Methods for the sequestration of carbon dioxide and carbon using natural substances

Abstract

A process for the carbonation of calcium silicates, or alternatively other mixed silicates containing calcium, by electrolysis of water with calcium sulfate as the electrolyte and optionally solid calcium sulfate as a diaphragm, is disclosed. The process uses and produces only natural minerals (lime and silica) that are readily suitable for landfill disposal. The electrolytic reaction can be carried out as a geochemical reaction or used to produce building materials.

Description

[0001] The present invention relates to an electrochemical process for binding and sequestering carbon dioxide and carbon, in which only natural substances are used and only mineral substances are produced in addition to hydrogen and oxygen. State of the art

[0002] Reducing greenhouse gases, especially carbon dioxide, in the atmosphere is a major challenge. Besides reducing carbon dioxide emissions, carbon capture and storage (CCS) is an important option. The injection of gaseous carbon dioxide naturally presents technical difficulties (Daval, npj Materials Degradation (2018) 11). Carbon dioxide can be captured from the atmosphere or seawater using alkalis, such as recyclable amines. Irreversible capture, for example, with industrial waste products like fly ash, forming carbonate, is also an option (Sanna et al. 2014, Chem Soc Rev. (CS-REV-01-2014-000035.R1); Xie et al. Engineering 1 (1), 2015, 150-157; Lu et al. 2016, Chemical Engineering Journal 306, 330-335). However, such alkalis are hardly available in the enormous quantities required. Natural silicates such as olivine, wollastonite or pyroxene are an alternative here.Geochemically, they are also naturally converted to the corresponding carbonates during weathering (Daval, npj Materials Degradation (2018) 11). The released silicic acid dehydrates to mineral silica / quartz, and is therefore, unlike salts of other acids, completely unproblematic and represents a natural mineral. However, this occurs very slowly and requires large surface areas, which poses a technical obstacle. Many studies therefore focus on the technically advantageous carbonation of silicates. These are, firstly, purely chemical methods.

[0003] Zhao et al. (Physical Chemistry Chemical Physics, Issue 36, 2013) described the positive effects of calcium ion chelating agents (acetic and gluconic acid) on the dissolution and carbonation of wollastonite.

[0004] Rau et al. (PNAS, 2013, 110 (25), 10095-10100) describe an electrolytic mechanism for hydroxide production and CO2 binding using saline water as electrolyte under reaction of metal silicates and formulate a general chemistry for such electrolyses.

[0005] Di Lorenzo et al. (Minerals 2018, 8(5), 209) test enzymatic and biomimetic catalysts for the carbonation of wollastonite.

[0006] Svensson et al. (SN Applied Sciences 1:318, 2019) report on the carbonation of natural and impure wollastonite using high temperatures and pressures in CO2 treatment.

[0007] Direct carbonation of gypsum via ammonium hydroxide (without silicates as a base) has already been described by Lee et al. (Energy 47 (1), 2012, 370-377).

[0008] Ding et al. (RSC Advances 6 (81), 2016) use ammonium for the carbonation of wollastonite.

[0009] Jin et al. (Brazilian Journal of Chemical Engineering 39, (2022), 661-669) use hydrochloric acid and ammonium in a (non-electrolytic) process for the carbonation of wollastonite, which is similar to the Solvay process.

[0010] Electrochemical methods have also become a focus in the carbonation of wollastonite (Xie et al. Chem. 4 (1), 2018, 24-26; https: / / doi.org / 10.1016 / j.chempr.2017.12.024). Of particular interest is the combination with electrolytic water splitting for hydrogen (and oxygen) production using surplus photovoltaic electricity or other sustainable sources. Here, the byproducts of cathodic hydrogen evolution—hydroxide ions—and anodic oxygen evolution—protons—are also utilized. The acidic environment of the anode chamber can be used to break down the silicate, releasing cations such as calcium and forming silicic acid. The alkaline environment of the cathode chamber is used for carbonate precipitation of the migrating calcium ions from the anode chamber.For both half-reactions, conditions are therefore considerably more favorable than for the direct reaction of wollastonite (calcium silicate) with the weak carbonic acid from carbon dioxide.

