The influence of high temperature on the discoloration and degradation of carbohydrates and on hydrogen peroxide bleaching of spruce TMP
Sammanfattning: When processing wood to obtain pulp, the fibers are subjected to high temperatureat moisture levels and to the influence of metal ions. Many process developmentshave resulted in utilization of higher temperatures that induce chemical reactionsin the wood fibers. This thesis deals with some aspects of using high temperaturesin pulping processes. The first part of this work concerns how carbohydrates areaffected, in terms of discoloration and degradation, by high temperature, moisture,and the presence of metal ions. The next part deals with the metal ion profiles inthermomechanical pulp (TMP) processes and the effects of the high temperaturetreatment of native wood under TMP processing conditions. The third sectionconcerns processing conditions present in the high‐temperature hydrogenperoxide bleaching of TMP.In experiments using cotton linter sheets impregnated with glucuronic acid anddifferent metal ions, increased 5‐(hydroxymethyl)furfural and 2‐furaldehydeconcentrations were accompanied by decreased viscosity and increaseddiscoloration when Mg2+ ions and, to some extent, Ca2+ ions were present. Thisindicates that under certain conditions, in this case, dry heat‐induced ageing underslightly acidic conditions and with access to oxygen, the presence of alkaline earthmetals can cause substantial degradation and discoloration of cellulose. Thisindicates that Mg2+ ions can promote the Lewis‐acid‐catalyzed degradation ofcellulose. When glucuronic acid was added to the cotton linter sheets, the presenceof Ca2+ or Mg2+ ions decreased the discoloration caused by the dry heat‐inducedageing of glucuronic acid and also led to a smaller decrease in viscosity comparedto that in cotton linter sheets containing only Ca2+ or Mg2+ ions. This may explainwhy the negative effect of Ca2+ or Mg2+ ions on viscosity is less pronounced incertain pulps. In mechanical pulping processes, the addition of sodium silicate and kaolin clayand re‐circulation of water from the paper machines increased the levels of ironand aluminum ions. In a mill using dithionite bleaching, the levels of iron andaluminum ions in the final pulp were approximately ten times higher than in a millusing hydrogen peroxide bleaching. This was probably due to extraction of moreiron and aluminum ions from the clay to the water due to low pH and to thecorrosion of process equipment induced by thiosulfate ions derived fromdecomposed dithionite. In laboratory trials, it was impossible to completelyremove iron from these pulps using diethylenetriaminepentaacetic acid orextraction with hydrochloric acid. This was probably due to the strong binding ofiron to the wood fiber or its presence as oxide/hydroxide clusters. The mostimportant issue is to avoid contaminating the pulp with iron ions; process waterand process chemicals with low iron contents should therefore be used. Especiallyin the dithionite bleaching process, iron contamination increases thedecomposition of the dithionite, causing corrosion of the process equipment andadding further iron to the pulp.The shavings method is a straightforward method for studying the formation andelimination of chromophores in wood. Light absorption coefficients, lightscattering coefficients, and brightness can easily be obtained using this method.The method was used for studying how temperature in the absence of mechanicalenergy affects wood brightness. This provided useful information about the effectof process conditions in mechanical pulping and its relationship to chromophoreformation. An advantage of using thin shavings is the rapid penetration ofchemicals into the fibers. The shavings method was therefore also used in studyingthe kinetics of hydrogen peroxide bleaching at high temperature.The high‐temperature (HT) hydrogen peroxide bleaching of a thermomechanicalpulp was carried out in a pressurized laboratory‐scale wing defibrator. Theexperiments demonstrate two ways to obtain high brightness under HT conditions.One way is two‐stage peroxide bleaching with a low total alkali/peroxide ratio inthe first stage to avoid alkaline darkening. The second way is to pre‐impregnatethe pulp with hydrogen peroxide before adding alkali in two stages. Themaximum brightness was reached after only 2.5 minutes at low alkali charge.Thus, the bleaching time can be short and the total alkali charge low under hightemperatureperoxide bleaching conditions. To achieve very high brightness usingHT hydrogen peroxide bleaching, other measures than increasing the alkali chargeand prolonging the bleaching time are necessary.
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