122 CHEMICAL MANUEES. 



contains 5 per cent of orthophosphoric acid it may contain up to 

 15 per cent of water, without being actually wet. According to 

 Stocklassa, acid phosphate of Hme heated to 100'' C. loses, in ten 

 hours, 1-83 per cent of water; in twenty hours, 2-46 per cent ; 

 in thirty hours, 5-21 per cent; in forty hours, 6-32 per cent; 

 in fifty hours and upwards, 6-43 per cent constant. At 260' C. 

 phosphoric acid changes slowly into pyrophosphoric acid. 



2H3PO, - H.O = H.PP-. 



Moist superphosphate may be dried by evaporating the water 

 which it contains, either by absorbing a part of the free phosphoric 

 acid w^hich it contains by inert material, such as calcined gypsum, 

 which combines chemically with the water, kieselguhr, peat dust, 

 sawdust, or in fact by combining a portion of the free phosphoric 

 acid. From an industrial point of view, we have to examine the 

 application of heat to drying, direct heating, and cold drying. 

 The drying machine, by direct heating, utilized for drying bones, 

 may be used as in the drying of raw phosphates, sulphate of 

 ammonia MP 140° C, beyond which it volatilizes, but it does 

 not suit for superphosphate. It is, moreover, very trying and 

 dangerous for the health of the workmen owing to the disengage- 

 ment of the acid gases of the superphosphates. Besides, the 

 material being pressed by the workmen during shovelling, readily 

 sticks to the plates, and undergoes a sort of combustion from which 

 a retrogradation of as much as 4 per cent may result. Moreover, 

 it is very difficult to regulate the heat of a coal fire, which requires 

 constant attention. The principal systems of drying in actual use 

 are the following : — 



Lambert's Dryer. — The inventor gives the following deliciously 

 laconic description : The apparatus consists of a masonry chamber 

 divided into three compartments : (1) A hearth of refractory 

 materials, in which there are mixed the hot gases from the generator 

 hearth and the air propelled by the blowers. (2) A chamber 

 succeeding the preceding one, into which pass the mixture of gas 

 and hot air before entering the drying machine. (3) A stove 

 heated by the heat radiated from the hearth, and in which the ap- 

 paratus described below moves. The dryer, properly so-called, has^ 

 the form of a highly-elongated truncated cone and is arranged 

 horizontally on rollers ; it is entrained by means of gearing in a 

 continuous system of rotation. In the interior this apparatus is 

 fitted with corners intended to raise the material in an unin- 

 terrupted manner and to project it into the hot current during its- 

 whole stay in the apparatus. At the extremity of the small diameter 

 of the apparatus is a feed hopper, at the other end are apertures, 

 the object of which is to drop the dried material and allow the 

 escape of hot air and steam during evaporation. . 



