Removing metal from acidic aqueous solutions

Abstract

The present invention relates to a process for removing metals from acidic aqueous solutions, said process minimizing environmental damage, in particular completely avoiding emissions of CO2 or gaseous halogens and optionally also being able to make a contribution to permanent fixation of CO2.

Description

[0001] Metal separation from acidic aqueous solutions

[0002] The present invention relates to a process for the separation of metals from acidic aqueous solutions, which minimizes environmental pollution, in particular completely avoids emissions of CO2 or gaseous halogens, is ideally completely wastewater-free, and can optionally also contribute to the permanent fixation of CO2.

[0003] Various processes using electrolytic metal deposition have already been proposed for the recovery of metals from metal-containing acidic solutions.

[0004] 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.

[0005] 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.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.

[0006] DE 44 35 232 A1 discloses a process for the regeneration of pickling solutions containing hydrofluoric acid for descaling stainless steels, i.e. for the separation of the unreacted acids, their reformation from the salts and their return to the pickling electrolyte by means of electrodialysis, reduction by membrane electrolysis or chemically and by means of electrolytic alloy deposition of the metals iron, nickel and chromium in membrane cells, wherein the effluent from the stainless steel pickling is subjected to an electrodialysis stage for the supply of sulphuric acid, a membrane electrolysis for the reduction of the trivalent iron compounds and a further membrane electrolysis for the deposition of the metals, and at the same time the acid reformed in the two membrane electrolyses is returned to the pickling bath.

[0007] EP 0 603 763 B1 relates to a process for purifying spent acidic metal pickling solutions containing specific inhibitors that can be eliminated during the processing of the used pickling acids using conventional methods. In this process, the inhibitor-activator system contained therein is removed before further processing. Against this background, the present invention aims to provide improved processes and devices for processing acidic aqueous solutions containing cations of metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U, and the noble metals, and metal cations that cannot be electrolytically deposited from aqueous solutions.

[0008] 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 10. Embodiments and developments of the invention emerge from the dependent claims and the description.

[0009] The invention relates to a process for the processing of an acidic aqueous solution which contains cations of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals and of aluminum and / or at least one alkaline earth metal, comprising i. electrolysis of the solution in at least one electrochemical cell containing a hydrogen diffusion electrode as an anode with deposition of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals on the cathode and production of acid at the anode,

[0010] II. Diffusion dialysis of the solution obtained after electrolysis to produce a retentate enriched in the metal ions remaining in the solution and a permeate depleted in metal ions. (iii) Precipitation of aluminum salt and / or alkaline earth salt from the retentate by adding a basic alkali compound to the retentate. (iv) Separation of solid and liquid phases. (v) Electrodialysis of the resulting liquid phase to produce alkali solution and acid. In one embodiment, the acidic aqueous solution contains hydrochloric acid. In another embodiment, the acidic aqueous solution contains hydrobromic acid. In yet another embodiment, the acidic aqueous solution contains sulfuric acid.

[0011] The process according to the invention comprises the process steps of electrolysis, diffusion dialysis, precipitation and electrodialysis:

[0012] In the first step of the process according to the invention, electrolysis of the solution is carried out in at least one electrochemical cell containing a hydrogen diffusion electrode as anode with deposition of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals on the cathode and production of acid at the anode.

[0013] In one embodiment of the process, the metal is selected from Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, or U. In one embodiment of the process, the metal is a noble metal. In a specific embodiment, the metal is iron. In another specific embodiment, the metal is selected from nickel, cobalt, and / or chromium. In a further specific embodiment, the metal is chromium. In a further specific embodiment, the acidic aqueous solution containing cations of at least one metal is a hydrochloric acid iron salt solution.

[0014] In the at least one electrochemical cell, metal cations present in the solution, selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U, and the noble metals, 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, with the acid anions present, form free acid. In one embodiment, the free acid is hydrochloric acid. In another embodiment, the free acid is hydrobromic acid. In yet another embodiment, the free acid is sulfuric acid.

[0015] In the process according to the invention, the hydrogen overvoltage of the cathode is higher than the potential required for the deposition of the metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals on the cathode. In the present 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 metal.

[0016] In one embodiment of the method, the cathode consists of the 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.

