| As in mineral processing in the 1980's, column flotation technology has great potential in de-inking of waste paper. Although column flotation shows excellent brightness gain and ink removal efficiency, one problem is the lack of accepted scale-up and design procedure as in mineral flotation. Laboratory and pilot columns were operated on paper pulps to evaluate the effects of gas dispersion and mixing on scale-up. Gas dispersion properties were measured by gas holdup (ϵg), gas rate (Jg ) and the derived parameter, bubble surface area flux ( Sb). To calculate Sb, bubble size (db) was estimated from drift flux analysis. Direct measurement of db using a photographic technique supported the estimation. Mixing was modeled by relating the vessel dispersion number (Nd), determined from residence time distribution measurements made using a conductivity technique, to design and operating variables. From the mixing model, the pulp zone flotation rate constant ( kc) was determined from measured ink recovery vs. time data. The kc was correlated with gas dispersion and showed a linear dependence on Sb, supporting recent findings in mineral flotation. A linear relationship was also found with ϵ g. The improved reliability of scale-up achieved by incorporating the kc α Sb relationship is demonstrated. |
