Coal fly ash activated by NaOH roasting: Rare earth elements recovery and harmful trace elements migration

Coal fly ash (CFA) has been considered an important alternative resource for rare earth elements (REEs) and other critical metals such as Ge, Ga, U, etc. Concerns regarding the migration and transformation of harmful trace elements (HTEs: Hg, As, Se, Pb, Cd, Cr) from CFA to the environment are of great significance for the technical design of the recovery of REEs from CFA as well as its economic evaluation and environmental impact assessment. Here, the migration and transformation of these HTEs during the recovery of REEs from CFA activated by NaOH roasting were investigated. The recovery of REEs from CFA was above 90% under the conditions of the alkali-to-ash ratio of 1.2 and roasting temperature of 318 degrees C. In the NaOH roasting process, mercury compounds, i.e., Hg2Cl2, HgCl2, and HgSO4, were converted into HgO, which was then partially reduced to elemental Hg (Hg-0) in the water generated during the roasting process and promoted the release of Hg from CFA. In addition, other HTEs, e.g., As, Se, Pb, Cd, and Cr, were also transformed into compounds during the NaOH roasting process that were easier to be leached out during the following water and acid leaching process compared to that without the roasting process. For example, As compounds (As2O5, As2O3, Ca-3(AsO4)(2), and FeAsO4) were converted into Na3AsO4. Se compounds (Se, SeO2, and CaSeO3) were converted into Na2SeO3. PbSO4 and PbCl2 were converted into PbO. CdCl2 and CdSO4 were converted into CdO. CrO3, CrCl3, and Cr-2(SO4)(3) were converted into Na2CrO4 and NaCrO2. The experimental results of this work provided insights into understanding the migration and transformation of these HTEs and the associated leaching behavior during the recovery of REEs from CFA activated using a NaOH roasting approach.

Automated summary

Coal fly ash (CFA) is considered an important alternative resource for rare earth elements (REEs) and other critical metals, but concerns about the migration and transformation of harmful trace elements (HTEs) are crucial. Research showed that during the recovery of REEs from CFA activated by NaOH roasting, HTEs were transformed into compounds that were easier to be leached out, providing insights into the behavior of these elements during the recovery process.