FLOCCULATION IN KAOLIN SUSPENSIONS INDUCED BY MODIFIED STARCHES .1. CATIONICALLY MODIFIED STARCH - EFFECTS OF TEMPERATURE AND IONIC-STRENGTH

Turbidity fluctuations in flowing kaolin suspensions (1 wt.%) were studied by means of a fibre-optical sensor technique. From these measurements a flocculation index (F) was calculated for each of the suspensions. A high value of F indicates a high degree of flocculation. A cationically modified and converted (by an oxidation process) starch was, even at very low starch concentrations, an effective flocculant in the temperature interval from 23 to 50 degrees C and at NaCl concentrations ranging from 1 to 100 mM. No flocculation was observed at a 1 mM NaCl concentration if the kaolin suspension was pretreated with sodium polyacrylate (NaPA). In general, the cationically modified and converted starch was a less effective flocculant if the kaolin suspension was pretreated with NaPA. The effect of temperature on the starch-induced flocculation was substantial only at certain concentrations of salt and NaPA. At NaPA concentrations normally prevailing in industrial applications (e.g. paper coating) a higher F value at higher temperature was observed only if both the salt and the starch concentrations were high. The most substantial effect of a higher temperature in suspensions without any NaPA was a higher F value at low salt concentrations. Adsorption measurements indicated that the cationically modified and converted starch flocculated the kaolin suspensions by a bridging mechanism.