SEQUESTRATION OF CO

Description

[0001] The invention relates to a method for the sequestration of CO2.

[0002] Carbon dioxide (CO₂) acts as a greenhouse gas in the atmosphere and is considered one of the main causes of human-induced global warming. In addition to the fundamental reduction of CO₂ emissions, efforts are also being made to capture CO₂ already present in the atmosphere. CO₂ capture can also be used to make manufacturing processes that generate large amounts of CO₂, such as cement production, CO₂-neutral.

[0003] CO₂ sequestration refers specifically to the removal of CO₂ from the atmosphere, ideally in such a way that the CO₂ binds to other substances so that it cannot escape again. Various proposals exist for this purpose. One possibility is the use of olivine.

[0004] Olivine is a mineral with the general composition A 2 [SiO 4 ], where A can be represented by various divalent ions such as magnesium (forsterite, Mg 2 SiO 4 ), iron (Fe 2 SiO 4 , fayalite ), manganese (Mn 2 SiO 4 , tephroite ) as well as other ions and a combination of the different cations, since olivine is a mixed crystal series.

[0005] The patent literature on accelerating carbon dioxide sequestration in a reaction with olivine is very extensive. Many CO₂ sequestration processes using olivine are based on the following principle and differ mainly in the method of reaction acceleration: Mg₂SiO₄ + 2 CO₂ → 2 MgCO₃ + SiO₂

[0006] According to WO 2007 / 069902, the reaction should be accelerated by grinding and adjusting specific pH values; WO 2008 / 140821 and WO 2008 / 061305 suggest accelerating the reaction by high temperatures, high CO₂ partial pressures, and high fineness of the olivine. WO 2008 / 101293 suggests the addition of ammonium, WO 2007 / 106883 the addition of a base, and US 4944928 the addition of hydrochloric acid.

[0007] Besides these methods, there are a large number of other methods that are able to accelerate the sequestration of carbon dioxide in the reaction with olivine, all of which use complicated technologies and expensive starting materials.

[0008] The CO2min project attempted to combine the carbonation of olivine with cement production. The goal is to capture CO₂ emissions from the cement industry through the carbonation of olivine. Accordingly, cement production continues unchanged, maintaining the existing high-temperature process. The resulting carbon dioxide is bound through a reaction with olivine in the form of magnesium carbonate and SiO₂. This byproduct can then be added to the cement and thus disposed of. However, it is evidently not possible to completely convert the olivine into magnesium carbonate (see D. Kremer, H. Wotruba: Separation of products from mineral sequestration of CO₂ with primary and secondary materials. Minerals, Vol. 10 (2020), pp. 1098 ff). .The project used very high CO₂ partial pressures (17 bar) and high temperatures (175°C). Despite autoclave treatment, the conversion rate of the olivine was only 23%, and very large quantities of olivine would be needed to compensate for this low conversion rate. The chosen process technology is also likely unsuitable for carbon dioxide sequestration on an industrial scale.

[0009] Another idea for binding carbon dioxide through a reaction with olivine was also developed in the Netherlands; see RD Schuiling, P. Krijgsman: Enhanced weathering: An effective and cheap tool to sequester CO2. Climate change, Vol. 74 (2006), 349-354. In this method, olivine is spread on arable land or distributed on beaches. If the olivine dissolves into magnesium ions, the precipitation or seawater could simultaneously absorb carbon dioxide in the form of HCO3- ions according to the following equation: Mg2SiO4 + 4 CO2 + 2 H2O → 2Mg2+ + 4 HCO3- + SiO2

[0010] However, a laboratory experiment at the University of Hamburg showed that the described reaction does not occur in the manner depicted. Even if the entire arable land of the world were sown with olivine, it would only capture 0.2% of global carbon dioxide emissions (see T. Amann et al.: Enhanced weathering and related element fluxes – a cropland mesocosm approach. Biogeosciences, Vol. 17 (2020), 103–109). .

[0011] WO 2008 / 061305 A1 discloses a step-by-step process for the sequestration of CO2 using serpentine as a starting material; the serpentine does not undergo the intermediate step of hydrothermal conversion before being dehydrated and subsequently carbonated.

