Managing inorganic element behaviour in New Zealand kraft pulp mills

Extensive mill sampling has enabled development of accurate process models predicting inorganic element behaviour for the two New Zealand kraft pulp m ills. This paper summarises the inorganic element behaviour determined at b oth mills and contrasts this with the limited international data available, In addition, some observations of behaviour unique to these New Zealand mi lls are discussed, Silicon was observed to be present in high concentrations throughout both m ills. This observation follows from the high silicon content of soils and w ater in the central Volcanic plateau where the mills are located. High sili con levels could induce excessive scale formation during black liquor proce ssing with increased process closure. For this reason innovative research i s required to identify effective silicon purge points from the liquor cycle , Potassium was also found to be present in greater concentrations than expec ted in comparison with international mills of similar age and design. Tight precipitator operation and spill control may be responsible for the potass ium content of white liquor reaching 6.5 to 7.1 mole%. Potassium was enrich ed in precipitator dust, which could be used as a purge point for potassium accumulation.