Having secured by one means or another the solution containing the precious metals, the next step is precipitation. This is accomplished by passing the liquid upward through a mass of zinc which has been finely divided so as to present a large area of surface. This is secured by reducing the zinc to thin and narrow shav ings or threads, or even to dust: the latter, however, has a dangerous tendency to explode. An improved form is known as "atomized zinc,' and this in some cases is mixed with atomized lead, or the zinc is alloyed with lead before atomizing. The double cyanide of gold (or silver) and potassium is apparently decom posed, the zinc taking the place of the precious metals in the combination. These metals in a finely divided condition fall to the bottom of the zinc-box, or adhere lightly to the zinc particles, from which they may be brushed.
This sedimentary deposit from the zinc-box contains, besides the gold and silver, a percent age of zinc, and perhaps also some lead, copper and iron and a variety of insoluble metallic salts. It is treated with weak sulphuric acid, which removes the zinc and several other im purities. Or the precipitate may be roasted in a reverbatory furnace. In some plants both the roasting and the sulphuric acid treatment are applied. It is then mixed with a flux and smelted in a graphite crucible, preferably lined with clay. In some plants an oxidizer such as
fluorspar or manganese dioxide is also added. When the smelting is complete the metal is cast as bullion — generally from 900 to 950 parts fine. The slag consists principally of silicate of zinc, and frequently contains enough gold to pay for working it over. See GOLD MINING.
The cyanide process was first employed on a commercial scale in 1889 at the Crown Mines at Karangahake, New Zealand. In that year the world's consumption of cyanide amounted to about 50 tons. By 1905 the mining industry was consuming 10,000 tons annually. Previous to the outbreak of the war in Europe the larger part of the world's supply of cyanide was pro duced in Germany. The complete elimination of this source threw a large burden upon American manufacturers of cyanides, but the demand has been met, the principal raw material used being the refuse beet pulp of the beet sugar industry.
Bibliography.— Bain, H. F.,
Recent Cyanide Practice' (San Francisco 1910) ; Ber newitz, M. W.,
Practice 1910 to 1913' (San Francisco 1913) ; Clennell, J. E.,