BLEACHING Eucalyptus grandis KRAFT PULPS USING A SHORT TCF SEQUENCE
María C. Area and Fernando E. Felissia
The objective of this paper is to evaluate the possibility of eliminating conventional acidic Q stage with EDTA, in a Totally Chlorine Free (TCF) bleaching sequence. This article studies
two short TCF bleaching sequences, applied to industrial Eucalyptus grandis kraft pulps. Results show that it is possible to eliminate the acidic Q stage, replacing it by incorporating
DTPMPA in brown stock washing, Op and P stages. The optical and mechanical properties of bleached industrial pulps show no significant differences when using both sequences, as final
pulp characteristics are independent of the properties of previous stages. DTPMPA addition eliminated the deleterious influence of metals on TCF bleaching, and in some cases, increased mechanical properties.
Keywords: Kraft pulps - TCF bleaching – chelating agents – phosphonates – Eucalyptus grandis
Traditional industry seems to set aside Totally Chlorine Free (TCF) bleaching in favour of ECF pulp production, as such pulp manufacture usually results in higher production costs and the
same or inferior pulp quality. ECF effluents cannot currently be easily recycled to chemical recovery due to the build-up of chloride ions (Cl-), potassium (K) in some cases, and
scaling/deposition of organic and inorganic compounds. Since TCF bleach effluents contain virtually no chloride, the problems associated with chloride in ECF bleaching plant effluent
recycling almost disappear. TCF bleaching technology installations will increase as kraft mills move, slowly but surely, toward closed water circuits with bleaching effluents reduced to 5-8 m3 per tonne of pulp1.
Regulations are also changing the emphasis on AOX, and are accentuating the interest on mill water closed-cycle, through introducing limits in COD (chemical oxygen demand) discharge values2.
This article presents the conclusions of the application of two short sequences of a totally chlorine free bleaching process, to industrial Eucalyptus grandis kraft pulps.
The objective of this work is to verify that the elimination of the conventional EDTA acidic Q stage, in a totally chlorine-free bleaching of industrial pulps, is possible.
EDTA (ethylene diamine tetraacetic acid), the most used chelating agent in the pulp and paper industry, requires acidic conditions. Phosphonates, on the contrary, work in strong
alkaline medium, and recently proved to be very effective in metal handling3 - 6.
In this work, DTPMPA (diethylene triamine penta(methylene phosphonic acid)) is applied in brown stock washing, Op and P stages (all in alkaline media). Its performance is compared
with the classic chelant EDTA, used in acidic media at the acidic Q stage.
An acidic Q stage involves the addition of sulphuric acid, requiring supplementary equipment and special materials and a higher manipulation costs. The elimination of the chelating
specific stage presupposes appreciable savings, lowering operational and investment costs. The shortened sequence, with all alkaline stages, also simplifies the handling of filtrates,
reducing water consumption. Direct chelant addition in the bleaching sequence allows a better reagent performance and is more economical, without extra capital investment.
The Capitán Bermúdez mill of Celulosa Argentina S.A. provided industrial unbleached pulp, and Solutia Inc. supplied the phosphonates (DTPMPA as Dequest 2066).
Figure 1 shows the experimental plan.
Figure 1: Alternatives studied for TCF bleaching of Eucalyptus kraft pulp
After the analysis of results of a previous work 4, an acidic Q stage with EDTA was chosen as a control sequence, adding DTPMPA in P stage. The option, without DTPMPA, produced
high consumption of peroxide, with the pulps obtained presenting lower brightness and strength properties.
Table 1 presents total chelant charge in the studied sequences.
Table 1: Total chelant charges applied in 1-A and 1-B sequences (numbers in Figure 1)
To perform I-1 and I-2 stages, industrial brown pulp was washed as received for 20 minutes, at 3.0% consistency and at 58ºC. After each stage, all pulps were centrifuged to 30% consistency.
