Method for capturing and storing carbon dioxide in minerals through microwave-driven catalytic processes

Abstract

The present invention relates to a method for capturing and storing carbon dioxide in minerals through microwave-driven catalytic processes.

Description

[0001] METHOD FOR CAPTURING AND STORING CARBON DIOXIDE IN MINERALS THROUGH MICROWAVE-DRIVEN CATALYTIC PROCESSES

[0002] Technical field of the invention

[0003] The present invention relates to a method for capturing and storing carbon dioxide in minerals through microwave-driven catalytic processes.

[0004] The invention is applied in the field of carbon capture, utilisation and storage technologies by carbonation of minerals, and in particular waste minerals from other processes.

[0005] State of the art

[0006] Carbon Capture, Utilisation and Storage (CCUS) plays a crucial role in the Green Transition, participating in a more sustainable and zero-emissions industrial framework and is a technological approach aimed at better managing anthropogenic CO2 emissions, also promoting the use of captured CO2 for the production of secondary raw materials, in a Circular Economy system.

[0007] Magnesium and calcium are among the most abundant metals in the Earth's lithosphere. Their ability to form hydrated hydroxy-carbonates and anhydrous carbonates make them key elements for the future development of CCUS methodologies. Among the various techniques, mineral carbonation (MC) represents a valid opportunity, since through this process a reaction takes place between CO2 and minerals rich in magnesium and calcium in an aqueous environment, during which CO2 is chemically converted into a stable and solid compound, usually in the form of carbonates.

[0008] High fractions of MgO are available in silicates such as olivine, orthopyroxene, some clinopyroxenes and serpentine. To date, data reports that serpentine polymorphs, especially antigorite, are excellent candidates for fixing CO2 since the reaction efficiency is about 92% compared to lizardite (40%) and olivine (66%). This is attributed to the specific surface area of approximately 18.7 m2 / g for dehydrated antigorite compared to 10.8 m2 / g for dehydrated lizardite and 4.6 m2 / g for olivine. On the other hand, important Ca contents are found in minerals such as portlandite, a component of many wastes from the steel, brick and cement industry.

[0009] Magnesium and calcium oxides, hydroxides and silicates can react with CO2 to produce carbonates with varying levels of hydration. These crystalline phases are stable (they do not tend to decompose returning CO2 back to the environment) and are used as secondary raw materials in the construction materials sector. The carbonation reaction, although it is in many cases spontaneous, is characterized by slow kinetics and therefore not compatible with continuous industrial processes. Furthermore, the most interesting carbonation products are represented by the anhydrous carbonates of magnesium (magnesite or MgCCE) and calcium (calcite or CaCCE), as they represent the most stable polymorphs among the known carbonates, as well as those with greater commercial value, and obtaining them does not involve the consumption of water. However, for kinetic reasons, water-mediated carbonation of magnesium minerals gives magnesium hydroxycarbonate hydrates (MHCH) products, such as nesquehonite [MgCO3’3H2O] and hydromagnesite [Mgs(CO3)4(OH)2 • 4H2O] .

[0010] These compounds are in any case valuable secondary raw materials used, for example, in the construction industry or in sports.

[0011] To obtain magnesite, it is necessary to overcome activation energy barriers and therefore carry out the carbonation process at temperatures above 200°C.

[0012] It is known an acid attack method performed in order to solubilize the magnesium present in magnesium-containing mineral particulate, followed by carbonate precipitation induced by a pH increase and mixing the solution with CCh-containing gases, or the activation of silica minerals by a heat treatment resulting from fuel combustion, or to sequester CO2 through gas flow and leaching liquid flow on a rock pile, or to use amino acids as carbonation additives of silicates, or to carbonate cementitious materials with gas from biomass combustion. However, the reaction times of the methods described above are of the order of tens of days.

[0013] The use of microwaves has long been exploited in chemical synthesis processes to make heating more efficient and to substantially accelerate some endothermic reactions, thanks to their ability to couple directly with polar molecules, such as water, and / or electrolytes. Energy transfer can be highly localized and reagent-specific, promoting accelerated reaction kinetics.

[0014] The use of microwaves in the carbonation reactions of magnesium-containing minerals has been experimented by White et al. (2004), "Reaction mechanisms of magnesium silicates with carbon dioxide in microwave fields", Final Report to the U.S. Department of Energy, National Energy Technology Laboratory. The study, however, does not report any product obtained under hydrothermal conditions.

[0015] Subsequently, in Campione et al. (2024), "Microwave-driven carbonation of brucite" Journal of CO2 Utilization, 80, 102700, the magnesium carbonate phases obtainable from microwave carbonation reactions carried out from brucite slurry (magnesium hydroxide suspended in water) in the absence of any type of chemical additive were identified.

[0016] In light of the above, there is still a need to provide a method for capturing and storing carbon dioxide through carbonation of waste minerals that allows anhydrous carbonates to be obtained at temperatures substantially below 200°C.

[0017] Summary of the invention

[0018] The invention relates to a method for capturing and storing carbon dioxide comprising or consisting of the steps of: a) providing a reactor containing water; b) dissolving carbon dioxide in water with the formation of a saturated solution of carbon dioxide; and c) dispersing mineral raw material in the saturated solution of step b), with precipitation of anhydrous mineral carbonates, or alternatively b’) dispersing mineral raw material in the water of step a), and c’) dissolving carbon dioxide in water with the formation of a saturated solution of carbon dioxide and precipitation of anhydrous mineral carbonates, characterised in that (i) steps b) and c) or b’) and c’) are carried out in the presence of anionic catalyst additives and (ii) microwaves are continuously applied throughout steps b) and c) or b’) and c’).

