WO2017158493A1

WO2017158493A1 - Electrolytic cells and uses thereof for the treatment of acid mine drainage and for sequestering co2

Info

Publication number: WO2017158493A1
Application number: PCT/IB2017/051446
Authority: WO
Inventors: Donald Lake; Lee Groat; Andrew Carne; William Douglas Eaton
Original assignee: Terra Co2 Technologies Ltd.
Priority date: 2016-03-14
Filing date: 2017-03-13
Publication date: 2017-09-21

Abstract

Described herein are electrochemical cells for environmental remediation, including the treatment of acid mine drainage and sequestration of carbon dioxide. The electrolytic cells may comprise two, three or four chambers, each chamber being separated by a cationic exchange membrane (CEM) or an anionic exchange membrane (AEM). Also described are methods for obtaining solid metal carbonate compounds from sulfate compounds, methods for sequestering CO₂, methods for obtaining solid metal carbonate compounds, methods for obtaining iron carbonate from iron sulfate, methods for reducing trivalent metal compounds to divalent metal compounds, and electrochemical systems for carrying out such methods.

Description

ELECTROLYTIC CELLS AND USES THEREOF FOR THE TREATMENT OF ACID MINE DRAINAGE AND FOR SEQUESTERING C0₂

FIELD OF THE INVENTION

[0001] The invention relates to the field of environmental remediation, and more particularly to treatment of acid mine drainage and sequestration of carbon dioxide.

BACKGROUND OF THE INVENTION

[0002] Industrial processes, including mine sites and power plants, often burn hydrocarbons such as coal, oil, or natural gas, which produce emissions that contain significant amounts of C0₂, a known greenhouse gas. Sequestration of C0₂, a process by which C0₂ is locked away in a form which removes it from the atmosphere, is becoming increasingly important as governments world-wide become concerned about climate change.

[0003] Mining is an essential industry, producing many valuable commodities that form the basis of the world economy. However, mining activities cay have negative environmental consequences if not managed properly. Mine waste streams from projects with sulfide ores are particularly detrimental as both unprocessed waste rock and processed tailings material typically contain significant amounts of unoxidized or partially oxidized sulfide minerals. Over time, these minerals will react with water and atmospheric oxygen to create sulfuric acid and dissolved metals, a problem known as Acid Rock Drainage (ARD) or Acid Mine Drainage (AMD). This can contaminate waterways and groundwater, leaving a long-term environmental problem. There is thus a need for better, more efficient and more economical processes of ARD mediation.

[0004] Electrolysis, or other electrochemical processes, have been proposed for the recovery of metals from mining wastes. For instance, patent publication US 201 1/0089045 discloses an electrochemical process for the recovery of metallic iron or an iron-rich alloy, oxygen and sulfuric acid from iron-rich metal sulfate wastes. Many other electrochemical processes have been described, including those of US patent 3,531 ,386; US patent 3,801 ,698; US patent publication Nos US 2013/0008354, US 2013/0034489; US 2010/0200419; US 2010/0140103; US 201 1/0083968; and those of International PCT patent publications WO 2010/008896 and WO 2015/058287. However, those processes and the electrolytic cells used therein do not apply to production of environmentally stable carbonates, and are either complex, not cost- efficient, limited to treatment of particular compounds and/or not amenable to the treatment of mine waste materials.

[0005] Accordingly, there is a need for an electrochemical process and electrolytic cells useful in the remediation of Acid Rock Drainage (ARD) or Acid Mine Drainage (AMD). [0006] There is also a need for effective remedial strategies that target the production of environmentally stable solid metal carbonate compounds from mine wastes comprising metal-rich sulfate compounds, often with sulfuric acid present.

[0007] There is also a need for electrochemical processes and electrolytic cells that can be used in conjunction with sequestration of carbon dioxide. [0008] The present invention addresses these needs and other needs as it will be apparent from review of the disclosure and description of the features of the invention hereinafter.

BRIEF SUMMARY OF THE INVENTION

[0009] The invention is concerned with the use of electrochemical cells, including electrolytic cells, brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells, for environmental remediation and more particularly for the treatment of acid mine drainage and sequestration of carbon dioxide.

[00010] According to one aspect, the invention relates to an electrolytic cell, comprising:

- at least three chambers, said chambers comprising an anodic chamber, a cathodic chamber and a central zone comprising at least one central chamber; - a cationic exchange membrane (CEM) and at least one anionic exchange membrane (AEM);

wherein, when said at least one central zone comprises one central chamber, said central chamber is separated from the cathode by an AEM and is separated from the anode by the CEM;

wherein, when said at least one central zone comprises at least two central chambers, a first central chamber is separated from the cathode by a first AEM and is separated from a second central chamber by a CEM; and said second central chamber is separated from the anode by a second AEM and is separated from said first central chamber by said CEM.

[00011] According to one particular aspect, the invention relates to an electrolytic cell, comprising:

- three chambers, said chambers consisting of an anodic chamber, a cathodic chamber and a central chamber;

- a cationic exchange membrane (CEM) and an anionic exchange membrane (AEM),

wherein said central chamber is separated from the cathode by an AEM and is separated from the anode by the CEM.

[00012] According to one particular aspect, the invention relates to an electrolytic cell, comprising:

- four chambers, said chambers consisting of an anodic chamber, a cathodic chamber, a first central chamber and a second central chamber;

- a cationic exchange membrane (CEM) and two anionic exchange membranes (AEM),

wherein said first central chamber is separated from the cathodic chamber by a first AEM and is separated from a second central chamber by a CEM; and

wherein said second central chamber is separated from the anodic chamber by a second AEM and is separated from said first central chamber by said CEM. [00013] Preferably, the anodic chamber comprises a gas-diffusion anode. More preferably, the anode is a hydrogen-oxidizing anode. Preferably, the cathodic chamber comprises a cathode producing hydroxide ions and hydrogen gas.

[00014] The chambers comprise an aqueous solution comprising at least one conductive electrolyte. The at least one conductive electrolyte may be selected from NaCI, KCI, CaCI₂, HCI, H₂S0₄, NaOH, KOH, Na₂S0₄, K₂S0₄, and mixtures thereof. At least one of the chambers may comprise at least one conductive electrolyte containing metal sulfate compounds from industrial waste. The industrial waste may comprise Acid Mine Drainage. At least one of the chambers may comprise at least one conductive electrolyte containing metal sulfate compounds from a reactor that treats solid metal sulfide and/or from a heap of metal-oxidizing bacteria that treats solid metal sulfides.

[00015] Preferably, the cathodic chamber is operatively connected to the anodic chamber for circulating hydrogen gas produced at the cathode to the anode. The cathodic chamber may be operatively connected to a C0₂ absorption reactor. The electrolytic cell may also comprise a S0₂ reduction reactor operatively connected to a chamber receiving metal sulfate compounds. In embodiments, the chamber receiving metal sulfate compounds is the anodic chamber or the second central chamber.

[00016] A related aspect of the invention concerns a method for obtaining solid metal carbonate compounds from sulfate compounds. In one embodiment, the method comprises the steps of:

- providing a sulfate solution comprising metal-containing sulfate compounds;

- electrolysing said sulfate solution in an electrolytic cell comprising an anodic chamber having an anode, a cathodic chamber having a cathode; and

- recovering a solid precipitate of metal carbonate compounds from said an electrolytic cell. [00017] Another related aspect of the invention concerns a method for obtaining solid metal carbonate compounds from sulfate compounds containing divalent metal cations. In one embodiment, the method comprises the steps of:

- providing a sulfate solution comprising metal-containing sulfate compounds;

- electrolysing said sulfate solution in an electrolytic cell comprising an anodic chamber having an anode, a cathodic chamber having a cathode and at least one central chamber, wherein said at least one central chamber is separated from the cathode by an anionic exchange membrane (AEM) and is separated from the anode by a cationic exchange membrane (CEM); and

- recovering a solid precipitate of metal carbonate compounds from said at least one central chamber.

[00018] Preferably, in these methods, said electrolyzing comprises increasing alkalinity of an aqueous solution contained in the cathodic chamber.

[00019] In these methods, the sulfate solution preferably comprises sulfate compounds containing divalent metal cations such that the precipitated solid metal carbonate compounds comprises carbonate compounds containing divalent metal.

[00020] In one embodiment, the sulfate solution is fed to the anodic chamber and the divalent metal cations traverse the CEM while sulfate anions remain in said anodic chamber.

[00021] In one embodiment, the cathodic chamber comprises an alkaline solution, and the method further comprises circulating said alkaline solution through a C0₂ absorption reactor to produce an alkaline carbonate solution comprising carbonate compounds. In addition, said alkaline carbonate solution is feed to the cathodic chamber, wherein carbonate anions traverse the AEM to precipitate with divalent metal cations in the central chamber.

[00022] In one particular embodiment, the central chamber comprises a first central chamber and a second central chamber; said first central chamber is separated from the cathode by a first AEM and is separated from said second central chamber by a CEM; said second chamber is separated from the anode by a second AEM and is separated from said first central chamber by said CEM; and wherein said solid precipitate of metal carbonate compounds is recovered from said first central chamber. Said sulfate solution may be fed to the second central chamber, such that divalent metal cations traverse the CEM into the first central chamber while sulfate anions traverse the AEM into the anodic chamber.

[00023] In another particular embodiment, the cathodic chamber comprises a cathodic alkaline solution, and the method further comprises the step of circulating said cathodic alkaline solution through a C0₂ absorption reactor to produce an alkaline carbonate solution comprising carbonate compounds and the step of feeding said alkaline carbonate solution to the cathodic chamber, wherein carbonate anions traverse the AEM to precipitate with divalent metal cations in the first central chamber. The C0₂ absorption reactor may be operatively connected to the cathodic chamber, and carbonate compounds are produced by reacting said cathodic alkaline solution with C0₂ gas circulating inside the C0₂ absorption reactor. Preferably hydroxide compounds are produced within the cathodic chamber (e.g. sodium hydroxide, potassium hydroxide, mixtures thereof), and wherein said hydroxide compounds are reacted with C0₂ gas circulating in the C0₂ absorption reactor to produce said carbonate compounds. [00024] In embodiments the alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof. In embodiments the sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate. Preferably the sulfate solution comprises iron sulfate and the solid precipitate comprises iron carbonate.

[00025] In embodiments the method further comprises the step of circulating hydrogen gas produced at the cathode from the cathodic chamber to the anodic chamber. Preferably, the method further comprises the step of recovering said solid metal carbonate compounds. Purified sulfuric acid may also be recovered from the anodic chamber. [00026] According to a more specific aspect of the invention concerns a method for obtaining solid metal carbonate compounds from sulfate compounds containing divalent metal cations, comprising:

- providing a sulfate solution comprising iron sulfate;

- recovering a solid precipitate of iron carbonate from said at least one central chamber.

[00027] A further aspect of the invention concerns an electrochemical system comprising :

- an anolyte solution in contact with an anode;

- a catholyte solution in contact with a cathode;

- an ionic solution separated from the catholyte solution by an anionic exchange membrane (AEM) and separated from the anolyte solution by a cationic exchange membrane (CEM). [00028] In one embodiment, the anolyte solution comprises sulfate compounds containing divalent metal cations, and wherein said divalent metal cations traverse the CEM into the ionic solution. Preferably, the anolyte solution comprises an acid mine drainage solution and this acid mine drainage solution may comprise FeS0₄ and H₂S0₄.

[00029] In one embodiment, the catholyte solution comprises at least one electrolyte and carbonate compounds in aqueous solution, and the carbonates anions traverse the AEM to react and precipitate with divalent metal cations in the ionic solution.

[00030] A related aspect concerns an electrochemical system comprising

- an anolyte solution in contact with an anode;

- a catholyte solution in contact with a cathode; - first and second ionic solutions positioned between the catholyte solution and the anolyte solution,

wherein said first and second ionic solutions are adjacent and separated from each other by a cationic exchange membrane (CEM), and wherein the first ionic solution is separated from the catholyte solution by an anionic exchange membrane (AEM) and wherein said second ionic solution is separated from the anolyte solution by an anionic exchange membrane (AEM).

[00031] In one embodiment the second ionic solution comprises sulfate compounds containing divalent metal cations, and said divalent metal cations traverse the CEM into the first ionic solution while sulfate anions traverse the AEM into the anolyte solution.

[00032] In one embodiment the catholyte solution comprises at least one electrolyte and carbonate compounds in aqueous solution, and carbonates anions traverse the AEM to react and precipitate with divalent metal cations in the first ionic solution.

[00033] In one embodiment the anolyte solution, the catholyte solution, the first ionic solution and the second ionic solution comprise at least one of a conductive electrolyte selected from the group consisting of NaCI, KCI, CaCI₂, HCI, H₂S0₄, NaOH, KOH Na₂S0₄, K₂S0₄, and mixtures thereof.

[00034] In one embodiment, at least one of the anolyte solution and second ionic solution comprises divalent metal ions reduced from trivalent metal ions subsequent to a pass through a S0₂ reduction reactor. In one embodiment, the anolyte solution comprises sulfuric acid produced therein.