[0012] The invention therefore lies in the Task The basis is to specify an efficient method for the sequestration of CO2.

[0013] This problem is solved according to the invention by a method having the features of claim 1.

[0014] Further advantageous embodiments are specified in the dependent claims, the further description and the exemplary embodiments.

[0015] According to claim 1, a starting product is provided which contains at least 20 wt%, preferably at least 40 wt%, more preferably at least 60 wt%, and even more preferably at least 80 wt%, of one or more of the following components. These components can be ultramafic rocks, such as dunite, weathering products of ultramafic rocks, such as serpentinite, olivine, or industrial by-products. It is essential that all these raw materials each have an MgO concentration or MgO content of at least 10 wt%, preferably 20 wt%, and even more preferably over 30 wt%, and ideally over 40 wt%. This starting product is provided with a fineness corresponding to a BET surface area of ​​0.1 m² / g or finer. A BET surface area of ​​0.5 m² / g is advantageous, and a BET surface area of ​​1.0 m² / g or finer is even more preferred.

[0016] Olivine is an important mineral in ultramafic rocks. It is a solid solution series consisting of fayalite (Fe₂SiO₄), forsterite (Mg₂SiO₄), tephroite (Mn₂SiO₄), and other minerals of the form A₂[SiO₄]. Natural olivine occurrences are documented, and the olivine is often a magnesium-rich material with iron content.

[0017] Examples of industrial waste materials that can be used within the scope of the invention are foundry sand or refractory materials.

[0018] After the raw material has been prepared, it is homogenized if necessary. The potential components of the raw material are often natural rocks or materials from natural deposits. Experience has shown that these are rarely found in pure or homogenized form. Homogenization can be carried out, for example, with a mixer or simultaneously during the grinding process to the desired fineness.

[0019] Following homogenization, the prepared starting material undergoes hydrothermal treatment. This takes place in a heat treatment unit at a temperature above 100 °C for at least 24 hours. The heat treatment unit can be, for example, a heat tunnel or an autoclave. An autoclave is generally understood to be a gas-tight, sealable pressure vessel that can be used for the thermal treatment of substances under pressure. It is preferred if the heat treatment unit is a vessel, a combination of devices such as a furnace and a sealed mold, or a device for enclosing a volume. The treatment preferably takes place at temperatures above 100 °C, particularly above 150 °C, and even more preferably above 250 °C. For optimal results, it is advantageous if the treatment is carried out for longer than 36 hours, and even more preferably for longer than 48 hours.Particularly good results can be achieved if the treatment is carried out for a longer period of time, for example 4, preferably 7 days or longer.

[0020] Furthermore, water is added to the homogenized starting material by direct addition before, after, and / or simultaneously with homogenization in the preceding step, and by mixing the water with the starting material. Alternatively or additionally, steam can also be introduced into the heat treatment unit.

[0021] During the hydrothermal treatment of the homogenized starting material, it is at least partially transformed into magnesium hydroxide (Mg(OH)₂) and magnesium silicate hydrates (Mg₃Si₂O₅(OH)₄, Mg₃Si₄O₁₀(OH)₂) by the presence of H₂O. The underlying reactions are as follows, simplified here starting with forsterite (Mg₂SiO₄): (1) 2 Mg₂SiO₄ + 3 H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂ (2) 3 Mg₂SiO₄ + 5 SiO₂ + 2 H₂O → 2 Mg₃Si₄O₁₀(OH)₂, with reaction (1) occurring primarily. This is also preferred within the scope of the invention, since Mg3Si2O5(OH)4 is better suited for binding CO2 than Mg3Si4O40(OH)2. Furthermore, it should be taken into account that the amount of the respective products and their ratio depend, among other things, on the exact composition of the starting material.

