6 INTRODUCTION 



and ore. The oxygen necessary for the purification of the pig-iron 

 is supplied to the extent of about 70 % by the action of the metalloids 

 on the oxide of iron, the balance 30 % coming from the oxidising 

 gases of the furnace. In the Bessemer process the oxygen comes 

 entirely from the air blast, and the combustion of the phosphorus, 

 silicon, and carbon generates sufficient heat to raise the temperature 

 of the steel to the required extent. The slag formed in the basic open 

 hearth process is much greater in volume and there is a corresponding 

 decrease in phosphoric acid content compared with the basic Bessemer 

 process. Tables III and IV show the changes in the composition of 

 the slag by the two processes. 



Commenting on Table IV Sillars says: 



The decrease in P 2 O 6 content becomes quite sharp after the fourth sample, 

 and this, it will be observed, coincides with the commencement of the period 

 at which carbon elimination becomes predominant. If high grade slag is 

 desired, it is removed at this stage, and after charging fresh lime and oxide 

 of iron, the carbon elimination is proceeded with. It will be noticed that the 

 phosphorus in the first metal sample is as low as in any of the four immediately 

 following, and it may be asked why the slag could not equally well be removed 

 at this stage instead of an hour later. The reason is that although the phos- 

 phorus is eliminated very rapidly (sometimes it is reduced to 3 % twenty 

 minutes after charging), yet it is necessary to delay the removal of the slag 

 until all frothing has ceased and until the whole of the lime and ore is dis- 

 solved in the bath and the heat is sufficiently high to allow the slag formed 

 to flow freely through the tap hole. Unless the slag is removed when it has 

 reached the maximum concentration of phosphoric acid, the further additions 

 of lime and ore, and the denudation of the furnace structure under heat, cause 

 an increase in the slag volume which reduces the phosphoric acid content until 

 at the termination of the process it will contain from 7 to 10 % only. As the 

 content of lime increases, the slag thickens and reaches a viscosity which slows 

 the progress of the 'boil.' This may be corrected by the addition of oxide of 

 iron in the form of scale, but if sulphur has to be eliminated from the metal 

 it is essential to keep the slag as basic as possible ; the slag is therefore thinned 

 by the addition of fluorspar, and it is this addition more than any other con- 

 dition which reduces the solubility of the phosphoric acid in 2 % citric acid. 

 In Table IV 1 cwt. of fluorspar was added after the sixth sample was drawn, 

 and the soluble phosphoric acid fell from 11-78 % to 5*38 % immediately 

 afterwards. 



High grade slag can be obtained by pouring the slag immediately 

 before the addition of fluorspar. 



In the basic open hearth process the steel and slag are tipped into 

 a ladle the steel ladle which is only large enough to hold the steel. 

 When the steel ladle is full the slag overflows into the slag ladle placed 

 immediately under the spout of the steel ladle (Plate I). 



The significance of the change may be better appreciated by a 



