ROMELT PROCESS - PROMISING TECHNOLOGY FOR IRONMAKING. Yury Pokhvisnev, Vladimir Romenets, Valery Valavin, Alexander Zaytsev, Semen Vandariev

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1 ROMELT PROCESS - PROMISING TECHNOLOGY FOR IRONMAKING Yury Pokhvisnev, Vladimir Romenets, Valery Valavin, Alexander Zaytsev, Semen Vandariev Moscow Institute of Steel and Alloys, Russia Abstract Nowadays the trend has been toward diminution ironmaking and blast furnaces are removed partially from service in the world. This allows to redirected metallurgists attention to other means of ironmaking which are do not high-efficiency as blast furnace production but using unconventional and more inexpensive raw materials and energy sources. Smelting reduction processes, among them Romelt technology may be used for this purpose. Romelt process was developed in Russia in Moscow Institute of Steel and Alloys. It designed for hot metal production from iron-bearing materials using non-coking steam coal. Metallurgical wastes including finely dispersed ecologically dangerous dusts and sludges may use as raw materials. Blast furnace and basic oxygen furnace dust, scale, vanadium sludge, Lead-and-Zinc Works slag were processing on the pilot Romelt plant at Novolipetsky Steel Works in a period of 14 years. The mechanism of behavior of non-ferrous admixtures and the possibility of their separation in the Romelt furnace has been determined. The model of material and heat balance of Romelt process was developed. 1. INTRODUCTION The deceleration of growth rate of steel and rolling production is observed in the last few years. It is worldwide tendency, though in some countries, for example in China, production is increased. Worsening of the market conjuncture, crisis of overproduction, the fall of the prices cause the world producers to plan decrease of steel production approximately on 1 billion tonnes by Therefore, metallurgist attention may be focussed on new ways of metals production, which using unconventional cheaper sources of raw material and energy. Firstly it concerns the primary production: pig-iron. Such way of development will allow to solve some problems. First, industrial wastes may be used as raw material. The content of valuable components in wastes often exceeds their contents in raw material. However, their composition, different from that of usual raw materials, their being finely dispersed, and high moisture content present difficulties under processing. Secondly, the ecological task associated with stored waste s destruction or converted into safe compact state may be solved. Thirdly, the completely new ways of metal production are developed. Perhaps they will be not economically effective, as in existence, but are perspective. Just now, when there is no necessity for increase of ironmaking, and on the contrary, blast furnaces the existing capacities are deduced from operation, there are reserves for development of new processes. Most of iron in wastes is in the form of oxides. Therefore, it is possible to process wastes only under reduction conditions, for example, in blast furnaces. To do this, finely dispersed wastes should be pre-pelletized. Besides, volatile non-ferrous metals have a negative effect on the process in a blast furnace. They circulate in a shaft and increase considerably the coke rate. Besides, these admixtures condense progressively on the walls of the furnace and destroy the refractory lining. That is why processes, which do not use shaft furnace, are actively developing nowadays. Absence of the burden column that condenses sublimates and creates condi- 1

2 tion for the evaporated elements to enter repeatedly into the melting zone makes it possible to use the process for the recovering of nonferrous metals from the wastes. One such way for ironmaking from wastes is smelting reduction or bath smelting processes. Despite the differences between these technologies, in all of them oxides final reduction takes place in a liquid slag or metal bath. Carbonic hot metal, usually pig-iron, is a product from these processes. Initial burden materials may be pre-reduced, or to be used without any preparation. The main suggested smelting reduction technologies without pre-reduction of burden materials are Romelt, HIsmelt, AusIron. The Romelt process was the first of them to appear and be developed [1,2]. The other technologies were declared later. Now they are approaching the Romelt process principal scheme, keeping some individual peculiarities. 2. ROMELT PROCESS DESCRIPTION Romelt process is the single-stage technique of continuous ironmaking from various ironbearing materials using inexpensive noncoking coal. The general scheme of the Romelt furnace is shown in Figure 1. Iron-containing materials, coal and flux, are fed, using weighhoppers, from relevant bins to the common conveyer. The charging into the furnace is performed through the aperture in the furnace roof. No preliminary mixing of charge materials is needed: it takes place directly in the slag bath due to its intensive agitating. Sluice arrangements, used in the units for other processes that operate under pressure, are not needed in the Romelt furnace. The working space of the Romelt furnace is under negative pressure of 1 to 5 mm w.c. which is ensured by induced draft fan. Figure 1 The Romelt furnace scheme: 1-agitated slag, 2-sump for slag, 3-sump for hot metal, 4-hearth with refractory lining, 5-channels for slag and hot metal, 6-feed tunnel, 7-gas-escape branch pipe, 8-lower tuyeres, 9-upper tuyeres, 10-calm slag, 11-water-cooled panels The bath of melting slag is blown with an oxygen-air mixture through the lower tuyeres positioned below the slag layer. The tuyeres have simple structure and proved to be reliable in operation. They ensure the required agitating power of the slag bath. Getting into the agitated slag that contains coal, iron-bearing materials are reduced. Iron produced by the reduction becomes enriched in carbon. Drops of melted metal precipitate under 2

