APPLICATION OF CATALYSTS IN TCF BLEACHING OF STRAW PULP

The polyoxometalates (POMs) as selective delignification catalysts were applied in TCF (totally chlorine free) bleaching. Brown pulp, straw pulp with a kappa number of 12.6, viscosity of 1076 ml/g, and ISO brightness of 33.8% was cooked. A TCF bleaching O(P)A(P)P(O) was performed with oxygen and hydrogen peroxide. O-P stage (oxygen delignification with hydrogen peroxide) achieved 71.4% degree of delignification with conditions as follows: alkali charge 4%, H2O2 1%, MgSO4 0.1%, Na2SiO3 1%, EDTANa(2) 0.1%, 120 min at 120 degrees C. The brightness of pulp could be improved by addition of H2O2 and polyoxometalate in A(P) stage. Compared with silicotungstic acid and phosphotungstic acid, phosphomolybdic acid was a better catalyst. The optimum conditions were: initial pH value 3, temperature 90 degrees C, H2O2 1.5%, phosphomolybdic acid 1.0%. After O(P)A(P)P(O) bleaching sequence, brightness was high to 84.5% ISO. And acceptable viscosity 669 ml/g, low kappa number 1.4 were obtained simultaneously. The results indicated that O(P)A(P)P(O) was a successful TCF bleaching sequence.