Insights into the synergistic calcination of spent pot lining and red mud for stabilization of fluorine and recovery of iron

Abstract

Spent pot-lining (SPL) and red mud (RM) are significant industrial solid wastes generated from the aluminum industry, holding valuable potential as secondary resources. This study addresses the pressing challenges of mitigating fluoride emissions resulting from the pyrometallurgical process of SPL and recovering iron (Fe) components from RM. To achieve these objectives, a synergetic reduction-stabilization process via co-calcination was proposed. During co-calcination, the carbonaceous content of SPL acts as a reducing agent, facilitating the reduction of Fe oxides in RM to magnetite (Fe3O4). The recovered Fe3O4 was efficiently separated through magnetic separation with 66.9% recovery efficiency. Additionally, the co-calcination process synergistically stabilizes fluoride emissions from SPL due to the presence of calcium oxide (CaO) in RM. This results in fluoride emissions reacting with CaO to form insoluble fluorite (CaF2), leading to significantly reduced fluoride leaching concentrations (18.3 mg L−1), well below national recommended standards (100 mg L−1). Mineral transformations revealed that Na3AlF6 from SPL decomposed into NaF, AlF3, and HF, with subsequent reactions with CaO leading to CaF2 formation. The selectivity of element combinations among Ca-Fe-F-O can be attributed to the distinct ionic potentials in the HF-CaO-Fe2O3-C system, where CaF2 arose from a hard acid (Ca(II))-hard base (F-) interaction, and Fe2O3 reduction was influenced by the weaker bonding between soft base O2- and borderline acid Fe(III)/Fe(II) repulsion, according to the Hard-Soft Acid-Base (HSAB) theory. This proposed synergistic utilization approach not only detoxifies harmful fluoride emissions but also offers a sustainable method for Fe recovery, holding significant potential for environmental and economic benefits in the context of aluminum industry waste management practices.