Metallurgical apparatus – Process
Patent
1992-12-28
1994-07-05
Kastler, Scott
Metallurgical apparatus
Process
266155, F27D 1700
Patent
active
053260813
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for cooling the exhaust gases from a molten phase furnace, such as a smelting furnace. The exhaust gases are conducted from the furnace via a vertical cooling shaft into a waste heat boiler, where heat is recovered from the gases either as saturated or superheated, pressurized steam. The steam is utilized for electricity generation.
The present invention is especially suitable for cooling of exhaust gases from smelteries, for example, from the melting processes of metal sulphides. It is also applicable to other processes in which hot, dirty gases have to be cooled and in which water-cooled surfaces may constitute a risk.
The exhaust gases from metal smelteries are typically hot gases of 1,100.degree. to 1,400.degree. C. containing solid particles, dust partly in a molten form, and gas components which condense to a solid phase when cooled to a temperature of, for example, 200.degree. to 400.degree. C.
To meet the environmental requirements, such gases usually have to be cooled to a sufficiently low temperature prior to further treatment. As smelteries use sulphide as a raw material and the sulphur contained therein is transferred to the gas phase as sulphur dioxide (SO.sub.2) in the oxidation stage after melting, the SO.sub.2 content of the gases normally rises significantly, to a level of 7 to 15% or even higher if air is replaced by oxygen in the melting stage.
A conventional gas treatment of such a process comprises the steps of sometimes superheated steam and electrofilter. The gas cleaned of solids is then conveyed to a sulphuric acid plant in which SO.sub.2 contained in the gas is used as a raw material. The steam boiler is used because it facilitates electricity generation for the smeltery by means of a steam turbine. Usually, electricity is generated in excess and the surplus is sold.
Most smelting processes of metal sulphides employ a smelting furnace structure in which the easiest and simplest way of discharging the exhaust gases is to lead them upwards and to discharge them through an opening in the furnace roof. For example, Finnish patent specification 65632 and U.S. Pat. No. 4,087,274 propose smelting furnaces where the exhaust gases are discharged through an opening in the furnace roof.
However, this arrangement involves a risk if the steam boiler and the first heating surfaces thereof are disposed above the smelting furnace, extending straight upwards form the opening in the furnace roof. Bursting of a boiler tube in such a structure and the leak of water caused thereby would expose the smelting furnace to a danger of explosion, as water spraying from the leaking point automatically comes into contact with the molten material in the furnace.
A boiler arranged above a furnace could be provided with a superheater, i.e. with steam not water cooled heating surfaces. In that case, the portion above the furnace would constitute a superheater and the dangerous evaporating surfaces containing boiler water could be located farther off. However, this is impossible in practice for the following reasons: particles stick to the heating surfaces, whereby the surfaces tend to become clogged and the heat transfer decreases remarkably. A rise in the surface temperature makes the phenomenon still worse. Therefore, the heating surfaces in such boilers are usually constructed to give an as high cooling effect as possible while generating saturated steam. They are not constructed as hot superheater surfaces. If necessary, the steam generated in these boilers is superheated in a separate superheating boiler arranged in front of the steam turbine. superheating is too low, when compared with the thermal energy for evaporation, to provide a cooling effect by superheaters only in that portion of the boiler which is arranged immediately above the furnace. A steam pressure exceeding 100 bar would, on the other hand, result in the temperature of the evaporating surfaces rising too high in view of cleaning.
A conventional boiler arrangement used in these smelteries
REFERENCES:
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patent: 4087274 (1978-05-01), Edenwall et al.
patent: 4114862 (1978-09-01), Denjean
patent: 4273074 (1981-06-01), Kuhlmann
patent: 4362129 (1982-12-01), Banfi et al.
patent: 4475947 (1984-10-01), Andersson
patent: 4541864 (1985-09-01), Ratschat
A. Ahlstrom Corporation
Kastler Scott
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