REMOVAL OF COPPER FROM CARBON-IRON MELTS

Transcription

1 REMOVAL OF COPPER FROM CARBON-IRON MELTS Yuri Kostetsky Alexander Troyansky Maxim Samborsky Donetsk State Technical University, Artyoma Str. 58, Donetsk, 83000, Ukraine Abstract During a heat or a ladle treatment copper can t be removed from steel melts via traditional refining methods. Among other methods, which can ensure copper removing from iron melts, refining with sulfide fluxes may be highlighted as very promising to be a base of industrial technology. The slag systems on the base of sodium sulfide and aluminium sulfide are the most appropriate for application. First stage results of laboratory investigations show that it is possible to remove copper from iron melts by treatment with soda and soda containing mixtures. The accumulation of tramp elements in the scrap, mainly copper, is a widely concerned problem. Until now the problem of copper removal from steel during the conventional steelmaking processes has not found an appropriate solution. Usually standards limit copper content in the finished steel up to 0,2-0,3%. Exceeding this level brings noticeable diminution of mechanical properties of metal and may account for hot shortness in the hot-rolling operation, cracks during cold forming and even during continuous casting operations. Copper comes to steel with the metal part of charge. Almost all of metal materials bring it (Fig.1) /1/. But really in steelmaking under control is only the copper level in the scrap. Table 1 Copper contamination of charge materials Type of charge material Percentage of copper, % Waste chips 0,23-0,58 Obsolescent scrap and electric-furnace bundles 0,14-0,42 Heavy home scrap 0,07-0,10 Miscellaneous home scrap (scrap, tedges, pours, deadheads) 0,16-0,18 Ferroalloy up to 1,2 Auxiliary aluminum up to 3 Special sorting and separation during scrap processing allows lowering a copper accumulation in the steelmaking products. For example cutting up of the scrap in the pieces up to 300x300 mm instead of 600x600 mm and subsequent copper impurities picking up with the electromagnetic separator gives additional copper extraction 1,5-2 times as much. Furthermore it is possible to reduce a harmful influence of copper presence in steel by nickel

2 or molybdenum addition in quantity close to copper concentration /2/. Rare-earth metal additions also afford a beneficial effect. But these methods do not solve the problem in whole. In this connection it is important to develop effective methods of iron melts refining from copper contamination. Principal peculiarity of copper as impurity is lower affinity to oxygen than that for iron in conditions of steelmaking processes. As a result it is not eliminated from liquid metal under oxidizing conditions during a heat. During a ladle treatment copper is not removed from steel too. For the last decades numerous ideas have been proposed for the removal of copper from molten iron /3/. Among them may be selected as promising methods for steelmaking:! copper removal from molten iron into the gas phase by evaporation in vacuum or during the treatment with the aid of special gas mixtures;! filtration of iron melts;! Slag treatment with sulfide fluxes. In this work the last method is discussed as the most appropriate and promising for a commercial application. It is based on the facts that copper has larger affinity to sulfur than that for iron and on existing of the immiscibility interval between liquid iron sulfite and ironcarbon melt in Fe-C-S system. A reaction of copper removal from metal to sulfide flux can be represented as follows: [ Cu ] + 1/ 2[ S] = ( CuS 0. 5) a K = a X ( CuS0.5 ) ( CuS0.5 ) ( CuS0. 5 ) = 1/ 2 1/ 2 a[ S ] X a[ S ] (1) (2) Where i and X i are the activity coefficients and mole fractions of compounds, and a i denotes the activity of compounds. An expression for the distribution ratio of copper can be derived from equation (2). After rearrangement and introducing the mass percent instead the mole fraction of copper in the metal and flux the desired expression may be written as L Cu (% Cu) = [% Cu] = C ( CuS0.5 ) a 1/ 2 [ S ] (3) Where C is the coefficient correcting for the conversion into mass percentages and including the equilibrium constant of the reaction (1). The analysis of the expression (3) shows that increasing of sulfur activity and copper coefficient of activity in metal leads to increase of the distribution ration of copper. Large growth of sulfur activity is not acceptable because it is tied with increasing of the sulfur concentration in the metal. As regards for activity coefficient of copper it increases with growing of carbon and silicon concentrations in metal and with decreasing of metal s temperature. However the influence of these factors is negligible from the practical point of

