Storage-stable cathode waste, method for manufacturing same, and use thereof as fuel

Abstract

The present invention relates to a cathode waste, comprising a spent cathode, in particular a spent cathode of an aluminum electrolysis cell, and at least one hydrophobic binder, wherein the hydrophobic binder is selected from waxes, wax-like compounds or mixtures thereof. Another subject of the present invention is a method for manufacturing a cathode waste, comprising the following steps: (a) providing a spent cathode, in particular a spent cathode of an aluminum electrolysis cell, (b) comminuting the spent cathode in at least one comminuting device, (c) classifying the spent cathode by means of a separating device, (d) mixing the spent cathode with at least one hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof in a mixing device, (e) portioning the mixture obtained in step (d), (f) removing the cathode waste, wherein steps (b) to (d) are carried out in an inert gas atmosphere. Furthermore, the present invention relates to the use of the cathode waste as a fuel, preferably in power plants and in the production of mineral wool, cement and steel.

Description

Technical Field

[0001] The present invention relates to a cathode waste, a method for manufacturing the same, and its application as a fuel, preferably in power plants and for the manufacture of mineral wool, cement, and steel. Background Art

[0002] The production of aluminum is generally carried out by molten salt electrolysis in an aluminum electrolytic cell by means of the Hall-Héroult process. When electrolyzing a molten mixture of alumina and cryolite, the aluminum formed is deposited on the cathode, and at the anode, oxygen reacts with the graphite of the anode to form carbon dioxide and carbon monoxide. Over time, the graphite anode is consumed and must be replaced.

[0003] The cathode lining, which consists mainly of graphite as well, is inert to aluminum. However, sodium from the molten bath is absorbed by the cathode lining and forms an intercalation compound, which changes the wetting behavior of the cathode lining with respect to the electrolyte. The molten cryolite and alumina salts can then more easily penetrate into the cathode lining through pores and cracks and completely soak the cathode lining over time, thereby impairing the productivity of the electrolytic cell and its energy consumption. In addition, the content of iron and silicon impurities in the aluminum increases.

[0004] Therefore, in industrial aluminum electrolytic cells, the average operating time of the cathode lining is generally between 4 and 7 years. If premature failure of the cathode lining occurs, which is usually caused by cracks in the cathode lining, the actual service life may also be significantly shortened.

[0005] To replace the cathode lining of an aluminum electrolytic cell, the cathode lining is mechanically broken and removed, for example, using a pneumatic hammer. In industry, the resulting waste cathode, also known as "spent pot lining (SPL)", is divided into a "first slice" containing the cathode lining material and a "second slice" containing a mixture of the cathode lining material and the refractory lining material.

[0006] Depending on the further treatment of the waste cathode, the first slice consisting of the graphite of the cathode lining is separated from the second slice consisting of a mixture of the graphite of the cathode lining and the refractory lining. Typically, the waste cathode consists of approximately 55% of the first slice and 45% of the second slice.

[0007] The spent cathode of the first slice mainly consists of graphite and sulfur with low contents of volatile components. However, during aluminum electrolysis, toxic compounds such as cyanides (e.g., in the form of sodium cyanide) and fluoride compounds accumulate on or in the cathode lining. These toxic compounds are highly reactive with water and / or air, especially oxygen, where, in particular, heat generation, toxic gas generation, and ignition may occur. Therefore, spent cathodes are classified as hazardous waste and dangerous goods in most countries, for example, according to the European regulations (ADR) on the international transport of dangerous goods by road. In particular, in the transport document, the spent cathode is described as UN3170 waste, by-product of aluminum production, 4.3, III, (E), and is classified as a dangerous good of Class 4.3: "Substances which, in contact with water, emit flammable gases".

[0008] The reactivity with water and / or air oxygen limits the availability of the spent cathode or results in more expensive storage conditions and disposal paths, and increases the associated costs.

[0009] Currently, worldwide, the vast majority of the spent cathodes generated in aluminum production are stored in stockpiles. On the one hand, in countries with less strict regulations, they are stored without any further treatment of the spent cathodes. In other countries, complex heat treatment or wet chemical treatment is considered necessary before storage and is the prior art. At the same time, due to the high carbon content, the spent cathode has a calorific value that is absolutely attractive thermally, so using the spent cathode as fuel is an expected disposal path.

[0010] However, the application of the spent cathode classified as a dangerous good as fuel is limited by complex transportation, storage, and processability. Additionally, the availability of the spent cathode as fuel and its disposal cost largely depend on the size of the fragments of the spent cathode.

