Metallurgy

Mining of Ores

Ore Concentration or Dressing

Metal Extraction

Mining of Ores

Mining Methods & TechniquesThere 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.

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.

Opencast 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.

Underground Mining

Other mining methods include

Dredging in Shallow waterIs 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 MiningIs 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.

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

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.

 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.

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).

Metal Extraction

The methods by which metals are extracted from ores are:

 

Pyrometallu· rgical methods

- Thermo-electrolytic or electrometallurgy methods

- Hydrometallurgical methods

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

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.

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)