3 gravity on the furnace hearth. Thus, three melted layers are formed in the furnace: the layer of metal on the furnace hearth, the layer of calm slag between the metal and the lower tuyeres, and the layer of agitated slag (the reaction zone). There is about 70 tonnes of metal and 100 tonnes of slag in the furnace. Two lined chambers (sumps) are situated each at one of the end sides of the furnace. They are used for separate tapping of metal and slag. The sumps are connected with the working space by channels of different heights. This ensures separate transportation of metal and slag into the metal and slag sumps. There are holes for tapping metal and slag, which are located at different heights. That ensures continuous free tapping of the melting products at the speed that matches the unit capacity. If the furnace has little capacity tapping may be arranged periodically. In the slag bath the heat expense on melting and reduction of the burden materials is greater than the heat received from the burning of carbon into CO near the lower tuyeres. Thus, the main peculiarity of the process is the post-combustion of CO, H 2 and coal-volatile matter evolving from the bath by the oxygen blown through the upper tuyeres. Post-combustion of gas up to CO 2 and H 2 O allows to return the heat into the slag bath and to maintain processing of raw materials. The hearth and the lower part of the furnace bath, which contains permanently metal and calm slag, are lined with refractory bricks. In this zone the refractory lining is under favorable conditions: at the suitable temperature and out of the oxidizing effect of the atmosphere. In the zone of agitated slag the furnace walls are constructed of copper water-cooled panels. Formation of the slag scull lining on them reduces the heat losses and rules out their wear. That allows to avoid destruction of the lining in the places of the most aggressive attack of gas-slagmetal emulsion. Above the slag bath walls are made of steel water-cooled panels. After post-combustion gases flow through the water-cooled gas-escape branch pipe at a temperature up to С into the waste-heat boiler. There they are burned completely with natural air inflow and cool to С. Off gases are transferred to the gas-cleaning system and discharged into the atmosphere through the chimney. Flue-dust ejection from the unit measured in the gas-escape branch pipe is, on average, about 3% of the weight of the charged materials. In Moscow Institute of Steel and Alloys development of the single-stage liquid-phase process started in The first patent in Russia was obtained in To test the process feasibility a pilot commercial plant with the hearth area of 20 m 2 was built at the Novolipetsky Steel Works in Lipetsk (Russia). During forty-one campaigns were performed, each of which included startup and slowdown, with full tapping of metal and slag from the furnace. More than 40 thousand tonnes of hot metal were melted and used further in basic oxygen furnace for steel melting. 3. WASTES PROCESSING IN ROMELT FURNACE The Romelt process extends significantly the capabilities of the direct usage of wastes. During the operation of the plant significant experience of processing various materials (Table 1), including ecologically dangerous, while being in storage fine grained and dusty sludges, was obtained [3]. Hot metal for using in basic oxygen furnace steelmaking was melted from all kinds of wastes. Blast furnace and BOF dust mixture were used in most campaigns. Average compositions of hot metal, slag and dust obtained are given in Tables 2-4. In addition, compositions of the end products of the experiment when melting Ust-Kamenogorsk Lead-Zinc Works slag (Kazakhstan) are given too. Samples for the hot metal analysis were taken during tapping; samples for slag analysis were taken directly from the slag bath. Dust samples were taken in two points after gas cleaning: total dust and fine dust downstream of the Venture tube. 3

4 Table 1 Iron Bearing Materials Composition (wt.%) Component Blast and BOF Dust Mixture Lead-and-Zinc Works Slag BOF Dust Mill Scale Scale from the Billet Continuous Casting Vanadium Sludge Fe total Fe metal Up to 3 Up to FeO Fe 2 O SiO Al 2 O CaO MgO MnO TiO K 2 O Na 2 O ZnO PbO Cu 2 O Cr 2 O V 2 O S P 2 O As 2 O C Up to Ag (g/t) Up to Table 2 Chemical Analyses of Metal (wt.%) C Mn Si P Cu S Zn Ag Blast Furnace and BOF Dust Mixture Lead-and-Zinc Works Slag <0.001 Table 3 Chemical Analyses of Slag (wt.%) Fe tot CaO SiO 2 MgO Al 2 O 3 MnO TiO 2 Cu Zn Na 2 O K 2 O P 2 O 5 S Blast Furnace and BOF Dust Mixture Lead-and-Zinc Works Slag

