煤矿酸性“老窑水”低Ca/Mg成因机制

Genetic mechanism analysis of low Ca/Mg value of acid goaf water in coal mine drainage

山西省阳泉市山底河煤矿“老窑水”循环系统多年水质监测数据计算结果显示,煤矿酸性“老窑水”的Ca/Mg值普遍偏低,且存在Ca/Mg值随酸化程度的增强(SO_(4)^(2−)含量增加或pH减小)而减小的规律。针对这一问题,结合研究区的地球化学物源条件,通过室内试验以及野外监测水样的石膏、方解石、白云石矿物饱和指数与pH变化关系,分析煤矿酸性“老窑水”低Ca/Mg值的成因机制。研究表明:区内石炭系-二叠系的煤系地层中碳酸盐岩夹层、分散状态分布的菱镁矿、黄铁矿是“老窑水”中Ca^(2+)、Mg^(2+)、SO_(4)^(2−)的物质来源;在黄铁矿氧化水解形成的以硫酸根为主导的酸性溶液中(pH为2.0~4.5),代表硫酸对石膏、方解石、白云石可溶解性的饱和指数排序为石膏>方解石>白云石,受石膏在高浓度硫酸活性降低并发生沉淀、方解石溶解受Ca^(2+)同离子效应抑制和饱和状态的平衡调节的综合影响,使Ca^(2+)相对含量减少,由于MgSO_(4)溶度积大于CaSO_(4),故Mg^(2+)含量未受上述约束(或较低),脱白云岩化反应可因Ca^(2+)含量随石膏沉淀而继续进行,加之区内有菱镁矿的溶解,使得Mg^(2+)相对含量增加,最终出现了镁矿酸性“老窑水”Ca/Mg值低的结果。Ca/Mg值可作为煤矿酸性“老窑水”的污染特征指标,应用于环境影响评价。

The calculation results of water quality monitoring of the acid mine drainage circulation system of Shandi river in Yangquan City,Shanxi Province for many years show that the Ca/Mg value of the acid goaf water in coal mine drainage is generally low.The average value of 257 groups of acid mine drainage samples was 1.14,the average value of 69 samples of mine drainage in the study area during the same period was 3.73,and the average value of 206 samples of karst groundwater was 5.30,and there is a law that the Ca/Mg value decreased with the increase of acidification degree(increases of SO_(4)^(2−)content or decreases of pH value).In response to this problem,combined with the geochemical source conditions of the study area,this paper analyzes the relationship between the saturation index of gypsum,calcite and dolomite minerals and pH value changes in laboratory tests and field monitoring water samples,so as to reveal the genetic mechanism of low Ca/Mg values of acid goaf water in coal mine drainage.The research results show that the scattered magnesite and pyrite in the carbonate interlayers in the Carboniferous-Permian coal measure strata in the study area are the material sources of Ca^(2+),Mg^(2+)and SO_(4)^(2−)in the goaf water;In the sulfate-dominated acidic solution formed by the oxidative hydrolysis of pyrite(pH value is in the range of 2.0 to 4.5),the saturation index representing the solubility of sulfuric acid to gypsum,calcite and dolomite is in the order of gypsum>calcite>dolomite,at the same time,with the increase of pH value,gypsum will occur in the precipitation reaction,the relative content of Ca^(2+)would decrease,and the relative content of Mg^(2+)would increase,which will eventually lead to the decrease of Ca/Mg value in the solution.Its chemical mechanism is,(1)When the pH value is in the range of 2 to 3,the dissolution of gypsum is inhibited by high concentration sulfuric acid,the solubility decreased rapidly with the increase of pH value,gypsum precipitation may occur,which have been confirmed in laboratory tests,and the content of Ca^(2+)decreased;When the pH value is in the range of 3 to 4.5,the gypsum in the samples is in a reaction equilibrium state with a slight change in the saturation index near zero.A slight increase in pH value will lead to the formation of gypsum precipitation from Ca^(2+)dissolved from calcite and dolomite.The content of Ca^(2+)cannot continue to increase,and the relative content of Mg^(2+)increases.(2)In the pH range of 2 to 4.5,the calcite is affected by the co-ion effect of Ca^(2+)and the ion-pair balance when the gypsum is saturated,the saturation index of all samples was maintained at about−4.4,irrespective of pH decrease.(3)However,the dissolution of dolomite was not affected by the above inhibition(the solubility of MgSO_(4) in sulfuric acid solution is greater than that of CaSO_(4)),the Mg^(2+)and SO_(4)^(2−)content of the monitored samples have a significant positive correlation(the linear correlation coefficient reached 0.83).While dolomite forms gypsum precipitation with Ca^(2+)in the solution,de-dolomitization reaction occured sustainably,and there is dissolution of magnesite,which eventually increased the content of Mg^(2+)in acid water relative to the content of Ca^(2+),and the Ca/Mg value decreased.The Ca/Mg value can be used as an indicator of pollution characteristics of acid goaf water in coal mine drainage and applied to environmental impact assessment.