Brightness reduction of calcium carbonate and calcium sulfate fillers in the wet end of a paper machine

This study is part of a broader research aiming at finding out mechanisms and phenomena causing the brightness reduction of mechanical pulp and fillers in the wet end of a paper machine. The main focus of this study was on the darkening of precipitated calcium carbonate (PCC) and precipitated calcium sulfate (PCS) fillers circulating in the short circulation. Brightness reduction of fillers was studied using a laboratory scale circulation device. Diluted TMP filtrate or deionized water was used as wire water in the circulations. Darkening of fillers in waters containing wood based dissolved and colloidal substances were assessed. Samples were taken during the circulation and optical properties of prepared sheets measured. In addition, the adsorption of wood based dissolved and colloidal substances on the fillers was studied using UV Raman spectrometry. The fillers were also circulated with the fines fraction of peroxide bleached TMP to study the brightness reduction of the fillers in the presence of fines. Both PCC and PCS fillers adsorbed wood based dissolved and colloidal substances from the water phase resulting in decreased brightness. PCC filler was more prone to adsorb these substances and was therefore more susceptible towards darkening. At low pH, dissolution of PCC results in formation of carbon dioxide. This was found to be very detrimental to brightness.