Effects of Reducing Time on Metallization Degree of Carbothermic Reduction of Tall Pellets Bed

Transcription

1 ISIJ International, Vol. 56 (2016), ISIJ No. International, 1 Vol. 56 (2016), No. 1, pp Effects of Reducing Time on Metallization Degree of Carbothermic Reduction of Tall Pellets Bed Xin JIANG, 1) * Shih-Hsien LIU, 2) Tsung-Yen HUANG, 2) Guangqiang ZHANG, 1) He GUO, 1) Gang-Herng SHIAU 2) and Fengman SHEN 1) 1) School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, China. 2) China Steel Corporation, No. 1, Chung Kang Rd., Hsiao Kang, Kaohsiung, Taiwan. (Received on September 2, 2015; accepted on October 19, 2015; J-STAGE Advance published date: November 26, 2015) Recently, more and more attention has been paid on alternative ironmaking processes due to the sustainable development. Aimed for the development of a new direct reduction technology, PSH process, the effects of reducing time on the metallization degree (MD) of carbothermic reduction of tall pellets bed at high temperature (1 500 C) are investigated at lab-scale in present work. The experimental results show, (1) In case of 50 min of reducing time, the MD of bottom layer DRI (Direct Reduced Iron) is lower, about 13%. MD of total pellets bed is about 58%. (2) In case of 60 min, the MD of bottom layer DRI increases to 63%. MD of total pellets bed increases to 80%. But for this case, the longer reducing time may result in some disadvantage, e.g. re-oxidation of top layer DRI, low efficiency of thermal energy, and low productivity. (3) Hot charge can obviously increase the metallization degree of DRI, especially for the bottom layers. In case of 50 min, compared with cold charge operation, the MD of bottom layer increases from 13% to 78%. Then the MD of total DRI bed increases from 58% to 85% by hot charge operation. Therefore, 50 min and hot charge are proposed based on the investigation in present work. These experimental results can give some theoretical references for the development of PSH process in future. KEY WORDS: metallization degree; carbothermic reduction; reducing time; tall pellets bed. 1. Introduction Aimed for the sustainable development and environmental concerns, the development of alternative ironmaking process is more and more necessary. Many alternative ironmaking processes have been developed in the past decades of years, including shaft furnace, rotary kiln, tunnel kiln, rotary hearth furnace (RHF), and some smelting processes, e.g. COREX, FINEX, and HiSmelt etc. 1 14) Each process may have its advantage and applicability, for example treating some waste oxides and special ores, but it also has disadvantage and inapplicability. So, none of these alternative ironmaking processes has the overwhelming advantage and could defeat others. Recently, a new direct reduction process (Paired Straight Hearth furnace, shorten as PSH) was proposed by Professor Wei-Kao Lu of McMaster University, Canada, 15) and developed in North America and China. 16) Similar with RHF process, PSH process may also be divided into oxidation compartment and reduction compartment (Fig. 1). There are two operational characteristics in PSH process, high temperature ( C) in oxidation compartment and tall pellets bed ( mm, 5 7 layers of pellets) in reduction compartment. It should be pointed out that there are two core technologies for application and achievement of PSH process, (1) prevention of newly formed DRI (Direct Reduced Iron) from re-oxidation by fully combusted gas. (2) Fast heat transfer from the top to bottom of pellets bed. (1) The newly formed DRI at the top of the bed is protected from re-oxidation by the upward gas flow (basically the gas is CO rich) generated during the reduction of pellets * Corresponding author: jiangx@smm.neu.edu.cn DOI: Fig. 1. Schematic diagram of the carbothermic reduction of tall pellets bed ISIJ 88

