Metallurgy of Some Important Metals: Definition, Ores, Roasting, Calcination

Metallurgy of Some Important Metals: The primary source of minerals and ores is the Earth. Because of their reactive tendencies, the majority of the elements are not found in the state. Metals found in combination include potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, etc. Minerals are the natural materials found on Earth that contain metals and their compounds. Ores are minerals from which metal can be extracted easily and profitably. These ores have a high metal content. In this article, we will study the metallurgy of metals- their process of extraction and purification.

What is Metallurgy?

Metallurgy refers to the various processes involved in extracting metals from their ores and refining them for use.

Various steps in the metal extraction or metallurgical process:

1. Crushing and grinding the ore.
2. The concentration of the ore or the enrichment of the ore.
3. Extraction of metals from the concentrated ore.
4. Refining or purification of the impure metals.

Ores

Ores are minerals from which metal can be extracted easily and profitably.

Types of Ores

Ores are classified into four types: Oxides ores, carbonate ores, sulphide ores, and halides ores.

1. Crushing and Grinding of Ore

The majority of ores found in nature occur as large rocks. Crushers are used to break them down into small pieces. These pieces are then ground into a fine powder using a ball mill or a stamp mill. This is referred to as pulverisation.

Enrichment of Ores

Ores mined from the Earth’s crust contain gangue and matrix, which are impurities such as soil, sand, and so on. The process of removing impurities from ore is known as enrichment or concentration of ore. The following methods are used for ore enrichment:

Gravity Separation Method

This method is based on the density difference between ore particles and gangue. A jet of water is used to wash the powdered ore. Water washes away the lighter, rocky, and earthy impurities, leaving the heavier particles to settle at the bottom. This procedure is also known as hydraulic washing.

This concentration method is typically used with oxide ore. Because ores such as iron, tin, and lead are very heavy, they are concentrated using this method.

Froth Floatation Method

This method is based on the differences in wetting properties of ore and gangue particles with water and oil. This method is most commonly used for sulphide ores like copper, zinc, and lead. The finely powdered ore is mixed with water and a small amount of oil in this method. The mixture is blown with air. The ore particle floats on the surface in the froth. The heavier impurities settle to the bottom, and the surface froth is transferred to another tank, where acid is added to break up the froth. Filtration is used to separate the concentrated ore particles, which are then dried.

Magnetic Separation Method

This method is based on differences in the magnetic properties of ore particles and gangue. With the help of this method, ores attracted by a magnet can be separated from non-magnetic impurities. It is made up of a leather belt that moves over two rollers.

At one end, the powdered ore is dropped over the moving belt. At the other, the magnetic portion of the ore is attracted by the magnetic roller and falls close to the roller, while the non-magnetic impurities fall further away. For example, this method is used to concentrate iron ores (Haematite).

2. Conversion of the Concentrated Ore into its Oxides

The production of metal from concentrated ore mainly involves the reduction process of metal oxides into metals, but first, the concentrated ores should be converted into metal oxides. This can usually be done by two processes known as calcination and roasting. The method depends upon the nature of the ore.

Calcination

Calcination is the process of heating concentrated ore in the absence of air. This procedure is used for the following modifications:

i. To transform carbonate ores into metal oxide.
ii. To extract water from hydrated ores.
iii. To remove the ore’s volatile impurities.

Zinc carbonate, for example, is a zinc ore that is strongly heated in the absence of air to convert into zinc oxide, and carbon dioxide gas is expelled.

\({\text{ZnC}}{{\text{O}}_3}\xrightarrow{{{\text{Calcination}}}}{\text{ZnO}}\left( {\text{s}} \right) + {\text{C}}{{\text{O}}_2}\left( {\text{g}} \right)\)

Similarly, in the case of dolomite ore-

\(\mathop {{\text{CaC}}{{\text{O}}_3}.{\text{MgC}}{{\text{O}}_3}}\limits_{{\text{Dolomite}}} \xrightarrow{{{\text{Calcination}}}}\mathop {{\text{CaO}}\left( {\text{s}} \right)}\limits_{{\text{Calcium}}\,{\text{oxide}}} + \mathop {{\text{MgO}}\left( {\text{s}} \right)}\limits_{{\text{Magnesium}}\,{\text{oxide}}} + 2{\text{C}}{{\text{O}}_2}\left( {\text{g}} \right)\)

Roasting

Roasting is the process of intensely heating concentrated ore in the presence of excess air. This method is used to convert sulphide ores to metal oxide. The following changes occur during this process:

i. Oxidation of sulphide ores results in the formation of oxides.
ii. Moisture is utilised.
iii. Volatile impurities are removed.

