THE MAjSTUFACTUBE OP ACID PHOSPHATE. 7 



aluminum, and carbonates of lime or magnesia. All of these impuri- 

 ties take up or are acted upon directly or indirectly by sulphuric acid, 

 the bases being converted into sulphates and the fluorides, carbonates, 

 and organic matter being decomposed with the evolution of gases. 

 It is very important that the manufacturer should be acquainted 

 with the effect that these impurities and the compounds produced 

 therefrom will have upon his acid phosphate, and he should be able to 

 calculate from the analysis of his raw material what quantity and 

 strength of sulphuric acid is required to satisfy these impurities. 

 The action of the sulphuric acid upon the various foreign substances 

 found in natural phosphates of lime, and the effect of these impurities 

 on the finished product are discussed below in some detail. 



ORGANIC MATTER. 



Practically all phosphates, with the exception of apatite, are of 

 animal origin and therefore contain a certain amount of organic 

 matter. When present in any quantity organic matter usually 

 imparts a dark color to the phosphate. The presence of very small 

 quantities can be detected by the putrid odor emitted on crushing or 

 grinding the rock. The phosphates of our western States, as well as 

 some of the Tennessee rock, contain considerable quantities of organic 

 matter, while most of the Florida phosphates are very low in this 

 material. 



The methods now employed in drying phosphate, either by calcining 

 it on ricks of wood or putting it through a rotary drier, burns out or 

 destroys a part of the organic matter; the remainder is carbonized by 

 sulphuric acid with the evolution of volatile or gaseous products. 

 The sulphuric acid is at the same time reduced to sulphur dioxide 

 (S0 2 ), or to hydrogen sulphide (4 2 S) if the reduction has proceeded 

 further. The production of these gases not only entails a loss of sul- 

 phuric acid, but they are both disagreeable and deleterious to health. 



In making acid phosphate the organic matter found in the rock is 

 not considered, since the amount present is usually small. Owing 

 to the various forms in which organic matter may occur, it is almost 

 impossible to judge except by actual experiment how much sulphuric 

 acid is required for its decomposition. 



SILICA AND SILICATES. 



Sulphuric acid has no direct action upon silica (Si0 2 ), but when 

 fluorides are present an indirect action occurs, which is described 

 below. Silicates are directly acted upon by sulphuric acid, but so 

 slowly that they need hardly be taken into account. The presence of 

 silica or silicate minerals in phosphate rock is not considered objec- 

 tionable except in so far as they act as diluents. Phosphates con- 

 taining high percentages of silica necessarily have a lower percentage 

 of phosphoric acid than the less siliceous or purer phosphates. 



