Electrostatic separator in mechano-chemical activation

Abstract

The invention relates to a device for mechanochemical activation. The device has a mill (10), said mill (10) having a material inlet (12) and a material outlet (14). The device has a first separating device (20) arranged downstream of the mill (10) in the material flow, and the device has a product outlet (30) arranged downstream of the first separating device (20) in the material flow. The invention is characterized in that the first separating device (20) is an electrostatic separator, wherein the electrostatic separator has a first outlet (22) for charged particles and a second outlet (24) for discharged particles. The first outlet (22) is connected to the product outlet (30), and the second outlet (24) is connected to a return line (40), said return line (40) being connected to the material inlet (12) of the mill (10).

Description

[0001] The invention relates to a method using an electrostatic classifier in mechano-chemical activation for separating the activated fraction.

[0002] Activated clays have established themselves as an additive, particularly in the cement industry. The current standard 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 often requires flue gas purification, for example, 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.

[0003] To reduce clinker and thus carbon dioxide emissions, cement aggregates are used today. 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, containing a mass fraction of 75% test cement and a mass fraction of 25% cement aggregate, and of 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 (supplementary cementitious material, SCM) being evaluated 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 achieve 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.

[0004] Therefore, so-called mechanochemical activation through intensive grinding is increasingly being discussed. The process of mechanochemical 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 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 a key 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 on, for example, the strength development and processing 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] From the subsequently published DE 10 2023 106 221, the combination of mechanochemical and thermal activation in at least one agitator ball mill is known.

[0009] The color optimization during the mechano-chemical activation of clays is known from the subsequently published DE 10 2023 106 222.

[0010] From the subsequently published DE 10 2023 123 525 a cement additive made from old concrete is known.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] To operate the activation process efficiently, the challenge arises to separate activated particles from those that have not yet been activated, in order to be able to recycle the particles that have not yet been activated. In mechanical-chemical activation, the primary particles are first crushed in a preliminary step until virtually no further grinding progress occurs (Rittinger stage). This is followed by activation, which leads to changes in the crystal structure, even to the amorphization of the (clay) minerals. Furthermore, agglomeration and aggregation effects of the particles can be observed, which is reflected in a decrease in the specific surface area. The usual concept of separating fine product particles from coarse grit cannot therefore be applied here. The inventive concept therefore proposes to consider the largest particles as activated and separate them, and to consider the finest particles as non-activated and recycle them.

[0015] EP 1 888 243 B1 discloses a device for producing dispersed mineral products.

[0016] The object of the invention is to provide a reliable separation of already activated and not yet activated particles during mechano-chemical activation.

[0017] 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.

[0018] The invention relates to a method for mechanochemical activation. The method comprises the following steps: a) Mechano-chemical activation in a mill, b) transfer of the activated material to an electrostatic classifier, c) separation of the material in the electrostatic classifier into a charged fraction and a discharged fraction, d) transfer of the charged fraction to the product outlet, e) return of the discharged fraction for further mechano-chemical activation.

[0019] Steps d) and e) logically run in parallel.

[0020] It is important to note that amorphization through the mechano-chemical activation of mineral materials, particularly clay minerals, requires increased energy expenditure beyond that for conventional fine grinding, which requires a mill with high energy density, and mechano-chemical activation is carried out with an energy input per grinding chamber volume of at least 100 kW / m3, preferably at least 200 kW / m3. The mechano-chemical activation takes place without the addition of water, i.e. not wet or in a slurry, but rather, similar to the drying process in step a), without the addition of moisture, which distinguishes it from conventional wet grinding processes. A typical value for a ball mill, as an example of a fine mill, is usually around 20 kW / m3 and thus significantly lower.The grinding chamber volume is understood to be the volume available inside the first mill, i.e., the free volume when there is no material or, for example, no grinding media in the mill. Components belonging to the mill, such as a shaft that is arranged to move inside, are therefore not included in the grinding chamber volume, since this volume cannot be occupied by material.

[0021] Mechanochemical activation consists of three phases or stages: In the first stage, the particle size decreases (more or less linearly) with the energy input (Rittinger zone). Simply put, the more you grind, the finer the product becomes. However, there is a limit to this, a particle size that is almost impossible to exceed. From this point on, a second stage follows, in which the particle size cannot be further changed with further energy input (activation and aggregation zone). In this stage, crystallographic structures are destroyed by the breaking of atomic bonds; individual atoms or entire groups of atoms are replaced by other atoms or groups of other atoms. Particularly on the particle surfaces, the initial crystal structure, as well as the bond type and oxidation states of atoms, are altered due to high energy transfer and subsequent chemical reactions.For economic reasons, the transition from the first stage to the second stage, which is necessary for mechanochemical activation, is avoided during normal grinding, where only the creation of surfaces is expected. If the energy input is increased even further, a third stage can be reached, in which an increase in particle size is again noticeable due to the agglomeration of nanoparticles (agglomeration zone), which has a positive effect on the workability of activated clay-cement concrete. This zone 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.

