MANUFACTUKE OF SOLUBLE PHOSPHATES. 81 



pletely dissolving the phosphate. The difficulty has been got over 

 by installing superphosj)hate drying machines, so that there now 

 need be no fear in using the requisite amount of acid to dissolve 

 the phosphate completely. To calculate the acid some proceed in a 

 purely empiric manner, by using 1 ton 4 cwt. of sulphuric acid of 

 50° B. for one ton of tribasic phosphate of lime, and adding the 

 surplus of acid necessary to saturate the carbonate of lime ; others 

 saturate two atoms of calcium with 550 grammes of sulphuric acid of 

 50° B. and increase this quantity by that necessary to saturate the 

 carbonate of lime and the sesquioxide, three of sulphuric acid of 

 50° B. for one of sesquioxide; others finally starting from the 

 analysis of the raw phosphate, use the amount of acid necessary to 

 saturate the sesquioxides and the total lime, less the portion of the 

 latter inherent to the formation of acid phosphate of lime. The 

 last method is evidently the best, seeing that it is based on 

 theoretical figures ; it was always used for treating guanos, which 

 owing to the presence of bibasic phosphate of lime, were dissolved 

 with the greatest ease. But as already mentioned, mineral 

 phosphates often contain small amounts of silicates, which likewise 

 absorb sulphuric acid. From the foregoing practical men have 

 concluded that the best method is to make a trial mixing. These 

 trials ought to be carried out each time on a sufficient amount of 

 material to fill the mixer. Accordingly the phosphate and acid are 

 mixed by the aid of the agitator and the process is conducted exactly 

 as in normal working. The reaction, which takes place in the 

 mixing den, does not consist solely in the heating of the materials, 

 as one would be inclined to imagine, because if that were the case 

 the same result would be attained by heating the sulphuric acid or 

 even the material on which it acts when working on the small 

 scale. In the case of certain mineral phosphates, such as Canadian 

 apatite, mixing test trials never succeed, because the phosphoric 

 acid formed in the first phase of the operation cannot immediately 

 exercise its action on the rest of the phosphate. The operation 

 drags, the substance remains pasty, the sulphuric acid attracts 

 moisture, its action gradually becomes weaker, and, finally, a bad 

 quality superphosphate results.^ 



It is thus necessary when new phosphates are to be used, the 

 behaviour of which in mixing is unknown, to make trial mixings 

 with a larger quantity of acid than that got by calculation, and to 

 adhere afterwards to the amount which gave the best results. With 



lA rough trial test giving useful indications may be made when new 

 phosphates of unknown behaviour are being bought in the slack season by 

 treating the phosphate with acid in an earthenware bread pan, not stirring too 

 much and setting aside with the pan Ud on and inspecting the result after 

 24 hours' standing in a warm place. If the mixture dries, then it will dry far 

 better on the large scale. — Tb. 



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