High-efficiency extraction of aluminum and lithium from coal fly ash using a novel sodium pyrosulfate mechanochemical activation - Sodium persulfate pressure leaching technology

A novel method of high-efficiency extraction of aluminum (Al) and lithium (Li) from coal fly ash (CFA) is proposed. The mixture of CFA and sodium pyrosulfate (Na2S2O7) was subjected to mechanochemical activation, followed by the addition of sodium persulfate (Na2S2O8) for pressure leaching. The results indicated that the leaching efficiency of Al and Li reached 95.58% and 71.59% for treatment at the molar ratios of Na2S2O7/Al2O3 of 3:1, ball milling of 60 min, molar ratios of Na2S2O8/Al2O3 of 8:1, liquid-solid ratio of 4:1, reaction time of 120 min and temperature of 200 °C, then continued to increase the temperature to 230 °C, 95.45% Li leaching can be further achieved. Leaching residues were studied by multiple solid phase analysis techniques to elucidate the leaching mechanism. It was found that the mechanochemical activation of Na2S2O7 effectively broke a portion of Si–O–Al bonds in CFA to form aluminum tri-sodium sulfate, the Na2S2O8 was decomposed into hydrogen sulfate (HSO4−) and hydrogen peroxide (H2O2) during the pressure leaching, the oxygen released by H2O2 together with water vapor would create a high-pressure environment, allowing HSO4− to further disintegrate the Si–O–Al and Si–O bonds, ultimately releasing Al and Li. The extraction kinetics confirmed that the extraction process could be expressed well by the combination of the chemical reaction and interior diffusion, and the apparent activation energies of Al and Li were 28.61 kJ/mol and 26.79 kJ/mol at the temperature range of 140∼200 °C. This finding provides insight into more efficient recovery of Al and Li from CFA.