Process for the recovery of sulphuric acid and base from inorganic sulphate salts

Abstract

A process for producing sulphuric acid and a base from an inorganic sulphate salt is disclosed. This process involves reacting the inorganic sulphate salt with carbon dioxide and ammonia in water, forming ammonium sulphate and a hydrogen 5 carbonate, carbonate, or basic carbonate salt. The ammonium sulphate is then converted into sodium hydrogen sulphate (sodium bisulphate) through a reaction with sodium sulphate, and the ammonia gas is recycled. Sulphuric acid is recovered from the sodium hydrogen sulphate by crystallising sodium sulphate decahydrate (Glauber's salt) at a low temperature, which leaves the sulphuric acid in solution. This Glauber's salt can be reused in the process. Alternatively, the sulphuric acid can be recovered from a concentrated sodium hydrogen sulphate solution through solvent extraction. The solvent used comprises a mixture of tris(2-ethylhexyl)amine (TEHA) and 1-octanol. The metal hydrogen carbonate, carbonate, or basic carbonate salt can be further processed into a basic metal oxide or a metal hydroxide. In a condensed 15 form, the disclosed process can be used to recover sulphuric acid and ammonia from ammonium sulphate. This process enables the conversion of the sodium sulphate, which is formed during the synthesis of precursor cathode active materials (pCAM) for lithium-ion batteries, into sulphuric acid and sodium hydroxide.

Description

[0001] PROCESS FOR THE RECOVERY OF SULPHURIC ACID AND BASE FROM

[0002] INORGANIC SULPHATE SALTS

[0003] FIELD OF THE INVENTION

[0004] The invention relates to the metallurgical and chemical industries. The present disclosure concerns a process for splitting sulphate salts that are formed as waste products in hydrometallurgical processes into sulphuric acid and a base. It is disclosed how to produce sulphuric acid and sodium hydroxide from sodium sulphate waste generated during the synthesis of precursors of cathode active materials (pCAM) for lithium-ion batteries. The invention provides also a process for the decomposition of ammonium sulphate into sulphuric acid and ammonia.

[0005] BACKGROUND OF THE INVENTION

[0006] Waste sulphate salts are generated in large amounts by metallurgical and chemical processes. Sulphuric acid (H2SO4) is the main acid in hydrometallurgy. It is added to metal ores and concentrates to solubilise the metals by acid leaching. It is also formed during bioleaching of sulphidic ores. After leaching, the excess acid of the pregnant leach solution (PLS) must be neutralised and the pH increased to remove hydrolysable impurity ions such as iron(III) and aluminium(III) by precipitation as their hydroxides. This neutralisation is typically done by adding lime or limestone to the PLS and this leads to the precipitation of poorly soluble calcium sulphate in the form of its dihydrate, CaSO4-2H2O (gypsum). Gypsum is produced also upon removal of sulphate from lithium-containing brines upon addition of lime. Oxidative ammoniacal leaching of sulphidic ore concentrates such in the Sherritt Gordon and Arbiter processes leads to the formation of large amounts of ammonium sulphate, (NH4)2SO4. Sodium sulphate (Na2SO4) and ammonium sulphate are generated during pH control and saponification of acidic extractants by sodium hydroxide and ammonia in solvent extraction processes of metals from sulphuric acid media. These salts are also formed as waste products in hydrometallurgical processes for the recycling of metals from the black mass of end-of-life lithium-ion batteries (LIBs), or by desulphurisation of lead paste (PbSC ) during the recycling of lead-acid batteries. Phosphogypsum is the main byproduct of phosphoric acid production by acid digestion of phosphate rock with sulphuric acid. Depending on the process conditions, phosphogypsum consists primarily of CaSO4-2H2O (gypsum) or CaSO4-1 / 2H2O (bassanite). About 5 tonnes of phosphogypsum are generated per tonne of phosphoric acid produced. It is estimated that yearly around 100 to 280 millions of metric tons are produced and stockpiled worldwide.

[0007] A new, but steadily increasing source of sulphate waste is the sodium sulphate generated during the production of precursors of cathode active materials (pCAM) for lithium-ion batteries. In this process, a mixed nickel-manganese-cobalt hydroxide is precipitated by adding a sodium hydroxide solution to an aqueous solution comprising nickel(II), manganese(II), and cobalt(II) sulphates, in the presence of ammonia that acts as a complexing agent to control the pH and stabilize the metal ions in solution, leaving dissolved sodium sulphate in the mother liquor.

[0008] In principle, these sulphate salts can be valorised in a variety of applications. For instance, gypsum finds applications in the construction industry. Sodium sulphate can be used as a filler and handling-aid additive in powdered detergents, for the production of glass, or in the textile industry. Ammonium sulphate is a valuable fertiliser. However, many issues prevent the valorisation of sulphate waste salts. Gypsum generated in neutralisation reactions and phosphogypsum contain many impurity metals, so that this contaminated gypsum is of too low a quality for use as a construction material. Such huge quantities of sodium sulphate are generated by the production of pCAM that there is simply no market that can absorb these large amounts. Companies that use sodium sulphate as a raw material already have reliable supply chains, so there is no need for them to take over the sodium sulphate from the pCAM producers. Transportation costs can also be a problem for the valorisation of the by-products. This is often the case for ammonium sulphate, where the production is typically far from the markets for fertilisers and the transportation costs can be higher than the intrinsic value of the salt.

[0009] Often, producers of the sulphate salts have no option other than to stockpile these waste salts or send them to landfills. An example are the huge stockpiles of phosphogypsum, which occupy valuable land area and often cause environmental issues due to dust generation and the presence of radioactive impurities. Sodium sulphate is a problematic waste because it is highly water-soluble in its solid form, sodium sulphate decahydrate, Na2SO4-10H2O {Glauber's salt), so it cannot be stored in the open air. The sodium sulphate cannot be discharged to surface waters, because of very strict sulphate emission norms.

[0010] The most intensively studied approach to solve the sodium sulphate waste problem is the salt splitting of sodium sulphate into sulphuric acid and sodium hydroxide. These methods are typically based on electrochemical methods, and three options of salt splitting are being investigated : (1) two-compartment electrolysis systems, (2) three-compartment electrolysis systems, and (3) bipolar membrane electrodialysis systems (BMED) [A. Burns and C. Brereton, in B. Davis et al. (eds.) Extraction 2018, pp 1867-1882]. Electrolytic salt splitting technologies have high capital expenditures (CAPEX) due to the expensive electrolytic cells and membranes, and high operating expenditures (OPEX) due to the high electricity consumption. These methods are economically feasible only in regions where large amounts of cheap, renewable electricity are available. The expensive membranes are easily clogged and fouled, requiring regular replacement. Moreover, the BMED systems can handle only relatively dilute feed streams (about 100 g / L Na2SO4), and generate dilute NaOH and H2SO4solutions [Bruinsma et al. (2021) Desalination 511, 115096]. The NaOH and H2SO4solutions that are generated by electrodialysis are too dilute for reuse as a reagent in hydrometallurgical processes or for other applications. Concentration of these dilute solutions by techniques such as reverse osmosis (RO), combined with evaporation of water, consumes too much energy, even if the heat released during the condensation of water vapour can be largely recuperated.

[0011] The efficient conversion of ammonium sulphate to sulphuric acid and ammonia remains a technological challenge. Prior art demonstrates that the thermal dissociation of (NH4)2SO4to NH3and H2SO4is not readily achievable beyond the stage of ammonium hydrogen sulphate (NH4HSO4) formation. Even the conversion of ammonium sulphate to ammonium hydrogen sulphate is cumbersome, as it is an energy-intensive process carried out at temperatures of about 350 °C. Ammonium hydrogen sulphate is produced in the molten state at these high temperatures. Molten ammonium hydrogen sulphate is highly corrosive, especially in the presence of water vapour, and it rapidly attacks most metallic construction materials used for chemical reactors. Borosilicate glass and quartz glass are suitable materials on a laboratory scale, but they are not practical for industrial-scale reactors. Moreover, the ammonium hydrogen sulphate melt should not be allowed to solidify in a glass reactor, not even in small amounts, because the glass reactor will crack due to mechanical stress upon remelting the solid. Tantalum metal can withstand molten ammonium hydrogen sulphate, but it is expensive. German patent DE2130036 discloses a process in which ammonium sulphate is first heated to a temperature of 355 °C to convert it to ammonium hydrogen sulphate and ammonia, with half of the ammonia being recovered :

[0012] 2(NH4)2SO4- NH4HSO4+ NH3(T = 260-360 °C) Next, the molten ammonium hydrogen sulphate is burned with sulphur-containing heavy fuel oil at a temperature of 1040 °C. The remaining half of the ammonia is destroyed, but the sulphur content is recovered in the form of a SCh-containing gas stream. SO2 is oxidized to SO3 in a contact process and further processed into concentrated sulphuric acid. Patent US 3,243,261 discloses how the ammonia present in the ammonium hydrogen sulphate can be split out by mixing the molten ammonium hydrogen sulphate with sodium pyrosulphate and sodium sulphate, followed by heating this mixture to 500 °C to form ammonia gas and more sodium pyrosulphate. In a third step, the sodium pyrosulphate is partially decomposed at 900 °C, releasing SO3 that can be converted into sulphuric acid. European patent EP 0 083 831 discloses a process to recover all the ammonia present in ammonium sulphate. The first step is again the energy-intensive thermal decomposition of ammonium sulphate to ammonium hydrogen sulphate and ammonia at 400 °C. In the second step, the ammonium hydrogen sulphate is mixed with water to form a concentrated aqueous solution. In this solution, the following chemical equilibrium occurs:

[0013] 2NH4HSO4 ±5 (NH4)2SO4+ H2SO4 (in water)

[0014] By removing the sulphuric acid from the equilibrium through solvent extraction, the equilibrium shifts to the right. The sulphuric acid could be stripped from the loaded organic phase by water in a countercurrent process with four stripping stages, yielding a 30 wt% H2SO4 solution. The ammonium sulphate that precipitates during the extraction of sulphuric acid can be returned to the thermal decomposition step for further recovery of ammonia. The fact that at maximum only 50% of the ammonia present in ammonium sulphate can be recovered in the thermal decomposition step is a disadvantage of this process, in addition to the high energy costs and large CO2 footprint if fossil fuel is used for heating.

