Method for the preparation of alkali carbonates and / or hydrogen carbonates from waste water containing alkali salts

Abstract

The invention relates to a process for the production of alkali hydrogen carbonates and / or alkali carbonates from an aqueous starting solution containing alkali salts and alkaline earth salts, comprising the steps of: a) precipitation of the alkaline earth salts in the starting solution by electrolysis to obtain a pretreated solution containing alkali salts; b) electrodialytic conversion of the alkali salts in the pretreated solution to alkali hydroxides by means of bipolar electrodialysis to obtain a concentrate solution containing alkali hydroxides; c) carbonation of the alkali hydroxides in the concentrate solution to obtain a product solution containing alkali hydrogen carbonates and / or alkali carbonates, wherein the alkali salts in the starting solution are selected from alkali sulfates and / or alkali chlorides, and wherein the total concentration of the alkali salts in the starting solution is 2 to 5 mol / l.wherein the alkaline earth salts in the initial solution are selected from alkaline earth sulfates and / or alkaline earth chlorides, wherein the total concentration of the alkaline earth salts in the initial solution is 0.001 to 0.5 mol / l.

Description

[0001] The invention relates to a process for producing alkali carbonates and / or hydrogen carbonates from an aqueous starting solution containing alkali salts and alkaline earth salts, wherein the alkali salts in the starting solution are alkali sulfates or alkali sulfates and alkali chlorides and the alkaline earth salts are alkaline earth sulfates or alkaline earth sulfates and alkaline earth chlorides, for example wastewater from mining.

[0002] Alkali carbonates and alkali hydrogen carbonates, especially potassium and / or sodium carbonate, represent an important raw material. For example, soda ash (sodium carbonate) and baking soda (sodium hydrogen carbonate) are used as important raw materials in numerous applications, including in the chemical, metal, paper, and glass industries. Approximately 1.6 million tons of light and heavy soda ash are produced annually in Germany. It is irreplaceable in many sectors, so the continued production of such products is expected.

[0003] The conventional production of alkali hydrogen carbonates, especially sodium hydrogen carbonate, as basic chemicals, releases approximately 0.8 tons of CO₂ per ton of product and also results in the generation of approximately 10 m³ / ton of sodium / calcium chloride liquor. This leads to severe mineralization of the receiving waters downstream of production facilities. Soda ash manufacturers are therefore facing increasing criticism regarding this significant water pollution.

[0004] Today, soda is mostly produced on an industrial scale using the Solvay process, based on the raw materials sodium chloride (NaCl), coal (carbon), limestone (CaCO3), and ammonia. The CO2 required for production is obtained from coal through combustion. The burning of limestone to recover the ammonia, followed by the removal of the ammonia by adding lime milk to the reaction mixture, produces a significant quantity of CaCl2, which is not usable and is discharged in dissolved form into surface water or stored in settling basins.

[0005] Since NaCl is only incompletely converted in the Solvay process, the wastewater also contains up to 50 g / l NaCl, which is lost.

[0006] In order to minimize the immense environmental impacts caused by the Solvay process, various proposals have been developed to improve the CO2 and energy balance of the process and to partially replace some of the process steps with electrochemical process steps.

[0007] US Patent 2012 / 0298522A1 describes such a process combination. It proposes introducing CO₂ into ammonia water or a soda solution, then producing bicarbonate by adding brine (NaCl solution), and finally converting the bicarbonate to carbonate via electrolysis (cathode reaction). Hydrochloric acid or chlorine can be formed in the anode reaction. While this process proposal can reduce CO₂ emissions, it cannot eliminate them entirely.

[0008] The possibility of producing soda via chlor-alkali electrolysis is known from the state of the art.

[0009] None of the methods overcome the disadvantages of the state of the art listed above.

[0010] Furthermore, ore and coal mining, and the associated generation of contaminated wastewater, poses an enormous ecological problem, as the mining processes often involve significant interventions in the ecology of water systems. They can negatively impact entire river basins. The separation of iron, other heavy metals, and arsenic, as well as the neutralization of the wastewater, is now achieved using standard procedures.

