This paper studies transmission behavior of La (III) in dispersed supported liquid membrane (DSLM) of dispersed phase constituted by dispersed supported liquid membrane solution and HCl solution with polyvinylidene fluoride membrane (PVDF) as support and kerosene as membrane solvent, with 2-ethyl hexyl phosphonic acid-single-2-ethyl hexyl ester (PC-88A) and two-(2-ethyl hexyl) phosphoric acid (D2EHPA) as mobile carrier. It also investigates the influence of La (III) transmission by the material liquid acidity, initial concentration of La (III), HCI concentration, membrane solution, and HCI solution volume ratio, resolving agent and carrier concentration, as well as concluding that the optimal transmission and separation conditions are dispersed phase of 4.00 mol/L HCl concentration, 30:30 volume ratio of membrane solution, and HCl solution, within 0.160 mol/L controlled carrier concentration and 4.00 pH value of material liquid. Under the optimal conditions, the La (III) initial concentration of material liquid phase is 8.00
Rare earth metals have wide usage; they cannot only be used alone but also are used in the form of mixed rare earth. Adding a moderate amount of rare earth metals or their compounds in alloy can greatly improve the performance of the alloy; thus, the rare earth elements are also known as vitamin of metallurgical industry. For example, adding some rare earth elements in steel can increase the plasticity, toughness, wear resistance, heat resistance, oxidation resistance, and corrosion resistance [
In order to overcome these above difficulties in the conventional LM systems, a new liquid membrane technique, namely, dispersion supported liquid membrane (DSLM) [
The operation of SLM system is simple, which does not require the introduction of expensive surface active agent [
At present, the literatures on dispersed supported liquid membrane separating and migrating rare earth metal are rare. This work mainly discusses and studies the feasibility of dispersed supported liquid membranes separating and migrating La (III), utilizes membrane module design, carrier optimization, and migration rate control to realize the migration and separation of rare earth metals, researches the migration process, and establishes new methods and a new system of dispersed supported liquid membranes migrating and separating rare earth metals, which is expected to be a breakthrough in the industrial application.
The homemade migration pool of DSLM consists of the material liquid pool, the dispersed pool, and the supported pool, where the capacities of the material liquid pool and the dispersed pool are both 80 mL, equipped with adjustable speed electric blender. The supporting body is hydrophobic porous polyvinylidene fluoride membrane PVDF (Shanghai Yadong Nuclear Grade Resin Co., Ltd.); the diameter of the pore is 0.22 mm, the film thickness 65 mm, the porosity
Schematic diagram of DSLM apparatus.
UV-1200 type spectrophotometer (Shanghai HuiPuDa Instrument Factory), JJ-1 type precise timed electric blender (Jintan City DanYangMen Quartz Glass Factory), UV-2102PC type ultraviolet-visible spectrophotometer (Unico(Shanghai) Instrument Co., Ltd.), AY120 type electronic scales (Shimadzu), 520MPT atomic emission spectrometer (ChangChun JiLin University Little Swan Instruments Co., Ltd.).
They are 1.00 mol/L HAc~ NaAc buffer solution; 1.00 mol/L NaH2PO4~Na2HPO4 buffer solution; 6.00 mol/L HCl; 4.00 mol/L H2SO4; 1.00 × 10−2 mol/L arsenazo III (C22H18As2O14N4S2). Except that the concentration of La (III) is diluted to 1.00 × 10−2mol/L by 1.00 mol/L H2SO4, the concentration of the other standard solutions of various rare earth metal ions should be diluted to 1.00 × 10−2 mol/L with 1.00 mol/L HCl.
Membrane solution is made of the flow carrier of PC - 88 A, the concentration of which is diluted to 0.230 mol/L with kerosene.
The experiment adopts the homemade liquid film transmission device, puts the supported PVDF membrane into the membrane solution to leach and absorb for a certain time (about 3 to 4 hours), then uses the filter paper to suck up liquid on the surface of the membrane, and fixes it in DSLM migration pool. There are two-phase solution in the two slots, respectively, with the isolation of PVDF membrane; one is the material liquid, the other the dispersed phase. A certain amount (5.00 ~ 10.0 mL) of 1.00 × 10−3 mol/L solution of rare earth metals and buffer solution (total 60.0 mL) is added to the material liquid phase, while the 60.0 mL mixture of membrane solution and HCl solution are added to the dispersed phase. Start the blender and time it and then take samples of 1.00 mL to 10.0 mL the colorimetric tube from the material liquid phase at a certain amount of time.
