Refining

A Brief Overview of Refining

In metallurgy, refining process against impurities refers to the purification of impure metal. In this process, the end product is chemically identical to the original material but purer in form. 

Reduction method of metals cannot produce the purest version of metals. In such case, refining technique assists to obtain the 100% pure metal in small quantity by removing impurities. 

However, this technique varies with different metals. Sometimes, a specific substance helps to bring out the desirable form of a metal. In some other cases, a particular metal is refined to acquire some valuable by-products. 

Generally, refining can be done in several methods. Some of them are discussed in details below. 

Distillation

It is a process for separating components from a mixture based on the varying boiling points. In the case of metals, this method helps to refine the metals which have low boiling points like zinc, mercury and arsenic. During this process, any of these metals receives heat beyond its boiling point until it vaporises completely, leaving behind the solid impurities. Once that is over, the vapour containing pure metal cools down and stores as a pure metal. 

This particular refining process against impurities is further classified into sub-categories. 

  • Simple Distillation- Usually liquids are separated from solids or other non-volatile substances. 

  • Steam Distillation- Mainly we get essential oils from flowers using this method. 

  • Fractional Distillation- This process aids to separate cadmium from zinc. In this method, zinc containing cadmium mixes with powdered coke and then heated. The first batch of condensate contains cadmium, whereas pure zinc we get from subsequent steps. 

Liquidation

This process separates the components of ore or alloy or a crude metal by partial melting. Unlike distillation, in this process, the pure metal melts instead of vaporising. During this process, the metal having low melting point melts on passing heat while the impurities having high boiling points remain solid. Therefore it becomes easier for separating the pure metal from the impurities. Usually, the entire process is conducted in a sloped container; hence the liquid metal runs down while the solid impurities stay behind in the container. 

Earlier it helped to separate silver from copper using lead as a solvent. Still, this method helps in refining tin. 

Poling

Poling method is used to refine the metals containing oxidised impurities. Primarily is used to purify the metals such as tin and copper that exist in the impure of tin oxide and cuprous oxide, respectively. 

Earlier a green branch of trees was necessary to stir the molten metal as the hydrocarbon organic components of that part helps in reducing oxide impurities. Usually, it is a copper refining process against impurities.   

Firstly, the impure copper is kept in an anode furnace for two steps of refining. Next, iron and sulphur release in the form of iron oxide and sulphur oxide by blowing air. In this step, the iron oxides release from copper and gaseous sulphur oxide emits from furnace by off-gas system. After the first oxidation step, the second step of poling begins. During the second step diesel or natural gas acts as a reducing agent to remove the oxide from cuprous oxide. Finally, we get pure copper. 

Oxidation or Cupellation

 Cupellation is a process of removing impurities from silver or gold in a cupel (a shallow and porous dish made of bone-ash or other refractory material). 

In this process, the impure gold or silver stays on the cupel, and then a blast of hot air passes through it within a special furnace. The impurities like tin, copper, lead, etc. are oxidised. The cupel absorbs some portions of the impurities, and the rest vaporises, leaving behind the pure silver or gold. 

This method also purifies molten pig iron within a Bessemer converter. 

Electrolytic Refining

The most common refining process against impurities is electrolytic process. Most metals such as gold, copper, zinc, chromium and silver are purified electrolytically. For example, electrolytic refining of copper takes place in the following steps.

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  • An electrolyte of acidified solution of copper sulphate is necessary. A thick rod of impure copper acts as an anode, and a thin rod of pure copper acts as a cathode. 

  • Then current passes through the solution. The acidified CuSO₄ splits into copper ions and sulphur ions. 

  • The copper ions from electrolyte moves towards cathode and gain two electrons from the cathode. Then deposit as pure copper atoms on the thin copper rod. 

  • At the same time, copper atoms from the anode lose two electrons and converts into copper ions. Then dissolve in the solution. Thus the concentration of the electrolyte remains the same. 