For example,

1. Zinc blende \(({\rm{ZnS}})\) is strongly heated in the presence of excess air to convert zinc oxide, and sulphur dioxide gas is expelled.
   \(\mathop {2{\text{ZnS}}\left( {\text{s}} \right)}\limits_{{\text{Zinc}}\,{\text{blende}}} + 3{{\text{O}}_2}\xrightarrow{{{\text{Roasting}}}}\mathop {2{\text{ZnO}}\left( {\text{s}} \right)}\limits_{{\text{Zinc}}\,{\text{oxide}}} + 2{\text{S}}{{\text{O}}_2}\left( {\text{g}} \right)\)
2. Iron pyrite is converted into ferric oxide.
   \(\mathop {{\text{4Fe}}{{\text{S}}_2}\left( {\text{s}} \right)}\limits_{{\text{Iron}}\,{\text{pyrite}}} + 11{{\text{O}}_2} \to \mathop {2{\text{F}}{{\text{e}}_2}{{\text{O}}_3}\left( {\text{s}} \right)}\limits_{{\text{Ferric}}\,{\text{oxide}}} + 8{\text{S}}{{\text{O}}_2}\left( {\text{g}} \right)\)
3. Cinnabar is roasted to convert directly into mercury.
   \(\mathop {{\text{HgS}}\left( {\text{s}} \right)}\limits_{{\text{Cinnabar}}} + {{\text{O}}_2} \to \mathop {{\text{Hg}}}\limits_{{\text{Mercury}}} + {\text{S}}{{\text{O}}_2}\left( {\text{g}} \right)\)

Reduction converts the metal oxide formed after calcination or roasting into metal. The method used to reduce metal oxide is determined by the metal’s nature and chemical reactivity.

The various steps are discussed below-

3. Extraction of Metals

The methods of extraction of a metal from its ores depend upon the reactivity of the metal. The metals like \({\rm{Fe}},\,{\rm{Zn}}\) etc., are reduced by the carbon reduction method followed by refining. Similarly, metals with low activity are refined by self-reduction methods like \({\rm{Hg,}}\,{\rm{Cu}}\) and \({\rm{Pb}}\) Metals with high reactivity like \({\rm{Al,}}\,{\rm{Na,}}\,{\rm{Mg}}\) cannot be reduced by normal reducing agents. Hence, they are reduced by electrolysis methods.

Extraction of Metals Low in the Reactivity Series

Metals at the bottom of the activity series are extremely unreactive. Heating alone can reduce the metal oxides to metals. For example, mercury is obtained from cinnabar by heating only.

\(\mathop {{\text{HgS}}\left( {\text{s}} \right)}\limits_{{\text{Cinnabar}}} + {{\text{O}}_2} \to \mathop {{\text{Hg}}}\limits_{{\text{Mercury}}} + {\text{S}}{{\text{O}}_2}\left( {\text{g}} \right)\)

Similarly, copper is obtained from its sulphide by roasting copper sulphide then heating the copper oxide formed again with copper sulphide.

\(2{\rm{C}}{{\rm{u}}_2}{\rm{O}}({\rm{s}}) + {\rm{C}}{{\rm{u}}_2}{\rm{S}}({\rm{g}})\xrightarrow{{{\text{Heat}}}}6{\rm{Cu(s)}}\,{\rm{ + }}\,{\rm{S}}{{\rm{O}}_2}({\rm{g}})\)

Extraction of Metals in the Middle of the Reactivity Series

Iron, zinc, lead, copper, and other metals are in the middle of the reactivity series. These metals are moderately reactive and are typically found as sulphides or carbonates. It is converted to an oxide ore through the calcination or roasting process.

