(248at) Fate of Rare Earth Elements during Lab-Scale Combustion of Lignite Coal
If rare earth elements are going to be extracted from coal and coal ash, it is important to determine their
fate during the combustion of coal. Coal is typically burned at power plants in twenty percent excess air at
temperatures of around 1370oC. It is assumed that the rare earth elements are concentrated in the coal ash after
the combustion of coal due to the typically high melting temperatures, and low volatility, of rare earth
compounds. However, a couple common rare earth salts do have relatively low melting temperatures compared
to the average combustion temperature reached in a coal-fired power plant. Additionally, it would be
informative to know what form the rare earth elements are in, after combustion at high temperature. This is
important since rare earth oxides have different solubility characteristics than rare earth phosphates or
This first step of this project was to determine the concentration of rare earth elements in lignite coal and
lignite coal ashed at different temperatures in air. The concentration of rare earth elements in a lignite coal and
lignite coal ash was determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The
assumption that the rare earth element compounds are non-volatile in coal was investigated by combusting
lignite coal at 750°C in air, and measuring the concentration of the rare earth elements in the resulting ash.
Increasing the combustion temperature to temperatures closer to typical boiler temperatures was also examined.
It was determined that even with the difficulty of measuring the small concentrations of rare earth elements in
the coal, that all of the rare earth elements are concentrated in the coal ash. Additionally, a mass balance shows
that most of the REE elements in the parent lignite end up in the resulting ash. Thermogravimetric analysis was
also conducted on the coal, as well as some rare earth phosphates, to determine if the rare earth phosphates
would decompose into rare earth oxides during heating at high temperatures ~900°C in air. By examining the
fate of rare earth elements compounds at elevated temperatures, novel separation strategies can be developed.