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Communication

Purification of Wet-Process Phosphoric Acid via Donnan Dialysis with a Perfluorinated Sulfonic Acid Cation-Exchange Membrane

Ministry of Education’s Research Centre for Comprehensive Utilization and Clean Process Engineering of Phosphorus Resources, School of Chemical Engineering, Sichuan University, Chengdu 610065, China
*
Author to whom correspondence should be addressed.
Membranes 2021, 11(4), 298; https://doi.org/10.3390/membranes11040298
Submission received: 18 February 2021 / Revised: 2 April 2021 / Accepted: 7 April 2021 / Published: 20 April 2021

Abstract

:
This work reports the application of an electromembrane process, Donnan dialysis (DD), for the purification of so-called wet-process phosphoric acid (WPA). Nitric acid is used as the stripping solution to remove metallic cations (mostly Fe3+, Al3+, and Mg2+) that are harmful to the further processing of WPA. The paper first presents a set of experimental data on the measurements of the metallic cation fluxes through a perfluorinated sulfonic acid cation-exchange membrane. Not only WPA, but also synthetic phosphoric acid solutions with mixed metallic cations (MPA) and with a single metallic cation (SPA) were studied. This confrontation confirms (1) that the order of metallic cations fluxes is Mg2+ > Al3+ > Fe3+; (2) that, compared with MPA, the purification effect of WPA causes only negligible change; (3) that, by comparing the DD processes with SPA and MPA solutions, the reason for the low transmembrane fluxes of Fe3+ and Al3+ could be explained by the large ionic charge and large hydrated ion radius. Furthermore, by analyzing the ion composition of membranes equilibrated in SPA solutions, we conclude that the forms of cations in the membrane are most likely Fe3+, Al3+, and Mg2+.

1. Introduction

Ion-exchange membranes (IEMs) are an important class of dense polymeric membranes that bear fixed charges and mobile counterions in the polymer matrix, in the presence of polar solvents (e.g., water). These membranes can selectively allow the passage of oppositely charged ions (counterions), while obstructing similarly charged ions (coions) [1,2]. Commonly used IEMs are anion-exchange membranes and cation-exchange membranes (CEMs). Due to the nice selectivity of IEMs and their simple operation, they are used in various electromembrane processes, such as Donnan dialysis (DD), electrolysis, (reverse) electrodialysis, and diffusion dialysis.
As a membrane-based equilibrium process, DD is employed as a useful separation technology for chemical analysis, water treatment [3,4], and drug separation or purification, and especially for the separation and recovery of precious metals [2]. In the field of wet-process phosphoric acid (WPA) purification, metallic cations also need to be removed because these metallic cations not only increase the density of phosphoric acid, but also easily lead to the formation of insoluble phosphates, especially for iron and aluminum ions that seriously affect the downstream processing of WPA [5]. However, due to the complex composition of WPA, harsh operation conditions, and severe scaling problems, the purification of WPA has been a challenging topic.
Until now, the purification of WPA in industry relied mainly on solvent extraction or chemical precipitation. The solvent extraction method has advantages like a relatively high extraction ratio of phosphoric acid, good selectivity, easy phase separation, good flow properties of streams, and it is cheap and easy to realize. Both academia and industry have done a lot of work on extractant screening and purification performance evaluation. However, the extraction method uses a large amount of organic extractants (e.g., dibutyl sulfoxide [6] and cyclohexanol [7]), and the process produces poorly water-soluble sludge acid. Alternatively, the chemical precipitation method uses chemical precipitation agents, such as ammonia [5], NaOH [8], ammonium nitrate with hydrofluoric acid [9], and lime milk to precipitate the metallic cations in WPA in the form of phosphoric acid (double) salts. This process introduces chemical reagents into the produced phosphoric acid and causes the waste of phosphorus resources. Besides, this process is lengthy. In short, both purification methods have their limitations.
There are some reports on the application of ion exchange or membrane technology in the purification of phosphoric acid. In terms of ion exchange, Tang et al. used ion-exchange resins to remove the aluminum cations from synthetic phosphoric acid solutions [10]. Amin et al. also investigated the removal of iron from WPA by Puromet MTS9570 resin [11]. However, the disadvantage of ion exchange is that the solid-to-liquid ratio used is large, and the resin is loose and porous, which will cause excessive entrainment. There are also some studies on promising membrane processes for the removal of target metallic cations. Sonoc et al. utilized Donnan dialysis to recover lithium, nickel, cobalt, and manganese from lithium-ion batteries [12]. Diallo et al. used MPF34 nanofiltration membranes to study the migration of iron, the main metal contaminant, in WPA [13]. Therefore, it is, in principle feasible to choose a suitable ion-exchange membrane to purify WPA via the DD process.
In this study, DD was applied to purify a typical WPA. Synthetic phosphoric acid solutions with the same metallic cation composition as WPA (MPA), each with only one type of cation as the corresponding component in WPA (SPA), were employed to study the removal ratios of different metallic cations in the DD process. The existing form of cations in the membranes was also studied by comparing the molar ratios of metallic cations and phosphorus in the membrane.

