Method for melting metal by means of an electric immersion heater

Abstract

The invention relates to a method for melting non-ferrous metal by means of at least one electric immersion heater, wherein: - at least one electric immersion heater is brought into contact or into close proximity with non-ferrous metal in the solid state placed inside a vessel, - during a so-called priming step, a so-called initial electrical power corresponding to a fraction of the nominal power of the electric immersion heater is supplied to the electric immersion heater and - during a so-called melting step, the electrical power supplied to the electric immersion heater is increased progressively or in steps so that the non-ferrous metal present in the vessel changes from a solid state to a liquid state without significant input from heat sources other than the at least one electric immersion heater. The invention also relates to a melting device for implementing the method and to a production unit comprising such a device.

Description

DESCRIPTION TITLE: METHOD FOR METAL MELTING USING AN ELECTRIC IMMERSIVE HEATER TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of foundry equipment and is particularly concerned with the smelting of non-ferrous metals with a relatively low melting point, typically below 1100 °C. It relates to a metal melting process using at least one electric immersion heater. More specifically, the invention aims to melt a batch of solid metal, particularly alloys or pure metals of aluminum, magnesium, or zinc.

[0002] The present invention also relates to a device for implementing said fusion process which includes an immersion heater. STATE OF THE ART

[0003] The non-ferrous metal casting industry requires a significant amount of liquid metal for its molding operations. This is the case, for example, in the aluminum industry, which manufactures large quantities of castings, for example, for the automotive industry. Among the most commonly used molding processes are gravity casting, high-pressure die casting, and low-pressure die casting.

[0004] In all cases, the preliminary step to molding is the melting of solid metal. This step is most commonly carried out on-site and more occasionally off-site, requiring delivery of the liquid metal by truck in the latter case.

[0005] In foundries, two different furnaces are usually used, or one furnace with two chambers: a melting furnace (chamber), where the molten metal is melted, and a holding furnace (chamber), where the molten metal is held at a controlled temperature until it is used by a molding machine. The melting of solid metal is primarily carried out in gas-fired melting furnaces. Solid metal, usually in the form of ingots, is placed in a chamber. of fusion where several gas burners operate a combustion which generates the heat to melt the metal.

[0006] The melting furnace (chamber) is connected to a holding furnace (chamber) in which the molten metal is contained, awaiting collection and delivery to a pouring point near a molding machine. Such a configuration is illustrated, for example, in European patent EP 1 136 778 A1 (Nippon Crucible Co.).

[0007] Smelting using gas-fired furnaces is preferred because it has a relatively low average cost per ton of metal. Gas smelting also offers a good smelting rate in tons per hour.

[0008] One drawback of gas-fired smelting is that the process is polluting on-site because it involves the combustion of gas and necessarily relies on the use of fossil fuels. Furthermore, a disadvantage of gas-fired smelting furnaces is the phenomenon of "loss on ignition," which corresponds to the oxidation of a portion of the solid metal.

[0009] To overcome this defect, induction melting and radiative heating of the metal, which rely on an electrical power supply, are also used.

[0010] The induction method allows for rapid melting, with very low loss on ignition and excellent metallurgical quality of the resulting molten metal. However, this method has the drawback of being very energy-intensive.

[0011] Radiation melting has the advantage of requiring only an inexpensive and easy-to-maintain furnace. However, this method has the drawbacks of being very slow, generating a large amount of oxides that degrade the metallurgical quality of the resulting metal, and being energy-intensive.

[0012] No known smelting method of the prior art offers a satisfactory solution that both limits the use of polluting gases and produces a metal of satisfactory metallurgical quality, without representing an exorbitant cost in electricity supply.

[0013] Furthermore, the melting furnace described above is typically a centralized system within a factory that includes several molding machines. This arrangement necessitates transporting the molten metal temporarily contained in the holding chamber to the molding machines; this is done, for example, through a channel made of ceramic material. To keep the metal in liquid form at a precisely controlled temperature, a tank equipped with an immersion heater can be placed near the pouring point. Such an immersion heater is described in particular in the applicant's French patent FR 2 907 353 B1.

[0014] Although transporting molten metal poses a risk and exposes the liquid metal to oxidation by air, this plant layout is favored according to the prior art because it has the advantage of limiting gas use to a portion of the plant and because larger furnaces offer better energy efficiency. However, in some molding workshops, it would be preferable to have smaller, decentralized furnaces without gas heating, thus avoiding the transport of liquid metal throughout the plant and between the various molding machines. This is particularly advantageous when a molding plant simultaneously uses a significant number of different alloys of the same base metal, since a single furnace cannot produce two different alloys at the same time. The prior art does not offer a simple solution to this problem.