Heating alone will not reduce the oxides of these metals. As a result, suitable reducing agents such as carbon, carbon monoxide, aluminium, sodium, or calcium are used to reduce these metal oxides to metal. This is referred to as smelting.

i. Zinc, iron, and lead are obtained by heating their oxides with carbon.

\({\rm{ZnO(s)}}\,\, + \,{\rm{C(s)}}\,\, \to \,{\rm{Zn(s)}}\, + \,{\rm{CO(g)}}\)

\({\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}({\rm{s}}) + 3{\rm{C}}({\rm{s}})\xrightarrow{{{\text{Heat}}}}2{\rm{Fe(s)}} + 3{\rm{CO(g)}}\)

\({\rm{PbO(s)}}\, + \,{\rm{C(s)}}\xrightarrow{{{\text{Heat}}}}{\rm{Pb(s)}}\,{\rm{ + }}\,{\rm{CO(g)}}\)

ii. Iron is also obtained from its oxide by heating with carbon monoxide.

\({\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}({\rm{s}}) + 3{\rm{CO(g)}}\xrightarrow{{{\text{Heat}}}}2{\rm{Fe(s)}}\,{\rm{ + }}\,{\rm{3C}}{{\rm{O}}_2}({\rm{g}})\)

iii. Chromium, manganese, titanium, and vanadium metals are obtained by the reduction of their oxides with aluminium powder.

\(3\,{\rm{Mn}}{{\rm{O}}_2}({\rm{g}}) + \,4{\rm{AI(s) }}\xrightarrow{{{\text{Heat}}}}3{\rm{Mn(s)}}\,{\rm{ + }}\,{\rm{2A}}{{\rm{l}}_2}{{\rm{O}}_3}({\rm{s}})\)

\({\text{C}}{{\text{r}}_2}{{\text{O}}_3}\left( {\text{g}} \right) + 2{\text{Al}}\left( {\text{s}} \right)\xrightarrow{{{\text{Heat}}}}2{\text{Cr}}\left( {\text{s}} \right) + {\text{A}}{{\text{l}}_2}{{\text{O}}_3}\left( {\text{s}} \right)\)

Extraction of Metals Towards the Top of the Reactivity Series

Metals at the top of the reactivity series, such as sodium, magnesium, calcium, and aluminium, are extremely reactive and cannot be extracted from their compounds by heating with carbon. This is due to the fact that these metals have a higher affinity for oxygen than carbon. Electrolytic reduction is used to obtain these metals.

These metals, including \({\rm{Na,}}\,{\rm{K,}}\,{\rm{Ca,}}\,{\rm{Mg,}}\,{\rm{and}}\,{\rm{Al}}\) cannot be reduced by coke or carbon monoxide. The only way to reduce these metals is through the electrolytic reduction method or electrolysis.

For example- Sodium metal is obtained by the electrolysis of molten sodium chloride.

\({\rm{2NaCl}}\xrightarrow{{{\text{Fused}}}}{\rm{2N}}{{\rm{a}}^{\rm{ + }}}{\rm{ + }}\,{\rm{2C}}{{\rm{l}}^ – }\)

\(2{\rm{C}}{{\rm{l}}^ – }\, \to {\rm{C}}{{\rm{l}}_2} + 2{{\rm{e}}^ – }\,({\rm{at}}\,{\rm{anode}})\)

\(2{\rm{N}}{{\rm{a}}^ + }\, + \,2{{\rm{e}}^ – }\, \to 2{\rm{Na}}\,{\rm{(at}}\,{\rm{cathode)}}\)

\({\rm{2NaCl}}\xrightarrow{{{\text{Electrolysis}}}}2{\rm{Na}}\, + \,{\rm{C}}{{\rm{l}}_2}\)

4. Purification of Metals

The metal obtained by any of the above methods is usually impure and is referred to as crude metal. The process of purifying crude metal is known as refining. The refining method is determined by the nature of the metal and the presence of impurities in the metal. The following are some of the most common methods for refining metals:

Liquation

This method is used to refine metals with low melting points. This method can be used to purify metals such as tin, lead, bismuth, and others. This method is based on the notion that the metal to be refined is easily fusible, but the impurities are not. The impure metal is gently heated on the furnace’s sloping hearth during this process. The hearth is kept at a temperature that is slightly higher than the melting point of the metal. The metal melts and flows to the bottom of the sloping hearth, leaving behind the impurities. The pure metal is collected at the hearth’s sloping bottom.

