Shaft Furnace Technologies Related To Materials and Environmental Challenges

Transcription

1 Shaft Furnace Technologies Related To Materials and Environmental Challenges E. Mousa, A. Babich, R. Klock, S. Khumkoa, D. Senk RWTH Aachen University, Department of Ferrous metallurgy Shaft furnaces based on the counter flow principle are widely used in steel and foundry industries as well as in non-ferrous metallurgy, e.g. blast furnace, Midrex, Corex, OxiCup furnace, cupola furnace or Imperial smelter. To keep and to enhance advantageous of the shaft furnaces as ideal counter-current reactors, research activities at Dept. of Ferrous Metallurgy, RWTH Aachen University are focused on improvement of iron bearing material quality, coke quality, increasing the shaft permeability, prevention of sticking during reduction in DRI facilities, injection of auxiliary reducing agents as well as internal and municipal wastes, coke and total energy saving and reducing the carbon dioxide emission. 1 Introduction Product quality, energy saving and environmental issues are main challenges in iron & steel industry as well as in foundry industry. Shaft furnaces are the most effective reactors for manufacturing the hot metal, cast iron and DRI products. The main reason for high efficiency and competitiveness of the shaft furnaces is counter flow principle of their operation. Charge materials descend under the influence of gravity; the gas that is generated in the front of the furnace tuyeres ascends to the top. All the basic processes in the shaft furnace (heating and decomposition of materials, reduction, melting of metal and slag) occur due to heat exchange between the descending materials and ascending gases. The biggest representative of shaft furnaces is blast furnace (BF); further examples are cupola, OxiCup furnace, Midrex and Corex. In this paper recently completed and current activities of IEHK focused on improvement of raw material quality, efficiency of shaft furnace operation, energy saving and CO2 mitigation as well as prevention of sticking phenomenon are reported. These works have been or are being performed in the scope of European and national research programmes as well as on the basis of bilateral cooperation with international partners. 1

2 2 Coke Quality for a Blast Furnace Modern blast furnace operation with low coke rate and high injection rate causes a change in coke quality requirements. Standard characteristics of coke quality and test methods seem to be insufficient to simulate changing conditions in a modern blast furnace [1]. Non-standard tests were developed at RWTH Aachen University: to investigate coke dissolution behaviour to quantify the anisotropic effects of carbon phases in coke (together with ThyssenKrupp Steel AG) to investigate the coke behaviour under simulating blast furnace conditions (changing gas composition, temperature and time scenarios). Specimens before and after the tests are undergo microscopic examination. Results of a study on the coke behaviour under simulating blast furnace conditions (Figure 1) allowed concluding that [2]: Alkali vapour accelerates coke gasification but its catalytic action decreases with rising temperature. Catalytic effect of alkali appears strongly when CRI is low and almost disappears when using high reactivity cokes. The strong poisoning effect of carbon monoxide on the reaction degree of coke was observed. It remains also under alkali load. The higher porosity of feed coke is, the higher the CRI value is. This dependence becomes flatter after the tests simulating BF conditions and actually disappears under alkali load. Coke optical anisotropy decreases with raise of CRI but after the reaction tests this dependence becomes flatter. PC char and ash may have dual effect on coke reaction and degradation behaviour: the negative catalytic effect and the positive effect of preventing coke from degradation. Furthermore a concept for coke use is related to nut coke effect on bell coke behaviour and overall BF performance. Operation of many BFs has confirmed the coke saving possibility using nut coke in the mixture with burden. Nut coke reactivity was investigated and compared with reactivity of bell coke [2]. Then, the change in streaming conditions when charging nut coke was studied both theoretically and experimentally (Figure 2). It was founded that BF productivity can be increased by % (calculation results) or by % (experimental results) when using wt. % of nut coke due to the positive effect of nut coke-sinter mixture on shaft permeability [4]. Current research activities at IEHK aim at study of the reducibility of sinter mixed with nut coke. 2

