Process for producing ammonium carbonate and sodium carbonate

Abstract

A process for producing (NH4) 2CO3 and Na2CO3 in an electrodialyzer membrane stack by feeding an aqueous solution comprising NH4Cl, NaCl and dissolved CO2 to a base chamber of the electrodialyzer membrane stack and obtaining an outlet solution comprising (NH4) 2CO3 and Na2CO3 from the base chamber wherein the outlet solution from the base chamber has a pH of at most 9.5.

Description

[0001] The present invention relates to a process for the production of (NH4)2CO3 and Na2CO3 from an aqueous solution comprising ammonium chloride, sodium chloride and dissolved CO2 using a stack of electrodialysis membranes. TECHNICAL FIELD

[0002] The main industrial process for the production of synthetic sodium carbonate is the soda ash process, also known as the SOLVAY process (ammonia-soda process), or in a modified embodiment as the HOU process. This process consists in treating an aqueous ammonia salt with a gas containing carbon dioxide. From this solution, sodium bicarbonate precipitates, which is recovered and calcined to obtain sodium carbonate. The mother liquor resulting from the separation of sodium bicarbonate contains ammonium chloride and sodium chloride. This mother liquor is then reacted with calcium oxide or calcium hydroxide to obtain ammonia, which is recycled in the ammonia-soda process back into the initial brine solution, or treated in the HOU process to precipitate solid NH4CI. More details on the process for the production of sodium carbonate according to the ammonia-soda process and the production of refined bicarbonate are described in Ullmann's Encyclopedia of Industrial Chemistry ["Sodium carbonate" chapter, volume 33, pages 299-317, 2012 edition, Wiley-VCH Verlag GmbH & Co, in particular in paragraphs 1.4.1 and 1.4.2].

[0003] It has been proven that the known soda ash process consumes a large amount of energy, in particular for the recycling of ammonia. In addition, there remains a need for improving the recycling of ammonia and reducing the undesirable by-products of the process. In US 2,209,681, electrolysis of ammonium chloride for the recovery or production of chlorine or ammonia or both is disclosed. In this process, an electrolytic current is passed through an electrolytic cell having an aqueous electrolyte containing ammonium chloride and sodium chloride, the concentration of which is at least equal to the concentration of the ammonium chloride. This process presents several drawbacks and is not used.

[0004] A process for obtaining chemicals from process wastewater of the soda ash process, and in particular from a mother liquor obtained after filtration of sodium bicarbonate and containing ammonium chloride and sodium chloride, is described in Austrian patent No. AT 96 / 01330. In this process, sodium and ammonium ions are separated from chloride ions and the ions thus separated are converted into sodium hydroxide, ammonium hydroxide, ammonia and hydrochloric acid, respectively, together with the relevant counterions that are hydrolyzed. The separation of sodium and ammonium ions from chloride ions is carried out by electrodialysis. This electrodialysis is carried out using at least three compartments (see Figure 2 of the above-mentioned reference) separated from each other by a single-polarity anion-selective membrane and a single-polarity cation-selective membrane, which themselves are separated from the corresponding next part by a bipolar membrane. An alternative embodiment of this disclosure (see Figure 3 of the reference) uses only two compartments, which are alternatively separated by a bipolar membrane and a cation-selective membrane, to obtain one stream containing hydrochloric acid, sodium chloride and ammonium chloride, and a second stream containing ammonium hydroxide and sodium hydroxide. In the above-mentioned disclosure, the solution entering the base compartment has been stripped of carbon dioxide.

[0005] Electrodialysis technology is well known and is mainly used in electroseparation processes such as production of potable water, seawater desalination, brackish water desalination, industrial wastewater treatment, acid or base recovery in the metal plating industry, food and pharmaceutical processes. The principle of electrodialysis is detailed in, for example, Ullmann’s Encyclopedia of Industrial Chemistry (2011 edition, Wendt, -VCH Verlag GmbH & Co, Vol. 12, pages 273-313, Electrochemistry) or Technique de l’Ingénieur Encyclopedia 2006 edition (chapter on electrodialysis, J2840 V1, 2006, pages 1-15 and technical appendix pages 1-3).

[0006] WO 2016 / 055367 in the name of SOLVAY describes a process for producing ammonia from a solution comprising ammonium chloride (NH4CI) which is fed into at least one base compartment of a membrane stack of an electrodialyser, the pH of said base compartment being maintained at least at 10, preferably in the range between 10.3 and 12.0. The examples in this disclosure show the possibility of converting at least 98.5% of ammonium chloride (NH4CI) into ammonia (NH4OH); under the same conditions, the conversion of ammonium chloride into ammonia decreases to 86.5% when the pH of the base compartment is 9.8.

