METAL REFINING METHOD AND METAL REFINING APPARATUS

Abstract

To provide a method capable of efficiently removing or recovering Zn from an Al-based molten metal which uses a scrap or the like as a raw material.SOLUTION: This metal refining method comprises: a local heating step of heating an aluminum-based molten metal (m) in a first region (s1) on the molten metal surface of the aluminum-based molten metal; and a local pressure reduction step of setting pressure in a second region (s2) different from the first region on the molten metal to be lower than that in the first region. Thus, specific elements are evaporated from the second region, and the aluminum-based molten metal is refined. The specific elements are, for example, one or more types of Zn, Mg, and Pb larger in saturated evaporation pressure than Al. This refining method is effective not only for removing the specific elements from the aluminum-based molten metal but also for recovering the specific elements which become resources from the aluminum-based molten metal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for purifying an aluminum-based molten metal by evaporating a specific element. [Background technology]

[0002] With the rise of environmental awareness, lightweight aluminum-based components are being used in a variety of fields. Reusing scrap aluminum, rather than using newly smelted (or even refined) aluminum, can promote the use of aluminum-based components while achieving significant energy savings and reducing the environmental impact.

[0003] However, various elements other than Al may be present in the raw material molten metal (also called "Al-based molten metal") obtained by melting scrap. To prepare the Al-based molten metal into a molten metal with the desired composition, it is necessary to remove or reduce unnecessary or excess elements. One such refining method is vacuum distillation (reduced pressure distillation, vacuum degassing, vacuum processing). Vacuum distillation of Al-based molten metal is a method for preferentially evaporating and separating (desorbing) elements with higher vapor pressures than Al (e.g., Zn, Mg, Pb, H, etc.) from the Al-based molten metal. Vacuum distillation is usually performed on Al-based molten metal below its boiling point. The following literature provides relevant information: [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 6-145832 [Patent Document 2] Patent Publication No. 7-41879 [Patent Document 3] JP 9-316558 [Patent Document 4] Patent Publication No. 11-256251 [Patent Document 5] Patent Publication No. 2001-294949 [Patent Document 6] Patent Publication No. 2002-339024 [Patent Document 7] WO2011 / 96170 [Non-patent literature]

[0005] [Non-Patent Document 1] Otaki, Satsukime, Mori, Kudo, and Tanaka: Furukawa Electric Review, 104 (1999), 25 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Documents 1 to 6 and Non-Patent Document 1 all propose removing impurities such as Zn from a raw material molten metal (Al-based molten metal) that has been uniformly heated as a whole by evaporating it into a vacuum atmosphere. For example, in Patent Document 1, evaporated contaminants are collected by suction into a vacuum-environment processing chamber installed partially above the Al-based molten metal. Patent Document 1 is based on the premise that an inert gas is blown into the Al-based molten metal below the processing chamber (i.e., bubbling). Note that the heater installed above the processing chamber does not actively heat the Al-based molten metal from the surface side.

[0007] In Patent Document 7, the area near the surface of molten aluminum in a furnace under a vacuum atmosphere is heated by arc discharge to evaporate and remove impurities such as Zn. With the refining method in Patent Document 7, the evaporated impurities adhere to the inner walls of the furnace or return to the molten metal, making it difficult to efficiently remove or recover the impurities.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for purifying an aluminum-based molten metal by efficiently extracting a specific element using a technique different from conventional techniques. [Means for solving the problem]

[0009] As a result of intensive research into solving this problem, the inventors have succeeded in locally heating the surface of an aluminum-based molten alloy (first region) and selectively evaporating a specific element from a vacuum region (second region) located near the surface of the molten alloy, separate from the heated region. By expanding on this result, the present invention, which will be described below, has been completed.

[0010] 《Metal refining method》 (1) The present invention is a metal refining method comprising a local heating step of heating an aluminum-based molten metal in a first region on the surface of the aluminum-based molten metal, and a local low pressure step of lowering the pressure in a second region on the molten metal surface, different from the first region, compared to the first region, and evaporating a specific element from the second region to refine the aluminum-based molten metal.

[0011] (2) According to the metal refining method (also simply referred to as the "refining method") of the present invention, specific elements contained in an aluminum-based molten metal (also referred to as an "Al-based molten metal" or simply as "molten metal") can be efficiently evaporated (extracted) from a specific region and removed or recovered (distilled). The reason for this is thought to be as follows.

[0012] The evaporation rate (removal efficiency, recovery efficiency) of a specific element is greatly affected by the temperature of the molten metal near the molten metal surface (evaporation interface) and the pressure (degree of vacuum) of the atmosphere above it. In other words, the higher the molten metal temperature and degree of vacuum, the more the evaporation of the specific element is promoted, and the greater the evaporation rate.

[0013] In the local heating process of the present invention, a first region, which is different from a second region where a specific element is evaporated, is first locally heated. Local heating makes it possible to efficiently heat the vicinity of the surface of the Al-based molten metal to evaporate the specific element while avoiding large amounts of energy consumption, excessive heating time, and wear (shortening of the lifespan) of the furnace body, such as the heating furnace (crucible). Furthermore, local heating allows greater freedom in selecting and arranging the equipment, and the position and area on the molten metal surface to be heated can be easily adjusted. For example, the second region where a specific element is evaporated and the first region to be heated can be placed close to each other (even adjacent to each other), or conversely, they can be separated by an appropriate distance, taking into account factors such as convection within the Al-based molten metal.