[0015] The sulphates of trivalent metal ions, such as iron(III) sulphate or aluminium(III) sulphate, can be thermally decomposed relatively easily. For instance, Fe2(SO4)s decomposes in air at temperatures of 550 to 650 °C to Fe20s and SO3, with virtually no co-formation of SO2, and the SO3 can be converted into sulphuric acid. However, the sulphates of the divalent metal ions, such as calcium sulphate and magnesium sulphate, can be thermally decomposed only at much higher temperatures. For instance, MgSC decomposes to MgO and a mixture of SO3, SO2, and O2 at temperatures between 900 and 1100 °C. Therefore, it is not straightforward to recover sulphuric acid from magnesium sulphate.

[0016] Another option considered for the valorisation of sulphate waste salts is to convert these low-value salts into more valuable products. An example is the conversion of Na?SO4 into K2SO4 by a metathesis reaction with KCL K2SO4 can be sold as a fertiliser, but a waste stream of NaCI is produced. Patent WO 2024 / 061890 discloses a method to convert Na2SO4 into K2SO4 by reaction with KOH. The NaOH produced could be reused as the base in the hydrometallurgical process that generated the Na2SO4 waste. However, this approach can only be used if the reagents for the metathesis reaction are cheap and readily available. Moreover, it must be avoided that a problem is transformed into another one by generating a different waste stream.

[0017] Unless there are efficient methods to convert sulphate salts back into sulphuric acid and a base, truly circular hydrometallurgical processes cannot be developed. Selling these salts as a by-product is merely a secondary option due to the typically insufficient demand for these by-products. Transportation costs are often prohibitive. Moreover, there is a risk that these by-products may contain undesirable impurities, preventing their use in certain applications or resulting in a product of inferior quality. If the salts are discarded without being regenerated into acids and bases, a significantly larger input of these acids and bases into the hydrometallurgical circuits becomes necessary. Therefore, the preferred option is to split the salts back into their corresponding acids and bases at the hydrometallurgical plant.

[0018] SUMMARY OF THE INVENTION

[0019] The invention provides a general process for converting an inorganic sulphate salt into sulphuric acid and a base. For instance, the invention describes how to convert sodium sulphate into sulphuric acid and sodium hydroxide, or how to convert ammonium sulphate into sulphuric acid and ammonia. The process comprises the following steps:

[0020] A method of producing sulphuric acid and a base from an inorganic sulphate salt, the method comprising the steps of:

[0021] (i) Converting said inorganic sulphate salt into ammonium sulphate and a hydrogen carbonate, or a carbonate or a basic carbonate salt, using a source of ammonia and carbon dioxide,

[0022] (ii) Separating the hydrogen carbonate, or carbonate or basic carbonate salt from the ammonium sulphate,

[0023] (iii) Reacting the ammonium sulphate obtained after step (ii) with sodium sulphate, thereby forming sodium hydrogen sulphate and ammonia gas, and removing the generated ammonia (gas), thereby shifting the chemical equilibrium to the side of the reaction products,

[0024] (iv) Converting the sodium hydrogen sulphate to sodium sulphate and sulphuric acid, by removing either sodium sulphate or sulphuric acid from the reaction medium, thereby shifting the chemical equilibrium to the side of the products with the exception that if the inorganic sulphate salt is ammonium sulphate, steps (i) and (ii) are not performed.

[0025] The process is applicable to any inorganic sulphate salt, provided that it can be converted directly or indirectly to ammonium sulphate, (NFU^SCU, by reaction with carbon dioxide (CO2) and ammonia (NH3) in water. Carbon dioxide and ammonia are added to the reaction mixture in the form of gases or, alternatively, in the form of solid ammonium hydrogen carbonate (NH4HCO3), or ammonium carbonate, (NH4)2CO3, or as a solution of these salts. The process is directly applicable to ammonium sulphate without any preceding conversion step.

[0026] In step (i) , the reaction of carbon dioxide with water introduces protons into the system, which are captured by the base ammonia in the form of ammonium ions. The metal ions precipitate in the form of a hydrogen carbonate, carbonate, or basic carbonate salt. In the case of indirect conversion, the metal sulphate is reacted in solution with sodium carbonate (Na2COs) or sodium hydrogen carbonate (NaHCOs), so that the metal is precipitated in the form of a carbonate or basic carbonate salt and a solution of sodium sulphate (Na2SO4) is formed. This sodium sulphate is subsequently converted into ammonium sulphate. In the case of direct conversion to ammonium sulphate, the metal sulphate is converted to ammonium sulphate without sodium sulphate as an intermediate compound.

[0027] The hydrogen carbonate, carbonate, or basic carbonate salt that precipitates during the reaction of the metal sulphate with carbon dioxide, ammonia, and water, is separated from the liquid reaction mixture by a solid-liquid separation unit operation, such as filtration, centrifugation or decantation. The recovered solid is washed with an aqueous solution of ammonium carbonate or ammonium hydrogen carbonate to remove entrapped metal sulphate, and the washing water is combined with the filtered, centrifuged or decanted aqueous reaction mixture. The washed solid hydrogen carbonate, carbonate, or basic carbonate salt is then dried. After this solid- liquid separation, the reaction liquid, combined with the washing water, is cooled to a temperature of 10 °C or lower to crystallise out most of the unreacted sodium sulphate. Part of the ammonium hydrogen carbonate may also crystallise out. The sodium sulphate and ammonium hydrogen carbonate solids can be recovered by solid-liquid separation and recycled back to the beginning of the flowsheet. For the remaining liquid, there are two options. The first option is to heat the liquid to decompose ammonium carbonate and ammonium hydrogen carbonate into ammonia and carbon dioxide. The expelled carbon dioxide and ammonia gases are recovered and can be reused for converting a subsequent batch of metal sulphate salt into ammonium sulphate and a metal carbonate, hydrogen carbonate, or basic carbonate salt. After the expulsion of ammonia and carbon dioxide from the liquid reaction mixture, ammonium sulphate is recovered from the solution by evaporation of the water, for instance, by spray crystallisation. The second option is to concentrate the liquid under reduced pressure or by heating, after removal of the largest part of the sodium sulphate. Above 50 °C, ammonium sulphate can be crystallised, whereas the remaining sodium sulphate remains in solution. This mother liquor can be recycled to the beginning of the flowsheet.

[0028] In step (ii), the hydrogen carbonate, or carbonate or basic carbonate salt is from the dissolved ammonium sulphate by filtration, centrifugation or decantation.

[0029] In step (iii) , the ammonium sulphate recovered in step (ii), or alternatively, ammonium sulphate from another source, is reacted with water and sodium sulphate to form sodium hydrogen sulphate (NaHSO4) and ammonia gas. The chemical equilibrium is shifted to the side of the products by removal of ammonia gas from the system, with recovery of the ammonia. The recovered ammonia can be reused in step (i). The chemical reaction in step (iii) is carried out at the boiling point of the solution or at temperatures close to the boiling point, typically below 100°C:

[0030] (NH4)2SO4+ Na2SO4s; 2NaHSO4+ 2NH3T (in water)

[0031] Preferably, a concentrated aqueous solution of ammonium sulphate and sodium sulphate is used, and even more preferably, a saturated solution. The molar ratio between ammonium sulphate and sodium sulphate is preferably 1 : 1, but a slight excess of sodium sulphate (up to a maximum of 20%) can be used. The sodium sulphate can be added to the reaction mixture in the form of anhydrous sodium sulphate or the decahydrate Na2SO4-10H2O (Glauber's salt). This sodium sulphate is not a consumable, as it is regenerated in step (iv) . The sodium hydrogen sulphate formed in the reaction can be transferred to step (iv) in the form of a solid or a concentrated aqueous solution.

[0032] Alternatively, step (iii) can be carried out by intensively mixing ammonium sulphate and sodium sulphate in a 1 : 1 molar ratio, followed by heating the mixture at a temperature lower than the melting point of the mixture. Ammonia gas is expelled from the mixture and is recovered for reuse in step (i).

[0033] In step (iv) , sulphuric acid is recovered from sodium hydrogen sulphate. A concentrated solution of sodium hydrogen sulphate in water is prepared by dissolving the salt in water at 80 to 100 °C, the solution is cooled to a temperature below 0 °C, and solid sodium sulphate decahydrate (Na2SO4-10H2O) crystallises out, leaving sulphuric acid in the mother liquor:

[0034] 2NaHSO4+ XH2O ±5 Na2S04 10H201 + H2SO4 + (x-10)H2O

[0035] For each molecule of Na2SO4 that crystallises out of the solution, 10 molecules of water are simultaneously removed from the solution. This removal of water has the effect of concentrating the sulphuric acid solution. Depending on the temperature and the acidity of the solution, the solid that crystallises is either Na2SO4-10H2O, an acidic sodium sulphate (mainly sodium hydrogen sulphate, NaHSC ), or ice. The preferred solid is Na2SO4-10H2O because this will yield the most concentrated sulphuric acid solution and will result in the lowest concentration of sodium impurities in the sulphuric acid. The Na2SO4-10H2O is separated from the liquid reaction mixture by a solid-liquid separation unit operation, such as filtration or centrifugation, and, optionally, the recovered solid is washed with an aqueous solution of sodium sulphate. The Na2SO4-10H2O crystals can be recycled to step (iii). The mother liquor left over after the removal of Na2SO4-10H2O crystals can be subjected to another crystallisation step at a lower temperature to recover more Na2SO4-10H2O crystals, or it can be further processed to recover the sulphuric acid. In many cases, the sulphuric acid obtained after step (iv) is already of a purity and concentration high enough for direct reuse in hydrometallurgical operations, and no further concentration or purification steps are required. The temperature at which the Na2SO4-10H2O crystallisation is carried out, depends on the composition of the starting solution, but will be between 0 and -40 °C.