2 in the tall bed, and enhanced by the high volatile coal in green ball over a longer period of the reducing time (refer to the C/O ratio in ore/coal composite pellets in section 2.1 Raw Materials ). So, tall pellets bed is one of the operational characteristics of PSH process. (2) From the view point of the process control, the radiation heat transfer from heating source to the bottom of bed is the critical step of the process. The temperature of heat source is the most important variable for enhancing the rate of heat transfer in the whole system, because the intensity of thermal radiation is proportional to T 4 of heat source. Therefore, high temperature is also one of the operational characteristics of PSH process, which can drastically improve heat transfer in the process. In addition, the shrinkage of the newly formed DRI in top layers at high temperature (1 500 C) is the key for radiation heat transfer, because the shrinkage can result in a larger space for the passage of radiative heat flux (Section 3.2 in this paper for details). Therefore, PSH process is considered to produce DRI with high DRI quality, low carbon rate, and consequentially low CO 2 emission. In present work, the effects of reducing time on metallization degree (shorten as MD) of carbothermic reduction of tall pellets bed at high temperature (1 500 C) are experimental investigated. 2. Raw Materials and Experimental Procedure 2.1. Raw Materials The chemical composition of iron ore concentrate used in this work is shown in Table 1. This iron ore consists mainly of magnetite. The iron ore concentrate is ground by shatterbox down to 200 mesh for pelletization. The proximate analysis and ash composition of the pulverized coal is shown in Table 2. The volatile matters (CO and H 2 rich) generated from coal can protect newly formed DRI from re-oxidation during its rising in the pellets bed. So the coal with high volatile matters (VM) can be used in tall pellets bed, in which the volatile matter is about 26%. For the preparation of ore/coal composite pellets, coal is ground down to 60 mesh for pelletization. Based on previous experiments, the diameter of pellet is mm, and there are 5 layers of pellets (about 80 mm in height of total pellets bed, void ratio of packing bed is about 14 18%) for each experiment. 17) The carbon addition is the gram-atomic ratio of the fixed carbon in the coal added to the combined oxygen in iron oxides, denoted as C/O (g-atomic / g-atomic). In this work, C/O = 0.95 (if C is calculated based on total carbon, C/O is 1.2). The upward reducing gas flow can protect newly formed DRI from reoxidation in tall pellets bed. So, tall pellets bed is one of the core technologies in PSH process. In order to ensure one dimensional radiation heat transfer from top to bottom in pellets bed, the crucible for holding pellets are surrounded by insulating materials with 20 mm in thickness Experimental Procedure PSH experiments can be carried out in an electric muffle furnace and a natural gas-fired furnace. In the muffle furnace, the advantage is accurate and even temperature distribution, and little raw materials are needed for each experiment. The disadvantage is that only temperature profile of PSH furnace can be simulated, but the atmosphere profile cannot be simulated due to heating by electric heating elements (MoSi 2 ). In the natural gas-fired furnace, the advantage is that both temperature profile and atmosphere profile of PSH furnace can be simulated, but more raw materials and more work are needed. In present work, the experiments of carbothermic reduction of tall pellets bed are carried out in electric muffle furnace. In the future work, the optimum conditions decided by the experimental results of muffle furnace will be used in natural gas-fired furnace. The temperature profile of electric muffle furnace is shown in Fig. 2, and the experimental procedure consists of the following steps: (1) Placing the dried ore/coal composite pellets into a special crucible (cold charge). The height of pellets bed is about 80 mm. (2) Pre-heat the furnace at C for 20 min in order to rapid heating in later stage (step 1 in Fig. 2). (3) Decrease furnace temperature to C, and then charge the crucible loaded by pellets into furnace (step 2 in Fig. 2). (4) Keep the furnace temperature at C for 5 minutes, then heat up to C and keep C for the residual time (step 3, 4, 5 in Fig. 2). (5) Discharge the crucible loaded by DRI at pre-determined reducing time, and take apart the surrounding insulating materials immediately to stop the reduction reaction. DRI is cooled down to ambient temperature in the flowing nitrogen. (6) DRI of each layer is analyzed for weight loss and metallization degree. From top to bottom, it s named 1st layer, 2nd layer,, 5th layer respectively. Table 1. Chemical composition of iron ore concentrate, mass%. TFe FeO SiO 2 CaO MgO Al 2O 3 LOI H 2O Table 2. Proximate analysis and ash composition of pulverized coal, mass%. Proximate analysis Ash analysis Fixed C Total C Ash VM SiO 2 Al 2O 3 Fe 2O 3 MgO CaO Fig. 2. Schematic diagram of temperature profile control of electric muffle furnace ISIJ

