Co-extraction of germanium and nitrogen-rich humic acid from germanium-rich lignite by ammonoxidation

Germanium, a scattered metal in nature, is generally extracted from coal fly ash produced by the combustion of germanium-rich lignite in industrial process. It is a direct approach for germanium enrichment from lignite, however facing the challenges of high carbon emission and losing valuable humic acid. In this work, a new concept and approach on utilization of germanium-rich lignite was proposed, i.e. co-extraction and utilization of germanium and humic acid. As a result, germanium and nitrogen-rich humic acid (NRHA) were co-extracted by ammonoxidation of lignite, affording “germanium-NRHA co-existence substance”. Ammonoxidation of lignite was performed at the condition of 80 °C for 3 h, then the single extraction rate of germanium was 26.74 wt% and yield of NRHA from lignite was 38.20 wt%, respectively. Sequential ammonoxidation was carried out in the same reaction conditions for 3 times, thereby the total extraction rate of germanium reaches to 42.05 wt%, and yield of NRHA reaches to 54.84 wt%. Instead of direct combustion, valuable NRHA was separated from lignite with the nitrogen content improved from 16.51 % to 21.50 % by ammonoxidation of lignite. The NRHA is a potential materials as biofertilizer or nitrogen doped carbon materials. Thereby, it is beneficial for low-carbon and valuable utilization of germanium-rich lignite.