Study on Dispersion and Mixing Mechanism of Coal Slime Particles in Jet Mixing Flow Field

Abstract

The jet flow field is characterized by the dispersion and mixing of multiphase flow, which is widely used in the field of coal slime flotation. In this paper, the behavior of coal slime surface modification, the behavior of material suspension and the effect of different jet fluids on coal slurry blending were studied. Based on the action mechanism of the jet flow field, a jet device suitable for coal slime graded mixing was proposed, and the mixing effect of the jet device was tested. The results show that the jet flow field has strong effects on material dissociation and dispersion, and the mixing effect of a single jet is equivalent to that of the laser particle size analyzer’s own agitation device after stirring for 2.5 min at 500 r/min speed. The SEM test of material surface morphology and the changes of Al and Si elements measured by EDS show that the jet flow field can effectively remove the fine mud wrapped on the surface of coal particles. The precondition of material suspension is to have the just-suspended capacity. The critical jet velocity of coal slime suspension is in the range of 6 m/s~9 m/s. The mixing ability of the jet stream has certain limitations. The increase or decrease of the jet height will cause the decrease of the suspension percentage of sampling points in the tank. The gas jet mode can promote the reagent acting on the surface of the bubble liquid film to form oil bubbles, which is more suitable for hydrophobic mineral flotation. The optimal speed of the gas jet is 0.86 m/s, and the shortest cycle period is 1.0 T (T is one material cycle period). The flotation perfection index of 0.5–0.25 mm and less than 0.075 mm coal slime increased by 2.67% and 26.78%, respectively, indicating that the overall idea of the jet mixing device proposed based on the experimental conclusion is feasible.

1. Introduction

Coal preparation is the precondition of clean coal technology, and flotation is the most widely used separation method in fine coal slime separation. Coal slurry blending is the starting point of coal slime flotation. It can activate the surface of suspended coal particles and improve the collision and adsorption probability of flotation reagents and bubbles, so as to achieve full adsorption of reagents and bubbles on the surface of hydrophobic coal particles. This process provides good initial conditions for subsequent flotation operations [1,2,3]. Jet flow field is widely used in the field of multiphase fluid mixing, which has a good dispersion and mixing effect on reagents and bubbles [4,5,6,7,8,9]. In a jet mixing flow field, the air is repeatedly sucked in and dispersed to form microbubbles, and the reagents adhere to the surface of the bubble liquid film, resulting in a strong coupling effect [10,11,12].

In recent years, many scholars have studied the coal slurry blending process before flotation and the application of the jet flow field in flotation fields. Ma et al. explored the influence of slurry blending on the coal slime flotation process and found that high-shear slurry blending can effectively increase the adsorption of reagents on a coal slime surface, and the flotation efficiency can be effectively improved by using high-shear blending [13]. Feng et al. explored coal slurry blending before flotation and found that the high-shear slurry mixing method can significantly improve the flotation index of oxidized coal, and significantly increase the flotation rate and recovery rate of the combustible body [14]. Verderrama et al. studied the influence of high strength conditions on the flotation of fine mineral carriers. The fine carriers adhere better to the surface of coarse particles at low shear energy values, and at higher shear energy values, they detach from the coarse ones because of the shear forces operating at the contact surface [15]. Li et al. simulated the practical pulp-mixing process through solid-liquid two-phase experiments, and used parameters such as cumulative concentration variance, interval concentration variance, just-suspended capacity (the ability of materials to suspend precisely) and effective range to characterize the suspension characteristics of materials, and found that material suspension is decided by the just-suspended capacity and effect range [16]. Wang et al. studied the influence of impeller stirring speed and effective power in a mechanical flotation cell on the dispersion efficiency of different size coal slimes and obtained that the dispersion efficiency increased as the effective power increased but decreased as the coal slime size increased, and both flotation recovery and matching degree generally increased as the impeller stirring speed increased [17]. Weng et al. designed a gas holdup measurement method suitable for microbubble flow field, and explored the mechanics of bubble formation, coalescence and collision [18]. Zhou et al. studied the influence of flow field distribution inside the jet device on the mixing and dispersion performance of multi-stream fluids by optimizing the jet device of the jet flotation machine [19]. Wu et al. studied the air-blast atomization with bubbles in the liquid jet by using a high-speed camera and a Malvern laser particle size analyzer, and obtained the influence rule of working parameters on bubble particle size distribution [20].

