MANUFACTURE OF SUPERPHOSPHATE 71 



The pan is made of cast iron, with 2 per cent of 

 a special alloy which renders it very resistant to 

 acid. The arms of the agitator and the blades as 

 well as the valves are of cast steel. The mixing 

 shaft makes sixty turns a minute; the mixing is 

 triturated until the pulverized phosphate is fine 

 enough to pass through a 70-mesh sieve. As the 

 mixture remains longer in the liquid state, the 

 the length of time occupied in mixing must be pro- 

 longed. The acid, contained in a lead-lined tank, 

 is drawn into a measuring tank by turning a valve; 

 it then flows through a 2-inch lead pipe into the 

 mixer in the form of a fine spray. At the same time 

 the crushed phosphate previously weighed and 

 laid on sacks on two inclined planes to right and 

 to left of the mixer is. run into the mixer. In 

 certain factories the phosphate is brought to the 

 mixer by an elevator, and received in buckets by 

 means of which it is run into the mixer. The bags 

 retain about 1 per cent of phosphate in the fabric. 

 The mixer can take a charge of about 495 to 550 

 pounds. When the phosphate is rich in carbonate 

 of lime the mixture froths and threatens to prime. 

 Such a mishap is obviated by diminishing the 

 amount of phosphate. The acid and phosphate 

 ought to be run in simultaneously and never after 

 each other. The mixer works continuously with no 

 stop except in case of a breakdown. When one 

 mixing is finished the sides of the mixer are rapidly 

 dusted with a little phosphate to neutralize any 

 free acid left there which might corrode the metal. 



