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|Scientific report of the LGP2
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|Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
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"
Chemical upgrading of cellulose pulps aims at focusing target material properties and it should be possible on mill sites. The new thematic started in our research group aims at developing on-site applicable chemical treatments to produce deep modifications on chemical pulp substrates and understanding the induced modifications.
A first case has been the development of sequences using catalysed hydrogen peroxide and oxygen applied on bleached chemical pulps. These reagents enrich the pulp in carboxylic acid groups that give new properties to the pulp. Among these, cross-linking reactions are made possible without any added reagent when a thermal treatment is applied. This occurs via esterification reactions between the acid and alcohol groups in the cellulose network. The new created linkages are resistant to moisture and induce deep changes in the physical properties of fibres and of mats. The use of oxygen-based reagents is an advantage for environmental safety. Chemical characterisations of the produced materials should be pursued to understand better the main features of the reaction mechanisms.
In a second study, we have been focused on studying the role played by Kraft cellulose on the electrification phenomena at the surface of high-voltage transformer boards submitted to oil circulation. We have established that carboxyl groups at fibre surfaces were the main chemical species involved in exchange ion processes and acid-base equilibrium that create charge separation between the oil and the board. Because of the insulating properties, the increase of the voltage leads to destructive discharges of the insulating parts of the transformers. This study gives rise to a Ph.D thesis with the partnership of EDF and with the collaboration of the University of Poitiers specialised of the electrification phenomena under flow. We have also contributed to the development of novel tests (ECT tests) and to the recommendation of some remedy to cut-off these phenomena.
The study of laccase-mediator systems leads to the development of new delignification processes and environmental friendly bleaching using reduced quantities of chemicals.
Studies have been devoted to this subject and research has been focused on mediators. The main interest of the mediator is its small size, lower than that of the laccases. As a consequence the mediator may oxidize the lignin when both are very close inside the fibre matrix. This work is carried out with the collaboration of the Thapar Center for Industrial Research and Development (Patiala, India) and the main objective is to find new mediators, more efficient than those already known. Using the cyclic voltametry technique new organic and inorganic mediators with a suitable redox potential (0.8 – 1.15 V(/NHE)) have been selected. These compounds are tested as mediator in laccase-mediator systems applied for pulp delignification. Currently, organic mediators seem to be more efficient than inorganic ones.
Ozone is known as an efficient delignification reagent for chemical pulps. Since 1992, the ozone bleaching process has been set up in 28 bleaching lines, where ozone is used in the middle of the bleaching sequence as a complementary delignifying agent after oxygen delignification and in some cases in combination with chlorine dioxide. Previous studies made by our group have shown that ozone could also be a good brightening agent when used at the end of a bleaching sequence, where the last points of brightness are usually difficult to gain when chlorine dioxide is used . In most cases 1 to 2 kg ozone per ton of pulp are sufficient to produce an instantaneous bleaching effect, increasing the brightness by 3-5 points. The process is working equally well at acidic or neutral pH. No cellulose degradation takes place under these conditions, and the strength properties of the ozonated pulps are not affected. However it has been shown that the efficiency of ozone in a final stage varies from pulp to pulp.
The main objective of the present project (Guillaume Pipon’s thesis) was first to explain the reasons why the efficiency of a final ozone stage could vary depending on the pulp type, and second to propose some solutions to improve the process. One explanation for the lack of activity of some pulps toward ozone treatment is the presence of hexenuronic acid groups. These groups are formed during the kraft delignification process and are located on residual xylans. Their reactivity with lignin is very high and since they are white structures their degradation by ozone does not participate to the bleaching effect and represents a waste of ozone. Selective removal of the hexenuronic acids by acidolysis prior to ozone application is one way to solve this problem.
Another reason for the poor response of some pulps is the presence of lignin fragments still attached to the cellulosic matrix. In fact a study on the ozonation of some lignin models showed that these models are first oxidised into coloured substances. Only an excess of ozone can destroy these newly formed compounds. Again this represents an over conssumption of ozone. Using efficient delignification reagents prior to final ozonation is a prerequisite to observe a good bleaching response.
A mill scale trial is under preparation, with the support of Wedeco (subsidiary of ITT).
Bleachability of kraft and, more generally, alkaline pulps has been the subject of numerous investigations during the last decade. A good knowledge of the reasons why some pulps are more difficult to bleach than others would allow to adjust the cooking conditions in a proper way or, if this is not desirable for any reason, to define the most appropriate bleaching sequence which would minimize both the chemical cost and the environmental impact. This project has been the subject of Nicolas Benattar’s PhD thesis defended in 2005.
Unbleached chemical pulps produced by alkaline processes (kraft, NaOH, NaOH-Anthraquinone, IDE, Novacell) were analysed for their content in quinone groups. Residual lignins were extracted by mild acidolysis and analysed by 19F NMR spectroscopy, after reaction with 1-(4-trifluoromethyl)- phenylhydrazine and by voltammetry. This latter method was performed on a glassy carbon electrode, by analysing the reduction wave of the quinone groups in water at pH 11.5 or in DMSO. This wave appeared in the -0.5 to -1.0V (vs SCE) range. The number of quinone groups measured by 19F NMR varied from 0.02 to 0.07 mol per 200g of lignin. Kraft pulps were generally poorer in quinone groups than the other alkaline pulps. The ranking was the same with the two analytical techniques. Some differences existed also among pulps from the same process, depending on the operating conditions and the degree of delignification. Since ClO2 is not very reactive with quinones, it is suggested that the bleachability differences between pulps upon ECF bleaching may arise from the variation of quinone groups in residual lignin. As practical consequence it is suggested to introduce in the bleaching sequence some chemicals which attack quinones readily, such as hydrogen peroxide.
This has been successfully implemented in two kraft mills belonging to the Tembec Canadian group.
|Figure 1 - Alizarine, a model molecule for residual
chromophores in kraft pulps
|Figure 2 - Cyclic voltametric plot of alizarine in water at pH 11.5|
|Figure 3 - Voltametric plots of residual lignins samples
obtained by the kraft and soda-anthraquinone
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