Effect of physico-chemical conditions on the properties of zeolite microparticles used in pulp and paper applications

Zeolites are framework aluminosilicates that have been proposed as a replacement for bentonite in microparticle retention systems. It has been claimed in many patents that zeolite can improve sheet formation in fine paper manufacturing. However, this improvement was not observed in mechanical grades. These experimental results, and also theoretical evidence, suggest that papermaking pH plays an important role in zeolite efficiency. In consequence, the effect of pH and temperature on the chemical properties of zeolite and bentonite microparticles was determined. Streaming potential, conductivity, and pH were measured while microparticle solutions were gradually acidified to pH 5. Samples were also taken at specific pH values to measure the cationic demand. Results showed that a gradual reduction of pH caused several modifications to zeolites. The first step was the neutralization of free hydroxyl ions in solution. The second step was an irreversible exchange of the charge-compensating sodium ions by hydrogen ions on the inner and outer surface of microparticles. Finally, dealumination of the zeolites occurred. It was determined that neutralization must be completed before ion-exchange can begin. However, ion-exchange and dealumination occurred simultaneously. Between pH 7.5 and 9, zeolites had more anionic sites than bentonite (around 0.8 meq/g vs. 0.5 meq/g). The higher surface charge of zeolites might partly explain their efficiency in microparticle systems. On the other hand, under pH 7.5, the anionic charge of zeolite samples rapidly decreased while that of bentonite was stable. The effect of temperature was also studied in the 25 to 55 °C range. Generally, a temperature rise promoted a greater dissociation of the surface hydroxyl groups and an acceleration of ion-exchange and dealumination processes.