Effect of Coal Type on the Reduction and Magnetic Separation of a High-Phosphorus Oolitic Hematite Ore

ISIJ International, Vol. 55 (2015),ISIJ International,No. 3 Vol. 55 (2015), No. 3, pp. 536–543

Effect of Coal Type on the Reduction and Magnetic Separation of a High-phosphorus Oolitic Hematite Ore

Wen YU, Tichang SUN,* Qiang CUI, Chengyan XU and Jue KOU

School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083 China. (Received on July 8, 2014; accepted on December 5, 2014)

Coal-based direct reduction followed by magnetic separation technique was employed to produce direct reduction iron powder (DRI powder) from a high-phosphorus oolitic hematite ore, and the effects of type and particle size of coal and C/O (Fixed carbon/Oxygen) mole ratio on this process were investigated. The results showed that when using coarse-sized and medium-sized coals, bitumite and lignite presented bet- ter iron recovery as compared with anthracite, while this advantage disappeared as the particle size of coals decreased. In addition, the iron content of the DRI powder increased with improving of the coal rank depended on fixed carbon contents, while decreased with the decrease of particle size of coals. Increasing the C/O ratio resulted in a sharp rise of P content of the DRI powder. X-Ray Diffraction (XRD) analysis revealed that more liquid phase was formed in the briquettes during reduction with anthracite as reductant. SEM (scanning electron microscope) observation confirmed that the size of the metallic iron grains formed in reduced briquettes decreased with decreasing of the rank of the coal and the particle size of coal.

KEY WORDS: oolitic hematite; direct reduction; magnetic separation; coal type.

A variety of carbonaceous materials such as graphite, 1. Introduction coke, coal, coal char, biomass, and plastics have been Oolitic iron ore is one of the most important iron employed as reductant for reducing high-grade iron ores by ore resources, which widely exists in France, Germany, researchers.8–11) It is generally recognized that the overall rate America, Canada, China etc.1,2) However, it has not been of carbothermal reduction reaction of hematite is controlled exploited around the world since its unique oolitic structure by the gasification of coal, i.e. Boudouard reaction, and the and generally contains high levels of phosphorus that resist reduction can be improved by using reductant with higher to conventional mineral processing methods. Along with the reactivity or adding Boudouard reaction catalyst.10,12,13) fast depletion of easy-to-process iron ores, exploitation of Although lots of studies have been done on the influence this refractory iron ore becomes more and more important. of reductant type on the reduction of iron oxides, few on Coal-based direct reduction followed by magnetic separa- the effect of reductant type on the reduction-magnetic pro- tion process has been employed to produce DRI powder cess of the low grade iron ores. The major concerns of that from oolitic iron ore in some research.3–5) This technology process is not only reducing iron oxides to metallic phase, combines metallurgy and mineral processing technology but also transforming the finely-divided metal into particles together, in which iron oxides are primarily reduced to which are amenable to separation.3) Xu7) and Li6) conducted metallic iron with grain growth, and then the reduction the experiments of the effect of coal type on direct reduc- roasted product was ground to liberation size followed by tion and phosphorus removal of high-phosphorus oolitic magnetic separation. The iron product obtained by this pro- hematite ore, but in their experiments the amount of coals cess generally contains more than 90 mass% Fe, which was was calculated on the mass ratio of ore/coal rather than the expected to be a good substitute of steel scrap in electric arc molar ratio of C/O (Fixed carbon/Oxygen). It is well known furnaces for steelmaking. Iron recovery, which is around 90 that the fixed carbon in the coal is the major content contrib- mass%, is much greater than that achieved by conventional utes to the reduction of iron ore, so in that case the effect means. Another attractive feature of this process is that of coal type cannot be revealed accurately. In the study of untreated non-coking coal was used as reductant instead of Zhang, the calculation method of coal consumption was not coke. It was shown in the concerned literatures that different described.14) Four types of reducing agent were employed by types of coal, from lignite to anthracite with the fixed carbon S. Weissberger and Y. Zimmels3) for the direct reduction of content range from 30 mass% to 90 mass%, have been used Ramin oolitic iron ore, and significant differences of the iron for the reduction of low grade iron ores.6,7) contents in concentrates and iron recoveries were observed, however the mechanism of the effect of reductant type on * Corresponding author: E-mail: [email protected] this process was not studied. DOI: http://dx.doi.org/10.2355/isijinternational.55.536 The objective of this paper is to explore the influence of

