Separation of acids or alkalis from acidic or alkaline aqueous solutions containing dissolved metal ions

Abstract

The present invention relates to methods and devices for separating acids or alkalis from acidic or alkaline aqueous mixtures containing dissolved metal ions

Description

[0001] Separation of acids or alkalis from acidic or alkaline aqueous solutions containing dissolved metal ions

[0002] Technical area

[0003] The present invention relates to processes and devices for separating acids or alkalis from acidic or alkaline aqueous mixtures containing dissolved metal ions.

[0004] Description of the state of the art

[0005] Both nanofiltration and diffusion dialysis are used to separate or recover acids and alkalis during metal removal from acidic and alkaline aqueous mixtures contaminated with metal ions. These processes are frequently used in surface technology, for example, for bath maintenance during the anodizing process or during the pickling of ferrous materials or aluminum.

[0006] These processes are used, for example, for acidic mixtures containing at least one acid, for example HCl, HNO3, HF, H2SO4 and at least one type of metal ion, for example Cu 2+ , Al 3+ , Fe 2+ , Fe 3+ , Cr 3+ , Cr 6+ or Pb 2+ and in the case of alkaline mixtures containing at least one alkali, for example KOH or NaOH, and at least one type of metal ion, for example Al 3+ , Fe 2+ , Fe 3+ or Zn 2+ contain.

[0007] Nanofiltration is a pressure-driven membrane process that retains dissolved molecules, heavy metal ions, and other small particles. Nanofiltration membranes, by definition, have a pore size of 2 nm or less. Nanofiltration membranes allow monovalent ions to pass through, while divalent or multivalent ions are partially retained. Therefore, the recovery of hydrochloric acid with monovalent ions from mixtures containing multivalent metal ions, for example, is an interesting application. Retention rates of up to 80% for multivalent metal ions can be achieved, while the acidity in the permeate decreases only slightly. Nanofiltration requires high pressures in the process and, due to the osmotic pressure, is advantageously used to recover acids and alkalis at low metal ion concentrations in the solution, since the required filtration pressure increases with the metal ion content.

[0008] CN 210103658 U describes the application of nanofiltration for acid recovery from an iron pickling plant using hydrochloric acid. The nanofiltration is carried out in multiple stages, and high pressures are required for nanofiltration. These high pressures result in significant operating costs due to electricity consumption and increase the investment costs for the pressure-stable system and acid-resistant pump with a discharge head of up to 2000 m.

[0009] For example, to remove 1 kg of Fe from a pickling solution containing 50 g / l Fe ions and 100 g / l HCl using nanofiltration, approximately 25 l of pickling solution must be treated. This yields approximately 5 l of wastewater containing 200 g / l Fe and 100 g / l HCl, and 20 l of permeate containing approximately 100 g / l HCl and 12.5 g / l Fe ions. The permeate is returned to the etching bath. This results in a power consumption of approximately 0.3 kWh / kg Fe ions, an acid loss in the wastewater of approximately 500 g HCl / kg Fe ions, and a deionized water consumption of approximately 5 l.

[0010] CN 110563090 A describes the application of diffusion dialysis for acid recovery from an iron pickling solution containing hydrochloric acid. In contrast to nanofiltration, the operating costs of acid recovery are minimal, but the investment costs are very high due to the large membrane areas required. With diffusion dialysis, the recovery rate of approximately 90% for hydrochloric acid and the metal rejection rate for polyvalent metal ions of over 95% are significantly better than with nanofiltration; however, the wastewater volume is significantly higher. For example, to remove 1 kg of Fe from a pickling solution containing 50 g / l Fe ions and 100 g / l HCl using diffusion dialysis, approximately 20 l of pickling solution must be processed. This results in approximately 25 l of wastewater with approximately 40 g / l Fe and 8 g / l HCl and 20 l of permeate with approximately 90 g / l HCL and < 1 g / l Fe ions, with an electricity requirement of < 0.01 kWh / kg Fe ions and approximately 25 l of demineralized water required.

