Methods and systems for converting metal ion-containing solids to useful products

Abstract

Systems and methods of converting metal ion-containing solids to useful products are disclosed. In some embodiments, the useful products comprise metal hydroxides. In some embodiments, the useful products comprise metal silicate hydrates. Some systems and methods pertain to a continuous production of metal hydroxides. Such continuous production may comprise a solid-liquid extractor and a filtration unit flowingly coupled to an electrochemical cell. Electrolyte may be caused to flow between the solid-liquid extractor, the electrochemical cell and the filtration unit.

Description

METHODS AND SYSTEMS FOR CONVERTING METAL ION-CONTAINING SOLIDS TO USEFUL PRODUCTS Cross-Reference to Related Applications

[0001] This application claims priority from US application No.63 / 626,743 filed 30 January 2024 and entitled METHODS AND APPARATUS FOR CONTINUOUS PRODUCTION OF METAL HYDROXIDES and US application No.63 / 626,692 filed 30 January 2024 and entitled METHODS AND APPARATUS FOR PRODUCING METAL HYDROXIDES AT LOW VOLTAGES USING AN ELECTROCHEMICAL CELL, both of which are hereby incorporated herein by reference for all purposes. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of US application No.63 / 626,743 filed 30 January 2024 and entitled METHODS AND APPARATUS FOR CONTINUOUS PRODUCTION OF METAL HYDROXIDES and US application No.63 / 626,692 filed 30 January 2024 and entitled METHODS AND APPARATUS FOR PRODUCING METAL HYDROXIDES AT LOW VOLTAGES USING AN ELECTROCHEMICAL CELL both of which are hereby incorporated herein by reference for all purposes. Field

[0002] The invention pertains to methods and systems for converting metal ion- containing solids to useful products, such as metal hydroxides and metal silicate hydrates. Background

[0003] The cement industry contributes approximately 8% of global CO2 emissions, largely due to the carbon-intensive process of producing cement clinker, the main component of Portland cement. These emissions are due to the decomposition of limestone (CaCO3) releasing stoichiometric amounts of CO2 (equation (1)), and because kilns, heated by fossil fuels, operate at temperatures exceeding 900 ºC and 1,450 ºC to decompose limestone and form clinker (2CaO·SiO2and 3CaO·SiO2; equations (2) and (3)), respectively.Limestone calcination (>900 ºC):CaCO3(s) → CaO(s) + CO2(g)(1)Clinker formation (>1,450 ºC):2CaO(s) + SiO2(s) → 2CaO^SiO2(s)(2)3CaO(s) + SiO2(s) → 3CaO^SiO2(s)(3)

[0004] The inventors have recognised a general need for improved systems and methods for converting metal ion-containing solids to useful products. There is a need for sustainable (higher energy efficiency and lower output of CO2) methods and apparatus for producing cement clinkers. The inventors have also recognised a general need for improved methods and systems for converting metal ion-containing solids to useful products such as metal hydroxides and metal silicate hydrates at high efficiency and low production costs and low carbon emission levels. Summary

[0005] This application has a number of aspects. These include, without limitation: ● systems and methods for producing metal hydroxides; ● systems and methods for producing of metal silicate hydrates; ● systems and methods for the continuous production of metal hydroxides; ● systems and methods for the continuous production of metal silicate hydrates; ● systems and methods for producing cement clinker precursors; ● systems and methods for producing cement clinker; ● systems and methods for converting waste cement to calcium hydroxides (Ca(OH)2) with high current density (e.g., of up to about 300 mA cm-2) and which high product yield (e.g., about or more than 80%), high product purity (e.g., 90% m / m) and low carbon dioxide emission levels can be achieved; ● systems and methods for converting metal ion-containing solids to metal hydroxides with relatively low applied potential at a relatively high current density, and with high Faradaic efficiency; ● systems and methods for converting waste cement to metal silicate hydrates(e.g., 2CaO·SiO2 and 3CaO·SiO2) with high current density (e.g., about or more than 80%), high product yield (e.g., >90%) and low carbon dioxide emission levels can be achieved; ● systems and methods for converting metal ion-containing solids to metal silicate hydrates with relatively low applied potential at a relatively high current density, and with high Faradaic efficiency; etc.

[0006] One aspect of the invention pertains to systems and methods for continuous production of metal hydroxides. The system comprises an electrochemical cell fluidly connected to an extractor and a filtration unit. The electrochemical cell is configured to electrolytically generate hydrogen ions (H+) for reaction with metal ion-containing solids to form metal ions, and to electrolytically generate hydroxide ions (OH-). The hydroxide ions react with the metal ions to form metal hydroxides. A flow of electrolyte solution is caused to circulate between the electrochemical cell, the extractor and the filtration unit.

[0007] One non-limiting example application is in cement production. The described systems and methods may use waste cement as the feedstock, thereby advantageously bypassing the need for virgin limestone as is conventionally used as the feedstock in the production of cement clinkers. Bypassing the need for virgin limestone as the feedstock could desirably reduce global carbon dioxide emissions while additionally assists to divert substantial waste cement from landfills.

[0008] Proof of concept experiments have demonstrated that a 99.8% reduction of carbon dioxide (CO2) emissions and 80% reduction of carbon dioxide emissions may be achieved by the continuous production of metal hydroxides using fresh waste cement and aged cement (enriched with absorbed carbon dioxide) respectively as feedstock in the systems and methods described herein. Calcium hydroxide product yields of greater than 80% and product purity of greater than 90% m / m may be achieved with a current density of up to about 300 mA cm-2.

[0009] Other aspects of the invention pertain to systems and methods for converting metal ion-containing solids to metal hydroxides with relatively low applied potential at a relatively high current density, and with high Faradaic efficiency.

[0010] In such aspects, the methods of converting metal ion-containing solids involve the use of a redox shuttle molecule, such as but is not limited to a(hydro)anthraquinone.

[0011] In some embodiments of the method, a shuttle molecule undergoes a reduction reaction at the cathode to produce a hydrogenated shuttle molecule and hydroxide ions. A hydrogenated shuttle molecule undergoes an oxidation reaction at the anode to produce a shuttle molecule.

[0012] Hydrogen ions are produced in the cell. In some embodiments, the hydrogen ions are electrolytic dissociation of water at the bipolar membrane. In some embodiments, the hydrogen ions are produced at the anode.

[0013] Metal ion-containing solids may be supplied to the cell. In embodiments in which a chemical reaction chamber is provided, the metal-ion containing solids are supplied to the chemical reaction chamber. In some embodiments, the metal ion- containing solids are supplied to the anode chamber.

[0014] The metal ion-containing solids react with the hydrogen ions produced in the cell to yield metal ions. The metal ions are removed from the cell. The hydroxide ions formed at the cathode are removed from the cell. The metal ions and the hydroxide ions react to yield metal hydroxides.

[0015] Proof of concept experiments have demonstrated that a Faradaic efficiency of 100% may be achieved with an applied potential of 0.38 V at a current density of about 100 mA cm-2in the conversion of metal ion-containing solids to metal hydroxides by engaging the reversible redox activity of a redox shuttle molecule to mediate oxidation and reduction in the electrochemical cell.

[0016] In some embodiments, the electrical potential applied between the cathode and the anode does not exceed 5 V. In some embodiments, the electrical potential applied between the cathode and the anode does not exceed 2.5 V. In some embodiments, the current density is maintained at 100 mA cm-2or greater. In some embodiments, the current density is maintained at 500 mA cm-2or greater.

[0017] Other aspects of the invention pertain to systems and methods for producing metal silicate hydrates. One non-limiting example application is in the production of calcium silicate hydrates from limestone. Another non-limiting example application is in the production of calcium silicate hydrates from waste cement. Another non-limiting example application is in the production of calcium silicate hydrates from raw mineral calcium silicates. Another non-limiting example application is in the production ofcalcium silicate hydrates from gypsum. A further non-limiting example application is in the production of calcium silicate hydrates from calcium-containing industrial and mine waste.

[0018] In some embodiments of the method, a hydrogen-containing reactant such as water undergoes a reduction reaction at the cathode to produce hydroxide ions. An oxidation reactant undergoes an oxidation reaction at the anode to form an oxidation product. Hydrogen ions are produced in the cell. In some embodiments, the hydrogen ions are formed at the bipolar membrane. In some embodiments, the hydrogen ions are formed at the anode. Metal ion-containing solids may be supplied to the cell. The metal ion-containing solids react with the hydrogen ions produced in the cell to form metal ions. Silicate-containing compounds may be supplied to the cell. The silicate- containing compounds react with the hydroxide ions produced at the cathode to yield silicate ions.

[0019] The metal ions and the silicate ions may be combined to yield metal silicate hydrates, such as those represented by the generic formula MXaSibOc · NH2O, wherein M and N is each an integer greater than or equal to 0; a, b, and c is each a number greater than or equal to 0 including a number that is greater than 0 and less than n where n is a positive number; X is any metal ion such as but is not limited to Ca2+, Mg2+, etc. In some embodiments, metal hydroxides are additionally produced.

[0020] Further aspects and example embodiments are illustrated in the accompanying drawings and / or described in the following description.

[0021] It is emphasized that the invention relates to all combinations of the above features, even if these are recited in different claims. Brief Description of the Drawings

[0022] The accompanying drawings illustrate non-limiting example embodiments of the invention.

[0023] FIG.1A is a schematic diagram illustrating a system for continuous production of metal hydroxides, showing reactions that may occur in the electrochemical cell according to an example embodiment of the invention.

[0024] FIG.1B is a schematic diagram illustrating a system for continuous production of metal hydroxides, showing reactions that may occur in the electrochemical cellaccording to another example embodiment of the invention.

[0025] FIG.1C is a schematic diagram illustrating a system for continuous production of metal hydroxides, showing reactions that may occur in the electrochemical cell according to another example embodiment of the invention.

[0026] FIG.2 is a flow chart illustrating the steps of a method for continuous production of metal hydroxides according to an example embodiment of the invention.

[0027] FIG.3A is a schematic diagram illustrating a system for production of metal hydroxides using redox shuttle molecules, showing reactions that may occur in the electrochemical cell according to an example embodiment of the invention.

[0028] FIG.3B is a schematic diagram illustrating a system for production of metal hydroxides using redox shuttle molecules, showing reactions that may occur in the electrochemical cell according to another example embodiment of the invention.

[0029] FIG.4 is a flow chart illustrating the steps of a method of production of metal hydroxides using redox shuttle molecules according to an example embodiment of the invention.

[0030] FIG.5A is a schematic diagram illustrating a system for producing metal silicate hydrates, showing reactions that may occur in the electrochemical cell according to an example embodiment of the invention.

[0031] FIG.5B is a schematic diagram illustrating a system for producing metal silicate hydrates, showing reactions that may occur in the electrochemical cell according to another example embodiment of the invention.

[0032] FIG.5C is a schematic diagram illustrating a continuous system for producing metal silicate hydrates according to an example embodiment of the invention.

[0033] FIG.6 is a flow chart illustrating the steps of a method of producing metal silicate hydrates according to an example embodiment of the invention.

[0034] FIG.7 is a schematic diagram illustrating a system for producing metal silicate hydrates according to another example of the invention.

[0035] FIG.8 is a schematic diagram of a cement recycler used in the Examples.

[0036] FIG.9 illustrates the effect of flow rate on pH and proton efficiency (PE) at 200 mA cm–2.FIG.9a is a schematic of pH sampling points: pHin, calcium, pHout, calcium, and pHin, lime. FIGs.9b, d, and f are plots which illustrate theoretical (calculated from Ksp and [Ca2+]) and actual (calculated from pH values) PE as a function of electrolyte flowrate. FIG.9c, e, and g illustrate pH dependence on the flow rate of the closed-loop electrolyte within the cement recycler.

[0037] FIG.10a is a plot of voltage measured as a function of current density of the cement recycler. FIG.10b illustrates the concentration of Ca2+in the closed-loop electrolyte, exiting the chemical chamber and before entering the calcium extractor, as measured using Inductively Coupled Plasma Optical Emission Spectroscopy during electrolysis at 200 mA cm–2. FIG 10c shows cell voltages (corresponding to the left y axis) and Ca2+concentration of the closed-loop electrolyte within the reactor at the outlet of the calcium extractor ([Ca2+]out, calcium, corresponding to the right y axis) from the cement recycler over time at 100 mA cm–2.

[0038] FIG.11a shows CO2concentrations ([CO2]calcium extractor) evolved from the calcium extractor during the electrolysis of fresh waste cement (St. Marys), aged waste cement, and limestone at current densities of 100 mA cm–2, 200 mA cm–2, and 300 mA cm–2with N2 as the carrier gas (flow rate = 200 sccm). Fig.11b are X-ray diffraction patterns of the solid Ca(OH)2 product isolated from the lime extractor after electrolyzing various fresh waste cements, aged waste cement, and natural limestone at 200 mA cm–2for 2 h.

[0039] FIG.12 is a schematic description of experimental streamline for the 2- Chamber Electrolyzer as discussed in the Examples. The electrochemical reaction was first performed to dissolve CaCO3(s) and produce OH–(Phase 1). The electrolytes were mixed to precipitate Ca(OH)2(s)in a Calcium Reactor (Phase 2). Finally, the Ca(OH)2(s) was isolated and the electrolyte was reused.

[0040] FIG.13a is a plot of voltage measured as a function of current density using aqueous or H2O / MeCN (4:1) mixed electrolytes set at 60°C for the 2-Chamber Electolyzer. The Control and 3-Chamber Electrolyzers used aqueous electrolytes at 20 ºC in the Examples. Fig.13b is a plot of voltage measured as a function of electrolyte temperature for electrolysis experiments performed at 100 or 1000 mA cm–2using either an aqueous or H2O / MeCN electrolyte. FIG.13c is a plot showing calculated Faradaic efficiency (FE) of i-CO2(g) from the anode chamber in a 2- Chamber Electrolyzer and the chemical chamber in a 3-Chamber Electrolyzer after 5 min of reaction at 100 mA cm–2.

[0041] FIG.14 illustrates electrochemical performance and product characterizationof the 2-Chamber Electrolyzer with a H2O / MeCN (4:1) mixed electrolyte at 60 ℃. FIG. 14a illustrates tracked pH changes in the electrolytes over a 30 min reaction campaign. FIG.14b illustrates voltage measured on the 2-Chamber Electrolyzer after 5 min of electrolysis during Phase 2 for five successive Cycles. The same electrolyte was used for all Cycles.

[0042] FIG.15 is a plot of electrolyzer voltages recorded as a function of time at 100 mA cm–2. The flow rate of the (H2)AQ electrolytes was slowed to 100 mL min–1to reduce the deformation of the CEM in long-term experiment. (Reaction conditions: 3- Chamber Electrolyzer, solvent: H2O at 20°C; 2-Chamber Electrolyzer, solvent, H2O at 60°C; 2-Chamber Electrolyzer, H2O / MeCN (4:1) at 60°C).

[0043] FIG.16 is a scanning electron microscopy image of calcium silicate hydrates (CSH, MCaaSibOc·NH2O(s)) produced by the cement electrolyzer used in the Examples.

[0044] FIG.17 are scanning electron microscopy images of belite (Ca2SiO4(s)) (left) and alite (Ca3SiO5(s)) (right) derived from calcining the calcium silicate hydrates (CSH, Ca2SiO4·H2O(s)) and hydrated lime (Ca(OH)2(s)).

[0045] FIG.18 illustrates Barrett-Joyner-Halenda (BJH) differential pore size distribution analysis (left) and (right) Braunauer-Emmett-Teller (BET) linear N2 sorption isotherm of calcium silicate hydrates (CSH, Ca2SiO4·H2O(s)) products prepared with the cement electrolyzer used in the Examples. BJH desorption cumulative pore volume: 0.218276 cm3 / g. BJH Desorption average pore width: 7.6153 nm. BET specific surface area of CSH: 91 m2 / g.

[0046] FIG.19 is a differential scanning calorimetry thermogram and trace of the derivatives of the weight percent of sample per temperature for calcium silicate hydrates (CSH, Ca2SiO4·H2O(s)) and hydrated lime (Ca(OH)2(s)) prepared with the cement electrolyzer used in the Examples, resulting in the formation of belite (Ca2SiO4(s)).

[0047] FIG.20 is plot of measured voltage as a function of current density at 60°C for the cement electrolyzer used in the Examples processing both calcium-rich feedstock and silica-rich feedstock to prepare calcium silicate hydrates (CSH, Ca2SiO4·H2O(s)) products.

[0048] FIG.21 show ex-situ X-ray diffractograms of reactive cement precursors with aCa:Si ratio of 3:1 (1 Ca2SiO4·H2O(s) : 1 Ca(OH)2(s)). This cement precursor enriched with calcium silicate hydrates (CSH, Ca2SiO4·H2O(s)) was calcined in an electric oven from 400 °C to 1,500 °C.

[0049] FIG.22 is a plot illustrating relative content (wt%) of lime (CaO(s)), belite (Ca2SiO4(s)), and alite (Ca3SiO5(s)) from the reaction of calcium silicate hydrates (CSH, Ca2SiO4·H2O(s)) and calcium hydroxide (Ca(OH)2(s)) calcined between 600 °C and 1,500 °C, as determined by Rietveld Quantitative analysis.

[0050] FIG.23 is an X-ray diffractogram of magnesium silicate hydrate product prepared with the cement electrolyzer used in the Examples processing magnesium carbonate and silica as feedstocks. X-ray diffractogram indicates the presence of forsterite (Mg2SiO4(s)) (36.5%), MgSiO3(25.9%), and enstatite (Mg2Si2O6(s)) (37.6%). Detailed Description

[0051] Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive sense. Definitions

[0052] “Bipolar membrane” or “BPM” is a membrane comprising plural layers including an anion exchange layer on one side and a cation exchange layer on another side. A bipolar membrane may comprise one or more layers between the anion exchange layer and the cation exchange layer. For example, an intermediate layer may comprise a catalyst which facilitates dissociation of water into protons and hydroxide ions. The anion exchange layer may conduct hydroxide ions. The cation exchange layer may conduct protons. An example bipolar membrane is Fumasep FBM™ available from FUMATECH BWT GmbH.

[0053] “Cation exchange membrane” or “CEM” is a membrane that is selectively permeable to cations. An example cation exchange membrane is NafionTM.

[0054] “Microporous polymer membrane” is a membrane that is made of a polymericatmospheric pressure in Pa, R the ideal gas constant of 8.314 J / mol K and T the temperature in Kelvin.

[0059] “Flow cell” refers to an electrochemical cell in which a catholyte and / or anolyte are flowed through the cell while the cell is in operation. A non-limiting example construction of a flow cell provides flow plates separated by an MEA. An anode flow plate is located at the anode side of the MEA and a cathode flow plate is located at the cathode side of the MEA. The anode and cathode flow plates comprise flow channels that respectively receive an anode feed and a cathode feed. A power supply is connected across the anode and cathode of the MEA in the flow cell to drive oxidation reactions at the anode and reduction reactions at the cathode.

[0060] “Ion exchange membrane” is a membrane that has a significantly higher permeability for certain dissolved ions than for other ions. Ideally an ion exchange membrane would pass ions of a selected species or type (e.g. cations or anions) while blocking other ions. A CEM is an example of an ion exchange membrane.

[0061] “Oxygen evolution reaction” or “OER” is the process of generating molecular oxygen by an electrochemical reaction. An example of an oxygen evolution reaction is the oxidation of hydroxide, in accordance with Equation 5. 4OH-(aq) + 4e- → 2H2O(l) + O2(g) (Eq.5)

[0062] “Hydrogen oxidation reaction” or “HOR” is an electrochemical reaction that involves the oxidation of hydrogen molecules. The reaction products depend on the environment in which the reaction occurs. The environment may be acidic or alkaline. An example of a hydrogen oxidation reaction is the oxidation of hydrogen gas, in accordance with Equation 6. H2(g) → 2H+(aq) + 2e- (Eq.6)

[0063] “Hydrogen evolution reaction” or “HER” is the process of producing hydrogen by an electrochemical process. An example of a hydrogen evolution reaction is the reduction of water, in accordance with Equation 7.2H2O(l) + 2e- → H2(g) + 2OH-(aq) (Eq.7)

[0064] “Hydrogenation” includes any reaction between hydrogen atoms (H) or hydrogen molecules (H2) and a reactant. Hydrogenation includes reactions which result in a hydrogen atom being added to a reactant to form a product of the reaction. For example, a hydrogenation reaction may reduce a double or triple bond in a hydrocarbon. One example of a hydrogenation reaction is adding hydrogen atoms to a shuttle molecule such as a suitable quinone compound to yield a hydrogenated shuttle molecule. Another example of a hydrogenation reaction is a reaction which adds hydrogen atoms to oxygen molecules to yield hydrogen peroxide.

