Yangyang Huai^a,* Yupeng Qian^a,b Yongjun Peng^a

^aSchool of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia

^bSchool of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China

Applied Surface Science

Volume 531, 30 November 2020, 147334

https://doi.org/10.1016/j.apsusc.2020.147334

In this work, the authors use Cryo-XPS to study an industrially relevant electrochemical reaction – sulfidisation on the surfaces of metal oxides.

Malachite (Cu₂(CO)₃(OH)₂) is an important mineral in the production of copper, particularly due to high demand and diminishing copper sulfates resources. Part of an efficient extraction process is called froth flotation and unlike the sulfate minerals, malachite requires the formation of a sulfate layer to enable the flotation process to occur successfully. The effectiveness of this process is inconsistent in industry and optimal concentration of hydrogen and sulfur ions difficult to establish. The authors have combined electrochemical methods and Cryo-XPS in order to improve the knowledge of this process.

Cryo-XPS

Malachite was analysed in the presence of several concentrations of Na₂S at 130K, which allowed an electrolyte layer of the solution to remain unvaporised on the surface within the UHV system. 0 mM, 1 mM, 2 mM and 4 mM droplets (matching those solutions used in the elecetrochemical studies) were deposited onto the surface of a sample of malachite and the sulfidisation process allowed to occur for 300s. Deionised water was used to rinse the sample and the system was transferred to the Cryo-XPS system.

The resultant XPS determined that while 2 mM of NaS₂ was sufficient to form a fully sulfated layer, malachite + 1 mM Na₂S was unable to sulfate all of the hydroxides and carbonates. These results plus a number of electrochemical studies enabled the authors to postulate new mechanisms for the sulfidisation process.

For a low concentration of sulfating agent, they suggest a poorly conductive mixture of Cu(II)OH∙Cu(I)S while increasing the concentration forms a semi-conductive Cu(I)S chalcolite layer above surficial elemental sulfur. It is hoped that these observations may enable a more efficient industrial flotation process.