Novel Alkali Extraction–Acid Precipitation–Chlorination Distillation Method for Co-extraction of Humic Substances and Germanium (Ge) from Ge-Rich Lignite

The traditional combustion-chlorination distillation process used for extracting germanium (Ge) from lignite not only causes significant environmental pollution but also wastes the high-value humic substances (HSs), especially fulvic acids (FAs) in lignite. This study attempts to comprehensively recover Ge, humic acids (HAs), and FAs from lignite using an alkali extraction-acid precipitation-chlorination distillation (AEAPCD) method. The optimized process parameters are obtained as alkali extraction at 100 °C for 2 h using 0.3 mol/L NaOH as the extractant and a liquid-solid ratio of 9:1, acid precipitation of extracted liquid using HCl to adjust the pH value to 1, and the chlorination distillation of Ge-rich FA solution at 95 °C for 30 min. In this case, the recovery yields of HAs and FAs in their respective products are 88.83 and 91.42%, respectively, and the Ge recovery yields of GeCl₄, HAs, FAs, and alkali extraction residues are 64.95, 5.18, 9.91, and 19.96%, respectively. The mechanism study shows that germanium in lignite is mainly bound to HSs, followed by other organic compounds (most likely lignin-like substances). During alkaline extraction of lignite, HSs (especially FAs) first undergo neutralization reaction with NaOH to form soluble sodium salts, and the bound germanium is also extracted. As the concentration of NaOH solution increases, lignin-like substances undergo hydrolysis, generating a large amount of small-molecule soluble substances, such as p-coumaryl, coniferyl, and sinapyl alcohols, or their derivatives, dimers, trimers, etc., and germanium bound to them is also extracted. In addition, purification of crude FAs and HAs was also attempted in this work.