[0007] However, when the above process operation is performed under the above pH conditions, ammonium ions (NH4 + ) will leak from the base compartment to the acid compartment through the selective membrane, and in turn produce acid solutions containing ammonium ions, which are detrimental for certain uses when they are valorized as acid or discharged to the environment after neutralization. Indeed, ammonia is toxic to most aquatic species and is difficult and costly to treat, either by steam stripping, or by oxidation through photocatalytic techniques, or by expensive biological treatment, when it is present in low concentrations. SUMMARY

[0008] The inventors found that by maintaining the pH in the base compartment at less than 9.5, the leakage of ammonia through the electrodialysis membrane of the base compartment, which is fed with an aqueous solution comprising NH4CI, NaCI and dissolved CO2, is significantly reduced or eliminated.

[0009] In particular, the presence of dissolved CO2may be provided by carbonating at least a part of the NH4CI and NaCI solution upstream of the base compartment with a CO2-containing gas, so that the resulting solution comprises NH4CI, NaCI and dissolved CO2, before feeding the aqueous solution into the base compartment.

[0010] The present invention thus relates to a process for producing (NH4)2CO3and Na2CO3in an electrodialysis membrane stack by feeding an aqueous solution comprising NH4CI, NaCI and dissolved CO2to the base compartment of the electrodialysis membrane stack, and obtaining an outlet solution comprising (NH4)2CO3and Na2CO3from the base compartment, wherein the pH of the outlet solution from the base compartment is at most 9.5.

[0011] The present invention also relates to a process for producing sodium carbonate by the soda ash process, comprising the step of producing (NH4)2CO3and Na2CO3using the above process.

[0012] Electrodialysis refers to an electrochemical process that is capable of at least partially or completely extracting salt ions from one solution to another through ion exchange membranes placed under an electric field. In the preferred mode of the present invention, electrodialysis is operated in an electrodialysis membrane stack, which comprises a combination of bipolar ion exchange membranes with anion and / or cation exchange membranes within a set of contiguous compartments. An anion membrane is an ion exchange membrane that is permeable to anions and ideally impermeable to cations. A cation membrane is itself permeable to cations and impermeable to anions. A bipolar membrane is an ion exchange membrane comprising a cationic face and an anionic face. Such membranes can be produced by joining a cationic membrane and an anionic membrane. Bipolar membranes can for example be produced by the method described in WO 01 / 79335 in the name of Solvay.

[0013] Within the bipolar membrane, under the influence of a sufficient local electric field, water that has been diffused into it dissociates into its H + and OH - ions, which then migrate on both sides of the membrane. As a result, acidification takes place in one of the chambers adjacent to the bipolar membrane and alkalization in the other adjacent chamber. Sequential bipolar membranes are separated by cation- or anion-monopolar membranes. When an electrodialyser has only bipolar membranes and one type of monopolar membrane (cationic or anionic), they are said to have two (types of) chambers. The electrodialyser used in the method of the invention preferably has only bipolar membranes and an anionic membrane.

[0014] The chamber located between the anionic face of the bipolar membrane and the anionic membrane constitutes the alkaline chamber. In this chamber there is a supply of OH - ions originating from the bipolar membrane. In the method of the invention, the alkaline chamber is supplied with an aqueous solution comprising ammonium chloride, sodium chloride and dissolved CO2. The OH - ions supplied from the bipolar membrane will react with the ammonium cations to produce the desired ammonia and water.

[0015] In the present invention, the dissolved CO2 in the solution comprising NH4CI and NaCI fed to the alkaline chamber (pH at most 9.5) is generally present as bicarbonate ions (HCO3 - ) and carbonate ions (CO3 2- ). The counterions of the bicarbonate ions are cations present in the aqueous solution, such as ammonium ions (NH4+), sodium ions (Na + ), and a small amount of hydronium ions (H3O + ).

[0016] The first advantage of the invention is that the amount of ammonium ions lost through the alkaline chamber of the electrodialyser membrane stack is reduced.

[0017] The second advantage of the invention, related to the first, is that the amount of ammonium ions transferred to the adjacent acid chamber is limited, which makes it easy to valorize the acid solution produced, with a reduced amount of ammonium impurities.

[0018] The third advantage of the invention is that it facilitates the use of the acid produced in the electrodialyser membrane stack to reduce the cost of the treatment of the acid solution after its use to remove ammonia, and also to reduce the environmental impact on fish and aquatic plants when the used acid, after said appropriate treatment, is discharged into rivers and seas, since ammonia is harmful to such biological species in nature.

