Cement additive made from old concrete

Abstract

The present invention relates to a method for treating mineral material such as old concrete, old hardened cement paste and the like, wherein the mineral material is carbonated and mechanically activated.

Description

[0001] Cement additive from old concrete

[0002] The invention relates to a method for safely binding carbon dioxide with old building materials, for example cement stone, and thus ensuring safe long-term storage and at the same time achieving activation in order to be able to use this material as a cement aggregate.

[0003] It is becoming increasingly necessary to conserve natural resources and utilize recycled materials. Carbon dioxide emissions are also a critical cause of global warming. Therefore, increasing emphasis is being placed on capturing carbon dioxide from exhaust gases and permanently storing or using it. One possible form of storage is injecting it into the ground as liquefied carbon dioxide. However, this process is not without controversy, as its permanent retention is not necessarily guaranteed, and any escape would reinforce the greenhouse effect, especially since further energy is required for capturing and storing it, potentially producing more carbon dioxide.

[0004] One of the most carbon-intensive industries is the cement industry. Firstly, the firing process requires a lot of energy, which leads to carbon dioxide emissions when using conventional fossil fuels. Secondly, carbon dioxide is released from the raw materials, mainly limestone or marl, as part of the process.

[0005] On the other hand, large quantities of old concrete are generated when concrete structures are demolished. Therefore, there is currently discussion about recycling concrete, for example to produce new cement and concrete. However, this is problematic because sand and the set cement, for example, are mixed and bonded together and are difficult to separate. The sand-free or at least sand-poor component of the old concrete is also known as old cement block. It is known that concrete can absorb carbon dioxide during its lifetime, but only a fraction of the carbon dioxide released from the limestone during production. After a long time, for example in very old buildings, this value can be around 25% based on the calcium content of the concrete. This means that it is reabsorbed very slowly and therefore over long periods of time, up to around 1 / 4 of the carbon dioxide originally released.

[0006] From WO 2020 / 058 247 A1 a method and a plant for processing material containing cement stone is known.

[0007] The use of carbon dioxide from and for cement is known from EP 3 656 750 A2.

[0008] From the subsequently published DE 10 2022 132 073, a method and a device for the efficient reduction of carbon dioxide emissions are known.

[0009] Activated clays have established themselves as an additive, particularly in the cement industry. The current method is drying and calcining the clays, i.e., thermal activation. This requires energy for heating, and the high temperature can also cause further, potentially undesirable, changes in the material. Furthermore, the thermal process requires flue gas purification to capture the resulting nitrogen oxide and sulfur oxide emissions, among other things. Furthermore, the thermal process will require the use of processes to capture and, if necessary, purify the carbon dioxide produced or released.

[0010] WO 2017 / 008 863 A1 discloses a method and a plant arrangement for processing and activating a raw material.

[0011] EP 3 909 682 A1 discloses a method and a roller mill for thermomechanically activating a clay mixture.

[0012] DE 10 2015 106 109 A1 discloses a process for the tribochemical activation of binders and additives.

[0013] WO 2020 058 247 A1 discloses a method and plant for processing material containing cement paste. Therefore, the idea is to activate clays by introducing mechanical energy during the grinding process, thus replacing thermal energy with green electricity and preventing the oxidation of iron, for example.

[0014] From the subsequently published DE 10 2023 106 210 a process for grinding and pozzolanic activation in a stirred ball mill is known.

[0015] From the subsequently published DE 10 2023 106 217, a process for grinding and pozzolanic activation in two separate stages of a stirred ball mill is known.

[0016] From the subsequently published DE 10 2023 106 221, the combination of mechanical and thermal activation in at least one agitator ball mill is known.

[0017] The object of the invention is to obtain a material of the highest possible quality from old concrete in order to bind carbon dioxide, particularly from the atmosphere, and to reduce the amount of clinker required for cement.

[0018] This object is achieved by the method having the features specified in claim 1. Advantageous further developments emerge from the subclaims, the following description, and the drawings.

