Storage-stable cathode breakout material, process for its production and its use as fuel

Description

[0001] The invention relates to a cathode breakout material, a process for its production and its use as fuel, preferably in power plants and in the production of mineral wool, cement and steel.

[0002] Aluminum is typically produced by molten-salt electrolysis in aluminum electrolysis cells using the Hall-Héroult process. During electrolysis of the molten mixture of aluminum oxide and cryolite, the resulting aluminum deposits at the cathode, and at the anode, oxygen reacts with the anode's graphite to form carbon dioxide and carbon monoxide. Over time, the graphite anodes become depleted and must be replaced.

[0003] The cathode lining, which also consists primarily of graphite, is inert to aluminum. However, sodium from the molten bath is absorbed by the cathode lining and forms intercalation compounds that alter the wetting behavior of the cathode lining toward the electrolyte. The molten cryolite and aluminum oxide salts can then more easily penetrate through pores and cracks in the cathode lining and, over time, completely impregnate it, thereby reducing the productivity of the electrolysis cell and its energy consumption. Furthermore, the content of iron and silicon impurities in the aluminum increases.

[0004] For this reason, the average service life of the cathode lining in industrial aluminum electrolysis cells is typically between four and seven years. The actual lifetime can be significantly shorter if premature failure of the cathode lining occurs, usually caused by cracks in the cathode lining.

[0005] To replace the cathode lining of an aluminum electrolysis cell, the cathode lining is mechanically broken up and removed, for example, using a jackhammer. The resulting cathode waste, also called "spent pot lining (SPL)," is industrially divided into a "first cut," containing cathode lining material, and a "second cut," containing a mixture of cathode lining material and refractory lining material.

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

[0007] The cathode waste from the first cut consists predominantly of graphite with a low content of volatile components and sulfur. However, during aluminum electrolysis, toxic compounds such as cyanides, for example in the form of sodium cyanide, and fluoride compounds accumulate on or in the cathode lining. These toxic compounds exhibit high reactivity with water and / or air, particularly oxygen, which can lead to, among other things, heat generation, toxic gas evolution, and ignition. Therefore, cathode waste is classified as hazardous waste and dangerous goods in most countries, for example, under the European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR). Specifically, cathode waste is designated in transport documents as UN3170 WASTE, BY-PRODUCTS OF ALUMINUM PRODUCTION, 4.3, III, (E) and is classified as a Class 4 dangerous good.3: "Substances which, in contact with water, emit flammable gases".

[0008] The reactivity with water and / or atmospheric oxygen limits the usability of the cathode breakout and requires more complex storage conditions and disposal routes, thereby increasing the associated costs.

[0009] Currently, the vast majority of cathode waste generated during aluminum production worldwide is landfilled. On the one hand, in countries with less stringent regulations, the waste is landfilled without any further treatment. In other countries, complex thermal or wet-chemical treatment prior to landfilling has been recognized as necessary and is state of the art. At the same time, cathode waste has a very attractive thermal calorific value due to its high carbon content, making its use as a fuel a desirable disposal option.

[0010] However, the use of cathode waste, classified as hazardous material, as a fuel is limited by its complex transport, storage, and processing requirements. Furthermore, the usability of cathode waste as a fuel, as well as its disposal costs, depend heavily on the size of the cathode waste fragments.

[0011] While larger fractions of cathode waste can be disposed of to a recycler for use as fuel at comparatively low cost, it is difficult to find a use for smaller grit-like fractions and dusts of cathode waste. Due to their large surface area and associated higher reactivity, these often cannot be used as fuel in recycling plants. The higher reactivity of the smaller fractions of cathode waste makes them more hazardous and thus more difficult to transport and handle. Disposal of these small fractions of cathode waste is therefore correspondingly complex and expensive.

[0012] WO 2014 / 026138 A1 describes a cathode waste stream with a carbon content of at least 65 wt.% and its use as a fuel. The high carbon content is achieved by using only the first cut of the cathode waste stream and, optionally, by adding a carbon-enriching compound to it. The carbon-enriching compound is selected from compatible metallurgical residues, such as by-products from graphite anode or graphite cathode production. Such by-products are not further specified in WO 2014 / 026138 A1, but could, for example, be dust, fragments, or offcuts from production scrap. Although the carbon-enriched cathode waste stream produced in this way has a higher calorific value, it remains a hazardous material due to the cyanides and fluorides contained in the cathode waste stream, which are highly reactive with water and / or atmospheric oxygen.