A stainless steel 4.6 L reactor was adapted for oxygen delignification, incorporating special agitation and heating systems. Liquor heating system involved: glycerine recirculation inside
a double shaft, heated by 4 electric resistances (400 W each) and activated by a digital temperature controller. The high shear agitation system (multiple blade propeller, specially
designed to fulfil the blending requirements of the O stage) rotates at a constant speed of 500 rpm. Oxygen is injected into the bottom of the reactor.
For the experiences, 300 g od of the washed and centrifuged pulps and the NaOH solution was preheated in a microwave oven to 95ºC. The stainless steel reactor was also preheated to 105ºC.
At the end of the stage, the pulps were centrifugated to 30% consistency and washed. Washing was performed by diluting pulps with hot water (60 ºC) to 3% consistency, mixing
manually during 15 minutes and centrifuging again.
In the chelating stage (Q), a 14 L plastic chest was used and manually agitated. We incorporated 0.05% and 0.1% of MgSO4 in both the oxygen stages and P treatments,
respectively. The second oxygen stage was reinforced with peroxide (Op).
Table 2 presents treatment conditions of all bleaching stages.
Table 2. Oxygen, Chelating stage and Peroxide stages conditions in both sequences
For the peroxide stage, 50 g (od) of pulp was treated in plastic bags. After bleaching, pulps were neutralised with sodium meta-bisulfite at 1.5 % consistency and thoroughly washed.
The bleached pulps were refined to different wetness values, using a PFI laboratory refiner.
TAPPI Standards were used in most determinations (Kappa number, viscosity, colour parameters L*, a*, b* and physical properties), except brightness (ISO 3688:1997), and
opacity (ISO 2471:1997). Brightness loss was determined by thermic aging, heating the handsheets for 1h at 100ºC.
Metallic ions content (Fe, Cu, Mg, Ca, and Mn) in pulps was analysed by atomic absorption spectroscopy after wet digestion with nitric acid in a microwave oven.
COD (Chemical Oxygen Demand) was determined in spent liquors and washing waters to establish the organic load that would be carried over to the following stage (CPPA st. H.3P).
For statistical analysis of the W, O and Op stages, Hypothesis t test was applied, concerning the difference between the means of two samples from normal distributions, while a paired
sample comparison for testing differences between P stages, after 2, 3 and 4 hours, was used.
The applied methodologies are best detailed in previous works3, 4.
Results and Discussion
Table 3 presents chemical characterization of pulps treated with sequences I-A and I-B.
Table 3. Chemical characterisation of pulps treated with sequences I-A and I-B
This shows that there are no significant differences between brown pulps treated with EDTA or with DTPMPA in viscosity, Kappa number and COD parameters.
Figure 2 shows the evolution of metals. Calcium values are noticeably high in industrial pulps (1000 ppm), probably due to tap water. Addition of DTPMPA in brown stock washing
produces a reduction of only 10% in Ca levels.
Figure 2: Metallic ions profile in different stages of complete sequences I-A and I-B
DTPMPA treatment in the OP stage significantly reduced all metallic ions but Ca and Mg, in both sequences (Table 3, Figure 2). Mg was incorporated as MgSO4 in the O and Op stages
to protect viscosity. Pulps treated with DTPMPA showed better Mg retention.
Pulp washed initially with DTPMPA (I-2), had low Mn content at the beginning of the process. Mn concentration was reduced by 43% after the O stage, only by washing in the control
sequence (pulp I-3), while DTPMPA addition in brown stock washing decreased Mn content by 60% (pulp I-4).
Fe and Mn presented higher values in the Op stage of sequence I-B, evidencing the absence of Q stage. Nevertheless, Mn levels near 1 ppm are acceptable for entering the P stage.
Table 4 shows the evolution of the physical properties of pulps in each sequence, while table 5 and figure 3, expose the results of the final refined pulps.
Table 4: Physical properties of industrial pulps in both applied sequences (numbers in Figure 1)
Table 5. Refining curves of TCF bleached pulps from both sequences, after 2h of P stage
Figure 3: Refining curves of TCF bleached pulps from both sequences, after 2h of P stage
There are no differences in mechanical properties of unrefined pulps after the P stage in both sequences. The evolution of all mechanical properties with refining is similar in both pulps
(Table 5, Figure 3).
