Electric furnace operation method

Abstract

To provide a method for effectively dissolving a solidified metal layer which remains in and is attached to a furnace bottom and a solidified slag layer attached thereon, and enabling efficient electric furnace operation, in operation of an electric furnace having an electrode.SOLUTION: A method for operating an electric furnace having an electrode removes a part of at least a solidified slag layer out of a solidified metal layer attached to and generated in a furnace bottom after end of previous operation and a solidified slag layer generated on the solidified metal layer, exposes the solidified metal layer as a lower layer, then charges a raw material into an electric furnace, and brings molten metal generated by melting the raw material into contact with the exposed solidified metal layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for operating an electric furnace that reduces and melts raw materials, including waste lithium-ion batteries, to recover metals, including valuable metals. [Background technology]

[0002] For example, in an operation to recover valuable metals from raw materials such as waste lithium-ion batteries using an electric furnace equipped with three-phase AC electrodes, when the operation is stopped, an alloy containing the valuable metals (hereinafter also referred to as "metal") is discharged from a metal hole by tapping, and then slag is discharged by tapping from a slag hole or by tilting the electric furnace.

[0003] At this time, to prevent slag from mixing with the discharged metal, the metal level cannot be lowered below the top of the metal hole. Furthermore, when the slag is discharged by tapping, the slag level cannot be lowered below the bottom of the slag hole. Alternatively, when the slag is discharged by tilting, if the slag layer reaches the metal layer due to tilting, there is a possibility that the metal will be mixed with the slag. For these reasons, metal and slag that could not be discharged remain in the electric furnace.

[0004] The metal remaining in the electric furnace solidifies on the hearth refractory, forming a solidified metal layer. The remaining slag also solidifies in the same way, forming a solidified slag layer on top of the solidified metal layer. The solidified slag layer adhering to the hearth is not easily removed even when operation is resumed. This is because the metal heating temperature during operation is lower than the melting point of the slag. Therefore, the solidified metal layer underneath the solidified slag layer is also not easily removed. This can result in metal holes remaining filled with the solidified metal and slag layers, making the furnace inoperable.

[0005] For example, Patent Document 1 discloses an electric melting furnace and melting method for solidifying radioactive waste that utilizes a magnetic field, in which an electromagnetic force acts partially on the molten glass in the area where the direct heating current and the magnetic field intersect, in accordance with Fleming's left-hand rule, resulting in a force that forcibly promotes convection in the molten glass in the melting furnace, and this convection increases the amount of heat transferred from the molten glass to the raw material layer on the melt surface, improving melting capacity and making it possible to suppress and prevent the settling and accumulation of deposits on the furnace bottom.

[0006] However, in an electric furnace equipped with electrodes such as a three-phase AC type, no technology is disclosed for removing the solidified metal layer adhering to the furnace bottom and the solidified slag layer adhering thereon. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 07-104436 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been proposed in view of the above circumstances, and aims to provide a method for effectively removing the solidified metal layer remaining on the bottom of an electric furnace and the solidified slag layer adhering thereto, thereby enabling efficient operation of the electric furnace. [Means for solving the problem]

[0009] The inventors of the present invention have conducted extensive research to solve the above-mentioned problems. As a result, they have discovered that the solidified metal and solidified slag layers can be effectively eliminated with simple operations by removing part of the solidified slag layer to expose the underlying solidified metal layer, and then charging raw materials into the furnace so that the raw materials melt and produce molten metal, which is then brought into contact with the exposed solidified metal layer. This has led to the completion of the present invention.

[0010] (1) The first aspect of the present invention is a method for operating an electric furnace equipped with electrodes, which involves removing at least a portion of the solidified slag layer formed on the bottom of the furnace after the previous operation and the solidified metal layer formed on top of the solidified slag layer to expose the underlying solidified metal layer, and then loading raw materials into the electric furnace, melting the raw materials to produce molten metal, which is then brought into contact with the exposed solidified metal layer.

[0011] (2) The second aspect of the present invention is a method for operating an electric furnace, in which the amount of raw materials charged into the electric furnace is adjusted so that the molten slag produced by melting the raw materials charged into the electric furnace comes into contact with the solidified slag layer remaining after removing part of the solidified slag layer.

[0012] (3) The third aspect of the present invention is a method for operating an electric furnace according to the first or second aspect of the present invention, in which it is determined that the solidified slag layer and the solidified metal layer have melted and become molten based on the measurements of thermometers installed in the hearth structure of the electric furnace.

[0013] (4) A fourth aspect of the present invention is a method for operating an electric furnace according to any one of the first to third aspects, wherein the raw material includes waste lithium-ion batteries.

