This study examined the brightness reduction of peroxide-bleached mechanical pulp and paper fillers in the wet end of a paper machine. The effect of different process variables was studied on a laboratory scale using a device that simulates the short circulation of a paper machine. The fines-containing wire water was circulated in the device and wire water samples were taken during this circulation. Fines sheets were prepared from the samples and their optical properties were measured to explore the darkening of the fines fraction during the circulation. In addition, the darkening of the fiber fraction and paper fillers was studied. The results indicated that the brightness reduction of the peroxide-bleached mechanical pulp in the wet end of a paper machine was partly due to the formation of light-absorbing chromophores in the fibers and fines and partly due to the adsorption of light-absorbing substances to the surfaces of fibers and fines from the water phase. Paper fillers do not contain lignin or other structures with a high darkening tendency and their brightness reduction was caused by the adsorption of light-absorbing dissolved and colloidal substances (DCS). Pulp washing was found to be an efficient way to reduce the tendency for darkening of the pulp itself and also the fillers, because it removes light-absorbing substances from the pulp suspension. In addition to the wood-based DCS, the most important factors affecting the brightness reduction of peroxide-bleached mechanical pulp were the residence time of the pulp in the wet end and the iron concentration of the white water. Other factors decreasing the brightness notably, but to a lesser extent, were high shear rate generated to the pulp, carbon dioxide gas, chloride ions, and calcium ions. No change was noted in the light absorption of the peroxide-bleached mechanical pulp between acidic and neutral test conditions. Thus, from brightness point of view, paper grades from peroxide bleached mechanical pulp can be manufactured equally well at neutral or at acidic process conditions. The Precipitated Calcium Carbonate (PCC) fillers were found to have somewhat higher tendency for darkening than the Precipitated Calcium Sulfate (PCS) filler.
|Translated title of the contribution||Peroksidivalkaistun mekaanisen massan ja täyteaineiden vaaleuden aleneminen paperikoneen märässä päässä|
|Publication status||Published - 2013|
|MoE publication type||G5 Doctoral dissertation (article)|
- brightness reduction
- mechanical pulp
- wet end