Paper Technology Journal - Voith

we had to reduce the originally specified
dirt speck area from 10 to under
6 mm2 /m2 for uncoated grades and to
under 4 mm2 /m2 for coated grades.
These revised levels of cleanness allow
up to 35% of the previously used hardwood
chemical pulp to be substituted by
deinked pulp. A further increase in the
DIP portion is foreseen for the near
future. It has been noted that coated
papers calendered in a supercalender
react very sensitively to residual dirt
specks. Under compression in the supercalender,
even the smallest of dirt specks
show up again.
The new recycling grades produced from
100% deinked pulp have excellent cleanness
and their visual qualities are very
convincing. Customer acceptance and
sales of this product are consistently increasing.
Brightness
Today, the original target for brightness,
85% ISO without UV luminance, is, in
fact, being exceeded. As shown in Fig. 3,
in November 1997 brightness was
86.5%.
Based on brightness with UV luminance,
values of approx. 100% ISO are
achieved. This high value is due to optical
brighteners in the recovered paper.
Since copy papers generally have a high
content of optical brighteners, their use
can be reduced considerably when using
ISO brightness without UV luminance
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12 ’95
Stock Preparation
deinked pulp. By using alternative bleaching
agents, such as ozone, for instance,
a greater fibre brightness is possible, but
there is the danger of destroying the effect
of the optical brighteners.
Stickies
1 ’96
2 ’96
3 ’96
4 ’96
5 ’96
6 ’96
7 ’96
8 ’96
9 ’96
10 ’96
11 ’96
The deinked pulp produced in the Kemsley
plant has never yet been the subject
of any complaints about stickies. The
wide range of process principles is the
main reason for an effective stickies
reduction. Particularly important here is
the 4-stage fine slot screening with
0.1 mm slot width and the two dispersion
stages, with their high specific
dispersion energy.
Fig. 3: Development in deinked pulp brightness
Still acceptable
Target
12 ’96
1 ’97
2 ’97
3 ’97
4 ’97
5 ’97
6 ’97
7 ’97
Colloidal-chemical cleanness
8 ’97
9 ’97
10 ’97
11 ’97
13
Colloidal-chemical disturbances when
using a high percentage of deinked pulp
are connected with
■ Conductivity
■ Cationic demand and
■ pH value.
To solve the colloidal-chemical problems,
individual solutions in the retention
systems of the paper machines and
in the deinking processes need to be
found. These solutions can differ greatly.
What is important is whether the paper
mill is producing coated or uncoated
papers. Experience has shown that it is
the broke systems that set off the colloidal-chemical
disturbances.
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