Method for manufacturing aggregate and aggregate

Abstract

The present invention includes the steps of providing a starting product comprising at least 20% by weight of magnesium silicate hydrate, 2 The present invention relates to a method for producing an aggregate, comprising the steps of crushing the starting product to a fineness corresponding to a BET surface area of ​​100000 / g or finer, homogenizing the starting product, at least partially dehydrating the starting product of bound water by means of heat treatment in a heat treatment unit, contacting the dehydrated starting product with CO2, in which CO2 reacts with the dehydrated magnesium silicate hydrates and binds to the resulting magnesium carbonate hydrates and / or magnesium carbonate, pressing and compressing the dehydrated starting product before or after contacting with CO2 into a solid mass for producing an aggregate. The present invention also relates to an aggregate.

Description

[Technical Field]

[0001] This invention relates to a method for producing aggregate in which CO2 is sequestrated, and to aggregate itself.

[0002] In the construction industry, natural and artificial aggregates are simply called aggregates. These may originate from natural deposits, be generated during the recycling of building materials, or be produced as industrial by-products. While not commonly used, alternative terms in the construction industry that have nearly the same meaning include concrete aggregate, mineral mixture, mineral blend, or mineral material.

[0003] In principle, aggregates can be processed with a binder (usually cement) and water to form concrete. However, asphalt is also a mixture of aggregates and bitumen. The particle shape, strength, and particle size curve of the aggregates can, among other things, influence the properties of the resulting building material.

[0004] Aggregates without binders are used, for example, to create unpaved roads, seepage and de-icing packs, capillary action interruption layers, and similar fillers.

[0005] Carbon dioxide (CO2) acts as a greenhouse gas in the atmosphere and is considered one of the main causes of anthropogenic global warming. In addition to fundamentally reducing CO2 emissions, efforts are being made in parallel to bind the CO2 already present in the atmosphere. By binding CO2, it is possible to make manufacturing processes that generate large amounts of CO2, such as cement production, CO2 neutral.

[0006] CO2 sequestration specifically refers to removing CO2 from the atmosphere, ideally by combining it with other substances so that it cannot escape again. Various proposals for this purpose are known. One possibility is the use of olivine.

[0007] Olivine is a mineral with a common composition of A2[SiO4], and various divalent ions of A exist, such as magnesium (forsterite, Mg2SiO4), iron (Fe2SiO4, fayalite), and manganese (Mn2SiO4, tefloite). Furthermore, since olivine is part of a solid solution series, other ions and various combinations of cations also exist.

[0008] The patent literature on accelerating carbon dioxide sequestration in reactions with olivine is very extensive. Many methods for sequestrating CO2 using olivine are based on the following principles, differing mainly in how the reaction is accelerated.

[0009] [ka]

[0010] According to International Publication No. 2007 / 069902, the reaction should be accelerated by grinding and adjustment to a specific pH value. International Publication Nos. 2008 / 140821 and 2008 / 061305 propose accelerating the reaction with high temperature, high CO2 partial pressure, and high olivine fineness. International Publication No. 2008 / 101293 proposes the addition of ammonium, International Publication No. 2007 / 106883 proposes the addition of a base, and U.S. Patent No. 4944928 proposes the addition of hydrochloric acid.

[0011] In addition to these processes, there are many other processes that can accelerate the sequestration of carbon dioxide during the reaction with olivine, but all of these processes use complex techniques and expensive starting materials.

[0012] The CO2min project attempted to combine olivine carbonation with cement production. Its objective was to capture CO2 emissions from the cement industry by carbonating olivine. As a result, cement production would continue without modification, maintaining the existing high-temperature process. The resulting carbon dioxide would be combined with olivine in the form of magnesium carbonate and SiO2. This generated material would then be added to the cement and disposed of. However, it is clearly impossible to completely convert olivine to magnesium carbonate (see D. Kremer and H. Wotruba, "Separation of products from mineral sequestration of CO2 with primary and secondary materials," Minerals, Vol. 10 (2020), pp. 1098 et seq.). This project utilized very high CO2 partial pressures (17 bar) and high temperatures (175°C). Despite autoclaving, the conversion rate of olivine was low, requiring a very large amount of olivine to compensate for the low conversion rate. The selected process technology is also likely unsuitable for carbon dioxide fusion on an industrial scale.

[0013] Another idea, developed in the Netherlands, involves binding carbon dioxide through a reaction with olivine (see RDSchuiling and P. Krijgsman, "Enhanced Weathering: An effective and cheap tool to sequester CO2," Climate Change, Vol. 74 (2006), pp. 349–354). Olivine is spread on arable land or coastlines. When olivine dissolves in magnesium ions, precipitation or seawater reacts with HCO3 according to the following equation. - It can simultaneously absorb carbon dioxide in the form of ions.

