Caro's Acid Activation of Sulfite Pulp

paper presented at TAPPI 2001 Pulping Conference Seattle
Introduction:
Caro’s Acid Activation of Sulfite Pulp
Rajan P. Sundara and John D. Kronis
Degussa Corporation
Allendale, NJ
In order to be in compliance with the EPA’s Cluster Rule by April 15, 2001, all the bleached
paper grade magnesium-based sulfite (magnefite) mills are required to adopt a totally chlorine
free (TCF) bleaching sequence. At present most of the magnefite mills have either converted
into TCF bleaching or may be in the process of being converted to TCF bleaching. Pulps from
the magnefite process can be easily bleached to a brightness as high as 90+% ISO in three
stages. However, some mills that do not require such high brightness could bleach their pulp
using two stage bleaching to about 83 – 86% ISO. Whether it is two stage or three stage
bleaching, the most common sequences include an oxygen-containing delignification stage (Op
or Eop) followed by medium or high consistency peroxide bleaching with or without an
interstage such as a chelation stage. Examples of such sequences include, OpP, EopP,
EopQP or OpQP (the peroxide stage could be a medium or high consistency stage).
In the oxygen-containing pre-delignification stage (Op or Eop) either sodium hydroxide or
magnesium oxide can be used as an alkali source. There are several publications (1-3) on use
of MgO in the pre-bleaching stage (Op or Eop) and its benefits over caustic such as less COD
and possible recycle of filtrate to the recovery process and partial closing of the mill effluent.
However, MgO in the pre-bleaching stage results in significantly lower delignification and lower
brightness pulp. In order to improve the delignification and final brightness from using MgO, use
of peroxy acids such as peracetic acid (Paa) or peroxymonosulfuric acid (Caro’s acid – Ca) has
been recommended following the pre-delignification stage.
Nils, et al (4), has reported on treating the pulp with peracetic acid in place of a chelation stage
in OpQP bleaching while using MgO as the source of alkali in the Op and P stages. Though
use of peracetic acid in place of Q-stage, and MgO in Op and P stages could improve the
delignification and brightness, the final brightness and viscosity were lower than obtained from
using caustic in the Op and P stages.
Devenyns, et al (5), reported their work on Caro’s treatment for the production of high
brightness sulfite pulp and mill closure using the sequences, OCaPP and EoCaP with MgO as
alkali source in the pre-bleaching stage. The Caro’s acid treatment was conducted to
compensate the loss in delignification and brightness from using MgO in the pre-bleaching
stage. However, no economic justification was made for Caro’s acid activation.
This study investigates the use of Caro’s acid treatment as an activating stage placed in
between the Eop and P stages using caustic as alkali source to produce higher brightness and
reduce bleaching cost.

Experimental:
Unbleached hardwood (aspen) sulfite pulp used in this study was obtained from a nearby
magnefite mill. The Kappa number of the pulp as received was high, due to a large amount of
carryover chemical. To be representative of mill bleaching, the as-received pulp having a
Kappa number of 44 was used in the Eop stage. Following the Eop stage, a peroxide bleaching
stage was conducted both at medium consistency, represented as [P(MC)], and at high
consistency, [P(HC)], at the conditions shown in the following table. In some experiments a
chelation stage (Q) using EDTA or an activation step using Caro’s acid or Peracetic acid was
conducted prior to the peroxide bleaching stage.
The conditions used in each stage are furnished in the following table.
Conditions Eop P(MC) P(HC) Q Paa Ca
Stage Stage stage
Consistency, % 12 12 30 4 12 12
Temperature, o C 80 85 85 65 70 70
O 2 -Pressure, MPa. 40
Time, min 15+60 180 180 45 30 30
pH 10.3 – 10.7 9 – 10 8 – 9 6.0 –6.5 4 – 4.5 5 – 7.0
NaOH, % 5 – 5.3 0.9 – 1.7 0.3 – 0.75 0.9 – 2.8
Peroxide, % 0.75 0.5 – 2.5 1 – 2.5
EDTA, % 0.2
Caro’s acid, % AO 0.1- 0.3
Paa, % AO 0.1-0.3
Peracetic acid used in this study is an equilibrium 35% concentration solution available from
laboratory chemical supply companies.
Caro’s acid (peroxymonosulfuric acid) was prepared in our laboratory using 70% peroxide and
concentrated sulfuric acid with a conversion rate of 75 – 79%.
Pulp washing following each stage of bleaching was conducted by diluting the pulp to 2%
consistency and filtering through a Buchner funnel. The brightness (% ISO) of the pulp from
each stage was measured by preparing two handsheets with 4.0 g OD pulp each and using a
Technidyne brightness meter. The residual peroxide and COD of filtrates and viscosity of pulps
from all the bleaching stages were measured using the corresponding Tappi Test Methods.
Results and Discussion:
The average brightness and Kappa number of Eop-bleached pulp were 66.4% ISO and 14.7,
respectively.
Effect of MC vs. HC Peroxide Bleaching and QP(MC) Bleaching:
Figure 1 shows the brightness obtained from peroxide bleaching of Eop pulp at medium and
high consistency conditions. Also, shown is the brightness obtained from bleaching the
chelated pulp with peroxide at medium consistency.
2