© 2015 ISIJ 536 ISIJ International, Vol. 55 (2015), No. 3 coal type on the reduction and magnetic separation of a high coal, and water (8~12 mass%) were mixed together, and phosphorus oolitic hematite ore, the effects of particle size then the mixture was pressed to form briquette using a die of coal and C/O mole ratio were studied as well. with a size of 30 mm diameter with the aid of hydraulic equipment. The amount of reductant was calculated based on the mole ratio of the fixed carbon to removable oxygen 2. Experiment of the iron oxides (C/O ratio). The mixing ratios of iron ore 2.1. Materials and each coal are shown in Table 2 at the C/O ratio of 1.0. The high-phosphorus oolitic hematite ore used in this The reduction roasting was carried out in a muffle fur- study was obtained from Hubei Province, Iron ore assayed nace with a temperature control programmer. In a run two 43.58 mass% Fe, 0.04 mass% FeO, 0.83 mass% P, 17.10 briquettes were put in a graphite crucible with a cup of 70 mass% SiO2, 9.28 mass% Al2O3, 3.58 mass% CaO and 0.59 mm in diameter and 75 mm in height. The graphite crucibles mass% MgO. The detail information of the iron ore has been with sample were put into the furnace after the furnace tem- described in a previous paper.4) The iron ore was crushed to perature reached 1 200°C and held for 40 min. The holding 100 mass% passing 1 mm. time was determined by referring to the previous papers Three kinds of coal with different rank, namely lignite, which revealed that 40 min of holding time was enough bitumite and anthracite in the ascending order, were used as for the reduction of the composite briquette at the tempera- reductant. These coals were crushed and screened to three ture of 1 200°C and extending holding time may cause the size fractions: coarse-sized (−0.9~+0.6 mm), medium-sized reoxidation of the reduced briquette.4,15) When the crucible (−0.45~+0.1 mm) and fine-sized (−0.074 mm). Each size was put in the furnace, the furnace temperature decreased of the three coals was analyzed, respectively, and the results about 100°C and it recovered in about four minutes. After are given in Table 1. It can be seen that the nature of differ- 40 min, the graphite crucible were taken out of the furnace ent particle sizes of the same coal were basically the same. and cooled to room temperature under ambient atmosphere. The schematic of the muffle furnace, crucibles and the bri- 2.2. Composite Briquette Formation, Reduction and quette used is given in Fig. 1. Separation After the reduced briquettes had cooled in the crucible, The mixture of iron ore and coal was pressed to form they were crushed to −2 mm and then treated by two-stage briquette before subjecting to reduction roasting by the fol- grinding and wet magnetic separation. The grinding experi- lowing procedures: first, 20 g iron ore, a certain amount of ments were conducted in a rod mill (RK/BM-1.0L, Wuhan Rock Crush & Grind Equipment Manufacture Co., Ltd, China) having ten Φ15 mm×120 mm rods at 60 mass% Table 1. The proximate analysis of coals (mass%). solid density and with a speed of 192 r/min. The first stage grinding time was 10 min. The XCGS-73 magnetic tube Coal type Particle size Cfix Moisture Ash Volatile S with a magnetic field intensity of 1 120 Oe was used to −0.9~ + 0.6 mm 83.78 1.21 6.20 8.81 0.07 recover metallic iron from the slurry. The magnetic products Anthracite −0.45~ + 0.1 mm 83.40 1.47 6.42 8.71 0.11 obtained from the first separation were reground for 40 min 15) −0.074 mm 81.86 1.22 7.56 9.36 0.12 and separation. The main evaluation indexes of test results −0.9~ + 0.6 mm 56.69 6.63 11.93 24.75 0.12 were the iron content, P content and the iron recovery of DRI powder. The iron recovery was calculated as follows: Bitumite −0.45~ + 0.1 mm 56.56 6.77 13.09 23.58 0.13 Iron recovery= −0.074 mm 54.92 6.55 14.4 24.13 0.12 Weight of DRI powderi∗ ronc ontento fD RI powdder −0.9~ + 0.6 mm 38.52 10.31 4.82 46.35 0.20 ∗100% Weight of rawi ore∗ ronc ontento fr aw ore Lignite −0.45~ + 0.1 mm 38.13 10.27 5.43 46.17 0.20 −0.074 mm 37.68 10.20 6.15 45.97 0.20