[0065] “Shuttle molecules” are molecules of a chemical compound which has reversible redox chemistry. A shuttle molecule is a compound that can be reversibly oxidized and reduced. In some embodiments, a shuttle molecule is an organic compound such as an unsaturated organic compound. In some embodiments, the shuttle molecule is a quinone compound or a quinone derivative. “Quinone” is any member of a group of compounds which comprises an unsaturated benzene ring to which two oxygen atoms are bonded as carbonyl groups (i.e., a functional group composed of a carbon atom bonded to an oxygen atom by a double bond). Any suitable quinone compound or quinone derivatives may be used as a shuttle molecule. The quinone may be non-substituted, or substituted with one or more functional groups such as hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl groups. Non-limiting examples of suitable quinones that may be used as a shuttle molecule include benzoquinones, diaziquone, indolequinone, naphthoquinones, anthraquinones, polyquinones, and their derivatives thereof. In one example embodiment, shuttle molecule 36 is a tert-butyl-anthraquinone. Other suitable compounds that can be used as a shuttle molecule include but are not limited to aromatic N-heterocycles (phenazines), TEMPO, ferrocene, transition metal complexes, and their derivatives and combinations thereof. The primary structures of these compounds may be modified by for example adding one or more sulfonic and / or carboxylic acids substituents to adjust one or more properties of the compound(e.g., increase solubility, tune reduction potential, improve chemical stability, and modify pKa) to create a compound which may be more suitable for the electrolysis. Systems and methods for continuous production of metal hydroxides from metal-ion containing solids

[0066] Aspects of the invention relate to systems and methods of continuously converting metal-ion containing solids to metal hydroxides. FIGS.1A, 1B and 1C are schematic diagrams that illustrate example systems 10 for such continuous production of metal hydroxides. The system 10 includes an extractor 12 configured to extract metal ions from the metal-ion containing solids and an electrochemical cell 14 configured to electrolytically generate hydrogen ions which may be used to extract metal ions from metal ion-containing solids, and hydroxide ions for reacting the metal ions to yield metal hydroxides. In some embodiments, a filtration unit 16 is provided. The filtration unit 16 may be configured to isolate the metal hydroxide. Electrolyte may be caused to circulate between the electrochemical cell 14, the extractor 12, and / or the filtration unit 16. Example Embodiments

[0067] Referring to FIGS.1A and 1B, the cell 14 comprises an anode chamber 18, a cathode chamber 20, and a chemical chamber 26. A cathode 22 is exposed to the cathode chamber 20. An anode 19 is exposed to the anode chamber 18. A separator 24 separates the chemical chamber 26 and the cathode chamber 20. A separator 28 separates the anode chamber 18 and the chemical chamber 26. In some embodiments, the cell 14 comprises three compartments. Referring to FIG.1C, the cell 14 comprises an anode chamber 18 and a cathode chamber 20. A cathode 22 is exposed to the cathode chamber 20. An anode 19 is exposed to the anode chamber 18. A separator 24 separates the anode chamber 18 and the cathode chamber 20. In some embodiments, the cell 14 comprises two compartments.

[0068] The separator 24 is adapted to block passage of the metal ion in the metal ion- containing solids that is fed to the system 10 for processing. A coating layer may be provided on the separator 24. In some example embodiments, the separator 24 is adapted to block the passage of calcium ions.

[0069] In some example embodiments, the separator 24 comprises an ion exchange membrane such as a cation exchange membrane. In some example embodiments, the separator 24 comprises a polyaniline-coated cation exchange membrane.

[0070] In some embodiments, the cell 14 comprises three compartments. In such embodiments, the cell 14 additionally comprises a chemical reaction chamber 26. In such embodiments, the separator 24 separates the chemical reaction chamber 26 and the cathode chamber 20. A bipolar membrane 28 may separate the anode chamber 18 and the chemical reaction chamber 26. The bipolar membrane 28 is adapted to dissociate water molecules into hydroxide ions 27 and hydrogen ions 29. The hydrogen ions are 29 may permeate through a cation exchange layer 31 into the chemical reaction chamber 26.

[0071] A power source 32 is connected to apply an electrical potential difference between the cathode 22 and the anode 19. A negative electrical charge is applied to the cathode 22. A positive electrical charge is applied to the anode 19. The power source may be configured to maintain a desired electric current between the cathode 22 and the anode 19 and / or to maintain a potential difference between the cathode 22 and the anode 19 at a desired level or in a desired range.

[0072] An oxidation reaction 34 takes place at the anode 19. A reduction reaction 36 takes place at the cathode 22.

[0073] A reservoir comprising a hydrogen-containing reactant 38 may be fluidly connected to supply the hydrogen-containing reactant 38 to the cathode chamber 20. In some embodiments, the hydrogen-containing reactant 38 participates in the reduction reaction 36 to generate hydroxide ions 30.

[0074] In some embodiments, the reduction reaction comprises a hydrogen evolution reaction (HER). In such embodiments, the hydrogen-containing reactant 38 comprises water.

[0075] In some embodiments, a shuttle molecule 40 is supplied to the cathode chamber 20. The shuttle molecule 40 participates in the reduction reaction 36 to yield a hydrogenated shuttle molecule 42 and hydroxide ions 30. The reduction reaction 36 may be performed in the presence of the hydrogen-containing reactant 38. The hydrogen-containing reactant 38 may serve as a source of protons in the reduction reaction 36.

[0076] An oxidation reactant 44 participates in the oxidation reaction 34 to form an oxidation product 46. In some embodiments, the oxidation reaction 34 comprises an oxygen evolution reduction (OER). In such embodiments, the oxidation reactant 44 comprises a solution containing hydroxide ions (OH-). The oxidation product 46 may comprise oxygen gas (O2) and water molecules.

[0077] In some embodiments, the oxidation reactant 44 comprises a hydrogenated shuttle molecule 48. In such embodiments, the hydrogenated shuttle molecule 48 participates in the oxidation reaction 34 to yield a shuttle molecule 52.

[0078] The cathode 22 may comprise any materials suitable for use as an electrode. Such material may, for example comprise a catalyst suitable for driving a hydrogen evolution reaction (HER).

[0079] The anode 19 may comprise any materials suitable for use as an electrode. Such material may comprise a catalyst suitable for driving an oxygen evolution reaction (OER).

[0080] The cathode 22 and / or anode 19 may be a gas diffusion electrode.

[0081] The cathode 22 and / or anode 19 may be made of one or more metal, alloy or a supported metal / alloy catalyst. The metal may be any transition metal, or combination of one or more transition metals. Suitable electrocatalyst that may be incorporated in the cathode 22 may, for example, comprise one or more of C, Pt, Fe, Co, Mo, and combinations thereof. Suitable electrocatalyst that may be incorporated in the anode 19 may, for example, comprise one or more of Pt, Rh, Ir, Ru, Pd, Ni, and combinations thereof.

[0082] The cathode 22 and / or anode 19 may be porous. An example of a porous electrode is an electrode comprising an electrically conductive foam such as a metal foam. In some example embodiments, the cathode 22 and / or anode 19 comprises a layer of porous nickel (Ni) foam. The nickel foam layer may be free-standing or supported (e.g. by other components of a membrane electrode assembly).

[0083] In some embodiments, a suitable anolyte 54 is supplied to the anode chamber 18. In some embodiments, the anolyte 54 comprises a base. In some example embodiments, the anolyte 54 comprises a solution containing hydroxide ions (OH-). In some embodiments, the anolyte 54 comprises the oxidation reactant 44.

[0084] In some embodiments, the anolyte 54 comprises a pH-neutral aqueoussolution. In some embodiments, the anolyte 54 has a pH in the range of from about 6 to about 8. In some embodiments, the anions and / or cations of the anolyte 54 are monovalent. The anions of the anolyte 54 may for example comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-), perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H). The cations of the electrolyte may for example comprise one or more alkali metal ions such as lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+), hydrogen (H+), ammonium (NH4+), hydronium (H3O+), tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

[0085] In some embodiments, a reservoir containing the anolyte 54 and / or the hydrogenated shuttle molecule 48 is connected to supply the anolyte 54 and / or the hydrogenated shuttle molecule 48 to the anode chamber 18.

[0086] A catholyte 56 is supplied to the cathode chamber 20. In embodiments in which a chemical reaction chamber 26 is present, an electrolyte 60 is supplied to the chemical reaction chamber 26. In some embodiments, the electrolyte being supplied to the cathode chamber 20 and the chemical reaction chamber 26 is supplied from the same source (although the electrolyte may be supplied from different sources). In such embodiments, a reservoir containing a supply of an electrolyte 64 is connected to flow between the cathode chamber 20 and the chemical reaction chamber 26 in a closed loop.

[0087] In some embodiments, the catholyte 56 and / or electrolyte 60, 64 are derived from a pH-neutral aqueous solution. In some embodiments, the catholyte 56 and / or electrolyte 60, 64 has a pH in the range of from about 6 to about 8. In some embodiments, the anions and / or cations of the catholyte 56 and / or electrolyte 60, 64 are monovalent. The anions of the catholyte 56 and / or electrolyte 60, 64 may for example comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-), perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H). The cations of the electrolyte may for example comprise one or more alkali metal ions such aslithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+), hydrogen (H+), ammonium (NH4+), hydronium (H3O+), tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

[0088] In some embodiments, a supporting electrolyte comprising the metal ions 76 that are desired to be extracted from the metal ion-containing solid 74 may be provided. Such a supporting electrolyte may be supplied to the cathode chamber 20 at or near the start of the electrolysis to provide a flow of the metal ions to drive an efficient production of metal hydroxides.

[0089] In some embodiments, a first outlet 68 is provided at the cell 14. The first outlet 68 is adapted to discharge a flow of electrolyte 54, 56, 60, 64 out the cell 14. The first outlet 68 may be fluidly connected to an inlet 70 of the extractor 12. The electrolyte 54, 56, 60, 64 discharged from the cell 14 may be caused to flow from the cell 14 to the extractor 12. The electrolyte 54, 56, 60, 64 may be anolyte 54 and / or catholyte 56, and / or electrolyte 60, 64 that has been acidified by the hydrogen ions that are formed at the bipolar membrane 28 and / or in the oxidation reaction 34 at the anode 19.

[0090] In some embodiments, the first outlet 68 is arranged at the chemical reaction chamber 26. In such embodiments, the first outlet 68 is arranged to discharge a flow of the electrolyte 60, 64 out of the chemical reaction chamber 26 into the extractor 12. In some embodiments, the flow of the electrolyte 60, 64 out of the chemical reaction chamber 26 additionally comprises shuttle molecules 40 and / or hydrogenated shuttle molecules 42. In such embodiments, the electrolyte 60, 64 may be reduced shuttle molecule containing electrolyte.

[0091] In some embodiments, the first outlet 68 is arranged at the anode chamber 18. In such embodiments, the outlet 68 is arranged to discharge a flow of the electrolyte 54 out of the anode chamber 18 into the extractor 12. In some embodiments, the flow of the electrolyte 54 out of the anode chamber 18 additionally comprises shuttle molecules 52 and / or hydrogenated shuttle molecules 48. In such embodiments, the electrolyte 54 may be oxidized shuttle molecule containing electrolyte.

[0092] In some embodiments, a reservoir containing metal ion-containing solids 74 is connected to supply the metal-ion containing solids 74 to the extractor 12. Theextractor 12 is configured to extract metal ions 76 from the metal ion-containing solids 74. The metal ions 76 may be dissolved in the electrolyte 54, 60, 64 supplied into the extractor 12, thereby forming a metal-ion-enriched electrolyte 82. An outlet 84 is provided at the extractor 12 for output of the metal-ion-enriched electrolyte 82 out of the extractor 12.

[0093] In some embodiments, the outlet 84 of the extractor 12 is fluidly connected to a first inlet 86 of the cell 14. The first inlet 86 may be arranged at the cathode chamber 20. In such embodiments, the metal-ion-enriched electrolyte 82 is directed to flow out of the extractor 12 to the cathode chamber 20. The metal-ion-enriched electrolyte 82 provides a source of the metal ions 76 to the cathode chamber 20 for reacting with the hydroxide ions 30 generated in the reduction reaction 36 to form metal hydroxides 90.

[0094] The extractor 12 can be any suitable apparatus configured to dissolute one or more components in a liquid solvent. The one or more components may comprise a solid compound, or a mixture comprising one or more solid components. In some embodiments, the extractor 12 is a solid-liquid extractor. A filter is typically arranged in the extractor 12 adapted to separate the solid component from the liquid component. The solid component retains on the filter while the liquid component is caused to flow out of the extractor 12.

[0095] In some embodiments, a second outlet 92 is provided at the cell 14. The second outlet 92 is adapted to discharge a flow of the metal hydroxide 90 out of the cell 14. The second outlet 92 may be adapted to additionally discharge a flow of the electrolyte 54, 56, 60, 64 out the cell 14. The concentration of the metal ions 76 in the flow of the electrolyte 54, 56, 60, 64 that is caused to flow out of the cell 14 at the second outlet 92 is less than the concentration of the metal ions 76 in the metal-ion- enriched electrolyte 82. In some embodiments, the second outlet 92 is arranged at the cathode chamber 20.

[0096] The second outlet 92 may be fluidly connected to an inlet 94 of the filtration unit 16 arranged for output of the metal hydroxide 90 and the electrolyte 54, 56, 60, 64 from the cell 14 to the filtration unit 16.

[0097] In some embodiments, the flow of the electrolyte 54, 56, 60, 64 out of the cell 14 additionally comprises shuttle molecules 40 and / or hydrogenated shuttlemolecules 42.

[0098] The filtration unit 16 is configured to isolate the metal hydroxide 90. The filtration unit 16 may be configured to separate the metal hydroxides 90 from the liquid 100. The liquid 100 may comprise the anolyte 54 and / or electrolyte 56, 60, 64. The liquid 100 may additionally comprise the shuttle molecules 40 and / or hydrogenated shuttle molecules 42 if present. The liquid 100 is free or substantially free of metal hydroxides 90.

[0099] The filtration unit 16 can be any suitable apparatus configured to separate desired one or more components, here metal hydroxides, from other components in a mixture. In some example embodiments, the filtration unit 16 comprises a filter. The filter is made of a porous material. The filter is adapted to separate components in a mixture by retaining one or more components on the filter, and allowing one or more other components to pass through the filter.

[0100] A second inlet 96 is provided at the cell 14. The second inlet 96 is fluidly connected to an outlet 98 of the filtration unit 16. The liquid 100 may be caused to flow out of the filtration unit 16 through the outlet 98 into the cell 14 through the second inlet 96.

[0101] In some embodiments, the second inlet 96 is arranged at the chemical reaction chamber 26. In such embodiments, the outlet 98 of the filtration unit 16 is fluidly connected to direct a flow of the aqueous solution 100 to the chemical reaction chamber 26.

[0102] In some embodiments, the second inlet 96 is arranged at the anode chamber 18. In such embodiments, the outlet 98 of the filtration unit 16 is fluidly connected to direct a flow of the aqueous solution 100 to the anode chamber 18.

[0103] One or more gas-liquid separators 102 may be provided between the cell 14 and the extractor 12 and / or filtration unit 16 configured to separate one or more gasses from the electrolyte 54, 56, 60, 64 before supplying the electrolyte 54, 56, 60, 64 into the extractor 12 and the filtration unit 16 respectively. In some embodiments, a first gas-liquid separator 102A is flowingly connected between the first outlet 68 of the cell 14 and the inlet 70 of the extractor 14. In some embodiments, a second gas-liquid separator 102B is flowingly connected between the outlet 98 of the filtration unit 16 and the second inlet 96 of the cell 14. However, any suitable number of gas-liquidseparators 102 between the cell 14 and the extractor 12 and / or filtration unit 16 may be provided.

[0104] Aspects of the invention relate to combining methods and apparatuses for converting metal-ion containing solids to hydroxides with downstream methods and apparatuses for making useful products. The one or more downstream methods and apparatuses may be configured to process the metal hydroxides 90 to yield one or more additional useful products.

[0105] In some embodiments, the metal hydroxides 90 are used in cement production. In some example embodiments, the metal hydroxides 90 are calcium hydroxides. Calcium hydroxides may be used as a cement clinker precursor.

[0106] The metal hydroxides 90 are supplied to a reactor (not shown), for example a kiln. An admixture may be added to the reactor to react with the metal hydroxides 90 to yield cement clinker under heat, for example at a temperature of about 1500oC.

[0107] In some embodiments, one or both oxygen gas and hydrogen gas is produced as gaseous byproducts from oxidation reaction 34 at the anode 19 and reduction reaction 36 at the cathode 22 respectively. In some embodiments, one or both oxygen gas and hydrogen gas are used as reactants in a combustion reaction which produces heat. The produced heat may be used to heat the reactor. The produced heat may be supplied to other downstream methods or apparatuses.

[0108] In some embodiments, the admixture comprises silicon dioxide (SiO2) or silica. Silicon dioxide may react with calcium hydroxides in reactor to form calcium silicates as cement clinker (e.g., 2CaO·SiO2, 3CaO·SiO2). The cement clinker may be further processed to form Portland cement in further one or more downstream apparatuses. Overview of method of continuous production of metal hydroxides

[0109] FIG.2 is a flow chart illustrating the basic steps of a method 500 of continuous production of metal hydroxides from metal ion-containing solids according to one example embodiment of the invention.

[0110] In block 502, an electrical current and / or potential is applied between an anode and a cathode.

[0111] In block 503, a flow of catholyte is supplied to the cathode chamber. In embodiments in which a chemical reaction chamber is provided, a flow of electrolyte60, 64 is supplied to the chemical reaction chamber. In some embodiments, an electrolyte is caused to circulate between the cathode and chemical reaction chambers in a closed loop. A flow of anolyte is supplied to the anode. In another embodiment, an electrolyte is caused to circulate between the cathode and anode chambers in a closed loop. The reduction reactant may be contained in the electrolyte or may be separately supplied to the cathode. The oxidation reactant may be contained in the anolyte or may be separately supplied to the anolyte.

[0112] In block 502, a reduction reactant undergoes a reduction reaction at the cathode to produce hydroxide ions (block 506).

[0113] In block 507, an oxidation reactant undergoes an oxidation reaction at the anode to produce an oxidation product.

[0114] In block 508, hydrogen ions are produced in the cell. In some embodiments, the hydrogen ions are produced by electrolytic dissociation of water at the bipolar membrane. In some embodiments, the hydrogen ions are produced at the anode in the oxidation reaction.

[0115] In block 510, a flow of electrolyte is caused to flow out of the cell to enter an extractor. The electrolyte may be electrolyte that has been acidified by the hydrogen ions formed in the cell.

[0116] In block 512, the acidified electrolyte extracts the metal ion-containing solids in the extractor to yield a metal ion-enriched electrolyte. The metal ion-enriched electrolyte comprises the metal ions.

[0117] In block 514, the metal ion-enriched electrolyte is supplied to the cathode chamber. The metal ions contained in the metal-ion-enriched electrolyte react with the hydroxide ions in the cathode chamber to yield metal hydroxides.

[0118] In some embodiments, a mixture comprising the metal hydroxides are caused to flow out of the cathode chamber to enter a filtration unit (block 516). The metal hydroxides are separated from one or more components contained in the mixture. In some embodiments, the other components comprise the electrolyte. The electrolyte may be free or substantially free of solids, and in particular, metal hydroxides.

[0119] In block 518, the electrolyte is returned into the cell for re-use in subsequent reactions.

[0120] In block 520, The produced metal hydroxides are removed from the filtrationunit.

[0121] The produced metal hydroxides may be supplied to a reactor, such as a kiln for reacting with an admixture such as silicon dioxide (SiO2) to yield cement clinkers. Example methods Continuous production of metal hydroxides using a three-chamber cell

[0122] In some example embodiments, a three-chamber electrochemical cell such as a cell illustrated in FIGS.1A and 1B is used in the continuous production of metal hydroxides from metal ion-containing solids.

[0123] In some embodiments, a hydrogen-containing reactant such as water is supplied at the cathode. The hydrogen-containing reactant undergoes a reduction reaction at the cathode to produce hydroxide ions.

[0124] In some embodiments, the anolyte comprises the oxidation reactant. The oxidation reactant may comprise a base which contains hydroxide ions. The hydrogen ions participate in an oxidation reaction to produce water and oxygen gas.

[0125] In some embodiments, the water molecules permeate into a bipolar membrane within which the water molecules are electrochemically dissociated into hydrogen ions and hydroxide ions. The hydrogen ions permeate through a cation exchange layer of the bipolar membrane to enter a chemical reaction chamber. The hydrogen ions acidify the electrolyte circulated in the chemical reaction chamber.

[0126] The acidified electrolyte may be caused to flow out of the chemical reaction chamber to the extractor within which the acidified electrolyte extracts the metal ion- containing solids to yield the metal ion-enriched electrolyte. The metal ion-enriched electrolyte may be supplied to the cathode chamber within which the metal ions react with the hydroxide ions formed at the cathode in the reduction reaction to yield metal hydroxides. The metal hydroxides may then be caused to flow out of the cathode chamber to enter the filtration unit within which the metal hydroxides are separated from the electrolyte. The separated electrolyte may be supplied to the chemical reaction chamber for re-use in subsequent reactions.

[0127] Redox shuttle molecules may be used as the oxidation and reduction reactants in the oxidation and reduction reactions respectively. The redox shuttle molecules may be contained in the respective anolyte and electrolyte solutions.Alternatively, the redox shuttle molecules may be supplied to the cell separately from the respective anolyte and electrolyte solutions.

