Fine tuning of wood extractives control

By Przemyslaw Pruszynski, Global Technical Specialist, Nalco Company

BRUSSELS, March 21, 2011 (RISI) - In part I of a two part article, the important area of extractives control from wood for pulp is addressed. Part II can be read in the next Mills & Technology newsletter

With the increased content of recycled fiber, decreased and highly varying quality of recycling raw materials, a trend towards neutral pH in mechanical grades and the increasing application of high-yield pulps (HYP) in typical fine paper furnish, issues related to controlling pitch and stickies are becoming increasingly more important to the overall economic results of many paper mills. These issues normally extend beyond typical wet end runnability, press section, dryers and calender stack deposits, reaching all the way to printing press deposits and printability complications. Blending virgin mechanical pulps and recycled fiber can also create additional opportunities for uncontrolled agglomeration of colloidal pitch and stickies particles, resulting in productivity and quality issues.

Natural wood extractives will remain a primary focus of this article, with some additional insight to colloidal fraction of synthetic stickies. Some fundamental information is also referenced to better understand how the roadmap to successful strategy for extractives control and related best practices were defined.

The trend towards neutral pH operations in mechanical grades has been driven primarily by the ability to utilize higher brightness calcium carbonate fillers^[1]. This important industry trend has redefined challenges related to extractives control by limiting an access to aluminum chemistry, utilized widely for pitch control under acid pH. Although small amounts of either alum or polyaluminum chloride (PAC) are still commonly used under neutral, and even alkaline pH, it is clear that pitch control under neutral pH is more challenging and requires careful system wide focus. Nalco's DC-PRO Contaminant Control Technology program provides such a system-wide approach based on an understanding of the nature of contaminants, effect of water chemistry, and an overall process stability and monitoring strategy that make extractives control more effective.

Removing extractives from the system

It needs to be stressed that the main strategy for any deposit control program should be reducing the amount of potentially detrimental, hydrophobic contaminants entering the system. Every opportunity to use any significant thickening stage in TMP and bleaching plants (hydrogen peroxide bleaching plants offer such opportunities) to divert rich contaminant pressate streams out from the process, should always be attempted and maximized. Optimizing pH at the thickening stage to increase the solubility of extractives, and application of dispersants to increase flow of extractives with the pressate stream, should also be evaluated through detailed studies of extractives mass balance. The compromise between reduction of extractives levels and potential fiber losses needs to be understood. Fiber recovery steps in terms of efficient screening or floatation units (DAF) are often introduced to conserve overall process yield.

Major forms of wood extractives

According to Back and Allen^[2], energy in mechanical pulping (refining or grinding) results in colloidal dispersion of wood extractives and unimodal distribution of all pitch components within colloidal particles. Individual pitch components, fatty and rosin acids, triglycerides and sterols all distribute themselves within the pitch particle in a way to expose more hydrophillic groups to water and to hide more hydrophobic components inside the core of colloidal particles, Fig. 1. Acid functions, mainly carboxylic groups from fatty and resin acids on the surface, provide particles with a surface anionic charge, responsible for its colloidal stability. Steric stabilization from sugars adsorbed on the surface provides some additional stability to colloidal particles.

Further, especially at higher pH solubility levels of some pitch components, mainly resin and to a lesser extent fatty acids, increases. Richardson^[3] studied solubility of various resin acids as a function of pH in distilled and process water. In most general terms he concluded that solubility of these species gradually increased with pH, especially when pH=7 was approached. Soluble resin and fatty acid salts may also be a source of deposits when they react with calcium ion to form insoluble calcium salts, or when their solubility is suddenly decreased by recirculation to areas of the process with lower pH.

Figure 1 - Structure of colloidal pitch particle and basic pitch components at various ph values

General extractives control strategy

Two major focuses of extractives control strategy can be clearly identified and defined:

• Preventing uncontrolled agglomeration of colloidal fraction;
• Managing soluble fraction of extractives by preventing its re-precipitation.

