Development of a database for tailing storage facilities for critical minerals recovery
Creators: name: Sarker, Shuronjit Kumar
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400599
subject: Civil engineering not elsewhere classified
classificationCode: 401903
subject: Hydrometallurgy

description: Society's growing demand for commodities has resulted in increased consumption of specialty minerals, some of which are deemed critical. These critical minerals hold varying degrees of importance for strategic purposes. However, their continuous depletion from primary sources, coupled with supply risks arising from geopolitical issues and geographical segregation, has become a significant concern. Consequently, there has been a heightened global interest in recovering these valuable elements from non-conventional sources, such as mining tailings. In fact, mine tailings are gaining attention because they often contain a higher amount of these elements compared to some primary deposits. Additionally, these tailings are the main source of pollution in mining operations, further emphasizing the importance of recovering critical minerals for environmental benefits too. The challenge, however, lies in identifying the potential tailings storage facilities (TSFs) containing these tailings. This challenge stems from the lack of a publicly available information on the tailings and TSFs. The objectives of this study were threefold: i) to identify and document the TSFs throughout Australia, creating a comprehensive database, ii) to conduct detailed characterizations of tailings obtained from selected TSFs to assess their potential for critical minerals, iii) to carry out hydrometallurgical experiments to recover the target critical minerals. By achieving these objectives, the study aims to contribute to the understanding and utilization of tailings, unlocking their potential as a source of critical minerals.

To develop the TSFs' database, this study employed visual inspection and satellite image interpretation using Google Earth. The objective was to identify and document the TSFs to make a functional database. A total of 1090 TSFs (331 active and 759 inactive) were identified across Australia. Among these sites, 98 TSFs were found within a 2-kilometer of urban centers. Coal and gold were the primary commodities associated with 27% of active mine sites with TSFs and 38% of inactive mine sites with TSFs, respectively. Additionally, approximately 16% of active mine sites with TSFs and 28% of inactive mine sites with TSFs were involved in the processing of copper as a major commodity. Considering the companionability matrix, it is apparent that many of these TSFs hold potential for the possible recovery of critical minerals, such as rare earth elements (REEs).

Two tailings samples were obtained from two TSFs identified using the developed database. One tailing sample was collected from a Zn-Cu-Pb mining site where sulfide minerals are processed. The tailing samples were analysed for physical, chemical, and mineralogical properties. The results revealed that the tailings were fine-grained (~50% below 63 μm). The tailings consisted of Si (~17 wt%), Ba (~13 wt%), and Al, Fe, and Mn (~6 wt%). Among these elements, Mn, a critical mineral, was the target element which was found to be predominantly present in rhodochrosite (MnCO3). Elemental distribution indicated that ~93% of Mn was distributed in -150+10 μm, accounting for 75% of the total mass. Furthermore, a mineral liberation analysis revealed that Mn grains were primarily liberated in sizes below 106 μm, suggesting the need for gentle grinding of +106 μm particles to unlock the trapped Mn minerals. Given the fine-grained nature of the sample, it is possible that direct hydrometallurgical processing could be a viable method for extracting Mn from this sample. 

The hydrometallurgical experiments involving leaching with H2SO4, HNO3, and HCl were assessed for their effectiveness in Mn recovery.  The leaching was optimized for the best-performing acid using multivariate leaching parameters. Leaching with H2SO4 resulted in a low Mn recovery and the process resulted in the formation of a gypsum paste at high acid doses. Leaching with HNO3 yielded good Mn recovery but selectivity over Fe (an impurity) was poor. In contrast, HCl acid leaching gave the best efficiency and selectivity for Mn recovery. Under the optimized conditions of 5% HCl, an L:S ratio of 5, 90-minute leaching time, and 50ºC temperature, the Mn recovery was an impressive 95%, with only 12% of the Fe being leached. The weight loss was 26% and phase changes occurring during leaching were confirmed by XRD and SEM analysis. Overall, the findings suggest that HCl acid leaching offers the potential as a highly efficient and selective method for Mn recovery from sulfide tailings.

The second tailings sample was sourced from a REEs mining site that processes carbonatite minerals. The tailing was chosen because most of the world’s REEs are produced from such deposits. The results revealed that the tailing consisted of mainly fine particles (50 wt% below 61 μm) and contained mainly Fe, and REEs accounted for ~50% and 9%, respectively. The major mineral phases were monazite containing REEs and goethite, a gangue mineral. The elemental distribution showed 70% of mass, total REEs, and Fe were concentrated in -63 μm size fraction. The mineral liberation analysis found that REE-minerals were primarily associated with goethite and were locked within it in +100 μm particles. However, -50 μm particles of REEs minerals were mostly liberated. The findings suggest that light grinding of +63 μm particles would liberate the locked REEs-minerals. Because the sample was fine-grained, direct hydrometallurgical processing could potentially be effective to recover REEs.

The hydrometallurgical experiments involving HCl acid leaching on the carbonatite-derived tailings were employed. The leaching conditions encompassed various factors such as acid concentrations, liquid-to-solid (L/S) ratio, temperature, and leaching time. The experiments also involved an alkali pre-treatment of the tailings to assess the impact of NaOH pre-treatment on the recovery rates. The results revealed a maximum 89% of REEs recovery of when the sample was leached without alkali pre-treatment, with 100% of Fe was also leached. With NaOH pre-treatment, the recovery reached a maximum of 95%. However, the leaching process exhibited a lack of selectivity, as nearly all Fe (99%) was also leached. To selectively remove Fe from the leachate, NH4Cl solution was added, resulting in a 95% separation of Fe. This study serves as a demonstration of the potential of tailings as a viable source for critical minerals, emphasizing the importance of reprocessing them to recover valuable resources, which not only addresses environmental concerns but also provides economic benefits.