FR2832733A1 - Utilization of video sensors for controlling the melting of aluminum in a furnace involves producing images of the bath surface or the inner surface of the furnace walls - Google Patents

Abstract

Video sensors are used for controlling melting of aluminum in a furnace by producing images of the bath surface or the inner surface of the furnace walls. Melting of aluminum in a furnace comprises: (a) introducing solid aluminum into the furnace; (b) melting the aluminum to form a molten bath; (c) production of images of the bath surface or of the inner surface of the furnace (2) walls; (d) treating these images to identify the presence of solids or the formation of oxides on the bath surface or for identifying the presence of hot spots or hot zones or zones having a certain minimal temperature on the furnace walls; and (e) modifying at least one furnace operating condition as a function of this treatment of the images. Independent claims are also included for the following: (a) a device for detecting the surface state of a bath of aluminum or the state of the furnace walls in an aluminum melting furnace; and (b) an aluminum melting furnace incorporating this detection device.

Description

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USE OF VIDEO SENSORS FOR ALUMINUM OVENS
Technical field and prior art
The invention relates to the field of aluminum smelting and furnaces for melting aluminum.

 It applies in particular to aluminum waste recycling processes.

 In the field of secondary aluminum smelting, the impossibility of measuring and controlling many phenomena and parameters during smelting constitutes an obstacle to understanding the smelting process, as well as to obtaining better performance and the development of rules allowing the elimination of fusion losses.

 Fusion occurs in rotary or reverberation ovens.

The process can be continuous but most ovens work discontinuously. Materials are loaded through huge doors that open in reverberation ovens and through the main door in rotary ovens. For the introduction of large volumes of waste, the furnace must be loaded two or three times per casting cycle.

 Such doors are also used for the skimming operation.

Some ovens have additional devices such as pumps or gas injectors. These devices are used essentially to ensure the homogeneity of the liquid bath, while allowing a more efficient fusion procedure.

 Aluminum or its alloys must be melted above the melting temperature. However, in order for the flow of liquid aluminum to be suitable during the subsequent treatment, for example in casting machines, it is preferable for the molten metal to reach a temperature level of 760 C. In addition, any overheating of the material is avoided. liquid bath, especially above 780 C, temperature above which the oxidation rate increases considerably, almost exponentially.

 An effective aluminum smelting technique involves applying as much heat as possible with a flame directly hitting the solid material.

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 However, when the solids have melted, the burner aimed at such a hot liquid quickly causes overheating.

 Good efficiency of the furnace is obtained at high melting speed, that is to say at a high power density, for two main reasons.

 Obtaining a good yield of aluminum from recycled waste is accompanied by a limitation of loss on ignition. In fact, a mixture of aluminum oxides and trapped aluminum floats on the fusion liquid. Limiting the time spent at high temperature limits the production of dross and scum.

 A reduction in the amount of heat lost to the surrounding environment also makes it possible to obtain a good energy yield.

Any reduction in the melting time contributes to the reduction of heat losses.

 The scum forms a layer of insulating material covering the liquid. As they are formed from alumina, they have a high melting temperature (above 1,300 C): they do not melt again but they increase.

 They can be removed entirely by scraping the surface: this is the skimming procedure. This operation is manual and requires the opening of the oven doors. There is no sensor to indicate when these doors should be opened for skimming.

 This opening allows the entry of a large quantity of air into the oven and therefore has two drawbacks: cooling the oven (returning to the previous temperature usually requires a time twice as long as the opening time of the door) and the introduction of oxygen from the air which easily oxidizes aluminum.

 The most advanced aluminum producers use certain control methods, implementing a verification of the temperature level of the refractory material by a thermocouple and reducing the combustion of burners or stopping one or more burners when certain temperature level is reached. However, these operations are primarily intended to ensure the safety of the refractory material.

 This technique has little effect on the temperature of the liquid metal: such a signal is too diffuse to allow the detection of points

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 hot. An insulating layer which varies over time and covers the liquid prevents any clear measurement of the behavior of the metal.

 Other producers verify the temperature of the liquid metal by immersing a thermocouple in it, and usually displaying the value in front of the operator who operates the loading device.

When a temperature of 760 C is displayed, the material loading operation can be accelerated while, at 730 C, the loading operation must be slowed down. However, the thermocouple only indicates a local temperature value: such a fixed device does not allow the detection of hot spots at distant and even relatively close locations.

 Another problem, specific to gas suppliers, is the lack of knowledge of the power density that an oven can withstand. In fact, most of the actions taken to find out this density are carried out when converting an existing air combustion furnace to an oxygen combustion furnace.