[0015] Furthermore, the gas furnace is the only commonly used furnace that can start dry, meaning with only a charge of solid metal. All other furnaces require a certain amount of liquid metal to operate; those skilled in the art call this liquid metal the "foot bath." For example, to melt solid metal in an induction furnace, a foot bath is first supplied, in which the metal is melted by gradually introducing it into the induction-heated metal. The liquid foot bath can come from another furnace operating within the plant. Consequently, a casting plant that does not have a gas furnace must always have a certain amount of liquid metal available to restart a furnace that has stopped operating. Alternatively, liquid metal can be delivered from an external foundry in a thermally insulated container (called a "ladle"), most often by road.This situation imposes a heavy management constraint on foundries without a gas furnace, given that the chemical composition of the bath starter used to start a furnace must obviously be compatible with the alloy that is about to be prepared in this furnace during start-up.

[0016] It follows from all that has just been said that it would be desirable to have an electric oven that can be started without a bath foot, that is to say, dry. OBJECTS OF THE INVENTION

[0017] The present invention aims to address all or part of the needs mentioned above or to remedy all or part of the drawbacks mentioned above.

[0018] According to known prior art practices, electric immersion heaters are not used for melting non-ferrous metals. More precisely, immersion heaters are only used in contact with liquid metal. Indeed, the very The low conductivity of air makes the uncovering of an immersion heater under load critical. "Uncovering" refers to the situation where an immersion heater's heating element is exposed to air. The applicant observed that, in the absence of contact with molten metal, electric immersion heaters reach their internal temperature very rapidly, reaching their maximum temperature in a matter of seconds. This problem does not occur when the heating element is immersed in molten metal and exchanges heat with it. Conversely, when the heating element of the immersion heater is exposed to air, the cooling is insufficient, causing the heater to quickly reach the maximum temperature its internal materials can withstand and resulting in damage.

[0019] It follows that according to the state of the art, immersion heaters are only used when they are permanently immersed in liquid metal; it is then possible to add solid metal to this liquid "bottom bath" to gradually increase the quantity of liquid metal, but it is not possible to start a furnace loaded with solid metal using an immersion heater.

[0020] The applicant has developed the control method, which is the subject of the invention, which allows the implementation of electric immersion heaters for the melting of solid non-ferrous metals.

[0021] Thus, according to a first aspect, the invention relates to a process for melting non-ferrous metal using at least one electric immersion heater, in which: - at least one electric immersion heater is placed in contact with, or in the immediate vicinity of, solid non-ferrous metal in a tank, - during a so-called priming stage, an initial electrical power corresponding to a fraction of the nominal power of said electric immersion heater is supplied to said electric immersion heater and - during a so-called melting stage, the electrical power supplied to the electric immersion heater is increased, in particular gradually or in stages, so that the non-ferrous metal present in the tank passes from a solid state to a liquid state.

[0022] Thanks to these measures, metal can be melted without consuming gas, unlike conventional melting processes. These measures improve safety within the plant, which does not need to maintain a gas distribution network, and eliminate fossil fuel consumption.

[0023] Preferably, the process for melting non-ferrous metal using at least one electric immersion heater operates without significant heat input from heat sources other than the at least one electric immersion heater. By "input" we mean Significant heat input is defined as a heat input that exceeds 1% of the total heat received by the metal to be melted during the process. In another embodiment, the furnace includes an electrical heating means other than the electric immersion heater. For example, the other electrical heating means is a heating plate, preferably located at the bottom of the furnace; said heating plate may include at least one heating element, and its heat input may be significant. In another example, the other electrical heating means is a heating plate or a high-flux heating element positioned near the metal to be melted, preferably positioned above the metal to be melted.

[0024] According to the provisions of the invention, an electrical power lower than the nominal operating power is supplied to the electric immersion heater during the initiation phase, then this power is increased, gradually or in steps, so that the maximum temperature tolerated by the internal materials of the electric immersion heater is not reached and it is not damaged.

[0025] Another remarkable feature of the invention is that the metal melting operation can be carried out in the same tank that is subsequently used to keep the metal in a liquid state. This has the advantage of simplifying the furnace and avoiding the cost and risk associated with transporting the molten metal within the plant and transferring it between furnaces or tanks.

[0026] The present invention is applicable to any non-ferrous metal and any non-ferrous metal alloy that has a melting point sufficiently low to be compatible with the use of an electric immersion heater. In particular, it is applicable to any metal and alloy with a melting point not exceeding approximately 1100°C. The base metal may, for example, be chosen from aluminum, zinc, magnesium, copper, tin, lead, lithium, or silver. It may also be an alloy based on one of these metals, such alloys potentially containing other alloying metals and impurities. It is understood that an Al-Fe alloy remains a non-ferrous alloy within the meaning of the present invention, despite the presence of iron in this aluminum-based alloy.

[0027] Solid metal refers to metal in any solid form, including ingots, chips (for example, those obtained by grinding metal sheets), granules, or powder. The metal to be melted may also be in various shapes, consisting of the skeletons or offcuts from machining, cutting, stamping, or sawing processes, or casting rejects. Some of these product forms may be waste (e.g., chips, stamping skeletons, offcuts). This solid metal may have been compacted in a press (for example batches of boxes, or used beverage cans, or batches of shavings).

[0028] Rated power refers to the power received by an appliance when it operates under normal conditions. It is expressed in watts (W) or kilowatts (kW) and is specified by the appliance manufacturer. In other words, the ideal electrical power supplied to an appliance corresponds to its rated power. Supplying power exceeding the rated power risks damaging the appliance, while supplying power below the rated power may result in reduced efficiency.

