Reaction Behavior of Iron Minerals and Metallic Iron Particles Growth

Article pubs.acs.org/IECR

Reaction Behavior of Iron Minerals and Metallic Iron Particles Growth in Coal-Based Reduction of an Oolitic Iron Ore Yongsheng Sun,* Peng Gao, Yuexin Han,* and Duozhen Ren College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China ABSTRACT: The reaction behavior of iron minerals and growth of metallic iron particles in coal-based reduction of an oolitic iron ore were investigated by X-ray diffraction (XRD) and scanning electron microscopy/enegry dispersive spectroscoy (SEM/ EDS), respectively. The results show that, the reduction of iron minerals occurred in the sequence of Fe2O3 → Fe3O4 → FeO → Fe. During the reduction, the main reducing reactions were indirect and the direct reactions that would also be crucial only occurred at the early stage. Because of high contents of SiO2 and Al2O3 in the ore, the solid phase reactions of FeO with SiO2 and Al2O3 took place. Small amounts of metallic iron (or FeO) were reduced from the ore in irregular shape and formed spherical particles when reduction time increased to 10 min. With reduction time increasing, the Fe diffused from small metallic particles toward the larger ones and coalesced with them. The whole reduction process could be analytically derived for the stages of chemical reaction, nucleation, and crystal growth.

1. INTRODUCTION Oolitic iron ore, which distributes mainly in France, the USA, Canada, Egypt, the former Soviet Union, and China, is an important existing form of iron ores.1 From the published literature, oolitic iron ore reserves amount to about 140 million tons in Europe, 10 billion tons in China, and 66 million tons in Pakistan.2,3 Efforts to exploite oolitic iron ore have been made for nearly 100 years. Several kinds of oolitic iron ores have been upgraded by washing and calcining, gravity, and high-intensity magnetic separation.4−6 However, no satisfactory mineral processing methods have been developed because of the lowgrade of iron (35−50 wt %), poor liberation of iron minerals, high grade of phosphor, and so on. In Europe, research into oolitic iron ore beneficiation was given up. In recent years, the increasing demand of iron ore and the dwindling trend of high grade iron ore reserves are so high that renewed interests in using refractory iron ore (especially oolitic iron ore). Therefore, research on exploiting oolitic iron ores were carried out actively. The studies indicate that the concentration of iron oxide minerals by traditional magnetic or flotation separation methods is impossible.7,8 Several researchers have reported technique of magnetizing processes by roasting-reduction in the temperature range of 973−1173 K, but most of them also do not have a satisfied recovery.9−11 Nevertheless, coal-based reduction followed by magnetic separation is proved to be an effective technology for recovering iron from oolitic iron ores, which was proposed by Northeastern University. According to this technology, coal is used as a reductant to reduce iron ore and the iron minerals in the ore are directly reduced to metallic iron. So the existential state of iron is completely changed, then metallic iron is concentrated by magnetic separation. Good results (recovery of iron ≥ 90% and content of iron ≥ 90%) have been achieved in laboratory batch tests when it was used to treat several refractory iron ores. For example, Li and Gao tested the recovery of iron from oolitic iron ore and oxide ores of Bayan Obo deposit by coal-based reduction and magnetic separation, © 2013 American Chemical Society

and determined the main factors influencing the reduction by test work.12,13 Although many literature works have used coal-based reduction followed by magnetic separation to recover iron from refractory iron ore resources, most of them focused on the conditions of reducing process and magnetic separation. In the current paper, the changing behaviors of iron minerals and particles growth of metallic iron during the processes were studied particularly. The objective of this paper is to reveal the mechanisms of the reduction of oolitic iron ore.

