Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
2001-01-23
2004-01-13
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S24000R, C423S243010, C502S407000, C502S411000
Reexamination Certificate
active
06676911
ABSTRACT:
TECHNICAL FIELD
This invention relates to a technology of treating acid gases, such as hydrogen chloride gas (hereinafter referred to as “HCl” for short) and sulfur oxides (hereinafter referred to as “SO
x
” for short), which are contained in exhaust gases in exhaust gas treatment equipment in a waste incineration system, for instance, and to a technology of stably fixing hazardous or toxic heavy metals occurring in soot and dust formed upon exhaust gas treatment and trapped in dust collectors.
PRIOR ART
Ordinary incineration equipment generally comprises an incinerator, a heat exchanger or water sprayer for cooling high temperature exhaust gases, an apparatus for blowing slaked lime (calcium hydroxide) into exhaust gases (in an exhaust gas guide pipe) for neutralizing acid gases such as HCl, and a dust collector and, optionally, further comprises an apparatus or equipment for eliminating hazardous components such as SO
x
, nitrogen oxides (hereinafter referred to as “NO
x
” for short) and dioxins. Soot and dust generated upon incineration of wastes contains dust, reaction products formed from slaked lime and acid gases such as HCl, unreacted slaked lime residue and, further, low-boiling compounds containing a toxic heavy metal such as Pb, and cyanides. Therefore, simultaneous soot and dust treatment is also necessary.
Today, in the disposal of industrial wastes containing hazardous heavy metals and cyanides, soot and dust is admixed with cement, water is added, and the mixture is kneaded, cured and solidified to thereby stabilize and prevent toxic heavy metals and cyanides from leaching out. However, the conventional method of industrial waste treatment which comprises mere solidification using cement such as mentioned above has various problems and, without strict management of treatment products, may cause a secondary environmental pollution.
The soot and dust collected in municipal refuse incinerators, for instance, contains hazardous heavy metals such as lead (Pb), cadmium (Cd), mercury (Hg), chromium (Cr) and copper (Cu). Currently, such kind of soot and dust is blended with bottom ash for landfill or solidified by means of cement. However, even solidification with cement involves the risk of leaching of Pb and the like. In recent equipment, in particular, slaked lime or quick lime (calcium oxide) is blown into exhaust gases in the exhaust route to catching acid gases generated upon incineration, for example HCl and SO
x
. The unreacted portion of such slaked lime or quick lime remains in fly ash and, therefore, the soot and dust collected has a high alkalinity not lower than pH 12. It is well known that Pb readily leaches out under highly alkaline conditions. Thus, highly alkaline soot and dust or a treatment product derived therefrom, without proper treatment, causes a serious problem, namely leaching of Pb.
Since the disposal by landfill of untreated soot and dust is prohibited by law, a chelating agent is added to soot and dust to thereby prevent Pb from leaching out. However, chelating agents are expensive. And, for some kinds of soot and dust, in particular highly alkaline soot and dust with a high concentration of Pb, they cannot suppress the leaching of Pb to a level below the legal limit (not more than 0.3 ppm) in some instances without adding them in large amounts (not less than 5% of the weight of soot and dust), hence imposing no light burden from the operation cost viewpoint.
Accordingly, the establishment of a novel exhaust gas treatment agent and a novel method of exhaust gas treatment with and by which acid gases such as HCl and SO
x
generated upon incineration of municipal and other wastes can be efficiently trapped and hazardous heavy metals can simultaneously be fixed has earnestly been desired.
In response to such a social demand, Japanese Unexamined Patent Publication H09-108538, for instance, discloses a treatment agent for reducing the content of alkaline compounds in soot and dust collected in dust collectors and preventing leaching of toxic metals such as Pb by using highly reactive calcium silicate hydrate in exhaust gas treatment systems.
Japanese Examined Patent Publication H07-63581 discloses an exhaust gas treatment agent which is prepared by dispersing a raw material composed of a substance capable of providing calcium oxide, silicon dioxide and aluminum oxide and a substance capable of providing calcium sulfate in water.
However, treatment of exhaust gases by contacting with calcium silicate hydrate can indeed reduce the alkaline substance levels in soot and dust but cannot be said to be better in acid gas collection efficiency as compared with slaked lime. Thus, when exhaust gases with a high acid gas concentration are treated, the neutralization agent is required in increased amounts and, as a result, the amount of soot and dust increases.
OBJECTS OF THE INVENTION
Accordingly, it is a main object of the present invention to provide a novel technology of treating exhaust gases as well as soot and dust by which acid gases, such as HCl and SO
x
, contained in exhaust gases generated upon incineration can efficiently be trapped and, at the same time, hazardous heavy metals can be stably fixed in the treatment of exhaust gases generated in incinerators for industrial wastes, municipal wastes and the like and in the treatment of soot and dust collected on that occasion.
DETAILED DESCRIPTION OF THE INVENTION
Paying attention to the state of the art as mentioned above, the present inventors continued studies and, as a result, found that a material comprising, as main constituents, calcium silicate hydrate particles having a controlled mean particle diameter and a controlled specific surface area can efficiently catch acid gases in exhaust gases and further can stably fix heavy metals in soot and dust without increasing the amount of soot and dust.
Thus, the present invention provides an exhaust gas treatment agent as specified below and a method of producing the same as well as a process for the treatment of exhaust gases and soot and dust as specified below.
1. An exhaust gas treatment agent characterized in that it comprises calcium silicate hydrate particles having an average particle diameter of 0.2 to 4 &mgr;m and a specific surface area of not less than 60 m
2
/g and calcium hydroxide particles having an average particle diameter of not more than 4 &mgr;m and the content of calcium hydroxide particles in both particle components is within the range of 20 to 60% by weight.
2. An exhaust gas treatment agent as defined in claim
1
occurring as a slurry.
3. An exhaust gas treatment agent as defined in claim
1
occurring as a dry powder.
4. A method of producing slurry-form calcium silicate- and calcium hydroxide-based exhaust gas treatment agents which comprises adding 100 to 2,000 parts by weight of water to a total of 100 parts by weight of a calcium silicate-containing raw material and a calcium hydroxide-generating raw material and carrying out the hydration reactions at 40 to 100° C. under wet grinding conditions.
5. A method of producing powder-form calcium silicate- and calcium hydroxide-based exhaust gas treatment agents which comprises adding 100 to 2,000 parts by weight of water to a total of 100 parts by weight of a calcium silicate-containing raw material and a calcium hydroxide-generating raw material and carrying out the hydration reactions at 40 to 100° C. under wet grinding conditions and then drying and classifying the reaction product.
6. A process for exhaust gas treatment which comprises contacting an exhaust gas with the exhaust gas treatment agent defined in claim
1
.
7. A process for the treatment of exhaust gases and soot and dust which comprises contacting an exhaust gas with the exhaust gas treatment agent defined in claim
1
and adding water to the soot and dust collected from the exhaust gas in a dust collector and kneading and solidifying the resulting mixture.
The exhaust gas treatment capacity of those exhaust gas neutralizing agents which are most frequently used and comprise slaked lime (calcium hydroxide
Jami Shingo
Kanai Kensuke
Kobayashi Shigehiro
Suzuki Hiroaki
Yamamoto Takanori
Knobbe Martens Olson & Bear LLP
Strickland Jonas N.
Sumitomo Osaka Cement Co. Ltd.
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