STEP 2: Very short questions
a. What is kept in the anode during electrorefining?
During electrorefining, the impure metal is always placed at the anode so that it slowly dissolves into the electrolyte solution.
b. Oxidation and reduction process is not necessary to extract gold.
Gold does not need oxidation or reduction because it is a very stable and unreactive metal that is already found in its pure form in nature.
c. What is the mistake in this experiment?
The mistake in the experiment is that silver sulphate solution is used as the electrolyte instead of copper sulphate solution, which is the correct electrolyte needed for refining copper.
d. Which step of metallurgy is shown in the diagram?
The diagram shows the process of electrorefining, which is the last step in metallurgy used to purify metals.
e. Which metal is extracted from haematite ore?
Iron is the metal that is extracted from haematite ore, which is its main natural source.
f. What mineral is the chief ore of aluminium?
Bauxite is the most important ore mineral from which aluminium is extracted.
g. What are ores? Give any two examples.
Ores are naturally occurring minerals from which metals can be extracted easily and at a reasonable cost. Two common examples are haematite and bauxite.
h. Why is argentite concentrated by froth floatation?
Argentite is a sulphide ore, so froth floatation is used to concentrate it. In this method, pine oil and air are used to separate the sulphide ore particles from the unwanted earthy impurities called gangue.
i. Which metal is mostly found in elemental form?
Gold is the metal most commonly found in its natural pure form inside the earth because it is highly stable and does not react easily with other substances.
STEP 3: Short questions
a. What is the advantage of oxidation reactions during metallurgy?
Oxidation reactions are very useful at several stages of metallurgy. Through calcination, oxidation converts carbonate and hydroxide ores into metallic oxides. Through roasting, it converts sulphide ores into metallic oxides as well. It also helps remove moisture and unwanted impurities like sulphur and phosphorus in the form of gases. Most importantly, metallic oxides produced by oxidation are much easier to reduce into pure metals than the original ore minerals.
b. Why is impure copper used as an anode and pure copper used as a cathode during the electrorefining of copper?
Electrorefining is the process of purifying metals using electricity in an electrolytic cell.
In this process, impure copper is placed at the anode so that copper atoms can lose electrons and enter the copper sulphate electrolyte as copper ions. The pure copper at the cathode acts as a surface where these copper ions gain electrons and get deposited as pure copper metal. This arrangement ensures that copper keeps moving from the impure anode to the pure cathode, while all impurities are left behind.
c. Write the molecular formula of the following ores.
i. Bauxite: Al2O3 · 2H2O
ii. Cryolite: Na3AlF6
iii. Felspar: K(AlSi3O8)
d. Differentiate between oxidation and reduction.
Oxidation is the process in which a substance gains oxygen or loses electrons, while reduction is the process in which a substance loses oxygen or gains electrons.
In metallurgy, oxidation is used to convert ores into metallic oxides, whereas reduction converts those metallic oxides into free metals.
The methods used for oxidation are calcination and roasting, while the methods used for reduction are smelting and electrolysis.
Oxidation is carried out by heating ores in the presence of air, while reduction requires the use of reducing agents such as coke or hydrogen.
e. Zinc sulphide is roasted instead of calcination. Why?
Roasting is the process of heating an ore strongly in excess air to convert it into a metallic oxide. Zinc sulphide is a sulphide ore that does not contain oxygen in its structure. Therefore, it must be roasted so that oxygen from the air can react with it to form zinc oxide, while sulphur dioxide gas is released. This is why roasting is the correct and suitable method for treating sulphide ores like zinc sulphide before further metal extraction.
f. All ores are minerals but all minerals are not ores. Give reason.
Minerals are naturally found inorganic substances that contain metals, while ores are only those minerals from which metals can be profitably and conveniently extracted. Many minerals contain metals in very small amounts or have too many impurities, making it neither easy nor economically worthwhile to extract metal from them. Therefore, although every ore is a mineral, not every mineral qualifies as an ore because only minerals that are useful for industrial metal extraction are called ores.
g. Write the molecular formula of the following ores.
i. Cuprite: Cu2O
ii. Feldspar: K(AlSi3O8)
iii. Copper glance: Cu2S
iv. Malachite: Cu(OH)2 · CuCO3
h. What types of ores are usually oxidized by roasting? Give an example.
