WO2024188710A1 - Cement additive made from old concrete - Google Patents

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

Cement additive from old concrete

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.

It is becoming increasingly necessary to conserve natural resources and to use recycled materials. Carbon dioxide emissions are also a critical cause of global warming. Therefore, there is an increasing focus on separating carbon dioxide from exhaust gases and storing or using it permanently. One possible form of storage is to introduce it into the ground as liquefied carbon dioxide. However, this process is not without controversy, as permanent retention is not necessarily guaranteed and if it were to escape, the greenhouse effect would be intensified again, especially since further energy is required for separation and storage, potentially producing carbon dioxide again.

One of the carbon dioxide-intensive industries is the cement industry. On the one hand, a lot of energy is required for the firing process, which leads to carbon dioxide emissions when using conventional fossil fuels. On the other hand, carbon dioxide is released from the raw material, mainly limestone or marl, as a result of the process.

On the other hand, large quantities of old concrete are produced when concrete structures are demolished. Therefore, there is currently discussion about recycling concrete, for example to produce new cement and concrete. However, the problem here is that sand and the hardened cement, for example, are mixed and bonded together in a way that is difficult to separate. The sand-free or at least low-sand component of the old concrete is also known as old cement stone. It is known that concrete can absorb carbon dioxide during its lifespan, 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, so it is absorbed very slowly and therefore over a long period of time. Periods of time approximately up to 1/4 of the carbon dioxide originally released is reabsorbed.

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

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

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

Activated clays have become established as an additive, particularly in the cement industry. The current method is to dry and calcine the clays, i.e. to activate them thermally. This requires energy for heating, and the high temperature can also cause other changes in the material, which may be undesirable. The thermal process also requires flue gas cleaning to separate the resulting nitrogen oxide and sulphur oxide emissions, etc. In addition, the thermal process will in future require the use of processes to separate and, if necessary, clean the carbon dioxide produced or released.

From WO 2017 / 008 863 A1 a method and a plant arrangement for processing and activating a raw material is known.

EP 3 909 682 A1 discloses a method and a roller mill for the thermomechanical activation of a clay mixture.

A process for the tribochemical activation of binders and additives is known from DE 10 2015 106 109 A1.

From WO 2020 058 247 A1 a method and plant for processing material containing cement stone is known. It is therefore being considered to activate clays by introducing mechanical energy during the grinding process in order to replace thermal energy with green electricity and to avoid the oxidation of iron, for example.

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

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.

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.

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.

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

The method according to the invention is used for carbonation of mineral material such as old concrete, old cement brick and the like. Such methods are known from the prior art. On the one hand, carbon dioxide can be bound and emissions can be avoided. 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 material carbonated and mechanically activated in this way is even more suitable as a cement additive. It is assumed that these two different methods have different positive effects on different components. change. Concrete and thus also old concrete and fractions produced from it, such as old cement brick, 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, are converted during carbonation, for example to CaCOs or MgCOs. This assumption from the well-known carbonation of old concrete can be seen as a probable reaction. It can also be assumed, for example, that a sand component, for example SiO2, can be changed by 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 various activation mechanisms, but it has turned out that this combination results in a particularly suitable cement additive.

Old concrete, old cement bricks and the like can also contain components of bricks, clinker bricks or bricks, for example. On the one hand, these components are often difficult to separate during demolition. On the other hand, they are made from clays, which can also be mechanically activated.

Furthermore, old concrete, old cement brick and the like can also contain asbestos minerals, for example. On the one hand, these are often contained in old building materials. On the other hand, these are fundamentally crushed by mechanical activation and embedded in the matrix, so that they can be processed safely and thus disposed of sustainably. In particular, the asbestos fibers can no longer be detected using an electron microscope after mechanical activation. The method according to the invention is therefore also suitable for the sustainable disposal of asbestos.

Mechanical activation is an effect that occurs during grinding with a very high energy input. During grinding, three phases can be separated from each other. The first phase is characterized by the particle size becoming smaller with increasing grinding energy. This is the normal grinding range in which all usual grinding processes. At the end of the first phase, a plateau is reached where additional grinding energy no longer changes the particle size. This phase of the grinding process is therefore avoided, as no additional profit can be achieved at higher costs. If more grinding energy is introduced, a third phase occurs in which the particle size actually increases again. This third phase is therefore avoided all the more, as 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. Inert materials in particular, such as SiO2, can be changed in this way so that it is no longer just a very stable and regular crystal lattice, but has reactive centers via imperfections and defects. One of the advantages is that this activation takes place (more or less) at room temperature and not, like the thermal activation of clays, at 800 °C to 1000 °C. 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 is currently the focus of attention, especially for clays.

