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  • Biorefinery concept in paper recycling : modelling, verification and prediction of bleaching process [Elektronski vir] : thesis submitted in fulfilment of the requirements for the degree of Doctor in Applied Engineering at the University of Antwerp to be defended by
    Tofani, Giorgio
    Recycled paper is mainly used to produce newsprints, magazines and packaging materials. Bleaching is one of the main paper recycling processes to improve the brightness of recovered paper. Hydrogen ... peroxide is used, with or without an additional step where sodium dithionite is applied. The recovered paper is composed of types of fibres having different reactivity to the bleaching process. Moreover, the relative quantities of the various fibres in recovered paper are not constant over time. It means that a standardised industrial bleaching process can lose its efficiency due to the change in the composition of recovered paper used until that moment. The presence of problems in bleaching efficiency can be observed only at the end of the industrial process by the brightness analysis of the final product. Furthermore, the paper recycling industry generates significant waste streams. Currently, the aqueous portion of the effluents is recirculated in the paper recycling process to reduce the environmental impact. The solid wastes are converted to low-value products (e.g. additives for construction) or disposed of in a landfill. Laboratory research is ongoing to recover chemicals and materials from the waste streams of recycled paper mills. The first part of this PhD is situated in the bleaching process of recycled paper. The main goal was to determine the influence of the fibre composition on the hydrogen peroxide and dithionite bleaching. With this knowledge, it could be possible to optimise the bleaching stage (e.g. load of hydrogen peroxide) before to start the industrial process because the brightness could be estimated knowing the composition of the recycled paper, in terms of types of fibres. For that purpose, a mathematical model was used to correlate the fibre composition to brightness. Afterwards, the hydrogen peroxide consumption during bleaching was followed and studied to evaluate different behaviours depending on the fibres. The second part of the project is on the valorisation of the effluents generated during the recycling of paper. The first waste stream was a sludge from the deinking process used to remove contaminants (e.g. inks and pigments) from recovered paper. The second waste stream was generated by the alkaline treatment of recycled cardboard. The first goal was to recover fillers from solid fractions of the two wastes. The second goal was to recover lignin with hydrocarbon derivatives from the liquid parts of the latter waste stream. The study was divided into four experimental parts: 1) The first experimental part was to correlate the fibre composition with brightness to develop a statistical tool able to estimate the latter parameter before that a specific recycled paper sample would go to the industrial process. In this doctoral project, a reproducible and easy lab-test was elaborated for bleaching of the fibres. The results of this bleaching were compared to a more complex lab simulation for the industrial bleaching process, said latter lab test being elaborated in the past by the paper mill. The basic reason is that dedicated industrial reference tests were not possible within the timeframe of the doctoral work. The four most representative types of fibres in the paper were chosen as references (unbleached fibres from chemical pulping, unbleached fibres from mechanical pulping, unbleached fibres from semimechanical pulping and bleached fibres from chemical pulping). Different bleaching sequences were tested to find the best reaction conditions for the four types of fibres. A bleaching sequence consisting of five additions of hydrogen peroxide with a further dithionite step was found to be the best compromise and selected to develop the mathematical model. Afterwards, several samples, prepared by making different ratios of the four reference fibres were subjected to this bleaching process. The same sequence was applied to real-life recycled paper samples. Multiple linear regression analysis was applied using the percentages of the fibres as independent variables and brightness as the dependent variable. 2) The aim was to give a first evaluation of peroxide consumption during the bleaching of the four reference fibres. The peroxide concentration was followed to observe and describe its trend. Afterwards, the differences between laboratory bleaching (low fibre’s concentration and high hydrogen peroxide amount) and industrial-like conditions (high fibre’s concentration and low amount of peroxide) were evaluated. 3) The first study on the waste valorisation was to evaluate the recovery of recycled fillers from deinking sludge and the solid parts of waste generated by alkaline treatment of recycled cardboard. The process consisted of incineration at 575°C. The resulting ashes were bleached with sodium dithionite. The bleached ashes from the deinking sludge were blended with pulp to evaluate their effects on the mechanical and optical properties. 