Process for selectively removing molybdenum from liquid mixtures containing it together with vanadium

The present invention relates to a process for selectively removing molybdenum from liquid mixtures containing it, together with vanadium.

The separation between molybdenum and vanadium from inorganic solutions containing them is generally not very easy. Among the most widely-known and applicable technologies currently used, the precipitation of molybdenum sulfide with alkaline sulfides or sulfuric acid, which leaves the vanadium in solution, or extraction and separation with organic solvents, can be mentioned.

In the former case, the process is relatively simple, leading to the formation of an inorganic compound insoluble in organic solvents and which cannot therefore be directly re-used in certain applications, in the latter case, the process is definitely more complex with the formation of an organic solution containing molybdenum.

We have found that by using alkaline xanthates in certain concentrations, it is possible to obtain good separation yields using a simple process which leads to the formation of molybdenum compounds having a solubility in organic solvents which allows them to be directly re-used in certain applications.

Xanthates of alkaline metals have been known for some time and are mostly applied, apart from the importance of cellulose xanthate for the production of rayon, as flotation collectors of metallic sulfides, in particular copper and molybdenum sulfides.

They are also used in the vulcanization of rubber, as herbicides and pesticides, additives for lubricants, etc.

Free acids are of the ROCS 2 H type, colourless, unstable and soluble in common organic solvents; they decompose at room temperature into carbon disulfide and into the corresponding alcohol according to the following scheme:

ROCS 2 − +H + =ROCS 2 H=ROH+CS 2

The corresponding alkaline salts, on the other hand, are relatively stable solids, soluble in water, alcohol and polar organic solvents. In aqueous solution and at room temperature, potassium ethyl xanthate, for example, hydrolyzes as follows:

6C 2 H 5 OCS 2 K+3H 2 O=6C 2 H 5 OH+2KSCS 2 K+K 2 CO 3 +3CS 2

by further hydrolysis of the carbon disulfide and trithiocarbonate, hydrogen sulfide is formed.

The salts of heavy metals are more soluble in organic solvents, chloroform, THF and benzene whereas they are not very soluble in water, alcohol and aliphatic hydrocarbons. A sufficient number of C atoms makes them more soluble in organic solvents.

The process, object of the present invention, for selectively removing molybdenum from liquid mixtures containing it, in a quantity greater than 200 mg/l, together with vanadium, is characterized in that it comprises the following steps:

bringing the liquid mixture to a pH of less than 3, preferably equal to or less than 2; adding a solution of an alkaline xanthate to the solution so that the molar ratio molybdenum/alkaline xanthate ranges from ¼ to ⅛, preferably from ⅕ to 1/7, maintaining the pH constant by means of the addition of an inorganic acid; stirring the mixture causing the precipitation of the molybdenum present in the mixture.

The alkaline xanthates recommended for the process according to the invention are alkaline ethylxanthates, in particular potassium ethylxanthate.

Some examples are provided for a better understanding of the present invention but should not be considered as limiting the scope of the invention itself.