Environmental Engineering Reference
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The viewing chamber is initially filled with plant process water, to provide a
clear image of the bubbles, in other words, the bubbles are washed before entering
the unit. Frother can also be added to the image area to insure that the bubble size
does not change while the bubbles are rising to be photographed. To obtain a bubble
distribution representative of the flotation cell content, a significant number of pic-
tures must be taken, and later processed by image analysis software. The result of
the whole process is a series of pictures as shown in Figure 6.2(a), and a histogram
of bubble diameters as shown in Figure 6.2(b), which is later transformed into an av-
erage diameter (usually the Sauter mean diameter). At the end of the picture taking
process, the device must be emptied and refilled with clear water, since the view-
ing chamber must not contain a dense pulp which would prevent light from going
through the chamber. Moreover, as bubbles accumulate in the container, pressure
continuously increases whereas the liquid level in the chamber decreases, which
poses a constraint to the time span available for continuous measurements. As a
consequence, the sensor is not adequate for process control purposes in its present
form. This device has been successfully used in several concentrators world-wide
for diagnosis of the air injection system of mechanical cells and columns, but al-
ways using the discontinuous procedure detailed above. COREM (Consortium de
Recherche en Mineralurgie, Quebec, Canada) researchers have automated this de-
vice in such a way that it has been used providing semi-continuous measures for an
extended period of time [52]. Despite its on-line measuring capabilities, no effort
has been made so far for its use in process control applications. This point will be
raised in Section 6.5.
A new sensor was proposed by Helsinki University of Technology researchers
[53] based on the McGill Bubble Viewer design and including some practical fea-
Figure 6.9
McGill Bubble Viewer
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