Methods for vessel heat management

Abstract

A method is provided for optimizing a heating process for a hot refractory vessel prior to tapping liquid molten metal from a furnace into the refractory vessel. The method comprises taking a first temperature measurement of a refractory lining of an empty vessel; waiting for a period of time; taking a second temperature measurement of the refractory lining of the empty vessel; measuring the thickness of the refractory lining; determining the heat energy content of the refractory vessel; modifying a heating parameter of a heating act based on the determined heat energy content of the refractory vessel for heating the refractory vessel; and heating the refractory vessel based on the modified heating parameter.

Description

[0001] METHODS FOR VESSEL HEAT MANAGEMENT

[0002] Field of Inventions

[0003] The inventions are generally related to methods for optimizing operation in metallurgical processes. More specifically, the inventions are related to methods for heating refractory lined vessels used in metallurgical processes.

[0004] Background

[0005] In metallurgical arts, furnaces are used to melt metals such as, for example, iron, steel, silver, lead, copper, and aluminum. Molten metal is then tapped into a series of ladles from the furnaces. After undergoing ladle treatments, such as alloying and degassing, the ladles are transported, in series, to a casting machine. For continuous casting one ladle is in the “on-cast” position (that is feeding the casting machine) while the other ladles are waiting in the “off-cast” position and are sequentially switched to the on-cast position once the lead ladle is emptied. The hot metal is transferred via a refractory shroud or pipe to a holding bath or a “tundish.” The tundish allows a reservoir of the metal to feed the casting machine while ladles are switched, thus acting as a buffer of hot metal, smoothing out flow, regulating metal feed to the molds, and cleaning the metal. Once a ladle is emptied, the ladle is transported back to the furnaces for refill, and the production cycle is repeated. As a part of the production cycle, the ladle can also undergo maintenance and repair prior to being refilled by the furnace.

[0006] Ladles heat up when filled in the liquid molten metal. Heat is absorbed from the metal by the ladle’s refractory lining. During off-cast and when the ladle has been emptied, the ladle cools down. The degree that the ladle cools down is based on the duration in which the ladle is empty and subject to ambient temperature and the duration in which the ladle must wait in the in off-cast position. The thermal state of a ladle is important. The use of a cooled ladle will cause a relatively high loss of liquid metal temperature. Prior to tapping liquid molten metal from a furnace into a ladle, it is critical for the temperature of the ladle to be at or above a threshold temperature. Having the ladle at its operating heated temperature, minimizes thermal stresses exerted on the ladle refractory when liquid molten metal is poured into an otherwise cooled ladle. Moreover, unwanted temperature loss of the liquid molten metal caused by a cooled ladle or the cooling of the ladle during transport from the furnace to the cast is curtailed or prevented. Casting can only be performed in defined narrow temperature range; thus once the ladle reaches casting, it is important for the molten metal not to be at a temperature below the requisite casting temperature. The cooling of the molten metal caused by a ladle that is not at its operating temperature can adversely affect the fluidity of the liquid metal for casting and will not allow for any compensation for heat losses before the metal is poured into a mold cavity. Accordingly, a reheating operation has been employed prior to tapping of the molten metal from the furnace into the ladle to optimize the operating temperature of the ladle that has otherwise cooled down during off-cast, transportation back to the furnace, and / or maintenance and repair, or to otherwise “normalize” the heat lost by the ladle.

[0007] Currently, a thermocouple, pyrometer or thermo-camera have been employed to measure the temperature of the ladle refractory and based on this temperature measurements the parameters for the reheating process is determined, including reheating temperature and duration. This method does not account for how much thermal energy is stored by the ladle. The amount of stored thermal energy can significantly affect the parameters of the reheating operation, as well as other operation parameters such as duration of off-cast or the conveyance time of the ladle to and back from the cast. Ladles that have a higher stored thermal energy need not be subjected to the same heat treatment, both in duration and intensity, as compared to ladles that have a relatively lower heat saturation and need not require a shorted off-cast time. To minimize cost and maximize production efficiency, it will be advantageous to derive the heat saturation profile of the ladle and to modify and improve manufacturing based on the degree to which the ladle stores thermal energy. These, and other advantages of the inventions will be apparent from the descriptions provided herein.

