Reduction of FeO in Molten Slags by Solid Carbon in. the Electric Arc Furnace Operation*

Transcription

1 Reduction of FeO in Molten Slags by Solid Carbon in the Electric Arc Furnace Operation* By Masatoshi OZA WA, * * Syuzo KI TAGA WA, * * Suguru NAKA YAMA* * and Yoshinori TAKESONO * * Synopsis In the course of operation of 70 t UHP furnace in Chita Plant, Daido Steel, solid carbon has been injected in order to facilitate the reduction of Fe0 in slag and to improve the energy saving and heating efficiency. In this paper, the effect of the injection method on the reduction rate of Fe0 in slag is discussed in relation to the quality and size of carbon powder injected, injection rate and nature of molten slag. Moreover the relation between the fundamental data of waste gas and the degree of slag reduction has been comfirmed. The results obtained are summarized as follows: (1) Volatile matter in solid carbon gives a large influence on the reducing reaction. In the coke of higher content of volatile matter, the reduction is controlled by the chemical reaction, while in the coke o f lower content of volatile matter, the reduction is controlled by the transport of Fe0 in slag. (2) The reduction rate depends on the basicity of molten slag. The reduction rate increases as the basicity of molten slag increases. (3) Estimation of carbon content of molten steel for a rapid operation is found to be possible by the use of the fundamental data of waste gas. I. Introduction A combined melting operation of " Oxygen enrichment and Carbon injection " applied at Chita Plant, Daido Steel Co., Ltd. (herein after it is described as " DAIDO ") brought about the improvement of productivity, the reduction of electric power consumption, etc. Most parts of the results were obtained by the intuition of the experienced operators, but the effective way of controlling the carbon injection in the electric arc furnace operation has not experimentally established yet. In general, refining of steel is mostly made by the reduction refining process which is largely influenced by the condition of molten steel obtained at the final stage of oxidation preceding the reduction period. In this sense, it is necessary to control the operation of carbon injection which is made at the final stage of oxidation in order to improve the yield, productivity, energy saving and cost reduction. For the establishment of such a control system, it is needed to estimate the content of carbon in molten steel which is in equilibrium with Fe0 in molten slag. This system is called "catch carbon control system " and the possibility of the establishment of this system has therefore been examined in the current work. The influences of the property of carbon and the nature of slag on the reduction of Fe0 in molten slag, the composition of reduced iron in slag, etc., were studied at first and the mechanism of reducing reaction of Fe0 in slag has been investigated on the basis of the results obtained. Further, the possibility of catch carbon control system has been discussed. The results obtained are described in the following chapters. II. Test Method 1. Furnace Used for the Measurement of Reduction Rate One of 70 t electric arc furnaces installed at Chita Plant of DAIDO was used for the measurement of the reduction rate of Fe0 in slag when carbon was injected. It seems that there are three major factors (quality of solid carbon, boundary area of reaction wherein solid carbon reacts with FeO, and nature of slag) which are controlling the reduction rate of Fe0 in slag when solid carbon is injected. In order to know the influences of these factors on the reduction rate of Fe0 in slag, the reduction rates of Fe0 were measured at different levels of these factors shown in Table 1. The reduction rate of Fe0 was determined by analyzing the composition of slag taken out in the course of carbon injection. 2. Determination of Carbon Content and EPMA Analysis of Reduced Iron Particle The amount of carbon existing in iron particle formed in the course of reduction was determined to clarify the mechanism of reduction of Fe0 by solid carbon. It is expected that when the reduction rate observed is low, the quantity of iron particle formed by reduction is small and the carbon content in iron particle increases because the diffusion of unreacted carbon into iron particle is facilitated, while it decreases when the reduction rate is high. Further, Table 1. Three major factors rate of Fe0 in slag. affecting the reduction ( 621)

