Inorganic Chemistry / Resource Chemistry
Complex formation, hydrolysis and precipitation of indium and germanium under bioleaching conditions
During the bioleaching process, there are a number of ligand ions interacting with indium and germanium strongly influencing solubility and chemical behaviour. The stability of the complexes that are formed, directly affects how target metals can later be separated from other metals, such as zinc, iron and copper, in the later stages of the bioleaching pathway.
The ligand with the highest concentration in the leachate solution is the sulphate ion, produced during the oxidation of the sulfidic ores. However, significant concentrations of other sulphur-containing ligands, as well as chloride and carbonate ions will also be present.
The aim of this sub-project is to develop a method to quantitatively determine speciation and complexation constants in process-relevant solutions. These constants will be used in geochemical and process engineering modelling to create optimal conditions for indium and germanium extraction from the leachate.
A methodology has been developed to determine the speciation and stability constants of various aqueous indium complexes using differential pulse voltammetry (DPV). In a titration where the titrant and sample contain equal concentrations of acid and In3+ ions and equivalent concentrations of ligand and supporting electrolyte anions respectively, small changes in ligand concentration can be made quickly and accurately while maintaining overall ionic strength. DPV measurements were made after each titration step and the half wave reduction potentials were recorded from the In3+ reduction peaks. From the change in the half wave potential as a function of ligand concentration, the value of n and the stability constant log ßn for the complex [InLn]3-x were determined. The following constants were found in good agreement with literature where available: InCl2+ log ß2 3.89 ± 0.11, InCl4- log ß4 4.49 ± 0.03, InNO32+ log ß1 0.40 ± 0.03.
We have also investigated the suitability of supporting electrolytes in these techniques. We have proven that at concentrations of nitrate anions used regularly in similar experiments (≥ 1 mol kg-1 NO3-) there is formation of InNO32+ complexes. In the light of these findings, previously published stability constants of indium using nitrate-based supporting electrolytes should be used cautiously. The similarity between the stability constant for InSO4+ we have determined (log ß1 = 1.77 ± 0.03) in 1 M NaNO3 and the IUPAC recommended stability constant (log ß1 = 1.78) implies the IUPAC constant is potentially incorrect.
- MANS-12, The effect of inorganic acids on the electrodeposition of indium, Freiberg, 2014
- MANS-13, The effect of ligand competition on the speciation and electrochemical behaviour of indium in acidic solutions, Chemnitz, 2015
- MANS-14, The electrochemically active speciation of indium in aqueous sulphate and chloride solutions, Halle, 2016
- GDCh-Wissenschaftsforum, The effect of ligand competition on the speciation and electrochemical behaviour of indium in acidic solutions, Dresden, 2015
- BHT, Untersuchungen zu Elektrochemie und Speziation des Indiums in sauren Laugungslösungen, Freiberg, 2015
- ISSP-17, Speciation and electrochemistry of indium in aqueous sulphate and chloride solutions, Geneva, 2016
- Ashworth, C.; Frisch, G. (2017) Complexation equilibria of indium in aqueous chloride, sulphate and nitrate solutions – a voltammetric investigation, Journal of Solution Chemistry (submitted)