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

Even if the problem of red discoloration, which is known from clays, is not a major problem 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 to be particularly suitable for obtaining a particularly good cement additive. A particularly good cement additive means that it can be used in particularly high proportions (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 saves on the burning of 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/1. Preferably, the mechanical activation is carried out by grinding with an energy input per ton of mineral material of at least 500 kWh/1.

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

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%.

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 vibrating mills, planetary ball mills and agitator ball mills. The micromill is particularly preferably an agitator ball mill. This type of mill has proven to be particularly suitable for achieving the necessary high energy inputs while at the same time being scalable to industrial scale.

In a further embodiment of the invention, the micromill is a stirred ball mill. The micromill is filled with a grinding media 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 dense ball packing, even with a theoretical grinding media filling level of 100% there is a corresponding free space, which can be taken up by the mineral material to be activated, for example. However, since the filling level of a grinding media bed depends very much on the shape and uniformity of the grinding media, it is practically easier to relate the grinding media filling level to the bulk volume and not to the actual (filled) volume. In a further embodiment of the invention, an agitator ball mill with a length-to-diameter ratio of 2.5 to 5 is selected.

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

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

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 from 3 m/s to 5 m/s, particularly preferably from 3.5 m/s to 4.5 m/s.

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 set 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.

In a further embodiment of the invention, the grinding takes place in a circuit. This means that the ground material coming out of the mill is fed back into the inlet of the mill. A size-selective separation takes place in the circuit, for example with a sifter. The coarse fraction of the size-selective separation is fed back into the circuit and the fine fraction of the size-selective separation is discharged from the circuit.

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

In a further embodiment of the invention, the carbonation is carried out in a mechanical fluidized bed reactor. It has been shown that it is particularly a mechanical fluidized bed reactor results in a very advantageous change in the finely ground mineral raw material. The relatively uniform size distribution of the agglomerated particles prevents both adhesion in a heat treatment device and the unwanted transfer of the product into the gas phase. The latter means that the product has to be filtered out of the exhaust gas flow and is thus practically circulated, which represents a burden on the overall process.

In a further embodiment of the invention, the mechanical fluidized bed reactor has a container arranged essentially horizontally. 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 vertically on the shaft. The mixing tools are particularly preferably designed in the shape of a plowshare. 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 in EP 0 500 561 B1.

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

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

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.

In a first further embodiment of the invention, the carbonation is carried out in a separate step before the 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% by weight, so that the reaction takes 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 removed and any clumps that may have been created by the water are dissolved again.

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

The advantage of this design is the particularly fine material used for carbonation. Due to the already increased surface area, the optimized accessibility of pores and the increased reactivity through mechanical activation, carbonation can be carried out more easily and efficiently. The surface is optimally prepared and reactive, so that the contact time / residence time is minimized and yet a very high to higher degree of carbonation can be achieved.

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.

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

In a further embodiment of the invention, the activated mineral material is examined to determine the activation. For the examination, 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. Depending on the result, for example, the energy introduced into the mill or the throughput through the mill can be adjusted.

In a further embodiment of the invention, the mineral material is moistened to a moisture content of 5 to 25% by weight, preferably 10 to 15% by weight, during carbonation. Moisture measurement and/or re-moistening can also be provided during carbonation in order to prevent drying out and thus insufficient carbonation, but at the same time to keep the moisture content as low as possible. Since the product must be stored dry at the end, any water introduced must ultimately be removed again during the process, which costs effort and energy.

In a further aspect, the invention relates to a device for carbonation and mechanical activation of mineral material such as old concrete, old cement brick 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 carbonates or mechanically activates, but that the device produces a new type of product which is both carbonated and mechanically activated. In a further embodiment of the invention, the high-energy mill is a stirred ball mill.

There are three basic concepts for the combination of carbonation and mechanical activation, each of which has its 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.