 In order to adapt the cost of oxygen to fuel savings, an increase in productivity is most often required.

Usually this means not only a reduction in the amount of heat lost in the flue gases by the hot nitrogen leaving the oven, but also an increase in the transfer of heat to the load. The power density is then increased. In addition, the ratio of energy transfer by convection and by radiation is modified, as is the spatial distribution of this energy.

 There is therefore a problem which is the use of burners having different power levels depending on the presence or absence of solid matter on the liquid bath.

 There is also the problem of performing some form of regulation of the melting process to reduce combustion losses.

 Furthermore, there is also the problem of determining the moment at which a skimming must be carried out.

 Another problem is to identify, in an oven, with or without a liquid metal bath, the regions which are more or less hot inside the oven and which allow a potential increase in power density or an adjustment of the system of oxygen burners.

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Statement of the invention
The invention firstly relates to a method for detecting solids or the formation of aluminum oxides on the surface of an aluminum bath of an aluminum melting furnace, characterized in that 'We take images of the surface of the bath, and we process the images to identify the presence of solids or the formation of oxides on the surface of the bath.

 The invention further relates to a process for melting aluminum in a furnace, comprising: - the introduction, into the furnace, of solid aluminum, - the melting of aluminum, to form an aluminum bath, - producing images of the surface of the bath, or of the interior surface of the walls of the furnace, - processing these images to identify the presence of solids or the formation of oxides on the surface of the bath, or to identify the presence, on the inner surface of the walls, of hot spots or hot zones or having a certain minimum temperature, - regulation of the melting, or modification of at least one operating condition of the oven, such as one or more operating parameters of one or more burner (s), or such as the stirring of the aluminum bath, depending on the treatment carried out in the previous step, or the detection of solids or the formation of oxides on the surface of the bath, or the detection of hot spots or hot areas or areas with a certain minimum temperature.

 The detection method as well as the above fusion method make it possible to obtain information on the state of the surface or the walls without having to open the oven to carry out a visual inspection.

 Image processing can be of the type by pattern recognition. It can also be a treatment by color reports.

 These treatments are well suited for the detection of unmelted solid masses, and for identifying the stages of oxidation of the surface of the aluminum bath.

 According to the invention, display means are used in the oven, such as a video camera, to detect the presence of solid materials or oxidized masses. Image processing can be implemented, such as processing by pattern recognition. We can

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 also perform detection using color ratio detectors.

 It is thus possible to detect cold regions, not melted, which are dark.

Such an analysis method, or the processing or identification carried out, can be used as a basis for regulating the fusion, in particular by: - switching from a high rate to a low combustion rate, - or by stopping operation burners or by reducing the number of burners operating in an oven equipped with several burners, - or by adjusting the flame length and the coverage of the regions covered with solid matter, - or by starting the stirring of the bath using pumps or by injecting a stirring fluid, - or also indicating whether the level of combustion power above the stirred bath can be increased or not - or by changing the stoichiometric ratio of the burners, - or by detecting the hot spots above the liquid metal,
Such an analysis or identification can also be used as a decision-making tool to optimize the skimming frequency based on variations in the reflectivity of the surface,
The analysis of images relative to colors according to the invention can also be applied to the visualization of the walls of an oven, in order to detect hot spots.

 We can then, depending on this information, change the orientation of the burners or the speed at which the gases are ejected from the burner or the flame pulse in order to modify the flame and the regions it reaches or covers. .

 Thus, during an oven conversion operation, it is possible to adjust the oxygen burner system (distribution of the heat flow and coverage by the flames).

 The invention therefore also makes it possible to detect hot spots on the refractory lining of an oven.

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 Brief description of the figures - FIG. 1 represents an aluminum melting furnace according to the invention, - FIG. 2 represents the installation of a camera in an oven, according to the invention.

 Detailed description of embodiments of the invention.

 FIG. 1 schematically represents an oven 2 in which an aluminum bath 4 is obtained by melting aluminum waste. A burner 14, supplied with an oxidizing fluid (for example oxygen) and a combustible fluid (for example natural gas), makes it possible to produce a flame 11 to reach the desired temperature in the oven.

 Fumes 6 escape from the oven through a chimney 13.

 The references 15 or 17 designate means for injecting the oxidant and the fuel into the burner.

 A video camera 18 is arranged so as to be able to view the surface of the molten liquid, inside the furnace. It is for example mounted in a hole drilled in the wall of the oven.

 Reverberation ovens are of the fixed or tilting type, but the tilting angle is small (less than 150). The electrical or optical connection lines of the camera can therefore be set up without any particular difficulty.

