TCF-gebleichte Hochausbeute-Magnesium-bisulfitzellstoffe mit hohen Festigkeiten

Increased pulp strength is urgently required for most of the sulphite pulp mills. An earlier stop of the cook at Kappa numbers around 50 yields a spruce pulp with much higher strength. Applying these conditions, pulp yield is still above 55 %, The magnesium bisulphite process renders possible better pulp strengths than the acid sulphite process. In particular the breaking length of resulting pulps is above 10 km, even at low beating degrees. These pulps are chlorine free bleachable in a sequence Q-P M G O -(A/Q)-P N A O H -FAS to more than 85 % ISO brightness. Resulting strength losses are low. However, in both P stages 5 % H 2 O 2 have to be applied, but only 50 % are consumed. Recycling of the corresponding filtrate is possible without brightness losses. If technical MgO is used, H 2 O 2 has to be stabilized with sodium silicate. Yield of fully bleached pulps is around 54 %. The bleaching sequence Q-P M g o -P A A -P N a O H yields similar results as the one mentioned before. In the P A A stage 0.5 % peracetic acid has to be applied calculated as H 2 O 2 This modification of sulphite pulping can be applied in mills without major investments.