Geochemical characterization of coal and waste rocks from a high sulfur bearing coalfield, India: Implication for acid and metal generation

Abstract

An integrated study on coal and mine wastes from the Jaintia Hills coalfield of Meghalaya, India involving mineralogy, acid base accounting (ABA), and net acid generation (NAG) potential and sequential leaching was undertaken to examine their potential in controlling the acid mine drainage (AMD). Mineralogical study revealed that pyrite is the major sulfide mineral in coal and mine waste, being more abundant in sandstone and carbonaceous shale; while, dolomite and calcite are abundant in a few shale and siltstones, and Fe and Al copiapites are enriched in sulfate salts. During the ABA test, all coals and >50% of mine waste showed paste pH<4, implying their acid generating nature. Further, the relations between net neutralization potential (NNP) and acid producing ratio (APR) revealed that blocky pyrite, pyritiferrous sandstone and efflorescent salts contribute acid much higher than coal, siltstone and carbonaceous shale. This inference is consistent with the NAG test. Partitioning of metals in mine waste indicates high proportions of them in the blocky pyrite being bioavailable than from other rocks. While Mn is highly bioavailable, major portions of Pb and Zn are in the reducible and oxidizable fractions. In efflorescent sulfate salts, >80% of metals are available in water soluble fraction; thus these minerals can be considered as the highest polluting residue in the mining environment. These minerals easily precipitate from AMD solution in dry periods and can re-dissolve under rain events because of their high solubility; therefore, they can play an important role in controlling the chemistry of mine drainage in regard to local climate change. In the case of coal, only small fractions of Mn, Ni, Zn, Cd, and Pb are released to the environment, though high proportions of them can become bioavailable under oxidizing conditions besides their other bioavailable forms.