The old process of barrel amalgamation is no longer used in this country, having been entirely replaced by pan amalgamation. Where the ore is base and needs a desul phurizing and ehloridizing roasting before the amalgamation can be successfully carried on. a dry-crushing mill (Fig. 1) is used, of which the general arrangement is as follows : After passing the rock-breaker the ore is dried either in a revolving drier or by means of a kiln, and the dried ore is fed by automatic feeders to the stamps. The pulverized ore from the stamps is raised to a bin in the upper part of the mill by a belt elevator (see ORE-DRESSING MaelmtEay). whence it is fed into a suitable roasting-furnace. In the fur nace the ore, with the addition of common salt, is desnlphurized and chloridized, the silver minerals of the ore being converted into silver chloride and thus prepared for the pans and settlers. After roasting, the ore is spread out on a cooling floor and is taken as required to the pans. Amalgamation follows on the same plan as in the wet-crushing mill. In some cases it is found desirable to concentrate the ore before amalgamating in wet-crushing mills, this combination never being made in a dry-crushing mill for obvious reasons. Ores subjected to this double treatment might be classed as semi-free milling, containing a portion of their silver value in base sulphides mid a portion in such form that it can be recovered by raw amalgamation. In such combination-mills, slime-washing machines, buddies, bumping-tables, or vanners, are interposed between the stamp-batteries and the settling-tanks. The silver-bearing base metals are thus concentrated and smelted, while the tailings run to the settling-tanks and are then treated in the usual manner•. This process was introduced quite successfully at the mill of the Combination Mining and Milling Co., Black Pine. Deer Lodge County. Montana, according to C. W. Goodale and W. Akers (Trans. A. I. 31. E., VOL xviii, p. 244 The ore was qnartzose with galena, lead, copper, zinc, and a little sulphur, a considerable proportion of the silver being carried by the base minerals. The mill had 10 stamps, the pulp from which was passed over four Prue Tanners. The tailings from the latter were settled and amalgamated. During the year ending May 31, 1889, 9,061 tons of ore, of an average assay of 22-67 oz. silver per ton, were crushed. There were pro duced 541 tons of concentrates, averaging 136 oz. silver per ton, which were sold to the lead smelters. The total saving by concentration and anudg,amation was per cent, and the cost of the combined process $4.35 per ton.
The Boss continuous process is a comparatively recent improvement upon the system of pan amalgamation. In this the pulp flows directly from the stamp-batteries to the pans, and the use of settling-tanks is done away with. The ore coming to the mill, it is dumped over grizzlies and passes through rock-breaker and ore-feeders into the batteries in the usual manner. The pulp from the batteries is then conveyed in pipes to the special grinding-pans placed immediately below and in front of batteries. In these ;tans, all grinding of the pulp is done, the product of ten stamps passing through two in succession. By their use the capacity of the stamps is increased, as coarser screens can be used in the batteries, and finer grinding is done in the pans. If the pulp needs chemical treatment before amalgamation it is run from the grinding-pans into a chemical mixer, where the proper chemicals are added, and thence runs to the first amalgamating pan of a series placed in hue. From the first pan the pulp flows continnonsly through the pans and settlers, they all being connected together by pipes near the tops of their sides, and one overflowing into the other through the line.
The amalgamating pans are charged with quicksilver by means of pipes leading from the distributing tank, and the amalgam is drawn oil through pipes to the strainers in front of pans. here a quicksilver elevator lifts the strained quicksilver back to the distributing tank, and all handling of that metal is thus avoided.
The pans and settlers are all placed upon the same frame and upon the same level, and each is driven by means of gears brought into action by a friction-elutch fitted to the gear on the main-line shaft. This arrangement, peculiar to the Boss process mill, is one of the leading features in the mechanical construction of this system. By it the pans and settlers are brought down close to their main driving-shaft., and receive their motion directly from it without the intervention of belts, tighteners, counter-shafts, and high-pan frames, as in the old-style mill. Each pan and settler being thus provided with a separate clutch, any pan or settler, or any number of either, can be stopped independently of the others, in case of acci dent or for cleaning-ont purposes. In order to secure the continuous flow of the pulp through the line when one or more pans or settlers are stopped, steam siphons are provided for carry ing the pulp past them and eutting out for repairs or cleaning.
In amalgamation by this method, the pulp comes to the pans in even and regular propor tions of sand and slimes. It all necessarily passes through the series of pans and settlers. and receives a uniform treatment. This uniformity and regularity, it is claimed, can not be attained when the pulp is settled in tanks and worked by charges. The amount of treatment the pulp receives in its passage is proportionate to the size of stream that flows into the pans; and the amount of treatment may be increased by decreasing the inflow, or vice versa. It is also claimed that the loss of quicksilver is less in this than in the ordinary process of pan amalgamation.
The process of lixiviation for the treatment of silver-ores was first introduced in this country, or rather in Mexico. by Mr. Ottokar Hofmann in 1868. the Patera process being used. During the past ten years great progress has been made in this direction, the decade having been marked by tire invention of the Russell process. The general process of silver lixiviation consists in roasting the are to convert its silver contents into the form of chloride, then dissolving the chloride of silver in hyposulphite of sodium, precipitating the silver as sulphide with sodium or calcium polysulphide, and refining the silver sulphide to bullion. The ore is first crushed by a rock-breaker, and then pulverized by stamps or rolls, the former being more generally employed than the latter ; although rolls have advocates. The crushed ore is subjected to a chloridizing roasting, as in amalgamation, the amount of salt used varying with the character of the ore, ranging from 4 to 10 per cent, or thereabouts. The roasted ore, after leaving the furnace, is spread on the cooling-floor, moistened with water, and charged into vats in lots of from 8 to 15 tons. These vats, which constitute the leaching-tubs, are provided with a central discharge, around which a filter bottom is arranged in the shape of it very flat funnel. The filter-cloth is kept in place by ropes driven into grooves around the discharge hole and the inner periphery of the vat near the filter bottom. The vat is furthermore pro vided with an outlet under the filter bottom, and has a slight inclination toward this outlet.