[0019] The process according to the invention is used for the carbonation of mineral material such as old concrete, old cement block, and the like. Such processes are known from the prior art. On the one hand, this allows carbon dioxide to be bound, thus avoiding emissions. On the other hand, it has been found that the material produced in this way is well suited as a cement additive. Essential to the invention, the mineral material is not only carbonated but also mechanically activated. It has been shown that the carbonated and mechanically activated material is even more suitable as a cement additive. It is assumed that these two different processes have different positive effects on different components. Concrete, and thus also old concrete and fractions produced from it, such as old cement block, consist of different components.Two essential components are hydrated cement, which, in simple terms, consists of calcium silicate hydrates and calcium silicate aluminate hydrates, and quartz sand (SiO2). It can be assumed that, for example, the metal oxides, such as CaO or MgO, convert during carbonation, for example to CaCO3 or MgCO3. This assumption, based on the well-known carbonation of old concrete, is considered a likely reaction. It can also be assumed, for example, that a sand component, such as SiO2, can be modified through mechanical activation, for example, towards silica gel, which in turn improves the setting properties and thus the suitability as a cement additive.Due to the complexity of the starting product, it is difficult to identify exactly the different activation mechanisms, but it has been found that this combination results in a particularly suitable cement additive.

[0020] Old concrete, old cement block, and the like can also contain components of bricks, clinker bricks, or other building bricks. These components are often difficult to separate during demolition. Furthermore, they are made from clays, which also have mechanical activation properties.

[0021] Furthermore, old concrete, old cement block, and the like can also contain, for example, asbestos minerals. These minerals are often present in old building materials. Furthermore, mechanical activation fundamentally crushes them and embeds them in the matrix, allowing them to be safely processed and thus sustainably disposed of. In particular, the asbestos fibers can no longer be detected by electron microscopy after mechanical activation. Therefore, the process according to the invention is also suitable for the sustainable disposal of asbestos.

[0022] Mechanical activation is an effect that occurs during grinding with very high energy input. During grinding, three phases can be separated from one another. The first phase is characterized by the particle size becoming smaller as the amount of grinding energy input increases. This is the normal grinding range in which all usual grinding processes take place. At the end of the first phase, a plateau is reached at which additional grinding energy no longer changes the particle size. This phase is therefore avoided in grinding operations, as no additional profit can be achieved at higher costs. If even more grinding energy is input, a third phase is reached in which the particle size actually increases again. This third phase is therefore avoided all the more because it produces a poorer result for higher costs.However, it has been shown that in this third phase there is a change in chemical bonds, i.e. the material itself is changed. In particular, inert materials such as SiO2 can be modified in this way so that they no longer have a very stable and regular crystal lattice, but instead exhibit reactive centers via vacancies and defects. One of the advantages is that this activation takes place (more or less) at room temperature and not, for example, at 800°C to 1000°C like the thermal activation of clays. In the case of clays, this has the advantage that iron centers, for example, are not oxidized to iron III, which happens during thermal activation and is undesirable due to the red discoloration. This is why mechanical activation, especially for clays, is currently the focus of attention.

[0023] Mechanical activation means that grinding takes place in the third phase, i.e. with a very high energy input.

[0024] Even if the red discoloration problem known from clays is not a major issue in old concrete, and the advantage of color optimization is therefore not significant in old concrete, the combination of carbonation and mechanical activation has proven particularly suitable for obtaining a particularly good cement additive. A particularly good cement additive means that it can be used in a particularly high proportion (with the same cement quality or the same properties of the concrete produced with it) and / or that other components can be replaced, for example and in particular clinker. This eliminates the need to burn clinker, which avoids both costs and carbon dioxide emissions. In a further embodiment of the invention, the mechanical activation is carried out by grinding with an energy input per ton of mineral material of at least 300 kWh / l.Preferably, mechanical activation is carried out by grinding with an energy input per ton of mineral material of at least 500 kWh / 1.

[0025] In a further embodiment of the invention, the mechanical activation is achieved by grinding with an energy input per mill volume of at least 100 kW / m 3 , preferably at least 200 kW / m 3 , carried out.

[0026] In a further embodiment of the invention, the material produced by the process has an activity index after 28 days according to DIN EN 450-1 of at least 85%.

[0027] In a further embodiment of the invention, the mechanical activation is carried out by grinding in a micromill. The micromill is selected from the group comprising vibratory mills, planetary ball mills, and agitated ball mills. The micromill is particularly preferably an agitated ball mill. This type of mill has proven particularly suitable for achieving the necessary high energy inputs while simultaneously being scalable to industrial scale.