[0128] In such embodiments, a shuttle molecule is reduced at the cathode to produce a hydrogenated shuttle molecule and hydroxide ions. A hydrogen-containing reactant such as water is additionally supplied at the cathode. The hydrogen-containing reactant may serve as the source of protons in the reduction reaction.

[0129] The hydrogenated shuttle molecule undergoes an oxidation reaction to produce a dehydrogenated shuttle molecule (or referred to here as “shuttle molecule”) and hydrogen ions.

[0130] In some embodiments, water molecules may be caused to permeate into a bipolar membrane within which the water molecules are electrochemically dissociated into hydrogen ions and hydroxide ions. The hydrogen ions permeate through a cation exchange layer of the bipolar membrane to enter a chemical reaction chamber. The hydrogen ions acidify the electrolyte circulated in the chemical reaction chamber. The acidified electrolyte may be caused to flow out of the chemical reaction chamber to the extractor within which the acidified electrolyte extracts the metal-ion containing solids to yield the metal ion-enriched electrolyte. The metal ion-enriched electrolyte may be supplied to the cathode chamber within which the metal ions react with the hydroxide ions formed at the cathode in the reduction reaction to yield metal hydroxides.

[0131] The metal hydroxides may be caused to flow out of the cathode chamber to enter the filtration unit within which the metal hydroxides are separated from one or more components contained in a mixture that is output from the cathode chamber. The metal hydroxides are separated from one or more components contained in the mixture. The other components may comprise the electrolyte and shuttle molecules and / or hydrogenated shuttle molecules.

[0132] The separated mixture comprising the electrolyte, the shuttle molecules and / or hydrogenated shuttle molecules may be returned to the chemical reaction chamber. The returned electrolyte, shuttle molecules and / or hydrogenated shuttle molecules may be re-used in subsequent reactions. Continuous production of metal hydroxides using a two-chamber cell

[0133] In some example embodiments, a two-chamber electrochemical cell such as a cell illustrated in FIG.1C is used in the continuous production of metal hydroxides from metal ion-containing solids. In such example embodiments, redox shuttle molecules may be used as the oxidation and reduction reactants in the oxidation and reduction reactions respectively. The redox shuttle molecules may be contained in the respective anolyte and catholyte solutions. Alternatively, the redox shuttle molecules may be supplied to the cell separately from the respective anolyte and catholyte solutions.

[0134] A shuttle molecule is reduced at the cathode in the presence of a hydrogen- containing reactant such as water to produce a hydrogenated shuttle molecule and hydroxide ions.

[0135] The hydrogenated shuttle molecule undergoes an oxidation reaction to produce a dehydrogenated shuttle molecule and hydrogen ions.

[0136] A flow of electrolyte (i.e., electrolyte containing oxidized shuttle molecules) may be caused to flow out of the anode chamber to enter the extractor. The flow of electrolyte may be anolyte that has been acidified by the hydrogen ions formed in the oxidation reaction. The acidified electrolyte extracts the metal ion-containing solids to yield a metal-ion-enriched electrolyte. The metal-ion-enriched electrolyte may be supplied to the cathode chamber within which the metal ions contained in the metal- ion-enriched electrolyte react with the hydroxide ions in the cathode chamber to yield metal hydroxides.

[0137] The metal hydroxides may be caused to flow out of the cathode chamber to enter the filtration unit. The electrolyte containing reduced shuttle molecules may additionally be caused to flow out of the cathode chamber to enter the filtration unit. In the filtration unit, the metal hydroxides are separated from one or more components contained in a mixture that is output from the cathode chamber. The metal hydroxides are separated from one or more components contained in the mixture. The other components may comprise the electrolyte and shuttle molecules and / or hydrogenated shuttle molecules.

[0138] The separated mixture comprising the electrolyte, the shuttle molecules and / or hydrogenated shuttle molecules may be returned to the anode chamber. The returned electrolyte, shuttle molecules and / or hydrogenated shuttle molecules may be re-usedin subsequent reactions.

[0139] The method 500 may be tuned to optimize one or more of current efficiency, applied electrical potential to achieve a desired current efficiency, product selectivity, efficiency and reaction rate of each of the reactions by adjusting one or more of: ● conditions of the flow cell such as temperature, pH, pressure, etc.; and / or ● characteristics of the separator and bipolar membrane (if present) such as the thickness, porosity, composition, etc.; and / or ● characteristics of the anode and / or cathode electrodes such as the material and method of fabrication; and / or ● nature of the cathode and / or anode catalyst; ● additional catalysts present; and / or ● the type of anolyte and electrolyte; and / or ● flow rate and / or composition and / or concentration of the oxidation reactants (e.g., hydrogenated shuttle molecules), reduction reactant (e.g., shuttle molecules and / or water) and / or electrolyte(s) and / or anolyte; and / or ● rate at which electrolyte is removed from the cell and / or returned to the cell; and / or ● rate at which metal hydroxides are removed from the cell; and / or ● rate at which electrolyte is supplied to the extractor; and / or ● rate at which the mixture comprising the metal hydroxides and electrolyte are supplied to the filtration unit; and / or ● size of the filters used in the extractor and / or filtration unit; and / or ● size and composition of the metal-ion containing solids; and / or ● presence of a chemical reaction chamber and if present, distance across the chemical reaction chamber between the bipolar membrane and the separator; etc.

[0140] In some embodiments, the electrolysis is operated at a temperature in the range of from 25oC to about 60oC. In some embodiments, the electrolysis is operated at a temperature less than about 60oC.

[0141] In some embodiments, the anolyte 54 and / or one or more of electrolyte 56, 60, 64 are heated to a selected temperature before being supplied to the anode chamber 18, chemical reaction chamber 26, and cathode chamber 20 respectively. In someembodiments, one or more of the anolyte 54 and / or one or more of electrolyte 56, 60, 64 are heated to a temperature in the range of from about 20oC to about 60oC.

[0142] In some example embodiments in which the bipolar membrane 28 separates the anode chamber 18 and the chemical reaction chamber 26, the thickness of bipolar membrane 28 is in the range of from about 25 to about 250 µm. In some embodiments, the bipolar membrane 28 is a product commercially available under the product name FumasepTMFBM. In some embodiments, the biopolar membrane 28 is fabricated by combining a cation exchange layer (such as a NafionTM211 membrane) and an anion exchange layer (such as an AemionTMCNN-8-25X membrane).

[0143] In some example embodiments, the separator 24 comprises an ion exchange membrane. The thickness of the ion exchange membrane may be in the range of from about 20 to about 200 µm. In some embodiments, the separator 24 is a NafionTM117 membrane.

[0144] In some embodiments, a flow rate at which the anolyte 54, the catholyte 56, and / or one or both of the electrolyte 60, 64 are supplied to the respective anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 may for example be in the range of from about 30 to 800 mL min-1for an electrode having a geometric surface area of 5 cm2.. The flow rate may be scaled according to the area of the electrode. In some embodiments, a flow rate at which the anolyte 54, the catholyte 56, and / or one or both of the electrolyte 60, 64 are supplied to the respective anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 is in the range of from about 100 to 350 mL min-1for an electrode having a geometric surface area of 5 cm2..

[0145] In some embodiments, the concentration of the anolyte 54, the catholyte 56, and / or one or both of the electrolyte 60, 64 are supplied to the respective anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 is in the range of from about 0.1 to about 5 M, and in some embodiments, less than about 5 M.

[0146] In some embodiments, the pH of the anolyte 54, the catholyte 56, and / or one or both of the electrolyte 60, 64 supplied to the extractor 12 is less than about 3, and in some embodiments, in the range of from about 0.2 to 3.

[0147] In some embodiments, the pH of cathode chamber 20 is maintained in the range of from about 9 to about 14 during electrolysis. In some embodiments, the pHof cathode chamber 20 is maintained in the range of from about 10 to about 14 during electrolysis.

[0148] In embodiments in which a closed-loop electrolyte 64 is arranged to circulate between the cathode chamber 20 and the chemical reaction chamber 26, the pH of the closed-loop electrolyte 64 is in the range of from about 4 to about 14.

[0149] In summary, one example aspect of the invention provides continuous systems and methods to convert metal-ion containing solids to metal hydroxides. This conversion may involve the conversion of calcium carbonate (CaCO3(s)) and / or waste cement (e.g., 2CaO·SiO2(s)(s) and 3CaO·SiO2(s)) to calcium hydroxide ((CaOH)2) for use in the production of cement clinkers. It will be understood that the systems and methods described herein may be used to convert any suitable metal-ion containing solids to metal hydroxides. One non-limiting example is in the conversion of magnesium-ion containing solids to (Mg(OH)2).

[0150] One aspect of the invention provides a “cement recycler” configured to electrochemically generate Ca(OH)2 from waste cement. The cement recycler integrates a “cement electrolyzer” as the cell 14, a waste cement digestion vessel as the extractor 12 (which may also be referred to herein as a “calcium extractor”), and a Ca(OH)2 isolation unit as the filtration unit 16 (which may also be referred to herein as a “lime extractor”).

[0151] In some embodiments, the cement electrolyzer comprises three chambers. In such embodiments, the three-chamber cement electrolyzer comprises an anode chamber, a chemical reaction reactor and a cathode chamber. A bipolar membrane may separate the anode chamber from the chemical reaction chamber, and a cation exchange membrane may separate the chemical reaction chamber from the cathode chamber. An oxygen evolution reaction (OER) may occur at the anode using a circulated anolyte (e.g., KOH), in accordance with Equation 8. The bipolar membrane may be adapted to produce hydroxide ions and hydrogen ions through water dissociation in accordance with Equation 9. The chemical reaction and cathode chambers may share a continuously circulated closed-loop electrolyte (e.g., CaCl2 and KCl). The electrolyte in the cathode reaction chamber, which has been acidified by the hydrogen ions formed at the bipolar membrane, is caused to output the chemical reaction chamber. The acidified electrolyte may be supplied to the calciumextractor within which acid digestion drives waste cement decomposition in accordance with Equation 10. Waste cement (e.g., in the form of chips) may be loaded into the calcium extractor. Once supplied into the calcium extractor, the acidic electrolyte (e.g., which has a pH of 0.5 to 2.0) from the cathode reaction chamber may dissolve reactive calcium from the waste cement, resulting in a flow of Ca2+-rich eluent out of the extractor. The Ca2+-rich eluent may be circulated to the cathode reactor chamber. A hydrogen evolution reaction (HER) may occur at the cathode which generates hydroxide ions. The hydroxide ions react with the calcium ions from the circulated Ca2+-rich eluent to yield Ca(OH)2 precipitate and H2 gas in accordance with Equation 11. The Ca(OH)2 precipitate and H2 gas may be directed into the lime extractor within which Ca(OH)2may be separated and isolated. Ca(OH)2isolation separates the solid precipitate from the electrolyte. The eluent may be recirculated to the chemical reaction chamber. The overall balanced equation of the example method is shown in Equation 12. Anode chamber:6OH–(aq)→ 3H2O(l) + 3 / 2O2(g) + 6e–(Eq. 8)Bipolar 6H membrane:2O(l)→ 6OH–(aq)+ 6H+(aq)(Eq. 9)3CaO·2SiO2·4H2O(s) + 6H+(aq) → 3Ca2+(aq) + Calcium extractor: (Eq.10) 2SiO2(s) + 7H2O(l) Cathode 3Ca2+(aq) + 6H2O(l) + 6e–→ 3Ca(OH)2(s) + 3H2(g) (Eq.11) chamber: Overall reaction:3CaO·2SiO2·4H2O(s) + 2H2O(l) → 3Ca(OH)2(s) +(Eq.12) 2SiO2(s) + 3H2(g) + 1.5O2(g)

[0152] In some embodiments, the cement electrolyzer comprises two chambers as discussed elsewhere herein.Example systems and methods for production of metal hydroxides which involve redox shuttle molecules

[0153] Aspects of the invention relate to particularly efficient systems and methods of converting metal-ion containing solids to metal hydroxides. Such particularly efficient methods utilize a redox shuttle molecule in the oxidation and reduction reactions at the respective anode and cathode, which generates hydrogen ions for decomposing the metal-ion containing solid into reactive metal ions, and hydroxide ions which may be used to react with the metal ions to form metal hydroxides. Example systems

[0154] FIGS.3A and 3B are schematic diagrams that illustrate example systems 1000 which include an electrochemical cell 14 configured to produce metal ions and hydroxide ions. The metal ions and hydroxide ions may be removed from the cell, and combined in a downstream reactor to produce metal hydroxides.

[0155] The cell 14 comprises an anode chamber 18 and a cathode chamber 20. A cathode 22 is exposed to the cathode chamber 20. An anode 19 is exposed to the anode chamber 18. A separator 24 separates the anode chamber 18 and the cathode chamber 20. In some embodiments, the cell 14 comprises two chambers.

[0156] The separator 24 is adapted to block passage of the metal ion in the metal-ion containing solids that is fed to the system 1000 for processing. A coating layer may be provided on the separator 24. In some example embodiments, the separator 24 adapted to block the passage of calcium ions. In some embodiments, the separator 24 comprises an ion exchange membrane such as a cation exchange membrane. In some example embodiments, the separator 24 comprises a polyaniline-coated cation exchange membrane.

[0157] In some embodiments, the cell 14 comprises three chambers. In such embodiments, the cell 14 additionally comprises a chemical reaction chamber 26. In such embodiments, the separator 24 separates the chemical reaction chamber 26 and the cathode chamber 22. A bipolar membrane 28 may separate the anode chamber 18 and the chemical reaction chamber 26. The bipolar membrane 28 is adapted to dissociate water molecules into hydroxide ions 27 and hydrogen ions 29. The hydrogen ions are 29 may permeate through a cation exchange layer 31 into thechemical reaction chamber 26.

[0158] A power source 32 is connected to apply an electrical potential difference between the cathode 22 and the anode 19. A negative electrical charge is applied to the cathode 22. A positive electrical charge is applied to the anode 19. The power source 32 may be configured to maintain a desired electric current between the cathode 22 and the anode 19 and / or to maintain a potential difference between the cathode 22 and the anode 19 at a desired level or in a desired range.

[0159] An oxidation reaction 34 takes place at the anode 19. A reduction reaction 36 takes place at the cathode 22.

[0160] An oxidation reactant 44 participates in the oxidation reaction 34 to form an oxidation product 46. In some embodiments, the oxidation reactant 44 comprises a hydrogenated shuttle molecule 48. In such embodiments, the hydrogenated shuttle molecule 48 participates in the oxidation reaction 34 to yield a shuttle molecule 52 and hydrogen ions 53.

[0161] In some embodiments, a shuttle molecule 40 is supplied to the cathode chamber 20. The shuttle molecule 40 participates in the reduction reaction 36 to yield a hydrogenated shuttle molecule 42 and hydroxide ions 30. The reduction reaction 36 is performed in the presence of a hydrogen-containing reactant 38. The hydrogen- containing reactant 38 may serve as a source of protons in the reduction reaction 36. In some example embodiments, the hydrogen-containing reactant 38 comprises water.

[0162] The cathode 22 may comprise any materials suitable for use as an electrode. The anode 19 may comprise any materials suitable for use as an electrode.

[0163] The cathode 22 and / or anode 19 may be a gas diffusion electrode.

[0164] The cathode 22 and / or anode 19 may be made of one or more metal, alloy or a supported metal / alloy catalyst. The metal may be any transition metal, or combination of one or more transition metals. Suitable electrocatalyst that may be incorporated in the cathode 22 may, for example, comprise one or more of C, Pt, Fe, Co, Mo, and combinations thereof. Suitable electrocatalyst that may be incorporated in the anode 19 may, for example, comprise one or more of Pt, Rh, Ir, Ru, Pd, Ni, and combinations thereof.

[0165] The cathode 22 and / or anode 19 may be porous. An example of a porouselectrode is graphite felt. Another example of a porous electrode is an electrode comprising an electrically conductive foam such as a metal foam. In some embodiments, the cathode 22 and / or anode 19 comprises a layer of porous nickel (Ni) foam. The nickel foam layer may be free-standing or supported (e.g. by other components of a membrane electrode assembly).

[0166] In some embodiments, the porosity of the foam electrode is about 50% to about 90%. In one example embodiment, the porosity of the foam electrode is about 80%. In some embodiments, the foam electrode has a thickness in the range of from about 200 to about 300 µm.

[0167] In some embodiments, a suitable anolyte 54 is supplied to the anode chamber 18. In some embodiments, the anolyte 54 comprises a pH-neutral aqueous solution. In some embodiments, the anolyte has a pH in the range of from about 6 to about 8. In some embodiments, the anions and / or cations of the anolyte 54 are monovalent. The anions of the anolyte 54 may for example comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-), perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H). The cations of the electrolyte may for example comprise one or more alkali metal ions such as lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+), hydrogen (H+), ammonium (NH4+), hydronium (H3O+), tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

[0168] In some embodiments, a reservoir containing the anolyte 54 and / or the hydrogenated shuttle molecule 48 is connected to supply the anolyte 54 and / or the hydrogenated shuttle molecule 48 to the anode chamber 18.

[0169] A catholyte 56 is supplied to the cathode chamber 20. In embodiments in which a chemical reaction chamber 26 is present, an electrolyte 60 is supplied to the chemical reaction chamber 26. In some embodiments, the electrolyte being supplied to the cathode chamber 20 and the chemical reaction chamber 26 is supplied from the same source. In such embodiments, a reservoir containing a supply of an electrolyte ((such as the one shown in FIGS.1A and 1B) is connected to flow between the cathode chamber 20 and the chemical reaction chamber 26 in a closedloop.

[0170] In some embodiments, the catholyte / electrolyte 56, 60 comprises a pH-neutral aqueous solution. In some embodiments, the catholyte / electrolyte 56, 60 has a pH in the range of from about 6 to about 8. In some embodiments, the anions and / or cations of the catholyte / electrolyte 56, 60 are monovalent. The anions of the catholyte / electrolyte 56, 60 may for example comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-), perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H). The cations of the electrolyte may for example comprise one or more alkali metal ions such as lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+), hydrogen (H+), ammonium (NH4+), hydronium (H3O+), tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

[0171] In some embodiments, the anolyte 54, the catholyte 56, and / or electrolyte 60 comprises water. In some embodiments, the anolyte 54, the catholyte 56, and / or electrolyte 60 additionally comprises an organic solvent, such as but is not limited to acetonitrile (MeCN).

[0172] In some embodiments, a reservoir containing metal-ion containing solids 74 is connected to supply the metal-ion containing solids 74 to the cell 14. The metal-ion containing solids 74 may be contained in the electrolyte 60. The metal-ion containing solids 74 may be provided as an aqueous slurry of microparticles.

[0173] Referring to FIG.3A, in some embodiments, the reservoir is connected to supply the metal ion-containing solids 74 to the chemical reaction chamber 20. In such embodiments, the metal ion-containing solids 74 is caused to react with the hydrogen ions 29 formed at the bipolar membrane 28 to yield metal ions 76.

[0174] Referring to FIG.3B, in some embodiments, the reservoir is connected to supply the metal ion-containing solids 74 to the anode chamber 18. In such embodiments, the metal ion-containing solids 74 is caused to react with the hydrogen ions 53 formed in the oxidation reaction 34 to yield metal ions 76.

[0175] In some embodiments, a reactor 120 is arranged downstream of the cell 14. The reactor 120 is configured to receive a flow of the metal ions 76 from the chemicalreaction chamber 20 and / or the anode chamber 18, and a flow of the hydroxide ions 30 produced in the reduction reaction 36 from the cathode chamber 18. The metal ions 76 may react with the hydroxide ions 30 to yield metal hydroxides 90.

[0176] The reactor 120 may in some embodiments be fluidly connected to the cell 14.

[0177] A first outlet 68 may be arranged at the chemical reaction chamber 20 and / or the anode chamber 18 for output of the metal ions 76 in the metal ion enriched electrolyte 82.

[0178] A second outlet 92 may be arranged at the cathode chamber for output of the hydroxide ions 30.

[0179] In some embodiments, a separator such as a centrifuge is arranged downstream of the reactor 120. The separator may be configured to separate the metal hydroxides 90 from other one or more components of a mixture which may additionally be supplied from the cell 14. In some embodiments, the other one or more components comprise shuttle molecules and 40 and hydrogenated shuttle molecules 48. A recycle stream 128 may be arranged to fluidly connect the separator to the anode chamber 18 and / or cathode chamber 20. The recycle stream 128 may be adapted to supply the shuttle molecules 40 and hydrogenated shuttle molecules 48 to the anode chamber and / or the cathode chamber 20 for re-use in subsequent reactions.

[0180] One or more downstream methods and apparatuses may be configured to process the metal hydroxides 90 to yield one or more additional useful products as previously described elsewhere herein which will not be repeated for brevity. Overview of method of producing metal hydroxides using redox shuttle molecules

[0181] FIG.4 is a flow chart illustrating the basic steps of a method 1500 of producing metal hydroxides from metal ion-containing solids using redox shuttle molecules according to one example embodiment of the invention.

[0182] In block 1502, an electrical current and / or potential is applied between an anode and a cathode.

