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

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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. 2.Schematic Schematic diagram diagram of of the the carbothermic carbothermic reduction reduction in in the the tall tall pellets pellets bed. bed.

ThereThere are are twotwo keykey pointspoints in the development development of of th thee PSH PSH process: process: (1) (1) In Inthe the tall tall pellets pellets bed, bed, the thenewly newly formed formed DRI DRI at the at thetop top of the of thebed bed can canbe prot be protectedected from from re-oxidation re-oxidation by the by upward the upward gas gasflow ﬂow(which (which is CO is rich) CO rich)generated generated during during the reduction the reduction in the intall the pellets tall pelletsbed, and bed, enhanced and enhanced by the high by thevolatile high volatilecoal in the coal green in the ball green over ball a longer over a period longer of period the reduction of the reduction time; (2) time; Efficient (2) Efﬁcient heat transfer heat transferfrom the from “oxidation the “oxidation compartment” compartment” (where (whereheat is generated) heat is generated) to the “reduction to the “reduction compartment” compartment” (where (whereheat is heatconsumed is consumed by the byendothermic the endothermic reaction reaction and sensible and sensible heat of heatsubstances). of substances). Therefore, Therefore, the PSH thewas PSH considered was considered as a possible as a possible process process to produce to produce DRI with DRI witha high a highquality, quality, low lowcarbon carbon rate, rate, and andconsequentially consequentially low low CO CO2 emissions.2 emissions. However,However, sincesince thethe PSHPSH processprocess waswas proposedproposed byby Lu,Lu, thethe reactionreaction characteristicscharacteristics ofof thethe carbothermiccarbothermic reductionreduction ofof ore-coalore-coal compositecomposite pelletspellets inin talltall pelletspellets bedsbeds havehave notnot beenbeen betterbetter understood.understood. Therefore, Therefore, the the present present work work was was done. done. The The aims aims of of this this study study are are as as follows: follows: (1) (1) common common rawraw materials materials were were used used to better to understandbetter understand the common the reactioncommon characteristics reaction characteristics of the carbothermic of the reductioncarbothermic of ore-coal reduction composite of ore-coal pellets composite in tall pellets pellet beds,s in tall which pellets have beds, not beenwhich found have in not the been literature found soin far;the (2)literature the effects so far; of some(2) the basic effects operation of some parameters basic operation on the parameters carbothermic on the reduction carbothermic were experimentallyreduction were investigated, experimentally including investigated, the amount includ ofing carbon the amount addition of carbon (denoted addition as C/O) (denoted and the as reductionC/O) and time;the reduction (3) comparison time; (3) on comparison the productivity on th ofe productivity metallic iron of among metall differentic iron among pellets different layers (differentpellets layers height (different in pellets height bed); in and pellets (4) bed); give someand (4) suggestions give some suggestions for choosing for reasonable choosing operationreasonable parametersoperation parameters in future commercial in future commercial development development of the PSH process.of the PSH process.

2. Experimental

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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 Table1. It should be noted that the moisture is 7.22% in the original concentrate. The chemical analysis was Minerals 2018, 8, x FOR PEER REVIEW 4 of 15 carried out under dry conditions (7.22% of H2O was dried), including TFe (Total Fe), FeO, SiO2, CaO, MgO, Al2O3, 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, X- X-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 Figure3. 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 Al2O3 LOI TFe FeO SiO2 CaO MgO Al2O3 LOI 63.2663.26 26.9926.99 6.68 6.68 0.15 0.15 0.12 0.12 0.20 0.20 −1.45−1.45

8000 ■Fe O ■ 3 4 ▲SiO 2 6000

4000 ▲ ■ ■

Intensity ■ 2000 ■ ▲ ▲ ■ ■ ▲ ▲ 0

10 20 30 40 50 60 70 80 90

Two-Theta (deg) FigureFigure 3.3. X-ray diffraction patternpattern ofof ironiron oreore concentrate.concentrate.

TheThe proximateproximate analysisanalysis andand ashash analysisanalysis ofof thethe pulverizedpulverized coalcoal isis shownshown inin TableTable2 2.. The The volatile volatile mattermatter (VM)(VM) inin thethe coalcoal isis aboutabout 26%.26%. ForFor thethe preparationpreparation ofof ore-coalore-coal compositecomposite pellets,pellets, coalcoal isis alsoalso ground,ground, andand 100%100% passedpassed −−0.2210.221 mm for pelletizing.

