Introduction - Advanced Control and Supervision of Mineral Processing Plants - page 5

scavenger section usually become the final circuit tailings, whereas the concentrate
is normally recirculated to the rougher feed for reprocessing. The rougher concen-
trate is processed in one or more cleaner sections (re-cleaners) where an improved
(optimal) concentrate grade is the aim. Since this typically implies getting rid of en-
trained gangue particles, cells having froth-washing capabilities (flotation columns)
are preferred. Columns usually outperform conventional mechanical cells in clean-
ing stages (better product grade) due to their particular froth-washing features and
thus, improved product grade. Whereas flotation columns do not have any mechan-
ical agitator for slurry suspension (air bubbles injected at the bottom keep the parti-
cles in suspension), conventional cells achieve pulp dispersion through agitation by
mechanically driven impellers. These cells employ one of two types of pulp flow and
aeration systems: cell-to-cell flow with adjustable weirs between cells or open-flow
without weirs and air intake via suction resulting from the operation of an impeller.
When dealing with multi-valuable species ores, the processing scheme can be of
two types: selective flotation, where all valuable minerals are separated sequentially
(one at a time in its own circuit, the tailings of the first mineral circuit becoming
the feed for the next mineral circuit, gangue is mostly found in the last circuit tail-
ings), or bulk flotation, where the separation of gangue from the all valuable mineral
is accomplished in a single circuit, whose concentrate (bulk concentrate) is selec-
tively processed in a separate circuit. Bulk flotation is typically used for Cu-Mo
ores, whereas selective flotation is used for Cu-Zn ores.
1.2.4 Dewatering Circuits
Depending on the degree of water elimination required, dewatering circuits can be
composed of one or more of the following processes: thickening (up to 65% solids),
filtration (up to 93% solids) and drying (up to 99% solids). Thickeners are a specific
type of sedimentation equipment which operates continuously, performing a wide
variety of tasks in the mineral industry. Their most common application is the en-
hancing of the solid content of mineral concentrates (for transportation purposes and
in respect of net smelter return contract clauses), as well as recovering water from
flotation tailings (reclaim of the excess of water for process re-use). The objective of
these units is to produce clean overflow water and maximum solid concentration in
the underflow. Flocculants are normally used to agglomerate the solids to increase
the settling rate and improve the overflow clarity. The use of synthetic flocculants
has increased the efficiency of sedimentation rates allowing increased production.
Filtration is used when even smaller amounts of water are permitted (typically the
case of concentrates); filters are then fed by the thickener underflow. Filters typi-
cally operate by passing the mineral slurry through a membrane (cloth, ceramics,
etc. ) whose pores retain the solid particles letting the liquid pass through. In some
cases, the solid discharge is continuous (vacuum filters such as belt-, drum- and
disk-filters) and in others the solid is discharged every few minutes (pressure plate-
and-frame filters). Further drying of the solid product (filter cake) can be achieved