[0011] Xie et al. (Environmental Earth Sciences (2018) 77:149) described the CO2 mineralization of natural wollastonite in silica and CaCO3 powder using membrane electrolysis with ammonium chloride as the electrolyte, thus avoiding the disadvantages of chlor-alkali electrolysis.

[0012] Scott et al. (Communications Earth & Environment, 2021, 2:25)2021 describe electrolytic CO2 sequestration using magnesium silicate minerals.

[0013] Gadioka (Communications Chemistry (2021), 4:23) summarizes known methods for mineral CO2 binding.

[0014] Feng and Hicks summarize methods for forced wollastonite carbonation (Journal of Cleaner Production 414, 2023, 137625).

[0015] Electrochemical carbon capture and concentration (eCCC) methods are summarized by Zito et al. (Chemical Reviews 2023, https: / / doi.org / 10.1021 / acs.chemrev.2c00681).

[0016] Some patent applications and patents found in the patent literature correspond to the cited publications. No patents that would impair novelty for the process described here have yet been found. Object of the invention

[0017] The challenge was to find an electrochemical reaction for carbon dioxide sequestration that did not use or produce any problematic substances. Ideally, only naturally occurring substances should be used and produced in addition to hydrogen and oxygen during electrolysis. The products would be exclusively solid and chemically stable. Furthermore, the substances used should be particularly inexpensive and available in large quantities. Description of the invention

[0018] The state of the art involved the electrochemical conversion of wollastonite to lime and silica with carbon dioxide, but this required the use of various electrolytes, primarily chlor-alkali electrolytes and ammonium ions. Both methods, if implemented on an industrial scale, could release volatile or problematic substances in solution, such as chlorine and ammonia, or at least require complex technical controls.

[0019] The disclosed invention relates to carrying out the electrochemical conversion of wollastonite with carbon dioxide to lime and silica using a special electrolyte, which solves these problems and opens up further and surprising technical possibilities.

[0020] The precipitation of alkaline earth carbonates as a stable solid form of carbon dioxide reservoir results from the presence of calcium and / or magnesium in the silicate starting materials such as wollastonite and has long been known to those skilled in the art. The use of alkaline earth ions, and especially calcium ions, in the electrolyte, however, is rather unusual. It offers the advantage that carbonate precipitation can also be achieved directly with ions from the electrolyte present in both chambers, thus becoming less dependent on the migration of calcium ions from the anode to the cathode chamber, which is technically advantageous for the process. The calcium cations are precipitated from the electrolyte as carbonate, but are simultaneously released again anodically from the wollastonite decomposition. Due to the identity of the electrolyte cations with those of the silicate, they are not consumed net and thus have only a catalytic function for the overall reaction.The anion used in the invention was chosen according to the requirement that it be the anion of a strong acid. Furthermore, it should not be easily oxidized anodically and should not form volatile gases. This rules out chloride, which would otherwise be a particularly obvious choice due to the good solubility of calcium chloride. The choice of sulfate is not obvious, as the various forms of calcium sulfate are known to be poorly soluble (anhydrite CaSO4, calcined gypsum CaSO4 × ½ H2O, gypsum CaSO4 × 2 H2O). However, gypsum has a modest solubility of 2.1 g / L, which explains both its natural occurrence as a mineral and its susceptibility to erosion in the presence of surface water. It can, however, be readily disposed of as industrial gypsum in the absence of water. With the decline in the operation of coal-fired power plants, industrial gypsum has recently become scarcer.More problematic due to the accompanying substances, usable for few uses and available on a large scale is the so-called phosphogypsum from phosphate production with sulfuric acid; this is often sent to landfills.

[0021] The behavior of this counterintuitive electrolyte Ca 2+ SO4 2- was found to be sufficient in experiments (Example 1). Thus, calcium sulfate was found to be an electrolyte that, like the other products lime and silica, occurs as a natural mineral and is unproblematic because it can be disposed of in landfills (Claims 1, 2, 3). The electrolyte effect can likely be further enhanced by known additives that increase the solubility of calcium ions, such as chelating agents, especially polyols.

