Extraction of Metals: Concentration of Ore, Refining of Impure Metal

Extraction of Metals: The purpose of Isolation of Elements in Chemistry Class 12 is to teach students about different methods of extracting metals from ores. Only a few metals, such as noble metals such as gold, silver, and platinum, are found in their natural metallic forms. Metallurgy is the branch of science that deals with extracting metals from ores that are present naturally in the environment. The majority of elements, particularly metals, are found in combination with other elements and are referred to as minerals. Although an element may combine with a number of other elements to form a variety of minerals, only a handful of them is viable sources of that metal. These sources are known as Ores.

Mining is the process of recovering metal ores from deep down. Metal ores can be found in variable quantities in the earth’s crust. We can utilise the minerals in the earth because of the extraction of metals from ores! The ores are not the same as the finished metals seen in structures and bridges. The desired metal complex, as well as impurities and earthy things known as Gangue, make up ore. Read further to find more.

Occurrence of Metals

The primary source of metals is the Earth’s crust. Seawater also contains some metal salts in an insoluble form. A metal may occur in its native state (state) or combined state depending upon its position in the reactivity series of metals.

The metals lying low in the reactivity series like gold and platinum occur in a state. Metals like copper and silver occur in a state and the combined state in the Earth’s crust.

Metals lying in the middle of the reactivity series like zinc, iron, lead, etc., are moderately reactive and occur in a combined state. These elements occur mainly as oxides, sulphides, or carbonates.

Metals lying high in the reactivity series like potassium, sodium, calcium, magnesium, and aluminium are highly reactive and always occur in a combined state. Therefore, these metals do not occur in a state.

Aluminium is the most abundant metal in the Earth’s crust. The second most abundant metal is iron, and the third one is calcium.

The naturally occurring chemical substances in the Earth’s crust are known as minerals. Ores are usually contaminated with earthly or undesired materials known as gangue. Therefore, all minerals are not ores, but all ores are minerals.

‘The minerals from which metals can be extracted profitably and economically are called an ore.”

Extraction of Metals

The extraction of metals from their ores involves various steps. The steps depend on the type of ore, the reactivity of the metal, and the nature of impurities present in the ore. These processes involved in the extraction and refining of the metal are called metallurgy. The ores of most of the metals should be brought to the Earth’s surface for extracting metal from them. This process is called mining. Generally, there are three main steps involved in the process of extraction of metals. These are:

1. Enrichment or concentration of an ore
2. Extraction of metal from concentrated ore
3. Refining of the impure metal

Enrichment or Concentration of Ore

An ore after mining from the ground contains many unwanted impurities like sand, rocky materials, etc. These unwanted impurities, including earthy materials, rocks, sandy materials, limestone, etc., are called gangue.

The first step in metallurgy is to remove these unwanted impurities from the ore, to get a concentrated ore containing a much higher percentage of the metal. The method used to remove gangue from ore depends on the physical or chemical properties of the gangue and ores. Some of the methods used in the concentration of the ore include,

1. Hand-picking: Lumps of the ore are broken into small pieces, the sand and mud adhering to the ore are washed away by a stream of water.

2. Hydraulic washing: This method is also called gravity separation or levigation. It is based on the difference in specific gravities of the ore and gangue particles.

3. Electromagnetic separation: The process is meant for the separation of magnetic ore from impurities. In this method, the powdered ore is placed over a leather belt which moves over two rollers, one of which is magnetic. When the crushed ore is passed over the magnetic roller, magnetic ore particles are attracted by it and fall below it while impurities fall away from the magnetic roller. Chromite from siliceous gangue, rutile from chlorapatite, and wolframite from cassiterite are separated by this method.

4. Froth floatation process: This process is commonly used for sulphide ores and is based upon different wetting characteristics of ore and gangue particles. The finely powdered ore is mixed with water, pine oil(frother), metal sulphide, and ethyl xanthate or potassium ethyl xanthate(collector) in a big tank. Then, the whole mixture is agitated with air. The ore particles wetted with oil come in the froth and are taken off, while impurities wetted with water settle at the bottom.

