Keywords: REEs, extraction, separation, purifications, REOs, chromatography, processing
Summary:An innovative chromatographic process for extracting REEs from low-grade source materials has been evaluated through a series of research projects funded by the Federal Government and from internally funded activities. The goal of these efforts was to develop a processing methodology that can produce mixed REO concentrates with purities exceeding 85% and low levels of actinide contamination from various REE source materials. Results show the developed chromatographic process utilizing proprietary stationary media coupled with optimized pre-processing of REE digestion solutions can produce an isolated mixed REO material with very low actinide residual. The process has been applied to low-grade REE waste ore materials containing apatite and monazite as well as a raw rhyolite ore. The simple process consists of a digestion stage that can include acid dissolution and caustic bake digestion followed by chromatography, column adsorption, and precipitation. Chromatography columns are integrated into an overall process scheme that involves the injection of the solution to be separated (the bolus) into the column followed by introduction of the mobile phase. Based upon the chromatographic response from the column, effluent from the column is diverted into specific holding tanks. These collected fractions can then be be further processed to produce desired products. This enhanced combination of processes is able to separate, isolate, and purify extracted REEs into a high-quality REO product while simultaneously removing residual contaminants such as thorium and iron. Processing results from source materials including iron mine waste tailings, beneficiated phosphate ore, and phosphogypsum waste containing between 0.1 to 1.5wt% REEs show processes efficiencies exceeding 90% while producing REO concentrates higher than 90wt%. For one waste ore, the process system was able to produce an REO solid that had 0.3wt% calcium, 2.3% phosphorous, 0.2wt% Fe, and 93wt% REEs given the fact that the influent to the chromatography system had calcium, phosphorous, iron, and REEs concentrations of 35 g-Ca/L, 65 g-P/L, 2 g-Fe/L, and 0.14 g-REEs/L respectively. In addition, the system removed virtually all of the thorium and uranium present with final the REO product having less than 0.003wt% total actinides. Although the current process has been optimized for REE extraction and ultimately the production of mixed REO concentrates for use by secondary processors that will separate the mixed REO material into individual REOs, observations show that the chromatographic process has the potential of separating mixed REEs into individual fractions. Given specific operating conditions, the separation of lanthanum and yttrium from neodymium and praseodymium approaches 40%. In one specialized test, the separation of lanthanum and yttrium from Nd/Pr was 100%. This result has potentially critical impacts on current employed REE separation techniques such as solvent extracting since a simple pass-through system employing chromatographic separation of individual REEs would greatly simplify processing, decrease costs of individual REO production, and limit the use of environmentally unfriendly chemicals.