CHAPTEK VI. 



MANUFACTURE OF SUPERPHOSPHATE. 



The manufacture of superphosphate comprises three principal opera- 

 tions: 1. Grinding the raw phosphate. 2. Rendering the ground 

 raw phosphate soluble by sulphuric acid. 3. The drying of super- 

 phosphate. 



Grinding Raw Phosphate. — Eaw phosphate should be carefully 

 ground, because it is found that the fineness of the phosphate con- 

 tributes to a great extent to a perfectly successful superphosphate. 

 Thus the powder should not leave more than 10 per cent of residue 

 on a 70 mesh sieve, and this residue should not exceed the size of 

 groats ; it is only at this cost that all the phosphoric acid is rendered 

 soluble. Certain phosphates are delivered ground, others in lumps 

 of the size of the fist. To lend itself well to grinding the phosphate 

 should be dry. Florida phosphate especially should not contain 

 more than 1 per cent of moisture, whilst Algerian phosphate grinds 

 very well with 5 per cent of w^ater. When dealing with phosphate 

 drenched with sea water in transit or accidentally in the warehouse, 

 it is extended on a drying platform of sheet iron heated by the com- 

 bustion gases [over the fiues from the boiler furnaces]. In the 

 grinding of phosphate at the present day ball-mills — continuously 

 feeding and discharging — are in general use, which owing to their 

 strong construction and their stable working answer well for the 

 purpose. In older factories flatstone mills are frequently used. 

 Griffin's crusher with walking beam has likewise some rare partisans, 

 but it is costly and requires frequent repairs w^hich become heavy 

 in the end. The material to be introduced into this mndins- 

 machme ought preferably to be reduced, and for that purpose edge 

 runners are suitable. The crusher, with blades — disintegrator — is- 

 likewise used, but only to crush phosphate in large nodules or rock 

 phosphate. These machines will now be briefly described, along with 

 the other grinding machines commonly used in the manufacture of 

 chemical manures.^ 



^ But superphosphate and chemical manures generally were made in Great 

 Britain by mechanical means for the last tifty years at least. However, 

 if France was originally twenty years behind the times in manure manu- 

 facture, it has more than made up for it by recent progress. But Great Britain 



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