Purification of peroxide-bleached TMP water by dissolved air flotation

Papermakers are continuously searching for better ways to overcome the problems related to accumulation of pitch and polygalacturonic acids (pectins) in process waters. Dissolved air flotation it a technique that could be,used as an internal cleaning stage to remove these substances. This study investigated, removal of pitch and galacturonic acids from peroxide-bleached, fiber-free thermomechanical pulp; (TMP) water by dissolved air flotation (DAF). Both inorganic coagulants and organic flocculants were applied to aggregate the detrimental substances before flotation. The smallest chemical doses that caused aggregation were determined before DAF experiments by turbidity measurements. We also used focused beam reflectance measurement (FBRM) to assess aggregation. Up to 90% of pitch could be removed by DAF at pH 5, determined by gas chromatography and turbidity assessment. At pH 8, the anionic charge of detrimental material increased, accompanied by increased consumption of cationic chemicals. In addition, resin acids dissolve at pH 8,impairing their removal. Aggregation and flotation should therefore be carried out at-acidic conditions to increase the aggregation of resin acids and to avoid excessive chemical consumption. Pectins were selectively aggregated. The results indicate that understanding the chemistry of aggregation and flotation is necessary for selection of flocculation chemicals, adjusting process conditions, and for obtaining an acceptable cleaning efficiency.