Wastewater-free treatment of acidic aqueous metal-salt-containing solutions

Abstract

The present invention relates to a process for separating metals from acidic aqueous solutions, in particular pickling solutions, which minimizes environmental pollution by wastewater.

Description

[0001] Wastewater-free treatment of acidic aqueous solutions containing metal salts

[0002] The present invention relates to a process for separating metals from acidic aqueous solutions, in particular metal pickling solutions, which minimizes environmental pollution from wastewater.

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

[0004] From CN 114 735 860 A, an integrated membrane system and a method for treating acidic waste liquids are known. The system comprises a diffusion dialysis unit and a selective electrodialysis unit arranged in series, with anion exchange membranes arranged in the diffusion dialysis unit and a chamber for liquid flow between the adjacent anion exchange membranes; the selective electrodialysis unit is equipped with a hydrogen-resistant anion exchange membrane and a monovalent selective cation exchange membrane arranged one behind the other in an overlapping manner, and a space for liquid flow is arranged between the hydrogen-resistant anion exchange membrane and the monovalent selective cation exchange membrane.

[0005] Against this background, the present invention aims to provide improved processes and devices for processing acidic aqueous solutions containing metal ions. This object is achieved according to the invention by a process having the features of claim 1 and a device having the features of claim 8. Further developments and refinements of the invention are set out in the dependent claims and the description.

[0006] The invention relates to a process for the processing of an acidic aqueous solution containing metal ions, comprising i. diffusion dialysis of the solution to produce a retentate enriched in metal ions and a permeate depleted in metal ions,

[0007] II. Precipitation of metal salt from the retentate by adding a basic alkali compound to the retentate, ill. Separation of solid and liquid phase, iv. Electrodialysis of the resulting liquid phase to produce alkali solution and acid.

[0008] 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 hydrofluoric acid. In yet another embodiment, the acidic aqueous solution contains sulfuric acid. In yet another embodiment, the acidic aqueous solution contains nitric acid.

[0009] The process according to the invention comprises the process steps of diffusion dialysis, precipitation and electrodialysis:

[0010] Before starting the process according to the invention, the acidic aqueous solution containing metal ions may be subjected to a pretreatment, for example filtration to remove suspended particles and / or nanofiltration to concentrate the solution.

[0011] In the first step of the process according to the invention, the acidic aqueous solution containing metal ions is subjected to diffusion dialysis, producing a retentate enriched in metal ions and a permeate depleted in metal ions. Diffusion dialysis separates the acid present in the solution from the metal salts contained therein. The metal-poor 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.

[0012] With spiral-wound modules for acid diffusion dialysis, such as those available from Spiraltec GmbH, 74343 Sachsenheim, Germany, 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.

[0013] In one embodiment of the process, the metal is a transition metal, for example 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 a further specific embodiment, the transition metal is chromium. In a further specific embodiment, the acidic aqueous solution containing cations of at least one transition metal is a hydrochloric acid iron salt solution.

[0014] In another embodiment of the process, the metal is an alkaline earth metal, for example Mg or Ca.

[0015] In one embodiment of the process, the permeate from diffusion dialysis obtained in step i. is fed to the acid chambers of an electrodialysis device used for electrodialysis in step iv. In step ii. of the process according to the invention, metal salts are precipitated from the retentate from diffusion dialysis by adding a basic alkali compound to the retentate.

[0016] 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.

[0017] In one exemplary embodiment of the process, metal salts are precipitated from the retentate of diffusion dialysis by adding NaOH or KOH solution, so that an alkali salt solution (Na or K salt) and precipitated salts (e.g., Fe(OH)2, Mg(OH)2) are obtained. In another exemplary embodiment of the process, metal salts are precipitated from the retentate of diffusion dialysis by adding Na2CO3 or K2CO3 solution, so that an alkali salt solution (Na or K salt) and precipitated salts (e.g., NiCO3, CaCO3) are obtained.

[0018] In one embodiment, the alkali salt is an alkali chloride. In another embodiment, the alkali salt is an alkali bromide. In another embodiment, the alkali salt is an alkali fluoride. In yet another embodiment, the alkali salt is an alkali sulfate. In yet another embodiment, the alkali salt is an alkali nitrate.

[0019] In a further embodiment of the process, a lye formed during electrodialysis in step iv. is used to absorb CO2, and the resulting carbonate solution is used in step ii. to precipitate carbonates. In one embodiment of the process, CO2 is absorbed from flue gases of combustion processes; in other embodiments, CO2 is absorbed 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 step iii. of the process according to the invention, the solid produced in step ii. is separated from the liquid phase.

[0020] 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.

[0021] In step iv of the process according to the invention, electrodialysis of the liquid phase from step iii 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 and a NaOH or KOH solution for precipitation.

[0022] In an exemplary embodiment, the liquid phase supplied to the electrodialysis contains approximately 10 wt.% NaCl, for example 5-15 wt.% NaCl. 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 caustic soda and hydrochloric acid. This requires approximately 1.2 kWh of electrical energy per kg of NaCl.

