THE MANUFACTURE OF ACID PHOSPHATE. 7 
aluminum, and carbonates of lime or magnesia. All of these impuri- 
ties lake 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. 