[0014] (5) A fifth aspect of the present invention is a method for operating an electric furnace according to any one of the first to fourth aspects, wherein the electric furnace is a three-phase AC electric furnace. [Effects of the Invention]

[0015] According to the present invention, a method can be provided for effectively removing the solidified metal layer remaining on the bottom of an electric furnace and the solidified slag layer adhering thereto, thereby enabling efficient operation of the electric furnace. [Brief explanation of the drawings]

[0016] [Figure 1]This is a schematic cross-sectional view of the inside of an electric furnace, illustrating how a portion of the solidified slag layer is removed to expose the solidified metal layer, and how the molten metal produced by charging raw materials is brought into contact with the solidified metal layer. [Figure 2] This is a schematic cross-sectional view of the inside of an electric furnace, and is used to explain the state when part of the solidified slag layer is removed and the amount of raw material charged is adjusted so that the resulting molten slag comes into contact with the solidified slag layer. DETAILED DESCRIPTION OF THE INVENTION

[0017] Specific embodiments of the present invention (hereinafter referred to as "present embodiments") will be described below. Note that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention.

[0018] The method according to the present embodiment is a method for operating an electric furnace, such as a three-phase AC electric furnace, equipped with electrodes, in which raw materials containing oxidized metals are heated and subjected to reduction melting.

[0019] In this operating method, raw materials containing oxidized metals are heated and reduced in an electric furnace to produce a molten mixture consisting of metal composed of the valuable metals contained in the raw materials and slag composed of impurity components. The molten mixture is then held and separated into upper and lower layers of low-density slag and high-density metal. The separated slag is then discharged and recovered by tilting the electric furnace or by tapping through a slag hole. The lower layer of metal containing valuable metals is then discharged and recovered by tapping through a metal hole located at the bottom of the electric furnace.

[0020] Electric furnaces are operated by repeatedly charging raw materials and discharging the metal and slag produced by reducing and melting the materials. The amounts of metal and slag discharged can be predicted from the amounts of raw materials charged. That is, the required amounts of metal and slag are discharged and recovered at the required times, depending on the levels (height levels) of the metal and slag produced.

[0021] After the required number of cycles of charging raw materials, reducing melting, and discharging metal and slag, the operation is terminated. When recovering metal and slag, it is desirable to discharge them from the furnace without mixing them as much as possible. However, in the case of metal, the metal level can only be lowered to the top of the metal hole. In the case of slag, if discharged through the slag hole, the limit is the bottom of the slag hole. Furthermore, if the slag is discharged by tilting the electric furnace, the limit is the point where the slag layer overlaps the metal layer. Therefore, a certain amount of metal and slag remain in the electric furnace. These remaining metal and slag layers then solidify, forming a lower solidified metal layer and an upper solidified slag layer.

[0022] The melting point of the solidified metal layer formed as described above is approximately 1300°C to 1400°C, and the melting point of the solidified slag layer is approximately 1500°C to 1600°C. Meanwhile, the reduction melting process, performed by charging raw materials into an electric furnace, is performed at a metal heating temperature of approximately 1350°C to 1450°C, and a slag heating temperature of approximately 1550°C to 1650°C. Therefore, if the solidified metal and solidified slag layers remain in the electric furnace when restarting operations after the previous operation, the molten metal located in the lower layer of the resulting molten layer cannot melt the solidified slag layer. Furthermore, if the solidified slag layer cannot be melted, the solidified metal layer cannot be melted either. Therefore, the metal holes remain filled, preventing the metal from being discharged, and operation must be suspended.

[0023] Therefore, in the method according to the present embodiment, at least a portion of the solidified slag layer formed on the bottom of the furnace after the previous operation and the solidified metal layer formed on top of the solidified metal layer is removed to expose the underlying solidified metal layer. After that, raw materials are charged into the electric furnace, and the exposed solidified slag layer is brought into contact with the molten metal produced by reducing and melting the raw materials.

[0024] As shown in the schematic diagram of Figure 1(A), at least a portion of the solidified slag layer is removed to expose the underlying solidified metal layer, which then comes into contact with the molten metal produced by melting the raw materials charged into the electric furnace. This allows the solidified metal layer to melt due to the relationship between the melting point of the solidified metal layer and the temperature of the molten metal (metal heating temperature). When the solidified metal layer melts, the entire metal layer, including the solidified metal layer, melts, allowing the metal to be efficiently discharged from the metal hole.

[0025] As shown in the schematic diagram of Figure 1(B), the metal level is lowered by gradually removing the metal, allowing the molten slag to come into contact with the solidified slag layer, melting it. Once the solidified slag layer has melted, the entire slag layer, including the solidified slag layer, melts, allowing the slag to be efficiently removed.

