Recycling the water of a Magenta pigment suspension was initiated with a 7 mL/L FeCl3 solution. The flocculation immediately starts. However it saturates and never reaches 100% where the pigment interface potential is expected to be zero (Fig.1).
To complete the flocculation by bringing the potential to zero, cationic polymer solution is added after a given dose of FeCl3. The cost of this process depends on the optimum switching moment from FeCl3 to the polymer.
In order to find the optimum, a titration with the selected polymer is required at various concentrations of FeCl3 (Fig. 1). Before the sample is filled into the 10 ml measurement cell of the Stabino® for the titration, the FeCl3 was allowed to react with the sample for 5 minutes.
In Fig. 2, where the polymer titrations at different levels of pre-treatment are shown, “100%” corresponds to the 7mL/L FeCl3, which was used in the process. Due to the high conductivity of > 50 mS/cm at 100%, a high sensitivity piston was used for titration. The starting potential decreases from 5% FeCl3 concentration to 80%. This is mainly due to the increasing salt conductivity. The decrease of the consumption is mainly related to the decrease of number of particles after reaction with FeCl3 .
By plotting the polymer consumption versus the concentration of FeCl3 (Fig. 3), the conclusion is obvious: It is not effective to do a FeCl3 pre-treatment at higher than 30 - 40% concentration. 2/3rd of the FeCl3 could have been saved. In view of the cost saving, the little investment into 5 – 15 minutes per titration is worthwhile.