Sulphide ores are the main type of ores that are oxidized using the roasting method. A well-known example is zinc sulphide, where roasting in excess air produces zinc oxide and sulphur dioxide gas according to the equation:
2ZnS + 3O2 → 2ZnO + 2SO2.
STEP 4: Long questions
a. What do you mean by the concentration of ore? Which type of method of concentration is useful for the following types of ores?
Concentration of ore is the process of removing unwanted earthy impurities known as gangue, such as soil, sand, and limestone, from the ore that is dug out from the ground.
Gravity separation, also called hydraulic separation, is useful for very heavy ores like gold, where flowing water is used to wash away lighter impurities and leave behind the heavier ore particles.
Froth floatation is useful for sulphide ores like argentite, where ore particles stick to oil bubbles and float to the surface while the gangue sinks.
Magnetic separation is useful for magnetic ores like magnetite, where a magnetic roller pulls the magnetic ore particles away from the non-magnetic gangue material.
b. How is reduction carried out? How are unstable, stable and highly stable metal oxides reduced? Explain with suitable examples.
Reduction is the process of removing oxygen from metallic oxides to obtain free pure metals. It is carried out either by simple heating or by using chemical reducing agents such as coke, carbon monoxide, or hydrogen, depending on how stable the metal oxide is.
Unstable metal oxides are those that decompose easily just by heating without needing any reducing agent. For example, when mercury oxide is heated, it simply breaks down into mercury metal and oxygen gas:
2HgO → 2Hg + O2
Stable metal oxides cannot be reduced by heat alone and require a reducing agent such as coke or carbon monoxide at high temperatures in a process called smelting. For example, iron oxide is reduced by carbon monoxide in a blast furnace:
Fe2O3 + 3CO → 2Fe + 3CO2
Highly stable metal oxides such as aluminium oxide cannot be reduced even by carbon because aluminium is too reactive. These oxides must be broken down using electrolysis. For example, when molten aluminium oxide is electrolysed, aluminium metal is deposited at the cathode:
2Al2O3 → 4Al + 3O2
Therefore, the method of reduction used for a metal depends entirely on how stable its oxide is, ranging from simple heating for unstable oxides to electrolysis for highly stable ones.
c. Describe the electrorefining of copper with a suitable diagram.
In electrorefining of copper, a large plate of impure copper is used as the anode and a thin strip of pure copper is used as the cathode. The electrolyte used is an acidified copper sulphate solution. When electric current is passed through the cell, copper atoms from the impure anode lose electrons, dissolve into the electrolyte as copper ions, and travel toward the cathode. At the cathode, these copper ions gain electrons and get deposited as 99.99 percent pure copper. The impurities from the anode either remain dissolved in the solution or settle at the bottom of the tank as anode mud.
d. What is oxidation? What types of oxidation are preferred for what types of ores? Explain with an example.
In metallurgy, oxidation is the process of converting concentrated ores into metallic oxides, which makes it easier to later reduce them into free metals. Calcination is the preferred method for carbonate and hydroxide ores such as calcium carbonate, where the ore is heated in limited air to drive off carbon dioxide and water, leaving behind the metallic oxide. Roasting is the preferred method for sulphide ores such as copper pyrite and zinc sulphide, where the ore is strongly heated in excess air so that sulphur is replaced by oxygen and sulphur dioxide gas escapes. For example, when iron carbonate undergoes calcination, it decomposes into iron oxide and carbon dioxide according to the equation:
FeCO3 → FeO + CO2
The correct choice of oxidation method ensures that the ore is properly converted into a metallic oxide that is ready for the next step of smelting.
e. What is the purpose of refining? Describes various methods of refining with an example.
The main purpose of refining is to remove the last traces of impurities from the metal obtained after reduction so that the highest possible purity can be achieved. Distillation is used for metals that have low boiling points, such as mercury and zinc, where the metal is heated until it vaporises and the pure vapour is then cooled and collected as pure metal. Electrorefining is the most widely used method, in which an electrolytic cell is used to transfer pure metal from an impure anode to a pure cathode, and this method is commonly used for purifying copper and silver. Poling is a method in which molten metal is stirred using green wood poles, and the gases released from the wood act as reducing agents to convert any remaining metallic oxides back into pure metal, and this method is often used for blister copper.