• Here, the foaming agent is pine oil, and the froth stabilizer is cresol and anisole. 
• Collectors used are ethyl xanthate and potassium ethyl xanthate.
• Activator used in \(\mathrm{CuSO}_{4}\) wild depressant used in \(\mathrm{KCN}\).

5. Liquation: This method is useful for such ore, which contains easily fusible mineral particles and high melting gangue.

6. Chemical separation (leaching): In this method, the powdered ore is treated with a suitable chemical reagent which dissolves the ore, while impurities remain insoluble in that reagent.

Bauxite is separated from \({\text{F}}{{\text{e}}_2}{{\text{O}}_3},\,{\text{Si}}{{\text{O}}_2},\,{\text{Ti}}{{\text{O}}_2}\) with the help of \({\text{NaOH}}\) in which \({\text{A}}{{\text{l}}_2}{{\text{O}}_3}\) dissolves while the rest are insoluble.

\({\text{A}}{{\text{l}}_2}{{\text{O}}_3} + 2{\text{NaOH}} \to 2{\text{NaAl}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}}\)

\({\text{NaAl}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} \to {\text{Al}}{({\text{OH}})_3} + {\text{NaOH}}\)

\(2{\text{Al}}{({\text{OH}})_3} \to {\text{A}}{{\text{l}}_2}{{\text{O}}_3} + 3{{\text{H}}_2}{\text{O}}\)

\({\text{A}}{{\text{g}}_2}{\text{S}} + 4{\text{NaCN}} \to 2{\text{Na}}\left[ {{\text{Ag}}{{({\text{CN}})}_2}} \right] + {\text{N}}{{\text{a}}_2}{\text{S}}\)

Extraction of Metal from Concentrated Ore

Different methods are employed for extracting metals from the concentrated ore. Based on the reactivity, the metals are classified into three groups, namely

a) Less reactive metals or metals of low reactivity
b) Moderately reactive metals or metals of middle reactivity
c) Highly reactive metals or metals of high reactivity

Let us discuss how less reactive metals are extracted.

Extraction of Less Reactive Metals

Metals that are least reactive such as mercury \(\left( {{\text{Hg}}} \right)\), gold \(\left( {{\text{Au}}} \right)\), platinum \(\left( {{\text{Pt}}} \right)\), are found in a state in nature and are placed at the bottom of the activity series. Therefore, these metals can be extracted by reducing their oxides by heating only.

Let us illustrate the principles behind the extraction of these low reactivity metals from concentrated ore.

Extraction of Mercury

The principal ore of mercury is cinnabar \(\left( {{\text{HgS}}} \right)\) which is a sulphide ore. The metal \(\left( {{\text{Hg}}} \right)\) can be extracted from the ore in the following steps.

Step 1-Roasting of concentrated mercury (II) sulphide ore in air

Roasting is the process in which a sulphide ore is strongly heated in the presence of air, converting the sulphide ore into its corresponding metal oxides. Thus, concentrated mercury (II) sulphide is roasted in air to form its corresponding oxide, i.e., mercury (II) oxide.

Step 2-Conversion of mercury (II) oxide ore to mercury metal

(a) Roasting of sulphide ore: The cinnabar ore \(\left( {{\text{HgS}}} \right)\) is heated in the presence of air to convert it into metal oxide \(\left( {{\text{HgO}}} \right)\).

\(2{\text{HgS}} + 3{{\text{O}}_2} \to 2{\text{HgO}} + 2{\text{S}}{{\text{O}}_2}\)

(b) Reduction of metal oxide to metal: The metal oxide so formed \(\left( {{\text{HgO}}} \right)\) gets reduced to the metal on further heating.

