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FAIR-CT96-1361
Papermaking Chemistry Aspects of Environmentally Friendly ECF and TCF Pulps |
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Contract No: | FAIR-CT96-1361 |
| Date Prepared: | September 2000 | |
| Source: | Final Report Executive Summary |
Introduction
The fear of AOX, potentially dangerous substances, was great a few years ago. This lead to the disappearance of the bleaching sequences containing a chlorine stage. Today, the pulp mills use the new Elementary Chlorine Free bleaching sequences (without Cl2, but ClO2 is used), producing much less AOX than conventional bleaching sequences, and increasingly Totally Chlorine Free bleaching sequences producing no AOX at all.
Chemical additives are increasingly important for an effective manufacture of high quality paper products. The background of this project is the importance of chemical additives (paper chemicals) in paper manufacture and the rapidly increasing use of new types of non-chlorine bleached pulps, i.e. elementary chlorine-free (ECF) and totally chlorine-free (TCF) pulps.
Objectives
The overall aim of the project was to find solutions to problems in the papermaking chemistry when using the new types of non-chlorine pulps, ECF and TCF pulps. To enable this there was need to build up a thorough knowledge of the systems involved. This project is an important step in the work towards an ecocyclic system in pulp and paper production. It contributes to exploiting natural resources within the tolerance limits of the earth, a big issue well in line with the EC policies. The project will strengthen the scientific base needed to protect the environment.
Activities
The working method in this project was rather broad, from development of new microscopic techniques for the analysis of residual substances on ECF and TCF pulps and laboratory investigations of the interaction between dissolved and colloidal substances from the pulps with process chemicals added during papermaking to pilot scale and mill scale trials of the build-up of disturbing substances.
Results
Cooking and bleaching was carried out in order to study the chemistry of ECF and TCF pulps. This was accomplished using a wide range of analytical techniques including microscopy. The CLSM offered the best possibilities for examining variation of lignin content of fibres taken from all stages of the bleaching process. Image analysis measurements of intensity (assumed to relate directly to lignin concentration) correlated reasonably well with the visual appearance of the images as well as with the amounts of lignin as determined by standard chemical analyses. A technique has been developed to determine the distribution of lignin in the cell walls of kraft pulp fibres at different stages of bleaching and to semi-quantify the changes in lignin content.
The amount and type of organic material released from the pulps during washing, slushing and refining with different energy inputs at different chemical environments (salt concentration) was characterised. Hardwood pulps released considerably more organic substances than softwood pulps. Another significant effect was that the pulps that are TCF- bleached released more organic substance. Furthermore, salt concentration has a big influence on the amount of material released during beating. The higher the salt concentration the less material is released.
Other investigations included a paper technological characterisation and a differentiated quantitative determination of the carry-over of model and commercial pulps in paper manufacture. Specifically, some typical European wood species together with typical pulping and bleaching sequences were taken into account.
The results obtained for market pulps correlated well with the results obtained with laboratory prepared pulps. Thus, TCF pulps released higher amounts of anionic trash than ECF pulps did. Also, their total anionic charges and surface charges were higher. This is true in particular for pulps that were subjected to ozone bleaching. The cationic demand indicates the charge character of the organics present and ultimately provides a measure for the disturbing effects of such organics in the papermaking cycle. The cationic demand of TCF pulps is considerably in excess of that of ECF bleached pulps. The levels of deleterious matter released were influenced by the ionic strength of the suspension water and by the conditions of treatment. In the trials, the individual pulps started to behave differently as early as within the first 20 minutes of treatment.
The wood species of the sulphate pulp had a great importance on the efficiency of the wet-end additives. If the additive brings a specific property such as wet strength to the pulp, its efficiency was higher if the pulp had a low specific surface. If the additive is a retention aid, its efficiency will be better on the pulp having a high specific surface. This is probably since the retention without retention aid is 'naturally' higher for pulps with high surface area. Another strong effect of the origin of the fibre has been found with the wet-strength agent of Urea-Forrnaldehyde type: the wet strength is much more easily obtained on the softwood pulps than on the softwood or eucalyptus pulps. Other effects than the different specific surfaces could exist to explain this difference.
