METHOD FOR TREATING PIG IRON

Abstract: A method for treating pig iron, i.e. crude iron containing more than 3% carbon, in order to remove silicon and sulphur therefrom, comprises charging a converter with the molten pig iron, said converter being provided with at least one tuyere in its bottom and/or its side and containing slag from the preceding cycle, said slag being rich in sulphur and calcium, said calcium being bound partly to the sulphur in the form of calcium sulfide, partly being present in the form of excess lime (calcium oxide). The silicon of the pig iron is oxidized and the silicon oxide formed is bound by the excess lime of the slag. The slag formed is poured out and an excess of lime is added to the remaining iron melt to react with and bind any sulphur present in the melt. Stirring gas is entered through said tuyere(s), and the melt, refined with respect to silicon and sulphur, is drawn off below the covering slag bed. The slag, rich in sulphur and lime, is kept in the converter to be used with the next charge.

Description: METHOD FOR TREATING PIG IRON TECHNICAL SCOPE The invention relates to a method for treating pig iron, i.e. crude iron containing more than 3% carbon, so as to remove silicon and sulphur therefrom. BACKGROUND ART It is theoretically a simple procedure to refine pig iron to remove silicon and sulphur. The thermodynamic conditions there¬ fore are favourable, but nonetheless no technical solution has been sufficiently efficient to become dominant, as is evident from the abundance of suggested and attempted methods and addi¬ tives. According to prior art, the pig iron is treated in an ordinary ladle, a torpedo ladle, a jar ladle or a rotary kiln. According to one conventional method, sulphur refining is carried out by simply adding a desulphurizing agent, such as sodium carbonate or lime and fluoritε, to the ladle before'it is charged or on top of the melt. This method is inefficient and the results inconsistent. Efficiency is improved by injecting the desulphurizer into the melt. This technique requires fairly complicated equipment, however, and in order to get efficient desulphurization comparatively costly desulphurization agents must be added, such as magnesium or calcium carbide in conjunc- tion with lime. Finally, refining in rotary kilns or in jar ladles requires very long desulphurization-times. DISCLOSURE OF THE INVENTION The object of the invention is to provide a better method of removing silicon and sulphur from pig iron melts. In particular, the invention aims at providing a method which comprises simple handling, quick reactions, cheap desulphurizers and lesser quantities thereof. 096 These and other objects are realized by the invention being characterized as defined in the appended patent claims. Further characteristics and objects of the invention will become apparent from the following description of a preferred embodiment. DESCRIPTION OF PREFERRED EMBODIMENT The reaction vessel according to the invention is a converter with one or several tuyeres in its bottom and/or its side. The converter should also be provided with a pouring hole at one side, so that the molten metal can be drawn off with the slag remaining in the converter. The method according to the invention is a cyclic process, in which the slag is kept in the converter from one charge to the next. This slag contains that amount of sulphur which was removed from the preceding melt. The sulphur is bound in the slag in the form of calcium sulfide. In addition, the slag contains enough of an excess of lime to keep the basicity, measured as the quotient Caθ/Siθ2 » constantly above 1, preferably above 2, after the oxygen blast following charging the converter. A working cycle starts with the charging of the converter with the molten pig iron to be refined, the converter already con- taining said slag from the preceding cycle. An oxidizing gas is let into the melt through the tuyeres, thereby oxidizing the silicon of the pig iron. The oxidizing gas may for example consist of air or water vapour or of a mixture of air and water vapour, or be some other gas mixture containing oxygen. It is also possible to accomplish oxidation by adding iron oxide ? the iron oxide being reduced by the silicon of the melt. The heat of reaction of the silicon oxidation may also be put to use to melt added scrap iron or pig iron according to prior art. The excess lime of the slag remaining after desulphurization has several functions in connection with the silicon oxidation: it serves to bind the silicon oxide formed, to prevent re-reduction of sulphur into the iron melt, and to impart the desired viscosity to the slag. If the sulphur content of the iron melt is comparatively small, the basicity should be considerably larger than 2, so as to prevent the re-reduction of sulphur to the iron melt. To keep the slag from becoming difficult to handle, a flux is also added in connection herewith, preferably in the form of fluorite, CaF2_ which also improves desulphurization. When the silicon oxidation is completed, but before any signi¬ ficant oxidation of carbon starts, the slag formed is drawn off, with the molten iron remaining in the converter- A desulphurizer, preferably lime, is added to the remaining iron melt in the con¬ verter, said lime or the like being added, to react with and bind any sulphur in the melt, in such a quantity that the condi¬ tion stated above regarding basicity is complied with. To speed up the reaction, gas is entered through the tuyeres for stirring. The stirring gas may be nitrogen, air, water vapour, and/or argon. The addition of lime may be done either by adding lump lime or lime powder from above or by injecting it through the tuyeres together with the stirring gas. In this way the melt is refined with respect to sulphur, which accumulates in the slag. The pig iron melt is drawn off through the pouring hole below the covering slag bed, the sulphur-rich and lime-rich slag being kept in the converter to be used with the next charge. The procedure described may be modified within the scope of the invention. Lime may thus be added in two or more batches, for example, one batch before the desulphurization of the de- siliconized iron melt and one after the iron melt has been drawn off. Further, lime may be added in several batches or continuous¬ ly during the oxidation of silicon, such as by injection.

Claims: CLAIMS 1. Method for treating pig iron, i.e. crude iron containing more than 3% carbon, for the purpose of removing silicon and sulphur therefrom, c h a r a c t e r i z e d i n that it comprises a cyclic procedure with the following steps: a) charging a converter with the molten iron, said converter being provided with at least one tuyere in its bottom and/or its wall and containing slag from the preceding cycle, said slag being rich in sulphur and calcium, said calcium partly being bound to sulphur as calcium sulphide, partly being present as an excess of lime (calcium oxide); b) oxidizing the silicon of the pig iron, the silicon oxide formed being bound by the excess lime of the slag; c) drawing off the slag formed; d) adding an excess of lime to the remaining metal melt to react with an bind any sulphur in the melt; e) stirring the melt by passing stirring gas through said »_ tuyere(s); f) drawing off the molten pig iron, refined with respect to silicon and sulphur, below the covering slag bed; and g) keeping the slag, rich in sulphur and lime, in the converter to be used with the next charge. 2. Method according to claim 1, c h a r a c t e r i z e d i n that gas containing oxygen is blown into the converter through said tuyere(s) for the oxidation of silicon. 3. Method according to claim 1, c h a r a c t e r i z e d i n that the oxygen needed for the oxidation of silicon is supplied entirely or partly by iron oxides added to the melt. 4. Method according to claim 1, c h a r a c t e r i z e d i n that the heat of reaction of the oxidation of silicon is utilized for melting added scrap iron and/or pig iron. 5. Method according to any of the claims 1 - 4, c h a r a c t e r i z e d i n that the basicity of the slag remaining in the converter from one charge to the next is >1, preferably ___2. 6. Method according to any of the claims 1 - 5, c h a r a c t e r i z e d i n that lime is added also before charging the converter with crude iron and/or during the oxidation of silicon.

Inventors: JOHNSSON MATS KLAES OLOF, LUNDSTROEM PER-AOKE, JOHNSSON, MATS, KLAES, OLOF, LUNDSTROEM, PER-AOKE
Applicants: UDDEHOLM INTERNATIONAL AKTIEBO, UDDEHOLM INTERNATIONAL AKTIEBOLAG
Application year: 1985
Main classification: C21C1
IPC codes: C21C1