| dc.description.abstract | Rare earth elements (REE) are now classed as ‘critical’ because of rising global demand as a result of their use in many new, green and emerging technologies. However, there is currently restricted supply, dominated by China (>90 per cent), and extremely low rates of recycling (less than 1 per cent). These factors increase the urgency to identify alternative REE resources. Bauxite, apart from being the main commercial source of aluminum, also has the potential to become an important resource for REE and other critical metals.
The Parnassus-Giona bauxite deposits (B1-B3 from oldest to youngest) in Greece were formed by lateritic weathering processes. The eroded material, derived from parent aluminosilicate rocks and ophiolites, accumulated in the karst topography of Upper Jurassic to Middle Cretaceous limestone to form large bauxite deposits. The crystallisation of authigenic REE-bearing minerals towards the base of the bauxite occurred due to the downward transport of relatively mobile elements (including the REE). Per descensum enrichment results occasionally in very high REE concentrations.
Aluminium S.A. is processing over 1.5 Mt of bauxite annually using the Bayer Process and as a result is producing approximately 700,000 t of red mud waste. Red muds from Greek B3 and B2 bauxite deposits contain an average of about 900 ppm ΣREE. The B3 bauxite deposits from which they are mostly derived usually contain lower ΣREE (approximately from 150 to 700 ppm), varying with location and depth. The red muds therefore contain approximately twice the levels of REE than the original bauxite, making them a potential alternative REE resource. However, compared to carbonatite REE deposits such as Mountain Pass that contains 8.24% REO on average and 5% REO cut-off grade, Greek bauxites and red muds commonly have less than 0.1% of average REO and cut-off grade. Moreover, Greek bauxites and red muds are also lagging in REE proportions compared to other similar REE resources such as the Jamaican red muds (0.23-0.38 wt.% ΣREE+Y) or the Chinese HREE enriched ion adsorption deposits (0.03-0.35% REO).
This study compares REE concentrations in bauxite and corresponding red mud waste from bauxite deposits in Greece to assess the feasibility of efficient REE extraction either by selective mining of REE-enriched bauxites or red mud waste stockpiles. In more detail, bauxite samples were collected from recently operating B3 underground bauxite mines and red mud waste samples from dehydrated stockpiles. These were examined for their major, minor (XRF) and trace elements (ICP-MS), main minerals (optical microscopy and XRD) and most importantly REE-bearing minerals (SEM and EPMA). Finally, various weathered REE-rich samples (bauxites, laterites and kaolinites) and their by-products (e.g. red muds) were analysed in order to determine if they contain any easily leachable phases.
The main outcome of the study is that both bauxites and red muds can be considered as REE potential resources, containing REE that their cut-off grade will not exceed the mining and processing costs, and in a form that is relatively easily recoverable using standard or modified leaching protocols. More precisely, bauxite could have a cut-off grade of 0.1% or more only at specific HREE enriched locations near the bedrock of the deposits. On the other hand, red mud as a waste of blended bauxite originating from various mines could contain 0.1% REO on average including LREE, HREE, Sc and Y. In the same context, red mud waste could be economically exploitable at a lower cut-off grade in comparison with a stand-alone bauxite mine because of the significantly lower mining costs. However, further work is required to determine the most economically and environmentally feasible method to extract REE from bauxites, before the Bayer process, or after it from red muds. | en_GB |