[0017] 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. A hydrogen diffusion electrode is used as the anode to form HCl and prevent the evolution of chlorine gas.

[0018] 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.

[0019] In another embodiment, the gas diffusion electrode is a plastic-bonded electrode, for example a PTFE-silver electrode or PTFE-carbon electrode.

[0020] 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.

[0021] 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 or from Gaskatel GmbH, 34123 Kassel.

[0022] 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.

[0023] In another embodiment, the hydrogen introduced into the gas diffusion electrode of the at least one electrochemical cell is generated by steam reforming, for example, methane or natural gas. In a further embodiment, the steam reforming is followed by a water-gas shift reaction, and a solution containing carbonate ions is generated from the carbon dioxide formed in the process according to the invention. In another embodiment, the hydrogen introduced into the gas diffusion electrode of the at least one electrochemical cell is generated by partial oxidation of hydrocarbons, for example, methane.

[0024] In another embodiment, the hydrogen introduced into the gas diffusion electrode of the at least one electrochemical cell is produced by a Kvaerner process, ie pyrolysis of hydrocarbons to form solid carbon and hydrogen.

[0025] In one embodiment of the process, in step i. the electrolysis is carried out as membrane electrolysis, wherein the aqueous solution is first fed to the cathode compartment of the electrochemical cell for metal deposition and then passed into the anode compartment for acid formation, and both compartments are separated by an ion exchange membrane.

[0026] In one embodiment, the acidic aqueous solution containing cations of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U, and the noble metals 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 a metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U, and the noble metals is deposited at the cathode. The 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 metal cations and more free acid than the solution introduced into at least one electrochemical cell.

[0027] In one embodiment of the method, in step i. the solution is electrolyzed in a plurality of series-connected electrochemical cells, each containing a hydrogen diffusion electrode as an anode, with deposition of a metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals on the cathode and production of acid at the anode, wherein the order of deposition of the individual metals is carried out according to the electrochemical voltage series of the dissolved metals and the voltage is increased from bath to bath to such an extent that the most noble of the still dissolved metals is deposited in each electrochemical cell.

[0028] In one embodiment of the method, an acidic aqueous solution containing cations of at least two different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals is passed successively through at least two electrochemical cells, and a first of the at least two different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals is deposited at the cathode of the first electrochemical cell and another of the at least two different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble 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 metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals has a more positive standard potential E° (at a temperature of 298.15 K, standard pressure of 101,325 kPa, and an activity of the dissolved particles of 1 mol / l) than the other of the at least two different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals.

[0029] In one embodiment of the process, the acidic aqueous solution, which contains cations of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals, is passed sequentially through several electrochemical cells. In one embodiment of the process, the acidic aqueous solution contains cations of at least two different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals. In a further embodiment of the process, the acidic aqueous solution contains cations of at least three different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals. In a further embodiment of the process, the acidic aqueous solution contains cations of more than three different metals selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals.

[0030] The solution obtained after electrolysis is subjected to diffusion dialysis in step II, producing a low-acid retentate enriched in the remaining metal ions and a high-acid permeate depleted in metal ions. Diffusion dialysis separates the acid present in the solution from any remaining mineral salts, such as Mg or Ca salts. The low-metal acid formed by diffusion dialysis can be recycled to the upstream process, optionally also to a downstream electrodialysis device. The diffusate from diffusion dialysis is fed into the acid chambers of the electrodialysis device, and the acid is then further concentrated by electrodialysis.

[0031] With spiral-wound modules for acid diffusion dialysis, such as those available from Spiraltec GmbH, 74343 Sachsenheim, a hydrochloric acid recovery of more than 90% can be achieved at a feed flow of 1.5-2 l per square meter of membrane area per hour at atmospheric pressure and a temperature range of 20°C to 50°C, provided the water flow is approximately 10% to 20% greater than the feed flow. A retention of multivalent cations in the retentate of >99% is achieved.

[0032] In one embodiment of the process, the permeate of diffusion dialysis obtained in step II. is fed to the acid chambers of an electrodialysis device used for electrodialysis in step v.

[0033] In step ill. of the process according to the invention, precipitation of aluminum and / or alkaline earth metal salts from the retentate of the diffusion dialysis takes place by adding a basic alkali compound to the retentate.