2. EXPERIMENTAL SECTION 2.1. Materials. The oolitic iron ore used for the study was taken from an iron mine in western Hubei Province, China. The output size of the run of mine was 200 mm. The chemical composition of the ore is shown in Table 1, which indicates that the primary element is iron and the predominant impurity is SiO2. Phosphorus and sulfur are the harmful elements. The iron minerals in the ore are listed in Table 2. It can be seen that Fe is mainly distributed in hematite and limonite. The scanning electrom microscopy (SEM) image of the iron ore is illustrated in Figure 1, the results of energy dispersive spectroscopy (EDS) measured at different points are also presented. As shown in Figure 1, it can be found that the surface of iron ore particles is smooth, and the iron minerals in the ore associated closely with gangue minerals. The iron ore particles can be seen as a homogeneous phase. A common bituminous coal was used as the reductant. The chemical composition of the coal is given in Table 3. It indicates that the soft coal used in this research is a suitable reductant with low contents of ash and harmful elements, namely S and P, and high contents of fixed carbon and volatile. Received: Revised: Accepted: Published: 2323

November 23, 2012 January 14, 2013 January 23, 2013 January 23, 2013 dx.doi.org/10.1021/ie303233k | Ind. Eng. Chem. Res. 2013, 52, 2323−2329

Industrial & Engineering Chemistry Research

Article

Table 1. Chemical Composition of Oolitic Iron Ore (mass %) Constituents

Fe

FeO

SiO2

Al2O3

CaO

MgO

P

S

content

42.21

2.46

19.80

5.47

3.03

3.59

0.83

0.11

At the same time, the composition analysis was carried out by EDS on an Inca X-ray spectrometer combined with the SEM.

Table 2. Phase Composition of Iron Minerals (mass %) iron phase

magnetite

pyrite

siderite

hematite and limonite

distribution of Fe

0.11

0.04

0.13

40.11

iron silicate

total

1.82

42.21

3. RESULTS AND DISCUSSION 3.1. Behavior of Iron Minerals in Reduction. The XRD patterns of samples of the ore and reduced materials investigated under five different reduction times are presented in Figure 3A−F, respectively. From Figure 3A, it can be seen that the predominant iron mineral in the ore is hematite (Fe2O3), which is consistent with the data in Table 2, and the main impurity is SiO2. Therefore, only the reduction behavior of hematite (Fe2O3) was studied in the paper. According to Figure 3B, the new peaks of Fe, Fe3O4, and FeO appear, and Fe has the strongest peak, but the Fe2O3 peaks are weak, which indicates that Fe2O3 in the ore was partially reduced to Fe, Fe3O4 and FeO. When the reduction time reaches 10 min (Figure 3C), the Fe2O3 peaks disappear, and at the same time the Fe peaks are enhanced. However, the peaks of Fe3O4 and FeO become very weak. These demonstrate that all the Fe2O3 was reduced to Fe, Fe3O4, and FeO. In addition, most of the Fe3O4 and FeO were reduced to Fe. When the reduction time increases to 20 min, all the peaks of Fe2O3, Fe3O4, and FeO fade away, and the iron element is mainly in the form of the metallic phase. The reduction process of iron oxide was studied by many researchers. Above 843 K, the reduction follows a series of stepwise reduction reaction (Fe2O3 → Fe3O4 → FeO → Fe).14−16 In this research, the reduction temperature was 1573 K, so the iron minerals in the ore were reduced following this sequence, and these step-by-step reactions also could be seen in Figure 3. According to the iron minerals reduction sequence and experimental methods, the reactions from iron oxides to metallic iron in the process of the reduction oolitic iron ore are given as follows:

