138 CHEMICAL FERTILIZERS 



inch. It may sometimes be found necessary with certain 

 rock phosphates in coarse lumps, e.g. Algerian phos- 

 phates, to put them first of all through a Blake crusher. 

 Where old-fashioned mills, which have no sieves attached 

 to them, are used, it is necessary to have a sifting machine. 

 This consists of a hollow octagonal prism rotating on an 

 axis slightly inclined to the horizontal. The phosphate 

 meal enters at the upper end, and, by rotating, gradually 

 works down to the lower end. The flat sides of the octagon 

 have framed sieves, which are fastened by any simple 

 device. 



Mixing the Acid and Phosphates. To blend the acid 

 with the phosphate, a " mixer " is used. The rock phosphate 

 and sulphuric acid are weighed out in appropriate amounts 

 and placed in the mixer. For the purpose of determining 

 the amount of sulphuric acid and rock phosphate that should 

 be used in the mixer, it is necessary for the manufacturer 

 either to know by actual practice the volume of acid con- 

 sumed by the phosphate in use at the time, or else to calculate 

 the proportions from the result of analysis. Table 18 gives 

 a statement showing the amount of acid of various strengths 

 that is consumed by the different ingredients in the rock 

 phosphates. It will be noticed in Table 18 that iron oxide 

 and alumina consume a large amount of acid in proportion 

 to other materials, a fact which explains why iron and alumina 

 are so little liked by the superphosphate manufacturer. 

 In modern appliances the reservoir for measuring the acid 

 and the weighing machine for weighing the phosphates can 

 be made partially automatic, so that once having been fixed 

 at definite weights and volumes, they will continue to send 

 the materials into the mixer at the correct rate. The mixer 

 itself may be formed according to several different designs. 

 Some of the old-fashioned mixers consist of lead-lined wooden 

 troughs, in which a spirally -grooved core mixes and propels 

 forward the materials as they are delivered into the trough. 

 For small mixers this method works satisfactorily enough, 

 but lead is a soft metal to withstand such rough usage. 

 To deal with several hundredweights of acid phosphate sludge 