[0008] Summary

[0009] In accordance to one embodiment, a method is provided for optimizing a heating process for a hot refractory vessel prior to tapping liquid molten metal from a furnace into the refractory vessel. The method comprises taking a first temperature measurement of a refractory lining of an empty vessel; waiting for a period of time; taking a second temperature measurement of the refractory lining of the empty vessel; measuring the thickness of the refractory lining; determining the heat energy content of the refractory vessel; followed by modifying a heating parameter of a heating act based on the determined heat energy content of the refractory vessel for heating the refractory vessel; and heating the refractory vessel based on the modified heating parameter.

[0010] In one embodiment, the waiting period is 10 seconds to 5 minutes. In one embodiment, the heating is conducted by a fuel gas flow into the vessel, and the modifying the heating parameter comprises optimizing the temperature and duration of the fuel gas into the vessel. In one embodiment, the modifying the heating parameter includes modifying a temperature of the liquid molten metal tapped into the refractory vessel from the furnace. In one embodiment, the heating is conducted by an operation selected from a group consisting of oxygen blowing, electrical heating energy input in an electric arc furnace, and by adding an alloy for chemical exothermal reaction.

[0011] In one embodiment, the method additionally includes determining the heat energy content of refractory vessel after heating of the refractory vessel followed by modifying a temperature of the liquid molten metal tapped into the refractory vessel from the furnace.

[0012] In accordance with one embodiment, a method of optimizing an operational parameter for continuous casting of molten metal is provided, comprising: measuring the enthalpy of a refractory vessel after molten metal is poured into a cast; and modifying and / or applying an operational parameter based on the measured enthalpy of the refractory vessel. In one embodiment, the operational parameter is selected from conveyance route of the refractory vessel, heat treatment of the refractory vessel prior to pouring molten metal into the refractory vessel from a furnace, temperature of the molten metal that is tapped from a furnace into the refractory vessel, and time allocated to the maintenance of the vessel. In one embodiment, the optimization of the heat treatment comprises duration of the heat treatment, applied temperature of the heat treatment, and / or the temperature profile (i.e., temperature overtime) for the heat treatment. In one embodiment, the heat treatment comprises an operation conducted by application of fuel gas, oxygen blowing, electrical heating, chemical exothermal reaction, or a combination thereof. In one embodiment, the heat treatment is conducted by increasing the temperature of the molten liquid metal tapped into the refractory vessel from a furnace. Descri ption

[0013] The inventions are related to methods for optimizing operation of vessels used in metallurgical processes. Some described embodiments of the inventions are directed to methods for optimizing heat management of refractory vessels used in metallurgical processes. In some embodiments, the inventions are related to methods for heating refractory vessels used in metallurgical casting. Heating can include preheating or reheating of the refractory vessel. Preheating can include heating the refractory vessel before performing another process, operational act, or manufacturing step. Reheating can include applying heat to increase or elevate the temperature of the refectory vessel that was previously heated but has cooled down to a lower temperature or to below a desired temperature. In some embodiments, the methods disclosed herein are directed to heating of refractory lined vessels prior to disposing molten metal from a furnace into the vessel. Heating can be conducted by, for example, by gas-fire burner, electrical power heating, chemical induced reaction, and / or increasing temperature of the melt that is tapped from a furnace into a refractory lined vessel.

[0014] In one embodiment, the vessel can be a container and / or transporter of a molten metal or a non-metallic material. The vessel can be, for example, a steel shell or receptacle. In one embodiment, the vessel is a ladle or a converter. The ladle or converter can be one used in metallurgy processes. Transport can be, for example, from a furnace to a secondary treatment processing station or to a forming operational site, such as continuous casting. The nonmetallic materials can include any chemical that is being subjected to high or elevated temperatures. Continuous casting requires several vessels that are rotationally used to carry the material from the furnace to the cast. Vessels can be used to contain and transfer molten metal from CONARC® furnaces, Basic Oxygen Furnaces (BOFs), Electric Arc Furnaces (EAFs), foundry furnaces, Submerged Arc Furnaces (SAF), and torpedo-ladle-cars and bottom blown furnaces (Q-BOP's).

