Is it worth, from an energy and environmental standpoint, to extract rare earth elements (REEs) from coal ash? China's domination of global REE supply, critical to the manufacturing of a plethora of products, may force that exploration.
The combustion of coal for power generation produces waste coal ash, also called fly ash, which must be captured, collected, and stored due to federal regulations. Coal ash has been historically known to contain trace amounts of strategic minerals and metals, including REEs. Extracting REEs from coal ash requires less time and effort than digging it out of the ground, by all accounts, but the environmental and safety costs of such a technique have yet to be fully determined.
Rare earth elements, e.g., the lanthanides, scandium, and yttrium, as well as the associated metals molybdenum and tungsten, are needed for the production of components for items ranging from cell phones and other consumer electronics, to batteries, to governing defense systems and missiles.
David Mayfield and Ari Lewis, environmental toxicologists for environmental and science consultancy Gradient Corp., wrote for Waste Management World
an article stating the recovery of metals from coal ash could be relatively more efficient than mining it from rock ores. They also noted that with the escalating financial and environmental costs of mine development, and with tight supply of rare earth minerals being controlled by China - driving up prices -- sustainable methods of REE extraction from coal ash deposits become more compelling to develop.
According to a report by Houston-based IHS Chemical
, production and consumption of rare earth minerals totaled over 100,000 metric tons in 2012. IHS's study estimates that from 2012 to 2017, global demand for rare earth products will grow by 7.6 percent annually and reach more than 150,000 metric tons, with China leading consumption growth at 8.3 percent annually. There are even more aggressive forecasts for REE consumption that are floating about.
According to Rare Earth Investing News
, there isn't a high density of rare earth elements in fly ash - a few parts per million. But better chemical engineering techniques are leading to more efficient metal extraction processes. And metal recovery from coal ash is inherently less intensive and complicated than that from mineral ores.
REE extraction methods typically involve initial low-pH acid leaching of the ash, followed by removal of undesired minerals and purification using solvent extraction, according to Mayfield and Lewis. In such a method, they wrote, extraction efficiencies depend on how many other elements are in the ash and what kinds of acids and extractants are used. The main problem is that separating out REEs is a hassle due to the "unique chemical similarity between this group of elements," meaning that "multiple physical and chemical extraction techniques are typically employed to purify each metal."
Accounting for all those makes it a longer and more expensive process.
There are other options to coal ash for REE extraction being explored. A recent report
illustrated a method of recycling rare earth elements from wastewater that was described in a study in the journal ACS Applied Materials & Interfaces. "Recycling REEs from wastewater not only saves rare earth resources and protects the environment, but also brings considerable economic benefits," the researchers were quoted.
China undercut the prices of REEs produced by other nations to become the world's dominant exporter of the strategic elements, but the country has paid the environmental price as a result of heavy mining. But according to Mayfield and Lewis, coal combustion wastes as strategic elements resources could have environmental costs, as well, writing, "...The process by which these materials are extracted results in the generation of multiple waste streams."
Some of these include large volumes of produced water containing contaminants. And "any residual naturally occurring radioactive or organic wastes from acids or extraction solvents will also require recovery and disposal... these processes have yet to be commercialized, thus our understanding of future waste streams is limited."
In addition to the environmental risks, health and safety standards for REEs and REE extraction remain underdeveloped due to lack of studies on human effects, so there may be regulatory costs and burdens that have yet to precipitate. Detailed knowledge of REE compositions in coal combustion wastes at storage facilities is also limited.
Mayfield and Lewis concluded that while coal combustion waste represents a potentially abundant source of strategic and rare earth minerals, which would "alleviate supply risks," the management of extraction process wastes will be a hurdle and need to be strict. The pair also recommended data gathering on the toxicological effects of coal ash processing.
"Coal combustion waste deposits that are identified as being potentially economically viable should undergo a full chemical characterization to determine which contaminants may require specialized waste handling measures," they wrote, while calling for further study on all aspects of the process.
--with David Sims