Question

Explain extraction of metals.

Answer 1

The process of separating a metal from its ore and preparing it for use is called metallurgy. Some common processes used in metallurgy are discussed below:

Concentration (removing the impurities)
Ores mined from the earth’s crust will contain impurities. In fact, the desired metal contained in the mineral ore will constitute a small % of the mined material. These impurities are known as gangue (i.e sand, & gravel)

Froth Flotation
Used if ore density is less than the density of impurities.
Magnetic Separation
Another method of concentrating Ores.
Magnetic ores (i.e Fe3O4) are separated from their impurities by being passed through a magnetic field. The field attracts the magnetic ore & repels the nonmagnetic impurities.

Roasting
Ore is crushed and heated to a high temp using a strong blast of hot air. The process converts the ores to their oxides which can then be reduced. Eg: The natural occurring ores of zinc are ZnS (sphalerite) & ZnCO3 (smithsonite). When roasted, smithsonite decomposes to ZnO(s) & CO2(g). The strong blast of hot air involved in roasting supplies O2(g) to the sphalerite, thus, becoming oxidized as a result. The oxidation produces ZnO(s) & SO2(g).
$ZnC{O}_3\left(s\right)+\left(heat\right)\to ZnO\left(s\right)+CO2\left(g\right)$
$2ZnS\left(s\right)+3O_2\left(g\right)+\left(heat\right)\to 2ZnO\left(s\right)+2S{O}_2\left(g\right)$

Reduction
C(s) & CO(g) are often used as reducing agents in simultaneous reactions. Oxides of Cr, V, & Mn are reduced using Al. During the reduction process, the metal oxide is heated to a temperature above its boiling point in order to vaporize it & condense as a liquid.
Eg: The ZnO(s) that is produced in the roasting process is combined with C(s)/CO(g) & heated a temperature of ${110}^{\circ }C$ allowing for reduction & vaporization to occur.
$ZnO\left(s\right)+C\left(s\right)+\left(heat\right)\to Zn\left(g\right)+CO\left(g\right)$
$ZnO\left(s\right)+CO\left(g\right)+\left(heat\right)\to Zn\left(g\right)+C{O}_2\left(g\right)$

Pyrometallurgical Process
Pyrometallurgy is the processes of roasting an ore a high temperature & then reducing its oxide product. Its characteristics include:
1. large amount of waste as a product of concentration.
2. high energy consumption to maintain high temps.
3. gaseous emissions that must be controlled $(i.eC{O}_2\&S{O}_2)$

Refining
The impurities contained in the metal product of the roasting/reduction process are removed.
Eg: The Zn(l) produced in the reduction process often contains impurities of Cd & Pb. Though fractional distillation of Zn(l) will work in the refining process, a more commonly used and efficient method is electrolysis. The ZnO(s) produced from roasting is dissolved in $H_{2}S{O}_4$(aq) & then Zn(s) powder is added allowing the impurities to be displaced. The solution is then electrolyzed & Z n2+ is reduced to its pure metallic Zn(s) as a result.

Hydrometallurgical Process
The method of refining zinc described above is an example of a hydrometallurgical process.
The hydrometallurgical process eliminates the need to control gaseous emissions that are often produced in roasting.
Hydrometallurgy is the process of extraction & refining that involves the use of water & aqueous solutions. It is carried out at moderate temps & is generally carried out in 3 steps:
1. Leaching: metal ions are extracted from their ore by water/acids/bases/salt solutions. Redox reactions that occur are often essential.
2. Purification & concentration: Impurities are separated either by absorption on the surface of activated charcoal, ion exchange, or water evaporation; leaving behind a more concentrated solution.
3. Precipitation: the process of electrolysis is often used to precipitate the desired metal ions in an ionic solid or reduce them to their free metal.

Zone Refining
A rod containing the desired pure metal & impurities is passed through a series of heating coils and cooled again. The process isolates the impurities that concentrate in the molten zones, leaving the portions behind them somewhat purer. This process is repeated until impurities are moved to the end of the rod and cut off, resulting in an almost completely pure metal rod.