Automatic Control of Flotation Columns - Advanced Control and Supervision of Mineral Processing Plants - page 252

the column overflow exhibits much greater air content (above 70%); thus it is named
the froth zone.
As mentioned before, water is usually sprayed on the overflowing froth. Provided
adequate conditions are met (water balance in the upper zone and gas rate value),
part of this water finds its way down to the bottom, washing down from the froth
zone any liberated hydrophilic particles that could have been hydraulically entrained
there by the ascending bubbles. In this form, the overflow concentrate will be freed
from undesired gangue particles, thus improving its final grade. As a result of this
washing action the froth zone is also called the cleaning zone.
The sine qua non condition for an effective flotation separation is the different
behavior with respect to water and air, of the various mineral components of the pro-
cessed ore. Some particles, typically the valuable mineral, will attach themselves
to air bubbles (hydrophobic) whereas others, typically the undesired minerals or
gangue, will remain in the water phase (hydrophilic). Almost all minerals are natu-
rally hydrophilic. To render some minerals hydrophobic, while keeping hydrophilic
the remaining ones, various chemical reagents are generally required: collectors, ac-
tivators, depressants and pH modifiers. These chemicals are normally added in the
preliminary stage of pulp conditioning. However, chemical reagents may be directly
added to the flotation device. The reagent addition has obviously an important im-
pact on the metallurgical performance as it will determine the ability of the valuable
particles to be “collected” by the bubbles and thus transferred to the concentrate. Air
bubble properties also play an important role in the collection process; for instance
smaller bubbles exhibit a greater specific area (area/volume) and thus have a greater
particle-carrying capacity. Bubble diameter also influences the ascending speed of
bubbles, whereas its volume affects the buoyancy characteristics of the particle-
bubble aggregates. The reagent used to modify the bubble properties, acting only
upon the air-water interface, is the so-called frother, typically a water-soluble al-
cohol. Preliminary laboratory testing is usually carried out to determine the proper
dosage of all these reagents.
Figure 6.1 Schematic repre-
sentation of a flotation column