Optimization of Atmospheric-Pressure Extraction Conditions for Recovery of Rare Earth Elements from Coal Fly Ash

Optimization of atmospheric-pressure extraction conditions for recovery of rare earth elements from coal fly ash

Isaiah Morones, Cesar A. Martinez Bejarano, Sergio Martinez-Monteagudo, Catherine E. Brewer

Chemical and Materials Engineering, New Mexico State University

Rare earth elements (REE) are metallic elements that include 15 of the lanthanides, as well as scandium and yttrium. The global need for these REE increases every year as they are used in many modern technological innovations, from microchips, electric cars, and solar panels to fighter jets and satellites. China controls 90-100% of the REE available on the commercial market. In recent years, the U.S. has increased its efforts to secure more stable domestic sources of REE. Coal fly ash (CFA) is a very fine, powdery material from the combustion of coal for heat and power. CFA is one of the largest types of industrial waste generated within the U.S., producing nearly 130 million tons in 2014. Some CFA contains as much, or more, of the desired REE than traditional REE ores.

Typical REE extraction processes are intense and hazardous, requiring strong acids. Researchers at Sandia National Laboratories have developed a lab-scale process to extract REE from coal fly ash under relatively mild conditions, using water, supercritical CO₂, and small organic acid chelators. The goal of this project is to determine the engineering parameters and, where possible, process simplifications to enable extraction process scale-up. Experiments have been conducted on two types of locally-sourced CFA (type C and type F) to quantify the return on investment for each increase in extraction temperature, pressure, time, and amount of chelator.

The first series of experiments were conducted as batch extractions at atmospheric pressure on a stirring hot plate, using water (control), acetic acid, lactic acid, or citric acid as the chelator. REE in the extracts were quantified using inductively coupled plasma spectroscopy. Results showed that extraction, measured as the wt.% of each element present in the CFA entering the extraction liquid, increased with both time and temperature. Extraction of REE was higher for acetic and citric acids compared to water or lactic acid. Data from the atmospheric batch extractions will be used to select the conditions for extraction trials in a 1.8-L autoclave reactor under nitrogen or CO₂ to quantify the effects of pressure.