The effect of the bleaching sequence on the efficiency of the wet-end additives was usually not significant, except for AKD sizing. The efficiency of this sizing agent seemed better on ECF pulps than on TCF pulps, and it was very difficult to size eucalyptus TCF pulp. This sizing agent seemed also to be sensitive towards soluble substances carried by hardwood and eucalyptus pulps.
The amount of transferred flocculant increased with increasing shearing time during the experiments. Furthermore, it was found that for the pulps with a higher amount of soluble charged substances, i.e. for the hardwood TCF pulp, there was a higher amount of C-PAM remaining in solution. This can be explained by the reaction between the cationic flocculant and soluble substances. If a part of the flocculant added react with soluble substances it is likely that the complexes formed will still remain in solution,
A method was developed to prepare a suspension with high concentrations of dissolved and colloidal material. It was found that it is possible to prepare at laboratory scale dissolving substances the amount of which was in industrially relevant level. The preparation contained refining procedure where pulp was refined in a filtrate that is repeatedly re-circulated. Extremely high dissolving substances levels were obtained by reverse osmosis treatment of solutions. The method was used in the work with the effect of disturbing substances on retention and sizing.
Retention studies in the presence of dissolving showed that anionic dissolved organic material had a negative influence on the retention of fine material when a cationic polymer was used as retention aid. It was also shown that, even though the anionic material was removed prior to the retention trials, a negative effect remained due to some kind of interaction between the uncharged organic material and the cationic retention aid. The main effect was, however, caused by the anionic material. In similar manner, the sizing efficiency decreased when there were dissolving substances presence in the white water. Thus, the higher the COD level was the higher Cobb-values were obtained. However, after anionic substance removal no significant detrimental effect of dissolving substances on sizing could be observed.
Two different types of experimental paper machine trials have been done:
Results from the laboratory experiments and the experimental paper machine trials were used to extend a computer simulation model for the white water system of a paper machine. Based on the results from the experimental paper machine trials, one of the pulp mills, where the samples for these trials were delivered, was chosen to perform the trials. By doing so the relevance of the experimental paper machine trials could be verified in the best way. Focus in the mill trials was on sizing efficiency.
Results from the first experimental paper machine trials can be summarised as following: It was found that the amount of transferred flocculant from pulp fibres to filler increases with increasing time. It means that if the distance (time) from the addition point to the headbox was high, this long reaction time lead also to a low filler retention, and to a high amount of C- PAM in white water. On contrary, if cationic polymer was added very late to thin stock only small amount of flocculant was transferred from pulp fibres to calcium carbonate. However, high amounts of C-PAM was found in white water (unreacted). It can be concluded that in that case addition point was too close to headbox. It was noticed that early thin stock addition was the best choice of addition point for retention aid.
Simulations show that it is possible to simulate variations in the amount of dissolving substances in the white water system to some extent. There are still problems with regard to prediction of dissolving substances, i.e. as shown in previous tasks the amount of dissolved material is depended on several factors, such as pulp type, pulp charge content, pH, salt concentration, etc. which means that both variation in pulp and process conditions influence the amount of dissolving substances in the paper machine, making simulation of these components extremely difficult.
The paper mill in which white water and paper samples were taken out uses TCF-bleached hardwood pulp and AKD sizing. A test was made in which an additional pulp washing was done in order to decrease the amount of dissolving substances in papermaking. It was clearly shown that the extra pulp washing decreased the amount of disturbing substances in white water. Similarly, higher cationic demand values were obtained for the pulp which had not been washed with an extra time. When comparing results from washed and unwashed pulps at a given AKD dosage, it was found that lower Cobb values are obtained when pulp was very well washed compared to normal washed pulps.
Discussion
The overall objectives of the project was to find solutions to problems in the papermaking chemistry when using the new types of non-chlorine pulps, ECF and TCF pulps. To enable this goal there was need to build up a thorough knowledge of the systems involved. The large amount of ECF and TCF-bleached pulps that were used by the participants were prepared from unbleached pulps taken from different pulp mills in a pilot bleaching unit. ECF and TCF-bleached sulphate pulps were prepared in laboratory. Furthermore a series of bleached sulphate pulps was prepared for microscopic purposes.