[0183] In block 1504, a flow of catholyte is supplied to the cathode chamber, anda flow of anolyte is supplied to the anode. In embodiments in which a chemical reactionchamber is provided, a flow of electrolyte is supplied to the chemical reaction chamber. In some embodiments, a flow of electrolyte is arranged to circulate between the chemical reaction chamber and the cathode chamber in a closed loop. A shuttle molecule may be contained in the catholyte or may be separately supplied to the cathode. A hydrogen-containing reactant such as water is additionally supplied at the cathode. The hydrogen-containing reactant serves as a source of protons for the reduction reaction. A hydrogenated shuttle molecule may be contained in the anolyte or may be separately supplied to the anode.

[0184] In block 1506, the shuttle molecule undergoes a reduction reaction at the cathode to produce a hydrogenated shuttle molecule and hydroxide ions.

[0185] In block 1508, the hydrogenated shuttle molecule undergoes an oxidation reaction at the anode to produce shuttle molecules.

[0186] In block 1510, hydrogen ions are produced in the cell. In some embodiments, the hydrogen ions are products of electrolytic dissociation of water at the bipolar membrane. In some embodiments, the hydrogen ions are produced at the anode.

[0187] In block 1512, metal ion-containing solids are supplied to the cell. The metal ion-containing solids react with the hydrogen ions produced in the cell to yield metal ions (block 1514). The metal ions are removed from the cell (block 1516).

[0188] In block 1518, the hydroxide ions formed from the reduction reaction is removed from the cathode chamber.

[0189] The metal ions and the hydroxide ions are supplied to a reactor arranged downstream of the electrochemical cell within which the metal ions participate in a chemical reaction by reacting with hydroxide ions to produce metal hydroxides (block 1520).

[0190] In some embodiments, the removal of the metal ions and hydroxide ions from the chambers additionally removes shuttle molecules and hydrogenated shuttle molecules. In block 1522, the shuttle molecules and hydrogenated shuttle molecules may be separated from the metal hydroxides by any suitable separation method such as centrifugation. The shuttle molecules and hydrogenated shuttle molecules are optionally returned to the cell for re-use in subsequent reactions.

[0191] In some embodiments, carbon dioxide is generated in the reacting of the metal ion-containing solids with the hydrogen ions to yield metal ions. In someembodiments, the carbon dioxide is removed from the cell. The carbon dioxide may be directly supplied to one or more downstream apparatuses and / or methods. In some embodiments, the removed carbon dioxide is supplied to a second electrochemical cell to participate in a CO2RR to yield carbon-containing compounds.

[0192] The produced metal hydroxides may be supplied to a reactor, such as a kiln for reacting with an admixture such as silicon dioxide (SiO2) to yield cement clinkers. Example methods Production of metal hydroxides using redox shuttles with a three-chamber cell

[0193] In some example embodiments, a three-chamber electrochemical cell such as the cell as illustrated in FIG.3A is used in the production of metal hydroxides from metal ion-containing solids.

[0194] In such embodiments, the shuttle molecule is reduced at the cathode in the presence of a hydrogen-containing reactant such as water to form a hydrogenated shuttle molecule and hydroxide ions. The hydrogenated shuttle molecule undergoes an oxidation reaction to produce a dehydrogenated shuttle molecule (shuttle molecules) and hydrogen ions.

[0195] Water may be caused to permeate into a bipolar membrane within which the water is electrochemically dissociated into hydrogen ions and hydroxide ions. The hydrogen ions permeate through the cation exchange layer of the bipolar membrane to enter a chemical reaction chamber.

[0196] Metal-ion containing solids, which may be in the form of an aqueous slurry, may be supplied to the chemical reaction chamber. The metal-ion containing solids react with hydrogen ions produced at the bipolar membrane to yield metal ions.

[0197] The metal ions are removed from the chemical reaction chamber. The hydroxide ions formed from the reduction reaction is removed from the cathode chamber. The metal ions and the hydroxide ions may be supplied to the downstream reactor within which the metal ions react with hydroxide ions to produce metal hydroxides. Production of metal hydroxides using redox shuttles with a two-chamber cell

[0198] In some example embodiments, a two-chamber electrochemical cell such asthe cell as illustrated in FIG.3B is used in the production of metal hydroxides from metal ion-containing solids.

[0199] In such embodiments, the shuttle molecule is reduced at the cathode in the presence of a hydrogen-containing reactant such as water to form a hydrogenated shuttle molecule and hydroxide ions. The hydrogenated shuttle molecule undergoes an oxidation reaction to produce a dehydrogenated shuttle molecule (or shuttle molecule) and hydrogen ions.

[0200] Metal-ion containing solids, which may be in the form of an aqueous slurry, may be supplied to the anode chamber. The metal-ion containing solids react with hydrogen ions produced at the anode to yield metal ions.

[0201] The metal ions are removed from the anode chamber. The hydroxide ions formed from the reduction reaction are removed from the cathode chamber. The metal ions and the hydroxide ions are supplied to the downstream reactor within which the metal ions react with hydroxide ions to produce metal hydroxides.

[0202] The method 1500 may be tuned to optimize one or more of current efficiency, applied electrical potential to achieve a desired current efficiency, product selectivity, efficiency and reaction rate of each of the electrochemical reactions and chemical reactions by adjusting one or more of: ● conditions of the flow cell such as temperature, pH, pressure, etc.; and / or ● characteristics of the separator and bipolar membrane (if present) such as the thickness, porosity, composition, etc.; and / or ● characteristics of the anode and / or cathode electrodes such as the material and method of fabrication; and / or ● nature of the cathode and / or anode catalyst; ● additional catalysts present; and / or ● the type of anolyte and electrolyte; and / or ● flow rate and / or composition and / or concentration of the oxidation reactants (e.g., hydrogenated shuttle molecules), reduction reactant (e.g., shuttle molecules and / or water) and / or electrolyte(s) such as presence of and / or concentration of one or more organic solvent(s); and / or anolyte; and / or ● rate at which metal hydroxides and / or metal ions are removed from the cell; and / or● size and composition of the metal-ion containing solids; and / or ● presence of a chemical reaction chamber and if present, distance across the chemical reaction chamber between the bipolar membrane and the separator; etc.

[0203] In some embodiments, the electrical potential difference applied between the cathode 22 and the anode 19 to maintain the current density at a level of at least 100 mA cm-2is about 0.38 V, and with 100% Faradaic efficiency. In some embodiments, the electrical potential difference applied between cathode 22 and anode 19 to maintain the current density at a level of at least 100 mA cm-2is in the range of from about 0.1 V to about 6 V.

[0204] In some embodiments, the electrolysis is operated at a temperature in the range of from 20oC to about 60oC. In some embodiments, the electrolysis is operated at a temperature less than about 60oC.

[0205] In some embodiments, the anolyte 54, the catholyte 56, and / or the electrolyte 60 are heated to a selected temperature before being supplied to the anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 respectively. In some embodiments, one or more of the anolyte 54, catholyte 56 and / or electrolyte 60 are heated to a temperature in the range of from about 20oC to about 60oC.

[0206] In some example embodiments in which bipolar membrane 28 separates the anode chamber 18 and the chemical reaction chamber 26, the thickness of bipolar membrane 28 is in the range of from about 25 to about 250 µm. In some embodiments, bipolar membrane 28 is a product commercially available under the product name FumasepTMFBM. In some embodiments, biopolar membrane 28 is fabricated by combining a cation exchange layer (such as a NafionTM211 membrane) and an anion exchange layer (such as an AemionTMCNN-8-25X membrane).

[0207] In some example embodiments, the separator 24 comprises an ion exchange membrane. The thickness of the ion exchange membrane may be in the range of from about 20 µm to about 200 µm. In some embodiments, the separator 24 is a NafionTM117 membrane.

[0208] In some embodiments, a flow rate at which the anolyte 54, the catholyte 56 an / or the electrolyte 60 are supplied to the respective anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 may for example be in the range offrom about 30 to 800 mL min-1for an electrode having a geometric surface area of 5 cm2.. The flow rate may be scaled according to the area of the electrode. In some embodiments, a flow rate at which the anolyte 54, the catholyte 56 an / or the electrolyte 60 are supplied to the respective anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 is in the range of from about 100 to 350 mL min-1for an electrode having a geometric surface area of 5 cm2..

[0209] In some embodiments, the concentration of the anolyte 54, the catholyte 56 an / or the electrolyte 60 are supplied to the respective anode chamber 18, chemical reaction chamber 26 and cathode chamber 20 is in the range of from about 0.1 to about 5 M, and in some embodiments, less than about 5 M.

[0210] In some embodiments, the concentration of the metal-ion containing solid 74 in the electrolyte 60 that is supplied to chemical reaction chamber 26, and / or of the metal-ion containing solid 74 in the anolyte 54 that is supplied to anode chamber 18 is in the range of from about 2 g / L to about 50 g / L per cm2of an electrode. The concentration may be scaled according to the area of the electrode.

[0211] In some embodiments, the anode chamber 18 is maintained at a pH of greater than about 6. In some embodiments, the pH of anode chamber 18 is maintained in the range of from about 4 to 8.

[0212] In some embodiments, the pH of cathode chamber 20 is maintained in the range of from about 9 to about 14 during electrolysis. In some embodiments, the pH of cathode chamber 20 is maintained in the range of from about 10 to about 14 during electrolysis.

[0213] In summary, one example aspect of the invention provides particularly efficient systems and methods to convert metal-ion containing solids to metal hydroxides which utilize a hydrogen-containing redox shuttle. Example applications of this particularly efficient method are in the field of decomposition of limestone (CaCO3(s)) in the production of calcium hydroxides which may be used in the production of cement clinkers. Processing of other metal-ion containing solids using the systems and methods described herein is within the scope of the invention.

[0214] One aspect of the invention provides a zero-gap, two-chamber electrolyzer. The zero-gap electrolyzer comprises an anode, a cathode and a Ca2+-blocking cation exchange membrane separating the anode and the cathode. The anode and cathodeare pressed against the cation exchange membrane. In such embodiments, the distance between the anode and cathode is the thickness of the cation exchange membrane. A hydrogenated shuttle molecule such as dihydroanthraquinone-2,7- disulfonic acid disodium salt (H2AQ) in an anolyte (e.g., potassium acetate (KOAc)) may be supplied to the anode. The H2AQ participates in an oxidation reaction at the anode to form hydrogen ions, in accordance with Equation 13. A shuttle molecule such as anthraquinone-2,7-disulfonic acid disodium salt (AQ) and a hydrogen containing reactant such as water may be supplied to the cathode. The AQ participates in a reduction reaction at the cathode to form hydroxide ions, in accordance with Equation 14. In some embodiments, calcium carbonate (CaCO3(s)) is suspended in the anolyte. The calcium carbonate may react with the hydrogen ions formed by H2AQ oxidation to yield calcium ions, in accordance with Equation 15. The calcium ions and the hydroxide ions may be removed from the electrolyzer and supplied to a calcium reactor downstream of the electrolyzer to form calcium hydroxide. Anode chamberH2AQ → AQ + 2H++ 2e– (Eq. 13)(PCET) Cathode chamberAQ + 2H2O + 2e–→ H2AQ + 2OH– (Eq. 14)(PCET) Anode chamber H2AQ + CaCO3(s)→ AQ + Ca2++ CO2(g)+ H2O + (Eq.15) overall reaction 2e–(PCET) Example systems and methods for producing metal silicate hydrates

[0215] Some aspects of the invention pertain to particularly efficient systems and methods for producing metal silicate hydrates from metal-ion containing solids. In some embodiments of the invention, the systems and methods pertain to a continuous production of metal silicate hydrates from metal-ion containing solids.

[0208] Referring to FIGS.5A, 5B, 5C, in some embodiments, the system 2000comprises an electrochemical cell 14. The cell 14 comprises an anode chamber 18 and a cathode chamber 20. A cathode 22 is exposed to the cathode chamber 20. An anode 19 is exposed to the anode chamber 18. A separator 24 separates the anode chamber 18 and the cathode chamber 20. In some embodiments, the cell 14 comprises two compartments.

[0216] The separator 24 is adapted to block passage of the metal ion in the metal-ion containing solids that is fed to the system 2000 for processing. A coating layer may be provided on the separator 24. In some example embodiments, the separator 24 is adapted to block the passage of calcium ions.

[0217] In some embodiments, the separator 24 comprises an ion exchange membrane such as a cation exchange membrane. In some example embodiments, the separator 24 comprises a polyaniline-coated cation exchange membrane.

[0218] In some embodiments, the separator 24 comprises an anion exchange membrane.

[0219] In some embodiments, the separator 24 comprises a bipolar membrane adapted to dissociate water molecules into hydroxide ions 27 and hydrogen ions 29.

[0220] In some embodiments, the separator 24 comprises a microporous polymer membrane.

[0221] In some embodiments, the separator 24 comprises a layer of electrolyte.

[0222] In some embodiments, the cell 14 comprises three compartments. In such embodiments, the cell 14 additionally comprises a chemical reaction chamber 26. In such embodiments, the separator 24 separates the chemical reaction chamber 26 and the cathode chamber 20. A bipolar membrane 28 may separate the anode chamber 18 and the chemical reaction chamber 26. The bipolar membrane 28 is adapted to dissociate water molecules into hydroxide ions 27 and hydrogen ions 29.

[0223] A power source 32 is connected to apply an electrical potential difference between the cathode 22 and the anode 19. A negative electrical charge is applied to the cathode 22. A positive electrical charge is applied to the anode 19. The power source 32 may be configured to maintain a desired electric current between the cathode 22 and the anode 19 and / or to maintain a potential difference between the cathode 22 and the anode 19 at a desired level or in a desired range.

[0224] An oxidation reaction 34 takes place at the anode 19. A reduction reaction 36takes place at the cathode 22.

[0225] A hydrogen-containing reactant 38 is supplied to cathode chamber 20. In some embodiments, the hydrogen-containing reactant 38 participates in the reduction reaction 36 to generate hydroxide ions 30. In some embodiments, the reduction reaction comprises a hydrogen evolution reaction (HER). In such embodiments, the hydrogen-containing reactant 38 comprises water.

[0226] An oxidation reactant 44 participates in the oxidation reaction 34 to form an oxidation product 46.

[0227] In some embodiments, the oxidation reaction 34 comprises an oxygen evolution reduction (OER). In such embodiments, the oxidation reactant 44 comprises a solution containing hydroxide ions (OH-). The oxidation product 46 may comprise oxygen gas (O2) and water molecules.

[0228] In some embodiments, the oxidation reactant 44 comprises water. In such embodiments, water is oxidized at the anode 19 to form oxygen gas and hydrogen ions 33.

[0229] The cathode 22 may comprise any materials suitable for use as an electrode. Such material may, for example comprise a catalyst suitable for driving a hydrogen evolution reaction (HER).

[0230] The anode 19 may comprise any materials suitable for use as an electrode. Such material may comprise a catalyst suitable for driving an oxygen evolution reaction (OER).

[0231] The cathode 22 and / or anode 19 may be a gas diffusion electrode.

[0232] The cathode 22 and / or anode 19 may be made of one or more metal, alloy or a supported metal / alloy catalyst. The metal may be any transition metal, or combination of one or more transition metals. Suitable electrocatalyst that may be incorporated in the cathode 22 may, for example, comprise one or more of C, Pt, Fe, Co, Mo, and combinations thereof. Suitable electrocatalyst that may be incorporated in the anode 19 may, for example, comprise one or more of Pt, Rh, Ir, Ru, Pd, Ni, and combinations thereof.

[0233] The cathode 22 and / or anode 19 may be porous. An example of a porous electrode is an electrode comprising an electrically conductive foam such as a metal foam. In some example embodiments, the cathode 22 and / or anode 19 comprises alayer of porous nickel (Ni) foam. The nickel foam layer may be free-standing or supported (e.g. by other components of a membrane electrode assembly).

[0234] In some embodiments, the porosity of the foam electrode is about 50% to about 90%. In one example embodiment, the porosity of the foam electrode is about 80%. In some embodiments, the foam electrode has a thickness in the range of from about 200 to about 300 µm.

[0235] In some embodiments, a suitable anolyte 54 is supplied to the anode chamber 18. In some embodiments, the anolyte 54 comprises a base. In some example embodiments, the anolyte 54 comprises a solution containing hydroxide ions (OH-). In some embodiments, the anolyte 54 comprises water. In some embodiments, the anolyte 54 comprises the oxidation reactant 44.

[0236] In some embodiments, a reservoir containing the anolyte 54 is connected to supply the anolyte 54 to the anode chamber 18.

[0237] An catholyte 56 is supplied to the cathode chamber 20. In embodiments in which a chemical reaction chamber 26 is present, an electrolyte 60 is supplied to the chemical reaction chamber 26. In some embodiments, the electrolyte being supplied to the cathode chamber 20 and the chemical reaction chamber 26 is supplied from the same source. In such embodiments, a reservoir containing a supply of an electrolyte 60 (such as the one shown in FIGS.1A and 1B) is connected to flow between the cathode chamber 20 and the chemical reaction chamber 26 in a closed loop.

[0238] In some embodiments, the catholyte / electrolyte 56, 60 comprises a pH-neutral aqueous solution. In some embodiments, the catholyte / electrolyte 56, 60has a pH in the range of from about 6 to about 8. In some embodiments, the anions and / or cations of the catholyte / electrolyte 56, 60 are monovalent. The anions of the catholyte / electrolyte 56, 60may for example comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-), perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H). The cations of the electrolyte may for example comprise one or more alkali metal ions such as lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+), hydrogen (H+), ammonium (NH4+), hydronium (H3O+),tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

[0239] In some embodiments, a reservoir containing metal-ion containing solids 74 is connected to supply a flow of metal-ion containing solids 74 to the cell. In some embodiments, the reservoir is connected to supply the metal-ion containing solids 74 to the anode chamber 18. In such embodiments, the metal-ion containing solids 74 may be suspended in the anolyte 54. In such embodiments, the metal-ion containing solids 74 may react with the hydrogen ions 33 formed in the oxidation reaction 34 to yield metal ions 76.

[0240] In some embodiments, the reservoir is connected to supply the metal-ion containing solids 74 to the chemical reaction chamber 26. In some embodiments, the metal-ion containing solids 74 may be suspended in the electrolyte 60. In such embodiments, the metal-ion containing solids 74 may react with the hydrogen ions 29 formed at the bipolar membrane 28 to yield metal ions 76.

[0241] In some embodiments, the reservoir is connected to supply the metal-ion containing solids 74 to the cathode chamber 20. In such embodiments, the metal-ion containing solids 74 may be suspended in the catholyte 56. In such embodiments, the metal-ion containing solids 74 may react with the hydrogen ions 29 formed in the bipolar membrane 28 to yield metal ions 76.

[0242] In some embodiments, a reservoir containing silicate-containing compounds 132 is connected to supply a flow of the silicate-containing compounds 132 to the cathode chamber 20. In some embodiments, the silicate-containing compound 132 is suspended in the catholyte 56. In such embodiments, the silicate-containing compound 132 may react with the hydroxide ions 30 formed in the reduction reaction 36 to yield silicate ions 134.

[0243] In some embodiments, a cathode product outlet 140 is arranged at the cathode chamber 20 for output of the silicate ions 134 formed from reactions in the cathode chamber 20. In some embodiments, hydroxide ions 30 are discharged from the cathode chamber through the silicate ion product outlet 140.

[0244] In some embodiments, a metal ion product outlet 144 is arranged at the anode chamber 18 for output of the metal ions 76 formed from reactions in the anode chamber 18.

[0245] In some embodiments, a metal ion product outlet 144 is arranged at the chemical reaction chamber 26 for output of the metal ions 76 formed from reactions in the chemical reaction chamber 26.

[0246] In some embodiments, a reactor 136 is arranged downstream of the cell 14. The silicate ions 134 formed at the cathode chamber 20 and the metal ions 76 formed at the chemical reaction chamber 26 and / or the anode chamber 18 may react in the reactor 136 to yield metal silicate hydrates 138.

[0247] The reactor 136 may be fluidly connected to the cell 14.

[0248] As illustrated in FIG.5C, in some embodiments, the cell 14 is flowingly connected to the extractor 12 configured to extract metal ions 76 from the metal-ion containing solids 74. The metal-ion containing solids 74 may be loaded into the extractor 12. In some embodiments, a reservoir containing a supply of the metal-ion containing solids 74 may be connected to supply the metal-ion containing solids 74 to the extractor 12.

[0249] In some embodiments, the first outlet 68 of the cell 14 is fluidly connected to an inlet 70 of the extractor 12 adapted to supply a flow of electrolyte 60, 64 from the cell 14 into the extractor 12. In some embodiments, the first outlet 68 is arranged at the chemical reaction chamber 26. In such embodiments, the electrolyte 60, 64 is acidified by hydrogen ions 29 produced at the bipolar membrane 28. The metal ions 76 may be dissolved in the acidified electrolyte 60, 64, thereby forming a metal-ion- enriched electrolyte 82. An outlet 84 is provided at the extractor 12 for output of the metal-ion-enriched electrolyte 82 out of the extractor 12.

