WO2012050530A1 - Carbon dioxide capture with regeneration of salt - Google Patents

Abstract

A process is disclosed. The process comprises (a) producing an acidic solution and an alkaline solution from water and a salt of a base by bipolar membrane electrodialysis, wherein the base is capable of reacting with carbon dioxide to form a carbonate, (b) contacting a source of carbon dioxide with the alkaline solution to produce a carbonate of the base, (c) contacting a substance comprising an alkaline earth metal with the acidic solution to produce a salt of the alkaline earth metal, and (d) reacting the salt of the alkaline earth metal and the carbonate of the base to produce a carbonate of the alkaline earth metal and regenerate the salt of the base. A system for performing the process is also disclosed.

Description

CARBON DIOXIDE CAPTURE WITH REGENERATION OF SALT

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of, and priority from, United States

Patent Application Serial No. 61/392,599, filed October 13, 2010, and entitled "A Process for Long-term CO2 Mineral Sequestration," the entire contents of which are incorporated herein by reference.

FIELD

[0002] The present invention relates generally to processes and systems for capture of carbon dioxide.

BACKGROUND

[0003] It is desirable to control carbon dioxide (CO2) emission into the atmosphere, and various carbon capture and storage techniques have been developed. In a process disclosed in WO 2008/018928A2 to House et al.

(hereinafter referred to as "House"), published February 14, 2008, CO2 is captured from a source of C0₂ using an alkaline solution. The alkaline solution is formed by processing a salt solution into an acidic solution and the alkaline solution. The separation of the salt solution into the acidic solution and the alkaline solution may involve membranes such as bipolar membrane electrodialysis. In some embodiments, the alkaline solution is reacted with CO2 from the source of CO2 for the purpose of capturing and storing the C0₂ as a carbonate (including

bicarbonate) species. The acid solution may be sold as a by-product or neutralized, such as by reacting the acid with a rock or mineral species including silicate mineral or rocks. It may be preferred that the rock or mineral source has reduced iron so that oxidation of the iron can be used to generate useful energy. [0004] The net reaction in the process of House described above is spontaneous in thermodynamics, and dissolution of silicate minerals in an acid solution such as HCI and absorption of C0₂ in an alkaline solution such as a NaOH solution are both relatively fast processes.

SUMMARY

[0005] According to one aspect of the invention, there is provided a process comprising (a) producing an acidic solution and an alkaline solution from water and a salt of a base by bipolar membrane electrodialysis, wherein the base is capable of reacting with carbon dioxide to form a carbonate, (b) contacting a source of carbon dioxide with the alkaline solution to produce a carbonate of the base, (c) contacting a substance comprising an alkaline earth metal with the acidic solution to produce a salt of the alkaline earth metal, and (d) reacting the salt of the alkaline earth metal and the carbonate of the base to produce a carbonate of the alkaline earth metal and regenerate the salt of the base, (a) to (d) may be repeated and performed continuously, and the regenerated salt of the base may be used in (a). The process may further comprise (c1) extracting a first liquid from a mixture produced at (c), wherein the first liquid comprises the salt of the alkaline earth metal, (c2) increasing the pH of the first liquid to from about 4 to about 7, to form a first precipitate, and removing the first precipitate from the first liquid to form a second liquid, and (c3) increasing the pH of the second liquid to from about 9 to about 10, to form a second precipitate, and removing the second precipitate from the second liquid to form a third liquid, wherein (d) comprises mixing the third liquid and the carbonate of the base in a solution having a pH from about 10 to about 11. The pH of the first liquid may be be increased to about 5 at (c2), the pH of the second liquid may be increased to about 9.5 at (c3), and the pH of the solution in (d) may be about 10.6. (c2) may comprise adding a portion of the alkaline solution produced in (a) to the first liquid to increase the pH of the first liquid, and (c3) may comprise adding a portion of the carbonate of the base produced in (b) to the second liquid to increase the pH of the second liquid. Hydrogen peroxide may also be added to the first liquid before (c2). A suitable base for the process may comprise an alkali metal and a suitable salt of the base may comprise a halogen. The alkaline earth metal may comprise magnesium. The substance comprising the alkaline earth metal may be a silicate mineral. The silicate mineral may be a serpentine, olivine, pyroxene, sepiolite, mafic, or talc mineral. Where the substance comprising the alkaline earth metal is a silicate mineral, the process may further comprise extracting a solid from a mixture resulting from (c), wherein the solid comprises silica, contacting the solid with a solution comprising an alkali metal to form a solution of a silicate of the alkali metal, and contacting the solution of the silicate of the alkali metal with carbon dioxide to form a carbonate of the alkali metal and a precipitate comprising silica. The precipitate comprising silica may be further treated with a solution of ammonium nitrate.

[0006] According to another aspect of the invention, there is provided a system comprising a bipolar membrane unit for producing a stream of an acidic solution and a stream of an alkaline solution from water and a salt of a base by electrodialysis, wherein the base is capable of reacting with carbon dioxide to form a carbonate, a first reactor in fluid communication with the bipolar membrane unit, for receiving the stream of the alkaline solution and contacting a source of carbon dioxide with the stream of the alkaline solution to produce a stream comprising a carbonate of the base, a second reactor in fluid communication with the bipolar membrane unit, for receiving the stream of the acidic solution and contacting a substance comprising an alkaline earth metal with the stream of the acidic solution to produce a stream comprising a salt of the alkaline earth metal, and a third reactor in fluid communication with the first and second reactors, for receiving and reacting the salt of the alkaline earth metal and the carbonate of the base to produce a carbonate of the alkaline earth metal and regenerate the salt of the base. The third reactor may be in fluid communication with the bipolar membrane unit for supplying the salt of the base regenerated in the third reactor to the bipolar membrane unit. The system may further comprise a separator in fluid

communication with the second reactor for receiving a product mixture produced at the second reactor and extracting a first liquid from the product mixture to form the stream comprising the salt of the alkaline earth metal, a fourth reactor in fluid communication with the separator, for receiving the first liquid, increasing the pH of the first liquid to from about 4 to about 7, to form a first precipitate, and removing the first precipitate from the first liquid to form a second liquid, and a fifth reactor in fluid communication with the fourth reactor, for receiving the second liquid, increasing the pH of the second liquid to from about 9 to about 10, to form a second precipitate, and removing the second precipitate from the second liquid to form a third liquid, the fifth reactor being further in fluid communication with the third reactor for supplying the third liquid to the third reactor. In some embodiments, the fourth reactor may be in fluid communication with the bipolar membrane unit for receiving a portion of the alkaline solution produced by the bipolar membrane unit. In some embodiments, the fifth reactor may be in fluid communication with the second reactor for receiving a portion of the carbonate of the base produced in the second reactor. The system may further comprise a sixth reactor for receiving a solid comprising silica extracted from the product mixture and contacting the solid with a solution comprising an alkali metal to form a solution of a silicate of the alkali metal; and a seventh reactor in fluid communication with the sixth reactor for receiving the solution of the silicate of the alkali metal and contacting the solution of the silicate of the alkali metal with carbon dioxide to form a carbonate of the alkali metal and a precipitate comprising silica. The system may also comprise a source of hydrogen peroxide for adding hydrogen peroxide to the first liquid.

[0007] Other aspects, features, and embodiments of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the figures, which illustrate, by way of example only, embodiments of the present invention:

[0009] FIG. 1 is a block diagram illustrating a process for C0₂ removal from a source, exemplary of an embodiment of the present invention;

[0010] FIG. 2 is a block diagram illustrating a system for carrying out the process of FIG. 1 , exemplary of an embodiment of the present invention; [0011J FIG. 2A is a schematic diagram showing a bipolar membrane electrodialysis unit for use in the system of FIG. 2;

[0012] FIG.3 is a line graph showing the fractions of different carbonate species at chemical equilibrium at 25°C;

[0013] F!G. 4 is a block diagram illustrating an alternative system for carrying out the process of FIG. , exemplary of an embodiment of the present invention;

[0014] FIG. 5 is an x-ray diffraction (XRD) diagram of a sample silicate mineral;

[0015] FIGS. 6A and 6B are data graphs of extraction fraction at different conditions;

[0016] FIGS. 7A and 7B show XRD diagrams of sample materials;

[0017] FIG. 8 is a data graph of oxide weight percentages in different samples;

[0018] FIG. 9 is a data graph of Mg removal efficiencies at different pH values in the first precipitation stage;

[0019] FIGS. 10, 11 and 12 are data graphs of temperature and pH dependence of various properties of samples;

[0020] FIG. 13 is an XRD diagram of coarse silica samples;

[0021] FIGS. 14A and 14B show data graphs of adsorption and pore size distribution of different samples;

[0022] F!GS. 15A, 15B and 15C show data graphs of silicon extraction rates at different conditions;

[0023] FIGS. 16A and 16B are data graphs of silicon precipitation efficiency at different conditions;

[0024] FIGS. 17A, 17B, 17C, 18A, 18B and 18C are bar graphs of pH dependency of different products; [0025] FIG. 19 shows data graphs of adsorption and pore size distribution of different samples; and

[0026] FIG. 20 is an XRD diagram of calcined silica samples.

