Recovery of metal salt solutions

Abstract

The invention relates to processes and devices for recovering solutions that contain transition metal salts and acids by way of membrane electrolysis in an electrochemical cell using a hydrogen gas diffusion electrode as the anode.

Description

[0001] Spiraltec GmbH S28.015 P-WO

[0002] 74343 Sachsenheim, DE July 5, 2024 / NP

[0003] Treatment of metal salt solutions

[0004] The invention relates to methods and devices for the treatment of solutions containing transition metal salts and acids by means of membrane electrolysis in an electrochemical cell with a hydrogen gas diffusion electrode as anode.

[0005] For the recovery of metals from metal-containing acidic solutions, such as acidic leaching solutions or spent pickling solutions, various processes have already been proposed using electrolytic metal deposition.

[0006] Japanese Patent Application No. 53 019 171 proposes a process for the treatment of old pickling liquors containing metal ions and acids. In this process, the free acids are recovered by dialysis and the metals are recovered by subsequent membrane electrolysis of the old pickling liquor depleted of free acids. This process is primarily used for the treatment of iron pickling liquors containing sulfuric acid.

[0007] EP 0 463 671 A1 discloses a process for processing used pickling liquor containing primarily nitric and hydrofluoric acid, as well as nickel, chromium, and iron salts, using acid dialysis and membrane electrolysis. To recover the free nitric and hydrofluoric acid from acid dialysis, the acid dialysis effluent, which is depleted in free acids, is subjected to membrane electrolysis to recover the metals. The acid dialysis overflow, which is enriched in free acids, and the membrane electrolysis effluent, which is depleted in metals, are subjected to evaporation.

[0008] US 8 784 639 B2 discloses an electrochemical process for the recovery of metallic iron and chlorine gas from an iron-rich metal chloride solution, in which the iron-rich metal chloride solution is electrolyzed in an electrolyzer comprising a cathode compartment equipped with a cathode having a higher hydrogen overvoltage than iron and containing a catholyte with a pH below about 2, and an anode compartment equipped with an anode and containing an anolyte and a separator which allows the passage of anions, wherein the electrolysis step comprises circulating the iron-rich metal chloride solution in a non-anodic chamber of the electrolyzer, whereby iron is electrolytically deposited at the cathode and chlorine gas is evolved at the anode, leaving an iron-poor solution.The electrolytically deposited iron and the chlorine gas are recovered separately and at least a portion of the iron-poor solution is recycled into the iron-rich metal chloride solution.

[0009] Against this background, the present invention has set itself the task of providing improved processes and devices for the processing of solutions containing transition metal salts and acids.

[0010] The object is achieved according to the invention by a method having the features of claim 1 and a device having the features of claim 14. Embodiments and developments of the invention emerge from the dependent claims and the description.

[0011] The invention relates to a process for processing an acidic aqueous solution containing cations of at least one transition metal, comprising i. transporting the solution through at least one electrochemical cell having an anode compartment containing a gas diffusion electrode as the anode, and a cathode compartment containing a cathode whose hydrogen overvoltage is higher than the potential required for deposition of the transition metal at the cathode, wherein the anode compartment and the cathode compartment of the electrochemical cell are separated by an anion or cation exchange membrane, and wherein the solution is introduced into an inlet into the cathode compartment of the at least one electrochemical cell, discharged from the cathode compartment through an outlet of the cathode compartment, and introduced into the anode compartment via an inlet of the anode compartment of the at least one electrochemical cell, and discharged from the anode compartment through an outlet of the anode compartment,

[0012] II. Electrochemical conversion of the solution by introducing hydrogen into the gas diffusion electrode and deposition of at least one transition metal on the cathode and formation of acid at the anode.

[0013] In the at least one electrochemical cell, in which an anion or cation exchange membrane is arranged between the cathode and anode, metal cations present in the solution are reduced at the cathode and deposited as metal. At the anode, designed as a gas diffusion electrode, elemental hydrogen is oxidized to protons, which, together with the present acid anions, form free acid.

[0014] In the process according to the invention, the hydrogen overvoltage of the cathode is higher than the potential required for the deposition of the transition metal at the cathode. In this application, the term overvoltage generally refers to the difference between the electrical potential of an electrode when current is passed and the thermodynamic value of the electrode potential without electrolysis for the same experimental conditions. With reference to a cathode, the term "hydrogen overvoltage" refers to an overvoltage associated with the release of hydrogen gas at the cathode. A cathode with a high hydrogen overvoltage minimizes hydrogen evolution during electrolysis and thus facilitates the electrochemical deposition of transition metal.