Add a moderate amount of the buffer solution and a certain amount of the 1.00 × 10-4 mol/L chromogenic agent arsenazo III (C22H18As2O14N4S2) into the taken samples and dilute it with the deionized water to 10 mL; after 10 min past the chromogenic reaction, use UV-1200 spectrophotometer to measure the absorbance of La (III) at 653 nm.
According to the relationship curve of the absorbance value and the concentration of rare earth metals, the mobility (such as formula (
At the end of the experiment, it is necessary to treat the membrane solution and remove residual rare earth metals in the solution. This experiment adopts 4.00 mol/L H2SO4 to resolve it, and the membrane solution can be recycled.
The processes of reaction and the migration of metal ions in the dispersed supported liquid membrane system are as follows.
The La (III) ions in material liquid phase diffused through the water between the material liquid phase and the membrane phase. On the interface of the water phase and the membrane phase, the metal ions La (III) will have the following complex reaction with the carrier of PC–88A (abbreviated to HR):
The metal ions with carrier complex generated by the reaction diffuse from the interface between the material phase and the membrane phase to the inside of the membrane [
By [
When the material liquid phase contains two kinds of ions, if the permeability coefficients of the two kinds of ions in the membrane are different, the ions can be separated by the liquid membrane system. The separation factor is defined as
Known from the mass transfer mechanism of rare earth metal ions in the DSLM, differential concentration of H+ in the material liquid phase and the dispersed phase is mass transfer power of rare earth metals in DSLM [
What is more, the pH value of material liquid phase can influence the existence of the rare earth metal ions; under the proper pH value, the rare earth metal ions can form complex carrier with the membrane carrier and enter into the liquid film. If the metal ions are transported, the separation effect is well; otherwise, the separation effect is poor. If the pH value becomes too low, the acidity difference between the material liquid phase and the dispersed phase is too subtle and the migration effect will not be satisfying; If the pH of the material liquid becomes too high, it may cause the rare earth metal ion hydrolyse or form hydroxy complex, which affects the migration rate [
The value ratio of the membrane and HCL solution in the dispersed phase of La (III) in DSLM migration system is selected as 30:30; the HCl concentration of the dispersed phase is 4.00 mol/L; the initial concentration of La (III) is 1.00×10-4 mol/L; the concentration of PC–88A in membrane solution is 0.160 mol/L. Under the condition, study the influence of material liquid pH on the migration behavior of La (III) in the DSLM; the experimental results are shown in Figure
Effect of pH in feed phase on transport of rare earths.
Rare earth metal | (min) Migration Time | Item | Results | ||||
---|---|---|---|---|---|---|---|
La(III) | 125 | pH | 3.00 | 3.30 | 3.60 | 4.00 | 4.30 |
−ln | 0.0503 | 0.483 | 1.40 | 1.67 | 1.71 | ||
| 4.47 | 4.29 | 1.25 | 1.49 | 1.52 |
Effect of pH in feed phase on transport of La (III).
In Figure
Continue to reduce liquid H+ concentration and the La (III) of material liquid hydrolyses, and the solution becomes muddy. From Table
During the migration process of La (III), the best material liquid pH is selected as 4.00, at 125 min, and the migration rate of rare earth metals in selected condition is 75.2%.
The concentration difference of H+ in material liquid phase and dispersed phase is the mass transfer dynamic in DSLM for rare earth metal. We can also change the mass transfer dynamic in DSLM for the rare earth metal via changing the concentration of resolution agent in the dispersed phase based on the determination of the pH value in the material liquid phase. If the concentration of the resolving agent increases, resolving rate increases and the mobility will increase, too. But when the resolving agent concentration increases to a certain extent, the difference of H+ concentrations between the dispersed phase and the material liquid phase becomes too large that the corresponding osmotic pressure difference enlarges; therefore, it is possible that H+ would osmose from the dispersed phase to the material liquid phase. In this case, the membrane solution on the support body and the carrier will run off due to this process, resulting in the phenomenon of decrease in the mobility or instability in the membrane phase. Therefore, we need to study the impact of the concentration of the resolving agent HCl in the dispersed phase on the migration of the rare earth metal.
The material liquid phase pH of La (III) in DSLM migration system was selected as 3.6, the volume ratio of membrane solution and HCl in dispersed phase was 30: 30, and the concentration of carrier PC-88A in membrane solution was 0.160 mol/L. The initial concentration of La (III) was 1.00×10−4 mol/L. In the research on the impact of the concentration of HCl solution in dispersed phase of migration behavior of La (III) in DSLM under this condition, the experimental results are shown in Figure
Effect of HCl concentration in the dispersion phase on transport of rare earths.