[0013] WO 88 / 06572 A1 describes processes for producing mineral wool in which, among other things, briquettes consisting of a mixture of cathode waste, hard coal coke, and coke pitch are used as part of the fuel charge of the mineral wool cupola furnace. This is intended to reduce unwanted silicon deposits in the furnace that arise during the production of the mineral wool. The briquette should preferably contain approximately 40% hard coal coke, 0.45% cathode waste, and 15% coke pitch. A disadvantage of these briquettes is that they contain only extremely small amounts of cathode waste, and thus only a small proportion of the cathode waste can be recycled and disposed of as fuel.Furthermore, coke pitch used as a binder is technically difficult to handle due to its high stickiness, the processing window at high temperatures and the partly carcinogenic ingredients, such as polycyclic aromatic hydrocarbons.

[0014] Against this background, the object of the invention was to provide a cathode breakout material that ensures safe storage and transport even of smaller-grained fractions and dusts of the cathode breakout and does not have the disadvantages mentioned in the prior art. A further object of the invention was to provide a simple process for producing such a cathode breakout material. This object is achieved by a cathode breakout material containing

[0015] Cathode breakout of aluminum electrolysis cells, and at least one hydrophobic binder, wherein the cathode breakout is present with the hydrophobic binder in agglomerated form, characterized in that the hydrophobic binder is selected from wax, a wax-like compound or mixtures thereof, wherein the cathode breakout material contains 30 to 90 wt.% cathode breakout and 10 to 70 wt.% hydrophobic binder, based on the total weight of the cathode breakout material.

[0016] The invention further relates to a process for producing a cathode breakout material and the use of the cathode breakout material according to the invention as fuel.

[0017] Surprisingly, it has been found that by adding a hydrophobic binder selected from wax, a waxy compound, or mixtures thereof to cathode breakout, a cathode breakout material is obtained that exhibits no significant reactivity toward water and / or atmospheric oxygen, thus ensuring its storage and transport stability. The cathode breakout is rendered inert by the addition of the hydrophobic binders provided according to the invention to such an extent that it is no longer considered waste to be transported as hazardous goods. In particular, the cathode breakout material according to the invention no longer emits flammable gases upon contact with water and therefore no longer needs to be transported as hazardous goods of subclass 4.3 and provided with appropriate transport documents.Thus, the cathode waste material according to the invention can be stored and transported without difficulty, making its disposal as a fuel, preferably in power plants and in the production of mineral wool, cement, and steel, more cost-effective and thus economically more attractive than landfilling. Utilization as a fuel is also preferable to landfilling from an ecological perspective.

[0018] A further advantage of the solution according to the invention is that the presence of the hydrophobic binder, selected from wax, a waxy compound, or mixtures thereof, in the cathode breakout material according to the invention further increases the calorific value of the cathode breakout in accordance with the added proportion of hydrophobic binder, so that fluctuations in calorific value between different batches of the cathode breakout are no longer significant when used as fuel. Surprisingly, it was found that with the cathode breakout material according to the invention, it is possible to adjust the combustion behavior within a large degree of freedom and thus to optimize it for the respective utilization process when used as fuel, so that it can be precisely adapted to the requirements of the corresponding further processing method.

[0019] An additional advantage of the invention is that even the small, grit-like fractions and dusts of the cathode waste, which are highly reactive with water and / or atmospheric oxygen due to their high surface area, can be agglomerated and largely rendered inert with the hydrophobic binder selected from wax, a waxy compound, or mixtures thereof. This allows these components of the cathode waste, which were previously difficult and expensive to dispose of due to their reactivity and size, to be used cost-effectively and safely as fuel. For example, the size of the cathode waste usable for mineral wool production is typically limited to fractions larger than 50 mm to ensure a certain gas permeability of the furnace feed.With the method according to the invention, it is possible to agglomerate even the small fractions below 50 mm of the cathode breakout by means of the hydrophobic binder, selected from wax, a wax-like compound or mixtures thereof, into a cathode breakout material whose size is adapted to the specifications of the respective further processing method.

