Enrichment of used hydrated cement from used concrete

Abstract

The present invention relates to a method for processing used concrete or a used concrete product, the method comprising the following steps: a) grinding the used concrete or the used concrete product in a stirred ball mill 10, b) separating the ground material from step a) according to size into a coarse fraction 40 and a fine fraction 50.

Description

[0001] Enrichment of old cement brick from old concrete

[0002] The invention relates to a method for removing quartz and thus enriching old cement stone from old concrete.

[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, there is a growing focus on capturing carbon dioxide from exhaust gases and storing or utilizing it permanently. One possible method is injecting liquefied carbon dioxide into the ground. However, this method is not without controversy, as its long-term retention is not guaranteed, and any escape would exacerbate the greenhouse effect. Furthermore, capturing and storing the carbon dioxide requires additional energy, potentially leading to further carbon dioxide production.

[0004] One of the most carbon-intensive industries is the cement industry. Firstly, the process requires a great deal of energy, which, when using conventional fossil fuels, leads to carbon dioxide emissions. Secondly, carbon dioxide is released from the raw material, such as limestone, as a result of the process itself.

[0005] On the other hand, large quantities of old concrete are generated when concrete structures are demolished. Therefore, the recycling of concrete, for example to produce new cement, is currently being discussed. However, a problem arises because components such as sand and hardened cement are often difficult to separate and are inextricably mixed and bonded together. The sand-free or at least low-sand component of old concrete is also referred to as recycled cement. It is known that concrete can absorb carbon dioxide during its service life, but only a fraction of the carbon dioxide released from the limestone during its production. After a long period, for example in very old buildings, this value can be around 20% relative to the calcium content of the concrete; thus, approximately one-fifth of the originally released carbon dioxide is reabsorbed very slowly and over long periods.From WO 2020 / 058 247 A1 a process and a plant for processing material containing cement stone are known.

[0006] The use of carbon dioxide from and for cement is known from EP 3 656 750 A2.

[0007] From the subsequently published DE 10 2022 132 073 a method and a device for the efficient reduction of carbon dioxide emissions are known.

[0008] From the subsequently published DE 10 2023 113 943 a method and a device for the efficient reduction of carbon dioxide emissions are known.

[0009] The activation of old concrete using a plowshare mixer and a mill is known from the subsequently published DE 10 2023 123 525.

[0010] From WO 2018 / 114951 A1 a method and a device for the production of a hydraulic binder by processing a construction waste material are known.

[0011] From JP 3 945717 B1 a separating plate is known with which the interior of a drum body of a mill for crushing an aggregate and so forth is separated using grinding media, such as a steel ball, and a mill.

[0012] From DE 10 2016124707 A1 a method for providing a hydraulically hardening binder and a use of the cement base material based thereon is known.

[0013] Scientific papers on cement grinding are known from Martin Reformat: "Cement Grinding", September 16, 2020 (2020-09-16), XP093163189, found on the internet: URL: https: / / www.db-thueringen.de / servlets / MCRFileNodeServlet / dbt_derivate00063561 / Dissertation_Reformat_Martin.pdf. For all these applications, it is advantageous to have the highest possible proportion of recycled cement and to minimize the amount of dead rock, especially SiC>2. The goal is therefore to reduce the SiCh content or to provide a fraction enriched with CaO and MgO. Only about 1 / 7 of recycled concrete is recycled cement; gravel and sand constitute the main component as inert SiCh constituents.

[0014] The purpose of the invention is to create a SiCh-enriched and a SiCh-depleted fraction from old concrete, thereby reducing the energy expenditure for the processing of the material in the further processing of the SiCh-depleted fraction.

[0015] This problem is solved by the method with the features specified in claim 1. Advantageous further developments are described in the dependent claims, the following description, and the drawings.

[0016] The process according to the invention serves for the reprocessing (or processing) of waste concrete or a waste concrete product, in particular for separating a fraction with a particularly high SiO2 content. Thus, a fraction enriched with recycled cement paste can be used for further processing, which, for example, reduces the energy required for its activation. The process comprises the following steps: a) grinding the waste concrete or the waste concrete product in a stirred ball mill, b) size-selective separation of the ground material from step a) into a coarse fraction and a fine fraction.

