Investigating the potential recovery of REY from metalliferous sediments in a seafloor analogue : the Troodos ophiolite, Cyprus

• Main Author: Josso, Pierre
• Format: Dissertation
• Language: English
• Published: University of Southampton 2017

The perceived supply risk for essential materials used in the development of green energy and other state-of-the art technologies creates the need for investigation of new sources for these raw materials. Many of these raw materials are characterized as “critical” given supply risks posed by geographic location, the economic and political stability of producing countries, potential substitution and opportunities for recycling [European Commission, 2014]. At present, 20 raw materials are listed by the EU as critical and this inventory is likely to grow in the coming years as the world population increases, driven by the development of India, China, Africa, Brazil and others. Among these critical elements, the rare earth elements and yttrium (REY) form a group of 15 metals essential for the development of wind turbines, cell phones and batteries among other applications and their production has been under Chinese domination for the last three decades. More than 95 % of the consumed REY worldwide originated in China during the last thirty years, a monopole that reflects economical constrains rather than the unequal distribution of REY resources across the world. Indeed, important proven reserves are known outside China though their extraction is expensive and energy consuming. In addition, most REY-rich deposits possess important concentrations of actinides (U and Th) problematic for waste disposal. This study therefore investigates the potential recovery of REY from umbers, metalliferous sediments of the Troodos massif in Cyprus, as an alternative to the dominant magmatic-related REY deposits. Field evidence and geochemical characterisation of umbers show strong similarities with high-temperature plume fall-out deposits observed in most mid-oceanic ridge settings. Umbers constitute fine-grained brown Fe-Mn-rich mudstones with an amorphous oxyhydroxides dominated mineralogy and total rare earth oxide contents of ≈0.05 wt. %. REY fractionation trends show excellent comparison with signatures of hydrothermal particles settling around active vents. The umbers display a negative Ce anomaly in a convex upward REE trend when normalized to chondrite, characteristic of a hydrothermal signal overprinted by seawater. From an economic perspective, although the REE content is low, the absence of mineralogical control on the distribution of these elements in umbers and the extremely low radioactive content (Th + U < 5 ppm) makes their potential extraction attractive. A protocol for the leaching of umbers is presented testing a variety of lixiviants used in the REY extractive industry. Results show a strong mobilisation of the lanthanides in the solution in comparison with non-targeted elements. Most importantly, the results presented highlight that 80 to 90 % of the initial REY content of umbers is leached out using weak acid concentration in a matter of hours at low temperature. Fractionation along the REY series during leaching usually favours the release of the middle and light REE with a decreasing trend towards the heavy REE, except for Yttrium. Ce recovery is minimal as a result of its tetravalent oxidation state allowing formation of acid-resistant Ce oxides. Furthermore, a process of selective precipitation is presented for the purification of the leach solution and extraction of a solid REY phase using ammonium oxalate as a complexing and chelating agent. Precipitation experiments show the precipitation efficiency is a function of pH, between pH values ranging from 0.7 to 3.2, with more than 96 % of REY precipitated at pH > 1.1. Purity of the precipitate is adjusted using precise pH buffering to avoid Ca-oxalate formation as the major impurity. Indeed, mass balance calculations and direct EDS measurement of the oxalate precipitate by SEM show maximal purity at pH 1.1 (66 – 94 % REY) while increasing Ca precipitation decrease purity below 10 % at pH > 1.5. The fractionation observed along the lanthanide series during the precipitation experiments was successfully reproduced via numerical modelling using PHREEQC software. REE distribution within the precipitate therefore reflects the interplay of aqueous and solid REY-oxalate complexes stability constants as well as incorporation of REY within the structure of co-precipitating Ca- and Na-oxalates. This study demonstrates the feasibility of extracting efficiently REY from Fe-Mn oxide-rich metalliferous sediments. These deposits constitute interesting alternatives to high-grade deposits and their processing for REY production could be valuated as a by-product of pigment production. Alternatively, the process presented here could be applied to other oxide-based formations including marine ferromanganese deposits, or industrial wastes containing comparable high-tech metals concentration and enrichment process.