EFFECT OF AMMONIUM CITRATE ON THE RHEOLOGY AND PARTICLE PACKING OF ALUMINA SLURRIES

The valence of the citrate ion changes from -1 to -3 as the ammonium counterions of tribasic ammonium citrate dissociate with increasing pH. The citrate anion is potential determining because it strongly adsorbs to the surface of oxide particles in aqueous suspensions, changing the surface charge. At pH 8, close to the ''natural'' isoelectric point of Al2O3, the fully dissociated, trivalent citrate ions produce a negative surface charge. At pH 3, where the natural surface charge of Al2O3 is positive, citrate ions can neutralize the surface. The greatest number density of citrate ions is adsorbed at pH 3 because they are only partially dissociated and the natural surface of Al2O3 contains a high density of positive sites. Attractive particle networks are produced at both the isoelectric point and when excess salt is added to a dispersed suspension. Rheological measurements are used to compare the strength of attractive, Al2O3 particulate networks produced with and without added citrate ions. The attractive networks produced at pH 3, the isoelectric point in the presence of citrate ions, are slightly weaker relative to those produced at pH 9, the natural isoelectric point of Al2O3. Much weaker networks are produced by dispersing slurries at pH 8 with small amounts of tribasic ammonium citrate and then adding excess salt (either tribasic ammonium citrate or NH4Cl). The strength of the weakly attractive networks produced at pH 8 appears identical to those produced when an indifferent electrolyte is added to Al2O3 slurries dispersed at pH 4, reported previously. These observations suggest that although the citrate ion is known to adsorb to the surface of Al2O3, as evidenced by its potential-determining effect, it does not produce a short-range repulsive potential of any significance.