[0014] [ka]

[0015] However, laboratory experiments at the University of Hamburg have shown that the described reaction does not occur in the described form. Even if olivine were spread across the entire world's arable land, it would only capture a mere 0.2% of global carbon dioxide emissions (see T. Amann et al., "Enhanced weathering and related element fluxes - a cropland mesocosm approach," Bio geosciences, Vol. 17 (2020), pp. 103-109). [Overview of the Initiative] [Problems that the invention aims to solve]

[0016] The fundamental objective of this invention is to provide an efficient method for producing aggregate that can also sequester CO2. [Means for solving the problem]

[0017] According to the present invention, this objective is achieved by means of the method having the features of claim 1 and by means of the aggregate having the features of claim 15. [Modes for carrying out the invention]

[0018] Further advantageous embodiments are described in the dependent claims and further description.

[0019] According to claim 1, first, at least 20% by mass, preferably at least 40% by mass, more preferably at least 60% by mass, and even more preferably at least 80% by mass of magnesium silicate hydrate (Mg3Si2O5(OH)4, Mg3Si4O 10A starting product containing (OH)2) is provided. One example of this is serpentinite. Serpentinite is a metamorphic rock formed by natural alteration, in particular weathering, of ultramafic rocks. Advantageously, the starting material should not contain any SiO2, and no substance that releases SiO2 during heat treatment should be added. SiO2 can react with magnesium silicate hydrate during subsequent heat treatment, thereby reducing the quality of the product.

[0020] An important mineral of ultramafic rocks is olivine. Olivine is a mixed crystal series of fayalite (Fe2SiO4), forsterite (Mg2SiO4), tefroite (Mn2SiO4) and other minerals of the A2[SiO4] type. Natural olivine deposits have been recorded, and olivine is often an iron-containing, magnesium-rich substance.

[0021] The basic reactions occurring during weathering are as follows. They are shown here in a simplified form starting from forsterite (Mg2SiO4).

[0022]

化

[0023] Magnesium silicate hydrate (Mg3Si2O5(OH)4, Mg3Si4O 10 (OH)2) can exist in the form of lizardite, antigorite, talc and other forms. It should be noted that the stoichiometric water content of antigorite may be lower (in the range of 13% by mass) than the range of 16% by mass to 20% by mass, which is the water content determined by testing. This can be explained by the fact that some materials are very fine, and water may also adhere to their surfaces.

[0024] Similarly, such deviations from stoichiometry also apply to the ratio between Mg and Si. Furthermore, heteroions such as Fe may be incorporated into the reaction products. However, other reaction products such as hydromagnesite, hematite, magnetite, or gibbsite may also be produced. This depends on the exact composition of the starting products in each case. All or part of the reaction products may contain iron, carbonates, alkalis, or other heteroions.

[0025] This starting product is 0.1m 2 The material is crushed to a fineness equivalent to or finer than the BET surface area per gram. The BET surface area can be measured using the BET method according to the standard DIN ISO 9277:2003-05 "Measurement of specific surface area of ​​solids by gas adsorption". 0.5m 2 The BET surface area per g is favorable, and 1.0m 2 A BET surface area of ​​1 / g or finer is even more preferable. This crushing can be achieved by grinding. Depending on the source of magnesium silicate hydrate, crushing in the sense of the present invention may also be performed during or as a result of mining, extraction, or more generally, manufacturing.

[0026] Once the starting products are prepared, they are homogenized as needed. Serpentinite, given as an example, is a material from a natural deposit. Empirically, it has been shown that it cannot be obtained in its pure form or in a homogenized state. Homogenization can be carried out, for example, using a mixer, or simultaneously with crushing to the desired fineness.

[0027] Next, the homogenized starting product is at least partially dehydrated from bound water by heat treatment means in a heat treatment unit. The heat treatment proposed herein is also called strengthening or calcination. Bound water is also called crystal water. Bound water must be distinguished from unbound water, which can be considered as free H2O. Complete dehydration can be achieved at considerable expense. According to the present invention, the water content of bound water should be reduced by at least 60%, preferably at least 80%, and more preferably at least 90%.

[0028] For heat treatment, the starting product can be heated to a temperature of 180°C to 1000°C. Depending on the degree of fineness present, heating for just a few minutes may be sufficient. A temperature of 300°C to 800°C is preferred, and a temperature of 500°C to 700°C is even more advantageous.

