Abstract
The present paper is concerned with a series of laboratory scale trials on recycling for reuse of waste liquor and washings in the bleaching of birch kraft pulp. The pulp was bleached in a sequence of oxygen-alkali, peracetic acid, and hydrogen peroxide (O-PA-P), and washed countercurrently as shown in Fig. 1. Pulp brightness could be maintained at 90 without any effluent during at least 10 recycles, if the pH of O-and P-bleaching liquor were kept optimum by addition of NaOH. The amount of fresh water used in this case was roughly estimated at 5 m3 per ton of pulp, but this system required 1.5% (on o. d. pulp) of additional NaOH at O-stage. The additional NaOH could be omitted by increasing in the amount of fresh washing water to 10 m3 per ton of pulp, which may, in turn, increase the load of evaporator. As compared with conventional sequence (O-D-E-D), in respect of pollutant loads, countercurrent washing reduced total colour and total COD into 1/9-1/10 and 1/8-1/9, respectively, after 10 recycles.
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