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Metallurgy is defined as the process by which metals are extracted from their naturally occurring ores. Ores are the natural aggregation of one or more valuable metals that are mined and processed for their use in various fields. Natural occurring metal compounds that are present with some other substances under the Earth’s core are known as metal ores. In the case of zinc extraction, the ore that is extracted from the earth’s crust by mining is called Zinc Sulphide (ZnS). Extraction of Zinc metal in its pure form is a bit complex task when compared to the extraction of other metals. Because at the melting point of other metals Zn becomes gas which allows Zn to escape with other gases. The ore of Zn which is generally ZnS contains very little concentration of the Zn and that is why the Froth Flotation process is performed to increase the concentration of Zn. After the concentration process, roasting is performed, and with roasting the main extraction of Zn metal starts.
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Key Terms: Zinc, Metallurgy, Metals, Metal Extraction, Ores, Froth Flotation, Roasting, Elements, Alloys, Zinc Sulphide, Concentration of Ore, Reduction
What is Zinc?
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Zinc is a silver greyish, brittle chemical element with an atomic number 30. It exhibits a +2 Oxidation state and is chemically similar to Magnesium in some respects. It has five stable isotopes and is the 24th abundant element in the earth's crust. Zinc blende is the most common ore of Zinc which is a Zinc Sulphide mineral. It is widely used in the automobile, electrical as well as hardware industries. It is also used in the formation of alloys with other metals. It is a transition earth metal and the fourth most used metal in the world.
Zinc
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Zinc Mining
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The process by which Zinc is extracted from its ore for the preparation of Zinc metal is known as Zinc mining. The most common ore of Zinc is Zinc blende which is a mineral of Zinc Sulphide. Other ores of Zinc are Calamine, Zincite, etc. Earlier, the extraction of Zinc was a very difficult process due to its low melting point. The temperature at which other metals begin to melt, Zinc becomes gas and gets escaped with exhaust gas. Zinc blende consists of very less percentage of zinc so it needs to be concentrated. The best method used for its concentration is the froth floatation method.
Zinc Mining
Procedure for Zinc Extraction
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The ore consists of a high amount of undesirable substance which is commonly known as gangue. In order to get highly pure metal, it needs to undergo multiple steps which involve various principles of chemistry. The steps which are involved in the extraction and isolation of metal are as follows-
- Concentration of Ore
- Isolation of the metal from its concentrated ore
- Purification of the metal
Concentration of Ore: Froth Flotation
A Slurry is prepared which is the mixture of water and fine powdered form of ore. Collectors and froth stabilizers are added to the slurry to increase the hydrophobic nature of the mineral. The collector will mainly act as the surfactant in this process.
Pulp is formed from the slurry which is added to the container containing water. For the formation of bubbles, an air jet is forced into the container. As mineral particles repel with water, they get attached to these bubbles and come to the surface. It leads to the formation of froth which is now collected and separated. The froth is further refined to get pure metal.
The principle of the wetting ability of particles is used in the Froth floatation process.
Minerals can only get attached to air bubbles when they are hydrophobic in nature. Greater the attraction between the bubbles and the minerals, the more the stability of the froth.
Isolation of the Zinc from its Concentrated Ore
After the formation of concentrated ore, metal is isolated from it. The method that follows is Roasting. In this process, concentrated ore is further ground into small pieces and suspended in a rising stream of air. This process leads to the reduction of Sulphur content and converts Zinc blend into Zinc Oxide. Zinc oxide is more reactive as compared to Zinc Sulphide.
2ZnS + 3O2 → 2ZnO + 2SO2
Reduction- Zinc Oxide is further reduced to Zinc metal after getting mixed with powdered coke and heated to 1673 K in fire clay. In this reaction, carbon from coke acts as the reducing agent, and the following reaction takes place-
2ZnO + C → 2Zn+CO2
Zinc gets vapourised immediately because of its boiling point which is and leaves behind impurities like iron, cadmium, etc after distillation. The gas containing zinc is collected and cooled to obtain crude metal which is known as spelter.
Purification of Metal
It involves the process of Electrolytic refining in which impure zinc is the anode and a pure thin sheet of Zinc is made the cathode. ZnSO4 acts as an electrolyte solution in which a small amount of Sulphuric acid is added. On passing the electricity, pure zinc gets deposited on the cathode side.
Uses of Zinc
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- In rubber manufacturing industries, Zinc oxide is used as a protective skin ointment.
- It is also used in the formation of alloys of different metals.
- Zinc is used in vehicles and other household appliances.
- In the human body, it is involved in maintaining the balance of enzymes.
Zinc Oxide Ointment
Process of Zinc Extraction
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Enriched zinc concentrates removed from the mine are transported all over the world in bulk quantity. They are temporarily stored and transported by conveyor belt. During the production of zinc, sulphuric acid is produced as a byproduct which is also shipped from the harbor. Multiple types of concentrates are picked by cranes which mix them and blend them to obtain the right mix. It ensures the highest quality of refined zinc.
The conveyor belt transports the concentrates to the roasting plant where it is introduced into a fluidized bed furnace with the addition of air. Here, sulfur is oxidized at about 1000 degrees Celsius and then purified to produce Sulphur dioxide. It is used to produce sulphuric acid.
Acid soluble roasted is zinc material is deposited at the bottom which is further cooled and is pumped to the leaching plant. It is purified along with volts oxide which is an important raw material for making zinc. Volt oxide is a powder produced from the recycling of zinc-coated Steel. Sulphuric acid purifies both products in the leaching plant precipitating the impurities. The solution further undergoes three faces for producing metallic zinc. The production of metallic zinc is based on electrolysis lead and Aluminium sheets are suspended in baths filled with zinc-containing leach solution or electrolytes.