[0019] The invention further relates to the use of anionic catalyst additives selected from potassium formate (K+HCOO ), potassium acetate (KCCH3COO ), potassium oxalate (K22+C2O42), potassium fumarate (FUM), potassium malate (MAL), potassium malonate (MAO), potassium succinate (SUC), potassium tartrate (TAR), potassium citrate (CIT), sodium ethylenediaminetetraacetate (EDTA), lactic acid (LAC), sodium bicarbonate (Na+HCO3-), potassium iodide (K+L), sodium sulphate (Na22+SO42‘), potassium nitrate (K+N03-), betaine (B), betaine hydrochloride (BHC), choline chloride (CHCL), preferably selected from potassium acetate (K+CEhCOO ), potassium malonate (MAO), potassium succinate (SUC), potassium tartrate (TAR), potassium citrate (CIT), sodium ethylenediaminetetraacetate (EDTA), sodium bicarbonate (Na+HCO3‘) and sodium sulphate (Na22+SO42‘), more preferably selected from potassium acetate (KCCH3COO ), potassium malonate (MAO), sodium citrate potassium (CIT) and sodium sulphate (Na22+SO42‘), in combination with continuous microwave treatment, in the catalysed precipitation of anhydrous metal carbonates, preferably magnesite or calcite. Brief Description of the Figures

[0020] Figure 1A-B: 1A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst. IB) X-ray diffraction (XRD) pattern of pristine brucite powder.

[0021] Figure 2A-B: 2A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of potassium fumarate (FUM) as an anionic catalyst. 2B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0022] Figure 3A-B: 3 A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of potassium malonate (MAO) as an anionic catalyst. 3B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0023] Figure 4A-B: 4A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of potassium succinate (SUC) as an anionic catalyst. 4B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0024] Figure 5A-B: 5 A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of potassium citrate (CIT) as an anionic catalyst. 5B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0025] Figure 6A-B: 6A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of potassium sulphate (SOF) as an anionic catalyst. 6B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0026] Figure 7A-B: 7A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of betaine hydrochloride (BHC) as an anionic catalyst. 7B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0027] Figure 8A-B: 8 A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 with addition of choline chloride (CHCL) as an anionic catalyst. 8B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 1 without anionic catalyst.

[0028] Figure 9 SEM of the sample obtained in Example 1 and analysed in Figure 7B.

[0029] Figure 10 SEM of the sample obtained in Example 1 and analysed in Figure 5 A. Figure 11 SEM of the sample obtained in Example 1 and analysed in Figure 6A.

[0030] Figure 12 SEM of the sample obtained in Example 1 and analysed in Figure 3A.

[0031] Figure 13A-C: 13A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 4 without anionic catalyst (160°C - 60 min). 13B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 4 without anionic catalyst (190°C - 30 min). 13C) X-ray diffraction (XRD) pattern of pristine portlandite powder.

[0032] Figure 14A-C: 14A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 4 with microwave treatment and without anionic catalyst. 14B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 4 without microwave treatment and without anionic catalyst. 14C) X-ray diffraction (XRD) pattern of pristine portlandite powder.

[0033] Figure 15A-B: 15 A) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 4 with added anionic catalyst of K-acetate (CH3COO ). 15B) X-ray diffraction (XRD) pattern of the solid precipitate obtained according to Example 4 without anionic catalyst.

[0034] Figure 16: SEM of the solid precipitate obtained according to example 4 starting from portlandite.

[0035] Figure 17: Raman spectra of the solid precipitate obtained according to Example 4 and reported in Figure 14A; of the solid precipitate obtained according to Example 1 with addition of potassium citrate (CIT) as an anionic catalyst; of the solid precipitate obtained according to Example 1 with addition of potassium sulphate (SOf) as anionic catalyst as an anionic catalyst; and of the solid precipitate obtained according to Example 1 with addition of potassium malonate (MAO) as anionic catalyst.

[0036] Figure 18: Magnification of the Raman spectrum of Figure 17 in the spectral range 1040-1180 cm1.

[0037] Detailed description of the invention

[0038] To address the problems of the prior art, a method for capturing and storing carbon dioxide by carbonation of minerals through microwave-driven catalytic processes is therefore proposed herein, which allows the capture, storage and utilisation of carbon, exploiting the reactivity of some minerals containing magnesium and calcium towards carbon dioxide (CO2).

[0039] In the present invention, the following terms are intended to mean:

[0040] "mineral raw material" preferably means a raw material from other processes, for example quarry waste, mining waste; waste from steel, brick and cement industry, said raw material comprising magnesium, calcium or mixtures thereof, for example serpentine minerals ([Mg3Si2Os(OH)4]), characterised by alternating layers of silica (SiCh) and brucite ([Mg(0H)2]), olivine ([Mg2SiO4]), portlandite (Ca(OH)2) and wollastonite (CaSiO3).

[0041] The anionic catalytic additives are anionic species that facilitate the precipitation of anhydrous mineral carbonates by non-covalent interaction with an alkaline-earth cation released by the mineral raw material. Ion-pairing mediated by Coulomb-type forces is an example of non-covalent interaction and Mg2+and Ca2+are preferred examples of alkaline-earth cations.

[0042] "Microwave" refers to electromagnetic waves with frequencies comprised between 300 MHz (0.3 GHz) and 300 GHz, preferably between 900 MHz and 30 GHz, more preferably at a frequency of 2.45 GHz.

[0043] "Carbonation" is the process of forming carbonates from mineral raw materials, such as metal hydroxides, by the action of carbon dioxide and with the possible mediation of a liquid such as water.

[0044] "Saturated solution of carbon dioxide" means a solution in which water (H2O) has dissolved the maximum possible amount of carbon dioxide (CO2), so that it is not possible to dissolve further CO2 without part of it remaining undissolved or in the form of a gas.

[0045] In a first aspect, the invention refers to a method for capturing and storing carbon dioxide comprising or consisting of the steps of: a) providing a reactor containing water; b) dissolving carbon dioxide in water with the formation of a saturated solution of carbon dioxide; and c) dispersing mineral raw material in saturated solution of step b), with precipitation of anhydrous mineral carbonates, or alternatively b’) dispersing mineral raw material in the water of step a), and c’) dissolving carbon dioxide in water from step b’) with the formation of a saturated solution of carbon dioxide and precipitation of anhydrous mineral carbonates, characterised in that (i) steps b) and c) or b’) and c’) are carried out in the presence of anionic catalyst additives and (ii) microwaves are continuously applied throughout steps b) and c) or b’) and c’).