Table 2.2. ProximateProximate analysisanalysis andand ashash analysisanalysis ofof pulverizedpulverized coalcoal (mass%).(mass%).

ProximateProximate Analysis Analysis Ash Ash Analysis Analysis Fixed C Total C Ash VM SiO2 Al2O3 Fe2O3 MgO CaO Fixed C Total C Ash VM SiO2 Al2O3 Fe2O3 MgO CaO 61.3161.31 77.577.5 9.389.38 26.0026.00 49.1449.14 29.9829.98 10.22 0.70 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 Figure4. 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 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.

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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 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. 3. A special crucible was made for holding composite pellets in mufﬂe furnace (Figure5). 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 3.upward A special gas incrucible a vertical was directionmade for in holding the pellets composite bed; and pellets (b) the in insulatingmuffle furnace materials (Figure surrounding 5). The thecrucible mullite consists ring to obstructof two parts: the heat(a) the transfer mullite from ring the (the horizontal inner diameter direction is 80 of mm) the pelletsto retain bed. the 4. Theupward special gas crucible in a vertical was put direction into the in furnace.the pellets Then, bed; and the (b) temperature the insulating of the materials furnace surrounding was heated to 1200the◦ Cmullite in air ring atmosphere, to obstruct and thethe heat crucible transfer was from pre-heated the horizontal to 1200 direction◦C in of the the furnace. pellets bed.

5. 4.TheThe furnace special door crucible was was opened, put into and the 500 furnace. g of the Then, dried the compositetemperature pellets of the furnace was charged was heated 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 6. Thethe furnace void ratio temperature of packing was bed keptis about at 1200 14–18%.C for 5 min. Then, it was heated to 1500 C in about ◦ ◦ 6.20 minThe furnace (15 C/min), temperature and kept was atkept 1500 at 1200C until °C for the 5 targetmin. Then, time. it was heated to 1500 °C in about 7. When20 min the (15 target °C/min), reduction and kept time at 1500 was °C reached, until the the target entire time. crucible was taken out of the furnace, 7.and When the surroundingthe target reduction insulating time materialswas reached, were the immediately entire crucible taken was taken away out from of the furnace, crucible to stopand the the reduction surrounding reaction. insulating Then, materials the hot DRIwere bed immediately is quenched taken in liquidaway from N2 to the prevent crucible the to DRI fromstop re-oxidation. the reduction reaction. Then, the hot DRI bed is quenched in liquid N2 to prevent the DRI 8. Dividefrom there-oxidation. DRI bed. The cooled DRI bed is shown in Figure6. From top to bottom, the ﬁrst

8.12–13Divide pellets the DRI are definedbed. The ascooled the 1stDRI layer, bed is then shown the in second Figure 12–136. From pellets top to arebottom, defined the first as the 12– 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. 9. 9.Eight Eight pellets pellets in in each each layer layer were were randomlyrandomly selected for for chemical chemical analysis. analysis. Total Total Fe Fe(TFe) (TFe) and and MetallicMetallic Fe Fe (MFe) (MFe) were were obtained. obtained. Then,Then, for each each la layer,yer, the the ratio ratio of of metallic metallic Fe Feto tototal total Fe Fewas was defineddefined as as metallization metallization degree (MD (MD = MFe/TFe), = MFe/TFe), which which was an was actual an value actual (not value average (not value). average value).The metallization The metallization degrees degrees (MD) (MD)of the of total the totalDRI DRIbed, bed,which which is an is actual an actual value value too, too,can canbe be calculatedcalculated by by the the following following equation: equation: MDMD (total (total bed) bed) = [MFe= [MFe (1st (1st layer) layer)× × Weight (1st layer) layer) + + MFe MFe (2nd (2nd layer) layer) × Weight× Weight (2nd (2nd layer) layer) + + . . .… + + MFe MFe (5th (5th layer)layer) ×× WeightWeight (5th (5th layer)]/[TFe layer)]/[TFe (1st (1st layer) layer) × Weight× Weight (1st layer) (1st layer) + TFe + (2nd TFe layer) (2nd× Weight layer) ×(2ndWeight layer) (2nd + … layer)+ TFe (5th + . .layer) . + TFe × Weight (5th layer) (5th ×layer)]Weight (5th layer)]

1600

1500 Discharge and

℃ 1400 Charge cooled under N2

1300

1200 Temperature,

1100 5min

1000 020406080 Time, min FigureFigure 4. Schematic4. Schematic diagram diagram of of the the temperaturetemperature profile proﬁle control control of of the the electric electric muffle mufﬂe furnace. furnace.

