consisting in the use of large quantities of copper vitriol (blue stone) and salt, has proved very successful in the reduction of refractory ores not otherwise amalgamable. The quicksilver, charged with amalgam, is washed, skimmed, and strained through a canvas bag, which retains the amalgam. This is then distilled in cast-iron retorts, the mercury being collected under water, while the “retort bullion” remains behind. About one sixth of the charge retorted, or 200 lbs. of bullion from 1,200 lbs. of amalgam, is usually obtained from the retort, to be broken up, melted, and cast into ingots; it loses 2 to 3 per cent. in melting. The ingots are assayed, and their fineness in thousandths of gold and silver is stamped upon them. The coin value of the Comstock bullion is $1 75 to $2, one third of which is due to the gold it contains. The pulp escaping from the apparatus in which the amalgam is collected is called “tailings.” The tailings are often concentrated upon blankets or otherwise, or are simply allowed to settle in reservoirs, for reworking. The “slimes” or “slums” comprise that part of the ore which is crushed under the stamps to an impalpably fine condition, and escapes in the battery water without ever getting into the pans. Since many silver ores yield much fine powder in crushing, the slimes are often far richer than the tailings, the value of the latter being largely in the particles of quicksilver and amalgam which they contain. The chemistry of the Washoe process is summed up by Mr. Arnold Hague as follows: that the ore submitted to it consists chiefly of native gold, native silver, and argentiferous sulphurets, associated with varying portions of blende and galena; that the action of sodium chloride and copper sulphate in the pan produces copper chloride, while the presence of metallic iron causes the formation of copper dichloride; that both the chlorides of copper assist in the reduction of the ore by chloridizing the sulphurets of silver and decomposing the sulphurets of lead and zinc; that sulphate of copper enhances the amalgamating energy of mercury, by causing the formation of a small quantity of copper amalgam, and also tends to expel the lead; but

that the quantities of chemical agents usually added in the Washoe process are too small to be effective, and that the principal agents in the reduction are in general mercury and the iron of the pan, aided by heat and friction. The essential condition in the amalgamation is the keeping of the mercury bright and pure, that it may come into direct contact with the iron and sulphide of silver. The consumption of mercury in the Washoe process may be considered chiefly a mechanical loss, and only to a limited extent a chemical one. The pan amalgamation of slimes and refractory ores, with the addition of large proportions of copper sulphate and salt, involves a greater loss of mercury.—Refractory ores, not suitable for “raw” amalgamation by the Washoe or the patio process, are treated in many localities by the Freiberg process, consisting in the chlorination of the ore by roasting with salt, and its subsequent amalgamation. At Freiberg in Saxony, where this method originated, it has been abandoned, the ores formerly amalgamated being now treated by smelting. But in districts where fuel is scarce and labor dear, and lead ores for smelting are not at hand (which is the case in many parts of Nevada, for instance), the Freiberg system is still successfully employed, though greatly modified as to apparatus. The ore is crushed in stamp mills, without water, and the fine powder is further dried, usually by spreading on the top of the arch or the dust chambers of the roasting furnace. Either in the battery, during crushing, or on the drying or the charging floor, 6 to 7 per cent. (for rich ores, up to 20 per cent.) of salt is mixed with the ore. The mixture is then roasted, to chloridize the silver; this was done abroad in reverberatory furnaces, which have been used in Colorado and Nevada also, but are now generally replaced in the west by Stetefeldt's showering furnace or Brückner's cylinder. From the roasting furnace the ore is conveyed to the pans, where it undergoes an amalgamation similar to that of the simple Washoe process, except that less grinding is necessary. The Freiberg amalgamation was performed in revolving wooden barrels, which are still employed at some places in the United States. Each apparatus has its partisans. A peculiar method of amalgamation pursued in Chili avoids the chloridizing roasting, substituting a humid chlorination by means of copper dichloride (Kröncke's process). It is highly praised, but not yet widely employed. The use for this purpose of copper chloride, which is of earlier origin, involves a loss of quicksilver as calomel.—The processes of humid extraction of silver are of two classes. Either the silver is converted into a soluble compound and separated by leaching and precipitation, or the baser metallic constituents of the ore are rendered soluble and removed by leaching, leaving an auriferous and argentiferous residuum for further treatment. The methods of the first class convert the silver