[0022] However, it has been found that with high energy inputs, i.e. in the second stage, changes in the material itself occur, which, for example, in the case of clays, just like thermal activation, leads to activation, i.e. to a reactivity that enables the use as a binder (and thus as a clinker substitute).

[0023] Therefore, with such high energy inputs, subsequent thermal treatment can be dispensed with.

[0024] The key is separation in an electrostatic classifier, rather than, as previously, in a size-selective separation device. This allows the different charge behavior of the activated and non-activated particles to be utilized to separate them and thus obtain a pure activated fraction. The mechanochemical activation in a mill leads to electrostatic charging of the activated particles in the mill, which can then be separated in the electrostatic classifier. This creates a synergistic effect between the mechanochemical activation in step a) and the separation in step c).

[0025] In a further embodiment of the invention, in a first alternative, a size-selective separation is carried out in a second separation device between step a) and step b). The coarse fraction from the size-selective separation is then transferred to the electrostatic classifier in step b). The fine fraction from the size-selective separation is returned to step a). Thus, a size-selective separation is carried out first, as before, followed by the electrostatic separation.

[0026] In a further embodiment of the invention, in a second alternative, a size-selective separation of the charged fraction is carried out in a second separation device between step c) and step d). Only the coarse fraction from the size-selective separation, which contains the most highly activated particles, is fed to the product outlet in step d). The fine fraction is returned to the size-selective separation in step a). Thus, the electrostatic separation is carried out first, and then, in a second step, the small, least activated particles are separated by the size-selective separation and returned for further activation.

[0027] Overall, the activity of the product can be increased, thus increasing its usability as a cement additive, reducing the demand for clinker and thus reducing the total amount of CO2 produced for the production of cement.

[0028] Furthermore, a device for carrying out the method according to the invention is shown. This means that the device must actually be suitable for carrying out the method. Many mills are only designed to operate in the first range, which has a nearly linear relationship between the input grinding energy and particle size, and are therefore not designed to achieve the high energy inputs required for mechano-chemical activation. The device is used for mechano-chemical activation. Conventionally, activation is thermal, with the mineral material being heated to, for example, 900°C to 1000°C. The objective of activation is achieved in mechano-chemical activation through very intensive grinding, whereby significantly more energy is input than is required for comminution.Rather, in this range of mechanical activation, particle growth can be detected as a result of grinding. The device comprises a mill. The mill is preferably an agitated ball mill. Such devices are known, for example, from DE 10 2023 106 210, DE 10 2023 106 217, DE 10 2023 106 221, DE 10 2023 106 222 or DE 10 2023 123 525. These known devices are being further developed in order to improve them. The device comprises a mill. The mill is preferably an agitated ball mill. The mill has a material inlet and a material outlet. The device has a first separation device downstream of the mill in the material stream. The device has a product outlet downstream of the first separation device in the material stream.

[0029] Typically, size-selective separators are used as the first separation device. Since the mill operates in a range where particle growth can be detected due to the energy input, the coarse fraction is the activated fraction, and the fine fraction is the non-activated fraction.

[0030] According to the invention, the first separation device is an electrostatic separator. The electrostatic separator has a first outlet for charged particles and a second outlet for discharged particles. "Discharging" is used in the context of the invention to refer to uncharged or non-charged particles. One outlet is connected to the product outlet, and the second outlet is connected to a return line. The return line is connected to the material inlet of the mill.

[0031] Surprisingly, it has been found that the separation method used in an electrostatic classifier is significantly better at separating activated and non-activated particles. The fraction containing the charged particles turned out to be more reactive and therefore the fraction containing the already activated particles. This separation thus avoids a major problem. With size-selective separation, it is not possible to distinguish between coarse particles that are not even fully ground (i.e., from the first grinding stage, with a roughly linear relationship between grinding energy) and particles that are already activated and have therefore already grown or agglomerated (i.e., from the third stage, where the particle size increases with further input of grinding energy). Therefore, the poorest fraction remains in the product fraction.This disadvantage can be overcome by using an electrostatic classifier.

[0032] In a further embodiment of the invention, the device has a second separation device. The second separation device is a size-selective separation device. Examples of a size-selective separation device are a normal classifier, a cyclone, or even a sieve. Such a size-selective separation device serves to separate a material stream into a coarse fraction (e.g., on top of the sieve) and a fine fraction (beneath the sieve). Each size-selective separation device has a different separation size and separation efficiency, so that although it is fundamentally impossible to say what is coarse or fine, in a specific case with a specific size-selective separation device this is unambiguously and immediately clear. The second separation device has a fine outlet and a coarse outlet. The fine outlet discharges the fine fraction, and the coarse outlet discharges the coarse fraction.