The ore and coal were both crushed by a laboratory rolls crusher to particles (100% passing 2 mm) for the experiments. 2.2. Experimental Methods. The reduction experiments of oolitic iron ore were performed in a muffle resistance furnace as shown in Figure 2. The iron ore samples and coal (the reductant) were thoroughly mixed by rigorous stirring for approximately 30 min. The amount of coal addition in the mixture was determined according to C/O molar ratio of 1.5 (i.e., molar ratio of fixed carbon in the coal to reducible oxygen in iron oxides was 1.5). Each reduction experiment used 50 g iron ore sample and 17.65 g coal. The mixture was loaded into a ceramic crucible, and then a layer of coal (about 1 mm thickness and mass of 2 g) was put on upper surface of the mixture in crucible in order to ensure the reducing atmosphere within the crucible. The crucible was put into the furnace rapidly when the temperature of the furnace reached the reduction temperature (1523 K). The temperature was kept for predetermined time ranging from 5 to 40 min. In addition, purge gases were not utilized and experiments were run at atmospheric conditions. After the reaction, the reduced materials were cooled to room temperature by water quenching, and then superfluous coals in the materials were taken off by tabling. The products were dried at 353 K in a vacuum oven for 2 h. The samples were obtained and analyzed by XRD and SEM. 2.3. Analytical Tests. The existent form of iron minerals at different reduction time was investigated by XRD. The XRD measurements were carried out on a Japan Science D/max-RB diffractometer with Cu Kα radiation (λ = 1.541 Å), at an operating voltage of 40 kV and a current of 40 mA. The diffraction angle (2θ) was scanned from 10° to 90°. The morphology and particles growth were studied using SEM on a SSX-550 scanning electron microscope, produced by Shimadzu.

3Fe2O3(s) + C(s) = 2Fe3O4(s) + CO(g) Δr Gmθ = [12442224.37(T /K )] J ·mol−1

(1)

Figure 1. SEM images and EDS energy spectra of iron ore. 2324

dx.doi.org/10.1021/ie303233k | Ind. Eng. Chem. Res. 2013, 52, 2323−2329


Article

Table 3. Proximate Analysis of the Coal (mass %)a

a

FCd

Vd

Ad

Mad

P

S

Al2O3

SiO2

CaO

56.10

30.40

5.44

1.48

0.003

0.022

0.57

1.27

1.83

Mad: Moisture (dry basis). Ad: Ash (dry basis). Vd: Volatile matter (dry basis). FCd: Fixed carbon (dry basis).

2FeO(s) + SiO2(s) = Fe2SiO4(s)

(8)

2FeO(s) + 5SiO2(s) + 2Al 2O3(s) = Fe2Al4Si5O18(s)

(9)

At 20 min, the peaks of Fe2SiO4 disappear and Mg2SiO4 peaks turn up (as shown in Figure 3D). The peaks of Mg2Al4Si5O18 and Ca2Al4Si5O18 appear and Fe2Al4Si5O18 peaks fade away, when the reduction time is longer than 30 min, as shown in Figure 3E and F. Those are because alkaline oxides (MgO and CaO) in the ore could displace the FeO in Fe2Al4Si5O18 and Fe2SiO4, then the FeO was reduced by carbon. These changes demonstrate that the following reactions occurred. Figure 2. Schematic diagram of the reduction apparatus: (1) ceramic crucible; (2) mixture of ore and coal; (3) heating element; (4) furnace; (5) protecting layer (coal); (6) thermocouple for measurement; (7) reaction room; (8) control cabinet.

2MgO(s) + Fe2SiO4(s) + 2C(s) = 2Fe(s) + Mg 2SiO4(s) + 2CO(g)

Fe3O4(s) + C(s) = 3FeO(s) + CO(g) Δr Gmθ

2MgO(s) + Fe2Al4Si5O18(s) + 2C(s) −1

= [196720 − 199.38(T /K )] J ·mol

(2)

= 2Fe(s) + Mg 2Al4Si5O18(s) + 2CO(g)

FeO(s) + C(s) = Fe(s) + CO(g) Δr Gmθ = [149600 − 150.36(T /K )] J ·mol−1

Δr Gmθ = [−42121 − 53.37(T /K )] J ·mol−1

= 2Fe(s) + Ca 2Al4Si5O18(s) + 2CO(g)

(5)

FeO(s) + CO(g) = Fe(s) + CO2(g) Δr Gmθ = [−16950 + 20.64(T /K )] J ·mol−1

(12)

