Mechanochemical activation with a heat coupling process

Abstract

The invention relates to a device for a mechanochemical activation of a humid starting material. The device has a drying device (10) and a mill (20), wherein the drying device (10) and the mill (20) are connected in order to transfer the material dried in the drying device (10), and the mill (20) and the drying device (10) are connected in order to transfer the exhaust heat of the mill (20) to the drying device (10).

Description

[0001] Mechano-chemical activation with heat coupling

[0002] The invention relates to the mechano-chemical activation of moist starting material.

[0003] 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. Furthermore, the thermal process will require the use of processes to capture and, if necessary, purify the carbon dioxide produced or released.

[0004] Cement aggregates are now used to reduce clinker and thus carbon dioxide emissions. According to DIN EN 450-1, the activity index describes the ratio (in %) of the compressive strengths of standardized mortar prisms tested at the same age, which contain a mass fraction of 75% test cement and a mass fraction of 25% cement aggregate, and standardized mortar prisms produced exclusively with test cement. The test cement used is a Portland cement (type CEM I) with a strength class of 42.5 or higher. The cement aggregate to be evaluated (supplementary cementitious material, SCM) can be less or more effective than the test cement. An SCM considered inert, such as limestone, results in an activity index of 75%, meaning the SCM makes no contribution to strength development. However, high-performance SMCs such as granulated blast furnace slag can also achieve activity values ​​of more than 100 up to about 120.If the activity index is above 100, this means that the clinker content in the binder can be further reduced, namely by exactly the amount necessary to restore an activity index of 100. The clinker content is usually replaced by an inert, finely ground filler such as limestone, which is considerably cheaper to produce than clinker. Therefore, so-called mechano-chemical activation through intensive grinding is increasingly being discussed. The process of mechano-chemical activation can be used to produce cement aggregates that can optionally replace other secondary cementitious materials, i.e., SCMs. Ideally, SCMs possess pozzolanic, latent hydraulic, or even hydraulic properties, so that these materials contribute to the strength development when the finished binder is mixed with water.Inert materials such as limestone do not exhibit this additional strength development when mixed with water.

[0005] During mechanochemical activation, previously crystalline water remains in the mineral material, for example, as inner-layer water (xerogels). This differentiation from thermally activated materials is an essential quality feature of mechanochemically activated materials when used as cement aggregates, as it results in improved binding properties, particularly low water requirements. This has an improved impact, for example, on the strength development and workability of the binder-containing mortar or concrete, without the need for expensive cement additives such as superplasticizers.

[0006] From the subsequently published DE 10 2023 106 210 a process for grinding and pozzolanic activation in a stirred ball mill is known.

[0007] 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.

[0008] The subsequently published DE 10 2023 106 221 discloses the combination of mechanochemical and thermal activation in at least one agitated ball mill. The subsequently published DE 10 2023 106 222 discloses color optimization in the mechanochemical activation of clays.

[0009] From the subsequently published DE 10 2023 123 525 a cement additive made from old concrete is known.

[0010] One advantage of mechanochemical activation is that even clays with a lower kaolin content, which are not suitable for thermal activation, can be mechanochemically activated. This broadens the available raw material base.

[0011] Because clays are a complex system (especially compared to the firing of limestone), different activation processes result in different products (activated clays) with different properties. Likewise, the diversity of the clays that can be used means that not every process is suitable for every clay.

[0012] Mechanochemical activation differs fundamentally from thermal activation in terms of the understanding of the processes involved. While thermal activation is primarily determined by temperature and time, mechanochemical activation in a mill appears to be considerably more complex and dependent on many more parameters. Furthermore, a large portion of the input grinding energy is converted into heat.

[0013] When grinding a material, three stages can be observed depending on the energy input. In the first stage, the particle size decreases (more or less linearly) with the energy input (Rittinger zone). Put simply, the more you grind, the finer the product becomes. However, there is a limit to this, a particle size that can hardly be exceeded. From this point on, a second stage follows, in which the particle size cannot be changed any further with further energy input (aggregation zone). For economic reasons, the transition from the first to the second stage is avoided during grinding, as there is no further effect for the additional effort. If the energy input is increased even further, a third stage can be reached, in which a further increase in the particle size can be observed (agglomeration zone).This area is therefore much more likely to be avoided during grinding, as a better result in terms of particle size distribution can be achieved with less effort.

