minerals Article
Carbothermic Reduction of Ore-Coal Composite Pellets in a Tall Pellets Bed Xin Jiang *, Guangen Ding, He Guo, Qiangjian Gao * and Fengman Shen School of Metallurgy, Northeastern University, Shenyang 110819, China;
[email protected] (G.D.);
[email protected] (H.G.);
[email protected] (F.S.) * Correspondence:
[email protected] (X.J.);
[email protected] (Q.G.); Tel.: +86-24-83681506 (X.J.); +86-24-83691266 (Q.G.) Received: 19 October 2018; Accepted: 21 November 2018; Published: 27 November 2018
Abstract: Recently, increasing attention has been paid to alternative ironmaking processes due to the desire for sustainable development. Aiming to develop a new direct reduction technology, the paired straight hearth (PSH) furnace process, the carbothermic reduction of ore-coal composite pellets in a tall pellets bed was investigated at the lab-scale in the present work. The experimental results show that, under the present experimental conditions, when the height of the pellets bed is 80 mm (16–18 mm each layer, and 5 layers), the optimal amount of carbon to add is C/O = 0.95. Addition of either more or less carbon does not benefit the production of high quality direct reduced iron (DRI). The longer reduction time (60 min) may result in more molten slag in the top layer of DRI, which does not benefit the actual operation. At 50 min, the metallization degree could be up to 85.24%. When the experiment was performed using 5 layers of pellets (about 80 mm in height) and at 50 min duration, the productivity of metallic iron could reach 55.41 kg-MFe/m2 ·h (or 75.26 kg-DRI/m2 ·h). Therefore, compared with a traditional shallow bed (one or two layers), the metallization degree and productivity of DRI can be effectively increased in a tall pellets bed. It should be pointed out that the pellets bed and the temperature should be increased simultaneously. The present investigation may give some guidance for the commercial development of the PSH process in the future. Keywords: carbothermic reduction; ore-coal composite pellets; direct reduced iron (DRI); paired straight hearth (PSH) furnace; productivity
1. Introduction In the coming decades, the blast furnace (BF) will be the dominant ironmaking reactor in the world. However, there have been continuing efforts all over the world to search for alternative ironmaking processes because of high capital investment, coke requirements, and environmental concerns associated with the preparatory steps of the raw materials for the BF, e.g., coke making and sintering. Many alternative ironmaking processes have been developed over the past decades. Chemically self-reducing green balls (ore-coal composite pellets) contain carbonaceous reductant, and only heat is needed to convert green balls to direct reduced iron (DRI). A hearth-type furnace is the proper choice for these ore-coal composite pellets to commercially produce DRI. There are some studies regarding the reduction of ore-coal composite pellets [1–15]. Kasai et al. carried out some reduction experiments in a combustion bed packed with the composite pellets. They found that re-oxidation could be significantly suppressed by admixing CaO-bearing material or coating it around the composites [1]. Murakami et al. evaluated the effect of pressure on the gasification and reduction of the composites, and found that the gasification temperature decreased and the weight loss fraction at the target temperature increased with an increase in pressure [2–4]. Yunus et al. investigated the reduction of low grade iron ore deposits mixed with oil palm empty fruit Minerals 2018, 8, 550; doi:10.3390/min8120550
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Yunus et al. investigated the reduction of low grade iron ore deposits mixed with oil palm empty bunch (EFB) as a substitution for coke. Then, they found that EFB char appeared to be a promising fruit bunch (EFB) as a substitution for coke. Then, they found that EFB char appeared to be a energy source for decreasing coal consumption in ironmaking, and reducing CO emissions [5]. promising energy source for decreasing coal consumption in ironmaking, and 2reducing CO2 Takyu et al. have fundamentally examined the reduction by volatile matter in a coal-hematite ore emissions [5]. Takyu et al. have fundamentally examined the reduction by volatile matter in a coalcomposite. They found that the reduction degree of hematite ore initially increased with increasing hematite ore composite. They found that the reduction degree of hematite ore initially increased with content of volatile matter in coal, and that control of the heating rate is a possible way to promote increasing content of volatile matter in coal, and that control of the heating rate is a possible way to reduction at a low temperature [6]. promote reduction at a low temperature [6]. Based on the above literatures, previous studies have mainly focused on reduction parameters Based on the above literatures, previous studies have mainly focused on reduction parameters for ore-coal composite pellets, including flux (CaO-bearing material), pressure, additive fuel (EFB), for ore-coal composite pellets, including flux (CaO-bearing material), pressure, additive fuel (EFB), volatile matter, and others. These studies have obtained much valuable and meaningful information volatile matter, and others. These studies have obtained much valuable and meaningful information on the carbothermic reduction of ore-coal composites. However, the height of the pellets bed was on the carbothermic reduction of ore-coal composites. However, the height of the pellets bed was seldom considered, because they were focused on the characteristics and fundamental mechanism of seldom considered, because they were focused on the characteristics and fundamental mechanism of the reaction between the ore and reductant. Therefore, in previous studies, usually a shallow pellets the reaction between the ore and reductant. Therefore, in previous studies, usually a shallow pellets bed (1–2 layers, 20–25 mm) and lower temperatures (1000–1300 ◦ C) were used to reduce ore-coal bed (1–2 layers, 20–25 mm) and lower temperatures (1000–1300 °C) were used to reduce ore-coal composite pellets. However, the lower metallization degree and lower productivity are inevitable composite pellets. However, the lower metallization degree and lower productivity are inevitable disadvantages due to the contradictory requirements of fuel in the oxidation compartment and the disadvantages due to the contradictory requirements of fuel in the oxidation compartment and the reduction compartment (Figure 1). In order to increase the energy efficiency of fuel, the ratio of reduction compartment (Figure 1). In order to increase the energy efficiency of fuel, the ratio of CO/CO2 should be lower, but it is easy to re-oxidize the newly formed DRI, which results in a lower CO/CO2 should be lower, but it is easy to re-oxidize the newly formed DRI, which results in a lower metallization degree. On the other hand, in order to avoid DRI from re-oxidation, a high ratio of metallization degree. On the other hand, in order to avoid DRI from re-oxidation, a high ratio of CO/CO2 is necessary, but the energy efficiency of fuel and the flame temperature are lower. As a CO/CO2 is necessary, but the energy efficiency of fuel and the flame temperature are lower. As a consequence, the radiation heat transfer, which is proportional to T4 of the heat source, will be lower consequence, the radiation heat transfer, which is proportional to T4 of the heat source, will be lower and result in a lower metallization degree too. Therefore, a lower metallization degree and lower and result in a lower metallization degree too. Therefore, a lower metallization degree and lower productivity are inevitable for a shallow bed [16]. productivity are inevitable for a shallow bed [16].
Oxidation Compartment (CO + 1/2 O2 = CO2): Mixture of CO/CO2, T = 1300–1350 ℃
Oxidizing Reducing
Unreduced Iron
Re-oxidized
Direct Reduced Reduction Compartment: Iron Ore + C + Heat → Fe + CO Figure Figure1.1.Schematic Schematicdiagram diagramofofthe thecarbothermic carbothermicreduction reductionininaashallow shallowpellets pelletsbed. bed.