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((a)) ((b))

Figure 5.5. SpecialSpecial crucible crucible for for the carbothermic reductio reductionn experiments experiments in in the the tall pellets bed in the mufflemufﬂe furnace: ( a)) appearance appearance of of insulation insulation crucible;crucible; ((bb)) toptop viewview andand sizesize dimensiondimension ofofof crucible.crucible.crucible.

FigureFigure 6. 6. AppearancesAppearances of of the the total total direct direct redu reducedcedced iron iron (DRI) (DRI) bed bed and and each each layer. layer.

3. Experimental Results and Analysis 3.1. Effects of Amount of Carbon Addition (C/O) 3.1. Effects of Amount of Carbon Addition (C/O) 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 isis thethe gram-atomicgram-atomic ratioratio ofof thethe fixedfixed carboncarbon inin 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 Figure7. 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 increasedincreased toto aboutabout 85%85% andand 86%,86%, whichwhich indicatesindicates thatthat thethe reductantreductant inin thesethese pelletspellets isis sufficient;sufficient; (3)(3) itit inevitable that part of the DRI is re-oxidized in the top layer, so the MD first increases from the 1st isis inevitableinevitable thatthat partpart ofof thethe DRIDRI isis re-oxidizedre-oxidized inin thethe toptop layer,layer, soso thethe MDMD firstfirst increasesincreases fromfrom thethe 1st1st layer to the 3rd layer. Then the MD decreases from the 3rd layer to the 4th layer, because radiative layerlayer toto thethe 3rd3rd layer.layer. ThenThen thethe MDMD decreasesdecreases fromfrom thethe 3rd3rd layerlayer toto thethe 4th4th layer,layer, becausebecause radiativeradiative 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) isis higherhigher thanthan thethe 4th4th layer.layer. TheThe reasonreason forfor thisthis isis thatthat thethe pelletspellets inin thethe 5th5th layerlayer cancan bebe heatedheated fromfrom thethe hothot bottombottom ofof thethe cruciblecrucible byby thethe heatheat pre-storedpre-stored inin thethe refractory.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 Minerals 2018, 8, x FOR PEER REVIEW 7 of 15 hotMinerals bottom 2018, of8, x the FOR crucible PEER REVIEW by the heat pre-stored in the refractory. 7 of 15

100 100 Reduce for 50min Reduce for 50min C/O=0.80 C/O=0.80 C/O=0.95 C/O=0.95 C/O=1.10 C/O=1.10 8080

6060 Metallization Degree, % Metallization Degree, %

4040 123456123456TotalTotal (Top) (Top) (Bottom)(Bottom) Layer No. Layer No. FigureFigureFigure 7. 7.7. Effect EffectEffect of ofof C/O C/OC/O on on metallization metallization degree degree of of direct direct reduced reduced iron iron (DRI). (DRI).

3.1.2.3.1.2.3.1.2. Metallographic Metallographic Analysis AnalysisAnalysis AAA typical typical metallographic metallographic metallographic picture picture picture and and and energy energy energy disp disp disperseerseerse spectroscopy spectroscopy spectroscopy (SEM-EDS, (SEM-EDS, (SEM-EDS, Ultra Ultra UltraPlus; Plus; Carl Plus;Carl ZeissCarlZeiss GmbH, Zeiss GmbH, GmbH, Jena, Jena, Germany) Germany) Jena, Germany) of of the the DRI ofDRI the specimen specimen DRI specimen is is sh shownown is in in shown Figure Figure in8. 8. FigureIt It can can be8 be. concluded Itconcluded can be concluded that that there there arethatare mainly mainly there three are three mainly phases: phases: three (1) (1) the phases:the white white (1) zone, zone, the as whiteas presented presented zone, by as by presentedpoint point A, A, is is bythe the point metallic metallic A, isiron iron the phase; phase; metallic (2) (2) ironthe the lightphase; grey (2) zone, the light as presented grey zone, by aspoint presented B, consists by pointmainly B, of consists fayalite mainly (2FeO·SiO of fayalite2) and the (2FeO slag·SiO phase) and of light grey zone, as presented by point B, consists mainly of fayalite (2FeO·SiO2) and the slag phase2 of siliconthesilicon slag oxide, oxide, phase iron iron of siliconoxide, oxide, and oxide, and aluminum aluminum iron oxide, oxide, oxide, and with aluminumwith the the former former oxide, being being with crystallized thecrystallized former from being from molten crystallizedmolten slag slag onfromon cooling. cooling. molten It It should slag should on be cooling.be noted noted Itthat that should the the DRI DRI be notedin in the the that2nd 2nd thelayer layer DRI is is better inbetter the protected 2nd protected layer from is from better re-oxidation re-oxidation protected than fromthan there-oxidationthe 1st 1st layer, layer, which than which the results results 1st layer, in in less less which liquid liquid results slag slag and inand less fayalite. fayalite. liquid Therefore, slagTherefore, and fayalite. the the slag slag Therefore, phase phase is is homogeneous; homogeneous; the slag phase (3)is(3) homogeneous;the the deep deep grey grey (3)zone, zone, the as deepas presented presented grey zone, by by point as point presented C, C, is is quartz. quartz. by point In In addition, C, addition, is quartz. the the In black black addition, zone, zone, the as as presented black presented zone, byasby point presentedpoint D, D, is is bythe the point area area of D, of pores ispores the inside area inside of the poresthe pellets. pellets. inside Both Both the in pellets.in the the metallic metallic Both in phase phase the metallic and and the the phase slag slag phase, andphase, the there there slag isphase,is the the element element there is carbon, thecarbon, element because because carbon, carburization carburization becausecarburization is is inevitable inevitable in isin ore-coal inevitable ore-coal composite composite in ore-coal pellets. pellets. composite pellets.

