THE MANUFACTURE OF PHOSPHORIC ACID. 161 



to take into account the advantages which the sulpho-phosphate 

 possesses in virtue of its acid nature. It may be used in larger 

 quantities in calcareous and heavy soils, either alone or mixed with 

 basic slag or powdered lime. We know that in virtue of its high 

 lime content, basic slag can be mixed neither with ammoniacal 

 salts nor with superphosphates, for in the first instance there would 

 be a loss of nitrogen, and in the second of phosphoric acid soluble 

 in water. Now, not only does the mixture of basic slag with the 

 sulpho-phosphate of ammonia exclude losses of this nature, but it 

 also has the effect of increasing the percentage of phosphoric acid 

 soluble, in w^ater. According to experiments made on this subject, 

 this increase reached 13 per cent of the total phosphoric acid in the 

 mixture. Another advantage arising from the acid character of the 

 sulpho-phosphates resides in the possibility of using particularly 

 impure phosphates, such as those containing oxide of iron and 

 alumina. It is precisely this fact which will bring the sulpho- 

 phosphates into good repute some day, when the abundance of pure 

 phosphates has given place to penury. 



But there is a class of mineral phosphates of low content in lime, 

 and the base of which is -alumina with more or less oxide of iron. 

 Redonda phosphate consists of important beds of this nature. These 

 phosphates can be treated neither by the ordinary dissolving process 

 nor by the process of extraction of the phosphoric acid, because the 

 latter is present in a form difficult of attack. Now the manufacture 

 of sulpho-phosphates gives us the means of profiting excellently by 

 these phosphates, otherwise unutilizable, w^hich explains their cheap- 

 ness. The method about to be described, and which is absolutely 

 unpublished, is based on the following equation : — 



2A1P0, + AmHSO, -f- H.SO, = 

 AU(S0,)2 + 2AmHS0„AmH,P0^ 



The ground phosphate is fused at a temperature of 120° to 140° C. with 

 super acid ammonium bisulphate. The duration of this operation 

 depends on the degree of resistance of the substance. Most often 

 it requires two to three hours. It is clear that the water evaporated 

 in that time must be replaced. The phosphate is dissolved much 

 more rapidly and more completely when done under pressure and 

 at an elevated temperature. The plant required by that style of 

 working is certainly more costly, but the work would be done better 

 and factory expenses run less. 



The product so obtained is a sulpho-phosphate of ammonia and 

 sulphate of alumina. The presence of the latter renders it very 

 hygrometric and little fitted for its intended use. But there is a 

 very simple method of remedying this drawback. If an equivalent 

 quality of sulphate be added thereto it combines with the sulphate 



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