2 (622) Transactions ISIJ, Vol. 26, 1986 Fig. 1. Schematic diagram showing the positions measured the temperature and the concentrations of CO and CO2 in the waste gas. the mechanism of the formation of iron particle was estimated on the basis of the concentration gradients of Fe, C, 0, Si, Mn and Ca observed in the range of the interface of slag and particle to the center of particle. V 3. Catch Carbon Control System Utilized for the Electric Arc Furnace Operation The temperature and the concentrations of CO and CO2 in the waste gas were measured to stop the carbon injection at the optimum condition by estimating the reduction ratio of molten steel and slag in the electric arc furnace. Figure 1 shows the positions measured the concentrations of CO and C02 and the temperature of waste gas. Variations of (%FeO) in moten slag and [%C] in molten steel were measured simultaneously to find the relation between the content of carbon in molten steel and the concentrations of CO and C09 in waste gas. Fig. 2. Relation between (% FeO) in molten slag and the time of carbon injection by the use of three kinds of cokes. III. Reducing Reaction of FeO in Molten Slag 1. Quality of Solid Carbon and Reduction Rate Figure 2 shows the reduction curves of FeO in molten slag obtained by injecting three kinds of solid carbon. The reduction rate of Coke "A" is constant indicating that the reduction is the zero order reaction while those of Coke " B " and Coke " C " are the first order reactions. It can be seen from the figure that the mechanism of the reduction of FeO by Coke "A" is different from those by Cokes " B " and " C " 2. Size of Carbon Powder and Reduction Rate The reduction curves of FeO molten slag obtained by injecting two different sizes of Coke "A", 1 mm diameter on the average (fine grain) are shown in Fig. 3. Though the reaction area when fine grain carbon is injected into molten slag is larger than that by injecting coarse grain carbon, there is almost no difference between the reduction rates determined. The reduction rate can be expressed as a linear function of the time of carbon injection. 3. Injection Rate and Reduction Rate of Solid Carbon Figures 4 and 5 show the reduction curves of FeO Fig. 3. Relation between (% FeO) in molten slag and the time of carbon injection by the use of two different sizes of Coke "A". in molten slag obtained by injecting Cokes "A" and " B " with different injection rates. From Figs. 4 and 5, it can be seen that the reduction rate obtained

3 Transactions ISIJ, Vol, 26, 1986 (623) Fig. 4. Reduction curves of Fe0 in molten slag obtained by injecting Coke «A" with two different injection rates. Fig. 6. Reduction curves of Fe0 in molten stags with different natures obtained by adding different amounts of lime. by injecting Coke "A" is expressed as the zero order reaction while that by Coke " B " is expressed as the first order reaction, as in the case of Fig. 2. As far as the effect of injection rate is concerned, the reduction rate becomes high as the injection rate increases in both cases of Cokes "A" and " B ". 4. Nature of Slag and Reduction Rate Figure 6 shows reduction curves of FeO in molten slags with different natures obtained by adding different amounts of lime to the slag. In this experiment solid carbon injected is Coke " B ". It can be seen from the figure that the reduction rate decreases as the amount of lime added becomes small. Iv. Fig. 5. Reduction curves of Fe0 in molten slag obtained by injecting Coke «B" with two different injection rates. Discussion 1, Influence of the Content of Volatile Matter in Coke on the Reduction Rate There is no available data on the chemical characteristics and the reactivity of coke in molten slag. Figure 7 shows the relation between the content of volatile matter and the reaction property index of solution loss reaction which is one of the solid-gas Fig. 7. Relation between the content of volatile matter in coke and the reaction property index of solution loss reaction. reactions. This figure shows that the solution loss reaction becomes more vigorous as the content of volatile matter decreases if the content of volatile matter is less than 30 %. The relation between the content of volatile matter in solid carbon and the capacity coefficient " K " obtained from this test is shown in Fig. 8. It can be seen from this figure that the capacity coefficient becomes high as the content of volatile matter decreases. This phenomena can well be explained if it is considered that the reduction reaction of FeO in molten slag is retarded by the gas film formed on the surface of coke by the volatile matter (H2 and N2) as illustrated in Fig. 9. Further discussion will be made in the following sections. 2. Rate Determining Step of Reduction 1. Coke The reduction curves of FeO in molten slag by Cake "A", indicates that the reduction is the zero order reaction as shown in Fig. 2. Since Coke "A" contains larger amount of volatile matter than the others, the formation of gas film on the surface of coke is facilitated, resulting in the retardation of the inter-

4 624 ) Transactions ISIJ, Vol. 26, 1986 facial reaction as can be seen from Fig. 9. From this point of view, it is possible to assume that the rate determining step of reduction is the chemical reaction. 2. Cokes " B " and " C " From the reduction curves of Cokes " B " and " C ", it can be seen that the reduction is the first order reaction. Terefore it is possible to assume that the transport of FeO into the interfacial area of coke is the determining step of reduction. If this assumption is correct, the reduction rate is expressed by Eq. (1). -d(%feo) d t = K[(%FeO)-(%FeO)*]...(1) and " C " are not controlled by the chemical reaction because of higher reactivity of coke injected, and are controlled by the transport of FeO in slag. 3. Influence of the Reaction Area on the Reduction Rate In general, the reaction rate of such a heterogeneous reaction as observed in the test can be expressed by the following equation. - do = K V where, (%FeO) : concentration of FeO in slag t : time K: capacity coefficient (%FeO)* : equilibrium concentration of FeO Integrating Eq. (1), one obtains In [(FeO)-(FeO)*] = Kt+C...(2) The reduction rates of FeO in molten slag by the injection of Cokes " B " and " C " can be expressed by Eq. (2) as shown in Fig. 10 and this fact clearly indicates that the transport of FeO into the interfacial area of coke is the rate determining step of reduction. The reduction rates by the injection of Cokes " B " Fig. 8. Relation between the content of volatile matter in solid carbon and the capacity coefficient. Fig. 10. Reduction rates of Fe0 in jection of Cokes " B " and molten slag by the in- «C>> Fig. 9. Schematic view of the gas film formed on the sur- Fig. 11. Change in the reduction rate with the boundary face of coke particle by volatile matter. area of reaction.

