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the successful testing of these sensors in various mineral processing and tar sands
plants in North America.
Other sensors have also been proposed [44-47], all consisting of the extraction of
an aerated pulp sample from the flotation device and letting the two phases separate,
after which the gas phase volume can be measured. Obviously, these batch methods
are inappropriate for control purposes and therefore they will not be discussed here.
Grau and Heiskanen proposed a semi-continuous version of this phase-separation
method [48], whereby a pulp sample is continuously withdrawn from the flotation
cell and sent to the separation chamber or recycled back to the cell, depending on the
position of a by-pass valve. The gas measurement is made by liquid displacement
in an inverted graduated cylinder, which is obviously not a convenient approach for
process control purposes either.
6.3.4 Bubble Size Distribution (BSD) Sensor
For close to two decades, the practical evaluation of bubble size in flotation de-
vices has become a privileged field of research in several universities around the
world. Noteworthy research was carried out in Cape Town University (South Africa)
in 1989, McGill University (Canada) in 2001, Helsinki University of Technology
(Finland) in 2002, Universidad Federal do Rio Grande do Sul (Brazil) in 2003, and
CETEM (Brazil) in 2009. The first BSD sensor was developed in the late 1980s at
Cape Town University [49]. The UCT bubble-sizing sensor comprises a riser tube
and a water reservoir where a glass capillary tube is placed. A fraction of the bub-
bles that reach the reservoir are sucked into the capillary where they are transformed
into cylinders and their length and velocity are measured by two pairs of photo-
transistor-LED detectors. The suction rate is controlled by a peristaltic pump. The
total volume of gas collected is utilized to estimate the absolute size of the bubbles.
However, the diameter of the capillary tube imposes a lower bubble size limit. In
addition, the suction rate and the internal diameter of the capillary tube seem to
impose a maximum bubble size limit [45, 50].
More recently (2001), McGill University researchers developed a sensor for mea-
suring the bubble diameter in flotation devices from bubble image processing [51].
The McGill Bubble Viewer consists of a vertical tube submerged in the pulp, con-
nected to an inclined viewing chamber located outside the cell and a digital camera
placed perpendicular to the upper chamber wall (Figure 6.9). The viewing chamber
has transparent walls and a relatively narrow gap; its inclination angle (typically
20 o ) can be adjusted for better picture quality. Since the system works under vac-
uum, bubbles rise through the vertical tube, to finally reach the viewing chamber,
where they continue rising, sliding under the upper chamber wall until they leave by
the chamber's upper end into an ad-hoc container, which needs to be cleared once in
a while. A light source under the bottom wall of the chamber provides the necessary
light for taking pictures of the sliding bubbles.
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