The cyanide process is based on the facts that dilute cyanide solutions will dissolve the precious metals, from their ores when finely crushed, and that when gold in solution is brought in contact with zinc it will be precip itated. It is also possible to separate the gold from the solution electrically by using iron anodes and sheet lead cathodes. The Siemens Halske process on this principle achieved some success, but is unable as a rule to compete com mercially with zinc precipitation. Patents for cyanide extraction were taken out in Great Britain as early as 1840 and in the United States in 1867, but it was not until 1887, through the investigations and experiments of J. S. McArthur and R. W. Forrest, that the cyanide process showed practical results. These investigatOrs demonstrated the benefit of using very weak- solutions of cyanide and the precipitation on zinc shavings confined in boxes through which the solution flowed. McArthur and Alfred James, in South Africa, worked out a successful system of economic treatment, which, though since greatly extended and im proved in detail, embodied the basic fun damentals. The benefit of fine grinding or crushing was clearly realized by them in the early days, though not so completely applied as later.
The cyanide process has been a great help in the extension of gold and silver mining. On the Witwatersrand it has been especially bene ficial in that the gold occurs in such fine parti cles that extraction of only 55 to 65 per cent could be obtained by amalgamation, with but small addition by concentration and chlorina tion. By amalgamation and cyanide treatment, however, about 94 per cent is now obtained. Most gold and silver ores are amenable to cyanide treatment, though antimony combina tions are very detrimental and the association with tellurium may require preliminary roast ing or very fine grinding. The field of the cyanide process has constantly widened since its introduction in South Africa in 1890, and there have been many developments in its details. The flow sheets of the mills and cyanide vary in the same as well as different mining districts. The pulp is classified into sands and slimes and separate treatment given to each. For details of the process see works mentioned in references.
Treatment of Amalgam, Zinc Precipitates, Bullion, Refining.— The gold in the amalgam is separated by retorting; that is, mercury is vaporized by heat, and condensed in cooled pipes. The gold is left in the form of a yellow sponge which is melted with suitable fluxes in graphite crucibles and poured into bullion molds. Gold bullion contains some silver, copper and other metals, which are separated at mints or private refineries, by sulphuric acid, chlorine gas or electrolytic treatments. See ELECTROCHEMICAL INDUSTRIES.
The zinc precipitate is in the form of im pure brown powder or filter-press cakes. It is treated in various ways; viz.: (1) By direct fusion with fluxes; (2) roasting followed by fusion; (3) treatment with acid, followed by fusion with or without roasting; (4) reverber atory furnace, lead fusion and cupellation; (5) blast furnace, lead fusion, then cupellation.
Both rich gold ores and concentrates, especially when associated with lead or copper, are treated directly in smelters. Considerable gold is also recovered from copper and lead smelt ing in which gold is a small by-product and incidentally recovered in refining.
Geographical Distribution of Gold Mining.
—The following table gives the distribution of gold production. It also indicates that the out put has reached its zenith during the past five years, and now has a downward tendency. The output for 1917, given at $423,590,200, is approximately 6.7 per cent less than the out put for 1916. The table gives the percentage of production for all countries for 1913, and shows that the territory of the Entente Allies pro duced 91.3 per cent and that of the Central Powers only 0.6 per cent of the world's gold.
The greatest gold minim district in the g_ world is the Witwatersrand, Transvaal, South Africa. The principal mines are in an area about 30 miles long by 5 miles wide. The ore is in conglomerate beds included in a sand stone and quartzite succession, which though somewhat faulted is so uniform as to permit one mine opening into another. Although the conglomerate deposits were originally of placer origin, the sharp crystalline nature of the gold indicates secondary deposition or reduction. Some shafts are more than 5,000 feet deep. Production in 31 years has been about $2,500, 000,000. The yield per ton has decreased from $12 in 1890, at cost of $10.25 per ton, to $6.40 in 1916, with cost of $4.50 per ton. The total dividends have been about 24 per cent of the output. The largest force employed has been about 200,000 natives and 26,000 Euro peans.
The United States, including Alaska, ranks next to South Africa in production. The out put in 1916 was $92,590,300. Of this about 25 per cent was from placer mining and more than half of which was from dredging operations. Yields and costs vary in different districts.
The greatest producers are mines of low or moderate values in large, massive ore bodies, with favorable conditions for economical work ing. A conspicuous example is the Homestake Mine in South Dakota, which, since 1875, has produced over $147,000,000, with a yield under $4 per ton, and costs of $2.50 to $3 per ton. Dividends of 27 per cent of the output have been paid. The Alaska Treadwell group of mines, on Douglas Island, Alaska, and the Alaska Gold and Alaska Juneau, which are situated opposite, have the record of the world for the lowest lode-mining costs. The Tread well group treated 25,000,000 tons, yielding $2.37 per ton, with total costs of $1.42 per ton. The Alaska Gold in 1916 produced about 2, 000,000 tons, with a yield of 97 cents, and oper ating costs of 73 cents. The Portland, a tellu ride mine, in Colorado, where working costs are high, has produced $40,000,000 worth of ore, averaging $27 per ton, and declared dividends of only 20 per cent. The phenomenally rich ore of the Goldfield Consolidated, Nevada, though producing $50,000,000, has fallen in seven or eight years from $38 per ton to $7.52 per ton in 1916.