[0026] In this way, the solidified metal layer that has formed on the bottom of the furnace since the end of the previous operation and the solidified slag layer that has formed on top of it can be easily and efficiently eliminated by removing at least a portion of the solidified slag layer to expose the solidified metal layer and bringing the molten metal formed by melting the raw materials into contact with the exposed solidified metal layer.

[0027] It is also possible to restart operations after the end of operation by having workers remove and eliminate all of the solidified slag and solidified metal layers. However, because the solidified slag and solidified metal layers have high melting points, are hard, or have a high specific gravity, their removal requires a great deal of time and effort. Removing the hard, sticky, and highly dense solidified metal layer is particularly time-consuming and labor-intensive. Therefore, when the cycle between the start of operation and the next operation is short, such as once a week or more but once every six months or less, it is desirable to effectively remove the solidified slag and solidified metal layers with minimal removal work. In this regard, the method according to the present embodiment requires only the removal of at least a portion of the solidified slag layer, thereby enabling the solidified slag and solidified metal layers to be effectively removed with simple operations.

[0028] In the method according to the present embodiment, as described above, after removing a portion of the solidified slag layer, raw materials are charged into the electric furnace and subjected to reduction melting to produce molten metal and molten slag. As shown in the schematic diagram of Figure 2, after removing a portion of the solidified slag layer, the raw materials charged into the electric furnace are reduced and melted to produce molten metal. If the molten metal comes into contact with the exposed solidified metal layer at the same time as the molten slag comes into contact with the solidified slag layer, the necessary heat can be distributed throughout the electric furnace more quickly. This allows for faster metal and slag discharge.

[0029] Therefore, after removing part of the solidified slag layer, when charging raw materials into the electric furnace, it is preferable to adjust the amount of raw materials charged so that the molten slag produced by melting the raw materials can come into contact with the remaining solidified slag layer.

[0030] Specifically, for example, after removing a portion of the solidified slag layer, the upper limit of the raw material charge is set to the amount of molten metal produced that can fill the volume of the removed portion. The raw materials are charged at this rate, and operation is started so that the molten slag produced on top of the molten metal comes into contact with the solidified slag layer. By adjusting the raw material charge rate in this way, the molten slag can efficiently and effectively melt the solidified slag layer with the molten metal while simultaneously melting the solidified metal layer with the molten slag, allowing the solidified metal and slag layers to dissolve more quickly.

[0031] In the method according to this embodiment, the melting of the solidified slag layer and the solidified metal layer and their respective molten states can be determined based on the measurements of the thermometers installed in the hearth structure of the electric furnace.

[0032] When the solidified slag layer and solidified metal layer melt and become molten slag and molten metal, respectively, a layer of molten metal with a metal heating temperature of approximately 1350°C to 1450°C forms in the lower layer of the electric furnace. When this happens, a thermometer installed in the hearth structure (firebrick or insulating brick) measures the temperature, and the measured value increases significantly. Therefore, by continuously measuring the temperature with a thermometer installed in the hearth structure over time, a significant increase in the measured temperature can be determined to indicate that the solidified metal layer and solidified slag layer have melted and disappeared.

[0033] In the method according to the present embodiment, the raw material to be treated is not particularly limited, but may be, for example, a raw material containing waste lithium-ion batteries containing nickel and cobalt as oxides. In the electric furnace operation method, for example, the raw material containing waste lithium-ion batteries is charged into an electric furnace, heated, and reduced and melted, thereby producing a melt (molten material) consisting of a metal (alloy) composed of valuable metals such as nickel (Ni), cobalt (Co), and copper (Cu) contained in the raw material, and slag composed of impurity components.

[0034] Furthermore, the electric furnace in operation can be, for example, a three-phase AC electric furnace equipped with graphite electrodes. An example of a three-phase AC electric furnace is a submerged arc furnace. A submerged arc furnace has multiple electrodes submerged (submerged) in the heated material, and utilizes Joule heat (electrical resistance heat) in addition to heating by arc discharge. Specifically, in a submerged arc furnace, an arc discharge occurs between the electrode tip and the heated material, and the heated material (raw material) is heated by the arc. At the same time, current flows between the electrodes (between the electrode, heated material, and electrode) through the heated material, and the heated material (slag) also heats due to Joule heat. A submerged arc furnace allows continuous heating of the slag, and the metal located below the slag is heated by heat transfer from the slag. Thus, a submerged arc furnace has the advantage of being able to heat efficiently with little input power. [Example]

[0035] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to the following examples in any way.

