Environmental Engineering Reference
In-Depth Information
Flotation column operation is characterized by specific variables, namely froth
depth, bias rate, gas rate, gas hold-up, bubble size and bubble surface area flux. The
last three variables, known as gas dispersion properties, partly define the hydrody-
namic conditions which govern the metallurgical performance, and as such, current
research has centered on gas dispersion as being the key hydrodynamic character-
istic of flotation. All these variables are defined hereafter and their measurement is
explained in more detail in Section 6.3.
6.2.1 Froth Depth
As its name indicates, this variable ( H f ) corresponds to the distance between the
top of the column (overflow) and the position of the pulp-froth interface. It deter-
mines the relative height of cleaning and collection zones, and consequently, the
mean pulp residence time in the collection zone of the column. As such, it strongly
contributes to the value of the collection zone recovery, although other variables
such as gas flow rate, bubble diameter, etc. also have an influence. As a result of its
integrator behavior, it must be closely monitored and controlled for stable column
operation. This variable is currently measured by devices described in Section 6.3
and its regulatory control loop is normally implemented in most mineral separation
plants (concentrators).
6.2.2 Bias Rate
Perhaps the most characteristic feature of flotation columns is the addition of a fine
spray of water, called wash water, at the top of the device, on the surface of the
overflowing concentrate stream. In addition to stabilizing the froth and facilitating
the removal of the bubble-particle aggregates from the concentrate launder, this
stream washes down to the pulp zone, hydrophilic particles that might have been
entrained with these aggregates. This washing action can only take place if a proper
water mass balance is assured in the lower part of the column, i.e. , the collection
zone, so that part of this wash water can make its way down the froth zone.
This net downward water stream is called the bias rate [5]. For scaling purposes,
it is commonly expressed as a superficial bias rate ( J b ), i.e. , the bias flow rate (cm 3 /s)
divided by the column cross-sectional area (cm 2 ). The bias rate is said to be positive
if water moves down the column, and negative otherwise. When this latter case
occurs, the concentrate becomes stained with entrained hydrophilic particles and
the grade decreases, which is a non-desirable situation. While some authors claim
that a slight, but positive, bias rate ensures an adequate cleaning action [5], others
have established that small values do not contribute to reducing the entrainment
of gangue [6]. On the other hand, high bias rates (above 0.4 cm/s) are detrimental
since they might generate increased froth mixing and thus, drop-back of collected
Search WWH ::




Custom Search