The potential danger of heat surface scaling during the evaporation of acidic bleach plant filtrates is discussed. The results of ECF and TCF filtrate evaporation tests are presented. The purity of condensates is analyzed and their potential use to replace fresh raw water in bleached pulp washing is evaluated.
No severe scaling was observed on the heat surfaces. The precipitation seems to happen for the most part in situ in the liquid phase. This means, however that the evaporator should be designed so that it tolerates suspended matter in the evaporated liquid. When the pH of the filtrate is properly adjusted the purity of the condensates meets, after slight stripping, the requirements set for the wash water of bleached pulp.