Extraction of tellurium as a volatile fluoride

Purpose and goal
In this project, we aimed at validating the concept of extracting tellurium by an innovative method called liquid-gas separation: the LIGEX process. This process requires fluorination of tellurium into a volatile fluoride. The goals were therefore to identify the most promising fluorination routes depending on the starting material and to perform fluorination of tellurium at lab scale. Both goals were achieved. A promising method to fluorinate tellurium oxide was identified and developed experimentally.

Structure and implementation
The project consisted of two parts: 1. Calculations of thermodynamic equilibria in order to evaluate the possibility of fluorinating tellurium in metallic (such as in Boliden’s copper telluride) or oxide form with hydrogen fluoride (HF) or fluorine. 2. An experimental study on the fluorination of tellurium oxide TeO2 with HF. Two different methods were tested, namely direct fluorination with anhydrous HF and dissolution in a diluted aqueous solution of HF containing fluoride salts of alkali metals. Only the second method was successful.

Results
Tellurium in the starting material must be already oxidised in order to ease fluorination. We managed to fluorinate tellurium oxide TeO2 into TeF5– ions in water solutions. However, it is not sufficient in order to obtain volatile TeF6 with the LIGEX process. Therefore, this method must be developed further by using TeO3 instead of TeO2 in an attempt to form TeF7– ions. It should be possible to extract these ions in aqueous solution by solvent extraction and to subsequently release TeF6 with high purity, which would be of high scientific and innovative impact.