Optimization of sponge iron (direct reduced iron) production with Box-Wilson experimental design by using iron pellets and lignite as reductant

2.1. Material

In this study, iron pellets taken from Divriği with a diameter of 10-16 mm, using 0.8% bentonite as a binder, sintered at 1250-1320 °C, with a compressive strength of 280 kg/pellet were used. The chemical analysis results of the iron pellets used in the experiments are given in Table 1. Also, Scanning Electron Microscopy (SEM) image analysis pictures are given in Fig. 1. The chemical analysis results of the lignite sample obtained from Dodurga Lignite Plant used as the reducing agent are given in Table 2. The coal sample was crushed and screened to the -11.20+4.75 mm particle size range.

The SEM images of the iron pellets in Fig. 1, show the hard agglomerates that have completed the grain formation. Solidified liquid phases are also at the junctions of coarse and fine grains.

Reduction experiments were carried out in a crucible with a lid using Protherm brand MoS-B 180/8 Model (1800 °C) Muffle Furnace without atmosphere control in Hitit University Faculty of Engineering Department of Metallurgical and Materials Engineering. Preheating was not used in the experiments, and the furnace heating and cooling were done at 20 °C·min^-1. Scanning Electron Microscopy (SEM) image analysis of iron pellets and obtained sponge irons were carried out by using FEI brand Quanta 450 FEG model scanning electron microscope at Hitit University Scientific Technical Application and Research Center (HÜBTUAM). X-Ray Diffraction (XRD) analysis were carried out by using Rigaku brand DMAX IIIC model XRD Device in Cumhuriyet University Faculty of Engineering Department of Geological Engineering Laboratories.

2.2. Methods

Studies that calculate the reduction degree differently are found in the literature. Eq. (11) was used in the studies of Geçim (2006)Geçim, M.K. (2006). The investigation of sponge iron production parameters by using iron oxide pellets with domestic lignite coal. İstanbul Technical University, Graduate School of Natural and Applied Sciences, İstanbul, Turkey., Ersundu (2007)Ersundu, A.E. (2007). The suitabilty investigation of domestic iron ores for sponge iron production. İstanbul Technical University, Graduate School of Natural and Applied Sciences, İstanbul, Turkey., Schenk (2011)Schenk, J.L. (2011). Recent status of fluidized bed technologies for producing iron input materials for steelmaking. Particuology 9 (1), 14-23. https://doi.org/10.1016/j.partic.2010.08.011., Chatterjee (2012)Chatterjee, A. (2012). Sponge Iron Production by Direct Reduction of Iron Oxide. 2nd ed., PHI Learning Private Limited, Delhi, India., Khaki et al. (2018)Khaki, J.V., Shalchian, H., Rafsanjani-Abbasi, A., Alavifard, N. (2018). Recovery of iron from a high-sulfur and low-grade iron ore. Thermochim. Acta 662, 47-54. https://doi.org/10.1016/j.tca.2018.02.010., and Eq. (12) was used in the studies of Babich et al. (2008)Babich, A., Senk, D., Gudenau, H.W., Mavrommatis, K.T. (2008). Ironmaking. RWTH Aachen University, Department of Ferrous Metallurgy, Aachen, Germany., Lee et al. (2012)Lee, Y.S., Ri, D.W., Yi, S.H., Sohn, I. (2012). Relationship between the reduction degree and strength of DRI pellets produced from iron and carbon bearing wastes using an RHF simulator. ISIJ Int. 52 (8), 1454-1462. https://doi.org/10.2355/isijinternational.52.1454., Chen et al. (2017)Chen, H., Zheng, Z., Chen, Z., Bi, X.T. (2017). Reduction of hematite (Fe₂O₃) to metallic iron (Fe) by CO in a micro fluidized bed reaction analyzer: A multistep kinetics study. Powder Technol. 316, 410-420. https://doi.org/10.1016/j.powtec.2017.02.067., and Turgut (2010)Turgut, E. (2010). Production of sponge iron by direct reduction. MSc thesis, Kocaeli University, Graduate School of Natural and Applied Sciences, Kocaeli, Turkey..

