Methods for converting non-biosoluble mineral fibers, for example asbestos, into a safe, valuable material

Abstract

The present invention relates to a process for converting mineral fibers or a mineral fiber-containing mixture as a starting material into a building material, wherein the starting material is processed in a high-energy mill 50 with a volume of at least 300 l and an energy density of at least 100 kWh / m³ 3 is transformed into a building material through mechano-chemical activation

Description

[0001] The invention relates to a process for converting non-biosoluble mineral fibers into a non-carcinogenic and non-hazardous substance, and then simultaneously converting this substance into a building material. The effort expended in this process is not only used for the disposal or detoxification of the mineral fiber-containing material, but also serves a value-adding process, so that no new source of emissions is created. The best-known example of non-biosoluble mineral fibers is asbestos.

[0002] Asbestos is a collective term for naturally occurring, fibrous, crystalline silicate minerals that, after processing, yield technically usable fibers of varying lengths. Asbestos possesses high strength, is heat and acid resistant, and has good insulating properties. This allowed asbestos to become prevalent in various industrial sectors. However, due to the identified health hazards, its use is now banned in many countries, including the USA, the EU, and Switzerland. Today, disposal is the primary focus.

[0003] Asbestos-like minerals occur in two fine-fibered silicate mineral groups. While the serpentine group consists only of (clino-)chrysotile (white asbestos), the amphibole group includes the minerals grunerite (brown asbestos), riebeckite / crocidolite (blue asbestos), tremolite, actinolite, and anthophyllite.

[0004] Asbestos can be used for short periods up to 1000 °C and without restrictions up to 400 °C (chrysotile) or 300 °C (crocidolite). With increasing heat exposure, the water of crystallization content decreases, and consequently, the strength diminishes, until the fibers become completely brittle and fall away as a powdery mass. Asbestos is rot-resistant and mixes very well with cement.

[0005] Since 1970, asbestos fibers have been officially classified as carcinogenic. In 1990, the production and use of asbestos were generally banned in Switzerland and Austria, and in 1993 in Germany. Within the European Community, all member states had introduced restrictions on the use and marketing of asbestos by 2004.

[0006] Improper handling of asbestos and the processing of asbestos-containing materials release asbestos fibers. If fibers with a length greater than 5 µm, a diameter of no more than 3 µm, and a length-to-diameter ratio of at least 3:1 are also produced, these fibers can enter the alveoli of the lungs and trigger asbestosis even at low levels of exposure. The critical fiber geometry is the reason for the health hazard. The risk of developing lung cancer is increased.

[0007] The primary health risk comes from inhaling asbestos fibers, which are released naturally or through abrasion or weathering. Products containing loosely bound asbestos, with a fiber content of 60% or more, are particularly dangerous in this regard, as they readily release these fibers. For several years now, there have been more deaths from asbestos exposure in Germany than fatal workplace accidents. In 2019, there were almost 1,700 deaths from asbestos-related occupational diseases in Germany. Furthermore, payments for rehabilitation or pensions were made in approximately 44,000 cases of asbestos-related occupational diseases.

[0008] According to the European Waste Catalogue, asbestos-containing waste is classified as hazardous waste. This entails stricter technical precautions and, in some cases, permit requirements for handling this waste. These include labeling and documentation obligations and the requirement to dispose of it exclusively at specially authorized waste facilities. Following the virtually total ban on the use of asbestos in Germany in 1993, the question of proper disposal arose. Most landfills were not permitted to accept asbestos because this substance was not covered by their operating permits. As a result, disposal costs for asbestos-containing material rose six to ten times the previous price, making the development of disposal methods by research institutions and industry particularly important.