[0011] The resulting acid loss in the wastewater is approximately 200 g HCl / kg Fe ions, less than 50% of the acid loss occurring during nanofiltration.

[0012] CN 101759250 B describes a combined process for the recovery of heavy metal salts from a steel pickling line. The pickling solution is first treated with diffusion dialysis, the dialysate is heated, and then concentrated in hot nanofiltration. This requires the entire pickling solution to be first treated with diffusion dialysis, which in turn results in very high investment costs. Additional investment and high operating costs arise in the downstream nanofiltration at very high pressures and with a liquid volume approximately equal to the original solution.

[0013] The fundamental disadvantages of current processes, namely the high operating costs and increased investments for nanofiltration at very high pressures up to 200 bar, as well as the high investments for diffusion dialysis due to the very large membrane area requirement, remain in all applications known today.

[0014] Object of the invention

[0015] Against this background, the invention aims to provide a process for acid or alkali separation from mixtures containing metal ions, which achieves the highest recovery rates of acid or alkali with moderate wastewater volumes, a low content of metal ions in the acid or alkali, and minimal overall costs, particularly significantly reduced investment costs compared to diffusion dialysis of the entire solution and significantly reduced operating costs compared to nanofiltration of the mixture. Solution to the problem

[0016] To solve the problem, an economically optimized process for acid or alkali separation from mixtures containing metal ions is used, combining nanofiltration at moderate pressures with downstream diffusion dialysis of the reduced retentate amount with increased metal ion concentration.

[0017] The invention relates to processes for separating acid from an acidic aqueous mixture containing metal ions and for separating alkali from an alkaline aqueous mixture containing metal ions.

[0018] In the process for separating acid from an acidic aqueous mixture containing metal ions, an acid with a reduced metal ion content is obtained as the permeate in a first process step by nanofiltration of the entire mixture. An acid with an increased metal ion content is obtained as the retentate. The retentate from the nanofiltration is passed to diffusion dialysis, and an acid essentially free of metal ions is recovered as the diffusate. The dialysate obtained from diffusion dialysis has a reduced acid content and an increased metal ion content.

[0019] In one embodiment of the method, diffusion dialysis is performed at atmospheric pressure. In another embodiment of the method, dialysis is performed at elevated pressure.

[0020] In one embodiment of the process, the acidic aqueous mixture is an anodizing bath containing aluminum ions and sulfuric acid.

[0021] In another embodiment of the process, the acidic aqueous mixture is an iron or steel pickling solution containing iron ions and sulfuric acid and / or hydrochloric acid.

[0022] In the process for separating lye from an alkaline aqueous mixture containing metal ions, a lye with a reduced metal ion content is obtained as the permeate in a first process step by nanofiltration of the entire mixture. A lye with an increased metal ion content is obtained as the retentate. The retentate from the nanofiltration is passed through diffusion dialysis, and a lye essentially free of metal ions is recovered as the diffusate. The dialysate obtained from diffusion dialysis has a reduced lye content and an increased metal ion content.

[0023] In one embodiment of the method, diffusion dialysis is performed at atmospheric pressure. In another embodiment of the method, dialysis is performed at elevated pressure.

[0024] In one embodiment of the process, the alkaline aqueous mixture is a pickling or etching solution containing aluminum ions and sodium hydroxide solution and / or potassium hydroxide solution.

[0025] In the context of the present application, "essentially free of metal ions" means a residual content of < 1 g / l of ions in the diffusate.

[0026] General applications for the processes according to the invention are the treatment of acidic mixtures with at least one acid, for example HCl, HNO3, HF, H2SO4 and at least one metal ion species, for example Cu 2+ , Al 3+ , Fe 2+ , Fe 3+ , Cr 3+ , Cr 6+ , Pb 2+, or alkaline mixtures with at least one alkali, for example KOH or NaOH, and with at least one type of metal ion, for example Al 3+ .

