Towards Closed-Cycle Kraft Processing: Mill Balance for Tasman Pulp and Paper Company LTD, 1998 International Chemical Recovery Conference Proceedings

Extensive mill sampling has led to the development of a series of detailed mass balances for Tasman Pulp and Paper Co. Ltd’s (TPPL) bleached kraft pulp mill at Kawerau, New Zealand. The balances were completed to determine the source, behaviour and fate of key inorganic elements within pulping, bleaching and chemical recovery process areas. Steady state computer models of the process were also developed to allow reliable prediction of behaviour for the components of interest.

Silicon concentrations were found to be high in many process areas when compared with similar international data. This behaviour was considered to be directly related to the mills location in an active geothermal region.

Elevated silicon concentrations are implicated in scale formation during black liquor evaporation. As a consequence, further process closure would require specific attention to silicon behaviour and purging to avoid excessive scaling.

Potassium was also observed in greater than expected levels throughout the process. No direct explanation was found for this. Periodic pulping of eucalypt species before sampling may have contributed to an apparent increase of potassium input with wood. Significant quantities of calcium, magnesium and manganese were present in the bleach plant filtrates. Although these elements are purged in green liquor dregs, process closure through filtrate recycle may require increased purge efficiency or development of additional purge points to prevent accumulation.