Experimental Study on Aqueous Phase Entrainment in a Mixer-settler with Double Stirring Mode

Wang Shuchan; Zhang Tingan; Zhao Qiuyue; Liu Yan; Wu Qiuyang

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

Experimental Study on Aqueous Phase Entrainment in a Mixer-settler with Double Stirring Mode

Wang Shuchan; Zhang Tingan; Zhao Qiuyue; Liu Yan; Wu Qiuyang

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

The mixer-settler is a core device of solvent extraction for separating rare earth elements. There are some adverse effects like high rare earth accumulation and poor production efficiency during industrial production. Current researches usually focus on changing the structure of the mixer-settler without making a breakthrough towards gravity clarification. In this paper, in order to improve the efficiency of clarification, a mixer-settler with double stirring mode was designed and manufactured by adding a stirring device in the settler after reducing the volume of the settler. The innovation of this research involves adopting the ultraviolet-visible spectrophotometer to investigate the quantity of aqueous phase entrainment at the settler outlet in order to measure the clarification degree. Experimental results show that the clarification effect with stirring is better than that without stirring. The clarification effect is ameliorated as the stirring speed increases. Generally, the clarification effect shows a best condition when the offset distance is 12.5 cm, making the phase entrainment reduced to less than 0.1%. When the clearance over the tank bottom is 7 cm and 10 cm, respectively, the quantity of aqueous phase entrainment is better than the case with a clearance of 4 cm. The results show that the stirring paddle close to the mixed phase zone can better promote the two-phase separation.

mixer-settler; aqueous phase entrainment; stirring speed; clearance over the tank bottom; offset distance

1 Introduction

With the rapid development of the rare earth industry, the solvent extraction has become a major hydrometallurgical process for the rare earth separation. Solvent extraction operation can be carried out in various types of devices. However, mixer-settler is a core device for solvent extractive separation of rare earth elements thanks to its advantages in better adaptability and simple operation. Its disadvantages are big floor space and high production accumulation[1-2].

However, the mixer-settler has some shortcomings. The volume ratio between single-stage settler and mixer in China’s rare earth industry is generally above 2.5:1. The mismatching of two-phase separation rate and mixing extraction rate leads to rare earth accumulation and poor production efficiency, which has become a bottleneck in the rare earth industry[3].

According to the literature[4-6], domestic and international patented technology regarding the separation process in the extraction tank is realized by changing the structure of extraction tank (such as separating the settler into different portions, adding baffles or return channel, etc.) to optimize the two-phase separation process. These classic methods cannot eliminate the bottleneck of gravity clarif ication, making it difficult to significantly improve the rate of two-phase clarification-separation and the production efficiency of the unit equipment[7-8]. Therefore, this paper puts forward a new concept “high-efficiency extraction and clarification tank with double stirring mode”, involving the insertion of a mixing device in the settler after reducing its volume. Appropriate stirring conditions are studied so as to improve the separation rate of aqueous phase and organic phase in the clarification/extraction process and enhance the efficiency of the extraction unit[9].

2 Experimental

2.1 Experimental set-up and procedure

When the volume of the settler is reduced, the time for separation of the aqueous phase and organic phase is shortened to match with the mixing time, so that the production efficiency can be greatly improved. This paper attempts to achieve this goal by using both the action of clarification by gravity and clarification by stirring.

A new type of mixer-settler was designed at a settler/mixer volume ratio of 1.25:1, which was far less than that of conventionally used in the industry volume ratio of 2.5:1. The mixer volume was 200 mm×200 mm×300 mm, whereas the settler volume was 250 mm×200 mm×300 mm. There is an overflow hole between these two rooms. The mixer-settler and two tanks were all made of transparent acrylics.

In accordance with the process of industrial production systems, an experimental mixer-settler device with double stirring mode was designed. The organic phase and the aqueous phase (lanthanum chloride solution) were fed to the mixer by two centrifugal pumps and were fully mixed at a rotary speed of 450 rpm by means of a turbine pump equipped with six-vertical-leaf disc turbine agitator, 10 cm in diameter. The organic phase flow rate was 80 L/h and the water phase flow rate was 40 L/h. The flow ratio between two phases was 2:1. The flow rates of both phases entering the mixer were measured by two glass flowmeters mounted vertically in the feed lines. Two phases in the settler after being separated by reverse stirring with an impeller (four-leaf paddle, equipped with 45°-inclined blades, 2 cm in width and 10 cm in diameter) exited the settler through overflow tubes into their respective feed tanks in order to achieve a two-phase circulation loop. The experimental flow diagram is shown in Figure 1.

