Biological removal of sulphur and ash from fine-grained high pyritic sulphur coals using a mixed culture of mesophilic microorganisms

•About 50% of sulphur and 55% of ash content were biologically removed from coal.•Sulphur and ash removal were enhanced by both, increasing pH and adding ferrous iron.•Sulphate and jarosite were precipitated at high levels of pH and after iron addition. Bioleaching of coal’s pyrite is a promising option to reduce sulphur content of high sulphur coals from an economical, technical and environmental point of view. In this research, a mixed culture of acidophilic iron- and sulphur-oxidizing mesophilic microorganisms including Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans was used to remove pyritic sulphur from the high sulphur coal of Mehr Azin, Tabas, Iran. The influence of various parameters, namely nutrient medium type (Norris and 9K), initial pH (1, 1.5 and 2) and the addition of ferrous sulphate on the pyritic sulphur removal and ash content of coal was investigated using an orthogonal array L-18 Taguchi design. Shake flasks experiments were carried out in pulp density of 5% (w/w), particle size of smaller than 500μm, and a stirring rate of 150rpm at 35°C for 30days. Analysis of variance (ANOVA) was used to determine the effects of the variables on sulphur and ash reduction from the coal. The maximum sulphur removal (50.3% total sulphur) was obtained at the initial pH of 1, the ferrous sulphate addition of 0.02M and Norris nutrient medium. Total sulphur and ash content values were decreased from 3.87% to 1.92% and 25.72% to 11.6%, respectively. SEM/EDS and XRD analyses showed that a high level of sulphate and jarosite precipitations could be occurred at the initial pH of 2 when 0.05M ferrous iron addition and 9K nutrient medium were used.