520 Experiments with Phosphates and Slags. [Sept., 



only a by-product there is no likelihood of it ever being worth 

 the steel maker's while to modify his processes in order to 

 produce a basic slag similar to that which has been in use in 

 the past.* 



Mineral or Eock Phosphates. — One of the possible substitutes 

 for the old type of basic slag is ground mineral phosphate. 

 Like basic slag, rock phosphates are basic in character; they 

 have the advantage of a very high content of phosphate — from 

 50-88 per cent., depending upon the source. On the other 

 hand they are considerably less soluble in citric acid than the 

 high-grade basic slags, but are better in this respect than the 

 new open hearth fluorspar slags. Experts differ as to the 

 extent to which citric solubility may be taken as a measure of 

 the relative value of such types of phosphates, and the matter 

 can only be settled by extensive field trials. 



It is important to remember that the various rock phosphates 

 are not identical in character — they differ not only in their 

 source but in chemical composition and in solubility in citric 

 acid. In the following table the various rock phosphates are 

 set out in the order of their solubility and their origin; their 

 approximate content of phosphate is also indicated : — 



Table I. 







Approximate 



Citric 

 Solubility. 



per cent. 



Name of Phosphate. 



Origin. 



content of 

 Phos23hate. 

 per cent. 



Gafsa 



North Africa 



56—64 



38 



Egyptian 



H ?) 



.56—60 



35 



Algerian 



?» !5 



58—66 



33 



Florida Soft 



United States 



48—54 



27 



Tunisian 



North Africa 



54—60 



24 



Tennessee 



United States 



30—60 



23 



Nauru ... 



Oceania 



82—88 



21 



Makatea 



Oceania 



82—86 



19 



Florida Pebble 



United States 



70—76 



18 



In general, the higher the percentage of phosphates in these 

 natural phosphates the lower is their citric solubility. In this 

 respect there is a great difference between Gafsa phosphate at 

 the head of the table and Makatea and Florida Pebble at the 

 bottom. It is, therefore, necessary to take such facts into 

 consideration when planning field experiments, as it may well 



* A Committee has been appointed by the Minister of Agriculture to study 

 the problem thus created. Two reports of the Committee have been issued, 

 the first of which was summarised in this Journal for September, 1921, 

 and the second of which follows this article at p. 530. These reports contain 

 among other information a summary of the results of field trials with slags 

 now obtainable. 



