Extracting Metals: An Introduction
Rocks and soil contain metals, an essential component of our universe. They are not actually found in their pure form and need to be isolated. They are discovered with some naturally occurring chemicals containing one or more elements or their compounds. They are referred to as minerals. Ores are minerals that can be used to extract elements at a profit.
Reactivity Series
The arrangement of metals in decreasing order of their reactivities is referred to as the "metal reactivity series," also known as the "activity series”. The metals near the top of the reactivity series are strong reducing agents. As you move down the series, the reducing power of the metal becomes less. Therefore, it was found that the metal at the top of the reactivity series was more reactive.
The rate of the displacement reaction depends on the reactivity series. The metal at the top of the reactivity series can easily displace the metal below it in its solution. Metals with low reactivity are easily displaced by those with high reactivity.
Metal Extraction in the Middle of the Reactivity Series
Metals in the middle of the activity series are moderately reactive, i.e.; they are neither very reactive nor less reactive. This may include manganese, iron, zinc, lead etc. In nature, they are typically found as metal sulphides or metal carbonates. It is easier to extract a metal from its oxides as compared to its sulphides and carbonates. Therefore, prior to the reduction, the metal sulphides and carbonates should be converted into metal oxides.
The sulphide ores are converted into oxides by heating strongly in the presence of excess air. This process is known as roasting. The carbonate ores are changed to oxides by heating strongly in limited air or completely in the absence of air. This process is called calcination. These metal oxides are then reduced to the corresponding metals using suitable reducing agents such as carbon. Zinc oxide, for instance, is transformed into metallic zinc when heated with carbon.
Sometimes displacement reactions can be employed instead of carbon (coke) to convert metal oxides to metals. Since they can displace less reactive metals from their compounds, highly reactive metals like sodium, calcium, and aluminium are used as reducing agents.
Extraction of Zinc (Zn)
Zinc has 2 common ores, Zinc blende (ZnS) and Calamine (ZnCO3). Therefore, zinc can be extracted by 2 methods:
1. Extracting Zn from Zinc Blend (ZnS)
In zinc blend (ZnS), it is very difficult to isolate zinc alone because the affinity for Zinc and Sulphur is greater. It should be converted to corresponding oxides by the process of roasting.
Roasting
2ZnS(s) + 3O2(g)+ Heat ⟶ 2ZnO(s) + 2SO2(g)
Zinc oxide, thus on reduction, gives metallic zinc.
carbon monoxide. The carbon replaces the zinc. Here, carbon acts as a reducing agent. Industrially, coke is used as a source of carbon.
Reduction
ZnO(s) + C(s) ⟶ Zn(g) + CO(g)
2. Extracting Zinc from Calamine (ZnCO3)
Calamine is heated with constrained air before being transformed into zinc oxide (calcination). Thus, when produced zinc oxide is reduced in the presence of carbon (coke), metallic zinc is created.
Calcination
ZnCO3(s)+ Heat ⟶ ZnO(s) + CO2(g)
Reduction
ZnO(s) + C(s) ⟶ Zn(g) + CO(g)
Extracting Manganese from Pyrolusite (MnO2)
Pyrolusite (MnO2) can be reduced directly. When pyrolusite is treated with carbon, there will be no reaction. It is because carbon is less reactive than manganese and cannot displace manganese. Hence, a more reactive element should be used to reduce the ore. Pyrolusite reacts with aluminium powder to give manganese in its molten state. This is because aluminium is highly reactive, and liberates a huge amount of heat when treated with pyrolusite.
MnO2+ C ⟶ No reaction
3MnO2(s)+ 4Al(s) ⟶ 2Al2O3(s)+ 3Mn(l) + Heat
This reaction is known as a thermite reaction. In a thermite reaction, aluminium powder is used to reduce a metal oxide, and the metal is extracted in the molten state because of the heat liberated during the reaction.
Extracting Iron from Haematite
Haematite can be directly reduced to form iron. It can be reduced by both carbon and aluminium powder. Here, coke served as the source of carbon. Hence, carbon is more highly reactive than iron; it can displace iron from the ore and form metallic iron and carbon monoxide.
Fe2O3(s) + 3C(s) ⟶ 2Fe(s) + 3CO(g)
Also, iron can be extracted by this process. When the aluminium powder is treated with haematite, iron in a molten state is obtained. This reaction has many industrial applications.
Fe2O3(s)+ 2Al(s) ⟶ Al2O3(s)+ 2Fe(l) + Heat
Interesting Facts
A Thermite reaction occurs when the aluminium powder is used to reduce a metal oxide, and the metal is extracted in the molten state as a result of the heat liberated during the reaction.
These displacement reactions are highly exothermic.
The amount of heat evolved is so large that the metals are produced in the molten state.
Iron (III) oxide (Fe2O3) is used to join railway tracks and crack machine parts. Therefore, this molten iron can be used in the welding railway cracks (Thermite welding).
Key Features
Through electrolysis or reduction, a metal can be separated from its ore.
Manganese, iron, zinc, and other metals in the middle of the reactivity series have a slightly lower reactivity than the top-ranking metals and a higher reactivity than the lowest-ranking metals.
Metal oxides are more easily reducible than metal sulphides or carbonates.
Prior to the extraction of the metal, roasting or calcination is used to convert the metal sulphides or metal carbonates to metal oxides.
FAQs on Metal Extraction in the Middle of the Reactivity Series
1. Explain the extraction of low-reactivity metals.
The low-reactivity metals include copper (Cu), mercury (Hg), silver (Ag), platinum (Pt), and gold (Au). These metals' inertness allows their oxides to be transformed into metals by simple heating. Gold, platinum, and silver are the three least reactive metals.
By heating alone, low-reactive metals can be reduced to metals. Thermal decomposition is the extraction process used to obtain these metals. The less stable an oxide or compound a metal forms, the easier it is to extract. Low-reactive metals, or metals found at the bottom of the reactivity series, require less energy to extract than metals found above hydrogen gas.
2. Explain the economic importance of metal extraction.
In contrast to silver ores and gold, which are rarer and hence would be a more expensive commodity, the ore is cheaper when it is abundant, such as in the case of iron ore. The cost of electrolysis to separate aluminium from its molten oxide is less expensive when ore is reduced using coke, which is derived from inexpensive coal, but different metals have varied qualities that make them best suited for particular and different uses. In general, more reactive metals (like Al) cost more to extract than less reactive metals (like Fe) because they require different amounts of energy and are easier to extract, which is occasionally a result of more expensive technology.
3. Zinc can displace copper from its salt solution. However, copper cannot remove zinc from its salt solution. Why?
The metals at the top of the reactivity series are significantly more reactive than the metals at the bottom of the series. Since copper is positioned just under zinc, it is evident that zinc is more reactive than copper. The least reactive components can be replaced by the more reactive ones. Therefore, an element's reactivity accelerates the displacement reaction. Because of this, zinc can easily replace copper in a solution, but copper cannot. Because of this, when a zinc rod is submerged in the copper sulphate solution, its bluish tinge disappears.