[0046] The method of the invention allows to carry out the carbonation process of waste mineral phases, preferably comprising magnesium or calcium, leading to the capture and storage of carbon dioxide, with lower times, energies and temperatures than traditional methods and reaching high yields of anhydrous mineral carbonates, such as for example magnesite and calcite.

[0047] It was surprisingly found that, in fact, the introduction of specific catalysts together with the continuous application of microwaves in steps b) and c) of the method, made it possible to achieve the objectives of: - substantially accelerating the carbonation process; - decreasing the energy required by the process; and - allowing to produce magnesite and calcite at low temperatures, specifically lower than 190°C, preferably lower than 130°C, more preferably equal to or lower than 100°C.

[0048] In fact, the anionic catalyst additives have the benefit of accelerating mineral carbonation and precipitation, for example, of magnesite and calcite, through the ability of the aforesaid ones to stabilize, for example, the sub -coordinated form of Mg2+in aqueous solution, facilitating the precipitation of the MgCCh anhydrous phase and the subsequent incorporation of anhydrous units in the precipitated phase.

[0049] Furthermore, thanks to the synergy between the electromagnetic waves in the GHz (microwave) range and the aforesaid catalysts in solution, it was possible to observe a substantial increase in the kinetics of the mineral carbonation reaction, both for the mineral raw material comprising magnesium and for that comprising calcium, mediated by water, improving both the dissolution and precipitation processes, obtaining a low temperature process, and without the use of high carbon dioxide pressures.

[0050] Moreover, it was possible to favour both dissolution and precipitation processes without the sequential addition of strong acids and / or bases, but maintaining the natural initial pH of the mineral material suspension. In fact, microwaves enhance the dissolution effect of anionic catalyst additives, and at the same time catalyse the precipitation of magnesite and calcite at low temperature without the addition of acids or bases.

[0051] In step a) preferably the reactor is a microwave reactor (SynthWave, Milestone) consisting of a 900 ml Teflon-coated resonant cavity filled with water. The microwave radiation is produced by an antenna placed under the base of the cavity and connected to a 1500 W magnetron.

[0052] In steps c) or c’) the carbonation of the mineral raw material, for example brucite or portlandite, takes place by means of the continuous use of microwaves and the addition of anionic catalyst additives.

[0053] Step b) or c’) comprises dissolving carbon dioxide in water, with formation of a saturated solution of carbon dioxide.

[0054] In the solution saturated in water, carbon dioxide is reacted with water to form carbonic acid.

[0055] In step b) or c’), preferably, the partial pressure of carbon dioxide (CO2) is comprised in a range from 1 to 14 bar, more preferably of 8 bar.

[0056] In step b) or c’) the final concentration of carbonic acid in the saturated solution of carbon dioxide in water is comprised in a range of 0.15 M to 0.25 M, more preferably it is 0.2 M.

[0057] Still preferably, the saturated solution of carbon dioxide of step b) or c’) is characterized by a pH comprised from 3.5 to 4.5, preferably of 4.0.

[0058] In a preferred and advantageous embodiment, the reaction environment is 40% nitrogen (N2), at a nitrogen partial pressure comprised in a range of 3 to 8 bar, preferably of 6 bar.

[0059] Step c) or b’) comprises dispersing mineral raw material in the saturated solution of step b) or in the water of step a), respectively.

[0060] Preferably, the dispersed mineral raw material comprises magnesium-rich minerals, calcium-rich minerals or mixtures thereof, more preferably it is serpentine ([Mg3Si2Os(OH)4]), brucite [Mg(OH)2] or portlandite [Ca(OH)2], still more preferably it is brucite or portlandite.

[0061] Also preferably, the formation of the dispersion takes place with the aid of an ultrasonic bath, more preferably with a treatment from 3 to 10 minutes, even more preferably at room temperature.

[0062] In a preferred embodiment, the formation of the dispersion with the aid of an ultrasonic bath takes place at 150 W.

[0063] Preferably, the mineral raw material of step c), for example brucite or portlandite, is dispersed with a concentration of brucite or portlandite comprised in a range from 0.1 to 0.5 M with respect to the final suspension, even more preferably of 0.34 M or 0.17 M.

[0064] Preferably, the water-dispersed mineral raw material is characterized by a pH comprised between 9 and 10.5.

[0065] Partial dissolution of water-dispersed raw material brings about the formation of alkali-earth solvated cations.

[0066] Preferably, said solvated alkali-earth cations are Mg^+ or Ca^.

[0067] Step c) or c’) comprises the precipitation of anhydrous mineral carbonates, preferably magnesium carbonate (MgCO3 s) or calcium carbonate (CaCO3 s).

[0068] The aforesaid precipitation takes place after combining the alkaline-earth cations dispersed in step c) or b’) with the HCO3‘ and CO32' anions of the saturated solution of step b) or c’) in an aqueous environment.

[0069] Preferably, in step c) or c’) of precipitation of anhydrous mineral carbonates in aqueous environment, the suspension is characterized by a neutral pH, more preferably comprised between 7.0 and 7.5.

[0070] Thanks to the simultaneous use of anionic catalyst additives and continuous microwave irradiation in carbonation steps b) and c) or b’) and c’), no intervention is needed to modify the pH of the solution, for example the addition of strong acids in step c) or b’), to promote the dissolution of the waste mineral raw material, nor the subsequent addition of bases in the same steps, to facilitate the precipitation of anhydrous mineral carbonates.

[0071] In fact, the continuous use of microwaves of the carbonation reaction environment in steps b) and c) or b’) and c’) allows not only ease of mineral dissolution but also the precipitation of the carbonates and therefore a more general improvement of the carbonation kinetics, resulting in a reduction of times and temperatures.

[0072] Furthermore, also the use of catalysts brings substantial advantages in terms of reaction time, energy expenditure, reduction of reaction temperature and increase of reaction yield in terms of anhydrous mineral carbonates obtained.

