Effects of Alkaline Treatment on Dissolved Carbohydrates in Suspensions of Norway Spruce Thermomechanical Pulp

The effects of alkaline treatment on dissolved carbohydrates in suspensions of Norway spruce thermomechanical pulp (TMP) were studied by agitating 1% consistency TMP suspensions at different pH values (5.4 to 12) for 3 h at 60°, followed by acidification and centrifugation to obtain water samples for analysis. It was found that at pH values above 8, dissolved galactoglucomannans were deacetylated, resulting in their substantial adsorption onto the TMP. Galacturonans began to dissolve from the TMP at pH values greater than 8. The amount of arabinogalactans apparently remained constant throughout the various alkaline treatments. At pH values above 10, arabino-(4-O-methylglucurono)xylans began to dissolve from the TMP.

The amount of dissolved and colloidal substances (DCS), measured by TOC, could be substantially decreased by alkaline treatment of filtrate from the 1% consistency suspension, followed by acidification and remixing the filtrate with its original pulp. At 10 m³/t specific water consumption, the TOC of laboratory-prepared circulation water dropped about 20% due to the alkaline treatment. The decrease in TOC was attributed mostly to the adsorption of deacetylated galactoglucomannans. Alkaline treatment of fiber-free mechanical pulp circulation water may be a useful technique to reduce the amount of organic substances circulating in process waters and leaving the pulp or paper mill in the effluent stream.