Effect of Stirring on Oil-Water Separation in Rare Earth Mixer-Settler

Wang Shuchan; Zhang Ting’an; Zhang Zimu; Lü Chao; Zhao Qiuyue; Liu Yan

(School of Materials and Metallurgy of Northeastern University, Key Laboratory of Ecological Utilization of Multi-metal Intergrown Ores of Education Ministry, Shenyang 110819)

Effect of Stirring on Oil-Water Separation in Rare Earth Mixer-Settler

Wang Shuchan; Zhang Ting’an; Zhang Zimu; Lü Chao; Zhao Qiuyue; Liu Yan

(School of Materials and Metallurgy of Northeastern University, Key Laboratory of Ecological Utilization of Multi-metal Intergrown Ores of Education Ministry, Shenyang 110819)

Oil-water separation is critical to solvent extraction process of rare earth, which can directly affect the yield and quality of the product. The experiments measure the two-phase separation time in a beaker, mixing uniformity of two phases in the mixer and the oil phase entrainment at oil exit by the Karl Fischer method and numerical simulation for the mixersettler to study the combined effect of gravity and stirring. Experimental results show that relative to the static situation, the separation efficiency resulted from low-speed stirring is increased by 25%. The water content in the oil is a minimum at an offset distanceLof 10 cm and the clearance off the tank bottomzof 10 cm is as low as 0.49%. Distribution images of oilwater separation at 2 s indicates that stirring is very conducive to the separation of the oil-water phase.

oil-water separation; water content; stirring; mixer-settler.

1 Introduction

Oil-water separation is critical to solvent extraction process of rare earth, which can directly affect the yield and quality of the product. With the rapid development of industry and the heightened environmental awareness, the oil-water separation technology is drawing more attention[1-2]. The oil and water system is composed of a mixture of two immiscible liquids. Tiny droplets of water dispersed in oil are known as “water in oil” (the symbol is W/O), in which water is the dispersed phase, and oil is the continuous phase. In the mixer-settler for rare-earth extraction, oil-water separation generally relies solely on the difference in density of the two phases[3]. The lack of twophase driving force restricts the development of mixersettler.

Based on the methods referred to in the literature[4-6], the patented technology at home and abroad about the separation process of the extraction tank is clarified by changing the extraction tank internal structure to strengthen the two-phase separation process. For example, the YL-1 type materials for gathering device can accelerate the aggregation of dispersed phase, while adding a grid plate in the clarification chamber entrance can form a mixed phase flow area, and a multiplicity of baffles horizontally placed can realize the inclined plate clarification. However, this paper puts forward a new idea, viz.: “Combined effect of gravity and stirring to achieve liquid-liquid separation”, that is to propose insertion of a stirring unit in the settler after reducing its volume[7-9]. If the separation time of the aqueous and oil phases is shortened to match with the mixing time, the production capacity can be greatly improved.

2 Experimental

2.1 Experimental materials and apparatus

The oil phase used in this study consisted of saponated P507 and sulfonated kerosene mixed at a ratio of 1:1. The density of oil phase was 890 kg/m3with its viscosity equating to 11.8×10-3Pa·s. The aqueous phase was a lanthanum chloride solution, with its density equating to 1 011 kg/m3and its viscosity—1.24×10-3Pa·s. The interfacial tension between oil and water was 27.2×10-3N/m. The mixer-settler is still widely used in industrial production as a kind of extraction equipment, which is a chain of contactive mass transfer equipment. The mixer-settlerconsists of a mixer and a settler. In the mixer, the feed solution and the solvent are in close contact for enhancing mass transfer, and then are routed to the settler for phase separation. The mixer is fitted with a stirrer for promoting droplets breaking. The settler is a large vacuum chamber with a horizontal cross-sectional area, sometimes equipped with a guide plate or a wire mesh to accelerate the droplet coalescence and demixing.

The experimental set-up consisted of a three-stage mixersettler, six agitators, feed tanks and pumps. Figure 1 is a three-stage mixer-settler with double stirring. The mixersettler is made of plexiglass. The single mixer was 20 cm in length, 20 cm in width and 30 cm in height. The single settler was 25 cm in length, 20 cm in width and 30 cm in height. There was an overflow port between two rooms. The ratio of the mixer volume and the settler volume was 1.25:1, far less than that of a ratio of 2.5:1 in the industry.

A six-flat-blade disc turbine was used in the mixer and its diameter was 10 cm. The four-pitched-blade turbine (four-leaf paddle, and reverse 45° -pitched blades, with a blade width of 2 cm and a diameter of 10 cm) was installed in the settler.

