Paper Technology International 2020 - Journal - Page 61
PAPERTECHNOLOGYINTERNATIONAL
Charge monitoring improves paper machine performance.
Ian Padley, Global Solutions Team Manager Tissue, BTG Eclépens SA
Manfred Sailer, Market Manager, BTG Instruments GmbH
INTRODUCTION:
Ionic charge variation or excessive charge demand are all too often the hidden enemy in papermaking wet end chemistry, impacting
all grades. Sheet defects, excessive chemical usage and poor runnability are all problems which can arise from issues with charge. In this
article, we review the basic chemistry of ionic charge and explain how electrokinetic effects can be harnessed to measure charge. Armed
with modern and convenient measurement technologies, both lab and online, we look briefly at strategies the papermaker can employ to
mitigate unwanted charge impacts and forecast how the latest developments in data management and modelling will allow us to control
more complex operational scenarios.
In 2011, BTG wrote about charge measurement as follows:
Imagine a typical scenario faced by a tissue machine
operator where chemistry has got out of control. Your machine is
using a recycled fiber furnish and making some towel grades; but
you are finding it very difficult to reach the wet tensile specification.
This important end-user property is delivered by a chemical, a
cationic polyamide-epichlorohydrin in fact, but you don’t know that
just that it is ‘wet strength resin’. So, if the lab test for wet tensile is
low, let’s just add some more, OK?
Well, no. Perhaps when the next shift arrives, there will be
foam overflowing the wire pit. Felt dewatering is suddenly quite poor,
but we don’t see that, what we see is wet streaky Yankee coating,
poor crepe, web breaks and repeated crepe blade changes. And still
the wet tensile is poor. Maybe the new shift operator has seen this
before, they shut down, clean the felt, drop the backwater, restart on
fresh water and less wet strength resin, and for a while things may
be better. Or maybe he just turns up the antifoam pump and things
can then get even worse!
What really happened here? The problem with chemistry
is that we tend to see only end effects, not visual things like when
troubleshooting the mechanical operation of the machine. If we
understood a little bit of paper mill chemistry, we might speculate
that we had some poor waste rich in anionic trash in the furnish.
This trash consumed the wet strength resin before it reacted with
the fiber, but by increasing the resin pump, we put more unreacted chemical into the system, filling the felt, creating foam and
destabilizing the Yankee coating. But we still don’t know for sure.
What we have is a chemical soup we need to unravel, not just to
save chemical cost, but to improve the machine operation, as every
tissue maker knows that any chemical imbalance will eventually end
up on his or her Yankee.
Ian Padley, ‘No more soup’ (2011)
So, in the intervening years, the problem is solved, right?
Actually no, if anything it has got worse, not just in tissue, but across
all paper grades. There are a number of reasons for this.
Firstly, the deteriorating and uncertain quality of wastepaper
in recycled grades. Then the imperative to further close the water
cycle of a paper machine and to reclaim every fiber which may
escape in effluent, leading to a cycle up of contaminating materials.
Finally, not to neglect the de-skilling of some industry sectors
and the loss of experienced technicians both from mill and suppliers
who previously kept chemistry issues in balance.
Fortunately, our understanding of the issues and our
ability to measure them and apply effective control measures has
increased in the intervening years, and now we also have effective
and innovative solutions for the scarcity of skilled and experienced
technical staff across the industry. But first, let’s look at some
papermaking chemistry basics.
Papermaking charge chemistry basics
Composition of the furnish
Let us consider what is in your stock pipe and chests; it’s
a mixture of fibres, fines, pigments, chemical additives and ions
in water. Quite different particles in composition and in reality, the
differences in size and charge are even bigger.
Fibers are huge but carry little charge. Fines - which can
still be seen by the naked eye - are smaller but with higher charge.
Natural fibers and fines carry a negative charge.
Pigments are small and highly charged, they might be
anionic or cationic.
Chemical additives can show very different charge
characteristics, most have a cationic charge to effectively react with
the anionic fibers and fines.
Figure 1: Whats in your furnish?
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