In a first further embodiment of the invention, a carbonation device is arranged in front 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 removed in a simple manner. The optional deagglomerator, for example a beater mill, can efficiently break up clumps formed by the moisture, which simplifies the subsequent grinding.

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

In a further embodiment of the invention, an intermediate storage facility 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.

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 the 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 vertically on the shaft. The mixing tools are particularly preferably designed in the shape of a plowshare. Examples of plowshare-shaped mixing tools can be found, for example, in DE 27 29 477 C2 or DE 197 06 364 C2. In the sense of the invention, substantially horizontal is to be understood as in EP 0 500 561 B1.

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

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 therefore fed directly into the high-energy mill.

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

Fig. 1 first embodiment

Fig. 2 second embodiment

Fig. 3 third embodiment

Fig. 1 shows a first embodiment in which carbonation is first followed by mechanical activation. The old cement brick is in a first storage unit 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% by weight is set. In addition, a carbon dioxide-containing gas is supplied via the CC inlet, which can be, for example, an exhaust gas from another process. A mechanical fluidized bed is created inside the ploughshare mixer, so the solid is not pressed by a gas flow but by the Mixing tools swirl the mixture. This ensures good mixing and thus good contact between the moistened old cement brick and the CO2. The carbonated product is fed to a drying device 30, while the CO2-depleted gas is released via the residual gas outlet 23.

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

A further storage facility 40 is arranged behind the drying device 30. From the storage facility 40, the carbonated material passes into a high-energy mill 10, for example an agitator ball mill. The agitator ball mill is operated with a grinding media filling level of 65%, with steel balls with a diameter of 4 mm being used 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 is operated at a peripheral speed of 4 m/s. The ratio of gas volume flow to material flow is 0.01 m ³ /kg. The carbonated product activated in this way leaves the high-energy mill 10 via the product outlet.

Fig. 2 shows a second embodiment in which mechanical activation and then carbonation take place first. Old cement stone is fed from a storage unit 40 to a high-energy mill 10, for example an agitator ball mill. The agitator ball mill is operated with a grinding media filling level of 65%, with steel balls with a diameter of 4 mm being used 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 is operated at a peripheral speed of 4 m/s. The ratio of gas volume flow to material flow is 0.01 m ³ /kg. The mechanically activated material is transferred to a further storage unit 40.

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

Fig. 3 shows a third embodiment in which carbonation and mechanical activation take place simultaneously. Old cement stone is fed from a storage facility 40 to a high-energy mill 10, for example an agitator ball mill. The agitator ball mill is operated with a grinding media filling level of 65%, with steel balls with a diameter of 4 mm being used 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 is operated at a peripheral speed of 4 m/s. The ratio of gas volume flow to material flow is 0.01 m ³ /kg. The gas flow fed in via the CC inlet 22 contains CO 2 , for example exhaust gas. Water is fed in via the water inlet 22 to set a moisture content 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.

Reference symbols

10 High-energy mill

11 Product outlet

20 Carbonation device

21 Water inlet

22 CO2 inlet

23 Residual gas outlet

30 Drying device

31 Hot gas inlet

32 Wet gas outlet 40 storage

Claims

Patent claims 1. Process for carbonating mineral material such as old concrete, old cement brick and the like, wherein the mineral material is carbonated and mechanically activated. 2. Process 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 of at least 200 kW/m ³ . 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. Process according to claim 3, characterized in that the micromill is a stirred ball mill, the micromill being filled with a grinding media filling level of 50 vol.% to 95 vol.%, preferably 60 vol.% to 70 vol.%, the bulk volume of the grinding media being related to the grinding chamber volume of the micromill. 5. Method according to one of claims 3 to 4, characterized in that the micromill is a stirred ball mill, wherein the micromill is operated at a peripheral speed of 2 m/s to 6 m/s, preferably from 3 m/s to 5 m/s, particularly preferably from 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 returned 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 plowshare 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. Process according to claim 9, characterized in that the carbonation is carried out in a mechanical fluidized bed reactor, wherein the mechanical fluidized bed reactor is 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 method or several methods are selected for the examination 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. Process according to one of the preceding claims, characterized in that during carbonation the mineral material is moistened to a moisture content of 5 to 25 wt.%. 14. Device for carbonation and mechanical activation of mineral material such as old concrete, old cement brick 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 an agitator 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.