[0036] Alternatively, step (iv) can be carried out by subjecting the concentrated solution of sodium hydrogen sulphate to solvent extraction for transfer of sulphuric acid from the aqueous phase to the organic phase. This transfer of sulphuric acid will also shift the chemical equilibrium to the right, causing more complete conversion of sodium hydrogen sulphate to sodium sulphate and sulphuric acid. During the solvent extraction, there is the possibility that Na2SO4-10H2O crystallises out in the aqueous phase. The extracted sulphuric acid can be recovered from the loaded organic phase by stripping with hot water. The solvent extraction is preferably done at room temperature, and the stripping of the acid from the loaded organic phase with hot water is done at a temperature of 60 °C or higher. The preferred solvent is the tertiary amine with a branched alkyl chain, such as tris(2-ethylhexyl)amine (TEHA), in 1- octanol. 1-Octanol acts both as a diluent and modifier. It is not inert, but has a strong effect on the extraction efficiency. Hence, it is an "active" diluent. The preferred composition of the solvent is 50 vol% TEHA in 50 vol% 1-octanoL

[0037] Optionally, if no solvent extraction is used in step (iv), the sulphuric acid obtained in that step is purified by extraction from the mother liquor using an organic solvent, followed by stripping of sulphuric acid from the loaded organic phase with hot water. This solvent extraction is similar to that performed in step (iv). The extraction is preferably done at room temperature, and the stripping of the acid from the loaded organic phase with hot water is done at a temperature of 60 °C or higher. The preferred solvent is the tertiary amine with a branched alkyl chain, such as tris(2- ethylhexyl)amine (TEHA), in 1-octanoL

[0038] The sulphuric acid solution is concentrated by evaporation of water. This process can be carried out in a concentrator (evaporator), where the necessary sensible and latent heat is ideally supplied by either steam or electricity. Preferably, a multi-effect concentrator is used. This concentration step allows obtaining sulphuric acid with a concentration of up to 98 wt%.

[0039] The hydrogen carbonate, carbonate, or basic carbonate salts recovered in step (ii) can be used in the same form as they have been obtained, or they can be further processed into other inorganic chemicals. The carbonates or basic carbonates of the alkaline earth metals can be heated to decompose them into the corresponding oxides and carbon dioxide. This process is known as calcination. For instance, calcium carbonate can be decomposed into calcium oxide and carbon dioxide. Calcium oxide (quicklime) can be reacted with water to form calcium hydroxide (lime or slaked lime), and limewater can be produced from the said calcium hydroxide. Magnesium carbonate can be decomposed into magnesium oxide and carbon dioxide. Sodium hydrogen carbonate can be converted into sodium carbonate and carbon dioxide by heating. The carbon dioxide released in these calcination steps can be recovered and reused in step (i) .

[0040] Optionally, the sodium carbonate obtained from sodium hydrogen carbonate is converted into sodium hydroxide by the reaction of the sodium carbonate with slaked lime in water (caustification or causticisation). The calcium carbonate formed in this reaction can be reconverted into quicklime and carbon dioxide by calcination, and the carbon dioxide can be recycled to step (i).

[0041] The sodium carbonate can also be converted into sodium hydroxide via the L wig process. Here solid sodium carbonate is heated with iron(III) oxide to form sodium ferrite, NaFeO?, with release of carbon dioxide, that can be recycled to step (i). The sodium ferrite is leached with water, to give an aqueous solution of sodium hydroxide that can be further concentrated. In the leaching step, the iron(III) is reformed and this compounds can be reused in the sodium carbonate to sodium ferrite conversion step.

[0042] Sodium hydrogen carbonate can be converted directly into sodium hydroxide by electrolysis in a two-compartment electrolysis cell, divided by a cation-exchange membrane. The anode compartment is filled with an aqueous sodium hydrogen carbonate solution and the cathode compartment with a sodium hydroxide solution. At the cathode, hydrogen gas and hydroxide ions are formed, whereas at the anode, oxygen gas and protons are formed. The protons react with the hydrogen carbonate ions to form carbonic acid, which dissociates into water and carbon dioxide. Sodium ions migrate through the membrane from the anode to the cathode compartment, combining with the hydroxide ions to form sodium hydroxide. The hydrogen, oxygen, and carbon dioxide gases released can be recovered. After the separation of carbon dioxide from oxygen gas, the carbon dioxide can be reused in step (i) of the process. The sodium hydroxide solution can be concentrated by evaporation of water. The unit operations of steps (i) to (iv) can be carried out independently of each other, either in batch or continuous mode. However, it is recommended to integrate the different steps (i) to (iv) into one integrated system to maximize the efficiency of energy and materials flows. For instance, the ammonia gas released in step (iii) should be captured for reuse in step (i). The sodium sulphate recovered in step (iv) is reused in step (iii). The carbon dioxide released during the calcination of the metal hydrogen carbonate, carbonate, and basic carbonate salts can be reused in step (i). If all the ammonia and carbon dioxide released in the different process steps are recovered and reused, no net input of ammonia and carbon dioxide is required, and no chemicals other than water are consumed. The protons needed to regenerate the acid and the hydroxides used to regenerate the hydroxide base all originate from water. Efficient use of energy must be made; for instance, the heat released by the cooling of the solution in step (iv) can be recovered to a large extent and used for heating the solution in step (iii). The heat released by the condensation of water in the evaporation steps can be reused for preheating aqueous solutions.

[0043] The disclosed process can be extended to the recovery of acids other than sulphuric acid from their sodium salts. For instance, sodium methanesulphonate, CH3SO3Na, can be converted into methanesulphonic acid (MSA), CH3SO3H, by adding sulphuric acid to a solution of sodium methanesulphonate in a 1 : 1 molar ratio, followed by cooling the solution to a temperature of 0 °C or lower to crystallise Na2SO4-10H2O. The crystals of Na2SO4-10H2O are filtered off. The remaining solution is an aqueous solution of methanesulphonic acid and can be further concentrated by evaporation of water. The general reaction can be represented by the equation:

[0044] 2CH3SO3Na + H2SO4+ xH2O ±5 Na2S0410H201 + 2CH3SO3H + (x-10)H2O

[0045] In a similar way, hydrochloric acid can be recovered from solutions of sodium chloride, and nitric acid can be recovered from solutions of sodium nitrate:

[0046] 2NaCI + H2SO4+ XH2O ±5 Na2S0410H201 + 2HCI + (x-10)H2O

[0047] 2NaNO3+ H2SO4+ xH2O ±5 Na2S0410H201 + 2HNO3+ (x-10)H2O The invention is further summarised in the following statements.

[0048] 1. A method of producing sulphuric acid and a base from an inorganic sulphate salt, the method comprising the steps of:

[0049] (i) Converting said inorganic sulphate salt into ammonium sulphate and a hydrogen carbonate, or a carbonate or a basic carbonate salt, using a source of ammonia and carbon dioxide,

[0050] (ii) Separating the hydrogen carbonate, or carbonate or basic carbonate salt from the ammonium sulphate,

[0051] (iii) Reacting the ammonium sulphate obtained after step (ii) with sodium sulphate, thereby forming sodium hydrogen sulphate and ammonia gas, and removing the generated ammonia (gas), thereby shifting the chemical equilibrium to the side of the reaction products,

[0052] (iv) Converting the sodium hydrogen sulphate to sodium sulphate and sulphuric acid, by removing either sodium sulphate or sulphuric acid from the reaction medium, thereby shifting the chemical equilibrium to the side of the products, with the exception that if the inorganic sulphate salt is ammonium sulphate, steps (i) and (ii) are not performed.

[0053] 2. The method according to statement 1, further comprising the step of extracting sulphuric acid with an organic solvent, and subsequently stripping of sulphuric acid from the organic phase by hot water of a temperature that is at least 20 °C higher than the temperature at which the extraction was performed, but preferably 35 °C higher.

[0054] 3. The method according to statement 1 or 2, further comprising the step of concentrating aqueous sulphuric acid solution by evaporation of water.

[0055] 4. The method according to statement 1, further comprising the step converting the hydrogen carbonate, carbonate or basic carbonate salt produced in step (i) to an oxide or hydroxide.

[0056] 5. The method according to statement 4, wherein carbon dioxide released in said conversion is recovered.

[0057] 6. The method according to statement 4, wherein sodium hydrogen carbonate is converted to sodium hydroxide by water electrolysis,

[0058] 7. The method according to statement 6, wherein the membrane-separated electrolysis cell comprises sodium hydrogen carbonate in the anolyte and sodium hydroxide in the catholyte. 8. The method according to statement 4, wherein sodium hydrogen carbonate is converted to sodium hydroxide by converting sodium hydrogen carbonate first into sodium carbonate by heating with recovery of carbon dioxide, followed by a causticisation reaction between sodium carbonate and calcium hydroxide in water.

[0059] 9. The method according to statement 1, wherein ammonia (gas) formed in step (iii) is recovered.

[0060] 10. The method according to statement 1, wherein in step (i) the source of ammonia and carbon dioxide is ammonia and carbon dioxide in water.

[0061] 11. The method according to statement 1, wherein in step (i) the source of ammonia and carbon dioxide is ammonium hydrogen carbonate in water.

[0062] 12. The method according to statement 1, wherein step (iii) is carried out by heating a solution of ammonium sulphate and sodium sulphate in water to the boiling point.

