194 AGRICULTURAL ANALYSIS 



For an average content of three per cent, of phosphorus in the 

 pig iron, from 15 to 20 pounds of lime are used for each 100 

 pounds of pig iron. As soon as the melted pig iron has been 

 introduced into the converter, the air-blast is started, the con- 

 verter placed in an upright position, and the purification of the 

 mass begins. The manganese in the iron is converted into oxid, 

 the silicon into silica, the carbon into carbon dioxid and oxid, 

 and the phosphorus into phosphoric acid. 



By reason of the oxidation processes, the whole mass suffers a 

 rise of temperature amounting in all to about 700 above the tem- 

 perature of the melted iron. At this temperature the lime which 

 has been added, melts and, in this melted state, combines with the 

 phosphoric acid, and the liquid mass floats upon the top of the 

 metallic portion, which has by this process been converted into 

 steel. 



As soon as the process, which occupies only about 15 min- 

 utes, is completed, the fused slag is poured off into molds, al- 

 lowed to cool, broken up, and ground to a fine powder. For each 

 five tons of steel which are made in this way, about one ton of 

 basic slag is produced. 



In another process, in order to make a slag richer in phos- 

 phoric acid, a lime is employed which contains a considerable 

 percentage of phosphate. Although the slag thus produced is 

 richer in phosphoric acid, it is doubtful whether it is any more 

 available for plant growth than that made in the usual way with 

 lime free from phosphoric acid. In other words, when a basic 

 slag is made with a lime free from phosphoric acid, nearly the 

 whole of the phosphoric acid is combined as tetrabasic calcium 

 phosphate. On the other hand, when the lime employed con- 

 tains some of the ordinary mineral phosphate, the basic slag pro- 

 duced becomes a mixture of this mineral phosphate with the 

 tetracalcium salt. The mineral phosphate is probably not ren- 

 dered any more available than it was before. 



It is easily seen from the above outline of the process of man- 

 ufacture that basic slags may have a very widely divergent com- 

 position. When made from pig iron poor in phosphorus, the slag 

 will have a large excess of uncombined lime and consequently the 