DETAILED DESCRIPTION

[0027] An exemplary embodiment of the present invention relates to processes and systems for removal, and capture, of carbon dioxide (CO2) from a source of C0₂.

[0028] In a selected embodiment, a process S100 is as illustrated in FIG. 1.

[0029] At S112, water and a salt solution are processed to produce an acidic solution and an alkaline solution by electrodialysis. The electrodialysis process may be any suitable bipolar membrane electrodialysis (BMED) process. The salt solution may also include water as a solvent.

[0030] The salt solution may be any suitable salt solution which includes a salt of a base that is capable of reacting with carbon dioxide to form a carbonate of the base. A carbonate may be any compound that contains the divalent anion CO3^2". A carbonate may be a salt or ester of the carbonic acid. In some embodiments, the carbonate may contain a HCO3^" group. In selected

embodiments, the base comprises an alkali metal and the salt of the base comprises a halogen. An inorganic salt solution of a strong base and a strong acid may be conveniently used in some embodiments. For example, alkaline cations such as Na⁺ or K⁺, may be used as the salt cations. The anions in the salt solution may be selected from CI^", F^", Br^", S0₄ ^2", N0₃\ P0₄ ^3", or the like. The salt solution may contain an inorganic salt, such as NaCI, Na₂S0₄, NaN0₃, KCI, K₂S0₄, KNO3, Na₃P0₄, or the like. Other salts, such as organic salts e.g. sodium acetate (CH₃COONa), may also be used. In a particular embodiment, a solution of NaCI may be used as the salt solution. The base may include an alkali metal or another base cation such as an ammonium cation, NH₄ ⁺ or the like. [0031] For ease of description, it is assumed below that NaCI is used as the salt of the base. However, the following description, with appropriate modification, can also be applied to other salts, as will be understood by those skilled in the art.

[0032] The acidic and alkaline solutions produced at S112 will depend on the salt solution used. For example, when NaCI is used as the salt in the salt solution, the acidic solution will be a solution of HCI and the alkaline solution will be a solution of NaOH, and the reaction at S112 can be represented by Reaction (1 ):

NaCI + H₂0→ NaOH + HCI. (1)

[0033] The B ED electrodialysis process will be further described below.

[0034] At S114, carbon dioxide from a source of carbon dioxide is captured by contacting the source with the alkaline solution to produce a carbonate of the base. As a result, at least some carbon dioxide in the source is captured and removed from the source.

[0035] The source of carbon dioxide may be any gas mixture that contains C0₂. For example, the source of carbon dioxide may be air or a flue gas from a plant. The plant may be a power plant, cement plant, steel plant, or any other plant that produces emission gases which include CO2. In some embodiments, the source of carbon dioxide may be subjected to pre-treatment before being used at S114. In other embodiments, the source may be used directly without pre- treatment. Possible pre-treatments include those typically used in conventional flue gas treatments, as can be understood by those skilled in the art. The pre-treatment may involve the removal of certain components contained in the source other than C0₂.

[0036] The net reaction at S114, when NaOH is used in the alkaline solution, can be represented by Reaction (2):

2 NaOH + C0₂→ Na₂C0₃ + H₂0. (2)

[0037] This reaction may be carried out in any suitable manner known to those skilled in the art. For example, known processes and reactors for reacting C0₂ with an alkaline solution to neutralize the alkaline species may be utilized. In some embodiments, a conventional NaOH scrubber, spray tower, or bubbling tower, may be used to react NaOH and C0₂ in the source.

[0038] The carbonate of the base produced at S114 is substantially dissolved in the processing solution and will be used downstream in the process, such as at S118, as will be described below. After the C0₂ removal, the residual gas, which has a reduced C0₂ content or is substantially free of C0₂, may be released, such as into the atmosphere, or may be subjected to further processing or treatment.

[0039] At S116, a substance comprising an alkaline earth metal is contacted with the acidic solution to produce a salt of the alkaline earth metal.

[0040] The substance may be any suitable substance containing one or more alkaline earth metals, for example, minerals and industrial wastes such as steelmaking slags, cement kiln dusts and the like. In some embodiments, the substance may be a silicate mineral containing one or more alkaline earth metals capable of reacting with C0₂ to form a carbonate of the alkaline earth metal. In some embodiments, the silicate mineral may be g-containing silicate minerals. For clarification, minerals are used herein in a broad sense and include rocks and other forms or mixtures of materials, and include natural minerals or processed minerals, such as particulate minerals or purified minerals, or the like. In some embodiments, the silicate mineral may include hydrous magnesium iron phyllosilicate, ( g, Fe)₃Si₂0₅(OH)4, or magnesium iron silicate, ( g,Fe)₂Si0₄. Other suitable silicate minerals may also be used. Examples of Mg-containing silicate minerals include serpentine group (antigorite, lizardite, chrysotile, clinochrysotile, and orthochrysotile etc.), olivine group (olivine, forsterite, monticeliite), pyroxene group (pigeonite, enstatite, ferrosilite, diopside), mafic minerals, ultramafic minerals, talc, sepiolite and so on.

[0041] Mixtures or combination of different silicate minerals may also be used. [0042] A typical reaction at S116, when HCI is used in the acidic solution and Mg₃Si₂05(OH)4 is included in the silicate mineral, can be represented by Reaction

(3):

Mg₃Si₂05(OH)4 + 6 HCI → 3 gCI₂ + 2 Si0₂ + 5 H₂0. (3)

[0043] When other substances are used, the chemical reactions may be different but the products will generally include a salt of the particular alkaline earth metal, such as gCI₂. For instance, when the substance comprises a silicate mineral which includes Mg₂Si04, it may also react with HCI to form MgCl2, silica, and water.

[0044] In different embodiments, other materials containing suitable cations, such as Mg²⁺, Fe²⁺, Fe³\ Ca²⁺, or similar elemental cations, may also be used to react with the acidic solution to form a corresponding salt, silica and water.

[0045] The MgCI₂ salt is soluble in water and may be provided in a solution to S118.

[0046] As can be understood, other output may also be formed at S116. For example, when the alkaline earth metal-containing substance is a silicate mineral, the other output may comprise coarse silica, which may be further treated to produce substantially pure silica, either in its anhydrous or hydrated forms (e.g. H2S1O4, also known as Si0₂2H₂0 or Si(OH)₄). Such treatment may comprise treating the coarse silica with a solution comprising an alkali metal to form a solution of a silicate of the alkali metal and contacting the solution of the silicate of the alkali metal with C0₂ to form a mixture comprising a hydrated silica and a carbonate of the alkali metal. The hydrated silica may be further treated using methods known to those skilled in the art to obtain a purified anhydrous silica. For example, the hydrated silica may be calcined at an elevated temperature, e.g. 500°C, to produce the anhydrous silica by releasing water. Prior to the clacination, the hydrated silica may be optionally purified by an ion exchange.

[0047] At S118, the salt of the alkaline earth metal and the carbonate of the base are reacted to produce a carbonate of the alkaline earth metal and to regenerate the salt of the base (i.e., NaCI in the example case). [0048] The main reaction at S118, when MgCI₂ and Na₂C0₃ are used, can be represented by Reaction (4):

MgCI₂ + Na₂C0₃→ MgC0₃ + 2 NaCI. (4)

[0049] One of the products of the reaction, MgC0₃, a carbonate of Mg, can be conveniently disposed of as a solid, or further processed. The regenerated salt, NaCI, may be conveniently used in the feed of the salt solution at S112, as illustrated in FIG. 1. The regenerated salt may be purified prior to being used in the feed of the salt solution at S112. For example, the regenerated salt solution may be passed through an ion-exchange device such as an ion-exchange tower to remove any trace amount of divalent and trivalent ions that may be contained in the solution. The purified solution may then be used in the feed of the salt solution at S112.

[0050] As can be understood, process S100 can be carried out on a continuous basis. As the salt solution, the acidic solution, the alkaline solution, a solution of the salt of the alkaline earth metal, and a solution of the carbonate of the base can all be transported and processed in liquid phase, the process can be conveniently run continuously, and streams of the solutions may be conveniently transported between different processing locations using fluid transport and control devices and systems.

[00511 Process S100 may be conveniently utilized for long-term C0₂ mineral sequestration.

