Flotation characteristics of high ash oxidised indian non-coking coal and its effects on cell flotation

S.K. Biswal1* and D.K. Acharjee2

1 Department of Mineral Processing Technology, Regional Research Laboratory (CSIR) Bhubaneswar-751 013, India
2 Department of Chemical Engineering, Indian Institute of Technology Kharagpur - 721 302, India

Received 6 June 2002; accepted 3 April 2003


It has been observed that freshly-mined coal floats better than coal which has been exposed to the atmospheric oxygen. It is well known that high rank coals are generally more hydrophobic than low rank coal or oxidised coal. Non-coking coal of Talcher coal field, India has very high ash content and is believed to be surface oxidised. This coal contains around 35-50% ash resulting in low gross calorific value. This coal is very difficult to float even at higher concentration of collector due to its oxidised surface and high ash, thus rendering less hydrophobic to attain complete air bubbles attachment. The investigations have been carried out to improve the floatability of this oxidised, high ash non-coking coal by pre-treating it with chemical reagents. Aliphatic alcohols were selected as pretreatment reagent for this type of coal to examine whether floatability of coal increases. The effect of methanol, ethanol, butanol and octanol were evaluated to determine any improvement in terms of yield and ash content. In the present work, the examination of oxidation property of coal by FTIR spectroscopy revealed a notable change in the hydrophobicity index of the coal due to ethanol treatment. Electro-kinetics study of macerals and bulk sample of coal has also been carried out to confirm the oxidation of coal surface.
Coal flotation depends on coal petrographic composition, particle size distribution, density and ash content and also type of flotation reagents. The flotation characteristics of non-coking coal with respect to mineralogical characteristics, flotation reagents and process variables in Denver D-12 sub-aeration flotation cell have been optimised. As coal is under non-polar substances, non-polar collector diesel oil is used to increase the contact angle. Short chain alcohol MIBC and sodium silicate have been used as frother and dispersant respectively. The effect of other process parameters like pH, conditioning time and rpm of impeller have also been studied. Consumption of chemical reagents have been found to be high in the case of non-coking high ash coal in comparison with coking coal due to low hydrophobicity of coal surface, high ash content and fine size of particles.

Keywords: Indian oxidised high ash non-coking coal; Floatability; Hydrophilicity index; Point of zero charge; Effect of flotation reagents

* Corresponding author
   E-mail : skbiswal@rrlbhu.res.in