WET END CHEMISTRY NEWS LETTER FOR April, 1999

WET END OPTIMIZATION IS ACCOMPLISHED BY

Seamless Control of the Physics and Chemistry of Papermaking

Introduction

Recent technical publications by leading suppliers of on-line consistency measurement instrumentation show how they can be used to help stabilize the papermaking process. In this exposition we shall go a step further and show that, in addition to consistency stabilization, process chemistry optimization can be simultaneously achieved, with much greater benefit.

It is widely, although not universally, recognized that chemistry plays a significant role in the papermaking process. For our part, we have been conducting leading edge wet end chemistry studies since 1991 that have become increasingly sophisticated as our understanding of this complex subject has increased.

In 1993 we first advocated widespread adoption of the microparticle process. It can provide the best compromise between retention and formation, provided that use of a conventional retention aid is strictly limited. We advocate use of a charge-neutralizing chemical, such as a cationic scavenger in conjunction with the highly anionic microparticle, typically colloidal silica or bentonite. The optimum balance between them is determined very simply, by balancing their feed rates to maintain the headbox zeta potential at which drainage is maximized. This can be accomplished on-line by the Distributed Control System (DCS).

The total amount of combined scavenger and microparticle is determined by the desired level of retention, as measured by a consistency sensor. This again can be done by the DCS. Process chemistry optimization is the happy result. The papermaking process is stabilized at precisely the level the papermaker desires, physical sheet properties are maximized at the highest level achievable with the chosen chemicals, at minimum chemical usage and cost.

This subject has been previously addressed in a web site technical paper entitled "When Does the Second Generation Shoe Drop?" It provides documentation of the many advantages of balancing chemical feed rates to optimize chemistry, in contrast to the simplistic practice of using a single flocculating chemical.

Measuring the Electrokinetics

The traditional, scientific method is to measure the charge on the surface of the particles, called the zeta potential (V ). The zeta potential is conventionally measured in the laboratory by a method known as "microelectrophoresis." Widely used instruments are called the "Delsa", "System 3000", "Malvern","Rank Brothers", "Laser Zee", "Mobility Meter", "Brookfield", etc.

The first three of those cited are semi-automatic and deserve special mention because they can generate zeta potential distribution curves, or histograms. The curves are occasionally distinguished by being skewed to the negative side because of a greater presence of a smaller (and therefore more negative) species, such as fillers or fines. Or they can be bimodal, because of special surface chemistry characteristics, as in the case of stabilizing monomers used in the emulsion polymerization of latex.

However, an anomalous shape of the histogram is purely of incidental interest. Regardless of its shape, in order to optimize wet end chemistry the addition of cationic chemical, to reach the optimum zeta potential, must inevitably follow.

A method appropriate for on-line use is called "streaming potential", as exemplified by Zeta Data technology. A pad is formed of the fibers, fines and fillers between two electrodes. When water is drawn through the pad, the difference in charge between the two electrodes is a precise measure of the charge on the surface of the particles. The streaming potential value is plugged into the Helmholtz-Smoluchowsky equation to calculate the zeta potential (V ).

The most recent process chemistry development work done in our laboratory has led to recent publications under a title which begins with the words "Process Chemistry Optimization, Coated Free Sheet (CFS)". They uniformly show a direct and strong dependence of both process parameters and physical property parameters on the zeta potential value.

In fact, the relationship is so powerful that it leads to the counter-intuitive result that optimizing the zeta potential results in higher sheet ash and simultaneously greater strength. Since the higher sheet ash results in lower cost, we are achieving better quality at lower cost. In the real world, the dollar savings can run well into seven figures annually.

Also, the relationship is so sensitive that a variation of only 2-3mV zeta potential from optimum can result in a 20-30% decrease in cost efficiency. Therefore, a large premium attaches to DCS implementation of closed loop zeta potential control of chemical feed rates.

There is actually one other commonly used method of measuring "charge", based on quite a different principle. Should this subject be of special interest, we have posted an in-depth analysis in the technical paper section of the web site, "Zeta Data vs Mutek Comparison."

Simultaneous Control of Consistency and Headbox Zeta Potential

Zeta Data technology enables process chemistry optimization under conditions of variability in feedstock quality provided only that the headbox is operated at the zeta potential which maximizes drainage. This is true regardless of the particular state of machine chemistry conditions that is prevailing at the moment. This responsiveness to feedstock quality ensures that the chemicals are used in the most cost-effective way.

Because "maximum" drainage is a relative value, it can cope effectively with feedstock variability. The DCS also needs to have the essential input of the absolute consistency values at which the papermaker wants to operate the machine. It has also been found that these data are invaluable in stabilizing quality during grade changes and in re-starting after a machine break.

Complete harmony is found only in nature. Therefore, it is not surprising that maximum drainage and maximum retention are both obtained at the same, optimum zeta potential.

A certain amount of cationic scavenger is required to neutralize the anionic trash, and a certain amount of microparticle (colloidal silica or bentonite) is customarily used to control retention, drainage and formation.

Adjusting chemical feed rates to maximize drainage at an appropriate pre-determined consistency level is an internally harmonious means of achieving maximum process efficiency on the machine while simultaneously maximizing chemistry efficiency. This is far superior to the simplistic practice of adding a single flocculating chemical that systematically degrades formation.

John G. Penniman