It could be known by overall analysis that, in the reduction process, the iron minerals in the ore were reduced to metallic iron under the step-by-step reactions sequence. The indirect reactions played an important role and the direct reactions only took place at the beginning. The reactions of iron oxides to metallic iron originated meanwhile the solid phase reactions between iron oxides and gangues occurred. 3.3. Growth of Metallic Iron Particles in Reduction. In order to understand the growth behavior of metallic particles in the reduction process, the tests at different reduction time were conducted. The morphological features of reduced products from SEM and EDS analyses are shown in Figure 4. From the analyses of spots 1−10, the predominant element of bright particles is Fe and the other part is mainly based on Si, Al, Mg, and Ca, which shows that the bright particles are metallic iron and other parts are matrix. Combined with Figure 1, it can be found that the homogeneous particles of oolitic iron ore particles were changed into metallic iron phase and slag matrix phase during the reduction. The metallic iron gradually grew into spherical metallic iron particles with the reduction time increasing. At the beginning of the reduction, metallic iron (or FeO) was reduced from the iron oxides in the ore and randomly distributed on the matrix (as shown in Figure 4A). With more reduction time, the metallic iron gradually moved together and took the shape of spherical particles (as shown in Figure 4B). This rearranged phenomemon was developed with maintaining the low surface energy of the particles.23 With the further extension of reduction time, the Fe gradually diffused from tiny particles into the larger one. The iron particles continuously grew up (as shown in Figure 4C−E).

(4)

Fe3O4(s) + CO(g) = 3FeO(s) + CO2(g) Δr Gmθ = [30170 − 29.38(T /K )] J ·mol−1

(11)

2CaO(s) + Fe2Al4Si5O18(s) + 2C(s)

(3)

3Fe2O3(s) + CO(g) = 2Fe3O4(s) + CO2(g)

(6)

C(s) + CO2(g) = 2CO(g) Δr Gmθ = [166550 − 171(T /K )] J ·mol−1

(10)

(7)

When coal was used as reductant, the solid phase reactions on the interface between coal and ore particles were direct.17 Reduction of iron oxides in a direct reduction system had been known to occur by gaseous phases, for example CO, rather than the solid carbon.18−21 Therefore, during the reduction, reactions 1−3 were direct, while reactions 4−6 were indirect. The indirect reactions between coal and iron oxides played a significant role, and the direct reactions only took place at the early stage but they would also be crucial. From Figure 3C, it is also found that, when the reduction time is 10 min, Fe2Al4Si5O18 and Fe2SiO4 were formed, which indicates that there were not only the reactions of iron oxides to metallic iron, but the solid phase reactions between iron oxides and gangues. The reason is that the content of SiO2 and Al2O3 in the ore was high, and the contacts between SiO2(Al2O3) and FeO were inevitable, as a result, the solid phase reactions occurred.22 The reactions are shown as follows: 2325



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Figure 3. XRD patterns of oolitic iron ore before and after reduction: (A) 0; (B) 5; (C) 10; (D) 20; (E) 30; (F) 40 min, where (▲) Fe; (■) Fe3O4; (★) FeO; (●) Fe2O3; (⧫) Fe2Al4Si5O18; (◊) Fe2SiO4; (☆) Mg2SiO4; (△) Mg2Al4Si5O18; (○) Ca2Al4Si5O18; (□) SiO2; (◎) Fe3.12((Si1.51Fe0.49)O5)(OH)4.

Dynamics studies had shown that reduction time was one of the dynamic conditions of metallic iron particles growth, and extending the reaction time was conducive to iron particles growth.24 According to the EDS energy spectra, it also can be found that, with the extension of reduction time, the iron peaks on the matrix surface gradually become weak and almost disappear when the reduction time is longer than 20 min, which shows that during the reduction process, the metallic iron particles grew up while the iron minerals were reduced to metallic iron.