[0023] In one embodiment of the process, a concentrated salt solution and water are obtained from the depleted salt solution obtained after electrodialysis in step iv. by means of reverse osmosis. In one embodiment, the concentrated salt solution is added to the precipitation bath, and the water is partly added to the lye chambers of the electrodialysis device and partly used for diffusion dialysis to produce acid.

[0024] The depleted salt solution (feed solution) is slightly acidified before being fed to the reverse osmosis system. This prevents any polyvalent 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

[0025] 10 wt%, for example 5-15 wt%.

[0026] The only starting materials required for the process according to the invention are the acidic aqueous solution, some water, and optionally CO2, and the products obtained are precipitated hydroxides or carbonates. Ideally, the process is completely wastewater-free.

[0027] A particularly interesting application for the process according to the invention is the preparation of pickling or etching baths containing acids such as hydrochloric acid or sulfuric acid and metal salts, for example iron pickling.

[0028] In a modification of the process according to the invention, if, for example, only Ca and / or Mg are present as metal ions in the acidic aqueous solution, e.g. from leaching of alkaline earth silicates, the process comprising diffusion dialysis, precipitation and electrodialysis can be used in combination with CO2 absorption for the permanent fixation of CO2.

[0029] The invention also relates to a device for processing an acidic aqueous solution containing metal ions, comprising a) at least one dialysis module which is designed to produce a retentate enriched in metal ions and a permeate depleted in metal ions from the acidic aqueous solution containing metal ions, b) 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, c) 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, d) 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.

[0030] The device according to the invention comprises a) at least one module for diffusion dialysis. The at least one dialysis module is configured to receive a liquid stream of the acidic aqueous solution containing metal ions and to produce therefrom a retentate enriched in metal ions and a permeate depleted in metal ions.

[0031] The device comprises b) 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.

[0032] The device comprises c) 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.

[0033] 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).

[0034] 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.

[0035] In one embodiment, the device d) 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.

[0036] 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.

[0037] Among the advantages of the solution according to the invention is that the processing of acidic solutions containing metal ions does not produce any wastewater that requires purification and / or disposal. Only solid basic metal salts are obtained as the end product. Further advantages and embodiments of the invention will become apparent from the description, the drawing, and the following example.

[0038] 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.

[0039] Example

[0040] Treatment of hydrochloric acid iron stains

[0041] 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.

[0042] A hydrochloric acid iron pickle containing ferric chloride and residual hydrochloric acid is fed to a dialysis unit (20), where diffusion dialysis is performed to produce a ferric chloride-enriched retentate and a ferric chloride-depleted hydrochloric acid permeate from the feed solution. For this purpose, water is added to the permeate side of the dialysis unit.

[0043] The retentate stream from the dialysis unit (20) is fed to a reactor (30) in which iron hydroxide is precipitated by adding an aqueous solution containing NaOH. 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 back into the pickling process. 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.

[0044] The sodium hydroxide solution produced by electrodialysis is fed back to the reactor (30) and used to precipitate iron hydroxide.

[0045] 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).

[0046] List of reference symbols

[0047] 20 dialysis units

[0048] 30 reactor

[0049] 40 electrodialysis unit

[0050] 50 reverse osmosis device

Claims

Patent claims 1 . A process for the processing of an acidic aqueous solution containing polyvalent metal ions, comprising i. diffusion dialysis of the solution to produce a retentate enriched in metal ions and a permeate depleted in metal ions, II. Precipitation of metal salt from the retentate by adding a basic alkali compound to the retentate, ill. Separation of solid and liquid phase, iv. Electrodialysis of the liquid phase to produce alkali solution and acid.

2. The process according to claim 1, wherein the permeate of the diffusion dialysis obtained in step i. is fed to the acid chambers of an electrodialysis device used for electrodialysis in step iv.

3. The process according to claim 1 or 2, wherein a concentrated salt solution and water are obtained from the depleted salt solution obtained in step iv. after electrodialysis by means of reverse osmosis.

4. A process according to any one of claims 1 to 3, wherein the basic alkali compound in step II is NaOH or KOH.

5. A process according to any one of claims 1 to 3, wherein the basic alkali compound in step II is Na2CO3 or K2CO3.

6. Method according to one of claims 1 to 3, in which the The lye formed by electrodialysis is used for CO2 absorption and the resulting carbonate solution is used in step II for the precipitation of carbonates.

7. A process according to any one of claims 1 to 6, wherein the acidic aqueous solution to be processed is an iron pickle.

8. A device for processing an acidic aqueous solution containing polyvalent metal ions, comprising a) at least one dialysis module (20) designed to produce a retentate enriched in metal ions and depleted in acid from the acidic aqueous solution containing metal ions, and a permeate depleted in metal ions and enriched in acid, b) at least one reactor (30) 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, c) at least one electrodialysis device (40) designed to receive a liquid stream from the at least one reactor (30) and to produce acid and alkali metal hydroxide solution from an aqueous solution containing alkali metal salts, d) optionally a reverse osmosis device (50) designed toreceiving a liquid stream containing alkali salt from the at least one electrodialysis device (40) and producing therefrom a retentate enriched in alkali salt and a permeate depleted in alkali salt.

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