[0015] In one embodiment of the process, the cathode consists of the transition metal to be deposited on it. Alternatively, cathodes commonly used for depositing metals from solutions containing metal ions can be used. In specific embodiments, the cathode is made of or coated with titanium, titanium alloy, zirconium, zirconium alloy, zinc, zinc alloy, cadmium, cadmium alloy, tin, tin alloy, copper, copper alloy, lead, etc. In other specific embodiments, the cathode consists of lead alloy, niobium, niobium alloy, gold, gold alloy, mercury, or metallic amalgam with mercury.

[0016] In the process according to the invention, a gas diffusion electrode is used as the anode. Gas diffusion electrodes are electrodes in which the three states of matter—solid, liquid, and gas—are in contact with each other, and the solid, electron-conducting catalyst catalyzes an electrochemical reaction between the liquid and gaseous phases. Both the liquid and gaseous phases exist simultaneously in the pore system of the electrodes.

[0017] In one embodiment, the gas diffusion electrode is a sintered electrode. In another embodiment, the sintered electrode comprises a covering layer made of fine-grained material, a working layer made of various fractions, and a gas-conducting layer made of coarse-grained material. The so-called gas-conducting layer is located in the center of the electrode. At only a slight overpressure, the electrolyte is displaced from this pore system. A small flow resistance ensures that the gas can spread unhindered along the electrode. At a slightly increased gas pressure, the electrolyte in the pore system of the working layer is also displaced, albeit only partially. The covering layer itself is designed to be so fine-pored that gas can pass through the electrode into the electrolyte even during pressure peaks.

[0018] In another embodiment, the gas diffusion electrode is a plastic-bonded electrode, for example a PTFE-silver electrode.

[0019] In one embodiment, the gas diffusion electrode comprises a noble metal such as platinum, ruthenium, iridium, and rhenium as a catalyst. In a specific embodiment, the solid catalyst is pressed into a porous film with a thickness of approximately 200 μm.

[0020] Suitable gas diffusion electrodes for carrying out the process according to the invention are commercially available, for example from Industrie De Nora SpA, 20134 Milan, Italy.

[0021] In the process according to the invention, the acidic aqueous solution containing cations of at least one transition metal is introduced into an inlet into the cathode compartment of the at least one electrochemical cell, discharged from the cathode compartment through an outlet of the cathode compartment, and introduced into the anode compartment via an inlet of the anode compartment of the at least one electrochemical cell, and discharged from the anode compartment through an outlet of the anode compartment. The solution is first passed through the cathode compartment of the at least one electrochemical cell, where transition metal is deposited at the cathode. The transition metal-depleted solution is then passed through the anode compartment of the at least one electrochemical cell, where it is enriched with protons generated at the anode. The solution leaving the anode compartment contains fewer transition metal cations and more free acid than the solution introduced into the at least one electrochemical cell.

[0022] In one embodiment of the process, the acidic aqueous solution containing cations of at least one transition metal is passed sequentially through several electrochemical cells. In one embodiment of the process, the acidic aqueous solution contains cations of at least two different transition metals. In another embodiment of the process, the acidic aqueous solution contains cations of at least three different transition metals. In another embodiment of the process, the acidic aqueous solution contains cations of more than three different transition metals.In one embodiment of the process, an acidic aqueous solution containing cations of at least two different transition metals is passed sequentially through at least two electrochemical cells, and a first of the at least two different transition metals is deposited at the cathode of the first electrochemical cell, and another of the at least two different transition metals is deposited at the cathode of the second electrochemical cell. In one embodiment of the process, the first of the at least two different transition metals has a more positive standard potential E° (at a temperature of 298.15 K, atmospheric pressure of 101.325 kPa, and a dissolved particle activity of 1 mol / l) than the other of the at least two different transition metals.

[0023] In one embodiment of the process, the acidic aqueous solution contains copper and zinc cations, and copper is deposited on the cathode of the first electrochemical cell, which operates as a galvanic cell, and zinc is deposited on the cathode of the second electrochemical cell, which operates as an electrolysis cell. In a specific embodiment, the acidic aqueous solution contains sulfuric acid and / or hydrochloric acid. This significantly reduces zinc contamination of wastewater during the processing of pickling solutions containing Cu and Zn.

