PHOSPHATIO MANURES 217 



revolving shaft with spikes attached to it for stirring up 

 the materials. 



The reaction between the acid and the phosphate does 

 not take place in the mixer, and the slight rise of tem- 

 perature which occurs is due mainly to the action of the 

 acid on carbonates, chlorides and fluorides which are 

 commonly associated with the native phosphates. When 

 the proportion of such impurities is large, a considerable 

 quantity of acid is used up in this way, and samples 

 which contain them are, therefore, of lower commercial 

 value. The additional quantity of calcium sulphate formed 

 by these reactions may, however, be useful as a drying agent. 



The mixing process does not occupy more than a few 

 minutes, and, when it is complete, the mixture, which is 

 of a thin fluid consistency, is run off. It is in the pit or 

 den that the principal reaction occurs, and the tempera- 

 ture rises to over 100 deg. C. When it is over, the mass 

 gradually cools down, the calcium sulphate absorbs the 

 water and the whole sets into a solid mass which requires 

 to be dug out with picks, but is found to be, if properly 

 made, in a very friable, easily powdered condition. 



Composition and Value of Superphosphates. It has 

 been shown that superphosphates consist essentially of a 

 mixture of monocalcic phosphate and calcium sulphate. 

 As it is not practicable to completely dissolve the whole 

 of the phosphate, a certain proportion usually from two 

 to three per cent. of tricalcic phosphate is always present. 

 Dicalcic phosphate and free phosphoric acid are also 

 commonly present, but as a rule, only in small propor- 

 tion. They may be formed in process of manufacture, 

 or subsequently from the monocalcic phosphate. Sand, 

 compounds of magnesia, iron, aluminium, and other 

 impurities originally present in the raw phosphate or in 

 the acid, are of course found in the manufactured pro- 

 duct. The proportion of sand and insoluble matters, 