Table 2. Mixing ratio of iron ore and each coal in the briquette at the C/O ratio of 1.0.

Coal type Coal size Iron ore : Coal (mass ratio) −0.9~ + 0.6 mm 100 : 16.72 Anthracite −0.45~ + 0.1 mm 100 : 16.80 −0.074 mm 100 : 17.11 −0.9~ + 0.6 mm 100 : 24.71 Bitumite −0.45~ + 0.1 mm 100 : 24.77 −0.074 mm 100 : 25.51 −0.9~ + 0.6 mm 100 : 36.37 Lignite −0.45~ + 0.1 mm 100 : 36.74 Fig. 1. Schematic of the muffle furnace, crucibles and the bri- −0.074 mm 100 : 37.18 quette used in the experiments.

2.3. Analysis and Characterization of the others’. It means that the effect of coal size on the The chemical analyses were conducted by China Uni- iron recovery varied as the coal type. When decreasing the versity of Geosciences (Beijing) analysis laboratory. X-ray particle size of anthracite from coarse-sized to medium- diffraction (XRD, Rigaku DMAX-RB, Japan) using Cu Kα sized and then to fine-sized, the iron recovery increased radiation and a secondary monochromator were used to significantly from 65.95 mass% to 83.10 mass% and then identify the formed phases; the samples were scanned over to 91.97 mass%; however, decreasing the particle sizes of the 2θ range of 10° to 90°. Scanning Electron Microscope bitumite and lignite from coarse-sized to medium-sized, iron with Energy Dispersive Spectrum (Carl Zeiss EVO18) recovery just increased slightly from 90.95 mass% to 91.81 analyses were carried out on reduced briquettes mounted in mass% for bitumite, and 88.33 mass% to 88.41 mass% for epoxy resin and polished. lignite, respectively, and the further decrease of particle size of those two coals resulted in slight decrease of iron recovery. 3. Results Figure 2(b) illustrates the effects of coal type and coal 3.1. Effect of Coal Types on the Reduction Followed by particle size on the iron content of the DRI powder. It is Magnetic Separation of Briquettes evident that when the same size of different coals were used, The effect of coal type on the reduction followed by mag- the iron content of the DRI powder resulted from anthracite netic separation of high-phosphorus oolitic hematite ore was performing the best result, while DRI powder obtained from investigated. The experiments were performed at conditions lignite reduction had the lowest iron content. This confirmed of roasting temperature 1 200°C, roasting time 40 min and that iron content of the DRI powder products increased C/O ratio 1.0. The experimental results are shown in Fig. 2. correspondingly with the rank of the coal depended on Figure 2(a) shows the influence of coal type and coal size fixed carbon contents. Furthermore, for these three types of on the iron recovery. Compared with anthracite, bitumite coal, decreasing particle size all led to the decrease of iron and lignite presented an advantage on iron recovery when content of DRI powder. DRI powder with the highest iron coarse-sized and medium-sized coals were used; however, content of 94.55 mass% was obtained with coarse-sized when fine-sized coals were used, the iron recovery obtained anthracite as reductant and the lowest iron content of 83.50 by using anthracite as the reductant was higher than that mass% was obtained with fine-sized lignite.

Fig. 2. Effects of coal type and particle size on the reduction and separation of briquettes.