[0024] In one embodiment of the method, the solution discharged from the anode compartment of the at least one electrochemical cell is subjected to nanofiltration. The nanofiltration separates the solution into a retentate enriched in metal ions and a permeate depleted in metal ions. In a further embodiment of the method, the solution discharged from the cathode compartment of the at least one electrochemical cell is subjected to nanofiltration, and the permeate is fed to the inlet of the anode compartment of the at least one electrochemical cell. In a further embodiment of the method, the retentate from the nanofiltration is at least partially returned to the cathode compartment of the at least one electrochemical cell. In specific embodiments of the method, at least 60%, for example more than 80%, in particular more than 95% of the retentate is returned to the cathode compartment.In a further embodiment of the process, the permeate from the nanofiltration is partially recycled into the cathode compartment of at least one electrochemical cell to increase the conductivity of the electrolyte. This is particularly suitable for solutions with a low content of metal cations and low acid concentration.

[0025] Suitable nanofiltration modules are commercially available, for example, from Koch Separation Solutions GmbH, 52072 Aachen, Germany. One example of a particularly suitable nanofiltration module is the SeIRO® MPS-34 elements from Koch Separation Solutions. They contain a pH-stable nanofiltration membrane with a molecular weight cut-off (MWCO) of 200 Daltons, as a spiral-wound element with a hard coating, and a polysulfone permeate tube. They have an operating pressure of 15–35 bar, a maximum operating temperature of 70°C, and a permissible pH range in continuous operation of 0–14. The maximum pressure drop is 0.7 bar per element and 3.5 bar per vessel.

[0026] In one embodiment of the method, the hydrogen introduced into the gas diffusion electrode of the at least one electrochemical cell is generated by electrolysis of an aqueous solution in an electrolyzer. In another embodiment of the method, the electrolyzer and the at least one electrochemical cell are electrically connected in series.

[0027] In one embodiment, the electrolyzer is an alkaline electrolyzer. In the alkaline electrolyzer, hydrogen is formed at the cathode and oxygen at the anode at a direct voltage of at least 1.5 volts. In one embodiment, an alkaline aqueous solution is used as the electrolyte. In one embodiment, the electrolyte is potassium hydroxide solution with a concentration of 20-40 wt.%. In one embodiment, titanium electrodes with a ruthenium oxide coating are used as electrodes. In another embodiment, the electrolyzer is an acidic or proton exchange membrane electrolyzer (PEM electrolyzer) containing a proton-permeable polymer membrane (PEM) coated on the cathode side with a porous electrode made of carbon-supported platinum and on the anode side with metallic or oxide-forming noble metals (usually iridium and ruthenium).An external voltage is applied to these electrodes and water is added to the anode side of the electrolyzer.

[0028] In embodiments of the process, the standard potential E° (at a temperature of 298.15 K, atmospheric pressure of 101.325 kPa, and a dissolved particle activity of 1 mol / l) of the transition metal has a positive value greater than 0 V. In these embodiments, only the electrodes of the at least one electrochemical cell need to be electrically connected when carrying out the process; no additional voltage source is required. The at least one electrochemical cell corresponds to a galvanic cell.

[0029] In one embodiment of the process, the transition metal is Au, Pt, Ir, Pd, Hg, Ag, Rh, Cu, Ru, Bi, or Re. In a specific embodiment, the transition metal is Cu. In another specific embodiment, the acidic aqueous solution containing cations of at least one transition metal is a hydrochloric acid copper salt solution.

[0030] In other embodiments of the process, the standard potential E° (at a temperature of 298.15 K, atmospheric pressure of 101.325 kPa, and an activity of the dissolved particles of 1 mol / l) of the transition metal has a negative value of less than 0 V. In these embodiments, a voltage must be applied to the electrodes of the at least one electrochemical cell when carrying out the process, thus requiring an additional external voltage source. The at least one electrochemical cell corresponds to an electrolyzer in which elemental transition metal and free acid are generated. The overall reaction corresponds to a reduction of the transition metal cations by hydrogen. In one embodiment of the process, the transition metal is Fe, Pb, Sn, Mo, Ni, Co, Cd, Cr, Zn, Ti, Nb, V, Mn, U, Zr or Hf. In a specific embodiment, the transition metal is iron.In another specific embodiment, the transition metal is selected from nickel, cobalt, and / or chromium. In another specific embodiment, the transition metal is chromium. In another specific embodiment, the acidic aqueous solution containing cations of at least one transition metal is a hydrochloric acid iron salt solution.

