Metallurgy

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Metallurgy
Mining of Ores Ore Concentration or Dressing
Metal Extraction
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Mining of Ores

• Mining Methods Techniques
• There are two basic methods of mining
• Open cast
• Underground

The open cast method is used when the ore is near
the surface and little overburden (waste rock)
needs to be removed. Deeper ore are mined from
shafts or tunnels by underground methods. While
equipment used underground is limited by the size
of the shafts and tunnels, opencasts have no such
limitation and may use very large mobile
machinery.
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Opencast Mining
In opencast mining the topsoil and overburden are
first removed and stockpiled clear of the
boundary of the final excavation. Topsoil is
stockpiled separately so that it may be used for
rehabilitation. As the opencast increases in
depth the walls of the excavation are left at an
angle to avoid collapse. In shallow deposits the
oxidised material is often soft enough to be
removed by excavators or back-hoes. With
increasing depth the rocks are harder and it
becomes necessary to drill and blast before
excavating. Large tip-trucks carry the ore and
waste to the surface stockpiles.
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Underground Mining
Underground mining operations are more complex
than opencasts. Access from the surface to
underground orebodies is either by vertical or
inclined shafts, horizontal tunnels (called
adits) or downward sloping tunnels (called
declines). Mining of the orebody itself is called
'stoping'. There are a variety of stoping
methods. All methods involve drilling a pattern
of holes into the rock, charging (filling) the
holes with explosive, blasting the rock, bogging
(digging) it out and transporting it to the
surface.
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Other mining methods include Dredging in
Shallow water Is used when ore deposits lie under
water, the dredge is a purpose built ship which
has a chain and bucket, ore is brought ashore for
further processing while waste is deposited in
previously dredged area. Solution Mining Is used
to recover minerals or any mineral which will
dissolve in water. Water is pumped into ore
deposits using drilling rigs and dissolved
solution pumped to surface for treatment.
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Ore Concentration or Dressing
Before ores are sent for extraction it is
necessary to remove as much waste as possible.
The concentration or ore is increased. To remove
waste the ore is Crushed Ground Concentrated  
Some methods of concentration include
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Gravity or wet mill concentration Some ores are
heavier than the waste, a type of jolting box is
used to separate wet particles into different
layers depending on their specific gravities.
Flotation Use to separate two ores in a mixture
e.g. lead and zinc, which are nearly always
mined together (TARA Mines, Navan). These
mixtures consist of extremely fine particles
which are swirled around in large tanks. Air is
blown in through then ore and chemical agents
(flocculants) are added which produce froth and
sticks to the separate ores which will rise to
the surface or sick to the bottom and can then be
separated.
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 Magnetic Separation Finely ground particles are
passed over a rotating magnetic drum, the ore is
held on the drum until waste has passed and then
scraped off.
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• Amalgamation
• Rock or sand is crushed, these particles are
  mixed with mercury which sticks to the surface of
  ore. Copper plates collect all the coated
  particles as mercury sticks to copper, waste is
  removed mercury is recovered by distilling and
  ore is purified. Largely used in gold mining and
  removing gold and silver from tailings (name
  given to mining waste).

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Metal Extraction
The methods by which metals are extracted from
ores are   - Pyrometallu rgical methods -
Thermo-electrolytic or electrometallurgy
methods - Hydrometallurgical methods
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Pyrometallurgical methods   As the name implies
involves heat. Used where the metal ore occurs as
an oxide, as iron or tin. Ore is mixed with a
reducing agent (for iron coke) and a flux. When
mixture is heated in a furnace molten metal, slag
and gas are produced. E.g. The Blast
Furnace   Ore Reducing agent Flux Metal
Slag Waste
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Hydrometallurgical methods   Ore is dissolved in
a solvent, to make solution, in a process known
as leaching, Solutions are treated by
electrolysis, which results in the separation of
the metal.
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Thermo-electrolytic or electrometallurgy
methods Used to extract metals from fused mineral
mixtures or from salt particles. It is carried
out using an electrolytic process similar to
electroplating. Where the electrolyte is molten
alumina mixed with cryolite. Al. collect at the
cathode and oxygen at the anode. Aluminium,
Magnesium and Beryllium are produced by this
method. It is very expensive and requires a lot
of electricity often refineries are near power
stations. (Aughinish Alumina in Limerick)