Flocculation of calcite dispersions induced by the adsorption of highly cationic starch

The interactions between highly cationic starch and likewise cationic calcite were investigated by determining the adsorption isotherms and the flocculation of the calcite dispersions at different concentrations of starch. Starch of varying degree of substitution (DS), 0.2, 0.35, and 0.5, and molecular weight (MW), ∼3800 and ∼2×106 g mol-1, were used. The experiments were in addition made at three different temperatures; 25, 35, and 45°C. It was found that the interactions in most cases were of fairly low character. Consequently, the entropy and low solubility of the starch are expected to be the dominant driving forces for adsorption. The cationic character of both the untreated calcite and the starch generate an electrostatic repulsion that counteracts a possible adsorption. There is also repulsion between the cationic substituents in the starch, which among others factors controls the solubility of the starch in the aqueous phase. The repulsion can be affected by adding electrolytes. NaCl was used for this purpose in this investigation. As a result of the changes in the electrostatic repulsions the adsorbed amount of starch onto calcite did also change. The adsorption did increase with increasing NaCl concentrations, as did the degree of flocculation. The high MW starches did in all the conditions examined adsorb to a higher degree than did the low molecular ones. Increasing the temperature from 25 to 45°C causes changes in the adsorption and flocculation behavior of the dispersions, especially in systems where starch with DS of 0.2 is present. The starches of higher charge are less affected by the temperature due to stronger internal repulsion. The low MW starches did not flocculate the calcite dispersions, while the starches of high MW initially flocculated the dispersions and restabilized them at higher concentrations. The high MW starch with DS 0.2 induced the strongest flocculation. © 2003 Elsevier Science B.V. All rights reserved.