[0052] When regenerated salt solution is used as input for the electrodialysis at S112, the requirement for new salt of the base is reduced, although make-up new salt may still need to be added in a continuous process. However, care should be taken to reduce the chance of membrane failure due to the use of the regenerated salt solution. Further, in some embodiments, useful by-products may be conveniently produced in process S100.

[0053] In this regard, the liquid output stream from S116 may be processed in the following manner to reduce the potential mineral ion content, including Mg, Fe, Ca, and other mineral ions, in the stream of regenerated salt solution to be supplied to S112, and to produce useful by-products.

[0054] A first liquid is extracted from the mixture produced at S116, which contains the salt of the alkaline earth metal. The liquid may be extracted and separated from solid components of the mixture by a suitable separation technique such as filtration. The liquid may also be treated, for example, with hydrogen peroxide to oxidize any divalent ions contained in the liquid, prior to being precipitated.

[00551 In a first precipitation stage, a base is added to the first liquid to increase the pH of the first liquid to a first pH range from about 4 to about 7 or about 9 to form a first precipitate. In some embodiments, the pH of the first liquid may be increased to about 5. Conveniently, a portion of the alkaline solution produced at S112 may be used as the base and added to the first liquid.

[0056] It has been found that, in selected embodiments, as the pH is increased to the above range, precipitates containing Fe will form from the first liquid. The g salt, however, is much less likely to precipitate at such a pH range. For example, the precipitate may contain more than 90 wt% of oxides of Fe (such as Fe₂0₃), and less than 0.5 wt% of oxides of Mg (such as MgO). The precipitate may be removed from the first liquid using a suitable separation technique, such as using a cyclone separator.

[0057] The separated precipitate may be useful as a raw material in some industrial processes such as steel production.

[0058] The resulting liquid after separating the precipitate is referred to as a second liquid.

[0059] In a second precipitation stage, the pH of the second liquid is further increased to a second pH range from about 9 to 10 to form second precipitate. In selected embodiments, the pH of the second liquid is increased to about 9.5. At such a pH range, residual mineral ions in the solution such as Fe or Cr ions or the like tend to precipitate, and thus such ions can be removed. However, Mg components are still less likely to precipitate and substantially remain dissolved in the solution. The further precipitates can be again separated and removed from the second liquid, to form a third liquid.

[0060] The pH of the second liquid may be conveniently increased by adding a portion of the carbonate of the alkali metal (such as Na₂COs) produced at S114.

[0061] In a third precipitation stage, the carbonate of the base such as Na₂C0₃ is mixed with the third liquid to form precipitates containing Mg, such as hydromagnesite or amorphous magnesium carbonate. For example, the precipitates from this stage may contain more than 94 wt% of magnesium oxide ( gO). The amount of Na₂C0₃ added to the third liquid may be adjusted to increase the pH of the third liquid to a third pH range from about 10 to about 11 , such as about 10.6. It has been found that in such a pH range, the content of hydromagnesite in the precipitate may be maximized in selected embodiments. It has also been found that the pH of the first liquid during the first precipitation stage also affects the amount of hydromagnesite produced at the third precipitation stage. The precipitates can be conveniently separated from the remaining solution.

[0062] The Mg precipitates from the third precipitation stage may be useful as flame retardant agents in polymers, additives for pigments and paper production, or raw material for MgO production, or may be used in pharmacological

applications.

[0063] In selected embodiments, a system 200 as illustrated in FIG. 2 may be suitable for carrying out a specific embodiment of process S100.

[0064] As depicted, a salt solution tank 212 is provided for preparing and holding a salt solution. For ease of description, the salt solution is assumed to be a solution of NaCI and the substance comprising an alkaline earth metal is assumed to be a silicate mineral such as (Mg, Fe)-silicate. A water tank 214 is also provided for feeding both the salt solution tank 212 and a BMED unit 216. The structure and construction of suitable BMED units for generation of acidic and alkaline solutions from a salt solution are known to those skilled in the art, and thus are not described in detail herein. [0065] The structure of an example BMED unit 216 is schematically illustrated in F!G. 2A. As depicted, a basic BMED structure may include an anion exchange membrane (AEM) and a cation exchange membrane (CEM), placed between two bipolar membranes (BPM). The basic BMED structure, also referred to as an elementary BMED cell, may be repeated in the BMED unit 218, to provide increased processing volume or throughput. For instance, a number of the elementary cells may be placed between a cathode and anode for applying an electric voltage. As can be appreciated, the BMED unit 216 may include stacked membranes.

[0066] The overall reaction of water splitting occurring in the middle interface layer can be expressed as in Equation (5):

2H₂0 = H₃0⁺ + OH^", (5)

[0067] The theoretical potential for the bipolar membrane electrodialysis can be calculated by the Nernst-Equation, The reversible Gibbs free energy required for the production of 1 M of HCI and 1 M of NaOH in a bipolar membrane can be calculated using Equation (6):

-AG = -nFE = -RTln

where ^an' + - ^aou- are the activities of H⁺ and OH^" ions in the bipolar membrane interface (10^"7 mol/L at 25°C); «Η+- ^ΩΟΗ- are the activities of H⁺ and OH^" ions outside the bipolar membrane; Kw is the dissociation constant of water; ΔρΗ is the pH difference of the HCI and NaOH solutions.

[0068] For 1 M HCI solution and 1 M NaOH solution, ΔρΗ=14. Thus, at 25°C, the theoretical cell potential ΔΟ0.828 V, which is far lower than that of the conventional chlor-alkali process, 2.19 V. The theoretical electricity consumption in a particular bipolar membrane electrodialysis may be expected to be about 560kW-h/t NaOH. In comparison, in a conventional chlor-alkali process, the expected electricity consumption is about 1468kW h/t NaOH. [0069] Returning to FIG. 2, the salt solution is to be dissociated in the BMED unit 216 to produce the acid, HCI, and the base, NaOH. As alluded to earlier, other salts may be used and the corresponding acids and bases may be produced.

[0070] BMED unit 216 is in fluid communication, through conduit 10, with an acid digestion Reactor R-1 for reacting HCI with an alkaline earth metal-containing substance such as a (Mg, Fe)-silicate, and is in fluid communication, through conduit 20, with a scrubbing Reactor R-2 for reacting NaOH with a flue gas. The flue gas contains C0₂, and may contain SO_x, NO_x, particulate matters, or other materials that may be found in a flue gas. As alluded to earlier, in different embodiments, a suitable alkaline earth metal-containing substance may react with HCI to produce a salt of the alkaline earth metal.

[0071] The output end of BMED unit 216 has an outlet for the diluted salt solution that has passed through BMED unit 216, which is connected to the salt solution tank 212, through conduit 30, for recycling the diluted salt solution.

[0072] Reactor R-1 has an inlet 40 for receiving the silicate mineral. Reactor R-1 is also in fluid communication with a precipitation Reactor R-3, through a Separator S-1 , and conduits 101 and 102.

[0073] Separator S-1 may include a cyclone separator (not separately shown). Separator S-1 is connected to a Reactor R-6 through conduit 103, and has an outlet 104 for discharging solid precipitates.

[0074] Reactor R-6 is connected with a Separator S-2 through another Reactor R-7, and conduits 301 and 303. Reactor R-6 has an inlet 22 for receiving an alkaline solution, and an outlet 302 for discharging solid precipitates.

[0075] Scrubbing Reactor R-2 is in fluid communication, through conduit 201 , with precipitation Reactor R-3. Reactor R-2 has an inlet 50 for receiving a source of carbon dioxide such as a flue gas, and a vent outlet 60 for releasing processed gases.

[0076] Reactor R-3 is connected to another Separator S-3 through conduit 203. [0077] Separator S-3 may be a cyclone separator, and has an outlet 205 for disposing solid precipitates and an outlet 206 for outputting the regenerated salt solution. Outlet 206 may be in fluid communication with salt solution tank 212, for recycling the regenerated salt solution. Optionally, an ion-exchange device such as an ion-exchange tower (not shown) may be provided and connected to outlet 206 for removing divalent and trivalent ions that may be present in the regenerated salt solution before the purified regenerated salt solution is transferred to the salt solution tank 212. Suitable ion-exchange devices and ion-exchange towers include those currently known to persons skilled in the art and may be selected and configured depending on the particular application. When the regenerated salt solution contains undesirable divalent and trivalent ions, directly feeding the regenerated solution into the BMED unit 216 might reduce the lifetime of the membranes. Thus, in some embodiments purification of the regenerated salt solution can prolong the lifetime of the membranes in the BMED unit 216.

[0078] Reactor R-7 has an inlet 51 for receiving a flue gas, and an outlet 61 for venting. Reactor R-7 can be a spray tower or bubbling tower, or any other suitable reactor.

[0079] Separator S-2 is in fluid communication with Reactor R-3 through a conduit 304 and has an outlet 305 for discharging solid precipitates. Separator S-2 may be a cyclone separator.

