Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/307

IRON 291 passing through the stage of cast iron ( 29-31); (3) those in which steel is formed from wrought iron by directly carbonizing it ( 32-35) ; and (4) those in which steel is finally prepared by intermixture of carbonized and wrought iron in the fluid state ( 36-41). The methods of class 1 include the preparation of pig iron; its purification by refining, and conversion into wrought iron by fining and puddling (both by hand and by machinery) and by inverse cementation (heating in contact with iron oxide) ; and the preparation of puddled steel and pneumatic steel and iron, i.e., steel prepared by Bessemer s original process, viz., decarbonization more or less complete by blowing air through molten pig iron, and also of Heaton s steel (pig iron decarbonized by nitrate of sodium), &c. Class 2 includes the Catalan forge and allied processes, and the " direct " methods of Clay, Chenot, Yates, Blair, Snelus, Du Puy, Siemens, and others. The processes included in class 3 are those of steel manufacture by cementation and partial acieration by case hardening, together with various other allied methods of producing steel from soft iron ; whilst class 4 includes the Bessemer-Mushet steel process, in which blown Bessemer metal is made into what is usually known as " Bessemer steel " by incorporating with it spiegeleisen ; and the allied open hearth steel processes, in which wrought and cast iron are melted up together, or iron is decarbonized in a Siemens hearth and then mixed with ferro-manganese, &c. ; together with various modifica tions of these processes, such as the Snelus-Thomas-Gilchrist method of blowing phosphorized pig, the Uchatius process, the Ponsard process, &<x IV. MANUFACTURE OF CAST IKON. IRON SMELTING. 9. Preliminary Treatment of Ores. Many kinds of ore are unsuitable for use in the blast furnace without some preliminary treatment, consisting either of washing with water and dressing in the ordinary way adopted with heavy minerals to wash out clay, &c. ; weathering by exposure to air and moisture for considerable periods of time so as to oxidize pyrites, &c., and wash out the soluble matters formed ; roasting, so as to expel carbon dioxide and water and burn off organic matter, peroxidizing the iron in so doing ; or a combination of some or all of these processes. Even with such ores as magnetic ironstone calcination is usually found to increase the ease with which the ores are subsequently smelted, the lumps being rendered somewhat porous, and hence more readily acted upon ; indeed, with compact Swedish magnetites previous calcination is indis pensable, otherwise great waste of fuel is occasioned. In the case of franklinite, a mangano-zinciferous magnetite, the mineral is first roasted with lime and anthracite so as to distil off the zinc, and the residue then smelted for spiegeleisen. Certain Westphalian and other ores are deprived of sulphur existing as pyrites by roasting in a kiln, into which superheated steam is admitted at intervals, whereby sulphuretted hydrogen is first formed and subse quently burnt to sulphur dioxide, whilst the iron of the pyrites is converted into oxide ; by using a mixture of Mghly carbonaceous blackband and other ores, the roasting is effected without the use of any fuel other than that con tained in the blackband. Where fuel is not an object, calcination of iron stone is frequently effected in heaps analogous to those produced in the burning of " ballast " (clayey soil) for foundations of houses, roads, &c.; a fire of slack being made, shovelfuls of ironstone are thrown on to it, and then more slack and more ironstone alternately, until a sufficiently large heap is prepared ; or the heap is first built up and subsequently fired, the spots where the fire comes visibly out of the heap being, when requisite, dampered over with moist email ore so as to prevent too rapid combustion, which might otherwise cause the ore to frit, Blackband usually requires only lighting with a little coal, &c., when properly heaped, furnishing its own fuel. When economy in fuel is desirable, a calcining Idln is employed, generally resembling a lime kiln in construction. Fig. 1 repre sents a kind of kiln (Gjers s) used extensively in the Cleveland district ; it is usually built of firebrick cased with iron plates, circular in section, wider in the middle than at top, and tapering down wards from the middle slightly more rapidly than upwards. An iron double cone AA sur mounts an orifice in the base connected with radiating flues B, B, whereby airis introduced into the interior. The whole superstructure rests on an annular cast iron entablature CO, supported on stout iron pillars D, D ; the cal cined ore is raked out between these pillars, the spaces between them serving for the admis- FIG. 1. Gjers s Kiln. sion of air at the base ; a further air supply is obtained from orifices E, E, E, E in the lower conical portion. A usual size is some 20 feet maximum diameter, and a little more in height, with a capacity of FIG. 2. Westmann s Kiln. upwards of 5000 cubic feet ; but considerably larger kilns arc often used ; the consumption of slack is from 4 to 5 per cent, of the. ore (Cleveland ironstone) calcined. Siemens has patented a somewhat analogous calcining furnace, combustible gas and air being led into the centre and distributed by a cone ; Rachette s calciner is oval,