[0017] It has surprisingly turned out that a stirred ball mill is particularly suitable. It is currently assumed that the grinding media, which are typically smaller in relation to the size of the mill, are subjected to a different load during the grinding process, resulting in the more easily ground old cement paste being crushed more quickly and the SiO2-containing fraction being crushed significantly less. Furthermore, it was found that the old cement paste adhering to the quartz grains was almost completely removed (attrition), so that the SiO2-containing fraction consists mainly of very pure quartz grains. It has been shown that with a starting material containing approximately 65 wt.% SiO2 (quartz content), an enrichment to approximately 90 wt.% in the coarse fraction is possible. At the same time, the proportion of a fine fraction with an SiO2 content below 50 wt.% increases significantly.

[0018] This makes the stirred ball mill significantly different from other mill types, allowing for a very simple enrichment of the old cement stone.

[0019] Typically, the old concrete undergoes initial processing on-site by separating a very coarse fraction with an extremely high SiO2 content. Such an intermediate product, as defined by the invention, would be a recycled concrete product, a product made from recycled concrete.

[0020] According to the invention, the grinding process in step a) is carried out with an energy input per mill volume of at least 100 kW / m³. 3 The grinding process is preferably carried out in step a) with an energy input of 100 kW / m³ per mill volume. 3 up to 200 kW / m 3 This process was carried out. This energy input appears to result in the primary crushing of the more easily ground component, the old cement stone, but not of the more difficult-to-ground quartz component.

[0021] According to the invention, the grinding in step a) is carried out with an energy input of at most 100 kWh / t, preferably at most 50 kWh / t, and particularly preferably at most 25 kWh / t. The aim is not complete fine grinding. Rather, the aim is to achieve the smallest possible grinding of the difficult-to-grind quartz component in order to achieve separation.

[0022] Both requirements initially appear contradictory, as the energy input per mill volume is as high as possible, while the energy input per ton of material is kept low. However, this very combination results in only the recycled cement paste being crushed, while the quartz component is only minimally crushed. This allows for a material separation between the recycled cement paste and the quartz component via size-selective separation; in particular, a significant enrichment of the quartz component is observed in the coarse fraction. This enrichment exceeds previously achievable levels. This, in turn, increases the usability of the recycled cement paste, for example, as a binder for CO2 or as SCM (solid composition material).

[0023] In a further embodiment of the invention, the size-selective separation in step b) is achieved by pneumatically conveying the fine fraction from the mixture. Thus, only the fine fraction is easily removed from the solids stream. The fine fraction can then be easily recovered, for example, in a cyclone or dust separator. Such a separation is extremely simple and low-wear.

[0024] In a further embodiment of the invention, the size-selective separation in step b) is carried out using a classifier or a sieve. The sieve, in particular, is characterized by extremely high separation efficiency. Furthermore, two sieves arranged one above the other allow for simple separation into coarse, medium, and fine fractions. Ideally, when using a classifier, all the classifier grit (coarse fraction) is separated due to the particularly high quartz concentration in the stirred ball mill. If necessary, only a partial stream of the classifier grit can be removed, and the remaining grit is reintroduced into the mill's cycle.

[0025] In a further embodiment of the invention, grinding media with a diameter of 1 to 10 mm, preferably 3 to 6 mm, are used in the grinding process in step a). With grinding media of this size, a particularly optimal enrichment effect of SiÜ2 in the coarse fraction was observed.

[0026] In a further embodiment of the invention, grinding media made of steel or ceramic are used in the grinding process in step a). The advantage of ceramic grinding media is that abrasion has no negative impact on the finished product. In contrast, steel grinding media are readily available and inexpensive.

[0027] In a further embodiment of the invention, the fine fraction is selected with a mean diameter of less than 125 pm. It has been shown that this fraction exhibits a particularly high concentration of the recycled cement component and a significantly reduced quartz content.

[0028] In a further embodiment of the invention, the coarse fraction with a mean diameter of more than 250 pm is selected. The highest enrichment of SiO2 was observed in this fraction.

[0029] For example, it may be planned to return a middle fraction, such as a fraction between 125 pm and 250 pm, to the mill and thus only remove a fine fraction smaller than 125 pm and a coarse fraction larger than 250 pm from the milling process.

[0030] In a further embodiment of the invention, the grinding in step a) is carried out with a grinding media filling level of 40 vol.% to 95 vol.%, preferably 50 vol.% to 70 vol.%. The high proportion of grinding media also appears to have a positive effect on the separation effect between old cement and quartz.

[0031] In a further embodiment of the invention, the grinding in step a) is carried out in a stirred ball mill with a length-to-diameter ratio of 2 to 5, preferably 2.5 to 5.