[0019] A fourth advantage of the present invention is to increase the sustainability of the process for the production of sodium carbonate according to the ammonia-soda ash process by reducing the CO2 emissions from fossil fuels. Indeed, the present invention enables to partially or completely replace the regeneration of ammonium chloride (NH4CI) into alkaline ammonia (as (NH4)2CO3 in the present invention, or as NH4HCO3 when further carbonated) using lime (CaO) by an electrodialysis process that can use 'green electricity' (i.e. produced by solar, wind, geothermal, biogas and low-impact hydroelectric sources).

[0020] Note: the lime (CaO) used in the conventional soda ash process is mainly manufactured in lime kilns where calcium carbonate (limestone) is calcined by using fossil carbon sources such as coke.

[0021] A fifth advantage of the present invention (which is an advantageous embodiment) is that no lime kiln is used in the process of the present invention to produce ammonium carbonate or sodium carbonate. This enables to avoid such costly built equipment operating at high temperature and to reduce the associated maintenance costs.

[0022] A sixth advantage of the present invention is to reduce the solid effluent from the ammonia distillation since when lime or lime milk is used in the distillation section, part of the unreacted lime or limestone remains in the solid effluent. In contrast, when the hot calcination of limestone is replaced by the electrogenesis of alkaline and acid solutions, the erosion of limestone with the acid solution enables to produce the required carbon dioxide and the unreacted limestone in the final solid effluent is minimal.

[0023] A seventh advantage of the present invention is also to increase the sodium utilization from 60% to 70% to more than 90% or 95% in the ammonia-soda ash process. Indeed, the use of an anion membrane in the process of the present invention enables to migrate the chloride ions from NH4CI and NaCI into the acid solution (i.e. the solution comprising HCI) while the sodium ions are retained in the lye compartment and a part of the outlet solution is recycled to the soda ash process. Thus, the process of the present invention enables to increase the recycling of raw materials such as sodium chloride or limestone for the production of sodium carbonate.

[0024] Definitions

[0025] In the present specification, the term "ammonia-soda ash process" refers to the process described in the above technical field as the first or second variant of the Solvay process for the production of sodium carbonate (soda ash) by the ammonia process.

[0026] In the present specification, the term "electrodialysis" refers to an electrochemical process that enables to at least partially or completely extract salt ions from one solution to another solution by ion exchange membranes placed under an electric field.

[0027] In the present description, the term "electrodialyzer cell stack" refers to an apparatus in which an electrodialysis process can be carried out. It generally comprises several cells delimited by ion exchange membranes (also called ion-selective permeable membranes).

[0028] The term "comprising" includes "consisting essentially of" as well as "consisting of".

[0029] In the present description, the terms "%", "wt%", "wt%", "wt. %", "percent by weight" or "percent by weight" are used interchangeably, unless the "%" term refers explicitly to other physical units (such as for example "mole%" or "mol. %", "volume%" or "vol. %" and the like).

[0030] If the term "about" is used in reference to a value, this corresponds to a variation of ± 10% of the nominal value, unless otherwise indicated. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 (Fig. 1) illustrates schematically the functioning of an electrodialyzer in one embodiment of the method of the present application. DETAILED DESCRIPTION

[0032] Electrodialysis is a technique which consists in combining different membrane types (such as bipolar ion exchange membranes, anion exchange membranes and / or cation exchange membranes) in a set of adjacent cells. An anion membrane is an ion exchange membrane which is permeable to anions and ideally impermeable to cations. A cation membrane is itself permeable to cations and impermeable to anions.

[0033] A bipolar membrane is an ion exchange membrane comprising a cationic face and an anionic face. Such membranes can be produced by joining a cationic membrane and an anionic membrane. Bipolar membranes can be produced, for example, by the method described in WO 01 / 79335 in the name of Solvay.

[0034] Within a bipolar membrane, under the effect of a sufficient local electric field, water which has penetrated into it dissociates into its H + and OH - ions, which then migrate on both sides of the membrane. Thus, acidification occurs in one of the cells adjacent to the bipolar membrane and alkalization in the other adjacent cell.

[0035] Consecutive bipolar membranes are separated by cationic or anionic unipolar membranes. When an electrodialyser has only bipolar membranes and one type of unipolar membrane (cationic or anionic), they are said to have two (types of) compartments. The electrodialyser used in the process of the present invention preferably has only bipolar membranes and anionic membranes.

[0036] The compartment between the anionic face of the bipolar membrane and the anionic membrane constitutes the base compartment. In this compartment there is OH - ions originating from the bipolar membrane. In the process of the present invention, the base compartment is fed with a solution containing ammonium chloride, sodium chloride and dissolved CO2. The OH - ions supplied by the bipolar membrane will react with the dissolved CO2 (as dissolved carbon dioxide (H2CO3) or as bicarbonate ions (HCO3 - ) present) and will form ammonium carbonate or sodium carbonate.

