Navy

Recovering heavy metals and rare earth elements from fly ash, coal ash, and unrefined mineral ores

Zero waste generating process - no acid contaminated water is produced

Environmental

(James T M Towill)

On December 22, 2008, the Tennessee Valley Authority Kingston Fossil Plant spilled 1.1 billion gallons of coal fly ash slurry. River testing of the contaminated area showed high levels of toxic metals (including arsenic, copper, barium, cadmium, chromium, lead, mercury, nickel, and thallium). The spill covered the surrounding land with up to six feet of sludge.

Currently, 30-40 million tons of fly ash are stored each year, commonly in these large slurry-filed dams that can rupture and cause widespread contamination and devastation.

Although some coal ash or fly ash can be diverted for use in other industries, such as the cement industry, much of it cannot be used due to the heavy metal contamination and associated radioactivity.

Current methods for treating fly ash or mineral ores use different types of acidification in water/slurry mixtures, followed by post-processing to remove the heavy metals. This process is known to generate significant amounts of contaminated water, which limits the applicability of these techniques. Additionally, there is a significant cost associated with these technologies owing to the need for handling and processing contaminated waste water.

For fly or coal ash, the other most common technique involves the use of either plasma heating the ash to temperatures in excess of 3,000° C., which reduces the heavy metals to their metallic state. Once in their metallic state, they can then be recovered from slurry mixtures since they settle out. However, the use of fossil fuels to create the heat required, make this technique too expensive for large-scale processing. Another technique involves the use of MgCl2 or CaCl2, which is fused with the fly ash at 1,000 to 1,400° C. Again, this requires large amounts of the chloride salt, which is not regenerated in the process, as well as the energy required to heat the fly ash to high temperatures.

In response to the above, Navy scientists have developed a new method for recovery of heavy metals and rare earth elements from fly ash, coal ash, and unrefined mineral ores. This process provides a new source of rare earth elements and at the same time removes the heavy metal and radioactive contamination from the ash, rendering it safe for other industrial applications. This recyclable ionic liquid process uses an organic acid, such as acetic acid, to solubilize the metals.

Once solubilized, components can be removed from the ionic liquid by electrochemical deposition. The ionic liquid is recycled and reused in this process without generating waste. The heavy metals and rare earth elements are deposited onto a copper electrode. After this, they can be further purified via electrochemical processing. The only other byproduct of this is the regeneration of the organic acid, which can be added back to the ionic liquid to begin the process again.

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