\(2{\text{HgO}} \to 2{\text{Hg}} + {{\text{O}}_2}\)

  Thus, \({\text{HgS}}\) can be converted into \({\text{Hg}}\) by heating alone.

Extraction of Moderately Reactive Metals

The metals lying in the middle of the reactivity series are moderately reactive and usually occur as sulfides or carbonates in nature. Therefore, extraction of these metals is also carried out in two steps:

The moderately reactive metals can be extracted by the reduction of their oxides with carbon \(\left( {{\text{C}}} \right)\), aluminium \(\left( {{\text{Al}}} \right)\), sodium \(\left( {{\text{Na}}} \right)\), or calcium \(\left( {{\text{Ca}}} \right)\). Some moderately reactive metals also occur in nature as their carbonates or sulfides. But we can say that metals can be more easily extracted from their oxide ores than carbonates or sulphide ores. The oxide ores can be directly converted into metals by heating, whereas the carbonate or sulphide ores should first be converted into metal oxide.

The concentrated ores can be converted into metal oxide by using calcination or roasting based on the nature of the ore. The process of heating carbonate ore strongly in the absence of air is called calcination.

Metals such as zinc \(\left( {{\text{Zn}}} \right)\), tin \(\left( {{\text{Sn}}} \right)\), lead \(\left( {{\text{Pb}}} \right)\), and iron \(\left( {{\text{Fe}}} \right)\) can be extracted by the process of calcination. The metal oxide formed is then converted into metal by heating it in the presence of reducing agents like carbon \(\left( {{\text{C}}} \right)\), aluminium \(\left( {{\text{Al}}} \right)\), sodium \(\left( {{\text{Na}}} \right)\), or calcium \(\left( {{\text{Ca}}} \right)\). The use of reducing agents depends on the chemical reactivity of the metal to be extracted.

Let us now study how these reducing agents are used.

Reduction of Metal Oxide with Carbon

Some moderately reactive metal oxides are reduced by using carbon as a reducing agent. In this reduction process, the metal oxide is mixed with carbon in the form of coke and heated in the furnace to form carbon monoxide \(\left( {{\text{CO}}} \right)\). Thus, the metal oxide is reduced to metal. This method is used to extract zinc and iron from their oxides \({\text{ZnO}}\) and \({\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}\), respectively. This process of reduction of metal oxides by heating with coke is called smelting.

Their reduction with carbon can also extract oxides of moderately reactive metals like tin \(\left( {{\text{Sn}}} \right)\) and lead \(\left( {{\text{Pb}}} \right)\).

Reduction of Metal Oxide with Aluminium

Some oxides of moderately reactive metals are reduced by using aluminium as the reducing agent. In this process of reduction, the metal oxides are displaced with aluminium, and the metal is extracted. This process is called aluminothermy. A more reactive metal can displace less reactive metal from its metal oxide to give the metal. The reduction of metal oxide with aluminium is highly an exothermic reaction as much heat is released during the reaction- for example, manganese dioxide \(\left({{\text{Mn}}{{\text{O}}_{\text{2}}}} \right)\), iron (II) oxide, etc. can be reduced with aluminium.

Usually, the carbonate ores are converted into metal oxide by the process of calcination, whereas sulphide ores are converted into metal oxide by roasting.

Let us illustrate the extraction of moderately reactive metals from their ore with examples.

Extraction of Zinc

(a) Conversion of ore into metal oxide: Zinc occurs in nature as sulphide as well as in carbonate form. Thus, these must be converted to zinc oxide before reduction.

Roasting of zinc sulphide:

\(2{\text{ZnS}} + 3{{\text{O}}_2} \to 2{\text{ZnO}} + 2{\text{S}}{{\text{O}}_2}\)

Calcination of zinc carbonate:

\({\text{ZnC}}{{\text{O}}_3} \to {\text{ZnO}} + {\text{C}}{{\text{O}}_2}\)

(b) Reduction of metal oxide to metal:
The zinc oxide is reduced to zinc metal by using carbon as a reducing agent.

