CRANE.| | Theory of Ore Concentration. 335 
screen, the greater the capacity up to a certain limit; for the 
higher a grain is carried before it rolls back, the more rapid is 
its discharge from the screen. The most common speeds are 
from sixteen to twenty revolutions per minute. Fora screen 
to work to best advantage, the material screened should roll 
through, not be forced or washed along. 
The capacity of revolving screens varies with the speed, in- 
clination, length of surface, and feed. ‘The first foot or two 
of the surface does most of the screening; so several short 
drums might be better than one long one, although the ex- 
pense would be higher; but this must not be carried too far, 
or undersizing and low capacity will result. 
Sizing may be accomplished by tables (stationary and shak- 
ing) and buddles, on which concentration is effected by min- 
eral moving on inclined surfaces where friction acts. 
Separation depends upon: 
1. The inclination, which determines whether the grains have the ability 
to overcome the friction of the table. 
. The velocity of the water current. 
. Viscosity of slimes. 
. The relative size of the ore particles. 
. Specific gravity of the mineral. 
Imparting to the table a shaking motion, thus allowing inertia to act. 
Dow ow bo 
Careful adjustment gives an inclination at which one min- 
eral moves on the table, while another will not, due to the 
difference in specific gravity of the two. 
The inclination decreases with the size of the particles 
treated. Of two grains of equal size, the lighter one is the 
more easily moved. 
The velocity also depends on inclination and the amount of 
water that is used. 
The relative size of ore grains has a ruling influence on their 
movement. A wide range of sizes is the best for separation. 
The action of water currents, flowing over inclined surfaces, 
on ore particles of different specific gravities, is to move the 
lighter; the action is partially due to sorting and sizing. 
By giving a jigging or shaking motion to the table, the ac- 
tion varies with the mass and not according to the size or 
density ; thus inertia is brought into play, modifying the 