Figure 2(c) demonstrates the effects of type and particle recovery increased from 50.84 mass% of 0.4 C/O ratio dis- size of coal on the P content of magnetic product. With the tinctly to 90.87 mass% of 0.8 C/O ratio, further increasing decrease of anthracite size, P content increased from 0.12 of C/O ratio to 1.0 did not contribute to significant increase mass% of coarse-sized to 0.21 mass% of medium-sized, of iron recovery. Using lignite as reductant, the recovery and then to 0.27 mass% of fine-sized; with the same size of iron increased from 50.25 mass% of 0.4 C/O ratio to decrease, P contents were 0.25 mass%, 0.23 mass% and 80.26 mass% of 0.6 C/O ratio, and further increase of C/O 0.25 mass% with bitumite, and 0.27 mass%, 0.20 mass% ratio led to slight improvement of iron recovery. At the and 0.24 mass% with lignite. These results supported that C/O ratio of 0.4 and 0.6, the iron recoveries obtained from the P of the DRI powder cannot be reduced to a low level bitumite and lignite were almost the same and considerably without additives, and the law of the effects of type and higher than that of anthracite; however, this advantage of particle size of coal on the P content of DRI powder are lignite and bitumite was gradually lost as C/O ratio further not obvious. increased with iron recoveries of bitumite kept higher than that of lignite. 3.2. Effect of Coal Dosage on the Reduction Followed Figure 3(b) shows that, (1) the iron content of the DRI by Magnetic Separation of Briquettes powder obtained by using anthracite as reductant was higher In order to evaluate the influence of coal dosage on the than that of the one prepared by using bitumite, and the DRI recovery of iron, a series of experiments were carried out powder achieved by using lignite as reductant performed by varying the C/O ratio from 0.4 to 1.0, fine-sized coals the lowest iron content, these results are in line with those were used, and the roasting temperature and time were of shown in Fig. 2(b); (2) when anthracite was used, the iron 1 200°C and 40 min, respectively. content increased from 94.01 mass% to 95.36 mass% along Figure 3(a) shows that, as the C/O ratio varied from 0.4 with C/O ratio rose from 0.4 to 0.8, and then dropped to to 1.0, the iron recovery exhibited a significant improve- 91.50 mass% as C/O ratio increased to 1.0; when bitumite ment, from 45.51 mass% to 91.97 mass%, when anthracite was used, the iron content of the DRI powder increased was used as a reductant. When bitumite was used, the iron from 92.75 mass% to 94.37 mass% with increasing of C/O

Fig. 3. Effects of coal dosage on the reduction and separation of briquettes.

539 © 2015 ISIJ ISIJ International, Vol. 55 (2015), No. 3 ratio from 0.4 to 0.6, and further increase of C/O ratio to 1.0 observed on all these three coals. In addition, the P content resulted in obvious decrease of iron content of DRI powder of the DRI powder resulted from lignite is similar to that to 85.39 mass%; (3) when using lignite as reductant, the from the bitumite, and their P contents are higher than that iron content of the DRI powder decreased linearly from from anthracite at the C/O ratio of 0.4 and 0.6. But opposite 92.94 mass% to 83.50 mass% with increase of C/O ratio result was achieved as the C/O increased. from 0.4 to 1.0. Figure 3(c) reveals that the P content of the DRI pow- 4. Characteristics of Roasted Briquettes der products increase with increasing of C/O ratio from 0.4 to 0.8 for all types of reductants. This phenomenon 4.1. XRD Analysis may stemmed from two aspects: 1) the reduced rate of XRD was used to study the effects of coal type and coal fluorapatite was improved by increasing the C/O ratio, and particle size on the phase transitions in the briquettes. The the increase of reduction rate promotes P melting into the C/O ratio was kept at 1.0, and the roasting temperature and metallic iron; 2) more P-containing slag was mingled with time were of 1 200°C and 40 min, respectively. The results concentrate in some case as the C/O ratio increased, such are given in Fig. 4. as when lignite was employed, the iron content decreased As shown in Fig. 4(a), when anthracite was used as a with increasing of C/O ratio throughout the range studied reductant, a broad band indicating the presence of glass which indicated that more of slag was mingled with con- phase materials was observed between 10° and 40° in XRD centrate. For bitumite used product, the same issue existed patterns. Therefore, it could be inferred that there was cer- as the C/O ratio excessed 0.6. As the C/O ratio increased tain amount of liquid phase formed in the briquettes during to 1.0, slightly drop of P content of the DRI powder were reduction process. Furthermore, as the particle size of coal