\({\text{ZnO}} + {\text{C}} \to {\text{Zn}} + {\text{CO}}\)

The choice of reducing agent used depends on the nature of metal oxide being reduced. For example, a more reactive metal can reduce the oxide of a less reactive metal.

Example 1: Manganese dioxide, \({\text{Mn}}{{\text{O}}_{\text{2}}}\) can be reduced by heating with aluminium powder. Aluminium reduces manganese dioxide to manganese metal.

\(3{\text{Mn}}{{\text{O}}_2} + 4{\text{Al}} \to 3{\text{Mn}} + 2{\text{A}}{{\text{l}}_2}{{\text{O}}_3} + {\text{Heat}}\)

Example 2: Iron (III) oxide, \({\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}\) can also be reduced using aluminium as a reducing agent.

\({\text{F}}{{\text{e}}_2}{{\text{O}}_3} + 2{\text{Al}} \to 2{\text{Fe}} + 2{\text{A}}{{\text{l}}_2}{{\text{O}}_3} + {\text{Heat}}\)

Extraction of Highly Reactive Metals

The metals lying high in the reactivity series are very reactive and cannot be obtained by reducing their oxides and other compounds using common reducing agents like carbon. The metal oxides of these metals are difficult to reduce as these metals have a high affinity for oxygen. Such highly reactive metals are extracted by the electrolytic reduction of their molten chlorides or oxides.

Electrolytic reduction: When metals are extracted from their molten chlorides or oxides by passing an electric current through them. This process of electrolytic reduction is also called electrolysis. In an electrolytic reduction method, metal ions on electrolysis move towards the cathode, gaining an electron to become metal atoms. Let us illustrate the electrolytic reduction method of metals of higher reactivity with an example.

Electrolysis of Molten Sodium Chloride

Sodium metal is extracted from the molten sodium chloride by the process of electrolytic reduction. When molten sodium chloride is electrolyzed by passing electric current, it decomposes into sodium \(\left( {{\text{N}}{{\text{a}}^{\text{ + }}}} \right)\) ions and chloride \(\left( {{\text{C}}{{\text{l}}^{\text{ – }}}} \right)\) ions. The sodium \({\text{N}}{{\text{a}}^ + }\) ions move towards the cathode (negative electrode) while chloride \({\text{C}}{{\text{l}}^ – }\) ions move towards the anode (positive electrode). These sodium \({\text{N}}{{\text{a}}^ + }\) ions gain electrons at the cathode and get reduced to sodium atoms, and chloride \({\text{C}}{{\text{l}}^ – }\) ions lose electrons at the anode and get oxidized to chlorine atoms. The reaction involved is as follows:

At cathode: \({\text{2N}}{{\text{a}}^{\text{ + }}}{\text{ + 2}}{{\text{e}}^ – } \to {\text{2Na}}\)

At anode: \({\text{2C}}{{\text{l}}^ – } \to {\text{C}}{{\text{l}}_{\text{2}}}{\text{ + 2}}{{\text{e}}^ – }\)

Overall reaction: \({\rm{2Na}} \to {\rm{2Na + C}}{{\rm{l}}_{\rm{2}}}\)

Thus, sodium metal is obtained at the cathode while chlorine gas is liberated at the anode.

Note: We cannot use an aqueous solution of sodium chloride to obtain sodium metal. The sodium metal obtained at the cathode reacts with water to form sodium hydroxide \(\left( {{\text{NaOH}}} \right)\) and hydrogen gas. Thus, instead of sodium metal, hydrogen gas is liberated at the cathode.

Refining of Impure Metals

The metal obtained by any method of reduction process usually contains some impurities, so they are impure. The metal obtained along with the impurities is called crude metal. Now, we need to remove these impurities to obtain \(99.9 \%\) pure metal. The process of purifying impure metals (crude metals) is called refining of the metal.

