Separation Analysis of New Magnetic Separator for Pre-Concentration of Ilmenite Particles

Abstract

To achieve the utilization of fine ilmenite (especially −0.075 mm) produced in the titanium-magnetite processing plant in Panzhihua, a radial turbulent outer-cylinder magnetic separator (RTOCMS), was developed in this study. After optimizing the conditions of rotation speed and water flow, an RTOCMS concentrate with TiO₂ grade of 22.84% and TiO₂ recovery of 66.93% was obtained through one-stage roughing pre-concentration flowsheet. Magnetic force and competing forces were calculated and analyzed to illustrate the pre-concentration mechanism, and the results revealed that the combination of high water flow and high rotation speed resulted in the most effective for pre-concentration of the fine ilmenite in the RTOCMS process. In addition, particle size analysis of the concentrate product indicated that the RTOCMS was effective for the recovery of medium particle sizes (−0.075 + 0.038 mm), with a continued enhancement for the recovery of fine-grained products (−0.038 mm). Hence, the RTOCMS provides an effective way to pre-concentrate fine ilmenite ore.

1. Introduction

Weakly magnetic mineral resources are characterized by low grade usable minerals, nonuniform embedded particle size, and complicated co- and association of minerals with various specific magnetism coefficients [1,2,3]. Pre-concentration equipment should be able to handle a wide particle size feed and simultaneous pre-concentration of minerals with varying specific magnetization coefficients in order to achieve high efficiency and low cost pre-concentration of these minerals [4,5,6,7]. It also combines the benefit of being low-cost and simple to maintain. The development of external magnetic cylinder-type magnetic separation equipment has been a significant topic for scientific researchers in recent years [8].

The benefits of using an external magnetic drum magnetic separator are various: [9,10] the material’s size range is broad, allowing it to recover fine minerals while separating coarse minerals; magnetic force, gravity, and centrifugal force can all work together to complete the separation, using the specific magnetization coefficient, density, and centrifugal force difference between mineral particles, and the separation efficiency is high [11,12,13,14]. For example, Beijing General Research Institute of Mining and Metallurgy conducts pre-concentration of a vanadium-titanium-magnetite in Panxi area [15], with the cylinder speed is increased from 13 r/min to 26 r/min, the pre-discarding tailings rate drops from 28.46% to 23.89%, and the pre-concentration coarse and fine the recovery rates of TiO₂ and TFe in the ore were increased by 2.10% and 3.15%, respectively. Changsha Institute of Mining and Metallurgy used an external magnetic drum magnetic separator to conduct a pre-separation test study on an ilmenite containing 9.11% TiO₂. The recovery of TiO₂ in the pre-concentration rough concentrate increased from 74.52% to 87.25%, and at the same time, the TiO₂ grade decreased from 18.24% to 16.12, with rotating speed going from 9 r/min to 15 r/min% [16]. It can be seen that with the increase of the rotating speed of the cylinder, the pre-concentration recovery rate of the outer magnetic cylinder type magnetic separator shows an upward trend. Although the enhanced centrifugal force is beneficial to the recovery of fine-grained weak magnetic minerals, the tail throwing rate and the grade of the pre-concentration concentrate decrease. As a result, the rotation speed of the cylinder is often assured to be at a lower rotation speed in order to ensure the tailing rate of the pre-concentration and increase the pre-concentration grade [17,18,19].

The centrifugal force on the minerals in the suspended layer increases as the cylinder’s rotating speed increases [20,21], the probability of particles migrating to the inner wall increases, the concentration and viscosity of the material in the enriched layer near the cylinder’s inner wall increases even more, and the turbulent effect of solid particles leads to interlayers [8]. The radial force reduces, the inter-particle dispersibility lowers, particle mechanical entrainment increases, and pre-concentration efficiency declines as a result [22,23]. To summarize, the key to improving the efficiency of pre-concentration of weakly magnetic minerals is to increase the centrifugal force while maintaining the pulp flow regime as well as the radial force, and to improve particle dispersion while maximizing the specific magnetization coefficient and particle density difference [24,25].

