Recovery of brightness in paper mill water

Abstract

This paper investigates the utilisation of nanofiltration (NF), agitation (by pump or sonication), freezing and acetone extraction methods to purify paper machine o-water. The results were judged by regained pulp brightness and reduction of various components. In the NF experiments the brightness was almost totally regained. Also the reduction of most measured components like lignin and carbon were retained well by the membrane (PES). The metal ions (Si, Al, Mg, Na, K, Mn, Ca, Fe and Cu) were reduced in the permeate (retained by the membrane) to the level found in the clear filtrate. This indicated that the metal ions (Si, Al, Mg, Na, K, Mn, Ca, Fe and Cu) were not brightness-reducing components. The flux was rather low for a commercial application because the membrane was a low-flux membrane. The reduction of sugar, colour and lignin residuals was quite high (>80%), and it was considered that these were possibly brightness-reducing components. It was found that by treating the fines with acetone and then returning them to the original water, the colour was reduced to a high enough degree that approximately 50% of the brightness reduction was recovered. It was considered that coloured contaminants that were absorbed on the fines were removed or destroyed. The brightness recovery was >50% for both the TMP and PK4 o-water. The brightness recovery was greater using NF, but in that case so many substances were reduced in the permeate that it was difficult to determine which were the brightness-reducing substances. However, in the case of acetone washing, the only component that was reduced was the colour. For both o-water PK4 and TMP the colour was reduced by 50% and the brightness recovery was 50%, suggesting a direct correlation. Agitation of the water samples by the means of a pump and sonication showed no change in the water, which included no changes in the brightness. The same was also found for water samples that were frozen. These experiments also showed that temperature changes did not affect water composition.