Magnesium Sulfate is used in the Oxygen
(O2)-delignification stage during pulp bleaching. O2-delignification is used
mainly in mills practicing ECF bleaching technology using no elemental chlorine
(CI2). In this application, the preferred form is Epsom Salt (MgSO4,7H20),
primarily because of low cost.
MgSO4 is used mainly to protect pulp. Highly reactive
free-radicals are formed during O2-delignification and also during H2O2 stage.
These free radicals attack the carbohydrates in the cellulose fibers yielding
weaker pulp grades. MgSO4 scavenges the harmful free-radicals effectively and
removes them out of the stock system, producing magnesium hydroxide, Mg(OH)2.
The net result is stronger pulp with pulp viscosities rising from the use of
magnesium sulfate. In an ECF bleaching protocol, the last stage is generally a
H2O2 stage to stop the color reversion. MgSO4 has also been successful in
removing some of the heavy metals from the stock system. These metals could
potentially break down H2O2.
Most mills using 1 or 2 stage O2-delignification,
particularly on the softwood line, will use some MgSO4. Unbleached pulp from the
final brown stock washer is fed to a press where the pulp is dewatered. At this
stage, caustic and MgSO4 are added. The pulp slurry is then fed to the O2
reactor where the steam and oxygen are added.
Industry standard is a 25% solution, because of solubility
limitations of magnesium sulfate used in Medium Density (MD) stock (10% to 14%
consistency). For O2-delignification one mill is an exception. It is using a 12
to 15% solution on High Density (HD) stock. The mill buys crystals of Epsom Salt
and makes their own solution.
Magnesium Sulfate is mostly used for O2-delignification in
pine or softwood. The reason for this difference is that hardwood has naturally
occurring magnesium compounds which are not present in softwood.
The dosage rate for MgSO4 generally varies
from 6lbs/Ton to 15 lbs/Ton as liquid on AD (Air Dried) pulp. The rate depends
on the pulping process ad the type of pulp finish being used. One mill is
currently getting a substantial pulp strength benefit with as low as 1/8 th of
1% of MgSO4 solution on AD pulp @ 2.5 lbs/Ton.