[0027] Particularly economically interesting applications for the processes according to the invention are, for example, the bath maintenance of anodising baths with sulphuric acid, in which the Al formed during the process 3+ -ions must be removed, and the bath maintenance of iron or steel pickling with sulphuric and / or hydrochloric acid, where the Fe 2+ and / or Fe 3+ -ions must be removed. In the alkaline range, for example, bath maintenance with minimal alkali loss of Al pickling or Al (deep) etching solutions with sodium hydroxide or potassium hydroxide is economically very interesting, where the Al 3+ -ions must be removed.

[0028] In a first step of the process according to the invention, nanofiltration of the entire acidic or alkaline aqueous mixture is carried out. Commercially available nanofiltration modules can be used for nanofiltration. Examples include SPIRA-CEL® nanofiltration modules based on the NADIR® NP030 filtration membrane, a polyethersulfone membrane, from Mann + Hummel Water & Fluid Solutions GmbH, 65203 Wiesbaden, which achieve a rejection of multivalent metal ions in the range of 80% and more. For acidic mixtures, SPIRA-CEL® OY NP030 nanofiltration elements can be used. These elements tolerate pH 0-12 and the maximum process temperature is 80°C. For alkaline mixtures, SPIRA-CEL® OX NP030 nanofiltration elements can be used. These elements tolerate pH 2-14 and the maximum process temperature is 80°C.

[0029] In one embodiment of the processes according to the invention, the nanofiltration is carried out at a pressure of no more than 150 bar, for example, no more than 125 bar, or no more than 100 bar. In another embodiment of the processes according to the invention, the nanofiltration is carried out at a pressure of no more than 40 bar. In one embodiment of the processes according to the invention, the nanofiltration is carried out at a temperature of no more than 80°C.

[0030] In one embodiment of the process, the nanofiltration retentate is passed through one or more flow restrictors upstream of the diffusion dialysis. This ensures a constant and uniform flow through the diffusion dialysis.

[0031] In one embodiment of the method, the pressure in the nanofiltration and thereby the permeate flow of the nanofiltration is adjusted by controlling (increasing or reducing) the power of a pump upstream of the nanofiltration for the acidic or alkaline aqueous mixture containing metal ions.

[0032] In combination with at least one flow restrictor upstream of the diffusion dialysis, the pressure in the nanofiltration and the permeate flow can be easily adjusted by increasing or decreasing the pump output. Suitable flow restrictors for use with acids or alkalis are commercially available, for example, BT-Maric flow restrictors from Bertfelt Teknik AB, Sweden. BT-Maric flow restrictors (sometimes referred to as flow rate limiters, control valves, or flow stabilizers) are designed to ensure a predetermined, constant flow of water and similar media, regardless of pressure fluctuations.

[0033] In one embodiment of the process, microfiltration of the entire mixture precedes nanofiltration. In contaminated solutions, solids must be removed prior to nanofiltration to ensure long-term, reliable operation of the nanofiltration. Microfiltration is used to remove particles and oils from the mixture. Commercially available microfiltration modules can be used for microfiltration, for example, SEPRODYN® tubular microfiltration modules from Mann + Hummel Water & Fluid Solutions GmbH, 65203 Wiesbaden, Germany. These modules tolerate pH values ​​from 0–14, and the maximum process temperature is 60°C.

[0034] The nanofiltration retentate is then fed into a diffusion dialysis process. Diffusion dialysis, also known as Donnan dialysis, is used, for example, to recover free acids and alkalis from spent treatment baths in surface and textile technology. The Donnan effect is based on the concentration difference between two solutions separated by a semipermeable membrane. Due to the low driving forces, the process can be carried out in strict countercurrent operation. It is robust, long-term stable, and requires only minimal energy. In one embodiment, diffusion dialysis is carried out at atmospheric pressure.

