CHARACTERIZATION AND LIBERATION ANALYSIS OF RARE EARTH ELEMENTS INCLUSIONS WITHIN THE APATITE-RICH TAILING FROM THE PEA RIDGE IRON MINE

In response to a presidential executive order 13817, the US Department of the Interior listed 35 minerals considered necessary for domestic production supply and defense security [2018]. Rare earth elements (REEs) are essential critical minerals because of their industrial applications in modern electronics, green energy, health care, transportation, and defense. As a result, the mining and metallurgy sectors have sought domestic resources of REEs to address the production supply challenge. This study investigates the Pea Ridge Iron Mine, located in Washington, MO, with historical evidence of REEs deposits.

Pea Ridge Iron Mine mainly processed iron ore, rich in apatite minerals [Nuelle, 1998]. Additionally, REEs were understood to occur as inclusions within apatite grains [Whitten and Yancey, 1990]. Literature assay values for the REE-breccias pipes in the mine showed >10,000 ppm of REEs [Grauch et al., 2010]. Therefore, this study assays the rare earth element (REE) content and locking behavior of the apatite minerals discarded into the tailings dam, using characterization methods such as XRF, ICP-MS, and TIMA.

XRF analysis of the samples showed P₂O₅ and CaO at concentrations of 25 and 39 wt%, respectively, and major gangue content was identified as Fe₂O₄ concentrations of 15 wt %. The ICP-MS showed that the relative abundance of total REEs in the sample was 12000 ppm. ICP-MS also highlighted the higher concentration of light REEs (9000 ppm) than heavy REEs (3000 ppm). Locking behavior and mineralogy were determined using TIMA that presented a high concentration of crystalline apatite (79 wt%), (Ca, REE)₅(PO4)₃(F, Cl, OH). In comparison, the main gangue minerals were iron oxides (10 wt %) and quartz (3 wt%). The liberation and locking study revealed that monazite, (Ce,La,Nd,Th)PO₄, was the main rare earth mineral (REM) which occurred as -10 µm inclusions in the apatite minerals.

Moreover, the characterization data revealed that REEs were anticipated to be locked in larger apatite crystals, making it possible to concentrate them further using conventional apatite beneficiation circuits that rely on the froth flotation process. Thus, this study will immensely contribute to the ongoing efforts to help impact the domestic production of REE as part of the USA’s critical minerals supply strategies.