Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/360

344 IRON hole, was patented ; not till some time later, however (after the in troduction of Mushet s improvement of adding spiegeleisen), was the present form of converter arrived at, consisting of a pear-shaped or bottle-shaped vessel with tuyeres at the base, and supported upon axes, one of which being hollow serves as the tube by means of which the blast is communicated to the tuyeres, whilst the vessel can be rotated into any desired position round the axis (fig. 60) ; the use of hydraulic power to work the converter into position, and to manipu late the "ladle" into which the finished metal is run so as to be poured from it into moulds, soon followed. Subsequently various improvements in details of arrangement and construction of the plant have been introduced, notably by Holley in certain American works in the first instance ; amongst these may more particularly be noticed the use of somewhat longer converters (fig. 61), with movable bottoms, so that, as the tuyeres wear out (which occurs much more rapidly than does the destruction of the lining), new previously constructed bottoms can be introduced without entailing large loss Fig. 61. of time, whilst the greater length diminishes less by splashing and forcible ejection during the boil ; the effect of these improvements is to increase the possible output from a given plant by at least 50 per cent. At first it was usual to melt the pig iron in a separate reverberatory furnace and then to run it into the converter ; a cupola furnace (saving much fuel) was then substituted, a little lime being added to diminish its tendency to sulphuration ; in this way scrap of all kinds can be utilized, being melted down with fresh pig in a cupola furnace much more readily than in a reverberatory, and being less decarbonized and desiliconized in so doing, which is somewhat important ( 27). Where the blast furnaces are not too far from the converters, the molten pig is often run direct from the furnace into the converter without solidifying into pig and remelting, an intermediate ladle running on a railway being employed instead of a long gutter, which would cool the metal too much and be other wise impracticable. In some few Swedish works the old immovable cylindrical converter is still employed, the metal being tapped out into a ladle from which it is run into the ingot moulds, but in most works, even in Sweden, movable converters are now in use. The spiegeleisen usually added subsequently is melted in another smaller cupola ; in some few works however, e.g., in Styria, spiegel is not used, but when the charge is blown some more of the original pig is added ; in others Bessemer s original process is adhered to ( 27) ; the use of this method, however, is comparatively limited, the combination process being usually adopted in preference. When ferro-manganese is used instead of spiegeleisen, it is usually not melted, but simply heated in a suitable vessel by the waste flame of the converter, and emptied into the casting ladle together with the blown metal so as to intermix the two, the ferro-manganese fusing as soon as it comes in contact with the blown metal. Converters. The improved form of converter in use at the present day is indicated by fig. 61. It consists of a vessel in shape resembling a claret bottle with the neck somewhat shortened and slightly bent over sideways ; this is made of stout boilerplate, lined internally with " ganister," a kind of sandstone possessed of peculiar phy sical properties, more especially of the power of binding together to a compact non-crumbly excessively infusible mass when ground to powder, moistened, and finally ex posed to a high temperature. The base of the bottle is removable, being kept In position by bolts and nuts ; this being removed, a core is fitted concentrically within the shell, and the moistened ganister rammed lightly in ; sometimes the shell is also made of two parts thus treated separately and afterwards bolted together. The bottom is double, constituting an air chamber or " blast box," the top portion of which is a perforated cast iron plate, into which are fitted tuyeres consisting of slightly conical elongated bricks or plugs perforated longitudinally with holes (between 15 and 3 inch diameter) and fixed in at the thicker end ; these are composed of a mixture of fireclay and ganister, the space between them being filled up with ganister, so that the inner bottom is substantially the same as the lining sides of the converter, only perforated by a number of holes. The total number of orifices through which the blast thus comes is considerable, in order to spread it into numerous streams, the actual number varying with the size of the converter; fora 5 -ton converter a dozen or more tuyere bricks, each perforated with about as many holes, are usually employed. The lower plate of the air chamber is removable for the purpose of examining the tuyeres from time to time without removing the entire bottom. The different portions of the vessel being put together, and the joints well grouted with ganister slip, the whole is gently dried by lighting a small coke fire inside, and by and by increasing this, the blast being turned on gently until the whole vessel is thoroughly dry and hot, when it is ready to receive a charge of molten pig. The blast is communicated to the blast-box by a curved tube reach ing thereto from the hollow trunnion box, inside which one of the trunnions of the converter lies ; the other trunnion has a cogged wheel or pinion attached, by means of which the vessel can be rotated, a toothed rack gearing into the pinion and being worked directly by a hydraulic press or ram ; the rack and press were formerly arranged horizontally, but now are usually placed vertically or slantwise to save space. By means of an automatic valve the blast is shut off when the converter is in an inclined position, so that the level of the mouth is not lower than the tuyere hole then lowest ; in this position the molten metal is run in, the elevation of the tuyere holes preventing it from running into them ; whilst erecting the converter by means of the rack and pinion before the tuyeres sink below the molten mass, the blast is turned on at a pressure of some 1 - 5 atmospheres or more (21 lb per square inch and upwards) ; the pressure due to the column of molten metal being less than this, the air is forced through it without it being possible for the fused matter to run down into the blast-box. A large hood over the mouth of the converter and connected with a chimney or Hue prevents the flame and ejected matter from being scattered about and injuring the workmen. The ganister preferred for lining is a peculiar silicious deposit found under a thin coal-seam near Sheffield, of almost conchoidal fracture, therein differing from ordinary sandstones, and containing a few tenths per cent., or sometimes a little more, of lime and about the same amount of alumina, with small quantities of iron oxide and alkalies, the rest being silica ; analogous substances, however, are found in various other localities, e.g., in the Yorkshire, Northumberland, South Wales, and Shropshire Coal Measures. A well-prepared lining carefully repaired every few days will last several months, and even upwards of a year ; the bottoms, however, wear away much more rapidly, the tuyeres either becoming melted or dissolved away by the oxidation of the iron in their vicinity, the oxide produced exerting a marked solvent action on the fireclay and ganister bricks ; hence the advantage of Holley s removable bottoms. A damaged bottom can be removed and a new one put in its place, the joint being made up by ramming in ganister into the crevice whilst screwing up the bolts, without stopping the action of the converter for any lengthened period : whilst with the older vessels it was requisite to cool down somewhat, knock out the damaged tuyere bricks, replace them by others, and ram in ganister between them from the inside, and finally to heat up again by an internal fire before use, the whole process causing much delay and waste of fuel, especially as the renewal of some at least of the tuyere bricks is requisite every few blows. In some American works, e.g., Bethlehem, instead of a rammed ganister lining, one composed of blocks of a peculiar sandstone is employed, set in ganister as mortar for the joints. Subsidiary Appliances. The precise mode of arrangement of Bessemer plant varies in different works, but usually two converters are arranged to be worked together, one casting ladle being used for the pair, worked by a crane. In most of the European works the two converters are on opposite sides of the casting ladle, so that a