Acid leaching of coal and coal-ashes

Twelve Turkish lignites and the corresponding ashes were leached in sulfuric acid (pH 1.0, 25 °C) for 14 days. Asphaltite from Silopi amended the coals. The conditions mimic treatment in the effluent from bioleaching of sulfidic mineral concentrates, but the results are equally valid for an isolated leaching process. The extended time meant that we approached equilibrium and maximum extraction.The coals have limited neutralizing capacity. H₂SO₄ (1.0-2.0 l, 1 M) was needed to stabilize 1 kg coal at pH 1.0 (liquid:solid ratio 10:1), but the coal-ashes required 18.0-24.0 l/kg dry solid, which is the neutralizing equivalent to CaO.Leaching of dominant inorganic phases consume acid, but our interest is merely to remove trace elements present as dopants. We removed large fractions of Mg and Mn, but Al, K and Na extractions were limited by the presence of stable minerals and bimetallic oxides. The formation of the latter is driven by combustion at high temperatures. Alumina, normally not stable at pH 1.0, was protected from the effluent by the organic phase in coal. Fe leaching varied and appeared to be a marker for different chemical occurrences in the solids.Cd, V, Zn, U and Th were leached to near 80% from the ashes, but considerably less from the coals. Co and Ni extractions were near 60%, but not always higher from the ashes compared with the coals. Cu yields increase following ashing and reached ca. 60%. Ti, Ba, and Cs were not leached.We suggest that direct acid leaching is of interest to limit the deleterious impact of ash deposits and to recirculate metals from the ash. Ash may partly replace limestone in hydrometallurgical processing, but, more importantly, metal ions extracted from ash may be fed into the metal recovery stages of such processes. It is particularly interesting to leach Co, Cu, Ni and Zn, besides Mn, V and the environmentally hazardous Cd, U and Th. Leaching of whole coals is well motivated for domestic use-lump sizes around 18-50 mm, or slightly smaller, 10 mm, if mandated by practical residence times—where generally no other measures are taken to protect the local environment.