[0073] In particular, the catalysts are able to influence the dehydration kinetics of the metal ions in solution, favouring their precipitation in step c) or c’).

[0074] Therefore, thanks to its steps and conditions, the method requires no pre-treatment of the mineral phase and the use of continuous microwaves in the presence of anionic catalyst additives in steps b) and c) or b’) and c’) of the method of the invention, allows to activate the aforesaid catalysts and substantially accelerate the carbonation reaction of the minerals, obtaining anhydrous mineral carbonates, such as magnesite and calcite, at a temperature lower than or equal to 190°C, preferably lower than 130°C, more preferably equal to or lower than 100°C.

[0075] On the one hand, in fact, the catalysts have an ability to desolvate the dispersed alkaline-earth cations, for example comprising magnesium and calcium, thus making magnesium and calcium more likely to precipitate in the form of anhydrous carbonates.

[0076] On the other hand, the microwaves, by interacting specifically with the catalysts, intensify the desolvation effect of these catalysts, accelerate the carbonate precipitation process, stabilizing anhydrous phases, such as anhydrous mineral carbonates, at low temperature and allowing considerable energy savings.

[0077] At the end of steps b) and c) or b’) and c’), the catalysts are also fully recoverable, since they are activated but not modified by microwaves, and therefore can be used again for other carbonation cycles.

[0078] Preferably, the anionic catalyst additives are selected from the group comprising or consisting of potassium or sodium salts of mono-, di-, tri- and tetra-carboxylic acids, such as potassium formate (K+HCOO ), potassium acetate (K+CH3COO ), potassium oxalate (K22+C2C>42+), potassium fumarate (FUM), potassium malate (MAL), potassium malonate (MAO), potassium succinate (SUC), potassium tartrate (TAR), potassium citrate (CIT), sodium ethylenediaminetetraacetate (EDTA) and lactic acid (LAC); inorganic salts, such as sodium bicarbonate (Na+HCO3-), potassium iodide (K+L), sodium sulphate (Na22+SO42‘), potassium sulphate (K22+SO42‘), potassium chloride (K+C1‘), sodium fluoride (Na+F‘), and potassium nitrate (K+NO3-); ammonium salts, such as betaine (B), betaine hydrochloride (BHC), choline chloride (CHCL); phosphate (PO43), hydrogen phosphate (HPO42), dihydrogen phosphate (H2PO4 ), fluoride (F ), chloride (Cl"), perchlorate (CIO4 ), carbonate (CO32), bisulfide (HS ), metasilicate (SiOs2), phthalate (CsHsCh ), taurate (C2H6NSO3 ), salicylate (C7H5O3 ), aspartate (C4H6NO42), aminophenolate (C6H4ONH2 ), glycinate (C2H4NO2 ), glutamate (C5H8NO4 ), hydroxide (OH ), phenolate (CeHeO ), ionic isopropyl alcohol (C3H7O2 ), polyethylene glycol (CsHsOie2), hexafluorosilicate (SiFe2).

[0079] More preferably, the anionic catalyst additives are selected from the group comprising or consisting of potassium or sodium salts of mono-, di-, tri- and tetracarboxylic acids, for example potassium formate (K+HCOO ), potassium acetate (K+CH3C00‘), potassium oxalate (K22+C2O42), potassium fumarate (FUM), potassium malate (MAL), potassium malonate (MAO), potassium succinate (SUC), potassium tartrate (TAR), potassium citrate (CIT), sodium ethylenediaminetetraacetate (EDTA), lactic acid (LAC); inorganic salts, for example sodium bicarbonate (Na+HCO3-), potassium iodide (K+L), sodium sulphate (Na22+SO42‘), potassium nitrate (K+N03-); and ammonium salts, for example betaine (B), betaine hydrochloride (BHC), choline chloride (CHCL).

[0080] Still more preferably, the aforesaid anionic catalyst additives are selected from the group comprising or consisting of potassium acetate (IUCH3COO ), potassium oxalate (K22+C2O42), potassium fumarate (FUM), lactic acid (LAC), betaine hydrochloride (BHC), choline chloride (CHCL), potassium malonate (MAO), potassium succinate (SUC), potassium tartrate (TAR), potassium citrate (CIT), sodium ethylenediaminetetraacetate (EDTA), sodium bicarbonate (Na+HCO3-), sodium sulphate (Na22+SO42').

[0081] Still more preferably, said catalysts are selected from the group comprising or consisting of potassium acetate (K+CH3COO ), potassium malonate (MAO), potassium succinate (SUC), potassium tartrate (TAR), potassium citrate (CIT), sodium ethylenediaminetetraacetate (EDTA), sodium bicarbonate (Na+HCO3-) and sodium sulphate (Na22+SO42‘).

[0082] Even more preferably, said catalysts are selected from the group comprising or consisting of potassium acetate (BUCH3COO ), potassium malonate (MAO), potassium citrate (CIT), and sodium sulphate (Na22+SO42‘).

[0083] The aforesaid catalysts are environmentally friendly and, as will be shown in the accompanying experimental part, effectively induce the dehydration of the magnesium or calcium ions in solution and allow their precipitation in the form of anhydrous carbonates at temperatures of less than or equal to 190°C, preferably less than 130°C, more preferably equal to or less than 100°C.

[0084] In one preferred embodiment, the mineral raw material is brucite and the catalyst is selected from the group comprising or consisting of FUM, MAO, TAR, SUC, EDTA, CIT, BHC, CHCL hydrogen carbonate and sulphate. In another preferred embodiment, the mineral raw material is portlandite and the catalyst is selected from the group comprising or consisting of acetate (BUCHsCOO") and nitrate (K+NO3-).

[0085] Steps b) and c) or b’) and c’) preferably occur at or below a temperature of 190°C, more preferably equal to or lower than 160°C, even more preferably lower than 130°C, even more preferably equal to or lower than 100°C.

[0086] In an advantageous embodiment, the anionic catalyst additive is citrate and steps b) and c) or b’) and c’) preferably occur at a temperature equal to or lower than 100°C.

