How
AquaVive Technologies solves Wastewater Problems plus
“Pitch Control” In Pulp and Paper
Production
by
Peter Yew
Peter Yew was a key
speaker at the UBCM "Water, Storm Water and Wastewater
” Management Conference at the Sheraton Wall Centre,
Vancouver, BC, Phone (604.844.1914) or
e-mail
anton_kuipers@gems1.gov.bc.ca
The
Problem
The content of wood pitch in tree
varies from 1.0% to 5.0% (of dry wood), depending on the
wood species. After bleaching, the pitch content is
reduced to approx. o.15% - 0.20% in the pulp. However,
for unbleached grades and mechanical pulp, the content of
pitch is considerably higher. The wood pitch is insoluble
in water and consists of free fatty acids, resin acids,
fatty acid esters of glycerol and sterols, and materials
such as sitosterol and betulinol, etc.
Some wood pitch may exist within
the fibres but may not cause pitch problems. Again, free
pitch will not cause problems, if it does not agglomerate
within the system. However, in pulp and paper systems,
pitch as colloidal particles has a tendency to
agglomerate and may be deposited in the system. The
mechanism of pitch deposition includes resin transfer
from fibres, fines deposition and deposition of pitch
with hydrodynamic shear, evaporation, creaming,
coalescence, etc.
The pitch problem is one of the
most important factors limiting the efficiency and
productivity with washers, bleach plants and paper
machines. This is because modern pulp and paper making
processes are very sensitive to disturbances. In the
washing and bleach plant, pitch can plug washer screens
and reduce the efficiency of the washer and increase
bleach chemical consumption. Pitch particles in pulp
products increase dirt count and reduce pulp brightness.
Pulp with big pitch counts leads to paper defects (such
as breaks, holes and spots very often caused by pitch
deposits) which represent some of the most common
complaints of the user/buyer of paper. Thus, pitch
control is a critical consideration in pulp and paper
production.
The
Solution
There are four types of commonly
used basic approaches to pitch control. These are :
dispersion, adsorption, flocculation and dissolution. The
dispersion approach is to stabilize the pitch particles
by adding dispersants to protect colloidal action. Pitch
particles are eventually washed from the
system.
The adsorption approach is to
adsorb pitch particles using inorganic minerals. The
pitch adsorbed will be carried through the systems as
part of the product.
The flocculation approach is to
flocculate pitch particles using pitch fixing agents.
Here, the pitch particles are retained in the product
during drainage.
The dissolution approach is to
dissolve pitch particles by surfactants, solvents or a
combination of surfactants and solvents. The pitch
concentration is reduced and the deposition of pitch is
minimized.
Some mills use one approach or a
combination of two, three or four approaches to treat
pitch problems. The dispersion method will obviously
suffer from the closure of the backwater system. The
concentration of dispersed pitch can increase to levels
at which agglomeration may take place. The pulp from
mills using the flocculation method may cause pitch
problem in paper machine operation because pitch will be
released from the pulp in paper making systems. Also, the
flocculation method can result in over-flocculation and
sheet formation will deteriorate. Many mills use talc as
a pitch adsorbent in brown stock and in paper machine
systems to reduce pith problems. However, talc reduces
the efficiency of bleaching chemicals since it contains
many trace minerals such as Fe, Cu, Al, Mg, etc. These
can adversely affect the efficiency of bleaching
chemicals, especially in the mills using H2O2. The
impurities in talc contribute to rapid decomposition of
H2O2 and brightness reversion. In addition, the removal
of talc/pitch entities depends on the level of retention,
and these entities retained in the products increase
product ash, and may cause other deposit problems in
paper machine systems. The filler content of the final
sheet will increase and therefore strength properties can
deteriorate. The dissolution method depends on the
selection of proper solvents and surfactants for
different systems.
Obviously, these methods for pitch
control possess their own limitations and
disadvantages.
Pitch Control Agent SF-11 is
designed to overcome these limitations and disadvantage.
SF-11 is a blend of biodegradable and environmentally
correct chemicals. The principal ingredients are natural,
non-regulated biomass alcohol's.
It has been demonstrated in North
America that mills using products similar to SF-11 for
pitch control have been cost effectively producing high
quality pulp and paper. SF-11 is being using in many
mills abroad and is unmatched in performance and cost
effectiveness evidenced by comparative lab bench
trials.
For information:
AquaVive Technologies
Suite 303, 7651 Vantage Way Delta BC
Canada V4G 1 A6
Telephone: 604-940.2995
Facsimile: 604.940.2895
e-mail : modco@intergate.ca