[0029] An electric immersion heater is a device that includes at least one heating zone intended to be in contact with the metal to be heated, and a non-heating zone that connects the heating zone to an electrical power supply. The heating zone contains one or more internal heating elements that are brought to a high temperature by the passage of an electric current; this is a resistive heating system. This internal heating element transfers its heat to a sheath that forms the body of the immersion heater, which in turn transfers the heat to the metal or alloy to be melted or kept in a liquid state. The mechanism for transferring heat from the sheath of the immersion heater to the metal in the furnace is most efficient, by conduction, when the heater is immersed in liquid metal or when its sheath is in contact with a significant portion of the solid metal.Outside of these direct contact areas, heat transfer between the sheath and the metal contained in the furnace is radiative and / or convective.

[0030] The process of the invention comprises at least two steps, namely a first step called priming, and a second step called melting. The priming step aims to create a molten bath from a charge of solid metal. The melting step aims to melt the remaining solid metal in said bath, which will then rise to engulf all the metal charged into the furnace.

[0031] It is at this point that we move to a third phase, known as the holding phase, which aims to obtain and maintain precise control of the liquid metal's temperature for use in a molding machine. This holding phase corresponds to the usual use of immersion heaters according to the prior art. During the holding phase, small quantities of solid metal can be added, preferably periodically or continuously, as liquid metal is consumed by the casting machine.

[0032] The initiation phase may be preceded by a so-called preheating phase, which aims to preheat the furnace, either empty or with a small load of solid metal, before loading it with the solid metal load intended for melting. Each of the phases named above is characterized by a certain electrical power supplied to the immersion heater, as will be explained below.

[0033] According to the invention, during the priming step, the immersion heater is brought into contact with solid non-ferrous metal, or in the immediate vicinity of solid non-ferrous metal placed in the tank. This means that no liquid metal is in contact with the immersion heater before the priming step, or that the immersion heater is at least partially submerged, i.e., that the heating element of the immersion heater is in contact with the air. More precisely, the solid non-ferrous metal is poured into, or already present in, the tank prior to the priming step, so as to at least partially bury the immersion heater. Naturally, the geometry of the pile of solid non-ferrous metal pieces may include air pockets so that the immersion heater is in direct contact with the non-ferrous metal in some areas and separated from the metal in others.

[0034] During this initial heating phase, a fraction of the electric immersion heater's rated power is supplied to it. This power must be high enough to allow the surrounding metal to heat up gradually, but low enough to prevent premature degradation. During this initial heating phase, the heat from the immersion heater is dissipated by conduction through the solid metal and / or by radiation towards the solid metal. Since conduction is more efficient at heating solid metal, it is preferable for the immersion heater to be in direct contact with solid metal.

[0035] During the melting stage, the immersion heater is at least partially in contact with molten metal; that is, at least part of its sheath is immersed in liquid metal. The heat delivered by the immersion heater, resulting from the initial electrical power supplied to it, is then at least partially, and preferably primarily, dissipated by conduction. During this melting stage, the amount of liquid metal will increase as the solid metal melts, either in contact with the liquid metal or in contact with the immersion heater.

[0036] During either or both of these two steps, solid metal can be added to the cell; this added solid metal (just like the solid metal initially present in the cell when the priming step begins) can be preheated outside the furnace to a temperature between ambient temperature and the metal's melting point. The addition of metal to the cell during the melting step can be done directly into the liquid metal.

[0037] According to one feature of the process according to the invention, the heat transfer from the immersion heater to the solid metal therefore occurs, initially, by Heat transfer occurs through radiation and convection, on the one hand, and / or conduction, on the other, with the solid metal in contact with the immersion heater, and then progressively through direct contact of the immersion heater with the liquid non-ferrous metal produced by melting. The process thus implemented therefore exhibits good energy efficiency, meaning that nearly all of the energy consumed is effectively transferred to the metal. For illustrative purposes, the applicant estimates that the process covered by this application requires half the energy of an induction melting process to melt the same quantity of metal. The initial power supplied to the immersion heater during the start-up phase depends advantageously on the starting load and the quality of the thermal contact between the immersion heater and the metal.When the immersion heater is in contact with metal powder, granules, or shavings, this value may be higher compared to when the heater is surrounded by sheet metal frames providing less effective thermal contact, and / or it may increase more rapidly. All electrical power values ​​supplied to the immersion heater are expressed here as a percentage of the heater's rated power, which is set by the manufacturer to ensure optimal operation without damaging the heater.

[0038] During the optional furnace preheating phase, the refractories and the furnace air are preheated, with the air reaching a temperature of around 600 °C, without solid metal or with a low solid metal content. This must be done at very low electrical power.

[0039] In some embodiments, the initial electrical power supplied to the electric immersion heater during the start-up phase is a non-zero power less than 30%, preferably less than 15%, and even more preferably less than 10%, of the electric immersion heater's rated power. This initial power can be increased subsequently, gradually or in steps.