Numerous studies have shown that in the jet mixing flow field, the stripping rule of fine slime on the surface of coal, the rule of material suspension and the action mechanism of jet reagents (bubbles) have an important influence on the effect of coal slurry blending. Based on previous studies, the behavior of coal slime surface modification, the behavior of material suspension and the effect of different jet fluid on coal slurry blending were explored in this paper. This study aims to design a jet device applied to coal slime-graded mixing, which enriches the idea of coal slime flotation technology.

2. Experimental Section

2.1. Instruments and Test System

As shown in Figure 1, the main components and instruments of the test system include jet device, screw pump, high speed camera (Olympus I-Speed 3) and test instruments (droplet Angle measuring instrument C20, laser particle size analyzer SALD-7101, energy dispersive spectrometer XFlash6130, scanning electron microscope VEGA3SBH and ultraviolet spectrophotometer UV-5500PC, etc.). The parameters of the jet device were taken within the optimized value range [21]: the nozzle diameter D_z = 10 mm, the throat tube diameter D_h = 18 mm, the throat-nozzle distance L_e = 10.8 mm (0.6D_h) and the length of throat tube was 90 mm. The basic working principle of this system is the coal pulp, ejected by the screw pump, is spewed out at a high speed through the jet device, and the air and reagents are introduced to realize the mixing of multiphase materials. The mixed materials are introduced into the test instrument or high-definition camera imaging system using the delivery pipe to explore the moving mechanics of coal particles.

The suspension state of the material in the jet mixing flow field directly affects the impact effect of the particles. The cumulative concentration variance ${\sigma }^2$ is used to characterize the uniformity of material concentration distribution in the mixing tank.

$$C=\frac{c_i}{c_j}\times 100\%$$

(1)

$${\sigma }^2=\frac{1}{n}{\sum }_{i=1}^n{\left(\frac{c_i}{c_j}-1\right)}^2$$

(2)

where the $C$ is the percentage of suspension, %; $c_i$ is the percentage of solid mass at the sampling point, %; $c_j$ is the average solid mass percentage of the whole tank, %; and n is sampling points. The cumulative concentration variance ${\sigma }^2$ varies in the range of 0~1. The closer the value is to 0, the better the suspension ability of the material is. Bohnet et al. divided the suspension into three states according to the concentration standard variance $\sigma$: $\sigma \le 0.2$ belongs to the uniform suspension state, $0.2<\sigma <0.8$ belongs to the preliminary suspension state and $\sigma \ge 0.8$ belongs to the incomplete suspension state [22]. The percentage of suspension at the corresponding sampling points is: $80\%\le C\le 120\%,$ which is consistent with the uniform suspension state, $20\%<C<80\%$ or $120\%<C<180\%,$ which is consistent with the preliminary suspension state and $C\le 120\%$ or $C\ge 180\%,$ which is consistent with the incomplete suspension state.

2.2. Test Methods

2.2.1. Stripping Test of Fine Slime on the Surface of Coal Slime

Low-ash cleaned coal (ash content A_d = 5.97%) and kaolin (whiteness ≥ 90%) with a particle size less than 0.045 mm were prepared and mixed evenly according to the mass ratio of 4:1 to make the artificially mixed coal sample (A_d = 21.63%). One gram of manually mixed coal sample was added smoothly into 100 mL water, and after standing for 1 min, it was poured into the laser particle size analyzer (SALD-7101, Shimadzu, Kyoto, Japan) and stirred at a speed of 500 r/min. The particle size distribution of the suspension was tested at 1 min, 1.5 min, 2 min, 2.5 min, 3 min, 3.5 min, 4 min, 4.5 min, 5 min and 5.5 min. A certain amount of artificially mixed coal samples was taken to prepare the pulp with a concentration of 10 g/L. After standing for 1 min, the jet mixing system was started (feed volume Q = 1.41 m³/h, nozzle outlet velocity V = 5 m/s). Random samples were taken in the test tank and divided into two parts; one part was tested by laser particle size distribution, and the other part was sieved with a 0.045 mm sieve. SEM (scanning electron microscope, VEGA3SBH, TESCAN ORSAY HOLDING, Brno, Czechia) and EDS (energy dispersive spectrometer, XFlash6130, Bruker, Billerica, MA, USA) tests were carried out after the oversize product was oven dried.

2.2.2. Material Suspension Ability Test

The fixed parameters of the test are as follows: the distance from the throat tube outlet to the bottom of the tank is 100 mm, the particle size of the coal sample (true density ρ = 1.35 kg/L) is 0.5~0.25 mm and the pulp concentration is 160 g/L. The length and width of the tank are 500 mm, the height is 400 mm and the volume of the tank is 100 L.