3 CO CO 2 Ar N Figure 1: Scheme of the Tammann furnace set: 1-flow controller, 2-electronic scale, 3- computer 4-gas analyser, 5-coke sample 6-alkali, 7-quartz glass, 8- gas supply Figure 2: Scheme of cold model [3]:1-cylindrical stack for packed material, 2-grid, 3-conic hopper, 4-tuyeres, 5-variable area flow meter, 6-pressure drop measuring points, 7-U-tube manometer 3 Sinter Reducibility Iron ore sinter contains impurities such as SiO 2, Al 2 O 3, CaO, MgO, MnO 2, etc. which influence the reduction behaviour of iron oxides and the controlling mechanism [5]. Work presented here is focused on the simultaneous influence of SiO 2 and MnO 2 on the reduction behaviour of Fe 2 O 3 with CO at temperatures C. To study the effect of impurities (SiO 2, and MnO 2 ) on the reduction behaviour of Fe 2 O 3, powders (<50 µm) were mixed in different ratios, pressed into the form of cylindrical shape (d=7.8 mm, h=12 mm) and sintered at 1200 C for 6 hrs. The fired sample were isothermally reduced with CO gas (1 l/min.) at C using vertical tube furnace and the oxygen weight loss was continuously recorded as a function of time. The microstructure of different samples sintered at 1200 C (Figure 3) shows that pure Fe 2 O 3 has a relatively dense structure with more or less homogenously distributed pores (a); the presence of MnO 2 leads to the formation of manganese ferrite MnFe 2 O 4 as a thin layer around the pores borders (b); there are no phases developed between SiO 2 and Fe 2 O 3 during firing (c); the presence of SiO 2 affects on the 3

4 MnFe 2 O 4 which become less coarse with the development of relatively small pores (d). Figure 3: Photomicrographs of compacts annealed at 1200 C (100x): (a) pure Fe 2 O 3 (b) 6% MnO 2 -doped Fe 2 O 3 (c) 7.5% SiO 2 -doped Fe 2 O 3 (d) (6%MnO % SiO 2 )-doped Fe 2 O 3 In order to clarify the influence of SiO 2 alone or with MnO 2 on the reduction behaviour of Fe 2 O 3, the reduction rate (dr/dt) at initial and final stages (reaching the 20 and 80% of reduction degree respectively) was plotted against the temperature (Figure 4). It is clear that the reduction rate increases with rising temperature. The reduction rate at the initial stage increases as SiO 2 increases due to the effect of SiO 2 on the original porosity of the samples (Figure 4 a). The rate of reduction at the final stage decreases with SiO 2 due to the formation of hardly reducible fayalite phase (2FeO.SiO 2 ) which converts the reduction mechanism from chemical reaction to solid state diffusion. For the samples contains both SiO 2 and MnO 2, the rate of reduction decreases at the initial as well as at final stages (Figure 4 b). The reason for this is the presence of MnFe 2 O 4 which is formed during sintering. This phase is hardly reducible compared to Fe 2 O 3 and in presence of SiO 2 leads to the formation of fayalite manganoan (Fe,Mn) 2 SiO 4 which affects the reduction rate negatively. 4

5 Figure 4: Effect of temperature on the reduction rate of hematite: (a) Fe 2 O 3 with SiO 2, (b) Fe 2 O 3 with (SiO 2 + MnO 2 ) The objectives of current work are study the influence of nut coke on the reduction behaviour of different types of iron ore sinter and the examination of the reduction kinetics and mechanism in order to optimise the blast furnace efficiency and sinter reducibility. 4 Improvement of Shaft Furnace Efficiency and Energy Saving Optimal characteristics in the zone in front of the tuyeres, where heat and reducing gas are generated and sufficient gas permeability in the shaft and dripping zone are pre-requisites for effective operation of the shaft furnace. Another condition for high furnace productivity is optimal oxygen offer; it means not only its total amount but also oxygen distribution and penetration. IEHK has a pilot plant COBESI (Coke Bed Simulator) to simulate the processes in the front of the shaft furnace tuyeres (Figure 5). Modular concept of the pilot plant provides flexible adjustment for simulation of mentioned zone in different shaft furnaces with and without raceway 5

6 formation. To quantify the test results four measuring points equipped with thermocouples and gas analyser are installed along each of two coke chambers. The dust feeder and the injection lance enable injection of coals, iron containing dusts and further solid substances. two coke chambers dust cyclone post combustion tuyere with O 2 lance Figure 5: Scheme of the modified COBESI pilot plant Tests simulating cupola furnace, OxiCup furnace and blast furnace conditions when using metallurgical and foundry cokes and bricks were performed [6-8]. Thereby combustion, oxidation and melting behaviour of different injected dusts were examined. Furthermore advanced measurement techniques to explore the raceway shape and extension as well as a laser based non-intrusive system for temperature and gas composition detection were tested at the pilot rig [9-10]. External pulsations of oxygen and further blast parameters in blast furnace, cupola and OxiCup furnaces were proposed to improve the gas penetration, peripheral gas distribution in lower zone and intensify the PC gasification in the raceway [8,11,12]. Pilot trials with automated Sequence-Impulse-Process are currently being conducted at IEHK using the coke bed simulator. Energy saving and CO 2 mitigation are main challenges for European steel industry. IEHK is a member of a ULCOS-program consortium and participates in the project activities related to New Blast Furnace Process with top gas recycling, which will operate with lower CO 2-6