[0033] In a further embodiment of the invention, in a first alternative, the second separation device is located downstream of the first separation device in the material flow. The first outlet of the first separation device is connected to the second separation device. Thus, only the charged particles, and thus the activated particles, are fed to the second separation device. The coarse outlet is connected to the product outlet. Thus, only the largest activated particles are discharged as product, since these have the highest activity. The fine outlet is connected to the material inlet of the mill. This material is therefore returned as not yet sufficiently activated.

[0034] In a further embodiment of the invention, in a second alternative, the second separation device is positioned upstream of the first separation device in the material flow. Thus, all material originating from the mill is first separated by size and only then electrostatically. The material outlet of the mill is therefore connected to the second separation device. The coarse outlet is connected to the first separation device to separate the non-activated, barely crushed particles from the activated and regrown particles. The fine outlet is connected to the material inlet of the mill.

[0035] In a further aspect, the invention relates to the use of an electrostatic separator for separating activated particles from non-activated particles after mechanochemical activation in a mill. As already explained, this has surprisingly proven to be a very suitable separation method, which can, in particular, prevent the unwanted discharge of coarse starting material as product.

[0036] The device for carrying out the method according to the invention is explained in more detail below with reference to embodiments shown in the drawings. Fig. 1Basic form Fig. 2First alternative Fig. 3Second alternative

[0037] In Fig. 1the basic form is shown. The material to be activated is introduced into the mill 10 via the material inlet 12 and is ground there so intensively that a mechano-chemical activation occurs, for example with an energy input of 600 kWh / t. The activated material leaves the mill 10 via the material outlet 14 and is transferred to the first separation device 20, an electrostatic classifier. Here, a separation into charged and uncharged particles takes place. The charged particles leave the first separation device via the first outlet and are fed as a finished activated product to the product outlet 30, which can be, for example, a silo, a filling station or the transfer point to another system. The uncharged, not yet activated particles leave the first separation device 20 through the second outlet 24 and are fed back to the material inlet 12 of the mill 12 via the return line 40.

[0038] Fig. 2 and Fig. 3 show the combination of a first separation device 20 in the form of an electrostatic separator with a second separation device 50 in the form of a size-selective separation device, for example, a cyclone. For simplicity, only the differences from the basic form will be discussed below.

[0039] Fig. 2 shows the first alternative, in which the second separation device 50 is arranged downstream of the first separation device 20. Thus, the charged particles from the first separation device are transferred from the first outlet 22 to the second separation device 50, where they are separated by size into a coarse fraction and a fine fraction. The coarse fraction is fed to the product outlet 30 via the coarse outlet 54, and the fine fraction is fed via the fine outlet 52, and is returned, for example, to the material inlet 12 of the mill 10 via the return line 40.

[0040] In Fig. 3The second alternative is shown, in which the second separation device 50 is arranged between the mill 10 and the first separation device 20. Thus, the activated material from the mill 10 is transferred through the material outlet 14 into the second separation device 50, where it is separated into a coarse fraction and a fine fraction. The fine fraction is fed back to the material inlet 12 of the mill 10 via the fine outlet 52 and, for example, the return line 40. The coarse fraction is fed through the coarse outlet 54 to the first separation device 20. Reference symbol

[0041] 10 Mill 12 Material inlet 14 Material outlet 20 First separator 22 First outlet 24 Second outlet 30 Product outlet 40 Return line 50 Second separator 52 Fine outlet 54 Coarse outlet

Claims

1. A method for mechanochemical activation, the method comprising the following steps: a) mechanochemical activation in a mill (10), b) transfer of the activated material into an electrostatic separator, c) separation of the material in the electrostatic separator into a charged fraction and a discharged fraction, d) transfer of the charged fraction to the product outlet (30), e) return of the discharged fraction for renewed mechanochemical activation.

2. The method according to claim 1, characterized in that between step a) and step b) a size-selective separation is carried out in a second separation device (50), wherein the coarse fraction of the size-selective separation in step b) is transferred into the electrostatic separator, and the fine fraction of the size-selective separation is supplied back to step a).

3. The method according to claim 1, characterized in that between step c) and step d) a size-selective separation of the charged fraction is carried out in a second separation device (50), wherein the coarse fraction of the size-selective separation in step d) is supplied to the product outlet (30), and the fine fraction of the size-selective separation is supplied back to step a).

4. The method according to one of the preceding claims, characterized in that the mechanochemical activation is carried out with an energy input of at least 100 kW / m3, preferably at least at least 200 kW / m3, per mill volume.

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