[0020] For the purposes of the present invention, a distinction is made between cathode breakout and cathode breakout material. "Cathode breakout material" within the meaning of the invention means that the cathode breakout is present in agglomerated form with the hydrophobic binder. "Agglomerated" means that the individual particles of the cathode breakout are bound together by the hydrophobic binder to form larger aggregates.

[0021] The cathode breakout material according to the invention contains cathode breakout from aluminum electrolysis cells.

[0022] In contrast to cathode breakout material, cathode breakout is understood according to the invention as the raw material obtained by mechanically breaking open and removing the cathode lining, in particular the cathode lining of an aluminum electrolysis cell. Cathode breakout within the meaning of the invention is also referred to as spent pot lining (SPL). The cathode breakout is free of a hydrophobic binder.

[0023] In practice, a distinction is made between the first cut and the second cut of a cathode breakout of aluminum electrolysis cells. While the first cut consists only of the material of the cathode lining of the electrolysis cell, and thus primarily of graphite, the second cut also contains parts of the refractory lining of the electrolysis cell.

[0024] The cathode breakout in the cathode breakout material according to the invention can consist of the first cut or the second cut, or a mixture of the first and second cuts. This allows the cathode breakout material to be tailored to the specifications of the respective further processing method. For example, when using the cathode breakout as a fuel in the production of cement, the first and second cuts are typically used, whereas in the production of mineral wool, only the first cut is typically used.

[0025] The first cut of the cathode breakout typically comprises 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% silicon dioxide, 1 to 6 wt% calcium oxide, 0.1 to 0.3 wt% sulfur, and up to 300 ppm polycyclic aromatic hydrocarbons.

[0026] The second cut of the cathode breakout typically comprises 0 to 20 wt% carbon, 4 to 10 wt% fluoride, 6 to 14 wt% sodium, 10 to 50 wt% alumina, 10 to 50 wt% silicon dioxide, 1 to 8 wt% calcium, and 0.1 to 0.3 wt% sulfur.

[0027] The composition of the first section of the cathode breakout varies depending on the service life of the cathode lining until its removal. The composition of the second section, which consists of the refractory lining and a small portion of the cathode lining, is less dependent on the service life of the cathode lining. However, its composition can also vary due to different ratios of the refractory lining to the cathode lining during the removal process.

[0028] A mixture of the first and second cuts of the cathode breakout typically comprises 25 to 35 wt% carbon, 12 to 18 wt% fluoride, 12 to 18 wt% sodium, 12 to 18 wt% aluminum, up to 0.28 wt% cyanide, up to 3.5 wt% silicon dioxide, up to 3.5 wt% calcium oxide, 0.1 to 0.3 wt% sulfur, and up to 165 ppm polycyclic aromatic hydrocarbons.

[0029] Preferably, a mixture of the first and second cuts of the cathode outbreak comprises 50 to 60 wt.% of the first cut and 40 to 50 wt.% of the second cut.

[0030] The cathode breakout contained in the cathode breakout material according to the invention can be in any shape and size that is, in principle, suitable for agglomeration with a hydrophobic binder. However, it has proven advantageous for the production of pellets, castings, briquettes, or extrudates if the cathode breakout is present in as homogeneous a grain size as possible. By using the most homogeneous grain sizes for the cathode breakout, the pellets, castings, briquettes, or extrudates produced with it are more stable, and consistent quality in terms of properties, such as calorific value, can be ensured from one individual pellet, casting, briquette, or extrudate to the next.

[0031] The cathode scrap therefore preferably has a grain size of less than 50 mm, in particular less than 30 mm, and most preferably less than 0.2 mm. The cathode scrap can be ground to the target fineness using a suitable mill. The individual finenesses can be separated into suitable fractions by classifying them using a sieving process. Depending on the desired end product (pellet, casting, briquette, or extrudate), different grain sizes can be advantageous. For example, the smallest and most homogeneous grain sizes are advantageous for the production of pellets and extrudates, whereas coarser grain sizes and less homogeneous grain size distributions can also be used for the production of castings and briquettes.

[0032] The cathode breakout material according to the invention contains at least one hydrophobic binder selected from wax, a wax-like compound or mixtures thereof.

[0033] A hydrophobic binder, as defined in the invention, is understood to be a binder that is immiscible with water. While hydrophobic binders are virtually insoluble in water, they dissolve in organic, non-polar media.

