Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Rare earth metal
Reexamination Certificate
1995-05-15
2002-09-24
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Rare earth metal
C423S022000, C423S027000, C423S028000, C423S038000, C423S039000, C423S040000, C423S041000
Reexamination Certificate
active
06455018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the recovery of precious metals such as platinum, palladium, and rhodium, and optionally of other valuable elements, such as the rare earths and in particular cerium, from various compositions/materials comprised thereof, especially waste or spent catalysts, for example motor vehicle postcombustion catalysts.
2. Description of the Prior Art
It is known to this art that precious metals such as platinum, rhodium, palladium or iridium are currently widely used, whether alone or in combination, in a variety of industrial compositions, in particular catalyst compositions, and this optionally in conjunction with other valuable elements, such as, rare earths, for example, cerium.
Particularly exemplary such compositions are the so-called multifunctional catalysts (or “three-way” catalysts), and especially the catalysts for the treatment of the exhaust gases of internal combustion engines (motor vehicles or otherwise). Essentially for environmental protection reasons, considerable research and development in respect of such catalysts is presently ongoing (imposition of increasingly strict antipollution standards vis-a-vis the reduction of deleterious emissions of nitrogen oxides, of carbon monoxide and, equally, of unburned hydrocarbons).
Thus, at present, nearly 80% of the rhodium and 40% of the platinum values which are consumed worldwide are actually employed for the preparation of catalysts destined for the motor vehicle postcombustion sector. In view of the proportionately very high cost generated merely by the use of these precious metals in relation to the total cost of the finished catalysts, the recovery of same from a spent or waste catalyst for purposes of downstream recycling for the production of a new catalyst is a major desideratum in this art.
Processes for the recovery of the precious metals present in spent catalysts are described in the literature.
Particularly representative thereof are those which are principally based on a stage of acidic leaching of the spent catalysts containing the species to be recovered, this stage typically being carried out in the presence of an oxidizing agent.
Even more particularly according to these processes, the catalysts to be treated, which may either be in the form of pellets (beads, extrudates or the like) generally consisting of alumina, in particular of &ggr; active alumina, containing the various precious metals (as well as, optionally, other valuable elements such as, for example, cerium), or in the form of monoliths consisting of a framework (or support) of the honeycomb type made of refractory ceramic, and in most cases based on cordierite (silicoaluminate) onto which a washcoat (or coating layer) of alumina, in particular of &ggr; active alumina, is deposited, and also containing the precious metals (and other valuable elements), are preground and converted into a finely divided state. The powder thus obtained is then leached, one or more times, with strong inorganic acids such as sulfuric acid, hydrochloric acid or nitric acid, optionally in the presence of an oxidizing agent (H
2
O
2
, Cl
2
, HNO
3
or other), whereby one or more leaching solutions are obtained containing the desired different precious metals in a dissolved form, these being subsequently recovered from such solutions by any technique which is appropriate and per se known to the art, in particular by selective precipitation.
However, processes such as those described above especially present the disadvantage of exhibiting solubilization, and hence recovery, yields in respect of the various desired precious metals (Pt, Pd, Rh and others) which remain insufficient, this being particularly more so in respect of the rhodium species. Furthermore, other than the precious metals, these processes only very rarely address the case of the recovery of the other valuable elements which may be present in the spent catalysts, such as, for example, the rare earths and, in particular, cerium.
SUMMARY OF THE INVENTION
Accordingly, a major object of the present invention is the provision of an improved process for the recovery, at very high efficiency, of the different precious metals, and in particular rhodium, which are present in various compositions, for example of waste or spent catalysts, as well as for the recovery of other valuable elements which may be present in said compositions, such as, for example, the rare earths and, in particular, cerium.
Briefly, the present invention features an improved process for the treatment of any composition or material containing species of the precious metal type and, optionally, rare earths such as cerium, for the recovery of such species, said process including the following stages:
(i) optionally, first converting the composition to be treated into a finely divided state to increase the reactivity thereof,
(ii) intimately admixing this composition with sulfuric acid,
(iii) calcining the mixture thus obtained at a temperature of from 150° C. to 450° C., and
(iv) finally, treating the product thus calcined, in aqueous medium, on the one hand, with H
+
ions and, on the other, with chloride ions, this being carried out simultaneously or separately, whereby there are ultimately obtained, first, a solid residue substantially exhausted in respect of precious metals and, optionally, of rare earths such as cerium, and secondarily, one or more solutions containing the aforesaid species.
DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENTS OF THE INVENTION
More particularly according to the present invention, the combination of the above stages (ii) and (iii) corresponds to an operation which, for simplicity, will subsequently often be deemed “sulfuric pasting.”
The stage (iv) can be carried out essentially according to two different embodiments: (1) either, this being to selectively recover certain of the desired species, subjecting the calcined product to a double leaching by treating the product, first, with an acidic solution (introduction of the H
+
ions) and then, in a second step, with a solution containing chloride ions, whereby there are obtained, respectively, a first solution containing, for example, most of the cerium and a proportion of the rhodium, and a second solution containing, for example, most of the platinum and the remaining proportion of the rhodium; or (2) an overall recovery of the desired species is carried out by leaching all of these species simultaneously by treating the calcined product with a single solution having, on the one hand, the necessary acidity (introduction of the H
+
ions) and containing, on the other, the chloride ions.
Lastly, in either of the above alternative embodiments, the treatment with chloride ions is preferably carried out in the presence of an oxidizing agent.
The process according to the invention thus permits a selective or overall recovery, this being with very high efficiencies, of all of the species of the precious metal type or other valuable elements, in particular rhodium and cerium, which are present in the starting composition. It can reasonably be considered that the exceptionally high efficiencies thus obtained are essentially due to the preliminary sulfuric pasting stage which facilitates the subsequent leaching of the desired species.
According to the present invention, by the expression “precious metals” are intended, whether alone or in combination, all of the elements of the platinum group, namely the elements rhodium, palladium, iridium, platinum, ruthenium and osmium. Furthermore, by the expression “valuable element” are essentially intended the rare earths, namely the elements of the lanthanide series whose atomic numbers range from 57 to 71, inclusive, and more particularly cerium of atomic number 58, as well as yttrium of atomic number 39. Finally, by the expression “composition” are intended any composition containing one or more precious metals, optionally in combination with one or more valuable elements, and from which the recovery and subsequent
Bos Steven
Burns Doane Swecker & Mathis L.L.P.
Rhone-Poulenc Chimie
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