When we say that gold is 24 carats, what does it mean? It means that the gold is completely absent from impurities, it is entirely pure. Now, this does not happen naturally. Metals extracted or mined always have some impurities in them. The process of removing such impurities is known as refining. Let us learn more about it.
Refining of Metals
In metallurgy refining of metals is the final process. Once the extraction process is complete we must ensure that the metal is free of any impurities. If you remember we have done a similar process before of removing impurities during concentration of ores. However, in the refining process, the chemical composition of the metal will remain unchanged.
There are various ways to make a metal pure. Which refining method is to be chosen will depend on the physical and chemical properties of a particular metal. Let us explore a few common methods of refining of metals.
Browse more Topics under General Principles And Processes Of Isolation Of Elements
- Occurrence of Metals
- Concentration of Ores
- Thermodynamic Principles of Metallurgy
- Extractions of Crude Metal from Concentrated Ore
- Uses of Aluminium, Copper, Zin, and Iron
Certain metals such as Zinc and Mercury have a very low boiling point. So on heating them they very readily vaporize. And of course, they leave behind their impurities. The impure metal is heated beyond its melting point in a furnace and the vapors are reconverted to metals once the impurities are separated.
Learn more about the Uses of Zinc, Aluminium, and Copper here.
Again suitable for metals with low melting points for example tin. In this process, we heat the impure metal and then we let it flow on a sloped surface. The impurities will remain behind and the pure metal will collect at the bottom of the slope.
This is one of the most common and widely used methods because it is applicable to most metals. In this method, we use the different electrochemical properties of the metals and the impurities to our advantage.
In this process, the impure metal is the anode. The cathode is a sheet of the pure metal. The electrolyte is the solution of the salt of the same metal. Then we pass an electric current through the solution. The pure metal from the anode will dissolve in the electrolyte bath and then collect at the anode. The impurities will either dissolve or be found in a heap at the base of the anode.
Copper refining is done by the electrolytic process. Its impurities of iron and zinc will dissolve in the copper sulphate solution. And its other impurities of gold or silver or platinum will remain behind.
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Zone refining is a special method we use to purify metals. It was the invention of William Pfann. It purifies metals to a very high degree. A rod of impure metal is placed in a container which we fill with inert gas. Then we place a circular heater around the rod at the top.
The impure metal heats up due to the circular heater. Then when the heater shifts to the next zone, the pure metal cools and crystallizes. The molten impurities move along with the movement of the heater and shift to the next zone. These impurities collect in the last zone and then we can separate them.
This follows the principles of chromatography. Chromatography deals with the movements of components at different rates in a mixture and/or differential absorption of an absorbent.
In this process, impure metal is put in a medium (liquid or a gas). Then we move the medium through an absorbent. Different components of the impure metal are will absorb at different levels. And then the components that were absorbed are removed by using a suitable solvent.
There are various forms of this method, like Column Chromatography (where we use Al2O3), Thin Layer Chromatography, Gas-liquid chromatography etc.
Learn more about Extraction of Metal from Concentrated Ore here.
Solved Example for You
Q: We can obtain highly pure metal by zone. True or False?
Ans: Zone refining or Fractional Crystallization is the method employed to get metal of very high purity i.e ultrapure samples of Ge, Si, B, Ga etc. This method is based on the fact that impurities are more soluble than the pure metal in the melt. So the above statement is True.