[0011] EP 1600426B1 describes a method and apparatus for separating sulfate ions from sulfuric acid waters, such as those commonly found in mining areas. The sulfuric acid water is introduced into the two cathode compartments of a three-part electrolysis cell with an anion exchange membrane positioned between the cathode and anode compartments, where it undergoes electrolysis and electrodialysis. The anions present are transported from the cathode compartments through the anion exchange membrane into the anode compartment and can thus be separated. The process is complex, and the three-part electrolysis cell requires extensive maintenance.

[0012] However, the problem of high salt loads entering receiving waters remains unresolved on a large scale. Depending on their origin, wastewater contains large quantities of sulfates (coal and ore mining) or chlorides (potash and salt mining).

[0013] Electrochemical membrane processes are known to solve this problem, such as reducing salt loads.

[0014] CN107298450B describes the production of alkali carbonate using lithium as an example, starting with lithium adsorption on a resin column, followed by bipolar membrane electrodialysis. Interfering magnesium is removed by precipitation with NaOH. The resulting LiOH is precipitated with CO₂ gas as lithium carbonate. The process is disadvantageously suitable only for low lithium concentrations of 0.5–1.5 M. In particular, the lithium adsorption step on a resin-filled column limits the applicability of the process on an industrial scale or with higher alkali salt loads. The described method already assumes a solution rich in carbonate salts (Table 1). The alkaline earth compound load in the solutions to be treated is not specified. A further disadvantage is the enormous amount of NaCl waste generated during the precipitation of magnesium with NaOH, especially on an industrial scale.

[0015] Pan et al. (2020) disclose a multi-stage membrane electrolysis process for solutions containing LiCl / MgCl₂. The aim is to recover magnesium from magnesium-rich solutions and to remove lithium residues. The process includes electrolytic precipitation of Mg(OH)₂, electrodialytic concentration of the Li solution followed by electrolytic carbonation to remove the lithium as solid lithium carbonate from the desired magnesium hydroxide (Mg(OH)₂). The process is only effective for low alkali salt concentrations (1.18 mol / L in the catholyte, Table 1). It is based on solutions with a very high alkaline earth concentration (1.05 mol / L for magnesium, Table 1) and an alkaline earth magnesium to alkali lithium ratio of 98:1.

[0016] For lithium hydroxide (LiOH) as an important industrial compound, DE102015203395A1 discloses an electrodialytic purification process for obtaining a LiOH-containing aqueous solution from a LiOH-containing diluate contaminated with foreign substances. RU 2157338 C2 discloses a process for producing lithium hydroxides, hydrocarbonates or carbonates from an aqueous starting solution containing alkali salts and alkaline earth salts (from natural brine), in particular 15 g / l LiCl and 0.5 g / l Mg / CaCl₂ (total concentration of the alkaline earth salts in the starting solution approximately 0.008 mol / l and the total concentration of the alkali salts in the starting solution 2 to 5 mol / l), Li₂SO₄ is mentioned as an equivalent alternative (paragraph 17). The pH value is between 10.5 and 11.5 and the current density during electrolysis and electrodialysis is between 3 and 30 A / dm², i.e., 30 to 300 mA / cm².

[0017] A disadvantage of most processes is the formation of large quantities of semi-concentrates, which then have to be disposed of.

[0018] The object of the invention is to overcome the disadvantages of the prior art and to provide an economically and ecologically improved process for the production of alkali carbonates and hydrogen carbonates from alkali sulfates, which also allows for the direct treatment of wastewater. This process is primarily intended for use with wastewater from coal, ore, potash, and salt mining. In particular, it aims to avoid large quantities of NaCl waste. The process should also function at higher alkali salt concentrations from approximately 2 mol / l (alkali sulfate and / or alkali chloride). Unlike other processes, it should not generate NH₃.

[0019] In particular, the invention is intended to allow the production of soda (Na 2 CO 3 ), where the concentrations of NaCl in the starting solutions to be treated are naturally very high (e.g. up to 5 mol / L).

