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A Study on the Thermal Stability of Sodium Dithionite Using ATR-FTIR Spectroscopy

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A Study on the Thermal Stability of Sodium Dithionite Using ATR-FTIR Spectroscopy A study on the thermal stability of sodium dithionite using ATR-FTIR spectroscopy Vijaya Lakshmi Vegunta This work has been carried out at INNVENTIA AB Master Thesis in Fibre and Polymer Technology KTH Royal Institute of Technology Stockholm, Sweden June 2016 1. Abstract Sodium dithionite (Na2S2O4) is a powerful reducing agent. It has therefore been suggested to be used as an additive in kraft pulping to improve the yield. However, sodium dithionite easily decomposes and it is thus important to determine the effect of different conditions. The aim of this thesis has been to investigate the thermal stability of sodium dithionite under anaerobic conditions using ATR-FTIR spectroscopy under different conditions, such as heating temperature, concentration of the solution, heating time and pH. The stability of sodium dithionite was found to decrease with increasing heating temperature, concentration of sodium dithionite, heating time and pH. Sodium dithionite was found to be relatively stable at moderate alkaline pH:s 11.5 and 12.5, while a rapid decrease in stability with time was noted at higher heating temperatures and concentrations of sodium dithionite. Based on this study on the thermal stability of sodium dithionite, the following conditions are suggested as the most promising, when adding sodium dithionite to the kraft cooking as an additive; pH 12.5, with 0.4 M concentration of the solution, at a heating temperature of 100 °C. 2 Table of Contents 1. Abstract ........................................................................................................................................ 2 2. Introduction .................................................................................................................................. 5 2.1 Kraft pulping .......................................................................................................................... 6 2.2 Stability of sodium dithionite ................................................................................................. 7 2.3 Decomposition of sodium dithionite ...................................................................................... 8 2.4 Effect of temperature and pH on decomposition of sodium dithionite solution .................... 9 2.5 Possible reaction mechanisms .............................................................................................. 10 3. Purpose of the study ................................................................................................................... 11 4. Experiment ................................................................................................................................. 12 4.1 Materials ............................................................................................................................... 12 4.2 Sample preparation ............................................................................................................... 12 5. Method ....................................................................................................................................... 13 5.1 ATR-FTIR spectroscopy ...................................................................................................... 13 5.2 Calculations .......................................................................................................................... 14 6. Results and discussion ................................................................................................................ 16 6.1 Effect of heating time ........................................................................................................... 16 6.2 Effect of heating temperature ............................................................................................... 20 6.3 Effect of concentration ......................................................................................................... 24 6.4 Effect of pH .......................................................................................................................... 28 6.5 Structural changes observed by infrared spectroscopy ........................................................ 31 6.6 Multivariate analysis ............................................................................................................ 33 6.7 Decomposition products ....................................................................................................... 34 7. Conclusions ................................................................................................................................ 36 8. Recommendation and future work ............................................................................................. 37 9. Acknowledgement ...................................................................................................................... 38 10. References ................................................................................................................................ 39 Appendix 1: Scatterplots for each pH of sodium dithionite solution using Simca ........................ 42 3 Appendix 2: MODDE .................................................................................................................... 45 Appendix 3: ATR-FTIR spectra of sodium dithionite solution at each pH ................................... 49 Appendix 4: IR frequencies of sulphur containing compounds ..................................................... 51 4 2. Introduction Sodium dithionite (Na2S2O4) is a strong reducing agent with great industrial significance. Due to its reducing ability it finds use in fields such as printing and dyeing of textiles, production of synthetic rubber, synthesis of stabilizers for polymeric materials and as a biochemical reductant. Sodium dithionite also has valuable environmental remediation applications. It is being used to treat heavy metal waste in aquifers and other environmental significant treatments. A study of Jayme and Wörner (1952) showed that improved strength, higher bleachability and improved lignin degradation in kraft pulping were seen in the system when using sodium dithionite as a pulping additive. Due to its reducing properties sodium dithionite is used as a bleaching agent in textile and paper industry. Sodium dithionite may be used for the preservation of hemicelluloses in pulping and as it will not introduce new elements to the recovery system and it may be used without changing the system. Sodium dithionite efficiently reduces carbonyl functionalities of functionalized aldehydes and ketones to corresponding alcohols in good yields without affecting other functional groups in water or dioxane system at 85 °C (De Vries and Kellogg 1980; Sigh et al. 1998). An experimental research has been done by using sodium dithionite as pulping additive in a pilot scale and the results were promising (Wang et al. 2014). It is cost effective when compared to other alternative additives, such as sodium borohydride (NaBH4). Sodium dithionite must be handled strictly in an anaerobic environment due to its sensitivity towards oxygen. Analytical procedures which require removal or transfer of its solutions is therefore susceptible to serious error, underscoring the need for an in situ analytical technique. Sulphur – oxygen (S-O) compounds have relatively intense stretching modes, readily observed in the 1350 - 750 cm-1 wavenumber region of the infrared spectrum. In ATR (Attenuated total reflectance)-FTIR (Fourier transform infrared) spectroscopy, the IR radiation interacts with the sample mainly at the sample interface, resulting in a very short and reproducible effective path length. This makes ATR-FTIR spectroscopy as an ideal candidate for the quantitative Z- determination of multicomponent mixtures of sulphur – oxygen (SxOy ) anions. 5 2.1 Kraft pulping This section describes an overview of the kraft pulping process. Kraft pulping is currently the dominant chemical method for pulping. The pulp produced using this process is stronger and darker and requires chemical recovery to compete economically. The main objective of kraft pulping is to liberate wood fibers from the wood matrix to obtain wood pulp. This is achieved by removing lignin and preserving the molecular weight of carbohydrates as much as possible. The kraft pulping process involves digesting of wood chips at elevated temperatures and pressure. The typical out-line of this process is wood handling (chipping, debarking, and screening), cooking in digester (kraft cooking), and oxygen delignification, washing of pulp, multi stage bleaching and drying of pulp. In kraft mills, the wood chips are fed into digester together with the cooking liquor. The cooking liquor is white liquor which is a mixture of sodium hydroxide (NaOH) and sodium sulphide - - (Na2S). The active species in this cooking process are hydroxyl (OH ) and hydrogen sulphide (HS ) ions. The hydroxyl ions are consumed during kraft pulping and neutralize acidic groups. There is only a slight decrease in hydrogen sulphide ion concentration during pulping. The first step in kraft cooking is an impregnation step, where the cooking liquor is transported through the surface of wood chips and followed by the penetration and diffusion into the wood chips. Depending on the amount of hydroxyl and hydrogen sulphide ions present in the liquor, cooking temperature and time; different compositions of pulp can be obtained (Kleppe 1970; Aurell and Hartler 1965). In modern kraft mills the cooking is carried
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