2.2 Experimental method and procedure

The performance of settler was evaluated according to the amount of entrained organic phase and aqueous phase. Formerly, I. E. Lewis used glass bottle with a capillary to determine the phase entrainment quantity[10]. The entrainment quantity was calculated from the height of organic phase or aqueous phase in the capillary. A bottle was filled with the sample through a feed tube, and was subjected to centrifugation for 30 minutes in an ultracentrifuge. In this paper, a new method was adopted. The organic phase entrainment was investigated in order to measure the separation efficiency by means of the Karl Fischer method.

The Karl Fischer reagent is a commonly used reagent for moisture content determination, which since 1935 was proposed by Karl Fischer. This method has the advantages of high sensitivity and simple operation. The analytical apparatus of this kind made in Shanghai was adopted in this experiment.

Firstly, 1 000 mL of oil phase and 500 mL of water phase were placed in a 2-liter beaker. The mixture was stirred at 750 r/min for 10 minutes. The mixture was allowed to settle and then was again stirred at 50 r/min alternatively. The time of liquid-liquid separation was obtained by determining the water content of the oil phase in a volume of 600 mL. The impeller type was also a four-pitchedblade turbine.

The operating procedure in the mixer-settler was as follows: The mixer was first filled with the oil phase and aqueous phase at a ratio of 2:1. The system was then allowed to run. The flow rate of oil phase and aqueous phase was 40 L/h and 80 L/h, respectively. The flow rate of both phases entering the mixer was measured by two glass flowmeters mounted vertically in the feed lines. Two phases in the settler were separated and discharged from the settler through overflow tubes into respective feed tanks. Once a steady state was reached in the system, the water content in the oil phase was measured by sampling at the oil phase exit.

In this paper, based on the Eulerian-Eulerian approach, a liquid-liquid two phase flow model is established to describe the basic control equation in the single-stage mixersettler. The control equations of the dispersed phase and continuous phase are similar in the form. The solution can also follow the single-phase solution method of computational fluid dynamics. The mathematical model for fluid flow in this two-liquid-flow system is based on the following assumptions:

(1) Since the dispersed phase and the continuous phase comprise the continuous media, the two phases co-exist in the same three-dimensional space, and the arbitrary control body in the flow field is dominated by two types of fluids at the same time;

(2) Two fluids follow the respective control equation;

(3) Fluid is an isothermal flow;

(4) The nature of the dispersed phase is similar to a rigid sphere having a uniform particle size; and

(5) The continuous and dispersed phases are incompressible fluids.

3 Results and Discussion

3.1 Experiment in a beaker

Under alternative conditions of static situation and lowspeed stirring, the water content in oil was measured at different time intervals. The results are shown in Figure 2.

Figure 2 Water content in oil phase at different time intervals

The process of oil-water separation can be divided into two stages. At first, the coarse droplets rapidly move to an interface, enter the overall phase and form an obvious phase interface. This process can be described as initial clarification. Secondly, the fine droplets gather into the interface slowly. This process can be described as secondary clarification.

The experiment completes the process of initial clarification in about 4 minutes. At this time, gravity separation plays a dominant role. Stirring separation hampers the rapid decline of water in the oil phase. After 4 minutes, only by depending on the density difference, the twophase driving force is insufficient. The combination of gravity and stirring is more conducive to the two-phase separation. Two-phase separation time is about 8 min without stirring. The time is about 6 minutes at low speed to achieve a 25% increase in efficiency.

3.2 Experiment in the mixer

The experimental measurement of conductivity in different locations is to determine whether the two phases are mixed uniformly. The equipment used is a DDSJ-308A conductivity meter. This experiment studies the conductivity in different radial and axial positions. When the conductivity probe is placed at an impeller offset distanceLof 4 cm, 10 cm, 8 cm and 12 cm, respectively, and the clearance under liquid surfacezis 2 cm, 5cm and 8 cm, respectively, then the relationship between conductivity and speed change can be obtained, as shown in Figure 3.

Figure 3 Relationship of conductivity and speed under different radial and axial position

By examining the relationship between conductivity and speed change in different positions, it can be seen that the average conductivity is stabilized in the mixer after the speed reaches 400 r/min. By considering the experimental conditions, 450 r/min is chosen as the mixer stirring speed adopted in this study.

3.3 Experiment in the settler

In the three-stage mixer-settler, the volume of settler is reduced, the mixing time is about 4 minutes and the separation time is around 5 minutes. The goal is basically reached so that the time for separation of the oil and water phase is shortened to match with the mixing time.

