A Comparative Study on Flotation of Coal Using Eco-Friendly Single Reagent and Conventional Dual-Reagent System

Abstract

Coal, an organic sedimentary rock, is the backbone on which electricity generation and steel making rest. Coal contains mainly carbon, hydrogen, oxygen, nitrogen and sulphur as well as trace amounts of other elements, including the mineral matter that refers to the inorganic constituents of coal [1]. Mineral matter is the principal source of the elements that make up the ash content while the organic matter or the coal macerals contributes to carbon constituents. Most of the high ash coals are subjected to beneficiation for reducing the ash levels so as to make them suitable for various applications. Coking coal or metallurgical coal are coals when baked in absence of air, form a grey, hard, carbonaceous porous residue called 'coke'. These are mainly used for iron and steel manufacturing. Generally coking coals are a part of the bituminous group. While non-coking coals are mainly used for power generation. The mined coal is associated with inorganic impurities during its formation that forms the ash residue in coal which is undesirable for effective utilization of coal for many applications. The high-rank coal with high carbon and less ash content is depleting rapidly. As high-rank coal resources are running out due to the rise in energy demand and steel production, it is therefore imperative to use low-rank, oxidized coal to meet the increasing need for coal [2, 3]. Hence, the need for utilizing low-rank coals with low carbon and high ash is of utmost importance. As these low-rank coal can be effectively utilized after cleaning or washing in order to reduce the ash-forming mineral phases and thereby improving the carbon content of the coal for further suitable utilization. A fine coal washery losses ten times as much high-quality coal as a coarse coal washery. Recovering good quality fines will improve the economics of coal washery [4]. Generally, coal washing involves crushing and screening of Run-of-Mine (ROM) coal into smaller fractions, separating the gangue and mineral matter by using phys­ical separation methods such as dense media separation/heavy media separation (HMS) or physico-chemical process, called froth flotation. As flotation process is based on differences in the ability of air bubbles to selectively attach to specific mineral surfaces in a mineral-water slurry and float to the top based on their degree of hydrophobicity [5-7]. Froth flotation is one of the beneficiation methods in fine coal washing by exploiting the surface hydrophobicity difference between coal macerals that are naturally hydrophobic and its associated ash forming minerals impurities that are mostly hydrophilic in nature. This naturally hydrophobic surface property of coal surface provides a high response while processed by flotation as it is a surface­phenomenon-based separation technique. Hence, coal particles have a natural affinity toward air bubbles [8, 9] as they are naturally hydrophobic.