Coordination mechanism of aluminum with oxalate and fluoride in aluminum crystallization from vanadium extraction wastewater

Abstract

Aluminum speciation is complex in a solution where organic and inorganic ligands coexist, and aluminum can form different complexes with various ligands. In this paper, the coordination mechanism of aluminum with oxalate and fluoride in aluminum recovery from vanadium extraction wastewater was investigated. By comparing the process of recovering aluminum with oxalate and fluoride precipitation, oxalate crystallization is found to perform better with vanadium extraction wastewater. Under optimum conditions, the crystallization percentage of aluminum was 87.10%. K3Al(C2O4)3·3H2O was obtained and characterized by XRD and FTIR, and the total recovery of aluminum was 80%. The coordination mechanism and migratory behavior of aluminum with oxalate and fluoride during crystallization were studied by thermodynamic simulation and quantum chemical calculation. Results showed that aluminum primarily exists as AlF2+, AlF2+, AlC2O4+, and Al(C2O4)2 in vanadium extraction wastewater. With the addition of K2C2O4 and an increase in solution pH, these complexes were coordinated by C2O42− and formed Al(C2O4)33– complex anions, which provided a saturated solution environment for K3Al(C2O4)3·3H2O crystallization. The DFT simulation showed that the Gibbs free energies of the coordination reactions for Al(C2O4)33– were negative, which indicates that these coordination reactions can proceed spontaneously. This study is important when enriching the coordination mechanism of aluminum and improving the comprehensive utilization rate of resources for vanadium-bearing shale.