At a given peroxide charge, the high consistency peroxide bleaching yielded an average of 2
points increase in brightness compared to medium consistency bleaching. Incorporating a
chelation step prior to medium consistency peroxide bleaching, on the other hand, increased the
brightness of the final pulp by about 2.5 – 3.0 points. Thus, a maximum brightness of 83% ISO
was achieved from P(MC) with 2.5% peroxide compared to P(HC) which yielded a maximum
brightness of 85.3% ISO with 2.5% peroxide. Incorporating a chelation increased the final
brightness to about 86.3% ISO at 2.5% peroxide charge.
Effect of Chelation vs. Caro’s acid (Ca) on Peroxide Bleaching:
Figure 2 shows the results obtained from bleaching the Eop pulp using P(MC), QP(MC) and
Ca(0.1%)P(MC) sequences.
92
Brightness (% ISO)
90
88
QP(MC)
86
P(HC)
84
P(MC)
82
80
78
0.5 1 1.5 2 2.5 3
Peroxide Charge (%)
Figure 1.
Brightness from Medium and High Consistency (MC and HC) Peroxide
Bleaching of Eop Pulp and Medium Consistency Peroxide Bleaching
of Eop-Chelated Pulp
Brightness (% ISO)
92
90
88
86
84
82
80
Ca(0.1%)P(MC)
QP(MC)
P(MC)
78
0.5 1 1.5 2 2.5 3
Peroxide Charge (%)
Figure 2.
Brightness from Medium (MC) Peroxide Bleaching of Caro’s acid
Activated, Chelated and Unchelated Pulps.
3

In the Caro’s acid activation step the Eop pulp was treated with 0.1, 0.2 and 0.3% Caro’s acid
(Ca) expressed as active oxygen (AO). The three Ca-treated pulps were then bleached using
0.5 – 2.0% peroxide in the subsequent stage at medium consistency. It is clear that including a
chelation step provided only an average brightness gain of about 3 points over no chelation. On
the other hand, treating the pulp with 0.1% Caro’s acid (as AO) boosted the brightness gain to
about 6 - 7 points in the following peroxide bleaching [P(MC) stage] compared to untreated pulp.
Alternatively, to reach a brightness of 84% ISO only 0.5% peroxide is required in the P stage
with 0.1% Caro’s acid activation compared to a peroxide charge of 1.5% with chelation
treatment. Thus, a reduction of about 1.0% peroxide was possible with 0.1% Caro’s activation
prior to the P stage. Besides, it is clear that activation of Eop pulp using Caro’s acid could
increase the brightness ceiling to as high as 89% ISO when additional peroxide is applied in the
following P stage.
Brightness (% ISO)
92
Ca(0.2%)P(MC)
90
88
QP(MC)
86
84
P(MC)
82
80
Brightness (% ISO)
92
Ca(0.3%)P(MC)
90
88
QP(MC)
86
84
P(MC)
82
80
78
0.5 1 1.5 2 2.5 3
78
0.5 1 1.5 2 2.5 3
Peroxide Charge (%)
Peroxide Charge (%)
Figure 3. Figure 4.
Figures 3 and 4. Effect of 0.2 and 0.3% Caro’s Acid (AO) Activation compared to Chelation and
no chelation on the following P-stage Brightness.
When the Caro’s acid charged in the activation step was increased to 0.2% (AO) an additional
brightness gain of about 2 points on average was obtained over 0.1% Caro’s acid treatment
(Figure 3). Further increasing the Caro’s acid charge to 0.3%, however, did not provide
significant further improvement in brightness gain (Figure 4).
Figure 5 shows the results obtained from using peracetic acid (Paa) as an activating agent in
place of Caro’s acid. Whether it is the activation by Caro’s acid or Paa, both yielded similar
brightness gains in the following P-stage bleaching when equal amounts of AO are applied.
This clearly indicates that both Caro’s acid and Paa at equal AO levels applied produced similar
final brightness. Also, it is clear that the brightness gained from activation step using either
peroxygen compound is far higher than those obtained from a chelation or no chelation step.
Effect of Activation on Filtrate COD
The total COD of the filtrates collected from the activation stage (Caro’s or Peracetic acid using
0.1% and 0.3% AO charge) and the following peroxide stage are shown in Figure 6. It is clear
that Caro’s activation of Eop pulp resulted in much lower COD both at 0.1 and 0.3% AO levels
compared to peracetic acid-treated pulps. Only 10 – 12% increase in COD is seen when the
amount of Caro’s acid applied was increased from 0.1% to 0.3% as AO. However, an increase
4