[0035] Using anion exchange membranes, unbound strong acids can be recovered in this way (acid dialysis). Using cation exchange membranes, alkalis can be recovered (alkali dialysis). In this process, the spent bath in one chamber, separated by a membrane, is brought into contact with a second chamber through which water flows in countercurrent.

[0036] In the processes according to the invention, spiral-wound modules, as described in WO 2017 / 001 060 A1, can be used for diffusion dialysis. Such modules are available from Spiraltec GmbH, 74343 Sachsenheim, Germany. These modules achieve a recovery rate of approximately 90%, for example, for hydrochloric acid.

[0037] In one embodiment, suitable membrane modules have at least two flow channels which are formed by winding at least a first section of at least one membrane film and at least a second section of the at least one membrane film onto a central body, wherein a winding of the at least one first section is spaced from a winding of the at least one second section by at least one spacer, whereby two separate flow channels are formed during winding, each of which is delimited by the at least one membrane film.

[0038] Due to the post-treatment using diffusion dialysis, nanofiltration can be operated at reduced operating pressures compared to a system without downstream diffusion dialysis, and the size of the diffusion dialysis system and its membrane area can be significantly reduced compared to an application without upstream nanofiltration. Furthermore, the wastewater obtained has minimal acid or alkali content and a significantly increased metal ion concentration compared to the original mixture, which also allows downstream wastewater treatment processes, such as neutralization, evaporation, and metal salt precipitation, to be implemented more cost-effectively. Due to the downstream diffusion dialysis, nanofiltration can also be used advantageously with reduced selectivity, for example, in sulfuric acid solutions with divalent sulfate ions, and reduces the required membrane area for downstream diffusion dialysis.

[0039] As an example of the advantages of the combined process, the removal of 1 kg of Fe ions from a pickling solution containing 50 g / l Fe ions and 100 g / l HCl is again considered. Using a combined nanofiltration and diffusion dialysis system, approximately 22 l of pickling solution containing 50 g / l Fe ions and 100 g / l HCl are processed in nanofiltration at a pressure of approximately 125 bar, yielding approximately 10 l of wastewater containing 100 g / l Fe and 100 g / l HCl, and 12 l of permeate containing approximately 100 g / l HCl and approximately 8.5 g / l Fe ions. The permeate returns directly to the pickling solution, and the wastewater (10 l) from the nanofiltration is subsequently treated in diffusion dialysis at atmospheric pressure. There, approximately 12 liters of wastewater with approximately 85 g / l Fe and 8 g / l HCl and 10 liters of diffusate with approximately 90 g / l HCl and < 1 g / l Fe ions are obtained. The acid loss in the wastewater is only approximately 100 g HCl / kg Fe ions, and the demineralized water requirement is 12 liters.The required membrane area of ​​diffusion dialysis is reduced by 50% compared to the process without upstream nanofiltration, and the operating costs of nanofiltration are significantly reduced at pressures < 150 bar compared to a pure nanofiltration process.

[0040] The processes according to the invention can of course be optimized for each application, depending on the acid or alkali content and the metal ion concentrations of the bath solution. At low metal ion concentrations in the mixture fed to the nanofiltration (feed), the ratio of permeate to wastewater is increased compared to the example calculation, while at high metal ion concentrations it is reduced. The respective process optimum for a specific application with the optimal permeate / wastewater (retentate) ratio in nanofiltration can be determined for given raw material, disposal and investment costs. The resulting wastewater from the processes according to the invention, which combine nanofiltration and diffusion dialysis for the recovery of acids or alkalis, can, due to the increased content of metal ions, the reduced acid or alkali content.Due to the low lye content and the low total quantity, it can be further treated cost-effectively, for example by neutralising and / or evaporating, and / or precipitation of metal salts can be carried out.

[0041] The invention also relates to a device for separating acid from an acidic aqueous mixture containing metal ions or alkali from an alkaline aqueous mixture containing metal ions. The device is primarily used for carrying out the processes according to the invention.

