|Huge quantities of coal combustion ash are stored globally. Laced with toxic substances, in December 2008 1.1 billion gallons (4.2 million cubic metres) of coal fly ash slurry was spilled from a solid waste containment area at the Tennessee Valley Authority's Kingston Fossil Plant
Credit: Brian Stansberry
by David Mayfield and Ari Lewis
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Recently, prices of strategic minerals and rare earth elements have risen dramatically due to global supply shortages and increasing demand. Strategic elements encompass a broad group of metals that are essential for emerging technologies, including those from the aerospace and green energy industries, but which have limited global supply chains.
Several strategic metals including rare earth elements, gallium, germanium, indium, and tellurium are critical for many components of energy efficient technologies. Rare earth elements are a group of chemically similar metals including the fifteen elements with atomic numbers 57 through 71, plus scandium and yttrium.
China is the primary global producer of many rare and strategic elements. However, global demands are outpacing current production volumes and many nations are initiating programs to identify alternative metal resources.
Coal Ash to the Rescue?
The occurrence of trace concentrations of strategically important metals in coal and coal ashes has been understood for decades. However, the economics of recycling these resources from coal ash have not been viable - until recently with the dramatic rise in prices.
Metal mine development, particularly for strategic elements, requires extensive capital investments and complicated regulatory oversight. The length of time taken to identify resources, complete environmental permitting, and move to production, can span many years.
Further, as observed in China, if not properly controlled, beneficiation of metal ore deposits can result in unintended environmental consequences due to the release of chemicals used during mining and improper mine management.
Coal combustion waste storage facilities offer a potential source of strategic metals that may limit some of the expenses and environmental hazards associated with typical mine development. Therefore, it is worthwhile exploring environmentally sustainable methods to leverage existing coal ash deposits.
TRACE ELEMENTS IN COAL ASH
|A polished cross section of coal fly ash embedded in epoxy at 750 times maginification Credit: Wabeggs|
Raw unprocessed coal contains a variety of metals, and in some cases, enriched concentrations of some strategic elements. Surveys of coal resources indicate that some deposits may contain economically viable concentrations of rare elements. In addition, the combustion process results in the enrichment of metal concentrations in the coal ash wastes, often several times the concentration found in raw coals.
While, under certain conditions, some metals may leach from coal ash wastes (e.g., As, B, Se), many strategic metals remain bound to the waste ash. The range of strategic metal concentrations in some coal ashes are similar to those from mineral ores, suggesting that coal ashes are possible resources for metal recovery.
Limited information exists to characterise the concentrations of strategic elements in global coal ash storage facilities. However, from their research in 2012 Seredin and Dai estimated the rare earth ash content in U.S., Chinese, and Russian coal sources contained concentrations within the range of mineral ore deposits. Thus, the potential for recycling coal combustion wastes to recover strategic elements is evident.
However, due to the lack of trace element data, there is a need to characterise strategic element concentrations in coal combustion storage facilities worldwide. Further, it will be necessary to evaluate and prioritise these resources to focus efforts on those deposits with the highest amounts of strategic elements and those that can be effectively extracted from the coal ash matrix.
RECOVERY OF STRATEGIC METALS FROM COAL ASH
|Coal ashes from Hong Kong's Castle Peak Power Station - one of the largest coal fired power stations in the world Credit: Flickr: CheshireCat|
The recovery of metals from mineral ores, particularly strategic elements, is a complicated multi-step process that consumes energy and results in a variety of waste products. This beneficiation process includes initial crushing and grinding of the ores to smaller particles, filtration and flotation to remove undesired minerals, and further conditioning prior to final metal purification. By contrast, initial metal recovery from coal ash may be more efficient than ore processing since the physical form is more amenable to processing with limited initial conditioning.
Methods for extraction and separation of individual strategic metals from fly ash are emerging and becoming more efficient as chemical engineering techniques are improved.
Several extraction techniques have been summarised for strategic metals. Generally, these processes include initial acid leaching of ash material, followed by removal (e.g., precipitation) of undesired minerals, and purification using solvent extraction. The leaching stage employs the use of low-pH acids, such as hydrochloric, nitric, sulfuric, or oxalic acid, and varying temperatures and leaching times - depending on the composition of the fly ash.
