Various Operational Aspects of the Extraction of Critical Minerals from Acid Mine Drainage and Its Treatment By-product
Date of Graduation
Statler College of Engineering and Mineral Resources
Harry O. Finklea
Acid mine drainage (AMD) is environmentally hazardous acidic water, which is continuously generated in large volumes at coal mining sites, typically when pyrite is exposed to oxygen and water. As its continuous migration under the geologic conditions, AMD tends to leach metal elements out from the surrounding minerals, leading to high concentrations of metal elements, like iron, aluminum, magnesium, manganese, lead, arsenic, as well as critical cobalt and rare earth elements (REEs). AMD must be treated and monitored to meet the effluent limits, under U.S.C §1251, the Clean Water act, before being discharged to receiving streams.
Recently, the research is interested in the feasibility of refining REEs and cobalt from AMD as well as other 33 critical mineral elements listed by the U.S. Geological Survey (USGS) since their strategic importance, economic potential, and environmental advantage. REEs have unique chemical and physical properties that can be applied in a variety of industries and technologies, such as clean energy and defense technologies. Cobalt is considered with strategic importance since it has many vital properties applied in various fields, especially in the renewable energy area. Recently, acid mine drainage precipitates (AMDp) as the by-product of AMD treatment, generated in the Appalachian region, was proved to be the potential alternative resources of REEs as well as other critical minerals, considering the high concentration of REEs contained, abundant volume, the convenience of access, fewer processing steps comparing to raw ores, and the benign processes to the environment.
This research is composed of two separate studies as operational aspects of the extraction of critical mineral elements from AMDp. Three AMDp samples with different concentrations of critical mineral elements, generated from different mine sites in the Appalachian region, were the original feed of this research. All AMDp samples were treated with nitric acid leaching processing to produce pregnant leach solution (PLS) as the feed of solvent extraction (SX) process.
The first part studied the origin of the crud formation during the REEs SX process, then coming up with a mitigation solution accordingly, considering that crud formation is very deleterious since it can decrease solvent extraction efficiency and cause the loss of organic extractants. A testing scheme was specially designed to investigate the crud formation during the REEs SX process. The aqueous and crud samples were collected and characterized to study the distribution of various metal elements in different phases following mass balance calculations. Afterward, an experimental protocol was developed and implemented to successfully mitigate crud formation with respect to different feedstocks.
The second part investigated and identified an optimum recovery process for extracting cobalt from the REE SX raffinate, considering cobalt stays in the remaining raffinate instead of co-extracted with REEs during the REE SX process. A cobalt extraction process was developed and optimized, and various unit processes were investigated, including selective precipitation, solvent extraction, scrubbing, and stripping. Under the optimized operating conditions, around 93 wt.% of cobalt (proportion of PLS feed) was extracted to the organic phase at one cobalt SX stage. Afterward, approximately 85 wt. % of cobalt was concentrated from the PLS into the strip solution. About 35 wt. % of aluminum, 53 wt. % of calcium, 45 wt. % of magnesium, 86 wt. % of manganese, 3 wt. % of nickel, and 4 wt. % of zinc, referring to the concentration in the PLS, was co-stripped into the solution, which comprises the major impurities. The test results also suggest a possibility of separating nickel from cobalt and manganese, while the latter two are coextracted, which may require further separation.
To validate that the developed testing protocol can be applied to different aqueous chemistry systems for cobalt recovery, a synthetic solution following the composition of the REE SX raffinate, but with an increased concentration of cobalt, was prepared and used during the study. The results obtained from the synthetic solution are relatively consistent, with certain variations being observed. Approximately 98 wt.% of cobalt was extracted at the cobalt SX stage. After stripping, about 88 wt.% of cobalt was concentrated into the aqueous solution, along with several other metals. The variation may be due to the chemistry difference between the real and synthetic systems, such as elemental compositions and concentrations. However, the test results obtained with the synthetic solution validate that the developed testing protocol can be applied to different systems for cobalt recovery.
Xiao, Zhongqing, "Various Operational Aspects of the Extraction of Critical Minerals from Acid Mine Drainage and Its Treatment By-product" (2021). Graduate Theses, Dissertations, and Problem Reports. 8229.