[0034] In one embodiment of the process, the basic alkali compound is NaOH or KOH. In another embodiment of the process, the basic alkali compound is Na2CO3 or K2CO3.

[0035] In an exemplary embodiment of the process, mineral salts are precipitated from the retentate of diffusion dialysis by adding NaOH or KOH solution, yielding an alkali salt solution (Na or K salt) and precipitated salts (e.g., Ca(OH)2 and / or Mg(OH)2 and / or Al(OH)3). Remaining traces of heavy metal cations are also precipitated.

[0036] In one embodiment, the alkali metal salt is an alkali metal chloride. In another embodiment, the alkali metal salt is an alkali metal bromide. In yet another embodiment, the alkali metal salt is an alkali metal sulfate.

[0037] In a further embodiment of the process, a lye formed during electrodialysis in step v. is used to absorb CO2, and the resulting carbonate solution is used in step iii. to precipitate carbonates. In one embodiment of the process, CO2 is absorbed from flue gases of combustion processes, in other embodiments from the atmosphere or ambient air or other sources. By using the resulting carbonate solution to precipitate carbonates, CO2 emissions from other technical processes can also be permanently avoided. In one embodiment of the process, CO2 originating from steam reforming with a downstream water gas shift reaction of methane is absorbed. In another embodiment of the process, CO2 originating from the partial oxidation of hydrocarbons is absorbed.

[0038] In step iv. of the process according to the invention, the solid produced in step ill. is separated from the liquid phase.

[0039] Devices and methods for the precipitation of metal hydroxides and / or metal carbonates from aqueous solutions by adding basic precipitants and for separating the solid formed from the liquid phase are known in principle to the person skilled in the art, for example from the field of wastewater treatment or the causticization of pulp.

[0040] In step v. of the process according to the invention, electrodialysis of the liquid phase from step iv. is carried out to produce alkali solution and acid. Electrodialysis of the alkali salt-containing liquid phase using bipolar membranes (EDBM) in the electrodialysis device yields an acid solution as well as a NaOH or KOH solution for precipitation or for CO2 absorption to produce a carbonate solution for precipitation.

[0041] In an exemplary embodiment, the liquid phase supplied to the electrodialysis contains approximately 10 wt.% NaCl, for example 5-15 wt.%. The electrodialysis device with bipolar membranes is, in an exemplary embodiment, supplied with 1.5 kg NaCl per m 2 Membrane area and per h and the current density is 80 mA / cm 2 Approximately 90% of the NaCl is converted into sodium hydroxide solution and hydrochloric acid. This requires approximately 1.2 kWh of electrical energy per kg of NaCl. In one embodiment of the process, a concentrated salt solution and water are obtained from the depleted salt solution obtained after electrodialysis in step v. by means of reverse osmosis. In one embodiment, the concentrated salt solution is added to the precipitation bath; some of the water is added to the lye chambers of the electrodialysis device and some is used for diffusion dialysis to produce acid.

[0042] The depleted salt solution (feed solution) is slightly acidified before being fed to the reverse osmosis system. This prevents any alkaline earth cations and / or metal cations still present in the solution from precipitating as salts in the reverse osmosis device. In one embodiment, the salt content of the feed solution is 2-5 wt.%, and the salt content of the concentrated salt solution leaving the reverse osmosis system is approximately 10 wt.%, for example, 5-15 wt.%.

[0043] The only starting materials required for the process according to the invention are the acidic aqueous solution, H2 for electrolysis, water, and optionally CO2. The products obtained are metals, precipitated hydroxides, or carbonates. No gaseous emissions are generated, and ideally, the process is completely wastewater-free.

[0044] In one embodiment of the process, a product solution from leaching processes of silicates or oxide minerals with HCl, HBr, or H2SO4 is used as the acidic aqueous solution to be treated. Particularly interesting applications for the process according to the invention are the treatment of chloride solutions from the leaching process of olivine with hydrochloric acid, which contain iron and magnesium ions as metal ions, or the treatment of chloride solutions from the leaching process of hedenbergite with hydrochloric acid, which contain iron and calcium ions as metal ions. Mixtures of silicates and oxides can also be used as starting materials for leaching in order to obtain sufficient quantities of solutions containing alkaline earth cations for CO2 fixation. The preparation of pickling or etching baths containing acids such as hydrochloric acid or sulfuric acid and metal salts, e.g., copper chloride, is also possible.

