Method and apparatus for Ga-recovery

Abstract

The present invention encompasses a method of selectively separating Ga from wastewaters with the aid of a dialysis method. This exploits the particular complexation behaviour of Ga, which forms an unstable tetrahalo complex. This forms only in the case of a sufficiently high halide concentration. Since the halide concentration becomes lower across the membrane, the Ga-tetrahalo complex breaks down in the membrane, as a result of which the Ga is retained. Other metals such as In and Fe do not show this behaviour, and therefore the tetrahalo complexes of these metals can pass through the membrane and hence can be selectively separated off.

Claims

1. A process for separating gallium from impurities in an aqueous solution, the process comprising: performing a dialysis procedure between a feed solution and a dialysate separated by an anion exchange dialysis membrane, wherein: the feed solution is acidic and comprises anionic halide ions and cationic gallium ions in the feed solution in respective concentrations suitable for formation of anionic gallium halide complexes and wherein the concentration of the anionic halide ions is at least two moles per liter; the impurities comprise one or a combination of two or more of: an arsenic species, an iron species, and an indium species; and wherein the anionic gallium-halide complexes are selectively retained in the feed solution by the membrane and the impurities pass through the membrane into the dialysate.

2. The process according to claim 1, further comprising: maintaining a sufficient halide concentration gradient at the membrane between the dialysate and the feed solution so that the anionic gallium-halide complexes formed in the feed solution disintegrate into constituent cationic gallium ions and anionic halide ions when sorbed in the membrane and prevent the gallium ions from passing through the membrane.

3. The process according to claim 2, wherein the impurities comprise the iron species and/or indium species.

4. The process according to claim 3, wherein the iron species and/or indium species comprise anionic halide complexes comprising cationic iron ions and/or cationic indium ions, wherein the anionic halide complexes remain stable when sorbed into the membrane and pass through the membrane into the dialysate.

5. The Process according to claim 4, wherein the anionic halide complexes comprise tetrahalogeno-complexes MX.sub.4− wherein X=Cl or Br and M=In or Fe.

6. The process according claim 1, wherein the impurities comprise the arsenic species.

7. The process according claim 6, wherein the arsenic species comprises an arsenic acid in an anionic form as H.sub.2AsO.sub.4—.

8. The process according to claim 1, wherein the concentration of the gallium ions in the feed solution is 0.3 moles per liter or less.

9. The process according to claim 1, wherein the anionic gallium-halide complexes comprise gallium-tetrahalogeno complexes.

10. The process according to claim 1, wherein the halide ions comprise bromide ions.

11. The process according to claim 1, wherein the halide ions comprise chloride ions.

12. The process according to claim 11, wherein the gallium-halide complexes comprise gallium-chloro-complexes.

13. The process according to claim 12, wherein the gallium-chloro-complexes comprise gallium-tetrachloro complexes.

14. The process according claim 1, wherein the feed solution comprises a hydrogen halide acid.

15. The process according to claim 14, wherein the feed solution further comprises a nitric acid.

16. The process according to claim 1, wherein the feed solution has a pH value of ≤3.

17. The process according to claim 1, wherein the feed solution has a pH value of ≤2.

18. The process according to claim 1, wherein the anion exchange dialysis membrane comprises: a backbone comprising a copolymer having a degree of crosslinking; and a modification of a membrane layer at least on the feed side of the membrane, the modification being one or a combination of two or more of: having a higher degree of crosslinking than the backbone of the membrane; impregnating a surface of the membrane with weakly basic anion exchange groups; and a targeted control of membrane synthesis.

19. The process according to claim 1, further comprising applying an external electric field to the feed solution and the dialysate to superimpose an additional electric potential gradient between the feed solution and the dialysate.