3 view. Therefore the more preferable way to increase the distribution ratio of copper between slag and metal is a decrease in value of copper sulfide activity coefficient. The distribution ratio of copper between carbon-saturated iron and liquid FeS approximately equal 9 at the temperature 1400 o C and sulfur concentration in the metal 1,9% /4/. An addition of sodium sulfide to FeS leads to increase of the distribution ratio. The maximum value L Cu =24 is observed at the mole fraction of sodium sulfide in slag 0,4 and it slightly changes due to further increasing of the sodium sulfide concentration. Besides the additions of sodium sulfide also provide the decrease of sulfur concentration in metal from 1,9% up to 0,04% at the mole fraction of sodium sulfide 0,8. Figure 1. Influence of alkali metal sulfide additions to FeS flux onto the distribution ratio of copper between carbon-saturated iron and slag. As experiments show additions of another alkali element sulfides and alkaline earth metal sulfides similarly influence on the L Cu magnitude (fig.1) /5/. In laboratory conditions for slag systems such as FeS-LiS 0,5, FeS-KS 0,5, FeS-SrS 0,5 and FeS-BaS it was fixed follows maximum values of the distribution ratio 30, 20, 22 and 19, respectively. Additions of Mg and Ca sulfides did not influence substantially onto L Cu value due to their restricted saturation ability in liquid FeS /6/. Encouraging result was reached for aluminum sulfide, according with the publications its additions also permit to increase the distribution ratio up to 30 /7, 8/. Therefore laboratory experiments show the ability to refine an iron melts from copper by a sulfide slag treatment. The slag systems on the base of sodium sulfide and aluminum sulfide can be highlighted as the most appropriate for application in commercial processes. As rule the ideas of such processes employ a counterflow operation to ensure effective interaction between metal and slag. For instance it was proposed a process for copper removal from molten iron which uses successive contacts between the Al 2 S 3 -FeS flux and metal in the course of the three stages counterflow operation /8/. This method requires the use of sulfide resources, aluminum, carbon, flux for desulfurization, electricity for heating, etc., and

4 specially designed equipment. So it is critical from the economical point of view. Clearly it is main reason why such ideas didn t reach an industrial realization until now. There is data about experiments in which soda was used for a ladle treatment of open-hearth pig iron /9/. In the experiments pig iron, contains 0,23-0,5% sulfur and 0,23-0,38% copper, was tapped from EAF into a ladle which was preliminary loaded with soda. It is suggested that sodium sulfide was produced according the reaction: Na [ = Na S + CO + CO 2 CO3 + C] + [ S] 2 2 (4) After treating copper percentages in pig iron was lower on 0,03%. The same operation gave more perceptible results under laboratory conditions. Authors explained that through higher sulfur content in the pig iron (up to 1,53%) during the laboratory experiments. But observed laboratory results were some unstable. It was tied with features of interaction between melt and soda. The interaction leads to strong gasification and as result appreciable volatilizing of sulfides. Thus chose method of the reactant introducing into molten metal can t be adopt as effective, but it is interesting to clear up more deeper possibilities of iron melts refining from copper by soda. Such investigations were started at the Donetsk state technical university. It is suggested introduce a refining reagent into liquid metal by powder injection or in the form of granules. The first stage of the work was devoted to elucidating fundamental possibilities of iron melts refining from copper by soda or refining mixtures containing soda in a laboratory environment. Refining mixture Iron melt Bell Crucible A master alloy containing copper was prepared by alloying of pig iron with copper chips in a graphite crucible in an induction furnace. Then in each experiment, 500 or 350g of prepared pig iron was melted also in a graphite crucible in a vertical resistance furnace and at 1523K refining materials were introduced in the iron melt under argon atmosphere. Like this several series of heats were made. In the course of experiments follows materials were examined: fused soda block; soda and sulfur powders; mixtures of soda, sulfur and carbon powders. Figure 2 Cheme of powder materials introducing in the course of the experimental heats In order to eliminate vigorous metal splashing and to lower ineffectual losses by evaporation during a treatment powder materials were introduced into metal in a special bell made with graphite or quartz glass (fig.2).