[0022] Further development work revealed that gypsum can also be used very advantageously as a material for the diaphragm separating the electrode chamber (Example 4). First, the structure of interlocking gypsum needles (as used in gypsum as a building material) provides a mechanically stable yet permeable structure. Second, the diaphragm thus simultaneously serves as a reservoir for the electrolyte, since calcium ions precipitate as hydroxide and then as carbonate—as intended—in the anode chamber, and especially at the interface with the diaphragm, and are therefore consumed. Third, the sulfate anions released in the cathode chamber and the calcium cations released in the anode chamber can each migrate to the diaphragm and recombine there as CaSO₄ × 2 H₂O, thus compensating for any dissolution losses in the diaphragm. The diaphragm material is therefore in dynamic equilibrium with the electrolyte of the same substance.Fourthly, the choice of gypsum as the diaphragm material also makes it depotable, which appears particularly advantageous for discontinuous process operation with depotting of the entire electrolysis device (claim 4).

[0023] The described process can be carried out with stoichiometric amounts of gypsum. In this case, the gypsum is cathodically carbonated, and the released sulfuric acid is neutralized in the anode chamber by wollastonite, releasing silica. However, this is only possible with an equimolar amount of gypsum and wollastonite at most. On the other hand, according to the invention (as with other electrolytes in pure water electrolysis), catalytic amounts of the electrolyte calcium sulfate are sufficient for the carbonation of wollastonite or other corresponding silicates. When using the described gypsum diaphragm, the quantity and concentration correspond to the solubility in the chamber volumes, provided the electrolyte is not already present in saturated solution. For process efficiency, this saturated electrolyte concentration should not be undercut, even if catalytic amounts would suffice.

[0024] The choice of the specific electrolyte, its material identity with the diaphragm material, and its unproblematic disposal properties also give rise to further, particularly interesting embodiments of the new process, namely a semi-solid form for a geochemical reaction at the disposal site. Here, a wollastonite / (calcined) gypsum mixture (or a corresponding other silicate) is suspended in water. Gypsum again assumes the function of the electrolyte and—without a distinct form—together with wollastonite, the diaphragm function, preventing convection but allowing ion migration (Example 5) (Claims 4, 5, 6). In this embodiment, it is particularly interesting to select the electrodes for the described process from graphitic material (Example 5), which is especially advantageously obtained by the pyrolysis of biological material such as wood or other materials, and can serve as an additional disposal form for carbon (Claim 7).The electrode material can also be a finely dispersed, graphitic material and part of the semi-solid suspension according to the invention. In this case, compared to conventional electrodes, it serves to increase the electrolytically active surface area and to promote a more evenly distributed gas evolution (hydrogen, oxygen), thus ensuring better contact with the electrolyte solution.

[0025] Another embodiment, based on a semi-solid mixture of wollastonite and (calcined or hydrated) gypsum, serves for carbon sequestration in building materials. For this purpose, the semi-solid mixture is formed into a sheet-like shape to create building material panels (claim 8). The cathodes and anodes are alternately positioned on the outer surface (or on both sides). The sheet-like shape must allow gas exchange, at least in the area of ​​the electrodes (hydrogen and oxygen from electrolysis; carbon dioxide for carbonation). It is technically advantageous for the cathodes to be encased in tubes closed towards the gas space, so that the hydrogen produced can be removed separately. Electrolysis and carbonation can also be carried out sequentially. Both the cathodic carbonation and the sheet-like gypsum conversion can be used for the mechanical strengthening of the building material panels. Definitions

[0026] Polyols are multiple alcohols, for example glycerol (glycerin) or sucrose.

