Treatment of rare earth wastewater by emulsion liquid membrane

Weinan is rich in rare earth resources, and its ion-adsorbed rare earth ore reserves rank first in the world. At present, most rare earth separation plants in southern Anhui are directly discharging rare earth wastewater, causing great waste of rare earth resources and causing serious pollution to the environment. Therefore, if a process for treating rare earth wastewater can be developed, it will greatly promote the comprehensive utilization of rare earth resources in China and greatly improve environmental pollution. Rare earth wastewater (C (Re ₂ O ₃ ) = 115 ~ 200 mg / L), provided by a rare earth separation plant in southern Anhui.

Liquid membrane because of its high efficiency, good selectivity, enrichment, has been widely used in the hydrometallurgical, wastewater treatment and other advantages over the art. A lot of researches have been carried out on the extraction of rare earths by liquid membrane method at home and abroad: Qin Qingwei and other studies have studied the enrichment of rare earths in emulsion membranes, and the emulsion liquid membrane method has achieved good results in the extraction and separation of rare earths. However, there are few studies on the treatment of rare earth wastewater by emulsion liquid membrane method. Therefore, if the process of treating rare earth wastewater by emulsion liquid membrane method can be studied, the utilization rate of rare earth will be further improved, and the comprehensive utilization process of rare earth resources in China will be accelerated.

First, the experimental part

(1) Reagents and instruments

Reagents: membrane with Cyanex272 mobile carrier, chemically pure; polyamine surfactant N ₂ O _5, L113B; membrane solvent coal oil, industrial grade; hydrochloride, sodium acetate, commercially pure.

Main instruments: CNC constant speed agitator, milk making device, demulsifier, HI-8424 type acidity meter, 722 spectrophotometer.

(2) Experimental methods

The oil phase membrane formulation used in this experiment was: 3% C ₂₇₂ + 2% L113B + N205 (1:1) + sulfonated kerosene. In the experiment, the membrane oil phase is mixed with the water phase, poured into a milking device, and then stirred at a high speed (rotation speed greater than 10000 r/min) to obtain a milky white liquid, which is a water-in-oil type (W/O) emulsion. liquid. Then, the emulsion and the rare earth wastewater are added to the constant-speed stirring mixer in a certain ratio, and stirred at a rotation speed of 250 to 300 r/min. After standing and stratification, the upper emulsion is transferred to the demulsification device, and the emulsion is broken by pulse high voltage electrostatic method. The oil phase after demulsification is returned to the milk, and the rare earth is recovered from the internal water phase.

Second, the results and discussion

(1) Effect of internal phase acidity on extraction rate of rare earth

The migration of rare earth ions by the mobile carrier follows the reverse mass transfer mechanism. Maintaining a high H ⁺ concentration difference on both sides of the membrane phase is a prerequisite for ensuring the continuous migration of Re ^{3 +} . To ensure this concentration difference, the internal water phase must be higher. The concentration of H ⁺ . In this experiment, HCl was used as the internal water phase, and the effect of the acidity of the internal aqueous phase on the extraction rate of rare earth is shown in Fig. 1 (external phase pH = 2.0; Roi = 3:1; Rew = 1:35; t = 10 min).

Fig.1 Effect of internal phase acidity on extraction rate of rare earth

It can be seen from Fig. 1 that when the internal water phase c (HCl) is less than 2 mol/L, the extraction rate of rare earth increases with the increase of acidity, because the difference of H ⁺ concentration on both sides of the membrane phase increases continuously, and the rare earth is transmitted. The mass velocity is increased, so the extraction rate of the rare earth is increased; if the acidity is too large, the stability of the emulsion film is affected, and the membrane damage is increased, so that the rare earth extraction rate is lowered. Therefore, the internal phase acidity is preferably 2 mol/L.