[0009] Over time, various recycling methods were tested: • Mechanical shredding methods assume that if the fibers are sufficiently reduced in size (below 1 µm fiber length), any risk can be ruled out. These methods work well with pure asbestos. However, conventional mills fail with the inhomogeneous mixtures that arise during asbestos disposal. • Thermal processes and vitrification raise the temperature of the asbestos above its transformation point, thereby producing a non-fibrous material. This process rearranges the silicate structure of the asbestos. • During tempering, the water of crystallization is removed from the asbestos fibers, transforming them into harmless minerals. These now harmless fibers can then be easily destroyed by mechanical stress. However, the high energy consumption and CO2 emissions make this process economically and ecologically questionable. Chemical processes rely on the use of fluoride-containing acids. They have the same problems as other methods with the inhomogeneity of asbestos-containing waste. Furthermore, the large quantity of hydrofluoric acid required is a critical issue. • Binding methods involve completely encasing the waste in cement or other binding agents, pouring it into drums, and then preferably disposing of the drums underground. These methods have the advantage of being readily available, as the asbestos does not need to be destroyed beforehand. This method is common for the disposal of loosely bound asbestos.

[0010] None of the aforementioned recycling methods have proven optimal or technically feasible, so the disposal of asbestos-containing waste currently only takes place via former municipal waste landfills or hazardous waste landfills, or via local recycling centers that then transport the asbestos cement to the landfill. At the landfill, the asbestos-containing waste is deposited and covered with mineral material to prevent any further fiber release. In Germany, the price depends on the respective local authority and varies considerably. Larger quantities of asbestos-containing waste must be reported to the landfill operator well in advance. Underground disposal is also considered state-of-the-art.

[0011] The mechanochemical treatment of asbestos fibers is known from the scientific publications listed below. The investigations described therein are limited to experiments on a laboratory to pilot plant scale and to pure asbestos. Examples include: P. Balaz; E. Dutkova. Fine milling in applied mechanochemistry. Minerals Engineering, 2009, 22, 681-694 and P. Plescia et al. Mechanochemical treatment to recycling asbestos-containing waste. Waste Manag. 2003, 23(3), 209-218.

[0012] As mentioned above, asbestos fibers belong to the group of natural, crystalline mineral fibers. Due to their specific properties, these are considered carcinogenic. However, other non-biosoluble mineral fibers can also penetrate deep into the lungs and are generally classified as potentially hazardous. Such mineral fibers can be natural or synthetic, glassy or crystalline.

[0013] From WO 2017 / 008 863 A1 a process and a plant arrangement for processing and activating a raw material are known.

[0014] From EP 3 909 682 A1 a method and a roller mill for the thermomechanical activation of a clay mixture are known.

[0015] From DE 10 2015 106 109 A1 a process for the tribochemical activation of binders and additives is known.

[0016] A milling process is known from US 8 783 589 B2.

[0017] A process for the production of sludge powders is known from RU 2 209 824 C2.

[0018] A method for producing activated clay is known from CN 109 954 485 A.

[0019] From the subsequently published DE 20 2023 103 367 a process for grinding and pozzolanic activation in a stirred ball mill is known.

[0020] The object of the invention is to convert mineral fibers, in particular respirable mineral fibers, for example and especially asbestos, as well as substances containing such mineral fibers, into a valuable material, in particular a building material, in a standard process, thereby simultaneously eliminating the hazard posed by the mineral fibers and making the value of the effort invested in this process. At the same time, the process should be suitable not only for pure mineral fibers in a laboratory environment, but especially for the industrial conversion of asbestos-containing waste materials.

[0021] 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 drawing.

[0022] The process according to the invention serves to convert mineral fibers or a mineral fiber-containing mixture as a starting material into a building material. A typical example of such mineral fibers is asbestos, which, due to its previous use in construction, now presents a challenge. The building material can be, in particular, cement. In a simpler application, the building material can also be a binder for the production of a geopolymer, for example, for landfill covering. The aim of the invention is therefore to utilize the necessary work that must be carried out anyway for the production of required building materials in such a way that, without or at least without significant additional effort, mineral fibers are simultaneously converted into a harmless and reusable form, thus avoiding the unnecessary consumption of energy and / or chemicals simply to achieve easier landfill disposal.This process utilizes mechanochemical activation, which is known, for example, from the subsequently published German patent DE 20 2023 103 367 for clays. Activated clays are currently produced in large quantities as a clinker substitute and added to cement as a clinker substitute to reduce CO2 emissions. The demand for clinker substitutes is therefore a given, and the energy required for their production is expended anyway. The starting material is processed in a high-energy mill with a volume of at least 300 liters and an energy density of at least 100 kWh / m³. 3 converted into a building material, in particular a clinker substitute, by means of mechano-chemical activation.

