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BETTER UNDERSTANDING OF OZONATION DURING BLEACHING PROCESSES
Keywords: Ozone, Dissolved ozone, Ozone consumption, Process water, Chemical pulp, Bleaching
Figure 3.3: Evolution of ozone solution stability with introduce ozone charge when different waters (A: tap water; B: soft water; C: mill process water) and temperatures were considered for dissolving ozone.
Process water consumed slightly more ozone than tap and soft water (about 10% more). Temperature seemed to have minor effect on process water compared to tap and soft water. It was interesting for these tests to estimate also the ozone absorbed by the water. These values were calculated on the basis of the introduced ozone quantity minus ozone effluent representing the dissolved ozone plus ozone degraded during the trials (figure 3.4).
There was no significant effect of the temperature on the absorbed ozone for tap and soft water. The decrease in absorbed ozone when the introduced quantity increased was due to the rapid saturation of the solution at atmospheric pressure and the consumption by reaction on dissolved compounds and/or degradation of the solution during the time of trials including ozone injection and diverse handlings. In the case of process water, temperature had an important effect in one case and not in the other one probably due to a different composition in soluble compounds.
Figure 3.4: Evolution of ozone absorbed by water with introduced ozone charge when different waters (A: tap water; B: soft water; C: mill process water) and temperatures were considered for dissolving ozone.
Figure 3.5: Evolution of ozone degraded in water with introduced ozone charge when different waters (A: tap water; B: soft water; C: mill process water) and temperatures were considered for dissolving ozone.
The analysis of these results gave an idea of the consumed and absorbed ozones but it was interesting to evaluate the portion of degraded ozone in these trials. The percentage of degraded ozone is given by the ratio : introduced ozone minus effluent minus dissolved / introduced minus effluent (figure 3.5).
These figures underlined clearly that temperature was responsible for the major part of the degradation for tap and soft waters. In the case of process water, the quality of the water was already an important parameter in the degradation of ozone: 70 - 80 % of ozone degraded in the cold process water test when the hot process water test led to 90 - 95 %. We noticed that the process water became pink at ambient temperature, but no colour appeared during the tests at higher temperature.
3.4. Incidence of the process water content in the pulp
Trials were carried out on a fully washed hardwood bleached pulp diluted with various amounts of process and tap waters to a consistency of 4%. An ozone charge of 0.082% of dissolved ozone at 70 mg/l was applied onto the pulp. Brightness was measured before and after SO2 washing (figure 3.6).
The increasing amount of process water in the treated pulp led to a decrease in the bleaching efficiency. Indeed the small quantity of applied ozone reacted preferably on the COD charge introduced by the process water. A higher ozone charge would react more on cellulose after reacting on the dissolved matter, which would mean that at industrial scale the ozone charge had to be increased.
Figure 3.6: Evolution of pulp brightness when the pulp was diluted with process water when ozonated water was applied onto the pulp.
3.5. Effect of the temperature and the water quality on the bleaching
Bleaching trials were carried out on a mixed hardwood kraft pulp after D1 and D2. The tests were performed on pulp at 3.5% consistency using process or deionised waters at room temperature and at 65°C. Comparison was done using either dissolved ozone or ozone gas (figure 3.7) for the D1-bleached pulp before and after SO2 washing. For the trials at 65°C, the pulp suspension was heated to 85°C and then ozone was added as solution at room temperature in case of dissolved ozone and in the reactor maintained at 65°C for ozone gas.
Temperature did not have significant effect on the pulp brightness what ever the quality of the water. The detrimental effect of the temperature was counterbalanced by an improvement of the kinetic. The process water led to a reduction in brightness gain in all cases. Ozone gas was less efficient than dissolved ozone.
3.6.Influence of the chlorine dioxide residual on the ozone stage.
In a mill process, ozone could be introduced as a dilution stage before or during washing. Some residual chlorine dioxide could be present. It is well known that there is no incompatibility between ozone and chlorine dioxide. On the washed D1 pulp, were added different charges of ClO2 from 0 to 0.1%. A charge 0.1% of dissolved ozone was applied onto the pulp at room temperature (figure 3.8)
Figure 3.7: Evolution of pulp brightness before and after SO2 washing with ozone consumption when ozone gas or ozonated water was applied onto an hardwood kraft pulp.
Figure 3.8: Evolution of pulp brightness and viscosity with residual chlorine dioxide concentration in the pulp when ozone gas or ozonated water was applied onto an hardwood kraft pulp.
The presence of residual ClO2 did not have detrimental effect on the brightness results. On the contrary, the brightness was improved for 0.03 and 0.06% of ClO2 but this effect was not observed for the SO2 brightness. The increase in viscosity was correlated with the ClO2 charge and could be explained by the elimination of carbonyl groups by ClO2 responsible for viscosity drop in the viscosity measurements.
Ozone solutions are not stable and in less than 5 minutes, 50% of the dissolved ozone is decomposed in both tap and soft waters. When ozone solution was added to tap or process waters, 50% of the ozone were consumed by the tap water when 70% was consumed by the process water. The temperature had no significant effect on these conditions.
During the dissolution of ozone, the consumption of ozone was equivalent for the different water qualities in higher temperature conditions. However, process water had the same consumption for both cold and hot conditions. With regard to this consumption, it was to consider absorbed and degraded ozones. Temperature had no significant effect on ozone absorption for tap and soft waters. In case of process water, it was difficult to conclude because of different behaviours for both tested process waters.
However, ozone degraded in the absorbed ozone was sensitive to the temperature for all water qualities but process water was more detrimental, even in cold conditions.
The amount of process water in the pulp had an incidence on the final brightness and 1% brightness gain obtained for a washed pulp could decrease to 0.5 % gain when a process water was used to dilute the pulp.
In pulp bleaching, for both D1 pulps, the temperature had no significant effect. Process water consumed part of the ozone leading to slightly lower result. Ozone gas was less efficient than dissolved ozone whatever the water quality.
Ozone could be applied on pulp at the end of a chlorine dioxide sequence without washing, the residual chlorine dioxide was not detrimental to the brightness neither to the viscosity.
The author acknowledges Nicole Garnier and Guy Mary for their excellent collaboration and doing the experiments, Carine Kuligowski for her scientific support and OZONIA for its help in supplying equipments.
1. Van Lierop B., Skothos A., Liegergott N. (1996) : The technology of chemical pulp bleaching –Ozone delignification, in Pulp Bleaching – Principles and Practise, Dence, C.W.and Reeve D.W. Editors, Tappi Press, p 323-345.
2. Chirat C. , Lachenal D., Mateo C. Brochier B. (2002) : Final bleaching with ozonated water, at International Pulp Bleaching conference, Portland, Vol p 245 – 252
3. Brochier B. (2006) Overview of the use of ozone in the pulp and paper industry, Ozone news Vol 34 n°6p 21 - 27
Centre Technique du Papier – InTechFibres
Domaine Universitaire, BP 251, 38044 Grenoble Cedex 9 – France
e-mail : bernard.brochier@webCTP.coma
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