[0014] In the local low-pressure process, the second region where the specific element is evaporated is locally subjected to low pressure. This reduces the installation and maintenance costs of large exhaust equipment, while selectively creating a high vacuum above the surface of the Al-based molten metal, allowing the specific element to be evaporated efficiently.

[0015] The synergistic action of the local heating process and the local low-pressure process makes it possible to efficiently vaporize specific elements contained in the Al-based molten metal. This makes it possible to refine the Al-based molten metal from which at least a portion of the specific elements have been removed, or to recover the specific elements, which are useful resources, from the Al-based molten metal (raw material). The specific elements to be recovered can be in any state (gas (vapor), liquid, or solid).

[0016] Incidentally, when evaporating a specific element according to the present invention, mechanical stirring of the Al-based molten metal is not essential. Localized heating near the molten metal surface (first region) generates convection in at least the upper layer of the Al-based molten metal, and the molten metal temperature (also simply referred to as "molten metal temperature") can be increased and the specific element can be continuously replenished in the second region connected to the first region. In other words, even without active stirring, excessively uneven temperature and concentration distributions are unlikely to occur between the first and second regions.

[0017] The first and second regions only need to be able to communicate with each other through the molten metal. In other words, the first and second regions may be located close to each other (or even adjacent to each other) or far apart, as long as the high-temperature molten metal heated in the first region flows into the second region and the specific element is easily evaporated from the molten metal surface in the second region.

[0018] Furthermore, the "evaporation" of a specific element in the present invention means that the specific element in a gaseous state is released from the surface of the molten metal (the surface of the molten metal). The vaporization of the specific element itself may occur on the surface of the molten metal or inside the molten metal. In other words, the case where the specific element boils in the molten metal and is released from the surface of the molten metal can also be considered to be included in the "evaporation" of the present invention.

[0019] 《Metal refining equipment》 The present invention can also be understood as a metal refining apparatus. For example, the present invention may be a metal refining apparatus that includes a local heating means for heating the aluminum-based molten metal in a first region above the surface of the aluminum-based molten metal, and a local low-pressure means for lowering the pressure in a second region above the molten metal surface, different from the first region, relative to the first region, and that can evaporate a specific element from the second region to refine the aluminum-based molten metal. The metal refining apparatus of the present invention may also include a recovery means for recovering the specific element evaporated from the second region. Furthermore, the metal refining apparatus of the present invention may also include a differential pressure control means for controlling the differential pressure (ΔP = P1 - P2) between the pressure on the first region side (P1) and the pressure on the second region side (P2) within a predetermined range.

[0020] <Recycling method (device) / Recovery method (device)> (1) The present invention may be understood as, for example, a method (recycling method) for obtaining recycled Al alloys from scrap raw materials by removing specific elements (e.g., Zn, Mg, etc.). The recycled Al alloys from which specific elements have been removed may be used as solidified materials (ingots, etc.) or as molten metal (including semi-molten state). Furthermore, recycling of Al-based scrap is not limited to cascade recycling, and may also be used as upgrade recycling into wrought materials, etc.

[0021] (2) The present invention may further be understood as a method or apparatus (method or apparatus for recovering a specific element) for recovering a specific element from an Al-based molten metal (raw material) obtained by melting a raw material (e.g., scrap), independently of the refining or regeneration of the Al-based molten metal.

[0022] "others" (1) In this specification, "step" and "means" can be interpreted interchangeably. For example, "step" can be interpreted as "means" to be a component of a "product" (e.g., a metal refining apparatus), and "means" can be interpreted as "step" to be a component of a "method" (e.g., a metal refining method).

[0023] In this specification, the order of the steps (chronological factors) is not important. For example, the local heating step and the local low pressure step may be performed consecutively in parallel, alternately, or discretely.

[0024] (2) The first and second regions referred to in this specification are convenient divisions on the surface of the Al-based molten metal. The first region may include the lower part of the molten metal surface (upper layer of the molten metal). The second region may include the upper part of the molten metal surface (space above the molten metal). The range of the molten metal that is locally heated (upper layer region) may be, for example, about one-third of the depth of the molten metal.

[0025] The pressure on the first zone side (P1) and the pressure on the second zone side (P2) are measured, for example, by instruments such as gauges and sensors installed in the processing chamber (tank) or pipeline (piping) above the molten metal surface. Since it is not easy to stably measure the pressure near the molten metal surface, which is the boundary between the liquid and gas phases, it is best to install the instruments in a location where stable pressure measurement is possible. During steady-state operation, the average of the measured values ​​can be used as the pressure of each zone. Unless otherwise specified, "pressure" refers to the total pressure of the atmosphere in a specific space (also simply called "atmospheric pressure"). Also, unless otherwise specified, "pressure" refers to absolute pressure. The difference obtained by subtracting the smaller pressure from the larger pressure (reference pressure) is sometimes referred to as the "vacuum level."

[0026] (3) The aluminum-based molten metal referred to in this specification includes a solid-liquid coexistence state (semi-molten state). The aluminum-based molten metal can have any specific composition as long as it is primarily composed of Al (the Al content of the entire molten metal is greater than 50 atomic %, 70 atomic % or more, or even 85 atomic % or more). The concentration of a specific element in the raw molten metal (the Al-based molten metal before refinement) is not important, but is usually about 10 mass % or less, or even 5 mass % or less, of the entire molten metal. Unless otherwise specified, the concentration and composition referred to in this specification are expressed as a mass percentage (mass % or simply "%") of the entire object (molten metal, etc.).