[0028] In a further embodiment of the invention, the micromill is a stirred ball mill. The micromill is filled with grinding media at a filling level of 50 vol.% to 95 vol.%, preferably 60 vol.% to 70 vol.%. The bulk volume of the grinding media is related to the grinding chamber volume of the micromill. Since the filling level is around 64% for a simple bed and only 74% for a densely packed bed, even a theoretical grinding media filling level of 100% results in a corresponding free space, which can be occupied, for example, by the mineral material to be activated. However, since the filling level of a grinding media bed depends heavily on the shape and uniformity of the grinding media, it is practically easier to relate the grinding media filling level to the bulk volume rather than to the actual (filled) volume.In a further embodiment of the invention, a stirred ball mill with a length-to-diameter ratio of 2.5 to 5 is selected.

[0029] In a further embodiment of the invention, ceramic grinding media are selected.

[0030] In a further embodiment of the invention, grinding media with a diameter of 1 mm to 10 mm are selected.

[0031] In a further embodiment of the invention, the agitator ball mill is operated at a peripheral speed of 2 m / s to 6 m / s, preferably 3 m / s to 5 m / s, particularly preferably 3.5 m / s to 4.5 m / s.

[0032] In a further embodiment of the invention, the agitator ball mill is operated with a gas volume flow and a material flow. The ratio of gas volume flow to material flow is adjusted such that the ratio of gas volume flow to material flow is between 0.0001 m 3 / kg and 5 m 3 / kg, preferably between 0.1 m 3 / kg and 2 m 3 / kg.

[0033] In a further embodiment of the invention, grinding takes place in a circuit. This means that the ground material leaving the mill is fed back into the mill's inlet. Size-selective separation takes place within the circuit, for example, using a classifier. The coarse fraction from the size-selective separation is recycled, and the fine fraction from the size-selective separation is discharged from the circuit.

[0034] Advantageously, the carbonation can be carried out using a carbonation process as known, for example, from the subsequently published DE 10 2022 132 073 or the subsequently published DE 10 2023 113 943.

[0035] In a further embodiment of the invention, carbonation is carried out in a mechanical fluidized-bed reactor. It has been shown that a very beneficial transformation of the finely ground mineral raw material occurs precisely in a mechanical fluidized-bed reactor. The relatively uniform size distribution of the agglomerated particles prevents both adhesion in a heat treatment device and the unwanted transition of the product into the gas phase. The latter leads to the product having to be filtered out of the exhaust stream and thus being essentially recycled, which represents a burden on the overall process.

[0036] In a further embodiment of the invention, the mechanical fluidized-bed reactor comprises a substantially horizontally arranged vessel. A shaft is arranged centrally along the longitudinal axis of the vessel, with mixing tools arranged radially on the shaft. In the simplest case, these mixing tools can be rod-shaped and arranged perpendicularly to the shaft. Particularly preferably, the mixing tools are plowshare-shaped. Examples of plowshare-shaped mixing tools can be found, for example, in DE 27 29 477 C2 or DE 197 06 364 C2. "Substantially horizontal" in the context of the invention is to be understood as defined in EP 0 500 561 B1.

[0037] In a further embodiment of the invention, the carbonation is carried out in a ploughshare mixer, a twin-shaft batch mixer or an entrained flow reactor.

[0038] Regarding the design as an entrained flow reactor, reference is made to DE 10 2022 132 073.

[0039] There are three basic concepts for the combination of carbonation and mechanical activation, each with its own advantages. These are: 1) first carbonation, then mechanical activation; 2) first mechanical activation, then carbonation; 3) combined mechanical activation and carbonation. These will be discussed below.

[0040] In a first further embodiment of the invention, carbonation is carried out in a separate step before mechanical activation. The advantage of this embodiment is that a drying step can be carried out between carbonation and mechanical activation. Carbonation requires a water content, for example 10 to 20 wt.%, for the reaction to take place reliably and quickly. However, for the activity of the product, it is advantageous if as little water as possible is present after mechanical activation in order to prevent premature setting and thus a loss of activity. If necessary, a deagglomerator with a riser dryer can be arranged after carbonation so that the water previously required for carbonation is discharged and any clumps formed by the water are dissolved again.