about 45 – 58% in COD is seen when peracetic acid applied was increased from 0.1 to 0.3%
AO.
Brightness (% ISO)
92
90
88
86
84
82
Ca or Paa (0.3%)P(MC)
Ca or Paa (0.1%)P(MC)
Ca or Paa (0.2%)P(MC)
80
0.5 1 1.5 2 2.5 3
Peroxide Charge (%)
Figure 5.
Effect of Caro’s acid vs. Peracetic acid Activation on
Brightness in the following P-stage Bleaching
100
80
Paa(0.1%)P
Paa(0.3%)P
82
87
94 94
100
80
Ca(0.1%)P
Ca(0.3%)P
COD (Kg/t)
60
40
51
57
62
65
COD (Kg/t)
60
40
39
35
54
49
46 47
52
58
20
20
0
0.5 1 1.5 2
0
0.5 1 1.5 2
Peroxide (%)
Peroxide (%)
Figure 6.
Total COD of Filtrates from Activation Stage (Paa or Caro’s acid)
and following P stage.
5

Effect of Activation Stage on Pulp Viscosity:
Figures 7 and 8 show the viscosity of the peroxide bleached pulps following the Caro’s acid and
Peracetic acid activation at 0.1% and 0.3% AO, respectively. Caro’s acid activation results in
much higher viscosity retention compared to peracetic acid activation at both 0.1 and 0.3%
levels of AO charges.
50
0.5% P 1.0% P 1.5% P 2.0% P
Viscosity (mPa.s)
40
30
20
10
33
28
25
22
41
38
28
29
0
Paa(0.1%)P
Bleach sequence
Ca(0.1%)P
Figure 7.
Viscosity of Peroxide Bleached Pulps from Caro’s Acid and
Peracetic acid Activation at 0.1% AO
50
0.5% P 1.0% P 1.5% P 2.0% P
Viscosity (mPa.s)
40
30
20
10
27
24
19
18
37
31
26
19
0
Paa(0.3%)P
Bleach sequence
Ca(0.3%)P
Figure 8.
Viscosity of Peroxide Bleached Pulps from Caro’s Acid and
Peracetic acid Activation at 0.3% AO.
6