[0027] (4) Unless otherwise specified, "x to y" in this specification includes a lower limit of x and an upper limit of y. Any numerical value included in the various numerical values ​​or ranges described in this specification may be used as a new lower limit or upper limit to create a new range such as "a to b." [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram showing a configuration example of a metal refining device. [Figure 2] 1 is a photograph showing recovery filters after purification treatment for Sample 1 and Sample C1. [Figure 3] 10 is a graph illustrating the relationship between the pressure in the treatment tank and the discharge voltage. [Figure 4] 1 is a graph illustrating the relationship between the vapor pressure of Zn contained in an Al-based molten metal and the molten metal temperature. [Figure 5] FIG. 1 is a schematic diagram showing a configuration example of a metal refining device that performs differential pressure management. [Figure 6] 1 is a graph illustrating the relationship between differential pressure and molten metal head. [Figure 7] 1 is a process diagram (example) of differential pressure control. [Figure 8] 10 is a graph illustrating an example of pressure changes related to differential pressure management. DETAILED DESCRIPTION OF THE INVENTION

[0029] One or more components arbitrarily selected from the present specification may be added to the above-described components of the present invention. The contents described in this specification may apply not only to the metal refining method but also to the apparatus. The components related to the method may also be components related to the object (apparatus, (recycled) Al alloy (molten metal), etc.).

[0030] 《Local heating》 The local heating is preferably performed using a heat source with a high energy density, which allows the molten metal in the first zone near the surface to be rapidly heated, and the molten metal in the second zone connected to the first zone to be efficiently heated to a high temperature.

[0031] (1)Heat source The type and output of the heat source (device) may be selected and adjusted depending on the molten metal tank (heating furnace, crucible, flow path, etc.) that contains the molten metal and the shape of the first region. Examples of heat sources include high-energy beam irradiation (laser irradiation, electron beam irradiation, etc.) and electrical discharge (arc discharge, etc.). Whichever heat source is used, it is possible to simplify the heating device, reduce its size, save energy, or reduce the load on the molten metal tank.

[0032] High-energy beam irradiation allows for flexible and highly accurate adjustment of the heating position (first region). Electrical discharge allows for rapid heating of the vicinity of the molten metal surface by supplying high-temperature plasma. Electrical discharge may be performed by placing a pair of electrodes on the molten metal surface, or by placing one electrode on the molten metal surface (above the first region) and the other electrode (counter electrode) on the Al-based molten metal (the molten metal surface in the first region). In the latter case, electrical discharge occurs between the Al-based molten metal (the other electrode) and an electrode (the one electrode) placed above the molten metal surface, efficiently and rapidly heating the molten metal in the first region. For example, by using arc discharge, which is used in welding, the molten metal near the first region can be rapidly heated, while reducing the burden on equipment.

[0033] (2) Energy density The amount of energy (energy density) per unit area applied to the surface of aluminum-based molten metal is 10 2 W / cm 2 That's it, 10 3 W / cm 2 More than 10 4 W / cm 2 In order to avoid excessive equipment size and bumping near the molten metal surface directly under the arc, the energy density should be 10 5 W / cm 2 The following may also be used.

[0034] (3) Arc discharge Heating by arc discharge (arc heating) is caused by a high-temperature arc column generated between electrodes. The temperature of the arc column varies depending on the location, but is at least higher than that of a flame (<3000°C), reaching, for example, 4000°C or even 5000°C (Reference: Tanaka, Journal of the Japan Welding Society, 77 (2008), 50). Heating near the molten metal surface by the arc column can be direct or indirect arc heating. Arc heating is thought to be primarily due to radiation and the transfer of kinetic energy of particles (electrons, plasma ions, etc.). Arc heating as referred to in this specification includes arc plasma heating, in which the arc discharge is confined by a nozzle or airflow, etc., to increase directionality and heating temperature. The power source for the arc discharge can be either direct current or alternating current.

[0035] Arc discharge occurs under atmospheric pressure to sub-atmospheric pressure (approximately 10 5 ~10 4 Pa), or under reduced (low) vacuum atmosphere (approximately 10 4 ~10 2 When arc discharge is performed in a vacuum atmosphere, it is possible to stabilize the arc discharge and to stably increase the amount of heat input to the molten metal.

[0036] As an example, using a metal refining apparatus D shown in Fig. 1 (described later), the relationship between the pressure (ambient pressure in the processing chamber v) and the discharge voltage obtained by arc discharge (constant current: 100 A) in the holding tank 1 is shown in Fig. 3. From Fig. 3, it is clear that when arc discharge is performed in a predetermined vacuum atmosphere, the discharge voltage increases stably, and the desired amount of heat input can be applied to the molten metal m.

[0037] Based on this result, if the first region is heated by arc discharge, the pressure (P1) should be set to, for example, 500 to 2000 Pa, 650 to 1750 Pa, or even 800 to 1500 Pa.

[0038] In addition, by increasing the degree of vacuum on the first zone side (i.e., by decreasing P1), oxidation of the Al-based molten metal heated to a high temperature can be suppressed. However, if the degree of vacuum becomes too large, the amount of evaporation from the first zone side also increases, and the evaporated material accumulates on the surroundings (furnace wall, tank wall, etc.), which can reduce maintainability.

[0039] Localized low pressure The local low pressure makes the second zone lower in pressure than the surrounding area (at least the first zone), which allows specific elements to be preferentially evaporated from near the molten metal surface in the second zone and improves the recovery of specific elements.