Based Basedon onthe thereduction reductionofofore-coal ore-coalcomposite compositepellets pelletsininaafurnace furnacehearth, hearth,and andaiming aimingtotosolve solvethe the problem problemof ofthe thecontradictory contradictoryrequirements requirementsofoffuel fuelin inthe theoxidation oxidationcompartment compartmentand andthe thereduction reduction compartment, LuofofMcMaster McMaster University in Canada proposed new reduction direct reduction compartment, Wei-Kao Wei-Kao Lu University in Canada proposed a newadirect process, process, named the pairedhearth straight hearth (PSH) furnace process [17]. Similar pellets to shallow which iswhich namedisthe paired straight (PSH) furnace process [17]. Similar to shallow beds, pellets beds, the PSH process may also be divided into an oxidation compartment and a reduction
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the PSH process may also be divided into an oxidation compartment and a reduction compartment (Figure 2). The(Figure PSH process operational (1) a tall pellets bedpellets (5–7 layers, compartment 2). The has PSHtwo process has two characteristics: operational characteristics: (1) a tall bed (5– ◦ 80–120 mm) in the reduction compartment; and (2) a high temperature (1500–1550 C) in the oxidation 7 layers, 80–120 mm) in the reduction compartment; and (2) a high temperature (1500–1550 °C) in the compartment, which is caused full combustion of CO to COof2 . CO to CO2. oxidation compartment, whichbyisthe caused by the full combustion
Oxidation Compartment (CO + 1/2 O2 = CO2): 100% of CO2, Oxidizing Gas Reducing
Reduction Compartment: Iron Ore + C + Heat → Fe + CO Figure2.2.Schematic Schematicdiagram diagramofofthe thecarbothermic carbothermicreduction reductionininthe thetall tallpellets pelletsbed. bed. Figure
Thereare aretwo two key key points points in the development InIn thethe talltall pellets bed, the There development of ofthe thePSH PSHprocess: process:(1)(1) pellets bed, newly formed DRI at the toptop of the bedbed cancan be protected from re-oxidation by the gas gas flow the newly formed DRI at the of the be protected from re-oxidation by upward the upward (which is CO generated during the reduction in the in tallthe pellets bed, and enhanced by the high flow (which is rich) CO rich) generated during the reduction tall pellets bed, and enhanced by volatile coal in the green ball over a longer period of the reduction time; (2) Efficient heat transfer the high volatile coal in the green ball over a longer period of the reduction time; (2) Efficient heat from the “oxidation compartment” (where(where heat is generated) to the “reduction compartment” (where transfer from the “oxidation compartment” heat is generated) to the “reduction compartment” heat isheat consumed by the by endothermic reactionreaction and sensible heat of heat substances). Therefore, the PSH (where is consumed the endothermic and sensible of substances). Therefore, was considered as a possible processprocess to produce DRI with a high quality, low low carbon rate,rate, and the PSH was considered as a possible to produce DRI with a high quality, carbon consequentially lowlow COCO 2 emissions. and consequentially 2 emissions. However,since sincethe thePSH PSHprocess processwas wasproposed proposedby byLu, Lu,the thereaction reactioncharacteristics characteristicsofofthe the However, carbothermic reduction reduction ofof ore-coal ore-coal composite composite pellets pellets inin tall tallpellets pellets beds bedshave havenot notbeen beenbetter better carbothermic understood.Therefore, Therefore,the thepresent presentwork workwas wasdone. done.The Theaims aimsofofthis thisstudy studyare areasasfollows: follows:(1) (1)common common understood. rawmaterials materials were to understand better understand the reaction commoncharacteristics reaction characteristics of the raw were usedused to better the common of the carbothermic carbothermic reduction of ore-coal composite pellets in tall pellets which haveinnot found reduction of ore-coal composite pellets in tall pellets beds, which havebeds, not been found thebeen literature the (2) literature so far; (2) thebasic effects of someparameters basic operation on the carbothermic soin far; the effects of some operation on theparameters carbothermic reduction were reduction wereinvestigated, experimentally investigated, including the amount of carbon addition (denoted experimentally including the amount of carbon addition (denoted as C/O) and theas C/O) andtime; the reduction time; (3) on theofproductivity metallic iron among reduction (3) comparison on comparison the productivity metallic ironofamong different pelletsdifferent layers pellets layers (different height in pellets bed); and (4) give some suggestions for choosing reasonable (different height in pellets bed); and (4) give some suggestions for choosing reasonable operation operation parameters in future commercial development of the PSH process. parameters in future commercial development of the PSH process. 2. Experimental 2.1. Raw Materials The chemical composition of the iron ore concentrate used in this work is shown in Table 1. It should be noted that the moisture is 7.22% in the original concentrate. The chemical analysis was carried out under dry conditions (7.22% of H2O was dried), including TFe (Total Fe), FeO, SiO2, CaO,
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2. Experimental 2.1. Raw Materials The chemical composition of the iron ore concentrate used in this work is shown in Table 1. It should be noted that the moisture is 7.22% in the original concentrate. The chemical analysis was Minerals 2018, 8, x FORdry PEER REVIEW (7.22% of H O was dried), including TFe (Total Fe), FeO, SiO ,4CaO, of 15 carried out under conditions 2 2 MgO, Al2 O3 , and LOI (Loss of Ignition). In the dried concentrate, a slight increase in LOI (−1.45) MgO, Al2O3, and LOI (Loss of Ignition). In the dried concentrate, a slight increase in LOI (−1.45) is is caused by the oxidation of magnetite. In order to better understand the minerals in the iron ore, caused by the oxidation of magnetite. In order to better understand the minerals in the iron ore, XX-ray diffraction (XRD, X’ Pert Pro; PANalyical, Almelo, The Netherlands) analysis was conducted, ray diffraction (XRD, X' Pert Pro; PANalyical, Almelo, The Netherlands) analysis was conducted, and and the pattern is shown in Figure 3. Based on the chemical analysis and X-ray diffract, the iron ore the pattern is shown in Figure 3. Based on the chemical analysis and X-ray diffract, the iron ore was was found to be primarily magnetite, and the main gangue is SiO2 . The iron ore concentrate was found to be primarily magnetite, and the main gangue is SiO2. The iron ore concentrate was ground ground by shatter-box, and 100% ore passed −0.074 mm for pelletizing. by shatter-box, and 100% ore passed −0.074 mm for pelletizing. Table 1. Chemical composition of iron ore concentrate (mass%). Table 1. Chemical composition of iron ore concentrate (mass%). TFe FeO SiO2 CaO MgO Al2 O3 LOI TFe FeO SiO2 CaO MgO Al2O3 LOI 63.26 0.15 0.12 0.20 0.20 −1.45 −1.45 63.26 26.99 26.99 6.68 6.68 0.15 0.12
8000 ■Fe3O4 ▲SiO2
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Two-Theta (deg) Figure 3. 3. X-ray diffraction diffraction pattern pattern of of iron iron ore ore concentrate. concentrate. Figure
The The proximate proximate analysis analysis and and ash ash analysis analysis of of the the pulverized pulverized coal coal is is shown shown in in Table Table2. 2. The The volatile volatile matter matter (VM) (VM) in in the the coal coal is is about about 26%. 26%. For For the the preparation preparation of of ore-coal ore-coal composite composite pellets, pellets, coal coal is is also also ground, 0.221 mm for pelletizing. ground, and and 100% 100% passed passed − −0.221 Table Table 2. 2. Proximate Proximate analysis analysisand andash ashanalysis analysisof ofpulverized pulverizedcoal coal(mass%). (mass%).