[0029] In this process, magnesium silicate hydrate (Mg3Si2O5(OH)4, Mg3Si4O) present in the starting product is removed. 10 (OH)2) is at least partially converted to dehydrated magnesium silicate hydrate, which can be expressed in a simplified form as xMgO·SiO2·yH2O by dehydration. Dehydration here refers to the reduction of crystal water or water in the crystal in the converted starting product.

[0030] The basic chemical process is again simplified as follows:

[0031] [ka]

[0032] Equation (3) produces a nearly amorphous reaction product with a Mg-to-Si ratio of 1.5:2 and a bound water content of approximately 3%. In the reaction product produced by equation (4), the Mg-to-Si ratio is even lower. Therefore, the variables a, b, c, x, y, and z depend on the exact composition of the starting products and the processing parameters in each case.

[0033] After dehydration, the amount of bound water in the converted and dehydrated starting product is preferably less than 10% by mass, more preferably less than 5% by mass, even more preferably less than 3.5% by mass, and particularly preferably less than 2.5% by mass.

[0034] Therefore, the dehydrated starting product exists as a multiphase product. Other possible secondary phases include hematite, magnetite, enstatite, feldspar, pyroxene, quartz, and amorphous phases.

[0035] Once the starting product is dehydrated, it can be brought into contact with CO2. Here, CO2 reacts with the existing dehydrated magnesium silicate hydrate (xMgO·SiO2·yH2O). CO2 is mainly bonded to the resulting magnesium carbonate (MgCO3) and / or magnesium carbonate hydrate (MgCO3*mH2O). Magnesium carbonate is known by its mineral name, magnesite. Examples of magnesium carbonate hydrates include valintonite (m=2), neskehonite (m=3), and lancefordite (m=5). Furthermore, basic magnesium carbonate hydrates such as altinite, hydromagnesite, and dipingite also exist.

[0036] The basic chemical processes are simplified and generalized as follows:

[0037] [ka]

[0038] Here, q, x, and y represent corresponding variables, which may be zero in some cases. It should also be noted that in equation (5), the dehydrated magnesium silicate hydrate appears only as xMgO·SiO2·yH2O because it is (incompletely) dehydrated. Ideally, the above reaction according to the present invention produces little to no Mg(OH)2.

[0039] Before or after the step of contacting the dehydrated starting product with CO2, it is intended to be pressed and compressed into a solid form in order to produce aggregate. With the optional addition of a binder, this can produce an essentially solid object that can be used as aggregate. SiO2 should not be added before pressing and compression, otherwise too much CaO, Al2O3, or other additives will be needed to bind the SiO2.

[0040] According to the present invention, it has been recognized that an intermediate material suitable for highly binding CO2 can be produced by removing crystalline water from natural materials such as weathered ultramafic rocks, for example, serpentinite.

[0041] In this way, a large amount of CO2 can be bound with a relatively small energy input. For example, by using the method of the present invention, approximately 0.6 tons of CO2 can be bound to approximately 1 ton of serpentinite within a few hours.

[0042] Preferably, the heat treatment of the starting product is carried out at a temperature of at least 550°C and / or up to 750°C, and the starting product is heat treated for at least 5 minutes, advantageously 15 minutes, preferably 30 minutes, and more preferably at least 60 minutes.

[0043] It is advantageous to maintain the dehydration temperature characteristics of a particular material as accurately as possible within a deviation of less than 20°C. If the temperature during heat treatment is too low, magnesium silicate hydrate (e.g., serpentinite) will not be dehydrated or will be insufficiently dehydrated. If the temperature is too high, the magnesium silicate hydrate will be mainly converted to poorly reactive olivine. Dehydration occurs only within a narrow temperature range, with little to no olivine formation. On the contrary, the dehydrated starting product is formed in the form of an X-ray amorphous phase with a low residual water content of 2-5 mass%. This phase is highly reactive and is the target product for heat treatment.

[0044] Therefore, it is preferable that the heat treatment unit has an essentially uniform temperature distribution. This allows for good dehydration without generating unwanted by-products. Thus, it is advantageous not to heat the furnace with a flame in order to maintain the desired dehydration temperature in the furnace as accurately as possible and over most of the material's residence time. In this case, the material is temporarily exposed to very high temperatures, leading to olivine formation. For example, the temperature distribution in a directly heated rotary kiln is too uneven. Therefore, it is advantageous to use a rotary kiln, especially an indirectly heated rotary kiln without direct flame in the reaction chamber, as a heat treatment unit. The temperature during heat treatment, also called the firing temperature, can be controlled with particular precision in an electric heating furnace. Electric heating should be used, in particular, to precisely maintain the target temperature in the furnace chamber. It is advantageous to heat the kiln with electric energy from a renewable energy source because it does not produce CO2 emissions or exhaust gases and does not consume fuel. In contrast, preheating can also be performed using other heat sources, particularly heat exchangers, which remove some of the heat from the dehydrated starting products, for example, from the calcined serpentinite, thereby cooling it while simultaneously supplying this heat to the uncalcined serpentinite. The use of exhaust gases from the combustion process should be avoided as much as possible, as it can lead to uncontrolled binding of CO2, for example.

