Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2002-06-17
2004-01-13
Sample, David R (Department: 1755)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C588S010000, C588S014000, C588S015000
Reexamination Certificate
active
06676915
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for conditioning wastes constituted of aqueous solutions of sodium hydroxide.
More precisely, it concerns the treatment of radioactive solutions of sodium hydroxide, obtained as waste from fast neutron nuclear reactors.
In fact, the development of fast neutron nuclear reactors, using metallic sodium as coolant, results in the production of radioactive sodium wastes. These wastes can come from the operation of industrial and experimental reactors, but also from research laboratories. They can contain radioactive elements, such as
22
Na and other radioactive elements resulting from electronuclear activity in general such as uranium, plutonium, caesium, cobalt etc.
In order to reduce the potential chemical risk represented by stocking sodium in its metallic form, this waste is generally converted into a concentrated sodium hydroxide solution by a water destruction method.
These concentrated sodium hydroxide solutions must be transformed into solid wastes, retaining the radioactive products they may contain, with a view to storage.
STATE OF PRIOR ART
The document U.S. Pat. No. 4,028,265 [1] describes a method for conversion of caustic liquid radioactive wastes containing sodium nitrate into an insoluble solid product. According to this method, a clay powder with a base of aluminium silicate reacts with an aqueous solution or a suspension of the radioactive liquid waste, which has a concentration of sodium hydroxide of 3 to 7 M and which contains sodium nitrate, to form a product of the cancrinite type, which is then transformed by calcination, at least at 600° C., into a mineral form such as nepheline
This conversion corresponds to the reaction diagrams below:
Al
2
SiO
2
.2H
2
O+2NaOH+0.52 NaNO
3
+0.68H
2
O→Na
2
O.Al
2
O
3
.2SiO
2
.0.68H
2
O.0.52NaNO
3
+3H
2
O (1)
Na
2
O.Al
2
O
3
.2SiO
2
.0.68H
2
O.0.52NaNO
3
&Dgr;
2NaAlSiO
4
. 0.26Na
2
O+0.68H
2
O+0.26N
2
O
5
(2)
The silico-aluminous clays suitable for use for this conversion belong to the group comprising kaolin, bentonite, dickite, halloysite and pyrophillite. Using the intermediary product (cancrinite) of diagram (1), during calcination either a free powder is formed, or objects moulded by compression of the intermediary product in the shape required, followed by sintering at less than 600° C.
In this method, it is thus necessary to manipulate the powders in order to obtain solid shaped products and to use mechanical or hydraulic presses to compress these powders.
Also known are methods for synthesizing zeolite 4A, from calcinated kaolins, by reaction of the latter with sodium hydroxide, as described in Ind. Eng. Chem. Res., 27, 1988, pages 1291-1296 [2], and in U.S. Pat. No. 4,271,130, [3]. In the reference [2] a gel is formed by dissolving calcinated kaolin in the sodium hydroxide solution and then, by heating the gel, a solid product is formed by crystallisation. With this aim, quantities of kaolin are chosen such that the molar ratio Na
2
O/SiO
2
is between 1.8 and 3.8, and preferably 2.8. The quantity of water is such that the molar ratio H
2
O/Na
2
O is in the range from 30 to 50, which corresponds to sodium hydroxide solutions of 2.23 to 3.7 M. Zeolite powders are thus obtained, usable in detergent formulae, the dimension of the particles being in the range from 1 to 10 &mgr;m for 99% by weight of the particles.
In reference [3], a zeolite A is obtained from the reaction of metakaolin with an alkaline aqueous medium, using a solution of 7 to 30% sodium hydroxide and a quantity of NaOH, representing 1.3 to 3 times the stoichiometric quantity required for the formation of zeolite A. This corresponds to a ratio Na
2
O/SiO
2
of 0.05 to 10 and a molar ratio H
2
O/Na
2
O of 15 to 70, that is a sodium hydroxide solution of 1.58 to 7.4 M. As before, this zeolite in powder form is intended to be used in detergents and this is why a brighter and less yellow zeolite A is sought, a result which is obtained by using metakaolin as the original product.
DESCRIPTION OF THE INVENTION
The aim of the present invention is a method for conditioning waste constituted of an aqueous solution of sodium hydroxide, which makes it possible to obtain solid products, of the nepheline type, without having to treat a powder and compact it.
According to the invention, the method for conditioning a waste, constituted by an aqueous solution comprising 3 to 10 mol/l of sodium hydroxide NaOH, comprises the following stages:
a) a metakaolin powder is added to the aqueous solution such that a suspension is obtained capable of solidifying and forming a crystalline phase of the zeolite A type;
b) the suspension is introduced into a mould;
c) the suspension is left to solidify in the mould in order to obtain a moulded solid product based on zeolite A;
d) the moulded product is dried; and
e) the zeolite A phase is converted into a nepheline type phase by heat treatment at a temperature of 10000° C. to 1500° C.
In the method according to the invention, the fact of starting from a concentrated aqueous solution of sodium hydroxide and adding the metakaolin powder in the appropriate quantity makes it possible to obtain a suspension able to solidify to form a zeolite A type crystalline phase.
According to the invention, this zeolite A can thus be obtained directly under the form of a moulded solid product, then transformed into a nepheline type phase by a heat treatment. This is very advantageous when the aqueous solutions of sodium hydroxide contain radioactive products, because the handling of powder, which could result in dispersion of radioactivity, is avoided. Investment in equipment is reduced since it is no longer necessary to use mechanical or hydraulic presses, and one can also reduce the size of the installation and make products moulded according to the dimensions of the waste containers. Furthermore, the use of a nepheline type phase enables the radioactivity to be confined in a stable phase, thus preventing lixiviation of the radioactive products trapped in this structure.
In the method according to the invention, the fact of starting from an aqueous solution of sodium hydroxide, comprising practically no sodium nitrate or nitrite, makes it possible to obtain a zeolite A type crystalline phase by reaction with metakaolin.
In document [1], this phase could not appear, because of the presence of sodium nitrate and nitrite, which would lead to the formation of a cancrinite type phase.
In the invention, the reaction of the metakaolin with the sodium hydroxide corresponds to the following global reaction diagram:
The possibility of obtaining solidification of the sodium hydroxide solution by this reaction results from the choice of metakaolin, the concentration of sodium hydroxide in the original aqueous solution and the quantity of metakaolin added.
In fact, the zeolite phase only appears when metakaolin is used. According to the reaction time, the quantity of metakaolin and the reaction temperature, a proportion of hydroxysodalite phase can be formed. However, this second phase, even in large proportions, does not hinder the solidification of the suspension and the conversion into nepheline.
In order to obtain solidification of the solution directly without exudation, it is important to have a concentration of sodium hydroxide solution higher than 3 mol/l, and preferably higher than 5 mol/l. When the concentration of sodium hydroxide is too low, the reaction takes place leaving behind a floating liquid and a dense solid. When the concentration of sodium hydroxide exceeds 10 mol/l, mixing problems are observed together with a very short solidification time, so that the suspension is then very viscous and the solidification can be too rapid to implement the method according to the invention. It is therefore important to choose a sodium hydroxide solution of 3 to 10 M to obtain this solidification.
Furthermore, the quantity of metakaolin added should be chosen such that i
Fiquet Olivier
Gibert Didier
Le Chenadec Ronan
Commissariat a l' Energie Atomique
Hayes & Soloway P.C.
Sample David R
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