400 FeKa Point A 400 FeKa Point A

300 300FeLa FeLa

200 CKa Counts 200 CKa Counts

CLa 100 CLa 100 CMa CMa

0 0510150 051015 Energy, KeV Energy, KeV (a()a ) (b(b) ) Figure 8. Cont.

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500 SiKa 500 SiKa 400 SiKa Point B SiKa Point C 400 Point B Point C 400 400 300 300 300 300

200

Counts FeLa Counts OKa 200 200 Counts OKa FeLa Counts AlKa FeKa 200 AlKa FeKa OKa 100 100 OKa 100 100 CaKa CaKa 0 0 00 5 10 15 00 5 10 15 0 5 10 15 0 5 10 15 Energy, KeV Energy, KeV Energy, KeV Energy, KeV (c) (d) (c) (d) Figure 8. Typical metallographic picture and energy disperse spectroscopy of direct reduced iron FigureFigure 8. 8.Typical Typical metallographic metallographic picture picture and and energy energy disperse disper spectroscopyse spectroscopy of direct of direct reduced reduced iron (DRI) iron (DRI) (C/O = 0.95, 50 min, 2nd layer): (a) metallographic picture; (b) point A; (c) point B; (d) point C. (C/O(DRI) = (C/O 0.95, = 50 0.95, min, 50 2nd min, layer): 2nd layer): (a) metallographic (a) metallographic picture; picture; (b) point (b) A;point (c) pointA; (c) B;point (d) pointB; (d) C.point C. Other than the typical metallographic picture shown in Figure 8, in the metallographic picture OtherOther than than the the typical typical metallographic metallographic picture picture shown show inn in Figure Figure8, in 8, thein the metallographic metallographic picture picture of of the 5th layer DRI of C/O = 1.1 (Figure 9), there is some black and loose powder. SEM-EDS is used theof the 5th 5th layer layer DRI DRI of C/O of C/O = 1.1 = 1.1 (Figure (Figure9), there9), there is some is some black black and and loose loose powder. powder. SEM-EDS SEM-EDS is used is used to to analyze the element in this powder area. Based on Figure 9, it can be seen that there is some carbon analyzeto analyze the the element element in this in this powder powder area. area. Based Based on on Figure Figure9, it9, can it can be be seen seen that that there there is is some some carbon carbon (solid C in coal) and silica (ash in coal). This proves that the coal addition (C/O) is excessive, and some (solid(solid C C in in coal) coal) and and silica silica (ash (ash in in coal). coal). This This proves proves that that the the coal coal addition addition (C/O) (C/O) is is excessive, excessive, and and some some residual carbon remains in the DRI, which does not benefit the actual DRI production. Therefore, in residualresidual carboncarbon remainsremains inin thethe DRI, which does does not not benefit beneﬁt the the actual actual DRI DRI production. production. Therefore, Therefore, in order to better understand the exact carbon content in DRI, a quantitative chemical analysis was used inorder order to tobetter better understand understand the the exact exact carbon carbon content content in DRI, in DRI, a quantitative a quantitative chemical chemical analysis analysis was wasused for the residual carbon in the 5th layer (bottom layer) DRI with different C/O, and the result is shown usedfor the for residual the residual carbon carbon in the in 5th the layer 5th layer(bottom (bottom layer) layer) DRI with DRI different with different C/O, and C/O, the and result the is result shown is in Figure 10. shownin Figure inFigure 10. 10. As shown in Figure 7, the metallization degrees of the total DRI bed are similar in cases of C/O AsAs shown shown in in Figure Figure7 ,7, the the metallization metallization degrees degrees of of the the total total DRI DRI bed bed are are similar similar in in cases cases of of C/O C/O = 0.95 and C/O = 1.10 (the difference may be within error). But the residual carbon of C/O = 1.10 is == 0.950.95 andand C/OC/O = 1.10 (the difference may be within error). But But the the residual residual carbon carbon of of C/O C/O = =1.10 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 much more more than than C/O C/O = 0.95 = 0.95 in Figure in Figure 10. Therefore, 10. Therefore, C/O = C/O 0.95 =is 0.95considered is considered to be the to optimal be the optimal amount of carbon addition in the next stage of experiments. amountof carbon of addition carbon addition in the next in the stage next of stage experiments. of experiments.

