Influence of Impurities on Dispersion Properties of Bayer Alumina

Abstract

Among the different impurities contained in Bayer alumina (Ca, Fe, Na, Si, Mg), calcium was found to greatly influence the dispersion of alumina particles in water. Up to 90% of calcium is dissolved at acidic pH, whereas calcium remains on the alumina surface in the basic pH range and screens the negative Al–O− charges. The presence of calcium, through reducing repulsive interactions between particles, has a negative effect on the dispersion of alumina. The adsorption of the Na+ salt of poly(acrylic acid) (PAA‐Na) is strongly influenced by Ca2+/PAA‐Na interactions in suspension with an increase of the maximum adsorbed amount of PAA‐Na in the presence of calcium. The amount of PAA‐Na needed to reach a high electrostatic repulsion and a minimum of viscosity is 2 times higher in the presence of 400 pm calcium than for a low calcium content (∼80 ppm). Finally, with an appropriate amount of PAA‐Na, a similar state of dispersion can be reached with or without the presence of calcium.

Adsorption of Polyelectrolytes on Calcium Carbonate – Which Thermodynamic Parameters are Driving This Process?

“…This difference is owed to the more positively charged CaCO 3 surface existing in the Ca 2+ loaded fluid system (see Table ).The impact of temperature on the saturated adsorbed amount of BNS differs, depending on the type of fluid system: At pH 9, saturated adsorption increases with increasing temperature whereas in the alkaline, Ca 2+ loaded fluid, the opposite trend is observed. A similar behavior has been reported for the adsorption of hydrolyzed modified polyacrylonitrile on sulfate cellulose, for the interaction of BNS with β‐CaSO 4 ·½H 2 O (gypsum plaster) and for the adsorption of other polyelectrolytes on oxide surfaces …”

“…There, inorganic binders including various clays, reactive oxides, and cement clinkers and nonreactive fillers such as e.g. calcium carbonate, quartz or aluminum oxide are suspended in water and treated with polyelectrolytes to modify their properties . Common admixtures used for this purpose are dispersants, water‐retention agents, flocculants, retarders etc.…”

Stability of Aqueous Barium Titanate Suspensions for MLCC Inkjet Printing

“…Then, when the PAA ratio increases, cations contents increase as well as interactions with dispersant molecules. The literature relates that interactions of Ba 2+ and Ca 2+ cations with PAA molecules allow increasing adsorption of molecules on powder particles by creating bridges between negative surface sites (S) and carboxylic groups of dispersant, according to the reaction: …”