Before the metal, which may be either employed direct from the blast furnace without intervening remelting, or, if for any reason this is not convenient, may have been remelted in a cupola, is run into the converter, from 15 to 18 per cent, of common " well-burnt" lime is thrown into the vessel. The metal is then introduced and the charge is " blown" in the ordinary way to the point at which the ordinary Bessemer operation is stopped—that is, till the disappearance of the carbon. as indicated by the drop of the flame.
The dephosphorizing process requires, however, to be continued for a further 100 to 300 seconds ; this period of so-called "after-blow," which would be prejudicial both to quality and yield in the ordinary process, being with phosphoric iron milder conditions permit ting of the removal of phosphorus) that in which the great bulk of the phosphorus, down indeed to its last traces, is removed. The termination of the operation is shown by a peculiar change in the flame, and checked by a sample of the metal being rapidly taken from the turned-down converter, flattened under the hammer, quenched, and broken, so as to indicate by its fracture whether the purification is complete. A practised eye can immediately tell whether this is the case or not. If the metal require further purification, this is effected by a few minutes further blowing.
The operation is thus, as will be seen, but little different from the ordinary Bessemer process. The differences that have been indicated, viz.: the lime lining, the lime addition, and the after-biow, are, however, sufficient not only to enable the whole of the phosphorus (which would be otherwise untouched) to be completely removed, but the silicon, of which inconvenient and even dangerous quantities are occasionally left in the regular Bessemer process, is also entirely eliminated, while at least CO per cent. of any sulphur (also untouched in the ordinary proeess) which may have been present in the pig is also expelled. It is found, too, that the once-dreaded phosphorus is of most substantial assistance in securing by its combustion the intense heat, necessary for obtaining a successful blow and hot metal. If it is desired to produce "ingot iron," or a metal differing only from puddleAl iron by its homo geneity and solidity, the usual addition of spiegel is omitted, or replaced by a half per cent. of rich ferromanganese. The i)llosphorns is oxidized by the blast, forming phosphoric acid, which, finding itself in presence of two strong bases, oxide of iron and lime, unites with the latter of them to form phosphate of lime, which passes into the slag. Whether or not there is a transitory formation 'of phosphate. tanking oxide of iron perform the function of • a carrier, is a matter (though interesting theoretically) which it is needless here to discuss. The basic Siemens and Siemens-Martin processes are carried out upon the same lines as the Bessemer process. The dephosphorization is very complete, hut the operation takes about 5 per cent, longer than when pure material is the proportion of lime required is less than in the Bessemer process, and the wear of the basic hearth, with suitable arrangements, is not excessive. In 1878 there was not even in existence any public record of successful
dephosphorization of pig iron. In 1884. 864.000 tons of basic steel were produced. In 1890 the production was 2,603,083 tons. Moreover, in this last year, too, there were also pro duced, together with the steel, 623,000 tons of phosphoric slag, most of which was used for fertilizing purposes.
The Darby Reearburizing Process.—This process, invented by Mr. John Henry Darby, of the Brymbo Steel Works, consists in a method for adding the required carbon to molten steel by means of pure pulverized carbon, in lieu of the spiegel hitherto used.
The addition of the carbon may be made by any of the following methods : (1) By the use of a special funnel-shaped filter, which is filled with carbon, and through which the molten metal flows.
(2) By means of a worm, working in a funnel, hanging or standing over the ingot molds, when it is desired to recarburize only a few ingots from a charge.
(3) In the Siemens furnace the carbon is added to the molten steel its it, flows from the furnace down the spout, and in the Bessemer process as the metal flows from the converter into the ladle, so that the recarburization takes place, partly luring the casting and partly in the ladle. The third method is now used, both on account of its cheapness and exactness. The ground carbon is placed in a sheet-iron funnel, which for a 10-ton charge should be capable of holding about 450 lbs. of good ground coke. The funnel is provided with a sliding valve, at the lower end of which a pipe is affixed, and through which the carbon falls into the stream of metal. The flow of the carbon should be so regulated that the whole of the car bon is in the ladle when two-thirds of the steel has been run into it. The slag must be kept well back, especially in the basic process, to prevent reduction of the phosphorus in the slag. The amount of carbon to be added must be 10 to 20 per cent. in excess of the theo retical quantity for a given percentage. Experiments have shown that in order to in crease the carbon 0.05 per cent. in a ton of steel, about lbs. of coke must be used. From this as a basis, a table of "charges" may easily be figured out for any given percentage of increase, By this process the use of spiegel is entirely done away with ; the amount of ferromanganese to be added, however, is about the same as by the older method of recur burizing. (See On the Darby Process of Recarburization," by M. A. Thielen, Journal of the from and Steel Institute, No. 2, 1890.) The Lash Open-hearth Furnace Plant is illustrated in Figs. 1, 2, 3. It is peculiarly adapted to the use of natural gas. There are 16 furnaces erected in Pittsburg on the Lash system, four of 40 tons, five of 30 tons, one of 20 tons, and six of 15 tons.