[00035] In one embodiment, the second ionic solution comprises an acid mine drainage solution and the acid mine drainage solution may comprise FeS0₄ and H₂S0₄.

[00036] Preferably the hydroxide ions and hydrogen gas are produced at the cathode. Preferably also, the hydrogen gas is collected from the catholyte solution to be fed to the anode. [00037] In embodiments, the catholyte solution is circulated through a C0₂ absorption reactor to create an alkaline carbonate solution. The electrochemical system may also comprise a filter and/or thickener for recovering said solid metal carbonate compounds.

[00038] An additional aspect concerns a method for sequestering C0₂ and for obtaining solid metal carbonate. In one embodiment the method comprises the steps of:

- providing a sulfate solution comprising sulfate compounds containing metal cations;

- electrolysing said sulfate solution in an electrolytic cell, said electrolytic cell producing an alkali-containing solution under electrolysis; - feeding said alkali-containing solution to a C0₂ absorption reactor operatively connected to said electrolytic cell and to a source of C0₂, said C0₂ absorption reactor receiving said alkali-containing solution and said C0₂ for forming an alkaline carbonate solution;

- feeding said alkaline carbonate solution to the electrolytic cell and allowing precipitation inside said cell of carbonate ions as solid metal carbonate compounds.

[00039] In embodiments the alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof. [00040] In embodiments the sulfate solution comprises sulfate compounds containing divalent metal cations and the precipitated solid metal carbonate compounds comprises carbonate compounds containing divalent metal.

[00041] Preferably, the sulfate solution comprises iron sulfate and the solid precipitate comprises iron carbonate. Accordingly, a particular aspect of the invention concerns a method for sequestering C0₂ and for obtaining iron carbonate from iron sulfate. In one particular embodiment the method comprises the steps of:

- providing a sulfate solution comprising iron sulfate;

- feeding said alkaline carbonate solution to the electrolytic cell and allowing precipitation inside said cell of iron carbonate particles as solid iron carbonate.

[00042] In these methods, the sulfate solution may comprise sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate. Preferably, the methods further comprise the step of recovering the solid metal carbonate compounds or the solid iron carbonate.

[00043] Another aspect of the invention concerns a system for obtaining solid metal carbonate compounds and sequestering C0₂. In one embodiment the system comprises:

- an electrochemical cell producing an alkali-containing catholyte;

- a C0₂ absorption reactor operatively connected to said electrochemical cell and to a source of C0₂, said C0₂ absorption reactor receiving said alkali-containing catholyte and said C0₂ for forming an alkaline carbonate solution;

- a vessel for receiving an acidic sulfate solution comprising sulfate compounds containing metal cations and for receiving said alkaline carbonate solution, said vessel allowing precipitation of solid metal carbonate compounds.

[00044] In embodiments the alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

[00045] In embodiments the acidic sulfate solution comprises sulfate compounds containing divalent metal ions and the precipitated solid metal carbonate compounds comprises carbonate compounds containing divalent metal ions. Preferably the acidic sulfate solution comprises iron sulfate and said solid metal carbonate compound comprises iron carbonate. More particularly, the acidic sulfate solution may comprise at least one of FeS0₄, and Fe(OH)₃.

[00046] Accordingly, a related aspect concerns a system for obtaining solid metal carbonate compounds and sequestering C0₂, comprising:

- an electrochemical cell producing an alkali-containing catholyte;

- a vessel for receiving an acidic sulfate solution comprising iron sulfate and for receiving said alkaline carbonate solution, said vessel allowing precipitation of solid iron carbonate.

[00047] Another related aspect concerns a method for obtaining solid metal carbonate compounds and sequestering C0₂. In one embodiment the method comprises the steps of:

- providing an electrochemical cell producing an alkali-containing catholyte;

- providing a C0₂ absorption reactor operatively connected to said electrochemical cell and to a source of C0₂;

- feeding said C0₂ absorption reactor with said alkali-containing catholyte for forming an alkaline carbonate solution;

- reacting in a vessel an acidic sulfate solution comprising metal ions with said alkaline carbonate solution for precipitating solid metal carbonate compounds. [00048] A particular aspect concerns a method for obtaining solid metal carbonate compounds and sequestering C0₂. In one embodiment the method comprises the steps of:

- providing an electrochemical cell producing an alkali-containing catholyte;

- providing a C0₂ absorption reactor operatively connected to said electrochemical cell and to a source of C0₂; - feeding said C0₂ absorption reactor with said alkali-containing catholyte for forming an alkaline carbonate solution;

- reacting in a vessel an acidic sulfate solution comprising iron sulfate with said alkaline carbonate solution for precipitating solid iron carbonate compounds.

[00049] Preferably, the alkaline carbonate compound solution in these methods comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof. Preferably, the carbonate compounds are produced by reacting said alkali-containing catholyte with C0₂ gas circulating inside the C0₂ absorption reactor.

[00050] In embodiments, the acidic sulfate solution comprises sulfate compounds containing divalent metal cations and the precipitated solid metal carbonate compounds comprise carbonate compounds containing divalent metal.

[00051] In embodiments, the methods further comprise a step of circulating the acidic sulfate solution through a S0₂ reduction reactor prior to said reacting, wherein said S0₂ reduction reactor reduces trivalent metal compounds present in the acidic sulfate solution to divalent metal compounds.

[00052] In these methods and systems, the acidic sulfate solution preferably comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate. Such sulfide leachates may have a pH of about 1 to 3. Preferably the acidic sulfate solution comprises iron sulfate and the solid carbonate comprises iron carbonate. Preferably, the method further comprises recovering said solid metal carbonate compounds or the solid iron carbonate.

[00053] In these methods and systems, the electrochemical cell may be selected from the group consisting of brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells. [00054] Additional aspects of the invention concern a system for reducing trivalent metal compounds to divalent metal compounds and sequestering C0₂. In one embodiment the system comprises:

- a S0₂ reduction reactor operatively connected to a source of C0₂ and S0₂ gas, and to a source of trivalent metal compounds, said S0₂ reduction reactor reducing trivalent metal compounds to produce a solution comprising divalent metal compounds;

- an electrochemical cell producing an alkali-containing catholyte;

- a C0₂ absorption reactor operatively connected to said electrochemical cell and to the S0₂ catalytic reduction reactor, said C0₂ absorption reactor receiving said alkali-containing catholyte from the electrochemical cell and C0₂ from the S0₂ reduction reactor for forming an alkaline carbonate solution;

- a vessel for receiving said alkaline carbonate solution and for receiving said solution comprising divalent metal compounds, said vessel allowing precipitation of solid metal carbonate compounds.

[00055] A related aspect concerns a method for reducing trivalent metal compounds to divalent metal compounds and sequestering C0₂. In one embodiment the method comprises the steps of:

- reducing trivalent metal compounds with (e.g. inside) a S0₂ reduction reactor to obtain an acidic sulfate solution comprising divalent metal compounds;

- providing an electrochemical cell producing an alkali-containing catholyte;

- circulating said alkali-containing catholyte into a C0₂ absorption reactor operatively connected to said electrochemical cell for forming an alkaline carbonate solution;

- reacting in a vessel said acidic sulfate solution with said alkaline carbonate solution for precipitating solid carbonate compounds containing divalent metal. [00056] In embodiments of the above systems and methods, the trivalent metal compounds contain trivalent (ferric) iron and, in the S0₂ catalytic reduction reactor, S0₂ reacts with said trivalent (ferric) iron to produce divalent (ferrous) iron in a sulfate form. The trivalent metal compounds may comprise at least one of Fe(OH)₃ or Fe₂(S0₄)₃. In one embodiment the trivalent metal compounds contain trivalent (ferric) iron and the solid metal carbonate compound comprises FeC0₃. [00057] In these systems and methods, the trivalent metal compounds may comprise sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate. The sulfide leachates may have a pH of about 1 to 3.

[00058] In these systems and methods, the alkaline carbonate compound solution may comprise at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

[00059] In these systems and methods, the S0₂ reduction reactor may comprises a catalyst selected from the group consisting of platinum, activated carbon, ruthenium, rhodium, and vanadium pentoxide.

[00060] In these systems and methods, the electrochemical cell may be selected from the group consisting of brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells.

[00061] Additional aspect of the invention involves the use of electrochemical cells in connection with environmental remediation and more particularly in connection with treatment of acid mine drainage and sequestration of carbon dioxide. [00062] An ancillary yet key aspect of preferred embodiments of the invention is that the C0₂-enriched gas mixture used herein may be sourced directly from hydrocarbon burning operations or other C0₂-producing industrial processes, thereby significantly reducing the greenhouse gas emissions of such operations.

BRIEF DESCRIPTION OF THE DRAWINGS

[00063] In order that the invention may be readily understood, embodiments of the invention are illustrated by way of example in the accompanying drawings. [00064] Figures 1A and 1 B are diagrams illustrating the use of a three-chamber electrolytic cell for processing iron-containing sulfide waste streams to obtain iron carbonate, according to the embodiment of Example 1 . Figure 1A concerns a three- chamber electrolytic cell wherein the conductive electrolyte is sodium chloride (NaCI) or sodium sulfate (Na₂S0₄). Figure 1 B concerns a three-chamber electrolytic cell wherein the conductive electrolyte is potassium chloride (KCI) or potassium sulfate (K₂S0₄).

[00065] Figure 2 is a diagram illustrating the use of a four-chamber electrolytic cell for processing iron-containing sulfide waste streams to obtain iron carbonate, according to the embodiment of Example 2. [00066] Figures 3A and 3B are diagrams illustrating the use of a one-chamber electrolytic cell for processing iron-containing sulfide waste streams to obtain iron carbonate, according to another embodiment.

[00067] Figure 4 is a diagram illustrating an overall process using an electrolytic cell as defined herein, the process incorporating sulfide leachates as an electrolytic feedstock, according to one particular embodiment.

[00068] Figure 5A is a diagram illustrating the functioning of a brine electrolytic cell comprising a hydrogen-oxidizing anode and a cation exchange membrane (CEM), according to one particular embodiment.

[00069] Figure 5B is a diagram illustrating the functioning of a brine electrolytic cell comprising a hydrogen-oxidizing anode and an anion exchange membrane (AEM), according to one particular embodiment.

[00070] Figure 5C is a diagram illustrating the functioning of a brine electrolytic cell comprising a hydrogen-oxidizing anode, a cation exchange membrane (CEM), and an anion exchange membrane (AEM), according to one particular embodiment. [00071] Figure 6A is a diagram illustrating the functioning of a bipolar membrane electrodialytic (BMED) cell, according to one particular embodiment. [00072] Figure 6B is a diagram illustrating the functioning of an electro-electrodialytic (EED) cell, according to one particular embodiment.

[00073] Figure 7 is a diagram illustrating a process incorporating a brine electrolytic cell for processing sulfide mine waste and sequestering C0₂, according to one particular embodiment.

[00074] Figure 8 is a diagram illustrating a process including a brine electrolytic cell and a catalytic S0₂ reduction reactor for processing sulfide mine waste and sequestering C0₂, according to one particular embodiment.

[00075] Figure 9 is a graph showing solubility of ferrous iron in carbonate-bearing waters. Reproduced from Singer & Stumm, 1970.

[00076] Figure 10 is a line graph demonstrating S0₂ reduction of ferric iron, according to Example 1. Square = catalyzed ferric reduction; Triangle = ferric reduction; Circle = control.

[00077] Further details of the invention and its advantages will be apparent from the detailed description included below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[00078] In the following description of the embodiments, references to the accompanying drawings are by way of illustration of an example by which the invention may be practiced. It will be understood that other embodiments may be made without departing from the scope of the invention disclosed.

General overview

[00079] The invention involves the use of electrochemical cells, including electrolytic cells, brine electrolytic cells, BMED cells and EED cells, in connection with environmental remediation and more particularly in connection with treatment of acid mine drainage and sequestration of carbon dioxide. [00080] According to particular aspects, the invention concerns systems, methods, processes, and electrochemical cells for obtaining solid metal carbonate compounds from metal sulfate compounds, including metal sulfate compounds containing divalent metal cations. [00081] As used herein, the term "electrochemical cell" encompasses electrolytic cells, brine electrolytic cells, BMED cells and EED cells as defined herein.

[00082] As used herein, the terms "electrolytic cell" refer to a cell or vessel, comprising two or more chambers, and containing one or more electrolyte solutions that enable a chemical reduction-oxidation reaction when electrical energy is applied. An electrolytic cell according to the invention comprises two electrodes, a cathode and an anode, each positioned in a cathodic chamber and an anodic chamber respectively, these two chambers being separated by one or more ion exchange membranes, and zero, one, two or more additional central reaction chambers. Electrolyte is present in all chambers, and an electrical potential is applied between the two electrodes. [00083] As used herein, the term "sulfate compound containing a divalent metal cation" refers to inorganic sulfate compounds composed of a sulfate anion (S0₄ ^2") and a divalent metal cation (a metal with ionic charge of 2⁺). Examples include, but are not limited to, iron sulfate (FeS0₄), nickel sulfate (NiS0₄(H₂0)₆), magnesium sulfate (MgS0₄), zinc sulfate (ZnS0₄), cupric sulfate (CuS0₄), lead sulfate (PbS0₄), calcium sulfate (CaS0₄).