[0045] Rotary kilns are preferred because they have a large volume and can therefore achieve high throughput in order to achieve a sufficient residence time at the target temperature. Furthermore, rotary kilns are characterized by good thermal efficiency.

[0046] Heat treatment is particularly efficient with small-grain magnesium silicate hydrate, such as crushed serpentinite, in which case water bound to the particles is removed more quickly and efficiently. At the same time, a low partial pressure of water vapor in a heat treatment unit, such as a furnace chamber, promotes the formation of a reaction phase for binding CO2. A low partial pressure of water vapor can be achieved by air purging the furnace chamber.

[0047] After heat treatment, the dehydrated starting product, for example reinforced serpentinite, is usually available in powder form. This powder can be brought into direct contact with CO2 or can be pre-pressed into a solid. No further crushing is required, and it has even been found that such crushing can adversely affect the further process.

[0048] The pressing and compacting of the dehydrated starting product can be carried out such that the solid is formed with a volume of 1 mm 3 to 30,000 mm 3 . These sizes are particularly suitable for use as aggregate.

[0049] Furthermore, it has been shown that pressing and compacting are preferably carried out such that the solid is formed with a porosity of less than 30% by volume, preferably less than 20% by volume, more advantageously less than 10% by volume. On the one hand, when subsequently brought into contact with CO2, CO2 can easily reach even material not present on the surface due to the porosity, and on the other hand, this type of aggregate can be used particularly advantageously.

[0050] The higher the degree of compaction of the solid and the lower the porosity of the solid, the higher the subsequent strength of the aggregate, also known as sand or gravel. The compacted solid should be firm so that no damage occurs in subsequent process steps.

[0051] If the dehydrated starting product is brought into contact with CO2 after pressing and compaction, the dehydrated starting product should be pressed into pellets for further processing.

[0052] Fully automatic tableting presses are particularly suitable for this purpose. During the pressing process, a small amount of the dehydrated starting product, available in powder form, is filled into a die and compressed with a suitable tool. After the pressing process, the porosity of the pressed pellet can be determined from the ratio of bulk density to pure density.

[0053] The porosity should be less than 30 vol%, preferably less than 20 vol%, more preferably less than 10 vol%, and even better less than 5 vol%. However, it is advantageous to have a low residual porosity so that CO2 can penetrate the pellet.

[0054] The compression process can be improved by adding water, organic liquids, and other substances to facilitate the compression of the powder. A low porosity after the compression process ensures high strength. Furthermore, it is preferable that the pellets be available at green strength after the compression process, which facilitates further processing such as transportation and CO2 treatment.

[0055] If the powder is mixed with other materials before the pressing process, additional chemical reactions can increase its strength, further enhancing the strength of the cured pellets. These materials include, for example, NaOH, KOH, Ca(OH)2, as well as other compounds that release alkalis and / or alkaline earth elements, sodium aluminate, coal fly ash, truss, reinforced clay, and other materials containing Al2O3, CaO, and / or alkalis in a form that can react with SiO2 during treatment with CO2 (preferably in an autoclave). Materials containing alite or belite should not be added as they may adversely affect curing.

[0056] After heat treatment, dehydrated magnesium silicate hydrate is available as a powder and can be compressed with other substances as needed. The compression process allows for the achievement of green strength, which facilitates further processing. In contrast, the compression process does not yield the permanent strength necessary for use as gravel.

[0057] To construct high compressive strength in compressed particles, chemical reactions are required to solidify and maintain the structure. Several reactions occur upon contact with CO2, and these contribute in various ways to the permanent strength of the gravel. One reaction is the formation of magnesium carbonate or magnesium carbonate hydrate from dehydrated magnesium silicate hydrate. This occurs along with the consumption of dehydrated magnesium silicate hydrate. Magnesium combines with CO2, and SiO2 is deposited as an amorphous phase. This results in an increase in volume as CO2 penetrates the compressed particles from the outside. If magnesium carbonate hydrate is formed, the increase in volume is even greater. The greater the increase in volume during the chemical reaction, the greater the porosity remaining after compression. The lower the porosity, the higher the strength. Furthermore, the formation of new phases solidifies all phases, thus constructing the permanent strength required for use as gravel.