[0040] In one embodiment, the initial electrical power supplied to the electric immersion heater during the priming phase is a non-zero power of between about 0.5% and about 5% of the rated power of the electric immersion heater.

[0041] In one embodiment, during the priming step, the power supplied to the immersion heater is increased from a very low value, typically between about 0.5% and about 5% (and preferably between about 0.5% and about 2.5%), at a rate of about 1% to 3% per hour, until the transition to the melting step.

[0042] During the melting stage, the electrical power supplied to the electric immersion heater can be increased gradually or in stages over a long period of time. For example, the long period of time is at least 8 hours.

[0043] In advantageous embodiments, the process of the invention further comprises a holding stage for the molten metal in a liquid state, in which the power supplied to the electric immersion heater is controlled according to a temperature measured in the tank or on the heater. During this holding stage, the liquid metal can be drawn from the tank and fed into the molding machine; to ensure the quality of the molding process, it is necessary to control the temperature of the liquid metal during the holding stage. In a particular embodiment of this holding stage, a well-defined quantity of solid metal is added, preferably periodically, to the liquid metal to replace the metal consumed by the molding machine.

[0044] In another embodiment, the solid non-ferrous metal in the tank is first melted and then solidified within the tank, in contact with the immersion heater. The prior art does not offer any possibility, in an electric holding furnace, of remelting solid metal that has "set" in a tank, that is, metal that has been melted and then solidified in place, such that the immersion heater is embedded in the solidified metal. According to particular features of the invention, this metal can be remelted to obtain a bath of liquid metal. The metal can solidify in a tank unintentionally, for example during a power outage or a strike, or intentionally, when it is decided to interrupt the heating and thus the holding of a metal bath in a liquid state, and then resume operations when necessary, without having to drain the metal bath or move the immersion heater.

[0045] In some embodiments, the nominal power of said immersion heater is determined according to the dimensions of the tank.

[0046] The transition from melting mode to holding mode typically results in a decrease in the electrical power supplied to the immersion heater to avoid overheating the liquid metal, since during the holding phase it is no longer necessary to supply the furnace with the enthalpy of fusion (except for the small fraction of metal which is added in solid form to replace the liquid metal which is consumed).

[0047] In an advantageous embodiment, the switch from the melting stage to the holding stage can be automatic. For example, the transition from the melting stage to the holding stage in the liquid state of the molten metal is performed automatically based on the measurement of a representative temperature value of the immersion heater or the tank, or based on the detection of whether at least part of the surface of the metal in the tank is smooth or irregular.

[0048] A device for measuring the temperature of the liquid metal can be provided for this purpose, detecting a temperature increase when the melting phase is complete. A temperature measuring device can be integrated into the immersion heater itself, but this solution is not preferred. Alternatively, a temperature measuring device can be installed in an area near the bottom of the tank, capable of detecting the presence of a molten metal base and measuring its temperature.

[0049] In an advantageous embodiment, the end of the melting stage is determined using a machine vision system typically combining a camera with a computer system suitably configured to detect the presence of a smooth metal surface uninterrupted by protruding pieces of solid metal. This machine vision system can also be configured to detect the level of liquid metal in the furnace.

[0050] According to a second aspect, the invention relates to a device for melting non-ferrous metal by means of at least one electric immersion heater, which comprises at least one electric immersion heater, a regulator of the power supplied to said immersion heater and a microprocessor configured to control said regulator and in which the microprocessor implements a program for melting the non-ferrous metal which comprises: - the supply of said electric immersion heater with an initial electrical power corresponding to a fraction of the nominal power of said electric immersion heater and - the gradual or stepwise increase of the energy supplied to said immersion heater, so that the non-ferrous metal present in the tank passes from a solid state to a liquid state without significant input from other heat sources than the electric immersion heater.

[0051] The aims, advantages and special characteristics of the metal melting device which is the subject of the present invention being similar to those of the process which is the subject of the present invention, they are not recalled here.

[0052] According to a third aspect, the invention relates to a production unit for molding non-ferrous metal objects characterized in that it comprises at least one molding machine and a non-ferrous metal melting device according to the invention and in which the point of entry of the liquid metal into the molding machine is in the direct vicinity of the non-ferrous metal melting device or connected to it by a means of conveying a flow of liquid metal.

[0053] According to prior art, a large-capacity melting furnace can supply molten metal to several production lines for molding. This arrangement requires to transport the liquid metal temporarily contained in the holding chamber to the production units distributed within a plant. A constraint associated with this arrangement is that the same metal or alloy is supplied to each production unit or molding machine fed with molten metal from the melting furnace; this is unsuitable if two production units or molding machines need to be supplied with alloys of different types.

[0054] According to the invention, the production unit comprises a non-ferrous metal melting device according to the invention, which melts the metal on-site, near the pouring point. Given that the production unit has its own means of bringing the metal to a liquid state, the metal or metal alloy can be specifically chosen for this production unit.