[00084] As used herein, the term "solid metal carbonate compound" refers to inorganic compounds in a solid form comprising a carbonate anion (C0₃ ^2") and a divalent metal cation (a metal with ionic charge of 2⁺). Examples include, but are not limited to iron carbonate (FeC0₃, the mineral siderite), nickel carbonate (NiC0₃), magnesium carbonate (MgC0₃), zinc carbonate (ZnC0₃), copper carbonates (Cu₂(OH)₂C0₃, the mineral malachite, or Cu₃(OH)₂(C0₃)₂, the related mineral azurite), calcium carbonate (CaC0₃, the mineral calcite), and lead carbonate (PbC0₃, the mineral cerussite). [00085] According to one particular embodiment, the present invention is adapted for processing iron (Fe)-containing sulfide waste streams and to convert iron sulfate (FeS0₄, Fe₂(S0₄)₃) to siderite (i.e. divalent iron carbonate or FeC0₃). According to that embodiment, the electrochemical cell: (i) takes iron sulfate (FeS0₄, Fe₂(S0₄)₃), sulfuric acid, carbonate compounds, water, an electrolyte (e,g, sodium chloride), and other reactants as inputs; (ii) processes these reactants; and (ii) produces stable iron carbonate compounds (FeC0₃) and purified sulfuric acid. The iron in the iron sulfate may be present as either Fe²⁺ or Fe³⁺.

[00086] One of the benefits associated with the process according to the invention is the reduced C0₂ emissions for the treatment/remediation of Acid rock Drainage (ARD) or Acid Mine Drainage (AMD). As is known, in order to conventionally treat tailings, a source of calcium oxide (lime) is required. Production of calcium oxide from limestone (CaC0₃) emits large quantities of C0₂. Electrolysis or electrodialysis of brine, and subsequent carbonation using the electrochemical cell described herein provides a feasible method of treatment that consumes C0₂ instead of producing it.

[00087] Another benefit envisioned with the electrolytic cells according to the invention is the possible reduced energy requirements for the treatment/remediation of Acid Rock Drainage (ARD) or Acid Mine Drainage (AMD). As is known, in order to produce carbonate minerals from a source of acid mine drainage, a large source of alkalinity is required and production of alkaline solution is the most energy-intensive and capital-intensive part. Electrolysis or electrodialysis of brine, and subsequent carbonation using the electrochemical cells described herein provides a feasible source of alkalinity that lowers energy requirements.

[00088] For instance, the following are typical characteristics of iron sulfide acid rock drainage (Blowes et al., 2003):

1 - Components of iron sulfide drainage waters:

H₂0, Fe^3+/2+, H⁺, S0₄ ^2" are introduced by these two simplified reactions (Rimstidt et al., 2003):

a. FeS₂ + 7/2 0₂ + H₂0 Fe²⁺ + 2 S0₄ ^2" + 2 H⁺ (Iron sulfide reacts with water and oxygen)

b. Fe²⁺ + ¼ 0₂ + H⁺ ^ Fe³⁺ + ½ H₂0 (Ferrous iron oxidizes to ferric iron) 2- pH of 1 -3

[00089] Alkalinity is low unless there is interaction with carbonate minerals. For carbonates to form, alkalinity must be increased by orders of magnitude. The invention addresses this problem using electrochemical cells as described in details herein.

[00090] With respect to the treatment/remediation of Acid Rock Drainage (ARD), one particular aspect of the present invention lies in the production of an aqueous solution having a high pH and a high C0₃ ^2" concentration in order to neutralize acidic components of ARD and to create suitable conditions for reaction of divalent metal cations to precipitate carbonates (e.g. Fe²⁺ to precipitate as siderite (FeC0₃)). Considering that ARD typically has a pH of 1 -3, this involves a significant increase in the pH.

[00091] The present invention addresses this problem by providing electrochemical cells employing selective ion membranes to control pH and the ions present in different chambers of the cell.

Electrolytic cells

[00092] One aspect of the present invention relates to electrolytic cells and uses thereof for the production of environmentally benign solid metal carbonate compounds from reactive metal-rich sulfate compounds. Related aspects relate to the use of electrolytic cells and electrochemical processes in conjunction with sequestration of carbon dioxide. In embodiments, the electrolytic cells according to the invention may comprise two, three or four chambers, each chamber being separated by an ion exchange membrane.

Referring to the embodiment illustrated in Figures 1A and 1 B, the electrolytic cell 1 comprises a plurality of side walls 3 defining an enclosure having three chambers separated by two semi-permeable ion exchange membranes. The three chambers in the electrolytic cell comprise a cathodic chamber 10, an anodic chamber 13, and a central chamber 1 1 . An anode electrode 4 is present in the anodic chamber 13 and a cathode electrode 2 is present in the cathodic chamber 10. An Anion Exchange Membrane (AEM) 6 is positioned between the cathodic 10 and central 1 1 chambers, and a Cation Exchange Membrane (CEM) 7 is positioned between the central 1 1 and anodic 13 chambers. The central chamber 1 1 is thus separated from the cathode 2 by the AEM 6 and it is separated from the anode 4 by the CEM 7. As explained hereinafter, the electrolytic cell 1 may be operatively connected to a C0₂ absorption reactor 30.

[00093] In the embodiment illustrated in Figure 2, the electrolytic cell 1 comprises a plurality of side walls 3 defining an enclosure having four chambers separated by three semi-permeable ion exchange membranes. The four chambers comprise a cathodic chamber 10, an anodic chamber 13, and a first 1 1 and a second 12 central chamber. An anode electrode 4 is present in the anodic chamber 13 and a cathode electrode 2 is present in the cathodic chamber 10. A first Anion Exchange Membrane (AEM) 6 is positioned between the cathodic 10 and first central 1 1 chambers and a second AEM 8 is positioned between the cathodic 13 and second central 12 chambers. A Cation Exchange Membrane (CEM) 7 is positioned between the first 1 1 and second 12 central chambers. Accordingly, the first central chamber 1 1 is separated from the cathode 2 by the first AEM 6 and it is separated from the second central chamber 12 by the CEM 7. The second central chamber 12 is separated from the anode 4 by the second AEM 8 and it is separated from the first central chamber 1 1 by the CEM 7. As explained hereinafter, the electrolytic cell 1 may be operatively connected to a C0₂ absorption reactor 30.

[00094] In another embodiment illustrated in Figures 3A and 3B, the electrolytic cell 1 comprises a plurality of side walls 3 defining an enclosure having only two chambers, separated by a single semi-permeable ion exchange membrane (see 6 in Fig. 3A and 7 in Fig. 3B). The two chambers consist of an anodic chamber 13 and a cathodic chamber 10. These two chambers 10, 13 are separated from each other by an ion exchange membrane, either an AEM 6 (Fig. 3A) or a CEM 7 (Fig. 3B). As explained hereinafter, the electrolytic cell 1 may be operatively connected to a C0₂ absorption reactor 30.

[00095] The sidewalls 3 of the electrolytic cells may be manufactured from any suitable non-reactive material, including but not limited to polytetrafluoroethylene, fluoropolymers, ceramic, coated metal, etc. Preferably the material is selected to provide structural strength and rigidity to the cell, and prevents ingress and egress of fluids and gases outside of the desired inputs and outputs from chambers.

[00096] The cell 1 may also comprise a mechanical barrier 20 partially enclosing the cathode 2, as illustrated in Figures 3A and 3B. The role of that optional mechanical barrier 20 is to reduce or limit fluid mixing in a region of the cathodic chamber 10 leading to the C0₂ absorption reactor 30. For instance, it might be preferable to limit any undesirable interaction between the metal cations (e.g. Fe²⁺) and OH^" because metal hydroxides (e.g. iron hydroxides) have low solubility and tend to precipitate from aqueous solution. Examples of suitable barriers 20 may include, but are not limited to, a thin mechanical barrier of non-reactive material such as polytetrafluoroethylene (PTFE), or ceramic. Alternatively, the mechanical barrier 20 can comprise a semi-permeable micro-porous membrane enclosing the cathode. Many existing microporous membrane materials are suitable for the purpose of preventing fluid-mixing around the cathode. Examples include, but are not limited to, microporous polypropylene membranes, Poreflon® or similar PTFE microporous membranes, or microporous alumina and ceramic materials.

Functioning

[00097] When sufficient voltage is applied across the electrodes electric current will circulate between the electrodes, through the conductive electrolyte solutions present in the various chambers. Half-cell reactions will occur at each of the anode and cathode. Aqueous hydrogen ions (H⁺) will form at the anode by oxidation of hydrogen gas. At the cathode, aqueous hydroxide anions (OH^") and hydrogen gas (H₂) will form. The hydrogen gas produced at the cathode is captured and circulated 15 to the anode for oxidation. The electrolytic cell may be operatively connected to a system for temporary hydrogen gas storage, supplemental supply and pressure regulation (not shown) for ensuring continuous supply of hydrogen gas to the anode. To circulate the hydrogen gas, the cell may be connected to pumps or other mechanisms, for instance serpentine graphite plates. Additional hydrogen may also be supplemented to the system if required to accommodate any losses. As described hereafter, certain configurations could also be made (e.g. BMED cell) to avoid gas production at electrodes and enable a proton transfer situation across ion exchange membranes.

[00098] In preferred embodiments, the alkali-containing catholyte of the electrolytic cell 1 is circulated outside the electrolytic cell through a C0₂ absorption reactor 30. In this reactor C0₂-rich gases react with the catholyte to produce bicarbonate and/or carbonate anions. The solution comprising the bicarbonate (HC0₃ ^") and carbonate (C0₃ ^2") ions produced inside the C0₂ absorption column is returned to the cathodic compartment. The C0₃ ^2" ions will traverse the AEM to combine with divalent metal cations and precipitate as solid metal-carbonate compounds. The solid carbonate compounds that have precipitated within the electrolytic cell may be pumped to a separate vessel for solid/liquid separation. In one embodiment the solid carbonate particles exit the electrolytic cell as a suspension in water being recirculated into the cell and the particles are separated with a filter and/or thickener 35. In addition to the C0₂ absorption reactor 30, the electrolytic cells may also be operatively connected with an optional S0₂ reduction reactor 40 as shown in Figure 4. More details about the C0₂ absorption reactor 30, the S0₂ catalytic reduction reactor 40 and the precipitation and thickening 35 are provided hereinafter.

[00099] The functioning of the electrolytic cells may be explained by describing the processing of iron-containing sulfide waste streams to reduce their acid generation potential and sequester carbon dioxide as geochemically stable iron-carbonate minerals. Referring more particularly to the three-chamber electrolytic cell 1 of Figure 1 , NaOH is produced in the cathodic chamber 10, then is circulated in solution to a C0₂ absorption reactor 30 and returns with bicarbonate or carbonate anions in solution. The carbonate anions traverse the anion exchange membrane 6 to the central chamber 1 1 . Divalent metal cations (e.g. Fe²⁺) traverse the cation exchange membrane 7 from the anodic chamber 13 to the central chamber 1 1 and react with the carbonate anions to precipitate stable metal carbonate compounds (e.g. FeC0₃). Sulfuric acid remains and concentrates in the anolyte. The solutions in each chamber are recirculated and supplemented with additional reactants as required. Products, including sulfuric acid, are bled off as part of the recirculation stream. [000100] More particularly, according to this particular example, an acid mine drainage solution comprising FeS0₄ and H₂S0₄ is fed into the input of the anodic chamber 13. A gas-diffusion electrode 4 present in this chamber is fed with hydrogen gas, and produces H⁺ ions. The H⁺ ions displace Fe²⁺ ions to produce H₂S0₄. The freed Fe²⁺ ions move through the CEM 7 to the central chamber 1 1 .

[000101] The cathodic chamber 10 contains a gas-evolution electrode 2 (i.e. cathode) which produces hydrogen gas (H₂) and hydroxide (OH^") ions from water. This hydrogen gas is recirculated 15 to the electrode 4 of the anodic chamber 13. In this example, carbon is received by the cathodic chamber 10 in the form of C0₃ ^2" ions and/or carbonate compounds produced by a C0₂ absorption reactor 30 operatively connected to the electrolytic cell 1 (more details about the C0₂ absorption reactor 30 are provided hereinafter). The C0₃ ^2" ions move through the selective AEM 6 into the central chamber 1 1 .

