E4 NERC New approaches to copper production: arsenic capture and solventless extraction

The capture and treatment of high levels of arsenic from copper-smelting flue dust, concentrates, and waste water that result from copper mining is important due to concerns over the environmental management of this potential carcinogen.[Refs. 1-3] Treatment technologies for arsenic-containing metallurgical wastes have been developed, including the oxidative precipitation of arsenic as its stable iron oxide mineral scorodite (FeAsO4.2H2O) from acidic copper solutions. However, these precipitation processes are time-consuming, yield poorly characterised materials, and can be compromised due to the cementation of heavy metals such as cadmium. Furthermore, the upstream production of copper by solvent extraction requires the use of environmentally damaging organic solvents. This project will seek to improve arsenic remediation from copper concentrates using new solvent extraction chemistry integrated with microbial precipitation to yield stable and environmentally benign waste arsenic materials. The use of sustainable solvents and the exploitation of solventless procedures in copper production will be explored.
Research questions

1. Can solvent extraction be applied to the selective separation of arsenic from copper and cadmium?

2. Can microbial transformation of the captured arsenic be optimised to produce stable solids at low temperature?

3. Can a designed, aqueous soluble reagent selectively precipitate copper from solution?

4. Can the modes of actions of these processes be understood such that generic improvements can be made in environmental remediation?
Methodology

This project will comprise research from the Schools of Chemistry and Geosciences and links with academics and industries in Chile, a country responsible for ca. 30% of the world’s copper production from mineral sources that contain arsenic and cadmium (Pontificia Universidad Católica de Chile, and Ecometales, Santiago, Chile) are already in place. Thus, a broad impact in the metal recovery and environmental remediation fields can be made.

Solvent extraction of arsenic: The solvent extraction of arsenic complexes will be evaluated in the first instance. Acidic leach solutions of arsenic (derived from copper concentrates) comprise the As(V) and As(III) complexes As(O)(OH)3 and As(OH)3, respectively and their related mono- and dianions. We will appraise their transport from an acidic aqueous into an organic phase using amide reagents L that we have developed for selective metalate extraction;[Ref. 3] it is anticipated that these reagents would become protonated and transport the arsenic as the anions, e.g. [HL][L]nAsO2(OH)2. The use of these reagents should allow for discrimination between As and Cu/Cd as the latter elements would be extracted as the Cu(II) and Cd(II) cations. We will study the efficacy of transport and its mechanism using a range of experimental, spectroscopic, and computational techniques.

Microbial precipitation of arsenic: Microbial precipitation of scorodite produces a stable solid but requires high temperatures (e.g. Vega-Hernandez et al., 2019),[ref. 4] which limits the process to thermophilic organisms. By contrast, several bacteria, including Desulfosporosinus auripigmenti and Desulfovibrio strain Ben-RB [ref.5] can simultaneously reduce As(V) to As(III) and S(VI) to S(-II), with consequent precipitation of orpiment (As2S3). However, this occurs over a narrow pH (6-7) and sulfide concentration range, with excess sulfide (> 1mM) solubilising the precipitate.[ref. 5] The key to optimising orpiment precipitation lies in controlling the rate of microbial sulfate reduction to limit the concentration of sulfide, and we will explore the use of consortia versus pure cultures, whilst also varying initial sulfate concentration to constrain As2S3 precipitation. A range of X-ray (spectroscopic and diffraction) techniques will be used to characterise the reaction products.

Solventless precipitation of copper: The use of organic solvents such as kerosene in organic/aqueous biphasic solvent extraction processes is becoming an increasing environmental issue. While advances are being made in the use of alternative solvents, for example ethylene glycol or mixtures of ionic liquids,[Ref. 6] aqueous only separations are poorly studied. We will therefore evaluate the separation of copper from acidic aqueous copper concentrates by selective precipitation or immiscibility using aqueous soluble, linked phenolic oximes (HL---LH) to form precipitates or oils in contact with Cu cations. On contact with Cu(II) these would form polymeric complexes (-L---LCuL---LCu-)n that would be insoluble in water. We will study the selectivity of coordination for a range of metals using a variety of phenolic oximes, characterise the solids/oils formed by spectroscopic and crystallographic techniques, and develop processes for the release of Cu and reagent recycling.

Timeline: 0-12 months Arsenic separation by solvent extraction; 8-20 months microbial arsenic precipitation; 14-26 months solventless copper precipitation; 20-36 months complete process optimisation.

Applications must be made directly to the E4 DTP www.ed.ac.uk/e4-dtp/how-to-apply by the deadline of 7 January 2021. Prior informal enquiries to the supervisors are welcome.