Process Optimization and Structural Analysis of Humic Acid Extracted from Low-Rank Coal in Xinjiang, China

Xinjiang, China, has abundant coal resources with a high proportion of low-rank coal; however, these resources are still mainly utilized as fuel, and their level of high-value utilization remains limited. Humic acids are important functional organic components in low-rank coal and exhibit significant application potential in the high-value-added utilization of coal resources. In this study, lignite samples collected from four regions in XinjiangAkesu, Naomaohu, Dananhu, and Santanghuwere used as raw materials for humic acid extraction via an alkaline hydrothermal method, and the extraction process was optimized by using an orthogonal experimental design. The effects of the extraction temperature, extraction time, alkali concentration, and solid-to-liquid ratio on humic acid yield were systematically investigated through intuitive analysis and range analysis. Under the optimized conditions, the humic acid yields obtained from Akesu, Naomaohu, Dananhu, and Santanghu lignite reached 76.31%, 64.98%, 41.33%, and 63.53%, respectively. Furthermore, the structural characteristics of the extracted humic acids were characterized by elemental analysis, acidic functional group determination, UV-vis spectroscopy, FT-IR, X-ray diffraction, and XPS. The results showed clear differences in aromaticity and structural condensation among humic acids from different regions, with Santanghu-humic acid exhibiting the highest aromaticity. This study provides an optimized alkaline hydrothermal extraction process for humic acid and a theoretical basis for the high-value and functional utilization of low-rank coal in Xinjiang.