The Extraction and Recovery of Rare Earth Metals from Coal Combustion Products

The Extraction and Recovery of Rare Earth Metals from Coal Combustion Products

Joseph P. Laurino, PhD, MBA

Introduction

Rare earth elements (REEs), also called rare earth metals, consist of 17 chemical elements: the 15 lanthanides, plus scandium and yttrium.1 These elements are critical to national security, the development of green energy, and advances in several high technology fields.  REEs are also vital to many traditional industries, including petroleum refining, automobile pollution control devices, phosphor manufacturing, and glass polishing.

Despite their relative abundance, most REEs are not found in sufficient concentrations to make them economically exploitable.2 The United States – once able to domestically produce REEs in sufficient quantities – has become totally reliant on imports with the closing of the Mountain Pass mine in California in late 2015. Currently, the United States’ primary REE resource is China.3

World demand for REEs is approaching 200,000 tons per year. To meet this demand, more efficient metal recovery processes, improved separation techniques, and improved recycling technologies will need to be developed and implemented. Additionally, alternative sources for REEs need to be found.

Coal fly ash has recently been examined as a resource for rare earth elements. One study4 found that the average REE content for various ashes derived from sources in the Appalachian Basin, Illinois, and the Powder River Basin were 591, 403, and 337 mg/kg, respectively. Additionally, the amount of critical REEs (Nd, Eu, Tb, Dy, Y and Er) in the fly ashes was 34-38% of the total. Since the United States generates approximately 115 million metric tons (mt) of coal combustion products (CCPs) annually, including 45 million mt of fly ash, CCPs could represent a significant domestic supply of REEs.

Clearly, there is a need to improve the extraction and recovery technology of REEs from CCPs. While numerous procedures have been reported, none have seen widespread commercial acceptance due to various limitations, such as high cost, low efficiency, and the economic infeasibility of extending the technology to large-scale operations. For example, several commercial operations are able to separate mixed REE solutions into individual REEs; however, a process to efficiently extract and recover a range of valuable REEs from the calcium- and silica-rich CCP matrix has not been reported.

One way to achieve a commercially viable separation scheme is to employ a material that will economically and selectively bind various REEs in the presence of potential interfering ions, such as sodium, calcium, and silicon. Ideally, this process should be efficient, scalable, and have minimal environmental impact.

In 2009, Periodic Products developed5 and subsequently patented a series of non-toxic water insoluble polymers exhibiting rapid second-order binding kinetics to many metal species. These heterogeneous polymer adsorbents can bind up to their weight in metals, making both their binding rate and binding capacity comparable to water-soluble homogeneous polymer agents.

A recent Florida Industrial and Phosphate Research Institute (FIPR) co-funded study utilizing one of these new polymer compounds confirmed that the Company’s technology successfully recovered rare earth elements from waste by-products of phosphate fertilizer production. Further, the study determined that
semi-optimized yields were equal to or greater than those currently obtained by the rare earth mining industry from raw ore.6

This Technical Bulletin describes the non-optimized extension of this technology to the extraction and recovery of rare earth metals from CCPs. It should be noted that the non-optimized extraction and recovery yields for CCPs were similar to the non-optimized yields for phosphate waste products. This suggests that further optimization of the extraction of REEs from CCPs could produce extraction and recovery yields equal to or better than those reported in the FIPR optimization study for phosphate waste products.

Prior to Periodic Products’ technology, an efficient and economically viable low environmental impact process to extract and recover these critical materials from CCPs was not possible. The financial and environmental costs associated with recovering REEs from mining wastes such as CCPs using this technology
are significantly less than the costs of obtaining these metals from mined ore. It is estimated that the cost of Periodic Products’ extraction and isolation technology is approximately $6 per kilogram of mixed REEs – approximately 40% of the current cost of mining and isolating these metals, making this process
economically viable.

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