Observations of the bubble dynamics in a pulp suspension using flash X-ray radiography

A better understanding of bubble dynamics in an air/water/fiber suspension is important to many paper processing areas, including flotation deinking and gaseous bleaching. However, using conventional visualization techniques to view gas bubbles in pulp suspensions is nearly impossible because these systems are opaque at consistencies relevant to the paper industry. Therefore, alternative tools and techniques must be developed to characterize bubble behavior in these complex systems. In this study, flash X-ray radiography (FXR) is used to visualize air flows in a suspension of unprinted old newspaper (ONP) at various consistencies and air injection rates. FXR provides stop-motion images of air bubbles rising through the opaque ONP suspension. The technique is effective because air has a significantly different X-ray attenuation coefficient than water or wood pulp, which have similar attenuation coefficients. Qualitative observations of these air/water/fiber flow conditions are reported and compared with those obtained in a simple air/water system. The radiographic images reveal that agas flow characteristics are substantially altered in the presence of fibers. The results demonstrate that gas flows in a simple air/water system are not the same as those in an air/water/fiber system.