[0034] Waxes are usually substances or mixtures of substances that are kneadable at 20°C, solid to brittle-hard, have a coarse to fine-crystalline structure, are translucent to opaque in color, but not transparent, and melt above 40°C without decomposition and are already thin or have a low viscosity slightly above the melting point, have a strongly temperature-dependent consistency and solubility and can be polished under slight pressure.

[0035] A waxy compound is a compound that has physical behavior similar to that of a wax.

[0036] According to the invention, all natural, semi-synthetic and synthetic waxes known to the person skilled in the art are suitable as hydrophobic binders.

[0037] Examples of natural waxes are wool wax, China wax, beeswax, preen, tallow, sugar cane wax, carnauba wax, candelilla wax, cork wax, guruma wax, ouicuri wax, Cuba palm wax, esparto wax, cotton wax, rice bran wax, flax wax, peat wax, rose wax, jasmine wax, peetha wax, myrtle wax, wax fig wax, petroleum wax, earth waxes, stuff wax, vein wax, montan wax, petroleum wax and paraffin wax.

[0038] Examples of semi-synthetic waxes are ester waxes made from long-chain wax acids with monohydric fatty or wax alcohols, amides of fatty and wax acids, amide waxes based on fatty acids, such as distearylethylenediamide, ethylene distearmide, stearic acid amide, behenic acid amide, erucic acid amide, oleic acid amide, soy wax, castor wax, rapeseed wax, phthalamide waxes and acylated amides of fatty and wax acids.

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

[0040] The wax is preferably selected from polyolefin waxes, in particular polyethylene waxes, or paraffin waxes.

[0041] The use of conventional wax-like compounds known to the person skilled in the art is also possible in principle according to the invention.

[0042] Preferably, the waxy compound is selected from esters of glycerol with fatty acids, preferably from linear carbon chains having 4 to 26, typically 12 to 22, carbon atoms, fatty acids, in particular linear aliphatic monocarboxylic acids having 13 to 21 carbon atoms, and mixtures thereof, preferably from stearin.

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

[0044] Hydrophobic binders with a dropping point in this range demonstrate a good balance between sufficient strength at ambient temperature and the most energy-efficient process for producing the cathode breakout material. Due to the comparatively low dropping point, the amount of energy required to liquefy the hydrophobic binder during the production of the cathode breakout material is lower than for hydrophobic binders with a dropping point according to DIN ISO 2176 of over 75°C or more.

[0045] The dropping point of the hydrophobic binder can be advantageously selected depending on the season and / or climate zone in which the cathode breakout material is to be stored and transported, ensuring that the hydrophobic binder is sufficiently solid at ambient temperature. For example, in colder seasons and / or temperate climates, a hydrophobic binder with a dropping point according to DIN ISO 2176 in the range between 35°C and 45°C is sufficient, while in warmer seasons and / or subtropical and tropical climates, a hydrophobic binder with a dropping point according to DIN ISO 2176 in the range between 45°C and 75°C may be advantageous.

[0046] It is also particularly advantageous if the hydrophobic binder contains the lowest possible proportion of functional groups, preferably free of functional groups. Functional groups, as used herein, are chemical groups that differ from pure carbon-carbon or carbon-hydrogen single bonds. Such a hydrophobic binder further significantly reduces the reactivity of the cathode breakout material according to the invention with water and / or atmospheric oxygen, thus further improving its storage and transport stability. Furthermore, the higher carbon content in the hydrophobic binder also increases the calorific value of the cathode breakout material when used as a fuel.

[0047] According to a particularly preferred embodiment of the invention, the cathode breakout material has a calorific value between 10,500 and 31,000 kJ / kg, determined according to the RAL-GZ 724 method of the Federal Quality Association for Secondary Fuels. The addition of the hydrophobic binder significantly increases the calorific value of the cathode breakout, which typically ranges from 7,500 to a maximum of 10,000 kJ / kg.

[0048] The cathode breakout material according to the invention can be adapted to the requirements of the respective further processing method. Depending on the further processing method, the cathode breakout material can advantageously be in the form of pellets, cocoons, castings, briquettes, or extrudates.

[0049] According to a preferred embodiment of the invention, the cathode breakout material is in the form of pellets or extrudates and contains 75 to 90 wt.% cathode breakout and 10 to 25 wt.% hydrophobic binder, based on the total weight of the cathode breakout material.