[0037] The pH in the base compartment is usually maintained in the desired range by controlling the flow rate of the feed solution and the current density applied to the membrane stack of the electrodialyser, which will induce the production of OH

[0038] In the present invention, the pH of the outlet solution from the base compartment is at most 9.5, preferably at most 9.0. The pH of the outlet solution from the base compartment is usually at least 6.5, preferably at least 7.0.

[0039] The compartment between the cationic face of the bipolar membrane and the anionic membrane constitutes the acid compartment. In this compartment, there is H + ions originating from the bipolar membrane, as well as CI - ions crossing the anionic membrane from the base compartment which is fed with an aqueous solution containing ammonium chloride and sodium chloride.

[0040] The present invention also relates to numerous additional embodiments of the process described hereinafter, and can be combined with any of the other embodiments, either alone or in combination with several embodiments, unless it is apparent that they are not compatible.

[0041] In one embodiment, the membrane stack of the electrodialyser used in the process of the present invention comprises at least four compartments separated from each other by alternating bipolar membranes and anionic membranes. Preferably, each anionic membrane separates a base compartment from an acid compartment.

[0042] However, on an industrial scale, it is preferred that the membrane stack of the electrodialyser comprises a plurality of alternating base compartments and acid compartments separated from each other by a plurality of alternating bipolar membranes and anionic membranes. For example, the membrane stack can consist of a series of acid compartments and base compartments bounded by consecutive anionic membranes and bipolar membranes. In this configuration, each anionic membrane or bipolar membrane has one face in a base compartment and another face in an acid compartment.

[0043] The electro-dialysis stack is closed on both sides by electrolyte compartments comprising electrode elements and degassing devices. The electrode elements are usually made of a corrosion-resistant material, such as a metal or a metal oxide.

[0044] The concentrations of ammonium chloride and sodium chloride salts and dissolved carbon dioxide in the aqueous feed solution are not particularly limited. However, too dilute solutions would lead to a large energy consumption for recovering or concentrating the subsequently produced ammonium carbonate or sodium carbonate.

[0045] Concentrated solutions can be suitable, up to the point where the ammonium chloride or sodium chloride and / or the ammonium bicarbonate and sodium bicarbonate, or ammonium carbonate and sodium carbonate in these concentrated solutions are not saturated, to avoid reaching the saturation limit of the salts.

[0046] In general, the aqueous solution fed into the base compartment comprises at most 6 mol NH4 + / kg, at most 6 mol Cl - / kg, and at most 3.5 mol Na + / kg, and at most 3.5 mol / kg of dissolved CO2, expressed in kg of aqueous solution.

[0047] In the aqueous solution fed into the base compartment, the concentrations of the various chemical species should follow the ionic electroneutrality: i.e. the sum of the cationic species is equal to the sum of the anionic species, expressed in equivalent electronic charges.

[0048] The aqueous feed solution comprises ammonium chloride and sodium chloride, dissolved CO2, and can comprise other components, such as ammonium bicarbonate and / or sodium bicarbonate. The aqueous feed solution is advantageously derived from a filtered solution, obtained by separating the crude sodium bicarbonate crystals from their mother liquor, upon leaving the Solvay carbonation tower.

[0049] In one embodiment, the aqueous solution comprising NH4Cl, NaCl, dissolved CO2 is the mother liquor obtained after sodium bicarbonate filtration (crude sodium bicarbonate) in the Solvay process. Optionally, the mother liquor is pre-treated to reduce or adjust the volatile ammonia (NH3) and carbon dioxide (CO2) content.

[0050] In another embodiment, the aqueous feed solution of the base compartment can advantageously comprise dissolved CO2 supplemented by injecting carbon dioxide in the mother liquor, i.e. carbonating the mother liquor.

[0051] In another embodiment, the outlet solution from the base compartment comprising dissolved (NH4)2CO3 and Na2CO3 is carbonated by injecting a gas comprising CO2 to produce a carbonated liquor or a carbonated slurry, wherein at least a part of the dissolved carbonate ions (CO3 2- ) from ammonium carbonate and sodium carbonate is converted into bicarbonate ions (HCO3 -) and at least part of the carbonated liquid or carbonated slurry is recirculated back into the base compartment. In this further embodiment, depending on the pH of the aqueous solution from the outlet of the base compartment and the amount of CO2 injected in the inlet aqueous solution before introduction into the base compartment, ammonium carbonate ((NH4)2CO3) can partially or completely be present as bicarbonate ammonium (NH4HCO3). The same is true for sodium carbonate (Na2CO3) which is partially or totally present as sodium carbonate (Na2CO3) or as sodium bicarbonate (NaHCO3) in the aqueous solution of the base compartment (inlet or outlet solution) when further carbonated with a gas comprising CO2.