[0032] In a further embodiment of the invention, the grinding in step a) is carried out in a stirred ball mill 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 1.0 m 3 / kg and 2.0 m 3 / kg.

[0033] The method according to the invention is explained in more detail below with reference to the exemplary embodiments shown in the drawings.

[0034] Fig. 1 first example

[0035] Fig. 2 second example. Fig. 1 shows a first exemplary device to illustrate the process. Waste concrete or a waste concrete product is fed to the agitator ball mill 10 via a waste concrete feeder 12 and processed there at 150 kW / m³. 3The material is ground with 65% by volume of ceramic grinding media for 10 minutes. The ground material is discharged from the stirred ball mill 10 and fed into a separating device 20, which in this case enables pneumatic conveying of the small particles. For this purpose, gas is supplied via a gas supply 22, thus conveying the small particles into a separator 30. The oversized particles fall to the bottom in the gas stream and are discharged accordingly as the coarse fraction 40. In the separator 30, the gas stream is separated from the solid stream, and the gas is released via the gas outlet 32, while the solid is removed as the fine fraction 50. The fine fraction 50 is low in SiO2 and enriched with recycled cement, while the coarse fraction 40 is enriched in SiO2 and can contain up to 90% quartz. Thus, the fine fraction 50 constitutes the valuable material, and the coarse fraction 40 the inert material.

[0036] Fig. 2 shows a second exemplary device to illustrate the method. In this example as well, waste concrete or a waste concrete product is fed to the agitator ball mill 10 via a waste concrete feeder 12 and processed there at 150 kW / m³. 3 The material is ground with 65% by volume of ceramic grinding media for 10 minutes. The ground material is discharged from the stirred ball mill 10 and fed into a separating device 20. In addition to the fine fraction 50 and the coarse fraction 40, the separating device 20 produces a medium fraction 24, which is then fed back into the stirred ball mill 10. For example, the separating device 20 can have two different screens, for example, with mesh sizes of 125 µm and 250 µm.

[0037] Reference sign

[0038] 10 Stirred Ball Mills

[0039] 12. Supply of old concrete

[0040] 20 separating device

[0041] 22 Gas supply

[0042] 24 Middle fraction

[0043] 30 separators

[0044] 32 Gas removal 40 Coarse fraction

[0045] 50 Fine fraction

Claims

Patent claims 1. A method for reprocessing waste concrete or a waste concrete product, comprising the following steps: a) grinding the waste concrete or the waste concrete product in a stirred ball mill (10), b) size-selective separation of the ground material from step a) into a coarse fraction (40) and a fine fraction (50), characterized in that the grinding in step a) is carried out with an energy input per mill volume of at least 100 kW / m³ 3 is carried out, wherein the grinding in step a) is carried out with an energy input of no more than 100 kWh / t.

2. Method according to claim 1 , characterized in that the size-selective separation in step b) is carried out by pneumatic conveying of the fine fraction (50) from the mixture.

3. Method according to claim 1, characterized in that the size-selective separation in step b) is carried out by means of a classifier or a sieve.

4. Method according to one of the preceding claims, characterized in that grinding media with a diameter of 1 to 10 mm, preferably 3 to 6 mm, are used in the grinding process in step a).

5. Method according to one of the preceding claims, characterized in that grinding media made of steel or ceramic grinding media are used in the grinding process in step a).

6. Method according to one of the preceding claims, characterized in that the fine fraction (50) is selected with a mean diameter of less than 125 pm.

7. Method according to one of the preceding claims, characterized in that the coarse fraction (40) is selected with a mean diameter of more than 250 pm.

8. Method according to one of the preceding claims, characterized in that the grinding in step a) is carried out with an energy input of at most 50 kWh / t, particularly preferably at most 25 kWh / t.

9. Method according to one of the preceding claims, characterized in that the grinding in step a) is carried out with a grinding media filling level of 40 vol.% to 95 vol.%, preferably from 50 vol.% to 70 vol.%.

10. Method according to one of the preceding claims, characterized in that the grinding in step a) is carried out in a stirred ball mill (10) with a length-to-diameter ratio of 2 to 5, preferably of 2.5 to 5.

11. Method according to one of the preceding claims, characterized in that the grinding in step a) is carried out in a stirred ball mill (10) with a gas volume flow and a material flow, wherein 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 1.0 m 3 / kg and 2.0 m 3 / kg.

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