Magnetic Field Characteristics of Wet Belt Permanent ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 9, SEPTEMBER 2017

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Magnetic Field Characteristics of Wet Belt Permanent High Gradient Magnetic Separator and Its Full-Scale Purification for Garnet Ore Luzheng Chen1,2 , Yongming Zheng1 , Jianwu Zeng1,2 , Yongxing Zheng1, and Jian Liu1 1 Faculty

of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China 2 Kunming Gauss-Tesla Technology Co., Ltd, Kunming 650093, China

Purification of non-metallic ores has received considerable attention in the recent decade. In this investigation, the magnetic field characteristics of the innovative plate-type permanent magnet in wet belt permanent high gradient magnetic separator (WBHGMS) were analyzed; then, its full-scale purification of a garnet ore was introduced, and its performance dependences on the key operational parameters, i.e., magnet length, belt rotation speed, and feed particle size, were respectively examined. The separator produced a high-quality non-magnetic product assaying 2.00% Fe at an iron removal rate of 95.00% from the ore assaying 13.10% Fe. It was thus concluded that this WBHGMS separator has provided a promising method for the purification of non-metallic ores. Index Terms— Garnet ore, high gradient magnetic separation (HGMS), magnetic field characteristics, non-metallic ores, purification.

I. I NTRODUCTION

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IGH grade non-metallic ores, such as quartz and feldspar, are important material resources for such industries as glass, computer, optical fibers, and semi-conductive production [1], [2], and garnet ore has wide applications due to its excellent hardness and abrasive resistance [3]. Over the decades, the purity of most naturally non-metallic ores has been degrading due to the excessive exploitation in many areas, so that the purification of these ores is inevitably required to improve their purities. In reality, iron and titanium oxides are two main resources degrading the quality of nonmetallic ores, and they are usually characterized by low content, weak magnetism, and fine dissemination with target minerals. Flotation and leaching methods are fully investigated to remove these magnetic impurities, but magnetic separation presents the most effective and economical method in the purification of non-metallic ores, due to its simple operation, renewability, environmental friendliness, and low operation cost [4], [5]. High gradient magnetic separation (HGMS) is effective in removing magnetic impurities from non-metallic ores [6], [7]; but in the most cases, the prior removal for strongly magnetic particles, such as magnetite and iron scraps produced in the grinding process is preferred even demanded to avoid matrix clogging in the following HGMS process, when these particles are contained in the ores. This is due to the fact that in the particles, the residual magnetic field is remained while they are departed from the magnetic field of an HGMS separator, and the matrix cannot be completely cleared with water. However, it should be noted that almost all the non-metallic ores are polluted with magnetite particles as encountered in China; Manuscript received July 8, 2016; accepted May 18, 2017. Date of publication June 6, 2017; date of current version August 17, 2017. Corresponding author: J. Liu ([email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2017.2710126

usually, these magnetites are finely disseminated with target minerals in the ores and cannot be effectively removed with drum and roll magnetic separators. Just recently, a wet belt permanent HGMS (WBHGMS) was reported effective in purifying quartz and feldspar. This kind of separator was developed on HGMS in an inclined slurry flow of several centimeters thick [8], and it is characteristic of high magnetic field force, no matrix clogging, and high processing capacity. In this investigation, the magnetic field characteristics of the innovative plate-type permanent magnet in the separator were analyzed; then, its full-scale purification of a garnet ore was introduced, and its performance dependences on the key operational parameters of the separator, i.e., magnet length, belt rotation speed, and feed particle size, were respectively examined. II. D ESCRIPTION ON F ULL -S CALE WBHGMS S EPARATOR AND I TS M AGNETIC F IELD C HARACTERISTICS A. Full-Scale WBHGMS Separator and Its Technical Parameters The PBC-2215 full-scale WBHGMS separator, as shown in Fig. 1, mainly consists of a wearable belt and tensioning mechanism, a plate-type permanent magnet and inclination angle adjusting mechanism, motor, driving roll, and product launders. The belt is 1.3 mm thick and is uniquely made into a shallow U-shaped chute with raised sidewalls, wherein slurry flows downward. Perpendicular to the belt rotation, bulged strips are arranged on the belt at a given interval, to retain the magnetic particles captured onto the belt by magnetic poles beneath as shown in Fig. 1. The magnet is made into a thin plate with high-energy NdFeB blocks, and these blocks are assembled with polarities aligned in the belt rotation and alternately aligned across the belt width. Long narrow magnetic poles are arranged between the NdFeB rows to produce high magnetic field and field gradient on the belt surface, to capture the magnetic particles from the slurry flow.

