The metalliferous rock or ore as broken out by the explosive is prepared for the ex traction of its valuable constituent by processes to which the designation "ore has been given.
Ore dressing consists of two processes crushing and separation, which employ two dis tinct types of machinery.
By crushing, the material is broken up into coarse fragments, or ground into a fine powder, and the valuable ore detached from the worth less rock. In some mines the coarse crushing is done underground.
By separation, the valuable ore is concen trated into smaller bulk and weight by being separated from the waste, or in the case of two valuable ores, they are separated from one another. The term "concentration' is spe cifically used for a separation by mechanical means as distinguished from that accomplished by hand.
The selection of the particular types of crushing and grinding machinery is always de termined by the character of the ore and the process of extraction for which the ore is prepared.
The processes of extraction may be briefly defined as follows: (1) Smelting processes in which the pure metal is extracted from its ore by fire; (2) amalgamation process in which the metal forms an amalgam with mercury from which it is subsequently separated by the dis tillation of the latter; (3) chemical processes in which the metal is dissolved by various solu tions and then precipitated in solid form by suitable precipitations; (4) electrolytic proc esses in which the extraction is accomplished by electrolysis.
The crushing and grinding machinery used in mining consists of the following classes of machines: Rolls.— These are the standard machines for crushing brittle ores which are being prepared for concentration except where fine crushing is required.
Steam stamps are the standard machines for crushing ores containing native copper.
Gravity stamps are the machines most ex tensively used for fine crushing.
Tube mills are also quite extensively used for fine crushing and are the most efficient com petitors of the gravity stamps, and in many gold mines are used to follow the latter for the finest grinding.
Pulverizers are used for crushing and grind ing ore as it comes from the crusher or rolls, for fine concentration or for the of middlings and tailings from jigs for subsequent treatment on concentrating tables or other con centrating machines.
The preliminary crushing is usually accom plished by jaw or gyratory crushers and then the material is passed over to the rolls, stamps or ball mills. The selection of screens having
the proper mesh to crush through is one of the most important details. For purposes of con centration it is usually advantageous to begin by crushing to a coarse size, separating as much of the waste as possible, then recrushing to a finer size and again separating the waste. In crushing gold ores for subsequent treatment by the cyanide process, fine crushing gives a higher extraction than coarse crushing.
The following examples have been selected to illustrate the various classes of these ma chines: Fig. 1 shows a general view of a Blake Crusher, a machine of the "jaw-crusher' type. They are made in dimensions which provide a capacity of receiving rocks ranging from inches by 3 inches to 15 inches by 24 inches in size, which they reduce to fragments ranging from 1 inch to 3 inches in size, at the rate of 15 to 250 tons per 10 hours of work, according to the size of the machines.
They range in power from one-half horse power in those weighing 1,000 pounds to 25 horse power in those weighing 26,000 pounds, Fig. 3 shows a general view of a set of crushing rolls. It consists of a set of station ary and a set of movable rolls; an automatic feed hopper built in sections, so that the wear ing parts may be easily replaced; a dust cover; and require a speed of 225 to 250 revolutions per minute for their operation.
Fig. 2 shows a general view of an Austin Gyratory Rock Breaker. It represents one of the strongest and most durable of this class of crushers. It is made in sizes which have re ceiving openings ranging from 4 inches by 5 inches to 18 inches by 63 inches, and have re duction capacities ranging from 2 to 200 tons per hour, according to the fineness to which and two extra heavy, band wheels for the driv ing belts. The main frame and the journals for the stationary rolls are cast in one piece. The movable journal is held in the centre of the main frame by means of a heavy steel shaft, and the swinging journals are held in place by tension rods which are attached to nests of powerful coiled springs between washers on the rods. These springs are stiff enough to resist the pressure imposed upon them by ordinary crushing without being compressed, and yield the material is reduced and the size of the ma chine. In weight they range from 3,000 to 100,000 pounds, and require from 4 to 150 horse Power to drive them at speeds ranging from MO to 500 revolutions per minute.