[0045] The invention also relates to a device for processing an acidic aqueous solution which contains cations of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals and of aluminum and / or at least one alkaline earth 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 the deposition of the metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals at the cathode, b) optionally at least one water electrolyzer which is designed to supply the gas diffusion electrode of the at least one electrochemical cell with hydrogen, c) at least one dialysis module which is designed toreceiving a liquid stream from the at least one electrochemical cell and producing therefrom a low-acid retentate enriched in metal ions and a high-acid permeate depleted in metal ions, d) at least one reactor which is designed to receive and mix a retentate stream from the dialysis module and an aqueous solution of a basic alkali compound, and which has means for separating solids and liquids, e) at least one electrodialysis device which is designed to receive a liquid stream from the at least one reactor and to produce acid and alkali solution from an aqueous solution containing alkali salt, f) optionally a reverse osmosis device which is designed toreceiving a liquid stream containing alkali salt from the at least one electrodialysis device and producing therefrom a retentate enriched in alkali salt and a permeate depleted in alkali salt.

[0046] The device according to the invention comprises 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 the deposition of the metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals at the cathode.

[0047] In one embodiment, 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 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 metal complexes, for example, iron or copper halide complexes, from the cathode compartment into the anode compartment.

[0048] Suitable acid-stable anion and cation exchange membranes are commercially available, for example from Fumatech BWT GmbH, 74321 Bietigheim-Bissingen.

[0049] 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.

[0050] In one embodiment, the device comprises b) at least one water electrolyzer which is designed to supply the gas diffusion electrode of the at least one electrochemical cell with hydrogen,

[0051] The device comprises c) at least one module for diffusion dialysis connected downstream of the at least one electrochemical cell. In a further embodiment, the device comprises at least one dialysis module, the inlet of which is connected to an outlet of the anode compartment of the at least one electrochemical cell. The at least one dialysis module is configured to receive a liquid stream from the at least one electrochemical cell and to produce therefrom a retentate enriched in metal ions and a permeate depleted in metal ions.

[0052] The device comprises d) at least one reactor connected downstream of the at least one dialysis module. The at least one reactor is configured to receive and mix a retentate stream from the dialysis module and an aqueous solution of a basic alkali compound. As a result, polyvalent metal ions contained in the retentate are precipitated as basic metal salts. The reactor has means for separating solids and liquids to separate the precipitated solids from the liquid phase. In one embodiment, the means for separating solids and liquids comprise solid filters, for example, chamber filter presses. In another embodiment, the means for separating solids and liquids comprise settling tanks or centrifuges.

[0053] The device comprises e) at least one electrodialysis device connected downstream of the at least one reactor. The at least one electrodialysis device is configured to receive a liquid stream (mother liquor) from the at least one reactor and to generate acid and alkali metal hydroxide solution from an aqueous solution containing alkali metal salts. Acid and alkali metal hydroxide solution are generated in the at least one electrodialysis device from the liquid phase containing alkali metal salts from the at least one reactor.

[0054] Suitable electrodialysis devices with bipolar membranes are known in principle to those skilled in the art and are commercially available, for example from Fumatech BWT GmbH, 74321 Bietigheim-Bissingen. The bipolar membranes (BPM) have a cation exchange layer (CEL) and an anion exchange layer (AEL).

[0055] In one embodiment, the electrodialysis device comprises three chambers separated from each other by membranes. Acid is generated in a first chamber (acid chamber), alkali is generated in a third chamber (lye chamber), and an alkali salt-depleted aqueous solution remains in a central second chamber. The first chamber is bordered on one side by the cation exchange layer (CEL) of a bipolar membrane (BPM) and on the other side by an anion exchange membrane (AEM). The central chamber is bordered on one side by the AEM and on the other side by a cation exchange membrane (CEM). The third chamber is bordered on one side by the CEM and on the other side by the anion exchange layer (AEL) of a BPM. The stacking sequence of the repeating units of the membrane stacks in the electrodialysis unit is therefore BPM, AEM, CEM.