[0020] The problem is solved by a process for the production of alkali hydrogen carbonates and / or alkali carbonates from an aqueous starting solution containing alkali salts and alkaline earth salts, wherein the alkali salts in the starting solution are alkali sulfates or alkali sulfates and alkali chlorides, wherein the total concentration of the alkali salts in the starting solution is 2 to 5 mol / l. wherein the alkaline earth salts in the starting solution are alkaline earth sulfates or alkaline earth sulfates and alkaline earth chlorides, wherein the total concentration of the alkaline earth salts in the starting solution is 0.001 to 0.5 mol / l, preferably 0.01 to 0.1 mol / l, comprising the steps of: a) as a first step: precipitation of the alkaline earth salts in the starting solution by electrolysis, whereby the pH in the cathode compartment is increased to 10 to 14, to obtain a pretreated solution containing alkali salts, b) as a next step: electrodialytic conversion of the alkali salts in the pretreated solution to alkali hydroxides, by means of bipolar electrodialysis, to obtain a concentrate solution containing alkali hydroxides, and c) as a further next step: carbonation of the alkali hydroxides in the concentrate solution, to obtain a product solution containing alkali hydrogen carbonates and / or alkali carbonates.where the current density for electrolysis in step a) is between 10 and 50 mA / cm² and the current density during electrodialysis in step b) is between 5 and 100 mA / cm².

[0021] According to the invention, the aqueous starting solution contains alkali salts, wherein the alkali salts are alkali sulfates or alkali sulfates and alkali chlorides.

[0022] In one embodiment, the alkali sulfates are selected from potassium sulfate, sodium sulfate, and / or lithium sulfate. In another embodiment, the alkali chlorides are selected from potassium chloride, sodium chloride, and / or lithium chloride.

[0023] In one embodiment, the aqueous starting solution is selected from wastewater or sewage from mining, in particular from coal, ore, potash or salt mining.

[0024] According to the invention, the total concentration of the alkali salts in the starting solution is 2 to 5 mol / l.

[0025] Advantageously, aqueous solutions containing both alkaline earth salts and high concentrations of alkali salt from 0.5 to 5 mol / l can be processed using the process according to the invention.

[0026] The treatment of this wastewater by separating the alkali salts has so far presented a technologically difficult task, as the alkaline earth salts also present quickly lead to wear and tear of the equipment.

[0027] According to the invention, the aqueous starting solution contains alkaline earth salts, which are alkaline earth sulfates or alkaline earth sulfates and alkaline earth chlorides.

[0028] In one embodiment, the alkaline earth sulfates are selected from calcium sulfate, magnesium sulfate, barium sulfate and / or strontium sulfate and / or mixtures thereof.

[0029] In one embodiment, the alkaline earth chlorides are selected from calcium chloride, magnesium chloride, barium chloride and / or strontium chloride and / or mixtures thereof.

[0030] According to the invention, the total concentration of the alkaline earth salts in the starting solution is 0.001 to 0.5 mol / l, preferably 0.01 to 0.1 mol / l.

[0031] According to the invention, the first step therefore involves precipitation of the alkaline earth salts in the initial solution by electrolysis, whereby the pH value in the cathode compartment is increased to 10 to 14, in order to obtain a pretreated solution containing alkali salts.

[0032] In one embodiment, a cell voltage of 3 to 10 volts, preferably 4 to 8 volts, is applied to the electrodes (anode and cathode) of the electrolysis cell.

[0033] In one embodiment, the starting solution is alkalized with CO2 during or after electrolysis.

[0034] In one embodiment, particularly at higher concentrations of alkaline earth salts, the electrolysis takes place in two stages, i.e., the solution obtained after the first electrolysis of the initial solution is subjected to a further electrolysis and thus depleted of alkaline earth salts.

[0035] In one embodiment, the electrolysis of the starting solution takes place in a three-part electrolysis cell comprising two cathode compartments, an anode compartment and an anion exchange membrane arranged between the cathode and anode compartments.

[0036] In one embodiment, the initial solution is introduced into the two cathode compartments and subjected there to electrolysis and, if necessary, electrodialysis.

[0037] In one embodiment, a cell voltage of 5 to 20 volts, preferably 7 to 15 volts, is applied to the electrodes (anode and cathode) of the electrolysis cell.

[0038] The current density for electrolysis is 10 to 50 mA / cm².

[0039] At the cathode, hydrogen ions are reduced to hydrogen, or water molecules are reduced to hydrogen and hydroxide ions.

