IN THE RUSSIAN ACADEMY OF NATURAL SCIENCES

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1 Metallurgist, Vol. 45, Nos. 3 4, 2001 IN THE RUSSIAN ACADEMY OF NATURAL SCIENCES THE ROLE OF COAL IN THE ROMELT PROCESS FOR THE LIQUID-PHASE REDUCTION OF IRON V. A. Romenets, A. B. Usachev, A. V. Balasanov, and V. E. Lekherzak The modern processes currently being developed for the coke-less production of liquid iron-carbon semifinished products can be tentatively divided into three groups: processes that involve the preliminary reduction of iron in the solid phase and its final reduction in the liquid phase (Corex); processes that mainly involve the liquid-phase reduction of iron (DIOS, Hlsmelt); processes in which iron is reduced entirely in the liquid phase (Romelt, AusIron). The Corex process has already been used [1] to produce more than 6 million tons of iron-carbon semifinished product. The shortcomings of this process are the need to pelletize ores and concentrates composed of dust-sized particles (which in turn results in significant economic and environmental losses), the complex equipment which is used, and the impossibility of processing the dust-sized metallurgical wastes. Use of the technology is further complicated by the fact that it includes two stages, thus requiring that the operation of the shaft and the reactor-gasifier be coordinated. In addition, coke is actually used in the Corex process, the amount of coke being ~10% of the weight of the coal. The processes which are based mainly on the liquid-phase reduction of iron (DIOS [2], Hlsmelt [3]) were the result of an attempt to better balance the chemical and energy aspects of the two reduction stages: solid-phase and liquid-phase. With this as a goal, a large part of the heating and reduction was transferred to the liquid-phase stage. The following techniques are used to ensure that the liquid bath is supplied with heat in reactors designed for liquid-phase reduction: the outgoing gases are reduced as much as possible, and the heat liberated by the reduction reaction is transferred to the slag bath; the raw material is heated and subjected to preliminary reduction; the blast is heated (with oxygen-air, nitrogen, etc.); coal, natural gas, and other gaseous and liquid fuels are burned in the slag. Processes in which iron is subjected to solid-phase reduction and subsequent additional reduction in a slag bath have been developed on the basis of the assumption that heat from the secondary combustion of the gases cannot be efficiently returned to the slag bath if the entire reduction process is carried out in the liquid phase. This explains the two-stage nature of the process and the complexity of the technology. The idea of producing an iron-carbon semifinished product in one stage has been successfully realized in the Romelt process developed by the Moscow Institute of Steel and Alloys. The AusIron process [4], involving the reduction of iron entirely in the liquid phase, has not gone past the stage of laboratory testing. Only the Romelt process and a single-stage variant of the Hlsmelt process have actually been introduced on commercial units. The Romelt process has now been sufficiently refined and is ready for commercial use. The publications [4, 5] presented a detailed look at the Romelt technology for processing various types of iron-bearing raw materials (including metallurgical wastes) and analyzed features of the reducing and thermal performance of the unit. Moscow State Institute of Steel and Alloys (Technological University). Translated from Metallurg, No. 3, pp , March, /01/ $ Plenum Publishing Corporation 101

2 Fig. 1. Location of the zones in the slag bath of a Romelt furnace. Compared to other coke-less units for making pig iron, the Romelt process has progressed the farthest and is the simplest to execute. Many of the technological and design innovations first introduced on this unit were subsequently used to develop other modern processes for the liquid-phase reduction of iron (separate siphon-type removal of the iron and slag from the furnace; the feeding of an oxygen blast into the slag bath; conducting the process with slight negative pressure; the use of water-cooled barriers in the slag-bubbling and secondary-combustion zones, etc.). Studies have shown that for the Romelt process in which the reduction of iron from slag by coal and the combustion of coal in oxygen supplied through lances both take place simultaneously in a bubbling slag