The application of radial disturbance of the minerals enrichment layer on the inner wall of the cylinder is proposed in this work. The control of radial water flow is adjusted to achieve the regulation of the slurry flow state in the sorting process [26,27]. The radial force on the minerals enrichment layer is enhanced to alleviate the dissipation effect of the high concentration of solid particles, to strengthen the inter-particle relaxation, and to achieve the technical idea of reasonable matching of the appropriate fluid properties of the pulp with the magnetic and centrifugal fields, so as to strengthen the pre-concentration efficiency of weak magnetic minerals.

2. Experimental

2.1. Materials and Samples

The ilmenite sample obtained from the product of crushing from a ilmenite processing plant in Panzhihua was used in this study. Subsequently, the ore particles was dried and sampled to analyze the sample properties. The results of chemical multi-element and X-ray diffraction (XRD) analysis are shown in

Table 1. Chemical multi-element analysis results of mineral samples.

Elements	Content %	Elements	Content %
Na2O	1.43	K2O	0.09
MgO	8.67	CaO	10.09
Al2O3	12.31	TiO2	10.33
SiO2	32.75	MnO	0.22
P2O5	0.06	Fe2O3	17.37
SO3	1.15	CuO	0.02
Cl	0.02	Others	5.49

and Figure 1, respectively. The TiO₂ content in the sample was 10.33%. The phase analysis of titanium in the sample are listed in

Table 2. Phase analysis results of titanium mineral samples.

Minerals	Titanomagnetite	Rutile	Ilmenite	Total Titanium	C	S
Ti content (%)	0.29	0.12	5.64	6.25	0.029	0.46

. The content of the ilmenite in the sample was 5.64%, and that of the gangue minerals titanomagnetite and rutile, accounts for 0.29% and 0.12%, respectively. In addition, the TiO₂ distribution with variation of the particle size in the sample was analyzed using the combination of wet sieving and chemical multi-element analysis, and the results are shown in

Table 3. Particle size, density and TiO₂ metal distribution of ore sample.

Size (mm)	Density (g/cm3)	Yield (%)	TiO2 Content (%)	TiO2 Distribution Rate (%)
+0.30	2.95	17.36	4.90	7.91
−0.30 + 0.150	3.28	35.61	10.36	34.33
−0.150 + 0.075	3.65	24.15	14.60	32.80
−0.075 + 0.038	3.38	9.70	13.76	12.42
−0.038	3.03	13.18	10.23	12.54
Total	3.35	100.00	10.75	100.00

. The results showed that approximately 67.1% of TiO₂ distributes in the minerals with particle size more than 74 μm, and particularly, 12.54% of TiO₂ distributes in the portion less than 38 μm.

The ore sample is sieved, filtered, dried, and the density is measured, and the relationship between the particle size distribution and density of the sample is obtained as shown in Table 3. The results showed that the average density of the ore sample is 3.59 g/cm³, and the higher the TiO₂ grade of each grade, the higher the average density of the particles of this grade. Among them, the density of +300 μm particles is the smallest, which is 2.95 g/cm³, and the density of the −150 + 75 μm particles is the largest, which is 3.65 g/cm³.

The specific magnetic susceptibility of ilmenite samples is determined by a vibrating sample magnetometer (VSM). The VSM test results are shown in Figure 2, the background magnetic induction intensity of the effective sorting area is about 0.65 T, and the specific magnetic susceptibility of the minerals is about 2.0 × 10⁻⁶ m³/kg.

2.2. Equipment and Method

The main equipment used in this study is the new radial turbulent outer-cylinder magnetic separator. Its structure is shown in Figure 3 and can be divided into four systems: magnet, sorting cylinder, booster pump and transmission. The cylinder material are non-magnetic 304 stainless steel, 5-layer screen structure on the wall of the sorting cylinder for water feeding and to prevent mineral particles from blocking, the rack can change the sorting cylinder angle. The introduction of radial turbulence to strengthen the pre-concentration of weakly magnetic particles is the biggest innovation compared to the typical outer magnetic cylinder type magnetic separator. Special attention should be paid to operation aspects such as the motor and booster pump switch, as well as the frequency converter control, to ensure the stable functioning of each test and reduce human error.

