(763b) Recovery of Rare Earth Elements from Coal Ash with a Recycling Acid Leach Process

Rick Peterson1, Mike Heinrichs1, Rachid Taha1, Darwin Argumedo1, Justin Glier1, Slawek Winecki1

1Battelle Memorial Institute, Energy Resource Group, 505 King Avenue, Columbus, OH 43201

KEYWORDS: rare earth elements, coal ash, coal liquefaction

ABSTRACT

Rare earth elements (REE) have a wide range of uses in catalysis, alloying, magnets, and optics, and have become critical for the batteries, motors, and generators that enable sustainable technologies and infrastructure. Despite these critical applications, the United States has developed very little in the way of REE resources, with China producing more than 80% of the global supply of the elements1. A technology that will allow for the economic recovery of REE from coal will reduce dependence on foreign supplies and ensure US capability for renewable and defense products.

1 U.S. Geological Survey, "Mineral Commodity Summaries 2017," 19 January 2017. [Online]. Available: https://minerals.usgs.gov/minerals/pubs/mcs/2017/mcs2017.pdf.

Battelle has investigated the application of its Acid Digestion Process (ADP) to recovery of REE from coal ash produced by pulverized coal combustors (PCC), fluidized bed combustors (FBC), and Battelle’s bio-based direct coal liquefaction process. The ADP was shown to recover over 90% of the nitric acid used in the process for recycle, greatly reducing chemical consumption costs. This presentation will cover results of a sampling and characterization effort for varying coal ash feedstocks, as well as a feasibility study that assessed the economics of the ADP process when considering available sources.

The sampling and characterization portion of the project analyzed coal byproducts to allow selection of a suitable feedstock for Battelle’s process, and selection of key target materials for recovery to drive the process economics. Within the REE in coal, dysprosium (Dy) represents the highest value, and combined with neodymium (Nd), terbium (Tb), and praseodymium (Pr), these four elements account for over 75% of the REE value in the ash. Accordingly, Battelle’s process will be tailored to focus on these REE components. Scandium (Sc) represents the highest overall value within the ash, and will be a primary byproduct. Vanadium (V), yttrium (Y), lithium (Li), and cobalt (Co) also represent a significant value, and will be evaluated as potential byproducts going forward.

Based on the analytical results, the feasibility study focused on a specific PCC fly ash as a feedstock for the ADP. The PCC plant fly ash had a higher Total REE+Y+Sc concentration than

all other operating plant ashes sampled, at 545 ppm +/- 13.4 ppm, as well as a higher Heavy REE/Light REE ratio at 0.37 +/- 0.01. Additionally, it contains significant amounts of scandium (36 ppm +/- 1.4 ppm), vanadium (279 ppm +/- 12 ppm), yttrium (104 ppm +/- 5.3 ppm), cobalt (44 ppm +/- 2.5 ppm), and lithium (~166 ppm), which can be valuable process byproducts with robust market outlets. Although the coal liquefaction ash had a greater concentration of total REE, there is some risk for implementation since Battelle’s liquefaction process is not yet commercialized. The low temperature ash appeared to be more leachable, but it also demonstrated lower selectivity due to the high calcium content common with FBC systems. Furthermore, the overall capacity of FBCs is much lower than PCCs. Therefore, there would be more risk in feedstock sourcing.

Feasibility testing compared leaching efficiency and selectivity of PCC ash with samples of liquefaction ash and FBC ash to ensure and validate assumptions regarding leaching kinetics, acid loading limits, and product purities. This information was used to refine a process model built in CHEMCAD modeling software and fed into overall process economics and unit sizing used in the technoeconomic assessment.

The feasibility study focused on construction of a REE recovery plant sized to process up to 30 tonnes of ash per hour, which is approximately the ash production rate of a 2,000 MW power plant operating at full capacity.

Operating costs for both a First-of-a-Kind and an Nth-of-a-Kind plant were determined. These costs to recover rare earths from coal ash were then compared to the value of rare earths present in a variety of coal ashes around the country. Values for coal ash were obtained through determination of the concentrations of rare earth elements in coal samples adjusted to account for changes in concentration associated with the combustion process. Current market pricing information for REEs was used to determine the value associated with each of the coal sources.

The results of the feasibility study indicate that when a First-of-a-Kind plant is considered, approximately 5% of U.S. coal sources contain sufficient rare earth material to cover the costs of the recovery plant. When an Nth-of-a-Kind plant is considered, over 20% of all coal sources reviewed contain sufficient recoverable rare earths to cover recovery costs at current REE market prices. When historical high REE prices are considered, more than 25% of coal sources can be treated with a First-of-a-Kind plant, and more than 47% of sources with an Nth-of-a-Kind plant. These results are based on the current understanding and developmental status of the recovery process. It should be noted that further downstream purification costs are not included in this assessment. A probabilistic sensitivity analysis was conducted which examines how the cost to recover rare earth materials may change due to technology developments, uncertainties in capital requirements for the plant, or prices of REEs due to shifting market dynamics. Under certain sensitivity scenarios, the recovery plant can be economically viable processing an even greater range of coal sources.

This project is ongoing, and the presentation will also cover pending results of experiments designed to improve REE leaching efficiency, produce additional valuable byproducts, and upgrade the REE concentration in process products. Additionally, it will briefly address the design of a continuous bench scale ADP for REE recovery from coal ash.

This project was performed with funding from the Department of Energy’s National Energy Technology Laboratory (DE-FE0027012) and the Ohio Coal Development Office (OER-CDO-D-15-10)