Different refining methods are used for different metals. The method to be used for refining an impure metal depends on the nature of the metal and the nature of impurities present in it. The most important and widely used method for purifying impure metals is electrolytic refining. Since the refining of the metal is done by electrolysis, this method is called electrolytic refining. Many ways are used to purify metals, out of which electrolytic refining is most widely used.

Electrolytic Refining of Copper

The electrolytic refining apparatus consists of a thick block of impure copper metal as an anode, a thin strip of pure copper metal as cathode, and an electrolytic tank consisting of acidified copper sulphate as electrolyte. The word ‘acidified copper sulphate solution’ means that copper sulphate solution is acidified by mixing it with dilute sulphuric acid.

Now, the impure metal is connected to the positive end of the battery, and the pure metal is connected to the negative end of the battery. When an electric current passes through the electrolyte solution containing copper sulphate with dilute sulphuric acid, the impure copper from the anode dissolves and enters the copper sulphate solution, and pure copper metal is produced on the cathode. In other words, on passing electric current, pure copper from the anode passes from the solution as copper \(\left( {{\text{C}}{{\text{u}}^{{{\text{2}}^{\text{ + }}}}}} \right)\) ions and an equal amount of copper \(\left( {{\text{C}}{{\text{u}}^{{{\text{2}}^{\text{ + }}}}}} \right)\) ions from the solution are deposited on the cathode as pure copper.

At anode: \({\text{Cu}} \to {\text{C}}{{\text{u}}^{2 + }} + 2{{\text{e}}^ – }\)

At cathode: \({\text{C}}{{\text{u}}^{2 + }} + 2{{\text{e}}^ – } \to {\text{Cu}}\)

Thus, as the electrolysis proceeds, the anode becomes thinner, and the cathode becomes thicker. This is because the soluble impurities go into the solution, whereas the insoluble impurities fall at the bottom of the anode as ‘anode mud’.

1. At the cathode, the positively charged copper ions from the copper sulphate solution enter the cathode and are reduced to copper atoms.

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

Pure copper is formed.

2. At the anode, the copper atoms lose an electron to form cupric ions \({\text{C}}{{\text{u}}^{{{\text{2}}^{\text{ + }}}}}\). These ions enter into the electrolyte.

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

It is observed that the electrolytic method of refining copper not only purifies the metal but also gives us metals such as gold \(\left( {{\text{Au}}} \right)\) and silver \(\left( {{\text{Ag}}} \right)\) in the native state (as anode mud).

Summary

All materials found on Earth are made up of chemical elements. Most elements found on Earth are metals and nonmetals. In this article, we have discussed the ores of metals and how they are present, methods of extraction of metals from their ores, and the methods of purification of impure metal.

FAQs on Extraction of Metals

Q.1. How is a metal extracted and processed?
Ans: Although other non-ferrous metals have lower melting points than aluminium and can therefore be processed at somewhat lower temperatures, the same process steps tend to be used: crushing, grinding, concentration by flotation, or other means smelting, refining, and electrolytic purification.

Q.2. What is the major source of metals?
Ans: Rocks and soils are the principal natural sources of heavy metals in the environment. The primary rocks, which are called magmatic or igneous rocks, crystallize from magma upon cooling down.

Q.3. What are the steps involved in the extraction of metals from ores?
Ans: The extraction of a metal from its ore involves mainly three stages. They are Enrichment or concentration of ore, Extraction of metal from concentrated ore, and refining the impure metal.

Q.4. What are the methods of extraction of metals?
Ans: There are three main methods of extracting metals from ore. They are by electrolysis, reducing an ore by a more reactive metal, reducing the ore with carbon.

Q.5. What type of chemical reaction is used to extract metals from ores?
Ans: The chemical reaction used to extract metals is decomposition reaction, carried out by electricity, extracts metals from their naturally occurring compounds like oxides or chlorides.

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