Rare earth metal | (min) Migration n time | item | Results | ||||
---|---|---|---|---|---|---|---|
La(III) | 125 | HCl (mol/L) HCl concentration | 2.00 | 3.00 | 4.00 | 5.00 | 6.00 |
−ln | 0.924 | 1.14 | 1.39 | 1.42 | 1.26 | ||
| 8.21 | 1.02 | 1.24 | 1.27 | 1.12 |
Effect of HCl concentration in dispersion phase on transport of La (III).
From Figure
From Figure
During migration of La (III), the required best HCl concentration is 4.00 mol/L, respectively, at 125 min, 75 min, 95 min, 130 min, 95 min, and 155 min and the migration rates of six kinds of rare earth metals in the selected condition are 75.2%, 91.2%, 73.5%, 80.6%, 73.5%, and 67.9%.
Because the dispersed phase is the solution of HCl being dispersed uniformly in the membrane solution, the volume ratio between the membrane solution and the HCl solution directly affects the extraction and resolution rates of rare earth metals [
The material liquid phase pH of La (III) in DSLM migration system was selected as 3.6. The initial concentration of La (III) was 1.00 × 10−4 mol/L. HCl concentration in dispersed phase was 4.00 mol/L. The concentrations of carrier PC-88A in membrane solution were 0.160 mol/L, 0.160 mol/L, 0.100 mol/L, 0.160 mol/L, 0.100 mol/L, and 0.160 mol/L, respectively. Researching the impact of the volume ratio of the membrane solution over the HCl solution on the migration behavior of La (III) in DSLM under this condition, the experimental results are shown in Figure
Effect of volume ratio of membrane solution and HCl solution on transport of rare earth.
rare earth metal | (min) Migration Time (min) | item | Data results | ||||
---|---|---|---|---|---|---|---|
La(III) | 125 | volume ratio | 10:50 | 20:40 | 30:30 | 40:20 | 50:10 |
−ln | 0.660 | 0.814 | 1.39 | 1.41 | 1.46 | ||
| 5.87 | 7.24 | 1.24 | 1.26 | 1.30 |
Effect of volume ratio of membrane solution and HCl solution on transport of La (III).
It can be seen from Figure
By studying the impact of the volume ratio between the membrane solution and HCl solution in the dispersed phase on the migration behavior of rare earth metals in DSLM system, we can understand the following: the migration of rare earth metals in DSLM system is codetermined by the chemical reaction and the diffusion dynamics and is a dynamic equilibrium process. The entire process is controlled by the chemical reaction, that is, extraction reaction and resolution reaction, when the volume ratio between the membrane solution and HCl solution in the dispersed phase is relatively tiny. According to the principle of the chemical equilibrium, increasing the volume ratio of the membrane solution and HCl solution favors the formation of the carrier complex; therefore the mobility of rare earth metal increases rapidly; but when the volume ratio reaches a certain level, the concentrations of the carrier, rare earth metal, and complex at interface close to saturation. The diffusion process will play a decisive role, so the increase of rare earth metals mobility gradually slows down with the increase of the volume ratio. If the volume ratio continues to increase, the proportion of the resolution agent reduces. And resolution rate will inevitably decline; hence the migration rate also reduces.
In DSLM system and PC-88A as carrier, the best volume ratio of the membrane solution over HCl solution was 30:30 in La (III) migration process and the mobility of rare earth metal La (III) was 75.8% at 125 min under the selected conditions.
In certain DSLM system, if the initial concentration of rare earth ions is too large, the rare earth metal is not fully migrated within a certain period of time [
From (
The volume ratios of the membrane solution and HCl in dispersed phase of La (III) in DSLM migration system were selected as 30: 30, 40: 20, 30: 30, 30: 30, 40: 20, and 40:20. The HCl concentration in the dispersed phase is 4.00 mol/L. The concentration values of carrier PC-88A in the membrane solution were 0.160 mol/L, 0.160 mol/L, 0.100 mol/L, 0.160 mol/L, 0.100 mol/L, and 0.160 mol/L, respectively. The pH values of the material liquid phase were 4.00, 1.00, 5.20, 4.20, 5.00, and 5.10 respectively. Studying the impact of the initial concentration of rare earth metal in the material liquid phase on the migration behavior of La (III) in DSLM under this circumstances, the experimental results are shown in Figure
Effect of initial concentrations on transport of rare earth.
rare earth metal | (min) | item | Data results | ||||
---|---|---|---|---|---|---|---|
La(III) | 125 | (mol/L) initial concentration (mol/L) | 5.00 | 8.00 | 1.00 | 1.50 | 2.00 |
−ln | ~ | 2.80 | 1.67 | 1.12 | 0.751 | ||
| ~ | 2.49 | 1.49 | 9.97 | 6.68 |
Description: “~” represents undetectable, namely, full migration.