[0056] In one embodiment, the device f) comprises at least one reverse osmosis device. The at least one reverse osmosis device is configured to receive a liquid stream containing alkali salt from the at least one electrodialysis device and to produce therefrom an alkali salt-enriched retentate and an alkali salt-depleted permeate.

[0057] Suitable reverse osmosis systems are known in principle to those skilled in the art and are commercially available. Reverse osmosis systems such as those used for seawater desalination can be used. Suitable systems are available, for example, from Lenntech BV, 2645 EG Delfgauw, Netherlands.

[0058] 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. When using an electrolyzer for hydrogen production, the process can be carried out without additional auxiliary materials. Further advantages and embodiments of the invention will become apparent from the description, the drawings, and the following examples.

[0059] 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.

[0060] Examples

[0061] Treatment of chloride solutions from the leaching process of olivine with hydrochloric acid, containing iron and magnesium ions as metal ions.

[0062] Example 1

[0063] An exemplary embodiment of the device according to the invention is depicted in Figure 1. Figure 1 also shows the material flows occurring in an exemplary embodiment of the process according to the invention.

[0064] A chloride solution from the leaching of olivine with hydrochloric acid, containing iron chloride, magnesium chloride, and residual hydrochloric acid, is fed to an electrolysis unit (10) containing an iron cathode and a hydrogen diffusion electrode as the anode. Gaseous hydrogen, which can be produced, for example, in an upstream water electrolyzer (not shown), is fed to the hydrogen diffusion electrode. In the electrolysis unit (10), iron is deposited from the chloride solution on the cathode, and HCl is formed at the anode.

[0065] A solution depleted in iron ions and enriched in hydrochloric acid leaves the electrolysis unit (10) and is fed to a dialysis unit (20), where diffusion dialysis is performed to produce a magnesium chloride-enriched retentate and a magnesium chloride-depleted hydrochloric acid permeate from the supplied solution. For this purpose, water is added to the permeate side of the dialysis unit.

[0066] The retentate stream from the dialysis unit (20) is fed to a reactor (30), where magnesium carbonate is precipitated by adding an aqueous solution containing Na2CO3. The solid is separated, and the liquid phase containing sodium chloride is fed to an electrodialysis unit (40), where hydrochloric acid and sodium hydroxide solution are produced from NaCl. The hydrochloric acid can be recycled to the leaching of olivine. In the example shown, the permeate from the dialysis unit (20) is fed into the acid chamber of the electrodialysis unit (40) and discharged from the electrodialysis unit (40) together with the hydrochloric acid produced by electrodialysis.

[0067] The sodium hydroxide solution produced by electrodialysis is fed from the electrodialysis unit (40) to a CO2 absorber (not shown), in which, for example, CO2 from flue gases is absorbed into the sodium hydroxide solution to form Na2CO3. The resulting aqueous solution of Na2CO3 is fed back to the reactor (30) and used to precipitate magnesium carbonate.

[0068] A reverse osmosis device (50) is connected downstream of the electrodialysis unit (40). The liquid stream leaving the electrodialysis unit (40), containing the residual alkali chloride, is slightly acidified and fed to the reverse osmosis device (50), where a sodium chloride-enriched retentate and a sodium chloride-depleted permeate are produced. The retentate can be returned to the reactor (30), and the permeate, consisting primarily of water, can be fed to the permeate side of the dialysis unit (20) and used to produce the sodium hydroxide solution in the electrodialysis unit (40).

[0069] Example 2

[0070] Treatment of chloride solutions from the leaching process of olivine with hydrochloric acid, containing iron and magnesium ions as metal ions, in the apparatus shown in Fig. 1.