[000102] The central chamber 1 1 is fed only with recirculated water. C0₃ ^2" ions move into this chamber 1 1 from the cathodic chamber 10 through the selective AEM 6 and these C0₃ ^2" ions combine with the Fe²⁺ ions that have traversed the CEM 7 to precipitate solid FeC0₃ particles. These solid particles exit the cell 1 suspended in the recirculated water and are separated with a filter and/or thickener (more details about precipitation and thickening are provided hereinafter). [000103] The four-chamber electrolytic cell 1 embodiment illustrated in Figure 2 is functionally similar to the three-chamber electrolytic cell of Figure 1A and it operates in a similar manner to the 3-chamber electrolytic cell described above. The inputs to the 4-chamber cell are the same as for the 3-chamber. The products are also the same, with the additional benefit that the 4-chamber cell provides for the production of decontaminated water and purified sulfuric acid. Briefly, an extra AEM 8 is placed between the CEM 7 and the anode 4 to create a second central chamber 12 which receives the metal-sulfate feed instead of the anodic compartment 13 as in Figure 1A. The output from the second central chamber 12 is water comprising a lowered concentration of ions. The output from the anodic chamber 13 is water with an increased concentration of sulfuric acid. This addition provides a mechanism for producing purified sulfuric acid, as well as decontaminating water.

[000104] The two-chamber electrolytic cell 1 embodiment illustrated in Figures 3A and 3B is functionally similar to three-chamber electrolytic cell of Figure 1A. In Figure 3A, the cathodic chamber 10 receives the sulfate feed instead of the anodic 13 compartment as in Figure 1A. S0₄ ^2" ions traverse the single AEM 6 where they form sulfuric acid with the H⁺ ions produced in the anodic chamber 13. C0₃ ^2" ions produced by the C0₂ absorption reactor 30 reacts with Fe²⁺ ions present in solution in the cathodic chamber 10 to precipitate solid FeC0₃ particles. In Figure 3B, the anodic chamber 13 receives the sulfate feed like in in Figure 1A. Fe²⁺ ions traverse the single CEM 7 to react with C0₃ ^2" ions produced by the C0₂ absorption reactor 30 and to precipitate solid FeC0₃ particles.

[000105] Membranes: The electrochemical cells 1 , 50, 60, 70 described herein may use any suitable commonly available ion exchange membranes that act to separate electrolytes and to allow the selective passage of certain ions as desired. In embodiments, the anion 6, 8, and cation 7, exchange membranes comprise gel polystyrene cross linked with divinylbenzene. In embodiments, the anion exchange functional group is quaternary ammonium and the cation exchange functional group is sulfonic acid. AEMs and CEMs are available from various suppliers such as Membranes International Inc., U.S.A.; FuMa-Tech GmbH, Germany; Asahi Glass, Japan; and other suppliers. Nafion® is one particular example of a commonly available cation exchange membrane based on perfluorosulfonic acid with PTFE copolymer in acid (H⁺) form. Fumasep® FAB and FAA are particular examples of commonly available anion exchange membranes suitable for embodiments. [000106] When employing electrodialysis as an alternative to brine electrolysis in the production of alkalinity, a bipolar ion exchange membrane 9 may be required (e.g. Fig. 6A). A bipolar membrane (BPM) consists of a cation selective layer, an anion selective layer, and a transition region joining these two layers wherein the dissociation of water takes place. As above, bipolar membranes are available from FuMa-Tech GmbH, Germany and other suppliers. An example of a suitable BPM is the fumasep® FBM.

[000107] Anode: The anode 4 consists of a three-phase junction between the electrode, hydrogen gas, and the anolyte solution. Preferably, the anode is a hydrogen- oxidizing anode for oxidizing hydrogen gas into H+ cations. More preferably, the anode is a gas-diffusion hydrogen-oxidizing electrode diffusing H₂ and it may have a catalyst- loaded surface (e.g. platinum). Although it might be envisioned to use more conventional metal electrodes like platinum or ruthenium (catalyst) coated titanium, hydrogen- oxidizing anodes are preferred since with such anodes it generally possible to operate at lower voltage and avoid undesirable gas production such as oxygen and chlorine.

[000108] In one particular arrangement the electrolytic cell is coupled to a serpentine flow-field graphite plate to deliver hydrogen and electricity evenly across a platinum- loaded gas-diffusion anode. The anode, taking the form of a conductive carbon cloth or paper substrate with catalyst-loaded surface is in contact on one surface with the liquid anolyte.

[000109] Because the anode is vulnerable to flooding by the anolyte(s), thereby blocking necessary reaction sites within the porous anode, it may be preferable to employ a mechanism to prevent flooding of the electrode with the anolyte(s). One such example comprises adding a water-resistant PTFE treatment to conductive carbon paper or cloth in the gas-diffusion electrode. Another method may involve adding of a proton exchange membrane between the gas diffusion electrode and the anodic chamber, thus preventing anolyte from flooding the electrode while still permitting hydrogen ions created at the anode to pass through said membrane into the anolyte as desired.

[000110] Cathode: The cathode 2 may be any suitable corrosion-resistant conductor including, but not limited to, stainless steel, nickel, nickel-aluminum alloy, iron, titanium, platinum and alloys thereof. Preferably, the cathode is made of a material that will help to lower voltage in the cell by minimizing or reducing over potential caused by the hydrogen oxidization reaction (HOR) at the anode. Examples of such materials include, but are not limited to, Raney® nickel, nickel molybdate, platinum and other porous nickel-aluminum alloy sponge-type electrocatalyst (Birry et al., 2004). [000111] Electrolytes: The various compartments of the cells must contain electrically conductive solutions for conducting electricity between the two electrodes. Various electrolyte compounds can be used in the electrically conductive solutions according to the present invention. The composition of the conductive solution will vary according to the design of the electrolytic cell and the particular chamber. For the chambers fed sulfate compounds containing divalent metal cations (e.g. sulfide leachate), sulfuric acid naturally present in leachate and metal sulfates should suffice as the electrolyte. For the chambers fed with water, the electrolyte is sodium chloride (NaCI) or sodium sulfate (Na₂S0₄) in the figures presented therein, except for Figure 1 B in which the electrolyte is potassium chloride (KCI) or potassium sulfate (K₂S0₄). Various electrolytes may be used according to the present invention, including, but not limited to, sodium chloride (NaCI), sodium sulfate (Na₂S0₄), potassium chloride (KCI), potassium sulfate (K₂S0₄) calcium chloride (CaCI₂), hydrochloric acid (HCI), sulfuric acid (H₂S0₄), sodium hydroxide (NaOH), potassium hydroxide (KOH) and mixtures thereof. In one embodiment the preferred electrolyte is potassium sulfate (K₂S0₄). Those skilled in the art can appreciate that the base being produced by the catalytic reactions occurring within the cell will vary according to the particular electrolyte (e.g. NaOH for NaCI or Na₂S0₄ (e.g. Fig. 1 A); KOH for KCI or K₂S0₄ (e.g. Fig. 1 B); Ca(OH)₂ for CaCI₂, etc.).

[000112] The concentration of the electro lyte(s) in the conductive solutions must also be sufficient for the solution to be properly conductive. The concentration may also vary according to the various chamber and the electrolyte used. For instance, in the anodic compartment it may be preferable to have a nearly saturated 5M to 6M NaCI brine solution to maximize conductivity. In other embodiments, it may be preferable to use filtered seawater as an electrolyte source. In such cases, a much lower concentration electrolyte may be used, for instance a concentration of 0.5M NaCI, which is the approximate concentration of NaCI in seawater. Any intermediate concentration of NaCI between that of seawater (~0.5M) and the saturation point of NaCI in water (6.14 moles/L at 25°C) may be desirable depending on cost and availability of electrolyte.

[000113] In the cathodic chamber, the concentration of electrolyte may be as high as possible, as long as precipitation of the salt inside the cell can be avoided (e.g. below 6.14 moles/L for NaCI at 25°C). Generally, the highest possible concentration of electrolyte will be desirable in the cathodic chamber to maximize conductivity. However, if seawater is used as the electrolyte source, concentrations as low as 0.5M NaCI may be used as above for the anodic chamber (and any intermediate value may be used between 0.5M up to the saturation point of NaCI). As the reactions progress and an acid and base are simultaneously created in separate chambers, the acid (e.g. H₂S0₄) and the base (e.g. NaOH) will increase conductivity within the cell beyond the ability of the electrolyte (e.g. NaCI, or KCI).

[000114] As can be appreciated, according the embodiment illustrated in Figures 1-3, the inputs to the electrolytic cells include electricity, concentrated aqueous sulfide leachate containing ferrous sulfate and sulfuric acid, and carbonate compounds deriving from C0₂ gas. The products include precipitated iron carbonate, and purified sulfuric acid by-product.

[000115] Operation of the electrochemical cells 1 , 50, 60 described herein may be assisted by one or more control systems. Such a control system may assist in obtaining optimal production parameters, for instance by regulating the electric current or voltage, by regulating the concentration of the electrolytes, by regulating the flow pumps for circulating the solutions and/or hydrogen, by regulating the other components of the treatment process (e.g. C0₂ absorption reactor, S0₂ reduction reactor), etc. Such systems may comprise a series of sensors to collect data (e.g. pH meters, flow rate meters, hydrogen pressure monitor, etc.), a process control mechanism (e.g. a computer, PID controller, etc.) and a set of electronically operated mechanisms (e.g. valves, pressure regulators, etc.) to adjust various reaction parameters.

[000116] As illustrated in Figures 1 to 3, the sulfide leachate provides metal cations, primarily Fe²⁺, to precipitate as carbonates. Sulfide mine waste is received into the cell as an aqueous solution containing iron sulfate (FeS0₄), ferric and ferrous iron ions, sulfuric acid, and other components of acid mine drainage (e.g. Fe(OH)₃, Fe₂(S0₄)₃).

[000117] With respect to the particular embodiment illustrated in Figures 1-3, the key chemical formulae used in these electrolytic cells are as follows:

Anode reaction : H_2(g) + 2e^" = 2H⁺ _(aq) + 2e^" Cathode reaction 2H₂O₀₎ + 4e- = H_2(g) + 20H-_(aq)

Figures 1A + 1 B:

Anodic chamber 13: 2H⁺ ₍a_q) + S0₄ ^2" _(aq) = H₂S0₄(aq)

Central chamber 1 1 : Fe²⁺ _(aq) + C0₃ ^2' _(aq) = FeC0_3(S)

Cathodic chamber 10: 2H₂0₍i) + 4e^" = H_2(g) + 20H^" _(aq) (i.e. no reaction

except reduction of hydrogen at the cathode)

Figure 2:

Anodic chamber 13: 2H⁺ _(aq) + S0₄ ^2" _(aq) = H₂S0_4(aq)

First central chamber 1 1 : Fe²⁺ _(aq) + C0₃ ^2" _(aq) = FeC0_3(S)

Second central chamber 12: deionization of sulfate solution FeS0_{4 (aq)} -

XFeS0_4(aq) = (1 -X)FeS0_{4 (aq)} ; wherein X represents the fraction of iron sulfate that has passed through the membrane

Cathodic chamber 10: 2H₂0₍i) + 4e^" = H_2(g) + 20H^" _(aq) (i.e. no reaction except reduction of hydrogen at the cathode)

Figure 3A:

Anodic chamber 13: 2H⁺ _(aq) + S0₄ ^2" _(aq) = H₂S0_4(aq)

Cathodic chamber 10: FeS0_{4 (aq)} + C0₃ ^2" _(aq) = FeC03_(s)

2H₂0₍i₎ + 4e- = H_2(g) + 20H^" _(aq)

Figure 3B:

Anodic chamber 13: 2H⁺ _(aq) + S0₄ ^2" _(aq) = H₂S0_4(aq)

Fe²⁺ _(aq) + C0₃ ² _(aq) = FeC0_3(S)

Cathodic chamber 10: 2Η₂Ο₀₎ + 4e^" = H_2(g) + 20H^" _(aq)

[000118] Those chemical formula are provided as for illustrative purposes only and are not intended to limit the scope of the concepts described therein. Those skilled in the art will appreciate that the elements of the chemical formulae can easily be interchanged for other metals, sulfate and carbonate compounds.

[000119] The efficiency and/or performance of the electrolytic cell will vary according to various factors such as the ohmic losses caused by different electrolytes, the number and types of ion exchange membranes, complication of reactions due to varying feedstock, etc. [000120] Without wishing to be bound by any particular theory, it may be possible to determine the electric potential of the half-cell reactions (reaction at one of the electrodes) occurring in the electrolytic cell using the Nernst equation:

E = E° - R T ln(Q)/ n F) [Nernst equation] with constants:

E°= standard reduction potential

R= 8.314 J/mol °K

T= absolute temperature (in Kelvin)

n= number of electrons involved in half-cell reaction

F= 96485 J/V mol

Q= reaction quotient

where E_totai = Ecathode⁺E_anode (these depend on pH of anode/cathode electrolytes)

[000121] For instance, in the case of a H₂ gas-supplied anode, and H₂ gas-producing cathode, the half-cell reactions will be the reduction of hydrogen at the cathode, and oxidation of hydrogen at the anode. With respect to the oxidation reaction and potential for the chamber comprising the anode :

H_{2 (}g) = 2 H⁺ _(aq) + 2e^"

Eanode = +0.059^*(pH_a) (pH of anodic electrolyte)

For simplicity, we consider the anode electrolyte to be acidic with pH=0, and therefore: E_an0de = 0V

[000122] With respect to the reduction reaction and potential for the chamber comprising the cathode:

2 H₂0(i) + 2e = H_2(g) + 20H _(aq) E_Cathode = -0.059^*(pH_c) (pH of cathodic electrolyte)

[000123] The reaction at the cathode should be allowed to progress to pH=14 in order retain a pH of >8 (a suitable pH for carbonate precipitation) when diluted by carbonic acid from C0₂ absorption as well as subsequent mixing with sulfide leachate.

