1I n paper manufacturing flocculation can be defined as the formation of aggregates or flocs from the raw materials presented in a papermaking furnish. These interactions include fibres-fibres; fines-fibres; fillers-fibres; fillers-fines; and dissolved and colloidal fraction with all other furnish constituents. Fibre flocculation is mainly due to mechanical processes and it has an important influence on formation. However, fines and fillers interaction is dominated by the electrostatic interactions between particles and chemicals, and are the main influence on retention. Adhesion and occlusion of fines and fillers in the mechanical network mainly affect drainage. Dissolved and colloidal material flocculation is also due to electrostatic interactions with the added polyelectrolytes and it will mainly affect the efficiency of the chemicals and the runnability of the process and the quality of the final product when deposits are formed. Flocculation is extremely important in papermaking and it can be considered the intermediate state between the raw materials and the final product. For this reason, flocculation control has become an increasingly important issue in the paper industry over the years. The goal is to achieve the optimum flocculation that allows papermakers to obtain good retention and drainage at the same time as good formation. In the past, the optimum was considered the right equilibrium between these opposite effects; however, nowadays, the aim is to optimize these opposing effects in a sequential way. In other words, manipulating the chemicals and the turbulence in order to first flocculate the suspension, then break down the flocs, and then to re-flocculate the suspension in an optimum manner. This approach facilitates low mechanical flocculation and high chemical flocculation. Considering that the raw materials and the papermaking conditions are fixed for a given product and paper machine, there are only a few variables that can be controlled to improve flocculation, from the point of view of floc properties, size and strength to shear stress. Among these variables are consistency, type of chemical additive and addition point (Gess, 1998;Blanco, 1994).On the other hand, trends in papermaking such as a move towards closed-water systems, a higher use of recovered paper, faster machines, etc., also affect flocculation. For example, accumulation of anionic trash, fluctuating pH and temperature, increase of conductivity and turbulence, and so forth. These changes may affect pulp fractions, polymer characteristics, flocculation mechanism and floc properties. Therefore, the efficiency of the polymer can be dramatically affected. These effects most often result in a degradation of the additive efficiency and sometimes hinder the effectiveness of the various additives (Blanco et al., 1998;Vendries and Pfromm, 1998 One of the most important parameters that influences flocculation and, therefore, floc properties, is the flocculant characteristics. Flocculant additives operate via different flocculation mecha...

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The increasing use of fillers makes filler retention a more critical issue. The effects of both shearing and flocculant overdose on filler flocculation kinetics and mechanisms and floc properties, in the absence of fibers, are presented. The flocculation by cationic polyacrylamide (CPAM) or poly(aluminum chloride) (PAC) was determined using a focused beam reflectance probe. The effect of shear forces on aggregation kinetics depends on particle size. PAC produces soft flocs. At 250 rpm, the number of counts decreases due to flocculation, while, at 750 rpm, flocculation of particles smaller than 0.5 µm increased the number of counts. Filler flocs induced by the optimal CPAM dosage, which was 3 times lower than the one that neutralized the potential, were partially reversible, although they are generally believed to be irreversible; an excess of CPAM inhibited the reflocculation of the particles and decreased the flocculation rate, but a moderate excess improved floc strength and floc stability. These observations are consistent with the hypothesis of polymer flattening.

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The electrochemical reduction mechanism of anthraquinone (AQ) has been studied in N,N-dimethyl formamide (DMF) and/or its water mixtures by means direct current polarography (DCP) and differential pulse polarography (DPP). The polarograms consisted of two well-defined waves in anhydrous DMF. The presence of small amounts of water does not effect the first wave but causes the second wave to shift toward more positive potentials; large amounts of water (solutions with at least 40% water) give a single wave. Half-wave potentials for the one-electron step in DMF are more negative than those in aqueous solutions and have been used to calculate the disproportionation constant, KD, of the radical anion in each solution. The reduction potentials in aqueous solutions have been analyzed on the basis of the Heyrovsky model. Results show that the electrode process is a two electron-two proton conversion of the AQ quinone to dihydroxyanthracene (in acidic media) or its dianion (in alkali media). From these results a reduction mechanism has been proposed for AQ in both aprotic and protic solvents. LIST OF SYMBOLS ACN acetonitrile AQanthraquinone, 9,10-dihydroxyanthracene AQ'-radical anion anthrasemiquinone AQ 2 dianion 9,10-dihydroxyanthracene AQH-anion 9,10-dihydroxyanthracene