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Mine tailings recovery can be value generator, says Council for Geoscience

9th June 2017

By: Ilan Solomons

Creamer Media Staff Writer

     

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Mine tailings have been identified as a major source of environmental impact for many mining operations and more stringent requirements have been placed on the management of these tailings.

An aspect which is often overlooked in the life cycle of sustainable mine residues management is that mine residues can also represent untapped secondary resources for valuable minerals and metals. “Therefore, finding routes for recovering value from these wastes will assist in reducing their disposal and in making treatment processes more economically viable,” says Council for Geoscience geologist Sameera Mohamed.

She points out that platinum-group metals (PGMs) tailings contain a significant portion of aluminium, iron, calcium and magnesium bearing silicates, which have the potential to serve as a secondary mineral source.

Mohamed comments that tailings can be reprocessed and converted into value-added products, provided that a suitable economically viable technological process can be developed.

“For instance, elements such as iron could be used by the iron and steel industry, or as feedstock for the synthesis of nano-based materials for wastewater treatment processing, or for dye removal in aqueous solution,” she highlights.

Further, Mohamed notes that aluminium could be used to prepare alumina and magnesium could be converted into magnesite, as a carbon dioxide sequestration mineral technology.

She elaborates that PGM tailings have been assessed for mineral carbonation technologies; however, efficient elemental extraction using direct acid leaching methods is problematic.

Mohamed says that elemental recovery from PGM tailings using a multistage process has recently been successfully demonstrated. She explains that the process involved thermochemical solid-solid treatment of the tailings using ammonium sulphate, a widely available, low-cost, recyclable chemical agent, as an extractive agent.

“Thermochemical treatment induced mineral transformations within the tailings and, in particular, promoted the formation of water-soluble metal species. “Quantification of the efficiency of the process was achieved by subjecting treated tailings to an optimal dissolution procedure to enable metal extraction from newly formed mineral species.”

Mohamed states that extraction efficiencies greater than 50% for iron and 60% for aluminium were achieved using this process. Key variables included temperature, duration and a solid-solid stoichiometric ratio used during thermochemical treatment.

The study investigated the synthesis of iron-based nanoparticles from PGM tailings. She says that, at first, synthesis of iron nanoparticles was achieved using a co-precipitation method from commercial iron-based reagents such as ferric- and ferrous-chloride and sulphate to gain insight in magnetic nanoparticles preparation and stability.

Mohamed mentions that the effect of reaction temperature, solution pH and sulphate concentration in solution on the particle size distribution, uniformity, stability, crystallinity and morphology of synthesised nanoparticles was investigated.

She comments that black magnetic particles were formed under all conditions, regardless of the precursor salt used. X-ray diffraction analysis confirmed the crystalline nature of iron nanoparticles and showed diffraction peaks corresponding to magnetite and/or maghemite phases.

Mohamed points out that the nanoparticles exhibited different properties in terms of hydrodynamic particle diameters as well as colloidal stability over the pH range studied.

Further, she highlights that synthesis of magnetic iron-based nanoparticles was unsuccessful when using the filtrates obtained after thermochemical treatment and dissolution of PGM tailings.

“The most plausible reason was that the leachate contained only Fe3+ cations, whereas both Fe3+ and Fe2+ are required to form iron oxide. An additional reduction step will therefore be required for co-precipitation to occur. A suitable reducing agent has been identified and experimental work is under way,” Mohamed reveals.

Nonetheless, she highlights that the research has shown that iron extracted from PGM tailings may be a suitable feedstock for the synthesis of iron nanoparticles, and a combination of co-precipitation and reduction processes will need to be used to achieve the necessary Fe3+/Fe2+ ratio for the formation of magnetic iron nanoparticles from tailings- derived filtrates.

“Further investigation regarding the influences of various reaction parameters during the synthesis of iron nanoparticles needs to be pursued to better understand the reaction mechanisms taking place during the co-precipitation and reduction steps,” Mohamed concludes.

Edited by Martin Zhuwakinyu
Creamer Media Senior Deputy Editor

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