Occurrence modes and stepwise chemical extraction experiment of lithium in No.20 coal seam in Pu’an mining area, Guizhou Province

Lithium (Li) is an important energy metal, and the global demand for Li has grown rapidly in recent years. The discovery of coal-associated Li deposits in China proves that coal mines may serve as a potential alternative of Li resources. No.20 coal seam in Pu’an mining area in Guizhou province of China was selected as the study object. The basic properties, mineral composition, elemental composition, and occurrence modes of Li of the raw coal and six density-graded samples (obtained through a float-sink test) were determined by using coal proximate analysis, optical microscope, X-Ray fluorescence spectroscopy (XRF), inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). The results show that the content of Li in the raw coal reaches 134.09 μg/g, and in the six density-graded samples the content of Li rises with the increase of density (e.g. the density-graded sample>1.8 g/cm³ has the highest Li content of 212 μg/g), which implies that gravity separation can preconcentrate Li in the coal to a certain extent. The main mineral components of raw coal include clay minerals, pyrite, calcite, quartz, etc. According to the correlation between Li and Al, Si, etc., as well as the elemental content in mineral micro-areas (by LA-ICP-MS), it is speculated that Li mainly occurs in aluminosilicate minerals in an adsorbed form. The results of the sequential chemical extraction test used to investigate the phase composition of Li in coal are as follows: water soluble form (20.96%), ion-exchangeable form (32.90%), silicate/aluminosilicate form (22.80%), carbonate/phosphate form (10.81%), disulfide form (3.92%) and residual/organic form (2.58%). This study provides a theoretical guidance for the extraction of Li resources from Li-rich coal.