[0040] This causes the pH value in the cathode compartment to rise. The pH value in the cathode compartment is increased to 10 to 14, preferably to 11 to 13.

[0041] In one embodiment, hydrolyzable cations of the alkaline earth salts are precipitated as hydroxides. In another embodiment, hydroxides of magnesium, calcium, and / or strontium and / or barium are precipitated.

[0042] Due to the local cation deficit in the cathode compartments, existing anions, i.e., sulfate or sulfate and chloride anions, are transported from the cathode compartments through the anion exchange membrane into the anode compartment. There, the anions react with the hydrogen ions generated in the anode reaction to form the corresponding acids. In one embodiment, the acid is separated.

[0043] In one embodiment, the electrolytically treated solution is contacted with gaseous carbon dioxide. This causes sparingly soluble carbonates to form from alkaline earth metals still present in the solution. In one embodiment, carbonates of magnesium, strontium, and / or calcium are formed. The sparingly soluble carbonates precipitate and can be separated as solids.

[0044] In one embodiment, the electrolysis and, if necessary, the contact with CO2 are repeated one to several times.

[0045] In one embodiment, any lithium that may be present in the initial solution is precipitated as lithium carbonate.

[0046] In one embodiment, the pretreated solution obtained in this way has a total alkaline earth salt content of < 100 mg / l, preferably < 50 mg / l. If the alkaline earth salts are mainly calcium and / or barium salts, the pretreated solution has a total alkaline earth salt content of preferably < 10 mg / l.

[0047] In one embodiment, the pretreated solution obtained in this way has a pH value of 6 to 8.

[0048] In one embodiment, the alkali sulfates are electrolytically reduced to sulfides. This preferably occurs during the electrolysis of the initial solution by reacting the sulfates with iron ions to form iron sulfide. In one embodiment, the iron sulfide is deposited and immobilized at the cathode, for example, made of pyrite. In another embodiment, iron ions are additionally added to the initial solution. In yet another embodiment, the deposition takes place at cathodes made of technical iron, pyrite, or NiS.

[0049] In one embodiment, the sulfates still contained in the pretreated solution are electrolytically reduced to sulfides.

[0050] According to the invention, in the next step the alkali salts in the pretreated solution are electrodialytically converted to alkali hydroxides by means of bipolar electrodialysis to obtain a concentrate solution containing alkali hydroxides.

[0051] Electrodialysis is a method known to those skilled in the art for removing ionic components from aqueous solutions by transporting them through ion exchange membranes (cation exchange membrane, anion exchange membrane). These ion exchange membranes are located in an electrodialysis cell, also called a membrane stack. The transport of the ions occurs under the driving force of an electric field.

[0052] The construction of a membrane stack used in a process, for example, and the processes taking place within it with a starting solution containing NaCl as an alkali salt, are carried out as described in Fig. 1 schematically represented.

[0053] Under the influence of the applied cell voltage, the ions migrate through the membranes, thus being depleted in the diluate streams (2, 5) and enriched in the concentrate streams (1, 4). This allows for extensive desalination of the diluate streams.

[0054] In one embodiment, KAM and AAM are selective for monovalent ions. Thus, monovalent ions are preferentially transported through the membranes.

[0055] At the surface of the bipolar membrane (BPM), a catalytic splitting of water into H+ and OH- ions takes place. This results in the splitting of salts into the corresponding acid or base, enabling their production.

[0056] The pretreated solution is carried in the diluate streams (2, 5). A neutral, saline solution, for example a 1M NaCl and / or NaSO₄ solution, is preferably carried in the concentrate stream (1).

[0057] The current density during electrodialysis is 5 to 100 mA / cm².

[0058] During electrodialysis, the alkali ions (e.g. sodium ions) are concentrated in the concentrate streams (1,4) by ion migration from the diluate to the concentrate stream.

[0059] This depletes the alkali ions (e.g. Na ions) in the diluate streams (2, 5).

[0060] In the concentrate stream (4) passed between BPM and KAM, alkali ions accumulate and react with the hydroxide ions formed there to form the corresponding alkali hydroxide, to obtain a concentrate solution containing alkali hydroxides, also called alkali.

