THE MANUFACTURE OF ACID PHOSPHATE. 13 
Mills of the first type occupy but little space, do not require auxil- 
iary screens and conveyors, and the grinding is all finished in one 
operation. The fact, however, that the pulverized phosphate is not 
separated from the coarser rock until forced through the screens 
within the mill cuts down somewhat the efficiency of this type of 
machine, since considerable space and power is always taken up by 
material already ground. Another disadvantage of this type of mill 
is that any clogging of the screens or break in the same necessitates 
the shutting down of the entire mill while the damage is repaired. 
The manufacturers of the o her type of mill claim to have overcome 
these disadvantages in their machines; the ground material is con- 
tinually discharged from the mill and separated from the coarser 
rock by passing through revolving, or over vibrating screens. Any 
trouble with a screen can be corrected without stopping the mill by 
simply cutting off the supply of rock to that screen. Since the mate- 
rial flows over the screens instead of being forced against them as in 
the case of the other type of mill, the repairs necessary on the screens 
are reduced to a minimum. This type of mill, however, with its 
auxiliary screens and conveyors, takes up considerable space, costs 
more to install, and requires a greater amount of supervision. In 
Plates I and II these two types of roller mills are shown. 
The amount of material which can be pulverized per hour depends 
on the size of the mill, the character of the phosphate rock used, and 
the degree of fineness to which it is ground. A mill of the size usually 
employed in fertilizer factories may grind from 10 to 12 tons per hour 
of Tennessee brown rock phosphate to pass a 60-mesh sieve, but this 
mill will probably not grind more than seven or eight tons of pebble 
phosphate to the same degree of fineness in that period of time. 
A very ingenious ball mill is that of Pfeiffer. 1 The grinding is done 
by means of steel balls or flint pebbles and the separation of the fine 
material from the coarse is effected by means of a current of air. 
All loss of time due to the clogging and repairing of screens is thus 
avoided and a product of any degree of fineness can be obtained by 
simply regulating the strength of the air current. 
The degree of fineness to which phosphate rock is ground often 
has a very important effect on the acid phosphate produced there- 
from. Phosphate rocks low in carbonates and high in iron and 
aluminum are but slowly acted upon by sulphuric acid, and should 
therefore be gound very fine. Phosphates containing large quan- 
tities of carbonate of lime are acted upon quite rapidly, and conse- 
quently do not require extremely fine grinding. In ordinary practice 
the rock is usually ground so that 80 to 90 per cent will pass a 60- 
mesh sieve, but in working with less soluble phosphates it is fre- 
quently desirable to grind them so that 80 to 85 per cent will pass an 
80-mesh sieve. 
1 German patent No. 110,195. 