20. The process according to claim 1, wherein the dialysis is performed in continuous countercurrent mode.

Description

DETAILED DESCRIPTION

(1) According to the invention, it was surprisingly found that Ga-containing complexes and species, in contrast to non-metal species and other metal species different from Ga, are retained with high reliability and selectivity and in an controllable manner by an anion exchange membrane, if halide ions in a concentration of at least 2 mol/l are present in the feed in the region flowing up to the anion exchange membrane in an acidic solution of the feed (feed stream) to be separated. In particular, it was surprisingly found that a gallium-halogeno-complex forming in the feed solution ([GaX.sub.4−] with X=Cl or Br)— provided that the above mentioned minimum concentration of the halide ions mentioned above is maintained and is in the correct ratio in combination with the concentration of the Ga ions in the feed stream—is first sorbed in the membrane, but cannot permeate, then selectively disintegrates in the membrane due to the halide gradient between feed and eluate at or in the membrane due to the halide gradient between feed and eluate, and therefore no Ga in the form of the complex passes the membrane, whereas other metal and non-metal species and complexes, as far as they are present with an anionic charge in the feed, remain stable and therefore pass the membrane. The reason for this behavior is the special complexing behavior of gallium, according to which gallium-tetrahalogeno-complexes are unstable below a certain chloride concentration and disintegrate into Ga.sup.3+ and halide ions without the formation of mixed ligand complexes. At 0.3 mol/l Ga, the minimum concentration for the stability of GaCl.sub.4— is e.g. 2 mol/l Cl. For correspondingly lower Ga concentrations, lower Cl.sup.− concentrations are possible. However, a particularly effective and currently reasonable economic limit is 0.3 mol/l Ga.

(2) A dialysis procedure with the features mentioned above can be used as diffusion dialysis as well as electrodialysis.

(3) For example, arsenic at pH values ≤3 is predominantly present as H.sub.2AsO.sub.4—. Metal cations other than Ga form stable halogeno-complexes. Despite the halide concentration gradient prevailing in the membrane, in contrast to the GaX.sub.4— complex, these are decomposed only gradually and to a limited extent, so that the solution of contaminating non-metal species such as arsenic acid and metal complexes such as InCl.sub.4— and FeCl.sub.4— pass through the membrane, whereas gallium is retained highly selectively. For the reasons described, the selectivity of the gallium retention is increased by at least one order of magnitude through the use of coated membranes according to embodiments of the disclosure (see membrane comparison in FIGS. 6A-6B).

(4) The respective substances can then be recovered by suitable conditions or measures. For example, the Ga in the purified feed can be obtained directly by electrolysis or can be precipitated as Ga hydroxide by increasing the pH value—which may be achieved by adding lye or, alternatively, automatically by diffusing the acid into the dialysate. On the other hand, the arsenic acid from the dialysate can be used for wastewater treatment or arsenic recovery, for example by adding iron salts in the low pH range, such as pH=2.

(5) Two different processes can take place at a membrane during dialysis:

(6) Ion exchange at functional groups is an exchange of different ions with charges that are opposite to the functional groups and balance them. Since charge neutrality must be maintained during ion transport, directed charge transport by ion exchange can only occur if an equivalent of opposite charges is also transported in the opposite direction. This would require strong external forces (e.g. the application of an electric field).

(7) During the sorption of ion pairs, the ion exchange resins absorb anions and cations in order to balance the activity of the ions inside and outside the resin. The repulsion forces of the functional groups against equally charged co-ions can be overcome to a limited extent. Anion exchange resins can sorb H.sup.+ ions at high concentrations due to their high mobility, which enables, for example, the sorption of large amounts of acid.

(8) The sorbed ions are dissolved in the swollen polymer network. Ion sorption requires strong resin swelling and takes place particularly in the case of slightly crosslinked membrane polymers. Sorption on one membrane surface results in a concentration difference in the membrane that causes diffusion of the ion pairs to the other membrane surface. The selectivity of the ion transport depends on the selectivity of the ion sorption at the membrane surfaces and on the diffusion rate of the ion pairs.

(9) The functional principle of dialysis will be explained more in detail below. The principle of dialysis is based on the selective transport of ions through non-porous membranes coated with ion exchange resins. They are loaded on one side of the membrane and regenerated on the other. Concentration differences between the solutions (diffusion dialysis) or electric fields (electrodialysis) can serve as driving forces for mass transfer. The ion exchange resins of the dialysis membranes are specifically modified in order to influence the membrane selectivity by the exchange equilibrium, the loading kinetics and the transport resistance. The modifications enable continuous separation processes for acids, neutral salts and differently charged ions.

(10) Diffusion dialysis is based on the passive diffusion of coion-counterion-pairs through special diffusion dialysis anion exchange membranes with weakly cross-linked polymers. It is used, for example, in the recovery of mineral acids and alkaline solutions.

(11) In addition to the membranes, which have a service life of several years in operational use, diffusion dialysis consumes water as a receiver medium (dialysate) and a small amount of energy for volume circulation. Furthermore, the membranes can be stacked into compact modules, so that the process works very economically in principle.

(12) In electrodialysis, coions diffuse along the functional groups from which they are exchanged. By applying an external electric field, the chemical potential gradient is superimposed by an electric potential gradient. The current flow through the membrane is effected by electromigration. The alternating separation of the feed and dialysate chambers with electrodialysis anion exchange membranes and electrodialysis cation exchange membranes allows selective separation of the anions and cations from the feed and their accumulation in the dialysate. The rate of migration depends on the current density, so that even small amounts of salt can be quickly removed from the feed.

