This paper describes our efforts focused at defining the formation of diphenylmethane moieties in lignin during conventional kraft and soda pulping conditions. This objective was realized by confirming, initially, the assignment of a 31P NMR signal as being due exclusively to the presence of phosphitylated diphenylmethane (DPM) phenolic hydroxyl groups. More specifically, softwood milled wood lignin (Picea mariana) was subjected to kraft pulping conditions in the presence and absence of varying amounts of formaldehyde. After quantitative recovery of the lignin, the 31P NMR spectra were recorded and the spectra revealed selective signal growth in the region confined between 142.8 and 144.3 ppm, in accordance with previous model compound work and detailed calculations based on the Hammett principles. To further substantiate our conclusions we also carried out two series of isothermal (120°C) kraft and soda pulping experiments followed by quantitative determinations of the DPM moieties, and correlated our findings with the differences in chemistry known to occur between the two processes. Finally, diphenylmethane phenolic moieties were determined in isolated residual lignins from two western hemlock kraft pulp samples produced via an EMCC® mill protocol and a laboratory batch digester, respectively. These structures prevailed amongst the condensed phenolic units of the conventional pulp, providing additional evidence to support the fact that modern modified pulping technologies beneficially alter the structure of residual kraft lignin.Key words: phenyl groups, kraft pulping, modified kraft process, nuclear magnetic resonance (NMR), phosphorus spectroscopy.

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The quality of mixing of a pulp suspension and chlorine dioxide by a static mixer in an industrial chlorine dioxide bleaching stage was evaluated using electrical resistance tomography (ERT) as a function of process operating conditions, including chemical flow rate, suspension flow rate, and suspension mass concentration. The uniformity was quantified by a mixing index based on the coefficient of variation of the individual conductivity values in each image pixel. An increase in the mixing index, indicating lower mixing quality, was observed when the chemical flow rate increased. In addition, the mixing quality decreased with a decrease in suspension flow rate. On the other hand, a decrease in the suspension mass concentration at a constant volumetric suspension flow rate gave better mixing quality. The results show that ERT can be used to evaluate industrial-scale mixer performance and to monitor the changes in the mixing quality as a function of process operating conditions. The results are in good agreement with those in the literature based on other measurement techniques for similar mixer installations.La qualité de mélange d'une suspension de pâte et d'une solution de ClO2 par un mélangeur statique dans un stade de blanchiment industriel aétéévaluéeà l'aide de la tomographie par résistanceélectrique (ERT) en fonction du processus d'exploitation de conditions, y compris le débit de solution, débit de suspension et la concentration de fibre. L'uniformité aété quantifiée par un indice de mélange basé sur le coefficient de variation des valeurs individuelles de conductivité dans chaque pixel de l'image. Une augmentation de l'indice de mélange, indiquant une qualité inférieure de mélange, aété observée lorsque le débit de solution a augmenté. En outre, la qualité du mélange diminue avec une diminution de la vitesse d'écoulement de suspension. En revanche, une diminution de la concentration de fibre dans la suspensionà un débit constant de suspension volumétrique a donné un mélange de meilleure qualité. Les résultats montrent que l'ERT peutêtre utilisée pourévaluer sur le plan industriel le rendement du mélangeur et pour surveiller la qualité du mélange sous différentes conditions d'exploitation. Les résultats sont en bon accord avec ceux de la littérature basée sur d'autres techniques de mesure pour des installations de mélangeur similaire