Grantee Research Project Results

Final Report: A Low-Cost Rare Earth Elements Recovery Technology

Description:

Physical Sciences Inc. (PSI) and the University of Kentucky Center for Applied Energy Research (UK/CAER) investigated in a Phase I project a unique technology that has the potential to significantly reduce U.S. dependency for Rare Earth Elements (REE) on foreign suppliers. This technology utilizes an extremely low-cost, abundantly available waste material in the United States—fly ash from coal-fired power plants—as the resource for REE extraction. Presently, there are no mineral resources within the United States (including the Molycorp Mountain Pass mine) with sufficient heavy rare earth element (HREE) concentrations to meet the current and projected demand to 2020. The reprocessing of coal ash to remove hazardous elements (such as arsenic and thorium) also will aid in beneficiation of the ash, a critical environmental consideration for the United States. In Phase I, PSI demonstrated feasibility of a chemical process for economical, environmentally responsible, high-yield recovery of the REE, particularly, the HREE from coal ash.

Summary/Accomplishments (Outputs/Outcomes):

In this Phase I EPA SBIR project, PSI investigated the extraction of rare earth elements from coal ash. The company successfully demonstrated the recovery of critical rare earth elements, particularly the heavy rare earth elements, from selected coal ashes from Kentucky (Cooper power plant) and New Mexico (San Miguel power plant). While PSI identified several high REE content coals in the United States, these two ashes were chosen because the company could obtain them within the period of performance of Phase I.

• PSI found that out of the 6,682 coals in the U.S. coal quality database, there are 179 coals with > 500 ppm critical REY and 214 coals with > 1000 ppm total REE. The majority of the top total REE coals are Pennsylvanian coals from the Appalachian or Illinois basins.
• The distribution of REE in fly ash does not seem to partition particularly by the ash collection stage electrostatic precipitator (ESP) or fabric filter (FF) row, as expected for a suite of elements considered to be more refractory than, for example, chalcophile elements such as arsenic. In the plant studied by Mardon and Hower¹, the average yttrium Y+REE for the ash collection rows ranged from 1,279 to 1,573 ppm with no particular trend with the flue gas temperature. The LREE/HREE ratio, however, decreases from 8.17 in the first mechanical (cyclone) collection row to 6.76 in the third, and last, ESP row. The latter trend was repeated in most of the power plants studied by Hower et al.²
• The distribution of REE does seem to partition by size, with smaller size particles having higher concentration of REE. In addition to the partitioning of the light and heavy REE, the total REE (LREE + HREE) progressively increases from 246 ppm in the > 120-mesh fraction to 583 ppm in the < 500-mesh fraction of the Jungar, China, fly ash.² All of the REE increase in concentration, so the HREE/LREE ratio only varied between 29.5-32.8 among the size fractions. As far as PSI knows, this is the only published sized fly ash fraction with accompanying REE determination.
• The baseline process was very effective in selective extraction of REE while suppressing extraction of arsenic and thorium. The extraction yields for heavy REE were particularly higher. The yields reported below refer to % of starting REE content by weight in the ash.
  • The total REE (TREE) extraction yield was 6.7% compared to ~ 0.2% for all elements and < 0.1% for Al, Fe, Ca, and Na.
  • The extraction yields for Dy, Eu, Tb, Y, Gd, Sm were > 10%.  The yields for Nd and Pr were > 5%.
  • The extraction yield for As and Th were ~ 1% and 1.5%, respectively.
• Within the space of process parameters, PSI was able to investigate in Phase I; the alternate process was effective in selective extraction of some REE, but not for total REE. This was due mainly to non-selectivity with respect to Ca and Na.
  • The TREE extraction yield was low, ~ 1%, whereas Ca and Na were extracted with yields of 25%-35%.  By comparison, the extraction yields for Al and Fe were immeasurably low.
  • The extraction yields exceeded 7.5% for Eu and Tb, and 3.5% for Dy, Pr and Sm.
  • The extraction yields for As and Th were ~ 0.2% and 4%, respectively.
• PSI expects the above REE yields for either process to be substantially higher when higher REE-content coal/ash is obtained as the starting material in Phase II.
• PSI has preliminarily determined that the ash cake resulting from the nitric acid digestion has the potential for use as cement substitute in concrete applications, just as ash currently is used. Furthermore, hazardous elements such as arsenic and thorium are not present in the processed ash cake. Thus, the company's process produces a value-added commercial byproduct, which will significantly improve the overall economy.

Conclusions:

Phase I has successfully demonstrated feasibility of PSI's baseline process to recover critical rare earth elements, particularly the heavy rare earth elements, from selected coal ashes from Kentucky (Cooper power plant) and New Mexico (San Miguel power plant). Now, the process needs to be optimized to enable its scale up to commercial production levels and establish its economics on commercial scale. Process optimization will involve screening ash raw materials for high REE content, selection of process parameters and plant equipment to increase REE yield, marketable byproducts and co-products yields, and material throughput, while minimizing capital and operational costs.

References:

Journal Articles:

Supplemental Keywords: