The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 191

The 'mill' in a mining operation
comprises all activities related to
ore benefi ciation.

Conventionally the 'mill' in a mining operation comprises all activities related to ore
benei ciation. While the interior of a mill may seem to the visitor to be a bafl ing maze
of tanks, pipes, pumps, conveyors, motors, chemicals, pulps, and solutions, this seeming
confusion is actually a carefully designed system constructed for one objective - to recover
the valuable minerals locked up in the ore. The end product from a mill is called a concen-
trate, or in the case of gold and silver, a doré bar of the metal itself.
The most common benei ciation processes include gravity concentration; milling and
l otation (used for base metal ores); leaching (used for tank and heap leaching); dump
leaching (used for low-grade copper and low grade gold ores); heavy media separation
and magnetic separation. Typical benei ciation steps include one or more of the following
(USEPA 1995a,b): milling; washing; i ltration; sorting; sizing; magnetic separation; pres-
sure oxidation; l otation; leaching; gravity concentration; and agglomeration (pelletizing,
sintering, briquetting, or nodulizing).
●

Milling processes. All milling and concentrating processes begin with a crushing and
grinding stage, which is the most energy-intensive stage and which forms the largest
part of the costs of ore processing. As many as three crushing/grinding steps may be
required to reduce the ore to the desired particle size. Milled ore in the form of a slurry
is then pumped to the next benei ciation stage.

All milling and concentrating
processes begin with a crushing
and grinding stage, which usually
represents most of the total cost
of processing the ore.

●

Magnetic separation is used to separate iron ores from less magnetic material, and
can be classii ed as either high-or low-intensity (requiring as little as 1,000 gauss or as
much as 20,000). The particle size and the solids content of the ore slurry determine
which type of magnetic separator system is used.

●

Flotation uses a chemical reagent to make one or a group of minerals adhere to air
bubbles for collection. From its beginning at the turn of the 20th century, l otation
has gradually become the preferred method of separation for suli de minerals. Unlike
other methods, l otation has almost no limitations in separating minerals. Chemical
reagents include collectors, frothers, antifoams, activators, and depressants; the type of
reagent used depends on the characteristics of a given ore. These l otation agents may
contain sulphur dioxide, sulphuric acid, cyanide compounds, cresols, petroleum hydro-
carbons, hydrochloric acids, copper compounds, and zinc fume or dust.

Flotation has gradually become
the preferred method of
separation for sulfi de minerals.

●

Gravity concentration separates minerals based on differences in their specii c gravity,
and is a modii cation of gold panning. Nature has long practiced this form of mineral
concentration by leaving heavy and relatively indestructible minerals such as gold or
diamonds in placer or beach sand deposits. Agricola's famous treatise ' De Re Metallica '
demonstrates that, by the 16th century, gravity concentration technology was already
well advanced and a large number of devices had been developed. Beside specii c grav-
ity, the size of the particles being separated is important. Since small, heavy minerals
will settle as fast as large, light ones, particle sizes need to be kept uniform with clas-
sii ers (such as screens and hydro-cyclones). Today gravity concentration is most often
used in combination with other treatment processes.

●

Thickening/i ltering removes most of the liquid from both slurried concentrates and mill
tailings. Thickening is dei ned as removing a portion of the water from a slurry or sus-
pension, thereby concentrating the solid particles into the remainder. Gravitational thick-
ening is achieved by allowing solids to settle under the force of gravity in a sedimentation
basin. Liquid recovered from a thickener is usually recycled to a process water storage for
reuse at the mill. Chemical l occulants, such as aluminium sulphate, lime, iron, calcium
salts, and starches, may be added to increase the efi ciency of the thickening process.

Leaching is the process of
extracting a soluble metallic
compound from an ore by
selectively dissolving it in a
solvent.

●

Leaching is the process of extracting a soluble metallic compound from an ore by selec-
tively dissolving it in a solvent such as water, sulphuric or hydrochloric acid, caustic
soda or cyanide solution. The desired metal is then removed from the 'pregnant' leach