[0080] Fluid flow control and transport devices and components, such as valves, pumps, regulators, or the like, and other necessary or optional components of the system, are not shown for simplicity, but it should be understood that such components will be provided as needed or desired.

[0081] In operation, water and NaCI are fed to the BMED unit 216, which may be operated as follows. The NaCI solution is fed and flows continuously between the AEM and the CEM, as depicted in FIG. 2A. Water is fed and flows between a BPM and the CEM, and between a BPM and the AEM, respectively. An electric voltage is applied across the anode and the cathode to generate a direct electric current between the anode and the cathode. Water is dissociated (split) in each bipolar membrane (BPM) to produce equivalent amounts of H⁺ and OH^' ions. The H⁺ ions permeate through the BPM towards the anode direction. The H⁺ ions can combine with CI^' ions diffused through the AEM to form HCI. The OH^" ions permeate through the cathode direction. The OH^' ions can combine with Na⁺ ions diffused through the CEM to form NaOH. The resulting effect is that the NaCI solution is electrodialyzed to NaOH and HCI. A diluted NaCI solution is also produced from the BMED unit 216, as can be understood. The dilute NaCI solution produced from the BMED unit 216 may be recycled back to the NaCI tank 212 through conduit 30. The dilute NaCI solution may be treated prior to being recycled back to the NaCI tank 212.

[0082] The electrodialysis process conveniently requires less electrical energy to run, as compared to electrolysis processes for generating acid and base.

[0083] The acid solution containing HCI produced in the BMED unit 216 is fed to acid digestion Reactor R-1 and reacts with the silicate mineral. The silicate mineral may include finely ground serpentine or olivine powder. The ground mineral powder may be suspended in the HCI solution in Reactor R-1 and vigorously stirred at an elevated temperature to allow the extraction of Mg from the mineral powder.

[0084] After acid digestion, the solid residues produced in Reactor R-1 , which may contain mostly coarse silica and magnetite, are transported to Separator S-1 and filtered in Separator S-1. The liquid component (also referred to as leachate hereinafter) generated in Separator S-1 may contain mainly MgCI₂ and FeCI₃. The leachate may be partially recycled back to Reactor R-1 for further processing.

[0085] The leachate from Separator S-1 is to be sent to Reactor R-3.

[0086] Magnetite in the solid output of Separator S-1 may be separated from the solid output of Separator S-1 in a magnetic separator (not shown in FIG. 2).

[0087] In Reactor R-2, the flue gas is passed through the NaOH solution produced from the BMED unit 216, to produce a Na₂C0₃ solution. The cleaned flue gas, which has reduced C0₂ content or is substantially free of CO2, may be released into the atmosphere. [0088] The Na₂C0₃ solution is sent to precipitation Reactor R-3 to react with the leachate from the Separator S- . The products (precipitates) of the reaction in Reactor R-3 contain mainly coarse magnesium carbonate. The precipitates are separated in the cyclone Separator S-3. The liquid product from Separator S-3 contains regenerated NaCI and may be recycled back to the NaCI tank 212.

Optionally, the liquid product may be passed through an ion-exchange device such as the ion-exchange tower described earlier to remove trace amount of divalent and trivalent ions that may be contained in the liquid. The purified liquid may then be recycled back to the NaCI tank 212.

[0089] The loss of permselectivity of the individual component membranes and diffusive transport in the membranes can become problematic when the concentration of HCI in the stream in conduit 10 and the concentration of NaOH in the stream in conduit 20 are too high. The current efficiency of the BMED unit 216 may decrease with increased concentrations of HCI and NaOH in conduits 10 and 20 respectively. Thus, in some embodiments, the concentration of HCi in conduit 10 and the concentration of NaOH in conduit 20 may be limited to a range from about 0.5 to about 1 M during electrodialysis. This may be accomplished by continuously removing the acid and base produced at sufficient rates.

[0090] According to Equation (6), the electricity consumption by the BMED unit 215 is dependent on the ΔρΗ in the solutions, or, in other words, the respective concentrations of the acidic and alkaline solutions. If the concentration of HCI in the conduit 10 and the concentration of NaOH in conduit 20 are assumed to be 1 M, the theoretical cell voltage is 0.828 V and the theoretical electricity consumption is 560kW-h/t NaOH at 25°C. The actual potential drop across the BPM may be in the range of 0.9-1.1 V.

[0091] In selected embodiments, the HCI concentration in conduit 10 may be maintained to be below 1M, such as about 0.5 M. In this case, the theoretical cell voltage can be reduced to 0.787 V even if the NaOH concentration in conduit 20 is 1M. The pH of the NaOH solution can be lowered to below 14 by keeping the concentration of NaOH in conduit 20 below 1M, or by bubbling flue gas in the path of the NaOH solution in the BMED unit 216. [0092] ft is noted that if the pH in the NaOH solution is lowered to below 10, formation of NaHC0₃ in the alkaline solution stream may significantly increase. NaHC0₃ is not expected to react with MgCI₂, and when the regenerated salt solution is recycled back to the BMED unit 216, the residue NaHC0₃ in the salt solution can produce C0₂, thus reducing the C0₂ capture or sequestration efficiency.

[0093] If the pH of the Na₂C0₃ solution is assumed to be 10 and the concentration of the HCI solution is assumed to be 0.5 M, the theoretical cell voltage in the BMED unit 216 is expected to be 0.55 V, and the theoretical electricity consumption is estimated to be about 372 kW h/t NaOH at 25°C.

[0094] The expected fractional contents of different carbonate species at chemical equilibrium at different pH values are shown in FIG. 3.

[0095] In some embodiments, the silicate mineral for input to Reactor R-1 may include Mg and Fe silicates such as ultramafic rocks, including serpentine and olivine minerals. The sheet structured serpentine is thermodynamically stable and has a chemically inert framework. In some embodiments, serpentine may need to be pre-activated by heating at above 600°C prior to being reacted with the acidic solution. In some embodiments, however, when a strong acid, such as HCI, is used, a heat pre-treatment of serpentine is not necessary.

[0096] The silicate mineral may be pre-processed to form particulates with desired particle sizes. Smaller particle sizes can increase reaction speed.

However, this may be balanced with the cost for reducing the particle sizes. In some embodiments, ball milling may be used to reduce the particle sizes of the silicate mineral feed.

[0097] The particle sizes of serpentine may also have an effect on the Mg extraction rate. For example, in some embodiments, when the particle size is in the range of 125-250 pm, the extraction degree of Mg may be about 75% after 1 hour in 2M HCI at 70°C. If the particle size is reduced to 74-125 pm, the extraction degree of Mg may be increased to near 100% after 1 hour in 2M HCI at 70°C. Thus, in some embodiments, the silicate mineral (such as ultramafic rocks) may be ground by ball milling to particle sizes from about 74 pm to about 125 μητ

[0098] Further, it is noted that the reaction temperature in Reactor R-1 may affect the g extraction rate. For instance, when particles of silicate mineral with sizes from 125-250 pm are reacted with a HCI solution at 80°C, about 64% of Mg can be extracted in a 0.5 M HCI solution after 2 hours of reaction, and about 66% of Mg can be extracted in a 2M HCI solution after 2 hours of reaction. When the extraction temperature is at 24°C, the extraction extent can be below 24% after 2 hours of reaction in a 2M HCI solution.

[0099] Thus, in selected embodiments, serpentine may be reacted with HCI in Reactor R-1 at an elevated temperature, such as at 40, 50, 60, 70, or 80°C. Conveniently, when a hot flue gas is available, the hot flue gas may be used to heat the HCI solution. For instance, in some applications, the flue gas to be fed to inlet 50 may be hot, as a typical flue gas may have a temperature in the range of 95- 120°C. This hot flue gas may be used in a heat exchange unit (not shown) to heat the HCI solution before the HCI solution comes into contact with the silicate mineral.

[00100] To increase reaction rate, the mixture in Reactor R-1 may be agitated or stirred.

[00101] After acid digestion, the liquid and solid components of the mixture received from conduit 101 are separated in the Separator S-1. The solid product, which may contain mainly coarse silica and magnetite, may be discharged. The solid discharges may be washed by water to remove residual HCI prior to further disposal.

[00102] The coarse silica may be discharged through conduit 103, and may be directly used as building materials, or landfill stuff for land reclamation. The coarse silica may be further purified as described herein.

[00103] The magnetite may be separated by magnetic separation and discharged through outlet 104. The discharged magnetite may be used as a raw material for steel production, abrasives, insulation for windows, and additives in toners. [00104] The liquid component separated from the mixture in Separator S-1 is supplied to Reactor R-3 through conduit 102. The liquid component may also be treated, for example, with hydrogen peroxide, prior to being precipitated.