[0031] In one embodiment of the process, the acidic aqueous solution contains hydrochloric acid, hydrofluoric acid, nitric acid, sulfuric acid, phosphoric acid, or a mixture of two or more of the aforementioned acids. In a specific embodiment, the acidic aqueous solution contains only hydrochloric acid.

[0032] In one embodiment of the process, the acidic aqueous solution containing cations of at least one transition metal is a metal pickling solution containing metal ions and acid. In another embodiment, the metal pickling solution contains hydrochloric acid and iron and / or copper ions. In a specific embodiment, the metal pickling solution is a hydrochloric acid copper pickling solution. In another specific embodiment, the metal pickling solution is a hydrochloric acid iron pickling solution.

[0033] The electrolysis of a hydrochloric acid iron pickle is usually carried out between approximately 20°C and approximately 110°C under galvanostatic control. The current density at the cathode is usually approximately 1,000 to 2,000 A / m 2Under these conditions, the evolution of hydrogen gas at the cathode is suppressed. In this particular embodiment, the Faradaic efficiency is typically greater than about 90%, the cell voltage is less than 1.5 V, and the average specific electrical energy consumption for metal deposition with hydrogen supply is in the range of < 1 kWh per kg of iron.The invention also relates to a device for processing an acidic aqueous solution which contains cations of at least one transition metal, comprising a) at least one electrochemical cell with an anode compartment which contains a gas diffusion electrode as anode, and a cathode compartment which contains a cathode whose hydrogen overvoltage is higher than the potential required for the deposition of the transition metal at the cathode, wherein the anode compartment and the cathode compartment of the electrochemical cell are separated by an anion or cation exchange membrane, b) at least one water electrolyzer which is designed to supply the gas diffusion electrode of the at least one electrochemical cell with hydrogen, and / or c) at least one nanofiltration module.

[0034] The device according to the invention comprises at least one electrochemical cell with an anode compartment containing a gas diffusion electrode as anode and a cathode compartment containing a cathode whose hydrogen overvoltage is higher than the potential required for the deposition of the transition metal at the cathode.

[0035] The anode compartment and the cathode compartment of the at least one electrochemical cell are separated by an anion or cation exchange membrane. In one embodiment, the anode compartment and the cathode compartment of the at least one electrochemical cell are separated by an anion exchange membrane. The anion exchange membrane prevents the migration of transition metal cations from the cathode compartment into the anode compartment. In another embodiment, the anode compartment and the cathode compartment of the at least one electrochemical cell are separated by a cation exchange membrane. The cation exchange membrane prevents the migration of negatively charged transition metal complexes, for example, iron or copper halide complexes, from the cathode compartment into the anode compartment.

[0036] Suitable acid-stable anion and cation exchange membranes are commercially available, for example from FUMATECH BWT GmbH, 74321 Bietigheim-Bissingen.

[0037] In one embodiment, the device comprises more than one electrochemical cell, for example, two, three, or four electrochemical cells. In one embodiment, the electrochemical cells are connected in series. In another embodiment, the cathodes of the series-connected electrochemical cells are made of different materials.

[0038] In one embodiment, the device comprises at least one nanofiltration module. In a further embodiment, the device comprises at least one nanofiltration module, the inlet of which is connected to an outlet of the anode compartment of the at least one electrochemical cell. In a further embodiment, the device comprises at least one nanofiltration module, the inlet of which is connected to an outlet of the cathode compartment of the at least one electrochemical cell, and the permeate outlet of which is connected to an inlet of the anode compartment of the at least one electrochemical cell. In a further embodiment, the device comprises at least one nanofiltration module, the retentate outlet of which is connected to an inlet of the cathode compartment of the at least one electrochemical cell.

[0039] The advantages of the solution according to the invention include significantly reduced energy requirements compared to conventional electrolysis and the avoidance of gas evolution, particularly chlorine gas evolution, during electrolysis. For the deposition of metals with a positive standard potential, such as Cu, Ag, and Au, no additional voltage source is required. When using an electrolyzer for hydrogen production, the process can be carried out without additional auxiliary materials. Further advantages and refinements of the invention will become apparent from the description.

