The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 207

mineral particles are picked up by the bubbles of foam. They l oat to the surface as froth,
hence the name l otation. The unwanted gangue (host rock) falls to the bottom and is
removed as tailings. The froth is skimmed off the surface and the enriched ore (mainly the
copper mineral) is dewatered and stored or shipped as concentrate for further processing.
The mixture of water, foaming agent and parafi n is recycled. In the concentrating opera-
tion, minerals are separated from the gangue material of the ore that might contain less
than 1% metals to form a concentrate containing more than 30% metals.
Flotation has become the principal means of separating most metallic minerals as well as
some non-metallic ones. By selective l otation, whereby appropriate reagents are used, and
the alkalinity (pH) of the solution is controlled, almost any suli de mineral can be sepa-
rated from its associates, both ore and gangue, no matter how complex. Native gold, tung-
sten, and certain non-suli de minerals can also be isolated by l otation, but suli de minerals,
especially those containing copper, lead, and zinc, are most amenable to this treatment,
with silicates the least amenable (Pearl 1973).
From an environmental viewpoint the main issue of concentrating minerals is the gen-
eration of large amounts of i ne ground waste materials of no commercial value - the
tailings. These often pose the biggest environmental challenge associated with a mine
operation. How to dispose of tailings material in a safe manner? Safe refers to the physical
integrity of the tailings containment structures as well as the long-term quality of water
emerging from the tailings containment over time. Finding benei cial uses for tailings
storage sites after mine closure also poses challenges. Tailings management is discussed in
detail in Chapter Eighteen.
A second issue of particular public concern is the environmental impact of chemicals
used in concentration. Most of these are harmless to the environment and most concentra-
tion processes, including l otation, basically rely on physics to liberate ore minerals from
gangue. Notable exceptions are amalgamation and cyanidation of gold, which attracts
public attention because the two processes rely on the use of chemicals associated in the
public mind with, respectively, environmental disaster and murder, that is, mercury and
cyanide. The use of mercury is discussed in Chapter Five in the section on artisanal min-
ing; the use of cyanide to leach gold from rock is discussed in Chapter Six.

Flotation has become the
principal means of separating
most metallic minerals as well as
some non-metallic ones.

From an environmental
viewpoint the main issue of
concentrating minerals is the
generation of large amounts of
fi ne ground waste materials of
no commercial value- the tailings.

Thickening
Thickening is usually accomplished by settling slurries in large tanks, known as thicken-
ers ( Figure 4.18 ). Thickeners are used to remove liquids from concentrate slurries and
from tailings slurries. Concentrates exiting the l otation circuit typically with water con-
tents of 60 to 80 percent, are thickened to reduce the moisture content and reclaim the
process water and l otation reagents before the i nal dewatering. Slurried tailings may be
thickened to reclaim water and reagents and to facilitate i nal tailings disposal. Mills usu-
ally employ a number of thickeners concurrently. Typically, mills use continuous thicken-
ers equipped with a raking mechanism to remove solids. Gravity causes the solids to settle
to the bottom of the thickener, where they are scraped to a discharge outlet by a slowly
rotating rake. Several variations of rakes are commonly used in thickener. The thickened
solids, whether mineral concentrates or tailings, exit from the thickener as underl ow.
Dewatering the concentrate in a thickener, then in disc, drum, or vacuum i lters for
i nal dewatering, produces a relatively dry product, the i nal ore concentrate, ready for
further shipping and processing (Weiss 1985). The liquid component removed during the
thickening process may contain l otation reagents, and/or dissolved and suspended min-
eral products. The liquid is usually recycled for reuse at the mill.
Thickening of tailings is a common step prior to pumping the thickened slurry to the
tailings pond and ultimately disposing of the thickened slurry. Thickening minimizes the