[0011] A relatively large part of the spent cathode can be disposed of as fuel at a relatively low cost at a recycling facility, while the smaller abrasive-like parts and dust of the spent cathode are difficult to use because, due to their large surface area and the associated higher reactivity, they generally cannot be used as fuel in a recycling device. The higher reactivity of this smaller part of the spent cathode makes them more dangerous and thus more difficult to transport and handle. Therefore, disposing of this small part of the spent cathode is correspondingly complex and expensive.

[0012] WO 2014 / 026138 A1 describes a spent cathode having a carbon content of at least 65% by weight and its use as a fuel. Here, a high carbon content is produced by using only the first slice of the spent cathode and optionally additionally adding carbon-rich compounds thereto, where the carbon-rich compounds are selected from suitable smelting residues, such as by-products of graphite anodes or by-products of graphite cathode production. These by-products are not described in detail in WO 2014 / 026138 A1, but can be, for example, dust, chunks or the footings of production waste. The carbon-rich spent cathode produced here has a higher calorific value, but is still a dangerous good due to the cyanides and fluorides contained in the spent cathode that are highly reactive with water and / or atmospheric oxygen.

[0013] WO 88 / 06572A1 describes a method for manufacturing mineral wool, in which, in particular, a briquette composed of a mixture of spent cathode, coke and coking pitch is used as part of the fuel feed for the mineralizing iron furnace. Thereby, the undesired silicon deposition in the furnace, which occurs during the production of mineral wool, should be reduced. Here, the briquette should preferably contain about 40% coke, 0.45% spent cathode and 15% coking pitch. The disadvantage of such briquettes is that they contain only a very small amount of spent cathode, so only a small amount of spent cathode can be recycled and disposed of as fuel. In addition, the coking pitch used as an adhesive is technically difficult to process due to its high viscosity, processing window at high temperatures and partially carcinogenic components, such as polycyclic aromatic hydrocarbons. Summary of the Invention

[0014] In this context, the object of the present invention is to provide a cathode waste that ensures the safe storage and safe transportation of the smaller particle fraction and dust of the spent cathode and does not have the above-mentioned disadvantages of the prior art. Another object of the present invention is to provide a simple method for manufacturing such cathode waste.

[0015] This object is achieved by a cathode waste comprising a spent cathode, in particular a spent cathode of an aluminum electrolytic cell, and at least one hydrophobic binder, where the hydrophobic binder is selected from waxes, wax-like compounds or mixtures thereof.

[0016] The subject matter of the present invention also lies in a method for manufacturing cathode waste and the use of the cathode waste according to the present invention as a fuel.

[0017] Surprisingly, it has been found that by adding a hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof to spent cathodes, cathode waste that is not significantly reactive towards water and / or atmospheric oxygen is obtained, thereby ensuring its storage and transport stability. By adding the hydrophobic binder according to the invention, the spent cathode is inerted to such an extent that it is no longer considered waste that needs to be transported as a dangerous good. In particular, the cathode waste according to the invention no longer emits flammable gases upon contact with water and thus no longer has to be transported as a dangerous good of subclass 4.3 and equipped with the corresponding transport documents. Thus, the cathode waste according to the invention can be stored and transported without difficulty, which makes its disposal as fuel, preferably in power plants and for the production of mineral wool, cement and steel, more cost-effective and thus more economically attractive compared to stockpiling. From an ecological point of view, recycling as fuel is also more preferable to stockpiling.

[0018] Another advantage of the solution according to the invention is that, due to the presence of a hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof in the cathode waste according to the invention, the calorific value of the spent cathode is further increased according to the addition share of the hydrophobic binder, such that fluctuations in the calorific value between different batches of spent cathodes are no longer important when it is used as fuel. Surprisingly, it has been found that the combustion characteristics can be adjusted within a large degree of freedom using the cathode waste according to the invention and thus optimized for the respective recycling process when used as fuel, so that the combustion characteristics can be accurately matched to the requirements of the respective further processing method.

[0019] An additional advantage of the present invention is that small abrasive-like parts and dust of the spent cathode, which are highly reactive towards water and / or atmospheric oxygen due to their high surface area, can also be aggregated and largely inerted with a hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof, so that these components of the spent cathode, which have hitherto only been disposable at high cost and with difficulty due to their reactivity and size, can also be used as fuel in a cost-effective and reliable manner. Thus, for example, the size of spent cathodes that can be used for mineral wool production is usually limited to parts larger than 50 mm to ensure a specific gas permeability of the furnace feed. Using the method according to the invention, it is also possible to aggregate small parts of the spent cathode smaller than 50 mm into cathode waste whose size matches the requirements of the respective further processing method with the aid of a hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof.

