Concentrated coal—water suspensions containing non-ionic surfactants and polyelectrolytes 1. A preliminary study using rheology and adsorption isotherms

Abstract The influence of addition of dispersing agents on the properties of coal—water suspensions has been investigated using adsorption isotherms, and rheological (steady state and oscillatory techniques) measurements. Four different dispersants were investigated - two non-ionic ethoxylated surfactants and two anionic polyelectrolytes (lignosulphonate and naphthalene formaldehyde sulphonated condensates). The non-ionic dispersants showed an initial increase in the complex modulus and viscosity, with further decrease on ??? the dispersant concentration. The initial increase was attributed to adsorption of the non-ionic dispersant with the polyethylene oxide (PEO) chain partly directed towards the surface (probably by hydrogen bonding with polar groups such as CO and -COOH) leaving the hydrocarbon chain pointing towards the bulk solution. This makes the coal surface initially more hydrophobic resulting in its enhanced flocculation. On further addition of the non-ionic dispersant, adsorption occurs by hydrophobic bonding between the alkyl group and the hydrophobic portions of the coal surface. These result in a hydrophilic surface and the PEO chains provide steric stabilisation. With the anionic polyelectrolytes, adsorption occurs via the hydrophobic groups present on the chains leaving the charged chains dangling from the surface. This results in a combination of electrostatic and steric stabilisation. Adsorption isotherms showed good correlation with the rheological results, namely a rapid reduction in absolute shear modulus and viscosity at the concentration at which the steep rise in adsorption occurs. A comparison between the various dispersants was obtained from the complex modulus, | G *|, and viscosity, η pl volume fraction curves. The non-ionic dispersants showed a steep rise in | G *| and η pl at a critical volume fraction of coal that was lower than that obtained with the anionic polyelectrolyte. A more quantitative comparison was obtained from the maximum packing fraction φ p which could be calculated from the plot of | G *| −0.5 and η −0.5 pl vs φ c . The maximum packing fraction was higher with the anionic polyelectrolytes when compared with the non-ionic dispersants. However,