Effects of White Water Recirculation on the Performance of Optical Brightening Agents

In this study, we investigated the effects of the number of white water recirculation rounds on the whitening performance of added optical brightening agents (OBAs). The results could be useful in practical mill application to determine the variability estimates when using OBAs in printing and writing paper production. The variables of this study included OBA types [disulfo-OBA (2S-OBA) and tetrasulfo-OBA (4S-OBA)], dosage (0, 0.3, 0.6, 1.2, and 2.4% of the dry pulp), and the number of white water recirculation (0~7) rounds. The particle charge (PCD), pH, OBA retention, etc. of the white water samples were tested. The brightness, apparent whiteness, and ash content of the handsheets prepared from each round were tested.. Results of this study indicated that during the stock preparation process, the bleached hardwood kraft pulp (BHKP) and calcium carbonate filler did not notably affect the electrical charge of white water; while the addition of OBA and cationic starch modified the white water charge balance. The addition of 4S-OBA significantly influenced the PCD; while the effect of adding 2S-OBA was less significant. Adding either OBAs had no appreciable effect on the white water pH value. In the 2S-OBA group, the OBA dosage and number of white water recirculation rounds had little effect on the PCD of the white water; whereas in the 4S-OBA experimental group, with an increasing dosage and number of white water recirculation rounds, the PCD moved toward more-negative charges. The recirculation retention rates of the 2S-OBA and ash (filler) increased with its dosage and the number of white water recirculation rounds; those of the 4S-OBA, however, decreased with increasing dosage and number of white water recirculation rounds. Under the same dosage, the 4S-OBA tended to have a better whitening effect than the 2S-OBA. Also, the tonal stability of the 4S-OBA was superior to that of the 2S-OBA.