[0050] Cathode waste material in the form of easily dosed pellets or extrudates is advantageous for use as fuel in calorific value-controlled combustion processes, for example, in cement production in rotary kilns or in power plant operations. Here, the target temperature can be reliably predicted by charging with a homogeneous fuel of known calorific value. The smaller the fuel dosage form, the more precisely the temperature can be controlled. Achieving a target temperature within certain limits can be important for maintaining the quality of the manufactured product.

[0051] According to an alternative preferred embodiment of the invention, the cathode breakout material is in the form of briquettes and contains 60 to 80 wt.% cathode breakout and 20 to 40 wt.% hydrophobic binder, based on the total weight of the cathode breakout material.

[0052] Cathode waste material in the form of briquettes is advantageous for use as fuel in combustion processes controlled by a rough calorific value, for example, in cupola furnaces used in the production of mineral wool, and in batch furnaces, such as electric arc furnaces used in steel production. Briquettes make it easier to achieve the required gas permeability of the feed due to their coarser form compared to pellets. At the same time, despite the coarser form, reliable adjustment of the target temperature is possible by charging with homogeneous fuel of known calorific value.

[0053] According to a further alternative preferred embodiment of the invention, the cathode breakout material is in the form of castings and contains 30 to 80 wt.% cathode breakout and 20 to 70 wt.% hydrophobic binder, based on the total weight of the cathode breakout material.

[0054] Castings of cathode waste material are advantageous for use as fuel in combustion processes with roughly controlled calorific values, for example, in cupola furnaces used in the production of mineral wool, as well as in batch furnaces, such as electric arc furnaces. As with briquettes, the casting molds allow for the introduction of a large amount of fuel into the process while still allowing gas permeability. At the same time, the homogeneous fuel with a known calorific value allows for reliable temperature control.

[0055] Another object of the invention is a method for producing a cathode breakout material, comprising the following steps: (a) Providing cathode outcrop, in particular from aluminum electrolysis cells, (b) Comminuting the cathode outcrop in at least one comminution device, (c) Fractionating the cathode outcrop by a separating device, (d) Mixing the cathode outcrop with at least one hydrophobic binder selected from wax, a wax-like compound or mixtures thereof in a mixing device, (e) Portioning the mixture obtained in step (d), (f) Removing the cathode outcrop material, wherein steps (b) to (d) are carried out under an inert gas atmosphere.

[0056] The process according to the invention is characterized by the fact that it ensures simple, cost-effective, and energy-efficient production of the cathode breakout material. The statements made above regarding individual technical features of the cathode breakout material according to the invention apply accordingly to the corresponding technical features of the process according to the invention.

[0057] Step (a) of the process according to the invention provides for the provision of cathode waste, in particular from aluminum electrolysis cells. The provision of the cathode waste in step (a) can be carried out in any shape and size and is limited only by the technical possibilities for transporting the cathode waste. Thus, according to the invention, both coarse pieces and plates of cathode waste over 1 m in length as well as extremely fine dusts of cathode waste, as well as mixtures with a wide variety of grain and plate sizes, such as those typically generated during the mechanical demolition of a cathode lining, can be used in step (a).

[0058] In step (b) of the process, the cathode breakout is comminuted in at least one comminution device. In principle, any comminution device known to those skilled in the art can be used as the at least one comminution device. Preferably, the at least one comminution device in step (b) is a mill or crusher. Ball mills, impact mills, hammer mills, vertical mills, or shredders can be used here, for example. The at least one comminution device reduces the plate and / or particle size of the cathode breakout.

[0059] In step (c) of the process according to the invention, the cathode waste is fractionated by a separation device. According to the invention, separation devices can be used that ensure a homogeneous grain size of the cathode waste material. The separation device in step (c) is preferably a sieve. However, other separation devices are also conceivable in principle, with which a fine fraction of a specific grain size of the cathode waste can be separated from the cathode waste comminuted in step (b). The fractionation in step (c) can preferably take place simultaneously with the comminution in step (b). However, it is also conceivable for the fractionation to take place after the comminution in step (b).