[0015] The inside surfaces of vessels (e.g., ladles) are lined with a refractory material for the protection of the vessels against high temperatures of the molten metal (or non-metallic materials). For example, an inner safety of refractory lining (e.g., ladles 8 to 12 cm, converters 20 to 25 cm in thickness) covered by an outer layer of the same or different refractory material (e.g., ladles 18 to 25 cm, converters 80 to110 cm in thickness) covers the inner side of the vessel. Commonly used refractory materials include, by way of example, magnesia carbon brick, high alumina brick, aluminum magnesia carbon brick, aluminum magnesium castable, zircon refractory brick, and ceramic fiber. With use, the refractory linings are subject to wear and deposits caused by the molten metal. Accordingly, adequate maintenance and servicing of the refractory lining is needed to achieve continuous, efficient, and safe operation of the receptacle, and production of a high-quality material. When the refractory lining’s thickness degrades below an acceptable threshold depth, or when there is an overgrowth of deposits, the refractory lining is replaced, repaired, or replenished. Inspection of the refractory lining should be carried out when the vessel is at or near operating temperature to minimize operational costs that would otherwise be associated with cooling of the vessel before inspection of the lining. For the safety of steel workers and to increase manufacturing efficiency, laser scanning devices (e.g., high-speed lasers) have been used for obtaining and determining refractory thicknesses of hot vessels. Laser scanning devices can also be used for taking the surface temperature or the temperature of the refractory lining by detecting the thermal radiation. Laser scanning devices can produce high- resolution temperature measurements of the hot inner surface of vessels. In some embodiments, temperature pyrometer or thermal imaging cameras can be used to measure the surface temperature or the temperature of the refractory lining.

[0016] Prior to tapping liquid molten metal from a furnace into a vessel, it is critical for the temperature of the vessel to be at the vessel’s operating temperature and for the vessel’s temperature to be uniform. Having the vessel at its operating temperature and with a uniform temperature distribution minimizes thermal stresses exerted on the vessel refractory when liquid molten metal is poured into an otherwise cooled vessel. Moreover, unwanted temperature loss of the liquid molten metal caused by a cooled vessel is curtailed or prevented (e.g., melt temperature loss by heat transfer to refractory). The cooling of the molten metal can adversely affect the fluidity of the liquid metal for casting and will not allow for any compensation for heat losses before the metal is poured into a mold cavity. Accordingly, a heating, preheating or reheating operation can be employed, prior to transferring the liquid molten metal from the furnace to the vessel, to optimize the operating temperature of the vessel that has otherwise cooled down during off-cast, transportation back to the furnace, and / or maintenance and repair, or to otherwise “normalize” the heat lost by the vessel.

[0017] When molten metal is poured into the vessel, the vessel is heated by thermal energy from the melt. This process increases the existing heat content. A full heat saturation will be achieved after several hours of melt contact or several production cycles. The thermal energy storage of vessels can be based on variables including the temperature of the molten metal used; the size, dimensions and the material from which the vessel is made; the material from which the refractory lining is made; the mass, volume, and thickness of the refractory layer; the change in refractory thickness or the amount of refractory lining that is lost (i.e. , thermal mass of the vessel changes when the working refractory lining is lost); and the number of refractory layers. Based on the amount of thermal energy stored, the heating, preheating, or reheating of the vessel can be optimized. In accordance with the embodiments of the present inventions, the thermal energy stored in the vessel is used as a variable in determining the operating or process conditions for heating, preheating, or reheating.

[0018] As used herein, enthalpy, heat energy content, thermal or heat energy saturation, and stored thermal energy are terms which are used interchangeably and refer to the total heat content of the vessel or heat “stored” in the vessel when in an empty state and at the time of measurement.

