148 Fertilisers [pt. hi 



to have sufficient phosphorus present ; if the ore does not 

 contain enough for the purpose foreign ores rich in 

 phosphorus, or Belgian phosphates, are added in the 

 blast furnace. All the phosphorus goes into the pig iron 

 and when this is blown in the converter a slag is "obtained 

 containing phosphate equivalent to 40 per cent, or more 

 tricalcic phosphate. 



The more recent method is known as the Open Hearth 

 Process, and being more convenient to the steel makers 

 it seems likely to displace the Bessemer process; im- 

 fortunately for the agriculturist it does not produce so 

 useful a slag. The p'ig iron is heated in a furnace by 

 superheated producer gas; the heat is therefore quite 

 independent of the amount of phosphorus present, and 

 there is no inducement to the open hearth steel maker 

 to ensure a high, or indeed any, phosphorus content in 

 his pig iron. The average phosphoric irons produced 

 in England contain only 1-5 per cent, or less phosphorus, 

 and when worked in the open hearth furnace yield a 

 slag containing only 7 to 14 per cent, phosphoric acid 

 (PgOg) equivalent to 15-4 to 31 per cent, tricalcic phos- 

 phate of lime. 



Further, the open hearth slags are not usually as 

 readily soluble in citric acid as those yielded by the 

 Bessemer process. This happens particularly when the 

 pig iron contains much sulphur. An excess of lime is 

 required to remove the sulphur, and as this makes the 

 slag infusible, fluorspar or calcium chloride is added to 

 increase the f usibihty. This treatment has the effect of 

 rendering the slag insoluble in citric acid, hence the slags 

 finally produced are more or less insoluble according 

 to the amount of lime and flux added. 



There are three classes of basic slag available : 