2. Materials and Methods

2.1. Chemicals and Membranes

The WPA solution came from a phosphate ore processing plant located in Inner Mongolia in China. The characteristic contents of each component in this WPA are shown in Table 1. To further understand the ion-exchange process of the Fe, Al, and Mg species in the purification process, three different kinds of synthetic phosphoric acid solutions with each single metallic cation (SPA) of the same concentration as in WPA were prepared.
A commercial phosphoric acid solution with a H3PO4 mass fraction of 85% was used in the preparation of synthetic phosphoric acid solutions. The NaCl, MgO, and Al2(SO4)3 used were from Chengdu Kelong Chemical Co., Ltd. Fe2(SO4)3 was purchased from Chengdu Jinshan Chemical Reagent Co., Ltd. (China). All chemicals were of analytical pure grade. MPA was prepared according to the contents of target components listed in Table 1.
A perfluorinated sulfonic acid cation-exchange membrane (CEM) was used in this study. Table 2 lists the properties of this CEM. This CEM (DMR100M) was supplied by the Shandong Dongyue Future Hydrogen Energy Material Co., Ltd. (China).

2.2. Analysis Methods

An inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 7000DV, Platinum Elmer, Waltham, MA, USA) and X-ray fluorescence spectroscopy (XRF, S8, Tiger, Bruker, Karlsruhe, Germany) were used to detect the contents of metallic cations in solutions. Fluorine and phosphorus contents in solutions were detected with an ion meter (PXS-270, Shanghai INESA Scientific Instrument, Shanghai, China) and UV spectrophotometer (UV-3600, Shimadzu, Kyoto, Japan), respectively. A scanning electron microscope (SEM, Vega 3 SBH, Tescan, Brno, Czech Republic) was used to observe the morphology of the membrane.

2.3. Donnan Dialysis Process

The experiment of using the DD process for WPA purification is schematically shown in Scheme 1. Fe3+ (Mg2+ and Al3+) is the target impurity cation and the driving ion is H+ in this case. The membrane area, the concentration of stripping (nitric acid solution), and the starting solution volumes of both the stripping solution and the WPA feed are 1.77 cm2, 5 mol L−1, and 9 mL, respectively. The phosphoric acid and nitric acid solutions were well mixed in Chambers I and II via magnetic stirrers, respectively. Intensive agitation was assured to diminish the resistance of the liquid films on both sides of the membrane surfaces. The perfluorinated sulfonic acid CEM was clamped between the two chambers of the cell. In this circumstance, the metallic cations in the phosphoric acid solution could be driven against their concentration gradients, as Moya described [14], and did exhibit uphill transport across the membrane. This uphill transport of metallic cations was caused by the proton concentration difference between Chambers I and II. The ratio of removal (towards metallic cations) was chosen to express the extent of purification, and it was calculated via the following equation:
Ratio   of   removal = C t , II V II C 0 , I V I 100 %
Among them, C0,I and Ct,II are the concentrations of cations at time zero in Chamber I and at time t in Chamber II, respectively; VI and VII are the volumes of electrolyte solutions in Chambers I and II, respectively.

2.4. Absorption Experiments

A certain amount of the CEM was pretreated and then soaked in SPA for 48 h to ensure complete ion exchange between the membrane and the synthetic solutions; then the CEM was washed with deionized water to remove the absorbed free salts and acids. Afterwards, the CEM was soaked in 50 mL of 1 mol L−1 NaCl solution for 48 h to allow sodium ions to replace the metal ions in the CEM. Lastly, the phosphorus content and the metal ion content in the desorbing solutions were measured by UV spectrophotometer and ICP-OES, respectively. For Fe3+-SPA, Al3+-SPA, and Mg2+-SPA, the membrane areas used in the absorption experiments were 83, 85, and 83 cm2, respectively.

