Refining of Metals
Understanding the purification processes for extracted metals
Introduction to Metal Refining
The metals extracted from their ores often contain impurities that affect their properties and limit their applications. Refining is the process of removing these impurities to obtain pure metals suitable for various industrial and commercial uses.
Why is refining necessary?
Even after extraction, metals may contain:
- Other metals as impurities
- Non-metallic impurities like carbon, silicon, phosphorus, or sulfur
- Trapped gases or slag particles
- Oxide layers formed during extraction
These impurities significantly alter the physical and chemical properties of metals, reducing their usefulness for specific applications.
Methods of Metal Refining
Different refining techniques are employed based on the nature of the metal, the type of impurities, and the level of purity required. These methods can be broadly classified into physical and chemical methods.
Physical Methods of Refining
1. Liquation
This method is based on the difference in melting points between the metal and its impurities.
Example: Refining of Tin
Tin (melting point: 232°C) can be separated from higher-melting-point impurities like iron by heating the impure tin just above its melting point. The molten tin flows away, leaving behind the solid impurities.
2. Distillation
This method is used for refining volatile metals with low boiling points by heating them until they vaporize, followed by condensation of the vapor.
Example: Refining of Zinc
Zinc has a relatively low boiling point (907°C) compared to many of its impurities. When impure zinc is heated strongly, it vaporizes, leaving behind non-volatile impurities. The zinc vapor is then condensed to obtain pure zinc.
Chemical Methods of Refining
1. Electrolytic Refining
This is one of the most widely used methods for obtaining high-purity metals. It involves using electricity to transfer metal atoms from an impure anode to a pure cathode through an electrolyte solution.
Example: Electrolytic Refining of Copper
In this process:
- Anode: Impure copper
- Cathode: Thin sheet of pure copper
- Electrolyte: Copper sulfate (CuSO₄) solution acidified with sulfuric acid (H₂SO₄)
When electric current is passed:
(Anode +)
solution
(Cathode -)
At the anode, copper dissolves into the solution:
At the cathode, copper ions are reduced to copper metal:
Less reactive impurities like gold, silver, and platinum do not dissolve and collect as anode mud below the anode. More reactive impurities like Fe, Zn remain in solution and do not deposit at the cathode.
2. Zone Refining
This method is based on the principle that impurities are more soluble in the molten state of the metal than in the solid state. It’s used to produce ultra-pure metals for semiconductor industry.
Example: Refining of Silicon and Germanium
A moving heating coil is passed slowly along a rod of impure metal from one end to the other. As the coil moves, a molten zone is created, and impurities concentrate in this molten zone and move to the end of the rod.
3. Vapor Phase Refining
In this method, the metal is converted into a volatile compound, which is then decomposed to give pure metal.
Example: Mond Process for Refining Nickel
This process is based on the formation of nickel carbonyl [Ni(CO)₄], which is a volatile compound that decomposes easily when heated.
This process can produce nickel with a purity of over 99.9%.
Industrial Refining Processes for Common Metals
| Metal | Main Refining Method | Key Features |
|---|---|---|
| Copper | Electrolytic refining | Produces 99.99% pure copper; recovers precious metals in anode mud |
| Aluminum | Hoopes process (electrolytic) | Three-layer electrolytic cell with molten electrolytes |
| Iron/Steel | Oxidation processes (Basic Oxygen Process) | Removes impurities by oxidation (C to CO₂, Si to SiO₂, etc.) |
| Gold | Miller process and Wohlwill process | Miller: Chlorine gas method; Wohlwill: Electrolytic refining |
| Silver | Parkes process and electrolytic refining | Separation from lead using zinc, followed by electrolysis |
| Zinc | Fractional distillation | Based on difference in boiling points |
| Silicon | Zone refining | For semiconductor-grade silicon (99.9999% purity) |
Exam-Focused Key Points
Important concepts to remember:
- Selection of refining method depends on:
- The chemical reactivity of the metal
- The difference in properties between the metal and the impurities
- The level of purity required
- Electrolytic refining is the most widely used method for obtaining high-purity metals because:
- It can produce very high purity metals (>99.9%)
- It allows recovery of valuable impurities (like silver and gold from copper refining)
- It’s applicable to many commercially important metals
- Zone refining produces the highest purity metals (up to 99.9999%) and is essential for semiconductor production
- The electrochemical series plays a crucial role in understanding which impurities will be removed during electrolytic refining
- During electrolytic refining, metals more reactive than the metal being refined remain in solution, while metals less reactive form the anode mud
Solved Example Problem
Question: During the electrolytic refining of copper, which of the following impurities would be found in the anode mud?
a) Iron b) Zinc c) Silver d) Manganese
Solution:
We need to compare the reactivity of these metals with copper. Looking at the electrochemical series:
Zn > Fe > Mn > Cu > Ag
(From more reactive to less reactive)
During electrolytic refining of copper:
- Metals more reactive than copper (Zn, Fe, Mn) will go into solution as ions and remain there
- Metals less reactive than copper (Ag) will not dissolve and will collect as anode mud
Therefore, silver (Ag) would be found in the anode mud.
Answer: c) Silver
Applications and Significance
The refining of metals is a crucial step in metallurgy for numerous reasons:
- Enhanced physical properties: Pure metals have improved mechanical strength, ductility, malleability, and conductivity
- Improved chemical resistance: Pure metals often have better corrosion resistance
- Specialized applications: Many modern technologies require extremely pure metals:
- Electronics industry needs 99.9999% pure silicon and germanium
- Aerospace industry needs high-purity titanium and aluminum alloys
- Medical implants require high-purity metals to avoid rejection
- Recovery of precious metals: Refining processes like electrolytic refining allow the recovery of valuable metals like gold, silver, and platinum that are present as impurities
- Economic value: The price difference between impure and high-purity metals can be substantial
Summary and Key Takeaways
- Metal refining is the process of removing impurities from extracted metals to enhance their properties
- Refining methods can be broadly classified as physical methods (liquation, distillation) and chemical methods (electrolytic refining, zone refining, vapor phase refining)
- The choice of refining method depends on the metal’s properties, the nature of impurities, and the desired level of purity
- Electrolytic refining is the most widely used method for commercial metals like copper
- Zone refining produces the highest purity metals and is essential for semiconductor manufacturing
- Understanding the position of metals in the reactivity series helps predict their behavior during refining processes
Exam Tips:
- Be able to explain the principles behind different refining methods
- Know which refining methods are appropriate for specific metals
- Understand the electrochemical principles in electrolytic refining
- Be familiar with the equations and reactions involved in major refining processes
- Practice problems related to the behavior of impurities during refining
- Remember the industrial applications and significance of obtaining pure metals