[0087] Steps b) and c) or b’) and c’) preferably take place in a time comprised from 30 to 120 minutes, preferably of 30 minutes or 5 minutes.

[0088] In light of the above, the invention further relates to the use of anionic catalyst additives selected from the groups identified above, in combination with continuous microwave treatment, in the precipitation of anhydrous mineral carbonates, preferably magnesite or calcite, from an aqueous dispersion of mineral raw material saturated with carbon dioxide. The following examples further illustrate the invention.

[0089] EXAMPLES

[0090] Materials and methods

[0091] Brucite powders were supplied by Sigma-Aldrich BioUltra >99.0%. Portlandite powders were supplied by Sigma-Aldrich ACS reagent >96.0%. All tested additives were supplied by Sigma-Aldrich.

[0092] The MCs were carried out in a microwave reactor (SynthWave, Milestone) consisting of a 900 ml Teflon-coated resonant cavity filled with 200 ml tap water (base load), or with 200 ml brucite or portlandite suspension in deionised water. In the former case, the free volume of the cavity was used to insert a support housing up to 15 tubes of 16 ml volume filled with 5.0 ml of brucite or portlandite suspension in deionized water and a varying amount of aqueous solution of the chosen additive. Each tube was stirred during the carbonation treatment with the aid of an integrated magnetic stirrer. In the latter case, the free volume of the cavity was used to insert a Teflon mechanical stirrer.

[0093] Powder X-ray diffraction (XRD) analysis was carried out by a X’Pert Pro (Panalytical), with Cu Ka radiation (X = 1.5417 A), X-ray tube powered at 40 kV and 40 mA.

[0094] The qualitative analysis of diffractograms was carried out through the Match! software. Quantitative analysis of the identified steps was performed with the GSAS software using the Rietveld refinement method.

[0095] SEM analysis for morphology was performed using a Gemini 500 (Zeiss) with an acceleration voltage of 15 kV, equipped with a Quantax energy dispersion spectroscopy (EDS) microanalysis system (Bruker).

[0096] Raman spectroscopy analysis for qualitative and quantitative analysis of crystalline phases was carried out using Horiba LabRam HR Evolution (600 lines / mm holographic gratings) equipped with an Olympus BX41 confocal microscope at a controlled temperature of 20(l)°C. Raman spectra were excited by the 532 nm line of a solid state laser (YAG). The laser power on the sample surface was varied between 10% and 100% (-1-10 mW). The spectrometer was calibrated on the silicon Raman peak at 520.5 The spectral resolution was -2 cm1and the instrumental reproducibility in determining the positions of the peaks was -0.5 cm1. The collected spectra were corrected for the baseline for the continuous luminescence background and normalized to the acquisition time.

[0097] The crystalline structure of brucite powder with which the XRD spectra have been compared is reported by M. Catti, G. Ferraris, S. Hull, A. Pavese, "Static compression and H disorder in brucite, Mg(0H)2, to 11 GPa: a powder neutron diffraction study", Phys. Chem. Miner. 22 (1995) 200-206.

[0098] The crystal structure of hydromagnesite with which the XRD spectra were compared is reported by M. Akao, S. Iwai, "The hydrogen bonding of hydromagnesite" , Acta Crystallogr. Sect. B. 33 (1977) 1273-1275.

[0099] The crystalline structure of magnesite with which the XRD spectra have been compared is reported by D.L. Graf, "Crystallographic tables for the rhombohedral carbonates" , Am. Mineral. 46 (1961) 1283-1316.

[0100] The crystal structure of portlandite with which the XRD spectra have been compared is reported by Henderson D. M. & Gutowsky H. S., American Mineralogist (1962), 47, 1231.

[0101] The crystal structure of calcite with which the XRD spectra have been compared is reported by Graf D L, American Mineralogist (1961), 46, 1283.

[0102] Example 1 - carbonation by means of different anionic catalyst additives starting from brucite

[0103] The brucite suspensions with 0.34 M concentration were prepared by dispersing the powder in deionized water with the aid of an ultrasonic bath. The anionic catalyst additives tested are listed in Table 1 below. The effect of catalysts was tested in parallel experiments by adding a different catalyst (concentration reported in Table 1) in different vials containing the brucite dispersion.

[0104] The microwave reactor is filled at room temperature with the rack containing the vials and 8 bar of CO2 (9.9995%) and 6 bar of N2 (9.9999%), as measured by an integrated pressure gauge, are injected in the reactor and the microwave irradiation is continuously emitted to increase initially the temperature of the mixture from room temperature to the setpoint temperature (heating stage) and to maintain it at the setpoint temperature (in the range 85 - 190°C, as measured by a temperature probe immersed in the base load, dwelling stage) throughout the treatment duration, under stirring. The set temperature was held for a period of 0.5 - 2 hours. The irradiated energy was recorded during the entire treatment period. At the end of the dwelling phase, the reactor was cooled to 35°C by a water flow at 8°C provided by a chiller; thereafter, the reactor was depressurized and opened to recover the products from the vials. The vial contents were centrifuged and the precipitated phase was separated from the supernatant solution.

[0105] Table 1 shows the tested conditions. Table 1

[0106] Example 2 - XRD analysis of the samples obtained according to Example 1

[0107] The samples obtained in Example 1 were analysed by powder X-ray diffraction (XRD). The anionic catalysts used in Example 1 were judged according to the criterion that an additive performs catalytic activities towards the reaction producing magnesite if for temperatures below or equal to 190°C and reaction times of 30 minutes a signal attributable to magnesite on the X- ray diffraction pattern can be detected in the precipitates.

[0108] The tests carried out with different anionic catalyst additives at different operating conditions (time and temperature) of step b) of carbonation are listed in Table 2. The table shows the weight percentages of the crystalline phases identified and the yield of conversion of the starting material into carbonate.

