Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B21/00—Obtaining aluminium
  • C22B21/0084—Obtaining aluminium melting and handling molten aluminium

Definitions

• the invention relates to a method to control a process for heating or melting a metal, in particular aluminium, comprising heating said metal in a fuel-fired furnace wherein a fuel is combusted with an oxygen containing gas, and measuring the concentrations of carbon dioxide and oxygen in the furnace atmosphere.
• the invention relates to the field of heating or melting of metals in fuel-fired furnaces.
• Liquid or gaseous hydrocarbon containing fuels may be used.
• the heating or melting process is carried out in rotary or reverberatory furnaces.
• the process may be continuous or a batch process.
• the material to be melted for example scrap or ingots, is charged through large doors into the furnace. Typically a furnace is charged two or more times during a process cycle.
• metal losses occur essentially due to the following phenomena: A part of the losses originates from direct oxidation of the metal with the furnace atmosphere. A second part of the metal losses comes from metal that is entrapped between the metal oxides formed through direct oxidation.
• the oxidation of aluminium is temperature dependent.
• the rate of oxidation increases with increasing temperature, especially at temperatures above 780 0 C the oxidation increases rapidly.
• the temperature of the metal charge and of the metal melt is homogenized by rotation of the furnace in order to avoid overheating.
• mechanical or electromagnetic stirrers are installed to get a more uniform heat distribution within the furnace.
• the dross layer comprises aluminium oxide which has a high melting point.
• the dross layer will not melt further, but functions as a heat insulator. If it is allowed to grow too much, it will insulate the metal melt from the burner flame. The dross will be more heated and more metal will be oxidized.
• thermocouple An alternative is to submerge a thermocouple into the melted metal. However, this is only a local indication and it does not give any information as to hot spots on other locations. Monitoring of the temperature is thus not a sufficient means to monitor how the metal oxidation proceeds.
• This object is achieved by a method to control a process for heating or melting a metal, in particular aluminium, comprising:
• the CO 2 concentration in the furnace atmosphere is measured. Then equation (1) allows to calculate the theoretical O 2 concentration in the furnace atmosphere.
• a reducing substance such as a metal or an organic material
• some oxygen will be consumed and the oxygen content in the furnace will be reduced.
• the O 2 concentration in the furnace atmosphere is measured and compared to the theoretical O 2 concentration. The difference between both values is an indicator for the amount of metal or material that has been oxidized.
• the O 2 concentration and the CO 2 concentration can be determined by direct measurement or detection of the respective concentrations within the furnace or, according to a preferred embodiment, the O 2 concentration and the CO 2 concentration are measured in the flue gas stream. More preferred a sample is taken from the flue gas stream and then analyzed in order to determine the O 2 concentration and the CO 2 concentration.
• reaction (3) By simultaneous analysis of the concentrations of carbon dioxide, carbon monoxide, and hydrogen the inventors could show that reactions (3) and (4) can be neglected compared to reaction (2). It can be concluded that in case aluminium is charged into the furnace the amount of oxygen deviating from the oxygen concentration calculated from equation (1) is mainly consumed by reaction (2). Thus, the amount of aluminium oxidized is proportional to the deviation of the measured oxygen content from the oxygen content given by equation (1).
• the invention utilizes this insight to control an aluminium melting process.
• the content of oxygen in the furnace atmosphere is detected several times and the relative amount of aluminium oxide is determined from the difference between the detected oxygen concentration and the theoretical oxygen concentration. This information is used to regulate and / or control the melting process, for example by changing the burner power.
• Reaction (5) would be dominating as long as free O 2 is present in the furnace atmosphere.
• the invention could be applicable for the heating of steel or steel alloys..
• the heating or melting process is controlled without using the temperature of the flue gases or the temperature in the furnace. It is further preferred that the control of the melting process is not based on carbon monoxide measurements or on measurements of the hydrogen content in the furnace atmosphere or in the flue gases. It is especially advantageous to base the control of the heating or melting process on the difference between the theoretical O 2 concentration and the measured O 2 concentration, only.
