Towards the Closed-Cycle Bleached Kraft Mill; Metals Management for Closed-Cycle Bleached Hardwood Kraft Mills, 1994 Pulping Conference Proceedings

P. Gleadow, C. Hastings, P. Lownertz

Closed-cycle operation with no process effluent discharged into receiving waters provides a definitive solution to concerns about the aquatic impact of bleached kraft mill effluent. Challenges in moving towards a closed-cycle goal include those involved with reducing the volume of water used and effluents generated; those surrounding the chemistry of pulping and bleaching with increased recycle of potential contaminants; and concerns on the costs and operability of mills. The opportunities for further bleached kraft development in Australia and the increased development activity in system closure prompted the Australian National Pulp Mills Research Program to include an assessment of closed-cycle technologies in a greenfield bleached eucalypt kraft pulp mill.

A case study approach was supported by extensive modelling of both process (pulp, water, energy, dissolved organics) and elemental (Na, S, K, Cl, CA, Mg, Si, Al, Fe, Mn and P) flows. Mature wood cases are based on ECF bleaching using a three stage D(EOP)D sequence. The fast grown plantation cases have TCF bleaching in a four stage QP(ZQ)(PO) sequence. Closed-cycle and open-cycle cases were developed in which alkaline bleach effluents were returned to post oxygen washing and acidic bleach effluents concentrated and added to the weak black liquor. Satisfactory chemical, energy and water balances are developed for all cases.