[0022] Magnesium hydroxide (Mg(OH)₂) can be present as brucite. Magnesium silicate hydrate (Mg₃Si₂O₅(OH)₄, Mg₃Si₄O₁₀(OH)₂) can be present as lizardite, antigorite, talc, and other forms. It should be noted that the stoichiometric water content is sometimes lower—around 13 wt% for antigorite—than the value determined experimentally—ranging from 16 wt% to 20 wt%. This can be explained by the fact that the materials are sometimes so fine that water can adhere to their surface.

[0023] Similarly, such deviations from stoichiometry also apply to the ratio between Mg and Si. Furthermore, foreign ions, such as Fe, can be incorporated into the reaction products. Other reaction products, such as hydromagnesite, hematite, magnetite, or gibbsite, can also be formed. This depends on the exact composition of the starting material. All or some of the reaction products may contain iron, carbonate, alkalis, or other foreign ions.

[0024] Subsequently, the transformed starting material is at least partially dehydrated of bound water by thermal treatment and / or reaction milling. Bound water is sometimes also referred to as water of crystallization. It must be distinguished from unbound water, which can be considered free H₂O. Complete dehydration can be achieved with considerable effort. According to the invention, the water content of bound water should be reduced by at least 60%, preferably by at least 80%, and even more preferably by at least 90%.

[0025] For thermal treatment, the converted starting material can be heated to a temperature between 180 °C and 1000 °C. Depending on the desired fineness, heating for just a few minutes is sufficient. Temperatures between 300 °C and 800 °C are preferred, and temperatures between 500 °C and 700 °C are even more advantageous. Alternatively or additionally, the converted starting material can also be subjected to reaction milling to rearrange the crystal structures. In this so-called reaction milling process, crystalline water can also be removed from the converted starting material by rearranging the crystal structures. For this purpose, additives such as quartz can be added during milling.

[0026] In this step, dehydration converts at least some of the magnesium hydroxide (Mg(OH)₂) present in the converted starting material into magnesium oxide (MgO), and at least some of the magnesium silicate hydrate present (Mg₃Si₂O₅(OH)₄, Mg₃Si₄O₁₀(OH)₂) into dehydrated magnesium silicate hydrate, which can be simplified as xMgO·SiO₂·yH₂O. Dehydration here refers to the reduction of the crystalline water, or water of crystallization, in the converted starting material.

[0027] The underlying chemical processes are, again simplified, as follows: (3) Mg(OH)₂ → MgO + H₂O (4) Mg₃Si₂O₅(OH)₄ → 2 xMgO·SiO₂ ·yH₂O + zH₂O (5) Mg₃Si₄O₁₀(OH)₂ → 2 aMgO·SiO₂ ·bH₂O + cH₂O where (4) yields a largely amorphous reaction product with a Mg to Si ratio of 1.5 to 2 and a bound water content of about 3%. The reaction product formed in equation (5) has an even lower Mg to Si ratio.

[0028] Hence the variables a, b, c, x, y and z. Each depends on the exact composition of the starting product and the treatment parameters.

[0029] After dewatering, the water content of the bound water in the converted, dewatered starting product is preferably below 10 wt%, advantageously below 5 wt%, more preferably below 3.5 wt%, and even more preferably below 2.5 wt%.

[0030] The transformed and dehydrated starting material thus exists as a multiphase product. Other possible secondary phases include hematite, magnetite, enstatite, feldspars, pyroxenes, quartz, and amorphous phases.

[0031] Following dehydration of the converted starting material, it is contacted with CO₂. The CO₂ reacts with the magnesium oxide (MgO), the dehydrated magnesium silicate hydrate (MgO·SiO₂·yH₂O), and / or the magnesium hydroxide (Mg(OH)₂). The CO₂ is primarily bound in the magnesium carbonate (MgCO₃) and / or magnesium carbonate hydrate (MgCO₃·mH₂O) formed.