 In FIG. 2, numerical references identical to those of FIG. 1 designate therein identical or similar elements. In this figure (in which a pile of aluminum 20 introduced into the furnace is shown before melting), the camera 18 is equipped with a tube 46 containing the optical lenses. The references 50 and 52 respectively designate a camera optic and a video camera body. The lenses are cleaned by air or by a gas such as nitrogen, argon, or any other non-oxidizing gas.

 The camera is cooled by air or by a gas or an organic liquid such as oil. However, neither the camera nor the tube is cooled by water, in order to avoid any risk of explosion resulting from water-liquid metal contact.

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 In FIG. 2, a line 40 of cooling and protection gas, supplied by a reserve 48 of such a gas, is divided into a line 42 of protection gas for the optics 50 and into a line 44 of camera cooling.

 Means 30 such as a computer, make it possible to manage sequences of acquisition of images of the surface of the molten metal, to store these images in memory. An operator can enter a setpoint signal via a line or input 38.

 In FIG. 2, the camera is connected to an image processing unit 56, provided with any appropriate processing program, and itself connected to a display screen 36.

 Solids on the surface of the metal can be detected in two ways.

 First, they can appear as large objects, especially when stacked in front of a burner. As long as the bath does not appear to have a flat horizontal surface, there remains solid matter to melt.

 In this case, the acquired images can be processed by a pattern recognition process, which compares the prerecorded intensities at a certain level of detection in order to obtain 2D images with high contrast allowing the detection of the shapes.

 These same shapes, corresponding to solids, generally also have dark surfaces which contrast with the reddish surfaces of the refractory material of the furnace and with the yellow and white flames which blaze.

 Therefore, in another aspect, these same shapes can be detected by using detection of color ratios, at different wavelengths.

 The image analysis is then independent of the overall intensity that reaches the camera. In addition, the temperature level and the intensity ratios being correlated; the temperature dependence of such ratios is monotonous, which allows unambiguous temperature detection.

 This treatment can also be used to detect oxidized areas on the surface of the bath.

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 Liquid aluminum reflects radiation, as it reflects heat.

 The slightly oxidized aluminum is dull, it absorbs heat.

 The strongly oxidized aluminum is covered with dross, or dross, or skeletons of alumina, which isolates it from the heat source.

 Each case therefore corresponds to an equilibrium of radiation and energy, as well as an equilibrium temperature of the furnace and the load.

 Each case therefore has a specific intensity spectrum.

 A modification of the spectrum can be appreciated by selecting 2 or 3 or n (n> 3) wavelengths for observation or analysis.

 At a certain temperature, a peak or maximum will be centered at a wavelength L1, the ratio L1 / L2 of the intensities at the wavelength L 1 and at another wavelength L2 being maximum, this ratio changing when passing from one temperature to another, and in particular when passing from a situation where the metal is liquid and not oxidized to a situation where it is oxidized.

 The processing of color ratios includes the steps of calculating, at the same point in space, intensity ratios at different wavelengths.

 More formally, the detection of color ratios can include the following steps: - taking images 11, 12, ... In at different times, each image Ik being taken at two different wavelengths, thus providing the minus two images Ik1 and Ik2; - take points M in each image Ik1, Ik2 and, for these points, calculate the intensity ratio at these points (lk1 (M) / lk2 (M)).

comprises entre 1, 5 et 1, 8 um ou entre 3 et 5 um ou entre 8 et 14 um. The wavelengths selected are preferably between 1.5 µm and 15 µm. Acquisition sequences allowing the acquisition of images at two or three wavelengths, for example

between 1.5 and 1.8 µm or between 3 and 5 µm or between 8 and 14 µm.

 Another example of a treatment method consists in comparing the shapes identified by shape analysis and the intensity ratios obtained by the second treatment.

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 This can for example be done to compare a new set of images to a set of reference images, in order to identify slow changes on the surface of the bath.

 For example, it is thus possible to compare temperature levels with initial reference levels.

 Such image analysis can be used as a basis for regulating a fusion process.

 The images obtained from the video camera 18 can be subjected to one of the treatments detailed above, then the result of this or these treatment (s) can be used as inputs to a regulation process or be a basis for decision for an operator.

 A possibility of regulation consists in reducing the power density of the burner 14 during the detection of the oxidation, the computer 30 giving for example a setpoint to the supply means 15 and 17.

 Thus it is then possible to switch from a high burn rate to a low burn rate and vice versa.

