DRESSING OF ORES 



85 



If the ore is of a massive character and of little value it may be selected from the 

 waste at the stone-breaker ; but if much intermixed with other minerals, it will be neces- 

 sary to submit it to the crushing, stamping, or grinding mill. In the employment of 

 either apparatus the object should be to secure free and unbroken grains of ore so as 

 to readily collect them in the subsequent dressing processes. 



The general application of reduction machinery may bo briefly stated: Stone- 

 breakers, for tin, ironstone, copper, and lead ore dradge ; size of stuff, from 1 to 

 9 inches. Crushing mills ; for copper, and lead ore ; in pieces, from 2 to 50 milli- 

 meters diameter. Stamps, for tin, lead, and copper ore, in grains, and pieces, from to 

 75 millimeters. Grinding apparatus, for tin, silver, and auriferous ore, in grains, 

 from to 2 millimeters size. 



Crushing Mill. This machine was introduced by the late Mr. Taylor at Wheal 

 Crowndale, near Tavistock, in the year 1804. In its simplest form it consists of 

 two rollers mounted in a strong iron frame, and kept in contact by means of screws ; 

 motion is communicated to one of the rolls, either by a water-wheel or steam-engine, 

 but the other is made to revolve by the friction generated between the moving roll 

 and the stuff to be crushed. This mill is usually employed for reducing mineral sub- 

 stances which have already received some mechanical preparation, but machines have 

 been contrived with a series of rolls, set below each other, into which the stuff is 

 introduced as brought from the lode underground. In order to effect this operation, 

 the upper rolls are fluted, and the lower ones have various speeds and diameters, but 

 it may be remarked that although this arrangement has been somewhat extensively 

 employed in the north of England, yet it has found few advocates either in "Wales or 

 Cornwall, or on the Continent. 



The practice of keeping the rolls together by screws acting on the bearings is ob- 

 jectionable, since the entrance of a piece of steel, or other hard substance of greater 

 width than the fixed opening between the rolls, immediately produces a stoppage and 

 strains the apparatus, or otherwise causes serious breakages to some of the parts. In 

 order to obviate these evils, the rolls are usually adjusted and kept in position by 

 weighted levers pressing on their axis. 



As the machines employed in Cornwall may be considered the most effective in 

 operation, as well as complete in their construction, that type is selected for repre- 

 sentation. 



B B (fig. 662), are the crushing rollers fitted in a strong frame-work of cast iron, 

 which is stayed by a wrought-iron bar b, and firmly bolted to longitudinal beams in- 



serted in the walls of the crushing-house. The rollers revolve in bearings, which are 

 so arranged as to slide in grooves, and therefore admit of the cylinders being brought 