• the oxygen concentration and the carbon dioxide concentration are continuously detected.
• the measured oxygen concentration will be essentially equal to the theoretical oxygen concentration calculated from the measured CO 2 concentration. With increasing temperature, at least at some local spots metal will be oxidized.
• said furnace is heated by one or more burners. Further it is preferred to measure the amount of fuel supplied to the burner(s). If the fuel flow is measured the absolute amount of CO ⁇ , for example the mass of CO 2 in kg, can be calculated from the chemical reaction equation. Further, that information allows to calculate the absolute amount of O 2 which has been consumed by oxidation of the metal in the furnace. That is, the absolute difference, for example in kg, between the theoretical oxygen content and the measured oxygen content can be given.
• the amount of oxidized metal is calculated using the absolute amount of oxygen consumed in that oxidation reaction and the formula weight of the metal oxide, for example the formula weight of aluminium oxide AI 2 O 3 .
• the metal may also be oxidized by H 2 O and CO 2 but the inventors could show that the oxidation with oxygen is dominating in an industrial furnace.
• the oxygen and the carbon dioxide concentration are detected in the flue gases.
• a flue gas analysis provides a direct information on the composition of the atmosphere within the furnace. For practical reasons it is preferred to determine the oxygen and the carbon dioxide content in the furnace atmosphere from a measurement in the flue gas duct.
• the measurement of the oxygen concentration can be carried out by any equipment for analyzing oxygen.
• a laser especially a diode-laser, is used to analyze the oxygen concentration.
• the so determined metal oxidation rate is used to control the heating or melting process.
• the heating or melting process is controlled by changing the power of the burner or of the burners which are used to heat the furnace and its charge.
• the amount of oxygen supplied to the burner is changed in order to influence the heating or melting process. For example, it may be switched from oxygen burners to air burners or vice versa.
• several charges of metal are melted in said melting furnace and for each charge the difference between the theoretical and the measured oxygen concentration is determined. These data are then stored, for example in a computer memory. By varying different process parameters or exchanging part of the furnace equipment and recording new curves, these new curves can be compared to the stored curves. The comparison of the new curves with the stored curves allows to further optimize the heating or melting process. In addition these data can be used for training of personnel operating the furnace.
• the invention is used to monitor the combustion of organic contaminants on the metal charge. For example, if the metal charged into the furnace is contaminated by organic matter, such as oil, lacquer, or plastics, these materials are evaporated and combusted and oxidized inside the furnace. This oxidation will also create a difference between the calculated and the measured oxygen in the flue gas or in the furnace atmosphere. The oxidation of the organic matter can then be studied in the same way as the oxidation of the metal. When oxidation of organic matter is detected, it can be controlled by adding excess oxygen to the furnace.
• organic matter such as oil, lacquer, or plastics
• the evaporation of organic matter dominates at the beginning of the process at temperatures below 500 0 C, especially between 400 and 500 °C.
• the oxidation of the metal dominates later in the process when the metal is at higher temperatures, especially above the melting point of the metal.
• the oxidation increases at temperatures above the melting point at 660 0 C and it may increase rapidly at temperatures above about 780 0 C.
• the oxidation starts to be significant above 900 0 C.
• the invention shows either oxidation of organic matter or oxidation of metal, but not the two at the same time.
• the process it is of interest to study oxidation of organic matter and at what part of the process it is of interest to study oxidation of the metal.
• the invention has several advantages compared to the state of the art technology.
• the inventive method provides a signal showing the oxidation rate of a metal, in particular of aluminium, that is independent from the amount of leak air entering the furnace.
• the inventive method is more reliable than methods based on flue gas temperature measurements or based on carbon monoxide measurements.
• the invention provides a method which is very appropriate for industrial furnaces, in particular for rotary furnaces and reverberatory furnaces used for heating or melting of metals.
• the user of the invention will be able to have a better process control and hence will be able to decrease the aluminium losses and to get a higher metal yield.
• the inventive method is easy to implement.
