Ultrafine coal dewatering: Relationship between hydrophilic lipophilic balance (HLB) of surfactants and coal rank

Dewatering process is one of the most costly steps in mineral processing and coal washing plants. This fact becomes an important industrial challenge in lignite cleaning. The presence of micropores and relatively low hydrophobic nature of lignitic coal lead to trapping of large quantities of water in the coal matrix especially in fine sizes. Since lignites are used as a common fuel in power stations, they need to be dried to a certain moisture level before feeding to the combustion chamber. Dump feeding in power plants cause inevitable loss of efficiency and may be deleterious for combustion sections. It is thus generally wise to dewater coal as much as possible by physical means before it is dried. However, further dewatering cost in some cases may be prohibitive particularly for fine and hydrophilic particles. In this study, various type and amount of surfactants have been tested to increase the efficiency of coal dewatering. As the hydrophobic characteristic of coals varies and increases with increasing the coal rank, dewatering of bituminous and anthracite type coals can be accomplished using low HLB surfactants (Oily), however, these surfactants are not efficient in the case of low rank coals like lignites and brown coals. A special set-up has been designed to study the effect of high HLB surfactants on dewatering of ultrafine lignite particles. It is shown that, while surfactants with lower HLB number and oily characteristics are the most convenient dewatering reagents for slightly hydrophobic surfaces such as hard coals, surfactants with higher HLB values especially those with HLB of around 10 are the most efficient reagents for dewatering of ultrafine lignite and lower rank coals. The governing mechanisms are discussed in the light of hydrophilicity index and modification of physicochemical conditions.