Molecular simulations of selective adsorption of lithium in the No.6 coal seam, Junggar Coalfield

Objective With the discovery of metal deposits in coal-bearing strata, the distribution and occurrence patterns of lithium in coals have attracted much attention. However, its partitioning mode between organic structures and minerals in coals remains poorly understood.

Methods This study investigated typical coals with a high lithium content in the Junggar Coalfield in Inner Mongolia. By integrating multiple simulation methods, including quantum mechanics, Monte Carlo simulation, and molecular dynamics, this study established the structural models of organic matter and inorganic minerals, as well as a visualized lithium-coal molecular structure model. Accordingly, it investigated the preferential adsorption sites on both organic matter and minerals for lithium and explored the impacts of mineral types and the metamorphic grade of organic matter on lithium adsorption during coalification and heating.

Results In organic matter of coals, carboxyl groups were identified as the preferential adsorption sites for lithium ions, with the adsorption capacities of different functional groups for lithium ions decreasing in the order of carboxyl groups, hydroxyl groups, pyrrole, carbonyl groups, pyridine, and benzene rings. In the kaolinite structure, the center of the six-membered ring containing oxygen atoms within silicon-oxygen tetrahedra emerged as the preferential adsorption sites for lithium. In the case where organic matter, kaolinites, and illites coexisted, kaolinites exhibited a stronger adsorption capacity for lithium than illites under the same temperature. Meanwhile, illites showed a higher capacity to bind organic matter than kaolinites. With increasing metamorphic grade of coals, the adsorption capacity of organic matter for lithium gradually decreased.

Conclusions During coalification and heating, lithium experiences repartitioning between organic matter and minerals. The evolution of its occurrence state is difficult to be directly revealed using experiments. In contrast, molecular simulations can provide abundant molecular-scale information that is difficult to obtain through experiments, thereby providing important theoretical support for the exploration, exploitation, and utilization of lithium resources in coal-bearing strata.