[0250] In some embodiments, the outlet 84 of the extractor 12 is fluidly connected to an inlet 150 of the reactor 136. In such embodiments, the metal-ion-enriched electrolyte 82 is transported from the extractor 12 to the reactor 136. The metal-ion- enriched electrolyte 82 provides a source of the metal ions 76 to the reactor 136 for reacting with the silicates 134 formed from reaction of hydroxide ions 30, that were generated in the reduction reaction 36, with silicate containing solids 132 in extractor 154 to form metal silicate hydrates 138. In some embodiments, the concentration of the hydroxide ions 30 is greater than the concentration of the silicate ions 134 in the reactor 136, resulting in reaction of hydroxide ions 30 with metal ions 76 to form metal hydroxides 90.

[0251] In some embodiments, a second extractor 154 is provided. The second extractor 154 may be configured to extract silicate ions 134 from the silicate- containing compounds 132. In some embodiments, a reservoir containing a supply of the silicate-containing compounds 132 may be connected to supply the silicate- containing compounds 132 to the second extractor 154.

[0252] In some embodiments, a second outlet 92 is provided at the cell 14. The second outlet 92 is adapted to output a flow of the hydroxide ions 30 in the electrolyte 56 formed in the reduction reaction 36 out of the cathode chamber 20.

[0253] The second outlet 92 may be fluidly connected to an inlet 156 of the second extractor 154. The hydroxide ions 30 in the electrolyte 56 react with the silicate- containing compounds 132 to form the silicate ions 134.

[0254] An outlet 158 of the second extractor 154 may be fluidly connected to the inlet 150 of the reactor 136 for output of the silicate ions 134 and / or unused or unreacted hydroxide ions 30 for supply into the reactor 136.

[0255] In some embodiments, a filtration unit 16 is provided downstream of the reactor 136. An outlet 160 of the reactor 136 may be fluidly connected to the inlet 94 of the filtration unit 14 for output of a mixture comprising the metal silicate hydrates 138 and / or metal hydroxides 90 to supply into the filtration unit 16. The filtration unit 16 may be configured to separate the metal silicate hydrates 138 and / or metal hydroxides 90 from other one or more components discharged from the reactor 136. The one or more components may comprise electrolyte 56, 60, 64. In some embodiments, the outlet 98 of the filtration unit 14 is fluidly connected to one or more inlets 86, 96 of the cell 14 for output of the separated electrolyte 56, 60, 64 to return to the cell 14. In some embodiments, the first inlet 86 is arranged at the cathode chamber 20. In some embodiments, the second inlet 96 is arranged at the chemical reaction chamber 26.

[0256] In some embodiments, one or more gas-liquid separators 102 may be provided between the cell 14 and the extractor 12 and / or extractor 154 and / or filtration unit 16 configured to separate one or more gasses from the anolyte 54 and / or electrolyte 54, 56, 60, 64 before supplying the anolyte 54 and / or electrolyte 54, 56, 60, 64 into the extractor 12, extractor 154, and the filtration unit 16 respectively. In some embodiments, a first gas-liquid separator 102A is flowingly connected betweenthe first outlet 68 of the cell 14 and the inlet 70 of the extractor 14. In some embodiments, a second gas-liquid separator 102C is flowingly connected between the outlet 160 of the reactor unit 136 and the inlet 94 of the filtration unit 16. In some embodiments, a third gas-liquid separator 102D is flowingly connected between the second outlet 92 of the cell 14, and the inlet 156 of the second extractor unit 154. However, any suitable number of gas-liquid separators 102 between the cell 14 and the extractor 12 and / or extractor 154 and / or filtration unit 16 may be provided.

[0257] The described systems may be used to produce any metal silicate hydrates, which may be represented by the generic formula MXaSibOc · NH2O, wherein M and N is each an integer greater than or equal to 0; a, b, and c is each a number greater than or equal to 0 including a number that is greater than 0 and less than n where n is a positive number; X is any metal ion such as but is not limited to Ca2+, Mg2+, etc.

[0258] In some example embodiments, the described systems are used to produce calcium silicate hydrates, which may be represented by the generic formula MCaaSibOc · NH2O, wherein M and N is an integer greater than or equal to 0; a, b, and c is a number greater than or equal to 0 including a number that is greater than 0 and less than n where n is a positive number.

[0259] Aspects of the invention relate to combining methods and apparatuses for converting metal-ion containing solids to metal silicate hydrate and metal hydroxides with downstream methods and apparatuses for making useful products. The one or more downstream methods and apparatuses may be configured to process the metal silicate hydrates 138 and / or metal hydroxides 90 to yield one or more additional useful products.

[0260] In some embodiments, the metal silicate hydrates 138 and / or metal hydroxides 90 are used in cement production. In some example embodiments, the metal silicate hydrates 138 are calcium silicate hydrates, and the metal hydroxides 90 are calcium hydroxides. Calcium silicate hydrates and calcium hydroxides may be used as a cement clinker precursor.

[0261] In some embodiments, the metal silicate hydrates 138 and metal hydroxides 90 are supplied to a reactor such as a furnace to yield one or more useful products under heat, for example at a temperature from 20oC to 1,500oC. The one or more useful products may comprise metal silicate.

[0262] In some example embodiments, the metal silicate hydrates 138 are calcium silicate hydrates, and the metal hydroxides 90 are calcium hydroxides. Calcium silicate hydrates and calcium hydroxides may be heated to a temperature in the range of from about 20oC to about 1,500oC to yield useful products such as Belite.2CaO · SiO2(s)and Alite, 3CaO · SiO2(s). The cement clinker may be further processed to form Portland cement in further one or more downstream apparatuses. Overview of method of producing metal silicate hydrates

[0263] FIG.6 is a flow chart illustrating the steps of a method 2500 of converting metal-ion containing solids to metal silicate hydrates according to one embodiment of the invention.

[0264] In block 2502, an electrical current and / or potential is applied between an anode and a cathode.

[0265] In block 2503, an electrolyte is supplied to the cathode chamber and / or chemical reaction chamber, and an anolyte is supplied to the anode chamber. The hydrogen-containing reactant and the oxidation reactant may be contained in the anolyte for supplying to the cell. The metal ion-containing solids and / or the silicate- containing compounds may be contained in the respective electrolyte for supplying to the cell. The hydrogen-containing reactant and the oxidation reactant, may be supplied to the cell separate from anolyte. The metal ion-containing solids and / or the silicate-containing compounds may be supplied to the cell separate from the respective electrolyte.

[0266] In block 2504, the hydrogen-containing reactant such as water undergoes a reduction reaction at the cathode to produce hydroxide ions.

[0267] In block 2506, the oxidation reactant undergoes an oxidation reaction at the anode to form an oxidation product.

[0268] In block 2508, hydrogen ions are produced in the cell. In some embodiments, the hydrogen ions are formed at the bipolar membrane. In some embodiments, the hydrogen ions are formed at the anode.

[0269] In block 2510, the metal ion-containing solids react with the hydrogen ions produced in the cell to form metal ions.

[0270] In block 2512, the silicate-containing compounds react with the hydroxide ionsproduced at the cathode to yield silicate ions.

[0271] In block 2514, the metal ions and the silicate ions are combined to yield metal silicate hydrates. In some embodiments, metal hydroxides are additionally produced.

[0272] The metal silicate hydrates and / or metal hydroxides may be supplied to downstream methods and apparatuses for further processing. For example, the metal silicate hydrates and / or metal hydroxides may be heated to a temperature in the range of from about 400°C to 1,500°C to yield useful products such as belite.2CaO · SiO2(s)and Alite, 3CaO · SiO2(s). Example methods Production of metal silicate hydrates using a three-chamber cell

[0273] In some example embodiments, a three-chamber electrochemical cell such as the cell as illustrated in FIG.5A is used in the production of metal silicate hydrates.

[0274] In some embodiments, the hydrogen-containing reactant such as water undergoes a reduction reaction to produce hydroxide ions.

[0275] The oxidation reactant, which may be contained in the anolyte, undergoes an oxidation reaction to produce an oxidation product. In some embodiments, the oxidation reactant comprises hydroxide ions. The hydroxide ions in anolyte may undergo an oxidation reaction to produce water and oxygen gas.

[0276] The water molecules may be caused to permeate into the bipolar membrane within which the water molecules are electrochemically dissociated into hydrogen ions and hydroxide ions. The hydrogen ions permeate through a cation exchange layer of the bipolar membrane to enter a chemical reaction chamber.

[0277] A feed containing a metal-ion containing solid is supplied to the chemical reaction chamber. The metal-ion containing solids react with the hydrogen ions produced at the bipolar membrane to form metal ions.

[0278] A feed containing a silicate-containing compound is supplied to the cathode chamber. The silicate-containing compounds react with the hydroxide ions produced in the reduction reaction to yield silicate ions.

[0279] The metal ions formed in the chemical reaction chamber are removed from the cell. The silicate ions formed in the cathode chamber are removed from the cell. The metal ions and the silicate ions may be supplied to a downstream reactor for reactionto yield metal silicate hydrate. Continuous production of metal silicate hydrates using a three-chamber cell

[0280] In some example embodiments, a flow of electrolyte is continuously circulated between the electrochemical cell, and one or more of a first extractor, a second extractor, a reactor, and a filtration unit. In such embodiments, the metal ion- containing solids and the silicate-containing compounds may be supplied to the first extractor and the second extractor respectively. In such embodiments, the metal ion- containing solids and the silicate-containing compounds may not be supplied to the cell.

[0281] In some embodiments, the hydrogen-containing reactant such as water undergoes a reduction reaction to produce hydroxide ions.

[0282] The oxidation reactant, which may be contained in the anolyte, undergoes an oxidation reaction to produce an oxidation product. In some embodiments, the oxidation reactant comprises hydroxide ions. The hydroxide ions in the anolyte may undergo an oxidation reaction to produce water and oxygen gas.

[0283] The water molecules may be caused to permeate into the bipolar membrane within which the water molecules are electrochemically dissociated into hydrogen ions and hydroxide ions. The hydrogen ions permeate through a cation exchange layer of the bipolar membrane to enter a chemical reaction chamber.

[0284] A flow of electrolyte is caused to flow out of the cell to enter an extractor. In some example embodiments, the flow of electrolyte is caused to flow out of the chemical reaction chamber. The flow of electrolyte may be electrolyte that has been acidified by the hydrogen ions formed at the bipolar membrane.

[0285] Metal ion-containing solids may be supplied to the first extractor. The acidified electrolyte extracts the metal ion-containing solids to yield a metal-ion-enriched electrolyte. The metal-ion-enriched electrolyte comprises the metal ions. The metal- ion-enriched electrolyte may be supplied to the reactor.

[0286] The hydroxide-ion containing electrolyte may be caused to flow out of the cathode chamber to enter the second extractor. Silicate-containing compounds may be supplied to the second extractor. The hydroxide-ion containing electrolyte extract the silicate-containing compounds to yield silicate ions.

[0287] The silicate ions and hydroxide ions in electrolyte are caused to flow out of the second extractor. The silicate ions and hydroxide ions in electrolyte may be directed to flow into the reactor. The silicate ions and hydroxide ions react with the metal ions supplied from the first extractor to yield metal silicate hydrates and / or metal hydroxides.

[0288] The metal silicate hydrates and / or metal hydroxides may be supplied to the downstream filtration unit. The metal silicate hydrates and / or the metal hydroxides may be separated from one or more components contained in the mixture. In some embodiments, the other components comprise the electrolyte. The electrolyte may be free or substantially free of solids, and in particular, metal hydroxides and / or metal silicate hydrates.

[0289] The separated electrolyte may be returned into the cell. The separated electrolyte may be caused to flow into the chemical reaction chamber and / or the cathode chamber for re-use in subsequent reactions.

[0290] The produced metal hydroxides and / or metal silicate hydrates are removed from the filtration unit. Production of metal silicate hydrates using a two-chamber cell

[0291] In some example embodiments, a two-chamber electrochemical cell such as the one illustrated in FIG.5B is used in the production of metal silicate hydrates.

[0292] In some embodiments, the hydrogen-containing reactant such as water undergoes a reduction reaction to produce hydroxide ions.

[0293] The oxidation reactant undergoes an oxidation reaction at the anode to produce an oxidation product. In some embodiments, the oxidation reactant comprises hydroxide ions. In such embodiments, the oxidation product may comprise water and oxygen gas.

[0294] In embodiments in which a bipolar membrane is arranged to separate the anode chamber from the cathode chamber, water molecules are caused to permeate into the bipolar membrane. The water molecules are electrochemically dissociated into hydrogen ions and hydroxide ions. The hydrogen ions permeate through a cation exchange layer of the bipolar membrane to enter the cathode chamber.

[0295] In some embodiments in which a bipolar membrane is provided, a feed of ametal-ion containing solids is supplied to the cathode chamber. The metal-ion contain solids react with the hydrogen ions produced in the bipolar membrane to form metal ions.

[0296] In some embodiments, the oxidation reactant comprises a hydrogen- containing reactant such as water. In such embodiments, the oxidation product comprises hydrogen ions and oxygen gas.

[0297] In some embodiments, an ion exchange membrane such as a cation exchange membrane, or a microporous polymer is arranged to separate the anode chamber from the cathode chamber. In such embodiments, a feed of a metal-ion containing solid is supplied to the anode chamber. In such embodiments, the metal- ion containing solids react with the hydrogen ions produced at the anode in the oxidation reaction to form metal ions.

[0298] A feed of silicate-containing compound is supplied to the cathode chamber. The silicate-containing compounds react with the hydroxide ions produced in the reduction reaction to yield silicate ions.

[0299] In some embodiments, the metal ions formed in the anode chamber are removed from the cell. The silicate ions formed in the cathode chamber are removed from the cell. The metal ions and the silicate ions may be supplied to a reactor arranged downstream of the cell for reaction to yield metal silicate hydrate.

[0300] In embodiments in which the metal ions and the silicate ions are both formed in the cathode chamber, the metal ions and the silicate ions may be combined to yield metal silicate hydrate in the cathode chamber. The produced metal silicate hydrates may be removed from the cell.

[0301] In some embodiments, an anode flow field layer is arranged at the anode chamber and the cathode flow field layer is arranged at the cathode chamber. In some embodiments, the electrochemical cell comprises a flow cell. The flow cell may for example comprise a zero-gap electrolyzer. In such embodiments, the anode and cathode are pressed against opposing surfaces of the ion exchange membrane. A cathode and anode flow field layer may be arranged to press against the surfaces of the cathode and anode respectively. The anode and cathode flow field layers may each comprise an inlet arranged to receive a supply of a feed, and an outlet arrange to output a flow of a feed. For example, an anolyte reservoir may be fluidly connectedto supply an anolyte and / or a metal-containing compound in the anolyte to the anode flow field. An electrolyte reservoir may be fluidly connected to supply an electrolyte and / or a silicate-containing compound in the electrolyte to the cathode flow field. In some embodiments, a layer of electrolyte separate the anode from the cathode. Example system and method of producing metal silicate hydrates

[0302] FIG.7 illustrates another example system and method of producing metal silicate hydrates. In some embodiments, the system 3000 comprises an acidic chamber 3002, an alkaline chamber 3004, a metal ion-enriched chamber 3006, a silicate ion-enriched chamber 3008 and a reaction chamber 3100.

[0303] The acidic chamber 3002 is configured to generate hydrogen ions from an acid. The acid may for example comprise an acid solution produced in an electrochemical cell (e.g., an acid that is electrolytically produced) and / or an acid solution produced in a chemical reaction.

[0304] A reservoir comprising metal-ion containing solids may be connected to supply the metal-ion containing solids 74 to the metal ion-enriched chamber 3006. The acidic chamber 3002 may be fluidly connected to the metal ion-enriched chamber 3006. The hydrogen ions formed in the acidic chamber 3002 may be supplied to the metal ion- enriched chamber 3006 for reacting with the metal-ion containing solids to yield metal ions 76.

[0305] The alkaline chamber 3004 is configured to generate hydroxide ions from a base. The base may for example comprise a base solution produced in an electrochemical cell (e.g., an acid that is electrolytically produced), or a base solution produced in a chemical reaction.

[0306] A reservoir comprising silicate-containing compounds may be connected to supply the silicate-containing compounds to the silicate ion-enriched chamber 3008. The alkaline chamber 3004 may be fluidly connected to the silicate ion-enriched chamber 3008. The hydroxide ions formed in the alkaline chamber 3004 may be supplied to the silicate ion-enriched chamber 3008 for reacting with the silicate- containing compounds 132 to yield silicate ions 134.

[0307] The metal ion-enriched chamber 3006 may be fluidly connected to the reaction chamber 3100. The metal ions formed in the metal ion-enriched chamber 3006 maybe supplied to the reaction chamber 3100.

[0308] The silicate ion-enriched chamber 3008 may be fluidly connected to the reaction chamber 3100. The silicate ions 134 formed in the silicate ion-enriched chamber 3008 may be supplied to the reaction chamber 3100 for reacting with the metal ions 76 formed in the metal ion-enriched chamber 3006 to produce metal silicate hydrates 138.

[0309] The invention is further described with reference to the following specific examples, which are not meant to limit the invention, but rather to further illustrate it. Examples Example 1 - Continuous production of metal hydroxides

[0310] An electrochemical cell of the type illustrated in FIG.1A and the method of performing electrolysis illustrated in FIG.2 were used to convert waste cement as the metal ion-containing solids 74 into metal hydroxides 90 where the metal is calcium. The system may be referred to as a “cement recycler”. In these experiments, the electrochemical cell 14 comprises a cement electrolyzer. The extractor 12 comprises a waste cement digestion vessel or a calcium extractor. The filtration unit 16 comprises a Ca(OH)2 isolation unit or a lime extractor.

[0311] Referring to FIG.8, as used in these examples, the anode 19 and cathode 22 comprises a free-standing nickel foam which comprises an active area of 5 cm2. The anode chamber 18 is separated from the chemical reaction chamber 20 by a bipolar membrane 28. A cation exchange membrane (Nafion™) 24 separates the chemical reaction chamber 20 from the cathode chamber 20. The anolyte 54 used in this example comprises 3 M KOH. The oxidation reaction 34 which occurs at the anode 19 comprises an oxygen evolution reaction (OER). The bipolar membrane produces OH⁻ ions through water dissociation to maintain a stable anolyte concentration. The chemical reaction chamber 26 and cathode chamber 20 share a continuously circulated closed-loop electrolyte 64 (0.2 M CaCl2and 1 M KCl, 100 mL) at a flow rate of 1 to 20 mL min–1. This closed-loop electrolyte 64 is believed to facilitate the transfer of H⁺ ions from the bipolar membrane 28 into the chemical reaction chamber 26, where acid digestion drives waste cement decomposition, and supports the hydrogen evolution reaction (HER) as the reduction reaction 36 in the cathode chamber 20 togenerate OH⁻ ions for Ca(OH)2 precipitation.

[0312] In these examples, the calcium extractor 12 and the lime extractor 16 were adapted from HempelTMdistillation columns, and were equipped with cotton filters to prevent precipitate from entering the electrolyzer 14. The calcium extractor 12 was loaded with fresh / aged waste cement chips 74. The calcium extractor 12 was fluidly connected to receive electrolyte from the chemical reaction chamber 26. The acidic electrolyte 64 discharged from the chemical reaction chamber 26 was measured to have a pH of 0.5–1.0. The acidic electrolyte 64 was arranged to flow into the calcium extractor 12. The acidic electrolyte 64 was believed to dissolve most of the reactive calcium from the waste cement 74. The Ca2+-rich eluent 82 from the column 12 was recirculated to the cathode chamber 20. Unreacted cement and solid byproducts (e.g., SiO2 verified by X-ray diffraction) were captured on cotton filters in the extractor 12. The Ca2+-rich solution 82 reacts in the cathode chamber 20 with caustic produced at the cathode 22 to form Ca(OH)2 precipitate and H2 gas. The Ca(OH)2 precipitate and H2 gas were transported to the lime extractor 16 for separation and isolation of Ca(OH)2. The eluent 100 from the lime extractor 16 was recirculated to the chemical reaction chamber 20 to close the loop for the electrolyte solution.

[0313] The electrolysis experiments were operated in galvanostatic mode at current densities of 100 mA cm–2, 200 mA cm–2, and 300 mA cm–2. When operated at a current density of 200 mA cm–2, the generation of H+and OH–ions in the closed-loop electrolyte 64 was tracked using a pH indicator which indicated an effective range of pH of 4 to 10. 1.1 - Correlation of flow rate to the pH of recirculating electrolyte

[0314] During electrolysis, the chemical reaction chamber receives alkaline electrolyte saturated with Ca(OH)2 from the lime extractor (Fig.9a). Based on the Ksp of Ca(OH)2 and the Ca2+concentration at room temperature (Ksp, Ca(OH)2 = 6.9 × 10-6at 20 ℃, [Ca2+] = 0.2 M), the pHtheoreticalis calculated to be 11.8 when entering the chemical chamber of the cement electrolyzer. The inventors believe that this strongly alkaline electrolyte needs to become acidic before it exits the chemical chamber and enters the calcium extractor. A lower electrolyte flow rate is predicted to enable a higher pH swing by providing a longer residence time of the electrolyte in the electrolyzer toreceive more H+ions from the bipolar membrane.

