Source: DOE. Click to enlarge.

REEs have been generally found in varying concentrations ranging up to 1,000 parts per million by weight in the following materials in the United States: coal mine roof and floor materials; run-of-mine coal; prepared coal; partings; pit cleanings; coal preparation refuse; and tailings. REEs can be found in coal byproducts, including ash, coal-related sludge, and mine drainage.

REEs are abundant in the earth’s crust but most often occur in low concentrations. They are not found in isolated form but in a variety of minerals where, in most cases, they exist in concentrations too small for economic extraction. The REEs are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The rare earths are also often taken to include the metals scandium (Sc) and yttrium (Y). Source: DOE. Click to enlarge.

Certain coals can contain a higher ratio of heavy (generally more valuable) REEs than found in other sources of REEs such as natural ores. Given the potentially low REE concentrations in the feed materials, and subsequent potentially low yield of REEs from any separation process, minimizing costs is a key challenge. Physical and chemical separations may be useful in recovering REEs from coal and coal byproducts. The forms in which REEs are present in these materials could drive the design of separation processes.

The new FOA has two specific areas of interest (AOI):

1. Bench-scale technology to economically separate, extract, and concentrate mixed rees from coal and coal byproducts including aqueous effluents.

2. Pilot-scale technology to economically separate, extract, and concentrate mixed rees from coal and coal byproduct solids.

For the purposes of the FOA, DOE defines bench-scale technology (AOI 1) as existing separations technology which has previously demonstrated success at the basic research level for recovery of REEs from coal and coal byproducts; is ready or near-ready for design at bench-scale to recover REEs, and should be ready at the completion of this project for scale-up to pilot-scale.

Pilot-scale technology (AOI 2) is defined as existing separations technology which has previously demonstrated success at bench-scale for recovery of REEs from coal and coal byproducts; is ready or near-ready for design at pilot-scale to recover REEs; and should be ready at the completion of this project for scale-up to commercial-scale.

The DOE says that adapting separations technology previously used for other mineral processing applications will provide a pathway for identification of technology for recovery of coal-based REEs. Thus, DOE is seeking innovative applications of existing separation technologies for recovery of REEs from coal and coal byproducts.

These technologies may include combinations of physical separations, such as gravity concentration, flotation, magnetic, and electrostatic separations, as well as combinations of physical and chemical separations, with the latter category including leaching and/or ion exchange from rock, underclay, or other material associated with a coal seam. Separation processes may also include hydrometallurgical processing and processes to co-recover mixed REEs and other useful materials from coal and coal byproducts.

The resulting total concentration of mixed REEs in the final product from the proposed recovery process should approach 2 wt% to be of interest to commercialization. The process designs proposed should minimize or reduce the environmental, safety, and health impact of radioactive and other byproducts, and optimize the overall economics of the separation and recovery process.