Proximate Analysis Ash Proximate Analysis Ash Analysis Analysis Fixed C Total C Ash VM SiO 2 Al 2O 3 Fe Fixed C Total C Ash VM SiO2 Al2 O3 Fe22O O33 MgO MgO CaO CaO 61.31 77.5 9.38 26.00 49.14 29.98 10.22 0.70 61.31 77.5 9.38 26.00 49.14 29.98 10.22 0.70 5.37 5.37
2.2. Experimental Set-Up 2.2. Experimental Set-Up In the present work, the carbothermic reduction experiments of ore-coal composite pellets in tall In the present work, the carbothermic reduction experiments of ore-coal composite pellets in tall pellets beds were carried out in an electric muffle furnace. The experimental procedure consisted of pellets beds were carried out in an electric muffle furnace. The experimental procedure consisted of the following steps, and the temperature profile of the furnace is shown in Figure 4. the following steps, and the temperature profile of the furnace is shown in Figure 4. 1. Mixing of raw materials. Effective mixing of ore and coal is an essential step to ensure proper homogeneous reaction throughout the pellets. Therefore, first the ore and coal were mixed in an intensive mixer to ensure the uniform mixing of ore and coal, and a homogeneous reaction in the ore-coal composite pellets. 2. Pelletizing. The diameter of the ore-coal composite pellet is about 16–18 mm. The total H2O
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Mixing of raw materials. Effective mixing of ore and coal is an essential step to ensure proper homogeneous reaction throughout the pellets. Therefore, first the ore and coal were mixed in an intensive mixer to ensure the uniform mixing of ore and coal, and a homogeneous reaction in the ore-coal composite pellets. Pelletizing. The diameter of the ore-coal composite pellet is about 16–18 mm. The total H2 O content in the wet green ball is about 8.5–9.0%. In order to avoid the crack of pellets in the high temperature furnace due to evaporation, the wet green balls should be dried before charging into the furnace. A special crucible was made for holding composite pellets in muffle furnace (Figure 5). Minerals 2018, 8, x FOR PEER REVIEW 5 of 15 The crucible consists of two parts: (a) the mullite ring (the inner diameter is 80 mm) to retain the gas in a vertical the pellets bed; and (b) the materials surrounding 3.upward A special crucible wasdirection made forinholding composite pellets in insulating muffle furnace (Figure 5). The thecrucible mullite consists ring to obstruct the heat transfer from horizontal direction the pellets bed. of two parts: (a) the mullite ringthe (the inner diameter is 80ofmm) to retain the in a vertical direction in furnace. the pelletsThen, bed; and the insulating materials Theupward specialgas crucible was put into the the (b) temperature of the furnacesurrounding was heated to ◦ C inof the◦ C mullite to obstruct thethe heatcrucible transferwas frompre-heated the horizontal direction thefurnace. pellets bed. 1200 in air ring atmosphere, and to 1200 the 4. The special crucible was put into the furnace. Then, the temperature of the furnace was heated The furnace door was opened, and 500 g of the dried composite pellets was charged into the to 1200 °C in air atmosphere, and the crucible was pre-heated to 1200 °C in the furnace. crucible. The height of pellets bed is about 80 mm (5 layers, and 12–13 pellets each layer), and the 5. The furnace door was opened, and 500 g of the dried composite pellets was charged into the void ratio of packing bed is about 14–18%. crucible. The height of pellets bed is about 80 mm (5 layers, and 12–13 pellets each layer), and ◦ C for 5 min. Then, it was heated to 1500 ◦ C in about Thethe furnace temperature at 1200 void ratio of packingwas bedkept is about 14–18%. ◦ ◦ (15 C/min), and kept 1500 C until the5 target time.it was heated to 1500 °C in about 6.20 min The furnace temperature wasat kept at 1200 °C for min. Then, When the(15 target reduction time was °C reached, the entire crucible was taken out of the furnace, 20 min °C/min), and kept at 1500 until the target time. the surrounding insulating were taken away out from crucible to 7.andWhen the target reduction time materials was reached, theimmediately entire crucible was taken of the furnace, and surrounding insulating werebed immediately taken away from crucible stop thethe reduction reaction. Then,materials the hot DRI is quenched in liquid N2 tothe prevent thetoDRI stop the reduction reaction. Then, the hot DRI bed is quenched in liquid N 2 to prevent the DRI from re-oxidation. from re-oxidation. Divide the DRI bed. The cooled DRI bed is shown in Figure 6. From top to bottom, the first 8. Divide the DRI bed. The cooled DRI bed is shown in Figure 6. From top to bottom, the first 12– 12–13 pellets are defined as the 1st layer, then the second 12–13 pellets are defined as the 2nd 13 pellets are defined as the 1st layer, then the second 12–13 pellets are defined as the 2nd layer, layer, then the 3rd layer, 4th layer, and 12–13 pellets on the bottom are defined as the 5th layer. then the 3rd layer, 4th layer, and 12–13 pellets on the bottom are defined as the 5th layer. Finally, Finally, the total DRI bed is divided into 5 layers from top to bottom. the total DRI bed is divided into 5 layers from top to bottom. pellets inin each for chemical chemicalanalysis. analysis. Total (TFe) 9.Eight Eight pellets eachlayer layerwere wererandomly randomly selected selected for Total Fe Fe (TFe) andand Metallic FeFe (MFe) for each eachlayer, layer,the theratio ratioofofmetallic metallic total Metallic (MFe)were wereobtained. obtained. Then, Then, for Fe Fe to to total Fe Fe waswas defined asasmetallization degree(MD (MD= MFe/TFe), = MFe/TFe), which anvalue actual value (notvalue). average defined metallization degree which was anwas actual (not average value). The metallization degrees the total which an actual value The metallization degrees (MD)(MD) of theoftotal DRI DRI bed, bed, which is anisactual value too,too, can can be be calculated byby the following calculated the followingequation: equation: (total bed) = [MFe(1st (1stlayer) layer)×× Weight (1st × Weight (2nd layer) + + MDMD (total bed) = [MFe (1st layer) layer)++MFe MFe(2nd (2ndlayer) layer) × Weight (2nd layer) … + MFe (5th layer) × Weight (5th layer)]/[TFe (1st layer) × Weight (1st layer) + TFe (2nd layer) . . . + MFe (5th layer) × Weight (5th layer)]/[TFe (1st layer) × Weight (1st layer) + TFe × Weight (2nd layer)(2nd + … layer) + TFe (5th × Weight (5th × layer)] (2nd layer) × Weight + . .layer) . + TFe (5th layer) Weight (5th layer)] 1600 1500
2 dN n r a e d en g r u a d hl e c s o i o Dc
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Figure 5. Special Figure 5. Special crucible crucible for for the the carbothermic carbothermic reduction reduction experiments experiments in in the the tall tall pellets pellets bed bed in in the the muffle furnace: (a) appearance of insulation crucible; (b) top view and size dimension of crucible. crucible.
Figure Figure 6. 6. Appearances Appearances of of the the total total direct direct reduced iron (DRI) bed and each layer.