[0020] Within the scope of the present invention, there is a distinction between spent cathodes and cathode waste. In the sense of the present invention, cathode waste means that the spent cathode and the hydrophobic binder are present in an aggregated form. Aggregation means that the individual particles of the spent cathode are bonded together by means of the hydrophobic binder to form larger complexes.

[0021] The cathode waste according to the present invention comprises spent cathodes, in particular spent cathodes of aluminum electrolysis cells.

[0022] Different from the cathode waste, according to the present invention, the spent cathode is understood as the raw material obtained when the cathode lining, in particular the cathode lining of an aluminum electrolysis cell, is mechanically broken and removed. In the sense of the present invention, the spent cathode is also referred to as spent pot lining (SPL). The spent cathode does not have a hydrophobic binder.

[0023] In practice, a first slice and a second slice of the spent cathode of an aluminum electrolysis cell are distinguished. The first slice consists only of the material of the cathode lining of the electrolysis cell and thus consists essentially of graphite, while the second slice also includes a part of the refractory lining of the electrolysis cell.

[0024] The spent cathode in the cathode waste according to the present invention can consist of the first slice or the second slice or a mixture of the first slice and the second slice. Thereby, the cathode waste can be adjusted to match the requirements of the corresponding further processing method. For example, when the spent cathode is used as fuel in cement production, usually the first and second slices are used, while in mineral wool production, usually only the first slice is used.

[0025] The first slice of the spent cathode typically contains 40 to 75 wt% carbon, 10 to 20 wt% fluoride, 8 to 17 wt% sodium, up to 10 wt% alumina, up to 5 wt% aluminum (metal), 0.01 - 0.5 wt% cyanide, up to 6 wt% silica, 1 to 6 wt% calcium oxide, 0.1 to 0.3 wt% sulfur, and up to 300 ppm polycyclic aromatic hydrocarbons.

[0026] The second slice of the spent cathode typically contains 0 to 20 wt% carbon, 4 to 10 wt% fluoride, 6 to 14 wt% sodium, 10 to 50 wt% alumina, 10 to 50 wt% silica, 1 to 8 wt% calcium, and 0.1 to 0.3 wt% sulfur.

[0027] The composition of the first slice of the spent cathode varies according to the operating duration of the cathode lining until its breakage. The composition of the second slice, which consists of the refractory lining and a low proportion of the cathode lining, depends less on the operating duration of the cathode lining. However, due to the different proportion of the refractory lining and the cathode lining at the time of its breakage, its composition can also vary.

[0028] The mixture of the first and second slices of spent cathodes generally contains 25 to 35 wt% of carbon, 12 to 18 wt% of fluoride, 12 to 18 wt% of sodium, 12 to 18 wt% of aluminum, up to 0.28 wt% of cyanide, up to 3.5 wt% of silicon dioxide, up to 3.5 wt% of calcium oxide, 0.1 to 0.3 wt% of sulfur, and up to 165 ppm of polycyclic aromatic hydrocarbons.

[0029] Preferably, the mixture of the first and second slices of spent cathodes has 50 to 60 wt% of the first slice and 40 to 50 wt% of the second slice.

[0030] The spent cathodes contained in the cathode waste of the present invention can be present in any shape and size that is in principle suitable for agglomeration with a hydrophobic binder. However, it has been found that for the manufacture of pellets, castings, briquettes or extrudates, it is technically advantageous for the spent cathodes to be present in as uniform a particle size as possible. By using as uniform a particle size of the spent cathodes as possible, the pellets, castings, briquettes or extrudates thus produced are more stable and can ensure uniform quality in terms of properties (such as calorific value) from one single pellet, casting, briquette or extrudate to the next.

[0031] Therefore, preferably, the spent cathodes have a particle size of less than 50 mm, especially less than 30 mm, and particularly preferably less than 0.2 mm. The spent cathodes can be ground to the target fineness with a suitable grinder. The individual finenesses can be separated into suitable parts by classification by means of a sieving method. Depending on the desired final product (pellets, castings, briquettes or extrudates), different particle sizes may be advantageous. For example, for the manufacture of pellets and extrudates, as small and uniform a particle size as possible is advantageous, while for the manufacture of castings and briquettes, coarser particle sizes and a more uneven particle size distribution can also be used.

[0032] The cathode waste of the present invention contains at least one hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof.

[0033] In the context of the present invention, a hydrophobic binder is to be understood as a binder that is not miscible with water. Hydrophobic binders are hardly soluble in water, while they dissolve in organic non-polar media.

