This paper evaluates different approaches for the (E*) stage of the DHT(E*)D sequence for eucalyp-tus kraft pulp bleaching to 90%–91% ISO brightness, considering kappa factors 0.12, 0.16, and 0.20 in the first
stage. These (E*) approaches included E, (EO), (EP), (EOP), (PO), PHT, O, and (O/O) stages run at two tempera-tures. Results were compared on the basis of total active chlorine demand, chemical cost, pulp brightness stability
and viscosity, and effluent load. We concluded that a DHT kappa factor of 0.12 and a (PO) stage run at 85°C–95°C appeared to be the most attractive approach for bleaching eucalyptus pulp with the DHT(E*)D sequence, consider-ing cost-effectiveness, pulp quality, and effluent load; however, when the steam cost is taken into consideration, the cost advantage was lost.
-The alternatives used for minimizing the usage of chlorine dioxide in bleaching sequences included a hot acid hydrolysis (A hot ) stage, the use of hot chlorine dioxide (D hot ) and ozone stages at medium consistency and high consistency (Z mc and Z hc ), in addition to stages with atmospheric hydrogen peroxide (P) and pressurized hydrogen peroxide (PO). The results were interpreted based on the cost of the chemical products, bleaching process yields and on minimizing the environmental impact of the bleaching process. In spite of some process restrictions, high ISO brightness levels were kept around 90 % brightness. Additionally, the inclusion of stages like acid hydrolysis, pressurized peroxide and ozone in the bleaching sequences provided an increase in operating flexibility, aimed at reducing environmental impact (ECF Light). The D hot (EOP)D(PO) sequence presented lower operating cost for ISO brightness above 92 %. However, this kind of sequence was not allowed for closing the wastewater circuit, even partially. For ISO brightness level around 91%, the A hot Z hc DP sequence presented a lower operating cost than the others.
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