[0071] In the leaching process, 4.6 mol HCl are used per 1.05 mol of olivine (idealized composition MgFeSiO^). The chloride solution to be treated then contains 1.05 mol Fe, 1.05 mol Mg and 0.4 mol HCl. In the electrolysis with gas diffusion electrode, 1 mol Fe is deposited at the cathode, with a requirement of 1 mol H2(Fe 2+ + H2-> Fe + 2H + ) and an ideal power requirement of approx. 87 kJ (U* n*z*F aIn the subsequent diffusion dialysis, up to 95% of the HCl is recovered, with almost complete retention of the metal ions Fe and Mg when spiral modules from Spiraltec GmbH in Sachsenheim are used. An acid containing 2.28 mol HCl is obtained, which is fed directly to the acid chamber of the electrodialysis, and a solution containing 0.05 mol Fe, 1.05 mol Mg, and 0.12 mol HCl is used for precipitation. To achieve additional benefits through CO2 fixation, Na2CO3 solution is used in the precipitation. Thus, 1.16 mol Na2CO3 is required for neutralization and precipitation, and a solution containing 2.32 mol NaCl is formed. Neutralization releases 0.06 mol of CO2, resulting in the permanent fixation of 1.1 mol of CO2 in the form of MgCO3 and 0.05 mol of CO2 in the form of FeCO3. Electrodialysis of the resulting salt solution thus yields 2.32 mol each of NaOH and HCl.

[0072] In the acid chamber of electrodialysis, the hydrochloric acid produced in diffusion dialysis is further concentrated with 2.28 mol HCl, resulting in an acid with 4.6 mol HCl. When this acid is returned to the leaching stage, exactly 1.05 mol each of Mg and Fe are dissolved, leaving 0.4 mol as excess in the solution. This results in a closed acid cycle. Since no acid losses occur, hydrobromic acid can be used instead of hydrochloric acid, which is otherwise rarely used in commercial chemical processes due to its cost.

[0073] The 2.32 mol of NaOH formed absorb 1.16 mol of CO2 and thus form exactly the amount of Na2CO2 required for precipitation and residual neutralization.

[0074] The idealized energy requirement for electrodialysis is approximately 80 kJ / mol NaOH. At an efficiency of 75%, this results in an electrical energy requirement of 107 kJ / mol NaOH. 2.32 mol of NaOH are required for each 1 mol of Fe. Electrodialysis therefore requires 250 kJ of electrical energy per mol of Fe. At an efficiency of 80%, approximately 110 kJ of electrical energy per mol of Fe is required for electrolysis, and an additional 1 mol of H2 is required. The calorific value of 1 mol of H2 is 242 kJ. In summary, this results in a total energy requirement of approximately 360 kJ / mol of electrical energy per mol of Fe. An additional 1 mol of H2 (242 kJ / mol) is required for electrolysis.

[0075] In the process, 1.1 mol of CO2 are fixed as carbonate. Since 1.1 mol of CO2 are fixed as carbonate, 1.1 mol of methane can be used in a CO2-neutral manner. If the hydrogen is produced from methane through steam reforming and a water gas shift reaction, 0.25 mol of methane are required per mole of iron, and 0.25 mol of CO2 are formed, which is absorbed and fixed as carbonate. The overall endothermic reaction requires approximately 80 kJ of additional heat energy (equivalent to 0.1 mol of methane). This leaves an additional 0.75 mol of methane (equivalent to 600 kJ) available for CO2-neutral electricity generation in a combined cycle power plant. At an efficiency of 60%, 600 kJ * 0.6 = 360 kJ of electrical energy can be generated, which corresponds to the amount of energy required for the entire process. Thus, 1.1 mol of CH4 (equivalent to 4.4 MWh / ton of Fe) is required to produce 1 mol of Fe. The process is therefore completely CO2-neutral.In the industry profile of the iron and steel industry prepared by Navigant Energy Germany GmbH in 2019 on behalf of the Federal Ministry for Economic Affairs and Energy (BMWi), the energy consumption of conventional crude steel production is stated as 5 MWh of fuel / ton and 0.4 MWh of electricity / ton. The inventive process thus reduces energy consumption while avoiding any emissions. A further advantage of the new process is the easy scalability of the membrane processes used. This allows even smaller deposits of silicates to be exploited. The precipitated hydroxides or carbonates are an additional raw material, for example, as an additive for the construction industry, with significantly lower health risks and environmental impact compared to blast furnace slag.

[0076] In addition, exothermic CO2 absorption (reaction enthalpy approximately 0.35 MWh / t CO2) can be carried out at elevated temperatures above 60 degrees Celsius. This allows for additional heat recovery, which can be significantly higher than 0.5 MWh / t CO2 when absorbed from flue gases using condensing technology.