[0032] The underlying chemical processes are simplified and generalized as follows: (6) MgO + CO₂ + m H₂O → MgCO₃ · mH₂O (7) xMgO·SiO₂ · yH₂O + q H₂O + xCO₂ → xMgCO₃ · (q+y) / xH₂O + SiO₂ (8) Mg₂SiO₄ + 2 CO₂ + 2n H₂O → 2 MgCO₃ · nH₂O + SiO₂ (9) Mg(OH)₂ + CO₂ + p⁻¹ H₂O → MgCO₃ · pH₂O where m, n, p, q, x, and y represent corresponding variables. Some of these may be zero. Furthermore, it should be noted that in equation (7), the dehydrated magnesium silicate hydrate appears only as xMgO·SiO₂·yH₂O, since it has been (incompletely) dehydrated. Equation (8) lists forsterite (Mg₂SiO₄), which may still be present. Forsterite from the starting material may still be present at this stage. It should also be noted that xMgO·SiO₂·yH₂O is very similar to forsterite and can be simplified as amorphous forsterite.

[0033] If a starting material is provided which already contains at least 20 wt% magnesium silicate hydrate, preferably at least 40 wt%, more preferably at least 60 wt%, and even more preferably at least 80 wt%, as is the case, for example, with serpentinite, the steps of adding water and hydrothermal treatment can be omitted (not according to the invention). Serpentinite is a metamorphic rock that forms through the natural transformation, in particular weathering, of ultramafic rocks.

[0034] According to the invention, it was recognized that by combining a hydrothermal treatment and the expulsion of water of crystallization from natural materials, such as ultramafic rocks, an intermediate substance can be produced which is suitable to bind CO2 to a high degree.

[0035] Examples of olivine-rich rocks that can be used according to the invention include dunite, wehrlite, and habsburgite. These rocks often have a low degree of weathering. However, weathered rocks with a similar chemical composition but a higher water content can also be used. Examples of such weathered rocks include serpentinite.

[0036] In this way, large quantities of CO2 can be bound with relatively little energy expenditure. For example, it is possible to bind approximately 0.6 tons of CO2 within a few hours using approximately 1 ton of forsterite and the inventive process.

[0037] The hydrothermal treatment of the homogenized starting material for conversion into magnesium hydroxide and / or magnesium silicate hydrate is one of the process steps that takes the longest according to the invention. Therefore, it is preferred to carry out one or more treatments to accelerate the reactions during the hydrothermal treatment. Various treatment methods are available for this purpose, which can be performed individually or in combination. These are explained in more detail below. All or only some of the listed treatment methods can be combined.

[0038] One possibility is to continuously or discontinuously crush or break up the homogenized starting material, in particular to grind it very finely, during the hydro-thermal treatment or between several hydro-thermal treatments in the heat treatment facility in order to accelerate the conversion.

[0039] Continuous or discontinuous comminution can prevent or reduce clumping or agglomeration of the materials during hydrothermal treatment. This ensures that a sufficiently large surface area remains for the processes described above to occur. Several options are available for precise execution.

[0040] One option is to interrupt the hydro-thermal treatment, remove the material from the heat treatment unit and crush or break it up, for example by grinding it, and feed it back into the heat treatment unit.

[0041] Alternatively, it is also possible to provide a suitable comminution plant in the heat treatment facility, which performs comminution continuously or discontinuously during the hydro-thermal treatment.

[0042] Another possibility is to operate a heat treatment device, particularly continuously, and during the hydrothermal treatment, to remove a portion of the material from the heat treatment device, grind it up, and then feed it back in. This is particularly suitable if the starting material is present in the heat treatment device as a suspension or at least in a pumpable form. For example, a line can be provided from an autoclave to a grinding device such as a mill and then back to the autoclave, which is an example of a heat treatment device according to the invention. This can be described as a closed-loop process without interruption.

[0043] Another option is to add nucleating agents, pH raisers, foreign ions and / or other additives to the starting product at the beginning, before or during homogenization, after homogenization, or even in the heat treatment unit, to accelerate the reaction process.

[0044] Examples of nucleating agents include brucite, lizardite, antigorite, prehydrated olivine-containing rock, or mixtures of these substances. The addition of at least 2% by mass of nucleating agents is preferred.

[0045] Substances can be added to raise the pH value of the solution in which the reaction takes place, which release NaOH, KOH, NaCl, KCl, Na 2 SO 4 , MgSO 4 , K 2 SO 4 , Na 2 CO 3 , Ca(OH) 2 and / or K 2 CO 3 after their addition, thereby increasing the pH value in the solution and speeding up the reaction.