After leaching, removal of non-target minerals such as silicates, iron, calcium can be conducted using chelating resins or other precipitates. Finally, the individual metal is purified from solution using chemical extraction solvents. Extraction efficiencies can vary depending on concentrations of other elements in the ash or due to the non-specific nature of some acids and extractants.
Furthermore, the chemical separation of rare earth elements can be more cumbersome. Due to the unique chemical similarity between this group of elements, multiple physical and chemical extraction techniques are typically employed to purify each metal. Therefore, it is necessary to optimise the extraction technique for each coal combustion product source.
Pursuit of coal combustion wastes as a resource for strategic elements should be balanced with consideration of potential environmental benefits and impacts. While rare earth and other strategic elements are necessary components of renewable energy and sustainable technologies, the process by which these materials are extracted results in the generation of multiple waste streams.
If these new waste streams are properly managed, it should be clear that the development of coal combustion wastes for strategic metals may provide an environmentally sustainable option to reduce the amounts of coal wastes in storage facilities. Thus, the management of wastes remains a necessary component of metal resource development.
Metal extraction and recovery, as described, is a chemically intense process. Specifically, the metal separation steps require the use of leaching acids, caustic precipitates, and organic solvents. Each of these chemical components will need to be strictly maintained to limit unintended environmental releases or exposure to the facility operators. During metal extraction, multiple secondary waste streams are generated.
Metal processing requires large volumes of water for the acid leaching stages. While some of the water may be recycled and reused, a portion will need to be treated for contaminants. For example, coal ash residuals contain many trace elements other than strategic elements, which are potentially toxic, such as arsenic.
The initial acid leaching process is generally non-specific to trace elements, therefore any remaining common elements will require recovery and disposal. Further, any residual naturally occurring radioactive or organic wastes (from acids, extraction solvents) will also require recovery and disposal. Since, the recovery of strategic metals from coal ash is still evolving and will need to be tailored to the specific characteristics of the coal ash source, the composition of the waste materials are likely to vary considerably. Further, these processes have yet to be commercialised, thus our understanding of future waste streams is limited.
Further, existing environmental regulations are generally limited for the strategic metals industry. Specifically, occupational and environmental health standards have not been developed for most strategic metals or the specialty extraction solvents. This is largely due to limited information on the toxic effects of strategic metals on public health and ecosystems.
Additionally, the health and environmental effects from the release of these metals into the environment from all phases of development (processing, use, and disposal) is not well understood.
Therefore, as this industry expands, further research efforts may be required to generate the necessary toxicological information to develop safety recommendations. The potential for exposure of workers, communities, or surrounding ecosystems to any of the chemical contaminants inherent in this process is unknown, so the monitoring of environmental exposures may be needed as coal combustion wastes are reclaimed.
The future of advanced technologies and sustainable and efficient energy generation is dependent on the availability of a number of strategically important elements. One possible untapped resource that may alleviate supply risks for strategic metals is the significant availability of coal combustion waste products from coal fired power plants.
A number of research organisations are currently evaluating the processes to recover these strategic elements from coal combustion wastes. As part of these investigations it is necessary to consider the environmental risks associated with developing this resource. Additional research is needed to identify environmentally sustainable solutions for processing strategic elements from coal ash.
Limited existing information is currently available to characterise the strategic element composition of existing coal ash storage facilities. Further efforts should be initiated to survey and identify the coal ash deposits that are economically viable for metal recovery.
Furthermore, coal combustion waste deposits that are identified as being potentially economically viable should undergo a full chemical characterisation to determine which contaminants may require specialised waste handling measures. This characterisation can be used as one metric to prioritise those resources that have minimal concentrations of hazardous substances that require treatment and disposal.
Further research and development is also required to optimise the metal recovery and extraction process to minimse the use (or maximise recycling) of hazardous acids and solvents.
Public health and environmental risks from contaminants generated by coal ash processing should also be undertaken. Specifically, additional data gathering on the toxicological effects from exposure to strategic metals or the chemicals used in their production would allow for informed development of safety recommendations.
Finally, a thorough understanding of the potential routes of exposure and exposure concentrations generated during the various stages of coal ash processing is needed to protect occupational, public and environmental health.
David Mayfield and Ari Lewis are environmental toxicologists with U.S. environmental consulting firm Gradient, which assists global clients with problems relating to chemicals in the environment.
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