Page:The American Cyclopædia (1879) Volume IX.djvu/414

 400 IRON MANUFACTURE and the mass of metal begins to "boil" from the escape of carbonic oxide, which burns with a blue flame on the surface. Finally the ebul- lition becomes less rapid, and little bright points of soft iron appear. The iron is then said to " come to nature." The stirring or rabbling is continued incessantly, to prevent the formation of lumps of imperfectly decar- burized iron. When the operation is complete, the puddler forms a number of balls of the pasty iron, of about 60 to 80 Ibs. each. As soon as the balls are formed, the damper is lowered in order to prevent the waste of iron by burning. During the whole course of the puddling the working door remains closed, and the rabbling tools are introduced through a small notch in the lower side of the door. When the balls are ready for removal, the door is opened and the balls are taken singly to the hammer or squeezer. The duration of the process is from 1 to If hour, the loss of iron 10 to 15 per cent., and the consumption of fuel from 16 to 30 cwts., according to charac- ter of fuel, size of charge, &c. Sometimes the term puddling is restricted to the working of white or nearly white iron in a furnace without fettling, and the term boiling is ap- plied to the process described above. Such boiled iron is, other things being equal, supe- rior to puddled iron. As the oxidation of the carbon and silicon is mainly effected by the oxide of iron of the fettling, there should be a gain of malleable iron over the pig iron charged, instead of a loss; since for every 18 parts of carbon removed from the pig iron 56 parts of iron are reduced from the oxide, and for every 42 parts of silicon removed there are also 56 parts of iron reduced. In the ordinary pud- dling furnace, however, with the strongly ox- idizing atmosphere, the loss of iron more than compensates for this gain. Siemens has suc- ceeded, in his gas furnaces, in obtaining a yield of wrought iron equal to the weight of pig charged ; but Banks, in his rotary puddler, de- scribed below, has obtained nearly the theo- retical amount of gain. The quality of the wrought iron produced by puddling depends on the composition of the pig iron used, and on the care and thoroughness of working. The removal of silicon and carbon is easily effected by good working ; but phosphorus and sulphur are never completely removed. According to Parry, 15 to 80 per cent, of phosphorus and 80 per cent, of sulphur is ordinarily elimi- nated. The manner of removal of phospho- rus is somewhat doubtful. Percy thinks it is mainly in the form of phosphide of iron, which is more fusible than the iron, and is hence carried off with the cinder. It has, however, been noticed that the more basic the cinder, the more phosphorus it contains ; which renders it probable that phosphorus is present in the cinder in the form of phosphoric acid. Sulphur is said to be principally removed in the latter part of the process, which is consequently prolonged when highly sulphur- ous pig iron is puddled. This prolongation of the operation tends to make a highly fibrous iron, probably owing to the fact (see IRON) that the cinder becomes very basic and more infusible, and is hence not so readily expelled by rolling or hammering. The purer the pig iron treated, that is, the less sulphur and phos- phorus it contains, the shorter may the opera- tion be, and the more granular and crystal- line the product. Puddled steel is made in the same way as wrought iron, but from pure pig iron, containing not too much silicon. The operation is stopped before all the carbon is oxidized, and a steely product is obtained. The presence of manganese is here an advantage ; since, as has been before mentioned, oxide of manganese in the cinder does not oxidize the carbon of the pig iron. Tap or puddling cin- der is composed mainly of silica and oxides of iron ; it may be considered as a tribasic sili- cate of ferrous oxide, containing also, at times, ferric and magnetic oxide. Phosphoric acid and sulphur are also generally present. Such cin- der is used either raw or roasted in the blast furnace, yielding when in large quantity an in- ferior quality of iron, known as cinder pig ; or it is used after roasting as fettling for the pud- dling furnace. The following analysis is of tap cinder made from common white iron: silica, 7'71 ; ferrous oxide, 66'32 ; ferric oxide, 8'27; manganese oxide, 1'29; alumina, 1'63 ; lime, 3-91; magnesia, 0'34; sulphur, 1-78; phosphoric acid, 8-07; total, 99-32. There have been proposed from time to time a great number of fluxes and purifying agents for use in the puddling process; they are mainly di- rected against sulphur and phosphorus, par- ticularly the latter. The following are a few of the substances proposed: oxide of manga- nese, common salt, iodide of potassium, nitre, litharge, copperas, chloride of calcium, lime, and fluoride of calcium. Manganese generally exerts a favorable influence in all iron and steel processes. Its action is not well understood, except so far as it hinders the decarburization of pig iron when it replaces iron in cinder. It is not probable that it affects eithur sulphur or phosphorus in the puddling. The volatilization of sulphur and phosphorus as chlorides has long been a favorite theory; but there is no proof that chloride of sodium or any other chloride acts in this way. The oxidation of phosphorus in pig iron takes place readily ; and, if the cinder present is sufficiently basic, the phosphoric acid will remain combined ; if not, the phosphorus will recombine with the iron. (See Bessemer process, under STEEL.) A highly basic iron cinder, as above mentioned, retains a large amount of phosphoric acid, but if in- stead of oxide of iron a stronger base is sub- stituted, the phosphoric acid will be still more firmly held in combination. The alkalies and alkaline earths have proved to be valuable de- phosphorizers ; and it is probable that salt, nitre, and chloride of calcium act, in this re- spect, solely by virtue of their respective bases.