[000124] At pH=14 in the cathodic chamber and pH=0 in the anodic chamber the difference between anode and cathode half-cell potential will be 0.83V (as calculated from E,otai = Ecathode+Eanode = 0^*+0.059 + 14^*-0.059 = -0.83V). In contrast, at pH=7 in the cathodic chamber and pH=0 in the anodic chamber the overall cell potential will be - 0.413V (0^*+0.059 + 7^*-0.059 = -0.413V). A negative value for E_totai in these examples implies the reaction requires input of the stated voltage to proceed. [000125] The theoretical minimum voltage required to force the electrolysis reaction to occur will therefore increase from 0.413V to 0.83V as pH increases from 7-14 respectively. As can be appreciated, this is a significantly lower voltage requirement than common membrane, diaphragm and mercury chloralkali cells that have the theoretical Ecei, = 2.19V. [000126] Accordingly, those skilled in the art can appreciate that the amount of the desired products to be obtained (i.e. the solid carbonate precipitate) is directly proportional to the electric current provided to the electrolytic cells. Accordingly, to maximize the current (i.e. to optimize production) while reducing energy cost, the voltage should be low and the current density high. Preferably, the voltage should is set lower than the theoretical minimum of 2.19 Volts required for generation of chlorine gas, as chlorine gas is not a desired product in these cells. Nevertheless, when necessary (e.g. additional resistance), voltage can be adjusted to get higher throughput. In embodiments the voltage is about 0.5 Volts to a maximum of 2.19 Volts, or about 1 V to 1.8 V. In embodiments the current density is about 30 mA/cm² to about 500 mA/cm², or about 30 mA/cm² to about 100 mA/cm². High current density may be obtained by minimizing the distance between the two electrodes, by using high performance membranes, by choosing appropriate catalyzed electrodes and/or by optimizing electrolyte composition and chamber pH. In embodiments the current density is about 100 mA/cm² and the voltage is about 1 V to 1 .6 V. Using the electrolytic cell for the treatment of acid mine drainage

[000127] According to particular aspects, the invention relates to methods, processes and systems for obtaining solid metal carbonate compounds from sulfate compounds containing divalent metal cations. These methods, processes and systems may be particularly advantageous for the treatment of acid mine drainage and the sequestration of carbon dioxide. [000128] A solution comprising sulfate compounds containing divalent metal cations is provided. According to one particular embodiment, sulfide leachates (e.g. from AMD) provides aqueous divalent metal cations (e.g. Fe²⁺) in solution with sulfate anions (S0₄ ^2") to precipitate solid metal carbonate compounds (e.g. iron carbonate). The divalent metal cations may also derive from other sources such as ferruginous sandstones (Fe), finegrained metal-rich clay sediment of glacial or fluvial origin (e.g. Fe, Mg, Na, Ca, or others), seawater (Na, Ca), or waste products from industrial processes (various metal ions).

[000129] In one embodiment, the aqueous divalent metal cations are derived from solid sulfide mine tailings or waste pyrite concentrate and these are leached with the aid of metal-oxidizing bacteria (e.g. iron-oxidizing bacteria) in a heap or in a reactor. Expected output from this step will be an aqueous solution containing sulfuric acid and iron concentrations on the order of 50 g/L. Alternatively, acid mine drainage (AMD) from polluting sites could be contained and used in processes described herein for remediating acid mine drainage as well as providing inputs for C0₂ sequestration (see hereinafter) without the requirement for reaction with iron-oxidizing bacteria. For sites without significant sulfide drainage or solid sulfide sources, or projects requiring emphasis on carbon dioxide sequestration, concentrated pyrite (FeS₂) may be transported to the site for leaching and treatment. [000130] Acid mine drainage commonly consists of solutions containing sulfate compounds containing divalent metal cations and, therefore, the present embodiments and figures refer to compounds containing divalent metal cations (e.g. sulfate compounds containing a divalent metal cation or solid carbonate compounds containing a divalent metal cation). However, those skilled in the art will readily appreciate that the present invention is amenable to monovalent metal compounds as well and that the chemical formula, methods, processes described herein can easily be adapted to monovalent metal compounds. For instance, the sulfate compounds to be fed to the electrolytic cell could possibly be potassium sulfate (K₂S0₄) or sodium sulfate (Na₂S0₄). Accordingly, the resulting solid carbonate precipitate could possibly be potassium carbonate (K₂C0₃) or sodium carbonate (Na₂C0₃), respectively. Therefore, certain embodiments the present invention encompass monovalent metal compounds. [000131] As shown in Figure 4, the electrolytic cells 1 according to the invention may be integrated into a larger and more complex processing system. The processing system may include, for instance, a source of sulfide mine waste runoff 41 , a source of C0₂-enriched gas 42, a C0₂ absorption reactor 30, and a thickener and/or filter 35. In the embodiment illustrated in Figure 4, the process uses an electrolytic cell 1 that incorporates sulfide leachates as an electrolytic feedstock. This allows carbonate precipitation to occur within the electrolytic cell.

[000132] In one embodiment, carbon dioxide 43 may be received by the process primarily from a source of C0₂-rich gases 42 (e.g. as a constituent component of flue gases from a carbon-dioxide producing industrial process). Accordingly, the electrolytic cell may be operatively connected to a C0₂ absorption reactor 30 or a carbonation column producing C0₃ ^2" ions. Such embodiment is advantageous as it provides the additional benefit of carbon dioxide sequestration. Details about the use of a C0₂ absorption column 30 are provided hereinafter. [000133] The electrolytic cell 1 may also be operatively connected to an optional S0₂ reduction reactor 40. The role of the S0₂ reduction reactor 40 is to improve the efficiency of carbonate production by reducing trivalent metal (3+ oxidation state) compounds to divalent metal (2+ oxidation state) compounds (e.g. reducing ferric iron to ferrous iron), the latter being the species available for carbonate precipitation. Details about the use of a catalytic S0₂ reduction reactor 40 are provided hereinafter.

Use of brine electrolytic cells and of electrodialvsis (BMED and EED) cells

[000134] Related aspects of the present invention concerns methods, process and systems using brine electrolytic cells for producing an alkali-containing catholyte that is subsequently used for treating sulfide mine waste and sequestering C0₂. As used herein, a "brine electrolytic cell" refers to an electrolytic cell wherein there is one single electrolyte (e.g. concentrated sodium chloride (NaCI), potassium chloride (KCI), or calcium chloride (CaCI₂)) and wherein the products are the corresponding acid and base (e.g. HCI and NaOH for a NaCI electrolyte). Typically, the concentration of the electrolyte (i.e. brine) is about 0.5M to about 5M with higher concentrations being more desirable. Examples of suitable brine electrolytic cells 50 are shown in Figures 5A, 5B and 5C. [000135] Figure 5A shows a cation exchange version of a brine electrolytic cell 50 comprising a hydrogen-oxidizing anode 4, a cathode 2, and a cation exchange membrane 7 (CEM). Sodium (Na⁺) cations traverse the membrane 7 to react with hydroxide (OH^") anions produced at the cathode 2. The output is NaOH and HCI. [000136] Figure 5B shows an anion exchange version of a brine electrolytic cell 50 comprising a hydrogen-oxidizing anode 4, a cathode 2, and an anion exchange membrane 6 (AEM). Chlorine (CI^") anions traverse the membrane 6 to meet protons (H⁺) produced at the anode 4. The output is NaOH and HCI.

[000137] Figure 5C shows a 3-chamber version of a brine electrolytic cell 50 comprising a hydrogen-oxidizing anode 4, a cathode 2, a cation exchange membrane 7 (CEM), and an anion exchange membrane 6 (AEM). The brine electrolytic cell 50 comprises a central chamber 1 1 which is separated from the anode 4 by the AEM 6 and which is separated from the cathode 2 by the CEM 7. Brine solution is fed into the central chamber 1 1 . Ions from the brine traverse membranes 6, 7 as in Figures 5A and 5B, creating the same products, but reducing the brine concentration in the central chamber 1 1 . The additional benefit is thus the production of fresh water in the central chamber 1 1. As such, this cell may be used for desalination applications and may be advantageous for processes and installations that include a desalination plant.

[000138] Additional brine electrolytic cells that may be used herein, include those known in the art such as those described in US patent 3,531 ,386; US patent 3,801 ,698; US patent publication Nos US 2013/0008354, US 2013/0034489; US 2010/0200419; US 2010/0140103; US 201 1/0083968; and those of International PCT patent publications WO 2010/008896 and WO 2015/058287. Accordingly, the present invention encompasses methods, process and systems using such known cells to the extent they may be use for producing an alkali-containing catholyte useful for treating sulfide mine waste and sequestering C0₂.

[000139] In some embodiments it may be desirable to replace the alkalinity-generating brine electrolytic cell with an alkalinity-generating cell operating by electrodialysis including, but not limited to, bipolar membrane electrodialytic (BMED) cells and electro- electrodialytic (EED) cells. Accordingly, the present invention encompasses methods, process and systems using such electrodialysis cells, particularly in producing an alkali- containing catholyte that is subsequently used for treating sulfide mine waste and sequestering C0₂.

[000140] BMED cells: BMED cells may employ three or more compartments separated by cation exchange membranes and anion exchange membranes and at least one bipolar ion exchange membrane, the latter being incorporated to perform the function of dissociating water into H⁺ and OH^" ions. The anode and cathode are constructed of any corrosion-resistant conductor such as platinum-coated titanium.

[000141] Figure 6A shows a 4-chamber version of a BMED cell 60 comprising an anodic chamber 13, first 1 1 and second 12 central chambers and a cathodic chamber 10. The anodic chamber 13 and the second central chamber 12 are separated by a CEM 7. The two central chambers 1 1 , 12, are separated by a bipolar membrane 9 (BPM) comprising an anion selective layer, a cation selective layer and an intermediate junction layer (not shown). The first central chamber 1 1 and the cathodic chamber 10 are separated by an AEM 6. Like in the brine electrolytic cells, a brine salt aqueous solution (e.g. NaCI typically between 0.5M and 5M with higher concentrations usually being desirable) is fed into the anodic 13 and cathodic 10 compartments of the cell 60. Sodium cations traverse the CEM 7 into the second central chamber 12. Chlorine anions traverse the AEM 6 to the first central chamber 1 1 . As electric current is applied to the cell 60, the BPM 9 dissociates water into OH- and H+ ions that pass into the second 1 1 and first 13 central chambers, respectively. Accordingly, aqueous NaOH and HCI are formed in second 12 and first 1 1 central chambers, respectively.

[000142] Although only one unit comprising four compartments is shown in Figure 6A, the present invention encompasses additional embodiments where the ion exchange membranes 6, 7 and the bipolar membrane 9 are configured differently. The invention also encompasses embodiments comprising a number of repeating functional units in a stack, all powered by the electric current and potential drop between two electrodes.

[000143] EED cells: EED cells may employ three or more compartments separated by cation exchange membranes and anion exchange membranes. Two electrodes are constructed of a corrosion-resistant conductor such as platinum-coated titanium. [000144] Figure 6B shows a 3-chamber version of an EED cell 70 comprising an anodic chamber 13, a central chamber 1 1 , and a cathodic chamber 10. The anodic chamber 13 and central chamber 1 1 are separated from each other by an AEM 6. The central chamber 1 1 and the cathodic chamber 10 are separated from each other by a CEM 7. Like in the brine electrolytic cells, a brine salt aqueous solution (e.g. NaCI typically between 0.5M and 5M with higher concentrations usually being desirable) is fed into the central compartment 1 1 of the cell 70. With applied electrical current, chlorine anions traverse the AEM 6 to the anodic chamber 13 where they encounter H⁺, whereas sodium cations traverse the CEM 7 to the cathodic chamber 10 where they encounter OH^". Oxygen gas evolves from the anode 4 and hydrogen gas evolves from the cathode 2. Accordingly, aqueous HCI is formed in the anodic chamber 13 and NaOH is formed in the cathodic chamber 10.

[000145] If repeating EED units are employed in a stack, two electrodes are required for each repeating unit, unlike the BMED cell. The EED cell evolves hydrogen gas from the cathode, and oxygen gas from the anode and these gases may be used as fuel in a hydrogen-oxygen fuel cell when it is advantageous, or for other purposes.

