Introduction - Advanced Control and Supervision of Mineral Processing Plants - page 4

1.2.2 Grinding Circuits
The crushed material is then processed by wet grinding circuits. In the case of the
most commonly used equipment (tumbling mills and tower mills) size reduction
is accomplished through a combination of impact, chipping, and abrasion events
caused by energy transferred from grinding media, such as balls, rods or large par-
ticles. The primary grinding stage can be carried out by rod mills, in which the
grinding media consists of iron rods, by autogenous mills, which use large ore parti-
cles, and by semi-autogenous mills, which use a small load of balls. The final stages
of grinding (secondary and rarely tertiary) are normally accomplished by ball mills,
where the grinding media is made of balls or slugs. In a very few cases, ball mills
are also used as a primary grinding stage. The discharge product of the ball mills re-
quires size classification via hydrocyclones or mechanical (rake, spiral) classifiers,
to produce a fine stream (cyclone overflow), which feeds the separation circuit, and
a coarse stream (cyclone underflow) which is sent back to the ball mill for further
grinding (liberation).
1.2.3 Separation Circuits
Only flotation separation processes will be considered in this topic. A flotation cir-
cuit is usually composed of various sub-circuits (sections) with each having a spe-
cific objective. Each section is composed of a series of interconnected tank cells or
banks of conventional cells, where flotation takes place. Tailings from the first cell
become the feed of the next cell of the bank and so on; the tailing stream of the
last cell in the bank is sent to a different bank or section for further processing, or
discarded from the circuit (to the tailings pond). Usually, the concentrate stream of
each cell in a bank or section are combined into an overall concentrate, which is then
directed to the next section for further processing, unless it is the final concentrate
of the circuit in which case it is sent to the dewatering stage. Sometimes, regrind-
ing or thickening units exist within the flotation circuit. Regrinding is necessary
when floating two minerals with very different optimal particle size distribution or
when final concentrate grade cannot be achieved because of gangue contamination
in non-liberated particles. Thickeners may be needed to increase the solid content
of slurry. Before the actual flotation process, slurries need to be conditioned in an
agitated tank, called a conditioner, where appropriate chemical reagents are added.
More additions might be needed further down the circuit. Three types of sections
are normally encountered in a separation circuit: rougher, scavenger and cleaner.
The rougher section, the first after the grinding circuit, is fed with the classifier fine
stream, the objective being the recovery of as much valuable material as possible
with some disregard for the product quality (grade). Here, most of the fast floating
valuable minerals are separated by flotation from the slurry and recovered from the
froth of the concentrate. Tailings from the rougher stage are usually exhausted in
the scavenger section, where maximum recovery is the target. The tailings of the