MULTISTAGE CAUSTIC LEACHING DE-ASHING OF NIGERIAN LAFIA-OBI COAL

Abstract

Fractions of the high ash Nigerian Lafia-Obi coal L±250 ground to pass the 250 µm sieve in three stages were subjected to proximate/ash composition analyses, hot aqueous leaching de-ashing with water and sodium carbonate in multiple stages and in a H2O-Na2CO3-H2O sequence (with initial solution homogenization). The results obtained showed that ash contents percent of 24.60, 14.70 and 24.07 were obtained for fractions L-250(1), L-250(2) and L+250(2); respectively as against 32.55% in the as-received coal. The ash reductions obtained translate to overall average ash removal of about 38.66% at the 19.90% ash content of the concentrates blend at a good 1:20 ratio of reagent to coal. The study also showed that a three stage leaching in the sequence H2ONa2CO3-H2O (HSH) produced a higher leaching rate than Na2CO3-H2O-H2O (SHH). The ash content of the concentrates blend at 19.90% is lower than 23.80% required for coal blends for Indian standard coking practice, but higher than the maximum of 10% upper limit for the conventional cokemaking practice. The reduction in ash content obtained at the atmospheric pressure treatment of Lafia-Obi coal was found to compare favourably with that of a high pressure elevated temperature autoclave leaching of an Illinois coal. Binary blend formulation between the prime coking western Canada coal and Lafia-Obi coal as-leached showed that inclusion of 16.63% of the latter was probable and this translates to reduction in cost per ton of about $23.67. A successful upgrade of the leaching route derived to industrial scale will make Lafia-Obi coal available as a blend component for economical cokemaking.