[0055] Other purposes, advantages and special characteristics of the metal melting device which is the subject of the present invention being similar to those of the process which is the subject of the present invention, are not recalled here. BRIEF DESCRIPTION OF THE FIGURES

[0056] Other advantages, purposes and special features of the invention will become apparent from the following non-limiting description of at least one particular embodiment of the process, device and production unit that is the subject of the present invention, with reference to the accompanying drawings, in which:

[0057] [Fig 1] represents, in the form of a flowchart, a particular sequence of steps in the non-ferrous metal melting process that is the subject of the present invention; [Fig 2] represents, in the form of a graph, a particular embodiment of a non-ferrous metal melting process according to the invention. [Fig 3] schematically represents, in cross-section, an immersion heater that can be used in a non-ferrous metal melting device according to the invention, [Fig 4] schematically represents, in cross-section, a non-ferrous metal melting device according to the invention useful for implementing the process that is the subject of the invention, illustrated at the beginning of the initiation phase, [Fig 5] schematically represents, in cross-section, the device of figure 4, illustrated at the end of the priming step and [Fig 6] represents, schematically and in cross-section, the device of figure 4, illustrated at the end of the fusion step. DETAILED DESCRIPTION OF THE INVENTION

[0058] The present description is given by way of non-limiting attribution, each feature of an embodiment being able to be advantageously combined with any other feature of any other embodiment.

[0059] It should be noted from the outset that, unless otherwise stated, the power supplied to an immersion heater is expressed as a percentage of the nominal power of said immersion heater.

[0060] Figure 1 shows, in the form of a flowchart, a particular sequence of steps of the process 100 for melting non-ferrous metal which is the subject of the present invention.

[0061] In some embodiments, the preheating process 100 includes a preheating step 103 for the non-ferrous metal melting device. This step is designed to preheat the furnace, either empty or with a small charge of solid metal, before loading it with the solid metal charge intended for melting. Preferably, the power supplied to the immersion heater during this preheating step is fixed and between 5% and 25% of the heater's rated power, for example, approximately 5% or 10%. Preferably, the duration of the preheating step is between 24 and 96 hours.

[0062] Process 100 includes a step 105 during which at least one electric immersion heater is brought into contact with, or in the immediate vicinity of, solid non-ferrous metal disposed of in a tank.

[0063] Then, during a step 110 called priming, an initial electrical power corresponding to a fraction of the nominal power of said electric immersion heater is supplied to said electric immersion heater.

[0064] In one embodiment, the initial power supplied to the immersion heater is zero. In other embodiments, the initial power supplied to the immersion heater is equal to 1%, 2%, 5%, 10%, 15%, 20%, 25%, or 30% of the heater's rated power. This initial power can be a fixed value for a given immersion heater, or it can be modified according to external parameters.

[0065] In some embodiments, the power supplied to the electric immersion heater during the priming step 110 is fixed. In other embodiments, the power supplied to the electric immersion heater during the priming step 110 is increased gradually or in steps. The duration of the priming step 110 is preferably over a long period. In one example, the priming step is carried out over a period of time exceeding 40 hours. In other embodiments, it is carried out over a period of time between 20 and 40 hours, or between 20 and 30 hours.

[0066] During the priming phase, the power supplied to the immersion heater can be increased gradually or in steps. For example, the power supplied to the immersion heater is increased by 5% compared to the initial power.

[0067] At the end of the priming stage, a bath foot, i.e. a base of liquid metal in the tank, is obtained.

[0068] Then, during a melting step 115, the electrical power supplied to the electric immersion heater is increased gradually or in steps. Preferably, the rate of increase of the electrical power supplied to the immersion heater is faster during the melting step than during the ignition step. For example, this rate of increase, expressed as a percentage of the rated power per unit of time, is five or ten times greater during the melting step compared to the rate of increase during the ignition step.

[0069] Advantageously, during the melting stage, a means of stirring the base of the aluminum bath is implemented. The stirring means is configured to stir the liquid aluminum in order to optimize heat exchange with the immersion heater and thus reduce the melting time and homogenize the temperature of the aluminum bath.

[0070] In some embodiments, the transition from the priming stage to the melting stage and the associated speed change are triggered based on a temperature measured in the tank.

[0071] The power increase, whether gradual or in stages, during the melting stage is preferably carried out over a long period. In one embodiment, the gradual power increase takes place over a period exceeding 12 hours. In other embodiments, it takes place over a period of between 12 and 24 hours. Preferably, the increase in power supplied to the electric immersion heater, expressed as a percentage of the heater's rated power, does not exceed 10% per hour. Preferably, the increase in power supplied to the electric immersion heater does not exceed 6% per hour.

[0072] In one example embodiment, the power during the melting stage is increased by 1% every 15 minutes. Thus, during the melting stage, the power supplied to the electric immersion heater increases from an initial power fixed between 5% and 25% of the rated power to 100% of the rated power, in the space of 1125 minutes to 1425 minutes.

[0073] At the end of steps 105, 110 and 115, the non-ferrous metal present in the tank is melted from a solid state to a liquid state, preferably without significant input from other heat sources than at least one electric immersion heater.