[0034] Wax generally refers to a firm to brittle-hard substance or mixture of substances that can be kneaded at 20 °C, which has a coarse to fine-crystalline structure, is translucent to opaque but not transparent, does not decompose but melts above 40 °C, and is already in a thin liquid state or has a low viscosity slightly above the melting point, has a consistency and solubility strongly dependent on temperature, and can be polished under slight pressure.

[0035] Wax-like compounds are to be understood as compounds having a physical appearance similar to wax.

[0036] According to the invention, all natural, semi-synthetic and synthetic waxes known to the person skilled in the art can be considered for use as hydrophobic binders.

[0037] Examples of natural waxes are wool wax, China wax, beeswax, tail gland fat, lipids, sugarcane wax, carnauba wax, candelilla wax, cork wax, Gurumawachs, Ouicuriwachs, Cuban palm wax, thatch wax, cotton wax, rice bran wax, linseed wax, peat wax, rose wax, jasmine wax, Peetha-Wachs, myrtle wax, wax fig wax, petroleum wax, ozokerite, Stuffwachs, Aderwachs, lignite wax, petroleum wax, and paraffin wax.

[0038] Examples of partially synthetic waxes are ester waxes made from long-chain wax acids and monohydric fatty alcohols or wax alcohols, amides of fatty acids and wax acids, amide waxes based on fatty acids, such as distearylethylenediamine, ethylene bis-stearamide, stearic acid amide, behenic acid amide, erucic acid amide, oleic acid amide, soybean wax, Rhizinuswachs, rapeseed wax, phthalic diamide wax, and acylated amides of fatty acids and wax acids.

[0039] Examples of synthetic waxes are hydrocarbon waxes, polyolefin waxes, such as polyethylene wax, EVA wax and polypropylene wax, polyester waxes, and Fischer-Tropsch waxes.

[0040] Preferably, the wax is selected from polyolefin waxes, especially polyethylene wax, or paraffin wax.

[0041] According to the invention, it is also possible in principle to use conventional wax-like compounds known to the person skilled in the art.

[0042] Preferably, the wax-like compounds are selected from esters of glycerol and fatty acids, preferably from straight-chain carbon chains having 4 to 26, typically 12 to 22 carbon atoms, fatty acids, especially straight-chain aliphatic monocarboxylic acids having 13 to 21 carbon atoms, and mixtures thereof, preferably selected from stearic acid.

[0043] It has proven to be particularly practical to use at least one hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof, having a dropping point between 35 °C and 75 °C according to DIN ISO 2176.

[0044] A hydrophobic binder having a dropping point within this range has a good balance between sufficient strength at ambient temperature and an as energy-efficient as possible method for manufacturing cathode waste. Due to the relatively low dropping point, the amount of energy that has to be input in order to liquefy the hydrophobic binder during the manufacture of cathode waste is lower than for hydrophobic binders with a dropping point above 75 °C and higher according to DIN ISO 2176.

[0045] The dropping point of the hydrophobic binder can advantageously be selected according to the season and / or the climatic zone in which the cathode waste is stored and transported in order to ensure that the hydrophobic binder is sufficiently firm at ambient temperature. Thus, in colder seasons and / or in temperate climatic zones, a hydrophobic binder with a dropping point in the range between 35 °C and 45 °C according to DIN ISO 2176 is already sufficient, while in warmer seasons and / or in climatic zones in the subtropics and tropics, a hydrophobic binder with a dropping point in the range between 45 °C and 75 °C according to DIN ISO 2176 may be advantageous.

[0046] Furthermore, it is particularly advantageous if the hydrophobic binder contains the lowest possible share of functional groups, preferably no functional groups. Here, functional groups are chemical groups different from pure carbon-carbon bonds or carbon-hydrogen bonds. Such a hydrophobic binder results in a further significant reduction in the reactivity of the cathode waste according to the invention with water and / or atmospheric oxygen, such that its storage and transport stability is further improved. In addition, due to the higher carbon content in the hydrophobic binder, the calorific value of the cathode waste when it is used as fuel also increases.

[0047] According to a particularly preferred embodiment of the invention, the cathode waste has a calorific value between 10,500 and 31,000 kJ / kg, which is determined according to the RAL-GZ 724 method for secondary fuels of the Bundesgütegemeinschaft. By adding a hydrophobic binder, the calorific value of used cathodes, which is usually in the range between 7500 and at most 10,000 kJ / kg, increases significantly.

[0048] The cathode waste according to the invention can be matched to the requirements of the respective further processing method. Depending on the further processing method, the cathode waste can advantageously be present in the form of pellets, cocoons, castings, agglomerates or extrudates.

