Technology For Increase In Concentration Of Effluent Recycling Of Chemical-mechanical Pulp

The solids concentration of effluent of chemical-mechanical pulp in the alkali recovery islow(0.8~2%), which causes more steam consumption in evaporation concentration. A newlyimproved process for P-RC APMP is studied in the paper: Comparing with the conventional flow,the characteristics of improved process is that four twin-screw extruders in series are addedbehind the bleaching tower, with the high consistency and countercurrent extraction of bleachingeffluent. One part of the extracted wastewater is sent to alkali recovery, the other part is used forcompounding bleaching solution in the high consistency mixer by closed circulation ofwastewater. The transformation method also is suitable for the existing BCTMP flow. Thecombustion value of solid matter and the content of organic matter are tested. The results show:1. The wastewater in the bleaching reaction of second stage pre-impregnation is extruded,and the wastewater that is regarded as clear water is used for preparing bleaching solutionrecycling in the second stage pre-impregnation. As the cycle number increases, solid content andCOD change similarly: at the beginning presenting an increasing trend, then being more stable;the content of residual hydrogen peroxide slowly increases, then becomes stable; along with theincrease of cycle number, brightness and strength of paper as well as pH mildly fluctuates; assolid content of effluent is concentrated from4.36%to6.70%, wastewater circulation has a slightinfluence on pH, residual hydrogen peroxide, and brightness and strength of paper, but CODlargely increases.2. In the unimproved P-RC APMP flow, because bleaching effluent is diluted in latencychest and multi-disc washer, large quantity of water(7930.4kg/t·pulp) is consumed for theextraction of bleaching effluent at the extraction rate of95%, and the dilution factor is4.43,solids concentration of the effluent is0.08%; in the newly improved P-RC APMP process, inletconcentration of every twin-screw extruder is about20%, and the output concentration is about40%, with the high consistency and countercurrent extraction of bleaching effluent; the extractionrate of effluent is95%, which is realized by using less amount of water of1910.84kg/t·pulp, andsolids concentration of the effluent nears to5%, dilution factor is0.81; steam consumption ofeffluent of solid content5%reduces by85.3%, comparing with that of effluent of solid content0.8%, when one ton of effluent of solid content50%is produced in evaporation concentration.3. In the improved BCTMP flow, inlet concentration of every twin-screw extruder is about20%, and the output concentration is about40%, with the high consistency and countercurrentextraction of bleaching effluent; the extraction rate of effluent is95%, with amount of extractionwater of1866.87kg/t·pulp, and solids concentration of the effluent nears to4%, dilution factor is0.78; steam consumption of effluent of solid content4%reduces by81.3%, comparing with that of effluent of solid content0.8%, when one ton of effluent of solid content50%is produced inevaporation concentration.4. The combustion value of solid matter and the content of organic matter are tested. Thecombustion value of effluent(12027kJ/kg) is close to that of carbohydrate; acids for exampleazelaic acid and benzonic acid account for50%of total organic, vinyl compounds take up30%；alkane compounds contribute to15%, number of carbon atoms of organic matter are centralizedin the C7,C9,C10,C11,C21, but relative content of C12,C15,C16,C18is low.