[0058] The second reaction that causes the permanent solidification of the compressed particles is the hydration of dehydrated magnesium silicate hydrate. In this process, phases such as antigorite, talc, and lizardite are formed. This hydration reaction can be compared to the hardening of cement paste, with the difference being that MSH is formed instead of CSH.

[0059] A third chemical reaction that causes solidification is the reaction between the substance added before the particles are compressed and the SiO2 from carbonation. In the presence of reactive aluminum and alkali, a NASH phase is formed, which also occurs during the solidification of geopolymers. In the presence of reactive CaO, SiO2 can also be converted to CSH.

[0060] All three reactions contribute to reducing porosity and solidifying the compressed particles, which can then be used as gravel for concrete production and other purposes. The degree of the three reactions can be controlled by external conditions such as pressure and temperature, as well as the availability of CO2 and chemical composition.

[0061] When pressing occurs after contact with CO2, the pressed particles do not contain dehydrated magnesium silicate hydrate, but rather magnesium carbonate and SiO2, and possibly other phases. This means that the formation of magnesium carbonate and magnesium carbonate hydrate can no longer contribute to strength formation, or only to a small extent. Instead, solidification is based on the chemical reaction of the substances added after contact with CO2. When reactive aluminum and reactive alkalis are added, they react with amorphous SiO2 from sequestration to form NASH, enabling strength formation. The same applies to the addition of reactive CaO and the formation of CSH. Dehydrated magnesium silicate hydrate can also be added, which is converted to MSH. Other organic and inorganic binders can also be used.

[0062] The dehydrated starting product, upon contact with CO2 before pressing and compression, remains in powder form or forms a powder. This process is described in more detail below. Therefore, to produce aggregate, the dehydrated magnesium silicate hydrate should be solidified by contact with CO2.

[0063] This solidification is advantageously based on the reaction between amorphous SiO2 (Equation 5) and the added substance. These substances include those that introduce Al2O3, alkali, or CaO in a reactive form, such as hard coal fly ash, blast furnace granulated slag, reinforced clay, sodium aluminate, NaOH, and KOH. Portland cement clinker should not be added. After addition, intensive homogenization is possible and advantageous. If the materials are mixed in a dry form, joint grinding can be considered. In addition to the materials mentioned above, other binders, such as organic adhesives, can also be used to bind the gravel particles.

[0064] Next, the pre-treated material can be compressed to form a solid. In this process, the porosity after the compression process should be less than 30 vol%, preferably less than 20 vol%, more favorably less than 10 vol%, and even better less than 5 vol%. The compressed solid may contain a small amount of water. This water can be added before or after the compression process. The gravel particles harden through the reaction of amorphous SiO2 from the sequestration with alkali and Al2O3 from the added material, forming the NASH phase. Simultaneously, the formation of the CSH phase occurs in the presence of reactive CaO or through other chemical or physical processes.

[0065] These reactions are facilitated by raising the temperature to 40°C, and even better to 60°C, for at least 2 hours, preferably 12 hours, and more preferably 24 hours, which increases the strength of the gravel particles. Therefore, the solids are heat-treated at a temperature of at least 40°C, and more preferably at least 60°C, for at least 2 hours, and more preferably at least 24 hours. The gravel particles should be protected from drying out. This can be done in an apparatus such as an autoclave at an appropriate level of humidity. Preferably, the atmosphere is saturated with respect to water vapor.

[0066] Whether the contact between the dehydrated starting product and CO2 occurs before or after the production of the solid, the contact can be advantageously carried out in a sealed container, particularly an autoclave, or an overpressurized container. In this way, the CO2 bonding process can be optimized, for example, by adjusting the partial pressure of CO2 in the autoclave, the temperature present, and / or the water content in the atmosphere.

[0067] CO2 is bonded particularly rapidly if the dehydrated starting product is brought into contact with CO2 after being pressed and compressed at a CO2 partial pressure of at least 0.1 bar, or if it is brought into contact with CO2 before being pressed and compressed at a CO2 partial pressure of at least 0.0003 bar, preferably 0.0010 bar. While the dehydrated starting product is in contact with CO2, the temperature during the reaction can exceed at least 30°C, preferably 50°C. The CO2 partial pressure can be less than 0.5 bar.