[0036] [Example 1] The raw material containing the waste lithium-ion batteries was subjected to a pulverization process, and then a sieving process was carried out to separately prepare the powder-like processed raw material (powder-like material) that fell below the sieve and the foil-like processed raw material (foil-like material) that fell above the sieve without mixing them.

[0037] The raw materials were subjected to reduction melting treatment using a submerged arc furnace, which is a type of three-phase AC electric furnace, as the melting furnace.

[0038] Specifically, as a preheating operation, raw materials were first charged into the electric furnace at a rate of 30 kg per hour (24 kg of powdered raw materials and 6 kg of foil-like raw materials) for 3 hours (total raw material charge: 90 kg), and then subjected to reducing melting treatment. For the raw material charging, 6 kg of foil-like raw materials and 24 kg of powder-like raw materials were first charged out of the total 30 kg of raw materials, followed by 1 kg of carbon powder, and then electricity was turned on. After confirming the formation of a molten metal pool, the swivel-type furnace lid was closed, and then the foil-like raw materials and powder-like raw materials were charged through the charging port located on the side of the electrode. This process was repeated for 3 hours to perform the reducing melting treatment. The furnace body was then heated and the resulting reduced melt was held for 21 hours to separate the slag and metal.

[0039] After this, steady-state operation consisted of charging 30 kg of raw materials per hour for three hours, followed by a nine-hour hold period, followed by another three-hour charge of 30 kg of raw materials per hour, followed by a nine-hour hold period. This cycle was repeated six times, with the raw materials being charged and the reduction melting process completed. After 48 hours, the molten metal produced by this operation was tapped out of the metal hole at a rate of 60 kg per tap, once per day. The molten slag produced was also removed by tilting the electric furnace at a rate of 60 kg per tap, once every 16 hours.

[0040] After the final slag discharge, a total of 72 hours of operation was completed.

[0041] After the operation was completed and the electric furnace was cooled, it was confirmed that a solidified metal layer (40 mm thick) had formed at the bottom of the electric furnace, and a solidified slag layer (70 mm thick) had formed on top of that solidified metal layer. Before the next operation could begin, a portion of the solidified slag layer (approximately 50%) was removed using a breaker, and the preheating operation described above was then started.

[0042] During preheating, a total of 90 kg of raw materials were charged into the electric furnace, allowing the raw materials to melt and come into contact with the solidified slag layer. This raw material charging and reduction melting process allowed the raw materials to melt and produce molten metal, which came into contact with the solidified metal layer exposed by removing the solidified slag layer, melting the solidified metal layer. At the same time, the newly produced molten slag came into contact with the solidified slag layer, melting it.

[0043] After the preheating operation, the temperature reading on the thermometer installed at the bottom of the furnace was confirmed to have risen from 220°C to 250°C. A measuring rod was inserted from the top of the electric furnace to the bottom of the furnace, and it was confirmed that the solidified slag layer and solidified metal layer had melted. Based on this, the furnace was shifted to normal operation.

[0044] [Comparative Example 1] In Comparative Example 1, the same procedure as in Example 1 was carried out, except that the formed solidified slag layer was not removed and the preliminary operation was resumed.

[0045] As a result, after preheating, a measuring rod was inserted from the top of the electric furnace, but it only reached about 110 mm from the bottom of the furnace, and it was estimated that the solidified slag layer and solidified metal layer remained. As a result, it was not possible to discharge the newly obtained metal from the raw materials, and operations were stopped.

Claims

1. A method for operating an electric furnace equipped with electrodes, comprising: At least a portion of the solidified slag layer formed on the bottom of the furnace after the end of the previous operation and the solidified metal layer formed on the solidified slag layer is removed to expose the underlying solidified metal layer; Thereafter, raw materials are charged into an electric furnace, and the raw materials are melted to produce molten metal, which is brought into contact with the exposed solidified metal layer. How to operate an electric furnace.

2. The amount of raw materials charged into the electric furnace is adjusted so that the molten slag produced by melting the raw materials is in contact with the solidified slag layer remaining after removing a portion of the solidified slag layer. The method for operating an electric furnace according to claim 1.

3. It is determined that the solidified slag layer and the solidified metal layer have melted and become molten based on the measurement values ​​of thermometers installed in the hearth structure of the electric furnace.

3. The method for operating an electric furnace according to claim 1 or 2.

4. The raw material includes waste lithium-ion batteries.

4. The method for operating an electric furnace according to claim 1.

5. The electric furnace is a three-phase AC electric furnace.

5. The method for operating an electric furnace according to claim 1.

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