a—4.

79

Although these mills are owned by four separate companies, they are all, with the exception of the last, under the management of the Homestake superintendent. Thus it follows that the working details of the six mills are all, as far as practicable, after one model—the Homestake. The Caledonia mill stands alone, and works on a very different rock. Therefore the details of the plant differ considerably from those of the others. The details will therefore be summarised under two heads—Homestake and Caledonia. " Outline of Plant and Process. '' The crushing is done by means of rock-breakers and stamps. The breakers first reduce the ore to a size suitable for stamps. The ore arriving at the highest level of the mill in the mine-cars, is discharged from the side or bottom of the car over grizzlies to the crushing-floor, or it goes direct to the crusher-hopper. The small-ore particles passing through the grates of the sizing-screen, and the coarse ore, which has been reduced by the rock-breaker, both drop into the same ore-bin, which reaches down to the cam-floor. Water is fed continuously into the mortars, and forms with the ore a liquid pulp, which passes through a screen at the front on to and over the apron-plates on the lower floor of the building. The Caledonia mill has blankets on the lower end of these plates to catch any coarse, heavy particles; in the other mills the pulp passes directly from the apron-plates to the mercury plates, and through them on to sluice-plates. From the traps placed at one end of these the pulp runs into one main sluice, which may again have one or more traps before the pulp is finally allowed to run to waste. Thus the entire process of passing the auriferous coarse rock from the ore-floor to the final discharge at the end of the main sluice is an automatic one. " Battery amalgamation is used to extract the gold. It begins in the mortar, where mercury is added at intervals—while the continuous fine crushing with stamps is taking place—and ends on the apron-plates, where nearly all the amalgam not retained by the inside amalgamated copper plates is daily collected, any deficiency in the collecting mercury and amalgam on the plates being supplemented by the various traps. As the mills on the belt have to treat low-grade ores, it is necessary to their profitable operations that large amounts should be put through as rapidly as may be, and that at the same time as much gold as possible should be saved by simple means. To effect this a compromise is made between the two extreme methods of gold-milling. One of these aims at extracting as much gold as possible in the battery at the expense of capacity ; the other, by amalgamating outside the battery, increases the crushing-capacity, but requires a number of expensive operations to recover the gold. In the Black Hills amalgamation is carried on both inside and outside the battery, thus combining the simple way of recovering the gold from the first method with the large capacity of the second. The aim is to crush rapidly to the desired fineness, and arrange the amalgamation so that it shall be adapted to the large amount of pulp produced. " General Features of the Mills. " The following table gives a comparative view of the dimensions, power, batteries, and product of the mills :—

" The following is an explanation of the letters employed in the table :— "A. Horizontal 155-horse-power engine, with Meyer's cut-off. B. 300-horse-power Corliss engine. C. 350-horse-power Corliss engine, Eraser and Chalmers pattern. D. Two 60-horse-power horizontal engines, with automatic cut-off. (a.) Length and width are always of mill proper, excluding the engine-room, (b.) The boilers in the table are all horizontal, tubular, 54in. in diameter, 16ft. long, with 16 tubes of 3|in. in diameter. Steam-pressure is kept at 901b. per square inch, (c.) Made up as follows : Wrought-iron stern—length, lift.; diameter, 3-Jin.; weight, 3101b. : cast-iron-head, 2101b.—height, 18in.; diameter at top, 9in.; at bottom, Bin.: cast-iron shoe, 1401b.—cylindrical butt, Bin. high and BJin. diameter; tapering shank, 4Jin. diameter at the base, 3|in. at top, and sin. long, the shoe being made of white iron, chilled for 6Jin. from the base, the next 1-Jin. and the shank being cast in sand and cooled slowly : cast-iron gib-tappet or disc—-diameter at ends, 9Jin.; in the middle cylindrical part, 6in.; wearing-faces, 2Jin. thick ; middle part, 7in. long; total length, 12in.; weight, 1301b. Total, 8501b, stem, head, shoe, and disc or tappet, being proportioned as 31, 24, 11, 13. (d.) Batteries in two rows, back to back, leaving this space between the rows for orebins and feeders, (c.) Prom June, 1887, to May, 1888, inclusive. (/.) Estimated from report of Homestake Company, June, 1888, when product of 200 stamps in company's mill is given as 213,355 tons for the year ending the 30th May, 1888, or 1,216-775 tons per stamp. (■&.) Batteries in single line. (I.) Batteries in two rows, but face to face, with this space between them, (m.) Intentionally slower, crushing on harder rock than the other mills, (n.) For year ending the 30th April, 1888.

Dim iens: ions (a). Boi ilera (6). Hginos. 'ive-Btamp iatl :criei roduct. Name of Mill. ft a jo is Swas ri [ TO ' Op4 Q gis-i! H O O, 0J w n H 3 % 'A -Is w 3 < £■£ O g53 . 'o%SK a H Cylinders. u a £ .s s o Inches. 20 42 20 60 26 60 20 42 18(A)42 20 42 20 30 i 6 3 'A II S ro I u ;l p a sag Hit IP Tons milled in One Year. Bullion produced. <1 to a .-1 Homesfcake .. Golden Star.. Highland Deadvvood .. Golden Terra Fath'rdeSmet Caledonia 100 120 112 112 112 68 84 80 88J 92 46 46 60 461 14J 2 28} 4 22$ 4 22|. 2 22f 2 40 2 2 11 14 14 11 11 11 A B 0 A A A .1) 16 24 24 16(i) 16(i) 20 12 Lb. 44J (d) 850 36 (d)850 46 cZ)|850 k 850i k 850] 36 (I) 850 A; l 850 9 9 9 9 9 9 12 85 85 85 85 85 85 71 4-5 4-5 4-5 4-6 4-5 4-5 8-3 96,790 («) 146,565 (e)\ 146,013 (/); £ 1184,445 83,902 s. d. 15 2 11 o| 216,361 (/)l 130,509 12 0j 73,422 (») 60,395 16 8