The operation flow of the radial turbulent outer-cylinder magnetic separator is shown in Figure 4. The magnetic and non-magnetic particles are fed to the bottom of the sorting cylinder through the feed hopper. The magnetic particles are attracted by centrifugal force, magnetic force, gravity and fluid traction to the inner wall of the sorting cylinder and rotate with the cylinder. The non-magnetic particles are only subject to centrifugal force, gravity and traction, although they can rotate with the cylinder surface, but the traction effect of the water flow out to the outer end of the sorting cylinder, and finally washed out by the water into the tailing tank. The concentrate remains in the sorting cylinder and is subsequently flushed into the concentrate discharge port by the flushing water when passing through the non-magnetic zone, thus achieving an effective separation of the mineral particles. Finally, the concentrate was dried, weighed, and sampled to detect the grade, and then the ilmenite recovery was calculated.

2.3. Ore Characterization Techniques

The XRD analysis of the raw sample was conducted using a PW3040/60 X-ray diffractometer (Netherlands). The VSM testing of analysis of the raw sample was conducted using a PHYSICAL PROPERTY MEASUREMENT SYSTEM (PPMS-9#VSM) from QUANTUM DESIGN Under the conditions 2–400 $\mathrm{K}$ and $-9$ T to 9 T. The laser particle size analysis was conducted on a Delsa 440sx with the following parameters, Analysis model: General purpose, Size range: 0.020 to 2000 μm, Sensitivity: normal, Particle RI: 2.700, Absorption: 0.1, Obscuration: 4.53%, Dispersant name: water, Dispersant RI: 1.330, Weighted residual: 0.793%.

2.4. Evaluation Method

The TiO₂ grade, recovery of ilmenite concentrates and separation efficiency were used for evaluating the separation performance. The TiO₂ recoveries (R) and separation efficiency were calculated using the following equation:

$$\epsilon =\frac{\beta \left(\alpha -\theta \right)}{\alpha \left(\beta -\theta \right)}·100\%$$

(1)

$$E=\epsilon \left\{1-\frac{\alpha \left({\beta }_{{}_{\mathrm{M}}}-\beta \right)}{\beta \left({\beta }_{{}_{\mathrm{M}}}-\alpha \right)}\right\}$$

(2)

where $\alpha$ is the feed grade, $\beta$ is the concentrate grade, $\theta$ is the tailing grade, $\epsilon$ is the titanium recovery of concentrate, ${\beta }_{{}_{\mathrm{M}}}$ is the maximum titanium grade of ilmenite mineral (52.66% TiO₂), E is the separation efficiency.

2.5. Capture Force Calculation

When the mineral particles are sorted in the radial turbulent outer-cylinder magnetic separator, the magnetic minerals can be captured by a magnetic force and separated from the non-magnetic minerals. They are simultaneously affected by a variety of forces, mainly including magnetic force, centrifugal force, gravity, fluid drag force, friction force, etc. When separating in a pulp, the interaction of the mechanical forces should be take into account, and the capturing behavior should be attributed to the sum of the forces present. The forces on the particles are connected to its shape and the shape of the mineral particles is considered as a sphere to make it easier to compute and analyze the forces on the particles. The formula for calculating the force on mineral particles is as follows:

2.5.1. Magnetic Force

During a magnetic separation process, ilmenite is subjected to magnetic force, while gangue are not affected by magnetic force. The magnetic force is provided by the applied magnetic field and the magnetic matrices. The force can be calculated using the equation below.

$$F_{\mathrm{m}}={\mu }_{0}KVHgradH={\mu }_0\chi \rho \frac{4}{3}\pi r^{3}HgradH$$

(3)

where $F_{\mathrm{m}}$—the magnetic force; r—particle radius, m; $\rho$—the particle density, $\mathrm{kg}/{\mathrm{m}}^3$; ${\mu }_0$—the vacuum permeability, $\mathrm{N}/{\mathrm{A}}^2$; $\chi$—the specific susceptibility of the mineral particles, ${\mathrm{m}}^3/\mathrm{kg}$; K—the volume magnetic susceptibility of the mineral particles; V—the volume of the particles, ${\mathrm{m}}^3$; H—the magnetic field intensity, $\mathrm{A}/\mathrm{m}$; $gradH$—the magnetic field gradient, $\mathrm{A}/{\mathrm{m}}^2$.