[0049] According to a preferred embodiment of the invention, the cathode waste is present in the form of pellets or extrudates and contains 75 to 90 wt% of used cathodes and 10 to 25 wt% of a hydrophobic binder, relative to the total weight of the cathode waste.

[0050] In the case of regulated calorific value combustion, for example in the production of cement in a rotary tube furnace or in the operation of a power plant, cathode waste in the form of easily metered pellets or extrudates is advantageous for using cathode waste as fuel. Here, the target temperature can be reliably predicted by charging a homogeneous fuel of known calorific value. The smaller the fuel dosage form, the finer the temperature achieved can be regulated. Achieving the target temperature within a specific limit may be important for maintaining the quality of the manufactured product.

[0051] According to an alternative preferred embodiment of the present invention, the cathode waste exists in the form of agglomerates and contains 60 to 80% by weight of used cathodes and 20 to 40% by weight of a hydrophobic binder relative to the total weight of the cathode waste.

[0052] In the case of roughly regulated calorific value combustion, for example in a cupola furnace used in mineral wool production and in a batch furnace, such as an electric arc furnace used in steel production, cathode waste in the form of agglomerates is advantageous for using cathode waste as fuel. Due to the coarser dosage form compared to pellets, the necessary gas permeability of the feed can be more easily achieved with agglomerates. At the same time, despite the coarser dosage form, the target temperature can be reliably regulated by charging a homogeneous fuel of known calorific value.

[0053] According to another alternative preferred embodiment of the present invention, the cathode waste exists in the form of castings and contains 30 to 80% by weight of used cathodes and 20 to 70% by weight of a hydrophobic binder relative to the total weight of the cathode waste.

[0054] In the case of roughly regulated calorific value combustion, for example in a cupola furnace used in mineral wool manufacturing, and in a batch furnace, such as an electric arc furnace, cathode waste in the form of castings is advantageous for using cathode waste as fuel. The same as in the case of using agglomerates, the casting allows many fuels to be introduced into the operation process still breathable, and at the same time, reliable temperature regulation can be achieved here due to a homogeneous fuel with a known calorific value.

[0055] Another subject of the present invention is a method for manufacturing cathode waste, comprising the following steps:

[0056] (a) Providing used cathodes, especially used cathodes of aluminum electrolytic cells,

[0057] (b) Crushing the used cathodes in at least one crushing device,

[0058] (c) Classifying the used cathodes by a separation device,

[0059] (d) Mixing the used cathodes with at least one hydrophobic binder selected from wax, wax-like compounds or mixtures thereof in a mixing device,

[0060] (e) Portion the mixture obtained in step (d),

[0061] (f) Remove the cathode waste,

[0062] wherein steps (b) to (d) are carried out in an inert gas atmosphere.

[0063] The method of the present invention is characterized in that it ensures simple, low-cost and high-energy-efficient production of cathode waste. Herein, what has been described above regarding the respective technical features of the cathode waste according to the present invention correspondingly applies to the corresponding technical features of the method according to the present invention.

[0064] Step (a) of the method of the present invention provides for the provision of used cathodes, in particular used cathodes of aluminum electrolytic cells. The provision of the used cathodes in step (a) can be achieved in any form and size of the used cathodes, and is only limited by the technical possibilities of transporting the used cathodes. Thus, according to the present invention, in step (a), not only thick blocks and plates with a length of the used cathode greater than 1 m can be used, but also extremely fine dust of the used cathode, as well as mixtures with extremely different particle sizes and plate sizes, such as those usually generated during the mechanical breaking of the cathode lining, can be used.

[0065] In step (b) of the method, the used cathode is crushed in at least one crushing device. Herein, in principle, a crushing device known to those skilled in the art can be used as the at least one crushing device. Preferably, the at least one crushing device in step (b) is a grinder or a crusher. Here, for example, a ball mill, an impact grinder, a hammer mill, a vertical grinder or a shredder can be used. The at least one crushing device causes a reduction in the plate size and / or particle size of the used cathode.

[0066] In step (c) of the method of the present invention, the used cathode is classified by a separating device. According to the present invention, a separating device that ensures a uniform particle size of the cathode waste can be used. Preferably, the separating device in step (c) is a sieve. However, in principle, other separating devices can also be envisaged, by means of which a fine fraction of a specific particle size of the used cathode can be separated from the used cathode crushed in step (b). Herein, the classification in step (c) can preferably be carried out simultaneously with the crushing in step (b). However, it is also conceivable that the classification is carried out only after the crushing in step (b).

