Page:The New International Encyclopædia 1st ed. v. 09.djvu/21

* GOLD. beds of existing streams consists either in divert- ing the stream and working the dried l)ed in the open or in dredging the gold-bearing sand and gravel. In the latter case the dredges are gener- ally mounted on flat-bottomed boats, which also carry the necessary appliances for extracting the gold. Gold-dredging is extensively practiced in some of the Western States, particuhirly along the Snake River in Idaho and the Fcatlicr Kiver in California, and also in New Zealand. Quartz mining, or the mining of gold in veins, is carried on by underground mining operations which do not differ materially from similar op- erations in mining other ores. ( See JIining. ) Gold-quartz ores are crushed in stamp mills or other forms of crushing machines, and the metal is extracted by one of the metallurgical processes described in a succeeding section. A view of a modern stamp mill and extraction plant is shown on the accompanying plate. In this mill the ore is first crushed to coarse size in gj'rating rock- breakers, and then passes to the storage bins, from which it is fed to the stamps. After fine crushing the ore is passed over concentrating tables, which remove a portion of the values as concentrates. The remaining material, connnonly referred to as 'tailings,' passes through settling tanks, and is then changed into amalgamating pans, where a further nortion of the values is recovered. For illustrations of stamp mill, see Grinding and Crush ixG iLvcHiNERY. Met.llurgy. Gold may be extracted from its ores either by direct washing or by combining it with mercury, lead, silver, or copper, or by .bringing it into aqueous solution. Extraction by Washing. Extraction of gold by washing alone is possible only in the case of ores carrying native gold. The process is not exten- sively used in civilized coinitries. on account of the heavy losses of gold entailed by it, which may amount to 50 per cent. In combination with amalgamation, it is, however, the method most commonly employed. Simple washing is per- formed by means of pans, cradles, long toms. and sluices, the last being the appliance most used in the United States. The operation called pan- ning is the simplest of all methods of gold-wash- ing and the most wasteful. Prospectors make use of it, however, in testing for gold, and it is still carried on to some extent as a commercial process. In panning, the dish, filled to about two- thirds of its capacity with the pay dirt, is placed under water until the lumps can readily be broken up. The operator then grasps the dish with both hands, raises it to nearly the level of the water, and gives it a gentle oscillatory mo- tion. The nuul and fine sand are washed over the far edge, which is gradually loAvered until little but coarse pebbles, heavy minerals, and gold remain. After removing the pebbles, the pan is lifted and shaken so as to spread oit the mate- rials, when the yellow specks or colors of gold are visible. The cradle is a box provided with a sieve in the upper part, the whole resting on rockers so that it can be rocked by means of a handle. The gold-bearing gravel is placed in the sieve and washed with water, the coarse par- ticles remaining in the sieve and the fine with the gold passing through and falling on the in- clined bottom, where the light material runs nfT with the water, the heavier gold sinking to the bottom, where it is retained by transverse slats known as riffles. The long torn consists of two r GOLD. troughs, of which the lower one has crossriflles on its inclined bottom. Roughly, the upper trough is about 14 feet long. 20 inches wide at the upper end, and 30 inches wide at the lower end, which is closed by a sieve. The lower end of the. upper trough discharges into the U|)i)er end of the lower trough, which is about 12 feet long and 3 feet wide. By means of a strong stream of water flowing in at the up]icr end of the upper trough, the material is carried llirovigh both troughs, the gold being caught in tlie rillles of the lower box. Sluices consist of a series of 12-foot troughs emptying into one another, and often forming a line of troughing several hundred feet long. The bottoms of the troughs have rif- fles, and at intervals the bottom of the box is replaced by a grating known as a grizzl}'. Under- neath the grating there is a broad trovigh running at right angles to the main trough and opening into another trough parallel to the main direc- tion and which forms the continuation of the sluice. The coarse gravel is caught by the gratings, and the gold is deposited in the riffles as previously described. Frequently mercury is sprinkled into the riffles, thus combining amalga- mation with washing. The sluice is considered the best device for washing gold-bearing gravels. Among the other apparatus ifor washing gold are puddling tubs or boxes and pugmills. Where water is scarce separation is performed by win- nowing or dry blowing. In America the separa- tion of gold by washing is practiced chiefly by prospectors and individual placer miners. Extraction in the Dry Way. Extraction of gold by alloying it with lead and cupeling is known as the dry process. It may be effected either by introducing ores into a bath of lead, or by smelting the ores with lead-bearing fluxes, or by first smelting the ores for a concentrated product, and then smelting the latter with lead fluxes. The process is practiced chiefly in Hun- gary and the Harz Jlountains. With auriferous co])per ores, such as ehalcopyrite. bornite, etc., the usual methods of copper-smelting are carried out, as the gold is retained by this metal through- out the diit'erent operations in the same way as with lead-smelting. In the Western United States gold ores too poor to pay for separate treatment are frequently sold to copper-smelters at a profit, owing to their added value for fluxing purposes. Amalgamation. The process of extraction by amalgamation consists in combining the gold with mercury, and in decomposing the gold, mer- cury alloy, or amalgam by heating it, by which the gold is obtained in metal, while the mercury is volatilized and again condensed for further use. Amalgamation w'ithout previous crushing of the gold-bearing material is practiced with auriferous sands and gravels, it being extensively used in California in connection with hydraulic mining. The gold-bearing gravel is washed through sluices of the form already described into which mercury has been placed, and where it is retained by the riffles. The amalgamation of artificially crushed materials is a more elaborate process, and is practiced with many variations. In most cases, however, the gold-bearing ore is crushed in stamp-mills. A portion of the gold is caught by the mercury in the stamp-boxes, and the remainder washed out with the sand is caused to flow over copper plates coated with mercury which hold the gold and allow the barren rock or