[0060] The process according to the invention provides that, in step d), the cathode breakout is mixed with at least one hydrophobic binder selected from wax, a wax-like compound, or mixtures thereof in a mixing device. Such mixing devices are generally familiar to those skilled in the art. Preferably, the at least one hydrophobic binder is metered into the mixing device in step (d) in liquid form or liquefied by heating in the mixing device. The metered addition in liquid form can be achieved, for example, by supplying the hydrophobic binder to the mixing device from a separate heated storage container. However, it is also conceivable for the mixing device itself to be heated or for the wax to be liquefied by the energy input of the mixing unit in the mixing device.

[0061] Steps (b) to (d) of the process according to the invention are carried out under an inert gas atmosphere. This is necessary because the cathode effluent, particularly the fine particles formed in step (b), are highly reactive with water and / or atmospheric oxygen. As already explained above, this can lead to, among other things, heat generation, toxic gas evolution, and ignition. Such a reaction must therefore be avoided at all costs for safety reasons. Noble gases such as helium, neon, argon, krypton, and xenon, as well as nitrogen, can be used as inert gases. For economic reasons, the use of nitrogen is preferred according to the invention.

[0062] In step (e) of the process according to the invention, the mixture obtained in step (d) is portioned. The portioning in step (e) is preferably selected from the group consisting of molding, briquetting, extruding, or pelletizing.

[0063] Furthermore, it is also conceivable to fill the mixture obtained in step (d) into previously prepared solid cocoons of the hydrophobic binder. The advantage of this variant is that even less homogeneous cathode effluent can be processed. Thus, fractionation in step (c) is not absolutely necessary for this embodiment. The hydrophobic binder cocoon can be a hollow geometric shape, in particular a hollow sphere or a hollow cylinder, with a hollow sphere being preferred.For the hollow sphere cocoon embodiment, the portioning in step (e) would proceed in such a way that the mixture obtained in step (d) is filled into a solid hollow hemisphere made of the hydrophobic binder and the other solid hollow hemisphere, after heating and thus softening the peripheral edge of the hollow hemisphere, is then placed like a lid on the hollow hemisphere filled with the cathode fragment, so that the cathode fragment is completely enclosed by the solid cocoon made of hydrophobic binder.

[0064] Depending on the desired portioning method in step (e) of the process according to the invention, different proportions of cathode breakout and binder are preferably mixed in step (d).

[0065] According to a preferred embodiment of the invention, the portioning is a pelletizing or extruding and in step (d) 75 to 90 wt.% of cathode outcrop and 10 to 25 wt.% of hydrophobic binder, based on the total weight of the mixture of cathode outcrop and hydrophobic binder, are mixed.

[0066] According to an alternative preferred embodiment of the invention, the portioning is a briquetting and in step (d) 60 to 80 wt.% of cathode outcrop and 20 to 40 wt.% of hydrophobic binder, based on the total weight of the mixture of cathode outcrop and hydrophobic binder, are mixed.

[0067] According to a further alternative preferred embodiment of the invention, the portioning is an in-mold casting and in step (d) 30 to 80 wt.% of cathode outcrop and 20 to 70 wt.% of hydrophobic binder, based on the total weight of the mixture of cathode outcrop and hydrophobic binder, are mixed.

[0068] The method according to the invention provides that the cathode breakout material is removed in step (f). The cathode breakout material removed in step (f) is preferably in the above-mentioned dosage forms.

[0069] The cathode breakout material can be treated with a release agent before removal in step (f) to prevent the cathode breakout material from sticking together during storage and transport. Powdered substances are conceivable as release agents. Examples of release agents are calcium carbonate, talc, or silicates.

[0070] The process according to the invention can be carried out semi-continuously or continuously.

[0071] Finally, the invention relates to the use of the cathode breakout material according to the invention as fuel, preferably in power plants and in the production of mineral wool, cement and steel.

[0072] Depending on the recycling process in which the cathode waste material is ultimately used as fuel, the combustion behavior of the cathode waste material must be adapted to the different requirements of the respective recycling process. This can be achieved with the cathode waste material according to the invention by varying the proportions of cathode waste to hydrophobic binder, by selecting the hydrophobic binder, and by using different forms of administration, such as granules, pellets, briquettes, cocoons, extrudates, and castings.

[0073] If increased strength of the fuel is required for the use of the cathode breakout material as a fuel, even at high temperatures, the cathode breakout material according to the invention can also be used in dimensionally stable fuel composite moldings, which are used, for example, in the production of mineral wool.