[0023] A key point is that, according to the invention, amorphization through mechano-chemical activation of the mineral fibers or the mineral fiber-containing mixture requires an increased energy input that goes beyond the usual fine grinding, which necessitates a mill with high energy density, and the grinding and mechano-chemical activation with an energy input per grinding chamber volume of at least 100 kW / m³ 3 The process is carried out preferably without the addition of water, i.e., not wet or in a sludge, but as dry grinding without added moisture, which distinguishes it from classic wet grinding methods. A typical value for a ball mill, as an example of a fine mill, is usually around 20 kW / m². 3and therefore significantly lower. Here, the grinding chamber volume refers to the volume available inside the first high-energy mill, i.e., the free volume when there is no material or grinding media present. Components belonging to the mill, such as a shaft that is movably arranged inside, are therefore not included in the grinding chamber volume, as this volume cannot be occupied by material.

[0024] Mechanochemical activation consists of three phases or stages: In the first stage, the particle size decreases (more or less) linearly with increasing 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 is almost impossible to reduce further. From this point onward, a second stage begins, in which the particle size cannot be changed further with additional energy input (activation and aggregation zone). In this stage, crystallographic structures are disrupted 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, normal grinding, where only surface creation is expected, avoids the transition from the first to the second stage, which is necessary for mechanochemical activation. Increasing the energy input further can lead to a third stage, in which the agglomeration of nanoparticles results in an increase in particle size (agglomeration zone), which positively affects the workability of activated clay cement concrete. This zone is therefore avoided even more during grinding, as a better result in terms of particle size distribution can be achieved with less effort.

[0025] However, it has been found that high energy inputs, i.e., in the second stage, lead to changes in the material itself. In the case of clays, for example, this results in activation, i.e., a reactivity that enables their use as binders (and thus as clinker substitutes), just as thermal activation does. Therefore, with such high energy inputs, subsequent thermal treatment for activation can be omitted.

[0026] The described process fundamentally alters the mineral fibers, particularly asbestos fibers, to such an extent that they are not only no longer hazardous but can also be reused as a valuable material. In particular, it has been found that the length of the mineral fibers, especially the asbestos fibers, is reduced to less than 5 µm, preferably less than 3 µm, and most preferably less than 1 µm. This also reduces the length-to-diameter ratio to less than 3:1. Furthermore, scanning electron microscopy, for example, reveals that the mineral fibers, especially the asbestos material, have transformed into approximately spherical secondary particles, with typically at least 50% of these secondary particles having a diameter of less than 5 µm.The original mineral fibers, especially asbestos fibers, can no longer be detected by scanning electron microscopy after the procedure has been carried out.

[0027] In a further embodiment of the invention, the starting material is dried to a residual moisture content of less than 5% before mechano-chemical activation. This residual moisture content comprises only volatile water and no bound water, for example, and in particular, water of crystallization. Preferably, the drying is carried out in a riser tube dryer.

[0028] In a further embodiment of the invention, the starting material is reduced to a size of less than 5 mm before mechano-chemical activation. Preferably, the reduction is carried out in a hammer mill.

[0029] Drying and comminution are preferably carried out in a fluidized bed dryer with an integrated hammer mill. In this setup, the fluidized bed dryer is positioned above the hammer mill, so that particles not carried by the gas flow and therefore too large are drawn downwards by gravity into the hammer mill. New material can then be fed either directly onto the hammer mill or, preferably, directly into the riser tube dryer. The latter option has the advantage that fine material is immediately removed and therefore never enters the hammer mill.

[0030] In a further embodiment of the invention, the starting material is processed in the high-energy mill with an energy density of more than 200 kWh / m³. 3 converted into a building material by means of mechano-chemical activation.

