ORE-CRUSHING MACHINES. Machines for crushing ore or rock may be divided into two classes, coarse-crushing and fine-crushing. The ore as received from the mine is usually in large pieces, requiring a preliminary breaking, which is accomplished by machines of the first class, before the succeeding and final crushing, which is done by machines of the second class. The coarse-crushing machines in general use are nearly all of the jaw type, most of them being modifications of the well-known Blake rock-breaker. Fine-crushing machines are of two kinds, those in which the rock is broken by direct pressure. or impact, as in the case of stamps, rolls, and mills, and those in which the comminution is effected by a grinding motion. The selection of any particular machine depends upon the purpose for which the rock or ore is to be ground. It is obvious that in pulverizing to any given size, a greater proportion of fine material will be made by the machines which have a grinding action, in which the distance to which (lie and shoe approach each other, as in the case of rolls and multiple-jaw crushers, can be regulated, than by those which act by a direct blow.
In some work, such as crushing ore for mechanical dressing by water, it is desirable to make as small a proportion of slimes as possible ; hence rolls are commonly used. In crushing ore for gold and silver milling, where it is necessary that the whole shall be crushed fine, and the proportion of slimes is not of so much consequence, stamps are used. For exces sively fine grinding, as is desirable in pulverizing phosphate rock for the manufacture of superphosphate, for comminuting ochre, etc., for mineral paint, and work of like kind, the grinding machines are well adapted and are generally employed.