[0046] Other auxiliary substances that also accelerate the reaction include magnesite, hydromagnesite, nesquehonite, dolomite, SiO₂, feldspars, pyroxenes, and mixtures thereof; however, the addition of these substances can lead to the formation of new reaction products. Examples of foreign ions are aluminum, sulfate, or alkalis. These can also lead to the formation of new reaction products.

[0047] To increase the purity of the resulting product for CO2 sequestration, these auxiliary substances can be at least partially removed after the hydro-thermal treatment.

[0048] In principle, the reaction can also be achieved by simply increasing the temperature. In particular, temperatures above 150 °C, preferably above 200 °C, and more preferably above 250 °C are possible.

[0049] An additional way to accelerate the reaction is to use a suspension of homogenized starting material in the heat treatment unit for hydrothermal treatment. This suspension is stirred continuously or intermittently during the hydrothermal treatment. For example, an agitator can be used to ensure the movement of the suspension.

[0050] In this context, grinding can also be used as an alternative or additional step – as described above. Wet grinding is particularly suitable here, as a portion of the suspension can be removed from the heat treatment unit, wet-ground, and then returned to it. However, wet grinding can also be carried out directly within the heat treatment unit.

[0051] Another alternative is ultrasonic treatment of the homogenized starting material. Similar to comminution, this process ensures that substances formed on the starting material, such as magnesium hydroxide and / or magnesium silicate hydrate, separate from the remaining components of the starting material, thus providing a sufficiently large surface area to allow the reaction to proceed rapidly. This can be achieved, for example, using an ultrasonic horn or similar device.

[0052] Depending on the desired further treatment and processing of the homogenized and transformed starting material, it may be advantageous to carry out a drying step to remove unbound water before dewatering, i.e., separating bound water. This is particularly advisable and beneficial if the hydrothermal treatment of the starting material was carried out in an aqueous suspension.

[0053] The dried starting material can then be subjected to the dewatering step. The proposed thermal treatment can also be called tempering or calcining. It can be carried out in a rotary kiln or using a circulating fluidized bed of hot gases. When using a fluidized bed, dewatering occurs within a few seconds. Alternatively, the necessary energy can also be supplied electrically, for example in a muffle furnace. This requires times of approximately 5 to 10 minutes. Generally, open systems, such as those using flames, are preferable because it is easier to remove the resulting water vapor, thus accelerating the reaction.

[0054] In principle, it is sufficient for the converted and dehydrated starting material to be contacted with CO₂, such as that present in the air, for it to bind with the CO₂. However, the binding process can be intensified and accelerated if the contact of the converted, dehydrated starting material with CO₂ in an aqueous suspension is carried out by injecting CO₂-containing gas, such as air. It has been shown that this process step allows for faster binding of the CO₂ than treatment with normal ambient air alone, in the absence of water.

[0055] The contact of the converted, dehydrated starting material with a CO₂-containing gas can be carried out at partial pressures of at least 200 ppm, 400 ppm, 1000 ppm, 10,000 ppm, 100,000 ppm, and 200,000 ppm, particularly at room pressure or in the range of a maximum of 2 bar, which corresponds to 2 million ppm. According to the invention, it is not necessary to provide high pressures for rapid and sufficient CO₂ binding to occur. However, higher partial pressures further accelerate the binding of the CO₂.

[0056] To provide the starting material with a fineness corresponding to a BET surface area of ​​0.1 m² / g or finer, it is preferred to subject the starting material to milling, in particular wet milling. The starting material is usually not present at a higher fineness, even if it is already very fine in some cases due to natural weathering. This fineness can easily be increased by milling. Wet milling is also preferred here, as it is often more energy-efficient than dry milling. Since the starting material is subsequently subjected to a hydrothermal treatment in which it comes into contact with water, the advantage of wet milling can already be utilized in this step, especially as the milled material does not need to be dried.

