By Lidia Fernández-Rojo and Carlos Echevarría (Cetaqua)
In the mining industry, Acid Mine Drainage (AMD) is generated in the tailings ponds when the sulfur-bearing minerals are exposed to the atmospheric oxygen, leading to the formation of sulfuric acid and high concentrations of dissolved metal(oid)s. In the global context of critical raw materials scarcity, tailings, traditionally considered as a waste, represent a secondary source of valuable metal(loid)s. As a result, new technologies are being developed to recover them. Some of them are selective biological or chemical precipitation, adsorption, membrane filtration, electrochemical reactions, solvent extraction and ion exchange [1,2]. This last technology consists of extracting metals ions from a solution by exchanging them with weakly bound ions in a resin. Resins employed for this application are synthetic, composed of polymers and have the form of small beads (Fig. 1). Ion exchange resins have the advantage over other technologies to be highly efficient and selective and be able to recover metals from very dilute solutions. Furthermore, they can be easily regenerated and reused, which makes them a cost-effective option for metal recovery.
Fig.1. Typical resins beads employed in ion exchange technology.
In LIFE REMINE WATER, ion-exchange resins are employed to recover Cu and Zn from AMD. Before this step, a chemical oxidation with hydrogen peroxide and a chemical precipitation with caustic soda is carried out to remove Fe and Al and separate them from valuable metals. Then, the solution passes through a sand filter and finally gets in contact with the ion exchange resins that are located in vessels (Fig. 2).
Fig.2. Ion exchange resins vessels employed in LIFE REMINE WATER.
First assays show that this technology is able to remove more than 99% of inlet Cu and Zn, which are retained onto resins. 6 kg of Zn and 2 kg of Cu have been extracted from 8 m3 of AMD. A concentrated sulfuric acid has been employed to regenerate these resins and recover Cu and Zn in concentrated solutions. This last step is currently under optimization phase and it is expected to achieve concentrations up to 20 g/L for Cu and 30 g/L for Zn. These recovered metals can be further used for other applications in the mining industry.
[1] Chen, G., Ye, Y., Yao, N., Hu, N., Zhang, J., & Huang, Y. (2021). A critical review of prevention, treatment, reuse, and resource recovery from acid mine drainage. Journal of cleaner production, 329, 129666.
[2] Mwewa, B., Tadie, M., Ndlovu, S., Simate, G. S., & Matinde, E. (2022). Recovery of rare earth elements from acid mine drainage: a review of the extraction methods. Journal of Environmental Chemical Engineering, 107704.
[3] K. C. Sole, M. B. Mooiman & E. Hardwick (2017): Ion Exchange in Hydrometallurgical Processing: An Overview and Selected Applications. Separation & Purification Reviews, 10.1080.