(1) Test the influence of jet velocity on suspension ability: the nozzle outlet velocities were 6 m/s, 9 m/s, 12 m/s and 15 m/s (flow rates were 1.7 m³/h, 2.54 m³/h, 3.39 m³/h and 4.24 m³/h, respectively). The sampling method was used to measure the concentration of the suspension in the mixing tank to explore the critical nozzle jet velocity of material suspension at a high concentration. The mixing tank has an axisymmetric structure and a rectangular bottom, so a quarter of it was taken as the representative. The center point at the bottom of the tank (i.e., the central impact point of the jet flow) was taken as the origin, and the corresponding directions of length, width and height were set to X, Y and Z axes, respectively. The locations of sampling points in the X-Y plane are shown in Figure 2; and the height of sampling points on the Z-axis were: 50 mm, 150 mm, 250 mm and 350 mm.

(2) Test the influence of jet height on suspension ability: the nozzle outlet velocity was set as 9 m/s, and the height between the throat tube outlet and the bottom of the tank was adjusted to 50 mm, 100 mm, 150 mm, 200 mm and 250 mm by moving the lifting platform up and down to explore the optimal value range of the jet height.

2.2.3. Jet Mixing Test of Different Jet Fluid

In the jet mixing flow field, gas or liquid is usually used to bring the quantitative reagent through the jet tube, and the jet fluid velocity and material circulation time have a significant effect on the mixing performance [23,24].

(1) Test the influence of gas jet velocity on reagent adsorption: start the jet mixing system (the low-ash cleaned coal is made into coal pulp with a concentration of 80 g/L, the feed volume is 2.03 m³/h, the nozzle outlet velocity V = 7.2 m/s, the tank volume is 13 L and the material cycle period T = 20 s), add the reagent by jetting the gas. The suction intensity was adjusted to 600 L/h, 500 L/h, 400 L/h, 300 L/h, 200 L/h and 100 L/h, and the collecting reagent n-dodecane (molecular formula is CH₃(CH₂)₁₀CH₃ and the dosage is 1200 g/t) was evenly added in a material cycle period. At the end of pulp mixing, 300 mL of pulp was taken. After standing for 24 h, 50 mL of supernatant was taken, 0.0031 g phthalocyanin blue and 0.375 g β-cyclodextrin (C₄₂H₇₀O₃₅) were added and the absorbance was measured after stirring for 10 min and standing for 5 min.

(2) Test the influence of liquid jet velocity on reagent adsorption: the specific test procedure is the same as (1), and the jet water volume is set as 180 L/h, 150 L/h, 120 L/h, 90 L/h, 60 L/h and 30 L/h.

(3) Test the optimal circulation time of jet pulp: under the optimal jetting conditions (minimum absorbance) of the above tests, 0.5 T, T, 1.5 T, 2 T, 2.5 T and 3 T cycle mixing time (the material cycle period T = 20 s) were added after the end of one jet mixing cycle to carry out the above tests again.

3. Results and Discussion

3.1. Stripping Rule of Fine Slime on the Surface of Coal Slime

After sample preparation, the contact angles of the artificially mixed coal sample were measured. It can be seen from Figure 3 that the contact angle of low-ash cleaned coal is about 112°, showing a spherical shape with good hydrophobicity. The contact angle of the artificially mixed coal sample is about 53°, showing a parabolic shapewith average hydrophobicity. It indicated that the sample preparation meets the requirements.

Figure 4 indicates the jet flow field has strong material dissociation and dispersion effects. The cumulative yield of fine particle material in suspension increased significantly after the action of the jet device, and the effect of single jet mixing was equivalent to that of the laser particle size analyzer with its own stirring device after stirring for 2.5 min at the speed of 500 r/min.

Figure 5 demonstrates that before jet cleaning, the surface of the artificially mixed coal sample is coated with a large number of particles of different particle sizes and variable shapes. After jet cleaning, these particles are greatly reduced. It shows that jet flow cleaning can effectively remove the fine mud wrapped on the surface of coal particles, and the effect is remarkable.

An energy dispersive spectrometer was used to test the changes of elements in the material before and after jet mixing, so as to visually judge the effect of jet flow field on the stripping of fine slime on the surface of coal particles. As can be seen from Figure 6, there are C, O, Al and Si elements in the EDS spectrum of artificially mixed coal samples before jet mixing, among which, C element has the highest peak value, and O, Al and Si also have obvious peaks. After jet mixing, the peak values of Al and Si elements decreased significantly, indicating that the kaolinite coated on the surface of coal particles was partially cleaned and stripped.

