BRIGHTNESS
“Premix”: A novel process for improved
bleaching of mechanical pulps using a
mixture of reductive agents
By J. Wasshausen, G. Rangamannar, R. Amyotte and B. Cordy
Abstract: The Stora Enso, Biron mill in Wisconsin Rapids, is a 475 BD TPD pulp mill producing
medium and high brightness grades of paper from a mixture of Thermo Mechanical (TMP) and
Groundwood (GWD) pulps. The medium brightness grade used to be bleached by traditional hydrosulfite process while the high brightness grade was bleached by “PY” process. Last year the mill
changed its hydrosulfite reductive bleach process to a new process called “premix” process for bleaching mechanical pulps. This “premix” process employs a mixture of chemicals prepared from two stable solutions, sodium bisulfite and sodium borohydride, directly into the pulp stream.
HE PREMIX PROCESS has been successfully running in Biron mill for the
past year. This paper gives the details
of premix process, laboratory, and
mill bleach performance results. The
mill bleach data showed superior performance of
premix chemistry at lower application cost for
both high and medium brightness grades. This
process accomplishes this goal by taking advantage of the unique characteristics of borohydridebisulfite reductive chemistry.
The Biron Mill is located in Central Wisconsin
and produces lightweight coated papers. Two
fiber sources are used at the Biron Mill: mechanical pulp and kraft pulp. The mill was using
J. WASSHAUSEN
hydrosulfite for the reductive bleaching process.
Stora Enso, Biron Mill
In the year 2003, an alternative reductive bleachWisconsin Rapids, WI, USA
john.wasshausen@storaenso.com ing process, known as “premix”, using a mixture
of two reductive chemicals, was proposed for
improved bleach performance. Several laboratory and mill trials were performed using this alternative bleach chemistry. Based on the superior
results achieved during the mill scale trial, the
mill converted their reductive bleach process to
the premix process in early 2004. This paper discusses the details of the premix process performance in laboratory and mill environments.
T
G. RANGAMANNAR
Rohm and Haas Company
North Andover, MA, USA
R. AMYOTTE
Rohm and Haas Company
North Andover, MA, USA
B. CORDY
Rohm and Haas Company
North Andover, MA, USA
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PREMIX PROCESS
Technologies based on bisulfite-assisted borohydride reductions are commonly applied to bleach
wood pulps, recycled fibers, Kaolin, and textile
materials. Kindron et al. found that, by impregnating the fibers with bisulfite and then treating
with borohydride, high brightness pulps could be
produced [1]. Kruger et al. reported that impregnating wood chips with borohydride in the presence of bisulfite has produced high brightness
pulp with good color stability [2]. Devic et al.
showed that a mixture of bisulfite and borohydride pretreatment helps to improve the efficiency of peroxide bleaching of high-yield pulps [3].
Employing bisulfite in conjunction with borohydride has been shown to dramatically improve the
efficiency of sodium borohydride in water treatment applications [4]. Wang et al. achieved high
brightness on recycled fibers using a synergistic
two-stage process, first with bisulfite and then with
borohydride [5]. The premix process takes advantage of the unique reductive characteristics of
borohydride-bisulfite chemistry, as seen by the references cited above, and provides superior performance for bleaching mechanical pulps.
In the premix process sodium bisulfite
(NaHSO3), a mild reducing agent, is added to sodium borohydride (NaBH4), a strong reducing
agent, to generate a mixture of reductive chemicals
suitable for bleaching wood pulp. As a portion of
the sodium bisulfite and borohydride react to form
sodium hydrosulfite, the combination of these
chemicals creates a powerful and cost-effective
bleaching environment. Sodium bisulfite solution
is widely available as a 38% aqueous solution and
sodium borohydride is commercially available as a
12% sodium borohydride / 40% sodium hydroxide
aqueous solution (i.e. Borol® solution).
BGH VS. PREMIX
Another technology called Borol Generated
Hydrosulfite (BGH) has been in use for decades to
generate fresh hydrosulfite on mill sites to bleach
mechanical pulps [6]. It is a proven conventional
technology for bleaching mechanical pulps. The
BGH process requires the mixing of Borol solution, sodium bisulfite (SBS) and sulfuric acid
chemicals at a fixed molar ratio and pH to generate sodium hydrosulfite at maximum efficiency.
Even though BGH and premix processes are
similar, as they both require Borol solution and
SBS, in the premix process Borol solution and
sodium bisulfite molar ratios are flexible and may
vary to meet the bleach performance criteria.
The chemicals are mixed prior to the application
points to generate bleach liquor at a higher pH
than BGH process, and the generated liquor is
not stored for a long time. Thus the process generates a mixture of reductive chemicals which
under certain bleaching conditions provide better bleaching performance than applying hydrosulfite alone.
Both the premix and BGH processes offer significant performance advantages over powder or
liquid hydrosulfite chemicals. Both processes minimize the risk of chemical decomposition and dissolving losses, and thereby offer cost savings and
improved bleach performances to the industry.
T 70 Pulp & Paper Canada
BRIGHTNESS
GWANDT
MP
FIG. 1. Biron bleaching process.