3. Results and Discussions

3.1. Membrane Morphology Analysis

Figure 1 shows the SEM graphs of the perfluorinated sulfonic acid CEM DMR100M. As shown in Figure 1a, there are no observable holes or defects on the surface of the membrane, confirming that the membrane surface is dense at dry state. It can be seen from the cross-sectional graph of the membrane in Figure 1b that the membrane cross-section does not have any finger-like pores or sponge structures as observed in porous membranes. In conclusion, this perfluorinated sulfonic acid CEM is a dense film at dry state.

3.2. Removal of Metallic Cations from Industrial Wet-Process Phosphoric Acid

Figure 2 shows the relationship between the removal ratios of different metallic cations from WPA and time in the DD process with nitric acid as the stripping solution. It is obvious that the order of removal ratios of metallic cations is Mg2+ > Al3+ > Fe3+. The removal ratio of Mg2+ is much greater than those of the other two metal ions. After 9 h, the removal ratio of Mg2+ reached 48.23%. At the same time, the removal ratios were only 14.64% and 3.35% for Al3+ and Fe3+, respectively; the accuracy in determining the ratios presented is 6%. This experimental result is in line with expectations. In DD, the exchange of ions is a chemical reaction, and ions react with the sulfonic acid groups on and in the CEM to replace protons or other metal ions that had previously been there. The number of such groups in the membrane is certain. The greater number of charges carried by the ions, the more fixed groups are involved, and the mass transfer ratio of the moving ion is correspondingly smaller. This is the main reason for the biggest mass transfer ratio, seen in Mg2+, which has a lower charge. Meanwhile, the hydrated radius of ions is also an important factor for explaining the mass transfer ratio. Although the CEM is treated as a homogeneous membrane at a macroscopic scale, it contains some “pores” filled with water for ions to pass through. The larger the hydrated radius of ions is, the more difficult it is for them to pass through the membrane. The charge numbers of Fe3+ and Al3+ are the same, but the hydrated radius of Fe3+ is larger than that of Al3+ [1], which means that the mass transfer ratio of Fe3+ in the DD process will be smaller than Al3+.

3.3. Removal of Metallic Cations from Synthetic Mixed Metallic Cations Phosphoric Acid

In order to investigate the mutual influence of impurity cations in phosphoric acid on metal removal in the DD process, MPA was used. The removal ratio data are shown in Figure 3. The removal ratios of MPA and WPA are very close, and the slight difference may come from the influence of other impurity ions (trace K+, Na+, and some Ca2+ ions) in WPA. Although most Al3+ will form stable AlF63− with F, the remaining Al3+ will form AlF2+ or AlF2+ with F [15], which leads to the reduction of the charge number of the complex ions and the increase in the flux of Al3+.

3.4. Removal of Metallic Cations from Synthetic Single Metallic Cations Phosphoric Acid

In this case, the feed is SPA and the other experimental conditions are kept unchanged. The experimental data of the removal ratios of three metallic cations are summarized, and the results are shown in Figure 4. For the Mg2+, Al3+, and Fe3+-SPA, the average mass transfer rates in the first 30 min were 0.0261, 0.0062, and 0.0026 mol m2 min1. The removal ratio of Mg2+ can be improved by 13% compared with that in the MPA, but the removal ratio of Al 3 + is only improved by 6.27%, possibly due to the slow mass transfer rate of Al3+ ions in the membrane. The removal ratio of Fe 3 + is improved by less than 1%. This shows that the mass transfer resistance of Fe 3 + and Al 3 + in the DD process is much greater than that of Mg 2 + . The result is consistent with the removal ratio order of metallic cations.
According to the comparison of the absolute removal amount of metallic cations in different experiments, as summarized in Table 3, the total removal amount of charges carried by the removed metallic cations in WPA is the largest, reaching 2.48 mequ. Secondly, the charge amount carried by removed metallic cations in MPA acid is 2.30 mequ. For the Mg 2 + , Al 3 + , Fe 3 + and -SPA, the charge amounts removed are 1.98, 0.72, and 0.30 mequ., respectively. For all experiments, the driving force for the impurity cation removal (proton concentration difference) was large enough, and the difference in the transfer amount of charges carried by removed cations may come from the different diffusion rates of cations in the membrane, instead of the metallic cations concentration in the feed. Base on this result, we can conclude that the order of metallic cation fluxes is Mg2+ > Al3+ > Fe3+.

