MANUFACTURE OF SUPERPHOSPHATE. 



107 



the mixer lasts 1-j minutes ; agitation takes two minutes, according- 

 to the nature of the phosphate ; discharge takes half a minute. 

 The " den " may be filled to | of its height ; the vacant space serves as. 

 a regulator for the evacuation of the gas. The decomposition of the 

 phosphate by acid is effected not in the mixer, but principally in the 

 "den" or "house". Cold acid is used, i.e. acid the temperature 

 of which varies between 25^ and 30° C. (77° to 86° F.), and of a 

 density between 50° and 55° B. (106' to 128° Tw.). When the- 

 acid is at a lower temperature, the mixing does not heat enough 

 to drive off the water, and yield a dry superphosphate. When the 

 acid is too hot the mixing thickens too much in the mixer, in w^hich 

 case the acid may be further diluted. 



Fig, 22, — A. Mixer. B. Counterpoise. C, Gas Escai:e Pipe. D. Cup- 

 Elevator bringing the Eaw Phosphate, E. Phosphate Reservoir. F. Balance 

 for Weighing the Phosphate. G. Screw Conveyor, H, Feed Hopper. I. Acid 



KK, Underground Conveyor. L. Exit of Toxic Gases. 



Measuring Tank 



Owing to the gas given off, the thick liquid effervesces, and forms- 

 air-bells which rise to the surface ; at the same time it heats up to 

 120" to 250° C. (248° to 302° 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 (carrying off the water) . It is only when this is- 



1 A more valid reason for the greater dryness, if any, appears to be that 

 magnesium sulphate crystallizes with seven molecules of water, and that is 

 an efdorescent salt, not a deliquescent one. However, it is not usual to send 

 a manure drier up the cups. — Tr. 