[0041] In a second further embodiment of the invention, the carbonation is carried out in a separate step after the mechanical activation.

[0042] The advantage of this design is the particularly fine material used for carbonation. Due to the increased surface area, optimized pore accessibility, and increased reactivity through mechanical activation, carbonation can be carried out more easily and efficiently. The surface is optimally prepared and reactive, minimizing contact time / residence time while still achieving a very high to high degree of carbonation.

[0043] In a further development of the first and second further embodiments of the invention, the carbonation is carried out in a mechanical fluidized-bed reactor. The mechanical fluidized-bed reactor is selected with a Werkzeug-Froude number of 3 to 10. This achieves good mixing and fluidization of the fluidized bed, which in turn enables very good exchange between the carbon dioxide-containing gas phase and the solid.

[0044] In a third further embodiment of the invention, carbonation and mechanical activation are carried out in a single step. For this purpose, the milling, which causes mechanical activation, takes place in a carbon dioxide-rich atmosphere. The advantage is that only one device is required, thus reducing space and time requirements. The disadvantage is that the material is ground while moist, which in turn means more mass in the mill and thus increases the energy input.

[0045] In a further embodiment of the invention, the activated mineral material is examined to determine its activation. One or more methods are selected from the group comprising IR spectroscopy, RAMAN spectroscopy, X-ray diffraction analysis, heat flow calorimetry, thermogravimetry, scanning electron microscopy, particle size and / or particle shape analysis, and NMR spectroscopy. Depending on the result, the energy input into the mill or the throughput through the mill can be adjusted, for example.

[0046] In a further embodiment of the invention, the mineral material is moistened to a moisture content of 5 to 25 wt.%, preferably 10 to 15 wt.%, during carbonation. Moisture measurement and / or re-moistening can also be provided during carbonation to prevent drying out and thus insufficient carbonation, while simultaneously keeping the moisture content as low as possible. Since the product must be stored dry at the end, any water introduced must ultimately be removed during the process, which is labor-intensive and energy-intensive.

[0047] In a further aspect, the invention relates to a device for carbonation and mechanical activation of mineral material such as old concrete, old cement block, and the like. The device is preferably used to carry out the method according to the invention. With the device, it is possible to produce a higher-quality cement additive from old concrete and the like than is possible with conventional devices and methods. The device has a high-energy mill for mechanical activation. This means that the device not only carbonation or mechanical activation takes place, but that the device produces a novel product that is both carbonated and mechanically activated. In a further embodiment of the invention, the high-energy mill is an agitator ball mill.

[0048] There are three basic concepts for the combination of carbonation and mechanical activation, each with its own advantages. These are: 1) first carbonation, then mechanical activation; 2) first mechanical activation, then carbonation; 3) combined mechanical activation and carbonation. These will be discussed below.

[0049] In a first further embodiment of the invention, a carbonation device is arranged upstream of the high-energy mill. A drying device is preferably arranged between the carbonation device and the high-energy mill. The drying device is, for example, a riser dryer, preferably equipped with a deagglomerator. This allows the moisture required for carbonation to be easily removed. The optional deagglomerator, for example, a beater mill, can efficiently break up clumps formed by the moisture, simplifying subsequent grinding.

[0050] In a second further embodiment of the invention, a carbonation device is arranged downstream of the high-energy mill. The advantage is that the material optimization achieved through mechanical activation allows the carbonation process to proceed much faster, more efficiently, and more completely.

[0051] In a further embodiment of the invention, an intermediate storage unit is arranged between the high-energy mill and the carbonation device. This embodiment is particularly preferred when the high-energy mill and / or the carbonation device operate discontinuously, i.e., in batch mode.