[0109] Table 2

[0110] * unquantifiable yield due to the presence of other phases

[0111] ** presence of magnesium oxalate hydrate

[0112] *** at this temperature most of the magnesium remains in solution x absence of precipitate Figure 1A shows the X-ray diffraction (XRD) pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 1 A shows the diffraction peaks corresponding to Miller indices 100, 110, 1 1 -1 characteristic of hydromagnesite, while the peaks corresponding to Miller indices 104, 2 -1 3 and 1 0 -8 characteristic of magnesite are undetectable. Figure IB shows the X-ray diffraction (XRD) pattern of pristine brucite powder.

[0113] Therefore, the microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the absence of anionic catalyst yields a solid formed by 100% hydromagnesite.

[0114] Figure 2A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with addition of 0.34 M potassium fumarate (FUM). Figure 2A shows the characteristic diffraction peaks of hydromagnesite, together with the peaks characteristic of magnesite. Therefore, the peaks confirm the formation of 98.5% hydromagnesite and magnesite at 1.5%.

[0115] Figure 2B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 2B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite.

[0116] Therefore, the microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as potassium fumarate (FUM), yields a solid formed by 1.5% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only the hydromagnesite.

[0117] Figure 3A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with the addition of potassium malonate (MAO) 0.34 M. Figure 3A shows the characteristic diffraction peaks of magnesite, while the peaks characteristic of hydromagnesite are absent. Thus, the peaks confirm the formation of 100% magnesite.

[0118] Figure 3B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 3B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite.

[0119] Therefore, the microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as potassium malonate (MAO), yields a solid formed by 100% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only the hydromagnesite.

[0120] Figure 4A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with addition of potassium succinate (SUC) 0.34 M. Figure 4A shows the characteristic diffraction peaks of magnesite and the characteristic peaks of hydromagnesite. Thus, the peaks confirm the formation of 57% magnesite and 43% hydromagnesite.

[0121] Figure 4B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 4B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite.

[0122] Therefore, the microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as potassium succinate (SUC), yields a solid formed by 57% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only the hydromagnesite.

[0123] Figure 5A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 100°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with addition of potassium citrate (CIT) 0.17 M. Figure 5 A shows the characteristic diffraction peaks of magnesite, while the peaks characteristic of hydromagnesite are absent. Thus, the peaks confirm the formation of 100% magnesite.

[0124] Figure 5B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 100°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 5B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite.

[0125] Therefore, the microwave carbonation of brucite at 100°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as potassium citrate (CIT), yields a solid formed by 100% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only hydromagnesite.

[0126] Figure 6A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 145°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with addition of potassium sulphate (K22+SO42‘) 0.34 M. Fig. 6A shows the characteristic diffraction peaks of magnesite and the characteristic peaks of hydromagnesite. Thus, the peaks confirm the formation of 10% magnesite and 90% hydromagnesite.

[0127] Figure 6B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 145°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 6B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite. Therefore, the microwave carbonation of brucite at 145°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as potassium sulphate (K22+SO42‘), yields a solid formed by 10% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only hydromagnesite.

[0128] Figure 7A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with addition of betaine hydrochloride (BHC) 0.34 M. Figure 7A shows the characteristic diffraction peaks of magnesite and the characteristic peaks of hydromagnesite. Thus, the peaks confirm the formation of 8% magnesite and 92% hydromagnesite.

[0129] Figure 7B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 7B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite.

[0130] Therefore, the microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as betaine hydrochloride (BHC), yields a solid formed by 8% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only hydromagnesite.

[0131] Figure 8A shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with addition of choline chloride (CHCL) 0.34 M. Figure 8A shows the characteristic diffraction peaks of magnesite and the peaks characteristic of hydromagnesite. Thus, the peaks confirm the formation of 3.5% magnesite and 96.5% hydromagnesite.

[0132] Figure 8B shows the XRD pattern of the solid precipitate obtained after microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 8B shows the characteristic diffraction peaks of hydromagnesite, while the characteristic peaks of magnesite are absent. Thus, the peaks confirm the formation of 100% hydromagnesite.

[0133] Therefore, the microwave carbonation of brucite at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, in the presence of anionic catalyst, such as choline chloride (CHCL), yields a solid formed by 3.5% magnesite, while the same conditions, in the absence of anionic catalyst, lead to the formation of only the hydromagnesite.

[0134] Based on the results illustrated in Example 1, it can be seen that anionic catalyst additives, such as for example CIT, allow a drastic reduction in temperature at which the magnesite precipitation in a short time is promoted. In fact, for example, with CIT it is possible to obtain a product containing 100% magnesite at a temperature of 100°C with a treatment time of 60 minutes. Other catalysts such as sulphate, allow at the same times to obtain fractions equal to or greater than 10% of magnesite at substantially reduced temperatures compared to a treatment carried out in the absence of anionic catalyst additives. Finally, anionic catalyst additives such as CHCL allow at the same times to obtain detectable magnesite fractions in the products (> 1%) at lower temperatures compared to a treatment carried out in the absence of anionic catalyst additives.

[0135] Example 3 - SEM analysis of the samples obtained according to Example 1

[0136] The samples analysed by SEM for the evaluation of morphology are listed in Table 3.

[0137] Table 3

[0138] Figure 9 shows rhombic crystallites on the right, which represent magnesite precipitates that are formed together with hydromagnesite, giving rise to flake-like crystallites on the left. The crystallites correspond to a composition of 100% hydromagnesite.

[0139] Figure 10 shows the solid precipitate corresponding to 100% magnesite. It is possible to note how crystallites of morphology similar to hydromagnesite are absent.

[0140] Figure 11 shows the solid precipitate corresponding to 96% magnesite. It is possible to note that crystallites of morphology similar to hydromagnesite (i.e. present at 4%) are almost absent.

[0141] Figure 12 shows the solid precipitate corresponding to 100% magnesite. It is possible to note how crystallites of morphology similar to hydromagnesite are absent.

[0142] Example 4 - carbonation by means of different anionic catalyst additives starting from portlandite

[0143] Portlandite suspensions (Sigma-Aldrich ACS reagent >96.0%) with 0.34 M concentration were prepared by dispersing the powder in 200 ml of deionized water with the aid of an ultrasonic bath. The suspensions were directly poured into the reactor's Teflon liner as a base load to maximize microwave irradiation, as the reactivity of portlandite allows carbonation to be conducted at near room temperature.

