Grade 9

Grade 9Metals and Nonmetals


Extraction of metals


Metals have various applications in our daily lives due to their exceptional properties such as conductivity, ductility, and strength. Before we can use metals in various industries and practical applications, they must be extracted from their natural state. In this guide, we will explore the fascinating world of metal extraction in simple terms. We will go deep into both the chemistry and processes involved in extracting metals from ores.

Understanding ores and minerals

Metals are usually found in the Earth's crust combined with other elements. These combinations are called minerals. When a mineral contains a sufficient quantity of a particular metal and can be extracted economically, it is called an ore.

For example, the mineral hematite (Fe 2 O 3) is an ore of iron, bauxite (Al 2 O 3 ·2H 2 O) is an ore of aluminum, and chalcopyrite (CuFeS 2) is an ore of copper.

Stages of metal extraction

Extraction of metals involves several main stages. Let's understand them in detail:

1. Concentration of ore

The first step is the concentration of the ore, also known as ore dressing. This is important because raw ore extracted from the earth often contains clay and other impurities.

Common methods for concentrating ores include:

  • Gravity separation: This uses the difference in density between the ore particles and the impurities. A substance such as water is used, causing the denser ore to sink and the impurities to float.
  • Magnetic separation: This is used when the ore or impurities are magnetic. When the crushed ore is passed over a magnetic roller, the magnetic ore particles are attracted and the non-magnetic particles are retained.
  • Froth flotation: This is used mainly for sulphide ores. The process involves mixing crushed ore with water and adding a froth-producing substance. The ore sticks to the froth and rises to the surface, causing the impurities to separate.
  • Leaching: In this method a suitable solvent, often a chemical, is used to dissolve the desired metal, while the impurities remain undissolved.

2. Extraction from concentrated ore

After concentrating the ore, the next step is the extraction of the metal. Different methods are used depending on the chemical properties of the metal.

Reduction of ores

Most metal ores are oxides or are found in combination with oxygen. To extract the metal we have to remove the oxygen. This method varies according to the reactivity of the metal.

For highly reactive metals: Metals such as sodium, potassium, calcium and aluminium are extracted through electrolysis.

For example, in the electrolysis of molten NaCl (sodium chloride), sodium is produced at the cathode (negative electrode), while chlorine gas is evolved at the anode (positive electrode).

2NaCl(l) → 2Na(l) + Cl 2 (g)

For moderately reactive metals: These metals like iron, zinc, lead, etc. are often extracted by carbon reduction (using carbon in the form of coke) or reduction by another chemical agent like carbon monoxide.

An example of this is the extraction of iron from hematite in a blast furnace, where coke acts as a reducing agent:

Fe 2 O 3 + 3C → 2Fe + 3CO

For less reactive metals: Metals like gold, silver and platinum are either found in the free state or require minimal reduction. Methods like physical separation or mild chemical reactions are sufficient for their extraction.

3. Purification of the extracted metals

Metals often contain impurities even after extraction. The process of removing these impurities and refining the metals is important to ensure that they meet the desired standards and quality.

The following are the techniques used for purification:

  • Distillation: Ideal for metals with low boiling points, such as zinc and mercury, where the metal is vaporized and then condensed into a pure form.
  • Electrolytic refining: Used for high-value metals such as copper and silver. An impure anode and a pure cathode are immersed in an electrolyte solution, causing the metal ions to be deposited in pure form at the cathode.
  • Field refining: This method, often used for semiconductors, involves passing an induction coil along a metal body, causing the metal to melt and purify it locally, with impurities being driven to one end.

Metallurgical processes: Phases and examples

Let us look at some major metallurgical processes and practical examples:

Example 1: Extraction of aluminium from bauxite

Ore: bauxite (aluminum oxide hydroxide), mainly Al 2 O 3 ·2H 2 O

Stage 1: Concentration: The ore is initially purified using the Bayer process, where bauxite is dissolved in sodium hydroxide, separating the aluminium from impurities.

Step 2: Reduction: Pure alumina is reduced through electrolysis using the Hall–Heroult process, where aluminum oxide is dissolved in molten cryolite.

2Al 2 O 3 + 3C → 4Al + 3CO 2

Example 2: Extraction of iron from hematite

Ore: Hematite Fe 2 O 3

Stage 1: Concentration: The ore is crushed and concentrated using gravity separation.

Step 2: Reduction: The concentrated ore is reduced in a blast furnace to extract iron.

Fe 2 O 3 + 3C → 2Fe + 3CO

Environmental considerations

Extraction of metals is not only a matter of chemistry and physics, but also of environmental responsibility. Extractive industries have a significant impact on the environment due to deforestation, soil erosion and pollution. Therefore, it is important to ensure that these processes are conducted with minimal damage to the ecosystem.

New methods, particularly relating to green chemistry and sustainable mining, are being developed to ensure that the industry moves towards a more environmentally friendly approach. This includes advances in the recycling of metals and the reduction of toxic by-products and gases released during extraction processes.

Conclusion

The extraction of metals is a complex but essential process that involves various steps including concentration, reduction, and purification. With advances in technology, new eco-friendly and efficient methods of extraction are being developed. Understanding these basic principles behind metal extraction can provide practical knowledge about how everyday materials are produced and refined.

In short, metallurgy, the science of the extraction and processing of metals, plays a vital role in the development of materials that contribute to technological advancements and everyday conveniences.


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