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              Applying LDS to Monitor Flocculation in Papermaking
                                   M.G. Rasteiro and F.P. Garcia
       Chem Eng Dep, Coimbra University, Polo II, Pinhal de Marrocos, 3030-290 Coimbra, Portugal,

Flocculation is a dynamic process important to several industrial areas. In fact, inducing aggregation of
particles can improve effluent treatment by sedimentation or can be used to separate a valuable product
from the carrying fluid, among many other applications.
In the case of papermaking, flocculation of the fine particles is essential to achieve adequate retention during
sheet formation. Several additives have to be added to the stock suspension of fibres that feeds the paper
machine, in order to produce a paper sheet with adequate characteristics (strength, whiteness, etc.).
However, those additives, usually finely dispersed materials like calcium carbonate, bentonite, clay, etc, can
be very easily lost during the sheet formation process if their flocculation is not induced. On the other hand,
the extent of flocculation has to be controlled, since too much flocculation can lead to poor drainage, and
thus higher energy costs, and also to irregular surfaces. This need for an accurate control of flocculation in
papermaking was the motivation for this work.
In this work we have decided to monitor the flocculation process by evaluating the particle size distribution as
flocculation proceeds. The technique selected to control the flocculation process was LDS, since it is a
particle size analysis technique easy to operate, fast and allowing repeated measurements. Moreover, LDS
allows us to extract also information about the fractal dimension of the flocs. In addition, it is well known that
LDS is being progressively adapted to on-line measurements, though no applications in papermaking have
yet been reported.
In this paper only results for the flocculation of the single components in the stock suspension will be
reported (bentonite and calcium carbonate). The influence of the polymeric material used to promote
flocculation and of its concentration will be studied. Since both bentonite and calcium carbonate are
negatively charged only cationic and non-ionic polymers have been tested. However, polymers with different
charge density and molecular weight were used, in order to identify the flocculation mechanisms that can
better promote aggregation of the materials studied.
The results obtained have shown that it is possible to use LDS to monitor the dynamics of flocculation
processes. Moreover, LDS allowed the detection of different flocculation kinetics depending on the flocculant
characteristics, dosage and pH. The LDS equipment used enabled also a qualitative evaluation of the flocs
resistance by submitting the flocs to different shear stresses or ultrasound frequencies, once flocculation was
considered terminated. In fact, resistance is one of the flocs characteristics of crucial importance in
papermaking due to the high shears the stock suspensions have to stand.

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