The charge of roasted ore is' leached with water, to remove the soluble base-metal salts. Water does not dissolve silver chloride, but a concentrated solution of base-metal chlorides does, and therefore it is advisable not to make the leaching-vats too deep, as otherwise a too concentrated base-metal solution is produced by the water in descending through a thick layer of ore. The base-metal leaching is completed when a few drops of calcium polysulphide poured into sonie of the outflowing solution does not produce a precipitate. This part of the process takes, according to the character of the ore, from 4 to 8 and 10 hours. The base metal salts being removed, a stream of diluted solution of sodium hyposulphite is allowed to enter on the top of the ore. Sodium hyposulphite readily dissolves silver chloride. When the outflowing solution shows indications of silver, which also can be determined by an addition of a few drops of calcium polysulphide, the stream is conveyed to special tanks (the precipitating tanks), in which the silver is precipitated as silver sulphide by an addition of calcium polysulphide. To facilitate and hasten the settling of the silver precipitates, the preeipitation-tanks are provided with stirrers, by which the solution can be vigorously agi tated. After precipitation, the sodium hyposulphite solution is again in its original condi tion, and is therefore, after the precipitate has settled, decanted from the latter into tanks placed on a lower level. From these tanks the clear solution is pumped up to storage-tanks, and is ready to be used again. When all the soluble silver is extracted, the tailings are sluiced out through the central discharge, and the tank is ready for another charge of ore. The time required for silver leaching varies according to the character of the ore from 8 hours to 2 and 3 days. When enough silver precipitates have accumulated on the bottom of the precipitating tanks, they are drawn off and strained through a filter-press, or through properly arranged filters made of cotton cloth. The precipitate is then charged into a small reverberatory furnace and the sulphur burned off. The roasted precipitate is melted with lead in a cupelling furnace and refined. The wear and tear of a lixiviation plant is insignifi cant. The base-metal salts penetrating the wood of the vats prevent them from decay and preserve them for many years.
Ilofmann's system of trough lixiviation is a continuous one. The roasted ore is fed mechanically first into a stream of water, which rapidly moves in a triangular trough, for the removal of the base-metal salts. The pulp drops into settling-vats in which the washed ore accumulates. The washed ore is then sluiced out with a stream of sodium hyposulphite solution and the pulp conveyed through a triangular trough to another set of settling-tanks. In these tanks the mineral drops desilverized as tailings, while the silver is in solution. Base metal salts and silver chloride dissolve almost instantly if the ore is charged into a rapid moving stream of the respective solvent. The chemical process of this method is the same as in tank lixiviation, but the time of leaching is said to be shortened, and the manipulations are much simpler and more labor-saving. This method is especially adapted for larger works, and for ore which on account of lead requires a long leaching. A 50-ton lixiviation-mill designed for this process is shown, in elevation, in Fig. 2.
The Russell process is a modification of the ordinary system of tank-lixiviation or Patera process, in which a certain proportion of cuprous sulphate is added to the solution of sodium hyposulphite, constituting the lixiviant. This process has been carried to a high degree of perfection at the Marsae mill of the Daly Mining Co., at Park City, Utah. With very many classes of ore an additional amount of silver it is claimed can be extracted by means of this extra solution.
The cost of milling silver-ores varies greatly with the locality and the character of the ore. In 1876 the expense at the hest-designed mills on the Comstock lode had been reduced to $2.47-i per ton, and at the present time figures are probably lower ; this ore is, however, perfectly free-milling. The cost per ton at the mills (amalgamation) of the Granite Mount ain Mining Co., Phillipsburg, Mont., in 1890 was $10,182, divided as follows: Labor—super intendence, •284 ; engineers. •159: firemen, -099; crusher-men. '122 ; roaster-men, •319 ; drier men. •160: hattery-men and helpers, -463; cooling-room. '250 ; pan-men and helpers, •4l8 ; retort-men, .046 ; assaying, .093; w•atc•hmen, .055 : millwrights, •113; repairing, •043: sundry labor, •715; total. $3.339. Supplies—fuel, 1•639: castings, •579; salt. 2•550; quicksilver, '970: blue-stone, 144; lye, .029; other chemicals, .038 : belting• •022; lubricating, •066; illu minants, .031 : sundry supplies. •430; total. $6.4J8. Miscellaneon—tramming, •120; water, •096; blaeksmithing, '120; total, The amount of ore crushed was 6:1.529 tons wet, or 60.212 tons dry : amount of salt used• 9,370 tons; average assay of ore. 70•04 ounces silver per ton : average percentage of saving• 92-17. The salt and ore were mixed before crushing.
The cost in one of the three mills belonging to the company was but $9.14, the average being raised by the other two.
The cost of milling at the works of the (amalgamation) Elkhorn Mining Co., Elkhorn, Jef ferson County, Montana, in 1890, was $8.73 per ton, divided as follows: Superintendence, 47.80 cents ; engineer, 26'41 ; crusher-man, 19.74 ; drier-men, ; battery-men, roaster men, 23.24; cooling-floor men, car-men, 37.52; amalgamators. 25.99; pan-helper, laborer, melter, assayer (proportion), storekeeper, 05.59; repairs. mechanics (proportion), 27.54; millwright, 06.64; teams and laborers, 32.18; watchman, 09.99; tailings, storage, 01.71; office (proportion), 08.25; chemicals. lubricants, illuminants, 01•24; quicksilver, 29.56; salt, 173.22; fuel, 165.70; freights, 11.80; castings, other supplies, 15.14; general expense, 87.17 ; total, The amount of ore crushed was 9,103 tons ; amount of salt used, 990 tons ; average assay of ore, oz. silver ; average assay of tailing, 6 oz. ; percentage of silver saved, The Ontario and Daly mills, at Park City, Utah, furnish an interesting comparison, as at the former the ore is worked by amalgamation and at the latter by the Russell process of lix iviation, the ores being very similar in character. The cost per ton at the Ontario in 1890 was $9.08, 24,450 tons of ore being reduced. The average assay of the ore was oz. silver per ton ; average assay of tailings, oz. per ton ; percentage of silver saved, In the same year 20,795 tons of ore were milled by the Daly Mining Co., at an expense of $6.38 per ton, 88.77 per cent of the silver being saved.