[0315] To evaluate the impact of flow rate on waste cement decomposition, the inventors calculated the theoretical proton efficiency (PE) in the chemical chamber as a function of flow rate (Equation (16), which can be substituted to yield Equation (17)): ^^^^ =(^^ெ ^^^^^^௧^ௗ ுశ) ି (ைுష^^^^ ^^^^௧^^^௬௧^) ^^ெ ^^^^^^௧^ௗ ுశ× 100% (16), F is the Faraday constant (96485.3321 s A mol–1), [OH–] is the concentration of OH–ions in the electrolyte (6.28×10–3M, calculated from pHtheoretical), Q is the flow rate of the closed-loop electrolyte (mL min–1). Importantly, a higher PE generates a more acidic electrolyte, and, in turn, increases the rate of waste cement decomposition in the calcium extractor. FIG.9b, d, and f shows theoretical PE values as a function of flow rate.

[0316] The actual PE was measured by analyzing the pH of the closed-loop electrolyte at different flow rates. The pH of the closed-loop electrolyte within the reactor at the inlet (pHin, calcium) and outlet (pHout, calcium) of the calcium extractor and the inlet (pHin, lime) of the lime extractor were measured (FIG.9a). No measurable change in pH between pHin, lime and pHout, lime at the outlet of the lime extractor was detected, so pHout, limewas not included. At 200 mA cm–2, a sharp increase in pHin, calciumfrom 0.61 to 11.28 when the flow rate was increased from 2.5 mL min–1to 20 mL min–1was observed (FIG.9e). Higher pHin, calcium values indicated less effective Ca2+extraction from waste cement at higher flow rates. The pHout, calciumand pHin, lime remained relatively stable at around 10 and 12, respectively. FIG.9d (pink line) compares the actual PE with the theoretical values, emphasizing the importance of maintaining a low flow rate.

[0317] FIGs.9b, c, f, g are plots showing that flow rates of 1.5 mL min–1, 5.0 mL min–1, and 10.0 mL min–1achieve a PE of 85–90% at current densities of 100 mA cm–2, 200 mA cm–2, and 300 mA cm–2, respectively.1.2 - Cement electrolyser performance

[0318] Key electrolyzer performance parameters for the cement recycler under different conditions were evaluated. Voltages of 4.6 V, 6.3 V, and 7.7 V were required to drive electrolysis at current densities of 100 mA cm–2, 200 mA cm–2, and 300 mA cm–2, respectively (FIG.10a). These voltages were measured at the idealized flow rates determined in the previous section (1.5 mL min–1, 5.0 mL min–1, and 10.0 mL min–1at 100 mA cm–2, 200 mA cm–2, and 300 mA cm–2, respectively.). As anticipated, the voltage increased with higher current densities.

[0319] The concentration of Ca2+in the closed-loop electrolyte over time was evaluated to ensure that steady-state conditions were maintained during electrolysis. Since an initial addition of 0.2 M Ca2+was made to the electrolyte to promote Ca(OH)2 precipitation, which is slightly soluble in water, it is essential that this Ca2+not be depleted during the process. To maintain steady electrolyte composition, the Ca2+consumed at the cathode must be replenished by the calcium extractor. To confirm the effective extraction of Ca2+from waste cement, aliquots of the closed-loop electrolyte at the outlet of the calcium extractor were collected at regular intervals. The aliquots were analyzed using inductively-coupled plasma optical emission spectroscopy (ICP-OES) to monitor Ca2+concentrations of the closed-loop electrolyte over time ([Ca2+]out, calcium, FIG.10b). During a 2-hour electrolysis experiment at 200 mA cm–2, stable [Ca2+]out, calcium with a variance of less than 2.5% was observed (FIG. 10b, dark blue). Experiments at 100 mA cm–2and 300 mA cm–2also showed stable calcium readings. A control experiment using an electrolyte without added Ca2+at 200 mA cm–2for 2 hours was performed (FIG.10b, light blue). In this case, the [Ca2+]out,calciumstabilized at 670 ppm (0.02 M) after 30 minutes, with only trace amounts of Ca(OH)2 formed (yield of 3.6%, Table 2). The inventors believe that this is likely due to the ion product being below the Ksp, Ca(OH)2.

[0320] The long-term stability of the voltage (FIG.10c, pink) and [Ca2+]out, calcium (FIG. 10c, blue) in the closed-loop electrolyte over a 15-hour period at 100 mA cm–2was explored. A gradual increase in voltage was observed. The gradual increase is believed to be primarily due to the deposition of Ca(OH)2on the cation exchange membrane and the cathode during electrolysis. However, this voltage increase could be mitigated by cleaning the cathode chamber with 0.5 M HCl for 5 minutes aftereach hour of operation. Following these hourly cleanings throughout the 15-hour duration, a voltage increase of approximately 40% was recorded. Notably, the [Ca2+]out, calcium in the closed-loop electrolyte remained stable under the same operational conditions. Overall, these results demonstrate that the described cement recycler can sustain the production of Ca(OH)2over prolonged periods without significant declines in voltage or [Ca2+]out, calcium. 1.3 - Characterization of gaseous and solid products in the cement recycler

[0321] The concentrations and purities of the gaseous (O2, CO2, and H2 within the anode chamber, calcium extractor, and lime extractor, respectively) and solid products (Ca(OH)2in the lime extractor, SiO2in the calcium extractor) in the cement recycler were measured.

[0322] Gas chromatography was used to confirm O2, CO2, and H2 gaseous products at the anode, chemical, and cathode chambers, respectively. Gas chromatography was then used to quantify CO2 concentrations emitted from the calcium extractor [CO2]calcium extractor containing limestone as a control experiment. When purging the calcium extractor with N2 gas (flow rate = 200 sccm) during 10-minute electrolysis at 100, 200, and 300 mA cm–2, high [CO2]calcium extractor values of 7,600, 14,000, and 21,000 ppm were measured (FIG.11a). The same experiments using aged waste cement yielded [CO2]calcium extractor values that were 75 - 80% lower than those with the limestone (FIG.11a). The same experiments at 100 - 300 mA cm–2with fresh waste cement yielded [CO2]calcium extractor concentrations below 280 ppm (FIG.11a). The lower [CO2]calcium extractor values are consistent with a lower carbonate content of the waste cement relative to limestone. Given that the atmospheric CO2concentrations are 400 ppm, the inventors include that the described cement recycler operates with effectively zero CO2 emissions when electrolyzing fresh waste cement.

[0323] After 2-hour electrolysis experiments at 200 mA cm–2, Ca(OH)2 was collected from the calcium extractor and the SiO2 from the lime extractor. Fourier-transform infrared spectroscopy confirmed the presence of Ca(OH)2 in the product collected from the lime extractor. Powder X-ray diffraction and reference intensity ratios (FIG. 11b) were used to identify and quantify the relative amounts of Ca(OH)2 and byproducts (e.g., CaCO3, CaSO4, and KHCO3; Table 1). The electrolysis productsusing both fresh and aged waste cement contained 79 to 92% Ca(OH)2 by mass. The solid in the calcium extractor was >90% SiO2.

[0324] To calculate the yield of the desired Ca(OH)2 product, the following equation was used: ^^^^^^^^^^ =^^(ைு)మ^௫^^^^^^^௧^^^௬ ^^^^^^௧^ௗ ^^(ைு)మ ௧^^^^^௧^^^ ^^^^^^௧^ௗ × 100% (18)gives: ^^^^ =ூ·௧ / ி ି [ைுష]·ொ ூ·௧ / ி× 100% (19)where ^^^^(ைு)మis the molar mass of Ca(OH)2in g mol–1. The results showed that the yield of Ca(OH)2 was 55% from aged waste cement, and 66–80% from fresh waste cement after 2 h of electrolysis at 200 mA cm–2(Table 2). Table 1. Composition of the Ca(OH)2 product collected from the lime extractor after 2 h of electrolysis at 200 mA cm–2. Raw material Ca(OH)2 (wt%) CaCO3 (wt%) CaSO4 (wt%) KHCO3 (wt%) White PortlandCementAll 79 1 2 12 Aged wasteTable 2. Calculated yield of Ca(OH)2collected from the lime extractor after 2 h of electrolysis. Current density (mA Ca(OH)2 actually Raw material–2Yield (%) cm ) collected (g) Aged waste cement 100 0.47 37.9Aged waste cementAged waste cement 300 1.40 37.6St. Marys 200 1.79 72.2CementAll 200 1.74 80.2

[0325] In this Example, the inventors demonstrate the electrochemical conversion of fresh and aged waste cement into Ca(OH)2 and SiO2. Both of which are key precursors for the production of cement clinker. Various cementitious feedstocks, including three commercial cement products (termed as fresh waste cement), and aged waste cement from a local building demolition were successfully recycled using the described cement recycler. High yields of up to 80% of Ca(OH)2 with purities ranging from 79% to 92% were produced (Tables 1 and 2).

[0303] The purity of the electrochemically produced Ca(OH)2 meets the requirements for direct use in kiln-based clinker production, as stipulated by European Standard EN 197-1 (required purity of CaCO3feedstock: >75%). The impurities present in Ca(OH)2, such as CaCO3, CaSO4, and KHCO3, are thermally decomposed into CaO and K2O during clinker production at temperatures exceeding 1,450 °C, and thus should not have a negative impact on the quality of the clinker.

[0304] Both fresh and aged waste cement produced Ca(OH)2 of equally high purity.However, the yield from aged cement was lower, likely due to the presence of inactive aggregates. When using aged cement as the feedstock, the highest yield (55%) was achieved at a current density of 200 mA cm–2(Table 2). The lower yield (38%) at lower current density (100 mA cm–2) suggests insufficient OH⁻ was generated at the cathode to efficiently form Ca(OH)2. At higher current density (300 mA cm–2), the inventors conjecture that the lower yield (38%) is attributed to a pH imbalance, such as parasitic H⁺ ion migration into the cathode chamber.

[0326] The cement recycler operation depends on the continuous supply of Ca2+ions in the supporting electrolyte, which precipitate as Ca(OH)2 product during electrolysis. The inventors observed the need to add Ca2+ions (via 0.2 M CaCl2) at the beginning of the experiment to drive effective Ca(OH)2production. In a control experiment without the additional CaCl2, significantly reduced yields (<4%) were observed. This suggests that a sufficient [Ca2+]out, calcium may be needed to drive the precipitation reaction at the cathode. If the [Ca2+]out, calciumis too low, OH⁻ ions will react with H⁺ ions from the chemical chamber and drive a rapid pH increase in the calcium extractor.

[0327] The described cement recycler system efficiently separates gaseous (O2, CO2, and H2) and solid products (Ca(OH)2and SiO2) (FIG.8). Based on the inventors’ estimates, the combustion of H2 and O2 produced by the electrolyzer at 88% efficiency is sufficient to fuel a kiln for clinker production, and could therefore eliminate outright the need for fossil fuels. This combined with the near-zero CO2emissions during electrolysis of fresh waste cement (<280 ppm at 100–300 mA cm–2) provides a clear opportunity for genuinely zero-emission cement. While aged waste cement will produce higher levels of CO2, the levels of 1,000 to 4,000 ppm are significantly lower than those emitted from natural limestone (~20,000 ppm).

[0328] The presently described electrochemical process offers a carbon-neutral pathway for recycling waste cement, significantly reducing CO2emissions compared to traditional methods, and providing the opportunity to reduce cementitious waste. Example 2 – Production of metal hydroxides involving redox shuttle molecules

[0329] An electrochemical cell of the type illustrated in FIGS.3A, 3B and the method of performing electrolysis illustrated in FIGS.5A and 4B were used to convert wastecement as the metal ion-containing solids 74 into calcium hydroxides 90.

[0330] Three different electrolyzer configurations were tested. They are referred to as the Control Electrolyzer, 3-Chamber Electrolyzer, and 2-Chamber Electrolyzer. The oxidation and reduction reactions that are postulated to occur in each of the electrolyzer configurations are shown in Table 3). Table 3. Summary of Differentiated Features of the Three Electrolyzers Tested in this Example. Control 3-Chamber 2-Chamber Electrolyzer Electrolyzer Electrolyzer Number of 3 3 2 chambers Membrane(s) BPM, CEM BPM, CEM CEM Anode reaction2OH–→ 1 / 2O2(g) +H2AQ → AQ + 2H+H2AQ + CaCO3(s)H2+2O + 2e–+ 2e–→ AQ + Ca + CO2(g)+ H2O + 2e–Cathode reaction2H2O + 2e–→ H2(g)AQ + H2O + 2e–→ AQ + H2O + 2e–→ + 2OH–H2AQ + 2OH–H2AQ + 2OH–

[0331] In these Examples, the three-chamber electrolyzer was purchased from Dioxide MaterialsTM. The three-chamber electrolyzer 14 comprises a bipolar membrane (BPM, Fumasep FBMTM) which separates the anode chamber 18 from the chemical reaction chamber 26. A cation exchange membrane 24 (CEM, NafionTMN117) separates the chemical reaction chamber 26 from the cathode chamber 20.

[0332] The anode 19 and cathode 22 selected for use in the Control Electrolyzer each comprises a Nickel foam. The active area of the anode and cathode is 5 cm2. The anolyte 54 that was supplied to the anode chamber 18 comprises 1 M KOH. The electrolyte 56 that was supplied to the cathode chamber 20 comprises 1M KCl.

[0333] The anode 19 and cathode 22 selected for use in the 3-Chamber Electrolyzer and 2-Chamber Electrolyzer each comprises graphite felt. The anolyte 54 andelectrolyte 56 that were supplied to the anode and cathode chambers 18, 20 respectively, comprises 0.1 M H2AQ and 0.1 M AQ in 1 M potassium acetate (KOAc) respectively.

[0334] The shuttle molecules 40 and hydrogenated shuttle molecules 42 comprise dihydroanthraquinone-2,7-disulfonic acid disodium salt (H2AQ) and anthraquinone- 2,7-disulfonic acid disodium salt (AQ).

[0335] The cation exchange membrane 24 comprises a Ca2+-blocking polyaniline layer. A polyaniline-coated CEM blocks the diffusion of Ca2+through the membrane. To test the efficacy of this membrane for blocking Ca2+, the inventors quantified the amount of Ca2+ions in the anolyte and electrolyte by inductively-coupled plasma optical emission spectroscopy (ICP-OES) for the 2-Chamber Electrolyzer. After 120 min (4 Cycles) of electrolysis at 100 mA cm–2, 0.3 ppm of Ca2+was detected in the electrolyte. This result indicates that >98.9% of Ca2+ions were prevented from crossing the membrane into the electrolyte over the course of each experiment.

[0336] The metal ion-containing solids 74 were suspended in the anolyte 54 (i.e., 0.1 M H2AQ in 1 M KOAc) for delivery to the anode chamber 18 for electrolysis in the 2- Chamber Electrolyzer.

[0337] The metal ion-containing solids 74 were suspended in the electrolyte (i.e., 1M KCl or 0.1 M AQ in 1 M KOAc) for delivery to the chemical reaction chamber 26 for electrolysis in the 3-Chamber Electrolyzer.

[0338] For all electrolyzer configurations, the flow rates at which the anolyte 54 and elecvtrolyte 56 are supplied to the respective chambers are 150 mL min-1.

[0339] The experiments were designed to electrolytically produce H+, convert limestone into Ca2+ions, and to produce OH–under continuous flow in the electrochemical reactors. The Ca2+ions and OH–were then mixed in a batch process within a separate reactor referred to as the “Calcium Reactor”.

[0340] Each experimental campaign consisted of three iterative phases (FIG.12). During Phase 1, electrolysis was performed for at least 30 minutes while recirculating the same electrolyte through each of the anode and cathode chambers. Because the anolyte and catholyte were not mixed during the electrolysis experiments, the concentrations of Ca2+and AQ would build up with time in the anolyte, while H2AQ and OH–formed with time in the catholyte. Electrolysis was then stopped. Phase 2involved manually stirring the Ca2+-enriched anolyte and the OH–-enriched catholyte in an independent round bottom flask (Calcium Reactor) to precipitate Ca(OH)2(s)(FIG.12). For Phase 3, the Ca(OH)2(s) product was isolated, and resultant electrolyte solution was then reused for electrolysis experiments. Half of the solution was used as the anolyte, the other half as the catholyte. Phase 1, 2 and 3 were then repeated in succession. Each repeat is denoted a “Cycle”. The experimental campaign reported here was performed for at least 4 cycles. 2.1 - Cement electrolysis experiments

[0341] In these experiments, electrolysis was performed at 100 mA cm–2and 20 °C for 30 minutes unless stated otherwise. An Ecell of 4.6 V for the Control Electrolyzer was recorded. An Ecell of 2.0 V and 1.3 V at 100 mA cm–2for 3-Chamber Electrolyzer and 2-Chamber Electrolyzer, respectively, and 13.0 V and 8.1 V at 1 A cm–2were recorded (FIG.13 and Table 4). Table 4. Cell Voltages Required to Drive Electrolysis at 100 and 1000 mA cm–2. All reported voltages are those measured at 5 min during Cycle 1.aFull cell voltage (V) J (mA cm–2) Control 3-Chamber 2-Chamber Electrolyzer Electrolyzer Electrolyzer 100 4.6 2.0 1.3 (0.3b; 0.4c) 1000 16.4 13.1 8.1 (5.0b; 4.2c)aData recorded at an aqueous electrolyte temperature of 20 °C.bData recorde n aqueous electrolyte temperature of 60 °C.cd at a Data recorded at an H2O / MeCN (4:1 v:v) electrolyte temperature of 60 °C.

[0342] The voltages for each of the electrolyzers did not stay constant over the course of the electrolysis experiments, likely due to the build-up of ions during electrolysis. Notwithstanding, the drift in voltage was not as significant for 2-Chamber Electrolyzer as it was for the three-chamber electrolyzers. The voltage increased by 0.3 V for the 2-Chamber Electrolyzer over 30 minutes of electrolysis at 100 mA cm–2(FIG.15), and by >1 V within 18 min of electrolysis in the Control Electrolyzer and the 3-Chamber Electrolyzer.

[0343] While all the electrolyzers are continuous flow reactors, the electrolyzers were not coupled to the Calcium Reactor to minimize complexity for convenience in these examples. However it is to be understood that some embodiments of the invention may comprise a complete continuous flow system incorporating the Calcium Reactor or a similar unit. As such, the precipitation of Ca(OH)2(s) was carried out as a batch process in the Calcium Reactor, and the filtrate (i.e., the electrolyte) was then reused in a successive cycle. The Ecell increased by <0.2 V over 4 electrolyte recycling cycles for each of the electrolyzers (FIG.14b).

[0322] For these experiments, a Faradaic efficiency (FE) of 100% for i-CO2(g)generation for the 2-Chamber Electrolyzer was measured (FIG.13c), a surrogate indicator of all electrochemically generated H+reacting with CaCO3(s). The FE was merely 30% for the 3-Chamber Electrolyzer. These measurements were qualitatively supported through video monitoring of the system, and observing a higher amount of i-CO2(g) being generated in the 2-Chamber Electrolyzer, where i-CO2(g) bubbles adhere to and bring the CaCO3(s) microparticles to the surface of the anolyte slurry. 2.3 - Identification of solid byproducts in the anode chamber

[0344] Solid byproduct formation was observed at the anode during electrolysis at room temperature using the 2-Chamber Electrolyzer. Specifically, a dark precipitate was found along the flow channels at the graphite felt surface. The formation of this precipitate was commensurate with increases in anode resistances and cell voltages during electrolysis (FIG.15). These solid byproducts were deemed to be dimers of the quinone species based on analysis with scanning electron microscopy with energy dispersive X-ray spectroscopy (EDX, ultraviolet–visible (UV-vis) spectroscopy, 1H NMR, and mass spectrometry (MS). At elevated temperatures of >60 °C, the formation of byproducts was not observed on the electrode surface, and the voltage readings remained relatively constant during electrolysis (FIG.15). 2.4 - Temperature-dependent electrolysis experiments

[0345] To address the aforementioned solid byproduct formation, the inventors ranexperimental campaigns with the electrolyte at progressively higher temperature using the 2-Chamber Electrolyzer. When the temperature was increased from 20 to 60°C, the Ecell was reduced from 1.31 V to 0.32 V at 100 mA cm−2, and from 11.44 V to 5.03 V at 1 A cm−2(FIG.13a). A further voltage improvement was not observed at the higher temperature of 80°C.

[0346] The inventors hypothesized that the decrease in Ecell was due to improved diffusion of (H2)AQ molecules. To validate this hypothesis, the inventors determined the diffusion coefficient by performing cyclic voltammetry measurements on aqueous (H2)AQ (1:1) electrolyte. This analysis showed that the diffusion coefficient increased from 2.5 × 10–4to 6.09 × 10–4cm2s–1when heating the electrolyte from 20°C to 60°C. 2.5 - Electrolyte engineering

[0347] The inventors attempted to further improve the electrolyzer performance by adding 20% MeCN to the electrolyte, and then measured the Ecell for the 2-Chamber Electrolyzer again over the 20 to 80°C range (FIG.13b). Experiments with a H2O / MeCN (4:1) mixed electrolyte at 60°C achieved an Ecell of 0.38 V at 100 mA cm−2, which was slightly higher than the 0.32 V measured with the aqueous electrolyte at the same temperature.