[0052] Since the ratio of bicarbonate ions (HCO3 - ) / carbonate ions (CO3 2- ) determines the pH of the solution, ammonium carbonate and sodium carbonate, or bicarbonate ammonium and bicarbonate sodium salts, usually coexist both in said aqueous solution.

[0053] The concentration of ammonium chloride in the aqueous feed solution of the present invention is not particularly limited. In a preferred embodiment, the feed solution comprises at least 2 g / kg, such as at least 5 g / kg, preferably at least 10 g / kg, more preferably at least 50 g / kg, even more preferably at least 100 g / kg, even more preferably at least 130 g / kg, and even more preferably at least 160 g / kg of ammonium chloride per kg of feed solution.

[0054] The outlet solution from the base compartment typically comprises at most 5.0 mol of total ammonia per kg. Advantageously, the solution comprises at most 2.0 mol of total ammonia per kg, more advantageously at most 1.5, or even more advantageously at most 1.0 mol of total ammonia per kg of outlet solution. In such embodiments, the total free ammonia is advantageously at most 0.5 mol of free ammonia per kg of outlet solution, or even more advantageously at most 0.3 mol of free ammonia per kg of outlet solution.

[0055] In a further embodiment, the outlet solution from the base compartment comprising (NH4)2CO3 and Na2CO3 is introduced into a soda ash process. Optionally, gaseous ammonia from the outlet liquid from the base compartment is recovered before introducing the outlet solution from the base compartment into the soda ash process. Such ammonia recovery can be performed by heating and stripping.

[0056] In a further embodiment of the above-mentioned embodiments, the method comprises the further step of dissolving sodium chloride in the outlet solution of the base compartment to obtain an ammonia brine and introducing the resulting ammonia brine into a soda ash process.

[0057] The separation of the coarse sodium bicarbonate crystals from their mother liquor (as it leaves the Solvay carbonation tower) is usually operated on a rotary filter, a belt filter or a centrifuge. As a result, the mother liquor contains ammonium chloride, which needs to be regenerated into ammonia in order to be recycled and reused in the ammonia-soda process.

[0058] Subsequently, an operation to regenerate at least part of the ammonium chloride (NH4CI) into alkaline ammonia (such as ammonium carbonate or ammonium bicarbonate) can be performed according to the present invention.

[0059] The concentration of sodium chloride in the aqueous feed solution is not particularly limited. In preferred embodiments, the aqueous feed solution can comprise at least 20 g / kg. The aqueous feed solution typically comprises at most 250 g / kg, preferably at most 100 g / kg sodium chloride.

[0060] The compartment located between the cationic face of the bipolar membrane and the anionic membrane constitutes the acid compartment. In this compartment, there is a supply of H + ions originating from the bipolar membrane and Cl - ions crossing the anionic membrane. The acid compartment can for example be fed with water or a dilute aqueous hydrochloric acid, in which case the H + ions and Cl - ions supplied by the membrane form hydrochloric acid, which is obtained as an outlet liquid from the acid compartment.

[0061] In further embodiments, the method of the present invention comprises, in a further step, recovering the hydrochloric acid-containing outlet liquid from the acid compartment.

[0062] The outlet liquid from the acid compartment can be partially recycled to the inlet of the acid compartment. The remaining part of the outlet liquid is withdrawn from the process and corresponds to a co-production of the method of the present invention.

[0063] The hydrochloric acid thus produced can be used as such or can be concentrated by removing at least part of its water content to produce a concentrated aqueous hydrochloric acid solution. Concentrations of up to 37.5% HCl by weight or higher are advantageous for the transport of such concentrated acids.

[0064] The hydrochloric acid or concentrated hydrochloric acid can also or alternatively be sold, used in other processes, or reacted, for example, with calcium carbonate (limestone) to produce carbon dioxide. This is particularly advantageous because the carbon dioxide gas obtained by this acid erosion has a high concentration, such as at least 80 vol% CO2, or at least 90 vol% or higher CO2. This can be achieved by tightening the air inlet in the used acid erosion equipment. In contrast, a conventional lime kiln for calcining calcium carbonate produces a gas comprising CO2 at a concentration of about 40 vol%. Furthermore, such a high CO2 concentration brings a high degree of flexibility for the use of the relevant CO2-comprising gas for further use in the soda ash process. It also limits the pipe size for transporting this gas and reduces the power for compressing the gas for injection into a bicarbonate of soda tower (for the production of crude bicarbonate) or into a sodium bicarbonate crystallizer.

