We have already discussed the three steps out of five for metal extraction in Metal Extraction: Part- 1. Now in this part, we will discuss the rest of the two steps.
4. Conversion of Metallic Oxides to Free Metals
The metallic oxides obtained from the calcinations and roasting need to undergo reduction reaction to extract the metal.
The reduction can be done in various ways like
- Reduction through electrolysis
- Carbon reduction method and
- Self-reduction etc.
The position of the metal in the series of metals’ reactivity determines the method required and a table is given as follows for your convenience to understand the methods easily:
Reactivity Series of Metal
1. Reduction Through Electrolysis
The top-ranked metals in the reactivity series like K, Na, Ca, Mg and Al undergoes reduction by electrolysis. The Aluminium is extracted from Aluminium oxide by electrolysis.
Firstly, a solid aluminium oxide is melted into liquid. Aluminium oxide melts at 2050°C. As it is tough to raise the temperature so high, aluminium oxide is added with cryolite (Na3AlF6). Then it melts at 800°C-1000°C and the melted Al2O3 consists of Al3+ and O2- ion.
A steel container with graphite carbon layer is necessary in this method. The mixture of aluminium oxide and cryolite is taken in the container and some carbon rods are dipped in the mixture in such a way that they do not touch the steel container anyway. The steel container is connected to the negative charge of the battery while the carbon rods are connected to the positive charge.
Once the electric ﬂow starts, the electrolysis starts too. In the electrolysis, O2- donates electron at the anode and produces O2 gas and Al3+ present in the solution accepts the electron at the cathode to turn into Al metal.
2. Carbon Reduction Method
This method is applied for the mid-level position holder metals in the reactivity series. Metal is extracted by heating carbon along with metallic oxide. This method is called carbon reduction.
For example, Zn from ZnO, Fe from FeO, Pb from PbO are extracted in this way.
3. Auto Reduction
This process is applied for the oxides of metals positioned at the lower part of the reactivity series like Cu, Hg and Ag. Only heating instead of any reductant can conduct the reduction reaction.
For example, the ore of mercury can be reduced in the following way:
5. Puriﬁcation of Metals
The metals obtained from the above reduction methods are not even fully pure. They also contain impurities and some methods are used to purify them.
The Method of Adding Smelting Agent
Generally, some impurity still remains with the metal extracted after reduction. The substance that is added to purify it from the impurity is called smelting agent. If the impurity is alkaline, acidic smelting agent (SiO2) will be added. And if they are acidic, alkaline smelting agent will be added. We know smelting agent and impurity combine together into slag. Since slag is insoluble, it is then separated from the upper layer of the solution of molten metal.
Even the smelting process does not give pure metal. The metal is further puriﬁed by electrolysis.
Puriﬁcation by Electrolysis
In this method, the impure metal is used as an anode and a rod of the same metal in pure form is used as a cathode. The salt solution of the metal that is to be puriﬁed is used as an electrolytic solution.
When electricity is passed through the cell, the atom of the metal from the anode donates electron which enters the solution as ion. On the other hand, metallic ion accepts the electron and adds to the pure metal at the cathode. The method is used for copper and other metals. As for example
Puriﬁcation of Copper by Electrolysis
Copper obtained from reduction method is 98% pure and electrolysis can yield 99.9% pure copper. For this, an aqueous solution of CuSO4 is taken in a container and the metallic rod that needs to be puriﬁed connected to the positive charge of a battery is put in the container. It acts as an anode. A rod of pure copper (mm) is connected to the negative charge of the battery and put in the cell as a cathode. Usually, the impure copper rod is thick while the pure rod is thin.
Now, as electricity is passed in the cell from the battery, the Cu2+ ion from the impure rod goes into the solution. From the solution, the Cu2+ goes to the cathode and gathers with the pure rod as Cu.
Purification of impure copper through electrolysis
Here, an oxidation reaction occurs in the anode and a reduction reaction occurs in the cathode.
*References- NCTB Books and Wikipedia