Design of experiments to optimize the extraction parameters of a power grade Indian coal

Abstract

Low grade high ash coals with difficult physical washability characteristics require chemical cleaning techniques such as solvent extraction to obtain super clean coal with lower ash contents. Kedla, an Indian low grade coal has ash content around 44%–50% and there is a need to maximize its solvent extraction yield so as to make this power grade coal industrially useful. This paper highlights the use of definitive screening design and response surface methodology to extract the maximum organic matter from Kedla coal using ethylenediammine (EDA) and N-methyl pyrollidone (NMP) as solvents under mild atmospheric pressure conditions. The coal was pre-treated with acetic acid before extractions were carried out. The parameters such as the acid pre-treatment time, acid concentration, particle size, the co-solvent concentration and the solvent concentration were optimized in this paper. With this design of experiments technique about 40% of the organic matter with minimal ash (less than 2%) content was rendered extractable unlike the already postulated designer solvent system—e, N system (i.e. NMP containing small amount of EDA) that uses coal to solvent ratio as 1:17 (wt/vol) and coal to co-solvent ratio as 1:1(wt/vol) thus only giving 19% extraction yield. Therefore, the present design of experiments technique helps in conserving the number of experiments for process intensification.