500 500 SiKa 400 SiKa 400

300 300 CKa

CKa FeKa

Counts FeKa 200 OKa Counts 200 OKa CLa 100 CLa 100 CMa CMa 0 0051015 051015 Energy, KeV Energy, KeV (a) (b) (a) (b) FigureFigure 9.9. MetallographicMetallographic and and element element analysis analysis of ofthe the dire directct reduced reduced iron iron (DRI) (DRI) in the in 5th the layer 5th layer (C/O Figure 9. Metallographic and element analysis of the direct reduced iron (DRI) in the 5th layer (C/O (C/O= 1.1): = ( 1.1):a) metallographic (a) metallographic picture; picture; (b) element (b) element analysis. analysis. = 1.1): (a) metallographic picture; (b) element analysis.

Minerals 2018, 8, x FOR PEER REVIEW 9 of 15 Minerals 2018, 8, 550 9 of 15

Minerals 2018, 8, x FOR PEER REVIEW 9 of 15 6 Reduce for 50min 6 DRI in 5th layer (bottom layer) Reduce for 50min 4 DRI in 5th layer (bottom layer)

Residual Carbon, % Carbon, Residual 2 Residual Carbon, % Carbon, Residual 0 0.80 0.95 1.10 0 C/O ratio in composite pellets 0.80 0.95 1.10 C/O ratio in composite pellets Figure 10. Residual carbon of the direct reduced iron (DRI) in the 5th layer (bottom layer). Figure 10. Residual carbon of the direct reduced iron (DRI) in the 5th layer (bottom layer). 3.2. CarbothermicFigure 10.Reduction Residual for carbon 50 min of andthe direct60 min reduced iron (DRI) in the 5th layer (bottom layer). 3.2. Carbothermic Reduction for 50 min and 60 min 3.2. CarbothermicBased on the Reductionpreliminary for experiments,50 min and 60 for min 5 layers of pellets, the reduction time should be 50min at least.Based So on in the this preliminary section, experiments,the reduction for time 5 layers of 50 of pellets,min and the 60 reduction min were time investigated, should be 50 minand Based on the preliminary experiments, for 5 layers of pellets, the reduction time should be 50min atexperimental least. So in this results section, are shown the reduction in Figure time 11. of From 50 min the and figure, 60 min it can were be investigated, seen that: (1) and with experimental a reduction at least. So in this section, the reduction time of 50 min and 60 min were investigated, and resultstime of are 60 shownmin, the in MD Figure in the11. 4th From and the 5th ﬁgure, layers it is can higher be seen than that: for (1) 50min, with but a reduction the MD timein the of 1st 60 layer min, experimental results are shown in Figure 11. From the figure, it can be seen that: (1) with a reduction theis lower MD in than the for 4th 50 and min; 5th (2) layers the MD is higher of the thantotal forDRI 50min, bed for but 50 the min MD and in 60 the min 1st are layer 85.24% is lower and than 86.03%, 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 50respectively. 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 C/O=0.95 50min 100 60min C/O=0.95 50min 80 60min 80