Fig. 4. XRD patterns of the reduced briquettes with different coals as reductant: (a) with anthracite; (b) with bitumite; (c) with lignite.

© 2015 ISIJ 540 ISIJ International, Vol. 55 (2015), No. 3 decreased, the broad band reduced, while the intensity of the 4.2. SEM Observation patterns of metallic iron and quartz increased significantly, The reduced briquettes studied in section 4.1 were pol- indicating that the amount of the liquid phase decreased and ished and observed by SEM. The results were shown in the reduction of iron oxides was promoted along with the Fig. 5. reduction of anthracite particle size. However, the broad It can be observed that the melting degree of reduced band was almost not observed in the XRD patterns of the briquettes went up significantly with increase of the rank reduced briquettes with bitumite (Fig. 4(b)) and lignite (Fig. and particle size of coal. These results were consistent with 4(c)) as reductants, and also the intensity of the patterns XRD results shown in Fig. 5. Moreover, the particle size of of metallic iron and quartz increased very slightly as the metallic iron (white) decreased as the reductants used in the particle sizes of those two coals decreased. These results order of anthracite (Fig. 5(a)–5(c)), bitumite (Fig. 5(d)–5(f)) confirmed that, compared with briquettes using anthracite and lignite (Fig. 5(g)–5(j)). When the same coal was used, as reductant, fewer of liquid phase formed in those when reducing particle size of coal results in the decrease of the bitumite and lignite were used as reductants. It also exhib- particle size of iron grains. These results explained why the ited that the reduction of iron oxides were almost complete iron content of the DRI powder decreased as the decrease when those coals were coarse-size. of the coal rank as well as the particle sizes of coals. Addi-

Fig. 5. SEM images of the reduced briquettes with different reductants: (a)–(c) coarse-sized, medium-sized and fine-sized anthracite; (d)–(f) coarse-sized, medium-sized and fine-sized bitumite; (g)–(i) coarse-sized, medium-sized and fine-sized lignite; (j) Partial enlarged view of the (i).

541 © 2015 ISIJ ISIJ International, Vol. 55 (2015), No. 3 tionally, it can be seen from Fig. 5(j) that, when fine-sized hindered since very little of low-melting substances gener- lignite was used as reductant, many ultrafine metallic iron ated (Fig. 4(c)). The reactivity of bitumite ranks between grains were formed which are difficult to recover via mag- that of lignite and anthracite, therefore the amount of low- netic separation process, and this may explain why using melting substances produced in the briquettes with bitumite fine-sized lignite led to a decrease of iron recovery. as reductant ranks between that with lignite and anthracite as reductants. These explained why the iron content of DRI powder increased as coal rank increased. 5. Discussion Additionally, Table 1 shows that the content of volatile It is well known that the reduction of hematite by coal decreased with increasing of coal rank, the volatile mat- proceeds mainly through gaseous intermediates of CO and ters contained in the coal also can strengthen the reducing 16) CO2. The reaction sequence can be represented as follows: atmosphere during reduction process. Whereas the contribu- tion of volatile matters on the reduction of iron oxides can 3Fe2O3+CO → 2Fe3O4+CO2 ...... (1) be negligible since they almost completely released before

