STOEING OF SUPERPHOSPHATE. 123 



Zimmermann's Drying Machine. — Zimmermann's drying 

 machine consists of an oven 8 metres (26 feet) high, heated by 

 combustion gases. The hot air comes in contact with the material 

 on iron plates suspended by chains and fitted with a system of 

 agitation. An oven of this kind, occupying a space of 50 square 

 metres (538 square feet), dries four wagons of superphosphate 

 in twenty-four hours. The latter only remains exposed two- 

 minutes to the heat of a very bright coke fire ; it there becomes- 

 heated to 80^ C. and loses 5 to 6 per cent of water. The motor 

 power to shake the plates is three horse, and the coke burnt, in 

 twenty-four hours, half a ton. Three men can work two ovens. The 

 iron plates have the drawback of not allowing the hot gases to pass,, 

 which thus take the shortest path along the sides of the oven. 

 Lately the inventor has replaced them by sieves. The combustion 

 gases and the acid gases pass into a dust chamber, where the- 

 latter is deposited, whilst the acid gases are purified in a con- 

 denser tower. The acid contained in the gas which is disengaged 

 from the dryer is chiefly hydrofluoric acid. This gas, as well as- 

 the hydrofluosilicic acid, exists dissolved in the superphosphate 

 water, and is partly re-formed when the substance is heated, the 

 sulphuric acid, as CaH^SO^, [?] acting then on the still undecomposed 

 calcium fluoride. The hydrofluosilicic acid is decomposed by heat, 

 into hydrofluoric acid and silicic fluoride. 



The drying machine forming the object of the German patent 

 85,273, only differs from the above oven in the arrangement of thfr 

 interior and in the method of agitation. The shaking plates are- 

 not suspended by chains, they rest on cross pieces and are moved 

 by manuals. 



MoUer and Pfeiffer's Drying Machine. — M oiler and Pfeiffer have 

 constructed a drum drying machine in which they have striven ta 

 avoid shocks and sudden motion, which are absolutely injurious to 

 the structure of superphosphate. On the other hand, these makers- 

 secure energetic ventilation by superheated air. This apparatus is 

 based on this principle, that each matter requires appropriate treat- 

 ment according to its nature. It consists of four principal parts, 

 which are : (1) A heating system for heating dry air. (2) An ex- 

 hauster for ventilating and absorbing combustion gas. (3) An 

 arrangement for mixing air with the combustion gas, and to receive- 

 the superphosphates. (4) The drum, properly so-called. The work- 

 ing is as follows : The superphosphate is fed from a cup elevator into- 

 a hopper where it falls into the drying machine, w^here it is seized 

 by a strong current of hot air, which dashes it into the rotating drum. 

 The current of hot air is produced by a powerful fan with a conical 

 aperture ; the jet of compressed air that issues from this aperture 

 drives an ejector which absorbs the combustion gases. The air and 

 the gas are mixed in varying proportions at the will of the operator,. 