3 3. Effects of Reducing Time on Carbothermic Reduction 3.1. Weight Loss and Metallization Degree When the reducing time is shorter, the metallization degree (MD) is lower, especially for the bottom layers. Even though the Fe 2 O 3 is reduced to Fe 3 O 4 or FeO, the MD is still zero. So, MD can not actually evaluate the reduction degree for shorter reducing time. Therefore, theoretical maximum weight loss and Degree of Complete Reduction (shorten as DCR) are defined in this paper, and DCR is used to roughly evaluate the reduction degree. Metallization degree is analyzed only if DCR is higher than 80%. (1) Theoretical maximum weight loss: The weight loss under the following two assumptions, a) for iron ore, all of the O combined with Fe is removed, b) for coal, only ash still exist in the pellets after reduction. (2) Degree of complete reduction (DCR) = (actual weight loss) / (theoretical maximum weight loss). The DCR of each layer pellets reduced by different time is shown in Fig. 3. From the figure, it can be seen that, (1) DCR of each layer pellets increases with the increase of reducing time. Pellets of 1st layer reach the about maximum value at 30 min, and 2nd layer pellets reach the about maximum value at 50 min. While the other three layers of pellets can not reach the maximum value under the experimental condition. (2) In case of 50 min and 60 min, the DCR of total pellets bed are relatively higher, and they are 81.28% and 89.82% respectively. Therefore, the metallization degree of pellets reduced by 50 min and 60 min should be higher than other reducing time, and they are shown in Fig. 4. It can be seen, (1) in case of 50 min, the MD of 1st layer, 2nd layer, 3rd layer are relatively higher (about 80%), while the 4th and 5th layers are lower, 35% and 13% respectively. The MD of total pellets bed is about 58%. (2) In case of 60 min, the MD of 1st layer, 2nd layer, 3rd layer are still relatively higher (about 85%), the 4th and 5th layers increase to 75% and 63% respectively. The MD of total pellets bed increases to 80% Shrinkage and Density of DRI From the view point of the process control, the radiation heat transfer from heating source to the bottom of pellets bed is the critical in most of reducing time. But in later stage, especially for the bottom pellets, both the radiation from heating source and conduction from upper DRI to bottom layer are the critical steps. In PSH process, the newly formed DRI can shrinke at high temperature (about C), and the shrinkage of pellets in each layer is shown in Fig. 5. D 1 /D 0 =(diameter of pellets after reduction)/ (diameter of pellets before reduction). From the figure, one can conclude that the diameter of reduced pellets in the 1st layer can decrease to 70%, and then the cross section can decrease to about 50%. Then the 2nd and 3rd layer of pellets shrink too. It s well know, the shrinkage of pellets may result in a large space (Fig. 6) for the passage of radiative heat flux (in present experiments, the average weight load of each layer green ball is about 100 g, and the shrinkage Fig. 3. Effect of reducing time on DCR of pellets in bed. Fig. 5. Effect of reducing time on the shrinkage of DRI. Fig. 4. Metallization degree of DRI in different layers reduced by 50 min and 60 min. Fig. 6. Appearance of pellets bed after reduction ISIJ 90

