Rotary drum furnace for remelting non-ferrous metals

Abstract

Rotary drum furnace for remelting non-ferrous metals, in particular secondary non-ferrous metals and preferably secondary non-ferrous heavy metals, with a furnace drum (1) rotatable about a horizontal axis (A), and with at least one stirring body (6) attached to the furnace shell (5) of the furnace drum (1) and projecting into the interior of the furnace relative to a lining (7), characterized in that the lining (7) and the stirring body (6) are made of a castable and hardening refractory material, wherein at least the stirring body (6) has additional reinforcement (8, 9).

Description

The invention relates to a rotary drum furnace for remelting non-ferrous metals, in particular secondary non-ferrous metals and preferably secondary non-ferrous heavy metals such as lead, zinc, aluminium, copper, nickel, titanium, manganese, tin and their alloys such as brass or bronze, with a furnace drum rotatable about a horizontal axis, and with at least one stirring body attached to the furnace shell and projecting into the interior of the furnace relative to a lining. Non-ferrous metals, as is generally understood, refer to all metals other than iron and its alloys in which iron is not a major component. These non-ferrous metals are characterized, in some cases, by their low weight, corrosion resistance, or electrical conductivity, which explains their widespread use in the production of a wide variety of products. This document primarily concerns the remelting of secondary non-ferrous metals, that is, non-ferrous metals obtained through the processing and comminution of products originally manufactured from them. Specifically, it focuses on secondary non-ferrous heavy metals, meaning the aforementioned products with particularly high mass or specific gravity. Such a rotary drum furnace is described by way of example in the preprint WO 2005 / 090620 A1. This refers to a rotary drum furnace for remelting aluminum in a protective bath consisting of molten salt. For this purpose, agitators protruding from the refractory lining of the rotary drum furnace and attached to the furnace shell are provided. The agitators are manufactured using a hydraulically or chemically setting refractory casting or stamping compound. The casting or stamping compound for manufacturing the agitator contains up to 50% by volume of steel needles made of a heat-resistant steel. In addition, DE 202 15 666 U1 describes a stirring element for a rotary drum furnace, which is again used particularly for remelting aluminum in a protective bath. For this purpose, the stirring element is equipped with a metallic core and a non-metallic coating made of a refractory material that completely or partially surrounds the metallic core. Such rotary drum furnaces can typically be operated by heating the furnace interior using at least one burner, the exhaust gases of which are discharged downstream via an exhaust pipe. The exact procedure may be as described in detail in EP 2 329 210 B1. However, in this document, iron is explicitly mentioned as a starting material in addition to non-ferrous metals. In connection with the recycling of secondary non-ferrous metals, the known process for treating lithium-ion waste batteries according to DE 198 42 658 B4 is also of interest. However, in this process, the primarily processed electrode material is subjected to an acid treatment solution, which amounts to a completely different procedure compared to the melt refining process under discussion here, in the sense of a chemical transformation. In a process for melting secondary metals, as described in DE 41 15 269 A1, the rotary drum furnace used is equipped with a primary burner and a secondary burner. The furnace and the resulting afterburner are maintained at temperatures exceeding 1100°C for the melting and combustion of PCB-containing secondary metals. This is intended to reduce the overall proportion of unburned components containing organic material. However, details regarding the design of the rotary drum furnace remain undisclosed. The state of the art has generally proven its worth, but still offers room for improvement. For example, in the processing and especially the remelting of secondary non-ferrous metals, it has become apparent that not only are the steel stirrers often used for aluminum recycling, according to DE 20 2008 001 480 U1, actually unsuitable in practice when dealing with heavy metals, particularly secondary lead. This can be attributed to the fact that the steel stirrers are attacked by the molten lead, just like the lining typically made of firebricks. This is essentially due to the particularly high density of such heavy metals in combination with their significant melting point of, for example, approximately 330°C. In fact, three different damage processes are observed in connection with steel stirrers and the processing of secondary lead: the dissolution of iron, liquid metal embrittlement, or diffusion at grain boundaries. First of all, at the high temperatures of molten lead (approximately 330°C), iron from the steel of the stirrer can dissolve into the liquid lead. The lead then acts as a solvent for the iron, with prolonged contact and high temperatures being particularly critical. Another effect is so-called metal embrittlement, in which the liquid lead penetrates the steel along the grain boundaries. This weakens the bonds between the individual grains, leading to cracking and ultimately to complete destruction over longer periods. Finally, diffusion and intermetallic effects are also observed because, although iron and lead do not form stable intermetallic phases, lead can disrupt the carbon bonding in the steel and thus locally alter the microstructure. Finally, the stirring process also results in significant mechanical wear of the steel stirrer due to considerable shear forces. Similar problems are observed with refractory linings, such as firebricks, during the remelting of secondary lead in the example case. Therefore, existing designs of rotary drum furnaces, which have proven advantageous for remelting aluminum or similar light metals, are unsuitable for remelting secondary non-ferrous heavy metals in practice and are thus unusable. This is particularly disadvantageous because the increasing use of batteries in connection with electromobility means that their processing and thus the recovery of non-ferrous heavy metals, in particular, is of growing and rapidly increasing importance in practice. The invention aims to remedy this situation. The invention is based on the technical problem of further developing such a rotary drum furnace in such a way that secondary