[0042] The device according to the invention comprises at least one nanofiltration unit with a permeate outlet and a retentate outlet, wherein the retentate outlet of the at least one nanofiltration unit is connected to at least one inlet of at least one difusion dialysis unit.

[0043] In one embodiment of the device, at least one microfiltration unit is arranged upstream of the at least one nanofiltration unit, the filtrate outlet of which is connected to at least one inlet of the at least one nanofiltration unit.

[0044] In one embodiment of the device, at least one flow restrictor is arranged between the retentate outlet of the at least one nanofiltration unit and the at least one inlet of the at least one diffusion dialysis unit.

[0045] In one embodiment of the device, at least one pump for an acidic or alkaline aqueous mixture containing metal ions is arranged upstream of the at least one nanofiltration unit, which pump is designed to regulate a pressure in the at least one nanofiltration unit and thereby a permeate flow of the at least one nanofiltration unit. The advantages of the solution according to the invention include that the nanofiltration can be operated at reduced operating pressures compared to a system without downstream diffusion dialysis, thereby reducing operating costs, and that the size or membrane area of ​​the diffusion dialysis system is significantly reduced compared to a system without upstream nanofiltration, which requires lower investment costs. Further advantages and embodiments of the invention will become apparent from the description.It is understood that the features mentioned above and those to be explained below 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 separating acid from an acidic aqueous mixture containing metal ions, in which, in a first process step, an acid with a reduced metal ion content is obtained as permeate by nanofiltration of the entire mixture, and the retentate of the nanofiltration is fed to a diffusion dialysis and an acid essentially free of metal ions is recovered as the diffused.

2. A process for separating lye from an alkaline aqueous mixture containing metal ions, in which, in a first process step, a lye with a reduced metal ion content is obtained as permeate by nanofiltration of the entire mixture, and the retentate of the nanofiltration is fed to a diffusion dialysis and a lye essentially free of metal ions is recovered as diffused saturate.

3. The method of claim 1, wherein the acidic aqueous mixture is an anodizing bath containing aluminium ions and sulfuric acid.

4. The method of claim 1, wherein the acidic aqueous mixture is an iron or steel pickling solution containing iron ions and sulfuric acid and / or hydrochloric acid.

5. The method of claim 2, wherein the alkaline aqueous mixture is a pickling or etching solution containing aluminium ions and sodium hydroxide and / or potassium hydroxide.

6. Method according to any one of claims 1 to 5, wherein the retentate from nanofiltration is passed through one or more flow restrictors upstream of the diffusion dialysis. 7 Method according to one of claims 1 to 6, in which the pressure in the nanofiltration and thereby the permeate flow of the nanofiltration is adjusted by controlling the power of a pump upstream of the nanofiltration for the acidic or alkaline aqueous mixture containing metal ions.

8. Method according to any one of claims 1 to 6, wherein the nanofiltration is preceded by microfiltration of the entire mixture.

9. Method according to any one of claims 1 to 7, wherein the diffusion dialysis is performed at atmospheric pressure.

10. Device for separating acid from an acidic aqueous mixture containing metal ions or alkali from an alkaline aqueous mixture containing metal ions, comprising at least one nanofiltration unit with a permeate outlet and a retentate outlet, wherein the retentate outlet is connected to at least one inlet of at least one diffusion dialysis unit.

11. Device according to claim 10, wherein at least one microfiltration unit is connected upstream of the at least one nanofiltration unit, the filtrate output of which is connected to at least one input of the at least one nanofiltration unit.

12. Device according to claims 10 or 11, wherein at least one flow limiter is arranged between the retentate outlet of the at least one nanofiltration unit and the at least one inlet of the at least one diffusion dialysis unit.

13. Device according to one of claims 10 to 12, wherein at least one pump for an acidic or alkaline aqueous mixture containing metal ions is connected upstream of the at least one nanofiltration unit, which is configured to control a pressure in the at least one nanofiltration unit and thus a permeate flow of the at least one nanofiltration unit.