Fixation and agglomeration: hybrid fixatives
Flow Cytometry has been used recently to study changes in size and surface tackiness of hydrophobic particles during fixation. Vahasalla, in the work on fixative treatment of coated broke latexes, determined conditions for successful fixation by avoiding complicating process of agglomeration of colloidal particles. In most general terms he concluded that aggressive treatment resulted in undesired agglomeration, while mild, gradual treatment led to desired attachment of particles to the fibers and fines.
Nalco in Canada introduced a class of fixative products known as HYBRIDS in 1993. HYBRIDS were defined as lower charge and higher molecular weight when compared with typical coagulants, Fig. 2. Their success in treatment of a wide range of hydrophobic materials (pitch, stickies, coated broke) could be well explained by their molecular structure. The special position that this group of fixatives occupies was confirmed by Richardsonin work on how various classes of fixatives interact with major components of natural pitch in terms of their ability to fix the fiber (desired) or deposit on other surfaces (undesired). Richardson indicated that these non-classical, lower charge fixatives are more specific in driving fixation of pitch components towards the fiber.
Figure 2 - Placement of hybrid polymers in charge – molecular weight technology space (blue shade on the background symbolizes contribution of charge neutralization)
This observation might well be explained in terms of a Patch Assisted Fixation mechanism illustrated in Fig. 3. Lower charge density of HYBRID fixatives have the built-in protection against aggressive charge neutralization and operate through mild charge neutralization as recommended by Vahasalla. At the same time, due to higher molecular weight combined with lower charge density, HYBRIDS assume less compact conformation on the fiber, with polymer loops and tails expanding further out from fiber surface and as a result form well-defined and persisting (slow penetration into fiber structure) cationic patch. Such cationic patch provides an attractive force towards anionic colloidal pitch particles, directing them towards the fiber surface while their residual negative charge limits the competitive and undesired agglomeration process.
Nalco is currently working towards a new generation of HYBRID polymers with charge neutralization and patch forming properties tuned further to specific extractives types and system conditions.
Figure 3 - Patch-assisted fixation mechanism with hybrid polymers
Managing soluble fraction: preventing precipitation
Preventing precipitation of insoluble salts of fatty and resin acids requires controlling concentration of calcium ion and understanding changes in solubility as a function of pH. Both seasonality in the fatty acids:triglycerides ratio and the effect of applying the hydrolytic enzyme, Lipase, significantly changes the situation related to soluble extractives management. There is little that can be done to reduce the concentration of fatty or resin acids as it is defined by chemical structure, pH and system temperature. Controlling hardness in papermaking systems utilizing calcium carbonate fillers requires a constant focus on pH control and optimization of saveall operations to limit the amount of calcium carbonate fillers recirculation back to acidic TMP operations. Any opportunity for a local pH drop needs to be reviewed and avoided, as it will result in local hardness generation. This also includes following best practices in broke management and preventing microbiological activity in broke chests. Monitoring hardness out of the broke chest, in addition to pH, is the best way to detect any symptoms of localized pH drops in the tower (pH measurement itself may not be sensitive enough). Application of CO2 directly in pH control or to increase system alkalinity through in-situ reaction with caustic (ADALKA) are also good options for pH management with low hardness generation.
Colloidal and soluble pitch found in clear white water stream generated by saveall operation often agglomerates and forms troublesome deposits in white water tanks and lines. Fixation in these streams is not recommended, since in the absence of fibers and fines it would result in pitch destabilization and deposits. Instead some mills benefited from stabilizing effect of either non-ionic of low charge anionic dispersants added to these lean water streams.
Blending different fiber sources: best practices
It has been generally observed that machines using mixed TMP and DIP experience more severe deposit issues than systems using a single pulp source. Typically, an acidic TMP stream mixed with calcium carbonate containing DIP pulp causes hardness development, destabilization of colloidal pitch and stickies fraction and, finally, precipitation of soluble fraction of extractives. Since reactions initiated with pulp blending involve complicated equilibria within the structure of the fiber, their impact on machine deposits continues beyond the blending stage. It is important to condition individual pulp streams properly before they are mixed. This includes fixation of colloidal fraction for each individual pulp stream and matching their pH values as close as possible. Fixing TMP and DIP streams prior to mixing allows the ability to treat them early, selecting best fixatives for their individual water chemistry and avoiding uncontrolled colloidal destabilization during blending. Typically ΔpH<0.5-1.0 should be attempted between TMP and DIP prior to blending. When deciding about strategy for correcting ΔpH, one should remember that dropping DIP pH below 6.5 is not recommended (hardness development) and that increasing TMP pH needs to take under consideration potential brightness losses. Due to a high pH in bleaching stage, TMP bleached with hydrogen peroxide may have a pH closer to that of DIP. Blending coated broke with TMP or GWD pulps requires particularly effective fixation of both streams prior to mixing since pitch was found to serve as a tackifier for coating latexes.
Recent work done in a newsprint mill in Australia[3, 8]illustrates the benefits of a comprehensive approach to contaminant control. This mill had been using blended TMP and DIP as furnish and under acid conditions using a pitch and stickies control program based on high charge and non-quaternary polyethylene imine chemistry. The mill observed a severe loss of machine efficiency related to calender stack deposition after converting its operation to neutral pH. Changing deposit control program to quaternary fixatives, implementing early and specific fixation of TMP and DIP streams prior to their blending, the application of HYBRID fixative to TMP pulp, adding polishing fixative treatment closer to paper machine and CO2 pH control all resulted in full recovery of machine efficiency lost as a result of conversion.
Extractives control requires a total system approach to be fully successful. Essential technical elements of these programs were discussed in this paper. A new mechanism of operation of HYBRID polymer was proposed. Further work in optimizing fixatives chemistry, on-line control and monitoring of deposit control applications and a better understanding and control of extractives distribution within the sheet structure (important to dryers, calender stack and printing press deposits) will remain major development efforts in this area.
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