Highly efficient conversion of low-rank coals to high-quality humic acids via ozone oxidation depolymerization at room temperature

Abstract

The oxidation depolymerization represents an essential process for the conversion of low-rank coals to high-quality humic acids. Herein, the low-rank coals including Shengli lignite (SL), Naomaohu coal (NMH), Xiaolongtan coal (XLT), Shendong powder coal (SDP), Shendong chunk coal (SDC) and Mengdong coal (MD) were used in ozone oxidation experiments at room temperature. The results show that these coals can be effectively depolymerized to solvent-soluble components (humic acids) by ozone oxidation with acetic acid as the solvent. Among them, NMH and SDP exhibit the high organic matter conversions, reaching 75.41% and 77.10% in only one oxidation process, and the high yields of solid products of drying are up to 52.60% and 70.36%, respectively. Furthermore, after multiple cycles, the total organic matter conversions of SL and NMH can reach 81.15% at the fifth cycle for SL and 91.17% at the third cycle for NMH, and the total humic acid yields without ash are greater than 58.42% for SL and greater than 82.44% for NMH, respectively. This work provides an effective strategy to convert low-rank coals to humic acids with a high conversion and a high yield under mild conditions.