[000146] As can be appreciated, all the configurations of the brine electrolytic cells 50, BMED cells 60 and EED cells 70 can produce an alkali-containing catholyte and an acid- containing anolyte. In particular, the illustrated cells produce sodium hydroxide (NaOH) and hydrochloric acid (HCI), which indicates that the electrolyte is NaCI. As is known, sodium hydroxide is a strong base. According to another aspect of the present invention, such brine electrolytic cells, BMED cells and EED cells are incorporated into processes for the treatment or remediation of Acid Rock Drainage (ARD) or Acid Mine Drainage (AMD) as explained herein after and illustrated in Figure 7 and Figure 8. Although theses illustrated processes refer to sulfide mine waste comprising iron (e.g. FeS0₄, Fe(OH)₃, Fe₂(S0₄)₃), the processes invention are amenable to treatment of other compounds comprising metallic divalent cations such as lead sulfate (PbS0₄), zinc sulfate (ZnS0₄), nickel sulfate (NiS0₄), copper sulfate (CuS0₄), etc.

[000147] Although in the present figures the input to the brine electrolytic cells, BMED cells and EED cells are a salt such as NaCI, various electrolytes may be used according to the present invention, including, but not limited to, potassium chloride (KCI), calcium chloride (CaCI₂), hydrochloric acid (HCI), sulfuric acid (H₂S0₄), sodium hydroxide (NaOH), sodium sulphate (Na₂S0₄), potassium sulfate (K₂S0₄) and mixtures thereof. Those skilled in the art can appreciate that the produced acid and base by the catalytic reactions occurring within the cells will vary according to the particular electrolyte. For instance NaCI produces HCI and NaOH, Na₂S0₄ will produce H₂S0₄ and NaOH, KCI will produce HCI and KOH, CaCI₂ will produce HCI and Ca(OH)₂, K₂S0₄ will produce H₂S0₄ and KOH, etc. Therefore the present invention encompasses such alternate embodiments. [000148] According to the embodiment illustrated in Figure 7, a C0₂ absorption reactor 30 is operatively connected to the brine 50, BMED 60 and/or or EED 70 cell in order to receive therefrom the alkali-containing catholyte and create a carbonate solution comprising sodium bicarbonate and sodium carbonate. Details about uses of a C0₂ absorption reactor are provided hereinafter. The carbonate solution produced in the C0₂ absorption reactor 30 is transported to another vessel 35 to react with aqueous sulfide leachates and precipitate solid iron carbonate and sodium sulfate. The relatively stable precipitate particles are removed through traditional solid-liquid separation methods such as thickening. Details about the C0₂ absorption reactor and the precipitation and thickening process are provided hereinafter. [000149] Compared to Figure 7, the process of Figure 8 further comprises a S0₂ reduction reactor 40. In one embodiment, the reactor is a S0₂ catalytic reduction column (show in the center of the diagram). This S0₂ reduction reactor acts to improve the efficiency of carbonate production by reducing trivalent metal ions (3⁺ oxidation state) to divalent metal ions (2⁺ oxidation state), the latter being the species desired for carbonate precipitation (e.g. reducing ferric iron to ferrous iron). In addition, the S0₂ reduction reactor 40 scrubs sulfur dioxide from a S0₂ containing flue gas 44. According to one embodiment, the S0₂ reduction reactor 40 is a S0₂ catalytic reduction column and that column is operatively connected to (i) a source of C0₂-rich gases comprising also S0₂ 44, and (ii) to a source of sulfide mine waste 41 so that it can reduce metallic compounds found in the sulfide mine waste 41 to produce a solution comprising reduced metal sulfate compounds. The flue gas scrubbed of S0₂ is fed to the C0₂ absorption reactor 30 and the solution comprising reduced sulfate compounds is transported to another vessel 35 for carbonate precipitation. More details about the S0₂ reduction reactor are provided hereinafter.

Precipitation and thickening [000150] As illustrated in Figures 7 and 8, the carbonate solution produced in the C0₂ absorption reactor 30 is transported to another vessel 35. The purpose of the vessel 35 is to allow proper mixing and a chemical reaction between (1 ) the carbonate solution exiting the C0₂ absorption reactor 30 (e.g. NaHC0₃ or Na₂C0₃, typically pH 10-14) and (2) the metal-containing acidic sulfate solution (typically pH 1-3). In one embodiment the vessel 35 is a conical thickener. As illustrated, the acidic sulfate solution may comprise acidic mine waste 41 (Figure 7) or a solution comprising reduced sulfate compounds and sulfuric acid (Figure 8). Such mixing will increase the pH of the metal-containing acidic solution, thereby causing precipitation of solid metal carbonate compounds and formation of a sulfate such as sodium sulfate (Na₂S0₄). For instance, as illustrated in Figure 9, for the formation of siderite (FeC0₃) a pH of 7-10 is ideal, as well as suitable containment in a vessel that limits exposure to atmospheric oxygen thereby preventing Fe²⁺ from oxidizing.

[000151] Any suitable recipient or vessel can be used for receiving the carbonate solution and the acidic sulfate solution for the precipitation and thickening steps. Preferably, the vessel should be made of, or coated with, a non-reactive material (e.g. high density polyethylene (HDPE), PTFE, acrylic, titanium, ceramic). The vessel may be operatively connected to the C0₂ absorption reactor and/or to the source of acidic sulfate using non-reactive pipes with flow regulators and/or pumps that can be controlled by the process control system. In the illustrated embodiments, the acidic sulfate solutions are sulfide mine wastes comprising FeS0₄, and, therefore, precipitates comprise FeC0₃ (siderite).

[000152] The solid metal carbonate compounds that have precipitated according to the processes described herein may be removed from the vessel or recovered using any suitable technique or method of solid-solid separation. In one embodiment, the precipitates are recovered using a filter (e.g. a standard filter paper and/or cloth, or a vacuum assisted filter press apparatus). In one embodiment, the precipitates are recovered by thickening, for instance by using a large diameter thickener vessel with a conical bottom and rake system like those commonly found in mineral processing operations. A flocculant (e.g. isinglass, sodium silicate) may also be added to promote thickening and separation of solid particles from the liquids.

CO? absorption reactor

[000153] As illustrated in Figures 1 to 8, the electrochemical cells 1 , 50, 60 and 70 described herein are a source of alkalinity and these cells may be operatively connected to a C0₂ absorption reactor 30. According to the illustrated embodiments, the C0₂ absorption reactor 30 receives the alkali-containing catholyte produced by the electrochemical cells 1 , 50, 60 and 70 (e.g. NaOH solution) and it receives also a C0₂- rich gas mixture 43, 44 (e.g. from an industrial plant 42) to create a carbonate solution (e.g. a solution comprising sodium bicarbonate and sodium carbonate). The carbonate solution may be returned to the electrolytic cell 1 (e.g. Figures 1-4) or used for precipitation and thickening (Figures 5-8). The carbon-dioxide gas mixture 43, 44 contains C0₂ and it may also contain additional gaseous compounds, including but not limited to, sulfur dioxide (S0₂), water in steam form, and nitrogen.

[000154] As is known, C0₂ gas does not react directly with hydroxide (OH^"). However, hydroxide will react with carbonic acid (H₂C0₃) formed in small amounts by reaching equilibrium with dissolved C0₂ gas in solution:

C0₂₍aq₎ = 650^«[H₂CO₃] (Lower, 1999)

[000155] With addition of hydroxide, carbonic acid will be consumed and C0₂ will continue to dissolve to maintain equilibrium:

a) C0_2(aq) + H₂0 H₂C0₃

[000156] It is apparent from the above equilibrium reactions that the addition of 2 OH^" from the electrolytic cells will react with 2H⁺ and push the equilibrium toward C0₃ ^2", thereby allowing for carbonate precipitation if divalent metal anions are present. The OH^" anions will increase the pH and, if C0₂ is made to react with aqueous OH^" in correct proportion, bicarbonate and carbonate-species will be dominant.

[000157] For example, from the cathode reaction we produce OH^":

2H₂0 + 4e^~ ^ ¾+ 20H^" [000158] Sodium ions in the cathode chamber associate with hydroxide anions:

Na⁺ + OH^" ^ NaOH

[000159] Sodium hydroxide reacts with carbonate anion in the C0₂ absorption reactor:

2NaOH + C0₃ ^2" + 2H⁺ ^ Na₂C0₃ + 2¾0

[000160] Divalent iron carbonate precipitates by combination of sodium carbonate and ferrous sulfate:

Na₂C0₃ + FeS0₄ FeC0₃ + Na₂S0₄

[000161] As is known, pressure and temperature have an important effect on the carbonic acid concentration. For instance, concentration of carbonic acid increases with higher C0₂ pressure. Also, lower temperature increases solubility of carbon dioxide (as with all gases) (Lower, 1999). Accordingly, the C0₂ absorption reactor is preferably operated at low temperature (e.g. at about 0°C to about 25°C), at high C0₂ pressure (e.g. about 100 kPa to about 700 kPa) and at high pH (e.g. about pH 10 to about pH 14). Hot flue gases may require cooling at a desired temperature (e.g. using a heat exchanger) to optimize C0₂ absorption. [000162] Preferably, the C0₂ absorption reactor should be made of, or coated with, a non-reactive material suitable for containing a strong base (e.g. polyvinyl chloride (PVC), PTFE, HDPE, or ceramic). Preferably, the C0₂ absorption reactor is configured to encourage rapid chemical reactions. Similarly, C0₂ and/or the alkali-containing catholyte should be introduced into the reactor in a format that encourages rapid chemical reactions. Examples of possible operating mechanisms of the C0₂ absorption reactor include, but are not limited to, spraying/misting the alkali-containing catholyte through a chamber of flue gas, bubbling flue gas C0₂ through the alkali-containing catholyte, injecting a flue gas into a packed column with concurrent inflow of the alkali-containing catholyte, ambient C0₂ absorption through high surface area interaction with the alkali- containing catholyte, etc. In one embodiment, the C0₂ absorption reactor is shaped like a cylindrical column. In one embodiment, the C0₂ absorption reactor is shaped like a cylindrical column and C0₂ is bubbled through the alkali-containing solution using a ceramic diffuser 32 of suitable length to allow maximum C0₂ absorption.

SO? reduction reactor

[000163] The electrochemical cells 1 , 50, 60 and 70 described herein may also be operatively connected with an optional sulfur dioxide (S0₂) catalytic reduction reactor 40. The role of the S0₂ reduction reactor is to improve the efficiency of carbonate production by reducing trivalent metal ions (3⁺ oxidation state) to divalent metal ions (2⁺ oxidation state) (e.g. ferric iron to ferrous iron). For instance, optional circulation of leachate feedstock through such reactor may increase concentrations of the species available for carbonate precipitation (e.g. maximizing ferrous ion concentration for precipitation as siderite (FeC0₃)). This reduction reactor also reduces harmful S0₂ concentrations in the flue gases.

[000164] In embodiments, the S0₂ reduction reactor is be made of, or coated with, a non-reactive material (e.g. HDPE, PTFE, PVC and other plastics may be suitable depending on temperature and concentration of sulfurous acid and sulfuric acid in solution). Preferably, the S0₂ reduction reactor is configured to encourage rapid chemical reactions. Similarly, S0₂ and/or the metal ion containing solutions should be introduced into the reactor in a format that also encourages rapid chemical reactions. Examples of possible operating mechanisms of the S0₂ reduction reactor include, but are not limited to, spraying/misting the metal ion containing solution through a chamber of flue gas, bubbling flue gas through the metal ion containing solution, injecting flue gas into a packed column with concurrent inflow of metal ion containing solution. Preferentially the S0₂ reduction column will include a catalyst compound to improve reaction rates.

[000165] In one embodiment, the S0₂ reduction reactor is shaped like a column or like a reactor bed. Preferably the column is dimensioned (sufficient length and/or width) for ensuring a high reaction rate and for ensuring short, efficient flow path of gases and/or for completely scrubbing sulfur dioxide from a constant flow of flue gas.

[000166] In embodiments, flue gases are bubbled through a metal ion containing solution. The flue gas may contain dominantly C0₂ and trace amounts of S0₂. In embodiments, the temperature of the flue gas is about 25°C to about 75°C or less, in order to maximize S0₂ solubility. However it may also be desirable to operate at the temperatures between 150°C and 600°C, depending on the temperature of exiting flue gas and depending on the working properties of the catalyst employed.

[000167] In embodiments, like the one illustrated at Figure 8, the S0₂ reduction reactor 40 comprises a catalyst 49 to improve the speed of the reaction, (e.g. by providing more nucleation sites for the reaction to take place). Examples of catalysts that may be used include, but are not limited to, platinum, activated carbon, and other common catalysts used in the industry like ruthenium, rhodium, vanadium pentoxide (at temperatures greater than 400°C). Preliminary experiments show that catalysis by activated carbon improves reaction speed by at least 500% (see Example 1). The catalyst may need to be recharged or reactivated after prolonged used. In one embodiment, the S0₂ reduction reactor comprises a chamber filled with porous, high specific surface area, activated carbon.