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TABLE I M AIN PARAMETERS OF PBC-2215 F ULL -S CALE WBHGMS S EPARATOR

Fig. 2. Capture of magnetic particles from inclined slurry flow in a WBHGMS process. Fm —magnetic force. Fg —gravity. F f —buoyant force. F0 —friction force. Fd —hydrodynamic resistance. Fig. 1. Schematic diagram of PBC-2215 full-scale WBHGMS separator. 1—support frame. 2—driven wheel. 3—water sprays for magnetic particles. 4—tensioning mechanism for belt. 5—rinsing water trough. 6—feed box. 7—belt. 8—plate-type permanent magnet. 9—steel backing plate. 10—motor. 11—driving roll. 12—inclination angle adjusting mechanism for magnet. W— rinsing water. F—feed. NMP—non-magnetic product. MP—magnetic product.

expressed as [3]: Bgr ad B ≥

μ(υ −v) μ0 18 + (δ −ρ)g · (sin α−μ cos α) p μp K d2 (1)

The main parameters of the separator are listed in Table I. When the separator is operated, the slurry enters through feed box onto the U-shaped belt, and flows downward at a uniform thickness of several centimeters. Magnetic particles in the slurry flow are captured onto the belt and carried upward by the bulged strips on the belt. Then, they are further carried to the rinsing area and are fully scattered due to the direct impinging of rinsing water onto the magnetic deposits, and the entrained non-magnetic particles are rinsed out of the deposits. While the magnetic particles are carried to the corner of belt, they are flushed into the magnetic product launder by water sprays. Non-magnetic particles flow downward with the slurry to produce a non-magnetic product. The magnetic field force required on the belt surface of the separators depends on the property of particles, such as permeability and size, the operating parameters of belt, such as rotation speed, and the characteristics of slurry, such as flow velocity on the belt. From Fig. 2, it is derivable that the magnetic field force Bgr ad B required for the capture of magnetic particles from the slurry flow of the separator is

where Bgr ad B is a comprehensive index indicating the intensity of magnetic field force of a magnetic separator, which is the product of magnetic induction B and field gradient gr ad B; μ0 is the permeability of vacuum; μ p is the friction coefficient between magnetic particles and belt surface; K is the volume permeability of magnetic particles; μ is the dynamic viscosity of fluid; υ and v are the linear velocities of belt and slurry flow, respectively; δ and d are the density and diameter of magnetic particles, respectively; ρ is the density of fluid; α is the inclination angle of belt. B. Magnetic Field Characteristics on Belt Surface The separator uses an innovative plate-type permanent magnet of 25–35 mm thickness depending on the desired magnetic field force on belt surface; it is mainly made of steel backing plate, high-powered NdFeB blocks, and narrow magnetic poles, as shown in Fig. 3. This magnet design produces a symmetrical magnetic field distribution in the perpendicular and longitudinal directions, to the belt rotation and slurry flow as shown in Fig. 4.

CHEN et al.: MAGNETIC FIELD CHARACTERISTICS OF WBHGMS AND ITS FULL-SCALE PURIFICATION FOR GARNET ORE

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B. Effect of Magnet Length on Purification Performance

Fig. 3. Narrow magnetic poles and NdFeB blocks in a plate-type permanent magnet.

From Fig. 4, the magnetic induction is regularly distributed over the magnet, both in the perpendicular and longitudinal directions. In the two directions, the induction is improved significantly from the edges of magnet, and then reaches the steady plateaus after the length and the width from the edges are increased to 100 and 50 mm, respectively; the lower induction near the these edges is due to the leakage and fringing of magnetic flux [9]. The induction reaches the highest value on the belt surface, and it fluctuates between the highest 1200 mT at the middle point of magnetic poles and the lowest 780 mT at the middle of NdFeB blocks. At the middle point of magnetic poles, as shown in Fig. 4 (right), the induction is reduced from 1200 to 1050 and to 800 mT while the distance to magnetic poles is increased from 0 mm (magnet surface) to 2 mm (near belt surface) to 5 mm in the slurry flow; at the middle of NdFeB blocks, they are decreased from 780 to 620 and 420 mT. III. F ULL -S CALE P URIFICATION FOR G ARNET O RE A. Description on Feed Conditions and Evaluation Methods Two PBC-2215 full-scale WBHGMS separators were installed at Heyuan area of Guangdong province in China, to purify a garnet ore. The main impurities in the ore are iron-bearing minerals with an iron grade of 13.10%, and they are magnetite, hematite, and limonite. In practice, the garnet was ground to 60% below 0.15 mm and evenly fed to the separators at a controlled solid concentration around 40% and a processing capacity of 32–33 t/h for each separator. The belt rotation speed is adjusted through an ac transducer and the belt inclination angle was determined at 8° through pilotscale trials. For all the separations, sufficient rinsing water was used to relax the captured magnetic deposits on belt surface of separator. Mass weight (Mnm ), iron grade, and iron removal rate (IRnm ) of non-magnetic products were used for evaluating the separation performance of the separators; and the iron removal rate was calculated using the following equation [8]: β Mnm × 100% (2) IRnm = 1 − · α 100 where, α and β are the iron grades for feed and non-magnetic product, respectively.