Comparison of Bleaching Cost: EopQP(MC) vs. EopCaP(MC)
In this study a two-stage, EopP(MC) did not even reach a brightness of 84% ISO. To reach a
brightness of 86% ISO using EopQP(MC) requires about 2% peroxide in the peroxide stage,
besides 0.2% EDTA in the chelation stage. On the other hand, to achieve the same brightness
it requires 0.1% Caro’s acid (as AO) with only 1% peroxide in the peroxide stage. Thus, a
saving of about 1% peroxide and 0.2% chelant can be achieved by replacing the chelant with
0.1% Caro’s and some amount of caustic to maintain the pH.
The savings in bleaching cost obtainable from using 0.1% Caro’s acid with 1% peroxide plus
some additional caustic clearly out weighs the cost of using 0.2% chelant and 2% peroxide to
achieve 86% ISO. Besides, use of Caro’s acid could provide flexibility to the process to reach
even higher brightnesses when needed with some additional peroxide.
Comparison of Bleaching Cost: CaP vs. PaaP
Figure 9 shows the cost ratio for bleaching using Caro’s acid vs. peracetic acid as activating
agent for equal level of brightness. The bleaching cost for only the last two stages of CaP and
PaaP are computed. The actual costs are then normalized using the cost of CaP bleaching = 1
to get a cost ratio. The cost ratio for Caro’s acid vs. peracetic acid at 0.1% AO level indicates
that the cost of peracetic activation is significantly higher than Caro’s acid activation at every
level of peroxide applied in the following P-stage. The increase in bleaching cost for using
peracetic acid as an activating agent is about 28% - 44% more than Caro’s acid activation,
corresponding to a cost ratio of 1.28 – 1.44. However, the brightness achieved from Caro’s acid
and peracetic acid at 0.1% AO are the same at each peroxide level (See Figure 5).
2
Ca(0.1)P
Paa(0.1)P
Cost Ratio
1.5
1
0.5
0
1.0% P 1.5% P 2.0% P
Figure 9. The Cost Ratio for Bleaching using CaP vs. PaaP sequence at 0.1% AO.
7

2
Ca(0.3)P
Paa(0.3)P
Cost Ratio
1.5
1
0.5
0
1.0% P 1.5% P 2.0% P
Figure 10. The Cost Ratio for Bleaching using CaP vs. PaaP sequence at 0.3% AO.
When the applied AO is increased to 0.3%, the cost ratio of peracetic acid to Caro’s acid still
follows the same trend but with an overall higher cost to achieve higher brightness (Figure 10).
The cost ratio ranged between 1.51 – 1.67, indicating that it cost 51% to 67% more to bleach
using peracetic acid than Caro’ s acid.
Summary:
1. Two-stage EopP bleaching using a medium consistency peroxide stage resulted in a
maximum brightness of 83% ISO, while EopP bleaching a using high consistency peroxide
stage resulted in about 2 points higher brightness to yield 85% ISO.
2. Incorporating a chelating stage (0.2% EDTA) prior to medium consistency peroxide
bleaching boosted the brightness achieved to 87% ISO, about 4 points higher than EopP
bleaching.
3. Substituting the chelation step with Caro’s or Peracetic acid activation, however, boosted
the final brightness to a maximum of 90+% ISO.
4. To achieve a brightness of 86% either EopQP bleaching with 0.2% chelant and 2%
peroxide, or EopCaP bleaching with 0.1% Caro’s acid and 1% peroxide can be used.
However, savings in bleaching cost from using 0.1% Caro’s acid with 0.1% peroxide clearly
out weighs the QP bleaching.
5. Further, Caro’s acid activation provides flexibility to the process to reach even higher
brightnesses when needed with some additional peroxide.
6. Both, Caro’s acid and Peracetic acid at all three levels of activation, 0.1, 0.2 and 0.3% AO,
yielded the same level of brightness increase.
8

7. However, the Caro’s acid activation resulted in a much lower filtrate COD compared to
peracetic acid activation for the combined filtrates from the activation and the following P
stages.
8. Also, the Caro’s acid activation resulted in higher final pulp viscosity compared to peracetic
acid activation.
9. The cost of bleaching the sulfite pulp with Caro’s acid is significantly lower than with
peracetic acid
Thus, Caro’s acid is a better activation agent for sulfite pulp bleaching to achieve higher pulp
brightness and viscosity and lower filtrate COD compared to peracetic acid or chelation
treatment. Above all the cost of bleaching using Caro’s acid is far less than peracetic acid.
References:
1. Kronis, J.D; Nimmerfroh, N., Süss, H.U., Böttcher, H., Lüttgen, W., Geisenheiner, A., 1994
International Pulp Bleaching Conference, P 247.
2. Reisner, T.B., and Schloffer, K., 1996 International Non-Chlorine Bleaching Conference,
Orlando, Paper 9-2.
3. Bokström, M., and Tigerström, A., Journal of Pulp & Paper Canada, Vol 92, No. 11: T279
(1991).
4. Nils, G., Johansson, and Fletcher, D.E., 1996 International Pulping Conference, p 481.
5. Devenyns, J., Troughton, N., and Desprez, F., 1994 Pulping Conference, p 481.
9