eckel.] UTILIZATION OF IRON AND STEEL SLAGS. 227 
The slags produced in steel plants using the open-hearth process are 
less valuable as fertilizers than those produced by basic converters, 
as the former contain less phosphoric acid and more silica and lime 
than do the basic Bessemer slags. A two-stage modification of the 
open-hearth process — the Bertrand-Thiel process — gives slags higher in 
phosphoric acid than ordinary open-hearth slags. It is even claimed 
by Thiel a that the Bertrand-Thiel process produces a greater value 
of slag, if both quantity and phosphoric content be considered, per 
ton of finished steel than does the Thomas-Gilchrist process. 
The slags resulting from processes other than those above noted are 
not sufficiently phosphatic for use as high-grade fertilizers. Elbers 
has, however, called attention 5 to the fact that highly calcareous 
blast-furnace slags might be profitably used as fertilizers in place of 
the other forms of lime (marl, shells, etc.) now used by farmers. 
MINERAL WOOL. 
Over half of the material marketed as "mineral wool" or "silicate 
cotton" is derived from slag, the remainder being manufactured from 
natural rocks of different types. 
Originally the process was carried out at the furnaces. At present, 
however, the slag is bought from the furnace companies and remelted 
in a small cupola. From this the molten slag issues in a small stream, 
into which is injected steam or air under pressure. The effect is to 
scatter the slag, small spherules of slag being blown out from the 
main stream, each spherule carrying behind it a thread of slag. 
The fluidity and composition of the slag and the pressure of air or 
steam are manipulated so as to give the greatest proportion of fiber to 
spherules, as the spherules are commercially unavailable and must be 
separated from the fiber if present in much quantity. 
No analyses of slags used in the manufacture of slag avooI are at 
present available. A mineral wool made from natural rock gave on 
analysis the following result: 
Analysis of mineral wool made from natural rock. 
Silica 37. 5 
Alumina and iron oxide 20 
Lime '.. 30.6 
Magnesia 11.8 
The presence of sulphur is a defect in most mineral wools made 
from slag, as they must be carefully protected against moisture to 
prevent the oxidation of the sulphur and the consequent destruction 
of the pipes or other metallic surfaces on which the wool has been 
used. 
The most important property of slag wool, from a commercial point 
of view,. is that it is a very poor conductor of heat. This property 
a Chem. Zeit., 1901, p. 371. &Eng. and Min. Jour., November 3, 1900. 