non-ferrous heavy metals in particular can be processed easily and permanently. To solve this technical problem, a rotary drum furnace of the generic type for remelting non-ferrous metals, and preferably secondary non-ferrous heavy metals, is characterized according to the invention in that the lining and the agitator are made of a castable and hardened refractory material, wherein at least the agitator has additional reinforcement. In most cases, a matching refractory material is used both as a replacement for the previous lining and as the agitator. The invention is based on the premise that the combined use of a castable and hardening refractory material for both the lining and the agitator can have a particularly positive effect on the previously described and destructive phenomena of dissolution, embrittlement, and diffusion, as well as mechanical abrasion, compared to previous embodiments. This is essentially due to the fact that such refractory materials, such as refractory concrete or ceramic materials, as well as combinations thereof, exhibit high resistance to mechanical abrasion, even when subjected to shear forces such as those exerted by molten lead. Of particular importance is the fact that, in this example, liquid lead has a very high surface tension, meaning it practically does not wet the concrete and instead "beads up" on it. This prevents the lead from penetrating the fine pores of the concrete. In other words, the fine pores in the concrete are too small for liquid lead to overcome due to its high surface tension and viscosity. Consequently, liquid lead cannot penetrate the concrete, and unlike the steel stirrers discussed earlier, neither embrittlement nor the described diffusion occurs. Furthermore, the mere pressure exerted by the high weight of the lead is insufficient to overcome the capillary forces associated with the pores in the concrete. Furthermore, there is no chemical affinity because lead does not react with concrete components, nor does it form a wetting compound. Finally, in the case of the lead melt under consideration, the relatively low melting point of approximately 330°C for lead means that the heat is quickly dissipated upon contact with concrete, and the lead solidifies relatively rapidly before penetration is even possible. However, this latter effect does not typically occur inside such a rotary kiln with continuous heating, for example, by a gas flame. In any case, it becomes clear that secondary non-ferrous heavy metals in particular can be easily remelted using the rotary drum furnace according to the invention, in order to recover the desired heavy metal from appropriately recycled products and feed it into further processing. This is where the main advantages lie. The general procedure is such that, in the case of the previously described lead recycling and lead processing by remelting secondary lead, various sources of secondary lead can be used. Typically, used motor vehicle batteries are employed at this stage. By remelting in the rotary drum furnace according to the invention, the lead can be separated from any impurities. Finally, the pure lead obtained from the rotary drum furnace is cast, for example, into new lead ingots, which can then be used for alloys or the like. Consequently, car batteries or, for example, radiation protection equipment and ammunition can be manufactured from this material as byproducts. As a preparatory step for processing the secondary non-ferrous metal, the usual procedure is to collect used lead-acid batteries and then crush them, for example, in a hammer mill. This separates the main components: lead, acid, and plastic. The lead components can then be removed, typically by separating the lead and plastic by weight through a sorting process. The acid can be collected beforehand as a liquid. The resulting recyclate, containing predominantly secondary lead, is then charged into the melting furnace. Here, refining takes place, separating the lead from associated materials such as slag and impurities. After refining, alloying elements such as antimony, tin, or calcium can be added if required.This may take place directly in the rotary drum furnace according to the invention or after the production of secondary lead or the lead ingots obtained in this manner. To achieve this in detail and with particular advantage, the additional reinforcement for the agitator is made of a metal, a fibrous material such as ceramic fibers, individually or in combination. The reinforcement of the agitator primarily ensures that it is equipped to withstand and absorb the shear forces previously described, which act upon the agitator in particular. The refractory material used for the lining and the agitator is made of refractory concrete or a ceramic material, or combinations thereof. Overall, comparable materials can be used, as already described in DE 202 15 666 U1. In particular, the refractory concrete in question consists of alumina cement, refractory aggregates such as chamotte, bauxite, chromium ore, and water, as well as chemical additives that make it heat-resistant up to over 1000°C. The alumina cement forms stable bonds, while the chamotte and aggregates give the concrete volume and heat resistance. Additional additives serve to improve flowability and resistance to thermal shock. The reinforcement of the agitator body features needles embedded in the refractory material, typically with a maximum volume fraction of 30 vol%. Advantageously, these needles are made of steel and / or ceramic, with a diameter of 0.5 mm to 2 mm and a length of 20 mm to 50 mm. This is, of course, not an absolute limitation. In any case, considering these dimensions, a particularly strong bond has been achieved between the needles and the castable and hardening refractory material. In addition, the agitator's reinforcement includes a metal mesh basket, and in particular a steel mesh basket, in addition to the needles. This means that, generally, the agitator's reinforcement is designed in two parts, namely the metal mesh basket on the one hand, and the needles, specifically steel needles and / or ceramic needles, on the other. The metal mesh basket is adapted to the outer shape of the agitator and arranged inside it. Generally, only a cover layer remains, covering the metal mesh basket on the outside. The needles are generally located inside the metal mesh basket, while the cover layer is predominantly free of embedded needles. This allows the cover layer to form a particularly strong bond with the underlying metal mesh basket, preventing any embrittlement of the cover layer. Furthermore, this prevents the formation of any cavities or capillaries in the cover