21:2 CHEMICAL MANUEES. 



be added to the cast-iron in lieu of lime, eventually mixed with 

 sand. Fused phosphatic chalk has the same solubility as the best 

 slags. (CaCOs melts at 1000° C, the phosphate at 1900° C.) 



Crushing of Basic Slag. — In the beginning of the basic slag 

 industry it was very difficult to crush. Cylindrical crushers and flat- 

 stone mills were used at that time. But as the basic slag is mixed 

 with grains of steel of larger or smaller size, the plant was rapidly 

 worn out. Attempts were then made to eliminate the pieces of 

 iron in the basic slag by means of magnetic separators. But these 

 methods did not give good results ; the separation of iron was incom- 

 plete and the cleaning of the machines involved frequent stoppage. 

 From 1888 mills with torched steel balls, the wear of which is very 

 slight, have been used. Ball mills, moreover, almost entirely do 

 away with dust, owing to the installation of a special chamber in- 

 tended to collect it, and of a draught chimney which draws it 

 thereto. The hot air entering the draught chimney entrains the 

 dust formed in the crusher ; about 1 per cent of dust is got for 100 

 of fine powder. When fans were used — useless with the present 

 system — 6 per cent of dust was obtained. The dust chamber is 

 emptied every eight to fifteen days, according as the work is by 

 day or both by day and by night. The blocks of basic slag are first 

 broken up by hand to eliminate pieces of iron, which are laid on one 

 side to return to the foundry. The basic slag so obtained is then 

 reduced to a fine powder (passing through a 100 sieve) in ball mills. 

 The powder is collected in the lower part of the mill and bagged up 

 in 2 cwt. (220 lb.) bags. Formerly the basic slag was allowed to 

 slake in the air, and the fragments after eliminating the iron were 

 ground in flatstone mills. But basic slag slaked after long ex- 

 posure to atmospheric agents sometimes contain excessively hard 

 pieces, resisting the most forcible grinding ; this method was aban- 

 doned. The treatment to which the basic slag is subjected as it 

 comes from the converter has a considerable effect on the grinding. 

 V. Eeis has examined the composition of l^asic slag and its con- 

 catenation with its resistance to grinding. From an outward in- 

 spection there are two kinds of basic slag, block slag and poured 

 slag. Block basic slag is got when it is allowed to solidify in the 

 wagons into which it is poured from the converter. It then cools- 

 slowly and uniformly, and may be removed in a single block which 

 is easily detached from the platform of the wagon. Poured basic 

 slag is obtained by pouring the contents of the wagon on the 

 ground. It spreads out in a thick layer and cools rapidly. This, 

 kind of basic slag is generally very hard ; its grinding is difficult 

 and requires a great expense of motive power. Basic slags in blocks, 

 on the contrary, are more easily ground, but they also contain very 

 hard lumps. The following table by Eeis gives the composition 

 of different slags and their resistance to grinding : — 