[0074] In addition to the cathode breakout material, such fuel composite bodies typically comprise a hydraulically hardening binder, such as cement, especially Portland cement. Due to the advantageous inertness of the cathode breakout material according to the invention, such fuel composite molded bodies can be produced by incorporating the cathode material according to the invention without the cathode breakout material reacting with the water used in the production of the fuel composite bodies.

[0075] It goes without saying that the cathode breakout material according to the invention is also suitable as a starting material for the production of other conceivable fuel composite molded bodies, each of which is adapted to the different requirements of the respective utilization process.

[0076] With regard to the use, the above statements regarding the cathode material according to the invention and the process for its production apply accordingly with regard to the technical features of the cathode breakout material according to the invention and its design and production.

[0077] The invention is described in more detail below using an exemplary embodiment with reference to the attached figure. This example serves merely to illustrate the invention and does not limit the scope of the invention. Fig. 1Schematic representation of an embodiment of the method according to the invention for producing cathode breakout material

[0078] In Figure 1An embodiment of the method according to the invention for producing cathode waste material is shown schematically. First, a nitrogen-inertized mill is fed with the provided cathode waste 2 with a grain size of less than 50 mm via a vibrating hopper 3. If fragments are too coarse or the range in the size distribution is too large to be fed to a mill, a crusher, also inertized with nitrogen, can be installed upstream. The provided cathode waste 2 simultaneously acts as a dust collector. Using a sieve 4 near the bottom, the final grain size at which the fine fraction of the cathode waste is released from the mill into the heated mixer 5 is determined. In the mixer 5, liquefied wax is added via a dosing unit 6 up to a concentration of 20 to 40 wt.% wax, based on the total weight of the cathode waste material.The wax was previously liquefied in a heated wax storage container 7. Once the correct mixing ratio of wax and cathode breakout is achieved in the mixer 5, the mill 1, any upstream crusher, and the wax addition via the dosing unit 6 are stopped. The mixing direction in the mixer 5 is reversed, and the cathode breakout material / wax mixture is poured into slightly conical casting molds 8. After cooling to room temperature, the finished cathode breakout material can be removed from the casting molds 8 and is thus available as a cast product. EXAMPLES

[0079] 1300 kg of cathode scrap were delivered to a vertical mill. The cathode scrap was pre-sorted, free of contaminants such as corundum or aluminum, and contained no pieces larger than 5 cm.

[0080] The milling process took place under a nitrogen atmosphere and was carried out with a target fineness of 10% > 90 µm. This means that after milling, 90% of the cathode breakout was smaller than 90 µm, and the remaining 10% was approximately between 150 and 200 µm. Pellet production - Variant A - Partial pelleting

[0081] 1800 g of the ground cathode scrap were placed on a pelletizing plate preheated to 70°C. By adding 10 to 15 wt.% wax, pellets were obtained that had a solid, round shell on the outside but contained almost dry ground material on the inside. Pellet production - Variant B - Full pelleting

[0082] In another example, 1800 g of ground cathode scrap was placed on a pelletizing plate preheated to 80°C, and 17 to 21 wt.% wax was added. Pellets were obtained that contained a mixture of wax and ground material across their entire diameter. The angle of the pelletizing plate during pellet production was 30° to the solder, and the rotation speed was 30 rpm.

[0083] Pelletizing was carried out in a semi-continuous process, with the ground cathode scrap being fed in proportion to the pellet removal (falling over the edge). The preheated, liquid wax was also metered (or sprayed) in the corresponding ratio. The angle of the pelletizing plate during pellet production was 30° to the solder, and the rotation speed was 30 rpm.

[0084] Calorific value and heating value analyses were subsequently carried out for both partial and full pelleting according to RAL-GZ 724. The results are summarized in Table 1.

[0085] The "original substance" listed in Table 1 refers to the pellets as they were removed from the pelletizing machine. "Dry substance" refers to the pellets that were dried according to DIN EN 14346 after removal from the pelletizing machine.

[0086] By varying the rotation speed from 20 to 40 rpm and the angle of attack from 15° to 30° to the vertical, pellets with different average diameters d = 8 to 17 mm can be obtained. Production of castings

[0087] In another example, castings were produced. A sieve fraction < 3 mm was separated from the fines of the cathode waste and used.

[0088] In a batch process, approximately 3 kg of paraffin with a melting point between 70 and 80 °C were liquefied and heated to approximately 100 °C. The kinematic viscosity at 100 °C is between 3 and 10 mm 2 / sec.