5 Table 4 Chemical Analyses of Total Dust (wt.%) Fe tot CaO SiO 2 MgO Al 2 O 3 MnO Cu Zn Pb Na 2 O K 2 O P 2 O 5 S Ag, g/t Blast Furnace and BOF Dust Mixture Lead-and-Zinc Works Slag n.d. n.d Table 4 Chemical Analyses of Fine Dust (wt.%) Fe tot CaO SiO 2 MgO Al 2 O 3 MnO Cu Zn Pb Na 2 O K 2 O P 2 O 5 S Ag, g/t Blast Furnace and BOF Dust Mixture Lead-and-Zinc Works Slag n.d. n.d > In the Romelt furnace iron is recovered as the main valuable component. The maximum possible copper content in the hot metal when melting Lead-and-Zinc Works slag after complete renewal of the metallic bath is about 2%. Other components are, at least, converted into safe compact state. However, this does not exhaust the possibilities of the process. Concentration of volatile metals in the dust is rather high, and it, therefore, can be used for further processing. The results of the experiment show rather good distribution of the components between the phases in the Romelt process. Table 5 shows distribution of various components between metal, slag and dust. The discrepancy as regards copper is caused by the presence in the furnace of large quantity of the copper-free hot metal left after previous experiment. Thus, easyreducible non-volatile components (copper, nickel) are almost completely reduced and dissolved in the hot metal. Therefore, choice of proper charge allows to produce alloy cast iron with needed properties. Table 5 Component Distribution Between Phases When Melting Lead-Zinc Works Slag Component Metal Slag Dust Fe Cu 53/88 * 3 9 Zn Pb S (considering sulfur in the gas phase) * The numerator is the current value of transition ratio, the denominator is the calculated transition ratio at total metal bath change. 4. ASPECTS OF COAL BEHAVIOUR IN THE SLAG BATH Irrespective of the mechanism of reduction coal is the only source of reducing reagent in Romelt process. There are no principal limitations on the range of the coal used for running the process under normal conditions. Any one of the coals with different content of fixed car- 5

6 bon, ash and volatile matter can be used as the reducing agent. However, the specific coal and oxygen rates will depend greatly on the composition of the coal used. The unprepared wet coal in Romelt process is falling from above into the slag bath. The volatile matter is generated in slag bath and has a stimulating influence on the processes proceeding in the same. Both the material and heat balance of the process are dependent on how and in which form volatile matter is generated and what is its role in the main processes taking place in the unit. That s why the point of behaviour of volatile matter of coal is one of the mostly critical points for Romelt process irrespective of the grade of the coal to be used. In metallurgy the points relating to volatile matter of coal are investigated for the coking production conditions only. In blast furnace process these points are not the priority ones, as content of volatile matter in coke is negligible (0.7 1%). Coal rate in Romelt process consists of the two parts. First, this is coal expenditure required for binding oxygen of the lower tuyeres to produce CO. Second this is coal expenditure for reduction of oxides. Insufficiency of coal can be the reason for the increase of the oxidizing potential of the slag bath, which can lead to the uncontrolled boiling of the same. However, the excessive coal rate in addition to the increase of hot metal production cost, deteriorates also the thermal conditions in the unit. At the first sight, the required quantity of coal depends only on the content of fixed carbon in coal. However, experimentally it has been shown that volatile matter can participate partially in the processes taking place in the liquid slag bath of Romelt unit. At that H 2, CO and N 2 undergo no changes in the slag bath and are evolving from the same to produce the gaseous phase. However, CH 4 and CO 2 can participate in the chemical reactions CH 4 = C + 2H 2 (1) CO 2 + C = 2CO (2) If the quantity of CO 2 is small and the same of methane is substantial, then these chemical transformations will lead to appearance of the additional quantity of carbon for reduction of oxides. Carbon produced by reaction (1) is fine-dispersed and highly active and improves the kinetics of reducing reactions. In addition to the above mentioned, under actual conditions at Romelt plant use is made of wet coal with moisture content of up to 10 12% by mass. That s why in the unit the processes are also proceeding of moisture evaporation and partial decomposition of the same following the reaction H 2 O+ C = CO + H 2 (3) It is apparent that an additional carbon is required for proceeding the reaction (3), as well as (2). All the three reactions (1) to (3) take place in the slag bath simultaneously and there is opportunity for none of them to be singled out for separate investigation. That s why the special experiments were carried out at Romelt plant for determination of the share of decomposition of volatile matter of coal and moisture in the slag bath. Compositions of using coals are given in Table 6. The results obtained are shown in Figure 2. We notice that actual coal rate per melting in all the four experiments is less than the theoretically calculated one under the assumption about the full decomposition of water following the reaction (3) and non-participation of volatile matter of coal in the reduction processes. For example, in case of melting BOF dust the low volatile coal charge has amounted to 11.5 t/h. Should the moisture of the charge have decom- 6