In this study; the Reduction Degree (%) values, which are a measure of how much of the total oxidized iron is reduced, were calculated using Eq. (13) given below.

The reduction (sponge iron production) experiments carried out within the scope of the study by using Box-Wilson experimental design. Proper optimization was made for the parameters effective in the reduction. In this context, independent parameters in Box-Wilson experimental design method were used; time: 10-90 min, temperature: 900-1200 °C, and [C_Fix/Fe_Total] weight ratio: 0.40-0.50 (Hughes et al., 1982Hughes, R., Kam, E.K.T., Mogadam-Zadeh, H. (1982). The reduction of iron ores by hydrogen and carbon monoxide and their mixtures. Thermochim. Acta 59 (3), 361-377. https://doi.org/10.1016/0040-6031(82)87158-x.; Lin et al., 2003Lin, H.Y., Chen, Y.W., Li, C. (2003). The mechanism of reduction of iron oxide by hydrogen. Thermochimica Acta 400 (1-2), 61-67. https://doi.org/10.1016/s0040-6031(02)00478-1.; Piotrowski et al., 2005Piotrowski, K., Mondal, K., Lorethova, H., Stonawski, L., Szymański, T., Wiltowski, T. (2005). Effect of gas composition on the kinetics of iron oxide reduction in a hydrogen production process. Int. J. Hydrog. Energy 30 (15), 1543-1554. https://doi.org/10.1016/j.ijhydene.2004.10.013.; Jozwiak et al., 2007Jozwiak, W.K., Kaczmarek, E., Maniecki, T.P., Ignaczak, W., Maniukiewicz, W. (2007). Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres. Appl.Catal. A-Gen. 326 (1), 17-27. https://doi.org/10.1016/j.apcata.2007.03.021.; Turgut, 2010Turgut, E. (2010). Production of sponge iron by direct reduction. MSc thesis, Kocaeli University, Graduate School of Natural and Applied Sciences, Kocaeli, Turkey.).

2.3. Box-Wilson experimental design

The Box-Wilson statistical experimental design method includes three types of combinations. These are the axial (A), factorial (F), and center (C) points. The independent variables are at five levels, which are determined based on the number and range of variables in the experiment. The details of the method can be found elsewhere (Davies, 1956Davies, O.L. (1956). The Design and Analysis of Industrial Experiments. Hafner Publishing Co., New York.; Crozier, 1992Crozier, R.D. (1992). Flotation. Pergamon Press.).

Three operating parameters i.e., time, temperature and the [C_Fix/Fe_Total] weight ratio were chosen as the most important independent variables. The time (X₁) was changed between 10 and 90 min, the temperature (X₂) between 900 and 1200 °C and the [C_Fix/Fe_Total] weight ratio (X₃) between 0.40 and 0.50. The experimental design consisted of six axial (A), eight factorial (F) and three center (C) points. The center point was repeated three times to estimate experimental error. The experimental conditions as coded values and real values used for the Box-Wilson statistical design are presented in Table 3.

The Reduction Degree (Y) was correlated with the other independent parameters (X ₁ , X ₂ , X ₃ ) using Eq. (14). Design Expert 8.0 computer program was used for the determination of the coefficients of Eq. (14) by regression analysis of the experimental data.

3.1. The effect of time and temperature

Variation of the Reduction Degree (%) in a constant [C_Fix/Fe_Total: 0.45] weight ratio depending on time and temperature is given in Fig. 2. In addition, SEM image analysis of sponge iron obtained from Experiment No.: C [Center] and Experiment No.: A1 is given in Fig. 3 and Fig. 4, respectively.

As it can be seen from Fig. 2, as the temperature rises up to 1,000 °C, the Reduction Degree (%) rose steadily during the course of around 80 minutes. Especially at temperatures below 950 °C, the maximum Reduction Degree (%) values reached in 80 minutes started to decrease again with increasing time. The Reduction Degree (%) values in the temperature range 1000-1050 °C did not change much depending on the time. However, the Reduction Degree (%) values decreased rapidly both at low (less than 40 min) and high (over 60 min) times, especially at temperatures above 1100 °C. The Reduction Degree (%) reached its maximum value in about 50 min in this temperature range.