[0019] When the vessel is empty - that is, after pouring the molten metal into the cast and prior to molten metal being poured from the furnace into the vessel for the start of a new cycle (which can be referred to as the “cooling down” period) -- the surface temperature or the temperature of the refractory lining of the vessel is measured. This is followed by a wait period. The wait period can be greater than or equal to 10 or 20 seconds. In one embodiment, it can be less than or equal to 15 minutes, preferably less than or equal to 10 minutes, more preferably less than or equal to 5 minutes. In one embodiment, the wait period is about 2 minutes, about 3 minutes, or about 2 to 3 minutes. During this wait period the thermal energy stored in the refractory decreases continuously by thermal radiation and heat transport by ambient air. Next, the surface temperature or the temperature of the refractory lining is taken again and the difference in temperature between the first and second measurements is ascertained (AT = absolute value of T2 - T1). In addition to taking the temperatures at two different points of time, a laser device is used to measure the thickness of the refractory lining for determining the thermal mass or volume of the refractory lining. AT is mainly dependent on physical values such as the temperature of the inner surface of the refractory lining or the refractory temperature and the enthalpy of the refractory lining. The thermal energy saturation, enthalpy, stored heat energy, or the total heat content of the vessel is determined based on the first temperature measurement, the second temperature measurement (or AT), and the thermal mass of the refractory lining (e.g., lining thickness). The heat energy saturation level is inversely correlated to the loss of melt temperature. In other words, a vessel with low heat energy saturation will decrease the melt temperature more than a vessel with relatively higher heat energy saturation. Based on the calculated thermal energy saturation, the operating or process parameters of heating, preheating, or reheating of the vessel is determined, optimized, and applied, including duration of heating, the temperature of the applied heat, and the temperature profile (i.e., change in temperature over time). Once heated to its designated temperature, the vessel is ready to receive molten metal from the furnace for the start of a new production cycle. This process can be conducted after any needed maintenance to the vessel, including maintenance to the refractory lining.

[0020] In one exemplary embodiment, a gas-fired burner is used in inject a flame into the interior of the vessel when the vessel is positioned on a heating, preheating, or reheating stand. In another exemplary embodiment, an electrical power heater can be used to heat, preheat, or reheat the vessel. For a gas-fired burner, a fuel gas is supplied to a vessel. During an initial heating time period, when the vessel is at its coolest stage, the fuel gas is supplied at a relatively high rate, followed by gradually decreasing the fuel rate when the vessel temperature gradually increases or after a set point temperature is reached. Application of the gas-fired burner continues until the selected operating temperature of the vessel is reached. In some embodiments, heating, preheating, or reheating is via converters by oxygen blowing, prolonging electrical heating energy input in EAF, and / or by adding an alloy for chemical exothermal reaction (e.g., addition of alumina).

[0021] Based on the methods of ascertaining the heat energy saturation of the vessel, the parameters of the heat, preheat, or reheat operation can be optimized, and heat applied, including the amount and rate of fuel gas or electrical input energy applied and the duration of the heating, preheating, reheating process, that otherwise could take as long as 24 hours to complete. The parameters of the exothermal chemical reaction can also be optimized. Additionally, the over-heating of the vessel can be avoided. Overheating of vessels can result in refractory damage and costly repairs.

[0022] Based on the heat energy saturation profile of the vessel, other operational parameters, other than or in addition to the heating operation parameter, can be modified. Such operational parameters can include altering the melt tapping temperature from the furnace into the vessel, the conveyance route of the vessel, the duration of the cast-off period, and / or the time period allocated for maintenance, including repairing or replenishing the refractory lining. For example, for vessels that maintain a relatively lower amount of heat saturation or storage, the conveyance route or the cast-off period can be shortened to minimize or prevent any undesirable cooling of the liquid metal.

[0023] In one embodiment, the thermal energy of the refractory vessel can be measured for a second time after the heating, preheating, or reheating treatment in the same manner described above. Based on this measurement, the sufficiency of the final casting temperature can be achieved by adjusting the melt tapping temperature to the thermal energy of the vessel in which the melt is being tapped from the furnace and into the vessel.