The premix process is a mill-specific process, and the bleach response for virgin
mechanical pulps depends on the mill
bleach process conditions, wood species,
and process pH. The choice of BGH or premix technology for a mill application
should be based on which technology offers
improved performance and higher cost savings under the mill process conditions.
The premix process was selected for
the Stora Enso Biron application because
it offered several advantages over BGH
process for this mill. The Biron mill pulp
responded well to premix chemicals. The
capital and the maintenance cost
required for a premix process was less
than for the BGH process. Unlike the
BGH process, the bleach liquor is prepared and added immediately into the
pulp stream, thus eliminating the need
for any bleach liquor storage.
BIRON MILL BLEACH
PROCESS OVERVIEW
The Biron mill uses groundwood pulp
(GWD) in combination with thermomechanical pulp (TMP) for the production of lightweight coated paper. These
two pulps are pumped to a blend chest at
approximately 4% consistency. Medium
and high brightness grades of pulps are
produced. The brightness target for medium brightness grade is 64 % GE, and for
high brightness grade, it is 75-78 % GE.
A schematic of the bleaching process
of the Biron mill is shown in Fig. 1.
High Brightness Grade
The pulp from the blend chest is pressed
to approximately 30% consistency in a
press and subjected to a peroxide bleaching process for the production of high
brightness grade. Sodium silicate and
sodium hydroxide are added to the pulp
to assist in the peroxide bleaching process. The pulp, after mixing with chemicals, is processed through retention towers for brightness development. After
neutralization, the pulp is subjected to the
premix bleaching process. The bleaching
is performed at 65-75°C. Typical
unbleached brightness varies from 54 %
GE to 60 % GE. The bleached brightness
Pulp & Paper Canada T 71
FIG. 2. Premix vs. hydrosulfite laboratory bleach performance.
after peroxide bleaching varies between
72% GE and 74 % GE. The bleached pulp
after premix bleaching varies from 75 %
GE to 78 % GE depending upon the
grade produced. The pulp is then sent to
paper machines for making high brightness grade papers.
Medium Brightness Grade
The blend chest pulp is treated with the
premix chemical and sent to a storage
tower. The storage time varies from 60 to
90 minutes. The bleaching is performed
at 65-75°C. The bleached pulp brightness
varies from 63 % GE to 66 % GE.
PREMIX —
LABORATORY RESULTS
A sample of TMP/GWD pulp was collected from the blend chest to perform a laboratory bleaching study using the premix
technology. The objective of this study was
to compare the performance of premix
against hydrosulfite. The pulp was
bleached at 3.9 % consistency and 55°C
for 45 minutes. The borohydride solution
dosage was varied from 0 to 10 lbs/ton
and the hydrosulfite dosage was varied
from 0 to 20 lbs/ton. The results are
shown in Fig. 2.
The laboratory studies showed that
premix bleach performance is superior to
hydrosulfite. Target brightness gains are
achieved at significantly lower dosage for
borohydride solution than for hydrosulfite. Based on these results, it was determined that a mill trial would be necessary
to validate the laboratory performance.
PREMIX —
MILL PERFORMANCE
Mill trials were performed on the medium
brightness line using a temporary totebased chemical dosing system for the premix chemicals. The borohydride and
bisulfite mixture was introduced to the
pulp at the same location as hydrosulfite.
Bleach process conditions such as consistency, temperature and pH of the pulp
were kept the same for both premix and
hydrosulfite chemicals. The results are
shown in Fig. 3.
The mill trial premix results were even
better than the laboratory studies. The trial showed that the mill pulp could be
bleached to gain eight points of brightness at a significantly lower dosage of premix chemicals. The maximum brightness
gained in the laboratory was 6.5 points.
This higher gain in the mill may be
because of the higher process temperature (74°C) at the mill. The mill premix
results indicated that, in terms of chemical usage for gaining the target brightness, the ratio of hydrosulfite to borohydride solutions is the same as for
laboratory results. The results show every
four lbs of hydrosulfite is replaced with
one pound of borohydride solution.
A cost curve was generated based on
the mill bleach response results. This is
shown in Fig. 4. Maximum brightness
obtained at the highest hydrosulfite
dosage is taken as a reference point for
cost comparison. The premix bleach cost
index included both borohydride and
bisulfite solution cost. The mill performance data concluded that a cost savings
of 20-30% is achieved using the premix
process, compared to hydrosulfite, for
gaining 4-5 points of brightness.
Mill Premix Process Optimization and
Permanent Conversion
Retention Time Optimization
Pulp was collected from the pipeline
immediately after the addition of the premix chemical for the retention study. The
collected pulp was kept in the water bath
for the specified amount of time and the
brightness was measured.
Figure 5 shows the effect of retention
time on brightness gain. It can be seen
from the results that approximately 80%
of the maximum brightness is developed
within 5 minutes of the reaction time. The
optimum retention time is between 60 and
90 minutes. The mill is currently running
the process under optimum conditions.
Premix Bleaching of Medium Brightness Grade
Based on the positive results obtained
during the mill trial, in early 2004 the mill
switched the reductive bleaching process
from hydrosulfite to the premix process.
