Measurement of pitch deposition by impinging jet microscopy: Effect of divalent salts.

Papermakers for many years have had to contend with the wood resins that are released during pulping and papermaking. As paper mills reduce water consumption by further process loop closure, there is potential for the problems to increase. Accumulated organic and inorganic material can lead to deposits on machinery, poor process control, loss in efficiency, and lower product quality. There are a number of factors, which have an impact on the deposition tendencies of colloidal particles as a result of different inorganic materials or salts. Along with the valency of the salt, the concentration of salt, the properties of the colloidal particles, and the chemical nature of the surface are important. Variations in colloidal stability and deposition are observed even for salts of the same valency. With the use of impinging jet microscopy (IJM) the deposition of pitch on to hydrophobic and hydrophilic model surfaces was measured and the effect on the rate of deposition was quantified with changing cation. On both model surfaces the pitch deposition was seen to be slightly faster when calcium at 800 mg/L was present than magnesium at the same concentration. This concentration is around twice the critical coagulation concentration. The concentration of calcium in process water in some paper mills can be 200 - 300 mg/L. With further process loop closure this could rise to levels being investigated in this study. The concentration of magnesium would normally be much lower than this unless magnesium based alkalis were to be used. The rate of deposition onto the model hydrophobic surface was far greater (up to a 2.5 times) than on the hydrophilic surface for both salts. Contact angle measurements inferred that in the air-surface environment the hydrophobicity of the surface doesn't affect its affinity for neat pitch suggesting that the pitch may be reforming on the surface. IJM results show variation in the pitch shape on the model surfaces. It is possible that molecular reorganization of the components occurs at the surface and water interfaces. On the hydrophilic surface the pitch particle size for both salts is about 0.33-0.35 ˜í¬¿m while for the hydrophobic surface the particle size is about 5 times more for the calcium salt than the magnesium salt. Film thinning or spreading of the pitch particles was observed to occur on the hydrophobic surface with calcium and to a lesser extent with magnesium salt. With time film thinning will affect the chemistry expressed by the surface and change the interaction of other process components in the paper making process.