[0040] The pressure (P2) on the second zone side can be adjusted as appropriate, for example, to 0.1 to 1000 Pa, 1 to 100 Pa, or even 5 to 50 Pa. The greater the degree of vacuum above the molten metal surface in the second zone (the smaller P2), the lower the partial pressure of the specific element above the molten metal surface, and the greater the evaporation and recovery amounts of the specific element.

[0041] However, if the pressure (P2) on the second zone side is too low compared to the pressure (P1) on the first zone side, i.e., if the differential pressure between the two (ΔP = P1 - P2) becomes too high, the molten metal column above the second zone (the molten metal column inside the tank body) will also become high, leading to an increase in the size of the entire equipment or damage. Therefore, the differential pressure (ΔP) should be, for example, 100 to 5000 Pa, 200 to 1000 Pa, or even 300 to 800 Pa. The pressure (P1) on the first zone side may be, for example, 100 to 10,000 Pa, 200 to 5000 Pa, or 400 to 2500 Pa, depending on the local heat source.

[0042] The pressure in the second zone can be reduced by, for example, a cylindrical or tubular tank body (or chamber) that surrounds the molten metal surface in the second zone and the space above it, and an exhaust means (such as a vacuum pump) that exhausts the inside of the tank body.

[0043] In order to stably refine the Al-based molten metal and recover specific elements, it is advisable to control (maintain) within a predetermined range the difference in height between the molten metal surface in the second zone and the molten metal surface in the first zone (referred to as the "molten metal column height" or "molten metal head"), or the pressure difference between the second zone and the first zone (ΔP = P1 - P2). Various specific methods for this are conceivable. Here, we will explain an example in which the molten metal column height is controlled (adjusted, controlled, etc.) to stay within a predetermined range (differential pressure control process, differential pressure control means).

[0044] The differential pressure is controlled by decreasing P1 and / or increasing P2. For example, differential pressure control can be easily achieved by connecting the second region to a higher pressure side (the first region, the atmosphere, etc.) at a predetermined time. Differential pressure control may be performed continuously or continuously, or only when the differential pressure, molten metal column height, etc., deviates from a predetermined range. Understanding (measuring, detecting, etc.) the differential pressure itself is not necessarily required for differential pressure control. For example, it is sufficient to simply interpose a pressure valve (switching valve / differential pressure control means) between the pressure circuits on the first and second regions, which is driven by the differential pressure and operates when the differential pressure deviates from the predetermined range.

[0045] Of course, differential pressure control may be performed while monitoring the differential pressure. For example, precise differential pressure control may be performed using a differential pressure control means including a differential pressure gauge (including a pressure gauge, vacuum gauge, compound gauge, etc.) that can directly or indirectly monitor the differential pressure, a control valve that can connect or block the second region (above the molten metal surface) to the high-pressure region, and a controller that operates the control valve based on the pressure (signal) monitored by the differential pressure gauge. The differential pressure may be a directly measured ΔP, the difference between measured values ​​P1 and P2, or a value estimated from either P1 or P2. For example, if either P1 or P2 is stable, the pressure of the other may be used as the differential pressure.

[0046] 《Specific elements》 Specific elements with a higher vapor pressure (saturated vapor partial pressure, described later) than Al evaporate from the molten metal surface in the second region into the vacuum atmosphere above (also simply referred to as "above") and are distilled.

[0047] (1) Vapor pressure Saturated vapor pressure (equilibrium vapor pressure) depends on temperature, and increases as the temperature increases. The saturated vapor pressure (P0) of a pure metal (liquid phase) is expressed as a function of temperature (T) by the following equation (1) (Source: Nagafune: Tsuyama National College of Technology Bulletin 13 (1975) 63). logP0=aT -1 +blogT+cT+D (1) where log is common logarithm, T is absolute temperature (K), and a, b, c, and D are constants.

[0048] In addition, the saturated vapor pressure (Pt) of the entire molten metal at temperature T, which is made up of multiple constituent elements, is calculated by dividing the saturated vapor pressure of each constituent element (P i : called saturated vapor partial pressure.) is expressed by the following equation (2) (Source: Nagafune: Tsuyama National College of Technology Bulletin 13 (1975) 63). Pt =ΣP i =Σ(α i P i0 ) (2) where α i : Activity of constituent elements in the molten metal, P i0 : The saturated vapor pressure of the constituent elements (liquid phase). Activity (α i ) depends on the concentration of the constituent elements in the molten metal and the molten metal temperature.

[0049] The saturated vapor partial pressure of the constituent elements contained in the molten metal (P i ) is greater than the partial pressure of the constituent element in the air above, the constituent element can evaporate (emit, dissipate) from the surface of the molten metal into the air. In the case of an Al-based molten metal, theoretically, each constituent element, including Al, can evaporate onto the surface of the molten metal depending on its respective saturated vapor partial pressure and the partial pressure above the molten metal surface.

[0050] However, the saturated vapor pressure of Al is negligibly smaller than the saturated vapor pressure of other elements contained in Al-based molten metal. This is also true when considering the saturated vapor partial pressure taking into account activity. For example, the saturated vapor pressure at 700°C is Al: 2×10 -5 Pa, Zn: 8.4 × 10 3 Pa, Mg: 7.5 × 10 2 Pa, Pb: 6.7 × 10 -1Pa. Therefore, even when considering the activity (concentration), the vapor pressure (saturated vapor partial pressure) of other elements relative to Al is 10 3 ~10 7 This is about twice as large. In other words, there is a large difference in vapor pressure between Al and the specific element in the Al-based molten metal. Therefore, in the first and second regions, Al does not substantially evaporate, and the specific element with a higher vapor pressure mainly evaporates. In this specification, unless otherwise specified, the saturated vapor partial pressure described above will simply be referred to as "vapor pressure."