Effect of initial concentrations on transport of La(III).
It can be seen from Figure
From the mass transfer mechanism of DSLM, the concentration difference of H+ in material liquid phase and dispersed phase is the mass transfer power for metal ions in DSLM [
The volume ratio of the membrane solution and HCl in dispersed phase of La (III) in DSLM migration system was selected as 30: 30. The H+ concentration in the dispersed phase was 4.00 mol / L. The concentration of carrier PC-88A in the membrane solution was 0.160 mol/L. The initial concentration was 8.00 × 10−5 mol/L. The pH value of the material liquid phase was 4.00. Researching the impact of different analytical agents in the dispersed phase on the migration behavior of La (III) in DSLM under this condition, the experimental results are shown in Figure
Effect of different stripping agents on transport of La(III).
The effects of HCl solution, H2SO4 solution, and HNO3 solution on La(III) migration were studied, respectively, in this experiment, keeping the acidity of the analytic phase in dispersed phase stable.
As shown in Figure
The process of DSLM migration of rare earth metals is jointly nominated by the chemical reaction between rare earth metal complexes and the carrier and its diffusion process after bonding [
The volume ratios of the membrane solution and HCl in dispersed phase of La (III) in DSLM migration system were selected as 30: 30, 40: 20, 30: 30, 30: 30, 40: 20, and 40: 20; the HCl concentration in the dispersed phase is 4.00 mol / L; the initial concentrations were 8.00 × 10−5 mol/L, 7.00 × 10−5 mol/L, 1.00 × 10−4 mol/L, 8.00 × 10−5 mol/ L, 8.00 × 10−5 mol/L, and 1.00 × 10−4 mol/L, respectively. The pH values of material liquid phase are 4.00, 1.00, 5.20, 4.20, 5.00, and 5.10, respectively. Researching the impact of the carrier concentration in dispersed phase on the migration behavior of La (III) in DSLM under this condition, the experimental results are shown in Figure
Effect of carrier concentration on transport of rare earths.
rare earth metal | (min) | item | Data results | ||||
---|---|---|---|---|---|---|---|
La(III) | 125 | (mol/L) carrier concentration (mol/L) | 0.036 | 0.065 | 0.100 | 0.160 | 0.230 |
−ln | 1.34 | 1.98 | 2.33 | 2.80 | 2.83 | ||
| 1.19 | 1.76 | 2.16 | 2.49 | 2.52 |
Effect of different carrier concentration on migration of La (III).
As can be seen from Figure
Hence, the selected optimum carrier concentration of the dispersed phase in La (III) migration process is 0.160 mol/L and the mobility of rare earth metal La (III) is 93.9% under the selected conditions at 125 min.
The experiments show that DSLM system of (PC-88A-) kerosene—HCl—has a significant role in enrichment and transmission of La (III). The acidity of the material liquid phase, the initial concentration of La (III), the concentration of HCl in the dispersed phase, and the volume ratio of HCI and the membrane solution will affect the transmission of La (III). During the rare earth metal migration process, the most appropriate analytical agent is HCl and the ionic strength of the feed phase has little effect on the migration behavior of rare earth metals in DSLM. The optimum mass transfer conditions for La (III) are that the concentration of HCl in dispersed phase is 4.00 mol / L, the volume ratio of membrane solution and HCl solution 30:30, the carrier concentration 0.160 mol / L, and the pH value of the feed phase 4.00. Under the optimal conditions, the migration rate reaches 93.9% after 125 min when the initial concentration of La (III) in the material liquid phase is 8.00 × 10–5 mol/L. Maintaining the dispersed phase acidity under the same premise, La (III) mobility is 93.9%, 94.0%, and 87.8%, respectively, after 125 min using HCl, H2SO4, and HNO3 as parsing agents. The HCl solution, H2SO4 solution, and HNO3 solution have some effect on La (III) resolving, in which the 4.00 mol/L HCl solution and 2.00 mol/L H2SO4 solution are better for resolving and then comes HNO3. La (III) is the best transmission condition in the separation of La (III) experiment when the concentration of the flow carrier PC-88A is selected at 4.00 mol/L. It proves that the best concentration value of HCl is 4.00 mol/L during the La (III) migration. At 125th min, 75th min, 95th min, 130th min, 95th min, and 155th min, the mobility of rare earth metal La (III) is 75.2%, 91.2%, 73.5%, 80.6%, 70.1%, and 67.9%, respectively, under the selected circumstances.
The authors declare they have no conflicts of interest in this work.
The research was funded by the National Natural Science Foundation of China (Grant nos. 51379219, 41371187) and Zhejiang province Funds for Distinguished Young Scientists (Grant no. LR15E090002).