[0061] In one embodiment, the concentration of alkali hydroxides in the concentrate solution in the concentrate stream (4), also called alkali, is 0.5 to 7 mol / l, preferably 1 to 3 mol / l.

[0062] In one embodiment, the purity of the lye is >90%, preferably >95%.

[0063] Alkali ions also accumulate in aqueous solution in the concentrate stream (1).

[0064] In one embodiment, the aqueous solution from the concentrate stream (1) is fed as a pretreated solution to a renewed electrodialysis in order to further deplete the solution of alkali ions.

[0065] In one embodiment, the alkali ion concentration after completion of the electrodialysis in the concentrate streams (1, 4) and / or in the concentrate solution is 0.5 to 7 mol / l, preferably 1 to 3 mol / l.

[0066] In the diluate stream (3) passed between BPM and AAM, the corresponding acid is formed from chloride anions and protons.

[0067] In one embodiment, the concentration of the acid is 0.5 to 5 mol / l, preferably 1 to 3 mol / l.

[0068] In one embodiment, the purity of the acid is >90%, preferably >95%.

[0069] In one embodiment, the residual alkali ion concentration in the diluate streams (2, 5) after completion of electrodialysis is 0.01 mol / l, preferably 0.1 mol / l.

[0070] Materials are used for electrodes and electrolyte, pumps and containers which are known from the prior art and especially to those skilled in the art.

[0071] According to the invention, in a next step the alkali hydroxides in the concentrate solution are carbonated to obtain a product solution containing alkali hydrogen carbonates and / or alkali carbonates.

[0072] Methods for the carbonation of alkali hydroxides are known to those skilled in the art.

[0073] In one embodiment, the alkali hydroxide-containing solution is contacted with CO2.

[0074] In one embodiment, the alkali hydroxides are first converted to alkali hydrogen carbonate.

[0075] In one embodiment, the alkali hydrogen carbonates are converted into their carbonates, for example, by crystallization. Such methods are known to those skilled in the art.

[0076] In one embodiment, the alkali hydrogen carbonate contains various alkali ions. In another embodiment, the different alkali hydrogen carbonates are separated from each other, for example, by fractional crystallization of their carbonates.

[0077] The invention also relates to the use of the inventive method for the treatment of wastewater and / or for the production of alkali carbonate and / or alkali hydrogen carbonate from wastewater.

[0078] In one embodiment, the wastewater is selected from wastewater from mining, in particular from coal, ore, potash or salt mining.

[0079] The invention also relates to the use of wastewater or sewage from mining as an aqueous starting solution for the production of alkali carbonate and / or alkali hydrogen carbonate using the process according to the invention.

[0080] For the realization of the invention, it is also advantageous to combine the embodiments and features of the claims described above. Furthermore, the invention is not limited to the specifically described combinations of features, but can also be defined by any other combination of certain features of all disclosed individual features, provided that the individual features are not mutually exclusive, or a specific combination of individual features is not explicitly excluded.

[0081] The invention will now be explained in more detail using exemplary embodiments, without these having a limiting effect. Example 1 - Reference example

[0082] A NaCl brine of concentration 60 g / l is placed in a bipolar ED stack with a membrane configuration according to Fig. 1The electrodes and membranes each have an area of ​​500 cm². The NaCl brine is guided in the diluate streams (2, 5) and subjected to electrodialytic desalination at i = 4 mA / cm² and a flow rate of 0.5 l / h (anodic and cathodic purge streams each 6 l / h). The introduced NaCl solution is desalinated to a residual concentration of 5 g / l. In the compartments (3, 4) immediately adjacent to the bipolar membrane, the corresponding acid / base solutions are formed in the form of 1.8 M NaOH (concentrate stream 4) and 1.5 M HCl (stream 3), each with a purity of 95%. The dilute brine with 5 g / l NaCl is then subjected to further electrodialytic desalination and desalinated to a residual concentration of 0.5 g / l NaCl. The concentrate from concentrate stream (1) with a concentration of 50 g / l NaCl is recycled into the process. The NaOH is subsequently converted to NaHCO3 by introducing CO2. Example 2 - Reference example

[0083] An industrial wastewater with a content of 25 g / l NaCl, 23 g / l CaCl 2 , 0.4 g / l MgSO 4 and 0.2 g / l SrSO 4 is first introduced into the cathode compartment of an electrolysis cell and electrolyzed at i= 50 mA / cm 2< until a pH value of 10 to 11 is reached in the effluent of the cathode compartment.

