Pulp Suspension Mixing

PULP FIBER SUSPENSIONS 20-3.1 Pulp Suspension Mixing... 

Mixing and reaction time constants are not readily calculated for pulp suspensions. Mixing rate is influenced by the complex suspension rheology. Bleaching rate is influenced by chemical diffusion into the fiber wall and can be controlled by mass transfer. Further, in laboratory experiments a net reaction rate is measured and may have been influenced by the mixing conditions during the test. Despite these concerns, a number of estimates can be made. 

Figure 20-17 Static mixer used in pulp suspension mixing applications. Shown is the Komax steam mixer used in medium consistency applications.

Bennington, C. P. J., G. Owusu, and D. W. Francis (1997a). Gas-liquid mass transfer in pulp suspension mixing operations. Can. J. Chem. Eng., 75, 53-61. 

NMR imaging techniques were applied to the measurements of velocity field in opaque systems such as tomato juice and paper pulp suspensions [58-60]. In both cases, the particle concentrations are sufficiently high that widely applied techniques such as hot film and laser Doppler anemometry could not be used. The velocity profile for a 6 % tomato juice slurry clearly showed a power-law behavior [58, 59]. Row NMR images for a 0.5 % wood pulp suspension provided direct visual of three basic types of shear flow plug flow, mixed flow and turbulent flow as mean flow rate was increased. Detailed analysis of flow NMR image is able to reveal the complex interaction between the microstructure of suspensions and the flow [60]. 

The interactions between different polysaccharide additives (such as guar gum or starch) and the components in a variety of pulp suspensions (including whole pulp and washed pulp) have been described as a four-step process (Fig. 7) mixing of polysaccharides into the pulp suspension complexation with DCS adsorption of polysaccharides (free or complexed) onto fibres and fines material and, finally, association of fines to the adsorbed polysaccharides on the fibres [48]. According to the authors, this agrees with the maximum fines retention, drainage and paper strength observed in industrial applications. 

Onabe [84] analyzed the drainage behavior of pulp suspensions with a two-component system, with attention to polyion complex formation. With cationic and anionic polyacrylamides (molecular weights 3,000,000 and 1,000,000 g/mol) added individually, a polyion complex with an irregular three-dimensional stmcmre was precipitated on the fiber surface. This reduced the homogeneity of the paper sheets. In contrast to this, a pre-mixed system of an anionic and cationic polyacrylamide [83] resulted in an improvement in pulp retention and paper quality (paper strength). Because of the very high molar mass and thus the big coil diameter, the formed PECs are very big and are therefore good flocculants. 

Bennington, C. P. J. (1988). Mixing pulp suspensions, Ph.D. dissertation, University of British Columbia, Vancouver, British Columbia, Canada. 

Francis, D. W., and R. J. Kra-ekes (1990). Measuranent of mixing in high-consistency pulp suspensions, J. Pulp Paper ScL, 16(4), J130-J135. 

Paterson, A. H. J., andR. J. Kerekes (1985). Fundamentals of mixing in pulp suspensions measurement of microscale mixing of chlorine, J. Pulp Paper ScL, 11(4), J108-J113. 

Prior to about 1920, flotation procedures were rather crude and rested primarily on the observation that copper and lead-zinc ore pulps (crushed ore mixed with water) could be benefacted (improved in mineral content) by treatment with large amounts of fatty and oily materials. The mineral particles collected in the oily layer and thus could be separated from the gangue and the water. Since then, oil flotation has been largely replaced by froth or foam flotation. Here, only minor amounts of oil or surfactant are used and a froth is formed by agitating or bubbling air through the suspension. The oily froth or foam is concentrated in mineral particles and can be skimmed off as shown schematically in Fig. XIII-4. 

RIP [Resin in pulp] A general term for hydrometalluigical processes in which an ion-exchange resin is mixed with a suspension of a ground ore in water. The desired metal is selectively extracted into the resin. See CIP (1). 

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