PHOSPHORUS FERTILIZERS 149 



sulphate produced by interaction with ferric phosphate and 

 sulphuric acid, is, however, capable of decomposing mono- 

 calcium phosphate, with the production of ferric phosphate. 

 Ferric phosphate commonly occurs in two forms, the hydrated 

 form and the dehydrated form. If the amount of water 

 present is insufficient, calcium sulphate, during hydration to 

 gypsum, may take the needed water from the ferric phosphate, 

 which is then much more difficult to attack by acid. Alumina 

 compounds usually react somewhat slowly, and are hence 

 distinctly objectionable, since they tend towards retro- 

 gression. 



Organic Matter. The smell alluded to on p. 27 which 

 is given out when some rock phosphates are ground is also 

 produced when these phosphates are acted upon by sulphuric 

 acid. This smell is due to the organic matter contained 

 in the rock phosphates. No serious importance need be 

 attached to this volatile substance ; even if the organic 

 matter becomes charred by sulphuric acid in the process 

 of mixing, no diminished fertilizing value need be feared. 



Emptying the Den. Much of the work of emptying 

 the den is still carried out by hand labour. After the dens 

 have been rilled from the mixer to perhaps two- thirds of their 

 height, the mass of acid and phosphate rises to the top of 

 the den in the course of an hour or two. Owing to the heat 

 produced by the reaction, both steam and gas are given 

 off, which are then drawn away by a fan ; but the mass, 

 although now dry and crumbly, is very hot and continues to 

 give off steam. The work involved in removing the super- 

 phosphate from the den is both arduous and unpleasant. 

 One man removes the stuff from the den with a pick, two men 

 fill barrows, and a fourth or even a fifth man is needed to 

 wheel the superphosphate away by a plank and tilt it on the 

 heap. Where works are slightly better equipped, there is 

 generally a cup elevator placed nearly in front of the den, 

 and the material is put straight away into the cups. Where 

 the manufacturer makes a great point of producing a good, 

 dry superphosphate, it is not uncommon for the material 

 to be passed through riddles at once, and as it passes through 