Figure 6 shows the performance of the
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45
BRIGHTNESS
FIG. 3. Premix vs. hydrosulfite mill performance.
FIG. 4. Premix vs. hydrosulfite cost savings.
FIG. 5. Retantion time optimization.
FIG. 6. Performance of medium brightness grade before and
after premix process conversion.
FIG. 7. Performance of high brightness grade before and
after premix conversion.
FIG. 8. Performance of high brightness grade reductive
stage before and after premix conversion.
medium brightness line before and after
conversion. The data, accumulated over a
period of eight months, show that the process change was accomplished successfully at the mill, and the conversion was very
smooth. With the premix process, the mill
is able to produce target brightness for
the medium brightness grade from even
lower unbleached brightness levels by providing higher brightness gains.
Performance of High Brightness Grade
A combination of oxidative and reductive
bleach processes produces the high
brightness grade pulp at the Biron mill.
Figure 7 shows the performance of the
high brightness line before and after the
premix conversion. The mill’s high
brightness target has been raised from 75
to 78 % GE this year. Premix results show
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that the combination of peroxide and premix processes enabled them to successfully meet the increased target, by gaining
more than eighteen points of brightness.
Figure 8 shows the brightness gains
achieved in the reductive stages of the
high brightness line. These results clearly
demonstrate the superior performance of
the premix process in achieving higher
brightness gains after peroxide bleaching.
SUMMARY
The premix process has been running at
the Biron mill for more than a year now.
Premix process performance can be summarized as follows:
It is a simple and unique reductive
bleach technology. This process outperforms the conventional hydrosulfite process and offers potential cost savings to the
mill. It uses two stable liquid products that
do not decompose during storage. It has
eliminated the need for nitrogen blanket
or refrigeration of chemicals. Because of
the availability of the bisulfite at higher
concentrations and the significantly lower
quantity of borohydride solution used
than the liquid hydrosulfite product, the
number of truck deliveries per year has
been reduced by at least 30%. The mill’s
premix performance results have demonstrated that the premix process is an efficient reductive bleaching process that
replaces the conventional hydrosulfite for
bleaching virgin mechanical pulps.
ACKNOWLEDGEMENTS
The authors thank the management of
Stora Enso, Biron Mill for granting permission to present this technical paper.
T 72 Pulp & Paper Canada
BRIGHTNESS
LITERATURE
1. KINDRON, R.R., PENNINGTON, N.J., CARTSUNIS,
P.L. Production of wood pulps including treatment of
cellulosic fibers with bisulfite ion followed by alkali metal borohydride. US Patent 3,284,283. Feb. (1964).
2. KRUGER, E., RUFFINI, G., GANDINI, G., GHISLANDI, F. Treatment of wood chips with an alkali metal borohydride solution followed by mechanical defibration. US Patent 3,981,765. Sept. (1976).
3. DEVIC, M. Method of peroxide bleaching of pulp pretreated with a sequestering agent then with a sulphite
and reducing agent. US Patent 5,143,581. Sept. (1992).
4. COOK, M.M. Sodium borohydride dye reduction in
wastewater. Environmental chemistry of dyes and pigments,
Reife, A., Freeman, H. (eds). New York, NY. John
Wiley and Sons, Inc. pp.33-41 (1998).
5. WANG, D., MEYERS, P. Sequential bleaching of deinked paper pulp with sodium bisulfite, dithionite and
borohydride. US patent 5,562,803A. Aug (1996).
6. MUNROE, D.C. High yield Sodium Hydrosulfite
generation. US patent 5,336,479, Aug (1994).
Résumé:
L’usine Biron de Stora Enso à
Wisconsin Rapids produit du papier léger
couché à partir d’un mélange de PTM et de
pâte mécanique. L’an dernier, l’usine est
passée du blanchiment à l’hydrosulfite à un
nouveau procédé appelé Premix pour le
blanchiment des pâtes mécaniques. Selon
ce procédé, on ajoute directement dans le
circuit de pâte un mélange de produits chimiques préparés à partir de deux solutions
stables, le bisulfite de sodium et le borohydrure de sodium. La présente communication donne les détails du procédé Premix,
ainsi que les résultats des essais en laboratoire et de la performance du blanchiment
en usine.
Reference: WASSHAUSEN, J, RANGAMANNAR, G., AMYOTTE, R., CORDY, B. “Premix”: A novel process for improved bleaching
of mechanical pulps using a mixture of reductive agents. Pulp & Paper Canada 106(3): T7073 (March, 2006). Paper presented at the 91st
Annual Meeting in Montreal, QC, Canada,
February 7-10, 2005. Not to be reproduced
without permission of PAPTAC. Manuscript
received November 24, 2004. Revised
manuscript approved for publication by the
Review Panel on September 22, 2005.
Keywords:
MECHANICAL
PULPS,
BRIGHTNESS, REDUCTION, BLEACHING,
DITHIONITES, PERFORMANCE EVALUATION, SODIUM BISULFITE, SODIUM BOROHYDRIDE.
Pulp & Paper Canada T 73
107:3 (2006) ❘❘❘
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