Mr. W. A. Wilson, superintendent of the Daly (Marsac) mill, in the Engineering and slfiuing Jon-saul, vol. 1, p. 444, gives the following items of comparison at these two mills: The percentage of silver recovered in the Daly mill has reached as high as 92.2 per cent.
ORE-DRIERS.—The revolving ore-drier, which is generally used for drying ores in dry crushing silver-mills, is a long east-iron cylinder with a stationary fire-box at one end and a stationary flue at the other, the flames drawing through the cylinder as in the Brtickner, Hofmann, and Howell-White roasting-furnaces, although, of course, only a moderate heat is used. This drier, together with the shelf-drier, has entirely replaced the old boiler-iron floor drier. The revolving cylinder is usually made in several sections for convenience of hand ling, having two tracks or tires on which it rotates, supported by rollers underneath. The motion is transmitted through gearing and pulleys. The cylinder is of larger diameter at the fire end than at the flue end, and ore from the rock-breaker is fed in at the smaller end. The cylinder's axis is placed horizontally, but owing to its conical form the ore must travel gradually toward the fire at the larger end. Shelves or wings arranged spirally inside raise the ore and shower it through the flames, assisting to quickly and thoroughly dry it. The size of the drier, as commonly used, is 44 in. diameter at the large end, 36 in. diameter at the small end, and 18 ft. long. Its capacity is 30 to 40 tons per 24 hours. It requires about 1,100
fire-bricks for lining this drier, and about 12,000 common bricks for a stack about 40 ft. high.
The Kiln consists of a number of inclined shelves, which are arranged zigzag above each other in a vertical shaft, having openings or slits where they meet, on which the ore rests in a stratum, the thickness of which is governed by the width of the slits and the in clination of the shelves. If a portion of the charge is removed at the end of the bottom shelf a sliding motion of the ore takes place on all shelves above, and the top shelf is replen ished from a hopper set over it. It will be seen that the shaft is divided by the shelves into a number of triangular prismatic spaces. Through these the hot gases from a fireplace are made to circulate, each space communicating with the next one by a flue arranged in the side-wall of the shaft. These flues being located on alternate sides of the shaft, a continuous passage is formed through the whole structure. The kiln is 21 ft. high from the discharge floor to the top of the feed-hopper; the shelves are 2 ft. 4 in. wide and 5 ft. long, inclined at an angle of 38°. The stack should not rise more than 30 ft. above the top of the kiln. A double kiln requires a draft area of 3 sq. ft., and has a capacity of from 30 to 50 tons per 24 hours. The quantity of brick required for a double kiln is 30,000, and of iron 25,000 lbs.