[0014] However, it has been shown that high energy inputs, i.e., in the third stage, lead to changes in the material itself. In clays, for example, just like thermal activation, this leads to activation—that is, to a reactivity that enables its use as a binder (and thus as a clinker substitute). This third stage is therefore the mechanochemical activation and differs from the grinding, which is carried out in the first stage. Thus, according to the state of the art, grinding is not mechanochemical activation.

[0015] During mechanochemical activation, only about 25% of the input energy is converted into chemical energy, while about 75% is released as heat. This heat is in the maximum range of 200°C to 300°C, thus representing only a low-value heat that cannot be used efficiently, for example, for electricity generation. At the same time, one of the key reactants for mechanochemical activation is clay, which has a high moisture content and must be dried before activation, which requires thermal energy.

[0016] DE 32 25 989 A1 discloses a method and a device for drying and grinding mineral raw material.

[0017] The object of the invention is to use the heat released during the mechano-chemical activation in an efficient manner for a preliminary drying process, for example of clays.

[0018] This object is achieved by the method having the features specified in claim 1 and by the device having the features specified in claim 7. Advantageous further developments emerge from the subclaims, the following description and the drawing.

[0019] The invention relates to a method for drying and mechanochemically activating a material, preferably using a device according to the invention. The method comprises the following steps: a) drying the material in a drying device, b) transferring the dried material from the drying device to a mill, and c) mechanochemically activating the material in the mill.

[0020] At least a portion of the heat generated during the mechanochemical activation is transferred to at least one heat exchange fluid. The heat exchange fluid is fed to the drying device, or the heat is transferred to the drying device via a heat exchanger or heat pump.

[0021] The first alternative is usually provided when the gas stream from the mill is fed directly to the drying device as a warm gas stream for drying. The second alternative is usually provided when a jacket heat exchange fluid, such as water, is used as the heat transfer medium. Preferably, a combination can also be used, where heat is transferred both directly to the gas and indirectly via a cooling liquid.

[0022] In a further embodiment of the invention, the heat generated during the mechanochemical activation is transferred as completely as possible (within the scope of the achievable efficiency) to at least one heat exchange fluid. This is sufficient to dry even clays with a moisture content between 20 and 30 wt.%.

[0023] In a further embodiment of the invention, at least 40% of the heat generated during mechano-chemical activation is transferred to at least one heat exchange fluid. This is sufficient for the lower moisture levels of conventional clays. In a further embodiment of the invention, the heat exchange fluid is a gas stream passed through the mill. The gas stream is fed to the drying device. This has two advantages. Firstly, the gas stream comes into direct contact with the milled material, thus having the highest temperature level. Secondly, the gas stream can be used directly for drying, i.e. also for removing the water vapor generated during drying. A change of media in a heat exchanger is not necessary. This makes this embodiment simple and efficient. The only disadvantage is that the heat capacity of the gas stream is limited and not sufficient to completely transport heat.

[0024] In a further embodiment of the invention, the heat exchange fluid or an additional heat exchange fluid (if the heat exchange fluid is a gas stream as mentioned above) is a jacket heat exchange fluid passed through a cooling jacket of the mill. The advantage is that very large amounts of heat can be efficiently dissipated, which is regularly necessary to maintain the temperature in the mill. The disadvantage is that such a cooling system regularly achieves a lower temperature level, especially when cooling water is used as the (additional) heat exchange fluid.

[0025] In a further embodiment of the invention, water is chosen as the jacket heat exchange fluid. To raise the lower temperature level, the water is preferably passed through a heat pump. This makes it possible to raise a gas stream used for drying in the drying device to a sufficiently high temperature.

[0026] In a further embodiment of the invention, a thermal oil is selected as the jacket heat exchange fluid. This has the advantage that significantly higher temperatures can be achieved with a thermal oil than with liquid water. The thermal oil is passed through a heat exchanger to transfer the heat through a gas stream supplied to the drying device. "Upgrading" the heat in a heat pump is generally unnecessary, as this is simpler in terms of equipment. However, thermal oil is more expensive than water. In a further embodiment of the invention, the mechanochemical activation is carried out at less than 300°C, preferably at less than 250°C.