[0067] According to the method provided by the present invention, in step d), the mixing of the spent cathode and at least one hydrophobic binder is carried out in a mixing device, and the hydrophobic binder is selected from wax, wax-like compounds or mixtures thereof. Such a mixing device is in principle familiar to those skilled in the art. Preferably, in step (d), the at least one hydrophobic binder is metered into the mixing device in liquid form or liquefied by heating in the mixing device. The metering in liquid form can be achieved, for example, by feeding the hydrophobic binder from a separately heated storage container into the mixing device. However, it is also conceivable that the mixing device itself can be heated, or the wax is liquefied in the mixing device by the energy input of the mixing equipment.

[0068] Steps (b) to (d) of the method of the present invention are carried out in an inert gas atmosphere. This is necessary because the spent cathode, especially its fine fraction generated in step (b), is highly reactive with water and / or atmospheric oxygen, and as already explained at the beginning, heat generation, generation of toxic gases and ignition are particularly likely to occur. Therefore, from a safety perspective, such reactions must be avoided. As the inert gas, for example, noble gases such as helium, neon, argon, krypton and xenon, as well as nitrogen can be used. Due to economic reasons, nitrogen is preferably used according to the present invention.

[0069] In step (e) of the method of the present invention, the mixture obtained in step (d) is portioned. Preferably, the portioning in step (e) is selected from in-form casting, briquetting, extrusion or granulation.

[0070] In addition, it is also conceivable to fill the mixture obtained in step (d) into a solid cocoon of a hydrophobic binder that has been previously provided. Advantageously, in this variant, less homogeneous spent cathodes can also be processed. Therefore, for this embodiment, the grading in step (c) is not absolutely necessary. The hydrophobic binder cocoon can be a geometric hollow shape, especially a hollow sphere or a hollow cylinder, with a hollow sphere being preferred. For the embodiment of the hollow sphere cocoon, the portioning in step (e) is carried out in such a way that the mixture obtained in step (d) is filled into a solid hollow hemisphere made of a hydrophobic binder, and after heating and thus softening the periphery of the hollow hemisphere, another solid hollow hemisphere is subsequently placed on the hollow hemisphere filled with the spent cathode like a lid, so that the spent cathode is completely surrounded by a solid cocoon made of a hydrophobic binder.

[0071] According to the portioning method required in step (e) of the method of the present invention, different fractions of the spent cathode and the binder are preferably mixed in step (d).

[0072] According to a preferred embodiment of the present invention, sizing is granulation or extrusion, and in step (d), 75 to 90% by weight of the spent cathode and 10 to 25% by weight of the hydrophobic binder are mixed relative to the total weight of the mixture of the spent cathode and the hydrophobic binder.

[0073] According to an alternative preferred embodiment of the present invention, sizing is briquetting, and in step (d), 60 to 80% by weight of the spent cathode and 20 to 40% by weight of the hydrophobic binder are mixed relative to the total weight of the mixture of the spent cathode and the hydrophobic binder.

[0074] According to another alternative preferred embodiment of the present invention, sizing is in-mold casting, and in step (d), 30 to 80% by weight of the spent cathode and 20 to 70% by weight of the hydrophobic binder are mixed relative to the total weight of the mixture of the spent cathode and the hydrophobic binder.

[0075] According to the method of the present invention, cathode waste is taken out in step (f). Here, the cathode waste taken out in step (f) preferably exists in the above-mentioned dosage forms.

[0076] Before taking out in step (f), a separating agent can be applied to the cathode waste to prevent the cathode waste from sticking during storage and transportation. Powdery substances can be considered as separating agents. Exemplary separating agents are calcium carbonate, talc or silicate.

[0077] The method of the present invention can be carried out semi-continuously or continuously.

[0078] Finally, the present invention relates to the use of the cathode waste according to the present invention as a fuel, preferably in power plants and in the production of mineral wool, cement and steel.

[0079] Depending on the final recycling process in which the cathode waste is used as a fuel, the combustion characteristics of the cathode waste must be adapted to the different requirements of the corresponding recycling process. This can be achieved by means of the cathode waste according to the present invention through different adjustments of the ratio of the spent cathode to the hydrophobic binder, by the selection of the hydrophobic binder and by different dosage forms, such as granules, pellets, briquettes, cocoons, extrudates and castings.

[0080] If for the use of the cathode waste as a fuel, an increased fuel strength is required even at high temperatures, the cathode waste according to the present invention can also be used for a shape-stable fuel composite molding body, which is used, for example, in the production of mineral wool.

[0081] Here, such a fuel composite generally includes a hydraulic binder, such as cement, especially Portland cement, in addition to the cathode waste. Due to the favorable inertness of the cathode waste according to the present invention, such a fuel composite molded body can be manufactured in combination with the cathode material according to the present invention, and the cathode waste does not react with the water used in the manufacture of the fuel composite.