(13) In diffusion dialysis and electrodialysis membranes, the exchange capacities and crosslinking degrees of the ion exchange resins are targetedly adjusted to generate permeabilities and selectivities for different fields of application. Diffusion dialysis membranes with degrees of crosslinking of 5-10% divinylbenzene (DVB) promote salt diffusion by a high absorption of coions. Electrodialysis membranes are designed to inhibit salt diffusion by crosslinking degrees above 20% DVB.

(14) It was surprisingly found that the selectivity and controllability of the retention of Ga-containing species against permeating through the membrane of other metal and non-metal species to be separated is particularly significantly improved when the anion exchanger membrane used is provided with a copolymer layer that is more cross-linked than the membrane skeleton in order to modify the membrane accordingly on the surface facing the feed. The modified surface membrane layer is relatively thin, preferably in the thickness range up to 100 μm, further preferably up to 10 μm.

(15) There are different types of these functional layers or modifications of the outer surface, and they are characterized by their property of having a weaker swelling than the rest of the ion exchange resin and of sorbing smaller amounts of ions due to the shifting the Donnan equilibrium. For example, long-chain “polyelectrolytes” can be applied (Neosepta series by Tokuyama Soda, see JP19970338354 19971209).

(16) According to Sata (Journal of Membrane Science 100 (1995) 229-238), functional layers can also be produced by impregnating the surface with weakly basic anion exchange groups, which are produced by polycondensation of strongly cross-linked layers on the surface or by partial decomposition of strongly basic anion exchange groups on the membrane surface.

(17) Another possibility for the generation of the functional layer is the production by targeted control of the membrane synthesis.

(18) It is assumed that these special modifications or layers strongly contribute to the fact that in the present invention gallium-halide-complexes are only sorbed at the membrane. Thus, it was surprisingly found that the permselectivity of the negatively charged GaX.sub.4− complex is significantly increased by functional layers compared to other simply negatively charged anionic complexes (e.g. InX.sub.4−).

(19) As shown in FIG. 1, an etching effluent containing Ga ions, As-ions, possibly further metal cations and one or more acids (e.g. HCl, HNO.sub.3) is to be separated by dialysis processes in one embodiment of the present invention, so that a purified Ga-containing solution is produced for further processing, e.g. by precipitation and recovery of gallium. In this embodiment the ligand is for example X=Cl. The pre-refined solution produced should have the highest possible Ga concentration and be free from impurities such as arsenic acid or metal ions. The separation of arsenic acid and metal ions can take place in several dialysis stages, in which one or more impurities are selectively separated, respectively. The anion exchange membranes mentioned above can be used to diffuse acid anions but retain metal cations and gallium-halogeno-complexes. Korkisch (Handbook of Ion Exchange Resins: Their Application to Inorganic Analytical Chemistry, Vol. VI, CRC Press Boca Raton, Fla., 1989) shows that Ga can form anionic chloro- and bromo-complexes that are exchanged by anion exchange resins.

(20) FIG. 2 shows the protolysis equilibrium of the arsenic acid in the feed as a function of the pH value. From pH=0, the concentration of H.sub.2AsO.sub.4− anions which are permeable through the anion exchange membrane increases. In a membrane, the diffusion rate of arsenic acid would increase significantly, since the pH gradient between feed and dialysate would cause a shift in the protolysis equilibrium within the membrane.

(21) One concept of the present invention is based on the pH value adjustment of etching waste water to pH≤3, preferably ≤2. if the pH value rises above 3 due to acid separation, gallium hydroxide, which sorptively binds arsenic acid, also precipitates in this embodiment. The precipitate can additionally cause a blocking of the membrane.

(22) The basic idea of the present invention is based on the fact that a GaCl.sub.4—-complex (chloro-complex) is formed in a solution with pH≤3 as well as—in this embodiment—with a chloride ion concentration of at least 2 mol/l. This anionic complex could generally pass through an anion exchange membrane. At the same time, anion-exchange membranes can sorb large amounts of polyvalent metal ions if these change their charge through complex formation in the autoprotolysis of the aqua-complexes.