60 60

Metallization Degree, % 40

Metallization Degree, % 20 123456Total 20 (Top) (Bottom) 123456Layer No. Total (Top) (Bottom) FigureFigure 11.11. EffectEffect ofof reductionreduction timetime (50(50 minmin andand 6060Layer min)min) onon No. thethe metallizationmetallization degreedegree ofofdirect direct reducedreduced ironiron (DRI).(DRI). Figure 11. Effect of reduction time (50 min and 60 min) on the metallization degree of direct reduced TheTheiron metallographicmetallographic(DRI). picturepicture andand energyenergy spectrumspectrum ofof thethe 1st1st layerlayer DRIDRI specimenspecimen forfor 6060 minmin isis shownshown inin FigureFigure 1212.. The The white white zone zone is is metallic metallic iron iron.. The The deep deep grey grey phase phase (presented (presented by by point point A) A) is The metallographic picture and energy spectrum of the 1st layer DRI specimen for 60 min is isquartz, quartz, which which is isstill still a zone a zone of un-melting of un-melting solid solid grains grains due due to its to high its high melting melting point. point. The light The lightgrey shown in Figure 12. The white zone is metallic iron. The deep grey phase (presented by point A) is greyphase phase (presented (presented by point by point B) and B) and the the middle middle grey grey phase phase (presented (presented by by point point C) C) are molten slagslag quartz, which is still a· zone of un-melting solid grains due to its high melting point. The light grey (mainly(mainly fayalite,fayalite, 2FeO2FeO·SiOSiO22). It It can be interpreted that in thethe laterlater stagestage ofof reduction,reduction, thethe up-wardup-ward phase (presented by point B) and the middle grey phase (presented by point C) are molten slag protectiveprotective gasgas evolvedevolved lessless andand less,less, soso aa littlelittle bitbit ofof newlynewly formedformed metallicmetallic ironiron waswas re-oxidizedre-oxidized toto (mainly fayalite, 2FeO·SiO2). It can be interpreted that in the later stage of reduction, the up-ward FeO.FeO. Then,Then, thethe oxidesoxides ofof silicon,silicon, aluminumaluminum andand somesome ironiron formedformed moltenmolten slag.slag. OnOn cooling,cooling, fayalitefayalite protective gas evolved less and less, so a little bit of newly formed metallic iron was re-oxidized to crystallizedcrystallized fromfrom the the molten molten slag andslag distributedand distri inbuted the ﬁnalin amorphousthe final amorphous slag phase afterslag solidiﬁcation.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

Minerals 2018, 8, x FOR PEER REVIEW 10 of 15 Minerals 2018, 8, 550 10 of 15 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

400

Counts 200

OKa

0 0 5 10 15

Energy, KeV (a) (b)

500 500 Point B Point C SiKa SiKa 400 400

300 300

AlKa OKa Counts Counts 200 200 OKa FeKa

100 100 C FeKa C C C 0 0 0 5 10 15 0 5 10 15 Energy, KeV Energy, KeV (c) (d)

FigureFigure 12.12. MetallographicMetallographic picturepicture andand energyenergy spectrumspectrum ofof thethe 1st1st layerlayer directdirect reducedreduced ironiron (DRI)(DRI) reducedreduced for for 60 60 min: min: ((aa)) metallographic metallographic picture; picture; ( b(b)) point point A; A; ( c(c)) point point B; B; ( d(d)) point point C. C.