3.5. Existing form of Metallic Cations in the Membrane

The basic formula for the quantitative treatment of ion transfer across the CEM, i.e., the Nernst–Planck equation, requires knowledge of the existing form of the metallic cations in the membrane. Equation (2) below describes the target ion flux in the DD process, with JA, DA, qA, and ZA being the flux, self-diffusion coefficient, concentration in the membrane, and charge of Ion A, respectively.
J A = D A D B Z A 2 q A + Z B 2 q B Z A 2 D A q A + Z B 2 D B q A dq A dx
Obviously, the charge of the ion has a large effect on the mass transfer rate in the mass transfer process in the membrane. In order to determine in which forms these metallic cations exist in the CEM, absorption experiments have been done, and the data are shown in Table 4.
According to the molar ratio of a metal ion and the corresponding phosphorus content in the same membrane specimen, it can be concluded that most of the iron, aluminum, and magnesium ions in the CEM are present in the form of Fe3+, Al3+, and Mg2+, respectively.

4. Conclusions

In this work, a Donnan dialysis process with a perfluorinated sulfonic acid cation-exchange membrane (CEM) as the separator, and nitric acid as the stripping solution, is proposed to remove metallic cations (Mg2+, Al3+, and Fe3+) from wet-process phosphoric acid (WPA) solutions. The possibility of employing the Donnan dialysis to purify WPA is verified. Based on the ion composition of membranes equilibrated with synthetic phosphoric acid solutions each with a single metallic cation, it is concluded that the existing forms of the metallic cations in the CEM are Mg2+, Al3+, and Fe3+. This work would be of considerable relevance for the application of the electromembrane processes toward the purification of WPA.

Author Contributions

Conceptualization: T.L., Z.Z., Z.Y.; Methodology: Q.Z., T.L., Z.Y.; Validation: Q.Z.; Formal Analysis: Q.Z., T.L.; Investigation: Q.Z.; Resources: T.L., L.Y., X.W.; Data Curation: Q.Z., T.L.; Writing-Original Draft Preparation: Q.Z.; Writing-Review & Editing: T.L., Z.Y.; Supervision: Z.Y., X.W.; Funding Acquisition: T.L., Z.Y., X.W.; Project Administration: T.L., Z.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Fundamental Research Funds for the Central Universities (China).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are openly available in Dryad at DOI https://doi.org/10.5061/dryad.31zcrjdk9, accessed on 9 April 2021.

Conflicts of Interest

The authors declare no conflict of interests.