[0077] List of reference symbols

[0078] 10 Electrolysis unit

[0079] 20 dialysis units

[0080] 30 reactor

[0081] 40 electrodialysis unit

[0082] 50 reverse osmosis device

Claims

Patent claims 1. A process for the processing of an acidic aqueous solution containing cations of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals, as well as aluminum cations and / or cations of at least one alkaline earth metal, comprising i. electrolysis of the solution in at least one electrochemical cell containing a hydrogen diffusion electrode as an anode with deposition of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals on the cathode and generation of acid at the anode, II. Diffusion dialysis of the solution obtained after electrolysis to produce a retentate enriched in the metal ions remaining in the solution and a permeate depleted in metal ions, ill. Precipitation of aluminum salt and / or alkaline earth salt from the retentate by adding a basic alkali compound to the retentate, iv. Separation of solid and liquid phases, v. Electrodialysis of the liquid phase to produce alkali solution and acid.

2. The method according to claim 1, wherein in step i. the electrolysis is carried out as membrane electrolysis, wherein the aqueous solution is first fed to the cathode compartment of the electrochemical cell for metal deposition and then passed on to the anode compartment for acid formation, and both compartments are separated by an ion exchange membrane are.

3. A process according to claim 1 or 2, wherein in step i. the solution is electrolyzed in a plurality of electrochemical cells connected in series, each containing a hydrogen diffusion electrode as anode, with deposition of a metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals on the cathode and production of acid at the anode, wherein the order of deposition of the individual metals is carried out according to the electrochemical voltage series of the dissolved metals and the voltage is increased from bath to bath to such an extent that the most noble of the still dissolved metals is deposited in each electrochemical cell.

4. A process according to any one of claims 1 to 3, wherein the permeate of the diffusion dialysis obtained in step II. is fed to the acid chambers of an electrodialysis device used for electrodialysis in step v.

5. A process according to any one of claims 1 to 4, wherein a concentrated salt solution and water are obtained from the depleted salt solution obtained in step v. after electrodialysis by means of reverse osmosis.

6. A process according to any one of claims 1 to 5, wherein the basic alkali compound in step iii is NaOH or KOH.

7. A process according to any one of claims 1 to 5, wherein the basic alkali compound in step iii is Na2CO3 or K2CO3.

8. Process according to one of claims 1 to 5, in which the lye formed during electrodialysis is used for CO2 absorption and the carbonate solution obtained is used in step iii. for the precipitation of carbonates.

9. A process according to any one of claims 1 to 8, wherein the acidic aqueous solution to be processed was obtained by leaching olivine with hydrochloric acid, hydrobromic acid or sulphuric acid, or by leaching hedenbergite with hydrochloric acid or hydrobromic acid.

10. A device for processing an acidic aqueous solution which contains cations of at least one metal selected from the group consisting of Fe, Pb, Sn, Ni, Co, Cd, Cr, Zn, Mn, U and the noble metals, as well as aluminum cations and / or cations of at least one alkaline earth metal, comprising a) at least one electrochemical cell (10) with an anode chamber containing a gas diffusion electrode as anode, and a cathode chamber containing a cathode whose hydrogen overvoltage is higher than the potential required for the deposition of the metal at the cathode, b) optionally at least one water electrolyzer which is designed to supply the gas diffusion electrode of the at least one electrochemical cell (10) with hydrogen, c) at least one dialysis module (20) which is designed toreceiving a liquid stream from the at least one electrochemical cell (10) and producing therefrom a retentate enriched in metal ions and depleted in acid and a permeate depleted in metal ions and enriched in acid, d) at least one reactor (30) which is designed to receive and mix a retentate stream from the dialysis module (20) and an aqueous solution of a basic alkali compound, and which has means for separating solids and liquids, e) at least one electrodialysis device (40) which is designed to receive a liquid stream from the at least one reactor (30) and to produce acid and alkali lye from an aqueous solution containing alkali salt, f) optionally a reverse osmosis device (50) which has the means for, is arranged to receive a liquid stream containing alkali salt from the at least one electrodialysis device (40) and to produce therefrom a retentate enriched in alkali salt and a permeate depleted in alkali salt.

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