Chemistry of inorganic compounds – Sulfur or compound thereof – Oxygen containing
Reexamination Certificate
1998-11-23
2001-01-30
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Oxygen containing
C423S542000
Reexamination Certificate
active
06180078
ABSTRACT:
The present invention relates to a method for thermally regenerating spent acid, said spent acid being formed in connection with the scrubbing of sulfur dioxide bearing exhaust gases created in the pyrometallurgical production of metals. According to the method, the spent acid is concentrated and fed into the gas space of a primary smelting furnace, in which case the energy needed in the thermal regeneration of the spent acid is obtained from the heat content of the hot gases created in the smelting process.
The hot gases created in the smelting processes of sulfidic ores and concentrates contain mainly sulfur dioxide, heavy metals, arsenic, halogens and other compounds. It is important that the concentrate burns as completely as possible, and that the oxygen content of the exhaust gases created in the smelting process is as low as possible, because a high oxygen content in the exhaust gases leads to an increased formation of sulfur trioxide and further to the creation of spent acid in connection with the gas scrubbing. It has been found out that dust, particularly copper-bearing dust, catalyses the formation of SO
3
. Most probably SO
3
is created in the waste heat boiler, and in order to prevent this, it is essential that the oxygen content of the exhaust gases is low and that the amount of exhaust air is as small as possible.
The scrubbing of created gases can be divided into dry separation and wet separation. Dry separation methods are realized in connection with a smelter, in which case the gases exhausted from the furnace gas space are generally first conducted into a waste heat boiler, where the gas heat content is recovered. Thereafter the gases are conducted to an electric filter. A remarkable proportion of the heavy metal compounds contained in the gases, excluding quicksilver and its compounds, are separated already in connection with the cooling. The created gas is often conducted to the production of sulfuric acid, and in that case the wet separation of gases takes place in the sulfuric acid plant, in scrubbers, scrubbing towers and wet electric filters.
The purpose of the wet scrubbing of gases is to further cool down said gases to a suitable temperature by means of direct water cooling, and simultaneously to separate from the gases both solid and volatile impurities, such as heavy metals, halogens, arsenic and selenium. In connection with the wet scrubbing, also the SO
3
contained in the gases is washed from the gases as spent acid, when it gets into contact with water. The quantity of created spent acid is of the order 2-4% of the amount of sulfur dioxide fed into the wet scrubbing, and its H
2
SO
4
content is of the order 10-30%.
Spent acid is considered as hazardous waste, because it contains the heavy metals and arsenic of the exhaust gases. If the spent acid cannot be fed into any other process, such as the production of zinc or fertilizers, or into mineral concentration, the spent acid is generally neutralized with lime and conducted to ponds, but spent acid neutralization and gypsum storage are expensive methods for taking care of the spent acid problem. Even this solution is further restricted by new and more stringent demands for environmental protection.
One of the methods employed for treating spent acid is spent acid regeneration in a separate furnace specially built for this purpose. This choice, particularly applied in chemical engineering but also in metallurgical industry, is described in the article “Regenerating Spent Acid”, Sander, U., Daradimos, G., Chem. Eng. Prog. (1978), 74, p. 57-67. The article explains that the regeneration of spent acid into sulfur dioxide and water is an extremely endothermic reaction which requires energy at least 275 kJ/mol and works best at temperatures over 1,000° C. The temperature in the regeneration furnace is raised up to a sufficient level by means of some fuel. In the same article, it is also maintained that it is advantageous to concentrate spent acid to a H
2
SO
4
content of 70% prior to combustion.
According to the present method, the spent acid to be separated from the exhaust gases created in connection with the smelting of sulfidic ores or concentrates is concentrated and fed back into the primary smelting furnace, into the gas space of said furnace, and regenerated there back into sulfur dioxide, water and oxygen according to the following reaction:
H
2
SO
4
→H
2
O+½O
2
+SO
2
(1)
As is seen from this equation, the spent acid is regenerated back into sulfur dioxide, water and oxygen. Now the sulfur dioxide created in the spent acid regeneration is recovered, in addition to the rest of the sulfur dioxide, in order to produce either sulfuric acid or elemental sulfur.
An essential feature of the method is that the spent acid is fed into the gas space of the primary smelting furnace, in which case the high heat content of the gas is made use of. When the spent acid is fed into the gas space, from where the gases are further discharged to a waste heat boiler, the temperature of the gases is high, usually of the order 1,000-1,600° C. The feeding of spent acid to the exhaust gases lowers the gas temperature, and from the point of view of the waste heat boiler, this is profitable because lowered temperature extends the working age of the boiler and reduces the boiler strain. The term primary smelting furnace refers to a furnace whereto the ore or concentrate is fed. For instance in the smelting of copper and nickel concentrate, it is a suspension smelting furnace, such as a flash smelting furnace, a reverberatory furnace or a converter. In the suspension smelting furnace, the spent acid is advantageously fed into an uptake shaft, through which the hot exhaust gases are conducted into the waste heat boiler, but the gases can also be fed to the gas space of the lower furnace, or in exceptional cases even into the reaction shaft. The essential novel features of the invention are apparent from the appended claims.
In general it can be maintained that if the spent acid were fed into the reaction space proper of the smelting furnace, where concentrate and oxygen react, the energy needed in the regeneration of the spent acid should be fed into the furnace separately, because for example the heat content of copper and nickel concentrate is consumed in the reactions between the concentrate and oxygen, and it is not sufficient for regenerating spent acid. In special cases, where the concentrate heat content is high, it is possible to feed concentrated spent acid into the reaction shaft, in which case the degree of oxygen enrichment in the created process gas can be substantially raised. In that case the quantity of gas removed from the suspension smelting furnace is reduced, and the capacity determined in relation to the waste heat boiler and the copper production of the gas scrubbing line is increased.
It has turned out that the feeding of spent acid into the gas space of the primary furnace—the smelting furnace—leads to the advantageous result that the conduit, i.e. the furnace throat, leading from the furnace to the waste heat boiler, is kept cleaner than before, because at a lower temperature the quantity of dust build-ups in the throat remains smaller. It has been experimentally proved that the feeding of spent acid into the gas space lowers the temperature of the gas going into the boiler for an average of 30-100° C. The only drawback caused by the lowered temperature is that the quantity of the vapor created in the waste heat boiler is smaller, i.e. reduced by a quantity corresponding to the temperature decrease, but the advantages achieved by means of said method is manifold compared to said drawback. The feeding of spent acid to the exhaust gases does not increase the quantity of SO
3
created in the boiler, because it has been found out that nearly all of the spent acid regenerates at the gas space temperatures.
FIG. 1
illustrates schematically the process in which spent acid obtained during the pyrometallurgical production of metals is converted or regenerated into sulf
Holmi Johannes
Jalonen Antti
Ojala Jari
Poij{umlaut over (a)}rvi Jaakko
Griffin Steven P.
Morgan & Finnegan , LLP
Outokumpu Oyj
Vanoy Timothy C
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