What is Extraction of metals
Extraction of metals,Types with examples
Introduction to Metal Extraction
The process of extracting metals from their ores and refining them for use is known as metallurgy. Different metals require different extraction methods based on their reactivity and the nature of their ores.
The choice of extraction method depends on the metal’s position in the reactivity series and economic factors such as cost, availability of resources, and environmental considerations.
Types of Metal Ores
Ores are naturally occurring rocks containing minerals from which metals can be extracted profitably. They can be classified into several types:
| Type of Ore | Composition | Examples |
|---|---|---|
| Native Ores | Metal in free state | Gold (Au), Silver (Ag), Platinum (Pt) |
| Oxides | Metal combined with oxygen | Hematite (Fe₂O₃), Bauxite (Al₂O₃·2H₂O) |
| Sulfides | Metal combined with sulfur | Galena (PbS), Zinc blende (ZnS) |
| Carbonates | Metal combined with carbonate | Calamine (ZnCO₃), Magnesite (MgCO₃) |
| Halides | Metal combined with halogens | Rock salt (NaCl), Fluorspar (CaF₂) |
General Steps in Metal Extraction
The extraction of metals typically involves the following steps:
- Concentration of Ore: Removing impurities from the ore
- Conversion to Oxide: Converting the concentrated ore to oxide form
- Reduction: Reducing the metal oxide to free metal
- Refining: Purification of the metal
Exam Tip
Remember that the extraction method depends on the metal’s position in the reactivity series. Highly reactive metals require electrolysis, while less reactive metals can be reduced using carbon or hydrogen.
Concentration of Ores
Before extraction, ores must be concentrated to remove impurities (gangue). Different concentration methods are used based on the physical and chemical properties of the ore:
1. Hydraulic Washing (Gravity Separation)
Used when there’s a significant difference in density between the ore and gangue. The crushed ore is washed with running water, where lighter gangue particles are washed away while heavier ore particles settle down.
Example: Concentration of hematite (Fe₂O₃)
2. Froth Flotation
Used for sulfide ores. The crushed ore is mixed with water, pine oil (froth stabilizer), and a collector reagent. When air is blown, the metal sulfides stick to the froth that floats, while gangue settles down.
Example: Concentration of zinc blende (ZnS) and galena (PbS)
3. Magnetic Separation
Used when either the ore or gangue has magnetic properties. The crushed ore is passed over a magnetic roller where magnetic particles are attracted and separated.
Example: Concentration of magnetite (Fe₃O₄) from non-magnetic impurities
4. Leaching
Chemical method where the ore is treated with a suitable reagent that dissolves the ore but not the impurities.
Example: Bauxite (Al₂O₃·2H₂O) is leached with NaOH solution
Extraction Methods Based on Reactivity
1. Extraction of Metals Low in the Reactivity Series
Metals like gold, silver, and platinum occur in free state and can be extracted by simple physical methods.
Example: Gold Extraction
Gold is extracted from its ore through cyanide leaching:
The gold complex is then reduced using zinc:
2. Extraction of Moderately Reactive Metals
Metals like iron, zinc, lead, and copper can be extracted by reducing their oxides with carbon (coke) or carbon monoxide.
Example: Iron Extraction (Blast Furnace)
Iron is extracted from hematite (Fe₂O₃) in a blast furnace using coke as the reducing agent:
Limestone (CaCO₃) is added as a flux to remove silica impurities:
CaO + SiO₂ → CaSiO₃ (Slag)
Example: Copper Extraction
Copper is extracted from copper pyrites (CuFeS₂) through a series of steps:
- Concentration by froth flotation
- Roasting to convert to oxide: 2CuFeS₂ + 3O₂ → 2CuO + 2FeO + 4SO₂
- Smelting with silica: FeO + SiO₂ → FeSiO₃ (slag)
- Conversion to blister copper: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
Cu₂O + Cu₂S → 6Cu + SO₂
3. Extraction of Highly Reactive Metals
Metals high in the reactivity series (K, Na, Ca, Mg, Al) are extracted by electrolysis of their molten salts or oxides.
Example: Aluminum Extraction (Hall-Héroult Process)
Aluminum is extracted from purified bauxite (Al₂O₃) by electrolysis:
- Bauxite is purified by the Bayer process to get pure alumina (Al₂O₃)
- Alumina is dissolved in molten cryolite (Na₃AlF₆) at about 950°C
- Electrolysis is carried out with carbon anodes and a carbon-lined steel container as cathode
Anode: 2O²⁻ → O₂ + 4e⁻
Overall: 2Al₂O₃ → 4Al + 3O₂
Example: Sodium Extraction (Downs Process)
Sodium is extracted by the electrolysis of molten sodium chloride (mixed with calcium chloride to lower the melting point):
Anode: 2Cl⁻ → Cl₂ + 2e⁻
Overall: 2NaCl → 2Na + Cl₂
Hydrometallurgy
Hydrometallurgy involves the use of aqueous solutions to extract metals from their ores.
Leaching
The ore is treated with a suitable chemical reagent that selectively dissolves the metal, forming a solution.
Example: Gold Cyanidation
Gold is leached from its ore using sodium cyanide solution in the presence of oxygen:
Precipitation
The metal is recovered from the solution by adding a more reactive metal or by changing the pH.
Example: Copper Recovery
Copper can be recovered from copper sulfate solution by adding iron:
Pyrometallurgy
Pyrometallurgy involves the use of high temperatures to extract metals from their ores.
Calcination
Heating the ore in the absence of air to drive off volatile components.
Roasting
Heating the ore in the presence of air to convert it to metal oxide.
Smelting
Heating the ore with a reducing agent and flux to obtain the metal.
Exam-Focused Summary
Key Points to Remember
- The extraction method depends on the metal’s position in the reactivity series:
- Highly reactive metals (K, Na, Ca, Mg, Al) → Electrolysis
- Moderately reactive metals (Zn, Fe, Pb) → Reduction with carbon
- Less reactive metals (Cu, Hg, Ag, Au) → Simple heating or chemical methods
- Carbon reduction method cannot be used for metals above carbon in the reactivity series.
- Hydrogen reduction is used for metals like tungsten and molybdenum.
- Fluxes are added to combine with gangue to form easily removable slag.
- The economic and environmental factors influence the choice of extraction methods.
Practice Questions
1. Why is electrolysis used to extract aluminum and not iron?
Aluminum is more reactive than carbon, so carbon reduction is not effective. Iron is less reactive than aluminum and can be reduced using carbon in a blast furnace.
2. What is the role of cryolite in the extraction of aluminum?
Cryolite (Na₃AlF₆) lowers the melting point of alumina (Al₂O₃) from about 2050°C to 950°C, making the electrolysis process more energy-efficient and economically viable.
3. Why is zinc extracted by first converting it to oxide and then reducing it with carbon, while copper can be extracted by direct smelting?
Zinc is more reactive than copper. Copper is less reactive and can be extracted by heating its sulfide ore in air, which supplies enough heat for the reaction to proceed. Zinc sulfide needs to be completely converted to oxide first and then reduced.
