Refining Definition
Refining is defined as the process of reducing the impurities of a substance. Metals that are extracted from its ore are usually impure in nature. The extracted impure metal is called crude metal. Refining of metals is the final process. After the extraction process the metal is free from impurities. The refining method to be chosen will depend on the physical and chemical properties of a particular metal. There are different methods of making a metal pure :
Distillation
Liquation
Poling
Electrolysis
Zone refining
Vapour Phase refining
Chromatographic Method
Distillation
Metals such as Zinc and Mercury consist of a very low boiling point. So zinc and mercury readily vaporise while heating them, leaving behind their impurities. The leftover impure of metal is heated beyond its melting point in a furnace and the vapours are reconverted to metals once the impurities are separated.
Liquidation
Liquidation is suitable for metals having low melting points like tin. In Liquidation process, the impure metal is heated and then it flows on a sloped surface. While flowing, the impurities will remain behind and the pure metal will collect at the bottom of the slope.
Poling
Poling is a method used for metals that have oxidised impurities to purify them. Poling is mainly used to purify metals that are in the impure form like copper or tin of copper oxide or tin oxide. In the poling method a log of wood is taken that is still green and then used to stir the liquid metal. The hydrocarbons of the green wood reduce the metal, and the oxygen leaves as CO2 gas.
Electrolytic Refining
Electrolytic Refining is the most common and widely used method because it is applicable to most metals. In Electrolytic Refining, different electrochemical properties of the metals and the impurities are used. In Electrolytic Refining, the impure metal is the anode and the pure metal is cathode. The electrolyte is the type of the salt solution of the same metal through which an electric current is passed. The pure metal extracted from the anode will dissolve in the electrolyte bath and then collect at the anode. The impurities found will either dissolve or be found in a heap at the base of the anode.
Vapour-Phase Refining
In Vapour-phase refining, For releasing a metal from impurities, it is first converted into a suitable volatile compound by heating it. And the heating requirements are:
The metal used for this process should form a volatile compound with a suitable reagent.
The volatile compound that is to be used should be easily decomposable so that the recovery is easy.
Ni + 4CO Ni(CO)4 Ni + 4CO.
Zone Refining
It is a special method used to purify metals. In this method, metals are purified to a very high degree. Impure metal rod is placed in a container filled with inert gas. Then a circular heater is placed around the rod at the top for heating the impure metal. And when the heater shifts to the next zone, the pure metal cools and crystallises. The impurities that are melted will move along with the movement of the heater and shift to the next zone. All these impurities are then collected to the last zone and then it can be separated.
Chromatographic Method
This method depends on the chromatography. Chromatography mainly deals with the movements of components at different rates in a mixture and/or differential absorption of an absorbent. In the chromatographic method, impure metal is put in either liquid or a gas medium. Then the medium is moved through an absorbent. Different components will absorb the impure metal at different levels. And then those components are removed by using a suitable solvent. There are different forms of chromatographic methods, like Column Chromatography, Thin Layer Chromatography, Gas-liquid chromatography etc.
Did You Know?
The distillation using a terracotta apparatus was prevalent in Indus valley civilization since 3000 B.C. Also, Babylonians practised this method in Mesopotamian civilisation.
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FAQs on Refining
1. What is refining in chemistry, and why is it a necessary step in metallurgy?
Refining is the process used to purify an impure metal, often called a crude metal, after it has been extracted from its ore. This step is essential because crude metals contain various impurities, such as other metals, non-metals, or leftover slag. These impurities can harm the metal's properties, like its strength or conductivity. Refining removes these unwanted substances to produce a metal with the desired purity and characteristics for its intended use.
2. What is the basic principle used in most metal refining methods?
The fundamental principle of refining is to take advantage of the differences in physical or chemical properties between the desired metal and its impurities. For instance, some methods rely on differences in melting points (liquation), boiling points (distillation), or how they react with other chemicals (vapour phase refining). By targeting these differences, the pure metal can be effectively separated from the impurities.
3. What kind of impurities are typically removed during the refining of a metal?
The impurities found in a crude metal that need to be removed can include:
- Unreduced oxides of the metal itself.
- Non-metals like carbon, silicon, phosphorus, and sulphur.
- Other foreign metals that were also present in the original ore.
- Leftover materials like flux or slag that got trapped in the metal during extraction.
4. What is the main difference between smelting and refining?
Smelting and refining are two different stages in producing a metal. Smelting is the initial process of extracting a metal from its ore, usually by heating it with a reducing agent. Its goal is to get the metal out of its mineral form. Refining is the next step, where the crude metal from smelting is purified. The goal of refining is to remove remaining impurities to achieve a high degree of purity.
5. How does the zone refining method work to create ultra-pure semiconductors?
Zone refining works on the principle that impurities are more soluble in the molten state of a substance than in its solid state. A mobile heater slowly moves across a rod of impure metal, creating a narrow molten zone. As the pure metal solidifies and crystallises behind the heater, the impurities are carried along with the molten zone to one end of the rod. This process is repeated to concentrate the impurities at one end, which is then cut off. This method is crucial for producing highly pure silicon and germanium for electronic devices.
6. What is the principle behind the electrolytic refining of copper?
Electrolytic refining uses electrolysis to purify copper. In this setup:
- An impure copper block is used as the anode (positive electrode).
- A thin sheet of pure copper is used as the cathode (negative electrode).
- The electrolyte is a solution of an acidic salt, such as copper sulphate.
When electricity is passed, pure copper ions from the impure anode dissolve into the solution and deposit onto the pure cathode. The less reactive impurities, like gold and silver, do not dissolve and collect at the bottom as anode mud.
7. What is the Van Arkel method and for which metals is it used?
The Van Arkel method is a type of vapour phase refining used to get ultra-pure metals. The process involves two steps: first, the impure metal is converted into a volatile compound (like an iodide), and second, this compound is decomposed at a higher temperature to deposit the pure metal. It is especially useful for refining reactive metals like zirconium (Zr) and titanium (Ti) to the high purity needed for space and nuclear applications.
8. What is an example of vapour phase refining used in industry?
A classic industrial example of vapour phase refining is the Mond process for purifying nickel. In this process, impure nickel is gently heated with carbon monoxide gas to form a volatile compound called nickel tetracarbonyl [Ni(CO)₄]. The impurities are left behind as solids. This gas is then heated to a much higher temperature, causing it to decompose and deposit very pure nickel.
9. In which situation is the liquation method suitable for refining?
The liquation method is suitable for refining metals that have a low melting point compared to their impurities. The impure metal is heated on a sloped surface to a temperature just above its melting point. The pure metal melts and flows down, leaving the solid, higher-melting-point impurities behind. This technique is effective for refining metals like tin (Sn) and lead (Pb).
10. How does chromatography help in refining certain elements?
Chromatographic methods are used for refining when elements have very similar chemical properties, making them hard to separate. This method relies on the principle of differential adsorption. A solution containing the mixture is passed through a column packed with an adsorbent material. The element that is least strongly adsorbed travels down the column fastest and can be collected first, allowing for a clean separation. This is particularly useful for separating elements like the lanthanoids.
