Water-soluble chlorine and associated major cations from the coal and mudrocks of the Cannock and North Staffordshire coalfields

Abstract

Samples of high-Cl coals and their immediate roof, floor and intraseam dirt bands were analysed for major water-soluble ions in order to predict Cl leachability. In organic-poor mudrocks the total CI is as NaCI groundwater solution which is 100% water-soluble. In the coals the absolute water-soluble CI levels are higher, but even with ultrafine grinding 100% extraction cannot be achieved. The relation of the water-soluble Cl to Na in the coarse fractions and Ca in the ultrafine samples suggests different mechanisms of Cl holding and release. Dull coal has a higher water-soluble Cl content that bright coal, which may be explained by the greater size of dull coal pores and greater accessibility of these pores to the groundwaters. The groundwater Cl is Na-associated and leached with relative ease from coarse samples; it accounts for approximately one-third of the total Cl. For ultrafine samples there is a close correlation between water-soluble Cl −, Ca 2+, HCO − 3 and pH. This Cl − is thought to be combined with the organic matter and released by ion exchange (OH − for Cl −), the acidity created being balanced by carbonate solution. Fine grinding is essential to create a maximum surface area for exchange reactions to take place. If the groundwater Cl concentrations are high enough, they may override the total Cl-coal type relationship, but if the water-soluble Cl is leached from coal thus affected, the ‘normal’ relationship of highest total Cl in bright coal and lowest in dull coal holds true.