[0027] Silica / silicic acid refers here, for formal reasons, to the anhydrite SiO2 of silicic acid H2SiO4, since no statement can be made about the degree of hydration within the scope of the invention, and the term quartz describes a specific crystal structure. The chemical equations are also formulated using the formula unit [SiO2]. Sources of chemicals plaster Chemically pure, fully hydrated CaSO4 × 2 H2O from Roth. >Industrial gypsum Knauf brand calcined gypsum (CaSO4 × 1 / 2 H2O). Unless otherwise stated, industrial gypsum was used in the examples and fully hydrated only during the experiment. Graphite pencils Faber Castell HB 1.0 lead pencils were used as graphite electrodes. hydrochloric acid Roth, own dilution. sulfuric acid Roth, own dilution. Water Distilled water, Kerndl Autochemie. Wollastonite Ca3[Si3O9], [CaSiO3] Andrea Wolbring GmbH & CoKG, finely powdered. Abbreviations g grams L L Examples Example 1: Water electrolysis with calcium sulfate as electrolyte

[0028] 0.5 liters of saturated calcium sulfate solution were prepared. For this, 1.5 g of calcined gypsum (CaSO₄ × 1 / 2 H₂O) was mixed with 0.5 liters of water, stirred, and the undissolved excess was allowed to settle (solubility 2.1 g / L). The 0.5 L supernatant was transferred to an electrolysis chamber. The electrical resistance of the filled chamber was 38 kΩ. The chambers were separated by a paper diaphragm (Figure 2). A voltage of 100 V was applied for 30 minutes. As expected, gas evolution (hydrogen) was observed at the cathode, twice as strong as at the anode (oxygen). Additionally, a spreading turbidity emerged from the cathode in the cathode chamber; the precipitation of calcium hydroxide was expected and observed (Figure 3). After completion of the electrophoresis, a pH of 11.3 was measured in the cathode chamber and 2.75 in the anode chamber. The contents of the cathode chamber were left exposed to air overnight.The precipitate reacted with 1 N hydrochloric acid to form gas bubbles, as would be expected from calcium carbonate.

[0029] The experiment was repeated with a saturated solution of chemically pure calcium sulfate (CaSO4 × 2 H2O) and a paper diaphragm. The current flow was also observed.

[0030] The initial pH of the electrolyte in the chamber was 6.66. A voltage of 100 V was then applied, and a current of 76 mA was measured. Strong gas evolution was observed at the electrodes, twice as strong at the cathode as at the anode. After one hour, the current was 38 mA, after two hours 25 mA, and after three hours and before termination, it was 20 mA. Simultaneously, the gas evolution gradually decreased. Calcium hydroxide, which had not yet carbonated, was again found floating in the cathode chamber. The cathode chamber had a pH of 11.51, and the anode chamber a pH of 2.56. Example 2: Reaction of wollastonite with sulfuric acid

[0031] 20 mg of wollastonite Ca3[Si3O9] or (CaSiO3) were mixed with 1 mL of 1 N sulfuric acid H2SO4 and incubated for 1 day at room temperature. Control reactions consisted of 20 mg of wollastonite with either 1 mL of water or 1 mL of 1 N hydrochloric acid HCl (Figure 4).

[0032] The supernatants of each reaction were pipetted onto microscope slides (0.1 mL each) and dried overnight (Figures 5, 6, 7). In the reaction with sulfuric acid, the formation of gypsum (CaSO4 × 2 H2O) can be detected by its characteristic needle-shaped crystal form. Example 3: Electrolytic carbonation of wollastonite with a saturated solution of calcium sulfate as electrolyte

[0033] 5 g of wollastonite were weighed out and placed in the anode chamber. The electrolysis apparatus was filled with 0.5 L of saturated calcium sulfate solution (1 g CaSO₄ × 2 H₂O) and a paper diaphragm was attached. The reaction was started with 150 V and the gas evolution was observed. As soon as the gas evolution subsided, the wollastonite in the anode chamber was stirred and left to stand for 5 minutes. With the application of voltage, strong gas evolution occurred again. A turbidity formed in the cathode chamber (Figure 8).

