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Researchers of the LGP2
(May 2011)
 
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Laboratory of Pulp and Paper Science and Graphic Arts

II - Chemical Processes

II - 2 - Advanced oxidation processes applied to cellulosic fibers

The cellulosic fibres used for papermaking are bleached by oxidative processes. In fact bleaching consists in the removal of the residual lignin still present in the fibres by ring opening of its phenolic groups and resulting formation of carboxyl groups. Lignin is made more hydrophilic and goes into solution. Chlorine dioxide is the universal oxidant used in pulp bleaching. It has no effect on cellulose and therefore the bleached fibres have excellent paper properties. However chlorine dioxide is rather expensive and generates chlorinated organics which are potentially toxic and are difficult to eliminate.

Our group works on the substitution of ozone for chlorine dioxide. Ozone is a powerful oxidant which does not form hazardous by-products. Five Ph.D. theses have been defended on this subject. Significant progress has been made and described in Guillaume Pipon and Shree Prakash Mishra’s theses which lead to one application for a patent and stimulated several industrial developments made by Wedeco (Germany) and Degrémont, our partners in this field.

Figure 1 shows that both chemicals generate new chromophores when they react with lignin, which are then entirely destroyed in the case of ozone, but only partially removed in the case of chlorine dioxide. This fact explains the higher efficiency of ozone in pulp bleaching.

Development of the absorbance at 457 nm of a lignin solution 
   as function of ozone or chlorine dioxide consumed.
Figure 1 - Development of the absorbance at 457 nm of a lignin solution
as function of ozone or chlorine dioxide consumed.

We have shown that the new chromophores formed by chlorine dioxide are quinonic groups. Figure 2 illustrates how two quinone models (parabenzoquinone and naphtoquinone) react with chlorine dioxide and ozone. The absorbance of their water solutions is followed. It is concluded that the superiority of ozone lies in the fact that, contrary to chlorine dioxide, it readily reacts with quinones. These results were granted the Weldon Award by PAPTAC (Pulp And Paper Association of Canada) in 2008.

Colour removal during ClO2 andO3 treatment of quinones in water solution
Figure 2 - Colour removal during ClO2 and O3 treatment of quinones
in water solution. (a) naphtoquinone – (b) para-benzoquinone.

The ozone treatment has a marked negative effect on the degree of polymerization of cellulose. This has been one factor which explains the slow development of ozone bleaching in the industry. In fact it was well admitted that the fibre strength properties are directly related to the DP of cellulose. In one recent Ph.D. thesis work, we have shown that this is not the case. We could prepare ozone bleached pulps with excellent mechanical properties despite their very low DP (up to 50% lower than for conventional bleaching). By comparing with a treatment by endo cellulases which gives rise to the same decrease in DP it is shown that contrary to ozone treatment, cellulase treatment leads to more degraded fibres, as illustrated on Figure 3 (SEM analysis), with lower mechanical properties. It is concluded that cellulose depolymerization by ozone is homogeneous whereas cellulases perform a much more localized degradation which takes place at fibre defects such as pits…and creates weaker zones in the fibres. Homogeneity of cellulose depolymerization would therefore be a more important factor than the extent of the DP loss.

SEM micrographs of the unrefined handsheets of bleached eucalyptus Kraft pulp
Figure 3 - SEM micrographs of the unrefined handsheets of bleached eucalyptus Kraft pulp
treated by cellulase (A) and of same pulp bleached with a sequence containing an ozone stage (B)
(same DP in both cases)

Another way to decrease the use of chlorine dioxide is to perform an oxygen treatment under pressure (5 bars, pH 12, 100°C). Oxygen can open the lignin phenolic rings in a similar way to chlorine dioxide. Oxygen delignification is essentially performed as a first stage in the bleaching process because it needs an alkaline pH. The cooking being also alkaline the two effluents can be combined and e ventually burned. Unfortunately the efficiency of oxygen as oxidant is lower than that of chlorine dioxide and therefore it cannot displace more than 50% chlorine dioxide. Several theses have addressed the search for catalysts in oxygen delignification. The more recent works (Basile Guéneau’s thesis) have focussed on the criteria that a good catalyst should meet. They were conducted in cooperation with the Département de Chimie Moléculaire de l’Université Joseph Fourier in the frame work of the Région Rhône-Alpes research clusters.

Several copper complexes were investigated. They belong to the phenanthroline and terpyridine families. The requirements for an efficient catalyst are the following:

The assessment of these criteria was made by electrochemical analysis (cyclic voltametry), UV-Visible spectroscopy, EPR spectroscopy, and some studies on the reactions given by lignin and cellulose models.

Figure 4 shows the effect of some of these complexes on lignin removal in the case of fibers containing native lignin (the lignin content is measured by the kappa number). It appears that complex 6 works better than simple phenantroline complex (3) which has been proposed in the literature, whereas complex 7 does not work. The oxidation of copper (I) to copper (II) would not take place in the case of complex 7, as shown by the absence of Cu(II) signal in the EPR spectrum of the pulp treated by oxygen [Figure 5].

Effect of copper complexes on oxygen delignification of a pulp containing native softwood lignin.   EPR spectrumof an oxygen treated pulp in the presence of complex (3) (A) 
   and of complex (7) (B). Cu(II) signal is visible in (A) only
Figure 4 - Effect of copper complexes on oxygen delignification
of a pulp containing native softwood lignin.
(a) no treatment (b) oxygen treatment without catalyst –
(3) (6) (7) treatment using the designated catalysts
  Figure 5 - EPR spectrumof an oxygen treated pulp in the presence
of complex (3) (A) and of complex (7) (B).
Cu(II) signal is visible in (A) only
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