Fe3O4+CO → 3FeO+CO2 ...... (2) the sample temperature reaching an active temperature of reduction reaction, and this also confirms that the reactiv- FeO+CO → Fe+CO2 ...... (3) ity of coal plays a critical role in the reduction process. Particle size reduction of coal has a great effect on increas- C+CO2 → 2CO ...... (4) ing reactivity,20) and consequently promotes the reduction The reduction of FeO to Fe, which requires a stronger of iron oxides and inhibits the growth of the iron grains, reducing atmosphere, is more difficult to process than and this explained why the iron content of the DRI powder the reaction (1) and (2).17) Furthermore, since the raw ore decreased as the particle size of coal reduced. In addition, contains a large number of SiO2, Al2O3, and CaO, part of the iron recovery was just 65.95 mass% when coarse-sized the FeO may react with SiO2 to form fayalite which has a anthracite was used at the C/O ratio of 1.0 as shown in Fig. low melting point and was harder to be reduced than FeO.5) 2(a). It is because the reduction of iron oxides was hindered Moreover, the fayalite will react with other oxides contained in the weak reducing atmosphere. in the ore to generate molten phase.18) The ash content of the bitumite was higher than that of It is worth mentioning that the low melting point materi- anthracite and lignite as shown in Table 1. The coal ash als play an essential role in the growth of the metallic iron mainly consists of SiO2, Al2O3 and CaO which is similar particles which can facilitate the transfer of iron phase. As to that of the gangue in the iron ore, the ash content of the noted above, the coarsening of the iron grains in the reduced coal was considered not conducive to the reduction of high ore is essential for the effective separation of iron and slag. grade iron concentrate since these oxides react with FeO to That is to say, although the formation of iron-rich slag form some refractory substances.17) However, the absolute decreased the metallization rate of the reduced ore, it can content of ash contained in these coals is much less when promote the separation of metallic iron and slag by improv- compared with the gangue in low grade iron ore. Therefore, ing the coalescence of iron grains. When fixing the roasting it can be inferred that the effect of coal ash on reduction of conditions and the composition of the briquettes, the amount oolitic iron ore can be negligible. of low-melting substances formed in the briquettes during Increasing the C/O ratio can also enhance the reducing reduction process depends on the environment atmosphere. atmosphere, and suppressed the production of low smelt- The stronger reducing atmosphere, the higher metallization ing point materials. It can be seen from Fig. 2(b) that when rate and smaller amount of low-melting substances will bitumite and anthracite were used as reductants, the iron form, and the coarsening of metallic iron will be inhibited, content of the DRI powder increased with increasing of which in turn will decrease the iron content of the DRI C/O ratio firstly and then decreased with further increasing powder. In the present study, the environmental atmosphere of C/O ratio. This may be explained as follows: when the surrounds the briquettes and within it was determined by C/O ratio keep at low level, the particle size of iron grain the gasification reactivity of the reductant, coal size, and increased with the C/O ratio increasing since more of metal- C/O ratio. lic iron was formed. However, as the C/O ratio excess some It has been widely accepted that the gasification reactivity level, the growth of iron grains will be inhibited because of of coal decreases as coal rank increases, namely the reactiv- the amount of low melting point materials decreased as the ity of lignite, bitumite and anthracite ranks in the descend- C/O ratio increased. ing order.19) Using the coal with high reactivity as reducing In summary, the major concerns of coal-based reduction- agent can improve the Boudouard reaction and provide a magnetic process for recovering DRI powder from the high- strong reducing atmosphere. Therefore, when anthracite, phosphorus oolitic hematite ore is reducing the vast most of which owns the lowest reactivity among these three reduc- iron oxides to metallic iron while promoting the coalescence tants, was used as a reducing agent, more FeO will react of iron grains by sacrificing a few amount of FeO to form with gangue to form low melting point materials since low-melting materials. the weak reducing atmosphere was provided. As a result, the coalescence of metallic iron particles was promoted, 6. Conclusions which improved the separation of metallic form slag consequently. However, when lignite, which presents the highest In this study, coal-based direct reduction followed by reactivity, was used as reductant, although the reduction of magnetic separation technique was employed to extract hematite was improved, the coalescence of metallic iron was DRI powder from high-phosphorus oolitic hematite ore,