[0065] Thus, in a further embodiment of the process of the application, the aqueous hydrochloric acid solution from the acid compartment, or the concentrated aqueous hydrochloric acid solution, is reacted with limestone to obtain carbon dioxide (CO2) and an aqueous calcium chloride (CaCl2) solution. Optionally, in the above further embodiment, the process of the application comprises the further step of at least partially recycling the aqueous calcium chloride (CaCl2) solution to the acid compartment. The carbon dioxide thus obtained can advantageously be injected and dissolved in the aqueous solution fed to the base compartment. This is particularly advantageous because it enables easy control of the pH at the outlet of the base compartment to at most 9.5 or at most 9.0.

[0066] Alternatively, the carbon dioxide (CO2) obtained above can be injected and dissolved in at least a portion of the outlet solution from the base compartment (comprising (NH4)2CO3 and Na2CO3), thereby forming dissolved ammonium bicarbonate (NH4HCO3) or dissolved sodium bicarbonate (NaHCO3), and the resulting solution is recycled to the base compartment as an aqueous solution comprising NH4CI, NaCI and dissolved CO2. This is particularly advantageous in a'recycle' operation of the electrodialyzer.

[0067] The carbon dioxide (CO2) obtained above can also advantageously be introduced in the soda ash process, optionally together with additional carbon dioxide from an external source, such as a steam generator using carbonated fuel, or such as carbon dioxide from other industries, such as the cement industry or the glass industry.

[0068] Thus, the process of the application also relates to a process for the production of sodium carbonate by the soda ash process, which comprises the steps of producing (NH4)2CO3 and Na2CO3 according to one of the embodiments of the application or one of its optional alternatives. Indeed, the advantage of the application is to regenerate the acidic form of ammonia, such as ammonium chloride (NH4CI), into a basic ammonia: such as ammonium carbonate or bicarbonate. Such dissolved basic ammonium salts can be usefully recycled in the brine upstream of the conventional soda ash process, so as to be able to reabsorb acidic carbon dioxide in one or all of the different steps using CO2. In these steps, advantageously in the ammonia absorption step, or in the bicarbonation column washing step, or in the crude sodium bicarbonate precipitation step, the outlet solution from the alkaline compartment is introduced, or a solution derived from the outlet solution from the alkaline compartment.

[0069] In a further embodiment, (NH4)2CO3 and Na2CO3 from the alkaline compartment outlet solution, or derived from said alkaline compartment outlet solution, are fed into the soda ash process, before the ammonia (NH3) absorption in the NaCI brine, as reported above in stage 3 of the chapter "Sodium carbonate" of Ullmann's Encycl. 2012.

[0070] In a further embodiment, (NH4)2CO3 and Na2CO3 from the alkaline compartment outlet solution, or derived from said alkaline compartment outlet solution, are fed into the soda ash process, before the crude sodium bicarbonate precipitation in the bicarbonation column (i.e. the 'preparation column'), as reported above in stage 4 of the chapter "Sodium carbonate" of Ullmann's Encycl. 2012, or directly into the bicarbonation column. As an alternative to the bicarbonation column, these substances can be fed before (or into) the sodium bicarbonate crystallizer.

[0071] In the present application, part of the hydrochloric acid can be used for specific uses other than the production of carbon dioxide acid attack of calcium carbonate. The hydrochloric acid can be concentrated and sold.

[0072] In the present application, advantageously, the CO2 source for the production of soda ash or for the production of refined sodium bicarbonate can be supplemented or entirely provided with CO2 from other industries producing CO2 gas, such as the cement industry, the glass industry, the iron industry, or waste-to-energy industries burning carbonaceous fuels (fossil or non-fossil fuels such as waste biomass), or from mineral or underground sources. Advantageously, the CO2 source can be derived from, or supplemented with, CO2- containing gases comprising a CO2 capture step, such as CO2 from flue gases, combustion gases, industrial gases, gaseous emissions, or even captured from the ambient air or the sea. Said gases can be used as is, or after re-concentration of CO2, generally to reduce their inert gas and air content, so as to reduce their transport constraints and increase their absorption yield.

[0073] In an alternative, or in addition to the aforementioned CO2 sources, it constitutes a further advantageous embodiment to recover a CO2 containing gas leaving the soda ash process or leaving the refined sodium bicarbonate process, such as flue gas from a steam generator or carbonation column. Said CO2 containing gas can also be subjected to a CO2 reconcentration prior to its use, in order to facilitate its use in the soda ash or refined sodium bicarbonate production. Examples of such reconcentration techniques are described in WO 2016 / 102568 in the name of Solvay. These CO2 containing gases or reconcentrated CO2 containing gases can also be used to provide the dissolved CO2 in the aqueous solution comprising NH4CI and NaCI feeding the base compartment of the electrodialysis stack of the present invention.

