Environmental Engineering Reference
In-Depth Information
particles [7]. During column operation, bias rate can become negative as a result
of an excessive gas flow rate, entraining most of the wash water over the top of
the column, diminishing the cleaning action of the froth zone. On the other hand,
increasing bias rate would affect the residence time of ascending bubbles and that of
settling particles in the collection zone, thus altering the recovery. Its action may also
reflect on the froth zone recovery. Unfortunately, all these observations are mostly
qualitative since bias rate measurements, particularly in three-phase systems, are not
very reliable, as shown later on.
6.2.3 Gas Hold-up
This represents the volumetric fraction of gas in the considered zone (froth or col-
lection). It is estimated through Equation 6.1, where the term volume might refer to
the whole zone (overall gas hold-up) or part of it (local gas hold-up):
Volume of bubbles
Total volume
ε g
(
%
)=
100
·
(6.1)
Since gas is often considered as a flotation “reagent”, and thus influencing the
concentrate recovery, plant metallurgists have been interested in monitoring the gas
dispersion within the column using the air hold-up. Its value strongly depends on
prevailing values of gas rate, frother concentration, bias rate and solid concentration.
6.2.4 Bubble Size
This is defined as the diameter of an equivalent spherical bubble ( D b ). Note that
bubbles in different sizes give rise to a bubble size distribution (BSD) as shown in
Figure 6.2(b) here represented by a histogram. The collection of mineral particles
by bubbles greatly depends on the amount of bubble surface available as they pass
through the column. Therefore, both their surface and their ascending speed are
important. Consequently, a more adequate way of monitoring the influence of gas
on the flotation process is to use the amount of bubble surface area per unit time of
a cross-sectional area of the column instead of the gas hold-up. This variable, called
bubble surface area flux, or S b , can be evaluated from the number of bubbles per unit
time ( n b ), the surface of one bubble ( A b ) of diameter ( D b ), and the cross-sectional
area of the column ( A c ) as shown in the left part of Equation 6.2 [5].
n b A b
A c
6 J g
D b
S b
=
=
.
(6.2)
Figure 6.3 illustrates the bubble surface area flux concept. Assuming a suitable
mean bubble diameter, it can be estimated by the right part of Equation 6.2, where
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