!30 The Phosphates of America. 



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All the phosphoric-acid liquor above 5 B. which has passed 

 through the filters is blown by an "egg" (similar to the one de- 

 scribed in the chapter on sulphuric acid) into an elevated tank, and 

 thence it runs by gravitation to the evaporators, a series of leaden 

 pans of any convenient form of construction, and heated either by 

 a direct fire from the top or from the bottom or by the waste 

 steam from the boilers. If any choice is to be awarded to either 

 of these modes of evaporation, it must, in our opinion, fall upon 

 top-heating ; for as the hot gas comes into direct and immedi- 

 ate contact with the acid and the vapors produced are at once re- 

 moved by the draught, it is evidently the quickest, while the 

 pans are much less acted upon and freer from the danger of being^ 

 burnt through than those which are fired from below. The vessel 

 must, however, be kept constantly full and at a uniform level, in 

 order to protect the lead from any direct contact with the flame ; 

 nor is this a matter of any difficulty, since the heavy concentrated 

 acid continuously sinks downward, and may be drawn off from the 

 bottom, in a stream directly proportionate to that in which fresh 

 acid from the tank above is allowed to run in at the top. 



In works where it is thought best to heat the pans from the 

 bottom, the latter are generally so arranged in sets, that the weak 

 acid flows in at one end in a regulated stream, and is transferred 

 from pan to pan by overflow-pipes. The pans themselves in this 

 case are placed on cast-iron plates, those at the fire end being very 

 thick, to protect them from the extra heat, and generally lined with 

 clay and fire-brick. The fireplace comes under the strongest of the 

 pans, and the flame gradually travels towards the weakest, such an 

 arrangement being required by the fact that the concentration be- 

 comes more difficult as the acid gains in strength. According to 

 extensive and perfectly trustworthy experiments, a series of pans 

 having a total area of 118 superficial feet, with a fireplace of 6| 

 superficial feet, can produce, when properly constructed, eight tons 

 of phosphoric acid in twenty-four hours, concentrated to 45 B., 

 with a consumption of no more than twelve to fourteen per cent, 

 of its own weight of coal. 



During the progress of the evaporation, the acid solution de- 

 posits a considerable quantity of sulphate of lime, and it is there- 

 fore generally necessary to decant off the fluid before the final 

 degree of concentration can be attained. The gypsum can be re- 

 moved to one of the filters already described, and washed out with 

 any liquid that may be running into them from the mixing-tuns. 



