MILLS, GOLD. Gold-Milling Machinery.—Auriferous ores are commonly worked by the amalgamation process. Very rich gold-ores are sometimes sold to the lead-smelters and their gold contents collected in the lead bullion ; but the ores from which nearly all of the gold of the world, excluding that from placer-mines, is produced are of altogether too low grade to be treated in that manner. In the typical gold-mill the ore coming from the mine is dumped over a grizzly, and the coarse lumps crushed by means of a Blake, Dodge, or Gates crusher to con venient size, say, so as to pass a 1-in, ring. The crushed ore is fed by automatic feeders into wet crushing stamp-batteries, where it is crushed to that degree of fineness necessary to free the particles of gold from the gangue. The stamp-batteries are lined with copper plates covered with mercury ; and as the pulp inside the batters- is splashed against these plates before being crushed fine enough to be thrown out through the slotted steel screen, which forms one side of the mortar, a part of the gold is amalgamated. When the ore is crushed fine enough to pass through the screen, it flows down over a table of the same width as the mortar, and 8 ft., 10 ft., or 12 ft. long, covered with copper plates coated with silver amalgam, by which the particles of gold not already amalgamated within the mortar are caught. The pulp which has passed over the plates, always carrying a small amount of gold not practicable or economical to save, is called tailings, and is allowed to run away. The gold in ores, however, is not always free—that is, occurring in separate particles—but is sometimes contained in a mineral, occasionally in galena, but generally in pyrites. The gold thus contained can not be amalgamated, and other means are necessary for its recovery. For this purpose the first step is to save the auriferous mineral, and this is accomplished by concentrating the tailings from the amalgamating plates. As the tailings are generally very fine, in most cases exceeding 40 mesh, slime-washing machines are used exclusively for concentration in gold-mills. As it is only necessary to make one separation—that is, headings and tailings—Frue vanner's or another of the belt machines are admirably adapted for the purpose and are almost invariably used, although end bump-machines are employed in some mills. The pyritic concentrates thus made are usually rich enough to be shipped to the lead smelters, and in many dis tricts whence freight rates to a smelting center are low are disposed of in that manner.
Another method of treating auriferous pyrites, and one in which great progress has been made during the past ten years, is by chlorination.
In this process the ore is roasted oxidizingly for the elimination of the sulphur, after which it is subjected to the action of chlorine gas, in covered vats or barrels, where by the gold is converted into chloride of gold, which is sol uble in water. The chloride of gold having been dis solved, the filtrate is run to suitable tubs, where the gold is precipitated by hydrogen sulphide or ferrous sulphate.
The fine precipitate is dried, and finally melted into bull ion. Ores containing both gold and silver, such as those of the Comstock lode, are usually treated by the process of pan amalgamation (see SILVER-MILLS), but this process is seldom used for ores carrying gold alone.
The cost of gold-milling varies with the character of the ore, the equipment of the mill, the method of milling, etc. The lowest figure ever reached was at the mill of the Spanish Gold-Mining Co., Washington, Nevada County, Col.; there, in 1886, ore was milled at a cost of but cents per ton. The ore consisted of about one third hard quartz, one third tough slate, and one third decomposed quartz and slate. The crushing machinery consisted of three 5-ft. Huntington mills and one 4-ft. mill, running at GO revolutions per min., consuming 22 horse-power, and discharging through a No. 6 slot screen. In a 4-months' run. 19,402 tons of ore were crushed ; the averaging cost of milling being, as before stated, cents per ton, divided as follows : Labor, 9 cents ; water, cents ; shoes, cents ; screens, 11 cents ; dies, 1.7 cents: caps, scrapers, and bolts, cent : renewal of working parts, 2 cents ; quicksilver (at $40 per flask), •5 cent ; oil for illumination and lubrication, .2 cent ; labor at rock-breaker. 2 cents ; wear and tear of rock-breaker, .5 cent ; depreciation, 1 cent. Later the cost was further reduced to cents per ton, of which cents was for labor and 9 cents for supplies. The ore which was worked at this mill averaged only 65 cents per ton, and was actually mined and milled for 52 cents per ton, the mine being opened as a quarry and worked under extremely favorable circumstances. The foregoing figures are from statements by Mr. F. W. Bradley, the superintendent of the company. The Plymouth Consolidated Gold-Mining Co. milled ore in 1886 at an expense of 39 cents per ton, and saved and reduced the sulphurets at an additional expense of 17 cents per ton of ore. The Plumas-Eureka Mining Co. milled ore in 1888 at an expense of 5si. cents per ton, and in the same year the cost at the Yuba and Hanks mills of the Sierra. Butte Gold-Mining Co. was but 261 cents and 35 cents per ton, respectively. In Montana. in 1888, at the 60-stamp mill of the Montana Co., Limited, low grade gold-ore was crushed and amalgamated on plates, and the sulphurets con centrated on Frue vanner's at a cost of $1.13 per ton. At the large mill of the Alaska Treadwell Gold-Mining Co.. Douglas Island, Alaska, the cost of milling ore and concentrating sulphurets, for the year ending May 31, 1891, was cents per ton, of which 19'4 cents was for labor and 22.66 cents for supplies. At the Golden Star mill (120 stamps) of the Home stake Mining Co., at Lead City, South Dakota, the cost of milling in 1887–'88 was, according to Mr. II. 0. Hofrnan, cents per ton. The best practice in gold-milling in this country at the present time is undoubtedly that of California, McDermott and Duffield state that, on a considerable variety of gold-ores in that State, the percentage of gold saved averages from 80 to 85 per cent, and careful daily tests in some of the best gold-mills using concen trators show from 85 to 90 per cent.