Mid Sweden University

Heavy metals are well-known environmental pollutants due to its potential impact on associated ecosystems and human health. Thus, it is important to monitor the levels of heavy metals in the environment. X-ray fluorescence (XRF) analysis is a powerful and effective screening tool in measuring the concentration of multi-elements simultaneously.

This thesis provides insight into development and implementation of XRF instruments for environmental monitoring and industrial process control. The XRF method was compared with a commercial scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) for fly ash samples. Qualitative analysis and semi-quantitative analysis of Na, S, Cl, K and Cd in incineration fly ash were performed with these two similar techniques. One of the challenges of using XRF is the scattering background noise from the primary beam, which decreases the detection limit and the sensitivity of the measurement system. Hence, an X-ray beam filter was chosen to suppress the background noise for a specific element, Cr, in leachate. Numerical simulations and experiments were developed to find the proper filter material and thickness by calculating the X-ray fluorescence intensities and the signal-to-noise ratio. The developed system is capable of online monitoring of Cr levels, to certify that the concentration is below the threshold level in leachate. An XRF prototype was built and calibrated for underwater Hg analysis in maritime wet sediment using a radioisotope source. The presented results show that it is possible to detect Hg by K-shell emission thus enabling XRF analysis for sediment underwater.

For non-homogeneous samples, an image revealing the elemental distribution can be achieved by micro-XRF (µ-XRF). XRF mapping of element distributions on a microscopic level was obtained by using scanning XRF microscopy and full-field XRF projection microscopy (FF-XRF). The spatial resolution of the scanning XRF imaging setup using an X-ray tube is in the order of 100 µm, but need to be further improved to measure the homogeneity of S on individual fiber level in pulp and paper industry. For the scanning technique, it is a tradeoff between resolution and measurement time. Another technique is FF-XRF imaging, and a setup was implemented using an energy resolving pixel detector and X-ray optics. The capabilities and limitations of using X-ray optics in XRF imaging systems have been identified. These microscopy measurements can guide further comprehensive environmental and industrial monitoring missions, utilizing elemental distribution information.

To improve the competitive advantages of pulp fibre-based materials for tissue and packaging over fossil-based products, it is essential to increase knowledge of the selectivity of the cooking and the chemimechanical processes by optimizing the unit operations of impregnation, cooking and refining. A general goal in pulping processes is to achieve as efficient and even fibre separation as possible. A key to achieving this is to improve impregnation uniformity. In the case of chemical pulping, we need to study how a more even distribution of lignin at the fibre level via easily impregnated wood chips can be achieved using classic measures such as equalized hydroxide ion concentration, increased initial sulphide ion concentration, low sodium ion concentration and a low cooking temperature combined with an oxidative and reductive environment. In the case of chemithermomechanical pulp (CTMP) manufacturing, we need to achieve as even a degree of sulphonation as possible at the level of the individual fibres by means of improved sulphite ion distribution within the wood chips before they are pre-heated prior to entering the refiner. 

Firstly, we have studied selective cooking systems for sulphate pulp manufacturing in oxidative (polysulfide) and reductive (sodium borohydride) environments. The yield increased from 48% to a maximum of 53%, which resulted in faster dewatering when mimicking a tissue papermaking process. This could explain how the advantage of the increased yield (fewer fibres and a more open sheet structure) outweighs the negative effects of the higher hemicellulose content on the dewatering properties. Moreover, the increased proportion of hemicellulose in the fibre walls resulted in improved bonding and increased tensile index at a certain refining energy. 

Secondly, we have studied the uniformity of impregnation at the fibre level by developing an accurate way of measuring sulphur and sodium content in measuring points that are 5-10 µm in diameter with miniaturized X-ray-based technology. This technology is considered cheap and efficient enough to be introduced in industrial labs and/or in online equipment. Our newly built miniaturized energy dispersive X-ray fluorescence (ED-XRF) demonstrates the capability of imaging sulphur and possibly sodium distribution in wood chip fibres or individual fibres on a micro scale. 

In parallel, to the above research we have studied a new catalytic lignin-selective cooking method where a substantial portion of the dissolved lignin can be extracted as vanillin, creating significant value and opportunities for new cost-efficient wood biorefinery systems. 

För att förbättra konkurrensfördelarna med massafiberbaserade material såsom mjukpapper och förpackningsmaterial, jämfört med fossilbaserade produkter, är det viktigt att förbättra kunskapen om massatillverknings-processers selektivitet genom att optimera enhetsprocesserna; impregnering, kokning och raffinering. Det övergripande målet för alla massatillverknings-processer är att erhålla så effektiv och jämn fiberseparation som möjligt. För att erhålla jämnare distribution av impregnering måste vi studera hur frisättning av lignin på fibernivå via lättimpregnerade träflis kan uppnås med klassiska åtgärder som; utjämnad hydroxidjonkoncentration, ökad initial sulfidjonkoncentration, låg natriumjonkoncentration och låg koktemperatur kombinerat med oxidativ och reduktiv miljö. Vid tillverkning av kemi­mekanisk massa (CTMP) behöver vi erhålla så jämn sulfoneringsgrad som möjligt ner till fibernivå. Här behöver vi förbättra fördelningen av sulfitjonerna i vedflisen innan förvärmningen före raffinören. 

Vi studerade först selektiva kokningssystem för tillverkning av sulfatmassa i oxidativ miljö, polysulfid, respektive i reduktiv miljö, natriumborhydrid. Utbytet ökade från 48% till i bästa fall 53%, vilket resulterade i snabbare avvattning vid betingelser som simulerar tillverkning av hygienprodukter. Vi kunde förklara det ökade utbytet med att den positiva inverkan av färre fibrer vid viss ytvikt och öppnare arkstruktur dominerar över den negativa inverkan av högre halt vattenabsorberande hemicellulosa på avvattnings­egenskaperna. Den högre halten hemicellulosa i fiberväggarna bidrog till förbättrade bindningsegenskaper och ökat dragindex vid en specifik raffineringsenergi. Därefter studerade vi sätt att utvärdera homogenitet i  impregnering på fibernivå genom att ta fram en metodik som kan mäta svavel- och natriuminnehåll i mätpunkter som är 5-10 µm i diameter. Mikroskopi baserad på röntgenfluorescens bedöms vara lämplig för detta ändamål samt tillräckligt billig och effektiv att framöver kunna användas i industrilaboratorier och/eller i online-utrustning. 

Parallellt har också forskning utförts rörande en ny katalytisk lignin-selektiv kokningsmetod där en signifikant andel av ligninet kan lösas ut som värdefullt vanillin. Detta skapar framtida möjligheter att ta fram en kostnadseffektiv bioraffinaderimetodik där högt värde skapas både i form av väl separerade fibrer och i form av värdefulla naturliga kemikalier.