Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
2000-12-28
2003-03-04
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S121000, C423S130000
Reexamination Certificate
active
06528028
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for treating bauxite, which is widely used as a raw material for producing alumina. More particularly, the present invention relates to a process for treating bauxite in an environmentally friendly manner, which process converts an insoluble residue (called “red mud”)—which is produced when bauxite is treated by means of Bayer's process—into a composition which is effectively employed, and enables recycling of an alkali component in Bayer's process, the alkali component being contained in the insoluble residue and inhibiting the effective use of the residue.
2. Description of the Related Art
Conventionally, an insoluble bauxite residue is produced in very large amounts in Bayer's process. In Bayer's process, bauxite is treated with an alkali aluminate solution at high temperature and high pressure, a bauxite residue is separated from the resultant solution and removed, aluminum hydroxide is precipitated in and separated from the aluminate solution containing an alkali component in large amounts, and the aluminum hydroxide is calcined, to thereby produce alumina. The amount of a bauxite residue which is produced in the process varies with the quality and the treatment process of bauxite. About 600 kg to 1,000 kg of a bauxite residue is produced for every 1 ton of alumina. A bauxite residue contains large amounts of iron oxide, silicon oxide, and aluminum oxide, and thus extensive studies have been performed in an attempt to use the residue as a material such as a raw material for producing iron or a material for construction, and to enable effective use of these components after extraction and separation.
For example, there have been proposed a process for using such a bauxite residue as aggregate and a process for using the residue as a raw material for producing cement. However, alkali contained in a bauxite residue reacts with cement and the cement may become brittle. Therefore, the amount of the residue which is used for cement is limited, and the residue has never been actually used. Incidentally, there has been proposed a process for converting a silicate compound contained in a bauxite residue into a zeolite, and using the zeolite as an adsorbent. However, such a silicate compound contains alkali, and thus after the zeolite which is produced from the compound is used, the thus-used zeolite must be disposed of. Meanwhile, additional waste which must be treated is produced during the conversion of the compound into the zeolite.
In addition to the aforementioned processes, in an attempt to enable effective use of a bauxite residue, extensive, long-term studies have been carried out. However, in many cases, a large amount of a residue after the treatment of bauxite is not effectively used and is disposed of, for the reasons described below. The size of particles of the residue is as small as 4 microns, and thus carrying out dehydration treatment and handling the particles are very difficult. In addition, the bauxite residue contains an alkali component in large amounts.
U.S. Pat. No. 4,045,537 discloses a process for removing alkali contained in a bauxite residue, which is the so-called “soda lime process.” In the process, a bauxite residue is mixed with a calcium compound, the mixture is sintered in a reducing atmosphere, and the sintered product is washed and alkali is recovered, to thereby use the product as a raw material for producing iron. However, the treated product contains Na
2
O in an amount of more than 1 wt % and the product is unsatisfactory as a raw material for producing iron, although large-scale equipment is necessary in the process since all of an insoluble residue is treated. Meanwhile, Japanese Patent Application Laid-Open (kokai) Nos. 6-172876 and 7-166252 disclose a process for subjecting a bauxite residue to mineral dressing or chemical treatment by use of an acid, to thereby produce a raw material composition for iron production. In the process, the amount of alkali contained in the residue is reduced, and the residue can be used as a raw material for iron production. However, the process is unsatisfactory in consideration that a solution after acid treatment and a desilication product containing alkali must be treated.
Substantially, bauxite does not contain Na
2
O, but red mud usually contains Na
2
O in an amount of 5 wt % or more, which amount varies with the form and the treatment process of bauxite. Such an alkali component is incorporated into red mud, for the reasons described below. An SiO
2
component (which is called reactive silica) of clay predominantly comprising kaolinite, which is contained in bauxite, reacts with alkali in a solution, producing an insoluble desilication product (sodium aluminosilicate) predominantly comprising sodalite. Therefore, a bauxite residue is an insoluble residue which predominantly comprises iron oxide and a desilication product containing an alkali component.
A bauxite residue is produced in large amounts, and thus the residue is used as a material for construction which is consumed in large amounts, such as a raw material of cement, aggregate, or subgrade, or a raw material for iron production. When the residue is used as a raw material of cement, iron oxide, silicon oxide, and aluminum oxide are added to a calcareous raw material, to thereby produce portland cement. In this case, the residue is standardized to contain Na
2
O in an amount of 1 wt % or less. When the residue is used as a raw material for iron production, the amount of Na
2
O in the residue must be 1 wt % or less, since problems in operation, such as deterioration of a refractory substance in a shaft furnace, may occur.
However, there has not yet been considered a process for producing a bauxite residue containing as little alkali as possible, and treating bauxite with an alkali aluminate solution without production of additional waste during the production of the residue.
In view that it will not always be possible to dispose of a residue which is produced after the treatment of bauxite, and that disposal of an insoluble residue containing an alkali component may become an environmental issue in the future, an object of the present invention is to develop a new process for treating bauxite, which process comprises removing an alkali component from a bauxite residue without production of additional waste, recovering the alkali component and recycling the same in Bayer's process, and effectively using the bauxite residue as material, which process is environmentally friendly and industrially effective. The present invention has been accomplished on the basis of the object.
SUMMARY OF THE INVENTION
In order to attain the above-described object, the present inventors have performed extensive studies, and have found that, in Bayer's process in which bauxite is treated with an alkali aluminate solution to thereby produce alumina, when a desilication product and an insoluble residue are separately precipitated without incorporation of the desilication product into a bauxite residue or production of a complex of the desilication product, the insoluble residue other than the desilication product becomes a bauxite residue containing no alkali component, and when the desilication product is treated with a calcium compound and an alkali component is solubilized to thereby separate the alkali component from the desilication product and recover the component, the alkali component can be recycled in Bayer's process almost completely. The present inventors have also found that the bauxite residue contains an alkali component in very small amounts, and thus the residue can be effectively used as a resource. The present invention has been accomplished on the basis of these findings.
Accordingly, the present invention provides the following process.
(1) A process for treating bauxite characterized by comprising:
a) a step for extracting an alumina component from bauxite with an alkali aqueous solution under conditions for suppre
Ishikawa Isao
Kawai Yasuo
Takenaka Yoshiyuki
Bos Steven
Showa Denko K.K.
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