The invention concerns a process for the selective separation of uranium from its accompanying metals and its extraction by means of treating an aqueous acid solution of uranium with a collector, foaming it by the introduction by flowing of an inert gas, removing the foam, isolating the uranium and recovering the collector from the foam.
The importance of uranium has been increasing steadily in recent years, because at the present time it is the most important nuclear fuel. The demand for uranium is therefore rising constantly.
In nature, uranium compounds are found in numerous rock formations, but their concentrations in uranium are very low and generally are under 0.04%. Uranium ores are concentrated by means of conventional processes, by flotation, magnetic separation and other processes, and then the uranium is generally extracted from the ores.
In the wet processing of ores containing uranium, liquors containing very low concentrations are obtained, particularly in comparison with the concentration of the accompanying metals, especially iron, aluminum and calcium. Available processes for the extraction of uranium, for example, the ion exchange methods or liquid extraction processes, are time consuming and expensive and comprise several stages.
The problem of recovering uranium in the wet metallurgical processing of spent fuel of nuclear power plants presents itself in a somewhat different but fundamentally similar manner. Here the relative proportion of uranium is significantly higher than in ores poor in uranium. There exists therefore a need for a simple process, whereby uranium may be separated selectively from its accompanying metals, especially iron, but also aluminum and calcium and other metals.
German Auslegeschrift (DT-AS) No. 1,166,113 describes a flotation process for inorganic ions, wherein, depending on the charge of the ions to be flotated, an anionic or cationic collector is added to the solution containing the ions, whereupon the insoluble reaction product formed is placed into suspension by means of blowing in a gas and is removed in the form of foam. The process is also suitable for the separation of uranium in solutions of uranyl sulfate. If the solution additionally contains iron, it must be specially treated prior to the flotation. The iron (III) ions must be precipitated with ammonia as iron hydroxide, and sulfuric acid must be added until the iron hydroxide just begins to dissolve. This takes place at a pH value of approximately 3.7. In such a solution, the iron is present as Fe.sup.+++ and the uranium as a mixture of UO.sub.2 (SO.sub.4).sub.2.sup.2-, UO.sub.2 (SO.sub.4).sub.3.sup.4- and UO.sub.2.sup.2+.
If the pH value falls below 3.7, sulfate complexes of Fe.sup.+++ are increasingly formed, which are capable of floating together with the sulfate complexes of UO.sub.2.sup.2+. It is therefore necessary to maintain the concentration of sulfuric acid during the entire flotation process constant at a pH value of 3.7. This requires an unnecessary amount of control. The value of the above-mentioned process is further diminished by the fact that the collector must be added below its critical micelle concentration (cmc), because otherwise the efficiency of the process will be strongly reduced.
The above-mentioned process does not recite the separation of uranium from solutions containing salts in addition to iron sulfate.
The lowest H.sub.2 SO.sub.4 concentrations required for the leaching of lean ores depend on the composition of the ore and are between 0.1 and 1.0 Molar. The H.sub.2 SO.sub.4 concentration therein is high enough so that the iron already forms sulfate complexes to a considerable extent. A pretreatment of such solutions by precipitating the iron with NH.sub.3 and subsequent acidification with H.sub.2 SO.sub.4 until the precipitate disappears, leads to the loss of the major part of the sulfuric acid, which is thus lost for reuse in the course of the process, and to additional expense for NH.sub.3.
The sulfuric acid-containing liquors of domestic lean ores are such that the neutralization of the sulfuric acid with NH.sub.3 results immediately in the precipitation of a voluminous deposit. Even though this precipitate is readily flotated, it contains only traces of uranium. During the redissolution of the precipitate with the least possible amount of sulfuric acid, a pH value of 1.5 to 2.0 is immediately attained.
The process known from DT-AS No. B 1,166,113 therefore cannot be applied on an industrial scale and is especially not suited to the processing of low concentration uranium ores.