Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1999-08-13
2001-08-07
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S902000
Reexamination Certificate
active
06270682
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for the removal of chlorate ions from solutions by the catalytic treatment at a noble metal catalyst in the presence of hydrogen.
It is known from WO 96/07617, that compounds of halogen and oxygen, which are formed as byproducts of the oxidative treatment of water and are contained in water in small concentrations, can be removed by the catalytic treatment at a noble metal catalyst in the presence of hydrogen.
Palladium on an oxide support is used as catalyst here, the porosity and abrasion resistance forming the criteria for selecting the support material.
The Japanese patent 63-514 describes a method for reducing the accumulation of chlorate ions in salt solutions of the chloralkali electrolysis, in that the circulating salt solution, in the presence of hydrogen, is passed over a catalyst layer.
Metals or metal oxides of the eighth subsidiary group of the periodic table of elements are used as catalyst.
Information concerning the effect of the support material on the catalytic effectiveness of the metals cannot be inferred from this publication.
SUMMARY OF THE INVENTION
It is an object of the invention to make available a catalytic method for the removal of chlorate ions from solutions, which contain anionic co-adsorbates, using a modified noble metal catalyst.
It was found that, for the removal of chlorate ions from solutions, which furthermore contain co-adsorbates, the nature of the active catalytic component as well as the support material are important. The effectiveness of the method can be influenced by the targeted selection of the support material as a function of the amount of the co-adsorbates contained in the solution to be treated, this also including the pH dependence.
Pursuant to the invention, the solution, which contains the chlorate ions to be removed as well as anionic co-adsorbates, is treated in the presence of hydrogen at a supported rhodium and/or platinum catalyst. The chlorate is decomposed catalytically to chloride in accordance with the following equation:
ClO
3
−
+3H
2
→Cl
−
+3H
2
O
The reaction rate depends on the amount of hydrogen gas available, the temperature, the pH, the nature of the catalyst as well as on the support material.
Within the sense of the invention, support materials are understood to be inorganic materials, which may or may not be oxides, such as ZrO
2
, SiC and activated charcoal.
It is also within the sense of the invention to modify the surface of the inorganic support material, for example, by silylation, fluorination, reduction or oxidation.
When selecting the support material, it should be noted that the porosity of the support material is not one of the criteria for selection.
Surprisingly, it was found that the support material must have a certain effective electronegativity. Moreover, the support material must be readily wettable.
Within the sense of the invention, co-adsorbates are understood to be, for example, chlorides, bromides, hydroxides, sulfates and phosphates, the co-adsorbates not being limited to those listed.
In the sense of the invention, solutions are chlorate containing effluent, process water and salt solutions of chloralkali electrolysis; there are no limitations here with respect to the chlorate concentration, which can be decomposed.
Pursuant to the invention, the solution is passed over a supported rhodium and/or platinum catalyst at a pH of 1 to 10 and preferably of 1 to 6 and/or a temperature of less than 100° C. and preferably of 40° to 90° C. and/or a pressure of 1 to 30 bar and preferably of 2 to 15 bar.
The method can be carried out continuously or discontinuously.
If the solutions to be treated have elevated chlorate concentrations (greater than 100 mg/l), the use of a three-phase reactor is appropriate. For this, the required hydrogen is made available to the reaction by material transfer
H
2
(gaseous)→H
2
(dissolved)
It has proven to be advantageous to use a packed bed catalyst in the three-phase reactor. By so doing, the mechanical load on the catalyst is less than in a moving bed catalyst and catalyst abrasion is thus minimized. This is important also from an economic point of view when a noble metal catalyst is used. A suitable three-phase reactor with a packed bed catalyst is, for example, a trickle bed reactor.
Surprisingly, it was found that the chemical reaction leading to the decomposition of the chlorate is limited not at the dissociation but by chlorate concentrations in the trace range (ppm) by to the adsorption of the chlorate. At high concentrations, the reaction is limited by the desorption of the reaction products.
While selecting the catalytically-active components, it was found that a catalyst, which contains rhodiumi and/or a rhodium compound, is particularly suitable for the reduction of chlorate.
When different noble metal catalysts are compared (same support material, same metal content in % by weight), the following activity sequence is obtained: Rh>Pt>Pd.
From the Journal of Catalysis, 136, pages 161 to 169, 1992, it is known that the influence of an oxide support material on a catalytic reaction in the gas phase (hydrogenation of CO on a supported Ru catalyst) can be correlated according to the Sanderson concept with an effective electronegativity (EN) of the support material.
TABLE 1
Effective Electronegativity of Support Materials
(Geometric Mean of the Allred and Rochow Atom Electronegativities)
Support Material
EN
SiC
2.06
ZrO
2
2.45
Al
2
O
3
2.49
Activated Charcoal
2.5
Graphite
2.5
A simple transfer of this concept to reactions in an aqueous phase is not possible, since the ionicity and the wettability of the support material must be taken into consideration.
It has been found that, for the inventive method of catalytically reducing chlorate, the activity of the Rh/support system increases as the effective electronegativity of the support material decreases, if small amounts of co-adsorbates are present As the effective electronegativity of the support material increases, the inhibiting effect of the co-adsorbates on the catalytic reduction of chlorate decreases, the ionicity of the support material having to be taken into consideration in addition. Accordingly, when selecting a suitable support material for the catalytic reduction of chlorate, the co-adsorbates concentration in the solution to be treated determines the effectiveness of the method.
In addition, it must be taken into consideration that different anionic co-adsorbates have differently pronounced effects on the catalytic reduction of chlorate. Hydroxide ions have a greater effect than bromide ions and these, in turn, have a greater effect than chloride ions.
When, comparing these effects, the following sequence may be noted:
OH
−
>Br
−>Cl
−
.
In a preferred embodiment of the invention, support materials with an effective electronegativity of less than 2.2, are used for co-adsorbate concentrations of a less than 5 g/l of chloride.
For a co-adsorbate concentration of, for example, greater than 5 g/l of chloride, a support material with an effective electronegativity of more than 2.2 is used. In addition, a further criterion for the choice of support material here is the value of the difference ofthe electronegativities ofthe atoms, contained in the support material, that is, the ionicity of the support. Pursuant to the invention, it must be less than 1, if the co-adsorbate concentration is greater than 5 g/l of chloride.
It was furthermore found that, if the solution to be treated contains an amount of co-adsorbate corresponding to less than 5 g/l of chloride, Rh/SiC proves to be the catalyst with the best chlorate decomposition result. For co-adsorbate amounts corresponding to more than 5 g/l of chloride, Rh with activated charcoal as support material proved to be the catalyst with the best chlorate decomposition values. Elevated temperatures and increased hydrogen pressures have a favorable effect on the catalytic reduction of chlorate.
It was furthermore found that the req
Klesing Armin
Neuenfeldt Gerhard
Ottmann Alfred
Santen Rutger Van
Cintins Ivars
Crowell & Moring , L.L.P.
Solvay Deutschland GmbH
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