Electrokinetic study of silica particles flocculated by two cationic polyelectrolytes; sequential and simultaneous addition

Two cationic polymers were used to flocculate a silica dispersion. One was a high molecular weight polyacrylamide with a low charge density and the other was a low molecular weight polyamine with a high charge density. The polymers were added both sequentially and simultaneously to the silica particles. By adding the polymers in sequence the effect of having sites blocked on the silica particles was investigated. The ESA (electrokinetic sonic amplitude) signal, which is proportional to the dynamic mobility, was measured as well as in some cases the resulting floc size. The effect of using different stirring speeds was also investigated. At almost every charge ratio the charge compensation of the silica particles was found to be more successful when only polyacrylamide was used compared with when simultaneous and sequential addition of both polymers were employed. However, at charge ratios of 2–2.5 polymer charges/silica charge both sequential and simultaneous addition was found to be favourable for high percentage of HMW compared with when only polyacrylamide was used. At these charge ratios there is an optimal use of the polymer charges. Sequential addition gave an improved charge compensation compared with simultaneous addition when smaller amounts of polyacrylamide was added than was needed to fully cover the silica particles. At simultaneous addition, polyacrylamide adsorbs first onto the silica particles. Therefore, with increasing amounts of polyacrylamide, simultaneous addition was favourable due to the hindered adsorption of polyamine. The pre-addition of polyamine to the dispersion gave a larger floc size due to the expanded conformation adopted by the later adsorbed polyacrylamide. At small additions of polymers this gave a better use of the polymer charges and, therefore, an increased charge compensation was obtained compared with simultaneous addition. However, the charge compensation was proportional to the amount of adsorbed charges. No synergetic effect was obtained until the charge ratio was raised to 2–2.5. Above this charge ratio, too much polymer was added for giving a synergetic effect on the charge compensation.