[0051] (2)Specific elements The specific element having a large difference in vapor pressure (saturated vapor partial pressure) from Al is, for example, one or more of Zn, Mg, and Pb. A typical example is Zn, which has a particularly high vapor pressure.

[0052] As an example, the vapor curve (relationship between vapor pressure and molten metal temperature) of Zn contained in molten Al alloy was calculated based on the above-mentioned formulas (1) and (2), and the results are shown in Figure 4. The activity (α i ) was calculated from the Zn concentration and temperature using thermodynamic calculation software (Thermo-Calc, manufactured by AB). The Zn concentration (mass%) was set to 0.2%, 0.5%, 0.7%, or 1.2%, as shown in Figure 4. As can be seen from Figure 4, the vapor pressure of Zn increased with increasing molten metal temperature. Furthermore, even when the molten metal temperature was the same, the higher the Zn concentration, the higher the vapor pressure.

[0053] The vapor pressure of such specific elements increases with an increase in the temperature of the molten metal, and by further reducing the pressure above the molten metal, they become more likely to evaporate from the molten metal, thereby increasing the removal or recovery efficiency (Reference: Otaki, Satsukime, Mori, Kudo, Tanaka: Furukawa Electric Review, 104 (1999), 25).

[0054] "purification" The refining process of evaporating specific elements from the Al-based molten metal may be performed as a batch process (lump-time process) or a continuous process (continuous process). Batch processing allows for greater flexibility in setting the pressure in the processing chamber (e.g., the space on the second region side), the temperature near the molten metal surface, the processing time, etc. Continuous processing allows for efficient processing of large amounts of Al-based molten metal and can be smoothly integrated with other impurity removal processes and subsequent casting processes.

[0055] "collect" The specific element may be captured and recovered and reused as a resource. Therefore, the present invention may comprise a recovery step (means) for recovering the specific element evaporated from the second region.

[0056] The specific element is recovered, for example, by condensing and solidifying its vapor using a filter or a cooler (such as a cooling coil). Although some of the specific element may evaporate from other regions (such as the first region) than the second region, most of the specific element evaporates in the low-pressure (or even high vacuum) second region. Therefore, by condensing and solidifying the vapor of the specific element generated from the second region, the specific element can be recovered efficiently. Note that the specific element may not be recovered in a solid state, but may also be recovered in a gaseous (vapor) state, a liquid state, or a solid-liquid coexistence state.

[0057] 《Local heating means》 An arc discharge, which is a good example of a localized heat source, will be described below.

[0058] (1) Electrode One of the electrodes may be, for example, a torch electrode having a tip facing the surface of the Al-based molten metal. When a current (voltage) is applied between the torch electrode and the Al-based molten metal, an arc discharge can occur between the tip of the torch electrode and the surface of the Al-based molten metal.

[0059] Electricity is applied to the Al-based molten metal, for example, via a molten metal tank made of a conductive material such as metal, or via a counter electrode at least partially immersed in the Al-based molten metal. The counter electrode immersed in the molten metal may be disposed above the Al-based molten metal, similar to a torch electrode, for example. This concentrates the components and functions required for local heating above the Al-based molten metal, thereby making the device more compact, improving maintainability, and improving the ease of supplying and replacing the Al-based molten metal (molten metal tank).

[0060] The outer circumferential surface of the torch electrode is preferably surrounded by an insulator. This prevents free discharge and stabilizes the arc discharge that occurs between the tip of the torch electrode and the surface (local) of the Al-based molten metal. Free discharge is a discharge that can occur between the torch electrode (including the outer circumferential surface) and the counter electrode surface, the wall of the molten metal, the surface of the Al-based molten metal, etc. The insulator may surround the outer circumferential surface of the torch electrode within a range that can suppress free discharge. It is usually preferable for the insulator to extend to the vicinity of the tip of the torch electrode.

[0061] The insulator may be cylindrical or tubular, into which the torch electrode is inserted, or may be a film covering the outer surface of the torch electrode. If the insulator constitutes at least a part of the flow path for the gas supplied to the surface of the Al-based molten alloy, the torch side can be made more compact and simplified.

[0062] The arc discharge may be either a hot cathode arc or a cold cathode arc. In either case, the torch electrode should be a cathode (negative electrode, cathode). At this time, the wall surface of the molten metal bath or treatment chamber and the Al-based molten metal (counter electrode) should be at approximately the same potential. The electrode (torch electrode, counter electrode) exposed to high temperatures should be made of a high-boiling point material such as carbon (graphite) or tungsten (W). The shape of the electrode is not important, but it is usually cylindrical or rod-shaped.

[0063] (2) Airflow The arc discharge may be performed while applying an airflow onto the surface of the aluminum-based molten metal (particularly the first region). The airflow may be, for example, a gas flow consisting of an inert gas such as Ar, He, or N2, or a mixture thereof, ejected from a nozzle or the like, or a gas flow or plasma flow ejected from the torch electrode. The airflow may be applied continuously or intermittently. [Example]

[0064] [First Example] The present invention will be described in more detail based on a specific example in which an Al-based molten alloy containing Zn was subjected to a local heating process and a local low pressure process, and Zn was fractionally distilled and recovered by vacuum distillation.

