Oxalic Acid Cleanses CaOH⁺ from Pyrrhotite Surfaces and Boosts Recovery: Microflotation, Bench Tests, and DFT

In industrial production, lime on the pyrrhotite surface need to be removed using inorganic acids for activated flotation. However, inorganic acid reagents need to be replaced due to their causing severe environmental disasters with large consumption. In this study, the effects of two green organic acids with structures similar to that of pyrrhotite were investigated by micro-flotation experiments and density functional theory (DFT) calculations.The flotation experiments of micro and actual ore confirmed that oxalic acid could effectively reduce lime from the pyrrhotite surface. In the micro flotation experiment, the pyrrhotite recovery increases from 10.98% to 83.7% when the lime dosage is 500   mg/L, oxalic acid dosage 8.0 × 10 -3   mol/L at an acidic environment with pH is 5. But, the recovery only increases from 6.15% to 19.24%, when the acetic acid was added with a dosage of 16 × 10 -3   mol/L and pH is 6. Further investigation using DFT and frontier molecular orbital theory revealed that oxalic acid could increase the charge exchange with Ca 2+ in CaOH + due to its double carboxyl group structure. Therefore, the resulting Ca-O bonds are stronger than the Ca-S bonds formed between CaOH + and the mineral surface. Consequently, the Ca-S bonds break. Due to its single carboxyl group structure, acetic acid exhibits a weaker charge exchange capability than oxalic acid and does not cause the complete breaking of the Ca-S bonds. The actual ore closed-circuit test further verified that the iron concentrate grade was 2.13% and the sulfur grade was reduced by 90.15%. These findings highlight the potential of oxalic acid as a more sustainable and environmentally friendly substitute for inorganic acids, offering improved flotation performance while reducing the environmental impact of minerals separation.