Design and Implementation of Large-Scale Hard Rock AMD Dewatering and REE Production

Abstract

To develop a domestic Rare Earth Elements (REE) supply chain the Hydraulic Preconcentrate (HPC) produced at the Horseshoe bend treatment plant (HSB) was evaluated as a potential source for feedstock. After the clarifiers were flipped to the desired pH set points, a total of 24 Geobags were repeatedly filled and allowed to passively dewater. The Geobags evaluated were comprised of a mix of Woven, Non-woven, and Capillary Channel Fiber (CCF). The bags were constructed with either a composite of woven and nonwoven or only woven geotextile. Half of the bags of both types also included “fins” made of CCF to evaluate their impact on dewatering. Column Filtration Tests (CFTs) were performed onsite to provide laboratory value for hydraulic conductivity. Throughout their dewatering, the geobags were sampled to determine their rate of dewatering. Influent samples from HSB were also tested to produce a Specific Gravity (G_s), via pycnometer testing and Grain Size Distribution (GSD), via sieve and hydrometer analysis.

The dewatering ability of the geobags was high regardless of bag material with a mean total solids percentage of approximately 40% across all samples after only one month of dewatering.  The GSD performed on the HPC is gap graded with most particles having a diameter of approximately 0.5-0.05mm. Hydraulic conductivity values for the filter cakes during Column Filtration Testing ranged from 0.5 to 2.5 meters/day. This data will help to guide development of future large-scale HPC dewatering and the parametric design of an REE oxide production plant at the HSB site.