ORE DRESSING AND MILLING (PART 1) ORE DRESSING ORE DRESSING MACHINERY ROCK BREAKERS 1. Classification.-Under the heading of ore dressing are included those processes by which the miner prepares his ore for milling, smelting, or sale. The old method of cobbing ore is practiced in the mine and on the dump, consequently does not enter into this discussion, since it is presumed that ore has been assorted before it reaches the mill. There are two classes of machines used in ore dressing: one class break coarse material and are called rock crushers; the other class crush fine material, that is, pieces smaller than inch in diameter, and are termed fine crushers. It is true fine crushers sometimes approach pulverizers in their product, but it is not usual or desirable to pulverize ore to impalpable powder for milling purposes, consequently pulverizers are seldom needed. 2. Rock Breakers.—Ore as it reaches the mill is usually in lumps varying in weight from a few ounces to many pounds. To reduce the size so that fine crushers can work it finer, it is usual to employ a class of machines known as rock breakers. These machines are limited to two classes, termed jaw and gyratory crushers, on account of the movement of their parts in performing work. For notice of copyright, see page immediately following the title page. 3. Blake Crusher.-The Blake crusher shown in Fig. 1 has the entire frame cast in one piece. The swinging. jaw b is pivoted at h, so that its greatest movement is at the discharge opening. FIG. 1 At each full throw of the jaw the discharge opening varies, thereby causing a corresponding variation in the size of the product, and the only control the operator can have over the latter is to set the jaw by means of the wedge w and setscrew s, thus fixing the maximum size of rock that shall pass through the opening. The pulley r, which travels at the rate of from 225 to 250 revolutions per minute, operates an eccentric on the main shaft g. As the pitman d is raised and lowered with each revolution of the eccentric, the toggle plates p impart a reciprocating motion to the jaw b. are lubricated through the tubes t. nects the jaw b with a coiled steel or rubber spring, insuring a rapid return of the jaw after each stroke. The jaw plates shown at are usually cast with vertical corrugations, in order to give them a larger crushing area and better bite. The stationary jaw k, against which the plate is held by the check plates i, is bedded in zinc inch thick, directly against the frame. TABLE I The steel bearings e A tension rod m con 4. Dodge Crusher.-The Dodge crusher shown in Fig. 2 has an eccentrice on the shaft g which gives a rocking motion to the lever /. The lever is cast in one piece with the jaw b and pivoted on the shaft h. The stationary jaw a, as in the Blake crusher, is cast as part of the frame. The jaw shaft rests in sliding boxes, provided with setscrews s to regulate the size of the product. The setscrews should always be set tight, with locknuts set, while crushing. Packing strips p are used between the frame and the shaft boxes to take up H wear. The shaft makes from 200 to 350 revolutions per minute. This crusher has the discharge opening near the center of motion of the movable jaw, hence the variation in the discharge opening is so slight that a practically uniform product is obtained. This feature of the Dodge crusher has led to its adoption as an intermediate crusher between the coarse and the fine crushers in large mills. The vibration caused by jaw crushers is sometimes destructive to the building. They should be therefore set on timbers or foundations independent of the rest of the framework, especially where there are leaching tanks; otherwise they may throb in unison with the crushers. FIG. 3 5. The capacity of the Blake crusher is much larger, on account of the variable discharge opening, which reduces the tendency of the ore to clog; but at the same time this renders crushing to any degree of fineness and uniformity impossible. The Blake crusher is therefore used where the |