These challenges are significantly higher at neutral and alkaline pH, in the cases of using blended fiber sources and in systems using calcium carbonate used as fillers.

In essence, successful deposit control strategy needs to include preventing agglomeration of colloidal fraction, and reducing the detrimental impact of fraction larger than colloidal. In the case of natural wood pitch control where most of extractives enter the system in colloidal form, preventing colloidal agglomeration seems to be a key to success. Stickies enter the papermaking process with a larger spectrum of initial sizes. Therefore, controlling the detrimental effect of large particles (detackification) is generally a part of every stickies control program. Even in stickies control applications however, focusing on colloidal fraction remains very important, as it prevents formation of very detrimental secondary stickies.

Factors affecting agglomeration of colloidal pitch

Controlling colloidal pitch particles requires a clear understanding of factors that affect their stability. Since colloidal stability is based on repulsion of anionic particles, all factors that either lower their charge level, lower repulsive forces between charges or increase energy of collisions between particles leads to increased agglomeration. This easily explains why the terms of shear, temperature and pH shocks come to mind as major destabilizing factors for colloidal materials. Conductivity change resulting in screening individual charges and in turn reducing repulsive forces between anionic colloidal particles is also less often considered, but can still be a significant trigger to poor colloidal stability. Papermakers should look for pitch deposits in cases when overdosing bleaching chemicals is required to reach certain brightness targets.

Changes in pH, especially around pH=pKa value of carboxylic function (pKa≈5), vary the amount of charge by changing degree of dissociation of carboxylic functions on the surface of colloidal particles. It is therefore well understood why sudden changes in pH or just localized pH drop, especially at high conductivity, already unstable, systems, which can trigger significant deposition problems. Factors affecting colloidal stability make it very clear that improving machine chemical stability^[4] needs to constantly be a goal of the papermaker. Such stability projects are especially important for mechanical grades of paper with their close integration of pulping, bleaching and papermaking operations creating large potential for instability.

Major strategies in controlling colloidal pitch

The cheapest and most abundant detackifiers for pitch and stickies particles remain fibers and fines. Attaching hydrophobic colloidal particles to cellulosic materials effectively prevents their agglomeration. Microscopic evaluation of the amount of colloidal pitch particles throughout the papermaking process remains the best and simplest way to monitor the amount of colloidal pitch. This easy evaluation provides the opportunity to assess the "health" of colloidal pitch dispersion by monitoring not only a number but also the size of its particles. Information collected systematically may provide a mill with important clues when things unexpectedly worsen.

Papermakers should actively take full advantage from initial colloidal dispersion created in the pulping process and prevent the natural process of colloidal agglomeration by applying fixation additives very early in the process. Delaying this action makes extractives control more expensive and often less effective. Increasing the diameter of typical colloidal pitch particle between 1 μm, 2 μm and 3μm represents an increase of relative mass from 1 to 8 and 27 (volume of sphere particle is proportional to r³). It does not require further explanation how such a seemingly small particle size increase affects colloidal stability. It is very important to catch such early stages of agglomeration to prevent formation of larger deposits.

Effectiveness of the fixation mechanism strongly depends on the proper selection of fixatives, configuration of feed points and effective program monitoring. A stable retention program and an adequate level of retention is also an indispensable element of a successful fixation-based pitch/stickies control program.

Fixation of colloidal particles to the elements of papermaking furnish has historically been done through the application of chemical additives operating solely by a charged neutralization mechanism. Negative charge of the controlled particles, cationic charge density of the polymeric additives, pH and the level of soluble anionic trash all affect the effectiveness of additives that operate through charge neutralization. An almost universal source of charge in cationic polymers is an amine function. Primary, secondary and tertiary amine functions develop a positive charge in water through attaching a proton (H⁺). Such a proton can be detached at higher pH (depending on amine structure and its basicity), leading to a reduction of cationic charge. Quaternary ammonium salts do not display such charge sensitivity related to pH, retain their charge regardless of pH and are recommended as a choice for neutral and especially alkaline papermaking.

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