2.5.2. Centrifugal Force

According to the flow field simulation results, a clockwise rotating flow field is generated when the cylinder is rotating. As a result, the particles are subjected to centrifugal forces as they rotate in the flow field and attach to the surface of the sorting cylinder, following the cylinder’s uniform rotation. According to Newton’s second law of motion, the centrifugal force $F_{\mathrm{c}}$ is applied to the particles when they are in uniform circular motion in the sorting cylinder as follows:

$$F_{\mathrm{c}}=\frac{4\pi }{3}{r}^3\rho \frac{{v_{\mathrm{t}}}^2}{R}$$

(4)

While the particles are also subject to the flotation of the water flow during sorting, the effective centrifugal force is:

$$F_{\mathrm{ce}}=\frac{4\pi }{3}{r}^3(\rho -\delta )\frac{{v_{\mathrm{t}}}^2}{R}$$

(5)

where $F_{\mathrm{ce}}$—the effective centrifugal force; $v_{\mathrm{t}}$—the tangential velocity of the particle, $\mathrm{m}/\mathrm{s}$; R—the radius of sorting cylinder, m; $\delta$—the density of water flow, $\mathrm{kg}/{\mathrm{m}}^3$; other symbols have the same meaning as above.

2.5.3. Gravity

When the particles move in the sorting cylinder, they are always under the action of gravity, and the gravity is:

$$G=\frac{4\pi }{3}{r}^3\rho g$$

(6)

where g—gravity acceleration, $\mathrm{m}/{\mathrm{s}}^2$; other symbols have the same meaning as above.

2.5.4. Fluid Drag

The particles are subjected to the drag force of the water flow in the flow field of the magnetic separator, and the drag force of the particles in the fluid satisfies the Stokes formula, and the size of the drag force can be calculated by the following formula:

$$F_{\mathrm{s}}=6\pi \mu r\left(v_{\mathrm{p}}-v\right)$$

(7)

where $\mu$—the viscosity, $\mathrm{Pa}·\mathrm{s}$; v—the velocity of fluid movement, $\mathrm{m}/\mathrm{s}$; $v_{\mathrm{p}}$—the velocity of particles movement; other symbols have the same meaning as above.

2.5.5. Friction

When the particles are in contact with the surface of the sorting cylinder, the particles will be subject to the friction force caused by the relative motion with the surface, and the friction force is calculated by the following formula:

$$F_{\mathrm{f}}={\mu }_{\mathrm{f}}{F}_{\mathrm{r}}$$

(8)

where ${\mu }_{\mathrm{f}}$—the friction coefficient; $F_{\mathrm{r}}$—the combined force in the radial direction; other symbols have the same meaning as above.

2.6. Design of the Orthogonal Experiment

An orthogonal experimental array design was constructed for this study. The degree of effects of two factors, namely $x_1$, $x_2$, and the best co-optimization combination of these evaluation indexes, such as grade, recovery and separation efficiency, was observed. Therefore three different levels of the speed of sorting cylinder $x_1$ (rpm) have been selected, ranging from 175 to 225, respectively: 175, 200, 225. Similarly, water velocity $x_2$ (${\mathrm{m}}^3/\mathrm{h}$) is divided in three levels are: 1.0, 1.5, 2.0. The factor levels of the orthogonal tests are shown in

Table 4. Arrangement and coding table of independent variables in orthogonal experiment.

Variables	Symbols	Variable Coding
		Low	Medium	High
		−1	0	1
Speed of sorting cylinder (rpm)	$x_1$	175	200	225
Water velocity (m3/h)	$x_2$	1	1.5	2

.