[0027] In a further embodiment of the invention, the mill is operated with a residence time of the material to be ground in the mill of at least 5 minutes, preferably at least 10 minutes, particularly preferably at least 20 minutes.

[0028] In a further aspect, the invention relates to a device for the mechanochemical activation of a moist starting material. An example of a moist starting material is, in particular, clay. However, it can also be, for example, moist old concrete. Moist is understood to mean water that is volatile up to 100°C; this does not mean water of crystallization or, for example, the water stored between the layers in clays, which is only released at higher temperatures. The device comprises a drying device and a mill. The mill is preferably a stirred ball mill. The drying device and the mill are connected to transfer the material dried in the drying device. It is essential that the mill and the drying device are connected to transfer the waste heat from the mill to the drying device.This allows for particularly effective use of the high proportion of energy introduced into the mill and converted only into heat, as the temperature level in the mill is typically below 300°C, meaning the released heat is available at a low energy level, yet is particularly well-suited for drying. In a preferred embodiment, the mill is designed to provide the entire heat requirement of the drying device. This is possible because the mill is not operated for grinding, but rather for mechanochemical activation, i.e., in a range that does not occur during normal grinding. Therefore, the energy input is significantly higher than during the normal grinding process.This means that during normal operation only the mill is connected to the drying device as a heat source and no additional heat generator is required. This has the advantage that using the waste heat eliminates the need for another energy source, be it electricity or fossil fuels. Of course, another heat generator can still be provided, for example for start-up, but this can then be switched off during normal operation. There are two preferred options for transferring the waste heat, which can be used alternatively or, preferably, in combination. These are described in detail below. In simple terms, one option is the gas stream that is passed through the mill and heated there, which is directly and immediately suitable for drying. The second option is the use of a fluid from the cooling jacket, which is integrated into the mill housing and / or the mill shaft.

[0029] The advantage is that the waste heat recovered from the mill can be used to dry a raw material with an initial moisture content (volatile water content, not bound) of up to 20 to 30%. This allows for optimal use of the heat. At the same time, even relatively moist clays can be efficiently activated without additional drying, for example, by using conventional fuels.

[0030] In a further embodiment of the invention, the mill and the drying device are connected via a mill gas line to transfer the waste heat from the mill to the drying device. The gas flowing through the mill interior, along with the material to be ground, is fed directly to the drying device via the mill gas line. With a typical temperature of 200 to 250 °C for the gas stream leaving the mill, it is also particularly suitable for use in the drying device.

[0031] In a further embodiment of the invention, the mill has a cooling jacket with a jacket heat exchange fluid. This is usually necessary because the heat capacity of the gas stream passed through the mill is generally insufficient to dissipate all of the heat generated. In addition, a jacket heat exchange fluid particularly suitable for heat transfer can be used in the cooling jacket, in particular one with a suitably high heat capacity. Due to its heat capacity and availability, water is often preferred, although this usually limits the temperature to 100 °C. A thermal oil, on the other hand, can also be operated at higher temperatures. The cooling jacket is connected to the drying device via a jacket heat exchange fluid connection to transfer the jacket heat exchange fluid heated in the cooling jacket. This allows a larger amount of heat to be transferred overall, but usually at a lower temperature level, compared to the gas stream from inside the mill.

[0032] In a further embodiment of the invention, the jacket heat exchange fluid connection or the drying device comprises a heat pump. For example, if cooling water is used as the jacket heat exchange fluid, a typical temperature level upon leaving the mill is 70 °C. To use this for drying, it is advantageous to raise it to a higher temperature level and then transfer it to a gas stream so that drying can then take place in the gas stream. A higher temperature level is advantageous in order to utilize the high evaporation enthalpy of the water (the moisture content of the material).

[0033] In a further embodiment of the invention, the drying device is an entrained-flow dryer. Alternatively, or upstream, a belt dryer can be used for very moist material.

[0034] In a further embodiment of the invention, the mill has an internal volume of at least 1 m3 on.

[0035] In a further embodiment of the invention, the mill has an energy density of at least 200 kW / m 3 on.

