Crushing.—Unless an ore occurs as a placer or gravel deposit, it must first be crushed to free the valuable minerals from the attached gangue and the amount of crushing necessary will depend on the fineness of dissemination of the mineral. Furthermore, the final separation of the crushed material usually involves pre liminary preparation by sizing or classification to yield a series of graded products going to different types of concentrating machines since no one machine can make a satisfactory separation when fed with a mixture of coarse and fine. When all the ore is reduced directly to a fine state, as for flotation, this pre liminary grading is unnecessary.
Crushing customarily proceeds in steps and the best practice is to remove from the feed to any one crushing machine all particles which are already as fine as the product of that machine.
Some machines, such as stamps, have a screen as an integral part of the machine to guarantee that the product shall all be below a specified maximum size while other machines, such as rolls and ball mills, require that a screen or classifier be con nected in "closed circuit" with the crushing device to size out the finished product and return the coarse unfinished material for further reduction. Roughly, the field of coarse crushing in volves the use of breakers of the Blake or gyratory type which receive lumps up to 5 or 6 ft. in diameter and reduce in a series
of steps, with a reduction in size of about one-quarter at each step, down to a final product i or II in. in size. For intermediate crushing, there are rolls, steam stamps, Symons disc crushers, both horizontal and vertical and Symons cone crushers. Rolls handle feeds up to i or 2 in., make about one-fourth reduction in size and can crush economically down to s or A inch. Steam stamps, which are limited to use on the special problem of native copper ore, strike an extremely powerful blow by a steam-driven pestle in a mortar. The feed is 2 or 3 in. in size and the product + or inch. The Symons machines are relatively new but are finding wide application in the field of crushing between 6 or 8 in.
feed and around I in. product. In the Symons cone, the central gyrating cone is very flat and is overlain instead of surrounded by the fixed ring. The Symons disc crushers have a wobbling or tipping disc and a fixed disc. At a given instant, a point on the circumference of the wobbling disc is approaching the fixed disc while a point 580° away or diametrically opposite, is receding. This motion progresses around the circle so that in a complete wobble the space between the discs has opened and closed at every point. Many devices have been used for fine crushing or grinding but the more important are gravity stamps, roller mills, grinding pans, ball mills, rod mills, tube mills, Chile edgestone mills, arrastras and hinged hammer mills. The gravity stamp is discussed under AMALGAMATION : In Metallurgy. The grinding pan is a modification of the amalgamation pan which is also referred to under Amalgamation. The arrastra or drag-stone mill grinds the ore by dragging flat stones around a circular stone pavement. Roller mills employ cylinders which roll around inside a circular ring and ore is crushed between the rollers and the ring by pressure developed through centrifugal force or springs. Chile mills have heavy horizontal rollers travelling around a circular path. Hinged hammer mills and other forms of beating mills utilize the crushing effect of blows delivered upon the ore in space by arms or blades revolving at very high speed. Ball mills, rod mills and tube mills are horizontal revolving cylinders in which are iron balls, steel rods or flint pebbles to grind the ore as it passes through the cylinder from end to end. These revolv ing mills are used almost universally in ore dressing for wet fine grinding of ore for tabling and flotation, in gold and silver milling for preparing ore for cyaniding and in reducing all sorts of com mercial products wet or dry, even down to impalpable powder.