[0068] Furthermore, it is preferable to add a substance such as sugar or other organic additive to the dehydrated starting product before or during contact with CO2, which reduces the formation of magnesium silicate hydrate. This has the effect of increasing the amount of magnesium carbonate produced.

[0069] When treatment or contact with CO2 occurs after pressing and compression, if the solid material, such as pellets, is moist, it is favorable for the formation of magnesium carbonate or magnesium carbonate hydrate. The reaction proceeds more quickly in the presence of water, as the activation energy for the reaction is lower.

[0070] Water introduction can be achieved by spraying water onto the compressed pellets. It is also possible to create an atmosphere within the autoclave saturated with water vapor. A combination of these two variations is particularly suitable for introducing water.

[0071] The presence of water facilitates the dissolution of dehydrated starting products, such as strengthened serpentinite, as well as the formation of magnesium carbonate and SiO2. If other starting materials providing CaO, Al2O3, and / or alkali are present, other silicon-containing phases may also form instead of SiO2.

[0072] On the other hand, the presence of water can also enable the formation of magnesium silicate hydrate. When this occurs, the formation of magnesium silicate hydrate also contributes to the strength of the aggregate. However, the formation of magnesium silicate hydrate is a competitive reaction with the formation of magnesium carbonate or magnesium carbonate hydrate, and therefore reduces the CO2 binding capacity.

[0073] Therefore, it is desirable to control and, if necessary, reduce the formation of magnesium silicate hydrate. This can be done by selecting reaction parameters or by adding substances that can suppress the formation of magnesium silicate hydrate. These include substances such as sugars, as the formation of magnesium silicate hydrate can be reduced or completely avoided in the presence of dissolved sugars. Certain organic additives are also suitable. All of these substances should be mixed before the formation of solids or added in dissolved form.

[0074] The reaction with CO2 occurs through the diffusion of the gas into the solid material and the formation of magnesium carbonate and / or magnesium carbonate hydrate. The diffusion of the gas into the solid material is facilitated by a high partial pressure of CO2. Therefore, the reaction is preferably carried out in a sealed container, otherwise the partial pressure of CO2 will decrease again.

[0075] A sealed steel container, also known as an autoclave, is particularly suitable because it can withstand high pressure and operate at high temperatures. CO2 can be supplied to an autoclave filled with solids, either in its pure or diluted state. For this purpose, it is beneficial if the CO2 originates from exhaust gases or exhaust air from a manufacturing plant, resulting in an increase in CO2 partial pressure compared to normal air.

[0076] Such flue gases are produced, for example, in steel mills, glass factories, cement kilns, fossil fuel-fired power plants, and other industrial processes. The CO2 content of flue gases is often between 5% and 30% by volume. The use of these gases is advantageous because increasing the CO2 concentration and compressing the gas is sufficient to allow the CO2 to bind to solids in an autoclave.

[0077] This makes it possible to achieve CO2 partial pressures of 0.1 to 15 bar within the autoclave. To accelerate the CO2 absorption process within the autoclave, CO2 can be separated from the air or the aforementioned exhaust gas and introduced into the autoclave in a nearly pure state. Separation can be performed using conventional methods such as amine washing or calcium looping. This means that higher CO2 partial pressures of 1 to 50 bar can be achieved within the autoclave. For ease of technical application, the pressure should be less than 10 bar.

[0078] The formation of magnesium carbonate can be accelerated by increasing the temperature. This also increases the partial pressure of CO2 in the autoclave. Another advantage of increasing the temperature is that it is preferable to form anhydrous magnesium carbonate (magnesite, MgCO3) instead of magnesium carbonate hydrate (neskehonite, MgCO3·3H2O).

[0079] The temperature should be at least 25°C, preferably 50°C, but below 70°C.

[0080] Magnesite formation is also facilitated when magnesite nuclei are added by mixing them with the dehydrated starting product before the solid is compressed. The solid should be treated in an autoclave for at least 4 hours, preferably 12 hours, and even better 24 hours, the atmosphere should be saturated with water, and / or the CO2 partial pressure should be maintained at a high level permanently, and bound CO2 should be replaced by supplying new CO2.

[0081] If it is desirable to bring the dehydrated starting product into contact with CO2 before pressing and compressing, this can be done by blowing a CO2-containing gas into an aqueous suspension.

[0082] Contact with CO2 can be carried out in a sealed container such as a scrubber or autoclave. A suspension of water (possibly containing additives) and a dehydrated starting product in powder form, such as strengthened serpentinite, are placed inside. Here again, the atmosphere can be saturated with respect to water vapor. Furthermore, substances can be added to accelerate the CO2 bonding reaction.