[00105] The flue gas is fed through inlet 50 into the NaOH scrubber Reactor R-2 for C0₂ capture and removal by reacting with NaOH solution.

[00106] In selected embodiments, C0₂ capture from the flue gas may also be carried out in the BMED unit 216, instead of in the scrubber Reactor R-2, to reduce the NaOH concentration in conduit 20. This may enhance the current efficiency of the BMED unit 216 and minimize the electricity consumption.

[00107] The molar ratio of NaOH and C0₂ in the reaction mixture in scrubber Reactor R-2 may be maintained at about 2.Ί , such as by regulating the flow rates of the flue gas and the NaOH solution, so that the main reaction product is Na₂C0₃.

[00108] After C0₂ capture, residual gas phase in scrubber Reactor R-2 may be substantially free of C0₂, and the clean gas may be released through outlet 60.

[00109] The remaining fluid stream after C0₂ capture in scrubber Reactor R-2 contains Na₂C03 and is supplied to the precipitation Reactor R-3 to react with the liquid component received from Separator S-1 through conduit 102.

[00 10] In the precipitation Reactor R-3, precipitates are expected to form upon the mixing of MgCI₂ and a minor portion of FeCI₃ solution with Na₂C0₃ solution.

[00111] Depending on the various input materials used and the reaction conditions at various stages, the precipitates produced in Reactor R-3 may include nesquehonite (MgC0₃-3H₂0) and Fe. The precipitates may have a yellowish color when Fe is present.

[001 2] In some embodiments, measures may be taken to facilitate substantial complete precipitation of divalent ions, such as Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and multivalent ions, such as Fe³⁺, Al³⁺, Cr³⁺, or the like, before the processing fluid stream is fed back to the BMED unit 216, as such ions may precipitate in the BMED membrane stack, resulting in scaling of the membranes and deterioration of the performance of the BMED unit 216. One such measure is to provide slight excess of Na₂CC>3 solution in Reactor R-3 until no further precipitation is observed. Another measure is to control the pH value of the precipitation reaction solution as discussed before. In some embodiments, the final pH of the reaction mixture in Reactor R-3 may be in the range of about 9 to about 10.

[00113] After the precipitation reaction in Reactor R-3, the resulting mixture is pumped to cyclone Separator S-3 through conduit 203 to allow the separation of the solid products. The solid products in the stream 205 comprising mainly nesquehonite (MgC0₃'3H₂0) can be used as building materials and landfill stuff for land reclamation.

[00114] The liquid product received in conduit 206 may include mainly NaCI, Na₂C0₃/NaHC0₃, and a trace amount of divalent and trivalent ions. As discussed before, this liquid may be passed through an ion-exchange device such as the ion- exchange tower described earlier to remove the divalent and trivalent ions. The purified solution may then be recycled back to salt solution tank 212.

[001 5] Loss of NaCI during the operation may occur, and make-up NaCI may be added to the salt solution tank 212 or at another suitable point in process loop. Make-up NaCI may be continuously added or added at selected intervals.

[00116] In selected embodiments, an alternative system 300 as illustrated in FIG. 4, may also be used. System 300 in many aspects is similar to system 200 and like components in the two systems are labeled with like reference numbers.

[00117] The main difference between system 200 and system 300 is that Reactors R-4 and R-5 are added and connected to Separator S-1 and Reactors R- 2, R-3 by conduits 102, 106, 201 and 203 as shown in F!G. 4, for carrying out the different precipitation stages described earlier. Reactor R-5 has an inlet 21 for receiving a base such as NlaOH and an outlet 105 for discharge precipitates.

Reactor R-4 has an outlet 204 for discharging precipitates. Separator S-3 may be omitted in system 300. [00118] In operation, system 300 may be operated as described above with reference to system 200, but with the following differences in the precipitation process for the leachate from Separator S-1.

[00119] In the first precipitation stage, an alkali solution (NaOH, KOH, or the like) is added to Reactor R-5 through inlet 21 to increase the solution pH (pHi) in Reactor R-5 to the first pH range of about 4 to 9, such as about 4 to 7.

[00120] The volumetric ratio of the added alkali solution (1 M) to the leachate may be 0.05 to 0.15, such as 0.08 to 0.1.

[00121] The value of pH| may be adjusted to promote removal of Fe ions, without substantial removal of Mg ions as discussed elsewhere herein. The value of pH| may be from about 4 to about 9, such as about 5 to about 7. The precipitate formed in Reactor R-5 may thus have a high concentration of Fe (such as [Fe₂0₃] > 93 wt%) and a low concentration of Mg (such as [MgO] < 0.5 wt%).

[00122] The precipitates may be separated using a continuous cyclone separator and collected through outlet 105. The liquid solution separated from Reactor R-5 is transferred to Reactor R-4 through conduit 106.

[00123] In the second precipitation stage, the solution pH in Reactor R-4 is further increased to pHn using the a₂C0₃ solution received through conduit 201 from Reactor R-2 to remove residual minor components such as Fe, Cr, or the like, without substantial removal of Mg ions. The value of pHn (second pH range) may be from about 9 to about 10, such as about 9.5. The volumetric ratio of the carbonate solution from Reactor R-2 to the clear solution from Reactor R-5 may be from about 0.7 to about 1.0, such as about 0.9. The precipitates in Reactor R-4 may be separated from the liquid components by a continuous cyclone separator and the separated liquid component may be transferred to Reactor R-3 through conduit 203. The solid precipitates from Reactor R-4 may be discharged through outlet 204.

[00124] In the third precipitation stage, the solution pH in Reactor R-3 is further increased to pHw using the a₂C03 solution received through conduit 202 from Reactor R-2 until no more precipitates will form. The value of pHw (third pH range) may be in the range of about 10 to about 1 1 , such as about 10.6.

[00125] The precipitates in Reactor R-3 may be separated from the liquid component using a continuous cyclone separator, discharged through outlet 205, and washed with deionized water. Depending on the input materiais used and the processing conditions, the precipitates from Reactor R-3 may contain a crystalline hydromagnesite (4MgC0₃ Mg(OH)₂-4H₂0), or amorphous magnesium carbonate. In some embodiments, the precipitates may also contain nesquehonite. The Mg concentration (as measured by MgO concentration) in the precipitates may be above 94 wt%.

[00126] The value of pHi may affect the yield of hydromagnesite. When pHi is below 3, formation of hydromagnesite in the third precipitation stage may be low. When pHi is above 7, the yield of hydromagnesite in the third stage may decrease significantly due to increased precipitation of Mg in Reactors R-5 and R-4. Thus, pH, may be selected to be from about 5 to about 7, such as about 5.

[00127] In some embodiments, the yield of hydromagnesite with pHi = 5 may be expected to be at least 60 wt% with respect to the total Mg content in the input silicate mineral.

[00128] The purity of the produced hydromagnesite can be improved by oxidizing the divalent ions, such as Fe²⁺, Mn²⁺, or the like, to multivalent ions, such as Fe³⁺, Mn⁴⁺, or the like prior to the first precipitation step. The improvement may be conducted in Reactor R-5.

[00129] For example, in a selected embodiment, H₂0₂ (30%) may be added to the leachate from Separator S- to oxidize the divalent ions according to the following Reactions (7) and (8):

Fe²⁺ + H₂0₂ + H⁺→ Fe³⁺ + H₂0 ; (7)

Mn²⁺ + H₂0₂ + OH→ Mn0₂1+ H₂0 . (8) The volumetric ratio of fed H₂0₂ (30%) to the leachate may be from 0.002 to 0.005, such as about 0.003. With the addition of H₂0₂, the minor components, such as Fe, Mn, or the like, can be removed more efficiently. The purity of hydromagnesite can thus be increased to about 98.5 wt %. Hydromagnesite of such high purity may be marketed and sold as a fine chemical.

[00130] In some embodiments, the coarse silica separated in Separator S-1 may be a highly porous material with a BET surface area up to about 396 m²/g, a total pore volume of up to 0.32 cm³/g and a pore size of 3.9 nm. Due to the porous structure, the coarse silica is expected to have a high chemical reactivity. The coarse silica may be purified at room temperature in Reactor R-6 by dispersing it in a dilute solution of an alkali metal, e.g. NaOH solution, received from conduit 22. The dilute solution of the alkali metal may be produced in the BMED unit 216. Si may be selectively leached to the solution as silicates from the coarse silica according to Reaction (9):

Si0₂ + NaOH→ Na₂Si0₃ + H₂0 . (9)

[00 31] The ratio of alkali (NaOH or KOH) solution ( M) to the coarse Si0₂ in Reactor R-6 may be about 83 L alkali solution per 1 kg coarse Si0₂.

[00 32] After the NaOH leaching, the resulting mixture in Reactor R-6 may be separated. The separated solid residue may be discharged through outlet 302, which may be disposed as a landfill material. The leached silicate solution may be transferred through conduit 301 to Reactor R-7 to obtain purified Si0₂.