[0084] The electrolysis cell contains two cathodes, each measuring 10 cm wide and 13 cm long, made of 1 mm thick nickel sheet, coated on the surface with a 2 micrometer-thin alloy layer consisting of 70% silver and 30% palladium. It also includes two identical anion exchange membranes that separate the cathode compartments from the anode compartment located centrally between them.

[0085] The anode consists of an expanded niobium metal measuring 6 cm * 8 cm with a geometric electrode surface area of ​​14.2 cm², which is coated on the surface with a layer thickness of 1 micrometer with synthetic boron-doped diamond.

[0086] The distance between the cathodes and the anion exchange membranes is 2.1 mm, and the distance between the anode and the membranes is 4.2 mm. To maintain these distances, spacers made of 2.1 mm thick polyester fabric with an open area (aperture) of 70% are used in one or two (anode space) layers, covering the membranes on both sides across their entire surface area.

[0087] A solution of 0.05 to 1.5 M sulfuric acid circulates through the anode space of the cell at the beginning of the treatment of the initial solution.

[0088] This solution is then contacted with technical-grade CO₂ in aeration columns with a centrally located porous aeration tube made of microporous PE or a microporous ceramic. The sparingly soluble carbonates precipitate and settle at the bottom of the column, from where they are periodically removed. The treated electrolyte is then electrolyzed again and contacted with CO₂ once more, as described, to precipitate further carbonates. The catholyte, now largely (concentration <50 mg / L) free of alkaline earth metals and nearly pH-neutral, is then processed to NaOH and HCl as described in Example 1. The chlorine produced in the anode reaction is converted to HCl in a chlorine-oxygen gas cell, recovering some of the energy used in its formation. The resulting NaOH, with a concentration of 2.5 mol / L, is reacted with CO₂ to form NaHCO₃, as previously described. Example 3 - Reference example

[0089] Instead of the industrial wastewater mentioned in Example 2, wastewater from a potash mining tailings pile with the following composition is treated: NaCl 62 g / l, KCl 1.5 g / l, MgSO₄ 4 g / l, LiCl 0.5 g / l. The treatment follows the same scheme as described in Example 2; however, in the electrolysis pre-stage, electrolysis is carried out until pH > 12, and the Mg is precipitated as Mg(OH)₂. Subsequently, the catholyte is reacted with CO₂ as described to precipitate the Li as Li₂CO₃. The resulting neutral solution is transferred to an electrolysis cell with a packed-bed cathode, where the sulfate is reduced to sulfide at i = 5 mA / cm² at cathodes made of technical iron, pyrite, or NiS. A steel sheet serves as the anode. The pre-treated water is then subjected to bipolar electrodialysis as described, and the resulting mixed lye of NaOH and KOH is converted to the corresponding hydrogen carbonates. These can then be processed, for example, by...fractional crystallizations can be separated from each other. Example 4

[0090] A sulfate-containing wastewater with a concentration of 6.5 g / l sulfate, 0.8 g / l chloride, 1.55 g / l sodium, 0.5 g / l Ca, 0.2 g / l Mg and 0.05 g / l Sr, and a pH of 2.9, is treated analogously to Example 2, whereby the alkaline earth metals are first separated as carbonates by electrolysis. In this stage, a chloride-containing sulfuric acid with a concentration of 0.35 mol / l with respect to H₂SO₄ is produced.

[0091] This sulfuric acid is reacted with ammonia water to form ammonium sulfate solution, which can be used as fertilizer, provided that the chloride content is <1g / kg of the sulfate produced (based on the dry mass).

[0092] Otherwise, the solution is evaporated and the NH₄Cl-containing solid is heated to 340°C to remove the NH₄Cl by sublimation. The sublimate is condensed or absorbed in a water receiver, from which ammonia water can be recovered by bipolar electrodialysis and recycled back into the process. The alkaline earth-free solution is then subjected to bipolar electrodialysis as described to obtain NaOH and HCl, yielding 1.8 M NaOH and 1.3 M HCl.

