Extraction of Metals

Introduction

Metals are not the readily available products in their original form. They are obtained in the form of ores. Ores are the impure and the raw form of metals that need a systematic process of extraction. It is the adulterated form of metal requiring some extraction through physical and chemical processing. The process of obtaining metals from its ores is called metallurgy. The naturally occurring compounds of metals are known as minerals, and they are usually extracted from the earth’s crust by mining. Although, seawater contains some soluble salts like sodium chloride, magnesium chloride, the earth’s crust is the major reservoir of metallic ores. In most of the cases, metals exist in a combined state however some are available in their free state.  A native metal is a natural metal available in the metallic state which is usually not very active some of them are gold, silver, platinum, etc are found in the native or free state. Reactive metals make up the maximum proportion of metals, and they react very quickly to  the chemical processes such as oxidation, corrosion, etc. This article describes the scientific steps involved in the process of extraction of metals.

 

Formation of Gangue 

Ore -appearance

 

 

Metals are available in the form of ores, but the ores often found along with guange. The gangue, also known as matrix; an unwanted substance that has no commercial significance. Matrix is the mixture of rocky and sandy material found along with ores. Ores are the primary source of majority of the metals. All ores are minerals, but not all minerals are ores. The picture shown above is the sample of an iron ore resembling dark coal with the rough edges. With the help of metallurgy matrix can be separated. Metallurgy includes  smelting, electrolysis, reduction, and oxidation. Different methods are used to extract different metals. Based on their reactivity, metals are classified into high reactivity metals, medium reactivity metals and low reactivity metals. The low reactivity metals are at the bottom of the activity series and some of them are gold, platinum, silver, and copper occurring in their free state. Most of the metals are in their active form except the copper and silver, as they occur in sulphide and oxide ores. Metals of medium reactivity series are Zn, Fe, and Pb among many others. The medium reactivity metals occur as oxides, sulphides or carbonates. Metals of high reactivity (K, Na, Ca, Mg and Al) are very reactive and thus found in combined states with many other substances.

 

Steps of Metallurgy (metal extraction process)

1. Enrichment of Ores

Enrichment involves making ores more concentrated, it is otherwise known as the ore dressing. The process of removing gangue (earthly impurities) from its ore is known as the concentration process or ore dressing.

2. Hydraulic washing

It is carried out based on the principle of density differentiation of different materials in the enriched ores. When a gush of water is passed through a chamber, the lighter gangue particles will be washed away and only the heavier minerals will settle at the bottom. Ores of tin and lead are concentrated by using this method because they are heavy in density.

3. Magnetic separation

Magnetic separation is used for the some metallic wastes that are easily attracted by magnets. Some of them are the ores of iron pyrites, FeS and magnetite, and Fe304. After the magnetic separation, the remains are concentrated. During the process, the crushed ore is allowed to pass through electromagnetic belts so that only the mineral particles are retained and gangue particles are washed away.

4. Froth flotation

It is most commonly used  mechanical process for sulphide ores. The powdered ore is mixed with water in a tank that contains pine or eucalyptus oil and then the whole components are agitated through a gush of air. As the ore particles are drenched, oil floats to the surface along with froth but the heavy hydrated impurities get settled at the bottom. Finally, the oil froth with mineral particles which floats on the surface of the water can be skimmed off.

5. Conversion of concentrated ore into metallic oxide

The concentrated ore is converted into metal oxide through roasting or Calcination. Roasting is done by heating the concentrated ore to a high temperature in the presence of the excess of air.  Calcination is done by heating the concentrated ore in the absence of air at a lower temperature that cannot melt the ore. The reaction can be written as ;

2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2 Ca

6. Reduction of metallic oxides to metal by smelting

This is performed by the electrolysis with the help of reducing agents. By using electrolysis, highly electropositive (reactive) metals such as K, Na, Ca, Mg, Al oxides/halides and also fused metallic salts are reduced. For example, Extraction of Aluminium from {Al2O3.2H2O}. By using reducing agents, like carbon (coke, charcoal), carbon monoxide and hydrogen the metallic oxide is reduced to metal. For example;

ZnO+CO  → Zn+Co2

7. Refining and purification

The metals produced by various reduction processes are not very pure as they still contain a lot of impurities. The most widely used method for refining impure metals is electrolytic refining. 

 

 

Read more

1.     

Metals and non-metals

2.

Alkali metals and alkaline earth metals

3.

Management of natural sources

4.

Water as a natural resource

 

Questions

  1. What is an ore?

  2. Why do we need to extract metals?

  3. Explain the steps of metallurgy.

  4. What is gangue and what does it contain?

  5. Differentiate between low reactive, medium reactive and high reactive metals.

  6. What is froth flotation?

 

 



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