The operating procedure was as follows. The mixer was at first filled with organic phase and aqueous phase at a ratio of 2:1 (by volume). The system then started its operation. Once a steady state was reached in the system, the oil content in the aqueous phase is measured through sampling at the aqueous phase discharge outlet. The total volume of samples was recorded. The separation of the two phases was studied at different values of stirring speed, offset distance and off-bottom clearance. The impeller offset distance was just the distance from impeller position to the overflow port.

Figure 1 Schematic diagram of the experimental mixer-settler unit

2.2 Experimental materials and method

The organic phase used in this study was a solution of saponated P507 and sulfonated kerosene mixed at a volume ratio of 1:1. The aqueous phase was a lanthanum chloride solution containing 0.083 mol/L of lanthanum chloride, the pH value of which was adjusted with hydrochloric acid to 5.46. After the extraction reaction, pH value of the aqueous phase dropped to 1.53 and the concentration of lanthanum chloride was 0.054 mol/L.

The performance of settler was evaluated according to the measured organic phase and aqueous phase entrainment data[10]. In this paper, a new method was adopted. By using the organic phase as the solute and petroleum ether as the solvent, a series of organic phase containing samples were accurately prepared. The absorbance of the abovementioned samples was determined by an ultravioletvisible spectrophotometer. The absorbance at 268 nm was analyzed and the results are shown in Figure 2.

Through linear fitting to the experimental data, a regression equation is established:

The correlation coefficient is 0.99838. The relative error is small and the results are reliable after experimental certification.

The sample entrained the organic phase, the total volume of which was measured (about 10 mL). After the sample was moved into a volumetric flask, 10 mL of petroleum ether was pipetted accurately in the flask, which was fullyoscillated for five minutes in order to enhance the dissolution of organic phase in petroleum ether. By measuring the organic phase concentration, the oil content in the sample could be calculated.

Figure 2 Curve of absorbance at 268 nm versus organic phase content

3 Results and Discussion

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 over the tank bottom are important variables for the double mixing mode of the rare earth extraction process. When the impeller was placed at an impeller offset distancelof 10 cm, 12.5 cm and 15 cm, respectively, the clearance over the tank bottomzwas equal to 4 cm, 7 cm and 10 cm, respectively, and the clarification effects at different stirring speed could be measured.

Figure 3 Effect of impeller offset distance at z=4 cm on aqueous phase entrainment

It can be seen from Figure 3 that stirring can promote liquid-liquid separation at a lower stirring speed.

Figure 4 Effect of impeller offset distance at z=7cm on aqueous phase entrainment

Figure 4 shows a trend similar to the case referred to in Figure 3. The stirring operation can break down the balance between the aqueous phase and the organic phase and help dispersion drops move vertically towards the active interface. However, when the impeller was placed atl=15cm, the aqueous phase entrainment increased suddenly. This might occur because the impeller was too close to the aqueous phase outlet. At this point, under the joint action of clarification by gravity and clarification by stirring, the two phases would not have enough space to separate and exit from the settler.

Figure 5 Effect of impeller offset distance at z=10 cm on the aqueous phase entrainment

It can be seen from curves presented in Figures 3-5 that basically as the stirring speed increased, the oil content in aqueous phase decreased gradually, indicating that the clarification effect gradually improved. When the distancefrom the bottom was 7 cm and 10 cm, respectively, at an impeller offset distance ofl= 12.5 cm the oil content in aqueous phase was the lowest and less than 0.1%. Meanwhile, the oil content decreased with an increasing stirring speed at an impeller offset distance ofl=10 cm. The oil content showed little change at an impeller offset distance ofl=12.5 cm. When the impeller offset distancelwas equal to 15 cm, the aqueous phase entrainment increased suddenly, because the impeller was too close to the aqueous phase discharge outlet.

4 Conclusions

In the mixer-settler with a double stirring mode, the separation of the aqueous and organic phases was studied by ultraviolet-visible spectrophotometry at different values of stirring speed, offset distance and off-bottom clearance, and the results are presented as follows.

The clarification effect with stirring was better than the case without stirring. In general, the clarification effect showed a best outcome when the offset distance was 12.5 cm and the off-bottom clearance was 4 cm, while the oil content in aqueous phase was less than 0.1%. When the clearance over the tank bottom was 7 cm and 10 cm, respectively, the aqueous phase entrainment was better than the case with a clearance over the tank bottom equating to 4 cm. This study has shown that the impeller in a mixed zone can better improve the two-phase separation process.

Acknowledgements:The project is financially supported by Hi-Tech Research and Development Program (863) of China (2010AA03A405) and the National Natural Science Foundation of China (No. 50974035, No. 51004033)

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Recieved date: 2012-12-06; Accepted date: 2013-04-16.

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