  • In this way, indirectly copper atoms from the anode stores on the cathode repeatedly. 

  • Eventually, the size of the cathode decreases, whereas the size of the cathode increases. 

  • Finally, all the pure copper from anode deposits on the cathode. Similarly, the impurities in the impure copper rod settle down as anode mud at the bottom of the container. 

Reactions that take place in anode and cathode as follows

In Anode: Cu(s) → Cu2+(aq) + 2e-

In Cathode: Cu2+(aq) + 2e- → Cu(s)

Zone Refining

Scientist W.G. Pfann discovered this refining process against impurities. Typically metals that are purified using this method are gallium, silicon, indium, germanium, boron and other metals. 

The impurities have higher solubility in the molten metal as compared to that in solid metal. The difference in solubility of impurities makes this process possible. 

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During this Process of Zone Refining

  • The rod of impure metal inserted inside a tuber zone refiner with an inert gas environment. 

  • A circular mobile heater stays around the impure metal rod. Along the rod, the heater moves from one end to another. 

  • As the heater shifts from one zone of metal rod to another, impurities also shift to the zone with molten metal. 

  • Finally, impurities concentrate at one end of the metal rod. 

This process can be repeated until purest form of metal comes out. 

Vapour Phase Refining

In this refining process against impurities, impure metal is heated with a specific reagent that produces a suitable volatile compound as product. This volatile compound is heated which undergoes decomposition producing pure metal. 

Following are the Requirements for this Process. 

  • Metal should form a volatile compound with a suitable reagent. 

  • The volatile compound should easily decompose so that the recovery of the metal is easy. 

For example, Nickel is refined using this method. It is also known as Mond process. Following are the steps of acquiring pure nickel without impurities. 

  • Impure nickel is heated with a stream of CO gas. 

  • A volatile compound Nickel tetra carbonyl is produced. 

  • Impurities are left behind. 

  • Volatile Nickel tetra carbonyl is heated strongly decomposes and produces pure nickel. 

Van-Arkel Method

It is used in purification of titanium and Zirconium. Impure Ti/Zr is heated in an evacuated vessel with Iodine. Next, volatile titanium tetra iodide/ zirconium tetra iodide is obtained as product. Finally, it is heated with a tungsten filament to acquire pure metal (Ti/Zr).  

 Chromatographic Method

This refining process against impurities of metals follows a principle where different components of a mixture get absorbed as an absorbent at different levels, depending upon different rates of absorption. 

During this process, impure metal exists in a medium (gas or liquid). Next, you have to run a medium via an absorbent. Different impure components of metals will leave at subsequent levels. Next, you can remove impure elements using an appropriate solvent. 

This process is divided into several categories, like thin layer chromatography, column chromatography, etc. 

Did You Know?

The distillation using a terracotta apparatus was prevalent in Indus valley civilisation since 3000 B.C. Also, Babylonians practised this method in Mesopotamian civilisation. 

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FAQ (Frequently Asked Questions)

1. What Kind of Metals is Purified Using Zone Refining Method?

Ans. Zone refining method is used to refine metalloids like silicon, germanium, boron, etc. Likewise, some metals such as tantalum and gallium are refined too. 

2. Where Does the Impurity go of Impure Metals after Refining?

Ans. After passing the electric current, the impure metals disintegrate from the anode and mixes with the electrolytic solution. Only the soluble impurities dissolve into the solution. On the other hand, the insoluble impurities are collected as anode mud at the bottom of it.  

3. What is the Fundamental Difference Between Smelting and Refining?

Ans. Smelting refers to the extraction of metals from ores using pyrometallurgy (heating and melting). Refining refers to the process of extracting valuable metals from ores and different raw materials. More specifically, refining denotes the processes of acquiring purer version of metal. 

4. Which Method is Used to Refine Copper?

Ans. Electrolysis is the process generally used to refine copper.