[0065] "Device" An outline of a metal refining apparatus D (simply referred to as "apparatus D") is shown in FIG. 1. For convenience of explanation, the directions of the arrows shown in the figure are referred to as up-down or left-right directions. This also applies to FIG. 5 described later.

[0066] The device D includes a holding tank 1 for heating and holding an Al-based molten metal m (simply referred to as "molten metal m"), a local heating section 4 for heating the vicinity of the surface s1 of the molten metal m (first region), and a local low pressure section 5 for creating a high vacuum in the vicinity of the surface s2 of the molten metal m (second region).

[0067] The holding tank 1 (heating furnace) includes a housing 11, a crucible 12 (molten metal tank) for accommodating the molten metal m, a heater 13 for melting raw metal (aluminum-based scrap, etc.) in the crucible 12 and adjusting the temperature of the molten metal m, and a lid 15 for closing the top of the housing 11 to form a sealed processing chamber v (upper space) within the holding tank 1. The crucible 12 is made of alumina, and the heater 13 is of an electric resistance type.

[0068] The processing chamber v is depressurized by an exhaust unit 31. The exhaust unit 31 includes an oil rotary vacuum pump 311 (first exhaust means), an adjustment valve 312 that adjusts the pressure (degree of vacuum) inside the processing chamber v, an exhaust filter 313 that traps steam, fine particles, etc. that are sucked from the processing chamber v, and an exhaust pipe 314 that communicates with the processing chamber v. The adjustment valve 312 operates based on the measurement value (P1) of the pressure gauge 10.

[0069] The local heating unit 4 (local heating means) includes a power supply 40 that generates an arc discharge a, a torch 41, and a counter electrode 42. The torch 41 includes an electrode 411 (torch electrode) and a gas pipe 412 that surrounds the electrode 411. An inert gas (Ar) is supplied to the gas pipe 412 from a gas source (such as a cylinder) located upstream. The gas pipe 412 is made of an insulating material such as ceramics.

[0070] A TIG (Tungsten Inert Gas) welding power supply was used as the power source 40. Both the electrode 411 and the counter electrode 42 were rod-shaped electrodes, and the tip of the counter electrode 42 was immersed above the molten metal m. When electricity was passed between the electrode 411 and the counter electrode 42 by the power source 40, an arc discharge a was generated between the molten metal surface s1 and the tip of the electrode 411 (near the tip surface) located above the molten metal surface s1. The arc discharge a continuously converted at least a portion of the gas supplied from the gas pipe 412 into plasma, and an arc column and plasma flow were stably generated.

[0071] Part of the inert gas released from the downstream side of the gas pipe 412 becomes a gas flow g along the outer periphery of the arc discharge a and the molten metal surface s1. The gas flow g stably heats the molten metal surface s1. The vapor generated above the molten metal surface s1 by the gas flow g diffuses along the surface of the molten metal m and is guided to the exhaust section 31. As the temperature drops, part of the vapor becomes liquid or solid and accumulates on the exhaust pipe 314 and filter 313.

[0072] The local low-pressure part 5 (local low-pressure means) includes a cylindrical body 51 with one end immersed near the liquid surface s2 (second region), a recovery filter 52 inserted into the cylindrical body 51, and a chamber 53 that hermetically holds the other end of the cylindrical body 51. The chamber 53 is depressurized by the exhaust part 32. The exhaust part 32 includes an oil rotary vacuum pump 321 (second exhaust means), a regulating valve 322 for adjusting the pressure (vacuum degree) inside the chamber 53, an exhaust filter 323 for trapping steam, fine particles, etc. that have passed through the recovery filter 52 and reached the chamber 53, and an exhaust pipe 324 communicating with the chamber 53. Note that the cylindrical body 51 is made of a heat-resistant insulating material such as ceramics. Also, the regulating valve 322 operates based on the measured value (P2) of the pressure gauge 50. In this embodiment, the exhaust part 31 and the exhaust part 32 are collectively referred to simply as "exhaust part 3".

[0073] Incidentally, the pressure (P1) in the processing chamber v and the pressure (P2) in the chamber 53 are usually such that P2 < P1 < P0 (atmospheric pressure). Therefore, a height difference (molten metal head: h) corresponding to the pressure difference (ΔP = P1 - P2) between them and the density (ρ) of the molten metal m occurs between the liquid surface s1 and the liquid surface s2.

[0074] 《Purification》 Using the above-described apparatus D, as shown in Table 1, various purifications (removal and recovery of specific elements) of an Al-based molten metal (raw material molten metal) containing Zn (specific element) were carried out. Specifically, it is as follows.

[0075] (1) Al-based molten metal As the Al-based molten metal (raw material molten metal) before purification, a molten metal with Al-1.2% Zn was prepared in the crucible 12. The Zn concentration is the mass ratio of Zn to the whole of the molten metal or alloy. Commercially available pure Al and pure Zn were used as the metal raw materials for the molten metal. The amount of Al-based molten metal used in each sample was 6000 g in all cases.

[0076] The molten metal temperature was measured at a depth position about 25 mm from the liquid surface s1. The molten metal temperature before local heating was 750 °C in all cases.