[0074] In any of the embodiments described above, the hydrochloric acid produced in the acid compartment can be reacted, in whole or in part, with limestone to obtain carbon dioxide. Said carbon dioxide can then be used to precipitate crude sodium bicarbonate crystals in a bicarbonation column; or alternatively, said carbon dioxide can then be used to precipitate crude sodium bicarbonate in a sodium bicarbonate crystallizer.

[0075] In the present invention, the hydrochloric acid (HCI) concentration in the acid compartment is preferably at least 0.1 mol / L, more preferably at least 0.5 mol / L. It has indeed been observed that the more concentrated the hydrochloric acid, the less ammonia leaks into the acid compartment.

[0076] The following examples are merely intended to illustrate the present invention and are not intended to limit the scope of the invention as claimed.

[0077] Example 1 - Non-compliance

[0078] The base compartment of an electrodialysis stack comprising bipolar membranes and anion membranes was fed with a mother liquor obtained after bicarbonate filtration in a soda ash plant, the mother liquor comprising ammonium chloride, sodium chloride and dissolved CO2. The solution contained the following dissolved species: 170 g / l of ammonium chloride, 70 g / l of sodium chloride, 60 g / l of ammonium bicarbonate, and 40 g / l of sodium bicarbonate. The solution was diluted with deionized water to test different concentrations of ammonium chloride, as reported in Table 2 & 3 below. The temperature of the electrodialysis stack was set to 50°C and several current densities were applied from 0 to 1.5 kA / m2. The pH of the compartment was measured at the outlet of the base compartment (i.e. representative of the pH in the compartment) and maintained at the specified strongly basic pH (10.5 and 13.0).

[0079] The ammonium ions (expressed as ammonium molar equivalents NH4 + The flow (or flux) is the amount of flow and is reported per unit surface area of the respective membrane per unit of time (expressed in hours), thus globally expressed in mol eq NH4 +The concentration of ammonium ( / m².h) is determined by increasing the ammonium concentration in each acid chamber.

[0080] - Through the anion exchange membrane (denoted as AEM) to the first acid chamber, and

[0081] - Through the bipolar membrane (represented as BPM membrane in the table) and to the second acid chamber.

[0082] Some tests were conducted in a small cell to quantify ammonia loss at pH greater than 10. The results in the table below indicate that, in some cases, loss through the anion exchange membrane can be controlled when a high current density is applied. However, this is not the case for ammonia loss through the bipolar membrane.

[0083] Example 2 – In accordance with the present invention

[0084] Example 1 was repeated under similar conditions, but the pH was controlled at 8.0 to 8.6 at the alkali chamber outlet.

[0085] In a 10-hour test, significantly lower ammonia leakage could be measured at comparable current densities through each membrane (regardless of which of the same applied current densities fell within the same 0–1.5 kA / m² range). ).

[0086] ( The detection limit for ammonia is 0.005 mol NH3 / kg solution. The test was performed twice (double the test) to verify the reliability of the results.

[0087] Comparing Table 1 (non-compliant, with high pH between 10 and 14), it can be seen that ammonia leakage (in the form of NH4) + The molar equivalents (expressed in h / m²) are significantly higher than those for ammonia leakage at pH below 9.5 in Table 2 (in accordance with the present invention).

[0088] This demonstrates an advantage of the present invention: when the pH of the alkali chamber is controlled at a maximum of 9.5, leakage of ammonium ions through the membrane is avoided and significantly reduced.

[0089] Example 3 – In accordance with the present invention

[0090] In this example, tests were carried out in an electrodialyser membrane stack comprising bipolar membranes and anionic membranes, fed with a mother liquor obtained after filtration of the sodium bicarbonate in a soda ash plant, comprising ammonium chloride, sodium chloride and dissolved CO2. The liquid was fed to the base compartment in 'bleed and feed' mode, i.e. the liquid of the base compartment was circulated within the electrodialyser circuit, the mother liquor was fed to the circulation circuit and a portion of the outlet aqueous solution from the base compartment was discharged to keep the volume of base liquid in the electrodialyser plant constant. The circulation circuit was carbonated with a gas comprising carbon dioxide before being fed again to the base compartment. The flow of carbonated gas comprising carbon dioxide was adjusted to control the pH of the outlet solution from the base compartment. The concentration of salt in the base compartment was as shown in Table 3 below.

