高锂无烟煤中不同赋存态锂同位素组成趋同特征及其机理

Convergence and its mechanism of lithium isotopic compositionwith different occurrence states in Li-rich anthracite

煤和含煤岩系中Li是全球Li资源潜在来源之一,引起了国内外高度重视。沁水盆地局部地区煤中Li异常富集,探讨其同位素组成特点与演化有助于深化锂富集分配机制认识。以盆地北部阳泉矿区新景矿8号无烟煤层为研究对象,利用电感耦合等离子体质谱仪(ICP-MS)和X射线衍射仪(XRD)测定Li质量分数和矿物组成,结合逐级化学提取试验,确定煤中Li的赋存状态。利用多接收器电感耦合等离子体质谱仪(MC-ICP-MS),测试煤样和不同赋存状态Li的同位素组成,探讨了煤化作用过程中Li同位素分馏特征。结果表明:煤样Li平均质量分数达138.04μg/g,高于世界硬煤中的平均质量分数(14μg/g);主要载体为高岭石,有少量Li以有机结合态的形式存在;原煤样中δ7Li的均值为-2.13‰,硅酸盐结合态δ7Li的均值为-1.15‰,有机结合态δ7Li均值为-1.53‰,不同赋存状态Li同位素组成相似。研究认为,无烟煤中Li同位素组成趋同机理为:①煤化作用过程中轻的6Li从有机结构中脱除,再次吸附到黏土矿物中,导致煤中有机结合态Li同位素组成变重,硅酸盐结合态Li同位素组成变轻;②在水-岩反应过程中,硅酸盐矿物中重的7Li很容易被溶液萃取,导致硅酸盐结合态中重的7Li流失,Li同位素组成变轻;③煤化作用升温过程有助于Li元素的扩散,促使Li同位素达到动力学分馏平衡。

Coal and coal-bearing strata are considered as a great potential source for lithium extraction,which has aroused wide attention.Extremely high concentrations of lithium were found locally in the coals of Qinshui Basin.The study of Li isotopic composition and evolution is helpful to deepen the understanding of lithium enrichment and distribution mechanism.In the present paper,No.8 anthracite from the Xinjing Coal Mine in the Yangquan mining district,the Qinshui Basin,was selected for analyzing the lithium concentration and mineral phases with the techniques of inductively coupled plasma mass spectrometry(ICP-MS)and X-ray diffraction(XRD),respectively.With the sequential chemical extraction analysis,the occurrence mode of lithium was discussed.Lithium isotopic composition of raw coal and fractions with different occurrence modes were determined by multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)to investigate lithium isotopic fractionations during coalification.The results reveal that the average concentration of coal samples reaches 138.04μg/g,which is higher than the average concentration of world hard coal(14μg/g).Kaolinite and minor organics are considered as the main carrier of lithium.The mean value ofδ7Li in raw coal sample is-2.13‰and the mean values ofδ7Li in silicate and organic fractions are-1.15‰and-1.53‰,respectively.The value ofδ7Li in fractions with different occurrence states are close.The lithium isotopic composition results show:①6Li removals from organics and their adsorptions onto clays;②7Li of silicate minerals is easily extracted into the fluid during the water-rock interaction process;and③the diffusion of Li with the aid of heat during coalification results in Li isotope kinetic fractionation equilibrium.