[000168] In the particular embodiments illustrated in Figures 4 and 8 the S0₂ reduction reactor 40 is fed with sulfide mine waste 41 comprising trivalent iron in the form of iron oxy-hydroxides (e.g. Fe(OH)₃) or sulfates (i.e. Fe₂(S0₄)₃) to produce divalent iron in a sulfate form (i.e. FeS0₄). The output of the reaction (e.g. FeS0₄, H₂S0₄) may be introduced into an electrolytic cell 1 (Figure 4) or to a precipitation and thickening vessel 35 as shown in Figure 8. [000169] According to particular embodiments (e.g. like those illustrated in Figure 4 and Figure 8), this optional step involves passing a S0₂-containing gas mixture 43, 44 (e.g. flue gas deriving from combustion of liquid natural gas, H₂S from sour natural gas production, diesel, and/or coal, or any other flue gas containing elevated levels of S0₂) through a metal sulfate solution (e.g. an iron sulfide leachate) in the S0₂ reduction reactor 40. S0₂ will react with trivalent metal ions (3+ oxidation state, e.g. Fe³⁺) to produce divalent metal ions (2+ oxidation state, e.g. Fe ) in solution. Divalent metal (e.g. Fe²⁺) is the required valence state for metal carbonate precipitation. Employing such a technique will have the benefits of: (1 ) scrubbing S0₂ from the waste gas; and (2) making more metal available in solution for metal carbonate precipitation, thereby improving downstream reaction efficiency in the electrolytic cell 1 (Figure 4) or improving downstream reaction efficiency of precipitation in a precipitation and thickening vessel 35 (Figure 8).

[000170] For instance, the S0₂ reduction reactor 40 may be fed acid rock drainage (ARD) or a similar acidic aqueous solution derived from iron sulfide tailings. Such acidic solution may have a pH of about 1 -3 and contain Fe²⁺, Fe³⁺, S0₄ ^2" and H⁺ as dominant species.

[000171] In the illustrated embodiment, the sulfide mine waste 41 comprises trivalent iron (e.g. Fe(OH)₃, Fe₂(S0₄)₃) to produce divalent iron in a sulfate form (e.g. FeS0₄). However, it is envisioned that the present invention may by useful for reducing other metals with both divalent and trivalent oxidation states such as Ti³⁺ to Ti²⁺, Cr³⁺ to Cr²*, Mn³⁺ to Mn²⁺, Co³⁺ to Co²⁺, Ni³⁺ to Ni²⁺, Cu³⁺ to Cu²⁺, Zn³⁺ to Zn²⁺, etc.

[000172] Without wishing to be bound by any particular theory, the following provides some details about the chemical reactions involved within the S0₂ reduction reactor. For instance, a small percentage of S0₂ in flue gas (largely composed of C0₂) may reduce Fe³⁺ ₍aq₎ to Fe²⁺ ₍aq₎ by the following suggested reactions:

a) The solution of sulfur dioxide in water:

S0_2(g) + H₂0 S0₂ ^«H₂0

b) Dissolved S0₂ gas forming sulfurous acid:

S0₂ ^«H₂0≠ H⁺ +HSO₃- c) The reduction of the ferric ion by hydrogen sulfite:

H₂0 + 2 Fe³⁺ _(aq) + HSO3^" _(aq)→ 2 Fe²⁺ _(aq) + S0₄ ^2" + 3 H⁺

[000173] Reaction a) involves the dissolving of S0₂ gas and reaction b) is the reaction of aqueous dissolved S0₂ with water . Finally, in reaction c) the HS0₃ ^" anion reduces ferric iron, producing ferrous iron, sulfate, and protons. For every two ferric iron atoms reduced to ferrous iron, three H⁺ are released and must be neutralized with OH^" later in the process (e.g. in the electrolytic cell 1 as shown in Figure 4 or in the thickening and precipitation vessel 35 as shown in Figure 8). Feasibility of the overall S0₂ reduction process is demonstrated in Example 1. [000174] Alternative methods and techniques can be envisioned to reduce trivalent metal ions to divalent metal ions, according to the principles of the present invention. One such alternative comprises the addition of H₂S from sour natural gas production to burning hydrocarbons such as natural gas or coal, to oxidize H₂S to S0₂ by the reaction H₂S _(g) + 3/2 0_{2 (g)} S0_{2 (}g) + H20 (I). Another alternative comprises exposing the metallic sulfate solution to UV light in order to reduce trivalent metal ions to divalent metal ions (for example, by the reaction suggested by Viganico et al., 201 1 : Fe³⁺ _(aq) + H₂0 _{i) + UV Fe²⁺ ₍aq) + H⁺ (aq) + OH^" _(aq)). Such alternative methods and techniques may be useful when the natural sulfur-abundance of fossil fuels consumed in power generation is insufficient for complete reduction of the trivalent metal and/or the complete capture of C0₂, or when it is not feasible to reduce ferric iron in a particular case.

[000175] As can be appreciated there are a number of benefits to the cells, methods and systems contemplated and described herein. For instance, these cells, methods and systems provide for an accelerated and more optimized set of reaction conditions for the treatment of acid mine drainage allowing faster and more effective treatment of sulfide mine waste. The present cells, systems and methods are also able to treat Acid Mine Drainage directly, instead of requiring unreacted metal sulfides like some other methods. The present methods and systems broadens applicability of existing remedial strategies and enables treatment of acid mine drainage and sequestration of carbon dioxide in situations where runoff is the primary negative environmental challenge but the reactive sulfides themselves cannot be treated (e.g. a closed underground mine with leakage out the access portal; or when sulfides are present in the remaining wall rocks of a closed open pit mine). Optionally, the present methods and systems utilizes S0₂, which is toxic and present in some fossil fuel fired industrial plant off-gases, to improve reaction rates and the overall process. EXAMPLE

Example 1 : Catalysed Iron-reduction experiment

[000176] An experiment was carried out for demonstrating feasibility a S0₂ reduction process as described herein. [000177] Briefly, the experiment used 250 mL Erlenmeyer flasks and constant stirring with magnetic stir bar. To each flask was added 0.2 g FeOOH powder and 150 mL tap water (< 0.5 mg/L iron). The catalyzed flask was filled and sealed by rubber stopper with 40 mL granular activated carbon (1 -2 mm grains as commonly used in water filtration), and 100 mL of C0₂ and S0₂ in equal proportion. A second flask received the gas mixture but no catalyst. A third flask was filled with water, FeOOH and catalyst to determine if, as expected, S0₂ was the reactant.

[000178] Measurements were taken by diluting known volumes of filtered fluid to suitable expected concentrations for testing. Concentrations were determined with colorimetric ferrous iron test strips with combined error on the order of ±10-50 mg/L. [000179] The results are presented in the Table 1 below and plotted in Figure 10.

Table 1 : Results of activated carbon catalyst for S0₂ reduction of ferric iron.

Fe²⁺ mg/ L

Time (h) Catalyzed Not Catalyzed Control

0 0 0 0

2 100 25 0

1 1 500 100 0

108 1000 30 0

[000180] As can be appreciated from the Table 1 and Figure 10, activated carbon clearly catalyzes this reduction reaction. REFERENCES:

[000181] L. Birry, and A. Lasia. "Studies of the hydrogen evolution reaction on Raney nickel— molybdenum electrodes." Journal of applied electrochemistry 34.7 (2004): 735- 749.

[000182] D. Blowes, C. Ptacek, J. Jambor, and C. Weisener, "The geochemistry of acid mine drainage," Treatise on geochemistry, vol. 9, pp. 149-204, 2003.

[000183] S. K. Lower, "Carbonate equilibria in natural waters," Simon Fraser University, vol. 544, 1999.

[000184] J. D. Rimstidt and D. J. Vaughan, "Pyrite oxidation: a state-of-the-art assessment of the reaction mechanism," Geochimica et Cosmochimica Acta, vol. 67, no. 5, pp. 873-880, 2003.

[000185] P. C. Singer and W. Stumm, "The solubility of ferrous iron in carbonate- bearing waters," Journal (American Water Works Association), pp. 198-202, 1970.

[000186] Viganico, E.M., Colling, A.V., de Almeida Silva, R. and Schneider, I.A.H. Biohydrometallurgical/UV production of ferrous sulphate heptahydrate crystals from pyrite present in coal tailings. Minerals Engineering, 24 \ \ ), pp.1 146-1 148, 201 1 .

[000187] Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein, and these concepts may have applicability in other sections throughout the entire specification. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. [000188] The singular forms "a", "an" and "the" include corresponding plural references unless the context clearly dictates otherwise. Thus, for example, reference to "a compound" includes one or more of such compound, and reference to "the method" includes reference to equivalent steps and methods known to those of ordinary skill in the art that could be modified or substituted for the methods described herein.

[000189] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, concentrations, properties, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". At the very least, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the present specification and attached claims are approximations that may vary depending upon the properties sought to be obtained. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the embodiments are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors resulting from variations in experiments, testing measurements, statistical analyses and such. [000190] It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the present invention and scope of the appended claims.

Claims

CLAIMS:

An electrolytic cell, comprising :

- at least three chambers, said chambers comprising an anodic chamber, a cathodic chamber and a central zone comprising at least one central chamber;

- a cationic exchange membrane (CEM) and at least one anionic exchange membrane (AEM);

wherein, when said least one central zone comprises one central chamber, said central chamber is separated from the cathode by an AEM and is separated from the anode by the CEM;

wherein, when said least one central zone comprises at least two central chambers, a first central chamber is separated from the cathode by a first AEM and is separated from a second central chamber by a CEM; and said second central chamber is separated from the anode by a second AEM and is separated from said first central chamber by said CEM.

An electrolytic cell, comprising :

- a cationic exchange membrane (CEM) and an anionic exchange membrane (AEM),

An electrolytic cell, comprising :

- a cationic exchange membrane (CEM) and two anionic exchange membranes (AEM),

wherein said first central chamber is separated from the cathode by a first AEM and is separated from a second central chamber by a CEM; and wherein said second central chamber is separated from the anode by a second AEM and is separated from said first central chamber by said CEM.

4. The electrolytic cell of any one of claims 1 to 3, wherein the anodic chamber comprises a gas-diffusion anode. 5. The electrolytic cell of any one of claims 1 to 4, wherein the anodic chamber comprises a hydrogen-oxidizing anode.

6. The electrolytic cell of claim 5, wherein the cathodic chamber comprises a cathode producing hydroxide ions and hydrogen gas.

7. The electrolytic cell of any one of claims 1 to 6, wherein said chambers comprise an aqueous solution comprising at least one conductive electrolyte.

8. The electrolytic cell of claim 7, wherein said at least one conductive electrolyte is selected from the group consisting of NaCI, KCI, CaCI₂, HCI, H₂S0₄, Na₂S0₄, K₂S0₄, NaOH, KOH and mixtures thereof.

9. The electrolytic cell of any one of claims 1 to 8, wherein at least one of said chambers comprises at least one conductive electrolyte containing metal sulfate compounds from industrial waste.

10. The electrolytic cell of claim 9, wherein, said industrial waste comprises Acid Mine Drainage.

1 1 . The electrolytic cell of any one of claims 1 to 8, wherein at least one of said chambers comprises at least one conductive electrolyte containing metal sulfate compounds from a reactor that treats solid metal sulfides.

12. The electrolytic cell of any one of claims 1 to 8, wherein at least one of said chambers comprises at least one conductive electrolyte containing metal sulfate compounds from a heap that treats solid metal sulfides.

13. The electrolytic cell of any one of claims 1 to 12, wherein the cathodic chamber is operatively connected to the anodic chamber for circulating hydrogen gas produced at the cathode to the anode.

14. The electrolytic cell of any one of claims 1 to 13, wherein the cathodic chamber is operatively connected to a C0₂ absorption reactor.

15. The electrolytic cell of any one of claims 1 to 14, further comprising a S0₂ reduction reactor operatively connected to a chamber receiving metal sulfate compounds.

16. The electrolytic cell of claim 15, wherein the chamber receiving metal sulfate compounds is the anodic chamber or the second central chamber.

17. A method for obtaining solid metal carbonate compounds from sulfate compounds, comprising:

- providing a sulfate solution comprising metal-containing sulfate compounds;

- recovering a solid precipitate of metal carbonate compounds from said an electrolytic cell. 18. The method of claims 17, wherein said sulfate solution comprises sulfate compounds containing divalent metal cations and wherein precipitated solid metal carbonate compounds comprises carbonate compounds containing divalent metal.

19. A method for obtaining solid metal carbonate compounds from sulfate compounds containing divalent metal cations, comprising:

- providing a sulfate solution comprising metal-containing sulfate compounds;

20. The method of claim 19, wherein said electrolysing comprises increasing alkalinity of an aqueous solution contained in the cathodic chamber.

21 . The method of claim 19 or 20, wherein said sulfate solution comprises sulfate compounds containing divalent metal cations and wherein precipitated solid metal carbonate compounds comprises carbonate compounds containing divalent metal.