[0082] It goes without saying that the cathode waste according to the present invention is equally suitable as a raw material for manufacturing other conceivable fuel composite molded bodies, and the fuel composite molded bodies are respectively matched to the different requirements of the corresponding recycling processes.

[0083] Regarding the technical features, design, and manufacture of the cathode waste according to the present invention, the content described above regarding the cathode material according to the present invention and its manufacturing method correspondingly applies to the said application. Brief Description of the Drawings

[0084] The present invention will be further described below with reference to the accompanying drawings by way of examples. This example is only for illustrating the present invention and does not limit the protection scope of the present invention.

[0085] Figure 1 A schematic diagram showing an embodiment of the method for manufacturing cathode waste according to the present invention is shown. Detailed Description of the Invention

[0086] Figure 1 An embodiment form of the method for manufacturing cathode waste according to the present invention is schematically shown. First, the provided waste cathode 2 with a particle size less than 50 mm is loaded into the nitrogen-inertized grinder through the vibrating funnel 3. If the fragments are too coarse or the span of the particle size distribution is too large, a similarly nitrogen-inertized crusher can be connected upstream to be suitable for the grinder. Here, the suitable waste cathode 2 also serves as a dust remover. The final particle size is determined by the screen 4 near the bottom, and the fine part of the waste cathode is released from the grinder to the heated mixer 5 at this particle size. In the mixer 5, liquefied wax is added via the metering unit 6 until the concentration of the wax relative to the total weight of the cathode waste is 20 to 40% by weight. The wax is liquefied in the heated wax storage container 7 in advance. When the correct mixing ratio of the wax and the waste cathode is reached in the mixer 5, the grinder 1, the crusher connected upstream if necessary, and the addition of wax through the metering unit 6 are stopped. The mixing rotation direction in the mixer 5 is changed, and the cathode waste / wax mixture is poured into the slightly conical mold 8. After cooling to room temperature, the manufactured cathode waste can be taken out from the mold 8 and thus exists as a casting.

[0087] Examples

[0088] Supply 1300 kg of waste cathode to a vertical grinder. The waste cathode is pre-sorted, free of impurities such as corundum or aluminum, and free of fragments larger than 5 cm.

[0089] The grinding process is carried out in a nitrogen atmosphere and to a target fineness of 10% > 90 μm. This means that after grinding, 90% of the waste cathode is less than 90 μm, and the remaining 10% is approximately between 150 and 200 μm.

[0090] Pellet manufacturing - Variant A - Partially granulated

[0091] Place 1800 g of the ground waste cathode on a granulation disk preheated to 70 °C. Granules are obtained by adding 10 to 15 wt% of wax, which form a solid round shell on the outside but contain almost dry ground material on the inside.

[0092] Pellet manufacturing - Variant B - Fully granulated

[0093] In another embodiment, 1800 g of the ground waste cathode is placed on a granulation disk preheated to 80 °C and 17 to 21 wt% of wax is added. Granules are obtained that contain a mixture of wax and ground material throughout the diameter. When manufacturing the granules, the placement angle of the granulation disk with respect to the plumb line is 30°, and the rotational speed is 30 rpm.

[0094] Granulation is carried out in a semi - continuous process, where the ground waste cathode is replenished, for example, in a way that the granule removal (via edge dropping) is carried out in proportion to the mass. The preheated liquid wax is also metered (or sprayed) in a corresponding proportion. When manufacturing the granules, the placement angle of the granulation disk with respect to the plumb line is 30°, and the rotational speed is 30 rpm.

[0095] For both partial and complete granulation, calorific value and combustion value analyses are subsequently carried out according to RAL - GZ 724. The results are summarized in Table 1.

[0096] Table 1

[0097]

[0098] The "initial material" shown in Table 1 is understood as the granules as they are taken out of the granulator. The "dry material" is understood as the granules that have been dried according to DIN EN 14346 after being taken out of the granulator.

[0099] By varying the rotational speed from 20 to 40 rpm and the placement angle from 15° to 30° with respect to the plumb line, in principle, granules with an average diameter d = 8 to 17 mm can be obtained.

[0100] Manufacture of castings

[0101] In another example, a casting is produced. For this purpose, the fraction sifted out with < 3 mm is separated from the fine fraction of the spent cathode and used.

[0102] In a batch process, about 3 kg of paraffin wax with a melting point between 70 and 80 °C is liquefied separately and heated to about 100 °C. The kinematic viscosity at 100 °C is between 3 and 10 mm 2 / s.

