144 CHEMICAL FERTILIZERS 



present in small amounts?, that is the ratio H 3 PO 4 : CaHPO 4 

 is large, say 10 : i ; anhydrous forms of mono-calcium and 

 di-calcium phosphate may be present in considerable quan- 

 tities. Such mixtures show very defective mechanical 

 properties, and may, in extreme cases, be so pasty that they 

 cannot be distributed. A correctly prepared superphos- 

 phate should be capable of being kneaded between the fingers 

 without working up like putty. To obtain such desirable 

 properties, the manufacturer must limit the proportion of 

 water and free phosphoric acid. The evaporation of water 

 in the den varies directly with the concentration and tem- 

 perature of the acid, the temperature of reaction, and the 

 excess of lime over phosphoric acid in the mineral phosphate 

 used. The use of hot acid in the mixer promotes too rapid 

 evaporation before the mass has become well mixed, and 

 it is therefore preferable to keep the temperature down 

 below 30 C. (88 F.), but there are some particular phos- 

 phates, such as the Somme phosphates, which many manu- 

 facturers prefer to treat with acid at a higher temperature. 

 It is essential that the sludge in the mixer should be kept in 

 a paste-like form for a sufficient length of time, so that 

 it can be uniformly distributed, both in the mixer and in 

 the den. 



The concentration of the acid to be used varies with the 

 nature of the mineral phosphates and the moisture that they 

 contain. Many makers prefer to use an acid of a fairly 

 low gravity, 1*57, but such superphosphates are often rather 

 damp. A better practice is to use an acid of a gravity of 1*6, 

 which is very suitable for lumpy phosphates. The result 

 of raising the temperature of the reaction is to increase the 

 concentration of free phosphoric acid in the- liquid phase, 

 whilst in the solid phase di-calcium phosphate increases in 

 equal proportion owing to increased decomposition of 

 mono-calcium phosphate. Indeed, mono-calcium phosphate 

 is hydrolyzed in the presence of an excess of sulphuric acid. 

 The duration of mixing should therefore be kept very strictly 

 to the time necessary for complete and uniform admixture 

 which, with good apparatus, is less than half a minute. The 