3. Experimental Experimental Results Results and and Analysis Analysis 3. 3.1. Effects Effects of of Amount Amount of of Carbon Carbon Addition Addition (C/O) (C/O) 3.1. 3.1.1. Metallization Degree 3.1.1. Metallization Degree The definition of the amount of carbon addition is the gram-atomic ratio of the fixed carbon in The definition of the amount of carbon addition is the gram-atomic ratio of the fixed carbon in coal to the combined oxygen in iron oxides, denoted as C/O (molar ratio). In this work, C/O = 0.80, coal to the combined oxygen in iron oxides, denoted as C/O (molar ratio). In this work, C/O = 0.80, 0.95 and 1.10. The reduction time is 50min in this section. 0.95 and 1.10. The reduction time is 50min in this section. The metallization degrees (MD) of DRI specimens with different C/O are shown in Figure 7. The metallization degrees (MD) of DRI specimens with different C/O are shown in Figure 7. It It can be seen: (1) where C/O = 0.80, the MD is relatively low (about 78% for the total 5 layers of DRI) can be seen: (1) where C/O = 0.80, the MD is relatively low (about 78% for the total 5 layers of DRI) because the reductant is not sufficient; (2) where C/O = 0.95 and C/O = 1.10, the MD is obviously because the reductant is not sufficient; (2) where C/O = 0.95 and C/O = 1.10, the MD is obviously increased to about 85% and 86%, which indicates that the reductant in these pellets is sufficient; (3) it is increased to about 85% and 86%, which indicates that the reductant in these pellets is sufficient; (3) it inevitable that part of the DRI is re-oxidized in the top layer, so the MD first increases from the 1st is inevitable that part of the DRI is re-oxidized in the top layer, so the MD first increases from the 1st layer to the 3rd layer. Then the MD decreases from the 3rd layer to the 4th layer, because radiative layer to the 3rd layer. Then the MD decreases from the 3rd layer to the 4th layer, because radiative heat transfer is difficult in a higher pellets bed; (4) the MD of the DRI in the 5th layer (bottom layer) is heat transfer is difficult in a higher pellets bed; (4) the MD of the DRI in the 5th layer (bottom layer) is higher than the 4th layer. The reason for this is that the pellets in the 5th layer can be heated from the hot bottom of the crucible by the heat pre-stored in the refractory.
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higher than the 4th layer. The reason for this is that the pellets in the 5th layer can be heated from the 7 of 15 heat pre-stored in the refractory. 7 of 15
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Figure Figure Effect of C/O reducediron iron(DRI). (DRI). Figure7.7. 7.Effect Effectof ofC/O C/Oon onmetallization metallizationdegree degreeofofdirect directreduced
3.1.2. Metallographic Analysis 3.1.2. 3.1.2.Metallographic MetallographicAnalysis Analysis typical metallographic picture and energy disperse spectroscopy (SEM-EDS, Plus; AA typical metallographic and energy disperse spectroscopy (SEM-EDS, Ultra Plus; A typical metallographicpicture picture and energy disperse spectroscopy (SEM-EDS, UltraUltra Plus;Carl Carl Carl Zeiss GmbH, Jena, Germany) of the DRI specimen is shown in Figure 8. It can be concluded Zeiss GmbH, Jena, Germany) of the DRI specimen is shown in Figure 8. It can be concluded that there Zeiss GmbH, Jena, Germany) of the DRI specimen is shown in Figure 8. It can be concluded that there that there three are mainly three phases: (1) the as white zone, by asbypresented point A, is thephase; metallic are phases: (1) zone, point the metallic iron (2) the aremainly mainly three phases: (1)the thewhite white zone, aspresented presented pointA, A,isisby the metallic iron phase; (2)iron the phase; (2) the light grey zone, as presented by point B, consists mainly of fayalite (2FeO · SiO ) and light grey zone, as presented by point B, consists mainly of fayalite (2FeO·SiO 2) and the slag phase 2 light grey zone, as presented by point B, consists mainly of fayalite (2FeO·SiO2) and the slag phaseofof the slag phase of silicon oxide, iron oxide, and with aluminum oxide, withcrystallized the former from being crystallized silicon oxide, iron oxide, oxide, being silicon oxide, iron oxide,and andaluminum aluminum oxide, withthe theformer former being crystallized frommolten moltenslag slag from molten slag on cooling. It should be noted that the DRI in the 2nd layer is better protected from on cooling. It should be noted that the DRI in the 2nd layer is better protected from re-oxidation on cooling. It should be noted that the DRI in the 2nd layer is better protected from re-oxidationthan than re-oxidation than the 1st layer, which results in less liquidTherefore, slag and fayalite. Therefore, the slag phase the results ininless liquid slag fayalite. the phase the1st 1stlayer, layer,which which results less liquid slagand and fayalite. Therefore, theslag slag phaseisishomogeneous; homogeneous; (3) the deep grey zone, as presented by point C, is quartz. In addition, the black zone, is homogeneous; (3) the deep grey zone, as presented by point C, is quartz. In addition, the black zone, (3) the deep grey zone, as presented by point C, is quartz. In addition, the black zone,asaspresented presented by point D, is the area of pores inside the pellets. Both in the metallic phase and the slag phase, there as presented by point D, is the area of pores inside the pellets. Both in the metallic phase and the slag by point D, is the area of pores inside the pellets. Both in the metallic phase and the slag phase, there isphase, because carburization isisinevitable pellets. there iscarbon, the element carbon, because carburization isinore-coal inevitable in ore-coal composite pellets. isthe theelement element carbon, because carburization inevitablein ore-coalcomposite composite pellets. FeKa FeKa
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(c) (d) (c) (d) Figure 8. Typical metallographic picture and energy disperse spectroscopy of direct reduced iron Figure8.8.Typical Typical metallographic picture energy disperse spectroscopy of direct reduced iron Figure metallographic picture andand energy disperse spectroscopy of direct reduced iron (DRI) (DRI) (C/O = 0.95, 50 min, 2nd layer): (a) metallographic picture; (b) point A; (c) point B; (d) point C. (DRI)=(C/O min, 2nd layer): (a) metallographic picture; (b)A; point A; (c)B; point B; (d)C. point C. (C/O 0.95, =500.95, min,502nd layer): (a) metallographic picture; (b) point (c) point (d) point
