CHAPTER III. 

 DRYING AND ENRICHMENT OF PHOSPHATES. 



Drying. — Phosphates, for all uses to which they may be put, must 

 be dried and finely ground. When the phosphates are massive 

 (rock phosphate) or in lumps, they ma\- be completely dried in 

 summer by storage in a warehouse with vents in the side ; in winter, 

 drying is finished in a drying machine. Formerly phosphates 

 were calcined to increase the percentage of phosphoric acid and 

 facilitate grinding (p. 85 et seq.). Drying is indispensable in the case 

 of the sandy phosphates of Mons, Somme, and Cambresis. This 

 operation is conducted in a naked fire drier, on cast-iron plates, about 

 two-fifths of an inch thick, in contact with the flames of one or more 

 furnaces, but protected from over-heating. The phosphate is spread 

 on the drier, in layers of 4 to 5^ inches thick, according to the 

 nature of the phosphate. The drying reduces this thickness about 

 one-third. An operation is finished in 6 hours over the fines above 

 the fiames ; in 8 hours on the flues, forminoj the first circuit of the 

 flames ; and in 10 to 12 hours on the flues, forming the second 

 and the third circuit. A drier of 250 square metres (820 square 

 feet) may yield 35 to 40 tons of dry phosphate per day of 24 

 hours. The consumption of coal is about 2 cwt. per ton of sand 

 obtained. The sands, so dried, pass into cylindrical sifting ma- 

 chines, with polygonal section like those of an ordinary mill covered 

 w4th wire gauze Nos. 70, 80, 90, 100, 110, and fitted with baskets 

 inside, in which to collect the larger fragments. The dry sand 

 passes successively through the different mesh sieves, commenc- 

 ing with the larger sizes. The exit of the products is thus re- 

 gulated m a uniform manner over the whole length of the ap- 

 paratus. The residues on the sieves are crushed in fiat stone mills 

 revolving at a speed of 100 turns a minute. Each pair of stones 

 absorbs 6 to 7 H.P. The sifted phosphate is generally 7 to 8 per 

 cent richer in tribasic phosphate than the ground phosphate : the 

 whole is mixed so as only to have one average quality of the ap- 

 pearance of finely ground pepper. When the phosphate is in 

 nodules, as in Auxois, they must be crushed or cracked before 

 grinding. The crushers and cracking machines, used in the jMeuse 

 and Ardennes, consist of two cast-iron cvlinders, studded with 



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