[0091] The equipment used to measure the pH in the outlet solution of the base was a Hamilton probe, which was calibrated at the start of each test with three standard calibration solutions of pH (4, 7, 9).

[0092] The current density applied in each test was varied to adjust the flow supplied, so as to maximise production without affecting the transfer phenomenon.

[0093] The results of the tests show that the pH in the base compartment has a significant effect on the ammonia leakage, since reducing the pH to less than 9.5 (a reduction of 0.7 pH units) reduces the leakage by 58%. The results shown in Table 3 are from samples taken under stable conditions (after 50 hours of operation).

[0094] It was also observed that the concentration of acid in the acid circuit has a significant effect on the ammonia leakage. By increasing the concentration of HC1 in the acid compartment from 0.01 M to 2 M, the possibility of ammonia transfer to the acid compartment was reduced by more than 60%.

[0095] This shows one of the advantages of the present application: when the pH of the base compartment is controlled to at most 9.5, preferably at most 9.0, the leakage of ammonium ions through the membranes is avoided and significantly reduced.

[0096]

[0097]

[0098] Table 3 - Ammonium leakage test from base to acid compartment (Example 3 - non- compliant and compliant with the present application)

[0099] .

Claims

1. A method for producing (NH4)2CO3 and Na2CO3 in an electrodialysis membrane stack by feeding an aqueous solution containing NH4Cl, NaCl and dissolved CO2 into an alkali chamber of the electrodialysis membrane stack, and obtaining an outlet solution containing (NH4)2CO3 and Na2CO3 from the alkali chamber, wherein the pH of the outlet solution from the alkali chamber is at most 9.

5.

2. The method according to claim 1, wherein, The pH of the outlet solution from the alkali chamber is at least 6.5, preferably at least 7.

0.

3. The method according to claim 1 or 2, wherein, The alkali chamber is confined between a bipolar membrane and an anion exchange membrane.

4. The method according to any one of the preceding claims, wherein, The electrodialysis membrane stack includes at least four chambers, which are separated from each other by alternating bipolar membranes and anion exchange membranes.

5. The method according to the preceding claim, wherein, Each anion exchange membrane separates the base chamber from the acid chamber.

6. The method according to the preceding claim, the method comprising the additional step of obtaining an outlet liquid containing an aqueous hydrochloric acid solution from the acid chamber and optionally concentrating the aqueous hydrochloric acid solution by removing at least a portion of its water content to produce a concentrated aqueous hydrochloric acid solution.

7. The method according to any one of the preceding claims, wherein, The aqueous solution containing NH4Cl, NaCl and dissolved CO2 is a mother liquor obtained by filtering sodium bicarbonate in the soda ash process, wherein the mother liquor may optionally be pretreated to reduce volatile ammonia (NH3) and carbon dioxide (CO2).

8. The method according to any one of the preceding claims, wherein, The outlet solution containing dissolved (NH4)2CO3 and Na2CO3 from the alkali chamber is carbonated by injecting a gas containing CO2 to produce a carbonated liquid or carbonated slurry, wherein dissolved carbonate ions (CO3-) from the ammonium carbonate and sodium carbonate are present. 2- At least a portion of it is converted into bicarbonate ions (HCO3-). - Furthermore, at least a portion of the carbonated liquid or carbonated slurry is recycled back into the alkali chamber.

9. The method according to any one of the preceding claims, the method comprising the additional step of introducing the outlet solution containing (NH4)2CO3 and Na2CO3 from the alkali chamber into the soda ash process, optionally after recovering gaseous ammonia from the outlet liquid of the alkali chamber.

10. The method according to any one of claims 7 to 9, the method comprising the further steps of dissolving sodium chloride in the outlet solution of the alkali chamber to obtain ammonia brine and introducing the obtained ammonia brine into the soda ash process.

11. The method according to any one of claims 5 to 10, the method comprising the additional step of reacting the aqueous hydrochloric acid solution or the concentrated aqueous hydrochloric acid solution from the acid chamber with limestone to obtain an aqueous solution of carbon dioxide (CO2) and calcium chloride (CaCl2).

12. The method according to the preceding claim, further comprising the additional step of at least partially recycling the aqueous calcium chloride (CaCl2) solution to the acid chamber.

13. The method according to claim 10, wherein, The carbon dioxide is introduced into the soda ash process, optionally along with additional carbon dioxide from an external source.

14. A method for producing sodium carbonate by the soda ash process, the method comprising the steps of producing (NH4)2CO3 and Na2CO3 as described in any one of claims 1 to 13.

15. The method according to any one of claims 6 to 14, wherein, The hydrochloric acid produced in the acid chamber is reacted with limestone to obtain carbon dioxide.

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