Other than the typical metallographic picture shown in Figure 8, in the metallographic picture Other picture shown in in Figure 8, in metallographic picture of Otherthan thanthe thetypical typicalmetallographic metallographic picture shown Figure 8, the in the metallographic picture is used of the 5th layer DRI of C/O = 1.1 (Figure 9), there is some black and loose powder. SEM-EDS the 5th 5th layer DRIDRI of C/O = 1.1 (Figure 9), 9), there is some black andand loose powder. SEM-EDS is used to is used of the layer of C/O = 1.1 (Figure there is some black loose powder. SEM-EDS element in this powder area. Based on Figure 9, it can be seenthat thatthere thereisissome somecarbon carbon to analyze the analyze the element in this powder area. Based on Figure 9, it can be seen to analyze the element in this powder area. Based on Figure 9, it can be seen that there is some carbon (solidCCinincoal) coal) andsilica silica (ashinincoal). coal). Thisproves proves thatthe the coaladdition addition (C/O)isisexcessive, excessive, andsome some (solid (solid C in coal)and and silica(ash (ash in coal).This This provesthat that thecoal coal addition(C/O) (C/O) is excessive,and and some residualcarbon carbon remains in in the DRI, which in residual which does does not notbenefit benefitthe theactual actualDRI DRIproduction. production.Therefore, Therefore, residual carbon remains remains in the the DRI, DRI, which does not benefit the actual DRI production. Therefore, in order to better understand the exact carbon content in DRI, a quantitative chemical analysis was used inorder order to better understand the exact carbon content in DRI, a quantitative chemical analysis was to better understand the exact carbon content in DRI, a quantitative chemical analysis was used for the carbon in thein5th (bottom layer)layer) DRI with C/O, and the result shown used forresidual the residual carbon thelayer 5th layer (bottom DRI different with different C/O, and the is result is for the residual carbon in the 5th layer (bottom layer) DRI with different C/O, and the result is shown in Figure 10. shown in Figure 10. in Figure 10. Asshown shown in Figure7,7,the the metallizationdegrees degrees of thetotal total DRIbed bed aresimilar similar in casesofofC/O C/O As As shownininFigure Figure 7, themetallization metallization degreesofofthe the totalDRI DRI bedare are similarinincases cases of C/O 0.95 and and C/O == 1.10 ===0.95 1.10 (the (the difference difference may may be be within within error). error). But But the the residual residual carbon carbonof of C/O C/O===1.10 1.10is 0.95 andC/O C/O = 1.10 (the difference may be within error). But the residual carbon of C/O 1.10 is much more than C/O = 0.95 in Figure 10. Therefore, C/O = 0.95 is considered to be the optimal amount ismuch muchmore more than C/O = 0.95 in Figure 10. Therefore, C/O = 0.95 is considered to be the optimal than C/O = 0.95 in Figure 10. Therefore, C/O = 0.95 is considered to be the optimal amount of carbon in the next stage experiments. amount of addition carbon addition in the nextof of experiments. of carbon addition in the next stage ofstage experiments. 500 500
Counts Counts
400 400 300 300 200 200 100 100 0 00 0
SiKa SiKa
CKa CKa OKa OKa
FeKa FeKa
CLa CLa CMa CMa 5 5
Energy, KeV Energy, KeV
10 10
15 15
(a) (b) (a) (b) Figure9.9. Metallographic thethe direct reduced ironiron (DRI) in the (C/O Figure Metallographicand andelement elementanalysis analysisofof direct reduced (DRI) in 5th the layer 5th layer Figure 9. Metallographic and element analysis of the direct reduced iron (DRI) in the 5th layer (C/O = 1.1):=(a) metallographic picture; (b) element analysis. (C/O 1.1): (a) metallographic picture; (b) element analysis. = 1.1): (a) metallographic picture; (b) element analysis.
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Residual Carbon, Residual Carbon, % %
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Reduce for 50min DRI in 5th layer (bottom layer) Reduce for 50min DRI in 5th layer (bottom layer)
4
2 2
0 0
0.80
0.95
1.10
C/O ratio in composite pellets1.10 0.80 0.95
C/O ratioreduced in composite pellets Figure 10. Residual carbon of the direct iron (DRI) in the 5th layer (bottom layer). Figure 10. Residual carbon of the direct reduced iron (DRI) in the 5th layer (bottom layer).
Figure 10. Residualfor carbon of and the direct reduced iron (DRI) in the 5th layer (bottom layer). 3.2. Carbothermic Reduction 50 min 60 min 3.2. Carbothermic Reduction for 50 min and 60 min Based on theReduction preliminary 3.2. Carbothermic for experiments, 50 min and 60for min5 layers of pellets, the reduction time should be 50min at least. section, experiments, the reductionfortime of 50 min and min were and BasedSoonin thethis preliminary 5 layers of pellets, the 60 reduction timeinvestigated, should be 50 min Based on the preliminary experiments, for 5 layers of pellets, the reduction time should be 50min experimental results are shown in Figure 11. From the figure, it can be seen that: (1) with a reduction at least. So in this section, the reduction time of 50 min and 60 min were investigated, and experimental at least. So in this section, the reduction time of 50 min and 60 min were investigated, and time ofare 60 shown min, the in the and layers higher than for(1) 50min, the MD time in theof1st results inMD Figure 11. 4th From the5th figure, it is can be seen that: with but a reduction 60 layer min, experimental results are shown in Figure 11. From the figure, it can be seen that: (1) with a reduction is lower than min; the MD of thethan totalfor DRI bed for 50the min andin60 min 85.24% andthan 86.03%, the MD in the for 4th50 and 5th(2) layers is higher 50min, but MD the 1stare layer is lower for time of 60 min, the MD in the 4th and 5th layers is higher than for 50min, but the MD in the 1st layer respectively. 50 min; (2) the MD of the total DRI bed for 50 min and 60 min are 85.24% and 86.03%, respectively. is lower than for 50 min; (2) the MD of the total DRI bed for 50 min and 60 min are 85.24% and 86.03%, respectively.
100
Metallization Degree, Metallization Degree, % %
100 80
C/O=0.95
50min 60min 50min 60min
C/O=0.95
80 60 60 40 40 20
1 20 (Top) 1 (Top)
2
3
4
2
3 Layer No.4
5 6 Total (Bottom) 5 6 Total (Bottom)
Figure min) the Figure11. 11. Effect Effect of of reduction reduction time time (50 (50 min min and and 60 60Layer min) on on the metallization metallization degree degree of of direct directreduced reduced No. iron iron(DRI). (DRI). Figure 11. Effect of reduction time (50 min and 60 min) on the metallization degree of direct reduced ironmetallographic (DRI). The picture The metallographic picture and and energy energy spectrum spectrum of of the the 1st 1st layer layer DRI DRI specimen specimen for for 60 60 min min is is
shown The white white zone zone isismetallic metalliciron. iron.The Thedeep deepgrey greyphase phase(presented (presented point shown in in Figure Figure 12. 12. The byby point A)A) is The which metallographic picture and energy solid spectrum of due the 1st layer DRI specimen forThe 60 min is is quartz, is still a zone of un-melting grains to its high melting point. light quartz, which is still a zone of un-melting solid grains due to its high melting point. The light grey shown in Figure 12. The whiteB) zone isthe metallic iron. The deep grey phase (presented point A) is grey phase (presented by point middle grey phase (presented bypoint point C) C) are areby molten phase (presented by point B) andand the middle grey phase (presented by molten slag slag quartz, which is still a zone of un-melting solid grains due to its high melting point. The light grey (mainly fayalite, 2FeO · SiO ). It can be interpreted that in the later stage of reduction, the up-ward 2 (mainly fayalite, 2FeO·SiO2). It can be interpreted that in the later stage of reduction, the up-ward phase (presented by pointand B) and the middlebitgrey phase (presented by point was C) are molten slag protective protective gas gas evolved evolved less less and less, less, so so aa little little bit of ofnewly newlyformed formed metallic metallic iron iron was re-oxidized re-oxidized to to (mainly fayalite, 2FeO·SiO 2). It can be interpreted that in the later stage of reduction, the up-ward FeO. Then, the oxides of silicon, aluminum and some iron formed molten slag. On cooling, FeO. Then, the oxides of silicon, aluminum and some iron formed molten slag. On cooling,fayalite fayalite protective gas evolved less slag and less,distributed so a little bit of newly formed metallic iron after was re-oxidized to crystallized thethe molten in the final slag phase crystallizedfrom from molten and slag and distributed in amorphous the final amorphous slagsolidification. phase after FeO. Then, the oxides of silicon, aluminum and some iron formed molten slag. On cooling, fayalite crystallized from the molten slag and distributed in the final amorphous slag phase after
