Waste-to-resource valorization of low-rank coal: CO2-K2CO3 synergistic pretreatment for simultaneous carbon emission reduction and enhanced critical metal extraction.

Low-rank coal represents a carbonaceous resource enriched in lithium and gallium with great utilization potential. However, carbon encapsulation and stable aluminosilicate crystal lattices form dual barriers that severely hinder the efficient extraction of lithium and gallium. Conventional pretreatment methods can only achieve single-function modification: thermochemical activation destroys lattices but fails to remove carbon shielding, while standalone CO2 decarburization eliminates carbon matrices but cannot activate inert minerals. To address this technical bottleneck, this work proposes a novel CO2-K2CO3 synergistic pretreatment strategy that realizes one-step integration of decarburization, carbon valorization, and synchronous mineral lattice reconstruction. Quantitative results from time-of-flight secondary ion mass spectrometry (TOF-SIMS) and the sequential chemical extraction procedure (SCEP) demonstrate that 91.53% of lithium is hosted in aluminosilicate phases, whereas gallium mainly occurs in aluminosilicate (56.58%) and sulfide (40.61%) phases. K2CO3 plays a dual role: it catalyzes the Boudouard reaction to consume CO2 and generate combustible CO, lowering the decarburization temperature by 277 °C relative to standalone CO2 treatment and enabling pre-enrichment of target metals; meanwhile, it transforms inert aluminosilicates into acid-soluble nepheline (KAlSiO4) to liberate lattice-bound lithium and gallium. Under optimal conditions, the decarburization efficiency reaches 93.86%, achieving 1.51-fold and 3.34-fold pre-enrichment of lithium and gallium, respectively. The final leaching efficiencies of lithium and gallium reach 98.73% and 86.20%, respectively. This work breaks the inherent limitations of conventional approaches and offers an efficient low-carbon route for simultaneous carbon valorization and critical metal recovery from low-rank coal.