[0144] The microwave reactor was filled at room temperature with 6 bar of N2 (9.9999%), as measured by an integrated pressure gauge, and the microwave irradiation was continuously emitted to increase initially the temperature of the mixture from room temperature to the setpoint temperature (in the range 30-60°C, as measured by a temperature probe immersed in the base load, heating phase) and to maintain it at the setpoint temperature throughout the treatment duration. The set temperature was 35°C and once reached, 8 bar of CO2 (9.9995%) were injected into the reactor. The temperature was then held for a period of 5 minutes (dwelling phase). The irradiated energy was recorded during the entire treatment period. At the end of the dwelling phase, the reactor was depressurized and opened to recover the slurry. It was centrifuged and the precipitated phase was separated from the supernatant solution. By virtue of the low temperature of the treatment, another test was carried out under the same conditions but in the absence of microwave irradiation, by pre-heating the reactor to 60°C in the presence of 6 bar of N2 and absence of CO2 and waiting for the thermalization of the system at 35°C. Once this temperature was reached, 8 bar of CO2 were injected and the treatment was maintained for 5 minutes making sure that the temperature within this time interval was held at 35°C. At the end of the dwelling phase, the reactor was depressurized and opened to recover the slurry. It was centrifuged and the precipitated phase was separated from the supernatant solution. To test the effect of additives, the reactor was filled with 200 ml tap water and portlandite suspensions with 0.34 M concentration in deionized water and equimolar additive solution were placed in vials inside the reactor. In this configuration it is possible to apply the same identical conditions for different samples, without additives being able to damage the Teflon liner to any extent during heating. However, the flow of microwaves is reduced compared to the direct use of the Teflon liner, as the radiation must first pass through the base load (4 cm thick) before reaching the vials.

[0145] The tested additive is potassium acetate (K+CH3COO ). Table 4 shows the conditions tested.

[0146] Table 4

[0147] Example 5 - XRD analysis of the samples obtained according to Example 4

[0148] The samples obtained according to Example 4 were analysed by powder X-ray diffraction (XRD).

[0149] The anionic catalysts used in Example 4 were judged according to the criterion that an additive performs catalytic activities towards the reaction producing calcite if at low temperatures, for example 35°C and reduced reaction times, for example 5 minutes, a substantially greater fraction of calcite can be detected through X-rays in the precipitates than that obtained in the same treatment in the absence of catalyst.

[0150] Table 5 shows the tests carried out with different anionic catalyst additives at different operating conditions (time and temperature) of steps b) and c). Samples without the addition of additive but with or without exposure to microwaves are also reported, to show the influence of the heating treatment in the formation of calcite from portlandite. The table shows the weight percentages of the crystalline phases identified and the yield of the carbonate conversion process.

[0151] Table 5

[0152] Figure 13 A shows the X-ray diffraction pattern of the solid precipitate obtained after microwave carbonation of the portlandite at 160°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst. Figure 13 A shows the diffraction peaks corresponding to Miller indices 100, 1 0- 2, 104, 006, 2 -1 0, 2 -1 3 characteristic of calcite, while the peaks corresponding to Miller indices 001, 100 and 101 characteristic of portlandite are absent. Thus, the peaks confirm the formation of 100% calcite.

[0153] Figure 13B shows the x-ray diffraction pattern of the solid precipitate obtained after microwave carbonation of the portlandite at a temperature of 190°C and 30 minutes dwelling time, with 8 bar CO2 + 6 bar N2, without anionic catalyst. Figure 13B shows the characteristic diffraction peaks of calcite, while the characteristic peaks of portlandite are absent. Thus, the peaks confirm the formation of 100% calcite.

[0154] Figure 13C shows the X-ray diffraction (XRD) pattern of pristine portlandite powder.

[0155] Figure 14A shows the X-ray diffraction pattern of the solid precipitate obtained after microwave carbonation of the portlandite at 35°C for 5 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst and in Teflon liner. Figure 14A shows the characteristic diffraction peaks of calcite, and the characteristic peaks of portlandite are absent. Thus, the peaks confirm the formation of 100% calcite.

[0156] Figure 14B shows the X-ray diffraction pattern of the solid precipitate obtained after carbonation of the portlandite at 35°C for 5 minutes, in the absence of microwaves, and in the presence of 8 bar of CO2 and 6 bar of N2, without anionic catalyst and in Teflon liner. Figure 14B shows the characteristic diffraction peaks of calcite, and the characteristic peaks of portlandite are still present. Therefore, the peaks confirm the formation of calcite at 64% and a portlandite residue of 36%.

[0157] Figure 14C shows the X-ray diffraction (XRD) pattern of pristine portlandite powder.

[0158] Therefore, from Figures 14A-B it is possible to confirm the influence of the continuous microwave treatment of steps b) and c) of the method of the invention, in order to lead to an effective carbonation of portlandite to obtain calcite, even at reduced temperatures and for short times (35°C for 5 minutes), i.e. going from a concentration of calcite in the final product of 64 to 100%.

[0159] Figure 15A shows the X-ray diffraction pattern of the solid precipitate obtained after microwave carbonation of the portlandite at 35°C for 5 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, with the addition of K-acetate (CH3COO). Figure 15A shows the characteristic diffraction peaks of calcite, and the characteristic peaks of portlandite are still present. The diffractogram corresponds to 45% calcite and 55% portlandite, with a carbonation yield of 31%.

[0160] Figure 15B shows the X-ray diffraction pattern of the solid precipitate obtained after microwave carbonation of the portlandite at 35°C for 5 minutes, in the presence of 8 bar of CO2 and 6 bar of N2, without the addition of anionic catalyst. Figure 15B shows the characteristic diffraction peaks of calcite, and the characteristic peaks of portlandite are still present. The diffractogram corresponds to 22% calcite and 78% portlandite with a carbonation yield of 16%.