7 emissions based on drastically reduced consumption of carbon containing input materials [13]. Innovative use of renewable energy sources such as biomass and charcoal is another option to reduce or to avoid the CO 2 emission. Nowadays the use of charcoal in metallurgy is limited to small blast furnaces in Brazil. Tuyere injection might be another way for biomass and charcoal use. In contrast to pulverised coal which has a dense and compact surface, charcoal is characterised by highly porous structure. Technology for charcoal injection into the large modern blast furnaces is being developed. 5 Injection and Waste Recycling in Shaft Furnaces The primary goal for injection of auxiliary reducing agents is decreasing the coke rate in the blast furnace, cupola furnace or char in Corex/Finex furnaces. Technologies with top gas and biomass injection aiming at CO 2 mitigations were mentioned in previous section. Injection of solid waste and non-combustible materials (plastics, shredder light fractions, flue dust, BOF slag, TiO 2 containing materials etc.) has become an issue due to the benefit of these technologies regarding hot metal quality, furnace service life as well as favourable economical and environmental effects. Some of non-combustible materials and waste accelerate auxiliary reducing agent conversion in the raceway. Therefore combined injection of reducing agents with such materials has certain advantages [14] Δt Conversion = 20 ms 300 Conversion degree, % substoechiometric area Anthracite 1 fine Anthracite mm Anthracite 2 fine Anthracite mm Injection rate Injection rate, kg/thm O/C atomic ratio Figure 6: Results of injection tests of two anthracites with different grain sizes under BF raceway simulating conditions 7 0

8 Numerous studies at IEHK using lab and pilot injection facilities as well mathematical modelling contributed to the clarification of the conversion mechanism and determination of degree of gasification of wide range of substances, under BF, cupola and OxiCup furnace simulating conditions [14]. An example of conversion degree of two coals depending on oxygen/carbon atomic ratio is shown in Figure 6. Further studies on reaction kinetics of waste plastics and carbon materials are foreseen. Apart from injection, another ways to utilise municipal and industrial wastes like organic materials, iron- and carbon-rich dusts and sludges are being studied at IEHK. They include among others agglomeration processes such as production of self-reducing pellets and briquttes [15, 16] 6 Prevention of Sticking Sticking of pellets or fine iron ores might occur in shaft and fluidised bed processes at temperatures of about o C. The formation of big clusters of adhered pellets or iron ore fines affects negatively smooth operation of the shaft furnace for direct reduction and its productivity. Decreasing the reduction temperature to avoid this problem may cause a significant drop in productivity. As an example, a temperature decrease from 850 o C to 750 o C could result in a decrease of 30-40% in productivity. The formation mechanisms of sticking during direct reduction of iron ore was studied at IEHK since 1980s and methods to minimise and to prevent the sticking were elaborated. It was found that type and expansion of sticking depend on type of iron precipitation during reduction. The iron precipitation depends on the temperature and on the reduction gas potential. There are three kinds of iron precipitation: porous, dense and fibrous (Figure 7). The porous type of precipitation is characterised by good penetration conditions for the reduction gas. This property affects high reduction velocities. The dense iron layer obstructs the reduction of the inner part of an iron ore grain. The characteristic surface of the fibrous morphology (iron whisker) is responsible for the highest sticking tendency of all three kinds of precipitation. In the cause of fibrous iron precipitate, sticking is a result of the growth of fibrous that hooks to each other. 8

9 Figure 7: Iron precipitation regions during the reduction of iron ore [17] Methods to minimise and to prevent sticking by considering the effect of gangue in iron ore, number of reduction steps and ore coating with inert material has been discussed. It was found out that ore with higher gangue content tend to have lower sticking, and higher reduction temperatures result in more remarkable sticking [18]. Further study performed at IEHK in a muffle furnace with layered iron ore and coal showed that gaseous products of coal conversion affect the reduction behaviour and precipitation of iron (Figure 8). It was found that sticking can be controlled by volatile matter composition of coal when using suitable combination of ore and coal types. 10 μm 20 μm Figure 8: Sticking by iron whisker growth (pictures taken after the reduction test) 9

