The furnaces used in metallurgical processes typically have a crucible consisting of a refractory lining, composed of either bricks, blocks or monolithic refractories, with an adjacent outer shell or some other means of support for the refractory lining. Such furnaces hold a bath of molten metal or matte, usually with an overlying slag layer.
Due to the aggressive nature of many slags produced in metallurgical processes, cooling is required to freeze a layer of slag on the inner surface of the vessel to maintain a stable side wall. As discussed in Voermann et al., Furnace Cooling Design for Modern High Intensity Pyrometallurgical Processes (Proceedings at the Copper 99—Cobre 99 International Conference, Phoenix, Ariz., U.S.A.), the cooling required is dictated by the process conditions in the vessel. To keep the crucible lining in equilibrium, the process heat flux imposed by the process must be matched by the cooling system's heat removal capacity.
In practice, a wide range of heat fluxes are encountered in various metallurgical furnaces. Heat fluxes are dependent on the intensity of the process and whether the containment is for slag or metal. Heat fluxes can typically range from a low value of about 5 kW/m2, which can be removed by natural air cooling, to over 2,500 kW/m2, which requires intense forced water cooling. Generally, for heat fluxes in the lower range, about 15 kW/m2 or less, forced air cooling of the furnace shell plate can be used. For heat fluxes above about 15 kW/m2, some type of water cooling is generally adopted to avoid overheating of the furnace shell plate and structural members.
Due to the potential risk of an explosion in the event that molten material from inside the furnace contacts water in the cooling system, it is desirable to avoid using water as the cooling medium wherever possible. For this reason it may be desirable to use a furnace cooling system which does not use water as a cooling fluid. Although typical air cooling systems cannot match the heat removal capacity of water cooling systems, they have a wider operating temperature range and hence offer significant advantages in cooling applications where adjustable heat removal rates are required.
A number of furnace cooling systems are known in which gaseous media is used as the cooling fluid. For example, U.S. Pat. No. 5,230,617 (Klein et al.) discloses a cooling arrangement in which a number of metal shrouds encircle a cylindrical furnace. Each shroud forms a hollow cooling chamber through which air is circulated, and into which water is atomized to enhance the cooling effect. However, the introduction of water vapour into the system will complicate the cooling air supply system, and create corrosion problems that will impact material selection. Both of these issues will increase complexity and cost.
U.S. Pat. No. 1,674,422 to Allen, Jr. et al. discloses an air-cooled furnace wall in which cast hangers support refractory walls separated by air circulation spaces. U.S. Pat. No. 3,315,950 to Potocnik et al. discloses a heating chamber wall for a furnace, in which the wall has an interior space through which air is allowed to circulate. U.S. Pat. No. 3,777,043 (O'Neill) discloses an annular air circulation channel formed within the refractory furnace wall. U.S. Pat. No. 4,199,652 (Longenecker) discloses J-shaped channels formed between the refractory side wall and the metal outer shell of a furnace. U.S. Pat. No. 6,251,237 (Bos) discloses localized jets blowing directly onto the shell with variable flow for Hall-Heroult aluminum electrolytic pots.
In the above-mentioned patent to O'Neill, and in U.S. Pat. No. 1,751,008 (La France), the structure of the refractory furnace side wall is modified to provide increased surface area for enhanced cooling. In La France, this is accomplished by forming vertical ribs and channels in the outer surfaces of the blocks making up the refractory walls. In O'Neill, the annular cooling channels can be made in the form of “tortuous paths” by using bricks of varying lengths. While these techniques can help to enable increased heat removal capacity, it is extremely difficult to distribute the air evenly over the wall, a problem which worsens as the furnace ages due to shifting and movement of the brickwork. Also, the addition of air into the brickwork behind the shell plate would not be feasible in many furnaces since air would react with the furnace products, e.g. CO gas, metals, etc. For some applications this method is of limited value as it does not provide sufficient cooling capacity.
Thus, known gaseous media-cooling arrangements for metallurgical furnaces generally provide insufficient cooling and/or are unduly complex, requiring specially constructed furnace side walls. Such systems are also relatively expensive and cannot be practically adapted to existing furnace installations.