[0057] The converted and dewatered starting material can be used as a binder, for example as a complete or partial cement substitute, for concrete production before the CO₂ is bound. The water-to-binder ratio is preferably in the range of 1:2 or less. This means that the ratio is 1:2.22, preferably 1:2.5, ideally 1:2.86, and even better 1:3.33 or less. It has been found that a higher ratio, i.e., a higher water content, prolongs the hardening process and reduces the strength. The concrete produced in this way binds CO₂ from the ambient air even at room temperature. In principle, the hardening process can be further accelerated by heat or pressure treatment.

[0058] In an alternative embodiment, the converted, dewatered, and CO₂-bound starting material can preferably be solidified and used as aggregate or filler in the production of concrete and / or mortar. After the CO₂ has bound, further drying can take place. However, this is not strictly necessary, as a solid already forms from the converted and dewatered starting material during the binding of the CO₂, or the strength of the hardened material continues to increase.

[0059] The resulting material is inert and suitable for further processing into concrete together with a hydraulic binder, such as cement clinker. Further comminution may be necessary for this. Generally, it is advantageous for the process according to the invention if at least the starting material is free of cement clinker. This can mean, in particular, that it contains no or hardly any (less than 0.1 wt%) alite and / or belite phases. Experience has shown that the materials present in cement clinker partially slow down the reactions described here, so their presence is undesirable. However, cement clinker is harmless in small quantities.

[0060] Optionally, additional substances can be added to improve reactivity or modify the properties of the hardened material. These substances include organic additives, especially flow agents, rock flours, particularly limestone, dolomite and olivine, pozzolanic additives such as trass, glass flour, coal fly ash and / or thermally activated clays.

[0061] The starting materials provided according to the invention are usually not pure substances, so impurities are present to a high degree. However, it is advantageous if at least the molar ratio of Mg to Ca is 10:1 or greater and / or the molar ratio of Si to Al is also 10:1 or greater. It has been shown that the presence of calcium and aluminum, respectively, slows down the reactions in relation to magnesium and silicon, respectively, or even brings them to a complete standstill. Therefore, it is important to shift the corresponding molar ratios significantly in the direction of magnesium and silicon, respectively. Preferably, the molar ratio of Mg to Ca is at least 20:1 and / or the molar ratio of Si to Al is at least 20:1.

[0062] The invention is explained in more detail below with reference to exemplary embodiments and the figures. The figures show: Fig. 1 shows a hydration profile of a product produced by the process according to the invention; and Fig. 2 shows a profile of a CO2 concentration during the binding of CO2 by a product produced by the process according to the invention.

[0063] To verify the invention, the investigations described in more detail below were carried out, among other things. Pure forsterite was used in the first investigation, and natural olivine in the second. Pure Forsterite

[0064] For the first investigation, pure forsterite (Mg₂SiO₄) was used, which was produced by calcining a mixture of magnesium hydroxide carbonate and amorphous SiO₂ in a laboratory furnace. After calcination, the starting material was ground in a disc mill. The specific surface area determined by the BET method was 1 m² / g.

[0065] A mixture of forsterite and 1 molar NaOH solution in a ratio of 1:2.2 was prepared and treated in an autoclave at a temperature of 200°C. The reaction was interrupted, and the material was dried and ground. After a treatment period of four weeks, no forsterite was detectable in the treated and washed material by X-ray phase analysis. The loss on ignition after completion of the autoclave treatment was 19.3 wt%.

[0066] Three grams of the dried and ground material were fired in a platinum crucible at temperatures of 450°C, 600°C, and 750°C in a muffle furnace for one hour each. The samples showed a loss on ignition of 13.5% (450°C), 2.8% (600°C), and 0.52% (750°C), respectively. Sequestration

[0067] This material was used for the sequestration of CO₂. For this purpose, 1 g of the sample, calcined at 600°C, was placed in a beaker containing 50 g of water, and the suspension was continuously stirred. Simultaneously, gaseous CO₂ (concentration 100%) was introduced into the suspension. After 6 hours, the suspension was separated by filtration, and the solid was dried and analyzed by X-ray diffraction and 29<Si MAS. NMR examined.