The aims of the microscopic study were to find a method for identifying and locating lignin in the cell walls of kraft pulp fibres at different stages of bleaching. To this end a number of the methods described above were investigated. A technique was eventually developed using the CLSM and image analysis to make semi-quantitative measurements. This technique is intended as a research tool for examining the variation of lignin content in individual fibres as an aid to optimising bleaching processes. However, further work needs to be done to verify the results and to standardise the procedure. Another technique has also been separately developed using UV microscopy.
The purpose of the work concerning dissolving substances was to elucidate and describe the release of organic substances from the project pulps. It can be concluded that there are two different main factors influencing how much material is released from fibres during mechanical treatment. The fibre charge content and ionic strength. Thus, the higher the fibre charge and the lower the ionic strength is the more material is released from the fibres during mechanical treatment. It means that the mechanism for material release during mechanical treatment is so-called "Jack in the box" effect. Based on this theory the driving force for a material release during beating is electrostatic, i.e. there is a repulsion between the negatively charged cellulose surface and the negatively charged dissolved carbohydrates. When increasing salt concentration this repulsion increase, and hence the driving force is lowered. Since most of the dissolved substances are located in the cell wall (in the pores), mechanical treatment will influence on the material removal. Thus, mechanical treatment both opens up and closes pores, leading to the fact that relatively high molecular mass material can also be released from the cell wall by beating.
Investigations of the sulphite pulp bleach cycle were conducted to describe the evolution of the pulp properties vs. carry-over. In targeted laboratory studies, existing potentials for optimisation by reducing anionic trash levels were identified. For this purpose, chemical- technological tests were performed in the water loop of a TCF sulphite bleach in which the pulps were simultaneously characterised both between the individual bleach sequences and after bleaching.
In the context of market sulphate pulp manufacture it was found that the carry-over of pulp may be effectively controlled by the pulp washing stage downstream of the last bleaching stage and by the washing effect of the dewatering machine. As lab-scale and chemical-technological trials in an industrial plant combined with computer simulations of the anionic trash distribution in sulphate pulp bleaching have shown, the composition of the backwater that is used as washwater in the pulp bleachery may influence the kind of substances present in the carry-over.
The results from the investigation of the commercial pulps suggests that there exists a relationship between AKD sizing efficiency and fibre charge where an increased charge density leads to a decreased sizing efficiency. However, it is concluded that the interaction between size particle and fibre surface is not entirely governed by the charge density of the fibre surface. The observed differences in reactivity and retention between ECF and TCF might be caused by different surface morphology created during the bleaching procedure. Different bleaching sequence will give pulps with different specific surface areas and different chemical surface composition. However, further investigations of the difference between ECF and TCF fibre surfaces are needed to be able to explain this behaviour.
Transfer experiments showed that total retention values were lower for the softwood pulps than for the hardwood pulps, which probably reflected a difference in both the transferred amount of flocculant from pulp to filter particles and a different amount adsorbed onto pulp fines that were not retained. For a pulp containing a higher amount of fines (hardwood) a larger part of the flocculant added will adsorb onto the fines and the retention of these fines will be higher (hardwood). It was concluded that the amount of flocculant from either pulp fibers or pulp fines to calcium carbonate filler would be different for different pulps depending on the fines content in the pulp and the amount of charged group on the fibrous material.
Extremely high concentrations of disturbing substances were produced at laboratory scale, and the influence of them on retention and sizing was studied. It was found that dissolving substances have a negative influence on retention. It is plausible to believe that the extent of this effect is related to the amounts of anionic detrimental substances present in the white water, i.e. the amounts that have been released from the pulp at the stock preparation (this has been shown in other investigations). The key factor when different pulps are compared with respect to function of retention aid are thus in all probability the amounts of dissolved substances (primarily the anionic material but also to some extent the uncharged material), that are released during the stock preparation and later in the paper machine.
It was concluded that the reason for a bad sizing with dissolving substances is mainly the lowered size retention. Hence, a reasonable sizing, was obtained at a high size addition levels also in the presence of dissolving Substances. it also means that more size is consumed because of the disturbing substances. However, when anionic components were removed from the white water the sizing, efficiency improved considerably. Thus, even though close to 600 mg/l COD levels were obtained after fixation of anionic components, the sizing degree was as good as in the pure water case.
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