Extraction of Zinc: Froth Floatation Process, Roasting, Refining, Uses

Zinc is currently the fourth most widely consumed metal in the world after iron, aluminium, and copper. It has strong anticorrosive properties and bonds well with other metals. Zinc is an abundant metal, found in the Earth’s crust, with a myriad of industrial and biological uses. At room temperature, zinc is brittle and blue-white in colour, but it can be polished to a bright finish. Zinc is being extracted out of its nature ores. The process of extraction involves many steps. In this article, we have coved the extraction process of Zinc in detail. Read on to know more.

Structure of Zinc

Zinc is a bluish-white transition metal. The chemical symbol of zinc is \({\rm{Zn}}.\) It belongs to group \(12\) and period \(4\) of the Modern Periodic Table. Its atomic number is \(30.\) Hence, it has \(30\) electrons distributed in \(4\) energy shells as \({\rm{2,8,18,2}}.\) It has an electronic configuration of \({\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{p}}^{\rm{6}}}{\rm{3}}{{\rm{s}}^{\rm{2}}}{\rm{3}}{{\rm{p}}^{\rm{6}}}{\rm{3}}{{\rm{d}}^{{\rm{10}}}}{\rm{4}}{{\rm{s}}^{\rm{2}}}.\) It has an atomic mass of \(65.38\;{\rm{g}}/{\rm{mol}}\). Though zinc is present in the d-block of the Periodic Table, it does not behave like other transition elements. This abnormality is due to the filled d-orbitals.

Ores of Zinc

Zinc is the \(23{\rm{rd}}\) most abundant element in the Earth’s crust and the fourth most widely consumed metal after iron, aluminium and copper. The important ores of zinc are-

Metallurgy of Zinc

The principal ore of zinc used for its extraction is Zinc blend \(({\rm{ZnS}})\). As Zinc blend is a sulphide ore, the process used for its concentration is the Froth Floatation Method. The concentrated ore is then roasted in excess air at \(1200\,{\rm{K}}\) to obtain zinc oxide \(({\rm{ZnO}})\). It is then reduced to \({\rm{ Zn }}\) by coke at a temperature of \(1673\;{\rm{K}}\). The impure zinc obtained after reduction is purified by electrolysis.

Dressing of Ore

The ore is initially crushed to about \({\rm{1}}{\rm{.9}}\) centimetres (\({\rm{0}}{\rm{.75}}\) inches). It is then combined with water and ground to less than \({\rm{0}}{\rm{.1}}\) millimetres in a ball mill. The finely ground particles and water form a slurry that flows from the mill to flotation cells or tanks.

The Concentration of Ore: Froth Flotation Method

Principle- The basic principle behind the Froth Flotation process is the difference in the wetting ability of the ore and the suspended impurities. The particles are categorized into two types based on their wetting ability:

1. Hydrophobic
2. Hydrophilic

If the suspended minerals are hydrophobic, they get attached to the froth and not to water. In contrast, the hydrophilic impurities remain suspended in water rather than with the froth.

The slurry obtained after the dressing of the ore is mixed with collectors. The main purpose of adding collectors is that it enhances the wettability of the ore and allows it to form a froth. Froth stabilizers such as cresols, aniline are also used to sustain the froth.

In the extraction of zinc from \({\rm{ZnS}}\), pine oil acts as a collector. The gangue and ore particles have a different degree of wettability with water and pine oil. The ore particles being hydrophobic are wetted by oil, whereas gangue particles being hydrophilic are wetted by water. The oil, along with the ore, floats on the surface of the water. A froth is generated by blowing air through this mixture. The froth is skimmed off to recover the concentrated ore. The gangue particles that are preferentially wetted by water settle at the bottom.

Depressants are used to prevent other metal sulfides from attaching to the froth.

Metal from Concentrated Ore

Roasting: After the Froth Floatation method, the next step in the extraction of zinc is roasting. Roasting is the process of heating a sulfide ore to a very high temperature in the presence of air. This process is generally applied to sulfide minerals. The sulfide is converted to an oxide during roasting, and sulfur is released as sulfur dioxide, a gas. The sulfur dioxide obtained is used in the manufacture of sulphuric acid.
\(2{\rm{ZnS}} + 3{{\rm{O}}_2} \to 2{\rm{ZnO}} + 2{\rm{S}}{{\rm{O}}_2}\)

Reduction: Zinc is moderately active and placed in the middle of the reactivity series. Hence, it can be easily reduced by carbon. Zinc oxide obtained after roasting of \({\rm{ZnS}}\) is ground into fine pieces and mixed with coke. This mixture is heated in fire clay containers at a temperature of about \(1673\;{\rm{K }}\). Carbon of the coke acts as the reducing agent and reduces zinc oxide \(({\rm{ZnO}})\) to Zinc \(({\rm{Zn}})\). The reaction of the reduction process is as follows-
\(2{\rm{ZnO}} + {\rm{C}} \to 2{\rm{Zn}} + {\rm{C}}{{\rm{O}}_2}\)

The zinc obtained after the reduction process immediately vapourises. This is because the reduction process takes place at \(1673\;{\rm{K }}\), whereas the boiling point of zinc is \(1180\;{\rm{K}}\). The zinc vapours so formed distils over, leaving behind impurities such as cadmium, lead, and iron. The gas vapours are cooled to yield solid zinc. The metal obtained is crude which is purified further by the electrolysis process.