5 Transactions ISIJ, Vol. 26, 1986 (625) the coke of finer grain size is injected. Figures 12 and 13 show the change in the reduction rate with the boundary area of reaction obtained by the injection of Cokes "A" and " B " with different injection rates, and it is noted that the reduction rate increases as the injection rate increases, in both cases of Cokes "A" and " B ". From the above fact, it may be concluded that the increase in the interfacial area between slag and coke is effective for the increase in the reduction rate but the reduction rate is not increased by the injection of finer coke. This fact may suggest that finer coke is adhered to rising CO bubble and is easy to away from the reaction boundary area. 4. Influence of the Basicity of Slag on the Reduction Rate Figure 14 shows the relation between the capacity coefficient and the Basicity (CaO/ Si02) of slag, indicating that the reduction rate increases with the increase in the Basicity. In the present test, the less violent slag foaming is observed in the course of carbon injection for the slag of lower basicity. Kondo and his coworkers2~ reported that the more violent slag forming was observed with the slag of higher viscosity and higher surface tension (Fig. 15). Figure 16 shows the isothermal curves of surface tension of the CaO- S102 FeO system.3) From this figure, it may be assumed that the slag of lower surface tension owing to the lower basicity will cause less violent slag foaming. When more violent slag foaming is observed, the reduction rate will be increased according to the following phenomena (1) Specific gravity of slag is decreased by foaming and the buoyancy acting on the coke particle in slag is decreased, and the contact time of coke with slag is therefore increased by the above two facts, resulting in the increase in residence time of coke in slag. Fig. 14. Relation between basicity of slag. the capacity coefficient and the Fig. 12. Relation between d(%fe0)/dt%s-1 jection rate for Coke "A". and the in- Fig. 15. Relation between slag forming and surface tension of slag. and the viscosity Fig. 13. Relation between the capacity coefficient and the injection rate for Coke " B ". Fig. 16. Isothermal surface tensions of the Fe0-Ca0- system. Si0 2

6 ( 626 ) Transactions ISIJ, Vol. 26, 1986 From these reasons the interfacial area becomes larger. (2) Many CO bubbles are existing in the slag and their surfaces become to serve as the nucleation sites of new CO bubbles. (3) Slag becomes to be stirred more vigorously by the generation of CO. V. Microscopic Observation Photograph 1 shows a scanning electron microscopic image of iron grain reduced by solid carbon existing in slag. The iron grain shows a spherical form and the scales thought to be formed during solidification are observed along the grain surface. In order to examine the mechanism of reduction through the iron grain formed, the analysis was made to determine the content of carbon in the iron grain and the EPMA analysis was made along the slagiron grain boundary. The results obtained are summarized as follows : 1. Results of Carbon Analysis Figure 17 shows the relation between (%Fe0) and [%C]. From this figure, it is noted that [%C] for Coke "A" is higher than that for Coke " B " irrespective of (%Fe0). This is due partly to the following facts : (1) In the case of reduction by Coke "A", the volatile matter contained in coke forms a gas film on the surface of coke and the reduction of Fe0 on the interfacial is disturbed by the film formed and the diffusion of unreacted carbon into iron grain becomes to be facilitated because many carbon particles remain unreacted. (2) In the case of reduction by Coke " B ", the reduction rate at the interfacial area becomes high and this will result in the decrease in the amount of unreacted carbon which will diffuse into iron grain. Therefore, the iron grain contains a smaller amount of carbon. 2. Results of EPMA Analysis Photograph 2 shows the results of the qualitative Photo. 1. Scanning electron microscopic image of the reduced by solid carbon existing in slag. iron grain Fig. 17. Relation between (%Fe 0) and [%C]. Photo. 2. Qualitative analyses of Fe, C, 0, Si, Mn and Ca by EPMA.

