THE MANUFACTUEE OF PHOSPHOEIC ACID. 157 



But all the manufacturing problems were not solved. An import- 

 ant point is the exact determination of the amount of sulphuric 

 acid required to decompose the phosphate. That amount cannot 

 be fixed, except by inference from the SO3 in the weak solution of 

 a preceding operation, and to determine that it is necessary to 

 make a quantitative analysis. If a sufficient amount of acid be 

 not used the solution contains lime in excess, and after evaporation 

 it is often converted into a thick broth, which it is impossible to 

 work in that condition. This drawback is remedied by introducing 

 sulphuric acid, but it would be better not to use that expedient 

 owing to the disengagement of hydrofluoric acid fumes. Another 

 drawback which arises from a deficiency of sulphuric acid, is that 

 the extraction of the phosphoric acid is incomplete. But an excess 

 of sulphuric acid causes even greater drawbacks. In fact, if the 

 phosphoric acid solution, with excess of sulphuric acid, be evapor- 

 ated, it becomes inactive towards the phosphate which it is intended 

 to dissolve, which is likewise the case with too concentrated sul- 

 phuric acid. The phosphate is not attacked by the acid, and the 

 manufacturer is at his wits' end to know what to do with this 

 dilute mixture of phosphate and phosphoric acid. At the Wetzlar 

 factory when this mishap occurred, the phosphoric acid was ab- 

 sorbed by powdered peat, in the ratio of three to one ; the product 

 contained about 33 per cent of P^O^, and when it could not be got 

 rid of directly, it was aftervv^ards mixed with double superphosphate. 

 Once finished, the percentage of phosphoric acid soluble in water 

 was thus considerably increased, but at the expense of its physical 

 properties. 



Afterwards, a very simple way was found to restore its activity 

 to the phosphoric acid which had become inactive. All that has to 

 be done is to treat it with finely powdered quicklime. But phos- 

 phoric acid so treated 'does not behave like normal acid in the 

 course of the work. As already stated, the residual gypsum left 

 in the filter presses forms a very cumbersome waste product. It 

 contains 4:0 to 50 per cent of water. It is run on to a heap. In 

 summer it dries on the surface, it is then turned over with the 

 plough, and passed through a Carr's disintegrator, and marketed 

 as a powder. In wet years it is dried in a simple drying machine, 

 when there is an outlet for it. This product is known in commerce 

 as phosphatic gypsum ; it contains 60 per cent of gypsum and 3 to 

 4: per cent of phosphoric acid, of which 1 per cent is soluble in 

 citrate and 0*25 soluble in water. It has been tried to have this 

 gypsum specially adopted as a preservative agent of the fertilizing- 

 principles of farmyard manure, and it has been proposed to spread 

 it in stables, and on farmyard manure. This use is very rational, 

 for precipitated gypsum combines very readily with the ammonium 

 carbonate of farmyard dung. But it is clear that this material 



