JP7760942B2 - Electric furnace operation method - Google Patents

Description

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.

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

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, there is a possibility that metal will be mixed into the slag if the slag layer reaches the metal layer due to tilting. For these reasons, metal and slag that could not be discharged remain inside the electric furnace.

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 located below 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 operation impossible.

For example, Patent Document 1 discloses an electric melting furnace and melting method for solidifying radioactive waste that utilizes a magnetic field. According to Fleming's left-hand rule, an electromagnetic force acts partially on the molten glass where the direct heating current and the magnetic field intersect. As a result, a force is generated that forcibly promotes convection in the molten glass inside the melting furnace. 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 preventing the settling and accumulation of deposits on the furnace bottom.

However, no technology has been disclosed for removing the solidified metal layer adhering to the furnace bottom or the solidified slag layer adhering thereon in electric furnaces equipped with electrodes, such as three-phase AC type.

Special Publication No. 07-104436

The present invention was proposed in light of these circumstances, and aims to provide a method for effectively removing the solidified metal layer remaining on and adhering to the bottom of an electric furnace equipped with electrodes, and the solidified slag layer adhering thereon, thereby enabling efficient operation of the electric furnace.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the solidified metal and slag layers can be effectively eliminated with a simple operation by removing a portion of the solidified slag layer that has formed to expose the solidified metal layer underneath, then charging raw materials into the furnace and melting the raw materials to produce molten metal, bringing the exposed solidified metal layer into contact with the molten metal. This led to the completion of the present invention.

(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 a solidified slag layer formed on the bottom of the furnace after the previous operation and a solidified metal layer formed on top of the solidified metal layer to expose the underlying solidified metal layer, and then charging raw materials into the electric furnace, melting the raw materials to produce molten metal, which brings the exposed solidified metal layer into contact with the solidified metal layer.

(2) The second aspect of the present invention is a method for operating an electric furnace according to the first aspect, 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 a portion of the solidified slag layer.

(3) A 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 entered a molten state based on the measurement values of thermometers installed in the hearth structure of the electric furnace.

(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.

(5) The 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.

The present invention provides a method for effectively eliminating the solidified metal layer that remains and adheres to the bottom of an electric furnace, and the solidified slag layer that adheres to it, during electric furnace operation, enabling efficient electric furnace operation.

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. 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.

A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described below. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the gist of the present invention.

The method according to this embodiment is a method for operating an electric furnace equipped with electrodes, such as a three-phase AC electric furnace, to heat and reduce and melt raw materials containing oxidized metals.

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

An electric furnace is operated by repeatedly charging raw materials and discharging the metal and slag produced by reducing and melting the raw materials. The amount of metal and slag discharged can be predicted from the amount of raw materials charged. In other words, the required amount is discharged and recovered at the required time, depending on the level (height) of the metal and slag produced.

After the necessary number of cycles of metal and slag discharge, starting with the charging of raw materials and going through the reduction melting process, the operation is terminated. When recovering the 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, and in the case of slag, if it is discharged through the slag hole, the bottom of the slag hole is the limit. 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.

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, which involves charging raw materials into an electric furnace, is carried out with 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 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. As a result, the metal holes remain filled, the metal cannot be discharged, and operations must be suspended.

Therefore, in the method according to this embodiment, at least a portion of the solidified slag layer formed on top of the solidified metal layer that has adhered to the furnace bottom since the end of the previous operation is removed to expose the underlying solidified metal layer. Then, raw materials are charged into the electric furnace, and the exposed solidified slag layer is brought into contact with the molten metal produced by subjecting the raw materials to a reducing melting process.

As shown in the schematic diagram of Figure 1(A), by removing at least a portion of the solidified slag layer to expose the underlying solidified metal layer, the exposed solidified metal layer is brought into contact with the molten metal produced by melting the raw materials subsequently charged into the electric furnace. This makes it possible to melt the solidified metal layer due to the relationship between the melting point of the solidified metal layer and the temperature of the molten metal (metal heating temperature). Then, when the solidified metal layer melts, the entire metal layer, including the solidified metal layer, melts, allowing the metal to be effectively discharged from the metal hole.