[0063] 13. The method according to statement 1, wherein step (iii) is carried out by heating a solid mixture of ammonium sulphate and sodium sulphate at a temperature between 80 and 300, between 80 and 275 , between 80 and 250 , between 80 and 225, between 80 and 200, or between 80 and 170 °C.

[0064] 14. The method according to statement 1, wherein step (iv) comprises cooling an aqueous solution of sodium hydrogen sulphate to a temperature between 0 and -40 °C, there crystallising sodium sulphate decahydrate (Na2SO4-10H2O), while leaving sulphuric acid in solution.

[0065] 15. The method according to statement 1, wherein step (iv) comprises removal of sulphuric acid from the aqueous phase by extraction with an organic solvent, followed by stripping of sulphuric acid from the loaded organic phase by hot water.

[0066] 16. The method according to statement 15, where the organic solvent comprises a tertiary amine and an alkanol with at least 5 carbon atoms.

[0067] 17. The method according to statement 15 or 16, where the organic solvent comprises between 0 and 50 vol.% tris(2-ethylhexyl)amine (TEHA), between 0 and 50 vol.% 1-octanol, and the third component being an aliphatic diluent.

[0068] 18. The method according to statement 1, wherein in step (i) sodium sulphate is converted into ammonium sulphate and sodium hydrogen carbonate by reacting the sodium sulphate with ammonia and carbon dioxide in water.

[0069] 19. The method according to statement 1, wherein in step (i) sodium sulphate is converted into ammonium sulphate and sodium hydrogen carbonate by reacting the sodium sulphate with ammonium hydrogen carbonate in water. 20. The method according to statement 1, wherein in step (i) calcium sulphate or calcium sulphate dihydrate is converted into ammonium sulphate and calcium carbonate by reacting the calcium sulphate or calcium sulphate dihydrate with ammonia and carbon dioxide in water.

[0070] 21. The method according to statement 1, wherein in step (i) calcium sulphate or calcium sulphate dihydrate is converted into ammonium sulphate and calcium carbonate by reacting the calcium sulphate or calcium sulphate dihydrate with ammonium carbonate in water.

[0071] 22. The method according to statement 1, wherein in step (i) magnesium sulphate is converted into ammonium sulphate and basic magnesium carbonate by reacting the magnesium sulphate with ammonia and carbon dioxide in water.

[0072] 23. The method according to statement 1, wherein in step (i) magnesium sulphate is converted into ammonium sulphate and magnesium carbonate by reacting the magnesium sulphate with ammonium carbonate in water.

[0073] 24. The method according to statement 1, wherein in step (i) lithium sulphate is converted into sodium sulphate and lithium carbonate by reacting the lithium sulphate with sodium carbonate in water, and converting sodium sulphate into ammonium sulphate and sodium hydrogen carbonate by reacting sodium sulphate with ammonia and carbon dioxide in water.

[0074] 25. The method according to statement 1, wherein in step (i) lead(II) sulphate is converted into ammonium sulphate and lead(II) carbonate by reacting the lead(II) sulphate with ammonium carbonate in water.

[0075] 26. The method according to statement 1, wherein in step (i) lead(II) sulphate is converted into sodium sulphate and lead(II) carbonate by reacting the lead(II) sulphate with sodium carbonate in water, and converting sodium sulphate into ammonium sulphate and sodium hydrogen carbonate by reacting sodium sulphate with ammonia and carbon dioxide in water.

[0076] 27. The method according to statement 1, wherein in step (i) lithium sulphate is converted into lithium carbonate by reacting the lithium sulphate with ammonia and carbon dioxide in water.

[0077] 28. The method according to statement 1, wherein in step (i) lithium sulphate is converted into lithium carbonate by reacting the lithium sulphate with ammonium hydrogen carbonate in water, followed by heating the solution to a temperature of at least 80 °C. BRIEF DESCRIPTION OF THE DRAWING

[0078] Figure 1. Conceptual flowsheet to recover sulphuric acid (H2SO4) and sodium hydrogen carbonate (NaHCO3) from sodium sulphate (Na2SO4).

[0079] Figure 2. Conceptual flowsheet to convert sodium hydrogen carbonate (NaHCO3) into sodium carbonate (Na2CO3) or sodium hydroxide (NaOH).

[0080] DETAILED DESCRIPTION

[0081] DEFINITIONS

[0082] The chemical formula and name of the main compounds considered in this disclosure are:

[0083] NH3ammonia

[0084] NH4HCO3ammonium hydrogen carbonate (= ammonium bicarbonate)

[0085] (NH4)2CO3ammonium carbonate

[0086] NH4HSO4ammonium hydrogen sulphate (= ammonium bisulphate)

[0087] (NH4)2SO4ammonium sulphate

[0088] CO2carbon dioxide

[0089] H2CO3carbonic acid

[0090] H2SO4sulphuric acid

[0091] NaHSO4sodium hydrogen sulphate (= sodium bisulphate)

[0092] Na2SO4sodium sulphate

[0093] Na2SO410H2O sodium sulphate decahydrate (=Glauber's salt)

[0094] The term „inorganic sulphate salt" relates to any metal sulphate salt or to ammonium sulphate. Organic sulphate salts, i.e. sulphates with an organic cation are excluded. Unless otherwise specified, the term "sodium sulphate" can refer to either the solid or dissolved form, and it can refer to anhydrous sodium sulphate or to any of the crystal hydrates. When a specific hydrate is meant, it is explicitly mentioned, such as Na2SO410H2O. "Glauber's salt" (or "Glauber salt") is a synonym for sodium sulphate decahydrate or Na2SO410H2O. The mineral name of Na2SO410H2O is mirabilite and that of Na2SO4is thenardite. The term "acidic sodium sulphate" refers to NaHSO4or to any other of the solid phase in the binary Na2SO4- H2SO4phase diagram, such as Na3H(SO4)2[Faust and Esselmann (1926) Zeitschrift fur anorganische und allgemeine Chemie, 157 290-298], The IUPAC terminology of "hydrogen sulphate" and "hydrogen carbonate" is used throughout this disclosure, rather than the trivial names of "bisulphate" and "bicarbonate", respectively. A basic carbonate is a compound that comprises both a carbonate and a hydroxide group. Basic carbonates often have a variable composition, such as Mg2CO3(OH)2-3H2O, Mg5(CO3)4(OH)2-4H2O,

[0095] Mg5(CO3)4(OH)2-5H2O, and mixtures thereof for basic magnesium carbonate.

[0096] PROCESSES

[0097] A general form of the process comprises the following steps:

[0098] (i) Converting said inorganic sulphate salt into ammonium sulphate and a hydrogen carbonate, or a carbonate or a basic carbonate salt, using a source of ammonia and carbon dioxide,

[0099] (ii) Separating the hydrogen carbonate, or carbonate or basic carbonate salt from the ammonium sulphate,

[0100] (iii) Reacting the ammonium sulphate obtained after step (ii) with sodium sulphate, thereby forming sodium hydrogen sulphate and ammonia gas, and removing the generated ammonia (gas), thereby shifting the chemical equilibrium to the side of the reaction products,

[0101] (iv) Converting the sodium hydrogen sulphate to sodium sulphate and sulphuric acid, by removing either sodium sulphate or sulphuric acid from the reaction medium, thereby shifting the chemical equilibrium to the side of the products with the exception that if the inorganic sulphate salt is ammonium sulphate, steps (i) and (ii) are not performed.

[0102] Step (i) involves converting any sulphate salt into ammonium sulphate, (NH4)2SO4, through a reaction with carbon dioxide and ammonia in water. It is evident that step (i) is omitted if the sulphate salt used as input is ammonium sulphate. The metal ion in the sulphate salt is captured in the form of a hydrogen carbonate, carbonate, basic carbonate salt, or a mixture thereof. The compound formed depends on the type of metal ions and the relative amounts of carbon dioxide and ammonia used. The purpose of step (i) is to distribute the metal ions and the sulphate ion present in the metal sulphate salt over two different compounds. Thus, the original metal sulphate salt is converted into two new salts, one being ammonium sulphate and the other being the metal hydrogen carbonate, carbonate, or basic carbonate salt. These two salts can be further processed, with the ammonium sulphate converted to sulphuric acid, allowing for the recovery of the ammonia for reuse in step (i), and the hydrogen carbonate, carbonate, or basic carbonate salt further processed to a basic oxide or a hydroxide, allowing for the recovery of the carbon dioxide for reuse in step (i). The further processing of the hydrogen carbonate, carbonate, or basic carbonate salt is optional because these salts already exhibit basic character on their own. The invention discloses a process to convert neutral sulphate salts into sulphuric acid and a base, but acidic sulphate salts can also be treated, with a modification of the process: the acidic sulphate salts are first converted into the neutral sulphate salt by reaction with ammonia before they are further processed by reaction with carbon dioxide or ammonia. Ammonium hydrogen sulphate is fed directly into step (iii) and sodium hydrogen sulphate directly into step (iv).