(23) Ga.sup.3+ forms and strong hydrate complexes besides stable tetrahalogeno-complexes. Brendler (I. Brendler et al.: “Untersuchung zur Chlorokomplexbildung des Gallium(III)-Kations in wässiger Lösung”, Monatshefte für Chemie 123, 1992, p. 285-289) investigated the characteristic Raman spectrum of Ga-tetrachloro complexes at peaks (116, 128, 348 and 381 cm.sup.−1) in 1,5-molar Ga solution with 0 to 6 mol/l chloride. It was found that with increasing Cl concentration the intensity of the characteristic chloro-complex peak at 348 cm−.sup.1 increases with chloride concentration, indicating the increase in the GaCl.sub.4— concentration. The Raman spectrum was not shifted by chloro-complex formation, indicating a lack of stable gallium transition complexes.

(24) Own Raman measurements within the scope of the present invention (see FIG. 3) have shown that GaCl.sub.4— is detectable already at a HCl content of 2 mol/l with a strong peak at 348 cm.sup.−1.

(25) Ocken (E. Ocken: “Untersuchung zur Protolyse und Chloro-Komplex-Bildung des Gallium(III)-Kations”, Dissertation at the TU Bergakademie Freiberg, Institut für Inorganische Chemie, 1990, p. 79 ff.) showed by .sup.71Ga—NMR measurements with 1,5-molar Ga(ClO.sub.4).sub.3-solution that from a concentration of 3 mol/l chloride a further, broad resonance develops beside the characteristic resonance of the aqua-complexes, which narrows with increasing chloride concentration, increases in intensity and changes into the resonance characteristic for the tetrachloro-complex. This suggests a very slow exchange of equilibria between hexa-aqua complexes and unknown mixed ligand complexes. The resonance of the tetrachloro-complex is only pronounced from 6 molar chloride solution. Ocken found that the exchange equilibrium with the mixed ligand complexes is faster than with the aqua complex. The equilibrium can be described as follows (see Brendler 1990, p. 99):

(26) ##STR00001##

(27) This special complex formation behavior in gallium influences the loading of anion exchangers in the GaCl.sub.3/HCl/H.sub.2O system. The NMR results of ocken are shown in FIG. 4.

(28) In FIG. 5 (from: E. Ocken: “Untersuchung zur Protolyse und Chloro-Komplex-Bildung des Gallium(III)-Kations”, dissertation at the TU Bergakademie Freiberg, Institute for Inorganic Chemistry, 1990), the exchange isotherms of Ga.sup.3+ and Fe.sup.3+ at the anion exchange membrane Dowex-1×10 are compared with quaternary amines of type 1, a crosslinking degree of the PS-DVB resin of 10% DVB and an exchange capacity of 3.5 meg/g). The resins were respectively loaded from 0.01 molar solutions. FIG. 5 shows that the exchange isotherm of Fe.sup.3+ is approximately parabolic, since the Fe-chloro-complexes formed are in equilibrium with each other over a wide Cl-concentration range. For Ga.sup.3+ the Ga-exchange isotherm only increases steeply from 4.5 mol/l Cl due to the formation of GaCl.sub.4— until the Ga-complexes are displaced by the excess Cl ions from approx. 6 mol/l Cl. The Ga-exchange, which starts sharply from a certain Cl.sup.−-concentration, coincides with the findings of the Raman and NMR measurements described above and is of high relevance for the selectivity of the invention-based membrane process. This invention exploits the described effect by decomposing GaCl.sub.4— into Ga.sup.3+ and Cl.sup.− in the membrane by falling below the Cl concentration required for complex formation. Due to the concentration gradient of the chloride ions in the membrane between feed and dialysate, the described effect is strongly pronounced, Ga.sup.3+ cations, which form in the membrane due to the decomposition of the tetrachloro-complex, cannot pass through the membrane and therefore cannot reach the eluate side.

(29) The effect described for Ga cannot be observed for chloro-complexes of other metal ions such as In.sup.3+ or Fe.sup.3+, since the chloro-complexes sorbed by the anion exchange membrane are in equilibrium with each other over a broad Cl concentration range. This allows their permeation of the tetracholoro-complexes through the membrane and into the dialysate. The exploitation of this effect allows the selective separation of Ga from other metals such as In and Fe.

(30) Non-porous, ion-permeable membranes can be used to achieve these effects. The ion exchange resins can be composed of divinylbenzene copolymers or sulfonated fluoropolymers. By chemical modification, anion exchange membranes contain functional groups (fixed ions) with cationic charges that make the polymer swellable and ion-conductive. For example, a membrane was used which contains quaternary amines with short alkyl moieties as functional groups. The ion exchange capacity (IEC) here is about 1.8 meq/g, based on a dry membrane. The membrane consists of PS-DVB polymer, the crosslinking degree of the matrix is below 10% DVB. In addition to the uncoated membrane, a coated anion exchange membrane made of a fluoropolymer with quaternary amines of type 1 is used. In addition to frequent PS-DVB-based membrane polymers, other polymer types are conceivable, e.g. polyacrylic-based polymers.