5 Figure 3 shows an average percentage change of carbon, copper and sulfur concentration in the metal reached by a treatment relative to them start concentration for the each series. In the course of the experiments two different scheme of treatment were employed. In I, II series of experiments a portion of sulfur was added to the metal, which contains 3,93% of carbon, 0,64% of copper, 0.043% of sulfur, and then a soda block is introduced into the melt within five minutes. In two minutes a sample for chemical test was taken away. In the other series pig iron, contains 4,13% of carbon, 0,53% of copper and 0,1% of sulfur, was treated by a mixture of sulfur, soda and carbon. Samples for chemical test were taken away after five minutes holding.

6 Figure 3. Carbon, copper and sulfur average percentage change due to a treatment relative to them start concentrations for the each experimental series. Only second variant of the treatment, III-VII series, brought decrease of the copper percentage in the metal and the best result η [Cu] =24% was observed in VII series. But the results show some instability in III-VII series. It is responsible for unstable assimilation of refining materials in the course of experiments due to vigorous gasification. Take attention a clear correlation between changing of carbon and copper concentrations in the melt, which is observed after treatment in III-VII series. It is an evidence the reaction (4) takes place. A

7 principal drawback of this variant of treatment is enough high final percentage of supfur in the metal. The sulfur concentration in the metal didn t change strongly in II series and I. But copper percentage increased appreciably. Though it is hard to guess that a sulfur potential was too low for copper slagging after sulfur addition. Approximate mass balance shows that whole mass of copper in the metal must be lowered on 10% at least. Observed increasing of copper percentage can be explained by large losses of iron for slagging without subsequent noticeable copper extraction into the slag. Great sulfide slag losses occurred during treatment by a soda block. Thus represented results of the first stage of the investigation may be a point for discussion. Unfortunately they didn t give a possibility to make a final decision as for real efficiency of the examined refining methods with soda, but it brought necessary information for subsequent experiments planning. A computer model of the physical-chemical processes, which take place in course of the treatment, is under development now. LITERATURE 1. КУДРИН, В. Перспективные способы удаления примесей цветных металлов из железоуглеродистых расплавов. I конгресс сталеплавильщиков. Москва С ЗИГАЛО, И, БАПТИЗМАНСКИЙ, В, ВЯТКИН, Ю, и др. Медь в стали и проблемы ее удаления. Сталь с JIMBO, I, SULSKY, M, FRUEHAN, R. Thermodynamics of Copper Removal from Carbon Saturated Iron with FeS-Na 2 S-Cu 2 S Matte. W. O. Philbrook Memorial Symposium Conference Proceedings, ISS-AIME, Toronto Canada, 1988, p WANG, C, HIRAMA, J, NAGASAKA, T, BAN-YA, S. Copper Distribution between Molten FeS NaS 0.5 Flux and Carbon Saturated Iron Melt. ISIJ Int., 1991, vol. 31, 11, рр WANG, C, NAGASAKA, T, HINO, M, BAN-YA, S. Copper Distribution between FeS Alkaline or Alkaline Earth Metal Sulfide Fluxes and Carbon Saturated Iron Melt. ISIJ Int., 1991, vol. 31, 11, рр LIU, X, JEFFES, H. Decopperisation of molten steel by various slags. Ironmaking and Steelmaking, 1989, vol. 16, 5, pp COHEN, A, BLANDER, M. Removal of Copper from Carbon-Saturated Iron with an Aluminum Sulfide/Ferrous Sulfide Flux. Metallurgical and Material Transactions B, 1998, v. 29B, 2, pp SHIMPO, R, FUKAYA, Y, ISHIKAWA, T, OGAWA, O. Copper Removal from Carbon- Saturated Molten Iron with Al 2 S 3 -FeS Flux. Metallurgical and Material Transactions B, 1997, v. 28B, 11, pp КАШИН, В, КАЦНЕЛЬСОН, А, ДАНИЛОВИЧ, А. Опытно-промышленное опробование процесса рафинирования железоуглеродистых расплавов от меди. Сталь с