(II) Effect of external phase acidity on the extraction rate of rare earth

The effect of external aqueous acidity (pH) on the extraction rate of rare earth is shown in Figure 2 (c(HCl) = 2.0 mol/L; Roi = 3:1; Rew = 1:35; t = 10 min). The rare earth extraction rate increases as the pH of the outer aqueous phase increases. When the pH of the external aqueous phase is 0.5~1.5, the extraction rate of rare earth is more obvious. When the pH is 1.5~2.0, the extraction rate of rare earth is 96.5%, and the pH is further increased. The extraction rate of rare earth is basically unchanged. At the same time, as the pH of the outer aqueous phase increases, the extraction of other impurities by the liquid film will also increase. Therefore, the pH of the outer aqueous phase is too large, which is detrimental to the separation of rare earth and other impurities. Since the pH of the rare earth wastewater after oxalic acid precipitation is about 1.5, the pH of the rare earth wastewater treated by NaAc is 1.5 to 2.0, so that the pH of the external phase is preferably pH=2.0.

Fig. 2 Effect of external phase acidity on rare earth extraction rate

(III) Effect of oil ratio on extraction rate of rare earth

Intra-oil ratio (Roi) is the ratio of the oil phase to the internal aqueous phase resolver volume. Roi affects the structure and stability of the emulsion film and also has an important effect on the migration rate of Re ^{3 +} . The effect of the oil ratio on the rare earth extraction rate is shown in Fig. 3 (c (HCl) = 2 mol / L, external phase pH = 2.0, Rew = 1: 35, t = 10 min).

As can be seen from Fig. 3, when Roi = 3.0, 1 or 2.5: 1, the extraction rate of rare earth is relatively high, close to 97%. When Roi is too large, the viscosity of the liquid film increases, the mass transfer rate decreases, the extraction rate of rare earth decreases, and the viscosity of the liquid film is too large, which may cause difficulty in breaking the milk; when the Roi is too small, the viscosity of the liquid film becomes small, and the mass transfer rate is small. Accelerate, but the breakage rate of the liquid film will increase, resulting in a decrease in the extraction rate of rare earth.

Fig. 3 Effect of oil ratio (Roi) on rare earth mobility

(4) Effect of milk water ratio on extraction rate of rare earth

The ratio of milk to water (Rew) refers to the ratio of the volume of the emulsion film system to the volume of the outer aqueous phase. Rew is an important economic and technical indicator reflecting the extraction of liquid film. The Rew is as small as possible under the premise of ensuring the separation effect. The effect of the ratio of milk to water on the extraction rate of rare earth is shown in Fig. 4 (C(HC1) = 2 mol/L, external phase pH = 2.0, Roi = 3:1, t = 10 min).

Figure 4 Effect of emulsion ratio (Rew) on rare earth mobility

As can be seen from Fig. 4, the extraction rate of rare earth decreases as the ratio of emulsion to water decreases. When Rew < 1:35, the extraction rate of the rare earth is remarkably lowered because the emulsion ratio is too small, the stability of the emulsion film is lowered, and the contact area of ​​the emulsion with the external water phase is reduced. If the ratio of milk to water is too large, the consumption of the emulsion is increased, the cost is increased, and the economic requirements are not met. Therefore, it is preferred to select a milk ratio of Rew = 1:35.

(5) Verification experiment under optimal process conditions

The acidity of rare earth wastewater after oxalic acid precipitation is relatively small. After treatment with NaAc solution, the pH of rare earth wastewater is 1.5~2.0, and then the actual rare earth wastewater is treated by emulsion film under the optimal process conditions. Membrane phase: 3% C272 + 2% L113B + N205 (1:1) + kerosene; internal phase c (HCI) = 2 mol / L; oil internal ratio Roi = 3: 1; milk water ratio Rew = 1: 35; extraction time t = 10 min; temperature is room temperature; stirring speed is 250-300 r/min. Under the process conditions, after the rare earth wastewater is treated by the emulsion liquid film once, the extraction rate of rare earth is about 96%, and the rare earth concentration in the rare earth wastewater after treatment is 6.7mg/L, which can reach the standard discharge. After the emulsion is broken, the internal aqueous phase solution has c(Re203)=18.4 g/L; the rare earth has a dense multiple of 100 or more.

Third, the conclusion

The optimum process conditions for the treatment of rare earth wastewater by emulsion liquid membrane method: internal phase c(HC1)=2mol/L, external aqueous phase pH=1.5～2.0, oil internal ratio Roi=3:1, milk water ratio Rew=1:35 .

It is feasible to treat rare earth wastewater by emulsion liquid membrane method. After a batch treatment, the extraction rate of rare earth is 96%, and the concentration of rare earth after treatment is 6.7 mg/L, which can meet the national emission standard.

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