Chemistry of inorganic compounds – Zeolite – Synthesized from naturally occurring product
Reexamination Certificate
2000-04-27
2002-09-17
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Synthesized from naturally occurring product
C423S709000, C423S710000, C423SDIG002, C423SDIG002
Reexamination Certificate
active
06451282
ABSTRACT:
TECHNICAL FIELD
The invention relates to a process for production of zeolites from raw materials containing alkali alumino-hydrosilicates.
BACKGROUND ART
The processes which are based on processing materials containing alkali alumino-hydrosilicates, first of all by-products of various industries are more and more important from the economical point of view. A process is known e.g. for manufacturing 4A zeolite from “red mud” obtained from the Bayer process which is widely used for making alumina from bauxites [Cardile C. M.: Removal and conversion of DSP in Red Mud to type 4A Zeolite (Int. Bauxite Tailing Workshop, 1992)]. The core of the process is the separation of particles smaller than 45 &mgr;m consisting mostly of alkali alumino-hydrosilicate with cyclones from particles greater than 45 &mgr;m, which consist mostly of Fe
2
O
3
. The alkalinity of the <45 &mgr;m fraction is reduced by washing with water, subsequently the slurry is acidified to pH≡6.2 using H
2
SO
4
and the Ca- and Mg-salts are removed by filtration or decantation. The filtrate is further acidified to pH≡2.3 in order to bring the silicate components into a relatively stable colloid-like solution. The alumina containing silica solution obtained in this way is separated from the residues of red mud and is subsequently treated with the necessary amount of NaOH. Subsequently NaA zeolite is crystallized. The NaA (LTA) product is washed to remove the mother liquor (to below a value of pH 10.5) and dried. The filtrate mother liquor contains alkali sulfates in fairly high concentrations, from which the alkali content can partly be recovered by treatment with slaked lime (causticisation) and subsequently utilised in the Bayer process. In case of causticisation with slaked lime the alkali recovery yield is low, in fact, it is less than 50%. The recovered alkali solution contains non-reacted lime particles (and some calcium sulfate), consequently, the utilization of the solution obtained from the causticisation has little relevance in the alumina manufacturing process. One great disadvantage of the process described here is that a significant amount of sulfuric acid is required to acidify the raw material to a solution of pH 2.3 and a substantial amount of sodium hydroxide is needed for neutralization and converting the slurry alkaline with a pH of greater than 13.5 that is needed for the crystallization. A further disadvantage is that the iron oxide content of the red mud at least partly dissolves at pH 2.3 and thereby the zeolite product becomes contaminated. Therefore the commercial application of this process is not profitable and the purity of the product does not reach the European Standards.
The WO 91/15427 A1 patent application document laid available for the public describes a process in which sodalite or cancrinite originating from the Bayer alumina manufacturing process is used as it is or together with red mud; the sodium alumino-hydrosilicates content is dissolved with diluted sulfuric acid, subsequently a precursor gel is made from the resulting solution by blending it with a sodium hydroxide or sodium aluminate solution at a pH greater than 12, usually at a pH of 13.5. The precursor gel is aged at a temperature of 70-95° C. and subsequently NaA zeolite is obtained by crystallization. This process is basically identical with the process described above and the disadvantages are similar as well. It is to be emphasized that the viscosity temporarily increases to a great extent when the pH of the acidic solution is gradually increased. In order to avoid this kind of difficulty with mixing, it was proposed that all the amount of alkali which is needed to form the precursor be added within a very short time period. Inverse blending was also proposed (i.e. the acidic sodalite solution is to be added to the sodium hydroxide or sodium aluminate solution so that the difficulties with mixing are eliminated).
There is a need to develop such a process by which zeolite(s) can be made from raw materials containing alkali alumino-hydrosilicate in which the acid and alkali consumption are moderate, the zeolite product(s) has/have such a purity which meet(s) the expectations of the users in the world market, and the synthesis of zeolite can be carried out in common industrial equipment.
DISCLOSURE OF INVENTION
In our investigations it was found that a precursor slurry that is suitable for the crystallization of zeolite can be directly formed from alkali alumino-hydrosilicates provided that the alkali alumino-hydrosilicates are transformed to an amorphous state by an appropriate acid treatment. In this case the crystalline structure of the alkali alumino-hydrosilicates is destroyed by the removal of the charge compensating (mainly Na
+
) cations. In order to reach this goal considerably less (sulfuric) acid is needed than for completely dissolving the alkali alumino-hydrosilicate. Furthermore, the process steps of dissolving the raw material in the acid and of making a strongly alkaline solution again from the highly acidic solution can be avoided. The gel formation, which is a difficult step of the previous processes can also be avoided since as per our invention, the precursor slurry, suitable for crystallization of zeolite can be obtained by blending the amorphous alkali alumino-hydrosilicates with silica, alkali and alumina in appropriate amounts. The amorphous solid zeolite precursor can be washed free of the excess acid. or even dried. After addition of (considerably less) sodium hydroxide (and other components) the slurry obtained (possibly after aging) is ready to be crystallized into various commodity zeolite products (like NaA, NaX, NaY or maximum aluminium P zeolite).
On the basis of this understanding, this invention provides an improved process for the transformation of alkali (preferably sodium) alumino-hydrosilicates into various zeolites by acid treatment to destroy their crystalline structure first, (an amorphous material is made in this way), followed possibly by purification, and a subsequent separation of the intermediate amorphous solid from the liquid phase; then the slurry for zeolite crystallization is prepared from this amorphous material by suspending it in water which possibly contains alkali (sodium hydroxide) and possibly after addition of alumina-, and/or silica- and/or Na
2
O-containg components and/or crystallization promoters thereby adjusting the proper zeolite slurry composition. The zeolite slurry is possibly aged and/or blended with zeolite nuclei. After these the crystallization, filtration and/or washing and drying are carried out according to the known processes.
According to the present invention, natural alkali alumino-hydrosilicates and/or industrial by-products or wastes containing mainly alkali alumino-hydrosilicates can be used as raw materials. Materials with at least 50 w % (on dry basis) alkali alumino-hydrosilicates in the form of crystalline or amorphous compounds are considered as raw materials. Further on materials with alkali alumino-hydrosilicate content as defined here are considered as raw materials.
Acceptable raw materials are the desilication products which are obtained in the first step of the desilication of the pregnant liquor that is carried out at 150-170° C. in the sintering alumina manufacturing process. Natural materials which contain alkali alumino-hydrosilicates such as nepheline with a low iron content can be used as raw material, too.
It is to be mentioned that “sodium alumino-hydrosilicate” materials of high purity can preferably be obtained from the Bayer alumina manufacturing process by an Improved Low Temperature Digestion Process (Gy. Bánvölgyi: Travaux Vol. 24, 1997 (No 28) ICSOBA, Budapest, Proc. of 8th International Congress, Apr. 16-18, 1997, Milan, Italy, pp 214-2281 or in combined Bayer/sintering processes.
The raw materials, which can be used in the embodiment of this invention, are desilication products of sodium alumino-hydrosilicate type originating from various alumina manufacturing processes or scales d
Bánvölgyi György
Fejes Pál
Kiricsi Imre
Siklósi Péter
Dubno Herbert
Myers Jonathan
Sample David
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