Table 6 shows properties of unbleached and TCF bleached refined pulps (42-43 ºSR).
Table 6: Mechanical properties of unbleached and TCF bleached pulps, refined to 42-43 ºSR
The strengths of unbleached pulps were slightly higher when they were washed with DTPMPA.
In previous laboratory work, an increase of 10% in strength was found when the sequence including an acid stage was replaced with a simpler, completely alkaline sequence 6. This
effect could be due to the combination of two factors: acidic groups generated in the P stage (that increase fibre bonding8), and DTPMPA incorporation. In the case of industrial
pulps, the results do not show the same benefit in strength increase.
As the swelling degree is related to the valency of cations present in pulps (in the Na+ form, pulps are stronger than in the Ca+2 one), the effect of changing an unrefined pulp from its
calcium form to a sodium form confers the same benefit of some amount of refining. Using different water supplies, pulps readily exchange ions and pick up the higher valency ions9.
The dissimilar effects of phosphonates action on laboratory and industrial pulps can be explained by ionic equilibrium. Since laboratory pulps were always treated with demineralized
water, their calcium content was low (200 ppm) and reduction produced by DTPMPA was significant (30% less). On the contrary, the industrial pulp had notably high calcium content
(about 1000 ppm), and the great amounts of Ca that remained bound to fibres, prevented the beneficial effect of Na on pulp strengths.
On the other hand, mechanical properties of unrefined and refined TCF bleached pulps obtained in this work were similar to those of commercial eucalyptus ECF bleached pulps,
while brightness was lower and opacity was 10% higher7.
After 2 hours of peroxide treatment, brightness of pulp I-8 was inferior by 1.5 % ISO to that of pulp I-7. As peroxide residual was high, the peroxide stage time was extended in order to
verify if pulps could reach similar brightness. Table 8 exhibits pulps properties after2, 3, and 4 hours of treatment with hydrogen peroxide.
Brightness efficiency (the relationship between brightness increase and the percentage of consumed H2O2) was calculated as the percentage of brightness change [(final brightness -
initial brightness of the stage) / initial brightness], divided by the percentage of peroxide consumption in each treatment (calculated in the same way).
Table 7: Chemical characteristics and optical properties of pulps after P stage, at different periods of treatment
After a P stage of 4 hours, brightness of both pulps attained 86%ISO. The gain in brightness between Op and P stages is higher in pulp I-8 than in pulp I-7, as I-8 value was very low
after the Op stage (Table 4).
Bleaching efficiency was notably superior in pulp I-8 (43%) than in pulp I-7 (26%), due to differences in consumed peroxide. Differences persisted after 3 and 4 hours.
As mentioned in the previous work6, since the spent liquor of the P stage has more than 1%odp of residual peroxide (as well as some free DTPMPA and magnesium), it could be
recycled to the Op stage. It is possible to close the Op-P circuit because P spent liquor has very low levels of detrimental metallic ions.
In a bleaching sequence O-Q-Op-P, it is possible to eliminate the acidic Q stage, replacing it with the incorporation of DTPMPA in the brown stock washing, Op and P stages. Optical and
mechanical properties of both pulps are similar.
When adding DTPMPA in the P stage, final pulp characteristics are independent of the properties of the previous stages. It eliminated the deleterious influence of metals on TCF
bleaching, and, in some cases, increased pulp strengths.
The elimination of the intermediate acidification simplifies the bleaching plant, reducing the costs of equipment, reagents and operation.
This work is part of the "Best Chelating Agents Management to Obtain TCF Bleached Kraft Pulps" applied research project financed by Solutia Inc.
Thank you to Ind. Chem Isabel C. Silva for her support and Dr. Alberto Venica for reviewing the manuscript.
S .E .Aguilar; J. Clermont; P. Meza; C. A .Pavlik; S. Wolfart; O. M .Barboza and D. I. Bengoechea are gratefully acknowledged for their collaboration.
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