[0103] The details of how to carry out step (i) are described by the example of sodium sulphate. Sodium sulphate is contacted in an aqueous solution with carbon dioxide and ammonia. The reaction of carbon dioxide with water introduces into the system protons, which are captured by the base ammonia in the form of ammonium ions. The sodium ions precipitate in the form of sodium hydrogen carbonate, NaHCO3. This conversion of sodium sulphate into ammonium sulphate and sodium hydrogen carbonate is similar to the ammonia - soda process with Na2SO4 as feed [Pak et al. (2021) Chemical Engineering & Technology 44, 1759-1767]. The overall chemical reactions of the ammonia - soda process using Na2SO4 as feed looks similar to that with NaCI as feed {Solvay process') :

[0104] Na2SO4+ 2NH3+ 2CO2+ 2H2O ±5 2NaHCO3+ (NH4)2SO4

[0105] NaCI + NH3+ CO2+ H2O ±5 NaHCO3+ NH4CI

[0106] However, the conversion of sodium sulphate into ammonium sulphate is more complicated than the conversion of sodium chloride into ammonium chloride. First of all, ammonium sulphate is more difficult to recover from the reaction mixture than ammonium chloride. In the conventional Solvay process, ammonium chloride is precipitated out of the aqueous solution by saturating the aqueous solution with sodium chloride and cooling this solution. This is possible because, at low temperatures, ammonium chloride is much less soluble in this solution than sodium chloride. Sodium sulphate, unlike sodium chloride, has a very low solubility in water at low temperatures (10 °C and lower), so that it can be precipitated as crystals by cooling. On the other hand, ammonium sulphate has a very high solubility in aqueous solutions, even at low temperature. Therefore, it is impossible to precipitate ammonium sulphate by saturating the solution with sodium sulphate and cooling the solution. Another complication is that Na?SO4 and (NH^SC have the tendency to form a (poorly soluble) double salt, Na2SO4 (NH4)2SO4-4H2O, whereas NaCI and NH4CI do not show this tendency.

[0107] The sodium sulphate feed solution should be as near to saturation as possible to achieve maximum utilisation of the sodium sulphate. It is recommended to purify the sodium sulphate solution before reaction with carbon dioxide, ammonia and water. More particularly, it is recommended to remove dissolved calcium and magnesium ions because they might cause formation of gypsum, carbonates or basic carbonates scales in the equipment and in the pipe works. The magnesium ions can be precipitated as Mg(OH)2 by addition of Ca(OH)2, followed by precipitation of the calcium ions as CaCOs upon addition of Na2COs. Alternatively, the sodium sulphate solution can be purified by addition of an ammonium carbonate solution, to precipitate calcium carbonate and magnesium as basic magnesium carbonate.

[0108] The conversion of sodium sulphate to ammonium sulphate can be carried out in several ways. In one approach, an aqueous solution of sodium sulphate is first saturated with ammonia gas. The ammonia-saturated solution is then subjected to a carbonation step with carbon dioxide gas. Alternatively, ammonia and carbon dioxide gases can be injected simultaneously into the sodium sulphate solution. This forms dissolved sodium carbonate, which is then converted to sodium hydrogen carbonate by stopping the injection of ammonia gas and continuing only with carbon dioxide gas, resulting in a solution with an excess of carbon dioxide. In another approach, a batch of water is first saturated with ammonia and carbon dioxide to form ammonium hydrogen carbonate in situ. This solution is then added to the sodium sulphate solution. The ammonium hydrogen carbonate solution can also be formed by first dissolving ammonium carbonate and saturating the ammonium carbonate solution with carbon dioxide gas. In yet another approach, ammonium hydrogen carbonate is added to the sodium sulphate solution, either in solid form or as a concentrated solution. Finally, another approach involves adding solid sodium sulphate to an aqueous solution of ammonium hydrogen carbonate.

[0109] Ammonium carbonate is unstable and undergoes hydrolysis in water, with release of ammonia. On the other hand ammonium hydrogen carbonate is very stable. This compound crystallises from ammonia solutions which have been saturated with carbon dioxide. Because of its stability, ammonium hydrogen carbonate is preferred as reagents over ammonium carbonate. Ammonium carbonate and ammonium hydrogen carbonate combine with one another to form a double salt, ammonium sesquicarbonate, (NH4)2CO3-2NH4HCO3, which is the main component of commercial ammonium carbonate.

[0110] The reaction with sodium sulphate, ammonia, carbon dioxide and water is carried out at temperatures between 20 and 35 °C. A compromise has to be found between the solubility of sodium sulphate, which increases with temperature up to the transition point of 32.384 °C, and the tendency of ammonium hydrogen carbonate to decompose at higher temperatures. Also the solubility of ammonia and carbon dioxide in the liquid decreases with increasing temperature. The reaction can be carried out at atmospheric pressure or at elevated pressures. Working at elevated pressures suppresses the decomposition of ammonium carbonate or ammonium hydrogen carbonate, and improves the reaction rates.

[0111] In step (ii), the hydrogen carbonate, carbonate, or basic carbonate salt that is precipitated during the reaction of the metal sulphate with carbon dioxide, ammonia and water, is separated from the liquid reaction mixture by a solid-liquid separation unit operation, for instance filtration, centrifugation or decantation, and the recovered solid is washed with an aqueous solution of ammonium carbonate or ammonium hydrogen carbonate to remove entrapped metal sulphate, and the washing water is combined with the filtered, centrifuged or decanted aqueous reaction mixture. The washed solid hydrogen carbonate, carbonate or basic carbonate salt is dried. After this solid-liquid separation, the liquid is cooled to a temperature of 0 °C or lower to crystallise out most of the unreacted sodium sulphate. Also part of the ammonium hydrogen carbonate can crystallise out. The sodium sulphate and ammonium hydrogen carbonate solids can be recovered by solid-liquid separation, and are recycled back to the beginning of the flowsheet. For the remaining liquid, there are two options. Either the liquid is heated to decompose ammonium carbonate and ammonium hydrogen carbonate to ammonia and carbon dioxide. The expelled carbon dioxide and ammonia gases are recovered and are reused for converting of a next batch of metal sulphate salt into ammonium sulphate and a metal carbonate, hydrogen carbonate or a basic carbonate salt. After expulsion of the ammonia and carbon dioxide from the liquid reaction mixture, ammonium sulphate is recovered from the solution by evaporation of the water, for instance by spray crystallisation. Alternatively, after removal of the largest part of the sodium sulphate, the liquid is concentrated under reduced pressure or by heating. Above 50 °C, ammonium sulphate is crystallised, whereas the remaining sodium sulphate stays in solution. This mother liquor is recycled to the beginning of the flowsheet.

[0112] Metal sulphate salts other than sodium sulphate can be converted directly or indirectly to ammonium sulphate. Calcium sulphate, including the dihydrate CaSO4-2H2O (gypsum), can be converted into ammonium sulphate and calcium carbonate, by reaction with ammonia and carbon dioxide gases in water. Alternatively, calcium sulphate can be reacted with an aqueous ammonium carbonate solution {Merseburg process'). Calcium sulphate can also be converted indirectly to ammonium sulphate, by first reacting it with a sodium carbonate solution to form calcium carbonate and sodium sulphate, followed by conversion of sodium sulphate into ammonium sulphate. Magnesium sulphate can be reacted with ammonium carbonate to precipitate MgCOs-SFhO, with formation of ammonium sulphate. Reaction of magnesium sulphate with ammonia, carbon dioxide and water will yield a basic magnesium carbonate and ammonium sulphate. Alternatively, magnesium sulphate can be converted into magnesium chloride by reaction with sodium chloride:

[0113] MgSO4 + 2NaCI ±5 MgCk + Na?SO4 (in water)

[0114] The equilibrium can be shifted to the right by working at temperatures below room temperature and crystallising sodium sulphate as Glauber's salt, Na2SO4-10H2O. Lithium sulphate can be reacted with a solution of sodium carbonate (soda ash) to form sodium sulphate, which can be further processed to ammonium sulphate.

[0115] U2SO4 + Na2COs -» Li2COsl + Na2SO4 (in water)

[0116] Lithium sulphate can be converted into lithium carbonate by reaction with ammonia and carbon dioxide in water:

[0117] U2SO4 + 2NH3+ CO2 + H2O - U2CO3I + 2(NH4)2SO4

[0118] Precipitation of lithium carbonate can also be achieved by addition of ammonium hydrogen carbonate to an aqueous solution of lithium sulphate. First, lithium hydrogen carbonate (LiHCOs) is formed in solution, but this compound is thermally unstable and decomposes upon heating of the solution, resulting in precipitation of IJ2CO3.

[0119] Lead(II) sulphate (PbSO4, anglesite) is converted into lead(II) carbonate (PbCOs, cerussite) and sodium sulphate by reaction with a solution of sodium carbonate (soda ash). Alternatively, the lead(II) sulphate is treated with a solution of ammonium carbonate.

[0120] In step (iii) , the ammonium sulphate recovered in step (ii), or alternatively, ammonium sulphate from another source, is reacted with water and sodium sulphate to form sodium hydrogen sulphate (NaHSO4) and ammonia gas. The chemical equilibrium is shifted to the side of the products by removal of ammonia gas from the system, with recovery of the ammonia. The recovered ammonia can be reused in step (i). The invention takes advantage of the tendency of ammonium sulphate to undergo hydrolysis in aqueous solutions, and that this tendency increases in the presence of alkali metal sulphates such as sodium sulphate [Dibbits (1874) Z. anal. Chem. 13, 395-408; Dibbits (1876) Z. anal. Chem. 15, 245-250; Naumann and Rucker (1906) J. prakt. Chem. 74, 249-275]. Ammonium sulphate has a much stronger tendency for hydrolysis than ammonium chloride or ammonium nitrate.

[0121] Although ammonium sulphate undergoes hydrolysis when an aqueous solution of this salt is heated, no more than 50% of the ammonia present can be driven off because the reactions ends when all of the ammonium sulphate is converted into ammonium hydrogen sulphate:

[0122] 2(NH4)2SO4S; NH4HSO4+ NH3T (in water)

[0123] The reaction is represented as an equilibrium equation, but the equilibrium can be shifted to the side of the products and driven to completion by removing the ammonia gas, for instance by swiping an inert gas stream over the surface. The reaction given above is only the overall reaction, and it does not show that the ammonium ion reacts with water to form a hydroxonium ion H3O+and neutral NH3 molecule, followed by reaction between H3O+and a sulphate ion to form a hydrogen sulphate and a water molecule. The products formed in this reaction are similar to those of the thermal decomposition reaction used in the prior art, but with the advantage that the reaction can be reaction can be carried out at a much lower temperature. The reaction is 11 fastest for saturated solutions at their boiling point. The boiling point of a saturated solution of ammonium sulphate is 108.9 °C and the solution contains 3.92 moles per 1000 grammes.