 For example, switching with a high combustion rate is carried out when solid materials are present, either in the form of a stack after loading by opening the door, or in the form of solid materials which float on a liquid pool during continuous loading. . The presence of these solids is indeed a sign of a temperature a little above the melting temperature (660 C), but at which the oxidation rate remains low and to which it is possible to heat without risk of oxidation. Switching at a low burn rate is achieved when the entire bath is in the liquid state.

 Other solutions can also be used, such as stopping burners or reducing the number of burners operating in an oven equipped with several burners, under the control of the computer 30 or an operator.

 Another possible control of the process is adjusting the flame length and the coverage of the surfaces carrying the solids. This result can be obtained by changing the kinetic energy of the burners or, in the case of combustion by oxygen, by incorporating a certain amount of air into the oxygen stream: the

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 heat transfer mode becomes rather of the convective type, and the flame length increases. Again, it is the computer 30 which will control, automatically or under the command of an operator, the supply or the operating conditions of the burner 14.

 Another possibility is to start stirring the bath with the aid of pumps 32 or by injecting a liquid when the fully liquid state is detected. The pumps 32 or the injection of liquid are controlled by the computer 30.

 In this case, the analysis of the color ratio can also indicate, according to the rules indicated above, whether the level of combustion power above the stirred bath can be increased or must be reduced.

 Another possibility lies in the modification of the stoichiometric ratio of the gases supplied to the burner 14, again by action on the means 15 and 17.

 When the analysis of the color ratio indicates that there are cold zones in the oven, we then seek to obtain in these zones a good thermal efficiency and a high power density: the stoichiometric ratio is adjusted so that it gives the maximum power, usually with a small excess of oxygen (oxygen content of the smoke between 0 and 2 to 3%).

 When the analysis of the color ratio indicates that there are hot zones in the oven, an attempt is made to obtain a reducing atmosphere in the oven: a burner with an oxygen-poor mixture gives a certain amount of CO and also reduces the amount of heat available.

 The analysis of the color ratio is a technique which gives indications relating to the surface for the detection of hot spots above the liquid metal, which constitutes an important improvement compared to the spot measurement localized with one or more thermocouples which are immersed in the bath at a fixed location a priori arbitrary.

 The variations in the reflectivity of the surface of the liquid bath, already explained above, can also constitute a useful indicator.

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 The selection of the optimal skimming time or instant can then be based on variations in the reflectivity of the surface.

 The optimization of the skimming frequency benefits the thermal efficiency (no cooling of the interior of the oven due to a door opening) and the metal efficiency (we remove the air access to the hot metal).

 Two other possible applications concern: - monitoring the walls of a furnace, whether or not it is a melting furnace, and whether or not there is molten metal in this furnace, - the conversion of a combustion furnace air to an oxygen combustion furnace; a comparison of the combustion conditions (location of hot spots, spatial distribution of the heat flow, coverage by the incident flame) between combustion in air and combustion in oxygen can indeed facilitate the adjustment of the new burner system .

 In both cases, the walls of the oven can be observed by the same video system as described above, and the same treatment, for example by analysis of color ratios, can be applied to distinguish the hot zones from the cold zones of the refractory wall.

 Pyrometers can also be used, but this is a more punctual measurement technique and requires averaging during operation, while video analysis gives a more global surface indication.

 Detection by color ratios can therefore be usefully applied to the detection, on the surface, of hot spots, the temperature of which exceeds for example 780 ° C., on the refractory lining of an oven. In this case, it is not the surface of an aluminum or metal bath which is displayed, but one or more walls of the oven, the image processing for the detection of hot spots being however the same in both. case.

 This technique is advantageous compared to the technique using a thermocouple located at a point in the refractory material. Maintenance is also easier because a video camera can be replaced much easier than a thermocouple buried under several refractory linings.

 Having this information, an operator can control, for example from the computer 30, the orientation of the burners, or

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 the flame pulse, or perform any operation on the burners already mentioned above.

The image processing used in the invention can be implemented using color ratio analysis software and / or shape recognition software, and / or using '' software for comparing shape surveys and color reports
All of this software can be stored in the computer 30, and be called by an operator, for example by selection from a drop-down menu.

Claims (29)

 (4), - making images of the surface of the bath, or of the interior surface of the walls (2) of the oven, - processing these images to identify the presence of solids or the formation of oxides on the surface of the bath, or to identify the presence, on the surface of the walls, of hot spots or hot zones or having a certain minimum temperature, - the modification of at least one operating condition of the oven as a function of the treatment carried out at previous step or the detection of solids or the formation of oxides on the surface of the bath, or the detection of hot spots or hot zones or having a certain minimum temperature.