4 is shown in Fig. 5). In the later stage of reduction process, the heat required for the bottom layers pellets is transferred by both conduction from top and by radiation from heating source. The sufficient reduction and shrinkage of upper pellets may result in a good heat conductivity of DRI. Therefore, high temperature is necessary for PSH process, which can promote the reduction and shrinkage of pellets and be benefit for heat transfer in the tall pellets bed. The reduction and shrinkage of pellets may result in the change of density of pellets. Basically, there are positive and negative effects during the process. Negative effects: the volatilization of coal, the remove of O in iron ore and C in coal may result in decrease for the density of pellets. Positive effects: shrinkage of pellets after reduction may increase the density of pellets. The effect of reducing time on the density of pellets is shown in Fig. 7. From the figure, Fig. 7. Effect of reducing time on the density of DRI. we can know that in the early stage, the positive effect and negative effect are similar. But in the later stage, the chemically volatilization and reduction reaction are basically finished, and only the physically shrinkage occurs, which may result in the increase for the density of pellets. Therefore, under the condition of high temperature and tall pellets bed, the density of pellets (DRI) may be up to 3.0 g/cm 3, which is benefit for storage and long distance transportation. The typical metallographic picture and energy spectrum of DRI specimen are shown in Fig. 8. It can be concluded that there are mainly three phases, (1) white phase, pointed by A, is metallic iron phase. (2) Light grey phase, pointed by B, is melting slag, consisting compound minerals of silicon oxide, iron oxides, and aluminum oxide (mainly fayalite, 2FeO SiO 2 ). (3) Deep grey phase, pointed by C, is quartz. In addition, the black phase in Fig. 8, pointed by D is the area of pores inside the pellets Discussion on the Reducing Time Based on above analysis, longer reducing time (60 min) may result in a higher metallization degree, but it also may result in the following four problems. (1) In case of 60 min, the metallographic picture and energy spectrum of 1st layer DRI specimen are shown in Fig. 9. The white phase is metallic iron. Deep grey phase (pointed by A) is quartz, which is still a zone of unmelted solid grains due to its high melting point. Light grey phase (pointed by B) and middle grey phase (pointed by C) are melting slag (mainly fayalite, 2FeO SiO 2 ). It can be interpreted that in later stage, the reduction of FeO to metallic iron is suppressed, and then FeO reacts with SiO 2 to form 2FeO SiO 2, which is liquid and corrosive slag. During the experiments, DRI is taken out of furnace at C, the Fig. 8. Typical metallographic picture and energy spectrum of DRI (50 min, 2nd Layer). (Online version in color.) ISIJ

5 Fig. 9. Metallographic picture and energy spectrum of 1st layer DRI reduced by 60 min. (Online version in color.) temperature sharply cooled down. When the content of FeO in slag is more than 60% and temperature is lower than C, light grey phase is precipitated from middle grey phase due to their different melting points (Fig. 10), and then forms the banding structure. Therefore, if the reducing time is too long, more melting slag formed. Even though the formed melting slag is benefit for the shrinkage of DRI (the melting slag formed in top layer is most, and the shrinkage of top layer is most too; on the other hand, there is no melting slag in bottom layer, and the shrinkage of bottom layer is least), but melting slag is not benefit for the actual operation of PSH process. (2) After 50 min, the metallization degree of top three layers will not obviously increase, only the pellets of 4th layer and 5th layer will continue increase (Fig. 4). So, during the period of min, the function of thermal energy for heating top three layers is only to maintain temperature (no or little reduction reaction occur). This is not benefit for effectively utilization of thermal energy. (3) One of the characteristics of PSH process is high productivity. Longer reducing time is not benefit for the high productivity. (4) Whatever the reducing time, the metallization degree of bottom layer is lowest. So, increasing the metallization degree of bottom layer is the key to increase the metallization degree of total pellets bed. Fig. 10. Binary phase diagram of SiO 2 FeO. 4. Hot Charge Operation Based on above experiments, even though longer reducing time (60 min) can increase the metallization degree of DRI, but it also has some disadvantages, e.g. re-oxidation of top layer DRI, low efficiency of thermal energy, and low productivity. In order to obtain high metallization degree at shorter time (50 min), especially for bottom layers, hot charge operation is carried out in present work (procedures shown in section 2.2 is cold charge). The ore/coal composite pellets are directly charged into a crucible which has been pre-heated at C in the furnace. Then the reduction reaction of bottom pellets can occur at early stage by the sensible heat of the bottom refractory. This operation is difficult at lab-scale experiments, but in PSH practice it is easy to be achieved, because the bottom of crucible is still hot after its discharging, and it may be directly and easily charged in the adjacent furnace. 15) The metallization degree of DRI with hot charge opera ISIJ 92

