152 CHEMICAL FERTILIZERS 



mechanical appliances for extracting the material. Improve- 

 ments have been made by having false bottoms which allow 

 the material to drop out on to a conveyer. This method 

 still necessitates a great deal of hand work. More complete 

 arrangements are employed in some of the modern forms of 

 mechanical dens. These may be divided into two types, one 

 in which a revolving cutter is pushed into the den, and the 

 other in which the den itself moves forward against rotating 

 knives. These modern dens must therefore be of a cylindrical 

 form, and consequently do not possess the capacity for 

 the same amount of floor space that the ordinary old- 

 fashioned cubical brick den possesses. Where the den is 

 fixed, the cutting knives advance in a spiral into it, and near 

 the end of their run a great strain is placed upon the spiral 

 mechanism propelling the cutting blades into the den. From 

 the engineering point of view it is better to have the den itself 

 moving ; this design gives a less complicated movement to the 

 cutting blades. Where the den moves, simplification is 

 generally effected by having a cylindrical den moving for- 

 wards on two screws placed near the ground. The knife blades 

 are set spinning, and the den moves forwards against them. 

 The superphosphate removed from the den is then immedi- 

 ately conveyed to an ordinary cup elevator and passed through 

 riddles. The whole of the cup elevator should in this case be 

 enclosed, and connected with the fans, so as to draw away 

 the steam and poisonous vapours. Not merely does the 

 current of air draw away acid fumes, but it also dries the 

 superphosphate. In any case of removing superphosphate, 

 great care must be taken not to compress it too much, 

 otherwise it becomes sticky. It is for this reason that 

 dusting the material over at an early stage is advisable. 

 Some of the machines constructed for excavating superphos- 

 phate also make arrangements for dusting on some fine 

 powder. 



In certain cases some of the superphosphates are arti- 

 ficially dried. This artificial drying may be conducted by 

 the simple plan of spreading the superphosphate on a hot 

 stone floor, which is heated by some waste flue gas. If 



