Graduation date: 1999

This thesis investigates the optimization of washing techniques before bleaching a pulp produced using ionic liquids. One pulp is from softwood and the other from wheat straw. Through experiments with varied temperatures and concentrations, the effectiveness of different washing conditions was evaluated, focusing on their impact on kappa number, viscosity, yield and reject amount for the softwood pulp. The wheat straw pulp was studied using the same optimal washing conditions that gave the best results for the softwood pulp. The results showed that washing at 75°C with 1.5% NaOH was the most effective. After washing, the softwood pulp had a kappa number of 20.0 and a viscosity of 162 mL/g. For the wheat straw pulp, the best result was a wash with 1.5% NaOH at 75°C, resulting in a kappa number of 16.2 and a viscosity of 446 mL/g. The washing process proved effective on a laboratory scale and the next step will be to conduct pilot-scale trials. The trials so far show that there is potential to scale up the washing of pulp produced by ionic liquid-based cooking. After washing, bleaching was performed using sodium chlorite. This method was suitable for laboratory-scale experiments, leading to a kappa number reduction from 20.0 down to 0.39 for the softwood pulp, with a viscosity loss from 162 mg/L down to 152 mL/g and a brightness of ISO 85%. For the wheat straw pulp, the kappa number decreased from 16.2 down to 0.62, the viscosity fall to 272 mL/g from 446 mg/L and the brightness reached ISO 76%. The study demonstrates that washing parameters can be optimized to achieve high brightness of the final bleached pulp without significantly altering its physical properties. The kappa number decreases while the viscosity remains relatively stable for the soft wood pulp, indicating that the process can consistently produce the same results. The wheat straw pulp lost viscosity from a high level. Loss of viscosity must be expected in some extent. The insights from this study reveal that pulp produced with ionic liquids can be washed and bleached effectively under control. The control of loss of viscosity is crucial for industrial applications. Future process development will require efficient chemical recovery. It is not possible to have wastes out of the process. It will be essential to close the process and it will ensure the sustainability of the process.

Massatvätt är ett viktigt steg i kraft-processen, då kokkemikalierna kan återvinnas och återanvändas i processen, vilket minimerar belastningen på miljön. Det är ett viktigt steg innan blekning av massan, då en låg kemisk syre-förbrukning (COD) är viktigt för att minska förbrukning av blekningskemikalier. Dessutom kan massatvätt även eliminera andra oönskade material, såsom metaller eller extraktivämnen. Syftet med studien var att utvärdera av tvätt-effektivitet av industriell massatvätt. Målen var att jämföra industriell tvättning med extensiv tvättning, att undersöka kvaliteten av tvättvätskor med analystekniker och att undersöka befintliga metoder för att beräkna tvätt-effektivitet. Den experimentella fasen var indelad i provtvättning och tvätt-serier, utförda med olika tvättvätskor, uppehållstider och industriell och extensiv utspädningsfaktor. Provtvättningen gav resultat som möjliggjorde det att utveckla tvätt-metoden. Resultaten för tvätt-serien visade att den extensiva utspädningsfaktorn generellt gav bättre resultat och en mer effektiv tvätt, och att en tvättvätska med en temperatur på 80°C generellt gav bättre resultat. Ljusheten påverkades inte av utspädningsfaktorn och kappanalysen visade en väldigt liten skillnad jämfört med originalmassan. Utvärderingen av befintliga metoder för att beräkna tvätt-effektivitet indikerade att alla undersökta metoder kunde ge felaktiga resultat, men beräkningarna tillsammans med andra metoder såsom COD, konduktivitet och med flera borde ge industrin en god indikation på tvätt-effektiviteten / Pulp washing is an important step in the kraft process, as it allows for the cooking chemicals to be recovered and reused in the process, which also minimizes the environmental load to the subsequent stages. It is also important before the bleaching plant, as a low chemical oxygen demand (COD) carry-over is important to minimize the consumption of bleaching chemicals. Furthermore, washing can also remove other unwanted materials, such as metals or wood extractives. The aim of the study was to assess washing efficiency of industrial washing. The objectives were to compare industrial washing with extensive washing, to investigate the quality of wash liquors using analytical techniques and to examine existing methods for calculating washing efficiency. The experimental phase was divided into trial washing and washing series, conducted with different wash liquors, residence times and dilution factors (DF). The trial washing yielded results that allowed for the washing methodology to be improved. The results from the washing series indicated the extensive DF in general gave better results and provided a more efficient washing, and that a wash liquor of 80°C gave better results. Brightness was not affected by the dilution factor and kappa number analysis showed that there was a very slight difference compared to the original pulp. The evaluation of existing methods for calculating washing efficiency indicated that all investigated methods could give inaccurate results, but that the calculations together with other methods such as COD and conductivity should give the industry a good indication of the washing efficiency.