An investigation of the iron-cyanide mineralization in gold mine dumps
Loading...
Date
2022
Authors
Welman-Purchase, M. D.
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Cyanide, in the form of NaCN/KCN, is still widely used world-wide in the gold extraction process. Success of this extraction is owed to the high affinity that cyanide has for gold. Post extraction, waste material is added to tailings dam facilities in the form of a slurry. This slurry is likely to contain cyanide, which is hazardous if it is not managed. Another element that cyanide has a high affinity for, is iron. Iron-cyanide compounds (such as Prussian and Turnbull’s blue) are CNSAD (strong acid dissociable cyanide), meaning that they are stable compounds that only dissociate at pH conditions that are very low (<1). UV-radiation also dissociates these compounds. The potential determination of iron-cyanide compounds to form in the environment and their subsequent dissociation, is an area that requires more investigation. The main aims of the study include the modelling of Prussian/Turnbull’s blue in the goldmine tailings environment. To determine in a lab investigation whether or not iron-cyanide are able to form in different samples associated with such environments. Different analysis of the different samples and the analysis of a blue sample found on a tailings dam close to Welkom. Further modelling the affects that pH and the presence of other complexes has on the production of Prussian blue. The last aim was to determine if there are bacteria present that are able to degrade cyanide. The research approach consisted of two branches, namely modelling and the analysis of samples. Modelling was preformed using PHREEQC. The samples mentioned in this research consist of two branches, namely the laboratory study of adding cyanide to different samples and analysing them, and analysing tailings samples. Methods used in the laborarory study include XRD, FT-IR, XPS and total cyanide analysis. 9 samples were collected from a tailings dam close to Welkom, which forms apart of the Free State goldfields. XRF, ICP-OES, total cyanide, CNS analyzer (LECO) and metagenome analysis were performed on these samples. The main findings show that Prussian and Turnbull’s blue are able to form in such tailings environments. Prussian blue forms in an oxic environment and Turnbull’s blue forms in both an oxic and anoxic environment. The main variables that affect the formation of these iron-cyanide compounds is oxygen, the pH and concentrations of cyanide and iron available in solution. A NaCN solution was added to a pyrite sample, a Witwatersrand reference material and a tailings sample, where pyrite and the Witwatersrand reference material produced a blue substance. The analysis of these two samples and a blue sample from a tailings dam were analysed with FT-IR, revealing the presence of an iron-cyanide bond. The cyanide concentrations of the 9 tailings samples ranged from 0.6 – 10 ppm, where the highest concentration was found in a sample containing a blue substance (2.1) and the lowest is deeper into the tailings below the blue sample (2.3), suggesting that iron-cyanide compounds/complexes immobilize cyanide. The metagenome analysis revealed that the naturally occurring bacteria in the tailings are able to degrade or assimilate cyanide in the oligotrophic tailings environment. Cyanide is a source of carbon for the bacteria and an energy source. In conclusion, iron-cyanide compounds/complexes are able to form in the goldmine tailings environment, where a blue sample from a tailings dam was analysed and determined to be an iron-cyanide compound. It was also determined that these compounds/complexes immobilize cyanide, which are naturally degraded by the bacteria on the tailings dam. Ultimately, geochemical risk assessments for mining projects may benefit from including a microbiological aspect, which has not previously been considered. It is recommended that the following options be followed for remediation of cyanide:
1) The addition of an iron-source for iron-cyanide formation, immobilizing the cyanide
2) Burial of the material, possible burial in a mine void (anoxic environments result in higher pH conditions and zero UV radiation) or the addition of a pH buffer e.g., dolomite
3) Determine which microbes are present in the environment
4) Nurture the bacteria colonies, if found to be necessary
Description
Thesis (Ph.D.(Geochemistry))--University of the Free State, 2022