Liquid purification or separation – Processes – Chemical treatment
Utility Patent
1999-12-17
2001-01-02
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S724000, C210S726000, C210S768000, C210S770000, C210S774000, C210S912000
Utility Patent
active
06168721
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART STATEMENT
This invention relates to a process for the treatment of arsenic-containing sludge wherein arsenic compound-containing sludge resulting from the flocculating settling treatment of arsenic-containing waste water is converted into a composition harmless to the environment.
As processes for the treatment of arsenic present in waste water, those based on adsorption, ion-exchange, sulfide precipitation, hydroxide coprecipitation and the like are known. Among these treatment processes, the hydroxide coprecipitation process involving treatment with a calcium compound, magnesium compound, iron compound or the like is the most representative one. An outline of this process is described below with reference to FIG.
2
.
Arsenic-containing waste water
24
discharged from various plants and the like is introduced into a reaction tank
21
. In the waste water, arsenic is present in the form of arsenite ion (AsO
3
3−
) containing As
3+
and arsenate ion (AsO
4
3−
) containing As
5+
. If the content of As
3+
is high, it may be possible to previously add an oxidizing agent such as a peroxide (e.g., hydrogen peroxide) or a hypochlorite to the waste water and thereby oxidize As
3+
to hardly soluble As
5+
. When a calcium compound or an iron compound
25
is added to the waste water, the reactions represented by the following reaction formulae (1) and (2) or (3) and (4) take place, so that the calcium or iron salts of arsenous acid and arsenic acid are formed and precipitated. In the process illustrated in
FIG. 2
, slaked lime is added as a typical one of such additives.
3Ca
2+
+2AsO
3
3−
→Ca
3
(AsO
3
)
2
(1)
3Ca
2+
+2AsO
4
3−
→Ca
3
(AsO
4
)
2
(2)
Fe
3+
+AsO
3
3−
→FeAsO
3
(3)
Fe
3+
+AsO
4
3−
→FeAsO
4
(4)
In addition to these reactions, the calcium compound and iron compound also act as flocculating agents. As a result, the arsenic compound-containing precipitate formed according to the above formulae (1) to (4) is gradually coarsened and becomes easier to settle.
Then, this reaction fluid is introduced into a settling tank
22
where solid matter is separated therefrom. The supernatant water is discharged out of the system as treated water
26
, while the settled sludge
27
containing arsenic compounds and the like is withdrawn from the bottom of the tank, dewatered in a dewaterer
23
, and discharged as a dewatered cake
29
. Part of settled sludge
27
is returned to reaction tank
21
as returned sludge
28
.
Moreover, in Japanese Patent Laid-Open No. 192677/'97, the present applicant has previously proposed a process for the treatment of waste water and sludge by two-stage flocculating settling which includes the steps of adding a calcium compound to waste water so as to convert arsenous acid and arsenic acid into their calcium salts and separate them by settling [see the above formulae (1) and (2)]; dewatering, drying and calcining the resulting sludge; and adding a ferric compound to the treated fluid so as to combine arsenous acid and arsenic acid with ferric ion and separate the resulting compounds by settling [see the above formulae (3) and (4)].
Furthermore, in Japanese Patent Laid-Open No. 128396/'98, the present applicant has also proposed a process for the treatment of sludge wherein, after sludge comprising the calcium salts of arsenous acid and arsenic acid is formed according to the treatment process of the aforementioned Japanese Patent Laid-Open No. 192677/'97 or the like, a solid calcium compound is added thereto in excess and then calcined to obtain a fired product which is so stable as to suffer from no arsenic redissolution.
However, the above-described conventional treatment processes have the following problems.
(1) The hydroxide coprecipitation process, when used alone, has low arsenic-removing efficiency and fails to meet the emission standards for harmful substances as prescribed by the Prime Minister's Office. Moreover, since this process is merely an example of a waste water treatment process and has not been as an integrated process, a suitable method for the treatment of sludge must be newly chosen according to the properties and components of the sludge produced. That is, if sludge containing arsenic compounds is simply dewatered, dried and dumped, a new source of environmental pollution will be produced. Accordingly, it is necessary to establish an integrated treatment process combining waste water treatment with sludge treatment.
(2) In the treatment process described in the aforementioned Japanese Patent Laid-Open No. 192677/'97, these problems are apparently solved. However, it can be imagined that a very small portion of the arsenic compounds pyrolyzed during the calcination of sludge may be volatilized into the atmosphere. In this respect, a problem still remains from the viewpoint of environmental health.
(3) The treatment process described in the aforementioned Japanese Patent Laid-Open No. 128396/'98 is effective in preventing the above-described volatilization of arsenic compounds into the atmosphere during calcination. However, since a solid calcium compound is added to dry sludge in this process, its operability is so low that the calcium compound may not be uniformly mixed with the sludge but gathered into lumps. In such a case, there is a possibility that only a slight amount of arsenic compounds may be volatilized during calcination.
OBJECT AND SUMMARY OF THE INVENTION
In order to solve the above-described problems of the prior art, an object of the present invention is to provide a process for the treatment of arsenic-containing sludge wherein arsenic-containing sludge obtained by treating waste water to precipitate arsenic present therein can be calcined to yield a calcined product which involves no risk of arsenic redissolution when it is dumped, and wherein there is no possibility that arsenic compounds may be volatilized during calcination.
The present invention has been made with a view to accomplishing the above object and has the following four constituents (1) to (9).
(1) A process for the treatment of arsenic-containing sludge obtained by adjusting arsenic-containing waste water to a pH of 12 or greater by the addition of a calcium compound and subjecting the waste water to solid-liquid separation, the process comprising the steps of adding a calcium compound to the arsenic-containing sludge slurry obtained by the solid-liquid separation, dewatering the resulting sludge, drying the dewatered sludge, and calcining the dried sludge.
(2) A process for the treatment of arsenic-containing sludge as described above in (1) wherein, when the number of moles of the calcium compound added in order to adjust the waste water to a pH of 12 or greater is represented by A, the number of moles of the calcium compound added to the arsenic-containing sludge slurry obtained by the solid-liquid separation is represented by B, and the total number of moles of the calcium compound required to neutralize the acidic waste water and the calcium compound reacting with the arsenic present in the waste water is represented by C, the degree of Ca excess as defined by (A+B)/C is in the range of 1.5 to 3.0.
(3) A process for the treatment of arsenic-containing sludge as described above in (1) or (2) wherein the calcium compound added to the arsenic-containing sludge slurry obtained by the solid-liquid separation is in the form of a slurry or a solution.
(4) A process for the treatment of arsenic-containing sludge as described above in any one of (1) to (3) wherein the dried sludge is calcined at a temperature of 650 to 900° C.
REFERENCES:
patent: 5378366 (1995-01-01), Yen
patent: 5651895 (1997-07-01), Gordon
patent: 09192677 (1997-07-01), None
patent: 10128396 (1998-05-01), None
patent: 411047764 (1999-02-01), None
Baba Hiroshi
Iwashita Koichiro
Kamiyoshi Hideki
Nishida Morikata
Shimizu Taku
Anderson Kill & Olick P.C.
Hoey Betsey Morrison
Mitsubishi Heavy Industries Ltd.
Simmons David A.
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