© 2015 ISIJ 542 ISIJ International, Vol. 55 (2015), No. 3 the effects of coal type, coal size and the C/O ratio on the promoted. process were investigated at the temperature of 1 200°C. From these investigations, the following conclusions can Acknowledgement be drawn. The authors wish to express their thanks to the Natural (1) Generally, the recovery of iron can be improved by Science Foundation of China (No. 51134002) for the finance using coal with high reactivity and smaller size. However, support for this research. these measures also decreased the iron content of the DRI powder for strengthen of the reducing atmosphere inhibited REFERENCES the coalescence of reduced iron, which in turn deteriorates 1) J. B. Maynardand and F. B. Van Houten: Descriptive model of oolitic the conditions of separation of metallic iron and slag. ironstones, U. S. Geological Survey, Washington, D.C., (1992), 39. 2) Y. Zhao and C. Bi: Mineral Deposits, 19 (2000), 350 (in Chinese). (2) The effect of coal size on recovering DRI pow- 3) S. Weissberger and Y. Zimmels: Int. J. Miner. Process., 11 (1983), der weakens with decrease of the coal rank. Particle size 115. decrease of anthracite has a considerable effect on increas- 4) W. Yu, T. Sun, J. Kou, Y. Wei, C. Xu and Z. Liu: ISIJ Int., 53 (2013), 427. ing the iron recovery, while decreasing the particle sizes 5) Y. Li, T. Sun, J. Kou, Q. Guo and C. Xu: Miner. Process. Extra. of bitumite and lignite results in slight changes of iron Metall. Rev., 35 (2014), 66. 6) Y. Li, T. Sun and C. Xu: Min. Metall. Eng., 32 (2012), 66 (in recovery. Chinese). (3) The major concerns of coal-based reduction-mag- 7) C. Xu, T. Sun, C. Qi, Y. Li, X. Mo, D. Yang, Z. Li and B. Xing: netic process for recovering DRI powder from the oolitic Chin. J. Nonferrous Met., 21 (2011), 680. (in Chinese) 8) G. Wang, Y. Ding, J. Wang, X. She and Q. Xue: Int. J. Miner. Metall. iron ore is reducing the vast most of iron oxides to metallic Mater., 20 (2013), 522. iron and promoting the coalescence of reduced iron by sac- 9) T. Sharma: Int. J. Miner. Process., 39 (1993), 299. 10) J. S. J. Van Deventer and P. R. Visser: Thermochim. Acta, 111 rificing a few amount of FeO to form low-melting materi- (1987), 89. als. If the reducing atmosphere is too strong, very little of 11) Y. Ueki, R. Mii, K. Ohno, T. Maeda, K. Nishioka and M. Shimizu: ISIJ Int., 48 (2008), 1670. low-melting materials will be formed, and also the metallic 12) T. Coetsee, P. C. Pistorius and E. E. de Villiers: Miner. Eng., 15 iron grains formed will be very small, which will not only (2002), 919. results in the decrease of iron content of DRI powder, but 13) J. Moon and S. Veena: Metall. Mater. Trans. B, 37 (2006), 215. 14) Q. Zhang, G. Qiu and Q. Xiao: J. Cent. South Univ. Technol., 28 also reduces the iron recovery. If the reducing atmosphere (1997), 126 (in Chinese). is too weak, iron recovery decreased since the reduction of 15) W. Yu, T. Sun, Z. Liu, J. Kou and C. Xu: ISIJ Int., 54 (2014), 56. 16) Y. K. Rao: Metall. Tran., 2 (1971), 1439. iron oxides was hindered. 17) R. J. Fruehan: Metall. Trans. B, 8 (1977), 279. (4) The law of the effects of type and particle size 18) M. T and I. T: J. MMIJ, 116 (2000), 141. of coal on the P content of DRI powder is not obvious. 19) K. C. Xie: Coal structure and its reactivity, Science Press, Beijing, (2002), 290 (in Chinese). Increasing the C/O ratio resulted in the increase of P content 20) E. Hippo and P. L. Walker, Jr.: Fuel, 54 (1975), 245. of the DRI powder since the reduction of fluorapatite was