Electrolysis is a process of chemical decomposition produced by passing an electric current through a liquid or solution containing ions. In this process, we generally pass a direct electrical current through an ionic substance which is either present in its molten form or is dissolved in any appropriate solvent, producing chemical reactions at the electrodes and causing material decomposition. The key process of electrolysis is the exchange of atoms and ions from the external circuit by removing or adding electrons. Often the desired electrolysis products are in a different physical state from the electrolyte and some physical processes can remove them. The main components needed for electrolysis are:
An electrolyte: A substance, often an ion-conducting polymer that contains free ions in the electrolyte that carries electrical current. If the ions, as in most solid salts, are not mobile, electrolysis will not be possible.
Direct current (DC) electrical supply: It provides the necessary energy to generate or discharge the electrolyte ions. Electrical current is transported in the external circuit by electrons.
Two electrodes: These are electrical conductors providing the physical interface between the electrolyte and the energy-providing electrical circuit.
1. Metal Extraction (Electrometallurgy)
Electrometallurgy is the process of electrolysis for extracting metal from the ore. Electrolytes of fused electrolytes obtain metals such as sodium, potassium, magnesium, calcium aluminum, etc. There are two methods of metal extraction based on ore’s physical state. The ore is treated with a strong acid in the first method to obtain a salt and such salt solution is electrolyzed to release the metal. The ore is in a molten state in the second process and electrolysed in a furnace.
(i) Extraction of Zinc:
Sulphuric acid is used to treat the zinc ore. During the electrolysis process involved in this reaction, the zinc sulphate solution is obtained as a result, which also acts as an electrolyte for the electrolytic reaction. The cathodes are made of aluminum in the electrolyte tank and the anodes are made of lead. The current density is maintained at 1000 A / m2 and the cathode deposits zinc. The consumption of energy is between 3000 and 5000 KWH per tonne.
(ii) Extraction of Aluminium:
Bauxite and cryolite are aluminum ores. These are chemically treated and reduced to aluminum oxide, then dissolved in fused cryolite and electrolysed in a furnace. Deposits of aluminum settle down at the bottom. The furnace temperature is about 1000 °C to keep the electrolyte in a fused state. To complete the process, the current of around 4000 ampere is needed and the energy consumed is around 20,000 to 25,000 kWH per tonne.
2. Production of Non-Metals
Electrolysis is used to obtain non-metals such as hydrogen, fluorine, chlorine. Hydrogen is obtained in the presence of electrolytes such as H2SO4 and KNO3 through the electrolysis of water.
3. Metal Refining
The main advantages of electrolytic processes of extracting a metal are that the purity of the product obtained is between 98 and 99 %. Electrolysis is used for further refining. The anode is made of metal that has been extracted. The cathode deposits pure metal. The electrolyte is made of metal solution such as copper, copper sulphate and nickel, nickel chloride. Electrolytic process energy consumption in copper refining is 150 to 300 kWH / ton of refined copper. Copper purity can be enhanced by electro-refining. The same electroplating or electrodeposition principle is used in this case. The electrolyte is a copper ion solution, like copper sulphate. The cathode is a strip of pure copper and the anode is an unclean copper lump. At the anode: copper atoms dissolve to form copper ions from the impure copper. This causes the size of the anode to decline. Impurities are gathered in the cell and fall off the anode. At the cathode: ions Cu2 + migrate to the cathode where they are deposited as solid copper on the cathode. The cathode is getting thicker (increasing in size).
4. Production of Chemicals
Many chemicals like caustic soda (NaOH) chlorine gas etc. are produced on a large scale through electrolysis. Large-scale electrolysis also produces potassium permanganate, hydrogen and oxygen etc.
Electroplating is an object's coating with a thin metal film deposited during electrolysis by an electrolytic solution. Electroplating is often used on items such as knives and forks (silver plate) to produce an attractive and durable finish. It is also used to protect metals susceptible to corrosion. For example, relatively unreactive cadmium metal is often placed on steel components to inhibit rusting. Electroplating is a very valuable industrial process, but its use requires expensive and consistently efficient treatment of the waste that it produces. It is possible to produce inexpensive silver-plated jewelry by electroplating. Gold rings, which cause the fingers to turn green, are in fact gold-electroplated copper rings.
The article to be plated is used as the cathode in the electroplating process and the metal plated on the article is used as the anode. The electrolytic solution or ‘bath’ contains a plated metal salt. A low-voltage electrical current causes metal ions from the bath to gain electrons from the cathode and deposit on the cathode (the object) as a metal coating. It also causes metal atoms to lose electrons on the anode and go as ions into the bath. The anode gradually disappears as the plating proceeds and maintains the concentration of the metal ion in the bath. Many toxic solutions are used in electroplating operations. Objects to be plated with concentrated acidic or basic solutions are thoroughly cleaned of all grease and dirt. Eventually, due to contamination, cleaning solutions become ineffective and must be disposed of.
A number of factors contribute to the quality of the metal coating formed by electroplating:
• The concentration of the cations to be reduced is carefully controlled in the plating solution. It is necessary to avoid unwanted side reactions.
• The electrolyte type and concentration should be carefully considered.
• Compounds to control acidity and increase conductivity must be included in the solution.
• Some compounds make it brighter or smoother to cover the metal.
• The anode must be shaped like a cathode object in many electroplating cells to achieve even metal coating
This is a process by which a process of electro-plating reproduces wood cuts etc. in copper. A mould is first made of the type in wax in this process, then coated with black lead to give it a metallic surface and then subjected to the electro-deposition process. Thus, on the prepared surface, a copper film is formed.
This is another electro-deposition application. Electro-deposition reproduction of objects on some kind of mould or shape is known as electro-forming. This is another useful example among many electrolysis applications. We must take the impression of objects on wax or other wax-like material first. To make it conductive, the surface of the wax mold that bears the object's exact impression is coated with graphite powder. Then the mold is immersed as a cathode in the electrolyte solution. The electrolyte metal will be deposited on the impressed graphite surface of the mold during the electrolysis process. The article will be removed after obtaining a layer of desired thickness and the wax will be melted to obtain the reproduced object as a metal shell. Reproduction of gramophone record disc is a popular use of electroforming. The original recording takes place on a wax composition record. This wax mold is then coated to make it conductive with gold powder. This mold is then immersed as a cathode in a blue vitriol electrolyte. Using a copper anode keeps the solution saturated. On the wax mold, the copper electroforming produces a master plate used to stamp a large number of shellac disks.
The item to be cleaned from oil and grease is the cathode and the solution is passed through the iron tank or vat filled with an electrolyte solution and heavy current. At cathode, caustic soda and hydrogen are produced which removes the grease from the article surface. The process is known as cathodic cleaning and is applied to zinc and aluminum. Article is made anode for anodic cleaning.
9. Determination of Equivalent Elemental Masses
The quantities of metals deposited on the cathodes of the two cells are proportional to their equivalent masses of the respective metals, according to the second law of electrolysis when the same quantity of electronic current is passed through salt solutions of two different cells. If the quantities of metals deposited on the cathodes are WA and WB, then
Knowing the equivalent mass of one metal, it is possible to calculate the equivalent mass of the other metal from the relationship above. This method can also determine the equivalent masses of those non-metals that evolve at anodes.
10. Thickness of any coated layer
Electrolysis method can also be used to calculate the thickness of the coated layer of any material. For that we just need to know the dimensions of the coated material and we will get its thickness.