HIGH-GRADIENT MAGNETIC SEPARATION OF ULTRAFINE ...

Mineral Processing & Extractive Metall. Rev., 34: 340–347, 2013 Copyright # Taylor & Francis Group, LLC ISSN: 0882-7508 print=1547-7401 online DOI: 10.1080/08827508.2012.695304

HIGH-GRADIENT MAGNETIC SEPARATION OF ULTRAFINE PARTICLES WITH ROD MATRIX Luzheng Chen1, Zhihua Qian2, Shuming Wen1, and Songwei Huang1 1

Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming, Yunnan Province, China 2 SLon Magnetic Separator Ltd., Ganzhou, Jiangxi Province, China The matrix plays a key role in determining the performance of a high-gradient magnetic separator; it provides the carrier for the magnetic particles to be captured and transported to the nonmagnetic field as magnetic product. High-gradient magnetic separation (HGMS) of ultrafine hematite with the finest 1 mm rod matrix has been investigated on a pilot pulsating HGMS separator. The results of this investigation indicate that this matrix achieves a significantly improved performance for the ultrafine hematites, compared to the coarser 2 mm one which is now widely applied in industry, due to its stronger magnetic capture to ultrafine particles. It was concluded that the 1 mm matrix has a powerful manipulation over ultrafine particles in a pulsating HGMS process, and is capable of achieving a higher separation performance at a considerably lower energizing cost. Keywords: fine particle processing, hematite, magnetic separation, rod matrix

INTRODUCTION The rod matrix, due to its high operational reliability, simplified combinatorial optimization, and resistance to clogging, is widely used in pulsating high-gradient magnetic separations (HGMS), for the beneficiation of fine weakly magnetic minerals such as oxidized iron ores and ilmenite (Xiong, Liu, and Chen 1998; Zeng and Xiong 2003). Although it is a common knowledge that a finer matrix in wire radius gives a shorter magnetic reach and thus tends to result in an inherent disadvantage in applications, the continual depletion of the world’s mineral resources justify the efforts to treat the increasingly lower grade and finer size ores with finer matrix in a HGMS process, due to its strong manipulation over fine and ultrafine magnetic particles; such efforts also results from the continual pressure in the mineral processing industry to cut costs and improve efficiency (Watson and Beharrell 2006). It has been reported by Chen, Xiong, and Huang in 2009 that a finest 1 mm rod matrix achieves a significantly improved separation performance for fine hematite in a pulsating HGMS process, compared to the coarser 2 mm one which is now widely

Address correspondence to Luzheng Chen, Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming, Yunnan Province, China. E-mail: [email protected] 340

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applied in industry; however, the inherent essence of this superiority over fine particles was not fully discussed. In recent years, this finest rod matrix has gained effective applications in recovering fine and ultrafine magnetic values, mainly from tailings; therefore, in the present investigation, an attempt has been made to elucidate the superiority of the 1 mm matrix over ultrafine particles, with theoretical analysis and experimental data, to develop a general understanding on the magnetic capture of fine rod matrix to ultrafine magnetic particles in a HGMS process.

MATHEMATICAL DESCRIPTION ON MAGNETIC FORCE Suppose a ferromagnetic rod wire of radius a is placed axially along the z-axis in a cylindrical coordinate system as shown in Figure 1. A uniform magnetic field H is applied in the x-direction and a paramagnetic particle of volume Vp and magnetic susceptibility Kp is carried past the rod by a fluid of velocity t0, at an angle h to the negative x-direction of the system. The magnetic capture force Fm acted upon the particle by the rod may be written as (Xiong 1998; Jin et al. 2000): 1 a2 M a2 Fr ¼ l0 Kp Vp MH 3 cos 2h þ ; 2 H r2 r

ð1aÞ

1 a2 Fs ¼ l0 Kp Vp MH 3 sin 2h; 2 r

ð1bÞ

where, Fr and Fs are the nonzero radial and tangential components of the magnetic force Fm, respectively; l0 is the permeability of free space; M is the induced magnetization on the particle; and r is the capture radius, the distance from the axis of rod to the center of particle.