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solidification. The banding structure then formed. The formation of more molten slag does not benefit the actual operation. Therefore, 50 min is considered to be the optimal reduction time for an 80 mm The banding structure then formed. The formation of more molten slag does not benefit the actual bed (5 layers) in the present work, because the MD had no real change at all between 50 min and 60 operation. Therefore, 50 min is considered to be the optimal reduction time for an 80 mm bed (5 layers) min. in the present work, because the MD had no real change at all between 50 min and 60 min. 600 Point A
SiKa
Counts
400
200 OKa 0
0
5
10
15
Energy, KeV
(a)
(b) 500
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300 OKa 200
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300 AlKa 200 OKa 100
C 0
Point C
SiKa
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Counts
Point B
SiKa
400
5
FeKa C
C
C 10 Energy, KeV
(c)
15
0
0
5
10
15
Energy, KeV
(d)
Figure12. 12. Metallographic Metallographic picture picture and and energy energy spectrum spectrum of of the the 1st 1st layer layer direct direct reduced reduced iron iron (DRI) (DRI) Figure reducedfor for60 60min: min:(a) (a)metallographic metallographicpicture; picture;(b) (b)point pointA; A;(c) (c)point pointB;B;(d) (d)point pointC. C. reduced
3.3. Productivity of 3.3.Productivity of the the Tall Tall Pellets Pellets Bed Bed 2 h is used to evaluate the productivity of DRI. But this unit ignores In Intraditional traditionalwork, work,kg-DRI/m kg-DRI/m2··h is used to evaluate the productivity of DRI. But this unit ignores 2 the quality of DRI (metallization degree). In the the present present work, work, kg-MFe/m kg-MFe/m used unit 2·h·h the quality of DRI (metallization degree). In is is used asas thethe unit to to evaluate the productivity of metallic iron. For this unit, both the quality of DRI (metallization evaluate the productivity of metallic iron. For this unit, both the quality of DRI (metallization degree) degree) and quantity of DRI (productivity) aresimultaneously. considered simultaneously. The productivity and quantity of DRI (productivity) are considered The productivity presented by kg2 ·h, which is an actual value (not average value), may be calculated by presented by kg-MFe/m 2 MFe/m ·h, which is an actual value (not average value), may be calculated by the following equation: the following equation: Productivity = [MFe (1st layer) × Weight (1st layer) + MFe (2nd layer) × Weight (2nd layer) + … Productivity (1st(5th layer) × Weight (1st (m layer) + MFe(h)] (2nd layer) × Weight (2nd layer) + . . . 2) × Time + MFe (5th layer)=× [MFe Weight layer)]/[Acreage 2 + MFe (5th layer) × Weight (5th layer)]/[Acreage (m ) × Time (h)] In this series of experiments, a 5min variant is adopted. The effects of pellets layers and reduction of experiments, a 5 min is adopted. The13. effects pelletsto layers timeInonthis theseries productivity of metallic ironvariant are shown in Figure It isof worthy noteand thatreduction a shorter time on the productivity of metallic iron are shown in Figure 13. It is worthy to note that a shorter reduction time does not necessarily generate higher productivity, because it may result in an MD that reduction time does not necessarily generate higher productivity, because it may result in MD is too low to satisfy the requirement of ironmaking production. Therefore, a productivity withan MD≥85% that is too low satisfybythe requirement of ironmaking production. Therefore, a productivity is selected andtocircled a ring in this figure. It can be seen, for certain layers, there are one orwith two MD ≥ 85% is selected and circled by a ring in this figure. It can be seen, for certain layers, there are points which can meet an MD≥85%. For example, in the case of the four-layers pellets bed, the MD of one or two points which can meet an MD ≥ 85%. For example, in the case of the four-layers pellets DRI reduced for 40 min and 45 min are about 87% and 89%, respectively. Both of them can meet with bed, the MD ofBetween DRI reduced min and 45 min about 87% and respectively. Both of them an MD≥85%. them,for the40productivity of 40are min is higher than89%, 45 min. So, the productivity of 40 min (the maximum productivity for the four-layers bed) is selected in Figure 14.
reduction time to 70 min in the case of 6 layers. So, in Figure 14, there is no data on 6 layers. From the figure, one can conclude that the productivity denoted by kg-MFe/m2 (with different layers and figure, one can conclude that the productivity denoted by kg-MFe/m2 (with different layers and different reduction times) increases with the increase of the height of the pellets bed. In lab-scale different reduction times) increases with the increase of the height of the pellets bed. In lab-scale experiments, under the condition of 5 layers (about 80 mm) and 50 min, the productivity of metallic experiments, under the condition of 5 layers (about 80 mm) and 50 min, the productivity of metallic iron can reach 55.41 kg-MFe/m22·h (or 75.26 kg-DRI/m22·h). Therefore, tall pellets beds (and iron can reach 55.41 kg-MFe/m ·h (or 75.26 kg-DRI/m ·h). Therefore, tall pellets beds (and Minerals 2018, 8, 550 high temperatures) can increase the productivity of DRI. The reason for this11isofthat 15 simultaneously simultaneously high temperatures) can increase the productivity of DRI. The reason for this is that more layers of pellets are simultaneously heated, and the reduction reaction occurs in different layers more layers of pellets are simultaneously heated, and the reduction reaction occurs in different layers at the same time. So the productivity of DRI can be significantly increased in the PSH process (tall can meet withtime. an MD ≥ 85%. Betweenof them, of 40 min is higher min. (tall So, at the same So the productivity DRI the can productivity be significantly increased in the than PSH 45 process pellets bed) compared with the traditional shallow bed (one or two layers). the productivity of 40 min (thethe maximum productivity for(one the four-layers bed) is selected in Figure 14. pellets bed) compared with traditional shallow bed or two layers).
2 2
Productivity, Productivity,kg-MFe/m kg-MFe/m.h.h
80 80
MD≥85% MD≥85%
60 60 40 40 20 20 0 00 0
10 10
20 20
1 Layer 1 Layer 2 Layers 2 Layers 3 Layers 3 Layers 4 Layers 4 Layers 5 Layers 5 Layers 6 Layers 6 Layers 60 70 80 60 70 80
30 40 50 30 40 50 Reduction Time, min Reduction Time, min Figure 13. Effects of pellets layers and reduction time on the productivity of metallic iron. Figure Figure13. 13.Effects Effectsofofpellets pelletslayers layersand andreduction reductiontime timeon onthe theproductivity productivityofofmetallic metalliciron. iron.