7 posed completely, as it is shown in the first column of the scheme, and volatile matter have not taken part in the reduction processes, then the required coal charge is to be 13.8 t/h. Table 6 Proximate and Ultimate Composition of Coals Coal with Low Content of Volatile Matter Coal with High Content of Volatile Matter Moisture Content С fixed Ash S V.M C H O N Low Volatile Coal High Volatile Coal - Theoretically Needed Coal - Actual Coal Charge Degree of the Volatile Matter Decomposition, % Degree of the Water Decomposition, % Figure 2 BOF Dust Ore Fines BOF Dust Ore Fines Theoretical and Actual Coal Charge Besides, the degree of decomposition of volatile matter in slag bath and the degree of decomposition of water by coal was calculated. With this aim the theoretical composition of flue gases under various degrees of decomposition of volatile matter and water had been calculated. The results of calculations were compared with the composition of flue gases measured experimentally. Thus, the theoretical model of decomposition was chosen. The best agreement of the results is given under the respective columns in Figure 2. The experiments strongly supported that carbon of volatile matter participates in the iron reduction processes in the slag bath. 7

8 5. PROSPECTS OF ROMELT PROCESS The Romelt process is single-stage so, there is no need to coordinate, in terms of technology and power, operations of several units, which form an integral production process. It is necessary, though, to coordinate them in multi-stage processes and this is an essential production problem. The Romelt furnace can operate in a wide range of production, according to the demand in the market. Such a flexibility of the unit does not involve any long time reorganization of the technological process. However, in the case of low output, the coal and oxygen consumption rates increase. The Romelt process, like any other alternative to the blast furnace process, does not claim to be competitive with blast furnaces in the world hot metal production. But it can fill a definite niche in the metallurgical industry. Firstly, in the nearest future, the Romelt unit can be constructed at the integrated metallurgical works in addition to the blast furnaces, rather than in lieu of the same, first of all, for processing the iron-containing wastes generated at the works. This will allow to bring zinc and alkali metals out of a metallurgical cycle, with the respective saving of the coke rate. Secondly, invention of highly productive machines for thin slabs continuous casting has opened a new possibility in the market of rolled products for hot and cold strip rolling miniplants. For most miniplants, capital investments on the construction of a blast furnace or of any other unit with a shaft are excessively high. A smelting reduction unit, the Romelt furnace, for example, is more preferable in the structure of a miniplant in its head part. Its liquid iron can be more economically used in converters or electric furnaces. The Romelt unit requires less capital investment. It produces cheaper hot metal. Usage of their own hot metal at miniplants eliminates their dependence on the supply of scrap and on the fluctuations of its market. At present National Mineral Development Corporation Ltd. (India) initiates a program for the construction of Romelt plant capacity t/a. 6. CONCLUSION As a result of scientific researches during the 14 year-period of operation of the pilot commercial unit in the manner of individual campaigns all technological regimes were fixed in respect to various compositions and consumptions of charge materials, supply of coal and oxygen, extent of the post-combustion. Nowadays the experiments are finished, and the process is ready for the commercial use. The smelting reduction Romelt technology has been developed for processing of metallurgical wastes containing non-ferrous metal components. Experiments at the Romelt plant proved the possibility of recovering metallurgical wastes and obtaining hot metal. Non-ferrous metal admixtures get concentrated in one of the formed phases that facilitates their further processing. The process can be applied to all types of iron-bearing wastes. There are no fundamental restrictions on the Romelt process. Its usage expediency depends only on the economic factors: the amount of the specific investments and the cost of the received product. Bibliography 1. ROMENETS V.A., VEGMAN E.F., SAKIR N.F. Liquid-Phase Reduction Process. Izvestiya VUZov. Chernaya Metallurgiya, 1993, number 7, pages 9-19 (in Russian); Steel in Translation, 1993, volume 23, number 7, pages 7-16 (in English). 2. ROMENETS V.A. Romelt - Complete Liquid-Phase reduction Ironmaking Process. Izvestiya VUZov. Chernaya Metallurgiya, 1999, number 11, pages (in Russian). 3. ROMENETS V. ET AL. Processing Industrial Wastes with the Liquid-Phase Reduction Romelt Process. Journal of Metals, 1999, volume 51, number 8, pages