CLOSING THE WASH LIQUOR CIRCULATIONS IN TCF BLEACHING - IMPACT ON THEOZONE STAGE

Closing of the wash liquor circulations in totally chlorine free (TCF) bleaching will cause dissolved organic material to accumulate in the recycled liquors. This organic material has been found to retard ozone breaching. The objective of this investigation was to acquire jnforma tion that would make it possible to quantitatively estimate this effec t. To this end, laboratory bleaching trials with filtrate recycling we re made in-order: a) to determine the amount of organic material that is released in various TCF bleaching stages, and b) to examine the eff ect of recycled dissolved organic material from various sources on ozo ne stages. In this connection, it was further found that a significant share of the dissolved organic material from alkaline stages is reads orbed onto the fibres if recycled to an acid ozone stage. this readsor ption of material affects the concentration of organic material in the filtrate flows and must be taken into account in material balances. T he results of the laboratory trials can be used for estimating the eff ect of system closure on the increased ozone consumption caused by the recycled dissolved organic material. An example is presented, in whic h the results of the laboratory experiments are used in a simulation p rogram for calculating the effect of closing a (Z/Q)(PO)(Z/Q)(PO) sequ ence to an effluent level of 3 m(3)/adtp.