3. Results and Discussion

3.1. Effect of Rotation Speed of Cylinder

The effect of rotation speed of the cylinder in the separation zone of the radial turbulent outer-cylinder magnetic separator was first studied, with water flow as 2.0 m³/h, feed size −0.075 mm, constant magnetic field intensity 0.65 T, and the inclination angle of the cylinder is 0°, respectively. As shown in Figure 5, the rotation speed of the sorting cylinder has a very significant effect on the concentrate grade, the titanium recovery of concentrate product and separation efficiency. Both recovery and separation efficiency increased with increasing rotation speed. When the rotation speed increased from 175 rpm to 250 rpm, the recovery increased from 18.28% to 72.34%. A maximum recovery and separation efficiency gained at the rotation speed of 225 rpm. Increasing the rotation speed will result in a decrease in separation efficiency. In terms of concentrate grade, it was found virtually negatively correlated with the rotation speed of the cylinder. When the rotating speed is 175 rpm, the concentrate grade is the highest, which is TiO₂ 26.88%; when the rotation speed is 250 rpm, the concentrate grade drops to 17.55%. The rotation speed of the cylinder determines the retention time in the separating cylinder, thus posing a significant impact on the separation performance of the separator. In terms of separation efficiency, in the speed range from 175 rpm to 200 rpm, the separation efficiency increased from 12.46% to 35.63%, but when the speed was further increased until 250 rpm, the separation efficiency gradually decreased to 23.85%. As can be seen from Figure 5, a rotation speed of 175–225 rpm was suitable for the tested minerals. A too slow rotation speed of cylinder, nevertheless, would inevitably decrease the solids throughput of the separator.

3.2. Effect of Water Flow

Rinse water was introduced in radial turbulent outer-cylinder magnetic separators to improve the quality of magnetic products by eliminating particles trapped in the captured magnetic particles. Therefore, adding rinse water can generally give higher concentration grades. One condition worth noting is that the equipment pores have a tendency to become clogged when the water flow rate is 0 m³/h. Therefore, water flow rate must be greater than 0 m³/h. The effect of water flow was also conducted, with the separator at a rotation speed of the cylinder 200 rpm, feed size $-0.075$ mm, constant magnetic field intensity $0.65$ T, the inclination angle of the cylinder was 0°, respectively. The effect of water flow in the separation zone of the radial turbulent outer-cylinder magnetic separator was conducted. As shown in Figure 6, the water flow has the opposite effects on the concentrate grade, the titanium recovery of concentrate product and separation efficiency compared to the rotation speed of the cylinder. Both grade and separation efficiency increased with increasing water flow. When the water flow increased from 0.5 m³/h to 2.0 m³/h, the concentrate grade increased from 16.43% to 25.68%. In terms of the titanium recovery of concentrate product, it was found virtually negatively correlated with the water flow. When the water flow is 0.5 m³/h, the recovery is the highest, which is 75.36%; when the water flow is 2.0 m³/h, the concentrate recovery drops to 53.52%. The water flow determines the reverse competitiveness of the ores in the sorting process, thus posing a significant impact on the looseness of the particles. In terms of separation efficiency, in the water flow from 0.5 m³/h to 1.5 m³/h, the separation efficiency increased from 27.17% to 39.51%, but when the water flow was further increased until 2.0 m³/h, the separation efficiency gradually decreased to 35.63%. As can be seen from Figure 6, a water flow of 1.0–2.0 m³/h was suitable for the tested minerals.

3.3. Orthogonal Experiment

To further verify the theoretical separability description, separation tests of the ore by the radial turbulent outer-cylinder magnetic separator were conducted using the orthogonal experimental design. The effects of the rotation speed of cylinder and water flow were studied, and the conditions for obtaining a concentrate with maximum TiO₂ grade and recovery, as well as separation efficiency were optimized. The number of test samples required for the two-factor, three-level orthogonal test was nine, and the tests were arranged and coded at the levels shown in Table 4. The tests were carried out according to an orthogonal experimental design and the results obtained are shown in

Table 5. Two-factor three-level orthogonal test results.