 CLAIMS 1. Process for melting aluminum in a furnace, comprising: - introducing solid aluminum (20) into the furnace, - melting aluminum, to form an aluminum bath  2. Method according to claim 1, the images being processed by a pattern recognition method.  3. Method according to one of claims 1 or 2, the images being processed by a method of detecting color ratios.  4. Method according to claim 3, in which some of the images are collected at at least a first wavelength and at at least a second wavelength.  5. Method according to claim 4, said wavelength being between 1.5 and 15 pm.  6. Method according to claim 5, said length

 of waves being between 1.5 and 1.8 µm or between 3 µm and 5 µm. or between 8 and 14 µm. <Desc / Clms Page number 14>  7. Method according to one of claims 4 to 6, wherein at least one point is selected in the images taken at two wavelengths, and the intensity ratio at this point to the two wavelengths is calculated.  8. Method according to one of the preceding claims, wherein, when solids are detected, or when cold spots are detected on the surface of the bath or the walls of the oven, the power of at least one burner, or the burn rate, is increased.  9. Method according to one of the preceding claims, in which, when the formation of aluminum oxide is detected, or when hot spots are detected on the surface of the bath or the walls of the oven, the power of at least a burner, or the burn rate, is reduced.  10. Method according to one of claims 2 to 7, wherein, when the image processing indicates that there are cold zones in the oven or on the surface of the walls, the stoichiometric ratio is adjusted in order to provide increased power. .  11. Method according to one of claims 2 to 7, in which, when the image processing indicates that there are hot zones in the oven or on the surface of the walls, the stoichiometric ratio is adjusted in order to provide reduced power. .  12. The method of claim 1 to 7 wherein, after detecting the formation of aluminum oxides, stirring of the aluminum bath is carried out.  13. Method according to one of claims 1 to 7, wherein, following a detection of the formation of aluminum oxides, a skimming operation is carried out on the surface of the bath. <Desc / CRUD Page number 15>  14. Method according to one of claims 1 to 13, wherein the images are produced using a camera cooled by a gas or by an organic fluid.  15. The method of claim 14, the camera comprising optical lenses cleaned by a gas.  16. Device for detecting the surface condition of an aluminum bath (4) in an aluminum melting furnace (2), or the state of the interior walls of an furnace, characterized in that '' it comprises means (18) for producing images of the surface of the bath or of the interior walls of the oven, and for processing these images in order to identify the presence of solids or the formation of oxides on the surface of the bath, or the presence, on the surface of the walls, of hot spots or hot zones.  17. Device according to claim 16, the means for processing the images making it possible to process them by a pattern recognition process.  18. Device according to one of claims 16 or 17, the means for processing the images making it possible to process them by a method of detecting color ratios.  19. Device according to claim 18, comprising means for collecting images at at least a first wavelength and at at least a second wavelength.  20. Device according to claim 19, said wavelength being between 1.5 and 15 pm.  21. Device according to claim 20, said lengths

 of waves being between 1.5 and 1.8 µm or between 3 µm and 5 µm. or between 8 and 14 µm. <Desc / Clms Page number 16>  22. Device according to one of claims 18 to 21, comprising means for selecting at least one point in the images taken at two wavelengths, and means for calculating the intensity ratio at this point at the two lengths d wave.  23. Device according to one of claims 16 to 22 comprising means (30, 15, 17) for, when solid materials or cold spots are detected, increase the power of at least one burner, or its burn rate , or, when aluminum oxide formation or hot spots are detected, reduce the power of at least one burner, or its burn rate.  24. Device according to one of claims 16 to 23, further comprising means for adjusting the stoichiometric ratio of at least one burner, - in order to provide increased power, when the analysis by color ratios indicates that it there are cold zones in the oven, - or in order to provide reduced power, when the analysis by color ratios indicates that there are hot zones in the oven.  25. Device according to claim 16 to 24 further comprising means (30,32) for, following a detection of the formation of aluminum oxides, effect stirring of the aluminum bath.  26. Device according to one of claims 16 to 23, further comprising means for, following a detection of the formation of aluminum oxides, perform a skimming operation of the surface of the bath.  27. Device according to one of claims 16 to 26, the means (18) for producing images comprising a camera (46,50, 52) as well as means (40,44, 48) for supplying cooling gas from the camera. <Desc / Clms Page number 17>  28. Device according to the preceding claim, the camera comprising lenses (50) and means (40, 42, 48) for supplying cleaning gas to these lenses.  29. Aluminum melting furnace, comprising walls (2), burner means (14), and comprising a device according to one of claims 16 to 28.