![]() ![]() ![]() The precipitates were washed with water, dried and roasted in trays at a low red heat, the aim being to oxidise the zinc. The coarser material was put back in the boxes, and the fine sediment settled in the trough. The original method of cleaning up was to tease the zinc filaments out in a trough of clear water, passing only the fine muds through a screen. At first the precipitated metals adhere rather firmly to the zinc filaments, but in course of time the latter are eaten out, and the metals and admixed impurities become converted into a black mud ferrocyanide of zinc, carbonate and other salts of lime tend to form a white crust, which on agitation makes a milky liquor. In addition to these elements, insoluble cyanides and ferrocyanides, sulphate of lime, carbonate of lime, silica, and organic compounds accumulate in the boxes. Selenium and tellurium are also dissolved in alkaline solution and thrown down. The metals precipitated by zinc in alkaline cyanide solutions are gold, silver, copper, antimony, arsenic, mercury, lead, and occasionally nickel, cobalt, and cadmium. Some such simple process will no doubt be discovered which will enable fine gold to be obtained as a separate precipitate from the silver. In course of time, owing to the mixing of all the material, this is lost sight of, but it would appear to be possible, were it worth while, to obtain precipitates of various grades by keeping the material from each compartment separate. The upper compartments at first contain but little, while the lower ones can be seen to be well coated. Copper in the same way appears to be thrown out after the precipitation of the gold and silver has commenced If a fresh zinc box is taken, and gold-silver cyanide solutions allowed to flow through it, the gold is thrown down in the head box with a little silver, and the rest of the silver precipitated further on. It is undoubted that gold is one of the first metals to be thrown down from such a solution. On the introduction of the cyanide process and the zinc shaving method of precipitation, the purity of the resulting bullion was dependent on the metals and elements which passed into solution with the gold, and which were precipitated upon the zinc. Amalgam from batteries, if properly worked up, could be obtained containing only a few parts of base metal per thousand. The gold precipitated from chlorine solutions by means of charcoal gave fine gold running up to 998 fine, while that thrown down by sulphate of iron could be rendered equally pure by first precipitating the sulphate of lead, and allowing it to settle. When alluvial gold was collected and smelted the product in gold and silver rarely contained five parts of base metal per 1000, yet some of this was introduced in the reduction of a little iron or other metal associated with the gold. When ores were smelted the litharge method of elimination of baser metals and the subsequent parting had been worked out in fine detail. Comparatively little trouble was experienced in gold milling with regard to the preparation of a pure bullion until the advent of the cyanide process.
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