DRESSING OF ORES 97 



Fig. 673 represents the elevation of a steam stamps employed in Cornwall. A, 

 axle ; B, cams for lifting heads ; c, tongue or projection on lifter ; D D, guides for re- 

 taining lifter ; E, the lifter ; F, head of pestle ; o, chest or coffer ; H, hopper ; j, pass 

 connecting coffer and hopper; K, launder discharging water into the coffer; L, stamps 

 grate ; M, launder receiving the stuff" which has been flushed through the grates ; N, 

 the bottom or bed of stamps. 



The stamping process is not so simple as it may appear at first sight. Many of its 

 particulars, such as the form of the coffer, mode of exit for the stuff, weight and 

 rapidity of the pestles, and quantity of water employed, must be varied to suit the 

 mode of dissemination and the structure and character of the ore, as well as of the 

 matrix. Fineness of reduction is by no means always a desideratum, for if some kinds 

 of stuff be reduced too low, much of the ore contained in it will be wasted, hence 

 considerable judgment is necessary in selecting the grate best adapted to the stuff 

 to be operated upon. Sometimes the grate is replaced by the ' flash,' which con- 

 sists of a small hopper-shaped box, fitted to the front of the grate-hole. This box 

 is provided with a shutter which is raised or lowered according as the ore is 

 required in a fine or rough state. In dry stamping the fineness of the powder 

 depends not on the grate, but on the weight of the pestles, the height of their 

 fall, and the period of their action upon the substances beneath them. The follow- 

 ing practical results are derived from the steam stamps at Polberro Tin Mines, 

 Cornwall : 



Cylinder of engine, 36 inches diameter. 



Diameter of the fly-wheels, 30 feet. 



Weight of ditto, with cranks, shaft, and bolts, 42 tons. 



Power employed, 55 horses. 



Eeduced in 12 months, 30,201 tons of vein-stuff. 



Average number of revolutions of stamp axles per minute, 8. 



Number of heads lifted per minute, 72, each 9 inches high. 



Weight of each head, 600 Ibs. 



Average number of blows made per minute by each head, 45. y ( 



Weight of heads collectively, 19 tons. 



Number of grates, 72. 



Exposed area of front grates, 9 x 6 = 54 inches. 

 Ditto of end grates, 8 x 6 =48 inches. 



Number of holes to the square inch, 140. 



One of the most powerful and best arranged stamping engines in Cornwall, designed 

 iiutl constructed by Messrs. West and Sons, is at the West Phoenix Mines. Two ver- 

 tical cylinders are employed, each 26 inches diameter, with 7-feet stroke. The weight 

 of fly-wheel is 11 tons ; speed of piston, 240 feet per minute. The length of stroke 

 given by the cranks is 84 inches. The stamp axles, right and left of the engine, make 

 12 revolutions per minute, and drive at one and the same time 96 heads. The 

 average weight of each stamp head is 600 Ibs. ; new, 800 Ibs. ; when worn out, 

 150 Ibs.; dimensions of head, 20' / x7"xll", giving a sectional area of 77 inches. 

 The number of drops per minute is 60 ; average drop, 10 inches. Number of heads 

 in a battery, 4. Area of grate per battery box, 336 square inches, or 84 inches per 

 head. Diameter of holes in grates, millimeter. Weight of tin-stuff stamped per 

 24 hours, 18 cwts. per head. Average duration of head, 3 months. Weight of stuff 

 stamped per head, 80 tons. Loss of iron per ton of stuff, from 9 to 10 Ibs. Cost 

 of stamp heads per cwt., 11s. Average weight of coal required to stamp 1 ton of 

 stuff, 1 cwt. 



The consumption of coal per 24 hours, is 4 tons, cost 71. 4s. Qd. Engine driver's 

 wages, 10s. Oil and grease, per month, 4. The direct approximate cost of stamping, 

 per ton of stuff, is therefore, 



s. d. 



Wear of stamp head . . . .10 

 Coal . 1 9J 



Oil and grease ''". ;: . ..' &- . ; ' 



Total . 2 10 



Fig. 674 represents a stamping mill arranged for the reduction of auriferous 

 quartz, silver, and other valuable ores. The base consists of a cast-iron plate A, upon 

 which is screwed a frame B, without end or side joints. A thick cast-iron bottom is 

 dropped into this frame, upon which the stuff is pulverised. The coffer c, is made 

 of cast-iron plates, bolted together at their edges. Each head is in connection with 

 a grate or perforated plate, The feed is effected by means of an Archimedean screw, 



VOL. II. H 



