Electrolytic Refining

The oldest industrial electrolytic process is electro-winning. Sodium metal was obtained for the first time in elemental form in 1807 by the electrolysis of molten sodium hydroxide by the English chemist Humphry Davy. Maximilian, the Duke of Leuchtenberg, first experimentally demonstrated the electro-refining of copper in 1847. 

Electro-winning is the process into which the metals get placed in the solution from their ores, ie, they get electro-deposited to liquefy the metals. On the other hand, electro-refining is the process in which the impurities are removed from the metal.

The main aim of electrolytic refining is to purify metals. It’s very important to purify metals as they exist freely in nature, so many impurities are also present in them. To know about this electrolytic refining, it’s essential to know about electrolysis. 

Electrolysis is the process by which a chemical change takes place by the passage of an electric current. When a substance loses or gains an electron, it’s said to have undergone a chemical change. If it loses electrons, it’s called oxidation, and if it gains electrons, it’s called reduction. 

Metals that are purified by electrolysis

Copper (Cu) , Zinc (Zn) , Nickel , Silver (Au) , Gold (Au) , etc, are some of the metals refined by the process of electrolysis. Anode is made up of metal ( impure ) , and cathode is made up of a thin strip of pure metal. The electrolyte is the solution made with a soluble salt of the same substance. 

The nature of the metals and impurities present in them decide the method to be used for refining an impure metal. Therefore, electrolytic refining Au (gold) and Cu (copper) are refined. 

In metallurgy, impure metal is purified by refining. Electrolytic refining can be differentiated from other processes like calcining and smelting. In these two, a chemical change happens to the raw material, while in refining, the final substance or material is generally chemically identical to that of the original one. Many types of processes are used, including pyro-metallurgical and hydro-metallurgical techniques. 

The two most common pyro-metallurgical processes in refining are oxidation and reduction. In the oxidation, the metals that have a greater affinity for oxygen selectively combine with it and form metallic oxides; these metallic oxides can be further treated to get a pure metal or be separated and discarded as a waste product. Reduction is the reverse of oxidation. A metallic oxide compound is put into a furnace with any reducing agent (such as carbon) in this process. 

The metal releases all the combined oxygen leaving the pure metal combined with carbon to form carbon oxides. Hydro-metallurgical processes can be defined as the metal recovering method, which is used to obtain metals from their respective ores and waste materials by using the aqueous medium, by combining H2O, O2 and other chemical reagents in a pressurised environment (may or may not be). 

An electrolytic cell is used to purify materials (which are frequently metals) in the process of electro-refining. Impure metal acts as the anode, and the pure sample of the metal acts as the cathode. In a solution containing cations of the metal, a sample of the impure metal and a cathode are immersed, and an electric current is passed in between them. The metal is removed from the impure components and gets deposited in pure form in the cathode. 

Electrolytic refining of Copper (Cu) 

In refining a metal such as copper by electrolysis, the crude metal is used as the anode that goes into the solution. The pure metal gets deposited on the cathode, which is called electrolytic refining. 

In an electrolytic tank, the electrolyte used is acidified copper sulphate (CuSO4 + dilute H2SO4). An anode is a block of impure copper connected with the positive terminal of a power supply (battery). The cell’s cathode is a thin strip of highly pure copper metal. 

For producing high-purity copper, electrolytic refining is the main method. Less than 20 ppm impurities are present in the copper after electro-refining—after being melted and casted— and oxygen level is controlled at 0.018% – 0.025%. Electro-refining involves electrochemically dissolving copper from the impure anodes of copper to the electrolyte containing copper sulphate (CuSO4) and sulphuric acid (H2SO4). Then electro-chemically, it deposits the pure copper (Cu from the electrolyte) onto the stainless steel or copper cathodes. This is a continuous process.

the following reactions takes place during the electrolytic refining of Copper (Cu) –

Cathode reaction :

Cu2+ + 2e– → Cu

Anode reaction :

Cu → Cu2+ + 2e–

Insoluble impurities stick to the anode or fall to the refining cell’s bottom as anode mud. They get removed from there and are sent to a copper metal recovery plant and a by-product metal recovery plant. All the soluble impurities left the cell and are dissolved into the flowing electrolyte. The main purpose of electro-refining is to make a cathode copper high-quality.

The main requirements for this are – well spaced and flat vertical anodes and cathodes, a constant and gently flowing supply of the warm, high Cu²+ electrolyte which is to be across all the cathode faces and a continuous and a controlled supply of grain-refining agents and levelling. 

Conclusion

Therefore, electrolytic refining plays a vital role in refining metals using electrolysis. In electro-refining, impure metal is made anode, and pure metal is made as a cathode. The anode is the place or electrode where oxidation takes place, or loss of electrons takes place, and the cathode is the place or electrode where reduction or gain of electrons takes place. 

The chemical change takes place by oxidation and reduction. The electrolyte is the solution made with a soluble salt of the same substance. 

This process is very critical to obtaining metals in their pure state. Many metals are refined by this process, such as copper and gold.