[0031] In a further embodiment of the invention, the high-energy mill is operated with a filling level of 50% to 70%. The filling level refers to the ratio of the total grinding chamber volume to the volume filled with material and grinding media. Here, the grinding chamber volume is understood to be the volume available inside the first high-energy mill, i.e., the free volume when there is no material or, for example, no grinding media in the first high-energy mill. Components belonging to the mill, for example, a shaft that is movably arranged inside, are therefore not included in the grinding chamber volume, since this volume cannot be occupied by material.

[0032] In a further embodiment of the invention, a stirred ball mill is selected as the high-energy mill. The stirred ball mill is preferably operated at a peripheral speed of 2 m / s to 8 m / s.

[0033] Preferably, the stirred ball mill is arranged horizontally, including an inclination of up to 20° from the horizontal. Alternatively, the stirred ball mill can also be arranged vertically.

[0034] In another embodiment of the invention, grinding balls with a diameter of 1 mm to 10 mm are used.

[0035] In another embodiment of the invention, grinding balls made of steel or ceramic are used.

[0036] In a further embodiment of the invention, a starting material with a mineral fiber content, in particular an asbestos fiber content, of at least 0.008 wt.%, preferably at least 0.1 wt.%, is selected. The process is therefore also very well suited for processing asbestos-contaminated construction waste, and not only for processing pure asbestos or other respirable, non-biosoluble mineral fibers.

[0037] In a further embodiment of the invention, triethanolamine or another grinding aid is added as a grinding aid.

[0038] In a further embodiment of the invention, size-selective separation takes place after the mechano-chemical activation. In the size-selective separation, at least one coarse fraction and one fine fraction are produced. The coarse fraction is fed to the mechano-chemical activation process together with the starting material. The coarse fraction comprises, for example, and in particular, particles with a particle size greater than 1 mm.

[0039] Size-selective separation can be achieved, for example, and preferably, using a classifier. To generate multiple fractions, several classifiers can be connected in series to produce different size fractions.

[0040] In a further embodiment of the invention, size-selective separation takes place after mechano-chemical activation. During this size-selective separation, a coarse fraction, a fine fraction, and a very fine fraction are produced. The coarse and very fine fractions are fed to the mechano-chemical activation process together with the starting material. The very fine fraction, for example, with a particle size below 3 µm, primarily comprises particles that have not yet been sufficiently activated for subsequent use, so that renewed mechano-chemical activation leads to higher activity and thus to a better product. In terms of size, the fine fraction lies between the coarse and very fine fractions.

[0041] In a further embodiment of the invention, the particle size distribution is determined by laser diffraction before the feed of the starting material to the high-energy mill and / or after the removal of the ground material from the high-energy mill. This is preferably used for the active control of the fill level of the high-energy mill, the residence time of the material in the high-energy mill, the energy input, or the pre-crushing.

[0042] In another embodiment of the invention, the mechano-chemical activation is carried out using wet processes. For clays and similar materials, dry mechano-chemical activation is advantageous. However, when using mineral fibers, such as asbestos, wet milling can exceptionally be advantageous to avoid, for example, the release of respirable dusts via ambient air and thus prevent health risks.

[0043] In a further embodiment of the invention, a clinker substitute is produced according to the inventive method. Clinker substitutes are added to cement in particular instead of clinker, since large quantities of CO2 are released during clinker production, which can thus be saved. Typically, the reactivity of the clinker substitutes is lower, so that the clinker can only be partially replaced.

[0044] In a further aspect, the invention relates to a binder comprising at least 5% by weight of the clinker substitute according to the invention. The binder is preferably a cement.

[0045] Alternatively, the material produced according to the inventive method can be used, for example, in road construction or in the field of landfill covering as a geopolymer.

[0046] The method according to the invention is explained in more detail below with reference to an embodiment shown in the drawing. Fig. 1 Schematic representation of the conversion of mineral fiber-containing materials into a clinker substitute

[0047] In Fig. Figure 1 illustrates the conversion of mineral fiber-containing material. The mineral fiber source 10 can be, for example, an asbestos-containing building or waste from a building renovation. This material is usually fed on-site to a pre-shredding unit 20. This step often also serves to remove foreign materials, such as steel reinforcement in concrete. The pre-shredded asbestos-containing material is then fed to a riser dryer 30. Small particles are carried along in the airflow, while larger ones fall down onto the hammer mill 40 and are shredded there to, for example, less than 5 mm, until they too are transported by the airflow through the riser dryer 30. In the riser dryer 30, the input material is dried to a moisture content of less than 5% by weight.