7 Transactions ISIJ, Vol. 26, 1986 (627) analysis of elements by EPMA. All the elements including Fe, C, and 0 in the iron grain are uniformly distributed. The unreacted core model which is used to explain the gaseous reduction of solid iron oxides, can not be applied to explain the mechanism of such uniform distribution. 3. Reduction Model of FeO Solid Carbon From the above analytical results, the difference in the mechanisms of reduction, which are observed when different types of carbon are injected can be explained as follows, by considering the model shown in Fig. 18. (a) Fe2+ and 02~ iron diffuse to the surface of solid carbon. (b) An iron nucleus is formed by the reduction of Fe2+ at the interface of slag and solid carbon. (Heterogeneous nucleation) In Case of Coke "A" (c) Much more carbon diffuses into the iron particle formed than in the case of Coke " B " or " C " because the reduction rate becomes low because of the disturbance by the film formed by volatile matter on the interface of slag and carbon. Therefore more unreacted carbon diffuses into the iron grain. (d) Iron particle containing more carbon is herefore formed in the slag. In Case of Cokes " B " and " C " (e) Only a small amount of carbon diffuses into the grain formed iron because the reduction rate becomes high since the film of volatile matter does not formed on the interface. (f) Iron particle containing small amount of carbon is formed in the slag. changes in the temperature of waste gas and in (%FeO) in molten slag with the time of carbon injection. It is noted from this figure that the temperature of waste gas reaches a peak (point " E ") when (%FeO) approaches to the equlibrium value with Fig. 19. Changes in the temperature and the concentrations of CO and CO2 in the waste gas with the time of carbon injection. 4. Catch Carbon Control System on the Basis of the Data for Exhaust Gas Figure 19 shows the changes in the temperature and the concentrations of CO and CO2 in the waste gas with the time of carbon injection. This figure indicates the relation between the temperature of waste gas and the concentration of CO in the waste gas generated as the result of successful reduction of FeO. Such combustion is facilitated as the temperature of waste gas increases. Figure 20 show the Fig. 20. Changes (%FeO) injection. in in the temperature molten slag with of waste the time gas and in of carbon Fig. 18. Models used of reduction for the explanation mechanism.

8 (628) Transactions ISIJ, Vol. 26, 1986 Fig. 22. Relation between (%Fe0) and [%C]. Fig. 21. Relation between the present test. (%Fe0) and [%C] obtained in [%C] in molten steel. Figure 21 shows the relation between (%FeO) in molten slag is reduced to the value which is equilibrated with [%C], and then changes along the equilibrium curve. The reduction of FeO in a combined system of slag, molten steel and solid carbon will be discussed on the basis of the above mentioned fact. In this system the following two reactions should be taken into consideration. FeO+C... Fe+CO...(I) FeO+C... Fe+CO...(II) Eauation (I) represents the reaction between FeO and solid carbon injected into slag. Equation (II) shows the reaction between FeO and carbon dissolved in molten steel. If it is assumed that solid carbon injected does not dissolve into molten steel, namely, [%C] is constant, the point " S " in Fig. 22 changes along the direction of arrow up to the point " M " showing the equilibrium value right after the beginning of carbon injection according to Eq. (I). The reaction rate is represented by the following equation. - d (%FeO) = K {(%FeO) - (%FeO)*} dt When (%FeO) reaches the equilibrium value, this reaction is completed, and the change in the system is controlled by Eq. (II). This implies that the carbon injected begins to diffuse into molten steel and (%FeO) decreases so as to be equilibrated with [%C] in molten steel. This phenomena agreed well with the test results shown in Fig. 21. It is assumed that the peak temperature of waste gas in Fig. 20 is the conversion point from the reaction of Eq. (I) to the reaction of Eq. (II). As discussed above, it is possible to estimate the variation of [%C] in molten steel in the course of carbon injection on the basis of the data of waste gas if [%C] in molten steel ([%C] at the point " M " shown in Fig. 22) is known just before the beginning of carbon injection by some method. VI. Conclusion (1) The results of the micro and macro structure analyses of the samples obtained by the reduction of FeO in molten slag by solid carbon injection are summarized as follows : (i) The reduction differs by the type and quality of solid carbon injected and is considerably influenced by the content of volatile matter in solid carbon. the reduction becomes to be influenced more significantly by the chemical reaction as the content of volatile matter in solid carbon increases. However, it becomes to be controlled by the transport of FeO to the interfacial area of solid carbon as the content of volatile matter decreases. (iii) The reduction rate of FeO in slag is also influenced by the nature of slag. The reduction rate increases as the viscosity and the surface tension of slag increase, in other words, the basicity of slag increases. (2) The establishment of "catch carbon control system " where the content of carbon is estimated by the measurements of both the temperature and the concentrations of CO and CO2 in the waste gas from electric arc furnaces seems to be possible. REFERENCES 1) T. Miyakawa :.Nenryokyokaishi, 58 (1979), ) S. Kondo, T. Sugiyama and M. Sugata: Tetsu-to-Hagane, 58 (1972), 11. 3) P. Kozakevitch: Rev. Met., 46 (1949), 505 & 572.