326 University Geological Survey of Kansas. 
shaking screens, are located at various points throughout the 
mill to regulate the size of material delivered to the various 
concentrating apparatus. The more common form is the 
revolving screen or trommel, which is usually interposed be- 
tween a set of rolls and a jig. These separate the coarser 
from the finer material, thus preventing a waste of time 
and energy in treating material (in a certain machine) which 
is undersized or oversized. 
CONCENTRATION. 
The ore on entering the mill may be hand sorted and sized, 
as referred to above, and is then fed to the crusher, usually 
the Blake type (9"x15”). From the crusher, the partly re- 
duced ore, ranging in size from one-half to one inch in di- 
ameter, is fed to the first set of rolls (usually twenty-four 
to thirty inches in diameter by fourteen to sixteen inches in 
width) placed below and in front of the crusher frame, and 
is fed by wooden spouts, often lined with iron, which extend 
from the crusher to the rolls below. ‘The ore is then delivered 
to an elevator, which in turn delivers it to the first revolving 
screen (one-half to five-eighths inch mesh). 
Any oversized material not passing through the mesh of 
the screen is returned to the second set of rolls, usually of the 
same size as the first set, but set closer together, and is again 
run through the screen. This second operation is repeated 
until all is reduced to the size of the mesh of the screen and 
under. This second set of rolls must be set to crush finer 
than the first set, in order to insure the passing of all the 
material through the screen on its second passage. The 
product passing the screen, or the undersize, is delivered to a 
five cell, or compartment, rougher jig. (Size of cells, 30x42 
inches. ) 
The compartments of the rougher jig are provided with 
grating of one-eighth inch openings. 
The shaft speed of the rougher is in the neighborhood of 
175 revolutions per minute. The free chert or tailings may 
pass from the jig to the tailings pile, but to insure against 
any possible loss of mineral they are usually sent to the 