[0036] In a further embodiment of the invention, the mill has a length of at least 2 m, preferably at least 2.5 m.

[0037] In a further embodiment of the invention, the mill has a cross-sectional area perpendicular to the longitudinal axis of at least 0.71 m 2 , preferably at least 0.75 m 2 In a further embodiment of the invention, the mill has a length-to-diameter ratio of at least 3, preferably at least 3.5.

[0038] The device according to the invention is explained in more detail below using an embodiment shown in the drawing.

[0039] Fig. 1 exemplary device

[0040] An exemplary device is shown in Fig. 1. A moist starting material, for example, clay, enters the drying device 10 via the reactant feed 32. It is transferred from the drying device 10 in a dried state to the mill 20, activated there, and then removed via the product removal 34.

[0041] The mill 20 has a cooling jacket 22, in which, in the example shown, water is used as the coolant. The water leaves the cooling jacket 22 at, for example, 80 °C. Since this temperature level is only suitable for drying to a very limited extent, the water is fed into a heat pump 50 of the drying device 10, where it is raised to a higher temperature level, and then heated in a heat exchanger 60 to form a dry gas stream 42. This then dries the material and leaves the drying device 10 as a moist gas stream 44.

[0042] If thermal oil were used in the cooling jacket 22 instead of water, the heat pump 50 would possibly be omitted, and the cooling liquid (the thermal oil) would be led directly from the cooling jacket 22 into the heat exchanger 60.

[0043] Reference symbol

[0044] 10 Drying device

[0045] 20 Mill

[0046] 22 Cooling jacket

[0047] 32 Educt feed

[0048] 34 Product removal

[0049] 42 Dry gas stream Wet gas stream

[0050] heat pump

[0051] heat exchanger

Claims

Patent claims 1. A method for drying and mechano-chemically activating a material, the method comprising the following steps: a) drying the material in a drying device (10), b) transferring the dried material from the drying device (10) to a mill (20), c) mechano-chemically activating the material in the mill (20), wherein at least part of the heat generated during the mechano-chemical activation is transferred to at least one heat exchange fluid, wherein the heat exchange fluid is fed to the drying device (10) or transfers the heat to the drying device (10) via a heat exchanger (60) or a heat pump (50).

2. The method according to claim 1, characterized in that the heat exchange fluid is a gas stream passed through the mill (20), the gas stream being fed to the drying device (10).

3. Method according to one of claims 1 to 2, characterized in that the heat exchange fluid or a further heat exchange fluid is a jacket heat exchange fluid guided through a cooling jacket (22) of the mill (20).

4. Method according to claim 3, characterized in that water is selected as the jacket heat exchange fluid, the water being passed through a heat pump (50).

5. The method according to claim 3, characterized in that a thermal oil is selected as the jacket heat exchange fluid, the thermal oil being passed through a heat exchanger (60) for transferring the heat by a gas stream supplied to the drying device (10).

6. Process according to one of claims 1 to 5, characterized in that the mechano-chemical activation is carried out at less than 300 °C, preferably at less than 250 °C.

7. A device for the mechano-chemical activation of a moist starting material, the device comprising a drying device (10) and a mill (20), the drying device (10) and the mill (20) being connected for transferring the material dried in the drying device (10), the mill (20) and the drying device (10) being connected for transferring the waste heat from the mill (20) to the drying device (10).

8. Device according to claim 7, characterized in that the mill (20) is designed to provide the entire heat requirement of the drying device (10), so that only the mill (20) is connected to the drying device as a heat source.

9. Device according to one of claims 7 to 8, characterized in that the mill (20) and the drying device (10) are connected via a mill gas line for transferring the waste heat of the mill (20) to the drying device (10).

10. Device according to one of claims 7 to 9, characterized in that the mill (20) has a cooling jacket (22) with a jacket heat exchange fluid, wherein the cooling jacket (22) is connected to the drying device (10) via a jacket heat exchange fluid connection for transferring the jacket heat exchange fluid heated in the cooling jacket (22).

11. Device according to one of claims 7 to 8, characterized in that the jacket heat exchange fluid connection or the drying device (10) comprises a heat pump (50).

12. Device according to one of claims 7 to 11, characterized in that the drying device (10) is an entrained flow dryer.

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