REE (re)cycle: a multi-sensor investigation from rocks to tailings

Abstract

Rare earth elements (REE) are key constituents in electronic devices (e.g. smartphones, batteries), being present in both end-user and industrial applications. The rapid innovation cycles of electronic devices, combined with the increasing demand for new technological applications (e.g. mobility and e-cars) pose a challenge for the supply of REE, which are considered as Critical Raw Materials (CRM). This scenario calls for rapid, non-invasive methods that enable the identification of new REE-rich mining resources. Furthermore, the high supply risks associated with CRM such as REE drive technological developments to compensate and overcome market fluctuations by turning previously not mined co-resources into valuable and economic modalities, such as re-mining materials. We present an investigation focused on the identification of REE in waste rocks and tailing materials from the mine of Siilinjärvi (Finland). The deposit in the area consists of alkaline-carbonatite rocks, with the most important REE-bearing minerals being apatite (average REE concentration: 0.4% (wt%)) and monazite (REE concentration: up to 67% (wt%)). Mining activities focus on extraction of phosphate from fluorapatite, and the chemical reactions involved in this extraction generate phosphogypsum (PG) as a by-product. Literature reports indicate that REE can be incorporated to the PG matrix in the crystallisation process, with the most relevant examples including Nd, Ce, La, Sm, Gd, Tb, Dy, and Eu.  Our goal is to highlight how the sequential acquisition by multiple optical methods (multi-sensor approach) can trace REE contents for individually identified REE from pristine rocks to processing waste dumped in tailings. Each material type was scanned by two fast hyperspectral imaging (HSI) sensors integrated in a conveyor-belt system:  a reflectance-based HSI sensor operating in the visible to near-infrared and short-wave infrared (Specim AisaFenix); and an innovative laser-induced fluorescence line scan sensor (HSI-LiF, Freiberg Instruments). The optical sensing results were validated by mineralogical methods (mineral liberation analysis (MLA)). MLA results for PG indicate the presence of REE-bearing minerals including gypsum, apatite, and monazite (respective abundances (wt%): 97.4, 0.6, and 0.08). Optical features characteristic of Nd were identified on rocks and tailings samples by both HSI-reflectance and HSI-LiF sensors. Spectral signatures were detected in HSI-LiF spectra for an additional REE group including Sm, Er, and Pm. We highlight that efficient, non-invasive optical sensing can detect and re-evaluate tailing materials as a baseline for economic considerations according to market needs. The results confirm that REE detected on the pristine rocks of the mine can be traced through the mineral processing route to be found again in the tailings material. The multi-sensor optical detection based on HSI-reflectance and HSI-LiF, accordingly, provides an efficient non-invasive tool for exploring both mining and re-mining potential by providing immediate results on REE types and their spatial abundance, when employed as scanning techniques. This investigation was performed within the scope of EIT-funded projects (inSPECTOR and RAMSES-4-CE).