[0089] While continuously stirring, 10 kg of the <3 mm sieve fraction separated from the fines of the cathode waste was slowly added in small portions. After everything was mixed into a homogeneous mass, an 8 kg casting was poured off. Another 3 kg of paraffin was added to the mass remaining in the mixer. After melting, another 10 kg of the <3 mm sieve fraction separated from the fines of the cathode waste was added while stirring. This was used to create the second casting. Additional castings were created by repeating the respective steps.

[0090] The castings of the sieve fraction < 3 mm separated from the fines of the cathode breakout dehomogenize slightly during the solidification process. This leads to a higher wax concentration near the surface of the casting.

[0091] It can be assumed that with increasing fineness of the cathode breakout, less wax is required for stable shaping, regardless of the dosage form chosen. Adding more wax than necessary for physical stability is a good way to increase the calorific value as desired and adapt it to the requirements of the subsequent application.

Claims

1. Spent potlining material containing spent potliner from aluminum electrolytic cells, and at least one hydrophobic binder, the spent potliner being in agglomerated form with the hydrophobic binder, characterized in that the hydrophobic binder is selected from wax, a wax-like compound, or mixtures thereof, the spent potlining material containing 30 to 90 wt.% spent potliner and 10 to 70 wt.% hydrophobic binder, relative to the total weight of the spent potlining material.

2. Spent potlining material according to claim 1, characterized in that the spent potliner has a grain size of less than 50 mm.

3. Spent potlining material according to either of the preceding claims, characterized in that the spent potliner consists of the first cut or the second cut or a mixture of the first and second cuts, the first cut consisting of the graphite of the cathode lining, and the second cut constitutes a mixture of the graphite of the cathode lining and the refractory lining.

4. Spent potlining material according to any of the preceding claims, characterized in that the hydrophobic binder has a dripping point according to DIN ISO 2176 between 35 °C and 75 °C.

5. Spent potlining material according to any of the preceding claims, characterized in that the wax is selected from natural wax, semi-synthetic wax or synthetic wax, and mixtures thereof, preferably from polyolefin waxes, in particular polyethylene waxes, or paraffin waxes.

6. Spent potlining material according to any of the preceding claims, characterized in that the wax-like compound is selected from esters of glycerol with fatty acids, preferably from linear carbon chains having 4 to 26, typically 12 to 22, carbon atoms, fatty acids, in particular linear aliphatic monocarboxylic acids having 13 to 21 carbon atoms, and mixtures thereof, preferably the wax-like compound is stearin.

7. Spent potlining material according to any of the preceding claims, characterized in that the spent potlining material is in the form of pellets, cocoons, castings, briquettes, or extrudates.

8. Spent potlining material according to any of the preceding claims, characterized in that the cathode material has a heating value between 10500 to 31000 kJ / kg, determined according to the RAL-GZ 724 method of the German Federal Quality Association for Secondary Fuels.

9. Method for producing a spent potlining material, comprising the following steps: (a) providing spent potliner from aluminum electrolytic cells, (b) comminuting the spent potliner at least in one comminuting apparatus, (c) fractionating the spent potliner through a separating apparatus, (d) mixing the spent potliner with at least one hydrophobic binder, selected from wax, a wax-like compound, or mixtures thereof, in a mixing apparatus, (e) portioning the mixture obtained in step (d), (f) withdrawing the spent potlining material, wherein steps (b) to (d) are carried out in an inert gas atmosphere.

10. Method for producing a spent potlining material according to claim 9, characterized in that the at least one comminuting apparatus in step (b) is a mill or a crusher.

11. Method for producing a spent potlining material according to either claim 9 or claim 10, characterized in that the separating apparatus in step (c) is a sieve.

12. Method for producing a spent potlining material according to any of claims 9 to 11, characterized in that the at least one hydrophobic binder is metered to the mixing apparatus in step (d) in liquid form or is liquefied by heating in the mixing apparatus.

13. Method for producing a spent potlining material according to any of claims 9-12, characterized in that the portioning in step (e) is selected from in-mold casting, briquetting, extruding, or pelletizing.

14. Use of the spent potlining material according to any of claims 1 to 8 as a fuel, preferably in power stations and in the production of mineral wool, cement, and steel.