[00133] In Reactor R-7, Reaction (10) may occur:

Na₂Si0₃ + H₂0 + C0₂→ H₄Si0₄ |+ Na₂C0₃ . (10)

[00 34] After the reaction (scrubbing), the resulting mixture in Reactor R-7 may be separated. The gas component may be vented and the liquid and the solid components may be transferred to Separator S-2 through conduit 303. A hydrated Si0_2t H₄Si0₄ (also commonly known as Si0₂2H₂0 or Si(OH)₄), is separated out in Separator S-2 and discharged through outlet 305. The discharged hydrated Si0₂ may be washed with deionized water. As can be understood by those skilled in the art, purified anhydrous Si0₂ may be readily obtained from the hydrated Si0₂, e.g. by calcining H₄Si0₄ at an elevated temperature, e.g. 500°C. in a furnace (not shown in FIG. 4). The Na₂C0₃ component separated out by Separator S-2 may be discharged through outlet 304 and may be recycled to Reactor R-3.

[00135] In selected embodiments, the hydrated SiOa rom Separator S-2 may be further purified before calcination by ion exchange. One suitable ion exchange may be between the NH ⁺ ion of ammonium nitrate and the Na7Fe³⁺ ions coordinated to the hydrated Si0₂.

[00136] After the purification process described above, the produced silica, either anhydrous or hydrated, can have a purity of up to about 97 %, which may be useful in various applications, such as fillers for rubber, plastics, paints and resins, insulating materials, raw material for silicon, et al.

[00137] It should be understood that the specific embodiments described herein are for illustration purposes. Modifications to these embodiments are possible.

[00138] Exemplary embodiments of the present invention are further illustrated with the following examples, which are not intended to be limiting.

[00139] EXAMPLES

[00140] Example I: Extraction of Mg from serpentine using HCI solution

[00141] Prior to the extraction, the compositions of the serpentine samples from China and Finland and the coarse silica samples were analyzed by X-ray fluorescence (XRF, Bruker™ AXS S4 Explorer WD) and X-ray powder diffraction (XRD, Bruker™ AXS powder X-Ray diffractometer D8 Advance). The results are summarized in Table I and FIG. 5. According to Table I, both serpentine samples contain greater than 40 wt% MgO. [00142] Table I. Chemical compositions of serpentine and coarse silica

[00143] In a typical experiment, 40 g of the serpentine sample with the particle size in the range of 125-250 pm was suspended in 1 L of HCI solution with different concentrations (0.5 M, 1 and 2 ). The mixture was stirred by a magnetic stirrer with a stirring rate of 1000 rpm. The extraction was conducted at room temperature (24°C) or in an 80°C oil bath with a water condenser. The resulting suspension was taken out at intervals and filtered using a 0.45 pm filter. The filtrates were collected in a glass vial and analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, VISTA-MPX CCD Simultaneous ICP-OES). After extraction, the solid products were filtered using a Buchner's funnel by suction filtration at a fast filtration speed. The solids on the filter paper after filtration had a grey colour and an appearance similar to that of the sea sand. The filtrate was collected and retained for further use.

[00144] FIGs. 6A and 6B show the g extraction results of serpentine using HCI at 24°C and 80°C respectively. At 24°C, the Mg extraction extent did not exceed 24% at all the HCI concentrations studied after 2 hours of extraction (FIG. 6A). At 80°C, the Mg extraction extent was significantly enhanced (FIG. 6B). The HCI concentration did not appear to have an observable effect on the extent of Mg extraction. After 2 hours of extraction, an extraction extent of above 64% was achieved for the HCI solution with different concentrations at 80°C. A conversion of up to 96% Mg was achieved when the serpentine particles were refluxed at 80°C in the 1 HCI solution for 6 hour.

[00145] Example II: Direct precipitation of MgCI₂/serpentine leachate solutions with Na₂C0₃/ NaHC0₃ solutions

[00146] Example ll-A: Reaction of MgCI₂ with Na₂C0₃

[00147] 25 mL of 1 M Na₂C0₃ solution was slowly dropped into 25 mL of 1 M MgCI₂ solution contained in a beaker under vigorous stirring at 24°C. Initially, no precipitate formed and the mixture was transparent. With the continuous addition of Na₂C0₃ solution into the MgCI₂ solution, white precipitates formed. The white precipitates slowly deposited at the bottom of the beaker and a transparent solution formed in the upper layer of the mixture.

[00148] Example ll-B: Reaction of MgCI₂ with NaHC0₃

[00149] 25 mL of 2M NaHC0₃ solution was slowly dropped into 25 mL of 1 M MgCI₂ solution under vigorous stirring at 24°C. Throughout the addition of the NaHC0₃ solution, no precipitates formed and the mixture remained transparent.

[00150] Example ll-C: Direct precipitation between serpentine leachate solution and a₂C03

[00151] 0.5M Na₂C0₃ solution was slowly dropped into 300 mL of serpentine leachate solution at 24°C, which was obtained after 6 hours of reaction between serpentine (40 g, particle size: 125-250 m) and 1 M HCI (1 L) at 80°C under vigorous stirring. The pH of leachate was 0.76. The addition of the Na₂C0₃ solution ceased until the solution pH increased to 9.5, where no more precipitates formed with further increase of pH. A total of 411 mL of 0.5 M Na₂C0₃ solution was added. Reddish precipitates formed. The precipitates were retrieved by suction filtration and dried at 8Q°C in an electric oven. 9 g of precipitates were obtained. The precipitates appeared to be light and occupy a large volume. The precipitates thus obtained were denoted as DP. The crystalline phase of the precipitates was analyzed by XRD and the chemical composition was measured using XRF. The indexing of the crystalline phases of the precipitates was done using a

DIFFRACplus EVA auxiliary software. The total carbon content in the precipitates was analyzed using a total organic carbon analyzer (Shimazu™ TOC-V CSN) with a solid sample module SSM-5000A.

[00152] FIG.7A shows XRD pattern of the sample (DP) obtained from the direct precipitation, which was a mixture of a nesquehonite phase (MgC0₃-3H₂0, JCPDS card no. 70-1433) and a sjoegrenite phase ( geFe₂(0H)ie(C03)- H2O, JCPDS card no. 86-0182). Chemical composition analysis results in Table 2 confirmed that the precipitates were a mixture and showed that the mixture contained mostly g ( gO, 69 wt%) and Fe (Fe₂0₃, 27 wt%). The TOC result showed that the C/Mg ratio in this mixture was about 0.93, which corresponded to capturing 450 kg C0₂ per ton of serpentine.

[00153] Table 2. Chemical composition of the precipitates from a direct precipitation between serpentine leachate solution and Na₂C0₃ (Example I l-C)

[00154] Example III: Three-stage precipitation of serpentine leachate solutions with NaOH and Na₂C0₃ solutions

[00 55] In the first stage, 1M NaOH solution was added to 100 mL leachate at 24°C to increase the solution pH from pH₍=1 to 9 and to obtain precipitate 1 (P1). The leachate was obtained after 6 hours of reaction between serpentine (40 g, particle size: 125-250 pm) and 1 M HCI (1 L) at 80°C under vigorous stirring. P1 was separated by centrifugation and the clear solution was transferred for further precipitation in the second stage. [00156] In the second stage, the solution pH was further increased to pHn= 9.5 using 0.5 M Na₂C0₃ solution and precipitate 2 (P2) was obtained. P2 was separated by centrifugation and the clear solution was transferred for further precipitation in the third stage.

[00157] In the third stage, the solution pH was further increased to pHm=10.65 using 0.5 M Na₂C0₃ solution until no more precipitates formed. The obtained precipitate 3 (P3) was separated using centrifugation, washed with deionized water and dried at 80°C in an electric oven. The final solution was analyzed using ICP- OES.

[00158] Table 3 lists the volume of 1 M NaOH and 0.5 M Na₂C0₃ solutions used for the above step-wised pH adjustment.