The basic principles of chemical thermodynamics and kinetics and the basic laws of diffusion predict that an increase in the reduction temperature or prolonged reaction time at low temperatures can improve the degree of reduction of iron oxide (El-Geassy, 1986El-Geassy, A.A. (1986). Gaseous reduction of Fe₂O₃ compacts at 600 to 1050 °C. J. Mater. Sci. 21 (11), 3889-3900. https://doi.org/10.1016/0168-7336(87)80032-3.; Pawlik et al., 2007Pawlik, C., Schuster, S., Eder, N., Winter, F., Mali, H., Fischer, H., Schenk, J.L. (2007). Reduction of iron ore fines with CO-rich gases under pressurized fluidized bed conditions. ISIJ Int.l 47 (2), 217-225. https://doi.org/10.2355/isijinternational.47.217.). The temperature greatly affects the Reduction Degree (%). Since the main reduction reactions [reduction of magnetite to wustite] are endothermic and facilitated by high temperature, the Reduction Degree (%) increases with increasing temperature. Thus, an increase in temperature caused an increase in diffusion and reduction rates. This is described by Liu et al. (2004)Liu, G., Strezov, V., Lucas, J.A., Wibberley, L.J. (2004). Thermal investigations of direct iron ore reduction with coal. Thermochim. Acta 410, 133-140. https://doi.org/10.1016/s0040-6031(03)00398-8., Wang et al. (2012)Wang, Z., Chu, M., Liu, Z., Chen, S., Xue, X. (2012). Effects of temperature and atmosphere on pellets reduction swelling index. J. Iron Steel Res. Int. 19 (10), 7-12. https://doi.org/10.1016/s1006-706x(12)60144-7., Yi et al. (2012)Yi, Ly., Huang, Zc., Peng, H., Jiang, T. (2012). Action rules of H₂ and CO in gas-based direct reduction of iron ore pellets. J. Cent. South Univ. 19, 2291-2296. https://doi.org/10.1007/s11771-012-1274-0., Zhang et al. (2014)Zhang, T., Lei, C., Zhu, Q. (2014). Reduction of fine iron ore via a two-step fluidized bed direct reduction process. Powder Technol. 254, 1-11. https://doi.org/10.1016/j.powtec.2014.01.004., Guo et al. (2016)Guo, D., Zhu, L., Guo, S., Cui, B., Luo, S., Laghari, M., Chen, Z., Ma, C., Zhou, Y., Chen, J., Xiao, B., Hu, M., Luo, S. (2016). Direct reduction of oxidized iron ore pellets using biomass syngas as the reducer. Fuel Processing Technology 148, 276-281. https://doi.org/10.1016/j.fuproc.2016.03.009., and Chen et al. (2017)Chen, H., Zheng, Z., Chen, Z., Bi, X.T. (2017). Reduction of hematite (Fe₂O₃) to metallic iron (Fe) by CO in a micro fluidized bed reaction analyzer: A multistep kinetics study. Powder Technol. 316, 410-420. https://doi.org/10.1016/j.powtec.2017.02.067.. The reduction reaction in the CO atmosphere [reduction of magnetite and wustite to iron] is exothermic, so dynamic conditions develop simultaneously, although thermodynamic conditions deteriorate with rising temperature (Wang et al., 2012Wang, Z., Chu, M., Liu, Z., Chen, S., Xue, X. (2012). Effects of temperature and atmosphere on pellets reduction swelling index. J. Iron Steel Res. Int. 19 (10), 7-12. https://doi.org/10.1016/s1006-706x(12)60144-7.). The increase in the Reduction Degree (%) with increasing time can be attributed to a large contact area between the reducing gas and the pellets. The samples produced in this study were pellets with an approximate porosity of 60%, giving rise to the contact of gas with more grains of iron oxide. The reducing gas readily reacted by diffusing into the pellets and the gases formed from the reactions were easily removed from the pellets. The reduction in the rate of increase of the Reduction Degree (%) as the time increases further can be attributed to the coating of the outer surface of the pellets with an iron “shell” and the reduction of penetration of the reducing gas into the pellets as the reactions progress. Simultaneously, it is difficult for the gases formed as a result of reactions to escape out of the pellets. Additionally, unreacted pellet cores were continuously reduced. After 70 min, the increase in the Reduction Degree (%) slowed down and reached a constant level over longer periods. This shows that the reduction reactions are almost completed, and a large proportion of Fe₂O₃s are reduced to Fe. These results are consistent with the results of Guo et al. (2016)Guo, D., Zhu, L., Guo, S., Cui, B., Luo, S., Laghari, M., Chen, Z., Ma, C., Zhou, Y., Chen, J., Xiao, B., Hu, M., Luo, S. (2016). Direct reduction of oxidized iron ore pellets using biomass syngas as the reducer. Fuel Processing Technology 148, 276-281. https://doi.org/10.1016/j.fuproc.2016.03.009..