[0048] In the high-energy mill 50, comminution and mechano-chemical activation then take place, for example, at a filling level of 60% using steel grinding balls with a diameter of 4 mm, with an energy input of 2000 kWh / t. This resulted in a reduction of the primary particles to completely below 3 µm. The length-to-diameter ratio was also between 1:1 and 2:1. The formation of secondary particles, i.e., agglomerates, was also observed, at least to some extent.

[0049] The material treated in this way is fed to a classifier 60 and the coarse fraction, in the example shown particles over 30 µm, are fed back to the high energy mill 50 via a return line 70, the fine fraction is fed to the product outlet 80. Reference sign 10 Mineral fiber source 20 Pre-shredding 30 riser pipe dryers 40 Hammermühle 50 High-energy mill 60 sifters 70 Return line 80 product outlet QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2017 / 008 863 A1

[0013] EP 3 909 682 A1

[0014] DE 10 2015 106 109 A1

[0015] US 8 783 589 B2

[0016] RU 2 209 824 C2

[0017] CN 109 954 485 A

[0018] DE 20 2023 103 367 [0019, 0022] Cited non-patent literature

[0000] P. Balaz; E. Dutkova. Fine milling in applied mechanochemistry. Minerals Engineering, 2009, 22, 681-694

[0011] P. Plescia et al. Mechanochemical treatment to recycling asbestos-containing waste. Waste Manag. 2003, 23(3), 209-218

[0011]

Claims

[1] Method for converting mineral fibers or a mineral fiber-containing mixture as a starting material into a building material, wherein the starting material is processed in a high-energy mill (50) with a volume of at least 300 l and an energy density of at least 100 kWh / m³ 3 is transformed into a building material through mechano-chemical activation. [2] Method according to claim 1, characterized by that the starting material is dried to a residual moisture content of less than 5% prior to mechano-chemical activation, whereby the residual moisture content includes only volatile water and no bound water. [3] Method according to any of the preceding claims, characterized by that the starting material is crushed to a size of less than 5 mm before mechano-chemical activation. [4] Method according to any of the preceding claims, characterized by, that the starting material in the high-energy mill (50) has an energy density of more than 200 kWh / m³ 3 is transformed into a building material through mechano-chemical activation. [5] Method according to any of the foregoing claims, characterized by , that the high-energy mill (50) is operated with a filling level of 50 to 70%. [6] Method according to any of the foregoing claims, characterized by , that a stirred ball mill is selected as the high-energy mill (50). [7] Method according to claim 6, characterized by , that grinding balls with a diameter of 1 mm to 10 mm are used. [8] Method according to any one of claims 6 to 7, characterized by that grinding balls made of steel or ceramic are used. [9] Method according to any of the foregoing claims, characterized by that a starting material with a mineral fiber content of at least 0.008 wt.%, preferably at least 0.1 wt.%, is selected. [10] Method according to any of the preceding claims, characterized by , that trethanolamine is added as a grinding aid. [11] Method according to any of the foregoing claims, characterized by , that after mechano-chemical activation a size-selective separation takes place, whereby at least one coarse fraction and one fine fraction are produced, wherein the coarse fraction is fed to the mechano-chemical activation together with the starting material. [12] Method according to claim 11, characterized by , that after mechano-chemical activation a size-selective separation takes place, whereby a coarse fraction, a fine fraction and a very fine fraction are produced, wherein the coarse fraction and the very fine fraction are fed together with the starting material to mechano-chemical activation. [13] Method according to any of the preceding claims, characterized by, that the particle size distribution is determined by laser diffraction before the feed of the starting material to the high energy mill (50) and / or after the removal of the ground material from the high energy mill (50). [14] Method according to any of the preceding claims, characterized by that the mechano-chemical activation occurs wet. [15] Clinker substitute manufactured according to any of the preceding claims. [16] Binder comprising at least 5 wt.% of the clinker substitute according to claim 15.

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