22. The method of any one of claims 19 to 21 , wherein said sulfate solution is fed to the anodic chamber and wherein said divalent metal cations traverse the CEM while sulfate anions remain in said anodic chamber. 23. The method of any one of claims 19 to 22, wherein the cathodic chamber comprises an alkaline solution, the method further comprising circulating said alkaline solution through a C0₂ absorption reactor to produce an alkaline carbonate solution comprising carbonate compounds and feeding said alkaline carbonate solution to the cathodic chamber, wherein carbonate anions traverse the AEM to precipitate with divalent metal cations in the central chamber.

24. The method of any one of claims 19 to 21 , wherein:

said central chamber comprises a first central chamber and a second central chamber;

said first central chamber is separated from the cathode by a first AEM and is separated from said second central chamber by a CEM;

said second chamber is separated from the anode by a second AEM and is separated from said first central chamber by said CEM; and

wherein said solid precipitate of metal carbonate compounds is recovered from said first central chamber.

25. The method of claim 24, wherein said sulfate solution is fed to the second central chamber, and wherein said divalent metal cations traverse the CEM into the first central chamber while sulfate anions traverse the AEM into the anodic chamber.

26. The method of claim 24 or 25, wherein the cathodic chamber comprises a cathodic alkaline solution, the method further comprising circulating said cathodic alkaline solution through a C0₂ absorption reactor to produce an alkaline carbonate solution comprising carbonate compounds and feeding said alkaline carbonate solution to the cathodic chamber, wherein carbonate anions traverse the AEM to precipitate with divalent metal cations in the first central chamber. 27. The method of claim 26, wherein said C0₂ absorption reactor is operatively connected to the cathodic chamber, and wherein said carbonate compounds are produced by reacting said cathodic alkaline solution with C0₂ gas circulating inside the C0₂ absorption reactor.

28. The method of claim 27, wherein sodium hydroxide is produced within the cathodic chamber, and wherein said sodium hydroxide is reacted with C0₂ gas circulating in the C0₂ absorption reactor to produce said carbonate compounds.

29. The method of any one of claims 19 to 28, wherein said alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

30. The method of any one of claims 19 to 28, wherein said sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

31 The method of any one of claims 19 to 30, wherein said sulfate solution comprises iron sulfate and wherein the solid precipitate comprises iron carbonate.

32. The method of any one of claims 19 to 31 , further comprising circulating hydrogen gas produced at the cathode from the cathodic chamber to the anodic chamber.

33. The method of any one of claims 19 to 32, further comprising recovering said solid metal carbonate compounds.

34. The method of any one of claims 19 to 33, further comprising the step of recovering purified sulfuric acid from the anodic chamber. 35. The method of any one of claims 19 to 34, wherein said electrolysing comprises applying an electric potential of about 0.5 V to 2.19 V across the anode and the cathode.

36. A method for obtaining solid metal carbonate compounds from sulfate compounds containing divalent metal cations, comprising:

- providing a sulfate solution comprising iron sulfate;

37. An electrochemical system comprising :

- an anolyte solution in contact with an anode;

- a catholyte solution in contact with a cathode;

- an ionic solution separated from the catholyte solution by an anionic exchange membrane (AEM) and separated from the anolyte solution by a cationic exchange membrane (CEM). 38. The electrochemical system of claim 37, wherein the anolyte solution comprises sulfate compounds containing divalent metal cations, and wherein said divalent metal cations traverse the CEM into the ionic solution.

39. The electrochemical system of any one of claim 37 or 38, wherein the anolyte solution comprises an acid mine drainage solution.

40. The electrochemical system of claim 38, wherein said acid mine drainage solution comprises FeS0₄ and H₂S0₄. 41 . The electrochemical system of any one of claims 37 to 40, wherein the catholyte solution comprises at least one electrolyte and carbonate compounds in aqueous solution, and wherein carbonates anions traverse the AEM to react and precipitate with divalent metal cations in the ionic solution.

42. An electrochemical system comprising :

- an anolyte solution in contact with an anode;

- a catholyte solution in contact with a cathode;

- first and second ionic solutions positioned between the catholyte solution and the anolyte solution,

43. The electrochemical system of claim 42, wherein the second ionic solution comprises sulfate compounds containing divalent metal cations, and wherein said divalent metal cations traverse the CEM into the first ionic solution while sulfate anions traverse the AEM into the anolyte solution.

44. The electrochemical system of claim 42 or 43, wherein the catholyte solution comprises at least one electrolyte and carbonate compounds in aqueous solution, and wherein carbonates anions traverse the AEM to react and precipitate with divalent metal cations in the first ionic solution.

45. The electrochemical system of any one of claims 42 to 44, wherein the anolyte solution, the catholyte solution, the first ionic solution and the second ionic solution comprise at least one a conductive electrolyte selected from the group consisting of NaCI, KCI, CaCI₂, HCI, H₂S0₄, Na₂S0₄, K₂S0₄, NaOH , KOH and mixtures thereof.

46. The electrochemical system of any one of claims 42 to 45, wherein at least one of the anolyte solution and second ionic solution comprises divalent metal ions reduced from trivalent metal ions subsequent to a pass through a S0₂ reduction reactor.

47. The electrochemical system of any one of claim 42 or 46, wherein the second ionic solution comprises an acid mine drainage solution. 48. The electrochemical system of claim 47, wherein said acid mine drainage solution comprises FeS0₄ and H₂S0₄.

49. The electrochemical system of any one of claims 37 to 48, wherein hydroxide ions and hydrogen gas are produced at the cathode.

50. The electrochemical system of any one of claims 37 to 49, wherein hydrogen gas is collected from the catholyte solution to be fed to the anode.

51 . The electrochemical system of any one of claims 37 to 50, wherein the anolyte solution comprises sulfuric acid produced therein.

52. The electrochemical system of any one of claims 37 to 51 , wherein the catholyte solution is circulated through a C0₂ absorption reactor to create an alkaline carbonate solution.

53. The electrochemical system of any one of claims 37 to 52, further comprising a filter and/or thickener for recovering said solid metal carbonate compounds.

54. A method for sequestering C0₂ and for obtaining solid metal carbonate, comprising:

- providing a sulfate solution comprising sulfate compounds containing metal cations; - electrolysing said sulfate solution in an electrolytic cell, said electrolytic cell producing an alkali-containing solution under electrolysis;

- feeding said alkali-containing solution to a C0₂ absorption reactor operatively connected to said electrolytic cell and to a source of C0₂, said C0₂ absorption reactor receiving said alkali-containing solution and said C0₂ for forming an alkaline carbonate solution;

55. The method of claim 54, wherein said alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

56. The method of claim 54 or 55, further comprising recovering said solid metal carbonate compounds.

57. The method of any one of claims 54 to 56, wherein said sulfate solution comprises sulfate compounds containing divalent metal cations and wherein precipitated solid metal carbonate compounds comprises carbonate compounds containing divalent metal.

58. The method of any one of claims 54 to 57, wherein said sulfate solution comprises iron sulfate and wherein the solid precipitate comprises iron carbonate.

59. The method of any one of claims 54 to 58, wherein said sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

60. A method for sequestering C0₂ and for obtaining iron carbonate from iron sulfate, comprising:

- providing a sulfate solution comprising iron sulfate; - electrolysing said sulfate solution in an electrolytic cell, said electrolytic cell producing an alkali-containing solution under electrolysis;

The method of claim 60, wherein said sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

A system for obtaining solid metal carbonate compounds and sequestering C0₂, comprising:

- an electrochemical cell producing an alkali-containing catholyte;

The system of claim 62, wherein said alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

64. The system of claim 62 or 63, wherein said acidic sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

65. The system of claim 64, wherein the sulfide leachates have a pH of about 1 to 3.

66. The system of any one of claims 62 to 65, wherein said acidic sulfate solution comprises sulfate compounds containing divalent metal and wherein precipitated solid metal carbonate compounds comprise carbonate compounds containing divalent metal.

67. The system of any one of claims 62 to 66, wherein said acidic sulfate solution comprises iron sulfate and wherein said solid metal carbonate compound comprises iron carbonate.

68. The system of claim 67, wherein said acidic sulfate solution comprises at least one of FeS0₄, Fe₂(S0₄)₃ and Fe(OH)₃.

69. The system of any one of claims 62 to 68, wherein said electrochemical cell is selected from the group consisting of brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells.

70. A system for obtaining solid metal carbonate compounds and sequestering C0₂, comprising:

- an electrochemical cell producing an alkali-containing catholyte;

71 The system of claim 70, wherein said acidic sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

72. An method for obtaining solid metal carbonate compounds and sequestering C0₂, comprising:

- providing an electrochemical cell producing an alkali-containing catholyte;

- reacting in a vessel an acidic sulfate solution comprising metal ions with said alkaline carbonate solution for precipitating solid metal carbonate compounds.

73. The method of claim 72, wherein said alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof. 74. The method of claim 72 or 73, wherein said carbonate compounds are produced by reacting said alkali-containing catholyte with C0₂ gas inside the C0₂ absorption reactor.

75. The method of any one of claims 72 to 74, wherein said sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

76. The method of any one of claims 72 to 75, further comprising recovering said solid metal carbonate compounds.

77. The method of any one of claims 72 to 76, wherein said acidic sulfate solution comprises sulfate compounds containing divalent metal cations and wherein precipitated solid metal carbonate compounds comprise carbonate compounds containing divalent metal.

78. The method of any one of claims 72 to 77, wherein said sulfate solution comprises iron sulfate and wherein the solid carbonate comprises iron carbonate.

79. The method of any one of claims 72 to 78, further comprising a step of circulating the acidic sulfate solution through a S0₂ reduction reactor prior to said reacting, wherein said S0₂ reduction reactor reduces trivalent metal compounds present in the acidic sulfate solution to divalent metal compounds. 80. The method of any one of claims 72 to 79, wherein said electrochemical cell is selected from the group consisting of brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells.

A method for obtaining solid metal carbonate compounds and sequestering C0₂, comprising:

- providing an electrochemical cell producing an alkali-containing catholyte;

- reacting in a vessel an acidic sulfate solution comprising iron sulfate with said alkaline carbonate solution for precipitating solid iron carbonate compounds

82. The method of claim 81 , wherein said sulfate solution comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate. 83. A system for reducing trivalent metal compounds to divalent metal compounds and sequestering C0₂, comprising:

- an electrochemical cell producing an alkali-containing catholyte;

84. The system of claim 83, wherein the trivalent metal compounds contain trivalent (ferric) iron and wherein in the S0₂ catalytic reduction column S0₂ reacts with said trivalent (ferric) iron to produce divalent (ferrous) iron in a sulfate form.

85. The system of claim 83 or 84, wherein the trivalent metal compounds comprises at least one of Fe(OH)₃ or Fe₂(S0₄)₃.

86. The system of any one of claims 83 to 85, wherein the trivalent metal compounds contain trivalent (ferric) iron and wherein said solid metal carbonate compound comprises FeC0₃.

87. The system of any one of claims 83 to 86, wherein the trivalent metal compounds comprises sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

88. The system of claim 87, wherein the sulfide leachates have a pH of about 1 to 3.

89. The system of any one of claims 83 to 88, wherein said alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

90. The system of any one of claims 83 to 89, wherein the S0₂ reduction reactor comprises a catalyst selected from the group consisting of platinum, activated carbon, ruthenium, rhodium, and vanadium pentoxide.

91 The system of any one of claims 83 to 90, wherein said electrochemical cell is selected from the group consisting of brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells.

92. A method for reducing trivalent metal compounds to divalent metal compounds and sequestering C0₂, comprising:

- reducing trivalent metal compounds into a S0₂ reduction reactor to obtain an acidic sulfate solution comprising divalent metal compounds;

- providing an electrochemical cell producing an alkali-containing catholyte;

- reacting in a vessel said acidic sulfate solution with said alkaline carbonate solution for precipitating solid carbonate compounds containing divalent metal.

93. The method of claim 92, wherein the trivalent metal compounds contain trivalent (ferric) iron and wherein in the S0₂ catalytic reduction column S0₂ reacts with said trivalent (ferric) iron to produce divalent (ferrous) iron in a sulfate form. 94. The method of claim 92 or 93, wherein the trivalent metal compounds comprises at least one of Fe(OH)₃ or Fe₂(S0₄)₃.

95. The method of any one of claims 92 to 94, wherein the trivalent metal compounds contain trivalent (ferric) iron and wherein said solid metal carbonate compound comprises FeC0₃. 96. The method of any one of claims 92 to 95, wherein the trivalent metal compounds comprise sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate.

97. The method of claim 96, wherein the sulfide leachates have a pH of about 1 to 3.

98. The method of any one of claims 92 to 97, wherein said alkaline carbonate compound solution comprises at least one of carbonate ions, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, calcium carbonate and mixtures thereof.

99. The method of any one of claims 92 to 98, wherein the S0₂ reduction reactor comprises a catalyst selected from the group consisting of platinum, activated carbon, ruthenium, rhodium, and vanadium pentoxide.

100. The method of any one of claims 92 to 99, wherein said electrochemical cell is selected from the group consisting of brine electrolytic cells, bipolar membrane electrodialytic (BMED) cells and electro-electrodialytic (EED) cells.