[0040] It is understood that the above-mentioned features can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

Claims

Patent claims 1. A process for processing an acidic aqueous solution containing cations of at least one transition metal, comprising i. transporting the solution through at least one electrochemical cell having an anode compartment containing a gas diffusion electrode as the anode, and a cathode compartment containing a cathode whose hydrogen overvoltage is higher than the potential required for deposition of the transition metal at the cathode, wherein the anode compartment and the cathode compartment of the electrochemical cell are separated by an anion or cation exchange membrane, and wherein the solution is introduced into an inlet into the cathode compartment of the at least one electrochemical cell, discharged from the cathode compartment through an outlet of the cathode compartment, and introduced into the anode compartment via an inlet of the anode compartment of the at least one electrochemical cell, and discharged from the anode compartment through an outlet of the anode compartment, II. Electrochemical conversion of the solution by introducing hydrogen into the gas diffusion electrode and deposition of at least one transition metal on the cathode and formation of acid at the anode.

2. The method according to claim 1, wherein the solution discharged from the anode compartment of the at least one electrochemical cell is subjected to nanofiltration.

3. The method according to claim 1, wherein the solution discharged from the cathode compartment of the at least one electrochemical cell is a nanofilter ration and the permeate is fed to the inlet of the anode compartment of at least one electrochemical cell.

4. The method according to claim 2 or 3, wherein the retentate of the nanofiltration is at least partially returned to the cathode compartment of the at least one electrochemical cell.

5. The method according to any one of claims 1 to 4, wherein the hydrogen introduced into the gas diffusion electrode of the at least one electrochemical cell is produced by electrolysis of an aqueous solution in an electrolyzer.

6. A method according to claim 5, wherein the electrolyzer and electrochemical cell are electrically connected in series 7. A process according to any one of claims 1 to 6, wherein the standard potential E° (at a temperature of 298.15 K, atmospheric pressure of 101.325 kPa, and an activity of the dissolved particles of 1 mol / l) of the transition metal has a positive value greater than 0 V.

8. The method of claim 7, wherein the transition metal is Au, Pt, Ir, Pd, Hg, Ag, Rh, Cu, Ru, Bi, or Re.

9. Process according to one of claims 1 to 6, wherein the standard potential E° (at a temperature of 298.15 K, atmospheric pressure of 101.325 kPa, and an activity of the dissolved particles of 1 mol / l) of the transition metal has a negative value less than 0 V.

10. The method according to claim 9, wherein the transition metal is Fe, Pb, Sn, Mo, Ni, Co, Cd, Cr, Zn, Ti, Nb, V, Mn, U, Zr or Hf.

11. A process according to any one of claims 1 to 10, wherein the acidic aqueous solution comprises hydrochloric acid, hydrofluoric acid, nitric acid, sulfuric acid, phosphoric acid, acid or a mixture of two or more of the aforementioned acids.

12. A process according to any one of claims 1 to 11, wherein the acidic aqueous solution containing cations of at least one transition metal is a metal pickling solution containing metal ions and acid.

13. A process according to claim 12, wherein the metal pickling solution contains hydrochloric acid and iron and / or copper ions.

14. Apparatus for processing an acidic aqueous solution containing cations of at least one transition metal, comprising a) at least one electrochemical cell with an anode compartment containing a gas diffusion electrode as anode, and a cathode compartment containing a cathode whose hydrogen overvoltage is higher than the potential required for deposition of the transition metal at the cathode, wherein the anode compartment and the cathode compartment of the electrochemical cell are separated by an anion or cation exchange membrane, b) at least one water electrolyzer configured to supply the gas diffusion electrode of the at least one electrochemical cell with hydrogen, and / or c) at least one nanofiltration module.

15. Device according to claim 14, which comprises at least one nanofiltration module, the inlet of which is connected to an outlet of the anode compartment of the at least one electrochemical cell.

16. Device according to claim 14 or 15, which comprises at least one nanofiltration module, the inlet of which is connected to an outlet of the cathode compartment of the at least one electrochemical cell, and the permeate outlet of which is connected to an inlet of the anode compartment of the at least one electrochemical cell.

17. Device according to one of claims 14 to 16, which comprises at least one nanofiltration module, the retentate outlet of which is connected to an inlet of the cathode compartment of the at least one electrochemical cell.