Research progresson geochemistry and extraction of lithium from coal and coal ash

The exploration and exploitation of lithium from salt lake and ore deposits are limited to meet the consumption demand in China，thus it is urgent to develop new alternative resources. Lithium in coal and coal ash in China is found to be highly enriched. This paper reviews the geochemical research on lithium in coal and coal ash，including its testing method，content and distribution and modes of occurrence，as well as lithium migration during coal combustion and extraction from coal ash. The results show that high-lithium coal is mainly distributed in Carboniferous Permian coal seam in North China，Permian coal seam in South China and Southwest. Among them，the content of lithium in coal reaches the industrial grade in Jungar of Inner mongolia，Pingshuo of Shanxi，Fusui of Guangxi，Nanwu and Nantong of Chongqing mines，which may become an alternative source of strategic critical metal lithium. ICP-MS is frequently used to determine lithium in coal and coal ash. The main carriers of lithium in coal are clay minerals such as kaolinite，chlorite and illite，partly related to organic matter. The enrichment of lithium is mainly controlled by the sediment-source rocks，low temperature hydrothermal fluids，depositional environments，regional geological and tectonic processes. The migration of lithium in the process of coal combustion is affected by coal type，modes of occurrence，combustion atmosphere，temperature，particle size and boiler type，etc. The secondary enrichment of lithium in coal ash mainly occurs in the glass phase，and also exists in the crystalline phase of minerals such as mullite. The available extraction technology of lithium from coal ash is introduced. In addition，the advantages and disadvantages of the present extraction process are briefly analyzed. At the same time，it is pointed out that the key to extracting lithium is how to break the silicon-aluminum bond，and the recovery，purification of lithium in the leaching solution. On the basis of mechanical/chemical synergistic activation and deep desiliconization，in line with the concept of green and high efficiency，we should learn from the lithium extraction technology of salt lakes，and enhance the research on aluminum-based adsorbents to establish the technological process of synergistic extraction of multiple elements. Finally，the future research directions of lithium associated with coal are proposed.