BETTER UNDERSTANDING OF OZONATION DURING BLEACHING PROCESSES

Bernard Brochier
Presented at 14th ISWFPC - Durban, South Africa

Keywords: Ozone, Dissolved ozone, Ozone consumption, Process water, Chemical pulp, Bleaching

ABSTRACT
The ozone stability and the ozone consumption were studied in different ways in order to determine the impact of the main influencing parameters such as time, temperature and water quality. Ozone consumption was evaluated after mixing ozonated water with tap water or mill process water at ambient temperature and at 55°C. For understanding which ozone part was actually consumed, ozone mass balances were established introducing known ozone charges in a stirred glass reactor and measuring ozone effluent and dissolved ozone concentration. Then experiments were performed on mixed hardwood bleached kraft pulp with different amounts of process water to measure the incidence of the COD content on ozone consumption. As at mill scale, ozone is applied on pulp containing some residual chlorine dioxide, it was interesting to check the effect on the pulp brightness and viscosity.

Ozone could be applied on pulp at the end of a chlorine dioxide sequence without washing, the residual chlorine dioxide was not detrimental for the brightness neither for the viscosity.

1. INTRODUCTION
The use of ozone in pulp bleaching is now widely implemented: more than 20 mills in the world use ozone as delignifying or bleaching agent. The reactions of ozone with lignin and carbohydrates have been investigated in the literature. Some authors have observed a dramatic degradation of pulp carbohydrates after ozonation, whereas other scientists show evidence of a limited detrimental impact of ozone on cellulosic constituents of fibres. Whether or not, ozone reaction during the bleaching process still need to be investigated. Ozone has a high potential to degrade chromophoric structures and more particularly lignin. Ozonation at different consistencies can be performed at different levels of the bleaching sequence (1). An emerging use of ozone consists in using the ozone not only as a delignification agent but also as a brightness booster at the end of a bleaching sequence to remove the last residual chromophores (2).

One of the main factors affecting the ozonation efficiency is the mass transfer from the gaseous phase to the fibres through different water layers (3). A better knowledge of what happens during the ozonation process, more particularly during the mass transfer, could help to optimize the industrial ozone stages. In the frame of this study, the ozone distribution during a bleaching stage was investigated. An ozone mass balance was carried out under various conditions to assess the ozone dissolved in the water phase, ozone consumed by side-reactions and ozone actually reacted with lignin. Several parameters have an influence on the ratio "introduced ozone / reacted ozone": the water quality, the temperature and the pulp washing efficiency.
 
The aim of this study was to use ozone at low consistency in mill conditions (temperature close to 70 – 75°C and white water containing some COD and BOD charges). The detrimental effect of temperature and COD containing water was evaluated on ozone stability and the effect of temperature on the bleaching reaction was investigated. The ozone charge to be used at mill scale, compared to laboratory, the ozone pressure and water flow for dissolved ozone preparation were determined.

2. MATERIALS AND METHODS
A designed glass vessel (figure 2.1) was used for the ozone mass balance experiments. Ozone concentrations in the gas were measured by UV spectroscopy. Dissolved ozone was determined by the convention iodometric titration method. Ozone effluent was absorbed using a potassium iodide (KI) solution and determined by iodometric titration method. Totally and partially bleached hardwood kraft pulp from a French mill was considered. Pulp brightness were measured according to NF Q 500-12 standard and pulp viscosity with the ISO 5351-1 standard.


Figure 2.1: Laboratory glass reactor used for the ozone reactions.

3.RESULTS AND DISCUSSION

3.1. Dissolved ozone stability.
The ozone solution stability was studied in a previous study. It was demonstrated that a poor stability was observed. Ozone solution could not be stored and needed to be used directly after the production (figure 3.1). The water quality had no significant effect on the dissolved ozone concentration, in the applied conditions. It was noted that after 5 minutes, more than 50% ozone was already decomposed.


Figure 3.1: Ozone solution stability with storage time.
 
3.2. Dissolved ozone consumption
Ozone was not stable in water solution but its high reactivity could also contribute to consume ozone by reacting with dissolved solids present either in deionised, tap or especially charged mill process waters. It is well known that ozone is sensitive to temperature and that the solubility of ozone is inversely proportional to the increase in temperature. It was therefore interesting to evaluate the ozone quantity destroyed by a single mixing when saturated dissolved ozone water, tap water or mill process water were considered at ambient temperature and 50-60°C. To proceed this experiment, 250 ml of ozonated water at 70 mg O3/l were added to :
250 ml cold tap water
250 ml hot tap water (55°C)
250 ml cold mill white water
250 ml hot mill white water

The residual ozone was measured after mixing for 30 seconds (figure 3.2). The water consumed some ozone and especially process water. The ozone consumption was not influenced by the temperature when tap water is considered. On the contrary, with process water, the ozone was more rapidly completely consumed at higher temperature. We noticed that the mixture of cold or hot process waters with dissolved ozone became pink, highlighting a reaction between dissolved compounds in the mill process water. The colour disappeared after 1.5 minutes. Figure 3.2 illustrates a relative consumption of ozone by water. However it was to consider that the ozone consumption was proportional to the water volume mixed with ozonated water. For instance, 250 ml of water mixed with 250 ml of 70 mg/l ozone in water led to 8.4 mg of residual ozone. This meant that 1 litre of water consumed 18.2 mg of ozone.


Figure 3.2: Evolution of ozone solution stability with storage time when different waters were considered for dissolving ozone.
 
3.3. Ozone mass balance
During the dissolution process, part of the introduced ozone was absorbed and part of the ozone is degraded by reaction with compounds dissolved in the liquid phase. Temperature was also an important parameter. Some tests were performed in a high speed stirring glass vessel in which was introduced ozone in known quantity. The gas was injected in the liquid phase through a porous glass and the effluent gas was neutralized by bubbling through a KI solution, as shown in figure 2.1.

The experiments were carried out with tap water, soft water and mill process water in cold (ambient temperature) and hot (60- 65°C) conditions (Figure 3.3). The ozone consumption was calculated based on the ozone quantity introduced minus ozone effluent minus ozone in the reactor. Ozone effluent was measured by absorption in KI solution and ozone in the reactor was measured after addition of KI solution in the reactor.

Part of the ozone was consumed in the dissolution process. The consumption decreased when the introduced ozone increased. Tap water, soft water and process water had comparable behaviours.


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.




4.CONCLUSIONS
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.

5.ACKNOWLEDGEMENT
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.

6.BIBLIOGRAPHIC REFERENCES
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

CONTACT
Centre Technique du Papier – InTechFibres
Domaine Universitaire, BP 251, 38044 Grenoble Cedex 9 – France
e-mail : bernard.brochier@webCTP.coma