[0348] At higher current densities and 60°C, the addition of MeCN in the electrolyte played an important role on electrolyzer performance. Specifically, at 1 A cm–2the voltage was 4.23 V and 5.03 V with and without MeCN in the electrolyte, respectively (Table 4). 2.6 - The effects of pH

[0349] The electrolyte pH was monitored as a function of time during each electrolysis experiment. When using a H2O / MeCN mixed electrolyte for the 2-Chamber Electrolyzer, the anolyte and electrolyte reached a steady-state pH of 6.3 and 13.2, respectively, after 30 min (FIG.14a). The high pH of the catholyte enabled rapid precipitation of Ca(OH)2(s) in the Calcium Reactor. The desired Ca(OH)2(s) product for each experiment was confirmed by Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) measurements. Notably, the same experiments with the aqueous electrolyte yielded a lower catholyte pH of 11.8, which proved insufficient forCa(OH)2(s) precipitation.

[0350] In summary, the experimental results demonstrate that the 2-Chamber Electrolyzer is capable of decomposing limestone Ca(CO3)(s) under continuous flow with high energy and Faradaic efficiencies. This electrolyzer produces independent product streams from the anode and cathode chambers that may be mixed in an independent reactor to react Ca2+and OH–ions to form the targeted Ca(OH)2(s) product. This solid product may be isolated and used for downstream processing into cement clinker, while the remaining electrolyte can be delivered back to the cement electrolyzer for successive reaction with limestone feedstock.

[0351] The zero-gap two-chamber configuration of this electrolyzer, coupled to the use of organic redox mediators and mixed solvents (e.g., adding organic solvents to the electrolyte such as a H2O / MeCN (4:1) mixed electrolyte system), enabled limestone decomposition at exceptionally low voltages and with high Faradaic efficiency. 2.7 - Materials and Methods Faradaic efficiency calculation

[0352] The Faradaic efficiency (FE) for gaseous product i-CO2(g) of limestone decomposition generated by the electrolyzers was measured while operating at constant current densities of 50–200 mA cm–2. The inventors quantified FE values by measuring the concentration of i-CO2(g)in the gaseous product stream using a gas chromatograph (GC) and determining the mole fraction of CO2 in the gaseous mixture, χ. The FE for each gaseous product k was then determined in accordance with Equation 20: (20) ^^^^ ^^^^^^^^^^^= ^^ where nk is the number of electrons exchanged (nCO2 = 2), F is Faraday’s constant (F = 96,485 C / mol), Fm is the molar flow rate in mol s–1, and I is the total current in A. The molar flow rate is derived from the volumetric flow rate Fv by the relation Fm = pFv / RT, where p is the atmospheric pressure in Pa, R is the ideal gas constant (8.314 J mol–1K-), and T is the temperature in degrees Kelvin.Other characterization methods

[0353] The X-ray diffraction (XRD) patterns of the Ca(OH)2 product were obtained on an Empyrean system using Cu Kα radiation (λ = 1.5405 Å) as the X-ray source with a θ−2θ model. Fourier-transform infrared (FTIR) spectra were measured in a Bruker Invenio-RTMFTIR Spectrometer with RT-DLaTGS detector. Ultraviolet-visible (UV-vis) spectra were measured in an Agilent Cary 7000 Universal Measurement Spectrometer. The concentration of Ca2+cations in the anolyte and catholyte was measured by ICP-OES.

[0354] Voltage (J–V) responses of electrolyzers to current densities from 50 to 1000 mA cm–2were measured using a KeithleyTM22805-32-6 DC Power Supply. In stability tests, the recording of electrolyzer voltages started after 5 min of stabilization. Cyclic Voltammetry (CV) measurements were obtained with an electrochemical workstation (CHI660E, CH InstrumentsTM, Inc) at a scan rate from 0.1 to 0.8 V s–1and a sample interval of 1 mV. Example 3 – Production of calcium silicate hydrate (CSH, Ca2SiO4·H2O(s)) and its conversion into the primary phases of ordinary Portland cement (OPC), belite (Ca2SiO4(s)) and alite (Ca3SiO5(s))

[0355] An electrochemical cell of the type illustrated in FIG.5A and the method of performing electrolysis illustrated in FIG.6 were used to produce calcium silicate hydrate from calcium carbonate and silica as feedstocks.

[0356] Referring to FIGS.16-22, the results of the experiments show that when the electrolysis was performed at 60°C, the calcium silicate hydrate (CSH or Ca2SiO4·H2O(s)) yield was 90%. When the electrolysis was performed at 20°C, the yield of CSH was 48%. Yields were determined from the moles of silica (SiO2(s)) input into the cement electrolyzer. The X-ray diffractogram (XRD) pattern only showed the presence of hydrated lime (Ca(OH)(s)) and CSH in the product (FIG.21). The obtained material is expected to be a mixture of CSH and Ca(OH)(s), which is intended for producing alite and belite. Some minor signals for calcium carbonate (CaCO(s)) were observed in the XRD, but the inventors postulate that such is likely due to the reaction of Ca(OH)₂with carbon dioxide (CO(g)) in the air during the sample transfer and XRDmeasurement. Example 4 – Production of magnesium silicate hydrate (MgSH, Mg2SiO4·H2O(s))

[0357] An electrochemical cell of the type illustrated in FIG.5A and the method of performing electrolysis illustrated in FIG.6 were used to produce magnesium silicate hydrate from magnesium carbonate and silica as feedstocks.

[0358] Referring to FIG.23, the results of the experiments show that when the electrolysis was performed at 60°C, the magnesium silicate hydrate (MgSH or Mg2SiO4·H2O(s)) and other derivative yield was 100%. The X-ray diffractogram (XRD) pattern showed the presence of forsterite (Mg2SiO4(s)) (36.5%), MgSiO3 (25.9%), and enstatite (Mg2Si2O6(s)) (37.6%). Interpretation of Terms

[0359] Unless the context clearly requires otherwise, throughout the description and the claims: ● “comprise”, “comprising”, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”; ● “connected”, “coupled”, or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof; ● “herein”, “above”, “below”, and words of similar import, when used to describe this specification, shall refer to this specification as a whole, and not to any particular portions of this specification; ● “or”, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list; ● the singular forms “a”, “an”, and “the” also include the meaning of any appropriate plural forms. These terms (“a”, “an”, and “the”) mean one or more unless stated otherwise; ● “and / or” is used to indicate one or both stated cases may occur, for exampleA and / or Β includes both (A and Β) and (A or Β); ● “approximately” when applied to a numerical value means the numerical value ± 10%; ● where a feature is described as being “optional” or “optionally” present or described as being present “in some embodiments” it is intended that the present disclosure encompasses embodiments where that feature is present and other embodiments where that feature is not necessarily present and other embodiments where that feature is excluded. Further, where any combination of features is described in this application this statement is intended to serve as antecedent basis for the use of exclusive terminology such as "solely," "only" and the like in relation to the combination of features as well as the use of "negative" limitation(s)” to exclude the presence of other features; and ● “first” and “second” are used for descriptive purposes and cannot be understood as indicating or implying relative importance or indicating the number of indicated technical features.

[0360] Words that indicate directions such as “vertical”, “transverse”, “horizontal”, “upward”, “downward”, “forward”, “backward”, “inward”, “outward”, “left”, “right”, “front”, “back”, “top”, “bottom”, “below”, “above”, “under”, and the like, used in this description and any accompanying claims (where present), depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.

[0361] Where a range for a value is stated, the stated range includes all sub-ranges of the range. It is intended that the statement of a range supports the value being at an endpoint of the range as well as at any intervening value to the tenth of the unit of the lower limit of the range, as well as any subrange or sets of sub ranges of the range unless the context clearly dictates otherwise or any portion(s) of the stated range is specifically excluded. Where the stated range includes one or both endpoints of the range, ranges excluding either or both of those included endpoints are also included in the invention.

[0362] Certain numerical values described herein are preceded by "about". In thiscontext, "about" provides literal support for the exact numerical value that it precedes, the exact numerical value ±5%, as well as all other numerical values that are near to or approximately equal to that numerical value. Unless otherwise indicated a particular numerical value is included in “about” a specifically recited numerical value where the particular numerical value provides the substantial equivalent of the specifically recited numerical value in the context in which the specifically recited numerical value is presented. For example, a statement that something has the numerical value of “about 10” is to be interpreted as: the set of statements: ● in some embodiments the numerical value is 10; ● in some embodiments the numerical value is in the range of 9.5 to 10.5; and if from the context the person of ordinary skill in the art would understand that values within a certain range are substantially equivalent to 10 because the values with the range would be understood to provide substantially the same result as the value 10 then “about 10” also includes: ● in some embodiments the numerical value is in the range of C to D where C and D are respectively lower and upper endpoints of the range that encompasses all of those values that provide a substantial equivalent to the value 10.

[0363] Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and / or acts with equivalent features, elements and / or acts; mixing and matching of features, elements and / or acts from different embodiments; combining features, elements and / or acts from embodiments as described herein with features, elements and / or acts of other technology; and / or omitting combining features, elements and / or acts from described embodiments.

[0364] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with thefeatures of any other described embodiment(s) without departing from the scope of the present invention.

[0365] Any aspects described above in reference to apparatus may also apply to methods and vice versa.

[0366] Any recited method can be carried out in the order of events recited or in any other order which is logically possible. For example, while processes or blocks are presented in a given order, alternative examples may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and / or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, simultaneously or at different times.

[0367] Various features are described herein as being present in “some embodiments”. Such features are not mandatory and may not be present in all embodiments. Embodiments of the invention may include zero, any one or any combination of two or more of such features. All possible combinations of such features are contemplated by this disclosure even where such features are shown in different drawings and / or described in different sections or paragraphs. This is limited only to the extent that certain ones of such features are incompatible with other ones of such features in the sense that it would be impossible for a person of ordinary skill in the art to construct a practical embodiment that combines such incompatible features. Consequently, the description that “some embodiments” possess feature A and “some embodiments” possess feature B should be interpreted as an express indication that the inventors also contemplate embodiments which combine features A and B (unless the description states otherwise or features A and B are fundamentally incompatible).This is the case even if features A and B are illustrated in different drawings and / or mentioned in different paragraphs, sections or sentences.

[0368] It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions, and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth inthe examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

CLAIMS 1. A method of producing metal hydroxides from metal-ion containing solids, the method comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode chamber, a cathode chamber, and a separator separating the anode chamber and the cathode chamber; oxidizing an oxidation reactant, at the anode, to form an oxidation product; reducing a reduction reactant, at the cathode, to form hydroxide ions; flowing electrolyte from the electrochemical cell to a solid-liquid extractor comprising the metal-ion containing solids; extracting, in the solid-liquid extractor, metal ions from the metal-ion containing solids, thereby forming a metal-ion-enriched electrolyte comprising the metal ions; flowing the metal-ion-enriched electrolyte from the solid-liquid extractor to the cathode chamber; reacting, at the cathode chamber, the metal ions with the hydroxide ions to form metal hydroxide; and flowing the metal hydroxide formed from reacting the metal ions with the hydroxide ions out of the cathode chamber.

2. The method as defined in claim 1, wherein the flowing of the metal hydroxide out of the cathode chamber comprises flowing a mixture comprising the metal hydroxide and electrolyte out of the cathode chamber.

3. The method as defined in claim 2, further comprising: supplying the mixture to a filtration unit; and separating, in the filtration unit, the metal hydroxide from the mixture comprising the electrolyte.

4. The method as defined in claim 3, further comprising recycling the separated electrolyte to the electrochemical cell.

5. The method as defined in any one of the preceding claims, wherein a distance between the anode and cathode is the thickness of the separator.

6. The method as defined in any one of the preceding claims, wherein the anode chamber comprises an anode flow field layer positioned adjacent to the anode, and the cathode chamber comprises a cathode flow field layer positioned adjacent to the cathode.

7. The method as defined in any one of the preceding claims, wherein the flowing of the electrolyte from the electrochemical cell to the solid-liquid extractor comprises flowing the electrolyte from the anode chamber to the solid-liquid extractor.

8. The method as defined in any one of claims 4 to 7, wherein the step of recycling the separated electrolyte to the electrochemical cell comprises supplying the separated electrolyte to the anode chamber.

9. The method as defined in any one claims 1 to 4, wherein the electrochemical cell additionally comprises a chemical reaction chamber and a bipolar membrane separating the anode chamber and the chemical reaction chamber, and wherein the separator is positioned between the chemical reaction chamber and the cathode chamber, wherein the method further comprises electrochemically dissociating water, within the bipolar membrane, into hydrogen ions and hydroxide ions.

10. The method as defined in claim 9, wherein the flowing of electrolyte from the electrochemical cell to the solid-liquid extractor comprises flowing the electrolyte from the chemical reaction chamber to the solid-liquid extractor.

11. The method as defined in claim 10, wherein the step of recycling of the separated electrolyte to the electrochemical cell comprises supplying the separated electrolyte to the chemical reaction chamber.

12. The method as defined in any one of the preceding claims, further comprising flowing a supply of an anolyte into the anode chamber.

13. The method as defined in claim 12, wherein the anolyte comprises a base.

14. The method as defined in any one of the preceding claims, wherein the oxidation reactant comprises hydroxide ions.

15. The method as defined in claim 14, wherein the hydroxide ions are contained in the anolyte.

16. The method as defined in any one of claims 1 to 13, wherein the oxidation reactant comprises a hydrogenated shuttle molecule.

17. The method as defined in claim 16, further comprising flowing a supply of hydrogenated shuttle molecules or hydrogenated shuttle molecules contained in the anolyte into the anode chamber.

18. The method as defined in any one of the preceding claims, wherein the reduction reactant comprises a hydrogen-containing reactant.

19. The method as defined in claim 18, wherein the hydrogen-containing reactant comprises water.

20. The method as defined in any one of claims 1 to 17, wherein the reduction reactant comprises shuttle molecules.

21. The method as defined in claim 20, wherein the reducing of the reductant reactant at the cathode to form hydroxide ions is performed in the presence of a hydrogen-containing reactant.

22. The method as defined in claim 20 or 21, further comprising flowing a supply of shuttle molecules or shuttle molecules contained in a solution comprising the hydrogen-containing reactant into the cathode chamber.

23. The method as defined in any one of the preceding claims, wherein the reducing of the reduction reactant at the cathode to form hydroxide ions is performed in a catholyte.

24. The method as defined in claim 23, further comprising flowing a supply of the catholyte into the cathode chamber.

25. The method as defined in any one of claims 9 to 24, further comprising flowing a supply of electrolyte into the chemical reaction chamber.

26. The method as defined in claim 25, further comprising circulating a flow of electrolyte in a closed loop between the cathode chamber and the chemical reaction chamber.

27. The method as defined in any one of claims 23 to 26, wherein the catholyte and / or electrolyte comprise a pH-neutral aqueous solution.

28. The method as defined in any one of claims 23 to 27, wherein the catholyte and / or electrolyte comprise a pH in the range of from about 6 to about 8.

29. The method as defined in any one of claims 23 to 28, wherein the anions and / or cations of the catholyte and / or electrolyte are monovalent.

30. The method as defined in any one of claims 23 to 29, wherein the anions of the catholyte and / or electrolyte comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-), perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H–).

31. The method as defined in claim 29 or 30, wherein the cations of the catholyte and / or electrolyte comprise an alkali metal ion.

32. The method as defined in claim 31, wherein the alkali metal ions comprise one or more of lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), cesium (Cs+).

33. The method as defined in claim 23 to 32, wherein the cations of the catholyte and / or electrolyte comprise one or more of hydrogen (H+), ammonium (NH4+), hydronium (H3O+), tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

34. The method as defined in any one of claims 23 to 33, wherein a concentration of the catholyte and / or electrolyte is in the range of from about 0.1 M to about 3 M.

35. The method as defined in any one of the preceding claims, further comprising maintaining the pH in the cathode chamber at not less than about 9.

36. The method as defined in any one of the preceding claims, further comprising maintaining the pH in the cathode chamber at not less than about 12.

37. The method as defined in any one of the preceding claims, further comprising maintaining the pH in the cathode chamber between about 9 and about 12.

38. The method as defined in any one of the preceding claims, wherein the metal- ion containing solids comprise limestone, cement, concrete, or combinations thereof.

39. The method as defined in any one of the preceding claims, wherein the metal- ion containing solids comprise metal carbonate, metal silicate, and / or metal oxide.

40. The method as defined in any one of the preceding claims, wherein the metal ions comprise calcium (Ca2+) ions.

41. The method as defined in the preceding claim, wherein the metal hydroxide comprises calcium hydroxide ((Ca(OH)2).

42. The method as defined in any one of the preceding claims, further comprising collecting silica dioxide (SiO2) from the solid-liquid extractor in the extracting step.

43. The method as defined in any one of the preceding claims, the electrical potential of the electrochemical cell has a magnitude that does not exceed about 8.5 V.

44. The method as defined in any one of the preceding claims, wherein the method comprises maintaining a current density of at least 100 mA / cm-2.

45. The method as defined in any one of the preceding claims, further comprising flowing the electrolyte into a first gas-liquid separator configured to separate one or more gasses from the electrolyte, before flowing the electrolyte into the solid-liquid extractor from the electrochemical cell.

46. The method as defined in any one of claims 3 to 45, further comprising flowing the separated electrolyte into a second gas-liquid separator configured to separate one or more gasses from the separator electrolyte, before recycling the separated electrolyte from the filtration unit to the electrochemical cell.

47. A method of continuously producing cement clinker products from metal-ion- containing solids, the method comprising: the steps of the method of producing metal hydroxides as defined in any one of the preceding claims; flowing the metal hydroxide from the filtration unit to a reactor comprising silicon dioxide (SiO2) after the separating step; and reacting, in the reactor, in the presence of heat, the metal hydroxide with the silicon dioxide to form a cement clinker product.

48. The method as defined in claim 47, wherein the cement clinker product comprises 2CaO·SiO2(s) and / or 3CaO·SiO2(s).

49. The method as defined in claim 47 or 48, further comprising drying the metal hydroxide after removing the metal hydroxide from the filtration unit and before supplying the metal hydroxide to the reactor.

50. A system of producing metal hydroxides from metal-ion containing solids, the system comprising: an electrochemical cell comprising: an anode exposed in an anode chamber, adapted to oxidize an oxidation reactant to form an oxidation product; a cathode exposed in a cathode chamber, adapted to reduce a reduction reactant to form hydroxide ions; and a separator separating the anode chamber and the cathode chamber; a solid-liquid extractor, adapted to extract metal ions from the metal-ion containing solids, thereby forming a metal-ion-enriched electrolyte comprising the metal ions, the solid-liquid extractor comprising: an extractor inlet flowingly connected to a first outlet of the electrochemical cell, for flowing an electrolyte from the electrochemical cell to the solid-liquid extractor; and an extractor outlet flowingly connected to a cathode inlet, for flowing the metal-ion-enriched electrolyte into the cathode chamber so that the metal ions in the metal-ion-enriched electrolyte react with the hydroxide ions to form metal hydroxide in the cathode chamber.

51. The system as defined in claim 50, further comprising: a filtration unit adapted to separate the metal hydroxide from a mixture comprising metal hydroxides and electrolyte, the filtration unit comprising: a filtration inlet flowingly connected to a cathode outlet for flowing the mixture comprising the metal hydroxide and electrolyte from the cathode chamber to the filtration unit; anda filtration outlet flowingly connected to a first inlet of the electrochemical cell for flowing a separated electrolyte to the electrochemical cell.

52. The system as defined in claim 51, wherein the separator is adapted to block passage of the metal ions.

53. The system as defined in claim 51 or 52, wherein the separator comprises an ion exchange membrane.

54. The system as defined in claim 53, wherein the ion exchange membrane comprises a cation exchange membrane (CEM).

55. The system as defined in any one of claims 50 to 54, wherein the first outlet of the electrochemical cell is arranged for flowing electrolyte out of the anode chamber.

56. The system as defined in claim 55, wherein the first inlet is arranged to introduce the separated electrolyte into the anode chamber.

57. The system as defined in any one of claims 50 to 56, wherein a distance between the anode and cathode is the thickness of the separator.

58. The system as defined in any one of claims 50 to 57, wherein the anode chamber comprises an anode flow field layer positioned adjacent to the anode, and the cathode chamber comprises a cathode flow field layer positioned adjacent to the cathode.

59. The system as defined in any one of claims 50 to 54, further comprising: a chemical reaction chamber between the anode chamber and the cathode chamber; and a bipolar membrane separating the chemical reaction chamber and the anode chamber, the bipolar membrane being adapted to electrochemically dissociate water into hydrogen ions and hydroxide ions, and wherein the separator separates the chemical reaction chamber and the cathode chamber.

60. The system as defined in claim 59, wherein the first outlet of the electrochemical cell is arranged for flowing electrolyte out of the chemical reaction chamber.

61. The system as defined in claim 59 or 60, wherein the first inlet of the electrochemical cell is arranged to flow the separated electrolyte into the chemical reaction chamber.