[0052] In a further embodiment of the invention of the first or second embodiment, the carbonation device is a plowshare mixer, a twin-shaft batch mixer, or an entrained-flow reactor. In a further embodiment of the invention, the mechanical fluidized-bed reactor in the form of a plowshare mixer has a substantially horizontally arranged container. A shaft is arranged centrally along the longitudinal axis of the container, with mixing tools arranged radially on the shaft. In the simplest case, these mixing tools can be rod-shaped and arranged perpendicular to the shaft. The mixing tools are particularly preferably plowshare-shaped. Examples of plowshare-shaped mixing tools can be found, for example, in DE 27 29 477 C2 or DE 197 06 364 C2. Essentially horizontal in the sense of the invention is to be understood as meaning according to EP 0 500 561 B1.

[0053] Regarding the design as an entrained flow reactor, reference is made to DE 10 2022 132 073.

[0054] In a third further embodiment of the invention, the high-energy mill has a feed for carbon dioxide-containing gas. The carbon dioxide-containing gas is thus fed directly into the high-energy mill.

[0055] The device according to the invention is explained in more detail below with reference to embodiments shown in the drawings.

[0056] Fig. 1 first embodiment

[0057] Fig. 2 second embodiment

[0058] Fig. 3 third embodiment

[0059] Fig. 1 shows a first embodiment in which carbonation is initially followed by mechanical activation. The old cement block is located in a first reservoir 40. From there, it is introduced into a carbonation device 20, in particular a ploughshare mixer. Water is supplied via a water inlet 21 so that a moisture content of, for example, 20 wt.% is set. In addition, a carbon dioxide-containing gas, which can be, for example, an exhaust gas from another process, is supplied via the CC inlet. A mechanical fluidized bed is created inside the ploughshare mixer, i.e. the solid is swirled not by a gas stream, but by the mixing tools. This ensures thorough mixing and thus good contact between the moistened old cement block and the CO2.The carbonated product is fed to a drying device 30, while the CO2-depleted gas is discharged via the residual gas outlet 23.

[0060] The drying device 30 has a deagglomerator, a riser dryer, and a separation cyclone at the bottom. Warm air is supplied via the hot gas inlet at the bottom, which is discharged as humidified gas through the humid gas outlet 32 ​​behind the separation cyclone.

[0061] A further storage tank 40 is located behind the drying device 30. From the storage tank 40, the carbonated material flows into a high-energy mill 10, for example, a stirred ball mill. The stirred ball mill is operated with a grinding media filling level of 65%, using steel balls with a diameter of 4 mm as the grinding media. The energy input is 350 kW / m 3The agitator ball mill has a length-to-diameter ratio of 4 and operates at a peripheral speed of 4 m / s. The ratio of gas flow to material flow is 0.01 m 3 / kg. The activated carbonated product leaves the high-energy mill 10 via the product outlet.

[0062] Fig. 2 shows a second embodiment, in which mechanical activation is first followed by carbonation. Used cement stone is fed from a storage tank 40 to a high-energy mill 10, for example, an agitated ball mill. The agitated ball mill is operated with a grinding media filling level of 65%, using steel balls with a diameter of 4 mm as the grinding media. The energy input is 350 kW / m 3The agitator ball mill has a length-to-diameter ratio of 4 and operates at a peripheral speed of 4 m / s. The ratio of gas flow to material flow is 0.01 m 3 / kg. The mechanically activated material is transferred to another storage unit 40.

[0063] From the additional reservoir 40, the activated material is introduced into a carbonation device 20, in particular a plowshare mixer. Water is supplied via a water inlet 21, so that a moisture content of, for example, 20 wt.% is set. In addition, a carbon dioxide-containing gas, which can be, for example, an exhaust gas from another process, is supplied via the CC inlet. A mechanical fluidized bed is created inside the plowshare mixer, i.e., the solid is swirled not by a gas stream, but by the mixing tools. This ensures thorough mixing and thus good contact between the moistened waste cement block and the CO2. The carbonated product is removed via the product outlet 11.

[0064] Fig. 3 shows a third embodiment in which carbonation and mechanical activation occur simultaneously. Used cement stone is fed from a storage tank 40 to a high-energy mill 10, for example, an agitated ball mill. The agitated ball mill is operated with a grinding media filling level of 65%, using steel balls with a diameter of 4 mm as the grinding media. The energy input is 350 kW / m 3 The agitator ball mill has a length-to-diameter ratio of 4 and operates at a peripheral speed of 4 m / s. The ratio of gas flow to material flow is 0.01 m 3 / kg. The gas stream supplied via the CO2 inlet 22 contains CO2, for example, an exhaust gas. Water is supplied via the water inlet 22 to achieve a humidity of 20 wt.%. The CO2-depleted residual gas is discharged via the residual gas outlet, and the finished mechanically activated and carbonated product is removed via the product outlet 11.