3.3.3.3.Productivity Productivity of the Tall PelletsPellets BedBed 2 InIn traditional traditional work, work, kg-DRI/m kg-DRI/m2··hh is used to evaluate the productivityproductivity ofof DRI.DRI. ButBut thisthis unitunit ignoresignores 2 thethe qualityquality ofof DRIDRI (metallization(metallization degree).degree). In In the the present present work, work, kg-MFe/m kg-MFe/m2·h· his isused used as asthe the unit unit to toevaluate evaluate the the productivity productivity of metallic of metallic iron. iron. For this For unit, this both unit, the both quality the quality of DRI of(metallization DRI (metallization degree) degree)and quantity and quantityof DRI (productivity) of DRI (productivity) are considered are simultaneously. considered simultaneously. The productivity The presented productivity by kg- 2 presentedMFe/m2·h,by which kg-MFe/m is an actual·h, value which (not is an average actual value), value may (not be average calculated value), by the may following be calculated equation: by the following equation: Productivity = [MFe (1st layer) × Weight (1st layer) + MFe (2nd layer) × Weight (2nd layer) + … + MFeProductivity (5th layer) =× [MFeWeight (1st (5th layer) layer)]/[Acreage× Weight (1st (m layer)2) × Time + MFe (h)] (2nd layer) × Weight (2nd layer) + ... + MFe (5th layer) × Weight (5th layer)]/[Acreage (m2) × Time (h)] In this series of experiments, a 5min variant is adopted. The effects of pellets layers and reduction timeIn on this the series productivity of experiments, of metallic a 5 min iron variant are shown is adopted. in Figure The 13. effects It is ofworthy pellets to layers note andthat reduction a shorter timereduction on the time productivity does not necessarily of metallic generate iron are higher shown productivity, in Figure 13. because It is worthy it may to result note thatin an a MD shorter that reductionis too low timeto satisfy does the not requirement necessarily of generate ironmaki higherng production. productivity, Therefore, because a productivity it may result with in anMD MD≥85% thatis selected is too low and to circled satisfy by the a ring requirement in this figure. of ironmaking It can be seen, production. for certain Therefore, layers, there a productivity are one or with two MDpoints≥ 85%which is selectedcan meet and an MD circled≥85%. by For a ring example, in this in ﬁgure. the case It can of the be four-layers seen, for certain pellets layers, bed, the there MD are of oneDRI or reduced two points for 40 which min and can 45 meet min an are MD about≥ 85%. 87% Forand example,89%, respectively. in the case Both of theof them four-layers can meet pellets with bed,an MD the≥ MD85%. of Between DRI reduced them, for the 40 productivity min and 45 minof 40 are min about is higher 87% andthan 89%, 45 min. respectively. So, the productivity Both of them of 40 min (the maximum productivity for the four-layers bed) is selected in Figure 14.

Minerals 2018, 8, x FOR PEER REVIEW 11 of 15 Minerals 2018, 8, x FOR PEER REVIEW 11 of 15 Similar, the maximum productivities for other different layers beds are selected in Figure 14 too. Similar, the maximum productivities for other different layers beds are selected in Figure 14 too. It should be noted that the minimum requirement of 85% of MD is not reached by extending the It should be noted that the minimum requirement of 85% of MD is not reached by extending the reduction time to 70 min in the case of 6 layers. So, in Figure 14, there is no data on 6 layers. From the 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/m2·h (or 75.26 kg-DRI/m2·h). Therefore, tall pellets beds (and iron can reach 55.41 kg-MFe/m2·h (or 75.26 kg-DRI/m2·h). Therefore, tall pellets beds (and Mineralssimultaneously2018, 8, 550 high temperatures) can increase the productivity of DRI. The reason for this11 is of that 15 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 canat the meet same with time. an MD So the≥ 85%. productivity Between of them, DRI thecan productivitybe significantly of 40increased min is higher in the thanPSH 45process min. (tall So, pellets bed) compared with the traditional shallow bed (one or two layers). thepellets productivity bed) compared of 40 min with (the the maximum traditional productivity shallow bed for (one the four-layersor two layers). bed) is selected in Figure 14.

80 80 MD≥85% MD≥85% .h

2 60 .h

2 60

40 40 1 Layer 1 Layer 2 Layers 2 Layers 3 Layers 20 3 Layers 20 4 Layers 4 Layers Productivity,kg-MFe/m 5 Layers

Productivity,kg-MFe/m 5 Layers 6 Layers 0 6 Layers 00 1020304050607080 0 1020304050607080 Reduction Time, min Reduction Time, min Figure 13. Effects of pellets layers and reduction time on the productivity of metallic iron. FigureFigure 13. 13.Effects Effects of of pellets pellets layers layers and and reduction reduction time time on on the the productivity productivity of of metallic metallic iron. iron.

80 80 .h

2 60 .h

2 60

40 40 1 Layer 1 Layer 2 Layers 2 Layers 20 3 Layers 20 3 Layers 4 Layers

Productivity, kg-MFe/m Productivity, 4 Layers 5 Layers Productivity, kg-MFe/m Productivity, 5 Layers 0 00 1020304050607080 0 1020304050607080 Reduction Time, min Reduction Time, min FigureFigure 14. 14.Maximum Maximum productivityproductivity ofof metallicmetallic ironiron andand corresponding corresponding reduction reduction time time for for certain certain Figure 14. Maximum productivity of metallic iron and corresponding reduction time for certain pelletspellets layers. layers. pellets layers. 4. DiscussionSimilar, the maximum productivities for other different layers beds are selected in Figure 14 too.4. Discussion It should be noted that the minimum requirement of 85% of MD is not reached by extending the4.1. reduction Optimal Parameters time to 70 min in the case of 6 layers. So, in Figure 14, there is no data on 6 layers. From4.1. Optimal the ﬁgure, Parameters one can conclude that the productivity denoted by kg-MFe/m2 (with different layers and4.1.1. different Carbon reduction Addition times) 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). Minerals 2018, 8, 550 12 of 15

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 Figures7 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. Minerals 2018, 8, 550 13 of 15 Minerals 2018, 8, x FOR PEER REVIEW 13 of 15

Figure 15. LargeLarge space created in the pellets bed due to the shrinkage of DRI.

