P
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346 Appita Journal
Vol 62 No 5
perature and normal pressure according to
the literature (27). They are spherical par-
ticles with mean size 6.5 nm. The col-
loidal suspension has isoelectric point of
6.67 (pH) and zeta potential of 14 mV ~
30 mV at pH 4.5 ~ 6.0 in the presence of
5.0 m mol
⋅L
-1
NaNO
3
electrolyte.
The DIP investigated was supplied by
Yuexiu Guangzhou Paper Group (China)
at consistency 19.37 %.
Anionic polyacrylamide (APAM) with
MW=6,000,000 and cationic polyacry-
lamide (CPAM), with MW=8,000,000 were
supplied by Zibo Greatwin Industry Co.,
Ltd (China). Amphoteric starch (AmS,
Degree of substitution of cationic groups
0.02) was supplied by General Starch Co.,
Ltd (Thailand). Nanosized SiO
2
was offered
by Yika Chemicals Co., Ltd (China).
Removal of DCS
Removal of DCS in DIP by the dual-com-
ponent system retention aids containing
nanosized TiO
2
were investigated according
to the following experimental procedures.
1) Preparation of DIP slurry: 2 % con-
centration of slurry was obtained by
diluting 290.0 g of DIP and then
stirred for 10 min at 3000 rpm in the
disintegrator. Sampling from the pulp
suspension (in 300 mL lots) was per-
formed under agitation of 400 rpm.
2) Addition of retention aids: 300 mL of
slurry was poured into 500 mL beaker.
The retention aid components were
sequentially added into the vessel under
agitation of 1000 rpm. Table 1 presents
the addition sequence and dosage of
retention aids for each test regime.
3) Recycle handsheet-making operation:
The sheet former was set to the recycle
whitewater mode and the water level
adjusted. 300 mL of slurry was used for
each trial. Following each sheet mak-
ing cycle, a 30 mL sample of whitewa-
ter was taken for turbidity and COD
analysis (potassium dichromate
method), and the rest of the whitewater
was pumped to recycle. The dewater-
ing time of each handsheet-making
operation was recorded following the
standard (Tappi T 221om-93). The
formed handsheets were pressed and
dried before measuring the residual ink
and whiteness. During the handsheet-
making operations, the forming screen
was washed ultrasonically for 3 min
after every 5th cycle.
4) Determinations of turbidity and COD:
The whitewater samples were used
directly for the determination of tur-
bidity, and then centrifuged for 15 min
at 3000 rpm. The supernatant was
used for determination of COD, so as
to characterize the concentration of
DCS remaining in the whitewater.
Interaction of nanosized TiO
2
and
DCS
1) Preparation of extracted DCS solu-
tion: A sample of extracted DCS solu-
tion was prepared from the DIP fol-
lowing a procedure previously
described in the literature (28) as fol-
lows - DIP slurry was squeezed and
then diluted with the pressate, repeat-
ing these operations eight times. The
eighth pressate was centrifuged for 20
min at 5000 rpm, and the supernatant
was used as the sample of extracted
DCS solution.
2) Titration of nanosized TiO
2
colloid to
DCS solution: The titration of nano-
sized TiO
2
colloidal solution (0.1 %) to
DCS solution (10 mL) was carried out
with a multi-purpose autotitrator (MPT-
2, Malvern, UK), concurrently measur-
ing the zeta potential and particle size.
Determinations of effective
residual ink and whiteness of
handsheets
After the handsheets were equilibrated in
a constant temperature and humidity
chamber for 24 h, their effective residual
ink (TAPPI T567 Pm-1997) and white-
ness (TAPPI T452 om 1987) were deter-
mined separately.
RESULTS AND DISCUSSION
Removal of DCS in DIP slurry by
dual-component system retention
aids containing nanosized TiO
2
Since more than 90 % of DCS in solution
can be oxidized by potassium dichromate,
the COD values of DCS solution (using
the potassium dichromate method) were
used as the measure of the concentration
of DCS. The change in the COD value of
the whitewater during progressive recycle
handsheet-making operations with vari-
ous retention aid systems were thus inves-
tigated. The results are shown in Figure
1a and Figure 1b. When no retention aid
was used, the COD of whitewater rose
with the increasing recycle count, indicat-
ing the accumulation of DCS. As shown
in Figure 1a, even if the retention aids
such as APAM, AmS or CPAM/Nano-
SiO
2
were used, COD values had similar
rising trends. However with the nanosized
TiO
2
colloid or its dual component sys-
tems were used as retention aids, as shown
in Figure 1b, COD values rose during the
initial stages of repeated recycling then
appeared to reach a relatively constant
value, which was around 67 mg/L after the
30th recycle for Nano-TiO
2
and Nano-
TiO
2
/CPAM, and around 42 mg/L after the
21st recycle for Nano-TiO
2
/APAM and
Nano-TiO
2
/AmS. These results imply that
the nanosized TiO
2
colloid has inhibited
the accumulation of DCS in whitewater,
apparently aiding in retaining the DCS
substances in the handsheets.
Changes of turbidity of whitewater
during recycle handsheet-making
Turbidity changes are shown in Figures
2a and 2b. As shown in Figure 2a, only
AmS seemed to show any ability to limit
turbidity increasing with increased recy-
cling. With no additives, or with APAM,
CPAM/Nano-SiO
2
, the turbidity values of
whitewater ascended with the increase of
recycle times, apparently associated with
build-up of DCS. The turbidity result with
AmS is at odds with the COD trend
shown in Figure 1a. Figure 2b showed the
changes of turbidity values of whitewater
with recycle times when Nano-TiO
2
and
its dual-component systems were used as
retention aids. The dual-component sys-
tem of Nano-TiO
2
with either APAM or
AmS were able to prevent the continued
ascent of turbidity, but Nano-TiO
2
alone
Table 1
Addition sequence and dosage of retention aids
Test Number
30 s (Stirring Time)
45 s (Stirring Time)
60 s (Stirring Time)
1
Handsheets made
2
APAM (0.03%)
Handsheets made
3
AmS (0.5%)
Handsheets made
4
Nano-TiO
2
(0.05%)
Handsheets made
5
CPAM (0.03%)
Nano-SiO
2
(0.05%)
Handsheets made
6
Nano-TiO
2
(0.05%)
CPAM (0.03%)
Handsheets made
7
Nano-TiO
2
(0.05%)
AmS (0.5%)
Handsheets made
8
Nano-TiO
2
(0.05%)
APAM (0.03%)
Handsheets made