[0074] In some embodiments, process 100 includes a step 125 of holding the molten metal in a liquid state. During this step, the power supplied to the electric immersion heater can be controlled according to a temperature measured in the tank. This ensures that the liquid metal has a controlled temperature in the holding furnace, which facilitates obtaining reproducible quality of the molded parts produced by the molding machine.

[0075] In some embodiments, during a step 120 concomitant with the liquid state maintenance step 125, solid metal is added to the tank.

[0076] Figure 2 shows a graph illustrating a non-ferrous metal melting process according to two specific embodiments of the process that is the subject of the invention. It should be noted that the melting process according to the invention relies on the use of an electric immersion heater.

[0077] This graph shows curve 210 illustrating the internal temperature of the immersion heater, expressed in °C, as a function of time, expressed in minutes. Additionally, curve 220 illustrates the variation in the temperature of the aluminum over time.

[0078] It should be noted that the values ​​illustrated by curves 210 and 220 are theoretical values ​​provided for illustrative purposes only. These temperatures are in no way limiting for the process of the invention, and the same applies to time.

[0079] Figure 2 also shows curves 250 and 260, which illustrate the power supplied to the electric immersion heater, expressed as a percentage of its nominal power, as a function of elapsed time, expressed in minutes. Hereafter, the term "power supply program" refers to a control of the power supplied to the immersion heater based on elapsed time and / or a temperature measured in the tank.

[0080] Curve 250 illustrates a first power supply program to the electric immersion heater, while curve 260 illustrates a second power supply program to the electric immersion heater.

[0081] According to the power supply program illustrated by curve 250, an initial power of 5% is supplied during a preheating stage for 3200 minutes. Then, during a priming stage, over a period of 1800 minutes, the power supplied is increased progressively and steadily to reach a total of 10%. Next, during a melting stage, over a period of 1000 minutes, the power supplied is increased progressively and steadily to reach a total of 100% power to the immersion heater. Then, still during the melting stage, 100% power is continuously supplied to the immersion heater for 2500 minutes. Following the melting stage, a cooling stage bath temperature maintenance is activated, during which the power supplied to the immersion heater is controlled according to a temperature measured in the tank.

[0082] It is noted that the transition from the priming stage to the melting stage can be carried out according to a pre-established power program or according to a temperature value measured in the tank.

[0083] It is noted that the transition from the melting stage to the holding stage can be carried out according to a pre-established power program or according to a temperature value measured in the tank.

[0084] According to the power supply program illustrated by curve 260, the power supplied to the immersion heater during the preheating stage is 25%. This initial power of 25% supplied to the immersion heater is increased to 30% during the ignition stage and then to 100% during the melting stage.

[0085] Note that curves 250 and 260 coincide from 6000 minutes onwards.

[0086] It should be noted that the power supply programs for the electric immersion heater illustrated by curves 250 and 260 are provided for illustrative purposes only. Other power supply programs may be implemented without deviating from the invention, particularly power supply programs whose power values ​​for a given time fall between the power values ​​of curves 250 and 260.

[0087] Figure 3 shows an immersion heater 300 suitable for use in a non-ferrous metal melting device. The immersion heater 300 has a connection zone 310 with a ceramic sheath and a heating zone 320 containing an electrical resistance. The immersion heater 300 has a power supply line 330 connected to a power supply 901.

[0088] Figure 4 shows a non-ferrous metal melting device according to the invention, useful for implementing the process of the invention, comprising an immersion heater 300 and a tank 400. The device is illustrated here at the beginning of the initiation phase. Aluminum ingots to be melted, 950, 951, are placed in the tank 400.

[0089] The same setup is illustrated in Figure 5 at the end of the priming stage. A bath foot formed by liquid metal 980 has formed in the tank while at least part of the ingots, 950, 951, are partially melted.

[0090] The same setup is illustrated in Figure 6 at the end of the melting stage. All the ingots initially present in the tank have at this stage been melted into liquid metal 980 and new ingots of metal to be melted, 952, 953, can be added to the tank.

[0091] The invention can be implemented in several ways. In a first embodiment, the aim is to improve the thermal contact between the external wall of the immersion heater and the solid metal in the furnace.

[0092] According to a first embodiment, a sufficient quantity of metal pieces of a size small enough to create a mass is loaded into the furnace. This mass comes into direct contact with, and preferably surrounds, at least part of the immersion heater. Thus, at least part of the metal is in direct contact with the immersion heater, ensuring heat transfer by conduction.

[0093] These pieces can be chips (obtained, for example, by grinding metal sheets), granules, or powder. This improves the thermal contact between the immersion heater and the solid metal. This allows for the selection of a higher heating power compared to an embodiment in which the immersion heater is not in direct contact with the metal.

[0094] In a first variant of this embodiment, the pieces are loaded into a metal container, such as a bucket or sleeve, of a chemical composition compatible with the metal in question, in order to reduce the quantity of pieces required. Typically, this container is positioned below the immersion heater so that the heater is immersed in the container, and the container is at least partially filled with the pieces so that the heater is, at least along part of its length, in direct contact with the metal, namely the pieces. The solid metal to be melted (which can then be of any shape) is positioned around the container so that it is advantageously in direct contact with the container; loose metal pieces may be added.