2 2
Productivity, Productivity,kg-MFe/m kg-MFe/m.h.h
80 80 60 60 40 40
1 Layer 1 Layer 2 Layers 2 Layers 3 Layers 3 Layers 4 Layers 4 Layers 5 Layers 5 Layers
20 20 0 00 0
10 10
20 20
30 40 50 60 70 80 30 40 50 60 70 80 Reduction Time, min Reduction Time, min Figure 14. Maximum productivity of metallic Figure 14. Maximum productivity of metalliciron ironand andcorresponding correspondingreduction reductiontime timefor forcertain certain Figure 14. Maximum productivity of metallic iron and corresponding reduction time for certain pellets layers. pellets layers. pellets layers.
Similar, the maximum productivities for other different layers beds are selected in Figure 14 4. Discussion 4. Discussion too. It should be noted that the minimum requirement of 85% of MD is not reached by extending the time to 70 min in the case of 6 layers. So, in Figure 14, there is no data on 6 layers. 4.1.reduction Optimal Parameters 4.1. Optimal Parameters From the figure, one can conclude that the productivity denoted by kg-MFe/m2 (with different layers and different 4.1.1. Carbonreduction Additiontimes) increases with the increase of the height of the pellets bed. In lab-scale 4.1.1. Carbon Addition experiments, under the condition of 5 layers (about 80 mm) and 50 min, the productivity of metallic iron can reach 55.41 kg-MFe/m2 ·h (or 75.26 kg-DRI/m2 ·h). Therefore, tall pellets beds (and simultaneously high temperatures) can increase the productivity of DRI. The reason for this is that more layers of pellets are simultaneously heated, and the reduction reaction occurs in different layers at the same time. So the productivity of DRI can be significantly increased in the PSH process (tall pellets bed) compared with the traditional shallow bed (one or two layers).
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4. Discussion 4.1. Optimal Parameters 4.1.1. Carbon Addition Generally, C/O should be adjusted to meet the minimum requirement of the reduction of ore-coal composite pellets. Addition of either more or less carbon does not benefit the production of high quality DRI, including a high metallization degree, high density and strength etc., and these are the reaction characteristics of carbothermic reduction in a tall pellets bed. If more excessive residual carbon remains in the DRI, there are the following disadvantages: (1) grade of total iron decreases; (2) ash content increases with increasing coal addition, which weakens the density and strength of the DRI, which in turn is not good for storage and long-distance transportation; (3) a carbonaceous resource is wasted. Therefore, C/O = 0.95 is considered as the optimal amount of carbon addition in the present work. 4.1.2. Reduction Time The longer reduction time (60 min in Section 3.2) may result in the following three problems: (1) more melting and corrosive slag formed; (2) it does not benefit the effective utilization of thermal energy, because the MD of the DRI does not obviously increase; (3) one of the characteristics of the PSH process is high productivity. A longer reduction time does not increase productivity. There are two reasons why 40, 30, and other shorter times cannot be optimal: lower metallization degree and lower productivity. (1) In Figure 13, where there are 5 layers (pink points), the MD is less than 85% when the reduction time is 45min. If the reduction time is shorter than 45min, the MD will be lower. (2) A lower MD means less metallic iron is produced, and this results in lower productivity of metallic iron. Therefore, 50 min is considered to be the optimal reduction time for an 80 mm bed (5 layers) in the present work. 4.2. Heat Transfer As mentioned above, there are two key points for the carbothermic reduction in a tall pellets bed: (1) prevention for newly formed DRI from re-oxidation; and (2) efficient heat transfer from top to bottom in the pellets bed. Based on above experiments, the newly formed DRI can be protected from re-oxidation by the upward CO-rich gas flow generated during the reduction in the tall pellets bed, and the total MD of the DRI bed can be up to 85%. So, the effective protection from re-oxidation is the basis for the high temperature and high heat transfer, because the radiation heat transfer is proportional to T4 of the heat source. From the viewpoint of the process control, the radiation heat transfer from heating source to the bottom of the bed is the critical step of the process. In the above experiments, the newly formed DRI in the top layer will shrink under the high temperature (1500 ◦ C), and the pellets shrinkage may result in a large space for the passage of radiative heat flux. Following this, the 2nd layer of DRI shrinks and generates the larger passage, and then the 3rd layer . . . . . . (Figure 15). Therefore, a high temperature can promote the shrinkage of the DRI, and it benefits the radiative heat transfer. Under the present experimental conditions, according to the MD shown in Figures 7 and 11, the heat transfer up to the 3rd layer is more effective. On the other hand, DRI in the 4th layer and the 5th layer are less heated due to the shielding of the radiation by the upper layer pellets. Therefore, in the PSH process, the heat transfer is by different mechanisms, mainly radiation and conduction in the solid phase.
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Figure DRI. Figure 15. 15. Large Large space space created created in in the the pellets pellets bed bed due due to to the the shrinkage shrinkage of of DRI.
4.3. Simultaneous Tall Pellets Bed and High Temperature Temperature A very veryimportant importantphenomenon phenomenonunder under the present experimental conditions is that the pellets the present experimental conditions is that the pellets bed bed and the temperature should be increased simultaneously, because (1)the forcase the case of apellets tall pellets and the temperature should be increased simultaneously, because (1) for of a tall bed bed only, without temperature, toplayer layercannot cannoteffectively effectivelyshrink shrinkat at aa lower only, without highhigh temperature, the the DRIDRI of of thethe top temperature. If the space created by the pellets shrinkage in the pellets bed is small, the radiation temperature. If the space created by the pellets shrinkage in the pellets bed is small, the radiation heat heat transfer through the bed be limited, then pellets thebottom bottomlayer layercannot cannotbe be effectively effectively transfer through the bed willwill be limited, then thethe pellets inin the reduced; (2) for the case of a high temperature only, without a tall pellets bed, the newly formed DRI will be due to the theof protection by the upward The likely bere-oxidized re-oxidized due to lack the of lack the protection by theCO-rich upwardgas. CO-rich gas.consequence The likely is to obtain DRI with a lower degree or meltingdegree and corrosive slag.and Therefore, it isslag. the consequence is to obtain DRImetallization with a lower metallization or melting corrosive basis that the tall pellets bed and a high temperature should be adopted simultaneously. For the Therefore, it is the basis that the tall pellets bed and a high temperature should be adopted reduction of ore-coal composite pellets, in the traditional of theoperation, pellets bedthe is simultaneously. For the reduction of ore-coal composite operation, pellets, in the theheight traditional ◦ C. In the only 1–2oflayers of pellets mm1–2 in height), and the reduction temperature is 1200–1300 height the pellets bed(20–30 is only layers of pellets (20–30 mm in height), and the reduction present experiments, 5 layers pellets were used (about 80 mm), and the reduction temperature temperature is 1200–1300 °C. Inofthe present experiments, 5 layers of pellets were used (about 80 mm), ◦ was C. Therefore, compared the°C. traditional operation parameters, the temperature and and 1500 the reduction temperature waswith 1500 Therefore, compared with the traditional operation the pellets bed simultaneously increased experiments. A more efficient process and a drastic parameters, theare temperature and the pellets by bed are simultaneously increased by experiments. A improvement productivity DRIimprovement quality (metallization degree and density) are the results of a more efficient in process and a and drastic in productivity DRI quality (metallization tall pellets and high temperature. degree andbed density) are the results of a tall pellets bed and high temperature. 