DRYING AND ENRICHMENT OF PHOSPHATES. 61 



projecting teeth in the form of pyramidal trunks, which mutually 

 intersect each other. At other times the crusher consists of a 

 grooved mohile blade which crushes the phosphate against a fixed 

 plate (see Fig. 12 and context). Where the nodules to be crushed 

 are not quite dry, the working of the mills produces steam, which 

 finds vent through wooden pipes fitted either to the boxes which 

 collect the ground phosphate, or to the mill covers. These pipes, 

 acting as evaporators, end on the roof. The steam carries with it 

 entrained phosphate dust. Of late mechanical driers have been in- 

 stalled for drying raw phosphates. 



Calcining the Nodules.— Before crushing the nodules, they were 

 at first only dried in the sun, or on iron plates heated artificially. 

 Lately thev are calcined in special furnaces to free them completely 

 from water and carbonic acid ; their weight per cubic metre is thus 

 lowered from 1000 kg. to 950 kg , say from 20 cwt. to 19 cwt., 

 and the percentage of phosphoric acid is increased prj rata. These 

 furnaces are i'O metres, about 14 feet 9 inches high, and about 25 

 cubic metres in capacity, with a diameter of 2-65 metres, say 8 feet 

 8 inches at the top, 1-8 metres, say 5 feet 11 inches at the base, 

 and 3 metres, say 9-84 feet in the centre. Their bottom consists of 

 a busk formed bv two incline ! planes ending in two wide discharg- 

 ing orifices. The phosphate rests on iron bars, supported at one of 

 their ends by the inclined planes of the busk and at the other by 

 the horizontal metallic lintels whi-h form the crown of the discharg- 

 ing orifice. These bars form two grates at the bottom of each 

 furnace, and thev are used to regulate at will the descent of the 

 products. It suffices to draw one to let the calcined nodules fall 

 through the opening. They can thus be withdrawn at any point 

 of their contact with the grate ; above each grate the side of the 

 furnace is pierced by an opening to inspect the progress of the 

 operation. The furnaces are built of bricks or masonry ; they are 

 filled with nodules in layers of 4 inches thick which are separated 

 by very thin layers of fuel, small coke, or anthracite. A furnace of 

 the above dimensions yields 4J cubic metres of calcined phosphate 

 in twenty-four hours. The matter thus remains in the apparatus five 

 to six days. The roasting of a ton of nodules takes 1 cwt. of fuel 

 costing about 5d. The nodules first go to a crusher with smooth 

 blades; they then pass by an elevator chain of cups to the top of 

 the building where thev are dropped into an iron trough which 

 feeds the mills underneath. The ground phosphate passes auto- 

 maticallv to an elevator which raises them to the level of the two 

 sifting machines between which they are fed. From the siftmg 

 machine the substance falls into a wooden reservoir fitted with 

 two bagging-up machines. Whilst in the Pas de Calais phosphates 

 are always washed after simple hand sorting from the bulk of the 

 gangue, in the Meuse and Ardennes they are now beginning— 