As shown in the schematic diagram of Figure 1(B), the metal level is then lowered by the gradual discharge of metal, allowing the molten slag to come into contact with the solidified slag layer, allowing the solidified slag layer to melt. Once the solidified slag layer melts, the entire slag layer, including the solidified slag layer, melts, allowing the slag to be effectively discharged.

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

It is also possible to restart operations after the end of operations by having workers remove and eliminate all of the solidified slag and solidified metal layers that have formed. However, because the solidified slag and solidified metal layers have high melting points and are hard or have a high specific gravity, their removal requires a great deal of time and effort. Removing the hard, sticky, and more dense solidified metal layer in particular requires a great deal of time and effort. Therefore, if the cycle between the start of operations is short, such as once a week or more and once every six months or less, it is desirable to effectively eliminate the solidified slag and solidified metal layers with the minimum necessary removal work. In this regard, the method of this embodiment only requires the removal of at least a portion of the solidified slag layer, allowing for simple and effective removal of the solidified slag and solidified metal layers.

In the method according to this embodiment, as described above, after removing a portion of the solidified slag layer, raw materials are charged into the electric furnace and reduced and melted 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, which comes into contact with the exposed solidified metal layer. At the same time, if the molten slag comes into contact with the solidified slag layer, the necessary heat can be distributed more quickly throughout the electric furnace. This allows for faster metal and slag discharge.

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.

Specifically, for example, after removing a portion of the solidified slag layer, the upper limit of the raw material charge amount is set to the amount of molten metal produced that can fill the volume of the removed portion. Raw materials are charged at this amount, 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 preferably adjusting the raw material charge amount in this way, the molten slag can efficiently and effectively dissolve the solidified slag layer in parallel with the molten metal dissolving the solidified metal layer, allowing the solidified metal and slag layers to dissolve more quickly.

In the method according to this embodiment, whether the solidified slag layer and solidified metal layer have melted and become molten can be determined based on the measurements of thermometers installed in the hearth structure of the electric furnace.

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, made of refractory bricks or insulating bricks, measures the temperature, and the measured value increases significantly. Therefore, by continuously measuring the temperature of the thermometer installed in the hearth structure over time, it can be determined that the solidified metal layer and solidified slag layer have melted and disappeared when the measured temperature increases significantly.

In the method according to this embodiment, the raw materials to be processed are not particularly limited, but examples include raw materials containing waste lithium-ion batteries containing nickel and cobalt as oxides. In the electric furnace operation method, raw materials containing waste lithium-ion batteries, for example, are loaded into an electric furnace and heated for reduction melting, thereby producing a melt (molten material) consisting of a metal (alloy) composed of valuable metals contained in the raw materials, such as nickel (Ni), cobalt (Co), and copper (Cu), and slag composed of impurity components.

The electric furnace used in the process is, 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 via 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 for continuous heating of the slag, and the metal located below the slag is heated by heat transfer from the slag. As such, a submerged arc furnace has the advantage of efficient heating with little input power.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

[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 powdery treated raw material (powder material) that fell under the sieve and the foil-like treated raw material (foil material) that fell over the sieve without mixing them.

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

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 three hours (total raw material charge: 90 kg), and then subjected to reducing melting treatment. For 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 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 three 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.

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 was repeated six times in total, with the raw materials being charged and the reduction melting process carried out. After 48 hours had passed, 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 discharged by tilting the electric furnace at a rate of 60 kg per tap, once every 16 hours.

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

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, with a solidified slag layer (70 mm thick) 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 initiated.

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, creating new molten slag. Through this raw material charging and reduction melting process, the molten metal created by the raw materials melting was brought 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 created molten slag was brought into contact with the solidified slag layer, melting it.

Furthermore, after the above-mentioned 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 also 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 transitioned to regular operation.

[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.

As a result, after preheating, a measuring rod was inserted from the top of the electric furnace, but it only reached approximately 110 mm from the bottom of the furnace, and it was estimated that the solidified slag and solidified metal layers remained. As a result, it was not possible to remove the newly obtained metal from the raw materials, and operations were halted.

Claims (5)

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. 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. 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. The raw material includes waste lithium-ion batteries.
4. The method for operating an electric furnace according to claim 1. The electric furnace is a three-phase AC electric furnace.
5. The method for operating an electric furnace according to claim 1.