[0077] (2) Depressurization The vacuum pumps 311 and 321 were operated to evacuate and reduce the pressure in the sealed processing chamber v and chamber 53. The pressure (P1: absolute pressure) in processing chamber v and the pressure (P2: absolute pressure) in chamber 53 were set as shown in Table 1. The processing time shown in Table 1 is the elapsed time (discharge time in the case of local heating) after P1 and P2 reached the pressures shown in Table 1. The reduction in pressure in chamber 53 corresponds to the local low pressure step referred to in the present invention.

[0078] A pipe (inner diameter: 60 mm) made of ceramics (aluminum titanate) was used as the cylindrical body 51. The lower end of the cylindrical body 51 was immersed about 10 mm into the molten metal m from the molten metal surface s1.

[0079] Incidentally, the height difference (h) between the molten metal surface s1 and the molten metal surface s2 due to the pressure reduction by the exhaust part 3 was about 3 cm for both sample 1 and sample 2.

[0080] (3) Arc discharge (local heating process) For Samples 1 and 2, electricity was applied to the power source 40, and the molten metal surface s1 was heated by arc discharge a from the start of treatment. The arc discharge a was a DC arc with the electrode 411 as the negative electrode (cathode) and the counter electrode 42 as the positive electrode (anode). The electrode 411 was a tungsten rod with a diameter of 3.2 mm, and the counter electrode 42 was a graphite rod with a diameter of 6 mm.

[0081] The lower end of the counter electrode 42 was immersed to a depth of about 50 mm from the molten metal surface s1. The discharge current accompanying the arc discharge a and the flow rate of Ar gas forming the gas flow g were as shown in Table 1. The discharge time was the treatment time shown in Table 1.

[0082] For sample C1, neither power supply from the power source 40 nor gas supply to the molten metal surface s1 was performed, and the molten metal temperature was kept at 750° C., and only the pressure in the chamber 53 was reduced.

[0083] Evaluation and Measurement (1) Recovery of specific elements The appearance of the recovery filter 52 after purification for sample 1 and sample C1 is shown in Figure 2. As is clear from Figure 2, deposits were observed at the bottom of the recovery filter 52 for sample 1, which was locally heated near the molten metal surface s1. On the other hand, no such deposits were observed in the recovery filter 52 for sample C1, which was not locally heated.

[0084] In this way, it was found that by creating a high vacuum on the molten metal surface s2 side (second region) while heating the molten metal surface s1 side (first region), Zn (specific element) can be evaporated and recovered from the molten metal m within a short period of time.

[0085] (2) Removal of specific elements For sample 2, after local heating (arc discharge) was completed, the molten metal m was cooled (furnace-cooled) for 600 seconds while maintaining the vacuum in holding tank 1 (treatment chamber v). The treatment chamber v was then opened to the atmosphere, and the cooled molten metal m was removed. A portion of the molten metal m was poured into a stainless steel analytical mold and allowed to solidify by natural cooling in the atmosphere. The chemical composition (Zn concentration) of the Al alloy thus obtained was measured by X-ray fluorescence spectroscopy. The Zn concentration was found to be 0.65% by mass. Therefore, it was found that the Zn concentration of the molten metal m was reduced from the initial 1.2% by mass to 0.65% by mass by the above-described refining.

[0086] As in the case of sample 1, Zn deposits were also found below the recovery filter 52 of sample 2. The increase in mass of the recovery filter 52 was approximately 80% of the Zn decrease in the molten metal m calculated from the change in Zn concentration described above. In other words, it was found that the recovery filter 52 installed inside the cylinder 51 on the molten metal surface s2 side recovers approximately 80% of the Zn evaporated from the entire molten metal m (i.e., the Zn recovery rate is 80%).

[0087] From the above, it has been found that, as in the present invention, by heating a first region of an Al-based molten metal and creating a high vacuum in a separate second region connected to the first region, specific elements can be fractionated or recovered efficiently from the Al-based molten metal in a short period of time.

[0088] [Second Example] The present invention will be described in detail below with reference to specific examples of differential pressure control associated with the above-mentioned metal refining (fractional distillation and recovery of specific elements).

[0089] "Device" An outline of the metal refining apparatus D1 (simply referred to as "apparatus D1") used in this example is shown in Fig. 5. Members, devices, etc. similar to those of the apparatus D shown in Fig. 1 are given the same reference numerals, and detailed descriptions thereof will be omitted.

[0090] Device D1 is configured such that a differential pressure control unit 33 and a leak valve 34 are added to device D. Differential pressure control unit 33 (differential pressure control means) includes a differential pressure gauge 330 and a control valve 331. Both differential pressure gauge 330 and control valve 331 are interposed between a pipe 315 located between adjustment valve 312 and exhaust filter 313 and a pipe 325 located between adjustment valve 322 and exhaust filter 323. Leak valve 34 is provided in pipe 315. When leak valve 34 is opened, the inside of pipe 315 is opened to the atmosphere.

[0091] The control valve 331 opens and closes based on the differential pressure detected by the differential pressure gauge 330. Specifically, when the detected differential pressure is equal to or less than a predetermined threshold, the control valve 331 blocks communication between the pipes 315 and 325, thereby generating a desired differential pressure (ΔP = P1 - P2) between the processing chamber v and the chamber 53. On the other hand, when the detected differential pressure exceeds the predetermined threshold, the control valve 331 connects the pipes 315 and 325, making the processing chamber v and the chamber 53 substantially equal in pressure (P1 ≒ P2, ΔP ≒ 0) and making the molten metal levels s1 and s2 substantially the same height (molten metal head: h ≒ 0).

