METAL INDUSTRY BY-PRODUCTS 63 



has also been accounted for by the difficulties of taking 

 samples of blast-furnace charges and products. 



Basic Slag. Whilst the flue gases passing out at the 

 top of the blast furnace contain potash, the phosphorus of the 

 furnace is almost completely removed in the pig iron. Early 

 forms of iron smelting by means of wood charcoal provided a 

 basic ash which prevented the phosphorus from entering 

 the pig iron, but as soon as coal fuel was used and the type 

 of slag in the blast furnace became acid, and not basic, all 

 the phosphorus entered the iron. As phosphorus exceeding 

 2 % renders iron brittle in the cold, ores containing large 

 amounts of phosphorus were little used until the discovery 

 of methods for removing phosphorus. Phosphorus varies 

 from about 0-02 % in the best Swedish iron to upwards of 3 % 

 in common cinder pig. With ores containing high percen- 

 tages of phosphorus, a basic process of purification is abso- 

 lutely essential. In the original process for producing steel 

 from pig iron the vessel was lined with acid materials, but 

 by lining the converters with a basic lining the phosphorus 

 difficulty was overcome. The special lining is manufactured 

 from dolomite, or, at any rate, a magnesium limestone, 

 which is calcined, mixed with tar and made into bricks for 

 lining the furnace. During the process of converting pig 

 iron into steel, lime is usually added, so as to prevent the 

 destruction of the lining, and because the resulting basic 

 slag is better without an excessive quantity of magnesia. 



Steel is prepared from pig iron by the Siemens or open- 

 hearth method in increasing quantities to-day ; consider- 

 able quantities of scrap iron and iron ores are also added 

 in these furnaces. As the temperature of these open-hearth 

 furnaces is lower than that of the converter, a certain amount 

 of calcium fluoride is added to make the slag flow more easily. 

 The open-hearth process differs from the Bessemer converter 

 in that, whilst in the latter the phosphorus remains in the 

 metal until practically all the carbon has been eliminated, 

 in the open-hearth method much of the phosphorus is 

 removed from the metal in the earlier stages of the operation. 

 The open hearth is more easily controlled than the converter, 