4.3. Simultaneous Tall Pellets Bed and High Temperature A very very important important phenomenon phenomenon under under the the present present experimental experimental conditions conditions is that is that the pellets the pellets bed andbed andthe temperature the temperature should should be incr beeased increased simultaneously, simultaneously, because because (1) fo (1)r the for case the caseof a tall of a pellets tall pellets bed only,bed only, without without high high temperature, temperature, the theDRI DRI of ofthe the top top layer layer cannot cannot effectively effectively shrink shrink at at a a lower temperature. If If the the space space created created by by the the pellets pellets shrink shrinkageage in the in thepellets pellets bed bedis small, is small, the radiation the radiation heat transferheat transfer through through the bed the bedwillwill be limited, be limited, then then the the pellets pellets in inthe the bottom bottom layer layer cannot cannot be be effectively effectively reduced; (2) for the case of a a high high temperature temperature on only,ly, without a tall pellets bed, the newly formed DRI will bebe re-oxidized re-oxidized due due to theto lackthe oflack the of protection the protection by the upward by the CO-richupward gas. CO-rich The likely gas. consequence The likely consequenceis to obtain DRI is to with obtain a lower DRI metallization with a lower degree metallization or melting degree and corrosive or melting slag. and Therefore, corrosive it isslag. the Therefore,basis that theit is tall the pellets basis bedthat and the a tall high pellets temperature bed and should a high be adoptedtemperature simultaneously. should be adopted For the simultaneously.reduction of ore-coal For the composite reduction pellets, of ore-coal in the traditionalcomposite operation,pellets, in thethe heighttraditional of the operation, pellets bed the is ◦ heightonly 1–2 of layers the pellets of pellets bed (20–30 is only mm 1–2 in height),layers of and pellets the reduction (20–30 mm temperature in height), is 1200–1300 and the reductionC. In the temperaturepresent experiments, is 1200–1300 5 layers °C. In of the pellets present were experiment used (abouts, 5 layers 80 mm), of pellets and the were reduction used (about temperature 80 mm), ◦ wasand 1500the reductionC. Therefore, temperature compared was with 1500 the °C. traditional Therefore, operation compared parameters, with the traditional the temperature operation and parameters,the pellets bed the are temperature simultaneously and the increased pellets bybed experiments. are simultaneously A more efﬁcientincreased process by experiments. and a drastic A moreimprovement efficient inprocess productivity and a anddrastic DRI improvement quality (metallization in productivity degree and density)DRI quality are the(metallization results of a degreetall pellets and bed density) and high are the temperature. results of a tall pellets bed and high temperature.

4.4. Economics and Challenges The PSH process also has some economic advantages compared with shallow bed bed processes, because (1) the metallization degree is higher, and th thee price of DRI is higher too; (2) the productivity of metallic metallic iron iron is is higher, higher, so sothe the cost cost per perunit unit should should be lower; be lower; and (3) and the (3)building the building cost of the cost straight of the furnacestraight furnaceis lower. is lower.However, However, as with as withany new any newproc process,ess, there there are are also also some some challenges challenges with with the development ofof thethe PSHPSH system.system. For For example, example, the the proper proper refractory refractory materials materials are are necessary necessary due due to the to thehigh high temperature temperature in the in oxidation the oxidation compartment. compartment. Therefore, Therefore, there is athere lot of is work a lot to of do work for researchers to do for researchersin the ironmaking in the ironmaking field before thefield actual before commercial the actual commercial application ofapplication the PSH system. 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) WhenWhen the the height height of of the the pellets pellets bed bed is 80 80 mm mm (16–18 mm each layer,layer, andand 55 layers),layers), C/OC/O = 0.80 0.80 may may resultresult in in a a lower lower metallization metallization degree due to an insufficientinsufficient reductant,reductant, andand C/OC/O = 1.10 1.10 may may result result in more residual carbon in the DRI of the bottom layer. Therefore, under the present

Minerals 2018, 8, 550 14 of 15

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. (2) 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). (3) 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

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