No 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.