Figure 1 Magnetic capture of a paramagnetic particle approaching a rod wire.

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Then, the value of the magnetic force upon the particle is calculated as: Fm ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Fr2 þ Fs2 :

ð2Þ

Using the curve of the induced magnetization versus the applied magnetic field for a paramagnetic hematite (Wills and Napier-Nunn 2006), the relative intensity of the magnetic force on a particle in a HGMS separator of rod matrix with different wire radiuses could be valued as: ðFm ÞM Rm ¼ ðFm ÞN

aM aN

2 aN þ d 3 ; aM þ d

ð3Þ

where, Rm is the ratio between magnetic force (Fm)M by a finer aM matrix arranged with M mm radius rods and magnetic force (Fm)N by a coarser aN matrix arranged with N mm radius rods, respectively; and d is the capture distance from the rod surface to the center of particle, as schematically shown in Figure 1. According to Equation (3), the ratios of magnetic force Rm between the finest 1 mm matrix and the coarser 2, 3, and 4 mm matrixes, respectively, as a function of the capture distance d is shown in Figure 2. As can be seen, a higher magnetic force is achievable with the finest 1 mm matrix within a given magnetic reach, compared to the coarser 2, 3, and 4 mm matrixes; this short magnetic reach increases with the increase of rod radius in the coarser matrixes, specifically, they are 0.70, 0.85, and 0.97 mm for the 2, 3, and 4 mm matrixes, respectively, such that the finer magnetic particles may be captured by the 1 mm matrix if only a sufficiently short magnetic reach is used in the combination of rods in the matrix. Beyond the magnetic reach, the coarser matrix increasingly dominates the 1 mm one in the magnetic capture on fine magnetic particles; this, however, would inevitably demand a higher magnetic

Figure 2 Ratios of magnetic force on hematite between finest 1 mm matrix and coarser 2, 3, and 4 mm rod matrixes.


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induction and thus results in a higher energizing cost, as experimentally confirmed later in this investigation, due to the drastic decrease in magnetic gradient on the rod surface of coarser matrix, with the increase in rod radius.

EXPERIMENTAL Pilot Pulsating HGMS Separator A SLon-100 pilot pulsating HGMS separator as schematically illustrated in Figure 3 was used to perform the investigation. This pilot separator is fed periodically and its detailed description on the separation mechanism is early reported (Xiong et al. 1998). The finest 1 mm and coarser 2 mm rod matrixes, both with combinatorial optimization, were comparatively used in the investigation. The most essential feature in pulsating HGMS differing from other HGMS methods is slurry in the separating matrix exposed to pulsation to loose particles, which, as shown in Figure 3, is achieved by the installation of a pulsating mechanism below the lower magnetic yoke in the pilot pulsating HGMS separator. While the pilot separator is being operated, a direct current flows through the energizing coils and a magnetic field is built up in the separating zone. Firstly, the separating zone is filled with flowing water so that the pulsating energy can be transmitted to the separating zone, in which the level of water and its flow rate is adjustable through the valve below the pulsating mechanism. Then, the slurry is fed into the matrix in the separating zone through the feeding box. Magnetic particles are attracted from slurry onto the surface of the matrix, while nonmagnetic particles pass through the matrix and go out through the product box to become tailings, under the combined actions of slurry pulsation, gravity, and hydrodynamic drag. The pulsating

Figure 3 Pilot pulsating HGMS separator (left) and rod matrix (right): 1 ¼ feeding box, 2 ¼ magnetic pole, 3 ¼ magnetic yoke, 4 ¼ energizing coils, 5 ¼ magnetic matrix, 6 ¼ pulsating mechanism, 7 ¼ product box, 8 ¼ valve.