The SEM image analysis of the sponge iron obtained in Experiment No.: C given in Fig. 3 shows that the sponge structure, which is highly reduced and clinging to the neck regions with the effect of temperature, can be seen. The structure is completely sintered and does not contain a different phase at the grain boundaries. It is seen that pore sizes are between 20-50 µm in agglomerates, as seen by big red circles, and intergranular pores are between 1-10 µm as illustrated by small yellow circles. It is observed that the round-edged particles close to the sphere are reduced and topo chemically FeO to Fe structures. The growth morphology of Fe in the body-centered cubic or face-centered cubic cage in the cubic crystal, as seen by big red circles and the presence of a small amount of Fe₂O₃ or FeO structure, were also determined by fractures. These features are indicated by arrows caused by sharp crack propagation, as seen by small yellow circles between sinter grains. It is readily apparent the sintering and neck formation of reduced metal-like structures both in deformed form and as sharp ceramic cleavage fractures. Regional hard agglomerates have been identified, where fine-grained Fe structures reduced from coarse-grained Fe-O structures are the driving force for sintering by liquidifying surface metals which are shown as long bridge-like metallic horns or deformed buckled pieces.

It is seen in Fig. 4 that the SEM image analysis of the sponge iron obtained in Experiment No.: A1 consists of sponge iron structures of metal character that are thought to contain mostly Fe, similar to the SEM image analysis of sponge iron obtained by Experiment No.: C illustrated in Fig. 3. It has a more homogeneously distributed pore size, and the average pore size, is identical across the entire structure in the range of 2-10 µm.

3.2. Effect of Time and [C_Fix/Fe_Total] Weight Ratio

The variation of the Reduction Degree at a constant temperature of 1050 °C, depending on the time and [C_Fix/Fe_Total] weight ratio, is given in Fig. 5. Also, SEM image analysis of sponge iron obtained from Experiment No.: A5 is given in Fig. 6.

Figure 5 shows that with the increase in time, the Reduction Degree (%) increases and reaches its maximum values in the 40-60 min range. At higher times, the Reduction Degree (%) again decreased. In the 0.44-0.46 [C_Fix/Fe_Total] weight ratio range, the Reduction Degree (%) did not change much depending on the time. Time’s effect caused the Reduction Degree (%) values to drop precipitously to their lowest points in terms of the higher value of [C_Fix/Fe_Total] between 40 and 60 min, then rise precipitously to their highest points again after 60 min. The highest Reduction Degree (%) values were reached at the maximum time values and [C_Fix/Fe_Total] weight ratio. In other words; The Reduction Degree (%) values increased due to the increase of the [C_Fix/Fe_Total] weight ratio in low and high times, whereas the Reduction Degree (%) decreased as the [C_Fix/Fe_Total] weight ratio increased in the medium periods (in the range of 40-60 min).