4.4. Economics Economics and and Challenges Challenges 4.4. The PSH PSH process process also also has has some some economic economic advantages advantages compared compared with with shallow shallow bed bed processes, processes, The because (1) the metallization degree is higher, and the price of DRI is higher too; (2) the productivity because (1) the metallization degree is higher, and the price of DRI is higher too; (2) the productivity of metallic metalliciron ironisishigher, higher, should be lower; and the building coststraight of the of soso thethe costcost perper unitunit should be lower; and (3) the(3) building cost of the straight furnace is lower. However, as with any new process, there alsosome somechallenges challengeswith with the the furnace is lower. However, as with any new process, there areare also development of the PSH system. For example, the proper refractory materials are necessary due to development of the PSH system. For example, the proper refractory materials are necessary duethe to highhigh temperature in theinoxidation compartment. Therefore, there is athere lot ofiswork for researchers the temperature the oxidation compartment. Therefore, a lottoofdowork to do for in the ironmaking field before the commercial application of the PSH system. researchers in the ironmaking fieldactual before the actual commercial application of the PSH system. 5. Conclusions 5. Conclusions In the present work, the reaction characteristics of the carbothermic reduction in a tall pellets bed In the present work, the reaction characteristics of the carbothermic reduction in a tall pellets are investigated. The main findings can be summarized as follows: bed are investigated. The main findings can be summarized as follows: (1) When When the the height height of of the the pellets pellets bed bed is is 80 80 mm mm (16–18 (16–18 mm 0.80 may may (1) mm each each layer, layer, and and 55 layers), layers),C/O C/O == 0.80 result in in aa lower lower metallization metallization degree degree due 1.10 may may result result result due to to an an insufficient insufficient reductant, reductant,and andC/O C/O == 1.10 in more residual carbon in the DRI of the bottom layer. Therefore, under the present
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(3)
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in more residual carbon in the DRI of the bottom layer. Therefore, under the present experimental conditions, the optimal amount of carbon to add is C/O = 0.95. Addition of either more or less carbon does not benefit the production of high quality DRI. The longer reduction time (60 min) may result in more melting and corrosive slag in the top layer of DRI, which does not benefit the actual operation of the PSH process. With a reduction time of 50 min, the metallization degree may be up to 85.24%. Therefore, 50min is considered to be the optimal reduction time for an 80 mm bed (5 layers). In the present work, kg-MFe/m2 ·h is used as the unit to evaluate the productivity of metallic iron. In lab-scale experiments, under the condition of 5 layers of pellets (about 80 mm in height) and 50 min, the productivity of metallic iron can reach 55.41 kg-MFe/m2 ·h (or 75.26 kg-DRI/m2 ·h). Therefore, compared with the traditional shallow bed (one or two layers), the productivity of the DRI can be effectively increased in a tall pellets bed.
Author Contributions: X.J., G.D. and H.G. contributed to the materials preparation, performed the experiments, data analysis and wrote the paper; Q.G. revised the paper and refined the language; F.S. contributed to the design of the experiments. Funding: This research was funded by National Science Foundation of China, grant number 51874080 and 51604069, and the Fundamental Research Funds for the Central Universities of China, grant number N162504004. Acknowledgments: The authors wish to acknowledge the contributions of research fellows in China Steel Corporation of Taiwan and Tangshan OTSK Science and Technology Co., Ltd. Conflicts of Interest: The authors declare no conflicts of interest.
References 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14.
Kasai, E.; Kitajima, T.; Kawaguchi, T. Carbothermic reduction in the combustion bed packed with composite pellets of iron oxide and coal. ISIJ Int. 2000, 40, 842–849. [CrossRef] Qi, Z.; Murakami, T.; Kasai, E. Gasification and reduction behavior of iron ore-carbon composite under high pressure. ISIJ Int. 2012, 52, 1778–1784. [CrossRef] Murakami, T.; Kasai, E. Utilization of ores with high combined water content for ore-carbon composite and iron coke. ISIJ Int. 2011, 51, 1220–1226. [CrossRef] Murakami, T.; Kasai, E. Reduction mechanism of iron oxide–carbon composite with polyethylene at lower temperature. ISIJ Int. 2011, 51, 9–13. [CrossRef] Yunus, N.A.; Ani, M.H.; Salleh, H.M.; Aborashid, R.Z.; Akiyama, T.; Purwanto, H. Reduction of iron ore/empty fruit bunch char briquette composite. ISIJ Int. 2013, 53, 1749–1755. [CrossRef] Takyu, Y.; Murakami, S.H.; Son, S.H.; Kasai, E. Reduction mechanism of composite consisted of coal and hematite ore by volatile matter at 700–1100 K. ISIJ Int. 2015, 55, 1188–1196. [CrossRef] Yi, L.Y.; Huang, Z.C.; Jiang, T.; Zhao, P.; Zhong, R.H.; Liang, Z.K. Carbothermic reduction of ferruginous manganese ore for Mn/Fe beneficiation: Morphology evolution and separation characteristic. Minerals 2017, 7, 167. [CrossRef] Yu, D.W.; Paktunc, D. Calcium chloride-assisted segregation reduction of chromite: Influence of reductant type and the mechanism. Minerals 2018, 8, 45. [CrossRef] Suzuki, H.; Mizoguchi, H.; Hayashi, S. Influence of ore reducibility on reaction behavior of ore bed mixed with coal composite iron ore hot briquettes. ISIJ Int. 2011, 51, 1255–1261. [CrossRef] Tanaka, Y.; Ueno, T.; Okumura, K.; Hayashi, S. Reaction behavior of coal rich composite iron ore hot briquettes under load at high temperatures until 1400 ◦ C. ISIJ Int. 2011, 51, 1240–1246. [CrossRef] Jiang, X.; Wang, L.; Shen, F.M.; Lu, W.K. Adiabatic carbon rate of alternative ironmaking processes to produce hot metal. Steel Res. Int. 2014, 85, 35–43. [CrossRef] Yu, W.; Sun, T.C.; Cui, Q.; Xu, Y.; Kou, J. Effect of coal type on the reduction and magnetic separation of a high-phosphorus oolitic hematite ore. ISIJ Int. 2015, 55, 536–543. [CrossRef] Xu, C.B.; Cang, D.Q. A brief overview of low CO2 emission technologies for iron and steel making. J. Iron Steel Res. Int. 2010, 17, 1–7. [CrossRef] Iguchi, Y.; Yokomoto, S. Kinetics of the reactions in carbon composite iron ore pellets under various pressures from vacuum to 0.1 MPa. ISIJ Int. 2004, 44, 2008–2017. [CrossRef]
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16. 17.
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Yu, W.; Wen, X.J.; Chen, J.G.; Huang, W.Q.; Qiu, T.S. Preparation of direct reduced iron and titanium nitride from Panzhihua titanomagnetite concentrate through carbothermic reduction-magnetic separation. Minerals 2017, 7, 220. [CrossRef] Jiang, X.; Liu, S.H.; Huang, T.Y.; Shen, F.M. Effects of reducing time on metallization degree of carbothermic reduction of tall pellets bed. ISIJ Int. 2016, 56, 88–93. [CrossRef] Sun, S.; Lu, W.K. A theoretical investigation of kinetics and mechanisms of iron ore reduction in an ore/coal composite. ISIJ Int. 1999, 39, 123–129. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