[0095] In a second variation of this embodiment, the immersion heater is surrounded by pieces of solid metal; these can include ingots (for example, ingots of new metal or remelted metal), or briquettes formed by compacting metal shavings, sheets, or metal cans (such as beverage cans). This arrangement of the solid metal pieces is preferably done by hand, forming a dense pile that makes close contact with the immersion heater, depending on the size and shape of the pieces. Loose metal parts and shavings of any kind can then be loaded around this pile.

[0096] In a third variant, the immersion heater is surrounded by ingots or massive pieces of metal in which a channel has been made with a width slightly greater than the external diameter of the immersion heater; these pieces are threaded around the immersion heater, and then metal pieces can be loaded around this pile, in contact with the immersion heater, pieces of metal, in any form. These solid pieces may be briquettes formed by compacting metal shavings, sheets, or metal cans, in which a channel of sufficient diameter has been cut to allow them to be threaded around the immersion heater. These solid pieces may also be ingots of new metal or remelted ingots, in which a channel of sufficient diameter has been cut.

[0097] In a second embodiment, which is compatible with all other embodiments and variants described in this patent application, pieces of metal that have been preheated outside the furnace are loaded into the furnace.

[0098] Besides accelerating the melting of the metal, such a preheating step of the pieces of metal outside the furnace has the advantage of evaporating residual water on the metal before introduction into the furnace or of eliminating any organic matter and thus preventing the appearance of smoke in the furnace.

[0099] In a first variant of this embodiment, preheated pieces of metal are loaded when the furnace does not yet contain molten metal.

[0100] In a second variation, preheated pieces of metal are loaded into the molten metal bath. This can be done at regular or irregular intervals. The pieces can be fed by a conveyor, such as a belt conveyor. These pieces may be finely divided or not; they may be, for example, chips, or crushed or uncrushed sheets, plates, or stamping blanks, or briquettes formed by compacting pieces or beverage cans, or even ingots of new metal or remelted ingots.

[0101] In a third embodiment, which is compatible with all other embodiments and variants described in this patent application, the method allows the initial power and / or the power evolution of the immersion heater to be adapted according to events or parameters detected by the device. Such an event could be, for example, the appearance of liquid metal in the tank as detected by a temperature measuring device at a point inside the tank. Such a parameter could be, for example, the degree of swarf coverage of the immersion heater as detected by a camera associated with suitably configured software.

[0102] In a fourth embodiment, which is compatible with all other embodiments and variants described in this patent application, the device according to the invention has a plurality of pre-configured programs that can be selected by the user. These programs allow the initial power and / or the power evolution of the immersion heater to be adapted to certain parameters related to the tank, in particular its size, and / or the degree of filling of the tank, or the nature of the solid metal placed in the tank.

[0103] In a fifth embodiment, the solid non-ferrous metal in the tank is first melted and then solidified within the tank, in contact with the immersion heater. In other words, the immersion heater is "trapped" within the solidified metal, thus closely conforming to its contours. This embodiment ensures excellent contact surface area between the immersion heater and the metal to be melted.

[0104] The melting process that is the subject of the invention is carried out using a non-ferrous metal melting device.

[0105] This device is equipped with at least one electric immersion heater that can be lowered into a tank designed to hold the metal to be melted. The device may consist of a single tank for melting the metal and then maintaining its temperature, or of multiple tanks connected together.

[0106] The non-ferrous metal melting device further comprises a regulator of the power supplied to said immersion heater and a microprocessor configured to control said regulator and in which the microprocessor is configured to implement a non-ferrous metal melting program of the type described in relation to the process of this application.

[0107] The device may include at least one air blower, and preferably several, which can be used during the device's preheating stage or during the ignition and melting stages. Its purpose is to improve the dissipation of heat generated by the immersion heater within the tank; this allows for an increase in the electrical power supplied to the immersion heater, particularly during the preheating phase, without the risk of localized overheating of the heater.

[0108] According to another embodiment, which is compatible with all other embodiments, an immersion heater is used that has at least two heating circuits configured to be supplied with electrical current independently. This allows for finer control of the electrical power supplied to the furnace. More specifically, two (or more) independent heating resistance circuits can be provided along the entire effective length of the immersion heater, and only one of the two is used when a low electrical current supply is required (for example, during the preheating or ignition stage), with the second being added when a high electrical current supply is needed (for example, during the melting stage).

[0109] Alternatively, two (or more) electrical resistance circuits can be provided in two different zones distributed along the effective length of the immersion heater, one of which is located near the lower end. This subdivides the immersion heater lengthwise into at least two thermal segments that can be controlled independently. In this embodiment, greater electrical power is supplied to the lower circuit at the beginning of the melting phase, when the height of the bath base covers only a portion, typically a small portion, of the immersion heater's effective length.

[0110] A feedback loop can be implemented using a microprocessor. This loop increases the electrical power supplied to the lower segment, at least for a certain duration, when the temperature measurement device located near the bottom of the tank detects the presence of a molten metal bath. This duration can be pre-programmed. It can also be programmed that as soon as the metal level in the tank reaches a certain point, the electrical power supplied to the upper segment is also increased.

