Folgueras, and 2 co-authors, reported in 2017 that rare earth elements together with Y and Sc (REY) are essential in the development of technology for clean and efficient use of energy. Apart from the potential of coal and sewage sludge as energy resources, the enrichment of REY in their ashes may provide a feasible source of them. The concentrations of REY in ashes obtained at 800 and 1100 ºC of a bituminous coal and three types of sewage sludge were determined. Also, their relationships with major and minor elements and minerals were studied. The concentrations of REY, excepting for La, are higher in the coal ashes than those of the sludge ashes. In both types of materials, REY are associated with the inorganic matter, although the associations are different for the coal than for the sludges. In the coal ashes, La, Ce, Pr and Nd are mainly associated with silicates and aluminosilicates in ashes at 800 and 1100 ºC. The rest of REY (excepting Sm and Yb) are associated with carbonates in ashes at 800 ºC and with sulfates in ashes at 1100 ºC. In the sludge ashes, Y is associated with carbonates, while other REY are mainly associated with phosphates, silicates and aluminosilicates.
The authors concluded that from the study of REY concentrations in the ashes of a bituminous coal with a high carbonate content and three sewage sludge ashes, from sludges that had undergone different waste water treatments, the following conclusions can be drawn. The concentrations of the critical elements Y, Eu, Dy, Er and Tb are especially high in the bituminous coal studied (higher than the Clarke values for hard coal ashes) due to its high content of carbonates. The REY concentrations in sludge ashes are lower than those in coal ashes, excepting La. Moreover, the concentrations of Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu in sludge ashes at 800 ºC are very low, lower than 4 ppm and approximately equal for the three types of sludge. The distributions of REY concentrations are similar in the three types of sludge ashes, but they differ from those of coal ashes in the cases of Y, La and Ce.
Not only the concentrations and distribution of REY in the sludge ashes but also their associations and behavior are significantly different from those of the coal ashes. In the bituminous coal ashes obtained at 800 ºC, the following 11 elements Sc, Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, show positive linear relationships with carbon (expressed as CO2), these elements being associated with the carbonates such as calcite (CaCO3). In coal ashes at 1100 ºC, due to carbonate decomposition, there are positive relationships between Sc, Y, Gd, Tb, Dy, Ho, Er and Tm and SO3, while for Yb there exist positive relationships with SiO2, Al2O3, Fe2O3, Na2O, K2O and TiO2. The behavior of the first group of elements may be due to the fact that REY-bearing alkali and alkaline earthoxides released by carbonate decomposition react with SO3 to form sulphates, such as anhydrite (CaSO4). On the other hand, Yb-bearing CaO reacts with SiO2, Al2O3, Fe2O3 to form ferrites, silicates and aluminosilicates, which explains Yb associations with SiO2, Al2O3 and Fe2O3 in ashes at 1100 ºC. There are also positive relationships between La, Ce, Pr and Nd concentrations in ashes at 800 and 1100 ºC and several oxides including SiO2, Al2O3 and Fe2O3, which indicates that they are associated with larnite (Ca2-SiO4) and gehlenite (Ca2Al2SiO7) and to calcium ferrite (Ca2Fe2O5) in the ashes.
For sludge ashes, unlike coal ashes, only Y maintains a good correlation with CO2 in the sludge ashes at 800 ºC, which indicates that it is associated with carbonates. Moreover, there exist positive linear correlations between La, Ce, Pr, Nd and Sm concentrations and several oxides including SiO2, Al2O and P2O5, such correlations also being found for Sc and Eu with Al2O and P2O5. Thus, these elements are probably associated with phosphates and certain types of aluminosilicates. As this study shows, ashes from coals with a high content of carbonates may be a suitable source for the critical elements Y, Eu, Dy, Er and Tb, especially yttrium. However, further studies are needed in order to find more conclusive results.
Managed & Built by I2M Consulting, LLC | © 2010-
