Demulsification Treatment of Spent Metalworking Fluids by Metal Cations: The Synergistic Effect and Efficiency Evaluation

In this paper, various metal ions were utilized for the demulsification of spent metalworking fluids discharged from an automobile parts workshop. Five types of metal ions, i.e., Fe³⁺, Al³⁺, Fe²⁺, Ca²⁺ and Mg²⁺, combined with coagulant were systematically evaluated, and the synergistic effect as well as the optimum operating conditions were studied. The results indicated that the Ca²⁺ as well Mg²⁺ possessed hardly efficiency for the demulsification, on the contrary, Fe²⁺ reduced the yield of the by-product sludge and lowered the SV₃₀ ratio, and Al³⁺ boosted the COD_Cr removal. Furthermore, Fe³⁺ and Al³⁺ had a significant synergistic effect to achieve a better transmittance and a higher COD_Cr/SV₃₀ ratio which revealed that more COD_Cr was removed, as well as less by-product sludge was generated. For a better demulsification of spent metalworking fluids, the optimum operating conditions were gathered as follows: the dosage of metal ions was 0.08 mol/L with Al³⁺:Fe³⁺ ratio was 1.5:1, the reaction pH was 6.00, the reaction time was 18.00 min and the temperature was 323.00 K. Based on this, the COD_Cr removal, the SV₃₀ ratio and the transmittance and COD_Cr/SV₃₀ ratio of the spent metalworking fluids were 80.21%, 40.00%, 95.20% and 128.33 mg/mL, respectively. This combined metal ion demulsification method possessed an advantageous minimization of spent metalworking fluids, which greatly benefited the automobile parts workshops in cutting down the operating cost in environmental protection.