[0065] Reference symbol

[0066] 10 high-energy mill

[0067] 11 Product outlet

[0068] 20 Carbonation device

[0069] 21 Water inlet

[0070] 22 CO2 inlet

[0071] 23 Residual gas outlet

[0072] 30 Drying device

[0073] 31 Hot gas inlet

[0074] 32 Wet gas outlet 40 Storage

Claims

Patent claims 1. A process for carbonating mineral material such as old concrete, old cement block and the like, wherein the mineral material is carbonated and mechanically activated.

2. A method according to claim 1, characterized in that the mechanical activation is carried out by grinding with an energy input per mill volume of at least 100 kW / m 3 , preferably at least 200 kW / m 3 , is carried out.

3. The method according to claim 2, characterized in that the mechanical activation is carried out by grinding in a micromill, wherein the micromill is selected from the group comprising vibrating mill, planetary ball mill and agitator ball mill.

4. The method according to claim 3, characterized in that the micromill is a stirred ball mill, wherein the micromill is filled with a grinding media filling level of 50 vol.% to 95 vol.%, preferably of 60 vol.% to 70 vol.%, wherein the bulk volume of the grinding media is related to the grinding chamber volume of the micromill.

5. The method according to any one of claims 3 to 4, characterized in that the micromill is a stirred ball mill, the micromill being operated at a peripheral speed of 2 m / s to 6 m / s, preferably of 3 m / s to 5 m / s, particularly preferably of 3.5 m / s to 4.5 m / s.

6. Process according to one of claims 2 to 5, characterized in that the grinding takes place in a circuit, wherein a size-selective separation takes place in the circuit, wherein the coarse fraction of the size-selective separation is recycled in the circuit and the fine fraction of the size-selective separation is discharged from the circuit.

7. Process according to one of the preceding claims, characterized in that the carbonation is carried out in a ploughshare mixer, a twin-shaft batch mixer or an entrained flow reactor.

8. Process according to one of the preceding claims, characterized in that the carbonation is carried out in a separate step before the mechanical activation.

9. Process according to one of claims 1 to 7, characterized in that the carbonation is carried out in a separate step after the mechanical activation.

10. The process according to claim 9, characterized in that the carbonation is carried out in a mechanical fluidized bed reactor, the mechanical fluidized bed reactor being selected with a Werkzeug-Froude number of 3 to 10.

11. Process according to one of claims 1 to 7, characterized in that the carbonation and the mechanical activation are carried out in a common step.

12. Method according to one of the preceding claims, characterized in that the activated mineral material is examined to determine the activation, wherein one or more methods are selected from the group comprising IR spectroscopy, RAMAN spectroscopy, X-ray diffraction analysis, heat flow calorimetry, thermogravimetry, scanning electron microscopy, particle size and / or particle shape analysis, NMR spectroscopy.

13. A process according to any one of the preceding claims, characterized in that during carbonation the mineral material is moistened to a moisture content of 5 to 25% by weight.

14. Device for carbonation and mechanical activation of mineral material such as old concrete, old cement block and the like, wherein the device has a high-energy mill (10) for mechanical activation.

15. Device according to claim 14, characterized in that the high-energy mill (10) is a stirred ball mill.

16. Device according to one of claims 14 to 15, characterized in that a carbonation device (20) is arranged in front of the high-energy mill (10).

17. Device according to claim 16, characterized in that a drying device (30) is arranged between the carbonation device (20) and the high-energy mill (10).

18. Device according to one of claims 14 to 15, characterized in that a carbonation device (20) is arranged after the high-energy mill (10).

19. Device according to one of claims 16 to 17 or 18, characterized in that the carbonation device (20) is a ploughshare mixer, a twin-shaft batch mixer or an entrained flow reactor.

20. Device according to one of claims 14 to 15, characterized in that the high-energy mill (10) has a supply for carbon dioxide-containing gas.

Images