[0034] As expected, the sediment in the cathode chamber emits carbon dioxide when exposed to 1 N HCl (hydrochloric acid). Example 4: Electrolytic carbonation of wollastonite with saturated solution of calcium sulfate as electrolyte and calcium sulfate (gypsum) diaphragm

[0035] The electrolysis chamber was fitted with a 1.5 cm thick gypsum diaphragm. A 0.5 L saturated calcium sulfate solution was used as the electrolyte. 5 g of wollastonite were placed in the anode chamber (Figure 9) and magnetically stirred during electrolysis.

[0036] The electrolysis was started at 100 V. Strong gas evolution was observed at the electrodes, and the initial current was 66 mA. After a few minutes, calcium hydroxide clouding appeared in the cathode chamber. After one hour, the voltage was increased to 200 V, and the current was then 53 mA. After another two hours, the electrolysis was stopped. The pH value in the cathode chamber was 11.46, and in the anode chamber, it was 3.52. After electrolysis, the cathode side of the diaphragm, containing 1 N HCl hydrochloric acid, produced carbon dioxide bubbles, while the anode side did not. Example 5: Electrolysis with semi-solid suspension of gypsum and wollastonite and graphite electrodes

[0037] One gram of calcined gypsum (CaSO₄ × 1 / 2 H₂O) and 10 grams of wollastonite were weighed out, mixed in a beaker, and transferred to a Petri dish. Distilled water was added until the mixture was fully saturated and slightly layered. The mixture was then incubated for two hours until the hydration of CaSO₄ × 1 / 2 H₂O to CaSO₄ × 1 / 2 H₂O had occurred; the excess water left the mixture mobile and pasty. Two graphite electrodes were connected with fine copper wires and positioned 4.5 cm apart on the surface of the semi-solid gypsum-wollastonite mixture (Figure 10).

[0038] A voltage of 25 V was applied. Gas evolution was immediately observed at the electrodes. After two hours, the electrolysis was stopped. The pH value of the mixture was measured in the space between the electrodes, 1 cm from each electrode. It was 10.59 next to the cathode and 7.30 next to the anode. Character description Figure 1: Reaction scheme of an electrolytic carbonation of wollastonite and gypsum with calcium sulfate as electrolyte and diaphragm material. The equations with CO₂ were simplified by omitting the hydrogen carbonate / carbonate equilibria with water. Similarly, the equilibria of H₃O in the anode chamber were also omitted. + with SO4 2- Not shown with hydrogen sulfate ions and sulfuric acid. Figure 2: Electrolysis chamber after electrolysis with calcium sulfate as electrolyte. Figure 3: Turbidity in the cathode chamber after electrolysis with calcium sulfate as electrolyte. Figure 4: Reactions of 20 mg wollastonite with 1 mL water (left), 1 mL 1 N hydrochloric acid (center), and 1 mL 1 N sulfuric acid (right). The supernatant clarified after the reaction with hydrochloric acid (center). A bulky solid was formed after the reaction with sulfuric acid (right). Figure 5: Dried supernatant of the reaction mixture with water. Image taken under a binocular microscope, image diameter 5 mm. No residue is visible. Figure 6: Dried supernatant of the reaction mixture with hydrochloric acid. Image taken under a binocular microscope, image diameter 5 mm. Droplets of hygroscopic calcium chloride solution are visible. Figure 7: Dried supernatant of the reaction mixture with sulfuric acid. Image taken under a binocular microscope, image diameter 5 mm. Typical gypsum crystals are visible. Figure 8: Electrolysis of wollastonite with calcium sulfate as the electrolyte. The turbidity caused by calcium carbonate can be seen in the cathode chamber (left). Figure 9: Prepared electrolytic carbonation with saturated solution of calcium sulfate as electrolyte and calcium sulfate (gypsum) diaphragm and stirred wollastonite in the anode chamber (right). Figure 10: Electrolysis with semi-solid suspension of gypsum and wollastonite and graphite electrodes. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0000] Sanna et al. 2014, Chem Soc Rev. (CS-REV-01-2014-000035.R1); Xie et al. Engineering 1 (1), 2015, 150-157; Lu et al. 2016, Chemical Engineering Journal 306, 330 - 335

