Valorisation options for discarded platinum tailings in South Africa

Abstract

Tailings valorisation in various mining industries have been attracting much attention with a focus on value recovery and re-use. The interest in tailing valorisation was spiked by the significant environmental and social risks related to tailings storage facilities (TSFs). TSFs are prone to pollution and possible dam failures that can cause harm to the surrounding communities. This reconnaissance study aims to assess the re-purposing of the PGM tailings in various applications.

PGM tailings from a platinum mine in Rustenburg, South Africa, have been classified based on physical, chemical and mineralogical properties to analyse the possibility of value recovery and re-use. Tailing characterisation was completed by analysing moisture through thermogravimetric analysis (TGA), particle size distribution (PSD), specific gravity and bulk density, pH, X-ray fluorescence spectroscopy (XRF), ICP-OES, ICP-MS and X-ray diffraction (XRD).

PGM tailings reported a total moisture content of 29.8% and inherent moisture of 14.97%. PSD analysis indicated that the tailings had d90 values of 186 μm. Specific gravity and bulk densities of 3.2 and 1.4 g.cm-3 were respectively recorded. The tailings were also classified as alkaline with an average pH of 8.9.

The XRF tests performed indicated that the PGM tailings were dominated by four main chemical elements namely SiO2 (42.6%), Al2O3 (13.3%), MgO (15.3%) and Fe2O3 (13.1%). Furthermore, CaO and Cr2O3 were present in small amounts (< 10%). The ICP-OES results indicate that 99.6% of the sample consists of major elements such as Al (37.5%), Ca (26.9%), Mg (13.2%) and Fe (12.9%). ICP-MS results indicated the sample contained platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir) and ruthenium (Ru). The highest concentration of PGMs present in the analysed sample is 0.3 g.ton-1 Pt and 0.18 g.ton-1 Pd

Lastly, the XRD tests indicated the presence of crystalline phases composed of mainly orthopyroxene (65.8%), plagioclase (20.3%), and smaller amounts of mica and quartz. The PGM tailings were classified as an inert material with properties suitable for use as a possible cement extender, brick extender, carbon mineralisation adsorbent and desulphurisation adsorbent.

Cement mortar prism tests with partial cement replacement of 5%, 15%, 25% and 50% tailings were completed. The results indicated that the replacement of cement with PGM tailings reduced the compressive strength but was not below the acceptable standard. A 5% replacement produced cement with lower strengths of 39.5 MPa, which is still within the specified standards. The 15% and 25% replacements had a 28-day compressive strength rating higher than 25 MPa which is suitable for previous no-fine concrete (NPF). Lastly, by replacing 50% of the cement with tailings a final compressive strength of 11 MPa was obtained which is suitable for plaster material.

Clay bricks were manufactured incorporating PGM tailings by weight of clay in 10%, 20% and 30% fractions. To determine the success of tailings as brick extender mechanical properties and durability tests were performed. Results showed that tailings increased the dried and fired weight of the bricks by 5.3% and 6%, respectively. Bricks manufactured with PGM tailings have lower compressive strengths compared to traditional clay bricks. However, burnt clay bricks containing up to 30% tailings by weight of clay still meet the minimum compressive strength requirements specified by SANS.

Carbon mineralisation and desulphurisation experiments were carried out and the results were benchmarked against adsorbents proven to be successful. The findings show that for both these experiments, insignificant amounts of adsorption occurred and that the tailings showed a similar trend as an empty reactor.

Based on the results, it can be concluded that utilising tailings materials as cement and brick extenders is a viable and beneficial approach to reduce the risks and management related to TSFs post life of mine. Unfortunately, the potential of PGM tailings as carbon mineralisation and desulphurisation agents requires further studies.