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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Chemical processes > Novel investigations on chlorine dioxide bleaching           Update: January 8th 2007
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Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
(November 2006)
 
Documents taken from the
"Scientific Report of the Laboratory of Pulp and Paper Science and Graphic Arts - UMR 5518
Grenoble - France
January 2002-November 2005"

II - Chemical processes

II - 2 - Novel investigations on chlorine dioxide bleaching
G. Mortha, N. Bénattar, Y. Hamzeh, D. Lachenal

Part 1 - Modelling full ECF bleaching sequences

The generalization of Elemental Chlorine Free bleaching sequences of the type (O)DEDED, using chlorine dioxide (ClO2) as a unique oxidant along the sequence, has lead to questions about how to manage an optimal utilization of ClO2 to decrease costs and environmental impacts. Mathematical modelling is one interesting approach that allows an overall understanding of a process, and possibly to its optimization, with the pre-requisite to feed the model with physical knowledge.
In the herein presented work, an overall literature review, followed by an experimental work, has lead to establishing coupled stoichiometric and kinetic equations for each individual bleaching stages, that can be combined to describe the full sequence.
If X denotes the reaction progress variable (related to lignin or coloured chromophores concentration in the pulp), R is the involved reactant concentration, and fi particular mathematical functions, equations for each individual stage can be established as follows:

dX / dt = f1(X - X).f2 (R, T, pH)

dX / dR = f3(R, T, pH)

dpH / dt = f4 (X, pH)

In these equations, the effects of temperature, pH and initial pulp characteristics on the kinetics and extent of bleaching were accounted for through the mathematical expressions of the fi functions, and the chemical constants input to the model.

Modelling chlorite and chlorate formation [Figure 1]

Some reaction conditions, and more particularly pH, impact significantly the formation of residual oxychlorine species such as chlorite and chlorate, which represents a loss of oxidizing power. Equations to predict the formation of the latter species were developed by consideration of the reaction mechanism:

 

The overall model was assessed by laboratory bleaching tests, and the proximity between predicted and experimental data was found good [Table 1].

One interesting application of the model was the search of optimal bleaching conditions , and more particularly, how to optimize the share of ClO2 in the full sequence. An example of kappa number optimization after the D0 stage is presented in [Figure 2]. A general prediction of the model was that brightness can be improved by lowering the charge of ClO2 in D0. This tendency was verified experimentally by laboratory and indutrial scale test.

Modelling the formation of oxychlorine species   Search of an optimal kappa number
Figure 1 - Modelling the formation of oxychlorine species
during a D0 stage; unbleached softwood kraft pulp
of kappa number 25; 1% ClO2/pulp, 60°C.
  Figure 2 - Search of an optimal kappa number after the D0 stage,
for different chlorine charge applied on the sequence
(SKF= sequence kappa factor) –
unbleached softwood kraft pulp of kappa number 25
ClO2 charge (% on pulp) Final brightness (% ISO)
D0 D1 D2 Experimental Predicted
1.9 1.5 0.6 87.7 87.7
1.9 1 0.9 87.3 87.2
1.4 1.2 1 86.5 86.5
1 1.2 1 85.2 85.6
1 1.5 1 86.4 86.7
1 1.8 0.7 86 86.7
2.3 1.2 0.7 88.6 89.3
2.3 1.5 0.3 88 87.7
1.5 1.8 0.6 88.8 88.7

Table 1 - DEDED bleaching of a kraft softwood pulp
of kappa number 25: predicted and experimental results

Part 2 - Hydrodynamics and performance of chlorine dioxide delignification in a flow-through reactor

Delignification pattern in a flow through reactor
Figure 3 - Delignification pattern
in a flow through reactor –
bottom zone: white delignified pulp;
 top zone: unbleached pulp (liquid
circulation in the upward direction)

Flow-through chlorine dioxide delignification has been investigated in the purpose of going beyond the limits of batch bleaching conventional systems.

It was shown that reproducible medium consistency fibre beds of high thickness could be formed by drainage. Both the pressure drop and axial dispersion measured across the bed were low, but an important effect of temperature on the latter, not much discussed in the literature, was found.

Delignification in a flow-through system was more performing than in a batch system at any reactant charge and temperature. Raising temperature was necessary to decrease the minimum kappa number attainable in such a system (a kappa value just above 2 was obtained on a softwood kraft pulp using only 2.5% ClO2/pulp at 70°C).

However, inefficient competing reactions leading to a loss of oxidising power because of chlorate formation were also much raised with temperature, and further investigations are currently pursued to optimise the process.

Variation of the axial dispersion coefficient   Delignification profile in the bed
Figure 4 - Variation of the axial dispersion coefficient (Dax) versus
superficial velocity (Uo) for the different pulps at low temperature
(18°C) and different pulp consistencies.
  Figure 5 - Delignification profile in the bed (measured by the kappa
number value) versus bed height at different chlorine dioxide
charges and temperatures.
Typical chlorinated species concentration and pH profiles at the outlet of the fibre bed
Figure 6 - Typical chlorinated species concentration and pH profiles
at the outlet of the fibre bed (1% ClO2/pulp, 30°C, flow rate 20 mL/min)
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