Number	Variables		Responses
	${\mathit{x}}_1$	${\mathit{x}}_2$	Grade (%)	Recovery (%)	Separation Efficiency (%)
1	1	1	18.91	68.51	31.66
2	1	0	16.81	76.11	24.85
3	1	−1	14.23	79.83	17.60
4	0	1	25.68	53.52	35.63
5	0	0	23.84	66.00	39.51
6	0	−1	20.18	68.73	34.48
7	−1	1	26.88	18.28	12.46
8	−1	0	27.14	22.69	15.12
9	−1	−1	26.58	32.77	23.01

.

In order to make a comprehensive evaluation of the effect of rotation speed of cylinder and water flow on $-0.075$ mm ilmenite separation, Matlab software was used to fit a binary polynomial to the separation efficiency values under different conditions, and the fitted binary polynomial was in the form shown in Equation (9):

$$y=a_0+a_1{x}_1+a_2{x}_2+a_{1}1x_1^2+a_{2}2x_2^2+a_{1}2x_1{x}_2+\epsilon$$

(9)

The experimental results in Table 5 were fitted by Matlab to obtain a model of the sorting cylinder speed and water flow rate as a function of separation efficiency as shown in Equation (10):

$$y_1=0.001-3.59x_1-88.65x_2+0.04x_1^2-2.75x_2^2+0.49x_1{x}_2$$

(10)

The goodness-of-fit value of the functional model, $R^2=0.9777$, indicates that over 97.77% of the test results can be predicted by this model. Separation efficiency increases with the increase in rotation speed of cylinder and then decreases. With the increase in water flow, separation efficiency slowly increases. In the high water flow and high rotation speed of cylinder conditions, the separation efficiency reaches a maximum. Solve the function Equation (10) can be obtained, when the rotation speed of cylinder for 208 rpm, water flow of 1.9 m³/h, separation efficiency reached a maximum of 39.82%.

In order to verify the accuracy of the beneficiation index predicted by the optimized operating parameters, a validation test was conducted using this operating parameter. The validation test results are shown in the

Table 6. Validation test beneficiation index table.

Product	Weight (g)	Yeld (%)	Grade (%)	Recovery (%)
Concentrate	20.54	35.05	22.84	66.93
Tailings	38.06	64.95	6.09	33.07
Total	58.60	100.00	12.36	100.00

. The separation efficiency of the validation test is 39.64%, which is basically consistent with the predicted value of 39.82% by the function model, indicating that this function model is more accurate and can be used to predict the effect of separation cylinder speed and water flow rate on the separation efficiency of −0.075 mm grain size ilmenite.

3.4. Concentrate Particle Size Distribution and Magnetic Properties

The comparison of the laser particle size results in Figure 7a,b shows that the raw ore is $d_{50}=$ $0.061$ mm, while the concentrate is $0.087$ mm. The distribution curve and cumulative curve indicate that the particle size in the concentrate is coarser than in the raw ore, and that during the sorting process, the fine-grained minerals are more affected by the dragging force of the water than by the magnetic and centrifugal forces and are difficult to be captured by the magnetic and centrifugal forces into the concentrate. As can be seen from the VSM test results in Figure 8, at a field strength of approximately 0.65 T at the surface of the sorting cylinder, the magnetic susceptibility of the concentrate is approximately 3.64 × 10⁻⁶m³/kg, compared to 2.96 × 10⁻⁶m³/kg for the raw ore, and the concentrate product is more magnetic.

In order to further investigate the loss of TiO₂ from the fine particles, the concentrate was sieved into −0.075 + 0.038 mm and −0.038 mm grades, filtered, dried, weighed and assayed for the TiO₂ grade of each grade and calculated the recovery rate, the results are shown in

Table 7. The grade and recovery of different particle grades in concentrate and tailings products.