3.2. Material Suspension Behavior in Jet Mixing Flow Field

It can be seen from Figure 7 that when the jet velocity is 6 m/s, all sampling points in the tank space are in a preliminary suspension state. When the jet velocity increases to 9 m/s, except for one sampling point, the other 55 sampling points are in a uniform suspension state, and the critical jet velocity is in the range of 6 m/s~9 m/s.

Figure 8 shows that the suspension percentage of sample point materials at different depths when the jet velocity is 6 m/s. It can be seen from the figure that in the preliminary suspension state, the closer the place to the bottom of the tank is, the higher the material concentration is, and the concentration in the high liquid level interval tends to be the same. Along the X-axis direction, the concentration of the material decreases as the distance from the center of the flow stream becomes more distant. It can be seen that the premise of material suspension is to have the just-suspended capacity, and the jet velocity restricts the material suspension ability.

As demonstrated in Figure 9, when the jet height is 150 mm, all sampling points in the tank space are in a uniform suspension state. The increase or decrease of the jet height will cause a decrease of the suspension percentage of the high liquid level sampling point. When the jet height increases to 250 mm, the suspension percentage of the high liquid level sampling point decreases rapidly, indicating that the mixing ability of the jet stream has certain limitations.

3.3. Effect of Different Jet Fluid on Mixing Performance

It can be seen from Figure 10 that the absorbance is the lowest when the jet velocity of gas and liquid is 0.86 m/s and 0.13 m/s, respectively, that is, the adsorption capacity of coal slime and reagent is at the maximum. In this case, the jet velocity is the optimal value, and the optimal value of the gas jet mode is better. When the jet velocity continues to increase, the adsorption effect of the reagent becomes worse. The main reason is that when the jet velocity increases, the reagent will be mixed instantaneously, and it is not conducive to forming the uniform blending of the working fluid and the jet fluid, indicating that the quantitative linear dosing method should be adopted in the jet flow field.

It can be seen from Figure 11 that the gas and liquid jet modes require a 1.0 T and 1.5 T cycle time, respectively, to achieve uniform mixing and full adsorption of the reagent and coal pulp. The action time of the gas jet mode is lower than that of the liquid jet mode. The main reason is that the collecting reagent n-dodecane is a hydrophobic reagent. In the ejection process, the reagent acts on the surface of the bubble liquid film to form oil bubbles, which is more conducive to interacting with the low-ash hydrophobic coal slime.

4. A Jet Device of Coal Slime Graded Mixing

4.1. Device Structure and Working Process

Based on the above experimental conclusions, a jet device applied to coal slime graded mixing is proposed. The structure is shown in Figure 12, which is mainly composed of three parts: jet mixing system, box body and sliding system.

The main working process of the device is as follows: after the coal pulp with certain pressure enters the jet mixing device, a high-speed jet stream is formed at the exit of the inner nozzle; because of the suction effect of the jet, a negative pressure is formed in the inner cavity of the device. The reagent is drawn into the inner cavity from the inlet on both sides, and is dispersed into tiny small oil droplets by high-speed jet impact and cutting, forming the reagent mixture liquid, which is fully mixed and dispersed again through the wavy mixing unit at the lower part of the throat tube, and finally discharged by the dispersion tube.

The coal pulp is evenly spilled over onto the first skid through the annular feeding weir, and it is collided and mixed with the reagent mixture liquid. The pulp jumps and falls continuously through the trapezoidal ridges of the skid, the coarse particles move closer to the skid and are blocked by the trapezoidal ridges, passing through each skid one by one.It promotes the coarse particles mixing for a long time, and can make them adhere to enough oil film. The fine particles move with the upper water current, and skip the second, third and fifth skids under the action of the first and fourth outlet parting baffle. The pretreatment time of fine particles is short, so the excessive adsorption of collecting reagent can be avoided, which is beneficial to slow down the floating speed of the slime in the flotation machine, and reduce the pollution of high-ash fine slime in the flotation-cleaned coal.