[0124] However, all of the ammonia present in ammonium sulphate can be recovered in one single step if the hydrolysis of ammonium hydrogen sulphate is carried out in the presence of an alkali metal sulphate that is converted into the corresponding alkali metal hydrogen sulphate:

[0125] (NH4)2SO4+ M2SO4S; 2MHSO4+ 2NH3T (in water) where M = Li, Na, K, Rb, Cs. However, it is advantageous to select sodium sulphate, not only because it is the cheapest of the alkali metal sulphates, but because sodium hydrogen sulphate is essential in the next step of the process. So the reaction with sodium sulphate can be represented by the following reaction:

[0126] (NH4)2SO4+ Na2SO4s; 2NaHSO4+ 2NH3T (in water)

[0127] From this equation, it is evident that both ammonia molecules present in ammonium sulphate can be recovered in one process step and that sodium hydrogen sulphate is formed. The sodium sulphate used in this process step is not a consumable since it is recycled in the next process step. The hydrolysis of ammonium sulphate in the presence of sodium sulphate is an equilibrium reaction, so it is recommended to adjust the process conditions in such a way that the equilibrium is shifted to the right. This can be achieved by working with concentrated solutions of ammonium sulphate and sodium sulphate, heating the solution and removing the ammonia gas from the equilibrium, for instance by an inert gas stream. Preferably, a concentrated aqueous solution of ammonium sulphate and sodium sulphate is used, and even more preferably, a saturated solution. The molar ratio between ammonium sulphate and sodium sulphate is preferably 1 : 1, but a slight excess of sodium sulphate (up to a maximum of 20%) can be used. The sodium sulphate can be added to the reaction mixture in the form of anhydrous sodium sulphate or the decahydrate Na2SO4-10H2O (Glauber's salt). The reaction is carried out in a reactor with facilities to capture the released ammonia gas, with a minimum loss of ammonia. The recovered ammonia can be reused in the ammonium sulphate synthesis in step (i). The hydrolysis reaction is carried out at the boiling point of the solution, or at temperatures close to the boiling point , typically below 110°C. The low process temperatures are a major improvement compared to the prior art in terms of energy efficiency. This low temperature is also in the range of industrial high-temperature heat pumps that can recover waste heat of industrial processes [Kosmadakis (2019) Applied Thermal Engineering 156, 287-298]. To facilitate the removal of ammonia gas, air or an inert gas stream can be bubbled through the solution. This air or inert gas stream is preferably preheated before it enters the reactor.

[0128] Alternatively, step (iii) can be carried out by intensively mixing ammonium sulphate and sodium sulphate in a 1 : 1 molar ratio, followed by heating the mixture at a temperature lower than the melting point of the mixture. Ammonia gas is expelled from the mixture and is recovered for reuse in step (i). This reaction is carried out at temperatures between 80 and 170 °C. Temperatures above 170 °C should be avoided because anhydrous sodium hydrogen sulphate melts between 177 and 180 °C, and molten sodium hydrogen sulphate is a very corrosive liquid. To facilitate the removal of ammonia gas, air or an inert gas stream can be swept over the surface of the solid mixture. This air or inert gas stream is preferably preheated before it enters the reactor.

[0129] The reaction between ammonium sulphate and sodium sulphate can also be carried out by as described in German patent DE 30198. Here, solid ammonium sulphate is intensively mixed with sodium sulphate in a 1 :1 molar ratio, or using a slight excess of sodium sulphate. This mixing can be done by ball milling. Alternatively, the ammonium sulphate and sodium sulphate are dissolved in water to prepare a homogeneous solution in a 1 : 1 molar ratio or with a slight excess of sodium sulphate, and the solution is evaporated to dryness. The dry solid mixture is heated to a temperature between 350 and 370 °C to convert the ammonium sulphate and sodium sulphate into sodium hydrogen sulphate and ammonia. The reaction can be carried out in the absence or presence of superheated steam, or in the presence of a preheated air or inert gas stream. This reaction is less preferred than the above described hydrolysis reaction in aqueous solution, because of the much higher temperatures and the corrosiveness of the molten salt mixture.

[0130] Step (iv) is the actual sulphuric acid recovery step in which sodium hydrogen sulphate is split into sulphuric acid and sodium sulphate, and the sodium sulphate recovered is reused in step (iii). A concentrated solution of sodium hydrogen sulphate in water is prepared by dissolving the salt in water at 80 to 100 °C, the solution is cooled to a temperature below 0 °C, and solid sodium sulphate decahydrate, Na2SO4-10H2O, crystallises out, leaving sulphuric acid in the mother liquor. Hence advantage is taken by the following equilibrium in water:

[0131] 2NaHSO4+ XH2O ±5 Na2S0410H201 + H2SO4+ (x-10)H2O

[0132] The equilibrium is shifted to the right by forcing sodium sulphate to crystallise. Sulphuric acid remains in solution and crystals of sodium sulphate can be recovered. This method is based on the unusual solubility behaviour of sodium sulphate in water. Below 32.384 °C, sodium sulphate crystallises as the decahydrate Na2SO410H2O (Glauber's salt or mirabilite), and above 32.384 °C as anhydrous Na2SO4(thenardite). Sodium sulphate has a maximum solubility at this transition point of 32.384 °C. Glauber's salt shows a very strong temperature-dependence for its solubility in water: at 0 °C its solubility is about 10 times less than at 32.384 °C. Expressed in terms of anhydrous salt Na2SO4, its solubility is 4.31 wt% at 0 °C and 33.2 wt% at 32.384 °C. Whereas the eutectic temperature of the binary system Na2SO4-H2O is -1.2 °C, the eutectic temperature of the ternary system Na2SO4-H2SO4-H2O is much lower, well below 0 °C (although the exact value seems to be still unknown). The presence of sulphuric acid further depresses the solubility of Na2SO410H2O. As reported in patent GB 127677, when aqueous solutions comprising sodium sulphate, sulphuric acid and water are cooled to a temperature below 0 °C, crystals of Na2SO410H2O, acidic sodium sulphates or ice can be formed, depending on the temperature and acidity of the solution. The process variables are chosen so that the crystallisation of Na2SO410H2O is favoured over that of acidic sodium sulphates. The solution should not exceed a certain threshold acid concentration, because otherwise there is the risk of crystallisation of acidic sodium sulphates. When sodium sulphate crystallises as Na2SO410H2O from an acidic solution of sodium sulphate, prepared by dissolving sodium hydrogen sulphate in water, for each mole of Na2SO4that crystallises in the form of Na2SO410H2O, ten moles of water are removed from the solution. Hence, the acidity of the solution, and thus the sulphuric acid concentrations, increases steadily with crystallisation of Na2SO410H2O. Removal of water from a solution via water of crystallisation of hydrated salts is an efficient way of concentrating aqueous solutions, without the need of evaporating the water. The temperature range of interest for crystallisation of Na2SO410H2O is between 0 °C and -40 °C. Therefore, by preparing a concentrated solution of sodium hydrogen sulphate in water, followed by cooling of the solution to a temperature below 0 °C, crystallisation of a large portion of Na2SO4-10H2O can be caused, leaving behind an aqueous solution of sulphuric acid, that is steadily increasing in concentration. The crystallisation can be enhanced by addition of seed crystals to the cold solution. Alternatively, a stream of cold air or cold nitrogen gas can be blown in the solution. The crystals of Na2SO4-10H2O can be separated from the mother liquor by centrifugation. Optionally, the crystals can be washed by a Glauber's salt solution. These crystals can be reused directly in the previous step for converting ammonium sulphate into sodium hydrogen sulphate. The Na2SO4-10H2O crystallisation can be carried out by techniques that are commonly used in the field of freeze crystallisation [Heist (1979) Chemical Engineering 86, 72- 82],

[0133] Although the decahydrate Na2SO4-10H2O is the thermodynamically most stable phase below 32.384 °C in the Na2SO4-H2O binary phase diagram, there exists only a metastable heptahydrate Na2SO4-7H2O. Therefore, it is theoretically possible that not Na2SO4-10H2O, but rather Na2SO4-7H2O or a mixture of Na2SO4-10H2O and Na2SO4-7H2O crystallise out during the freeze crystallisation of a concentrated NaHSO4 solution in water. After removal of the first crop of Na2SO4-10H2O crystals, the solution can be subjected to further cooling in order to recover more Na2SO4-10H2O. The maximum concentration of sulphuric acid that can be obtained is between 20 and 30 wt%, depending on the temperature. The mother liquor remaining after removal of the Na2SO4-10H2O contains an aqueous solution of sulphuric acid, with a small amount of dissolved sodium sulphate. The sulphuric acid has a purity that is high enough for reuse in hydrometallurgical processes.

