Metals and Metallurgy - SS2 Chemistry Past Questions and Answers - page 1

The extraction of metals from their ores involves several principles and techniques to separate the desired metal from the ore matrix. The process typically includes the following steps:

a.    Mining: The first step is to obtain the ore from the Earth's crust through mining. Ores are rocks or minerals that contain a high enough concentration of the desired metal.

b.    Crushing and Grinding: The mined ore is crushed into smaller pieces and then ground into a fine powder. This increases the surface area of the ore, facilitating subsequent chemical reactions.

c.     Concentration: The ore is concentrated to remove impurities and increase the metal content. Techniques such as froth flotation, magnetic separation, or gravity separation are used to separate the ore particles based on their physical or chemical properties.

d.    Roasting or Calcination: Some ores require roasting or calcination, which involves heating the ore in the presence of air. This step helps in the removal of volatile impurities, moisture, and other unwanted substances.

e.    Reduction: The reduced metal is obtained by applying various reduction techniques. Common methods include smelting, where the ore is heated with a reducing agent (such as carbon) to remove oxygen and obtain the metal in a more pure form.

Examples of extraction processes include the extraction of iron from its ore (hematite) using the blast furnace method and the extraction of aluminium from bauxite through the Bayer process and electrolysis.

Refining of metals is crucial after extraction to purify the metal and remove impurities that may affect its properties and applications. The refining process involves the following techniques:

a.    Smelting: Smelting is often the first step in the refining process, where the extracted metal is heated with suitable fluxes or other chemicals to remove impurities through chemical reactions or volatilization.

b.    Electrolysis: Electrolysis is widely used for refining metals that are highly reactive or have high melting points. In this process, an electric current is passed through an electrolyte containing a dissolved metal compound. The metal ions are reduced at the cathode, while impurities settle as anode mud or are removed by selective anode dissolution.

c.     Distillation: Distillation is employed for refining metals with high vapour pressures, such as zinc and mercury. The metal is heated to its boiling point, and the vapours are collected and condensed to obtain pure metal.

d.    Zone Refining: Zone refining is a technique used for obtaining ultra-pure metals, such as silicon and germanium. A molten zone is passed through a rod of impure metal, causing impurities to move along with the zone. This process is repeated several times to concentrate impurities at one end, leaving behind a highly pure metal.

For example, copper is refined through electrolysis, where impure copper is used as the anode and pure copper as the cathode. During electrolysis, copper ions from the anode are reduced at the cathode, resulting in the deposition of pure copper. Overall, refining processes ensure that the extracted metals meet the required purity standards and have the desired properties for various industrial applications.