2 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 



the whole process, including the preparation of the raw materials 

 used, the methods of manufacture with the chemical reactions 

 involved, the equipment of the modern factory, the disposal of 

 obnoxious gases, the methods of drying, storing, and disintegrating 

 the superphosphate, and the cost and disposal of the finished product. 



RAW MATERIALS. 



The raw materials used in the manufacture of acid phosphate are 

 bone, guano, apatite, phosphate rock, and sulphuric acid. 



Before the discovery of the vast deposits of phosphorites or natural 

 phosphate rock in this country bone was one of the farmer's chief 

 sources of phosphoric acid. The bones were either steamed, charred, 

 or burned and applied directly to the field, or after grinding were 

 made into acid phosphate by treating with an approximately equal 

 weight of sulphuric acid. 



Ground bone, however, has considerable agricultural value with- 

 out being acidulated, and besides, the cost of the phosphoric acid 

 contained therein is so much greater than that contained in phosphate 

 rock that it is obviously not economical to use the former material 

 in the manufacture of acid phosphate. The amount of this substance 

 now derived from bone is therefore relatively small. 



Guano is another substance which has been extensively used in 

 the manufacture of acid phosphate. This material consists essen- 

 tially of the excrements of birds and sometimes of bats, and at one 

 time was found in large quantities. There are two types of guano 

 deposits: (1) The unleached deposits which are usually found in 

 caves or other sheltered places where the droppings have been pro- 

 tected from the leaching effect of percolating water. Such a deposit 

 not only contains phosphoric acid in a readily available form, but 

 also carries considerable quantities of nitrogen, the fertilizer constit- 

 uent commanding the highest price. (2) That which has been 

 leached of its more soluble constituents by exposure to the weather. 

 It contains practically no nitrogen and its phosphoric-acid content, 

 though usually high, is relatively insoluble. Deposits of guano have 

 been eagerly sought, and accessible and valuable ones are now rather 

 scarce. Only those containing high percentages of nitrogen, or situ- 

 ated in regions having excellent transportation facilities, are able to 

 .compete with other and cheaper sources of phosphate. 



At one time apatite was largely used in the manufacture of acid 

 phosphate. This mineral is very widely distributed, and occurs in 

 rocks of vaiious kinds and ages. It is most common, however, in 

 rocks of the metamorphic crystalline variety, such as limestone, 

 gneiss, mica, schist, beds of iron ore, etc. There are two main vari- 

 eties of apatite, namely, chlor-apatite (CaClCa^PgO^) and fluor- 