melt it is crucial to maintain the necessary parameters for the slag-coal suspension. In this article, we generalize the results of an investigation of the behavior of coal during the Romelt process. Study of the Slag Coal Metal System. The slag coal metal system was studied on the Romelt unit at the Novolipetsk Metallurgical Combine [7]. Samples of slag were removed from the central part of the bath during melting. The samples were taken from the following characteristic zones (Fig. 1): sub-lance zone (I) a poorly mixed layer of slag located under a layer of pig iron that has accumulated on the bottom; lance zone (II) a transitional region between the slag being mixed anddd the slag undergoing bubbling; bubbling-slag zone (III). The samples were taken with the use of a vertical probe that allowed us to simultaneously collect five samples of the slag melt from different levels over the height of the bath. It was established that the slag-coal suspension has two characteristic zones. The amount of coal in the surface layer is an order of magnitude greater than at the lower levels. Toward the lower boundary of the surface layer, the coal content of the slag suddenly decreases to ~0.5% (mass). Coal content then declines further to ~0.2% (mass) in the top part of the layer of quiescent slag. Here, the quantity of coarse coal particles also decreases toward the lower levels of the slag bath, where only fine coal particles are seen. The middle and lower layers of slag in zone I contain almost no coal particles. Under the conditions of the study, the total surface area of the coal in the furnace reached m 2. The mass of coal in the slag was kg. Here, the surface layer of the slag contained 1/3 2/3 of all of the coal in the furnace. In other words, the surface layer of the slag accounted for of the total surface area. The data we obtained on the granulometric composition and specific surface of the metal drops and coal particles in the slag bath was used in a kinetic analysis of iron reduction in the process. This allowed us to refine the role of coal as a reducing agent. 102

3 TABLE 1. Interfacial Areas in the Slag Bath of a Romelt Furnace Zone (see Fig. 1) Surface area, m 2 coal particles metal drops bubbles splashed and sprayed slag Spray 970 III (c) II-III (a, b) I Total area TABLE 2. Calculated Values of the Rate of Reduction of Iron by Different Reducing Agents in a Romelt Furnace, tons/h Surface of agent participating in the reduction, fraction Reducing agent Coal Gas Carbon in the drops of metal 1.08 Features of the Reduction of Iron in a Romelt Furnace. The reduction of iron from its oxides in slag can be done by coal particles and by carbon dissolved in metal inclusions in the slag. There are two ways that coal is involved in the reduction of iron in a Romelt furnace [8]: reduction occurring on the surface of gas bubbles that contain coal particles; the role of these particles is to regenerate the reducing atmosphere in the bubbles (the thermodynamic conditions which exist in the Romelt process make it difficult for gas bubbles that do not contain coal particles to reduce the iron oxides); reduction occurring with the coal particles in direct random contact with the slag; here, reduction takes place under conditions similar to those which exist when iron is reduced by a rotating carbon-bearing sample and gas bubbles are forcibly removed from the sample s surface. We evaluated the size of the reaction surface in the bath. To do this, we experimentally determined the surface area of the coal particles and the drops of metal. Samples of slag were obtained during periods of operation in which iron was reduced under roughly the same conditions. The gas slag interfacial area in the furnace zones with predominantly reducing conditions was evaluated by methods described in the literature (Table 1). To calculate the rates of reduction of iron by all of the reducing agents, we used the results of studies in which the behavior of the reducing agents was examined under the conditions most closely resembling the actual conditions in the Romelt process. Since the fraction of the surface of the coal particles that was in direct contact with the slag was not known beforehand, in the calculations we varied it from 0.1 to 1.0 in increments of 0.1. The fraction of the total surface of the bubbles containing coal particles was varied in the same manner. The average rate of formation of metal during the experiment was 9.14 tons/h. According to the data in Table 2, such a rate is possible when the fraction of the surface of the coal particles that is in direct contact with the slag is equal to and the fraction of the surface of the bubbles that contain coal particles is equal to About 60% of the iron was reduced in the surface layer of the slag bath and in the splash and spray zones. The participation of H 2 and CO in the reduction of the iron was roughly the same. 103