4) The second case focussed on the liquid fraction of the waste stream produced during the alkali treatment of recycled cardboard. An acid fractionation with sulphuric acid was applied to recover lignin and hydrocarbon derivates. The results can be summarised: 1) In the first experimental part, the 100 % unbleached fibres from chemical pulping showed the most significant increase in brightness. The fibres from mechanical and semi-mechanical pulping showed a lower bleachability. A multiple linear regression having ISO brightness as a dependent variable and two independent variables (unbleached fibres from chemical pulping and the combination of unbleached fibres from mechanical pulping and unbleached fibres from semi-mechanical pulping) was developed. A difference between measured brightness and the values predicted using the regression was observed in the real-life samples. This difference was on average, 7 points in brightness with a confidence interval of 2 points. Studying the literature, the difference can be due to the presence of different parameters in the recycled samples such as collapsed fibres or contaminants. These phenomena have a significant impact because the internal or external layers of the fibres are no more available to the bleaching agents. As far as we know, it is the first study that describes the correlation between brightness and fibre composition, using a mathematical model. 2) The peroxide concentration during bleaching was followed. The main observations were; the kraft fibres bleaching shows a rapid and significant peroxide consumption during the first 30 min. and afterwards, the bleaching seems to follow first-order kinetics. The bleaching of Bleached fibres, TMP fibres and CTMP fibres seems to follow first-order kinetics and its consumption rate increases with the progress of the reaction. The causes can be the decomposition of cellulose by the attack of hydrogen peroxide and the sidereactions of lignin at bleaching conditions. Interactions between fibres were also observed when the reference fibres were mixed. Moreover, the electron exchange was evaluated using the kappa number. It permitted to observe that the decomposition and side reactions of hydrogen peroxide are the primary cause of peroxide consumption rather than the bleaching reactions. In conclusion, it was also observed that the bleaching is more efficient at industrial-like conditions than the laboratory conditions used in this doctoral work. 3) The recycled fillers consist of a mixture of inorganic chemicals, mainly composed of calcium carbonate. During incineration of the deinking sludge and solid from alkaline treatment from recycled cardboard, the structure of the calcium carbonate was preserved using 575°C as incineration temperature. The ashes were bleached with sodium dithionite to reach a brightness suitable for magazine paper production. With this approach, it is scientifically and technologically possible to reduce the mass loss occurring during the paper recycling. However, further studies must be done to optimise the process and evaluate industrial scale-up. It is the first study that describes the recovery of recycled fillers, preserving their molecular structure. 4) Lignin and a mixture of hydrocarbon derivatives were recovered by acid fractionation of the liquid from the alkaline washing of recycled cardboard followed by solvent extraction. The recovered lignin showed similar behaviour to kraft lignin, the primary commercial type, on infrared and gelchromatography analysis. This work shows that the recovery of lignin and hydrocarbon derivatives has the potential for future application. However, further optimisation of the process should increase the yields of recovered lignin and hydrocarbon derivatives. Moreover, industrial scale-up must be evaluated. The recovery of lignin and hydrocarbon derivatives from the effluent generated by alkaline treatment of recycled cardboard using an acid fractionation is a new approach that needs further elaboration. As part of the thesis, fast and quantitative analysis of hydrogen peroxide using MQuant strips by Merck was developed measuring their optical cyan density, after immersion in hydrogen peroxide solutions, using a densitometer. In conclusion, this thesis adds to a better understanding of the recycled paper bleaching. It is possible to estimate the brightness than can be reached knowing the fibre composition of recovered paper samples despite some limitations. Moreover, a first approach to determine the peroxide consumption during bleaching and estimate its effectivity was made. To improve this part of the work, a more detailed description of the chromophores present in the fibres is necessary by their characterisation to quantify the functional groups that can react with bleaching agents. It is also essential to work at high consistency to increase the bleaching efficiency. Apart from the new insight in the bleaching process, the valorisation of the sidestreams was studied. The results show the possibility to recover recycled fillers, lignin and hydrocarbon derivatives from waste streams generated during the recycling of paper and cardboard.
    Type of material - dissertation ; adult, serious
    Publication and manufacture - Antwerp : [G. Tofani], 2021
    Language - english
    COBISS.SI-ID - 118640899

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