[0083] These substances, such as citric acid, acetic acid, or KH2PO4, can promote CO2 bonding by buffering the pH to less than 8.0, preferably less than 7.0, and even better less than 6.0, because of the low presence of magnesium silicate hydrate. Furthermore, substances that inhibit the formation of magnesium silicate hydrate and thus promote the formation of magnesium carbonate or magnesium carbonate hydrate can be added. These include, for example, sugars and certain organic additives. NaCl should not be added to the solution as it slows down or interferes with the reaction. The addition of magnesite nuclei is recommended.

[0084] The suspension is preferably moved continuously. This can be done by using a stirrer. Furthermore, a gas containing CO2 can be blown into the suspension. This can be air, flue gas, other waste gas from industrial processes, or pure CO2. A partial pressure of CO2 of 0.0003 bar, preferably 0.0010 bar, even more preferably 0.0100 bar, and especially preferably 0.3000 bar is sufficient for the reaction. The total pressure should be less than 10 bar, preferably less than 2 bar, so as to reduce the requirements for the mechanical stability of the vessel. The partial pressure of CO2 should be less than 0.5 bar to reduce the effort required to concentrate CO2 in the introduced gas.

[0085] In suspension, the dehydrated starting product reacts with introduced CO2 and magnesium carbonate or magnesium carbonate hydrate to produce amorphous SiO2 (Equation 5). In some cases, the formation of magnesium silicate hydrate may occur as a competing reaction.

[0086] The operating temperature of the processing unit, particularly the scrubber, is advantageous when it is higher than room temperature, as this accelerates the chemical reaction. This heating can be achieved, for example, by introducing a high-temperature flue gas. The temperature inside the scrubber should be at least 30°C, preferably 50°C. However, since the solubility of CO2 decreases with increasing temperature, the temperature rise should be limited to 70°C. This process can preferably be carried out at a very low CO2 partial pressure of less than 0.5 bar.

[0087] Preferably, contact with CO2, formation of a suspension, and precipitation of the resulting magnesium carbonate or magnesium carbonate hydrate can be carried out ideally simultaneously in a single vessel, also known as a reactor. Thus, a single-stage process can be provided, where the reaction preferably takes place in only one vessel, such as an autoclave. This simplifies the entire process, particularly in industrial applications, because, on the one hand, multiple reactors, and possibly different reactors, are not used in downstream processes, and on the other hand, the time-consuming separation of individual intermediate products is not required. Furthermore, there is no need to pump the suspension.

[0088] The dehydrated starting product, after contact with CO2, is subsequently separated from the aqueous suspension and optionally subjected to heat treatment (preferably in an autoclave), which is particularly advantageous when more than 20% by mass of magnesium carbonate hydrate, such as neskehonite, is present.

[0089] The remaining solution can be reused to continue the isolation process with a fresh dehydrated starting product. The solid material in the form of the dehydrated starting product, after contact with CO2, can be further processed in either a dried or undried form.

[0090] The dehydrated starting product, also called the sequestration product, obtained by contact with CO2, preferably contains anhydrous magnesium carbonate (magnesite), and the proportion of magnesium carbonate hydrate, such as neskehonite, should not exceed 20% by mass of the total substance, or more preferably less than 10% by mass. If the proportion of magnesium carbonate hydrate after sequestration is too high, heat treatment can be performed after the reaction with CO2 to convert the magnesium carbonate hydrate to anhydrous magnesium carbonate. This can be done, for example, by treatment in an autoclave at 100°C to 200°C, or by drying at 150°C to 300°C.

[0091] As already described, before pressing and compression, and after separation from the aqueous suspension, substances introducing Al2O3, alkali or CaO in a reactive form, dehydrated magnesium silicate hydrate and / or organic adhesives can be added to homogenize the mixture.

[0092] 0.1m 2 To provide a starting product having a fineness equivalent to or finer than the BET surface area per g, it is preferable to grind the starting product, particularly by wet grinding. The starting product is obtained without higher fineness, at least partially, even if it is already very fine partially due to natural weathering. This fineness can be easily increased by grinding. Wet grinding is also preferred here because it is often more energy efficient than dry grinding.

[0093] Furthermore, the present invention relates to an aggregate containing magnesium carbonate hydrate and / or magnesium carbonate, manufactured by the method according to the present invention.

[0094] Aggregates are inert and can be further processed into concrete with hydraulic binders such as cement clinker. This may require further crushing. In principle, the order of the method according to the present invention is advantageous if the starting products do not contain cement clinker. In particular, this can mean that the alite phase and / or belite phase are present in very little (less than 0.1 mass%). Empirically, the presence of materials present in cement clinker is undesirable because they partially slow down the reaction described herein. However, in small amounts, cement clinker is harmless.