• the invention is in particular useful to control a process for melting aluminium.
• figure 1 an aluminium melting furnace with the equipment to carry out the inventive control method and figure 2 the on-line flue gas analysis measured with the arrangement according to figure 1.
• Figure 1 shows an aluminium melting furnace 1 of the rotary type.
• the aluminium melting furnace 1 has been charged with aluminium scrap 2.
• Melting furnace 1 is heated with an oxy-fuel burner 3 which can be supplied with fuel, oxygen and / or air.
• the amount of fuel, oxygen and air provided to burner 3 is regulated by flow control valves 4 and can be measured by flow measurement means 5.
• Burner 3 generates a burner flame 6 which heats the aluminium charge 2.
• the flue gases 7 which are produced during the heating and melting of charge 2 leave the furnace 1 through a flue gas duct 8.
• Flue gas duct 8 is provided with an oxygen analyzer 9 and a carbon dioxide analyzer 10.
• Oxygen analyzer 9 and CO 2 analyzer 10 provide signals 11 , 12 which are proportional to the concentration of oxygen and carbon dioxide in the flue gases 7. These signals are sent as input to a process computer 13. From the flow measurement means 5 process computer 13 further receives input signals 14, 15, 16 proportional to the measured flow of fuel, oxygen and air, respectively. Any of the data 11 , 12, 14, 15, 16 can be shown on a computer monitor 17. Computer monitor 17 is also used to visualize the analysis of the data 11 , 12, 14, 15, 16.
• Process computer 17 calculates from the data 11 , 12, 14, 15, 16 a signal 18 which is used to control the melting process by varying the flow of fuel, oxygen, and / or air supplied to burner 3. These calculations are made on-line and can be shown on computer monitor 17 in a real time graph.
• CO 2 analyzer 10 continuously measures the CO 2 concentration in the flue gas stream 7. The measured values are sent to process computer 13 and are recorded. For example, every minute one measured value is recorded. By using equation (1) process computer 13 calculates the theoretical oxygen concentration for every measured CO 2 value. Thus, for every minute a measured CO 2 concentration and the corresponding theoretical O 2 concentration is recorded.
• Oxygen analyzer 9 continuously measures the O 2 concentration in the flue gases. The measured values are also stored every minute in the process computer 13.
• the measured oxygen value and the theoretical oxygen value should be equal.
• the furnace 1 contains aluminium and when this aluminium starts to oxidize, the oxidation of aluminium will consume some of the oxygen in the furnace atmosphere.
• the measured oxygen concentration will then be lower than the theoretical oxygen concentration.
• the difference between both values is an indication of aluminium oxidation. This difference is also calculated and stored in process computer 13.
• Figure 2 shows a typical graph recorded by process computer 13. At 14:50 a rapid increase in aluminium oxidation is detected and this information is used to control the melting process by changing the burner power.
• the inventive method is independent of leak air into the furnace, since the influence of leak air variations is compensated by repeating the calculation according to equation (1) for every measurement - in the example above, every minute. Of course the data can be calculated more or less frequent than every minute.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Organic Chemistry (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Manufacturing & Machinery (AREA)
• Waste-Gas Treatment And Other Accessory Devices For Furnaces (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Muffle Furnaces And Rotary Kilns (AREA)
• Gasification And Melting Of Waste (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)
• Liquid Deposition Of Substances Of Which Semiconductor Devices Are Composed (AREA)

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• 2005
  • 2005-11-29 AT AT05026027T patent/ATE404703T1/de active
  • 2005-11-29 EP EP05026027A patent/EP1790738B1/de not_active Not-in-force
  • 2005-11-29 DE DE602005008994T patent/DE602005008994D1/de active Active
• 2006
  • 2006-11-17 WO PCT/EP2006/011062 patent/WO2007062753A1/en active Application Filing
  • 2006-11-17 US US12/095,018 patent/US20090218736A1/en not_active Abandoned
  • 2006-11-17 BR BRPI0619375-7A patent/BRPI0619375A2/pt not_active IP Right Cessation

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