Metallurgy

Metallurgy refers to a process that is used to extract or obtain metals from their ores in their pure form. Metals are generally not found in their pure form in nature, they are found mixed with soil, limestone, sand, rocks, etc., which are called minerals. The metals are required to be extracted from ores or minerals at a low cost and with minimum effort. So, metallurgy deals with the purification and extraction of metals.

Process of Metallurgy

Metallurgy involves the concentration of ores, we can say that concentration of ore is performed. In this method, the impurities attached to the ore such as dust, sand, rocky or clay material including dead particles and other chemicals are removed. This method is also called enrichment of ore. These impurities attached to the ore are called gangue or matrix. Whereas, the substance which is added in the furnace to separate the gangue from the metal ore is called flux. It is a cleaning agent, which is used in the purification of ores.

So, we can say that in the concentration of ore, we remove gangue from the ore mined from the Earth's surface. This process is called concentration of ores as the removal of impurities increases the percentage of metal. It is also known as dressing of ores. This process is done by using the physical and chemical methods;

The methods used in the concentration of ore are described below;

Physical methods for concentration of ores

i) Crushing and Grinding: This process involves crushing and grinding of ores into a fine powder. It is done in a crusher or ball mill. It is also called pulverization.

ii) Hand-picking: It is a traditional method of concentrating metal ores. In this method, impurities are picked by hand and the unwanted rocky materials are separated with the help of a hammer.

iii) Hydrolytic method: In this method, the ore is poured over a sloping table with grooves. This table is grooved and keeps vibrating. Then, a jet of water is made to flow over the surface. The dense particles of ore settle in the grooves while the impurities are washed away with water. This method is also known as hydraulic washing, gravity separation method or levigation. It is based on the difference in the gravity or density of metal and impurities present in the ores. So, it is mostly used when the ore particles are larger than the impurities, e.g., heavy oxides ores of lead, tin, iron, etc.

iv) Magnetic Separation: This method is used when either metal or impurities in an ore is magnetic in nature, which means any one of them is attracted to the magnet. So, it can be used for magnetic ore like iron ore with non-magnetic impurities and for a non-magnetic ore that contains magnetic impurities. For example, non-magnetic ore of Tin (cassiterite - SnO₂) is separated from magnetic impurities such as iron and chromium by magnetic separation.

Pyrolusite (MnO₂) and chromite (FeO.Cr₂O₃) are also few magnetic ores that are processed by this method. In this method, the powdered ore is kept over a conveyer belt that rotates around two wheels. Out of two, one wheel is a magnetic wheel, so it attracts the magnetic particles and thus the magnetic particles get separated from the non-magnetic particles.

v) Froth floatation: In this method or process, the crushed or powdered ore is placed in a large tank filled with oil and water. This tank is known as floatation tank. So, crushed ore is mixed with water, oil in this method. Then, the compressed air is passed or blown into the tank through a pipe, which produces froth. This method is based on the difference in the wetting character of the ore and impurities (gangue particles).

The ore becomes wet due to oil and the froth containing the ore is lightweight so it rises above and impurities are left behind towards the base of the tank. The froth that contains the sulphide ore is separated and then it is washed and dried. Thus, ore gets separated from impurities.

This method is mostly used for removing impurities from sulphide ores. This is because sulphur has a tendency to go with oil. So, sulphur ore is attracted towards oil, whereas, the impurities tend to stay with water. As we know oil and water do not mix and thus they move away from each other.

Furthermore, oil is lighter than water, so it moves upward towards the top of the tank whereas the water with impurities tend to stay at the bottom. Oil is removed from the ore in the froth, which eventually separates the ore from impurities. Some examples of sulphide ore that are concentrated by this method are copper pyrites (CuFeS), galena (PbS), and zince blende (ZnS).

Chemical methods for concentration of ores

i) Leaching

It is a chemical method used for the purification of metal ores. It is used for the ores that are soluble in an appropriate solvent. The powdered ore is treated with specific reagents that can dissolve the ore but cannot dissolve the impurities. So, the impurities are left behind and separated by filtration. An example of leaching is given below;

Bauxite ore is purified by leaching. The chemical formula of bauxite ore is Al₂O_3.2H₂O and it contains ferric oxide (Fe₂O₃) and silicon dioxide (SiO₂) as impurities.

The powdered ore is treated with an aqueous solution of sodium hydroxide at a temperature of 150 to 170 degrees centigrade. Al₂O₃ is an amphoteric oxide, which means it has properties of both acids and bases. So, depending on the situation, it can act as an acid or base. For example, when it reacts with a base like NaOH, it will behave as an acid.

The alumina Al₂O₃ present in the ore is dissolved in NaOH to form sodium aluminate as shown below;

Al₂O₃ + NaOH → NaAlO₂ + H₂O

Whereas the other oxide Fe₂O₃ do not react with NaOH as ferric oxide has basic character and NaOH is also a base and we know a base does not react with base. So, these oxides do not dissolve as they do not take part in any reaction and can be separated by filtration.

However, Silicon in SiO2 is a metalloid, it has properties of both metals and non-metals. So, SiO2 is amphoteric in nature or has both acidic and basic character, so it will react with NaOH to form sodium silicate as shown in the below reaction;

SiO₂ + NaOH → Na₂SiO₃ + H₂O

Now, we have sodium aluminate and sodium silicate, which in the next step can be reacted with water or carbon dioxide. Let us see, their reaction with water as shown below;

Sodium aluminate when reacted with water, it forms NaOH and aluminium hydroxide in the form of precipitate, so, aluminium hydroxide can be separated easily by filtration method. See the below reaction;

NaAlO₂ + H₂O → NaOH + Al (OH)₃

Now, the sodium silicate does not react with water to form hydroxide as it is a metalloid. As it does not react, it is separated easily.

Na₂SiO₃ + H₂O → no reaction

Now, the aluminium hydroxide that we get in the form of precipitate is heated to form aluminium oxide as shown in the below reaction;

Al (OH) ₃ → Al₂O₃ + H₂O

Water is again added to Al₂O₃ as water of crystallization, which is known as recrystallization as shown below;

Al₂O₃ + 2H₂O → A_l2O₃.2H₂O

So, in the above reaction, we get the concentrated bauxite ore which is free from external impurities.

ii) Roasting

In this chemical method, the concentrated ore is heated strongly in the presence of an excess of oxygen at a temperature below the melting point of the metal in the ore. This process is called roasting and is mostly used for sulphide ores. For example;

2ZnS + 3O₂ → 2ZnO + 2SO₂

iii) Calcination

The ores that contain carbonate or hydrated oxides are heated in the absence of air. This process of melting the ore in the absence of air is called calcination.