[0068] X-ray phase analysis showed nesquehonite (MgCO₃ · 3H₂O) as the only crystalline reaction product. Using 29< Si MAS NMR It was shown that all the silicon existed as amorphous SiO₂. Therefore, the following reaction can be demonstrated, where only the initial and final states are shown and the intermediate steps are omitted: Mg₂SiO₄ + 6 H₂O + 2 CO₂ → 2 MgCO₃ · 3H₂O + SiO₂

[0069] Accordingly, carbon dioxide sequestration can be carried out using the method according to the invention. binder

[0070] The reactivity of the material as a binder prior to sequestration was investigated by determining its water binding capacity after 7 days of hydration. For this purpose, materials fired at different temperatures were ground in a hand mortar and mixed with water using a water-binder ratio of 0.50 (1:2) and stored in sealed containers at 22°C for 7 days. Hydration of the samples was then stopped by drying at 60°C, and the loss on ignition was determined by thermal analysis.

[0071] The resulting loss on ignition was 20.5% (450°C), 25.3% (600°C), and 7.1% (750°C), respectively. Accordingly, all three binders hydrated and bound water.

[0072] The hydration process of the sample fired at 600°C was investigated by calorimetry (DCA) at 25°C. A very rapid reaction was observed, with a maximum in the main hydration phase after approximately 2 hours, and the reaction was complete in less than 24 hours. A total heat of approximately 450 J / g was released, as shown in Fig. 1 shown.

[0073] Consequently, the binder reacts faster than most conventional cements. An examination of the hydrated and dried sample after DCA analysis using 29<Si MAS NMR Spectroscopy showed that all the silicon was present as magnesium silicate hydrate. Natural olivine

[0074] For the second investigation, natural olivine from a deposit in Norway was used. Its chemical analysis yielded the following composition: 41.9% SiO₂, 49.9% MgO, 6.9% Fe₂O₃, 0.6% Al₂O₃, 0.1% CaO, 0.5% loss on ignition.

[0075] The material was milled in a ball mill to a fineness of 7300 cm² / g Blaine and mixed with a 1:2 ratio of 1 M NaOH solution. It was then treated in an autoclave at 200°C for 22 days, with one interruption for re-milling.

[0076] After autoclave treatment, the intermediate product was dried, ground and the loss on ignition was analyzed (16.3%). Sequestration

[0077] The sequestration of carbon dioxide was then investigated. For this purpose, 10 g of an intermediate product calcined at 600°C was used, which, after thermal treatment, had been milled for 4 minutes in a disc mill at 700 revolutions per minute with the addition of triethanolamine. The olivine pretreated according to the invention was added to 1 liter of water and stirred continuously. Normal ambient air was blown into this suspension using a simple aquarium pump. Ambient air has a CO₂ concentration of approximately 400 ppm. In enclosed spaces, the CO₂ concentration is usually somewhat higher, typically between 500 and 600 ppm.

[0078] During the experiment, the CO₂ concentration in the air rising from the suspension was measured. This concentration decreased rapidly and continuously throughout the experiment to approximately 200 ppm. Carbon dioxide could be removed from the introduced air over several days. Accordingly, the material is able to react with CO₂ from the air and permanently bind it in the form of magnesium carbonate. This is demonstrated in Fig. 2 depicted. binder

[0079] Individual batches of the intermediate product were calcined at various temperatures, and water binding was investigated by hydration for 7 days at 22°C. Before hydration, the loss on ignition (6.8% after 550°C, 3.1% after 600°C, 2.2% after 650°C, 1.7% after 700°C) was lower than after reaction with water and subsequent drying at 60°C (20.1% after 550°C, 23.7% after 600°C, 24.7% after 650°C, 22.8% after 700°C).

[0080] Therefore, natural materials such as olivine can also be used for binder production, and hydration could be demonstrated after the described pretreatment.

[0081] It has therefore been shown that the inventive method makes it possible to bind CO₂ efficiently. Additionally, it is possible to produce a binder for concrete production as a byproduct, which can also be used for CO₂ sequestration.