(31) The process can take place, for example, in multi-chamber cells or plate modules or tube winding modules. The transport resistances of the diffusion boundary layers on both sides can be kept low by a frontal inflow or turbulent overflow.

(32) As already shown in FIG. 1, the dialysis cells can be arranged in cascades. This enables selective separation of the substances to be removed for different concentrations of the complex-forming halides. The individual stages of the cascade can be operated with specific process parameters. These include the selection of the membrane type and membrane area, the volume flows of feed and dialysate and the temperature and, in the case of electrodialysis, the current density.

(33) The following examples are intended to further illustrate the functionality of the disclosed process:

Example 1

Diffusion Dialysis for the Separation of Ga and as in a Batch Plant

(34) FIGS. 6A-6B compares the concentration curves of gallium and arsenic in dialysate using uncoated and coated diffusion dialysis anion exchange membranes in a batch plant (150 ml feed with 10 g/1 Ga (≈0.143 mol/l) and 11 g/l As (≈0.147 mol/l) and an initial chloride concentration of 2.2 mol/l; 1; 230 ml dialysate; 8 cm.sup.2 membrane area). The coated membrane is of the Selemion APS4 type. It is based on a polysulfone skeleton with quaternary amines and is coated on the feed side. The uncoated membrane is of the Neosepta AFN type and is based on a PS-DVB resin with quaternary amines of type 2.

Example 2

Ga—as Separation in a Continuous Countercurrent System

(35) FIG. 7 shows the design of a continuous countercurrent dialysis system. The gallium concentration in the dialysate remains below 5 mg/l (≈20.072 mmol/l). For a feed flow of 280 l per day and 10 g/l (≈0.143 mol/l) Ga and 10 g/l As (≈20.133 mol/l) and at an initial chloride concentration of 2 mol/l, the target concentration of 0.5 g/l (≈0.0067 mol/l) As can be undershot if the membrane area is 100 m.sup.2 and the dialysate wastewater flow is twice as high as the feed volume flow.

Example 3

Separation of Ga and in in Chloride-Containing Solution

(36) FIG. 8 shows the dependence of the mole fraction of the indium(III)-chloro-complexes on the Cl concentration in an equilibrium. The initial concentration of In was 0.25 mol/l at 25° C. and a pH value <2. The values are taken from: P Kondziela, J. Biernat (1975): “Determination of stability constants of Indium Halogenide complexes by Polarography”, Electroanalytical Chemistry and Interfacial Electrochemistry 61, pp. 281-288, and I. Puigdomenech (2013): “Hydra-Medusa”, software with database for the calculation of chemical equilibria, software version August 2009, database version January 2013.

(37) Here it becomes clear that an indium-tetrachloro-complex is already formed at a chloride concentration of 0.5 mol/l and above.

(38) FIG. 9 shows experimental results of a separation of Ga and In. Here the concentration curves of indium and gallium in the dialysate are compared.

(39) In both experiments an HCl solution with a concentration of 5 mol/l HCl was used as feed, with indium and gallium present as chloro-complexes. In the experiment with Ga an initial concentration of 0.15 mol/l Ga was used, in the experiment with indium an initial concentration of 0.06 mol/l In was used. InCl.sup.4− is already stable in 0.5 mol/l HCl (cf. stability diagram of indium chloro complexes in FIG. 8) and thus has a greater stability than GaCl.sub.4—. Thus, InCl.sub.4− can pass through the membrane much more easily than the corresponding gallium complex. The Ga transport from hydrochloric acid solution is therefore much slower than the In transport.

Example 4

Separation of Ga and Fe in Chloride-Containing Solution

(40) FIG. 10 shows experimental results of a separation of Ga and Fe. Here the concentration curves of iron and gallium in the dialysate are compared. The experiment was carried out in a two-chamber cell equipped with a Selemion DSV membrane with an area of 25 cm.sup.2. The feed volume was 200 ml and the dialysate volume 300 ml. The following initial concentrations were present in the feed:c.sub.Fe=4 g/l (≈0.072 mol/l), c.sub.Ga=0.5 g/l (≈0.0072 mol/l), c.sub.Cl−=1.8 g/l (≈0.005 mol/l). However, the number of chloride ions already present in the feed solution as complexes of Ga and Fe cannot be quantified here.

(41) In this example, lower Ga concentrations are used than in the above example. Hence, also the minimal concentration of Cl.sup.− is respectively lower.