layer through which, for example, liquid lead could penetrate into the interior of the agitator. According to the invention, this is precisely what prevents the cover layer from being predominantly or even completely free of embedded steel needles. This can easily be achieved in the manufacture of the agitator by first producing the metal mesh basket containing the refractory material and the steel needles embedded within it. Finally, the outer layer, which is predominantly or completely free of embedded steel needles, is applied and forms the desired bond with the metal mesh basket and the refractory material containing the embedded steel needles. The metal mesh basket is generally attached to a metal base plate and, for example, screwed to it. It has also proven effective to attach the metal plate, along with the agitator mounted on it, to the furnace casing, also screwing it in place. The furnace casing itself has an internal coating extending all the way to the agitator. This means the coating only leaves space around the agitator, otherwise completely covering the inside of the furnace casing. The thickness of the coating layer on the furnace casing, in the form of the refractory concrete layer, is generally selected and adjusted to be approximately 10% to 20% of the height or length of the agitator. This ensures the agitator is properly embedded in the lining, which essentially forms the refractory lining.A replacement layer of concrete is embedded, and in this way the concrete layer ensures that any shearing stresses of a lead melt inside the rotary drum furnace do not attack the base of the agitator, which in turn is screwed to the metallic furnace shell. In this way, a rotary drum furnace is provided that is particularly suitable for remelting non-ferrous metals, and preferably non-ferrous heavy metals or secondary non-ferrous heavy metals. The remelting of lead is especially successful in this context. All of this is ensured without the refractory concrete lining or the one or more agitators, also made of refractory concrete, being damaged by the remelting process. This is essentially due to the fact that, for example, liquid lead cannot penetrate the concrete layer or the agitator, and no chemical interaction takes place. To prevent shearing abrasion of the agitator, it is equipped with special reinforcement, which, according to the invention, consists on the one hand of the metal mesh basket and on the other hand of embedded needles for additional reinforcement.This is where the main advantages lie. The invention is explained in more detail below with reference to a drawing that merely illustrates an exemplary embodiment; the figures show: Fig. 1 schematically in section the rotary drum furnace according to the invention and Fig. 2 a detail from Fig. 1 in the area of ​​a stirring element, also in section. The figures depict a rotary drum furnace particularly suitable and designed for remelting secondary non-ferrous heavy metals, especially lead. The rotary drum furnace, shown in the overview, has a furnace drum 1 rotatably mounted about a horizontal axis A. For this purpose, the rotatable furnace drum 1 is equipped with a burner 2 at an opening on one end. Through an opposite opening, the secondary non-ferrous heavy metal to be processed, in this embodiment secondary lead, can be introduced into the interior using a charging device 3. There, it forms a bed 4 visible at the bottom of the furnace drum 1. This bed 4 consists of previously crushed lead residues or lead recyclate and is heated by the burner 2 until it melts. In this process, the furnace drum 1 is rotated horizontally about its axis A, as indicated by a double arrow in Fig. 1. This brings one or more stirrers 6, attached to a furnace shell 5 of the furnace drum 1 and shown in detail in Fig. 2, into contact with the forming molten lead. These stirrers ensure that the molten lead is homogenized and that any unmelted components are forced into the melt, thus converting it into a liquid state. To prevent damage to the agitator 6, as well as to the lining 7 on the inside of the furnace drum 1 and as a layer on the furnace shell 5, during this process, both the lining 7 and the agitator 6 are made of a castable and hardening refractory material, specifically refractory concrete, according to the exemplary embodiment. Furthermore, at least the agitator 6 has additional reinforcement 8, 9. The reinforcement 8, 9 can best be understood by referring to the schematic sectional view in Fig. 2. In fact, the reinforcement 8, 9 consists of two components: a metal mesh cage 8 and steel needles 9 embedded in the refractory material. In this embodiment, the refractory material is refractory concrete. In principle, a ceramic material can also be used here, as long as the refractory material is hardenable and castable. Furthermore, needles made of, for example, ceramic fibers can be used instead of steel needles 9 as part of the reinforcement 8, 9. In any case, the steel needles 9 have the dimensions specified in the introductory section of the description. It can be seen that the metal mesh basket 8 is adapted to the outer shape of the agitator 6. Furthermore, the metal mesh basket 8 is designed to fill the interior of the agitator 6, except for a covering layer 10 provided on its surface. This covering layer 10 is largely free of the embedded steel needles 9 in order to prevent any cracking or embrittlement on the surface of the agitator 6 and thus, in any case, to prevent any penetration of the liquid lead into the interior of the agitator 6. As can be seen in Fig. 2, the metal mesh basket 8 is connected to a metal plate 11, and in the exemplary embodiment, is screwed to the plate 11. The metal plate 11, like the metal mesh basket 8, is screwed to the steel furnace casing 5. This allows the metal plate 11, together with the stirring element 6 located on it, to be connected to, or screwed to, the furnace casing 5. Furthermore, the furnace shell 5, with the exception of the agitator 6, is coated with the refractory material or refractory concrete, which forms and defines the lining 7. The refractory concrete coating or lining 7 has a layer thickness S, indicated in Fig. 2 5, which is approximately 10% to 20% of the length L of the agitator 6. This ensures that the agitator 6 is securely embedded in the coating or lining 7, and any shear forces acting on it are absorbed by the circulating liquid lead and transferred into the furnace shell 5. In this way, continuous operation during the production of secondary lead is guaranteed in the example case. QUOTES INCLUDED IN THE DESCRIPTION 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 WO 2005 / 090620 A1

