Environmental protection requirements have led producers of paper pulp to discontinue the use of chlorine in the bleaching of kraft and sulphite cellulose pulps. Chlorine has been replaced by chlorine dioxide ClO2, which has become the universal bleaching agent in ECF (elemental chlorine-free) sequences.
These sequences generally comprise three to five stages, of which two or three are treatments with chlorine dioxide (D). The other reagents used in these sequences are oxygen (O), hydrogen peroxide (P), soda (E) and, less frequently, ozone (Z), which is the latest bleaching agent to appear in the industry. These sequences most frequently take the form D(EO)DED, D(EO)DD, D(EO)D(EP)D, etc.
More than two 2 million tonnes of ClO2are consumed every year by the pulp industry. Since chlorine dioxide has no other massive scale, one can measure the significance of the pulp industry for producers of sodium chlorate, which is the raw material used in the manufacture of ClO2. The benefit of replacing chlorine with chlorine dioxide resides chiefly in the fact that it generates five times less organic chlorine (AOX) than chlorine when it reacts with lignin
Replacing chlorine by chlorine dioxide was an expensive operation for the industry. Efforts have thus been made in the interim to discover bleaching processes that will permit the cost of the reagents used to be reduced. The cheapest reagent is oxygen. It has been used successfully to reduce the residual levels of lignin following cooking and thus reduce the quantities of chlorine dioxide. More than 67% of factories worldwide use oxygen in this manner, i.e. directly after cooking. However, oxygen is not effective enough by itself to achieve high levels of delignification. In addition, oxygen partially depolymerises cellulose. Thus, delignification rarely exceeds 50%. The subsequent sequence is a conventional ECF sequence in which the quantities of chlorine dioxide are reduced accordingly.
Ozone is a very strong oxidant. It has very extensive delignifying and bleaching powers. Since it is a non-chlorinated reagent, it was a natural choice for replacing chlorine or for enhancing the action of oxygen as part of an ECF sequence. Its first industrial application (Z stage) was in 1992, in an American mill, as part of an OZED sequence. However, ozone also reacts with cellulose, resulting in a significant drop in its degree of polymerisation (as measured by a viscosity test). This observation led a large number of paper manufacturers, for whom the viscosity of the pulp is the key quality criterion, to inquire as to exactly how harmless sequences containing an ozone stage were. This question, which has still not been answered, has considerably slowed the adoption of ozone in the paper industry. Today, only around 30 mills use ozone in bleaching.
The aim of this study is to examine the effect of bleaching sequences containing an ozone stage on the bleaching economy and the main characteristics associated with the suitability of bleached pulp for use in paper. Two industrial kraft pulps were used. The former is a Brazilian eucalyptus pulp (mixture of E. grandis and E. saligna), delignified by oxygen in one stage (kappa number 11.9). The latter is a pine pulp (Pinus Radiata) from Chile, also delignified by oxygen, in two successive stages (kappa number 10.6). These two pulps thus cover the scenarios of hardwoods and softwoods.
Study on sequences with an ozone stage
Based on prior studies carried out in the laboratory, several sequences were selected and applied to the two industrial kraft pulps. ZDED sequences were applied to the eucalyptus pulp. In this sequence, the quantities of ozone applied were 0.2, 0.4 and 0.8%, expressed in mass of ozone as a percentage of the mass of the pulp sample. The D0ED1D2sequence was taken as a reference for the conventional ECF sequence. ZEpD1D2, (ZD0)EpD1D2and D0Ep(D1Z)D2sequences were applied to the radiata pine kraft pulp. Ep designates an alkaline extraction in the presence of hydrogen peroxide. In these sequences, the quantities of ozone were 0.6%, 0.3% and 0.1% respectively. The control sequence was D0EpD1D2. Treatment with ozone was performed in a rotating spherical reactor at room temperature. The pulp was pre-acidified to pH 2.5 using 4N sulphuric acid at a fiber concentration (consistency) of 1-2% then centrifuged to a consistency of 35-40%, before finally being fluffed. In the sequence containing (ZD), no wash was carried out between Z and D. The same applies for the (DZ) treatment. For this latter treatment, the pH of the pulp was not modified prior to ozonation. The D stages were performed at 10% consistency in a sealed polyethylene bag placed in a thermostat-controlled water bath. D0 was carried out at 50°C for 1 hour for the eucalyptus pulp and at 58°C for 40 min for the pine radiata pulp. D1and D2were both carried out at 80°C for 2.5 hours. Stages E and Ep were performed at 70°C and 80°C respectively for 1 hour. The quantities of chlorine dioxide were calculated so as to obtain a brightness approaching 90% ISO. The degree of polymerisation of the cellulose (DP) was meadured The various levels of ozone used in the sequences studied provided a representative range of the ozonation conditions liable to be applied in industrial sequences.
The results in Table 1 set out the values for brightness, cellulose DP and consumption of bleaching reagents. It can be seen that the sequences containing an ozone stage allow savings in the quantity of chlorine dioxide used that exceed expectations. It is remarkable to notice that the factors of replacement (expressed in g of ClO2replaced by 1g of O3) exceed the theoretical value based on the fact that the two reagents perform the same reaction on the phenolic groups of lignin represented below and during which a phenolic unit is opened by one mole of reagent to form a derivative of muconic acid, Fig. 1, which makes the lignin more hydrophilic. This theoretical factor is 1.7.
Table 1 - Brightness, viscosity and reagent consumption for the sequences studied
Figure 1 - Oxidation of phenolic units in lignin by O3 and CLO2
The factors set out in Table 1 are all considerably higher than this value, and in certain cases reach extremely high levels. Considering the respective costs of ozone and chlorine dioxide, the conclusion is that substantial savings in the costs of reagents can be achieved by introducing an ozone treatment into a conventional ECF sequence.
The reason for these unexpected performances lies most probably in the secondary reaction mechanisms of chlorine dioxide with lignin. Several studies have shown that chlorine dioxide reacts with the phenolic groups of lignin, following a secondary reaction leading to the formation of quinones.1In another study, performed in our laboratory2, it was clearly shown that the action of chlorine dioxide on lignin leads to the creation of new coloured groups that cannot be completely degraded, even using a significant excess of chlorine dioxide. The same study confirmed that quinones react very difficultly with chlorine dioxide. All of these results suggest that the bleaching reaction from chlorine dioxide is hindered by the formation of quinones and that large excesses of chlorine dioxide must be used in order to degrade them sufficiently. These excesses of chlorine dioxide are partially consumed by the muconic acids initially created by the reaction of ClO2with lignin and present in the environment.3. This reaction has no effect on the bleaching of the pulp. Ozone, which is much more reactive, was not added in excess quantities and would thus be consumed more effectively.
Another reason for the overconsumption of chlorine dioxide is the fact that it is partially converted to chlorate4, which causes it to lose its oxidizing power.
The results of the bleaching also show that the viscosity of the pulp is lower when the sequence contains an ozone stage. This drop is particularly pronounced at high ozone levels and in particular for eucalyptus pulp treated with 0.8% ozone (ZDED sequence), which loses 50% of its viscosity. Other tests with even higher levels of ozone show that loss of viscosity continues to grow. A study of the resistance of brightness to heat has furthermore shown that this resistance was better than that of the control pulps, Table 1. This is even more pronounced when the quantity of ozone used is high. The explanation may lie in the greater purity of the pulps bleached in this manner (lower quantity of residual extractives) and possibly in the lower proportion of residual hexenuronic acid groups, known to contribute to yellowing under heat in bleached hardwood pulps.
Part II of this article can be read in the next Mills and Technology newsletter