6 Fig. 11. Effect of hot charge on metallization degree of DRI. tion is shown in Fig. 11. It can be seen that the metallization degree of each layer DRI with hot charge operation is relatively higher than cold charge, especially for the bottom layers (4th layer and 5th layer). Compared with cold charge, the MD of 4th layer increases from 35% to 77%, and the MD of 5th layer increases from 13% to 78%. Then the MD of total DRI bed increases from 58% to 85%. The reason for the different MD between hot charge and cold charge is that the pellets bed can be heated from the bottom of crucible by the heat pre-stored in the refractory. In fact, the hot charge can not only increase metallization degree of DRI, but also increase productivity. The productivity can be increased up to 85 kg-dri/m 2 h or 59 kg-mfe/m 2 h by the hot charge operation. 17) The productivity of tall ore/coal composite pellets bed reduced at high temperature will be discussed in the future paper. DRI produced by hot charge can obviously shrink too, and its density and strength are high enough for storage and long distance transportation. 5. Conclusions In present work, the effects of reducing time on the metallization degree of carbothermic reduction of tall pellets bed are experimental investigated. The main findings could be summarized as follows: (1) In case of 50 min of reducing time, the MD of 1st layer, 2nd layer, 3rd layer are relatively higher (about 80%), the 4th and 5th layers are lower, 35% and 13% respectively. The MD of total pellets bed is about 58%. (2) In case of 60 min of reducing time, the MD of 1st layer, 2nd layer, 3rd layer are still relatively higher (about 85%), the 4th and 5th layers increase to 75% and 63% respectively. The MD of total pellets bed increases to 80%. But for this case, the longer reducing time may result in some disadvantage, e.g. re-oxidation of top DRI, low efficiency of thermal energy, and low productivity. (3) Hot charge can obviously increase the metallization degree of DRI, especially for the bottom layers. In case of 50 min, compared with cold charge operation, the MD of 4th layer increases from 35% to 77%, and the MD of 5th layer increases from 13% to 78%. Then the MD of total DRI bed increases from 58% to 85% by hot charge operation. Acknowledgment The authors wish to acknowledge the contributions of associates and colleagues in Northeastern University of China and China Steel Corporation of Taiwan. Also, the financial support of National Science Foundation of China (NSFC and NSFC ) and the Fundamental Research Funds for the Central Universities (N ) are very appreciated. REFERENCES 1) X. Jiang, L. Wang, F. M. Shen and W. K. Lu: Steel Res. Int., 85 (2014), 35. 2) M. Balat: Energ. Source., 29 (2007), ) T. Gentzis: Int. J. Coal Geol., 43 (2000), 1. 4) X. Jiang, L. Wang and F. M. Shen: Adv. Mater. Res., 805 (2013), ) W. K. Lu, X. Jiang and J. L. Yang: J. Iron Steel Res. Int., 16 (2009), ) R. J. Longbottom and O. Ostrovski: ISIJ Int., 46 (2006), ) X. Jiang, F. M. Shen, L. G. Liu, X. G. Li and L. Wang: ISIJ Int., 53 (2013), ) T. Murakami and E. Kasai: ISIJ Int., 51 (2011), 9. 9) Y. Kobayashi, H. Sonezaki and R. Endo: ISIJ Int., 50 (2010), ) T. Ariyama and M. Sato: ISIJ Int., 46 (2006), ) B. Sarma and R. J. Fruehan: Ironmaking Conf. Proc., ISS, Warrendale, PA, US, (1998), ) K. Sun and W. K. Lu: Metall. Mater. Trans. B, 40 (2009), ) B. X. Zhu, G. Wei, X. Jiang and F. M. Shen: J. Northeastern Univ. (Natural Science), 33 (2012), ) M. C. Grimston, V. Karakoussis and R. Fouquet: Clim. Policy, 1 (2001), ) W. K. Lu and D. B. Huang: USA Patent, No. 60, ) X. Jiang, G. Wei, F. M. Shen and W. K. Lu: Research and Development of New Direct Reduction Process (PSH), ISCC, Beijing, (2011), ) Y. Q. Li: Northeastern University, China, 2014, Master thesis ISIJ