4.2. Effect Test Results of the Jet Mixing Device

The proposed jet mixing device was used to carry out the coal slurry blending process, and then the flotation test was carried out to test the performance of the device. The flotation perfection index and recovery rate of cleaned coal combustibles were used as the evaluation index.

$${\eta }_{\omega f}=\frac{100{\gamma }_j}{100-A_{d,y}}\times \frac{A_{d,y}-A_{d,j}}{A_{d,y}}$$

(3)

$$\epsilon =\frac{{\gamma }_j\left(100-A_{d,j}\right)}{100-A_{d,y}}\times 100\%$$

(4)

where ${\eta }_{\omega f}$ is the flotation perfection index, %; $\epsilon$ is the recovery rate of cleaned coal combustibles, %; ${\gamma }_j$ is the yield of flotation cleaned coal, %; $A_{d,j}$ is the ash content of flotation cleaned coal, %; and $A_{d,y}$ is the ash content of flotation raw coal, %.

The test results are shown in

Table 1. Test results of coal slime flotation mixing performance.

Jet Mode	Particle Size/mm	Flotation Cleaned Coal		Flotation Tailings		Calculation Results
		Yield/%	Ash Content/%	Yield/%	Ash Content/%	Calculate Ash Content of Feed/%	Flotation Perfection Index/%	Recovery Rate of Cleaned Coal Combustibles/%
Optimal batch flotation test	0.5–0.25	55.87	7.34	44.13	63.76	32.24	63.68	76.40
	0.25–0.075	59.25	9.59	40.75	69.32	33.93	64.33	81.08
	−0.075	38.93	12.93	61.07	45.94	33.09	35.45	50.66
Fluid jet mode	0.5–0.25	65.65	10.79	34.35	76.06	33.21	66.35	87.69
	0.25–0.075	62.44	12.29	37.56	68.97	33.58	59.60	82.46
	−0.075	63.57	11.80	36.43	71.80	33.66	62.23	84.51
Gas jet mode	0.5–0.25	64.66	11.21	35.34	72.80	32.97	63.69	85.66
	0.25–0.075	63.00	13.09	37.00	71.16	34.58	59.83	83.69
	−0.075	62.66	11.80	37.34	70.77	33.82	61.65	83.50

, and the comparison of the coal slime mixing test and the batch flotation test is shown in Figure 12.

It can be seen from Table 1 and Figure 13 that the flotation perfection index and the recovery rate of the cleaned coal combustible body show an overall increasing trend after the pulp adjustment by the jet mixing device. The flotation perfection index of 0.5~0.25 mm and less than 0.075 mm increased by 2.67% and 26.78%, respectively, but the flotation perfection index of 0.25~0.075 mm decreased by 4.73%. The overall effect of the coal slime- graded mixing jet device is good; however, there is the problem of material separation deviation in the 0.25~0.075 mm particle size. A part of them moves with the upper water flow and skips the second, third and fifth skids, resulting in a short pretreatment time.

5. Conclusions

• The jet flow field has strong material dissociation and dispersion effects, and the effect of single jet mixing was equivalent to that of the laser particle size analyzer with its own stirring device after stirring for 2.5 min at the speed of 500 r/min. The result of SEM test and EDS tests show that jet cleaning can effectively remove the fine mud wrapped on the surface of coal particles, and the effect is remarkable.
• The material suspension rule in the jet mixing flow field shows that the premise of material suspension is to have the just-suspended capacity, and in the coal slime flotation application field, the critical jet velocity of material suspension is in the range of 6 m/~9 m/s. The increase or decrease of the jet height will cause the decrease of the suspension percentage of the high liquid level sampling point. When the jet height increases to 250 mm, the suspension percentage of the high liquid level sampling point decreases rapidly, indicating that the mixing ability of the jet stream has certain limitations.
• The effect of different jet fluids on mixing performance shows that the optimal jet velocity of gas and liquid are 0.86 m/s and 0.13 m/s, respectively; and the minimum contact cycle time of full adsorption are 1.0 T and 1.5 T. The quantitative linear dosing method is conducive to the uniform mixing of coal slime and reagent (air bubbles). The gas jet method can promote the reagent to act on the surface of bubble liquid film to form oil bubbles, which is more suitable for hydrophobic mineral flotation.
• Based on the mixing mechanism of multiphase materials in the jet mixing flow field, a jet device suitable for coal slime graded mixing was proposed. The experiment result showed that the flotation perfection index and the recovery rate of cleaned coal combustibles show an overall increasing trend after the pulp adjustment by the jet mixing device. The flotation perfection index of 0.5~0.25 mm and less than 0.075 mm increased by 2.67% and 26.78%, respectively, but the flotation perfection index of 0.25~0.075 mm decreased by 4.73%. The overall effect of the coal slime-graded mixing jet device is good; however, there is the problem of material separation deviation in the 0.25~0.075 mm particle size. In the future, the precise hierarchical regulation mechanism should be further studied.