Upconversion Fluorescence in Naturally Occurring Calcium Fluoride

Fluorine can negatively interfere with leach and smelting processes during mineral processing. Real-time knowledge of the concentration and mineral hosts of fluorine in a mineral processing ore stream is important to protect process line equipment and product. Currently only offline methods of detection are available. Online sensors that determine specific fluorine-bearing mineral concentration in real-time would enable improved efficiency in processing decisions during mine production. Common excitation wavelengths used for fluorescence studies in minerals frequently provide signals that are not clearly host-specific, and hence of limited utility for mineral identification. We show that upconversion fluorescence, a process in which two or more photons are absorbed and one higher-energy photon is emitted, provides a more host-specific fluorescence output, minimizing spurious signals in complex environments and therefore greatly improving detection thresholds. Natural samples of fluorite (CaF₂), a major fluorine host at many mine sites, have been analyzed by near-infrared excitation and have revealed upconversion fluorescence from rare earth inclusions. Upconversion fluorescence was detected in samples with rare earth concentrations as low as one part per million and is therefore considered a potential new sensing modality for real-time fluorite monitoring.