70 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



combined iron and aluminum expressed as oxides, dO' not exceed 

 a given small percentage of the whole, from 2 to 4 per cent being 

 allowable. Iron when present in excess of about 2 per cent brings 

 about reactions which result in the formation of a gelatinous sub- 

 stance injurious to the mechanical condition of the mixture, occa- 

 sioning also a loss of soluble phosphoric acid. A first step in the 

 reaction with the iron is probably as follows : 2FeP04+3H2S04= 

 2H3P04+Fe2( 504)3. Of the sulphate of iron thus formed, a 

 part according to Fritsch*, reacts on acid phosphate of lime, thus 

 forming the objectionable gelatinous precipitate. Owing to the 

 demand of calcium sulphate for water, hydrated iron phosphate, 

 which is a product of these reactions, may subsequently become de- 

 hydrated and insoluble, thus causing the loss of available phos- 

 phoric acid. 



Aluminum, existing as a silicate in phosphate rock, is likely to 

 be injurious, since, according to Fritsch, if not decomposed by 

 the acid, it may cause a part of the phosphoric acid to retrograde. 

 However, when existing in the rock in small amounts as a phos- 

 phate, the aluminum is not supposed to occasion a loss of phos- 

 phoric acid, both the hydrated and non-hydrated phosphate being 

 soluble in the precipitated condition in phosphoric acid. 



Carbonates of calcium, when existing in small quantities in 

 phosphate rock, are beneficial rather than injurious. When the 

 ground rock is treated with acid the carbonate is the first of the 

 ingredients to be attacked, and the heat thus engendered promotes 

 subsequent reaction among the other constituents. Moreover, the 

 carbon dioxide gas, given off from the carbonate, lightens the 

 mixture and facilitates drying. Phosphate rock low in, or lacking, 

 carbonate develops little heat in mixing, and reacts slowly. In 

 such cases this constituent must be added. It is true that the pres- 

 ence of the carbonate necessitates the use of an increased amount 

 of acid, which in turn results in the formation of an increased 

 amount of calcium sulphate or gypsum. 



The amount of carbonate that is desirable is sometimes given 

 as 5 per cent, but the limits are not strict, and manufacturers do 

 not as a rule find it necessary to specify directly the amount of 

 the carbonate that the rock must or must not contain. Indirectly, 

 however, an excess of the carbonate is guarded against by other 



*T. Fritsch, The Manufacture of Chemical Manures, p. 79, IQH- 