4 Under actual conditions, reduction takes place when the coal particles are in direct contact with the slag (60 80%) and when carbon is in direct contact with the metal drops (10 15%). Reduction also takes place on the gas slag interface (10 25%). Thus, during the study period, 85 90% of the iron was reduced with the direct participation of the coal particles. This differentiates the liquid-phase reduction that occurs in the Romelt process from other combination processes in which carbon dissolved in the metal plays a substantial role (DIOS) or the main role (Hlsmelt) in the reduction operation. The productivity of the furnace was low during the study period. Even when all of the coal particles are entrained by the slag, furnace productivity can increase to only 10.3 tons/pig. For the same total surface of the coal particles in the slag bath, a 100 C increase in the temperature of the process makes it possible to approximately double the reduction rate. Increasing the FeO content of the slag from ~2 to ~4% has a similar effect. Thus, there is a realistic possibility of increasing the unit productivity of the Romelt process to 2 tons/h m 2. This finding is consistent with the conclusions reached in [9]. When used to process a pre-reduced (to wustite) iron-bearing raw material, the DIOS process can reach a maximum unit productivity of 2.5 tons/h m 2 if the temperature of the layer of dense slag is 1500 C, the temperature of the foamed layer of slag is C, and the slag contains 4 5% FeO [10]. Using data [11] that characterizes the changes in the productivity of an Hlsmelt unit with a change from the smelting of partially reduced material to the processing of an unprepared oxide (the productivity of the reactor increases with the degree of preliminary reduction of the iron-bearing raw material), we can evaluate the productivity of a DIOS unit when only liquid-phase reduction is performed. The productivity will be 1.9 tons/h m 2. The unit productivity of the Hlsmelt process in the case of reduction solely in the liquid phase (smelting of fine ore) is 1.47 tons/h m 2 when the temperature of the metal and slag is 1450 C and the slag contains 4.4% FeO [11]. Thus, while organized on the basis of different principles of reductive refining, all three of the most advanced technologies for the liquid-phase reduction of iron have roughly the same unit productivity. Regime Involving the Use of Coal to Block the Surface of the Slag Bath in a Romelt Furnace. Experience in the operation of the Romelt furnace at the Novolipetsk Metallurgical Combine has shown that there are certain optimum values for the content of coal particles in the slag bath, although this parameter can vary within a broad range of values. However, the furnace cannot be overloaded or underloaded with coal [12]. Charging of a suboptimal amount of coal in several of the trial heats led to over-oxidation of the slag melt and its uncontrollable frothing. Thus, coal in excess of the planned amount is often charged into the furnace to prevent over-oxidation, and this has helped stabilize the process. However, studies have shown that there is a limit for the excess number of coal particles in slag. Going past this limit may also cause disruptions in the process: a decrease in the temperature of the slag bath, an increase in the content of iron oxides in the slag, a reduction in the degree of secondary combustion of the outgoing gases, and the release of more heat in the waste-heat boiler. Feeding additional oxygen into the furnace did not promote secondary combustion because the oxygen did not completely react with the coal floating on the surface of the slag. This dense layer of coal was formed as a result of overcharging of coal or undercharging of the oxide-bearing raw material. The presence of the layer suppresses spraying and adhesion of the slag to the walls, which adversely affects the transfer of heat from the primary gas combustion zone to the slag bath (heat transfer takes place mainly through drops of slag and a slag film that flows down the