70. CHEMICAL MANURES. 



of the carbonic acid was disengaged owing to a too long stay in the 

 mixer. Then the system of closed chambers or dens was adopted, 

 because in dissolving very hard mineral phosphates, such as 

 Canadian apatite, pebbles, Florida phosphate, etc., a higher tempera- 

 ture was required, which it was easier to attain and maintain in 

 the closed chamber, where the superphosphate remained for about 

 twelve hours ; further, because in connecting it with a well-con- 

 structed flue for the disengagement of the acid vapours, the health 

 of the workmen was safeguarded. When the chamber possesses 

 two flues for the disengagement of gas each 1 metre (3 "28 feet) wide, 

 and when these flues are each fitted with a fan, and can be put in 

 connexion with the chimney stalk of the factory at will, or with 

 a condensation plant, it is easy to eliminate steam and acid gases. 

 A more dry superphosphate is thus obtained, and during the 

 emptying of the chamber or " house " the workers can enter without 

 running the risk of getting "gassed '". But in actual practice 

 hitches frequently occur. As already mentioned, as far back as 

 1880 it was necessary to instal plant to condense the gaseous 

 chlorine and fluorine compounds ; they consisted mostly of wooden 

 or masonry towers drenched with water, by a Koerting's vaporizer. 

 The hydrofluosilicic acid obtained under these conditions can be 

 converted into silico- fluoride of sodium, the selling price of which 

 covers the cost of condensation, and leaves even a slight margin. 

 This product, unfortunately, has only a limited use, and ends by 

 becoming cumbersome. The mixer installed above the closed 

 "dens " or "houses " is that of Lorenz, which is universally used.^ 

 Lately, dissolving chambers have i^een installed at a higher level, 

 and belt conveyors passed underneath to facilitate emptying. This 

 system has been adopted bv certain factories. 



1895 to 1908.— Such was the situation from 1880 to 1891:. 

 Superphosphate factories possessed mills, machines for mixing acid 

 with phosphate, installed above a closed den, into which the mixture 

 is discharged and left to itself for twelve hours ; arrangements for 

 elimination and condensation of acid gases, disintegrators, or roller 

 mills, armed with teeth for grinding superphosphates. Mean- 

 while ball mills appeared, then ball mills combined with tube 

 crushers, then Gritfln's pendulum crusher, etc. ; dust chambers, on 

 different plans, were installed ; new systems of transport, such as 

 aerial conveyors, belt conveyors, to bring the phosphate to the 

 crushers, coal to the boilers, the crushed and screened superphos- 

 phate to the storehouse. The adoption of aerial conveyors was 

 a real step in advance in the supeiphosphate industry, for inde- 

 pendently of the economy in hand labour, they enabled the super- 

 phosphates to be thrown on the heap from above, thus ensuring the 

 good preservation of the product. The installation of aerial con- 



^ Possibly on the continent, but not in Great Britain. — Tr. 



