92 CYANAMID — MANUFACTURE:^ CHEMISTRY AND USES 



above a certain minimum, the proportion of citrate insoluble 

 phosphate formed is approximately a logarithmic function of 

 the temperature. The quantity of Cyanamid that can be safely 

 mixed with acid phosphate varies greatly with the nature of the 

 acid phosphate, particularly its content of free acid and of 

 iron and alumina. For some grades of acid phosphate it may 

 be as much as 120 pounds of powdered Cyanamid, for the 

 poor grades of acid phosphate as low as 70 pounds of pow- 

 dered Cyanamid to 1,000 pounds acid phosphate in a ton 

 of complete mixture. 



By the process of granulation, in which the powdered 

 Cyanamid is formed into particles which pass through 15-mesh 

 and over 50-mesh standard screens, the chemical activity of 

 the Cyanamid with acid phosphate is greatly decreased. This 

 is mainly due to the fact that the specific surface exposed by 

 particles of different sizes varies inversely as their diameters. 

 The number of particles per unit of weight varies inversely 

 as the cubes of the diameters. One thousand particles one- 

 hundredth of an inch in diameter, for instance, would be 

 required to make one granule one-tenth of an inch in diameter, 

 and the total surface exposed would be one-tenth as much as 

 before granulation. Since chemical action can take place only 

 on the exposed surface of the solid Cyanamid (the acid phos- 

 phate having very little fluidity) it is evident that the localiza- 

 tion in a few places of a comparatively large number of widely 

 scattered small particles will greatly decrease the amount of 

 action that can take place. 



Practically, it has been found that the chemical activity of 

 the granulated Cyanamid now being manufactured is 

 about one-half the activity of the powdered Cyanamid; 

 hence, about twice as much granulated Cyanamid can 

 be used in acid phosphate mixtures to produce the same effect 

 as a given quantity of powdered Cyanamid. With improve- 

 ments in the process of granulation the safe amount will be 

 probably further increased. 



