MANUFACTURE OF SUPERPHOSPHATE 73 



The charging of the mixer lasts about two minutes; 

 agitation takes two minutes, according to the na- 

 ture of the phosphate; discharge takes half a min- 

 ute. The "den" may be filled to three-fourths 

 its height; the vacant space serves as a regulator 

 for the evacuation of the gas. The decomposi- 

 tion of the phosphate by acid is effected not in the 

 mixer, but principally in the "den" or "house." 

 Cold acid is used, that is, acid the temperature of 

 which varies between 25 degrees and 30 degrees C. 

 and of a density between 50 degrees and 55 de- 

 grees Be. When the acid is at a lower temperature 

 the mixing does not heat enough to drive the water 

 off, and yield a dry superphosphate. When the 

 acid is too hot the mixing thickens too much in the 

 mixer, in which case the acid may be further di- 

 luted. 



Owing to the gas given off, the thick liquid effer- 

 vesces, and forms air-bells which rise to the sur- 

 face; at the same time it heats up to 248 to 302 

 degrees F. Gradually it settles in the "den" and 

 after an hour it sets. An addition of dolomite 

 (carbonate of lime plus carbonate of magnesia) 

 keeps it liquid for some time longer, so that the 

 water evaporated is then much greater. 



All the heat given off by the reaction ought to 

 be utilized with that end in view, that is, carrying 

 off the water. It is only when this is done that 

 perfect solution is realized, and that a superphos- 

 phate that will behave well on subsequent manipu- 

 lations is obtained. Of recent years attempts 