62. The system as defined in any one of claims 50 to 61, further comprising a reservoir connected to supply a flow of anolyte into the anode chamber.

63. The system as defined in claim 62, wherein the anolyte comprises a base.

64. The system as defined in claim 63, wherein the base comprises hydroxide ions.

65. The system as defined in any one of claims 50 to 61, wherein comprising a reservoir connected to supply a flow of hydrogenated shuttle molecules or hydrogenated shuttle molecules contained in an anolyte into the anode chamber.

66. The system as defined in any one of claims 50 to 65, further comprising a reservoir connected to supply a flow of catholyte into the cathode chamber.

67. The system as defined in any one of claims 59 to 66, further comprising a reservoir connected to supply a flow of electrolyte into the chemical reaction chamber.

68. The system as defined in any one of claims 59 to 67, further comprising a reservoir connected to circulate a flow of the electrolyte in a closed loop between the cathode chamber and the chemical reaction chamber.

69. The system as defined in any one of claims 66 to 68, wherein the catholyte and / or electrolyte comprises a pH-neutral aqueous solution.

70. The system as defined in any one of claims 66 to 69, wherein the catholyte and / or electrolyte comprises a pH in the range of from about 6 to about 8.

71. The system as defined in any one of claims 66 to 70, wherein the anions and / or cations of the catholyte and / or electrolyte are monovalent.

72. The system as defined in any one of claims 66 to 71, wherein the anions of the catholyte and / or electrolyte comprise one or more of fluoride (F-), chloride (Cl-), bromide (Br-), and iodide (I-), hydroxide (OH-), nitrate (NO3-), nitrite (NO2-), bicarbonate (HCO3-), acetate (CH3COO-), trifluoroacetate (CF3COO-),perchlorate (ClO4-), chlorate (ClO3-), hypochlorite (ClO-), bisulfate (HSO4-), cyanide (CN-), and hydride (H).

73. The system as defined in any one of claims 66 to 72, wherein the cations of the catholyte and / or electrolyte comprise an alkali metal ion.

74. The system as defined in claim 73, wherein the alkali metal ions comprise one or more of lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), cesium (Cs+).

75. The system as defined in any one of claims 66 to 74, wherein the cations of the electrolyte comprise one or more of hydrogen (H+), ammonium (NH4+), hydronium (H3O+), tetramethylammonium ((CH3)4N+), tetraethylammonium ((C2H5)4N+), trimethylammonium ((CH3)3H+), silver (Ag+), and thallium (Tl+).

76. The system as defined in any one of claims 66 to 75, wherein a concentration of the catholyte and / or electrolyte is in the range of from about 0.1 M to about 3 M.

77. The system as defined in any one of claims 50 to 76, wherein the reduction reactant comprises a hydrogen-containing reactant.

78. The system as defined in claim 77, wherein the hydrogen-containing reactant comprises water.

79. The system as defined in any one of claims 50 to 76, wherein the reduction reactant comprises shuttle molecules.

80. The system as defined in claim 79, further comprising a reservoir connected to supply a flow of the shuttle molecules or shuttle molecules contained in a solution comprising the hydrogen-containing reactant into the cathode chamber.

81. The system as defined in any one of claims 50 to 80, further comprising a first gas-liquid separator arranged between the first outlet of the electrochemical cell and the extractor inlet, the first gas-liquid separator being adapted to remove one or more gasses from the electrolyte before flowing into the solid- liquid extractor.

82. The system as defined in any one of claims 50 to 81, further comprising a second gas-liquid separator arranged between the filtration outlet and the firstinlet of the electrochemical cell, the second gas-liquid separator being adapted to remove one or more gasses from the separated electrolyte before being recycled into the electrochemical cell.

83. The system as defined in any one of claims 50 to 82, further comprising a solid collector connected to receive a supply of silicon dioxide (SiO2) discharged from the solid-liquid extractor.

84. The system as defined any one of claims 50 to 83, wherein the electrochemical cell further comprises a power supply configured to drive the electrochemical cell with a potential difference that does not exceed above 8.5 V.

85. The system as defined in claim 84, wherein the potential difference introduces a current density of at least 100 mA cm-2.

86. The system as defined in any one of claims 50 to 85, wherein the metal-ion containing solids comprise limestone, gypsum, calcium silicates, cement, concrete, or combinations thereof.

87. The system as defined in any one of claims 50 to 86, wherein the metal-ion containing solids comprise metal carbonate, metal silicate, and / or metal oxide.

88. The system as defined in any one of claims 50 to 87, wherein the metal ions comprise calcium (Ca2+) ions.

89. The method as defined in any one of claims 50 to 88, wherein the metal hydroxide comprises calcium hydroxide ((Ca(OH)2).

90. A system for continuously producing cement clinker products from metal-ion- containing solids, comprising: the system of producing metal hydroxides as defined in any one of claims 50 to 89; and a reactor arranged downstream of the filtration unit connected to receive a flow of the metal hydroxide discharged from the filtration unit, the reactor being configured to bring the flow of metal hydroxide into contact with silicon dioxide (SiO2) in the presence of heat so that the metal hydroxide reacts with the silicon dioxide to yield a cement clinker product.

91. The system as defined in claim 90, wherein the cement clinker product comprises 2CaO·SiO2(s) and / or 3CaO·SiO2(s).

92. The system as defined in claim 90 or 91, further comprising a dryer arranged downstream of the filtration unit and upstream of the reactor connected to receive the flow of metal hydroxide from the filtration unit, the dryer being configured to dry the metal hydroxide before being supplied to the reactor.

93. A method of producing metal hydroxide comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode chamber, a cathode chamber, and a separator separating the anode chamber and the cathode chamber; oxidizing a hydrogenated shuttle molecule, at the anode, to form a shuttle molecule and hydrogen ions; reducing a shuttle molecule in the presence of a hydrogen-containing reactant, at the cathode, to form hydroxide ions and a hydrogenated shuttle molecule; supplying a metal-ion containing solid to the anode chamber; reacting, at the anode chamber, the metal-ion containing solid with the hydrogen ions to form metal ions; removing the metal ions from the anode chamber; removing the hydroxide ions from the cathode chamber; reacting, in a reactor, the metal ions and hydroxide ions to form a mixture comprising metal hydroxide.

94. The method as defined in claim 93, further comprising: separating the metal hydroxide from the shuttle molecule and the hydrogenated shuttle molecule in the mixture; recycling the shuttle molecule and the hydrogenated shuttle molecule to the anode chamber and the cathode chamber for use in the reducing and oxidizing steps respectively.

95. The method as defined in claim 93 or 94, wherein the separator is adapted to block passage of the metal ions.

96. The method as defined in any one of claims 93 to 95, wherein the separator comprises an ion exchange membrane.

97. The method as defined in claim 96, wherein the ion exchange membrane comprises a cation exchange membrane (CEM).

98. The method as defined in any one of claims 93 to 97, wherein the shuttle molecule comprises a quinone compound or a quinone derivative.

99. The method as defined in any one of claims 93 to 98, wherein the hydrogen- containing compound comprises water.

100. The method as defined in any one of claims 93 to 99, wherein the oxidizing of the hydrogenated shuttle molecule at the anode is performed in an anolyte.

101. The method as defined in any one of claims 93 to 100, further comprising suspending the metal-ion containing solid in the anolyte.

102. The method as defined in any one of claims 93 to 101, further comprising supplying a flow of the anolyte comprising the hydrogenated shuttle molecule and / or the metal-ion containing solid into the anode chamber.

103. The method as defined in any one of claims 93 to 102 wherein the reducing of the shuttle molecule at the cathode is performed in a catholyte.

104. The method as defined in claim 103, further comprising supplying a flow of the catholyte comprising the shuttle molecule into the cathode chamber.

105. The method as defined in any one of claims 100 to 104, wherein the anolyte and / or catholyte comprises a pH-neutral aqueous solution.

106. The method as defined in any one of claims 100 to 105, wherein the anolyte and / or catholyte comprises a pH in the range of from about 6 to about 8.

107. The method as defined in any one of claims 100 to 106, wherein the anolyte and / or catholyte comprises an organic solvent.

108. The method as defined in claim 107, wherein the organic solvent comprises acetonitrile (MeCN).

109. The method as defined in any one of claims 100 to 108, wherein the anolyte and / or catholyte comprises water.

110. The method as defined in any one of claims 100 to 109, further comprising heating the anolyte and / or catholyte to a temperature in the range of fromabout 20oC to about 60oC before supplying into the anode chamber and the cathode chamber respectively.

111. The method as defined in any one of claims 93 to 110, wherein the metal-ion containing solid comprises metal carbonate salt.

112. The method as defined in claim 111, wherein the metal carbonate salt comprises calcium carbonate (CaCO3).

113. The method as defined in any one of claims 93 to 112, wherein the metal ions comprise calcium ions (Ca2+).

114. The method as defined in any one of claims 93 to 113, wherein the metal hydroxide comprises calcium hydroxide (Ca(OH)2).

115. The method as defined in any one of claims 93 to 114, wherein a distance between the anode and cathode is the thickness of the separator.

116. The method as defined in any one of claims 93 to 115, wherein the anode chamber comprises an anode flow field layer positioned adjacent to the anode, and the cathode chamber comprises a cathode flow field layer positioned adjacent to the cathode.

117. A method of producing metal hydroxide comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode chamber, a cathode chamber, a chemical reaction chamber, a bipolar membrane separating the anode chamber and the chemical reaction chamber, an a separator separating the chemical reaction chamber and the cathode chamber; electrochemically dissociating water, within the ion exchange membrane, into hydrogen ions and hydroxide ions; oxidizing a hydrogenated shuttle molecule, at the anode, to form a shuttle molecule and hydrogen ions, the hydrogen ions being supplied to the chemical reaction chamber; reducing a shuttle molecule in the presence of a hydrogen-containing reactant, at the cathode, to form hydroxide ions and a hydrogenated shuttle molecule;supplying a metal-ion containing solid to the chemical reaction chamber; reacting, at the chemical reaction chamber, the metal-ion containing solid with the hydrogen ions to form metal ions; removing the metal ions from the chemical reaction chamber; removing the hydroxide ions from the cathode chamber; reacting, in a reactor, the metal ions and hydroxide ions to form a mixture comprising metal hydroxide.

118. The method as defined in claim 117, wherein the separator is adapted to block passage of the metal ions.

119. The method as defined in claim 117 or 118, wherein the separator comprises an ion exchange membrane.

120. The method as defined in claim 119, wherein the ion exchange membrane comprises a cation exchange membrane (CEM).

121. The method as defined in any one of claims 117 to 121, wherein the shuttle molecule comprises a quinone compound or a quinone derivative.

122. The method as defined in any one of claims 117 to 121, wherein the hydrogen- containing compound comprises water.

123. The method as defined in any one of claims 117 to 122, wherein the oxidizing of the hydrogenated shuttle molecule at the anode is performed in an anolyte.

124. The method as defined in any one of claims 117 to 123, further comprising supplying a flow of the anolyte comprising the hydrogenated shuttle molecule into the anode chamber.

125. The method as defined in any one of claims 117 to 124, wherein the reducing of the shuttle molecule at the cathode is performed in a catholyte.

126. The method as defined in any one of claims 117 to 125, further comprising supplying a flow of the catholyte comprising the shuttle molecule into the cathode chamber.

127. The method as defined in any one of claims 123 to 126, wherein the anolyte and / or catholyte comprises a pH-neutral aqueous solution.

128. The method as defined in any one of claims 123 to 127, wherein the anolyte and / or catholyte comprises a pH in the range of from about 6 to about 8.

129. The method as defined in any one of claims 123 to 128, wherein the anolyte and / or catholyte comprises an organic solvent.

130. The method as defined in claim 129, wherein the organic solvent comprises acetonitrile (MeCN).

131. The method as defined in any one of claims 123 to 130, wherein the anolyte and / or catholyte comprises water.

132. The method as defined in any one of claims 123 to 131, further comprising heating the anolyte and / or catholyte to a temperature in the range of from about 20oC to about 60oC before supplying into the anode chamber and the cathode chamber respectively.

133. The method as defined in any one of claims 117 to 132, wherein the metal-ion containing solid comprises a metal carbonate salt.

134. The method as defined in claim 133, wherein the metal carbonate salt comprises calcium carbonate (CaCO3).

135. The method as defined in any one of claims 117 to 134, wherein the metal ions comprise calcium ions (Ca2+).

136. The method as defined in any one of claims 117 to 135, wherein the metal hydroxide comprises calcium hydroxide (Ca(OH)2).

137. A method of producing metal silicate hydrates comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode chamber, a cathode chamber, a chemical reaction chamber, a bipolar membrane (BPM) separating the anode chamber and the chemical reaction chamber, and a separator separating the chemical reaction chamber and the cathode chamber; dissociating, in the bipolar membrane, water to produce hydrogen ions; permeating the hydrogen ions through a cation exchange layer of the bipolar membrane into the chemical reaction chamber;oxidizing an oxidation reactant, at the anode, to form an oxidation product; reducing a hydrogen-containing reactant, at the cathode, to form hydroxide ions; supplying a metal-ion containing solid to the chemical reaction chamber; reacting, at the chemical reaction chamber, the metal ion-containing solid with the hydrogen ions to form metal ions; supplying a silicate-containing compound to the cathode chamber; reacting, at the cathode chamber, the silicate-containing compound with the hydroxide ions to form silicate ions; removing the metal ions from the chemical reaction chamber; removing the silicate ions from the cathode chamber; reacting the metal ions formed at the chemical reaction chamber and the silicate ions formed at the cathode chamber to yield metal silicate hydrate.

138. The method as defined in claim 137, wherein the separator is adapted to block passage of the metal ions.

139. The method as defined in claim 137 or 138, wherein the separator comprises an ion exchange membrane.

140. The method as defined in claim 139, wherein the ion exchange membrane comprises a cation exchange membrane (CEM).

141. The method as defined in claim 137 or 138, wherein the separator comprises a microporous polymer membrane.

142. The method as defined in any one of claims 137 to 141, further comprising flowing a supply of the metal-ion containing solid or the electrolyte comprising the metal-ion containing solid to the chemical reaction chamber.

143. A method of producing metal silicate hydrates comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode chamber, a cathode chamber, and a bipolar membrane separating the anode chamber and the cathode chamber; dissociating, in the bipolar membrane, water to produce hydrogen ions;permeating the hydrogen ions through a cation exchange layer of the bipolar membrane into the cathode chamber; oxidizing an oxidation reactant, at the anode, to form an oxidation product; reducing a hydrogen-containing reactant, at the cathode, to form hydroxide ions; supplying a metal-ion containing solid to the cathode chamber; reacting, at the cathode chamber, the metal ion-containing solid with the hydrogen ions to form metal ions; supplying a silicate-containing compound to the cathode chamber; reacting, at the cathode chamber, the silicate-containing compound with the hydroxide ions to form silicate ions; reacting, in the cathode chamber, the metal ions and the silicate ions to form a metal silicate hydrate.

144. The method as defined in claim 143, wherein the oxidation reactant comprises hydroxide ions.

145. The method as defined in claim 143 or 144, wherein the hydroxide ions are contained in an anolyte.

146. The method as defined in claim 145, wherein the anolyte comprises a base.

147. The method as defined in any one of claims 143 to 146, further comprising flowing a supply of the anolyte into the anode chamber.

148. The method as defined in any one of claims 143 to 147, wherein the oxidation product comprises oxygen gas.

149. The method as defined in in any one of claims 143 to 148, wherein the hydrogen-containing reactant comprises water.

150. The method as defined in any one of claims 143 to 148, further comprising flowing a supply of water and / or water comprising the silicate-containing compound to the cathode chamber.

151. The method as defined in any one of claims 143 to 150, wherein the reacting of the metal-ion containing solid with the hydrogen ions is performed in an electrolyte.

152. The method as defined in any one of claims 143 to 150, wherein the metal-ion containing solid comprises limestone, gypsum, calcium silicate, cement, concrete, or combinations thereof.

153. The method as defined in any one of claims 143 to 150, wherein the metal-ion containing solids comprises metal carbonate, metal silicate and / or metal oxide.

154. The method as defined in claim 153, wherein the metal carbonate comprises calcium carbonate (CaCO3).

155. The method as defined in any one of claims 143 to 154, wherein the metal ions comprise calcium ions (Ca2+).

156. The method as defined in any one of claims 143 to 155 , wherein the silicate- containing compound comprises calcium silicate hydrate.

157. A method of producing metal silicate hydrates comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode chamber, a cathode chamber, and a separator separating the anode chamber and the cathode chamber; oxidizing a first hydrogen-containing reactant, at the anode, to form an oxidation product comprising hydrogen ions; supplying a metal-ion containing solid to the anode chamber; reacting, at the anode chamber, the metal ion-containing solid with the hydrogen ions to form metal ions; reducing a second hydrogen-containing reactant, at the cathode, to form hydroxide ions; supplying a silicate-containing compound to the cathode chamber; reacting, at the cathode chamber, the silicate-containing compound with the hydroxide ions to form silicate ions; reacting the metal ions formed at the anode chamber and the silicate ions formed at the cathode chamber to yield metal silicate hydrate.

158. The method according to claim 157, wherein the separator comprises a microporous polymer membrane.

159. The method according to claim 157, wherein the separator comprises a membrane electrode assembly.

160. A method of producing metal silicate hydrates comprising: applying an electrical potential between an anode and a cathode of an electrochemical cell, wherein the electrochemical cell comprises an anode flow field layer adjacent to the anode and a cathode flow field layer adjacent to the cathode; oxidizing a first hydrogen-containing reactant, at the anode, to form an oxidation product comprising hydrogen ions; supplying a metal-ion containing solid to the anode flow field layer; reacting, at the anode flow field layer, the metal ion-containing solid with the hydrogen ions to form metal ions; reducing a second hydrogen-containing reactant, at the cathode, to form hydroxide ions; supplying a silicate-containing compound to the cathode flow field; reacting, at the cathode flow field layer, the silicate-containing compound with the hydroxide ions to form silicate ions; reacting the metal ions formed at the anode flow field layer and the silicate ions formed at the cathode flow field to yield metal silicate hydrate.

161. The method as defined in claim 160, wherein an electrolyte layer separates the anode flow field layer and the cathode flow field layer.

162. The method as defined in claim 160 or 161, wherein the first hydrogen- containing reactant comprises water.

163. The method as defined in any one of claims 160 to 162, wherein the first hydrogen-containing reactant is contained in an electrolyte.

164. The method as defined in any one of claims 160 to 163, further comprising flowing a supply of the electrolyte into the anode chamber or the anode flow field layer.

165. The method as defined in any one of claims 160 to 164, wherein the oxidation product additionally comprises oxygen gas.

166. The method as defined in any one of claims 160 to 165, wherein the second hydrogen-containing reactant comprises water.

167. The method as defined in any one of claims 160 to 166, further comprising flowing a supply of water and / or water comprising the silicate-containing compound to the cathode chamber or the cathode flow field layer.

168. The method as defined in any one of claims 160 to 167, wherein the reacting of the metal-ion containing solid with the hydrogen ions is performed in an electrolyte.

169. The method as defined in any one of claims 160 to 168, wherein the metal-ion containing solid comprises limestone, gypsum, calcium silicate, cement, concrete, or combinations thereof.

170. The method as defined in any one of claims 160 to 169, wherein the metal-ion containing solids comprises metal carbonate, metal silicate, and / or metal oxide.

171. The method as defined in claim 170, wherein the metal carbonate comprises calcium carbonate (CaCO3).

172. The method as defined in any one of claims 160 to 171, wherein the metal ions comprise calcium ions (Ca2+).

173. The method as defined in any one of claims 160 to 172, wherein the silicate- containing compound comprises calcium silicate hydrate.

174. A method of producing metal silicate hydrates comprising: supplying an acid to an acid chamber; reacting, in the acid chamber, the acid with a hydrogen-containing reactant to form hydrogen ions; supplying the hydrogen ions formed in the acid chamber to a metal- enriched feedstock chamber, wherein the metal-enriched feedstock chamber contains a metal-ion containing solid; reacting, in the metal-enriched feedstock chamber, the hydrogen ions with the metal-ion containing solid to form metal ions; supplying a base an alkaline chamber;reacting, in the alkaline chamber, the base with a second hydrogen- containing reactant to form hydroxide ions; supplying the hydroxide ions formed in the alkaline chamber to a silicate ion-enriched chamber, wherein the silicate-ion enriched chamber contains a silicate-containing compound; reacting, in the silicate-enriched feedstock chamber, the hydroxide ions with the silicate-containing compound to form silicate ions; removing the metal ions from the metal-enriched feedstock chamber; removing the silicate ions from the silicate-enriched feedstock chamber; reacting, in the reaction chamber, the metal ions and the silicate ions to form a metal silicate hydrate.

175. The method according to claim 174, wherein the first hydrogen-containing reactant and / or the second hydrogen containing reactant comprises water.

176. A method of producing metal silicates, the method comprising: producing a metal silicate hydrate according to any one of claims 137 to 175 ; heating the metal silicate hydrate in a reactor to form metal silicate; removing the metal silicate from the reactor.