Characterization of the flocculation dynamics in a papermaking system by non-imaging reflectance scanning laser microscopy (SLM)

Non-imaging reflectance scanning laser microscopy (SLM) was used to characterize fiber-filler slurries and the flocculation processes in paper furnishes typical of those used for alkaline fine papermaking. The measurement produced a distribution of chord lengths characteristic of the size and shape of the furnish fiber and filler particles. Flocculation induced by a cationic polyacrylamide (C-PAM) flocculant resulted in significant changes in the chord length distribution which was monitored as a function of time. Upon addition of C-PAM to the furnish, the mean chord length increased to a maximum value followed by a decay in magnitude at longer times. The SLM data was compared to dynamic drainage jar first pass retention data obtained at equivalent polymer dosages. At longer times, the shear instability of the flocs caused the subsequent decrease in the mean chord length. The time dependent decay of the percentage of chord lengths greater than 40 mu m was fit to an exponential decay, yielding a decay time constant, tau. tau increased slightly with C-PAM dosage, but decreased dramatically with increased mixing intensity due to more rapid flee degradation under the more turbulent shear conditions. The advantages and limitations of the SLM technique in comparison with conventional flocculation analyzers are also discussed.