[0003] DE 202 15 666 U1 [0004, 0024]EP 2 329 210 B1

[0005] DE 198 42 658 B4

[0006] DE 41 15 269 A1

[0007] DE 20 2008 001 480 U1

[0008]

Claims

Rotary drum furnace for remelting non-ferrous metals, in particular secondary non-ferrous metals and preferably secondary non-ferrous heavy metals, with a furnace drum (1) rotatable about a horizontal axis (A), and with at least one stirring body (6) attached to the furnace shell (5) of the furnace drum (1) and projecting into the interior of the furnace relative to a lining (7), characterized in that the lining (7) and the stirring body (6) are made of a castable and hardening refractory material, wherein at least the stirring body (6) has additional reinforcement (8, 9). Rotary drum furnace according to claim 1, characterized in that the reinforcement (8, 9) is made of a metal or a fibrous material such as ceramics, either individually or in combination. Rotary drum kiln according to claim 1 or 2, characterized in that the refractory material is made of refractory concrete or a ceramic material or combinations thereof. Rotary drum furnace according to one of claims 1 to 3, characterized in that the reinforcement (8, 9) of the stirring body (6) comprises needles (9), in particular steel needles or ceramic needles, embedded in the refractory material with a volume fraction of 30 vol% at most. Rotary drum furnace according to claim 4, characterized in that the reinforcement (8, 9) of the stirring body (6) has a metal mesh basket (8) in addition to the needles (9). Rotary drum furnace according to claim 5, characterized in that the metal mesh basket (8) is adapted to the outer shape of the stirring body (6) and fills the interior of it except for a cover layer (10). Rotary drum furnace according to claim 6, characterized in that the needles (9) are arranged inside the metal mesh basket (8), while the top layer (10) is predominantly free of embedded needles (9). Rotary drum furnace according to one of claims 5 to 7, characterized in that the metal mesh basket (8) is connected to a metal plate (11) as a base, for example screwed to it. Rotary drum furnace according to claim 8, characterized in that the metal plate (11) together with the stirring body (6) located thereon is connected to the furnace shell (5), for example by screwing it on. Rotary drum furnace according to claim 9, characterized in that the furnace shell (5) is coated on the inside with the refractory material as lining (7) up to the stirrer (6).

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