The Effect of Washing Between Stages on Brightness Reversion for Sodium Hydrosulfite on Unbleached High Yield Pulps

The objective of the project is to determine the effect of bleaching with washing between stages on high yield unbleached pulp. The bleaching chemical used for this experiment is sodium hydrosulfite. Sodium hydrosulfite is a lignin preserving bleaching agent, which is necessary due to the high yield pulp. The lignin concentration in high yield pulp is extremely high compared to the normal amount found in other papers. The experiment will be done using a two stage bleaching sequence. The control sequence will consist of a direct charge of sodium hydrosulfite and the other two sequences will be done with washing between stages. After the bleaching stages are complete, they will be analyzed for brightness. The brightness pads that are made will then be placed into an oven in order to artificially age the sheet. This will allow for the testing of brightness reversion. When completed the results will show the effect of the washing between stages. This is done by comparing the amount of brightness reversion that has been done due to aging. It will also show the best concentration of the sequence. The effect washing has will be that as the pulp is washed the paraquinones that are formed will be partially washed out, this will allow for a reduction in the yellowing effect. As for the concentration, it can be seen that an optimal level of one to three percent on oven dry fiber is best. As the amount of sodium hydrosulfite in increased there is only a minimal effect on the overall brightness that is obtained. During the experiment there was an increase in brightness reversion after the washing stages. This is discussed further in the discussion section. The optimal charge was determined to be 1% sodium hydrosulfite. It was also determined that a 4% brightness reversion would be possible if oven aged for 18 hours