References

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Scheme 1. Schematic diagram of the experimental Donnan dialysis process for the purification of WPA. PFSA is short for perfluorinated sulfonic acid.
Scheme 1. Schematic diagram of the experimental Donnan dialysis process for the purification of WPA. PFSA is short for perfluorinated sulfonic acid.
Membranes 11 00298 sch001
Figure 1. Scanning electron micrograph (SEM) picture of (a) the surface of the perfluorinated sulfonic acid cation-exchange membrane (CEM) DMR100M and (b) the cross-section of the membrane.
Figure 1. Scanning electron micrograph (SEM) picture of (a) the surface of the perfluorinated sulfonic acid cation-exchange membrane (CEM) DMR100M and (b) the cross-section of the membrane.
Membranes 11 00298 g001
Figure 2. Removal ratios of different metallic cations from WPA as a function of time.
Figure 2. Removal ratios of different metallic cations from WPA as a function of time.
Membranes 11 00298 g002
Figure 3. Removal ratios of metallic cations (filled symbols) as a function of time, from MPA that is a synthetic phosphoric acid solution with the same cation compositions as WPA. The removal ratios for WPA (hollow symbols) are plotted as references.
Figure 3. Removal ratios of metallic cations (filled symbols) as a function of time, from MPA that is a synthetic phosphoric acid solution with the same cation compositions as WPA. The removal ratios for WPA (hollow symbols) are plotted as references.
Membranes 11 00298 g003
Figure 4. Removal ratio of (a) Mg2+, (b) Al3+, and (c) Fe3+ as a function of time, from SPAs that are synthetic phosphoric acid solutions, which each only contain one type of cation, as corresponding content of the same cation in WPA. The removal ratios from WPA and MPA are plotted as references.
Figure 4. Removal ratio of (a) Mg2+, (b) Al3+, and (c) Fe3+ as a function of time, from SPAs that are synthetic phosphoric acid solutions, which each only contain one type of cation, as corresponding content of the same cation in WPA. The removal ratios from WPA and MPA are plotted as references.
Membranes 11 00298 g004
Table 1. Characteristic composition of the wet-process phosphoric acid (WPA) solution employed.
Table 1. Characteristic composition of the wet-process phosphoric acid (WPA) solution employed.
Target ComponentsP2O5(H3PO4)
[wt.%]
Fe3+
[wt.%]
Al3+
[wt.%]
Mg2+
[wt.%]
26.57 (36.67)0.660.270.39
Other componentsCa2+
[wt.%]
K+
[wt.%]
Na+
[wt.%]
F
[wt.%]
0.080.030.011.47
Table 2. Characteristics of the perfluorinated cation-exchange membrane used in this work.
Table 2. Characteristics of the perfluorinated cation-exchange membrane used in this work.
Product CodeDMR100M
DimensionsDry film thickness [μm]15 ± 1
Wet film thickness [μm]19 ± 1
ParametersConductivity [mS cm−1]68.6 (H+ form)
Areal density [g m−2]28 ± 1
Ion exchange capacity [mmol g−1]1.57
MaterialsPerfluorinated sulfonic acid polymer, polypropylene reinforcement
Table 3. The charge amount carried by metallic cations removed from different samples in the Donnan dialysis process.
Table 3. The charge amount carried by metallic cations removed from different samples in the Donnan dialysis process.
Sample Acid TypesRemoved Fe3+
[mg]
Removed Al3+
[mg]
Removed Mg2+
[mg]
Total Charges
Removed
[mequ.]
WPA2.794.7621.912.48
MPA4.864.2719.072.30
Fe3+-SPA5.60//0.30
Al3+-SPA/6.49/0.72
Mg2+-SPA//24.041.98
Table 4. Element contents in the perfluorinated sulfonic acid CEM equilibrated with the synthetic SPA solutions.
Table 4. Element contents in the perfluorinated sulfonic acid CEM equilibrated with the synthetic SPA solutions.
Sample Acid TypeP
[mg]
Fe3+
[mg]
Al3+
[mg]
Mg2+
[mg]
Target Metal/P
[mol/mol]
Fe3+-SPA0.0100.2300.0100.10013.180
Al3+-SPA0.0100.0011.0300118.366
Mg2+-SPA0.01000.0153.091398.581
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MDPI and ACS Style

Zhong, Q.; Luo, T.; Yan, Z.; Yang, L.; Zhang, Z.; Wang, X. Purification of Wet-Process Phosphoric Acid via Donnan Dialysis with a Perfluorinated Sulfonic Acid Cation-Exchange Membrane. Membranes 2021, 11, 298. https://doi.org/10.3390/membranes11040298

AMA Style

Zhong Q, Luo T, Yan Z, Yang L, Zhang Z, Wang X. Purification of Wet-Process Phosphoric Acid via Donnan Dialysis with a Perfluorinated Sulfonic Acid Cation-Exchange Membrane. Membranes. 2021; 11(4):298. https://doi.org/10.3390/membranes11040298

Chicago/Turabian Style

Zhong, Qin, Tao Luo, Zhengjuan Yan, Lin Yang, Zhiye Zhang, and Xinlong Wang. 2021. "Purification of Wet-Process Phosphoric Acid via Donnan Dialysis with a Perfluorinated Sulfonic Acid Cation-Exchange Membrane" Membranes 11, no. 4: 298. https://doi.org/10.3390/membranes11040298

APA Style

Zhong, Q., Luo, T., Yan, Z., Yang, L., Zhang, Z., & Wang, X. (2021). Purification of Wet-Process Phosphoric Acid via Donnan Dialysis with a Perfluorinated Sulfonic Acid Cation-Exchange Membrane. Membranes, 11(4), 298. https://doi.org/10.3390/membranes11040298

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