[0002] Daval, npj Materials Degradation (2018) 11

[0002] Zhao et al. (Physical Chemistry Chemical Physics, Issue 36, 2013

[0003] Rau et al. (PNAS, 2013, 110 (25), 10095-10100

[0004] Di Lorenzo et al. (Minerals 2018, 8(5), 209

[0005] Svensson et al. (SN Applied Sciences 1:318, 2019

[0006] Lee et al. (Energy 47 (1), 2012, 370-377

[0007] Ding et al. (RSC Advances 6 (81), 2016

[0008] Jin et al. (Brazilian Journal of Chemical Engineering 39, (2022), 661-669

[0009] Xie et al. Chem. 4 (1), 2018, 24-26; https: / / doi.org / 10.1016 / j.chempr.2017.12.024

[0010] Xie et al. (Environmental Earth Sciences (2018) 77:149

[0011] Scott et al. (Communications Earth & Environment, 2021, 2:25)2021

[0012] Gadioka (Communications Chemistry (2021), 4:23

[0013] Journal of Cleaner Production 414, 2023, 137625

[0014] Zito et al. zusammengefasst (Chemical Reviews 2023, https: / / doi.org / 10.1021 / acs.chemrev.2c00681

[0015]

Claims

[1] Electrochemical process for the carbonation of wollastonite, pseudowollastonite, rock flours of skarn, or other calcium silicates such as pyroxenes, as well as mixed silicates containing calcium, or alternatively magnesium silicates, and gypsum, in which carbon dioxide can be used and bound in various forms, calcium carbonate is precipitated in the cathode chamber, silica is released from the silicates in the anode chamber, and calcium sulfate, preferably in the form of gypsum CaSO4 × 2 H2O, is used as a special electrolyte (the electrolyte solution may also contain complexing agents or polyols to increase the solubility of the calcium sulfate), and in which water is electrolyzed in the electrolysis chambers in parallel with hydrogen and oxygen. [2] Method according to (1) in which the carbon dioxide, for example in the form of air or technical gases from combustion or production processes, is used in free gaseous form, as dissolved in water or in chemical equilibrium form as hydrogen carbonate or carbonate dissolved in water. [3] Method according to (1, 2) wherein solid calcium sulfate, preferably in the form of gypsum CaSO4 × 2 H2O, is also used as material for the diaphragm of the electrolysis device; the diaphragm may also consist of mixtures of calcium sulfate and wollastonite (or the substitute silicates according to claim 1). [4] Method according to (1, 2, 3) in which no fixed electrolysis chamber is used, but a semi-solid water-moist suspension of a calcium silicate and calcium sulfate, preferably in the form of gypsum CaSO4 × 2 H2O, which performs the functions of both the electrolyte and the diaphragm, and the whole apparatus can be designed to be depositable. [5] Method according to (1, 2, 3, 4) wherein the method is carried out as a geochemical reaction on or in the ground, in boreholes or in shafts. [6] The process according to (1, 2, 3, 4, 5) which is carried out in a discontinuous form and in which all the substances used can be disposed of together after the process has been completed or the entire electrolysis apparatus can be disposed of. [7] Method according to (1, 2, 3, 4, 5, 6), especially according to (4), wherein a pyrolyzed biological material is used as the carbon electrodes and the electrode material serves as an additional bound form of carbon and, according to (6), is preferably also landfillable; the pyrolyzed carbonaceous electrode material may additionally be added to the suspension in finely dispersed form. [8] Another embodiment based on a semi-solid mixture of wollastonite and (calcined or hydrated) gypsum serves for carbon deposition in building materials, in which the semi-solid mixture is formed into a flat shape to create building material panels; the cathodes and anodes are alternately placed on the outside of the surface (or on both sides of the surfaces); the flat shape allows gas exchange (hydrogen and oxygen from electrolysis; carbon dioxide for carbonation) at least in the area of ​​the electrodes; it is particularly advantageous for the cathodes to be encased in tubes closed towards the gas space, so that the hydrogen produced can be carried out separately; electrolysis and carbonation can also be carried out sequentially.

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