[0111] It can be predicted that the total power supplied to the immersion heater will decrease again when the melting stage is complete; this end of the melting stage can be detected by an optical method such as a camera, as described above, and / or by a liquid metal temperature measurement means that detects a temperature increase when all the solid metal has melted.

Claims

CLAIMS A process (100) for melting non-ferrous metal using at least one electric immersion heater, characterized in that: - in a first step (105) at least one electric immersion heater is placed in contact with, or in the immediate vicinity of, solid non-ferrous metal arranged in a tank, - during a so-called priming step (110), an initial electrical power corresponding to a fraction of the nominal power of said electric immersion heater is supplied to said electric immersion heater, and - During a so-called melting step (115), the electrical power supplied to the electric immersion heater is increased gradually or in steps, so that the non-ferrous metal present in the tank changes from a solid to a liquid state. A method according to claim 1, wherein the initial electrical power supplied to the electric immersion heater is a non-zero power less than 30% of the rated power of the electric immersion heater, preferably less than 15%, and even more preferably less than 10%, of the rated power of the electric immersion heater. A method according to claim 1 or 2, wherein the temperature increase, gradually or in steps, during the melting step is carried out over a period of time of at least 8 hours.A method according to any one of claims 1 to 3, wherein the increase in power supplied to the electric immersion heater, expressed as a percentage of the heater's rated power, does not exceed 10% per hour. A method according to any one of claims 1 to 4, further comprising a step of maintaining the molten metal in a liquid state, wherein the power supplied to said electric immersion heater is controlled according to a temperature measured in the tank or on the heater. A method according to claim 5, wherein the transition from the melting stage to the holding stage of the molten metal in a liquid state is carried out automatically based on a temperature measured in the tank or on the immersion heater. A method according to claim 5, wherein the transition from the melting stage to the holding stage of the molten metal in a liquid state is carried out automatically based on the detection of a value representative of the appearance of at least a portion of the surface of the metal present in the tank, for example, when a smooth surface uninterrupted by protruding pieces of solid metal is detected. A method according to any one of claims 1 to 7, wherein the non-ferrous metal is selected from aluminum, zinc, magnesium, tin, copper, or an alloy based on one of these metals, said alloys being able to contain other alloying metals and impurities.A method according to any one of claims 1 to 8, wherein the non-ferrous metal in solid form disposed of in the tank has been previously melted and then solidified in the tank, in contact with the immersion heater. A method according to any one of claims 1 to 9, wherein the rated power of said immersion heater is determined according to the dimensions of the tank. A method according to any one of claims 1 to 10, wherein a stirring means is implemented during the melting step (115) in order to stir the liquid aluminum. A method according to any one of claims 1 to 11, which comprises, before the ignition phase, a preheating phase (103) during which the empty tank or the tank with a small charge of solid metal is heated by means of the immersion heater, before loading the furnace with the charge of solid metal intended for melting.A method according to any one of claims 1 to 11, wherein, during the priming step, a sufficient quantity of metal pieces of a size. sufficiently small to create a mass that comes into direct contact with at least a portion of the immersion heater. A method according to claim 13, wherein the pieces of metal are loaded into a metal container, such as a seal or a sleeve, of chemical composition compatible with the metal in question, and wherein said container is at least partially filled with said pieces, so that the immersion heater is, at least along part of its length, in direct contact with said pieces. A method according to any one of claims 1 to 12, wherein, during the priming step, the immersion heater is surrounded by ingots or solid pieces of metal in which a channel has been formed with a width slightly greater than the external diameter of the immersion heater.A method according to any one of claims 1 to 15, wherein at least one air blower is used during the preheating step or during the ignition and melting steps, said air blower being configured to generate an airflow to improve the dissipation of heat generated by the immersion heater within the tank. A method according to any one of claims 1 to 16, wherein at least one immersion heater has at least two heating circuits configured to be independently supplied with electricity.A method according to claim 17, wherein the immersion heater comprises at least two independent heating resistance circuits and wherein the number of heating resistance circuits energized under low electrical current, for example during the preheating or ignition stage, is less than the number of heating resistance circuits energized under high electrical current, for example during the melting stage. A device for melting non-ferrous metal using at least one electric immersion heater, characterized in that it comprises at least one electric immersion heater, a power regulator for said immersion heater, and a microprocessor. configured to control said regulator and in which the microprocessor implements a non-ferrous metal melting program which includes: - the supply of said electric immersion heater with an "initial" electrical power corresponding to a fraction of the nominal power of said electric immersion heater and - the gradual or stepped increase of the energy supplied to said immersion heater, so that the non-ferrous metal present in the tank passes from a solid state to a liquid state. . Production unit for molding non-ferrous metal parts, characterized in that it comprises at least one molding machine and a non-ferrous metal melting device according to the preceding claim, and in which the mold pouring point is in the direct vicinity of the non-ferrous metal melting device or connected to it by a means for conveying a flow of liquid metal.

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