[0092] <Differential pressure control> (1) Molten metal head The relationship between the pressure difference (ΔP=P1-P2) between the processing chamber v and chamber 53 and the height difference (molten metal head) between the molten metal surfaces s1 and s2 was calculated. An example in which the molten metal m was the aforementioned Al-based molten metal (Al-1.2% Zn) is shown in Figure 6.

[0093] (2) Process FIG. 7 shows an example of the differential pressure control process. Specifically, first, the vacuum pumps 311 and 321 are operated to reduce the pressure in the processing chamber v and the chamber 53 (time t0 / step S0). Next, after the processing chamber v and the chamber 53 reach a predetermined vacuum level, the adjustment valves 312 and 322 are operated to reduce the pressure (P2) in the chamber 53 below the pressure (P1) in the processing chamber v (time t1 / step S1). After the differential pressure (ΔP) between the two chambers falls within a predetermined range (below a predetermined threshold), the leak valve 34 is slightly opened. This causes an intentional disturbance, causing the pressure (P1) in the processing chamber v to suddenly increase (time t2 / step S2). The differential pressure gauge 330 detects an abnormality in which the differential pressure (ΔP) exceeds the predetermined threshold, and the control valve 331 is opened. As a result, the pipes 315 and 325 are connected, and the pressures in the processing chamber v and the chamber 53 are equalized (time t3 / step S3).

[0094] (3) Experiment The results of an experiment using the apparatus D1 in accordance with the above-described steps are shown in Fig. 8. Here, the target pressures of the processing chamber v and chamber 53 after time t1 were set as follows: P1 = 800 Pa, P2 < 50 Pa, and the threshold value of ΔP: 900 Pa, respectively.

[0095] As can be seen from FIG. 8, 180 seconds after the start of depressurization (t0), P1 = P2 = 1000 Pa (time t1). From then on, P1 remained approximately 800 Pa and P2 < 15 Pa, and 500 seconds after the start of depressurization (time t2), the above-mentioned disturbance was applied. Almost simultaneously, P1 and ΔP increased rapidly. When ΔP reached 900 Pa (threshold), control valve 331 opened, causing ΔP to decrease rapidly. The time required from the sudden increase in P1 due to the release of leak valve 34 to the sudden decrease in ΔP due to the operation of differential pressure gauge 330 and control valve 331 was extremely short.

[0096] The device D1 was stopped, and the processing chamber v and chamber 53 were returned to atmospheric pressure. The removed collection filter 52 was then observed. When differential pressure control (equalizing pressures P1 and P2 or reducing ΔP) was performed after the above-mentioned disturbance was applied, no molten metal adhesion to the collection filter 52 was observed. On the other hand, when differential pressure control was not performed after the disturbance was applied, molten metal adhered to the collection filter 52.

[0097] From the above, it was found that the introduction of differential pressure control can prevent damage to the equipment even in abnormal situations, and enables stable fractionation and recovery of specific elements.

[0098] [Table 1] [Explanation of symbols]

[0099] D Metal refining equipment 3 Exhaust section 4 Local heating section 5 Local low pressure area 33 Differential pressure control section m Al-based molten metal s1 Hot water surface (first area) s2 Hot water surface (second area) a. Arc discharge g Gas flow

Claims

1. a local heating step of heating the aluminum-based molten metal in a first region on the surface of the aluminum-based molten metal; a local low pressure step of evacuating the inside of a cylinder immersed in the aluminum-based molten metal from above the molten metal surface to a vacuum, thereby making a second region above the molten metal surface, which is different from the first region, lower in pressure than the first region; A metal refining method for purifying the aluminum-based molten metal by evaporating a specific element from the second region.

2. 2. The metal refining method according to claim 1, wherein the local heating step and the local low pressure step are carried out in parallel.

3. The metal refining method according to claim 1 or 2, further comprising a recovery step of recovering the specific element evaporated from the second region.

4. 4. The metal refining method according to claim 1, wherein the pressure (P2) on the second region side is set to 0.1 to 1000 Pa.

5. 5. The metal refining method according to claim 1, wherein the pressure (P1) on the first region side is set to 100 to 10,000 Pa.

6. 6. A metal refining method according to claim 1, wherein the differential pressure (ΔP=P1-P2) between the pressure on the first region side (P1) and the pressure on the second region side (P2) is set to 100 to 5000 Pa.

7. The metal refining method according to any one of claims 1 to 6, further comprising a differential pressure control step of controlling the differential pressure (ΔP = P1 - P2) between the pressure on the first region side (P1) and the pressure on the second region side (P2) within a predetermined range.

8. 8. The metal refining method according to claim 1, wherein the local heating step is performed by arc discharge.

9. 9. The metal refining method according to claim 1, wherein the specific element is at least one of Zn, Mg, and Pb.

10. a local heating means for heating the aluminum-based molten metal in a first region on the surface of the aluminum-based molten metal; a local low pressure means for making a second region on the molten metal surface different from the first region lower in pressure than the first region, A metal refining device capable of evaporating a specific element from the second region to refine the aluminum-based molten metal.

11. The metal refining apparatus according to claim 10 , further comprising a recovery means for recovering the specific element evaporated from the second region.

12. The metal refining apparatus according to claim 10 or 11, further comprising a differential pressure control means for controlling the differential pressure (ΔP = P1 - P2) between the pressure on the first region side (P1) and the pressure on the second region side (P2) within a predetermined range.

13. 13. The metal refining apparatus according to claim 12, wherein the differential pressure control means is a switching valve or a control valve capable of increasing the pressure (P2) on the second region side.

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