Effect of Aluminum Sulfate and Sodium Alumniate on the Drainage and Retention Properties of Fibrous Suspensions

The individual contribution of aluminum sulfate and sodium aluminate for improving drainage and retention properties of fibrous suspensions lias showed limited success as a retention aid. In industrial practice, alum and sodium aluminate additions are often not optimized for retention chemistry, but rather are added to control pH and to precipitate rosin for internal sizing. In this thesis work, the simultaneous addition of these alumin~unc ompounds was invesligated to study its effect on drainage and fine particle retention. Significant improvements in drainage rates and fines retention were observed for simultaneous aluminum addition compared to the individual contribution from each compound. Drainage rates were measured for recycled and mechanical pulps. Drainage rates were characterized by (1) measuring the freeness using the Canadian Standard Freeness (CSF) test and also by (2) determining the viscous drainage resistance coefficient using a dynamic drainage apparatus. For both methods, maximum drainage and retention occurred when a 40160% aluminum sulfate/sodium aluminate ratio was introduced into the pulp. The pH of each system also affected the drainage characteristics and degree of retention. Optimum drainage and retention occurred when the system operated at a pH of 4.5. At this pH level, a soluble alumina +4 polynler, A18(OH)20+4, was formed and had been shown to function as an effective retention and drainage aid. To utilize simultaneous aluminum sulfate (alum) and sodium aluminate as a drainage and retention system, it is recommended that both aluminum compounds are added to a level such that the headbox stock is at or near the isoelectric point. Since the pH of the system will vary with the level and ratio of alum and sodium aluminate added to the system, it is recommended that the pH of the system be controlled separately with the addition of sulfuric acid, hydrochloric acid, carbonic acid, or caustic. The alum-sodium aluminate system is thought to function by having the system at the isoelectric point of the stock. To achieve the desired pH it is important that the pH be controlled independently. Viscous drainage resistance and retention coefficients were measured for four drainage apparatus configurations and stirring speeds ranging from 0 to 1500 rpm. A baffle-and-impeller configuration provided the best drainage geometry and the highest retention compared to the other systems. The drainage resistance coefficients increased with the basis weight of the deposited mat for all systems. Fine particle entrapment in the mat during the drainage process may contribute to the increase in resistance