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Carbothermic Reduction of Ore-Coal Composite Pellets in a Tall Pellets Bed

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Carbothermic Reduction of Ore-Coal Composite Pellets in a Tall Pellets Bed 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 www.mdpi.com/journal/minerals MineralsMinerals 20182018, ,88, ,x 550FOR PEER REVIEW 2 2of of 15 15 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 CO2 emissions [5]. promising energy source for decreasing coal consumption in ironmaking, and reducing 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 coal- composite. 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 (Figure1). 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 FigureFigure 1. 1. SchematicSchematic diagram diagram of of the the carbothermic carbothermic reduction reduction in in a ashallow shallow pellets pellets bed. bed. BasedBased on on the the reduction reduction of of ore-coal ore-coal composite composite pell pelletsets in in a a furnace furnace hearth, hearth, and and aiming aiming to to solve solve the the problemproblem of of the the contradictory contradictory requirements requirements of of fuel fuel in in the the oxidation oxidation compartment compartment and and the the reduction reduction compartment,compartment, Wei-Kao LuLu of of McMaster McMaster University Universi inty Canadain Canada proposed proposed a new a directnew reductiondirect reduction process, process,which is which named is the named paired the straight paired hearth straight (PSH) hearth furnace (PSH) process furnace [17 process]. Similar [17]. to shallowSimilar pelletsto shallow beds, pellets beds, the PSH process may also be divided into an oxidation compartment and a reduction Minerals 2018, 8, 550 3 of 15 Minerals 2018, 8, x FOR PEER REVIEW 3 of 15 the PSH process may also be divided into an oxidation compartment and a reduction compartment (Figurecompartment2). The (Figure PSH process 2). The has PSH two process operational has two characteristics: operational characteristics: (1) a tall pellets (1) a tall bed pellets (5–7 layers, bed (5– ◦ 80–1207 layers, mm) 80–120 in the mm) reduction in the compartment;reduction compartment; and (2) a high and temperature (2) a high temperature (1500–1550 (1500–1550C) in the oxidation °C) in the compartment,oxidation compartment, which is caused which by is the caused full combustionby the full combustion of CO to CO of2. CO to CO2. Oxidation Compartment (CO + 1/2 O2 = CO2): 100% of CO2, Oxidizing Gas Reducing Reduction Compartment: Iron Ore + C + Heat → Fe + CO FigureFigure 2.
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