Product	Size (mm)	Grade (%)	Recovery (%)
Concentrate	−0.075 + 0.038	25.22	75.20
	−0.038	20.86	46.73
Tailiings	−0.075 + 0.038	6.08	24.80
	−0.038	6.27	53.27

. It can be seen that the concentrate yields of −0.075 + 0.038 mm and −0.038 mm particle size were 38.35% and 18.17%, respectively, and the concentrate recoveries were 70.25% and 46.73%, respectively. The results show that the yield and recovery of each particle size decreases as the mineral size becomes finer. The operating parameters at optimum separation efficiency tend to result in the ilmenite particles in the fine-grained minerals being subjected to strong current dragging forces, weakening the effect of the trapping force and entering the tailings.

3.5. Capture Force and Effect Factor Analysis

A computational analysis of the influence of the radial disturbance on the forces on mineral particles was carried out to investigate the mechanism of the effect of the radial disturbance on the movement of mineral particles during the sorting process. When the speed is 200 rpm, the relationship between the combined force on different types of particles in the radial direction and the particle size and radial water flow rate is shown in Figure 9a,b.

The forces on the ilmenite and gangue mineral particles in the radial direction are shown in Figure 10a. When the particles in the fluid movement has not settled to the wall of the sorting cylinder, the particles in the axial direction only by the fluid axial drag force, with the water flow from the inside to the outside, the axial flow velocity of the fluid from the inner surface of the sorting cylinder to the outer surface gradually increased. When the speed is 200 rpm, the relationship between the axial force and the axial water flow velocity for particles of different particle sizes is shown in Figure 11. As the axial flow velocity increases, the axial drag force on the particles increases, and the larger the particle size, the greater the drag force. In other words, when the particle size is the same, the greater the axial flow velocity of the fluid can make the particles move faster from the sorting cylinder to the outside of the sorting cylinder, improving the sorting efficiency of the magnetic separator.

Mineral particles in the axial direction is not only by the water flow axial traction, but also by the action of friction on the surface of the sorting cylinder Figure 10b. Among them, ilmenite by the size of the friction force and the radial direction by the size of the combined force, and because the particles are in the sorting cylinder wall with the sorting cylinder to do uniform circular motion, at this time the particles by the size of the radial combined force is equal to its centrifugal force. When the speed is 200 rpm, different particle sizes of the axial force and axial water flow velocity of the relationship was showed between Figure 12a,b. As the axial flow velocity increases, the axial force on the particles increases; when the particle size increases, the axial force on the particles increases and then decreases, and the increase in the process of increasing is becoming smaller and smaller, indicating that the same axial water flow velocity, the larger the particle size is, the more difficult it is to move from the inner surface of the sorting cylinder towards the outer surface of the outlet under the action of the water traction, and it is easier to stay in the sorting cylinder with the rotation of the sorting cylinder.

4. Conclusions

The present study was designed to determine the effect of radial disturbance material dispersion. For this purpose, we have developed a new type of test equipment, the external magnetic cylinder type magnetic separator. The influence factors were systematically investigated combined with the capture force analysis. The specific conclusions can be summarized as follows.

• A concentrate with TiO₂ grade of 22.84% and TiO₂ recovery of 66.93% and maximum separation efficiency 39.82% was obtained through one roughing circuit, under the condition of rotation speed 208 rpm, water flow 1.9 m³/h, feed size −0.075 mm, constant magnetic field intensity 0.65 T; and the inclination angle of the cylinder was 0°.
• The capture force analysis indicated that the effective pre-concentration of fine ilmenite by radial turbulent outer-cylinder magnetic separator was ascribed to its high water flow and high rotation speed, if there is sufficient magnetic force;
• The concentrate product particle size analysis indicated that the RTOCMS was effective for the recovery of medium particle sizes (−0.075 + 0.038 mm), with continued enhancement for the recovery of fine-grained products (−0.038 mm).
• The major limitation of this study is that the magnetic field intensity cannot be adjusted and the field intensity is too small to capture minerals in the −0.038 mm grain size.
• The impact on the recovery rate may be due to the use of a magnetic system with a hard magnetic material arrangement, the magnetic intensity of which does not meet the requirements for the recovery rate of fine-grained minerals.