CHAPTER rx. 



THE MANUFACTURE OF PHOSPHORIC ACID, DOUBLE SUPER- 

 PHOSPHATES, AND VARIOUS PRODUCTS. 



Historical Review. — The manufacture of phosphoric acid has lost 

 much of its former importance. The market for raw materials and 

 finished products constitutes one of the most important factors in the 

 development of an industry. Formerly, when pure phosphates of 

 high strength were rare, whilst phosphates of low stiength abounded, 

 the phosphoric acid industry and its derivatives were in an excellent 

 condition for living and prospering. The extensive deposits of 

 phosphorite in Germany induced manufacturers to devote them- 

 selves to the manufacture of a product which, separated from the 

 impurities which accompany it in the raw material, was admirably 

 adapted for the manufactuie of one of the most esteemed manures, 

 which is the double superphosphate. The discovery of the Lahn 

 phosphorite beds took place at the same epoch as the Strassfurt 

 deposits of potash salts, when Liebig had just formulated his 

 mineral manure theory. Lahn phosphorite, which, as is known, 

 contains a large proportion of iron and alumina, was received with 

 open hands by manufacturers, Init great was their astonishment 

 when they realized that this material was absolutely unfit for super- 

 phosphate manufacture. Dissolved by sulphuric acid, it yielded a 

 product, the soluble phosphoric acid content of which, already low 

 enough, diminished still further in such great pi'oportions, owing to 

 retrogradation, that superphosphate manufacturers had to abandon 

 its US9. Two large chemical factories, which owned a great part of 

 the Lahn phosphorite deposits, tried to utilize the material in 

 phosphoric acid manufacture. 



The fundamental idea of the process ot manufacture is con- 

 tained in Graham's method of analysis. It consists in digesting the 

 pulverized phosphate with 5 per cent sulphuric acid, in titrating 

 the dissolved phosphoric acid by uranium solution, leaving the oxide 

 of iron intact. The principle of the process is thus represented by 

 the following equation : — 



(151) 