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mechanism drives the slurry in the separating zone up and down, keeping particles in the matrix in a loose state so that magnetic particles can be more easily captured by the matrix and nonmagnetic particles can be more easily dragged out through the matrix. This pulsating HGMS method achieves a higher performance in separating fine weakly magnetic minerals, in comparison with traditional HGMS methods, due to the innovative introduction of slurry pulsation during the separating process. Sample. The material used in the present investigation contains 29.45% Fe, with the hematite as the dominant target mineral and the quartz as the primary gangue in the material, respectively. From Table 1, the material is characterized by ultrafine particle size distribution, with 78.43%, 64.72%, and 47.48% of the material less than 30 mm, 20 mm, and 10 mm, respectively; these fractions are rich in iron, accounting for 86.41%, 70.34%, and 49.13% of the iron values, respectively. Methods. The material was fully mixed in a stir beaker and evenly fed to the separator within 10s. Before experiments, operating variables of the pilot separator including pulsating stroke and frequency, feed velocity, etc., except for rod matrix and magnetic induction were all optimized through single factor optimizing test, that is, pulsating stroke and frequency at 5 mm and 225 r=min, respectively, feed weight of 200 g, feed velocity around 7.0 cm=s, and feed % solids around 10%. Evaluation Methods. Concentrate grade, recovery, and separation efficiency were used for evaluating the separation performance. Separation efficiency (E) was calculated using the following equation (Chen 2011): f ð cm cÞ E ¼R 1 ; c ð cm f Þ

ð4Þ

where, R is the recovery, f is the feed grade, cm is the maximum iron grade of hematite (70% Fe for pure hematite), and c is the concentrate grade. Table 1 Size-by-size analysis of concentrates from 1 mm and 2 mm rod matrixes Size range (mm) Concentrate from 1 mm matrix (0.4 T) Concentrate from 2 mm matrix (0.5 T) Feed

Weight (%) Grade (% Fe) Iron recovery (%) Weight (%) Grade (% Fe) Iron recovery (%) Weight (%) Grade (% Fe) Iron distribution (%)

þ30

30 þ 20

20 þ 10

10

Sum

28.67 32.70 68.58 30.44 36.73 71.17 21.57 18.55 13.59

17.83 59.01 65.05 17.72 58.46 55.73 13.71 34.53 16.07

28.56 59.61 79.78 26.07 56.02 59.55 17.24 36.23 21.21

24.94 49.72 25.09 25.77 50.88 23.08 47.48 30.47 49.13

100.00 49.32 49.02 100.00 49.26 42.60 100.00 29.45 100.00


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RESULTS AND DISCUSSION Separation Performance of Rod Matrix The separation performance of the pilot HGMS separator with the 1 mm and 2 mm rod matrixes, respectively, are shown in Figure 4. As can be seen, although the two matrixes produce two different sets of performance criteria for different magnetic inductions, they follow the same trend. An increase in the induction improves the iron recovery but reduces the concentrate grade; however, the 1 mm matrix achieves a much higher recovery for almost the same concentrate grade as that of the 2 mm one, and this does not inevitably require a high magnetic induction as that of the 2 mm matrix in applications; for instance, the 1 mm matrix produces a concentrate assaying 46.99% Fe with 58.64% recovery at a magnetic induction of 0.5 T, and the 2 mm one produces almost the same concentrate with a lower recovery of 51.03%, but at a much higher induction of 0.9 T as illustrated in Figure 4. This superiority in the recovery achievable with the 1 mm rod matrix results in the significantly improved separation efficiency of 37.79%, much higher than that (32.81%) of the 2 mm rod matrix. Obviously, this finest 1 mm matrix is capable of achieving a higher separation performance, at a considerably lower energizing cost in the pulsating HGMS process; specifically, the energizing power of the pilot pulsating HGMS separator is sharply increased from 1.08 to 6.02 kilowatt, while the magnetic induction is increased from 0.5 T to 0.9 T. Size-by-Size Analysis of Concentrate Products Two concentrates with almost the same grade, from the 1 mm matrix operating at a lower magnetic induction of 0.4 T and from the 2 mm matrix operating at a relatively higher magnetic induction of 0.5 T, respectively, were analyzed with results as

Figure 4 Comparison of separation performance between 1 mm and 2 mm rod matrixes.

