Grade 12 ↓
General principles and processes of separation of elements
Extracting metals from their natural sources is one of the most important activities carried out by humans throughout history. Metals are usually found as ores in the Earth's crust, mixed with other elements, primarily with rock and soil in the Earth. While we often use some metals in their pure form, many metals are more useful as alloys or compounds. The process of obtaining metals from their ores is called metallurgy, and it involves a series of steps to isolate the element into its pure form.
Ores and minerals
To understand metallurgy, it is important to distinguish between minerals and ores. Minerals are naturally occurring substances found in the Earth's crust with an ordered internal structure and a definite chemical composition. However, not all minerals are suitable for extracting metals. An ore is a type of mineral that contains a metal in sufficient quantity for economic extraction.
Examples of ores
Hematite (Fe 2 O 3 ): Iron OreBauxite (Al 2 O 3 ·2H 2 O): Aluminum ore.Galena (PbS): Lead OreCinnabar (HgS): mercury ore
Steps of the extraction process
The extraction of metals involves several major steps:
1. Concentration of ore
It is the process of removing impurities and unwanted substances from the ore. There are several ways to obtain it:
a) Gravity separation
Gravity separation uses water to wash away lighter impurities, based on the difference in density between the metal and the gangue (unwanted material).
b) Froth flotation
This process, commonly used for sulfide ores, involves mixing the ore with water and small amounts of chemicals called collectors and froth stabilizers. The mixture is then stirred to create bubbles. Metal particles stick to the bubbles and float to the surface so they can be removed.
Visual example
c) Magnetic separation
This method is used when the ore or impurities are magnetic. The magnetic material is separated from the non-magnetic material using a magnet.
2. Reduction of ore
Once concentrated, the next main step is to reduce the ore to recover the metal in its free state. This can be accomplished in several ways:
a) Reduction by carbon
In this process, carbon is used as a reducing agent to convert metal oxide into metal. This process is commonly used for the extraction of iron.
2Fe 2 O 3 + 3C → 4Fe + 3CO 2
b) Electrolytic reduction
This complex but effective process is often used for metals that cannot be reduced by carbon, such as aluminum. The metal ore is dissolved in a suitable solvent and then subjected to an electric current which causes the metal ions to migrate and deposit at the cathode.
Visual example
c) Reduction using hydrogen
Hydrogen can also be used to reduce metal oxides. When heated in a stream of hydrogen, the oxide turns into metal and the hydrogen turns into water. This method is mostly used for tungsten, molybdenum and other less reactive metals.
W O 3 + 3H 2 → W + 3H 2 O
3. Shodhana or purification
Finally, the extracted metal may need to be refined to remove any remaining impurities. Common methods of refining include:
a) Distillation
Useful in the purification of low melting point metals such as zinc and mercury, distillation involves heating the impure metal until it vaporizes, and then cooling the vapor to obtain the metal in its pure form.
b) Electrolytic refining
This is a common technique in which the impure metal acts as the anode and a strip of the same metal in pure form acts as the cathode. The electrolyte is a suitable salt of the metal used. Metal ions from the anode go into solution and get deposited at the cathode, thereby purifying the metal.
CuSO 4 (aq) + H 2 O → Cu + O 2 + H 2 SO 4
Example of a refinement process
Thermodynamics principles of metallurgy
The principles of thermodynamics are widely applied in metallurgy. Extraction of metal depends on the suitability of the reduction conditions determined by factors such as temperature and partial pressure of gases such as oxygen. The Gibbs free energy change (ΔG) plays an important role in determining the feasibility of a particular extraction process.
Ellingham diagram
These are graphical representations showing the variation in Gibbs free energy with temperature for reduction reactions of various oxides. The lower the position of the oxide line, the more stable the oxide.
Real-world applications
Metallurgy has a profound impact in the real world, as it affects sectors such as the automotive industry, construction, electronics, etc. These sectors rely heavily on the proper separation and refining of metals to achieve the desired quality in materials.
Conclusion
The principles and processes involved in the extraction and refining of metals are complex and demand a balance between chemical knowledge and engineering practice. As technology advances, these processes become more efficient, contributing to the availability and utility of metals in diverse applications. Whether achieved by traditional or innovative methods, metallurgy remains a cornerstone in the development of human civilization.
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