[00159] Table 3. Volume of 1 NaOH and 0.5 M Na₂C0₃ solutions used for pH adjustment

[00160] The crystalline phase of P1 , P2 and P3 was analyzed by XRD. The XRD patterns in FIGs. 7A and 7B indicate that P2 obtained at pH|=1 and 2 had a similar crystalline phase as that was obtained from the direct precipitation process in Example ll-C, i.e. a mixture of a nesquehonite phase (MgC0₃-3H₂0, JCPDS card no. 70-1433) and a sjoegrenite phase (Mg₆Fe₂(OH)i₆(C0₃)-H₂0, JCPDS card no. 86-0182). When 3≤ pHi≤ 5, the P3 obtained was a crystalline hydromagnesite phase. Further increasing pH|≥7, the P3 obtained was an amorphous magnesium carbonate phase with a small amount of crystalline nesquehonite. [00161] Chemical compositions of P1 , P2 and P3 were analyzed using XRF and the results were shown in F!G. 8. When pH| < 5, almost no P1 was obtained because of the highly acidic solution. When pH| = 5, P1 obtained had a very high concentration of Fe (Fe₂0₃ > 93.1 wt%) and trace concentration of Mg (MgO < 0.5 wt%). Further increasing pHi > 5 resulted in the increased concentration of Mg in the P1 , implying that an increased amount of Mg was co-precipitated with Fe. For the P2, when 1≤ pHi≤ 6, it comprised mostly Mg and a considerable amount of Fe, suggesting that the second stage may be necessary for the complete removal of Fe. When pHi≤ 3, there was no magnesium carbonate precipitate, suggesting that all Mg ions were removed during the first 2 stages. When pHi > 3, all the P3 obtained had a high concentration of Mg (measured as MgO > 94 wt%).

[00162] . FIG. 9 shows the Mg removal efficiency and the yield of

hydromagnesite at different pH,. When 1≤ pHi≤ 2, Mg removal efficiency was up to 90%. With the increase of pH₍, Mg removal efficiency decreased with the maximum of 80% at

pH₍ also appeared to have an effect on the yield of hydromagnesite. The maximum yield of hydromagnesite was obtained at pH|=5, up to 60 wt%.

[00163] From the morphology of the solid magnesium carbonates synthesized at different pHi values, which were observed using scanning electron microscopic (SEM) images of the different samples, it was observed that the precipitation pH (pHi) of the first stage may have a strong effect on the morphology of the P3. When pHi was 3, P3 exhibited a uniform sphere-like morphology, which comprised numerous thin flakes wreathing around a central axis. The thickness of thin flakes ranged from 20-30 nm and the diameter of the spheres was up to 16-18 pm. With an increase in pHi to 5, the diameter of the spheres increased up to 70 - 85 pm, and the central axis disappeared. The nanoflakes were connected to each other, forming a honey-comb like structure, a typical structure of the hydromagnesite material, and this was consistent with the XRD results. The big spheres remain at pHi up to 6, however they were found to be adhered with some amorphous particles. When pHi was increased up to 7, P3 was mainly made up of amorphous particles without a well-defined morphology. Some slab-like particles with a length in the range of 0-20 pm and a width of several microns coexisted in the amorphous ones. Further increasing pH| above 7, amorphous P3 with a particle size of tens of nanometers were produced.

[00164] FIGS. 10 and 11 show the TG-DTG curves of the magnesium carbonates (P3) obtained at different pH|. As can be seen, the sample obtained at pH| = 5 displayed a similar TGA profile as a typical hydromagnesite sample. The TG curve exhibited a small weight loss in 187-280°C and a big steep weight loss in 330-460°C. The first weight loss may be attributed to the release of the crystalline water in the hydromagnesite according to Reaction (11 ):

4MgC0₃-Mg(OH)₂'4H₂0→ 4MgC0₃ Mg(OH)₂ + 4H₂0 . (11 )

The second weight loss is ascribed to the dehydroxylation and decarbonation as represented in Reactions (12) and (13), respectively:

4 gC0₃ Mg(OH)₂→ 4MgC0₃ MgO +H₂0 ; (12)

4MgC0₃-MgO→ 5 gO +4C0₂ . ( 3)

The total weight loss measured was 56.4 %, close to the theoretical decomposition value of hydromagnesite (57%). By increasing the pHi to 6, the sample showed a slower decomposition rate and the total weight loss measured was 51.7%, lower than that obtained at pH| = 5. Further increasing pHi to 7, the sample prepared had the lowest decomposition rate among all the P3 samples. Also, this sample had a weight loss of ca. 10% in the temperature range of 750-900°C, which was too hot to coincide with major polymer decomposition and flame production stage upon ignition, thus not useful for the flame retardant application. The samples obtained at pHi = 8 and 9 exhibited a large and fast decomposition step in 330-460°C. However, they also displayed a significant loss below 200°C, ca. 20%, higher than that of hydromagnesite sample (8%), which contributes little to the reduction of the burning rate of polymers, like polyolefins.

[00165] The differential thermal analysis curves in FIG. 12 show that the hydromagnesite sample produced at different pH|S exhibited two evident endothermic peaks at 250 and 400°C, which is beneficial for the flame retardant application. The sample obtained at pHi above 7 did not show distinguished endothermic peaks.

[00166] Example IV: Further purification of hydromagnesite using H₂0₂

[00167] A small amount of H₂0₂ was added to the leachate with the volumetric ratio of H₂0₂/leachate = 0.002-0.005. Otherwise, the experimental steps followed those described in Example III, except that the pH|, pHn and pHm were set at 5, 9.5 and 0.65 respectively. The P3 thus obtained was analyzed for chemical composition using XRF. The MgO content was 98.5 wt%.

[00168] Example V: Purification of coarse silica

[00169] The coarse silica produced in Example I above was characterized using XRF, XRD and N₂ adsorption techniques. The XRF results in Tab!e 1 above show that the solid residue obtained from the acid leaching of serpentine in Example I was a coarse Si0₂, having a Si0₂ purity of 94 wt% and various impurities, such as Mg, Al, Fe, Ni, Cr, et al., thus needing further purification in order to become a value-added product. The XRD patterns in FIG.13 show that the coarse silica contained amorphous silica phase and a significant amount of remaining serpentine phase even after extraction at 80°C for 6 hours in 2 M HCI solution. The N₂ adsorption isotherms in FIG. 14A indicate that the coarse silica was a highly porous material. The pore size distribution (PSD) in FIG. 14B shows the coarse silica had a pore size centered at ca. 3.9 nm. The coarse silica had a very high BET surface area up to 396 m²/g with a pore volume up to 0.32 cm³/g. The porous structure may be favourable for the extraction of Si from the coarse silica.

[00170] Example V-A: Extraction of Si from the acid-leached residue of serpentine

[00171] In a typical procedure, 10 g of coarse silica, which was obtained after 6 hours of reaction between serpentine (40 g, particle size: 125-250 pm) and 1 M HCI (1 L) at 80°C under vigorous stirring, was mixed with 4 L of NaOH solution of different concentrations (FIGs. 15A ([NaOH] = 0.1 M),15B ([NaOH] = 0.2 ) and 15C ([NaOH] = 0.3 )). The suspension was refluxing at different temperatures shown in F!Gs. 15A, 15B and 15C for a certain period under vigorous stirring. The suspension was separated using centrifugation and the clear solution was kept for further use.

[00172] The NaOH concentration and extraction temperature may be the key factors in determining the Si extraction rate. FIGs. 15A, 15B and 15C show the Si extraction efficiencies obtained using NaOH solutions of different concentrations at different temperatures. As can be seen, the Si extraction rate was fast even under ambient conditions. At room temperature, 87% Si conversion was achieved in 30 minutes using 0.1 M NaOH solution. When the temperature was slightly elevated to 30°C, the Si conversion was increased to 94% after 10 minutes of extraction using the same NaOH solution. For the extraction carried out at the same temperature, the higher the NaOH concentration was, the shorter the time needed to achieve 100% of Si conversion. For example, for the 0.3 M NaOH, 6 minutes were needed to achieve almost 100% Si conversion at 40°C.

[00173] Example V-B: Precipitation of Hydrated Si0₂ using C0₂

[00174] C0₂ bubbled through the clear solution and a transparent silica gel was obtained. The silica gel was separated by centrifugation, washed with deionized water 3 times and dried at 80°C overnight. The residue solution was analyzed using ICP-OES to measure the Si precipitation efficiency.

[00175] FIGs.16A and 16B are data graphs of silicon precipitation efficiency at different conditions. The Si precipitation efficiency decreased with an increase in the extraction temperature. 98.5% of Si precipitation efficiency was obtained at room temperature when the solution pH was set at 7.7 (FIG. 16A). At 24°C, the higher the solution pH was, the lower the Si precipitation efficiency (FIG. 16B).

[00176] The purity of the hydrated Si0₂ samples was measured using XRF. FIG. 17A shows that the samples obtained at different pH at 24°C all exhibited a Si0₂ content above 96%, About 4% Na₂0 remained in the silica (FIG. 17B). Fe₂0₃ content was below 0.06% (FIG. 17C). The lower the pH was, the lower the Fe₂0₃ content. As shown in FIGs.18A, 18B and 18C, when the temperature was elevated above 24°C, the Si0₂ content increased up to 97.7%, Na₂0 content decreased to below 3%, and Fe₂0₃ content decreased below 0.05%. The hydrated Si0₂ sample obtained at 60°C was free of Fe.

