An imaging technique for sizing bubbles in flotation systems /

A sampling-followed-by-imaging technique for sizing bubbles in flotation systems has been developed and validated. The technique is based on directing a sample of bubbles into a viewing chamber where they are exposed and digitally imaged using incoherent backlighting. The images are automatically analyzed by means of a threshold criterion. The chamber is sloped to spread the bubbles over the surface of the viewing (glass) window. This configuration reduces bubble overlap and facilitates the definition of the focus plane.To validate the images, measurements were compared with standardized bubble sizes obtained with the displaced-volume method. It was observed that when thresholding at half the intensity level, sizes were systematically underestimated and the bias was correlated to bubble size, increasing from 0.3% at ca. 0.7 mm to 2% at ca. 4 mm. Experiments were designed to evaluate the bias due to system optics (i.e., the contribution of reflected/refracted rays and the defocus distance to image formation) and due to the sloped window. The results showed that reflected/refracted rays slightly biased towards underestimated sizes whereas the inclined window caused a small overestimation. The overestimation was due to the bubble flattening caused by the gravity force. These two effects were of similar magnitude and, therefore, tended to cancel. Consequently, the underestimation trend was attributed to the defocus distance which, in this unique system, is small and unambiguous (i.e., it is invariably negative since bubbles flow "behind" the focus plane, the underside of the window). A semi-empirical correction procedure is suggested which uses the fundamental method of Bongiovanni et al. (1997) to correct the bias due to system optics along with an empirical approach to compensate for bubble flattening.To verify whether the computed size distributions were representative of the actual dispersion, bubble populations with significantly different size classes were mixed in a bubble column (0.1 m x 2.5 m). The column was equipped with two independently aerated spargers with nominal porosities of 0.5 mum (generating bubbles from 0.2 mm to 1 mm) and 100 mum (generating bubbles from 1 mm to 5 mm), respectively. Various proportions of airflow rates were injected through each sparger. The results showed bi-modal distributions in which the real and calculated proportions corresponded with a deviation of less than 15%, considered good given the severity of the test. However, the deviation was correlated to the superficial gas velocity (J g) and bias was consistently against smaller bubbles. The data were not sufficient to reveal the cause of the bias, but several possibilities were entertained.The technique was demonstrated in an industrial-scale flotation machine, where it was compared with a capillary technique. It was also used to determine the bubble surface area flux (Sb) in a micro-flotation cell, setting up a test of the proposed relationship between the flotation rate constant k and Sb for the pulp zone