Coal ash as a source
As a byproduct of burning coal to make electricity, the U.S. produces nearly 80 million tons of coal ash a year. About 43 percent of that is used as an ingredient in cement and to enrich soil with macro- and micronutrients. The rest goes to landfills or gets mixed with water and stored in open ground pits called containment ponds.
That coal ash contains reasonable amounts of rare earths, particularly scandium and neodymium. In ash from some types of coal, the concentrations of rare earths are as much as 100 times higher than when they’re found naturally in the Earth’s crust.
Research, funded in part by government money, has found at least three ways to extract rare earths from coal ash, though none is yet commercially viable. The first two methods, using acids and special bacteria, are either too expensive or too time-consuming and therefore unlikely to be practical at an industrial scale.
A third route, which our research explores, uses heated, compressed carbon dioxide to selectively dissolve and extract the rare-earth elements. This form of carbon dioxide, called “supercritical,” has several applications, including in the food industry, removing caffeine from coffee beans.
Using carbon dioxide
Our research has found a potential route to profitability for extracting rare earths from coal ash using supercritical carbon dioxide. There are still significant challenges, though.
First, the process needs other chemicals. Carbon dioxide is relatively cheap, but it can’t help rare earths clump together and separate from the other elements in coal ash. Doing that will likely require an expensive chemical such as tributyl phosphate. Researchers will need to find ways to use as little of that as possible to keep costs down.
Second, the process has to be especially efficient at extracting scandium and neodymium, which are particularly valuable. Many coal ashes have lots of those elements – but not all, which raises the third challenge.
Some coal sources – and therefore the ash when that coal is burned – have more rare earths than others. We found that the value of rare earths in a single ton of coal ash can vary from US$99 at a coal plant in Ohio to $534 at a West Virginia plant. With extraction costs expected to range between $380 and $1,200 per ton, not every coal plant’s ash will be a profitable place to find rare earths.
Dealing with the leftovers
After the rare earths are extracted where possible – and profitable – the rest of the coal ash would not need to be handled any differently than it is already. But it would help the coal industry take a step toward a “circular economy,” in which anything one process generates as waste can be used as a raw material in another process. Further, the carbon dioxide and other chemicals needed for rare-earth extraction can be reused, reducing waste from the process itself.
This method would give the coal industry an additional stream of income – which would not likely reverse its decline, but could give coal a different strategic role in the country’s economy, even as its use as an energy source ends.
Given increasing demand and importance of rare earth elements (REE), exploration is underway to find alternatives to ore-extracted product. With REEconcentrations varying between 270 and 1480 ppm, coal ash has been deemed as one such potential source. A number of research groups are exploring technologies to separate REEs from coal ash and supercritical extraction has emerged as a high yield contender. Estimating the economic viability of this lab-scale process at the industrial scale is both important and challenging. In this study the researchers estimate industrial scale cost and revenues of production of REEs from coal ash by combining prior laboratory results, scaling models, combinatorial scenarios and sensitivity analysis. The processing cost of extracting REEs from one ton of coal ash using supercritical CO2 and tributylphosphate (TBP) is found to vary between $380 and $1200 for 550 g of REE. The value of REE oxides that may be obtained per ton of ash is estimated to vary between $6 and $557, with a median of $250. Scandium is the most expensive REE and can account for up to 90% of the value of the yield. The results suggest that factors critical to the economic viability of the process include scandium content & yield, reagent use, and processing time.