Now the lead sheet is made anode and the aluminium sheet is made the cathode. It makes the zinc gets deposited on aluminium sheets during electrolysis which is further peeled off, washed, and transported to the melting furnace in the casting plant. It is melted at about 420 degrees Celsius and cast into slaps. During the casting process, strict quality control rules are followed.
Things to Remember
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- Metallurgy is the field of Science that deals with the extraction of metals from their ores.
- The minerals from which metals are economically and conveniently extracted is known as Ore.
- The commonly found ore of Zinc is Zinc blende which is a mineral of Zinc Sulphide. Other ores are Calamine, Zincite, etc.
- Extraction of Zinc metal from its ore takes place in three steps- Concentration of Ore, Isolation of metal from the concentrate and Purification of metal.
- Concentration of Zinc Sulphide takes place by froth flotation method which is based on the principle of the wetting ability of ore as well as impurities.
- Metal is isolated from its ore by the process of roasting and reduction which produces an impure crude metal.
- Crude metal is further purified by electrolytic refining.
- Zinc has many applications such as the formation of Alloy, in rubber industries, industrial applications, etc.
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Sample Questions
Ques. How Zinc is extracted from its ore? (5 Marks)
Ans. Zinc blende is the Chief Ore of Zinc metal. Zinc is obtained from its ore in the following steps-
- Concentration- The ore is crushed into a fine powder and is converted into a slurry mixture after adding water. Collectors and froth stabilizers are the main ingredients in this process. Based on the wetting ability of ore and impurities, the concentration of ore takes place.
- Roasting- In this process, concentrated ore is roasted in the presence of air which leads to the formation of Zinc Oxide.
- Reduction- Reduction of Zinc Oxide takes place in this process in the presence of powdered coke.
- Purification- Electrolytic refining is used in this process in which impure Zinc is made anode and pure zinc is made the cathode. The Electrolytic solution is ZnSO4 in which a small amount of dilute sulphuric acid is added. On passing electricity, pure zinc is deposited on the cathode.
Ques. How Calcination plays an important role in the extraction of Zinc metal? (3 Marks)
Ans. Calcination takes place only when Zinc is present in Carbonate form. In this process, Zinc Carbonate is strongly heated in the absence of air which leads to the formation of Zinc Oxide.
The reaction involved in Calcination is as follows-
ZnCO3 → ZnO + CO2
The Zinc Oxide obtained is further reduced to get Zn metal. Zinc oxide is made to react with Carbon so that it can get reduced. It leads to the formation of Zinc metal-
ZnO + C → Zn + CO2
Ques. How does distillation play an important in the extraction of Zinc? (3 Marks)
Ans. Distillation plays an important role in the extraction of Zinc in the following ways-
- The metals which have a low boiling point are usually purified by this method. Zinc is one of those metals.
- Impure metal is heated above its boiling point so that it can form vapours leaving behind the impurities. Impurities don’t get vapourised with metals.
- The vapours formed by the pure metal are collected and cooled down. It is condensed to form pure metal.
Ques. What are the advantages and disadvantages of the Froth floatation method? (3 Marks)
Ans. The advantages of the froth floatation process is as follows:
- This process is applicable for almost all types of minerals.
- Surface reagents control and alter surface properties.
- Sulphide ores get the best results from this method.
The disadvantages of this method are as follows:
- This method is highly complex and expensive.
- Slime affects the result in this process.
Ques. What are the uses of Zinc? (5 Marks)
Ans. Zinc is used in the following ways-
- It is used in the galvanization of iron to prevent it from rusting. It is also used in the galvanization of steel which is used in car bodies, lamp posts, suspension bridges, etc.
- It is also used to produce die castings for electrical, automobile, and hardware industries.
- Many products such as paint, rubber, soaps, cosmetics, pharmaceuticals, etc are formed by zinc.
- Zinc Sulphide is also used to make X-ray screens, luminous paint, and fluorescent lights.
- Zinc is also used in the formation of alloys such as brass, aluminium solder, etc.
- Zinc is also involved in maintaining the balance of enzymes in the human body.
Ques. What are different alloys formed by Zinc metal? (5 Marks)
Ans. The alloys formed from the Zinc metal are as follows-
- Zamak – This alloy is by the combination of Zinc as a base metal with aluminium, magnesium, and copper.
- Tombak – A brass alloy consisting of a high amount of copper and a low amount of zinc.
- Zinag Alloy – It is formed by the combination of Zinc, Aluminium, and Silver.
- Nickel Silver – It consists of Nickel, Copper, and Zinc but doesn’t consist of silver.
- Lead-free solder – In commercial use, lead-free solder consists of tin, copper, silver, bismuth, Zinc, indium, antimony, and traces of other metals.
Ques. How crude zinc metal is isolated from the concentrated ore? (3 Marks)
Ans. The isolation of crude metal takes place in the following ways-
- Roasting – In this process, Zinc Sulphide is reduced into Zinc Oxide. The concentrated ore is ground into small pieces which are further suspended in the stream of air so that it reacts with Oxygen present in the air. Following reaction takes place-
2ZnS+3O2 → 2ZnO+2SO2
- Reduction – Above formed Zinc Oxide is further reduced to form Zinc. Now the Zinc Oxide is made to react with powdered coke-
2ZnO+C → 2Zn+CO2
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