[0095] Optionally, other substances may be added to the mixture to improve reactivity or modify the properties of the hardened material. These substances include organic additives, especially high-performance water-reducing agents, rock powders, especially limestone, dolomite and olivine, pozzolanic additives, such as truss, glass powder, hard coal fly ash and / or thermoactivated clay.

[0096] The starting products provided by the present invention are not usually pure substances and therefore contain a large amount of impurities. However, it is advantageous that the molar ratio of Mg to Ca is at least 10:1 and / or the molar ratio of Si to Al is also at least 10:1. It has been shown that when calcium and aluminum are present relative to magnesium and silicon, respectively, the reaction slows down or, in some cases, stops completely. Therefore, it is not important to shift the corresponding molar ratios significantly toward magnesium or silicon. 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.

Claims

1. a) Provide a starting product containing at least 20% by mass of magnesium silicate hydrate, and 0.1 m ２ A process of crushing to a fineness equivalent to or finer than the BET surface area per gram, b) A step of homogenizing the starting product, c) A step of at least partially dehydrating the starting product from bound water by means of a heat treatment unit, and during the heat treatment, the converted starting product is treated at a temperature of 180°C to 1000°C. After step c), the magnesium silicate hydrate present in the dehydrated starting product is at least partially dehydrated and thereby converted to dehydrated magnesium silicate hydrate, and after dehydration, the content of bound water in the converted and dehydrated starting product is less than 5% by mass. d) The dehydrated starting product is converted to CO ２ The process of bringing it into contact with the CO ２ It reacts with the dehydrated magnesium silicate hydrate and binds to the resulting magnesium carbonate hydrate and / or magnesium carbonate. e) a step of pressing and compressing the dehydrated starting product before or after step d) to solidify it and produce aggregate, A method for producing aggregate, characterized in that a rotary kiln with indirect heating and no direct flame in the reaction chamber is used as the heat treatment unit.

2. The heat treatment of the starting product is carried out at a temperature of at least 550°C and / or up to 750°C. The method for producing aggregate according to claim 1, wherein the starting product is heat-treated for at least 15 minutes.

3. The method for manufacturing aggregate according to claim 1, wherein the heat treatment unit has an essentially homogeneous temperature distribution.

4. The method for producing aggregate according to claim 1, wherein the rotary kiln is electrically heated.

5. 1 mm ３ ~30,000 mm ３ The method for manufacturing aggregate according to claim 1, wherein pressing and compression are performed to form the solid having the volume of the solid.

6. The method for manufacturing aggregate according to claim 1, wherein pressing and compression are performed to form the solid having a void ratio of less than 20% by volume.

7. The method for producing aggregate according to claim 1, wherein the solid is heat-treated at a temperature of at least 40°C for at least 2 hours.

8. The dehydrated starting product and the CO ２ The method for manufacturing aggregate according to claim 1, wherein the aforementioned contact is carried out in a sealed container.

9. The dehydrated starting product and the CO after the pressing and compression ２ The aforementioned contact with CO2 is at least 0.1 bar. ２ This is done using partial pressure, or Said contacting of the dewatered starting product prior to said pressing and said compression with said CO ２ is carried out at a CO ２ partial pressure of at least 0.0003 bar, the method for producing an aggregate according to claim 1.

10. The dehydrated starting product and the CO ２ The method for producing aggregate according to claim 1, wherein the contact with is performed at a temperature of at least 30°C.

11. The dehydrated starting product and the CO ２ The method for producing aggregate according to claim 1, wherein a sugar or other organic additive is added to the dehydrated starting product before or during the contact with the product, thereby reducing the formation of magnesium silicate hydrate.

12. The dehydrated starting product and the CO ２ The aforementioned contact with the CO is performed in an aqueous suspension prior to step e). ２ A method for producing aggregate according to claim 1, which is carried out by blowing in a gas containing the above.

13. The aforementioned CO ２ The method for producing aggregate according to claim 12, wherein the dehydrated starting product, which has been brought into contact with the aqueous suspension, is separated from the aqueous suspension, and if more than 20% by mass of magnesium carbonate hydrate is present, it is subjected to heat treatment.

14. Before pressing and compressing in step e) and after separation from the aqueous suspension, the reactive form of Al ２ O ３ A method for producing aggregate according to claim 13, comprising adding an alkali or a substance that introduces CaO, dehydrated magnesium silicate hydrate, and / or an organic adhesive, and homogenizing the mixture.

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