[0024] The method of deriving thermal energy from two subsequent temperature measurements can be based on the fundamental heat conduction equation. Model solutions of this differential equations can be computed based on known boundary conditions, such as heat loss by thermal radiation at the inner and outer vessel surface, and known refractory material parameter like thermal conductivity, density, and specific heat capacity. The local measurement of refractory lining thickness is essential to complete the set of parameters. The model allows simulation of the local dynamics of heat transfer especially through the inside vessel surface, on which the subsequent temperature measurements have been made. The dynamic output of the model, i.e. the development of local surface temperature by time, is directly depending on the start condition, i.e., the initial temperature measurement and the assumed temperature distribution along the refractory thickness. A linearly increasing temperature profile, from vessel outside towards vessel inside, is connected to high heat saturation grade, whereas a low heat saturation corresponds to a temperature profile, where only a small portion close to the inner surface is at high temperature level. The start temperature profile of the lining can be varied between high or low heat saturation. For each point of the surface, the dynamic model is able to forecast the dynamics of temperature drop, depending also on local lining thickness. With the measured time difference between the two temperature measurements, the local temperature drop can be related to the start temperature profile or heat saturation grade. Because of the monotonically decreasing dependence between local heat saturation grade and local drop of surface temperature, obtained by the two subsequent measurements, the heat saturation heat energy content can be computed. The local results can be accumulated to full heat energy content of the vessel refractory. The known heat saturation allows estimation of the necessary amount of heat transfer to be supplied at the heating, preheating, or reheating station. Thus, the necessary duration of heat treatment (e.g., fuel gas) can be controlled. Moreover, if the measurements are made after heating and right before tapping, the loss of melt temperature can be estimated by the known heat saturation. For both purposes, the same model can be used, but with different boundary conditions at the vessel inside surface: Estimation of gas amount - heat energy transport into lining surface by heated atmosphere; and estimation of melt temperature loss - heat energy transport into lining surface by direct melt contact.

[0025] While several particular forms, variations, and embodiments of the inventions have been illustrated and described, it will also be apparent that various modifications can be made without departing from the scope of the inventions. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the inventions.

Claims

Claims1. A method of heating a hot refractory vessel prior to tapping liquid molten metal from a furnace into the refractory vessel, comprising:(a) taking a first temperature measurement of a refractory lining of an empty vessel;(b) waiting for a period of time;(c) taking a second temperature measurement of the refractory lining of the empty vessel;(d) measuring the thickness of the refractory lining;(e) determining the heat energy content of the refractory vessel based on steps (a), (c), and (d); followed by(f) modifying a heating parameter of a heating act based on the determined heat energy content of the refractory vessel for heating the refractory vessel; and(g) heating the refractory vessel based on the modified heating parameter.

2. The method of claim 1 , wherein the waiting period is 10 seconds to 5 minutes.

3. The method of claim 1 , wherein the heating is conducted by a fuel gas flow into the vessel, and the modifying the heating parameter comprises optimizing the temperature and duration of the fuel gas into the vessel.

4. The method of claim 1 , wherein the modifying the heating parameter includes modifying a temperature of the liquid molten metal tapped into the refractory vessel from the furnace.

5. The method of claim 1 , wherein heating is conducted by an operation selected from a group consisting of oxygen blowing, electrical heating energy input in an electric arc furnace, and by adding an alloy for chemical exothermal reaction.

6. The method of claim 1 , additionally including determining the heat energy content of refractory vessel after heating of the refractory vessel followed by modifying a temperature of the liquid molten metal tapped into the refractory vessel from the furnace.

7. A method of optimizing an operational parameter for continuous casting of molten metal, comprising:(a) measuring the enthalpy of a refractory vessel after molten metal is poured into a cast; and(b) applying or modifying an operational parameter based on the measured enthalpy of the refractory vessel.

8. The method of claim 7, wherein the operational parameter is selected from conveyance route of the refractory vessel, heat treatment of the refractory vessel prior to pouring molten metal into the refractory vessel from a furnace, temperature of the molten metal that is tapped from a furnace into the refractory vessel, and time allocated to the maintenance of the vessel.

9. The method of claim 8, wherein the optimization of the heat treatment comprises duration of the heat treatment, applied temperature of the heat treatment, and / or the temperature profile for the heat treatment.

10. The method of claim 9, wherein the heat treatment comprises an operation conducted by application of fuel gas, oxygen blowing, electrical heating, chemical exothermal reaction, or a combination thereof.11 . The method of claim 9, wherein the heat treatment is conducted by increasing the temperature of the molten liquid metal tapped into the refractory vessel from a furnace.