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illustrated in Table 1, to obtain a deep understanding on the magnetic capture of fine matrix to ultrafine magnetic particles in a pulsating HGMS process. It can be seen from Table 1, compared to the 1 mm matrix, the 2 mm matrix achieved a slightly higher separation performance for the coarser size range over 30 mm, with weight 1.77% higher, grade 4.03% higher, and iron recovery 2.59% higher, respectively. However, with the decrease in size range from 30 þ 20 mm to 20 þ 10 mm, the 1 mm matrix achieved an increasing predominance over the 2 mm matrix in separation selectivity and iron recovery, due to its strong magnetic capture to ultrafine magnetic particles as theoretically analyzed afore; it achieved 0.55% and 3.59% higher grades and 9.32% and 20.23% higher recoveries for these two ultrafine size ranges, than did the 2 mm matrix. From Table 1, the iron values in the finest size range below 10 mm cannot be effectively concentrated with pulsating HGMS method operating at low magnetic induction of 0.4 T, even with the finest 1 mm matrix, as a result of the increase in the relative importance of hydrodynamic drag in comparison to magnetic force. For the 1 mm matrix, with the increase in magnetic induction from 0.4 T to a high level of 0.9 T, the recovery for this finest size range is greatly improved from 25.09% to 35.63%, at the expense of deterioration in concentrate grade from 49.72% Fe to 44.36% Fe; however, for the 2 mm matrix, this recovery is slightly improved from 23.08% to 25.27%, at the expense of drop in concentrate grade from 50.88% Fe to 47.45% Fe. It is, therefore, concluded that this finest 1 mm matrix has a powerful manipulation over ultrafine particles, compared to the 2 mm matrix. However, the effective recovery for the mostly ultrafine particles below 10 mm is still a challenge to be solved, as currently encountered in other HGMS practices.

CONCLUSIONS The matrix plays a key role in determining the performance of a high-gradient magnetic separator; it provides the carrier for the magnetic particles to be captured and transported to the nonmagnetic field as magnetic product. For the ultrafine particles, the finest 1 mm rod matrix is capable of achieving a higher separation performance at a considerably lower energizing cost, compared to the 2 mm matrix widely applied in industry, due to its powerful manipulation over ultrafine particles. Compared with the 2 mm matrix, this finest 1 mm matrix achieved an enhanced manipulation over the finest size range below 10 mm, as the magnetic induction is increased from 0.4 T to a high level of 0.9 T; however, the effective recovery for this mostly ultrafine particles is still a challenge to be solved, as encountered in other HGMS practices.

ACKNOWLEDGMENTS This research work was supported by the National Natural Science Foundation of China (Grant No. 51104076), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20115314120006), and the Major Program of the National Natural Science Foundation of China (Grant


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No. 51090385). The pilot HGMS separator is manufactured by SLon Magnetic Separator Company Ltd. REFERENCES Chen, L., 2011, ‘‘Effect of magnetic field orientation on high gradient magnetic separation performance.’’ Minerals Engineering, 24, pp. 88–90. Chen, L., Xiong, D., and Huang, H., 2009, ‘‘Pulsating high-gradient magnetic separation of fine hematite from tailings.’’ Minerals & Metallurgical Processing, 26, pp. 163–168. Jin, J. X., Liu, H. K., Zeng, R., and Dou, S. X., 2000, ‘‘Developing a HTS magnet for high gradient magnetic separation technique.’’ Physica C., 341–348, pp. 2611–2612. Watson, J. H. P., and Beharrell, P. A., 2006, ‘‘Extracting values from mine dumps and tailings.’’ Minerals Engineering, 19, pp. 1580–1587. Wills, B. A., and Napier-Munn, T. J., 2006, Mineral Processing Technology, 7th Edition, Amsterdam: Elsevier Science & Technology Books, pp. 354. Xiong, D., 1998, ‘‘Dynamic analysis of weakly magnetic mineral particles in high gradient magnetic field of rod medium.’’ Metal Mine, 8, pp. 19–22 (in Chinese). Xiong, D., Liu, S., and Chen, J., 1998, ‘‘New technology of pulsating high gradient magnetic separation.’’ International Journal of Mineral Processing, 54, pp. 111–127. Zeng, W., and Xiong, D., 2003, ‘‘The latest application of SLon vertical ring and pulsating high-gradient magnetic separator.’’ Mineral Engineering, 16, pp. 563–565.