Assessing the Hydrophobicity of Petrographically Heterogeneous Coal Surfaces

Abstract

Wettability and hydrophobicity of coal surfaces are technologically important but their accurate determination as a function of coal composition still remains a challenge. In this paper, a new experimental methodology has been developed to characterize coal surface hydrophobicity. This methodology combines the assessment of the extent of the surface heterogeneity by semiautomated petrography and image analysis, with contact angle measurements, using the axi-symmetric drop shape analysis technique. The measured distribution of petrographic components of macerals and minerals and their fractional areas on the coal surface where contact angles had been measured were correlated with the contact angle measured at that position. Contact angles for vitrinite and minerals were estimated independently, and those of the finely dispersed inertinite and liptinite were estimated from the contact angles of the heterogeneous surfaces and the fractional areas of the maceral and mineral components via Cassie's equation. The established link between the discrete petrographic components that define the coal heterogeneity and the coal surface hydrophobicity is significant for estimating the hydrophobicity of composite coal particles and for predicting the performance of the flotation process which is used for cleaning fine coal particles.

The effect of rank, lithotype and roughness on contact angle measurements in coal cleats

“…The results are similar to previous studies that report larger contact angles for vitrinite rich coals compared to higher inertinite coal surfaces using sessile drop measurements and coal flotation, meaning that the dull band is more hydrophilic compared to the bright band (Arnold and Aplan, 1989;Ding, 2009;Ofori et al, 2010) The contact angle depends only on the coal in contact (some bright, some dull); the mineral matter and the surface voids (detailed in Table 3). Roughness is taken to be zero since the surface is polished.…”

“…The Cassie equation has been used in several sessile drop studies to explain and predict the contact angle of a heterogeneous surface, such as coal (Ding, 2009;Drelich et al, 1996;Fuerstenau et al, 1990;Gosiewska et al, 2002;He and Laskowski, 1992;Keller, 1987;Rosenbaum and Fuerstenau, 1984). Studies have demonstrated that lithotypes have unique wetting surfaces, with inertinite rich coal creating a mean contact angle of 70°, in vitrinite rich the angle is 80º (Ofori et al, 2010) and liptinite rich the angle is 110° (Keller, 1987). However, Keller (1987) Gamson et al (1993) reported that coal possessed a range of different surface textures, predominately based on the lithotype.…”

“…The value (70˚) is based on literature values from (Keller, 1987) and (Ofori et al, 2010) while was assigned a value of 90˚ based on regression analysis using 6.…”

“…cos ( ) is the contact angle of minerals, which equals 45˚ based on literature values from (Keller, 1987) and (Ofori et al, 2010) ( ) is the contact angle of gas, which equals 180˚…”

“…Although core flooding methods rarely consider the effect of the coal heterogeneity in determination of relative permeability curves, other experimental studies using the sessile drop and captive bubble techniques have demonstrated that coal wettability is dependent on rank (Keller, 1987;Tampy et al, 1988), maceral composition (Arnold and Aplan, 1989;Ding, 2009;Fuerstenau et al, 1983;Ofori et al, 2010), mineralisation (Gosiewska et al, 2002;Susana et al, 2012) and the roughness (Drelich et al, 1996;Li et al, 2013). The sessile drop technique is convenient for the collection of contact angle data on a large number of coal samples, or sites on a sample, but there are limitations when coal wettability effects flow in cleats.…”