200 



CHEMICAL MANUEES. 



constituting the slag, which contains the major part of the impurities 

 combined with the cashtie. 



As to the gases which escape in consequence of this de-oxidation, 

 they are collected and their heat utilized, either to heat the air which 

 enters the blast furnaces to a temperature of 750° C, or to produce 

 the steam necessary to drive the blowing engine, or finally to pro- 

 duce electricity. The charging of the blast furnace is done con- 

 tinuously through the top, and its discharge through the bottom 

 (about every six hours). The temperature of the low^er zone where 

 the molten iron frees itself by difference of density from its floating 

 impurities is about 1200° C. The slag is utilized industrially for 

 making bricks, cement, etc. Let us see what becomes of the cast- 

 iron, which at this point is still phosphorusetted, as the follow^ing 

 analysis taken as an example shows : — 



TABLE LIV.— ANALYSIS OF CAST-IRON SHOWING PHOSPHORUS 



CONTENT. 



Per cent. 

 Iron 91-20 



Carbon 

 Phosphorus 

 Magnesia 

 Silicon 



3-60 

 2-75 

 3-20 

 0-25 



100-00 



The conversion of the cast-iron into steel is done in special pear- 

 shaped appliances, movable round a horizontal axis, and made of 

 steel lined with refractory stone. These appliances, termed con- 

 vertors, are in the Thomas process lined in the interior with lime 

 and magnesia. They are open on the top and pierced in the lower 

 part by holes through which air at a high temperature can be blown. 

 To charge them they are turned around on their axes to an angle of 

 90° and the liquid cast-iron is run in through the upper opening. 

 Then the pear is raised at the same time that air is driven through 

 the lower holes at a sufficient pressure for the fused metal to remain 

 in the convertor and not pass through the holes. Under the in- 

 fluence of the high temperature prevailing in the convertor, and of 

 the air injected, the silicon, the sulphur, the phosphorus are burnt 

 at the same time as a part of the carbon. The heat which is dis- 

 engaged inside the convertor is such that the metal, which had at 

 the moment of its introduction a temperature of 1200°, rises to 

 2100°, all within thirteen minutes. The phosphorus is oxidized and 

 is converted into ortho-phosphoric acid, and as it is in the presence 

 of excess of lime, it combines in a peculiar form tetrabasic phosphate 

 associated with calcium silicate and fotms w^hat is called basic slag. 

 When the operation is finished, which is ascertained by the appear- 

 ance of the gas given off* from the top of the convertor, the latter is 