It is seen that the sponge iron obtained with Experiment No.: A5 has a grain size as in SEM image analysis of the sponge iron obtained with Experiment No.: C illustrated in Fig. 3 and Experiment No.: A3 illustrated in Fig. 8, and due to the similarity of the reductant, the particles are partially attached to each other, and the sintered structure is developed. The stratified growth pattern is similar to the body-centered cubic or face-centered cubic growth morphology of Fe. The amount of oxide is also understood from the electron charge. As it is very well known, since the electron charge takes place on the edges of ceramic-like materials even if it was gold-coated, the higher the amount of Fe₂O₃ or FeO, the lighter the image. The sharp edges are also an indication of oxide formation due to high stratigraphy of grown material. As it is seen from previous SEM images, the reduced Fe is elongated and deformed bars.

3.3. Effect of [C_Fix/Fe_Total] Weight Ratio and Temperature

The change in the Reduction Degree (%) depending on [C_Fix/Fe_Total] weight ratio and temperature at 50 min fixed time is given in Fig. 7. In addition, SEM image analysis of sponge iron obtained from Experiment No.: A3 is given in Fig. 8.

As it can be seen from Fig. 7; at temperatures below about 1050 °C, the Reduction Degree (%) values tend to decrease due to the increase in the [C_Fix/Fe_Total] weight ratio. The Reduction Degree (%) values did not change much due to the increase in [C_Fix/Fe_Total] weight ratio at the temperatures above this value, but the maximum Reduction Degree (%) was reached at the highest temperature and [C_Fix/Fe_Total] weight ratio values. In other words, the Reduction Degree (%) values increased due to the increase in temperature. This increase in the Reduction Degree (%) values was faster up to a temperature of about 1050 °C, especially at high [C_Fix/Fe_Total] weight ratio values. The rate of increase of the Reduction Degree (%) values decreased due to the temperature increase above about 1050 °C.

The increase in the Reduction Degree (%) due to the increase in temperature is consistent with the results obtained by Ersundu (2007)Ersundu, A.E. (2007). The suitabilty investigation of domestic iron ores for sponge iron production. İstanbul Technical University, Graduate School of Natural and Applied Sciences, İstanbul, Turkey. and Lee et al. (2012)Lee, Y.S., Ri, D.W., Yi, S.H., Sohn, I. (2012). Relationship between the reduction degree and strength of DRI pellets produced from iron and carbon bearing wastes using an RHF simulator. ISIJ Int. 52 (8), 1454-1462. https://doi.org/10.2355/isijinternational.52.1454.. Furthermore, it can be said that the effect of [C_Fix/Fe_Total] weight ratio on the Reduction Degree (%) at constant temperature and time remains weak among other parameters. Similar results are expressed by Ersundu (2007)Ersundu, A.E. (2007). The suitabilty investigation of domestic iron ores for sponge iron production. İstanbul Technical University, Graduate School of Natural and Applied Sciences, İstanbul, Turkey. and Khaki et al. (2018)Khaki, J.V., Shalchian, H., Rafsanjani-Abbasi, A., Alavifard, N. (2018). Recovery of iron from a high-sulfur and low-grade iron ore. Thermochim. Acta 662, 47-54. https://doi.org/10.1016/j.tca.2018.02.010..

In Fig. 8, Fe-grains reduced by topochemical reaction and fast-growing fully concentrated sinter structure can be seen. Sintering may have occurred during the Fe reduction process, since the resulting hard agglomerates had a compact structure devoid of pores. This procedure continues up to all surfaces are grown in the certain directions of maximum packing density and again produces surface oxides. Fe crystals grow towards the surface as a result of a reduction that develops from the outside to the inside. During this process, the crystals encounter surface oxides and are reduced once more. The most important proof of this situation is the brittle crack advances on the surface. In certain regions, the crack is stopped, which can be thought to be due to the energy absorption of the metallized regions.

3.4. Optimization

The sponge iron obtained by Experiment No.: A3 with a Reduction Degree (%) of 71.91% contains a total of 97.12% Fe, of which 7.12% is oxidized. Optimum time (X₁; 10-90 min), temperature (X₂; 900-1200 °C), and [C_Fix/Fe_Total] weight ratio (X₃; 0.40-0.50) provided that selected parameters are the most important independent variables in the production of sponge iron, respectively, 82.59 min, 996.73 °C and 0.49 were determined, and the highest Reduction Degree (%) value was calculated as 96.46%.