THE MANUFACTURE OF ACID PHOSPHATE. 17 
the mixture can be dumped without fear of its cooling before the final 
chemical changes take place. 
MIXING THE ACID AND ROCK. 
When the manufacture of acid phosphate was first suggested by 
Liebig, and for many years afterwards, the mixing process was all 
done by hand. Certain proportions of acid and rock were dumped 
into shallow, open pits or troughs and worked with hoes in a manner 
similar to that in which mortar is mixed. The material was then 
allowed to stand in the pit until the chemical reactions were complete 
and the superphosphate dry enough to be dug out. The modern 
factory process has almost entirely displaced this pioneer method of 
making acid phosphate. Machines are now employed which are 
capable of mixing efficiently, in two to five minutes, quantities of 
rock and acid weighing from 1 to 2 tons. 
There are several types' of mixers or acidulators used in this country, 
and the following general description applies to almost all of those 
ordinarily employed: A cast-iron revolving pan from 4 to 8 feet in 
diameter and from 1 to 2 feet deep is driven by pinions. The pan is 
equipped with either one or two agitators or stirring devices which 
consist of heavy iron spiders having four arms fitted with steel plows. 
These stirrers are driven by bevel gears. In the center of the pan is 
the discharge hole, which is closed by a valve operated by a lever. 
This same lever also controls a scraper which is lowered into the pan 
as the plug is raised. Figures 1 and 2, Plate IV, show the construc- 
tion of an efficient acidulator. 
The weighed charges of acid and rock are run into the revolving 
^ acidulator at the same time and stirred for at least two minutes or 
until an intimate mixture is obtained. The lever which controls the 
scraper and discharge valve is then lowered and the material ejected 
into the "den" directly below or into a car which hauls it to the 
storage shed and dumps it on a pile. 
Both the "den" and "open dump" systems of making acid phos- 
phate are employed in this country. Each has points to recommend 
it and each has certain objectionable features. 
THE DEN SYSTEM. 
This system was devised in order that the reactions between the 
phosphate rock and sulphuric acid might take place rapidly and 
yield a dry, pulverulent product of high availability (so called) in the 
least possible time. 
As fast as the charges of acid and rock are mixed they are dropped 
into a closed, brick-lined chamber (den), which is filled to within a 
short distance of its top. Here the chemical reactions taking place 