[00177] Example V-C: Further purifying hydrated Si0₂ using ion-exchange method

[00178] The hydrated Si0₂ sample precipitated at pH 7.7 at room temperature in Example V-B was used for the ion exchange. The wet precipitate was washed using deionized water twice and was then washed with 1 M NH₄N0₃ once to allow the ion exchange between Na⁺/Fe³⁺ and NH₄ ⁺ to occur. XRF results show that the hydrated silica sample thus obtained had a Si0₂ content as high as 98.4 %, a Na₂0 content as low as 1.3% and was free of Fe.

[00179] Example V-D: Textural properties of the purified silica

[00 80] Prior to the analysis of the texture properties, the hydrated Si0₂ samples obtained from Example V-C were calcined at 500°C for 4 hours at a ramping rate of 2°C in an electric furnace. FIG.19 shows the N₂ adsorption isotherm and pore size distribution curves of a typical purified silica sample. The sample had a typical type iV isotherm, characteristic of mesoporous structure. Its BET surface area was up to 349 m /g with a pore volume of 1.1 cm³/g. The PSD curve shows that it had a narrow pore size distribution centered at 9.6 nm. The mesoporous structure of silica suggests that the silica may be used as a catalyst support or an adsorbent. FIG. 20 indicates that the purified silica was essentially amorphous.

[00181] Example VI: Design and operation of a demonstration plant

[00182] A demonstration plant was built to show the proof-of-concept of the whole process. The demonstration plant was a continuous flow system with a C0₂ sequestration capacity of one ton C0₂/year (2500 KWh/a). The ground fine serpentine particles were fed to a 5-L batch reactor to react with the HCI solution produced from a BMED unit. After 1-2 hours of leaching, the mixture was poured to a separation tank, where the coarse silica was separated using a hydrocyclone. The liquid was pumped to a precipitation tank for the three-stage precipitation. Iron hydroxide (P1), nesquehonite /sjoegrenite (P2) and purified magnesium carbonate (P3) were continuously precipitated in the three tanks and separated using hydrocyclones. According to the demonstration-scale experiment results, as shown in Table 4, to capture 1 ton of C0₂, 2.8 ton of serpentine was needed. As a result, 1.2 tons of coarse silica, 200 kg of iron hydroxide (P1), 70 kg of nesquehonite /sjoegrenite (P2) and 50 kg of magnetite were produced. Furthermore, 2.6 tons of purified magnesium carbonate (P3, MgO > 99% analyzed using XRF), a valued- added product, was obtained as well.

[00183] Table 4. List of raw material needed and products engendered for capturing 1 ton of C0₂.

[00184] It will be understood that any range of values herein is intended to specifically include any intermediate value or sub-range within the given range, and all such intermediate values and sub-ranges are individually and specifically disclosed.

[00185] It will also be understood that the word "a" or "an" is intended to mean "one or more" or "at least one", and any singular form is intended to include plurals herein.

[00186] it will be further understood that the term "comprise", including any variation thereof, is intended to be open-ended and means "include, but not limited to," unless otherwise specifically indicated to the contrary. [00187] When a list of items is given herein with an "or" before the last item, any one of the listed items or any suitable combination of two or more of the listed items may be selected and used.

[00188] Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.

Claims

WHAT IS CLAIMED IS;

1. A process comprising:

(a) producing an acidic solution and an alkaline solution from water and a salt of a base by bipolar membrane electrodialysis, wherein the base is capable of reacting with carbon dioxide to form a carbonate;

(b) contacting a source of carbon dioxide with the alkaline solution to

produce a carbonate of the base;

(c) contacting a substance comprising an alkaline earth metal with the acidic solution to produce a salt of the alkaline earth metal; and

(d) reacting the salt of the alkaline earth metal and the carbonate of the base to produce a carbonate of the alkaline earth metal and regenerate the salt of the base.

2. The process of claim 1 , wherein (a) to (d) are repeated and the salt of the base regenerated in (d) is used in (a).

3. The process of claim 2, wherein each of (a) to (d) is performed continuously.

4. The process of any one of claims 1 to 3, further comprising:

(c1) extracting a first liquid from a mixture produced at (c), wherein the first liquid comprises the salt of the alkaline earth metal;

(c2) increasing the pH of the first liquid to from about 4 to about 7, to form a first precipitate, and removing the first precipitate from the first liquid to form a second liquid; and

(c3) increasing the pH of the second liquid to from about 9 to about 10, to form a second precipitate, and removing the second precipitate from the second liquid to form a third liquid;

wherein (d) comprises mixing the third liquid and the carbonate of the base in a solution having a pH from about 10 to about 1 1.

5. The process of claim 4, wherein the pH of the first liquid is increased to about 5 at (c2), the pH of the second liquid is increased to about 9.5 at (c3), and the pH of the solution in (d) is about 10.6.

6. The process of claim 4 or claim 5, wherein (c2) comprises adding a portion of the alkaline solution produced in (a) to the first liquid to increase the pH of the first liquid, and (c3) comprises adding a portion of the carbonate of the base produced in (b) to the second liquid to increase the pH of the second liquid.

7. The process of any one of claims 4 to 6, comprising adding hydrogen peroxide to the first liquid before (c2).

8. The process of any one of claims 1 to 7, wherein the substance comprising the alkaline earth metal is a silicate mineral.

9. The process of claim 8, wherein the silicate mineral is a serpentine, olivine, pyroxene, sepiolite, mafic, or talc mineral.

10. The process of claim 8 or claim 9, further comprising:

extracting a solid from a mixture resulting from (c), wherein the solid comprises silica;

contacting the solid with a solution comprising an alkali metal to form a solution of a silicate of the alkali metal; and

contacting the solution of the silicate of the alkali metal with carbon dioxide to form a carbonate of the alkali metal and a precipitate comprising silica.

1 1. The process of claim 10, further comprising contacting the precipitate

comprising silica with a solution of ammonium nitrate.

12. The process of any one of claims 1 to 1 1 , wherein the base comprises an alkali metal and the salt of the base comprises a halogen.

13. The process of any one of claims 1 to 12, wherein the alkaline earth metal comprises magnesium.

14. A system comprising: a bipolar membrane unit for producing a stream of an acidic solution and a stream of an alkaline solution from water and a salt of a base by

electrodialysis, wherein the base is capable of reacting with carbon dioxide to form a carbonate;

a first reactor in fluid communication with the bipolar membrane unit, for receiving the stream of the alkaline solution and contacting a source of carbon dioxide with the stream of the alkaline solution to produce a stream comprising a carbonate of the base;

a second reactor in fluid communication with the bipolar membrane unit, for receiving the stream of the acidic solution and contacting a substance comprising an alkaline earth metal with the stream of the acidic solution to produce a stream comprising a salt of the alkaline earth metal; and a third reactor in fluid communication with the first and second reactors, for receiving and reacting the salt of the alkaline earth metal and the carbonate of the base to produce a carbonate of the alkaline earth metal and regenerate the salt of the base.

15. The system of claim 14, wherein the third reactor is in fluid communication with the bipolar membrane unit for supplying the salt of the base regenerated in the third reactor to the bipolar membrane unit.

16. The system of claim 14 or claim 15, further comprising:

a separator in fluid communication with the second reactor for receiving a product mixture produced at the second reactor and extracting a first liquid from the product mixture to form the stream comprising the salt of the alkaline earth metal;

a fourth reactor in fluid communication with the separator, for receiving the first liquid, increasing the pH of the first liquid to from about 4 to about 7, to form a first precipitate, and removing the first precipitate from the first liquid to form a second liquid; and

a fifth reactor in fluid communication with the fourth reactor, for receiving the second liquid, increasing the pH of the second liquid to from about 9 to about 10, to form a second precipitate, and removing the second precipitate from the second liquid to form a third liquid, the fifth reactor being further in fluid communication with the third reactor for supplying the third liquid to the third reactor.

17. The system of claim 16, wherein the fourth reactor is in fluid communication with the bipolar membrane unit for receiving a portion of the alkaline solution produced by the bipolar membrane unit.

18. The system of claim 16 or claim 17, wherein the fifth reactor is in fluid

communication with the second reactor for receiving a portion of the carbonate of the base produced in the second reactor.

19. The system of any one of claims 16 to 18, further comprising a source of

hydrogen peroxide for adding hydrogen peroxide to the first liquid.

20. The system of any one of claims 16 to 19, further comprising

a sixth reactor for receiving a solid comprising silica extracted from the product mixture and contacting the solid with a solution comprising an alkali metal to form a solution of a silicate of the alkali metal; and

a seventh reactor in fluid communication with the sixth reactor for receiving the solution of the silicate of the alkali metal and contacting the solution of the silicate of the alkali metal with carbon dioxide to form a carbonate of the alkali metal and a precipitate comprising silica.