The separation of the two phases was studied by varying the parameters, such as the stirring speeds, the offset distance and the distance from the bottom. The impeller offset distance is just the distance from the impeller position to the overflow port. The results are shown in Figure 4.

Figure 4 Water content in oil phase at different impeller position

The impeller offset distanceL(the distance from the impeller axis to the partition plate which is located between the mixer and the settler) and the clearance off the tank bottomzare important variables in the double mixing operation of the rare earth extraction. When the impeller is placed at an impeller offset distanceLof 7.5 cm, 10 cm, 12.5 cm and 15 cm, respectively, and the clearance off the tank bottomzis 4 cm, 7 cm and 10 cm, the separation effects at 50 r/min are measured.

When the offset distance increases, the water content of the oil phase first decreases and then increases. It reaches a minimum atL=10 cm, and the water content in the oil phase is as low as 0.49%. This occurs because the offset distance is too small, when the mixture just enters the settler. The process belongs to a kind of initial clarification. The gravity plays a dominant role and stirring separation is not obvious.

When the offset distance is too large, the impeller is too close from the oil phase exit. At this point, under the joint action of gravity and stirring, two phases would not have enough space to separate from each other except for being discharged from the settler.

When the clearance off the tank bottom is 7 cm and 10 cm, the water content of the oil phase is better than the case with a clearance of 4 cm. This shows that the impeller in a mixed zone can more effectively promote the twophase separation.

3.4 Numerical simulations

In this work, numerical analyses were performed in the mixer-settler with double stirring. Using standardk-εturbulence model coupled with the Eulerian granular multiphase model, the distribution images of oil-water separation at 2 s are obtained, as shown in Figure 5. The blue line at the left side represents the water entrance, while the red line represents the oil entrance. The upper line at the right side is the outlet of oil phase and the lower line at the right side is the water outlet.

Figure 5 Distribution images of oil-water separation at 2 s

Figure 5 is the inside view of the liquid pool showing the separation of oil and water under different stirring speeds at the same time. The difference is evidenced upon comparing Figure 5(a) with a settler with no stirring and Figure 5(b) with a stirring speed of 20 r/min, so it is obvious that the main difference stems from different stirring speeds. Figure 5 shows that the flow state in the two mixers is similar, and the oil and water phases are wellmixed. However, the difference of oil-water separation is very obvious in the settlers.

Figure 5(a) relies mainly on water-oil separation by gravity, the oil phase and the aqueous phase are stratified andevenly distributed at the top and bottom of the liquid pool, respectively. Figure 5 (b) indicates the separation by combination of gravity and stirring, and therefore the oil phase at the top of the stirring blade followed a vortexlike distribution, while the oil phase underneath the stirring blade followed an inverted vortex-like distribution due to the stirring effect. By contrast, Figure 5 (b) shows that the pure oil phase distribution area is significantly greater than that in Figure 5 (a), indicating that stirring is very conducive to the separation of the oil-water phase.

4 Conclusions

The experiments measure the water content in the oil phase by means of the Karl Fischer method, compare the liquid-liquid separation time between the static situation case and the low-speed stirring condition, design their own three-stage mixer-settler, and study the influence of the two-phase separation of oil and water on the impeller position. The results have shown that:

(1) The two-phase separation time is about 8 min without stirring. The time is about 6 minutes at low stirring speed, leading to a 25% increase in efficiency.

(2) By taking into account the relationship of conductivity and speed under different radial and axial positions, 450 r/min is considered as the appropriate value for specifying the mixer stirring speed.

(3) With the increase in the offset distance, the water content of the oil phase firstly decreases and then increases. It reaches a minimum atL=10 cm, with the water content equating to as low as 0.49%.

(4) When the impeller distance from the bottom is 7 cm and 10 cm, respectively, the water content is better than the case with an impeller distance of 4 cm. In general, the water content reaches a lowest value atL=10 cm.

(5) By using the standardk-εturbulence model coupled with the Eulerian granular multiphase model, distribution images of oil-water separation at 2 s can be obtained, indicating that stirring is very conducive to the separation of the oil-water phase.

Acknowledgment:This research was financially supported by the National 863 Plan (2010AA03A405, and 2012AA062303), the National 973 Plan (2012CBA01205), the National Natural Science Foundation of China (U1202274, 51204040), the National Science and Technology Support Program (2012BAE01B02) and Fundamental Research Funds for the Central Universities (N130702001 and N130607001).

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Received date: 2014-02-17; Accepted date: 2014-08-06.

Dr. Zhang Ting’an, Telephone: +86-24-83690459; E-mail: zta2000@163.net.