10 7 Conclusions Research activities at IEHK, RWTH Aachen University cover shaft furnace processes in iron and steel as well as in foundry industries for hot metal, cast iron and direct reduction iron production. Coke quality for a modern blast furnace with low coke rate and high injection rate was investigated using non-standard test methods. Sinter reducibility mixed with nut coke is being studied to clarify the nut coke effect on the reduction behaviour and kinetics of different types of iron ore sinter. To improve the shaft furnace efficiency, to reduce coke and energy consumption and consequently the impact on the environment, and to recycle municipal and industrial waste, various injection and agglomeration technologies as well as pulsations of oxygen and further blast parameters have been and are being developed and tested using lab and pilot facilities. Sticking of pellets or fine iron ores might occur in shaft and fluidised bed processes. The formation mechanisms of sticking during direct reduction of iron ore and methods to minimise and prevent this phenomenon were elaborated. 8 References [1] Babich, A.: Internet lecture Coke Quality for a Modern Blast Furnace, [2] Babich, A.; Senk, D.; Gudenau, H.W.: Coke Quality for a Modern Blast Furnace, Proc. 4th Int. Congress on the Science and Technology of Ironmaking.(ICSTI 06), Osaka, Japan, (2006), pp [3] Marinakis, N.: Durchgasungswiderstand im Schachtofen, Master Thesis, RWTH Aachen, (2005) [4] Babich, A.; Senk, D.; Yaroshevskiy, S.; Chlaponin, N.; Kochura, V. and Kuzin, A.: Effect of nut coke on blast furnace shaft permeability, Proc. 3rd International Conference on Process Development in Iron and Steelmaking (SCANMET III), 8-11 June 2008, Luleå, Sweden, vol. 2, pp [5] El-Geassy, A.A; Nasr, M.I.; Omar, A.A. and Mousa, E.A.: Reduction kinetics and catastrophic swelling of MnO2-doped Fe2O3 compacts with CO at K, ISIJ Int., Vol. 47, No. 3, (2007), pp

11 [6] Wieting, T.: Feststoffinjektion und Einsatz von Erdgas/Sauerstoff- Brennern zur Verbesserung der Wirtschaftlichkeit des Kupolofen- Schmelzprozesses, Dr.-Ing. Thesis, RWTH Aachen, (2005) [7] Babich, A.; Senk, D.; Gudenau, H.W.; Hirsch, A.; Klock, R.: Investigation of conversion of injected coals and other dusts in shaft furnaces, Metallurgical Processes and Equipment (Ukraine), 11 (2008), No. 1, pp [8] Klock, R.: Neue Methode zur Sauerstoffinjektion in Koksgefeuerte Schachtöfen, Dr.-Ing. Thesis, RWTH Aachen, forthcoming [9] Sandlöbes, S.; Senk, D.; Lischke, F.; Clausen, S.; Diaz, A.; Sancho, L.: Online measurement of concentrations and temperature of metallurgical off-gases, Proc. 3rd International Conference on Process Development in Iron and Steelmaking (SCANMET III), 8-11 June 2008, Luleå, Sweden, vol. 2, pp [10] Sandlöbes, S.; Senk, D.; Klock, R.; Babich, A.: Online measurement techniques in material production, this conference. [11] Patent application WO 2007/ A2, publ Method for the operation of a shaft furnace, and shaft furnace suitable for said method [12] BMBF Project 01RI0617E: Automatisierter Sequenz-Impuls-Prozess für Großschachtöfen [13] Schmöle, P. ; Lüngen, H.B.: La Revue de Metallurgie-CIT, Marth (2005), pp [14] Babich, A.; Senk, D.; Gudenau, H.W.; Mavrommatis, K.Th.: Ironmaking Textbook, Mainz GmbH Aachen, (2008), 402 p. [15] Wang, S.: Verhalten von kohlenstoffhaltigen Eisenerzagglomeraten bei der Reduktion, Dr.-Ing. Thesis, RWTH Aachen, (2004) [16] Ohler-Martins, K.: Metallurgische Mechanismen bei der Direkt- und Schmelzreduktion von Manganerz und Eisenerz, Dr.-Ing. Thesis, RWTH Aachen, (2008) [17] Schiller, M.: Die Mikromorphologie der Eisenphase als Folge der Reduktion von Eisenoxiden, Dr.-Ing. Thesis, RWTH Aachen,(1987) [18] Khumkoa, S.: Plating-Probleme während der Direktreduktion von Eisenerzen, Dr.-Ing. Thesis, RWTH Aachen, forthcoming 11