walls in the secondary combustion zone). Calculations showed that if the coal content of the surface layer of the slag is ~20 30% (mass), the process may transit to an undesirable regime in which coal blocks heat transfer from the secondary combustion zone to the bath. The occurrence of this regime depends not on the amount of coal that has accumulated in the slag and that coal s fractional composition, but also on the rate of turbulent circulation of the slag which determines the efficiency with which coal is mixed with other components of the slag melt. In the case of overcharging of coke, the following steps are recommended to return the furnace to normal operation: decrease the amount of coal and oxide-bearing raw material charged into the furnace and increase the consumption of oxygen in the blow. The simplest solution is to temporarily reduce the amount of coal charged, since the process of combustion of excess coal from the slag bath takes place relatively quickly. 104

5 Conclusions. Below are the main findings from a study of the behavior of coal in the slag bath of a Romelt furnace: 1. An experimental determination was made of the content, fractional composition, and specific surface of the metal drops and coal particles in the slag bath. It was established that most of the coal is concentrated in the surface layer of the bath. There is no coal in the slag discharged from the furnace. 2. A kinetic analysis was made of the contributions of the main reducing agents to the integral rate of reduction of iron in a Romelt furnace. It was determined that the coal particles which are in direct contact with the slag play the main role in the reduction of iron. A significantly smaller role in reduction is made by the gas phase and the carbon dissolved in the drops of iron. The carbonization of the drops in the bubbling slag occurs in advance of their decarbonization. The productivity of the furnace is determined mainly by the reduction of iron in the surface layer of the slag bath. 3. A study was made of a technologically nonoptimum regime for the Romelt process the regime in which coal blocks the surface of the slag bath. We determined the conditions that lead to this regime and gave recommendations for safely transitioning the furnace from the blocking regime to the normal regime. 4. The results of the studies were used to develop a technology for a commercial Romelt unit in India with a capacity of 320,000 tons of pig iron a year. REFERENCES 1. E. Aumayr, C. Bohm, H. Freydorfer, et al. The Corex process update 2000, in: Commercializing New Hot Metal Processes Beyond the Blast Furnace: International Conf., Atlanta, Georgia, U.S., June, 2000, pp T. Kitagawa, Compact, economical, and ecological ironmaking process DIOS, in: Commercializing New Hot Metal Processes Beyond the Blast Furnace: International Conf., Atlanta, Georgia, U.S., June, 2000, pp P. Bates and A. Muir, Hlsmelt: low-cost ironmaking, in: Commercializing New Hot Metal Processes Beyond the Blast Furnace: International Conf., Atlanta, Georgia, U.S., June, 2000, pp J. Fogarty, K. Hamilton, and J. Goldin, Ausiron a new direct reduction technology for pig iron production, Skillings Mining Review, No. 5, 4 8 (1998). 5. V. A. Romenets, Romelt a completely liquid-phase process for producing metal, Izv. Vyssh. Uchebn. Zaved. Chern. Metall. 6. V. A. Romenets, The Romelt process the production of metal in ferrous metallurgy by a coke-less method without a blast furnace, in: Fundamental Problems of Russian Metallurgy at the Threshold of the XXI Century: Sb. RAEN (1998), Vol. 1, pp A. B. Usachev, A. V. Balasanov, V. E. Lekherzak, et al., Study of the slag coal metal system in a Romelt furnace, Izv. Vyssh. Uchebn. Zaved. Chern. Metall., No. 11, 6 9 (1997). 8. A. B. Usachev, V. E. Lekherzak, and A. V. Balasanov, Reduction of iron in the Romelt process, Chern. Metall., No. 12, A. B. Usachev, Physico-chemical laws governing the reduction of iron in the Romelt process, Izv. Vyssh. Uchebn. Zaved. Chern. Metall., No. 8, 3 6 (1998). 10. K. Iwasaki, M. Kawasaki, and A. Kitagawa, Results of the use of a semi-commercial DIOS unit, Dzaire to Prosesu, 9, No. 4 (1996). 11. R. Dry, C. Bates, and D. Price, Hlsmelt the future in direct ironmaking, in: ICSTI: 58th Ironmaking Conf. Proceedings, Chicago, Illinois, U.S. (1999), Vol. 58, A. B. Usachev, V. A. Romenets, A. V. Balasanov, et al., Controlling the Romelt process for the liquid-phase reduction of iron, Chern. Metall., No. 8, (2000). 105