Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1998-08-14
2001-01-30
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S199000, C423S243060, C423S243080, C423S244070, C423S244080, C023S296000, C023S30200R
Reexamination Certificate
active
06180074
ABSTRACT:
The present invention relates to a processing of flue gases essentially comprising sulphur oxides, and, to a lesser extent, metals considered as being toxic, as well as acids, of formula HX, in which X represents a halogen.
Many processes produce flue gases containing sulphur oxides and other organic or inorganic contaminants, as well as metals considered as being toxic. This is more particularly the case with power stations and glassworking furnaces, depending on the quality of the fuel and the starting materials used.
On account of the increasing pressure exerted in terms of environmental regulations regarding installations which produce such emissions, methods for processing acidic flue gases are undergoing considerable development.
The standard processing methods by neutralization with lime, either by a dry or semi-wet route, generate a large amount of solid waste composed mainly of gypsum polluted with metals considered as being toxic, such as lead, cadmium and aluminum, for example.
U.S. Pat. No. 4,385,039 describes the processing of gases comprising compounds of the SOx type with a reagent which can be regenerated, and chosen from sodium carbonate activated by heat treatment, sodium bicarbonate or trona, alone or mixed. The solid residues obtained from the processing of the gases are then dissolved using an alkaline solution comprising a boron compound and aqueous ammonia. The resulting solution is then carbonated so as to precipitate sodium carbonate and/or sodium bicarbonate, which are then reused in the processing of the gases. However, this method always produces a large amount of solid waste (gypsum) which is polluted with metals considered as being toxic and cannot be upgraded. The reason for this is that this method involves a step of precipitation, with lime, of the sulphates remaining in the solution after separating out the precipitated carbonate and/or bicarbonate.
Given the composition of this waste and the highly water-soluble nature of the polluting metal elements, legislation requires them to be placed in a controlled dumping site, after immobilization.
This is a considerable inconvenience since not only is such processing expensive, but also a huge tonnage of waste is produced. Furthermore, there is no means for easily upgrading one or more products resulting from such processing.
The subject of the present invention is thus to propose a method for processing flue gases mainly comprising compounds of the SOx type, and metals considered as being toxic, this method not having the abovementioned drawbacks.
Thus, the method according to the invention makes it possible to process such flue gases with a reagent which is more effective than lime, hence used in lower molar amounts, and chosen such that the waste produced during the neutralization of these gases can be recycled easily and economically.
These aims and others are achieved by the present invention, which relates to a method for processing flue gases comprising compounds of the SOx type, by a dry or semi-wet route, this method consisting in carrying out the following steps:
the said flue gases are treated with a reagent chosen from sodium carbonate, sodium bicarbonate and mixtures thereof,
the sodium sulphates produced are selectively dissolved in water,
the solids remaining in suspension in the solution are separated out,
and the purified sodium sulphate present in the abovementioned solution is crystallized.
However, other advantages of the present invention will emerge more clearly on reading the description and the examples which follow.
As has been mentioned previously, the method according to the invention is directed towards the processing of flue gases mainly comprising gases of the SOx type. The term SOx as used here refers to the species SO
2
and SO
3
. The flue gases can also comprise sulphites and/or bisulphites.
The flue gases processed by the method according to the invention also comprise metals considered as being toxic. The expression “metals considered as being toxic” is used here to refer to metals considered as being toxic and/or polyvalent metals. More particularly, but without any intention of being limiting, mention may be made of mercury, cadmium, lead, arsenic, selenium, silver, tin, antimony, zinc, aluminum, silicon, calcium, titanium, chromium, manganese, iron, nickel, copper, molybdenum, bismuth, vanadium and cobalt.
The flue gases processed according to the method of the invention can also comprise, to a lesser extent, compounds of the Nox type and acids of formula HX in which X represents a halogen atom. More particular, the halogen atom is chlorine or fluorine.
The flue gases can also contain organic contaminants, in particular such as dioxins or furans.
Besides the contaminants which have just been mentioned, the said flue gases can also comprise dusts or fly ash.
The flue gases are thus treated, by a dry or semi-wet route, with a reagent chosen from sodium carbonate, sodium bicarbonate and mixtures thereof.
The term dry route is used here to denote more particularly processing in which the reagent is introduced in the solid state into the flue gases.
When the processing carried out is the dry route, two types of implementation can be distinguished, the first variant being free of water and the second variant being carried out in the presence of water.
According to the first variant of the dry-route method, anhydrous reagents are used more particularly.
According to the second variant of the dry-route method, the flue gases are placed in contact with an abovementioned reagent, in the presence of water but with an amount such that the reagent retains its powdery appearance. The reagent used can be in anhydrous form or can comprise water of crystallization.
The water can be provided in the method in several ways. In particular, it can be present intrinsically in the reagent or can be provided extemporaneously, before processing the flue gases or during their processing. Needless to say, combining these possibilities can be envisaged.
When the reagent comprises water of crystallization, it can be envisaged not to provide the water extemporaneously and to carry out the processing only in the presence of the reagent's water of crystallization.
However, according to a preferred embodiment of the invention, the water is provided extemporaneously, whether the reagent comprises water of crystallization or not. In this case, the water content is between 2 and 15% by weight relative to the reagent.
The term semi-wet route is used here to denote a method in which the reagent is introduced in the presence of water in an amount such that the products obtained after processing are in a solid form. In this case, the reagent is used in the form of a suspension or a solution.
The water content required to obtain a solution or a suspension of the reagent can be determined by a person skilled in the art on the basis of the technological constraints of the method. More particularly, this content depends on the processing temperature of the flue gases, the evaporation capacity of the apparatus and the fact that it is desired to obtain a solid product after the said processing.
When the reagent is in solid form, it may be preferable to use a product of fine and controlled particle size, so as to improve the kinetic of the neutralization processing.
As a guide, it is preferred to use a reagent whose average particle size is less than 50 &mgr;m. According to a more specific embodiment, a reagent whose average particle size is between 5 and 30 &mgr;m is used.
If a reagent with a particle size greater than or about 50 &mgr;m is used, it may be advantageous to carry out, prior to the processing of the flue gases, a step of grinding of the said reagent in order to reach a particle size of about 5 to 30 &mgr;m. Such a step can be carried out by any standard means known to those skilled in the art.
The carbonates and bicarbonates present on the market are suitable for carrying out the present invention.
The sodium carbonate used in the method is more particularly light sodium carbonate. The l
Fourcot Fabrice
Pouxviel Jean-Claude
Burns Doane Swecker & Mathis L.L.P.
Griffin Steven P.
Novacarb
Vanoy Timothy C
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