October – December 2009

345

P

e e r  

R

e v i e w e d

tion during the papermaking process. A
reduction can be achieved by the recircu-
lation of various water streams in a mill,
but this “white water closure” can make
the dissolved and colloidal substances
(DCS) accumulate in the water systems
This can interfere with papermaking by
reducing paper quality, such as decreasing
wet strength, sheet whiteness and paper
strength, by degrading paper machine
runnability, such as increasing drainage
time and deposit formation, and by con-
suming more chemicals (1-5). DCS origi-
nating mainly from mechanical pulps and
secondary fibres, can include a wide vari-
ety of chemical species, such as hemicel-
luloses, lipophilic extractives, lignin-like
compounds (from mechanical pulps) (6-
8), as well as pressure-sensitive adhe-
sives, coating binders, ink, wood resin,
rosin size, wet strength resins (from sec-
ondary fibres) (9).

The methods used for removal of DCS

involve membrane filtration (10-11), bio-
treatment (12-14), evaporation (15) and
freezing crystallization (16-17), which all
are performed in the equipments outside
the papermaking process. When the mem-
brane filtration techniques, including ultra-
filtration (UF), nano-filtration (NF) and
reverse osmosis (RO), are used for the
treatment of papermaking whitewater,
DCS in whitewater are concentrated into a
small volumetric stream. The process is
effective in removing DCS except for the
cost and the difficulty of occasional mem-
brane blocking. If membrane filtration and
bio-treatment are combined to form a
membrane-biological reactor, all of the
fatty acid and resin acid can be removed
(13). There is no doubt that the evaporation
and freezing technologies are simple and
effective in separating DCS from whitewa-
ter, but they require high energy-consump-
tion, which restricts their applications. In
addition these processes need large invest-
ment and result in secondary solid-wastes.

An alternative route for removing DCS

from papermill waters is to ensure that the

DCS substances are retained in the paper
sheet itself, rather than passing through
with the drainage waters of the papermak-
ing process. The option would involve, in
addition to the traditional retention aids
used in papermaking, some auxiliary
chemical that can interact with DCS sub-
stances to form complexes that flocculate
or adsorb on fibres, thus staying in the
paper sheet. This route appears to be both
cost saving and protective of the environ-
ment , as it can take full advantage of
materials, reduce the treatment load of
wastewater, and avoid the cost of external
equipment and their consumption. Some
researches (18-22) have shown that DCS
in whitewater could be flocculated by a
PEO (polyethylene oxide)/cofactor dual-
component system retention aid. The best
cofactor was the polyphenol-based poly-
mer (20). Xiao and co-workers (23)
reported on the removal of DCS in pulp
and paper wastewater by a dual-compo-
nent system consisting of a cationic-mod-
ified microporous zeolite HY and an
anionic polyacrylamide-based polymer. 

Previously, we studied the flocculation

of DCS in TMP (thermo-mechanical pulp)
and DIP wastewaters with nanosized TiO

2

,

and also carried out a preliminary study on
the application of nanosized TiO

2

in the

retention system of newsprint (24-26). In
the present work described here, our
researches focused mainly on the removal
and control of DCS (including the interac-
tion mechanism between nanosized TiO

2

and DCS ) using  dual-component system
retention aids containing nanosized TiO

2

in a laboratory based recycling handsheet-
making operation with DIP.

MATERIALS AND EXPERI-
MENTAL METHODS

Materials and chemicals

Nanosized TiO

2

colloidal particles were

synthesized by the hydrolysis of titanyl
organic compounds [TiO(OOCCH

3

)

2

and

Ti

2

O(OC

4

H

9

)

2

(OOCCH

3

)

4

] at low tem-

Study on removal of dissolved and colloidal
substances in deinked pulp by dual-component
system retention aids containing nanosized
TiO

2

colloid

XIAO-QUAN CHEN

1

, WEN-HAO SHEN

2

, HUAN-BIN LIU

3

AND XUN-LI KOU

4

1

Associate Professor and corresponding author

(xqchencn@scut.edu.cn), 

2

Associate Professor,

3

Professor, 

4

Previously Ph.D. student

State Key Laboratory of Pulp and Paper
Engineering 
South-China University of Technology
Guangzhou, P. R. China

SUMMARY 

The use of a dual-component system
retention aid containing nanosized TiO

2

for

removal of dissolved and colloidal sub-
stances (DCS) when using deinked pulp
(DIP) was investigated by simulated recy-
cling whitewater in the laboratory. When
either Nano-TiO

2

/AmS or Nano-

TiO

2

/APAM was used in this repeated

handsheet-making process, the COD value
of the whitewater increased and then
approached an equilibrium level with
increased recycling. Whereas Nano-
TiO

2

/AmS had a small effect on dewatering

times and Nano-TiO

2

/APAM had a signifi-

cant negative effect, the nanosized TiO

2

alone can improve the drainage.
Measurements of the effective residual ink
and whiteness indicated that the nanosized
TiO

2

possessed the ability to retain ink par-

ticles, permitting the whiteness to approach
a steady value after repeated recycle hand-
sheet-making operation. By contrast, the
common dual-component retention aid
CPAM/Nano-SiO

2

decreased the white-

ness. . Nanosized TiO

2

appeared to induce

agglomeration and flocculation with DCS.

KEYWORDS
Dissolved and colloidal substances,
nanosized TiO

2

, deinked pulp, hand-

sheet-making, dual-component sys-
tem, retention aids

INTRODUCTION

For a variety of reasons, papermakers are
working towards reduced water consump-