[0134] Alternatively, step (iv) can be carried out by solvent extraction. Here the equilibrium of the splitting of sodium hydrogen sulphate into sodium sulphate and sulphuric acid is shifted to the right by removal of sulphuric acid from the equilibrium mixture, for instance by solvent extraction. The preferred solvent composition is the tertiary amine tris(2-ethylhexyl)amine (TEHA) and 1-octanoL 1-Octanol acts both as phase modifier and diluent, and is therefore called an "active" diluent. The organic solvent comprises between 0 and 50 vol.% TEHA, between 0 and 50 vol.% 1-octanol, and the third component being an aliphatic diluent. The optimal composition of the solvent is 50 vol% TEHA and 50 vol% 1-octanol. The extraction is carried out preferably at room temperature (20 to 25 °C). The extracted sulphuric acid can be stripped from the loaded organic phase by hot water of a temperature that is at least 20 °C higher than the temperature at which the extraction was performed, but preferably 35 °C higher. Therefore, when the extraction of sulphuric acid is performed at 25 °C, stripping with water of 60 °C is recommended. However, the tertiary amine TEHA can be replaced by other tertiary amines such as Alamine 336, trioctylamine (TOA) or isooctylamine. Primary or secondary amines can be used instead of tertiary amines, but are less preferable. Alternatives for the amines are other basic extractants such as the commercial mixture of trialkylphosphine oxides Cyanex 923, trioctylphosphine oxide (TOPO) or tri-n-butyl phosphate (TBP). 1-Octanol can be partly or fully replaced by another higher alcohols such as 1-pentanol, 1-hexanol, 2-ethylhexanol, 1- heptanol, 2-octanol, isooctanol, 1-nonanol, 1-decanol, isodecanol. Besides the extractant and the alcohol, the solvent can comprise an aromatic, aliphatic or a mixed aromatic-aliphatic diluent. It is recommended to carry out the solvent extraction in batch mode in a stirred reactor, because Na2SO4-10H2O might crystallise out during the extraction and it is difficult to treat a slurry in mixer-settlers, especially in countercurrent mode. However, solvent extraction with simultaneous formation of a solid is known to a person skilled in the art since this situation is similar to what occurs during stripping precipitation of a solid from a loaded organic phase by a precipitation reaction. After solvent extraction, the loaded organic phase is separated from the liquid and from the precipitated solids, and the loaded organic phase is subjected to a stripping operation with hot water (60 °C or higher). The stripping can be carried out in mixer-settlers in continuous countercurrent mode. Alternatively, the stripping can be carried out in other types of contactors, for instance centrifugal contactors or an extraction column. In order to obtain a more concentrated sulphuric acid solution, the stripping is carried out with an organic-to-aqueous (O / A) phase ratio that is as high as possible.

[0135] Optionally, if no solvent extraction is used in step (iv), the sulphuric acid obtained in that step is purified by extraction from the mother liquor using an organic solvent, followed by stripping of sulphuric acid from the loaded organic phase with hot water. This solvent extraction is similar to that performed in step (iv). The extraction is preferably done at room temperature, and the stripping of the acid from the loaded organic phase with hot water is done at a temperature of 60 °C or higher. The preferred solvent is the tertiary amine with a branched alkyl chain, tris(2- ethylhexyl)amine (TEHA), in 1-octanoL Since in this case there is not risk of precipitation of Na2SO4-10H2O during the extraction, both the extraction and stripping steps can be carried out in mixer-settlers in continuous countercurrent mode. For many applications, the concentration of the sulphuric acid obtained from the mother liquor after crystallisation of Na2SO4-10H2O is sufficiently high for direct reuse, for instance as a sulphuric acid lixiviant in hydrometallurgical processes. If a higher concentration is needed, the concentration of sulphuric acid can be further increased by evaporating water until the desired concentration is achieved. This process can be carried out in a concentrator (evaporator), where the necessary sensible and latent heat is ideally supplied by either steam (in a steam concentrator) or electricity (in an electrical concentrator). Typically, the concentration process is done in two stages. In the preconcentration stage, the concentration of the feed stream is increased to approximately 85 wt% through evaporation under atmospheric pressure and vacuum, using a horizontal, falling film, forced circulation evaporator. Following preconcentration, the sulphuric acid is further concentrated in a steam-heated horizontal evaporator or a forced circulation evaporator made of tantalum, to achieve final concentrations of up to 98 wt% sulphuric acid. The evaporation process is conducted at reduced pressure to lower the boiling temperature sufficiently, allowing the use of steam as a heating medium. Operating at a lower temperature also reduces the risk of corrosion. To enhance the efficiency of the evaporation process, a multipleeffect evaporator is recommended. The diluted feed acid can be preheated with hot concentrated acid in a heat exchanger to improve the energy efficiency of the process. For concentrating smaller quantities of sulphuric acid, an electrically heated vessel concentrator, which operates under high vacuum, can be used. Alternative techniques for concentrating sulphuric acid include vacuum membrane distillation or spray evaporation.

[0136] The invention discloses a novel method for the recovery of sulphuric acid and ammonia from ammonium sulphate, (NFU^SC . Compared to the prior art, the invention offers the several advantages. It is carried out at lower temperatures than other processes. The maximum process temperature corresponds to the boiling point of a saturated aqueous solution of a mixture of ammonium sulphate and sodium sulphate. The boiling points of the saturated solution of the individual salts are 108.9 °C for ammonium sulphate and 102 °C for sodium sulphate. Therefore, the maximum temperature in this process step is less than 110 °C. This is much lower than the temperatures of 350 °C or higher for the conversion of ammonium sulphate into ammonium hydrogen sulphate via the convention thermal conversion process, and much lower than the high temperature processes that decompose ammonium sulphate into NHs and SO3 gases. The process temperature can be reached by using industrial high-temperature heat pumps. The waste heat of the cooling process for the crystallisation of Glauber's salt from an aqueous solution of sodium hydrogen sulphate could be recovered and used to heat the solution for the ammonium sulphate to sodium hydrogen sulphate conversion process. The process described in this invention also avoids the use of highly corrosive ammonium hydrogen sulphate melts. The process can generate sulphuric acid solutions with a concentration of 25 wt%, which is about 3 mol / L. This is sufficient for most applications in hydrometallurgy. If sulfuric acid concentrations with a higher concentration are needed, conventional methods for concentrating sulfuric acid solutions can be applied.

[0137] The hydrogen carbonate, carbonate, or basic carbonate salts recovered in step (i) can be used in the same form as they have been obtained, or they can be further processed into other inorganic chemicals. The carbonates or basic carbonates of the alkaline earth metals can be heated to decompose them into the corresponding oxides and carbon dioxide. This process is known as calcination. For instance, calcium carbonate can be decomposed into calcium oxide and carbon dioxide. Calcium oxide (quicklime) can be reacted with water to form calcium hydroxide (lime or slaked lime), and limewater can be produced from the said calcium hydroxide. Magnesium carbonate can be decomposed into magnesium oxide and carbon dioxide.

[0138] Sodium hydrogen carbonate can be converted into sodium carbonate and carbon dioxide by heating to a temperature between 160 and 180 °C:

[0139] 2 NaHCO3- Na2CO3+ H2O + CO2

[0140] The carbon dioxide released in these calcination steps is recovered and reused in step (i) . Optionally, the sodium carbonate obtained from sodium hydrogen carbonate is converted into sodium hydroxide by the reaction of the sodium carbonate with slaked lime in water (caustification or causticisation) :

[0141] Na2CO3+ Ca(OH)2- CaCO3+ 2NaOH

[0142] The NaOH solution formed in the causticisation reaction is further concentrated by evaporation of water. In this way a concentrated aqueous NaOH solution is obtained. Alternatively, NaOH can be crystallised from the concentrated solution. The calcium carbonate formed in this reaction is reconverted into quicklime and carbon dioxide by calcination at a temperature of 900 °C or higher, and the carbon dioxide is recycled to step (i):

[0143] CaCOs - * CaO + CO2

[0144] Alternatively, the Lowig process can be used to convert sodium carbonate to sodium hydroxide (Patents DE 21593 and US 274,619). Here, solid sodium carbonate is heated with solid iron(III) oxide to red heat, with formation of sodium ferrite (NaFeO?) and release of carbon dioxide. After cooling, the sodium ferrite is leaching with hot water at a temperature of around 90 °C. The advantage of the Lowig process is that sodium hydroxide solutions are obtained with a higher concentration than in the classical causticisation reaction with slaked lime. It is a cyclic process since the iron(III) oxide used in the process is reformed during the leaching step, and can reused for preparation of a new batch of sodium ferrite. The carbon dioxide released during the synthesis of sodium ferrite can be captured and reused in step (i) of the process.

[0145] Sodium hydrogen carbonate can be converted directly into sodium hydroxide by electrolysis in a two-compartment electrolysis cell, divided by a cation-exchange membrane (membrane-divided electrolysis cell). The anode compartment is filled with an aqueous sodium hydrogen carbonate solution and the cathode compartment with a sodium hydroxide solution. At the cathode, water is decomposed in hydrogen gas and hydroxide ions, whereas at the anode, oxygen gas and protons are formed. The protons react with the hydrogen carbonate ions to form carbonic acid, which dissociates into water and carbon dioxide. Sodium ions migrate through the membrane from the anode to the cathode compartment, combining with the hydroxide ions to form sodium hydroxide. The hydrogen, oxygen, and carbon dioxide gases released are recovered. After the separation of carbon dioxide from oxygen gas, the carbon dioxide is reused in step (i) of the process. The reactions are:

[0146] Cathode: 2H2O + 2e_-» H2(g) + 2OH’(aq)

[0147] Anode: H2O -» 1 / 2 02(g) + 2 H+(aq) + 2e_

[0148] H++ HCO3’ - H2CO3

[0149] H2CO3 - H2O + CO2 The ion-selective membrane separating the cathode and anode compartment is a cation-exchange membrane, such as Nation® perfluorosulphonic acid (PFSA) membranes based on a PFSA / polytetrafluoroethylene (PTFE) copolymer. Examples are Nation® N-117, N-115 or NR.E-212. As anode, a dimensionally stable anode for an oxygen-evolution process (DSA-O2) is used, which typically consists of a mixed metal oxide coating on a titanium substrate. As cathode materials, nickel or nickel- plated steel is used.

[0150] The sodium hydroxide solution produced in the cathode compartment is circulated to concentrate the solution, and it can be further concentrated by evaporation of water.