[0093] The NaOH is subsequently reacted with CO₂ to form NaHCO₃. Further concentration is achieved through processes known to those skilled in the art. The generated HCl can also be concentrated. In this case, it was used with energy recovery to produce AlCl₃ and FeCl₃ solutions, which in turn can be used as precipitation reagents in wastewater treatment in a known manner. For the preparation, sheets of aluminum or steel, each 250 cm² in size and 10 mm thick, were immersed in a container with the HCl and electrically connected to a second sheet of stainless steel of the same size, which served as the cathode. The dissolution of the less noble metals (steel, aluminum) occurs spontaneously under the experimental conditions. A voltage of 0.4 V and 1.8 V, respectively, was measured at the electrode terminals. Non-patent literature cited:

[0094] Pan, Xi-Jiu; Dou, Zhi-He; Meng, De-Liang; Han, Xiu-Xiu; Zhang, Ting-An; Titel: "Electrochemical separation of magnesium from solutions of magnesium and lithium chloride", Hydrometallurgy, Vol. 191, 2020, 105166. Ghanbari Maharloo, Ali Darvishi, Razieh Davand, Majid Saidi, Mohammad Reza Rahimpour mit dem Titel: "Process intensification and environmental consideration of sodium bicarbonate production in an industrial soda ash bubble column reactor by CO2 recycling", Journal of CO2 Utilization, Vol. 20, 2017, 318-327. Friedrich, H. - J.: Wasser-behandlung im Bergbau mittels Membranelektrolyseverfahren, Wissym 2015, Bad Schlema, Proceedings, Seiten169 ff.). Electrosynthesis Inc., USA "Electrochemical Salt Splitting", Autor D. Genders, Watts New 1995, Vol 1; No 1.

Claims

1. Method for the production of alkali hydrogencarbonates and / or alkali carbonates from an aqueous starting solution containing alkali salts and alkali earth salts, wherein the alkali salts in the starting solution are alkali sulfates or alkali sulfates and alkali chlorides, wherein the total concentration of the alkali salts in the starting solution being 2 to 5 mol / l, wherein the alkali earth salts in the starting solution are alkali earth sulfates or alkali earth sulfates and alkali earth chlorides, wherein the total concentration of the alkali earth salts in the starting solution being 0.001 to 0.5 mol / l, with the steps: a) as a first step: Precipitation of the alkali earth salts in the starting solution by electrolysis, wherein the pH in the cathode compartment is increased to 10 to 14, to obtain a pretreated solution containing alkali salts; b) as a next step: Electrodialytic conversion of the alkali salts in the pretreated solution to alkali hydroxides by means of bipolar electrodialysis, to obtain a concentrate solution containing alkali hydroxides; and c) as a further next step: Carbonation of the alkali hydroxides in the concentrate solution to obtain a product solution containing alkali hydrogencarbonates and / or alkali carbonates, wherein the current density for the electrolysis in step a) is 10 to 50 mA / cm2, and the current density during electrodialysis in step b) is 5 to 100 mA / cm2.

2. Method according to claim 1, wherein the alkali sulfate is lithium sulfate and / or the alkali chloride is lithium chloride, and lithium contained in the starting solution is precipitated as lithium carbonate.

3. Method according to one of claims 1 or 2, wherein the aqueous starting solution is selected from mining wastewater.

4. Method according to claim 3, wherein the aqueous starting solution is selected from wastewater from coal, ore, potash, or salt mining.

5. Method according to claims 1 to 4, wherein the electrolysis of the starting solution is carried out in a three-compartment electrolysis cell comprising two cathode compartments, one anode compartment, and a between the cathode and anode compartments arranged anion exchange membrane.

6. Use of a method according to any one of claims 1 to 5 for the treatment of wastewater and / or for the production of alkali carbonate and / or alkali hydrogencarbonate from wastewater.

7. Use of mining wastewater as the aqueous starting solution for the production of alkali carbonate and / or alkali hydrogencarbonate by means of the method according to any one of claims 1 to 5.

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