[0103] With continuous stirring, the fraction sifted out with < 3 mm, which is 10 kg and separated from the fine fraction of the spent cathode, is added in small portions slowly. After all the substances are stirred into a homogeneous mass, 8 kg of the casting is first poured. 3 kg of paraffin wax is added to the material remaining in the mixer, and after melting, 10 kg of the fraction sifted out with < 3 mm, which is separated from the fine fraction of the spent cathode, is added again under stirring. The second casting is thus produced. The other castings are produced correspondingly by repeating the respective steps.

[0104] The casting of the fraction sifted out with < 3 mm, which is separated from the fine fraction of the spent cathode, is slightly inhomogenized during the curing process. This results in a higher wax concentration near the surface of the casting.

[0105] One can start from the fact that with increasing fineness of the spent cathode, less wax is required for stable shaping, regardless of the embodiment chosen. Adding more wax than required for physical stability is a good means to arbitrarily increase the calorific value and adapt it to the requirements of the subsequent application.

Claims

1. A cathode waste, the cathode waste comprising a spent cathode of an aluminum electrolytic cell, and at least one hydrophobic binder, wherein, The waste cathode and the hydrophobic binder are present in an aggregated form, characterized in that the hydrophobic binder is selected from waxes, wax-like compounds or mixtures thereof, wherein, relative to the total weight of the cathode waste, the cathode waste comprises 30 to 90% by weight of the waste cathode and 10 to 70% by weight of the hydrophobic binder.

2. The cathode waste according to claim 1, characterized in that, The waste cathode has a particle size of less than 50 mm.

3. The cathode waste according to any one of the preceding claims, characterized in that, The waste cathode consists of a first slice or a second slice or a mixture of the first slice and the second slice, wherein the first slice consists of graphite of the cathode lining, and the second slice consists of a mixture of graphite of the cathode lining and a refractory lining.

4. The cathode waste according to claim 1, characterized in that, The hydrophobic binder has a dropping point between 35 °C and 75 °C according to DIN ISO 2176.

5. The cathode waste according to claim 1, characterized in that, The wax is selected from natural waxes, semi-synthetic waxes or synthetic waxes and mixtures thereof.

6. The cathode waste according to claim 5, characterized in that, The wax is selected from polyolefin waxes.

7. The cathode waste according to claim 6, characterized in that, The wax selected from polyolefin waxes is polyethylene wax or paraffin wax.

8. The cathode waste according to claim 1, wherein The wax-like compound is selected from esters of glycerol and fatty acids.

9. The cathode waste according to claim 8, characterized in that, The wax-like compound is selected from esters of glycerol and fatty acids from straight-chain carbon chains having 4 to 26 carbon atoms, and mixtures thereof.

10. The cathode waste according to claim 9, wherein, The wax-like compound is an ester of glycerol and a fatty acid from a straight-chain carbon chain having 12 to 22 carbon atoms.

11. The cathode waste according to claim 8, wherein, The wax-like compound is stearin.

12. The cathode waste according to claim 1, wherein, The cathode waste is present in the form of pellets, cocoons, castings, agglomerates or extrudates.

13. The cathode waste according to claim 1, characterized in that, The cathode material has a calorific value between 10500 and 31000 kJ / kg, which is determined according to the RAL-GZ 724 method for secondary fuels of the Federal Quality Organization.

14. A method for manufacturing cathode waste, comprising the following steps: (a) providing a waste cathode of an aluminum electrolysis cell, (b) crushing the waste cathode in at least one crushing device, (c) classifying the waste cathode by a separation device, (d) mixing the waste cathode with at least one hydrophobic binder selected from waxes, wax-like compounds or mixtures thereof in a mixing device, (e) portioning the mixture obtained in step (d), (f) removing the cathode waste, wherein steps (b) to (d) are carried out in an inert gas atmosphere.

15. The method for manufacturing cathode waste according to claim 14, characterized in that, The at least one crushing device in step (b) is a grinder or a crusher.

16. The method for manufacturing cathode waste according to claim 14 or 15, characterized in that, The separation device in step (c) is a sieve.

17. The method for manufacturing cathode waste according to claim 14, characterized in that, In step (d), the at least one hydrophobic binder is metered into the mixing device in liquid form or liquefied by heating in the mixing device.

18. The method for manufacturing cathode waste according to claim 14, characterized in that, The portioning in step (e) is selected from in-mold casting, briquetting, extrusion or granulation.

19. Use of the cathode waste according to any one of claims 1 to 13 as a fuel.

20. Use of the cathode waste according to any one of claims 1 to 13 as a fuel, wherein the cathode waste is used in power plants and in the production of mineral wool, cement and steel.

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