[0151] The disclosed process has a high circularity. At the optimum process conditions, all the ammonia which has been used to convert the starting metal salts in the ammonium sulphate can be recycled and reused for a new conversion. If the metals are removed from the circuit in the form of hydrogen carbonate, carbonate or basic carbonate salts, there is a net removal of carbon dioxide from the system and this loss of carbon dioxide must be compensated. However, if the hydrogen carbonate, carbonate or basic carbonate salts are converted to the corresponding metal oxides or metal hydroxides, the carbon dioxide released can be captured and reused for the carbonation reaction in the first step of the process. The process of this invention can also be part of a carbon dioxide sequestering process, for instance when CaSC is transformed into CaCOs and MgSC into basic magnesium carbonate or MgCO3-3H2O. In such cases, there is a net consumption of CO2.

[0152] The disclosed process can be extended to the recovery of acids other than sulphuric acid from their sodium salts. For instance, sodium methanesulphonate, CHsSOsNa, can be converted into methanesulphonic acid (MSA), CH3SO3H, by adding sulphuric acid to a solution of sodium methanesulphonate in a 1: 1 molar ratio, followed by cooling the solution to a temperature of 0 °C or lower to crystallise Na2SO4-10H2O. The crystals of Na2SO4-10H2O are filtered off. The remaining solution is an aqueous solution of methanesulphonic acid and can be further concentrated by evaporation of water. The general reaction can be represented by the equation:

[0153] 2CH3SO3Na + H2SO4 + xH2O ±5 Na2S04 10H201 + 2CH3SO3H + (x-10)H2O

[0154] Hydrochloric acid can be recovered from solutions of sodium chloride, and nitric acid can be recovered from solutions of sodium nitrate: 2NaCI + H2SO4+ xH2O ±5 Na2S0410H201 + 2HCI + (x-10)H2O

[0155] 2NaNO3+ H2SO4+ xH2O ±5 Na2S0410H201 + 2HNO3+ (x-10)H2O

[0156] The calcium salt of MSA can be converted indirectly into MSA by reaction with sulphuric acid, with precipitation of calcium sulphate. This calcium sulphate can be converted into ammonium sulphate as described elsewhere in this disclosure.

[0157] APPLICATIONS

[0158] The invention can be used to recover sulphuric acid from waste sulphate salts from the following industrial processes, although the invention is not limited to these metal sulphates or processes:

[0159] • Sodium sulphate generated during the production of cathode active materials (pCAM) for lithium-ion batteries, by reaction of sodium hydroxide with sulphate salts of cobalt, nickel and manganese, in the presence of ammonia as complexing agent;

[0160] • Sodium sulphate produced by hydrometallurgical processes for the recycling of lithium-ion batteries;

[0161] • Sodium sulphate produced during the purification and separation of cobalt and nickel by solvent extraction with acidic extractants in sulphate media;

[0162] • Sodium sulphate produced by the desulphurisation of lead paste with sodium carbonate solution in processes for the recycling of spent lead-acid batteries;

[0163] • Sodium sulphate produced by the conversion of anglesite (PbSO4) into cerussite (PbCO3) by sodium sulphate solution in lead hydrometallurgy;

[0164] • Sodium sulphate generated by precipitation of lithium sulphate by sodium carbonate;

[0165] • Sodium sulphate produced by neutralisation of excess sulphuric acid by sodium hydroxide in industrial processes using sulphuric acid;

[0166] • Sodium sulphate produced by precipitating heavy metals by addition of sodium hydroxide as hydroxides from aqueous sulphate streams in waste water treatment processes (metal hydroxide sludges);

[0167] • Ammonium sulphate produced by conversion of gypsum (CaSO4-2H2O) into calcite (CaCO3) by reaction with an aqueous solution ammonium carbonate, or with carbon dioxide and ammonia in water; • Sodium sulphate waste from boric acid production;

[0168] • Ammonium sulphate produced by treatment of phosphogypsum waste by reaction with an aqueous solution ammonium carbonate, or with carbon dioxide and ammonia in water; • Ammonium sulphate produced by conversion of magnesium sulphate to magnesium carbonate or basic magnesium carbonate by reaction with an ammonium hydrogen carbonate solution, an ammonium carbonate solution, or by carbon dioxide, ammonia and water;

[0169] • Ammonium sulphate generated by oxidative ammoniacal leaching of sulphidic ore concentrates;

[0170] • Ammonium sulphate generated by reductive leaching of deep-sea manganese nodules by sulphur dioxide in the presence of aqueous ammonia;

[0171] • Ammonia sulphate produced by scrubbing gas streams containing ammonia gas with a sulphuric acid solution

Claims

CLAIMS1. A method of producing sulphuric acid and a base from an inorganic sulphate salt, the method comprising the steps of:(i) Converting said inorganic sulphate salt into ammonium sulphate and a hydrogen carbonate, or a carbonate or a basic carbonate salt, using a source of ammonia and carbon dioxide,(ii) Separating the hydrogen carbonate, or carbonate or basic carbonate salt from the ammonium sulphate,(iii) Reacting the ammonium sulphate obtained after step (ii) with sodium sulphate, thereby forming sodium hydrogen sulphate and ammonia gas, and removing the generated ammonia (gas), thereby shifting the chemical equilibrium to the side of the reaction products,(iv) Converting the sodium hydrogen sulphate to sodium sulphate and sulphuric acid, by removing either sodium sulphate or sulphuric acid from the reaction medium, thereby shifting the chemical equilibrium to the side of the products, with the exception that if the inorganic sulphate salt is ammonium sulphate, steps (i) and (ii) are not performed.

2. The method according to claim 1, wherein step (iii) is carried out by heating a solution of ammonium sulphate and sodium sulphate in water to the boiling point.

3. The method according to claim 1, wherein step (iii) is carried out by heating a solid mixture of ammonium sulphate and sodium sulphate at a temperature between 80 and 300 °C.

4. The method according to claim 1, wherein step (iii) is carried out by heating a solid mixture of ammonium sulphate and sodium sulphate at a temperature between 80 and 170 °C.

5. The method according to any one of claims 1 to 4, further comprising the step of extracting sulphuric acid with an organic solvent, and subsequently stripping of sulphuric acid from the organic phase by hot water of a temperature that is at least 20 °C higher than the temperature at which the extraction was performed, but preferably 35 °C higher, or further comprising the step of concentrating aqueous sulphuric acid solution by evaporation of water.

6. The method according to any one of claims 1 to 5, further comprising the step converting the hydrogen carbonate, carbonate or basic carbonate salt produced in step (i) to an oxide or hydroxide.

7. The method according to claim 6, wherein carbon dioxide released in said conversion is recovered.

8. The method according to claim 6, wherein sodium hydrogen carbonate is converted to sodium hydroxide by water electrolysis, or wherein sodium hydrogen carbonate is converted to sodium hydroxide by converting sodium hydrogen carbonate first into sodium carbonate by heating with recovery of carbon dioxide, followed by a causticisation reaction between sodium carbonate and calcium hydroxide in water.

9. The method according to any one of claims 1 to 8, wherein ammonia (gas) formed in step (iii) is recovered.

10. The method according to any one of claims 1 to 9, wherein in step (i) the source of ammonia and carbon dioxide is ammonia and carbon dioxide in water, or wherein in step (i) the source of ammonia and carbon dioxide is ammonium hydrogen carbonate in water.

11. The method according to any one of claims 1 to 10, wherein step (iv) comprises cooling an aqueous solution of sodium hydrogen sulphate to a temperature between 0 and -40 °C, there crystallising sodium sulphate decahydrate (Na2SO4-10H2O), while leaving sulphuric acid in solution, or step (iv) comprises removal of sulphuric acid from the aqueous phase by extraction with an organic solvent, followed by stripping of sulphuric acid from the loaded organic phase by hot water.

12. pThe method according to any one of claims 1 to 11, wherein in step (i) sodium sulphate is converted into ammonium sulphate and sodium hydrogen carbonate by reacting the sodium sulphate with ammonia and carbon dioxide in water.

13. The method according to any one of claims 1 to 11, wherein in step (i) sodium sulphate is converted into ammonium sulphate and sodium hydrogen carbonate by reacting the sodium sulphate with ammonium hydrogen carbonate in water.

14. The method according to any one of claims 1 to 11, wherein in step (i) calcium sulphate or calcium sulphate dihydrate is converted into ammonium sulphate and calcium carbonate by reacting the calcium sulphate or calcium sulphate dihydrate with ammonia and carbon dioxide in water, or in step (i) calcium sulphate or calcium sulphate dihydrate is converted into ammonium sulphate and calcium carbonate by reacting the calcium sulphate or calcium sulphate dihydrate with ammonium carbonate in water.

15. The method according to any one of claims 1 to 11, wherein in step (i) magnesium sulphate is converted into ammonium sulphate and basic magnesium carbonate by reacting the magnesium sulphate with ammonia and carbon dioxide in water, or in step (i) magnesium sulphate is converted into ammonium sulphate and magnesium carbonate by reacting the magnesium sulphate with ammonium carbonate in water.

16. The method according to any one of claims 1 to 11, wherein in step (i) lithium sulphate is converted into sodium sulphate and lithium carbonate by reacting the lithium sulphate with sodium carbonate in water, and converting sodium sulphate into ammonium sulphate and sodium hydrogen carbonate by reacting sodium sulphate with ammonia and carbon dioxide in water, or in step (i) lithium sulphate is converted into lithium carbonate by reacting the lithium sulphate with ammonia and carbon dioxide in water, or in step (i) lithium sulphate is converted into lithium carbonate by reacting the lithium sulphate with ammonium hydrogen carbonate in water, followed by heating the solution to a temperature of at least 80 °C.

17. The method according to any one of claims 1 to 11, wherein in step (i) lead(II) sulphate is converted into ammonium sulphate and lead(II) carbonate by reacting the lead(II) sulphate with ammonium carbonate in water, or wherein in step (i) lead(II) sulphate is converted into sodium sulphate and lead(II) carbonate by reacting the lead(II) sulphate with sodium carbonate in water, and converting sodium sulphate into ammonium sulphate and sodium hydrogen carbonate by reacting sodium sulphate with ammonia and carbon dioxide in water.

Images