Refining of Zinc

The final step in the extraction of zinc is the purification of the crude zinc obtained after the reduction process. The purification of crude zinc takes place by electrolytic refining.

In the electrolytic refining of zinc, we have an electrolytic cell consisting of impure zinc as anode and a pure thin sheet of zinc as the cathode. The electrolyte used here is zinc sulfate \(\left( {{\rm{ZnS}}{{\rm{O}}_4}} \right)\) with a small amount of sulphuric acid \(\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right)\). On passing an electric current, the following reactions occur, resulting in the deposition of pure zinc over the cathode.

At cathode: \({\rm{Z}}{{\rm{n}}^{2 + }} + 2{e^ – } \to {\rm{Zn}}({\rm{ pure }})\)

At anode: \({\rm{ Zn (impure) }} \to {\rm{ Zn}}{{\rm{ }}^{2 + }} + 2{e^ – }\)

Extraction of Zinc from Calamine \(\left( {{\rm{ZnC}}{{\rm{O}}_3}} \right)\)

Zinc can be extracted from its carbonate ore (calamine) through calcination. The steps are-

1. Concentration of Ore

Calamine is converted into its oxide form by heating the ore strongly in the absence of air. This process is known as calcination and produces zinc oxide and carbon dioxide gas.
\({\rm{ZnC}}{{\rm{O}}_3}(\;{\rm{s}}) \to {\rm{ZnO}}({\rm{s}}) + {\rm{C}}{{\rm{O}}_2}(\;{\rm{g}})\)

2. Reduction

Zinc oxide obtained after calcination of \({\rm{ZnC}}{{\rm{O}}_3}\) is ground into fine pieces and mixed with coke. This mixture is heated in fire clay containers at a temperature of about \(1673\;{\rm{K}}.\) Carbon of the coke acts as the reducing agent and reduces zinc oxide \(({\rm{ZnO}})\) to Zinc \(({\rm{Zn}}).\) The reaction of the reduction process is as follows-
\(2{\rm{ZnO}}({\rm{s}}) + {\rm{C}}({\rm{s}}) \to 2{\rm{Zn}}({\rm{s}}) + {\rm{C}}{{\rm{O}}_2}(\;{\rm{g}})\)

Uses of Zinc

1. Zinc is the most commonly used anti-rusting element. Galvanization involves the coating of iron surfaces with zinc to prevent corrosion by rusting.
2. Zinc is extensively used as an alloy, such as brass, bronze and is widely used in the dye casting industry, household appliances, etc.
3. Zinc is used in the production of zinc oxide, widely used as an anti-rash crème.
4. It is used for the negative plates in some electric batteries.
5. Zinc is a trace element that is essential for human health. When people absorb too little zinc, they can experience a loss of appetite, decreased sense of taste and smell, slow wound healing and skin sores.
6. An adult human body contains \({\rm{2 – 3}}\) grams of zinc to function properly for the body’s enzyme.

Summary

The most generally accessible ore utilized in the extraction of Zinc is called Zinc Blende, also known as Zinc Sulphide \(({\rm{ZnS}}).\) In this article, we learnt the various processes involved in the extraction of zinc. We also learnt one of the major concentration processes, i.e., the Froth Floatation Method and its uses in our routine lives.

FAQs on Extraction of Zinc

Q.1. Which ore is used to extract zinc?
Ans: The chief ore used to extract zinc is zinc blende \(({\rm{ZnS}}).\)

Q.2. How is zinc extracted from zinc blend?
Ans: The steps involved in the extraction of zinc from zinc blende is:
1. The concentration of the ore through the Froth Floatation Method.
2. Conversion of zinc blende to zinc oxide through the roasting process.
3. Conversion of zinc oxide to crude zinc by reduction through coke.
4. Electro refining of crude zinc to pure zinc metal through electrolysis.

Q.3. What is the main ore of zinc?
Ans: The main ore of zinc is zinc blende \({\rm{ (ZnS) }}.\)

Q.4. What is the first process of zinc extraction?
Ans: The first process in the extraction of zinc is the concentration of the ore, which is done by the Froth Floatation method. It is based on the principle that the gangue and ore particles have different wettability degrees with water and pine oil. The ore particles being hydrophobic in nature are wetted by oil, whereas gangue particles being hydrophilic in nature are wetted by water.

Q.5. Where is zinc used?
Ans: Most of the zinc is used in the galvanization of iron. It is also used for the production of zinc oxide \(({\rm{ZnO}})\) and for alloying purposes, for example, brass.

Q.6. What is the role of zinc metal in the extraction of silver?
Ans: In the extraction of silver, zinc metal acts as the reducing agent and reduces silver from its cyanide complex to its elementary form.
\(2{\left[ {{\rm{Ag}}{{({\rm{CN}})}_2}} \right]^ – }({\rm{aq}}) + {\rm{Zn}}({\rm{s}}) \to 2{\rm{Ag}}({\rm{s}}) + {\left[ {{\rm{Zn}}{{({\rm{CN}})}_4}} \right]^{2 – }}({\rm{aq}})\)

Q.7.What are 3 properties of zinc?
Ans: Zinc is a trace transition element. It is brittle and crystalline at ordinary temperatures but ductile and malleable between \({110^ \circ }{\rm{C }}\) to \({150^ \circ }{\rm{C }}.\) Though it is present in the d-block of the Periodic Table, it does not behave as d-block elements due to completely filled d-orbitals. It does not exhibit d-d transition due to which zinc salts are white in colour.

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