Distillation

This method is used to purify metals that have a low boiling point, such as mercury and zinc. The impure metal is heated above its boiling point in this process, allowing it to form vapours. Because the impurities do not vaporise, they are separated. The pure metal vapours are then condensed, leaving the impurities behind.

Electrolytic Refining

This is the most common method for refining impure metals. This method is used to refine many metals, including copper, zinc, tin, nickel, silver, and gold. The impure metal in this process is composed of the anode and a thin strip of pure metal in the cathode. As an electrolyte, a solution of the metal salt is used. The impure metal from the anode dissolves in the electrolytic solution when an electric current is passed through it. On the cathode, an equivalent amount of pure metal is deposited. The soluble impurities dissolve in the solution, while the insoluble impurities settle at the bottom of the anode and are referred to as anode mud.

For example- Copper is purified by this method.

At anode:  

\({\rm{Cu}}\, \to \,{\rm{C}}{{\rm{u}}^{2 + \,}} + {\rm{2}}{{\rm{e}}^ – }\)

At cathode:

\({\rm{C}}{{\rm{u}}^{2 + \,}} + {\rm{2}}{{\rm{e}}^ – } \to {\rm{Cu}}\,\)

Zone Refining

This method removes impurities from metals such as germanium, silicon, gallium, indium, and boron. The impure metal is attached to a circular mobile heater at one end of this process. The pure metal crystallises as the heater moves, and the impurities pass to the adjacent part of the metal. As a result, the impurities accumulate at the other end of the rod, which is cut to obtain the pure metal.

Vapour Phase Refining

In this type of purification, the metal should form a volatile compound in the presence of a reagent and easily decompose for the metal to be recovered. The metal transforms into its volatile compound. This volatile compound is then decomposed to yield pure metal. Nickel, for example, is purified in this manner.

Chromatographic Method

The crude mixture is placed in a liquid or gaseous medium using this method. An adsorbent is used to move this medium. At different levels of the column, different components of the mixture are adsorbed. Suitable solvents are used to remove these components of the mixture.

Summary

We can conclude that metallurgy refers to the various processes involved in extracting metals from their ores and refining them for use. Ores are minerals from which metal can be extracted easily and profitably. We also studied the following point-
1. Metals of high reactivity are obtained by electrolytic reduction.
2. Metals of middle reactivity occur as ores of carbonate and sulphide, which are converted into oxides and then reduced and purified.
3. Metals of low reactivity occur as sulphide ores and are converted to metals by roasting and are then refined.
We also studied some of the purification processes like liquation, distillation, and electrolytic refining.

FAQs on Metallurgy of Some Important Metals

Q.1. What are ores? Name some important ores.
Ans: The majority of metals are too reactive to exist in the ground in the state. Instead, they coexist with other elements in the form of compounds known as ores. Ores are raw materials used in the production of metals.
Some important ores are-

Q.2. What are the three types of metallurgy?
Ans: The three branches of metallurgy are as follows-
i. Extractive metallurgy
ii. Mechanical metallurgy
iii. Physical metallurgy

Q.3. What are the different processes involved during metallurgy?
Ans: The different processes involved for obtaining pure metal from impure ores are:
i. Crushing and grinding the ore.
ii. The concentration of the ore or the enrichment of the ore.
iii. Extraction of metals from the concentrated ore.
iv. Refining or purification of the impure metals.

Q.4. Name the different processes involved in the concentration of ore.
Ans: The different processes involved in the concentration of ore are-
i. Electromagnetic separation
ii. Froth flotation method
iii. Hydraulic washing method

Q.5. Differentiate between roasting and calcination.
Ans: Roasting involves heating ores in the presence of air, whereas calcination involves heating ores in the absence of air.

Q.6. What is the importance of metallurgy in metal works?
Ans: Metallurgy, or the extraction of metal from ores or metal compounds, is a vital industry that produces metallic materials with unique properties for use in a variety of fields.

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