The magnet length from the feed area over belt to the right endpoint of magnet as shown in Fig. 1 determines the capture probability of magnetic particles by the magnetic force of the separator, and thus, it produces an effect on the purification performance. This investigation was first performed at a relatively low belt rotation speed of 4 r/min and at a feed particle size of 60% below 0.15 mm. It can be seen from Fig. 5 that the magnet length has a significant effect on the separation performance. The iron removal rate of nonmagnetic product (IRnm ) is greatly increased with increase in the length from 0.3 to 0.8 m, beyond which it approaches the maximum around 93%; accordingly, the iron grade of nonmagnetic product is reduced to the minimum around 2.85%. From Fig. 5, when the magnet length was less than 0.8 m, most of the fine magnetic particles cannot be captured by magnetic force, due to the insufficient separation time while they flow down the belt surface; thus, the separator obtains a low iron removal rate with a high iron grade in non-magnetic products. As the length is increased, the separation time becomes increasingly sufficient, and the capture probability for magnetic particles is greatly improved. As shown in Fig. 5, the iron removal rate and iron grade of non-magnetic products both reached the optimum values, when a magnet length no less than 0.9 m is adopted. C. Effect of Belt Rotation Speed on Purification Performance As shown in Eq. (1), the increase in the belt rotation speed enlarges its linear velocity with respect to that of slurry flow on the belt surface, which increases the hydrodynamic resistance upon particles in the flow. This, in turn, demands an improved magnetic field force Bgr ad B to capture the fine magnetic particles from the flow; or, for a fixed Bgr ad B as in the present separator, it may be counteracted using a lengthened magnet length or a reduced slurry flow velocity, for the effective capture of magnetic particles. As shown in Fig. 6, the performance of the separator remains stable while the belt rotation speed is controlled in the range of 4–8 r/min; when the rotation speed is further improved, the iron grade of non-magnetic products is significantly increased, with visible reduction in the iron removal rate and increase in the mass weight. This is due to the fact that an excessive belt rotation speed results in an increased hydrodynamic resistance to magnetic particles as discussed earlier. This deteriorated separation performance is also resulted from the shortened separation time for the particles in the slurry flow on belt surface, and the mechanically entrained particles in magnetic deposits cannot be sufficiently relaxed and released on the surface. D. Effect of Feed Particle Size on Purification Performance The feed particle size presents one of the most important feed conditions, as it determines the liberation degree between magnetic and non-magnetic components in the feed. From Fig. 7, with a full magnet length of 1.5 m and a relatively high belt rotation speed of 8 r/min (for ensuring a high processing

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Fig. 4.


Magnetic field distribution over magnet at longitudinal (left) and perpendicular (right) directions to belt rotation and slurry flow.

Fig. 5. Effect of magnet length on purification performance. (1) Belt rotation speed = 4 r/min. (2) Feed particle size ≈ 60%, −0.15 mm.

Fig. 6. Effect of belt rotation speed on purification performance. (1) Magnet length = 1.5 m. (2) Feed particle size ≈ 60%, −0.15 mm.

capacity), the separator achieved the lowest iron grade of 2.0% and the highest iron removal rate of 95%, while the garnet is

Fig. 7. Effect of feed particle size on purification performance. (1) Magnet length = 1.5 m. (2) Belt rotation speed = 8 r/min.

ground to around 44% below 0.15 mm. Departing from this grinding fineness, the iron grade of non-magnetic product is greatly deteriorated. It is easy to understand that, while the feed material was insufficiently ground, the iron impurities cannot be fully liberated from coarse garnet particles. In such case, the separator produces a small mass weight of non-magnetic product with high iron grade, as a part of garnet particles with iron impurities are lost in the magnetic product; and its high iron grade is resulted from the iron impurities of insufficient magnetism, which are concentrated in the magnetic product together with garnet particles. However, while the material was excessively ground, the separator produces a high mass weight of non-magnetic product with high iron grade, as many fine iron impurities are concentrated in non-magnetic product, as a result of the insufficient magnetic force to capture such particles.

CHEN et al.: MAGNETIC FIELD CHARACTERISTICS OF WBHGMS AND ITS FULL-SCALE PURIFICATION FOR GARNET ORE

IV. C ONCLUSION 1) The WBHGMS separator uses an innovative plate-type permanent magnet and operates on principle of HGMS in an inclined thick slurry flow, allowing a high processing capacity. This magnet produces a symmetrical magnetic field distribution over the belt and generates a significant effect on the purification performance of the separator. 2) The key operational parameters of the separator, such as belt rotation speed, determine the hydrodynamic resistance acting onto particles flowing in the inclined slurry flow and the separation time of particles on the belt surface, thereby producing a significant effect on the purification performance of the separator. 3) The separator achieved a very high iron removal rate in purifying the garnet ore, and it has provided a promising method for the purification of non-metallic ores. ACKNOWLEDGMENT This work was supported in part by the National Natural Foundation of China under Grant 51564028 and in part by the Key Program for Applied Basic Research of Yunnan Province under Grant 2016FA051. The authors would like to thank Kunming Gauss-Tesla Technology Co., Ltd., for developing WBHGMS separators.

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