[0161] Therefore, from Figures 15A-B it is possible to confirm the influence of the addition of the anionic catalyst in steps b) and c) of the method of the invention, in order to lead to an effective carbonation of portlandite to obtain calcite, even at reduced temperatures and for short times, i.e. going from a calcite concentration in the final product of 22 to 45%. Example 6 - SEM analysis of the samples obtained according to Example 4

[0162] The sample obtained according to Example 4, starting from portlandite in Teflon liner by carbonation at 35°C for 5 min, 8 bar CO2 and 6 bar N2 in the absence of anionic catalysts, characterized by XRD and shown in Figure 14 A, was shown in Figure 16.

[0163] Example 7 - Raman analysis of the samples obtained according to Examples 1 and 3

[0164] The samples obtained according to Examples 1 and 3 were analysed by Raman spectroscopy for the qualitative and quantitative analysis of the crystalline phases.

[0165] In Figure 17, the spectral magnification 1040-1180 cm1was analysed, where the symmetrical stretching vibration of the CO32group is present, which provides most of the information on the composition and structure of the sample. The peak at 1085 cm1is characteristic of pure calcite, that at 1090 cm1of pure magnesite, and that at 1115 cm1of hydromagnesite. From the spectrum it is evident that, following carbonation starting from portlandite in Teflon liner according to Example 4, at 35°C and 5 minutes dwelling time, with 8 bar of CO2 + 6 bar of N2, without anionic catalyst, the material obtained is 100% calcite, confirming the XRD of Figure 14A.

[0166] Following carbonation of brucite according to Example 1 at 100°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with the addition of 0.17 M potassium citrate (CIT), as confirmed by Figure 5A, the peak of hydromagnesite is totally absent, while the material proves to be 100% magnesite.

[0167] Following carbonation of brucite according to Example 1 at 190°C for 30 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with the addition of 0.34 M potassium mal onate (MAO), as confirmed by Figure 3 A, the peak of hydromagnesite is totally absent, while the material proves to be 100% magnesite.

[0168] Following carbonation starting from brucite according to Example 1 at 145°C for 60 minutes, in the presence of 8 bar of CO2 and 6 bar of N2 with the addition of 0.34 M potassium sulphate (SOT), as confirmed by Figure 6A, the peak of hydromagnesite is present, and the material proves to be 10% magnesite and 90% hydromagnesite.

Claims

- 25 -CLAIMS1. A method for capturing and storing carbon dioxide comprising or consisting of: a) providing a reactor containing water; b) dissolving carbon dioxide in water with the formation of a saturated solution of carbon dioxide; and c) dispersing mineral raw material in the saturated solution of step b), with precipitation of anhydrous mineral carbonates, or alternatively b’) dispersing mineral raw material in the water of step a), and c’) dissolving carbon dioxide in water from step b’) with the formation of a saturated solution of carbon dioxide and precipitation of anhydrous mineral carbonates, characterised in that (i) steps b) and c) or b’) and c’) are carried out in the presence of anionic catalyst additives and in that (ii) microwaves are continuously applied throughout steps b) and c) or b’) and c’).

2. The method of claim 1 wherein said anionic catalyst additives are anionic species that facilitate the precipitation of anhydrous mineral carbonates by non-covalent interaction with an alkaline-earth cation released by the mineral raw material.

3. The method of claim 2, wherein the anionic catalyst additives are selected from the group comprising or consisting of potassium or sodium formate, acetate, oxalate, fumarate, malate, malonate, succinate, tartrate, citrate, ethylenediaminetetraacetate (EDTA), hydrogen carbonate, iodide, sulphate, nitrate; lactic acid, betaine, betaine hydrochloride, choline chloride.

4. The method of claims 1-3, wherein the mineral raw material is brucite and the catalyst is selected from the group comprising or consisting of potassium or sodium fumarate, malonate, tartrate, succinate, ethylenediaminetetraacetate, citrate, hydrogen carbonate and sulphate; betaine hydrochloride; choline chloride.

5. The method of claims 1-3, wherein the mineral raw material is portlandite and the catalyst is selected from the group comprising or consisting of potassium or sodium acetate, tartrate and nitrate.

6. The method of claim 1, wherein microwaves are applied with frequencies comprised between 300 MHz (0.3 GHz) and 300 GHz, preferably between 900MHz and 30 GHz.

7. The method according to claim 1, wherein the carbon dioxide (CO2) partial pressure in step b) or c’) is comprised in a range from 1 to 14 bar, preferably of 8 bar.

8. The method according to claim 1, wherein the saturated solution of carbon dioxide of step b) or c’) is characterized by a pH comprised from 3.5 to 4.5, preferably of 4.

9. The method according to claim 1, wherein the mineral raw material comprises magnesium- rich minerals, calcium-rich minerals, or mixtures thereof, preferably it is serpentine ([Mg3Si2Os(OH)4]), brucite [Mg(0H)2] or portlandite [Ca(OH)2], more preferably it is brucite [Mg(0H)2] or portlandite [Ca(OH)2].

10. The method according to claim 1, wherein in step c) or b’), the mineral raw material is brucite or portlandite which is dispersed with a concentration comprised in a range from 0.1 to 0.5 M with respect to the final dispersion, preferably of 0.34 M or 0.17 M.

11. The method of claim 1, wherein the water dispersion of mineral raw material is characterized by a pH comprised between 9 and 10.5.

12. The method of claim 1, wherein the carbonation steps b) and c) or b’) and c’) take place at a temperature equal to or lower than 190°C, preferably equal to or lower than 160°C, more preferably lower than 130°C, even more preferably equal to or lower than 100°C.

13. The method of claim 1, wherein the carbonation steps b) and c) or b’) and c’) take place in a time comprised from 30 to 120 minutes, preferably of 30 minutes or 5 minutes.

14. Use of anionic catalyst additives as defined in claims 2-5, in combination with continuous microwave treatment, in the precipitation of anhydrous mineral carbonates, preferably magnesite or calcite, from an aqueous dispersion of mineral raw material saturated with carbon dioxide.

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