Claims

1. Method for sequestering CO2, comprising the steps of: a) providing a starting product comprising at least 20 % by mass of one or more of the following components: • ultramafic rocks, especially dunite, • weathering products of ultramafic rocks, especially serpentinite, • olivine, • industrial waste materials, each with an MgO concentration of at least 10 % by mass and with a fineness corresponding to a BET surface area of 0.1 m2 / g or finer, b) homogenizing the starting product, c) hydro-thermally treating the homogenized starting product in a heat treatment apparatus at a temperature of more than 100 °C for at least 24 hours, d) adding water to the homogenized starting product by adding water directly before, after and / or simultaneously with the homogenization in step b) and mixing the water with the starting product and / or introducing steam into the heat treatment apparatus, wherein after step c) the starting product was at least partially converted into magnesium hydroxide Mg(OH)2 and / or magnesium silicate hydrate in the presence of H2O, e) at least partial dewatering of the converted starting product from bound water by means of thermal treatment and / or reaction milling, • wherein the converted starting product is treated at a temperature between 180 °C and 1000 °C during thermal treatment, • wherein a rearrangement of the crystal structures in the converted starting product occurs during reaction grinding, wherein, after step e), magnesium hydroxide present in the converted, dewatered starting product is at least partially dewatered into magnesium oxide, and magnesium silicate hydrate present is at least partially dewatered and can thereby be converted into dewatered magnesium silicate hydrate, f) contacting the converted, dewatered starting product with CO2, wherein CO2 reacts with the magnesium oxide, the dewatered magnesium silicate hydrate and / or the magnesium hydroxide and the CO2 is bound in the resulting magnesium carbonate and / or magnesium carbonate hydrate.

2. Method according to claim 1, characterized in that for at least partial conversion of the starting product in step c), one or more treatments are carried out to accelerate the reactions taking place during the hydrothermal treatment.

3. Method according to claim 1 or 2, characterized in that during the hydro-thermal treatment or between several hydro-thermal treatments in the heat treatment apparatus in step c), the homogenized starting product is continuously or discontinuously crushed, in particular finely ground, to accelerate the conversion.

4. Method according to any one of claims 1 to 3, characterized in that nucleating agents, agents for raising the pH value, foreign ions and / or auxiliary substances are added to the starting product in step a), b), c) and / or d).

5. Method according to claim 4, characterized in that after step c) the auxiliary substances are at least partially removed again.

6. Method according to any one of claims 1 to 5, characterized in that for hydro-thermal treatment of the homogenized starting product in the heat treatment apparatus, the homogenized starting product is present in a suspension which is stirred continuously and / or discontinuously during the hydrothermal treatment.

7. Method according to any one of claims 1 to 6, characterized in that in step c), an ultrasonic treatment of the homogenized starting product is carried out.

8. Method according to any one of claims 1 to 7, characterized in that the contacting of the converted, dewatered starting product with CO2 in step f) is carried out in an aqueous suspension by blowing in CO2-containing gas.

9. Method according to any one of claims 1 to 8, characterized in that the contacting of the converted, dewatered starting product with a gas containing CO2 is carried out at partial pressures of at least 200 ppm, in particular at room pressure.

10. Method according to any one of claims 1 to 9, characterized in that the converted starting product is subjected to drying to remove unbound water before step e).

11. Method according to any one of claims 1 to 10, characterized in that, in order to provide the starting product with a fineness corresponding to a BET surface area of 0.1 m2 / g or finer, the starting product is subjected to grinding, in particular wet grinding.

12. Method according to any one of claims 1 to 11, characterized in that after step f), the converted, dewatered and CO2-bound starting product is / becomes solidified and is fed as an aggregate or as a filler for the production of concrete and / or mortar.

13. Method according to any one of claims 1 to 12, characterized in that after step e) the converted, dewatered starting product is used as a binder for the production of concrete, wherein the water-binder ratio is 1:2 or less.

14. Method according to any one of claims 1 to 13, characterized in that the starting product has a molar ratio of Mg to Ca of 10:1 or greater and / or a molar ratio of Si to Al of 10:1 or greater.