It can be found by comprehensive analysis that the growth of metallic iron particles is as follows. First, small amounts of metallic iron (or FeO) were reduced from the iron oxides in the ore with irregular shape. Then, the metallic iron gradually moved together and formed spherical particles, and Fe diffused from the tiny particles toward the larger one. Finally, the metallic iron particles existed in the form of larger size spherical particles. During the growth of the metallic iron particles, the iron oxides were also reduced to metallic iron. 3.4. Analysis of Reduction Mechanism. On the basis of the analysis of behaviors of iron minerals and metallic iron 2326



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Figure 4. SEM images and EDS energy spectra of reduced materials at different reduction times: (A) 5; (B) 10; (C) 20; (D) 30; (E) 40 min. 2327



Article

chemical reaction stage, nucleation stage, and crystal growth stage.

particle growth in the reduction, the process of iron minerals reduced to metal and the particle growth could be divided into three stages: chemical reaction stage, nucleation stage, and crystal growth stage. In chemical reaction stage, at the beginning, the iron minerals on the surface of the ore were reduced to Fe (or FeO) by C and CO. When the iron minerals on the surface of the ore were almost reduced, because the CO was easier to go through the solid surface than C, the iron minerals located inside the ore was further reduced by CO. At this time, the chemical reaction stage was characterized by indirect reactions. In the nucleation stage, the metallic iron particle nucleation formed. Because of heterogeneous distribution of iron minerals in the ore, the distribution of metallic iron that earliest appeared on surface of the ore were not homogeneous, which reduced the surface energy barrier of nucleation. Therefore, some metallic iron atoms assembled and formed atomic clusters on the matrix. As the reaction proceeding, a large amount of metallic iron atoms were produced. According to the principle of minimum free energy, Fe diffused to the atomic clusters and small amounts of Fe particles formed. These particles would become the nuclei of iron crystal. In crystal growth stage, the metallic iron particle grew quickly. Two driving forces are responsible for this transition. First, according to the Ostwald equation (eq 13), the smaller particles have higher concentration of solute. Therefore, the concentration gradient existed between particles with different sizes. The second driving force is the principle of minimum free energy and larger grain size tending to decrease surface free energy. According to these driving forces, Fe diffused from tiny particles to bigger particles. Therefore, small iron particles decreased and disappeared, while large particles reunited and grew up. RT ⎛ C1 ⎞ 2σ ⎛ 1 1⎞ ln⎜ ⎟ = ⎜ − ⎟ M ⎝ C2 ⎠ ρ ⎝ r1 r2 ⎠

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 24 8368 0162. Fax: +86 24 8368 8920. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China for funding this research (Grant No. 51134002).

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REFERENCES

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(13)

where M is the molecular weight; r1 and r2 are the radii of particles 1 and 2, respectively; C1 and C2 are the equilibrium concentration of the solute on the surface of different particles with radii of r1 and r2; σ is the surface tension of the solution; ρ is the density of the solution.

4. CONCLUSIONS The results of the present study indicate that the primary iron mineral in the ore was hematite, which was reduced to metallic iron in the series of stepwise sequence, and the reduction process was very fast, metallic iron appeared in the product in less than 5 min. In the reduction, the indirect reactions played an important role and the direct reactions only took place at the beginning. Because of the high contents of gangues (predominantly SiO2 and Al2O3) in the ore, the reactions between iron oxide and gangues originated. Small amounts of metallic iron (or FeO) were reduced from the iron oxides at the beginning, and then, the metallic iron moved together and formed spherical particles. With reduction time increasing, the Fe gradually diffused from tiny particles toward the large ones and coalesced with them, so the large particles grow larger and tiny particles dissolved. The growth of metallic iron took place together with the iron oxides were reduced. The reduction process, which contained both the reduction of iron minerals to metal and metallic iron particles growth, could be divided into 2328



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Reaction Behavior of Iron Minerals and Metallic Iron Particles Growth

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