Applications of Electrolysis: Top 12 Applications | Electrical Engineering

The following points highlight the top twelve applications of electrolysis. The applications are: 1. Extraction of Metals 2. Refining of Metals 3. Production of Chemicals 4. Electroplating 5. Deposition of Alloys 6. Electroforming 7. Electrotyping 8. Electrofacing 9. Electrometallisation 10. Electrodeposition of Rubber 11. Anodizing 12. Electropolishing.

Application # 1. Extraction of Metals:

Extraction of metal is an electrochemical process used for the production of metal with commercially acceptable purity. There are two methods of extraction of metals depending upon the physical state of the ore. In one of the processes the ore is treated with a strong acid to obtain a salt and the solution of such a salt is electrolysed to liberate the metal. The second process is employed when the ore is available in molten state or can be fused and in this process the ore, which is in a molten/fused state, is electrolysed in a furnace.

The details of the processes adopted for some of the metals are given in Table 8.2.

The methods adopted for extracting zinc and aluminium are explained below:

1. Extraction of Zinc:

This is an example in which an aqueous solution of the salt is used. The ore, consisting largely of zinc oxide, is treated with concentrated sulphuric acid, roasted, and passed through various chemical processes in order to remove impurities (such as cadmium, copper etc.) by precipitation. The zinc sulphate solution so obtained is then electrolysed. The electrolysis of zinc sulphate is accomplished in large lead-lined wooden boxes having a number of aluminium cathodes and lead anodes.

Zinc gets deposited on the cathodes and is removed periodically (once or twice a day). The current density on the cathodes is about 1,000 A/m². The voltage per cell is about 3.5 V and usually 100 to 150 cells are used in series requiring a pressure of roughly 500 V. Energy consumption is 3,000 to 5,000 kWh/tonne.

2. Extraction of Aluminium:

This is an example of fused electrolyte process. Aluminium is produced from bauxite containing aluminium oxide or alumina (70% in case of high grade bauxite), silica (silicon oxide) and iron oxide. The bauxite ore is first reduced to aluminium oxide by chemical treatment and then it is dissolved in fused cryolite. Cryolite is a solution of aluminium fluoride and fluoride of either of sodium, potassium or calcium. The mixture thus obtained is electrolysed.

The fusion and electrolysis are accomplished in a large shallow rectangular steel bath lined with carbon; carbon anodes projecting downwards into the bath and the bottom of the bath forms the cathode. The charge is melted by the arc struck between the carbon anodes and cathode and is then maintained in a molten state by the heating action of the electric current flowing through the charge. The liquid metal deposits at the cathode and settles at the bath bottom and is periodically siphoned out into large ladles from which it is poured into pig or ingot moulds.

Fresh alumina is fed into the bath at short intervals to replace that which has been decomposed by the current and the process is, therefore, a continuous one. The aluminium obtained by this process is 99.5% pure. A furnace having an area of about 15 square metres will need a voltage of about 6 volts and a current of about 40,000 amperes.

Energy consumption is 20,000 to 25,000 units/tonne. Almost the whole of the aluminium required in the present day’s industry is produced in this way. As the electrolytic process requires large amount of electric power and process is continuous, so such plants are installed near hydroelectric power stations. The high temperature (1,000°C) necessary to keep the ores in a fused state is maintained by the ohmic losses due to the current flowing through the electrodes and electrolyte.

Application # 2. Refining of Metals:

Refining is the process whereby a highly concentrated mixture of metals is subjected to electrochemical treatment for recovering not only the principal metal in pure form, but also the precious metals like gold, silver, bismuth etc., which may be present in the form of minute traces.

Some of the processes are explained below:

1. Refining of Copper:

The impure copper is cast into plates which are made the anodes in a bath of copper sulphate acidified with sulphuric acid. The cathodes consist of sheets of pure copper which are coated with graphite so that the electrolyte deposit of copper may be easily stripped off. When the current is passed, copper ions are discharged at the cathode and are deposited as pure copper while hydroxyl ions discharged at the anode bring an equivalent amount of cupric ions into the solution.

Thus there is a transfer of pure copper from the anode to the cathode while the impurities like nickel, zinc and iron, pass into the solution as sulphates and impurities like silver and gold, which are not affected by sulphuric acid—copper sulphate solution settles down as the anode sludge or mud. At regular intervals the deposit is stripped from the cathode and the anode is replaced. Copper extracted from its ore, known as blister copper, is 98 to 99 per cent pure. Copper of purity as high as 99.95% is obtained from blister copper by electrolytic process. Energy consumption in copper refining by electrolytic process is 150—300 kWh/ tonne of refined copper.

2. Refining of Gold:

Blocks of impure gold are made the anodes in an electrolytic cell containing a solution of gold chloride (AuCl₃) in hydrochloric acid. Thin plates of pure gold constitute the cathodes. On electrolysis pure gold is deposited on the cathode. Gold thus obtained is 99.9% pure. For a higher purity, if desired, the metal may be put to a second electrolytic refining. The energy consumption is 300-350 kWh/tonne of refined gold.

3. Refining of Silver:

Solution of nitric acid and silver nitrate is required for refining of silver with electric energy consumption of 400-425 kWh/tonne of refined silver. The process is similar to that for refining of copper.

4. Refining of Nickel:

The matter is first roasted and treated with dilute sulphuric acid at 80°C to remove copper. The residue consisting mostly of nickel compounds is made the anode in a solution of nickel ammonium sulphate. Iron plates are made the cathode. On electrolysis nickel deposits on these plates. The energy consumption is 2,500-4,000 kWh/tonne of refined nickel.

5. Refining of Lead:

The desilverised lead can be purified further by electrolysing a solution of lead fluorosilicate (PbSiF₆) and hydrofluorosilicic acid (H₂SiF₆) containing a little gelatin. The desilverised lead is cast into anodes while the cathodes are sheets of pure lead. Gelatin facilitates the formation of a coherent deposit of lead at the cathode. The metallic impurities which are more electropositive than lead, such as iron and tin, go into solution while the rest of the Impurities are thrown down as anode mud. The energy consumption is 100-120 kWh per tonne of refined lead.

6. Refining of Zinc:

The crude ore is roasted and leached with dilute sulphuric acid. The resulting solution is purified from elements present such as copper, cadmium, silver, cobalt, and antimony, arsenic by treatment with the powdered zinc and with ferric hydroxide. The solution of purified zinc sulphate is then electrolysed in large concrete tank lined with lead or with rubber, using chemically pure lead as anode and aluminium cathode. The electrolysis is carried at 35-40°C using a current density of 300-400 A/m² and a cell voltage of 3.25-3.5 volts. The zinc is stripped from the aluminium cathodes and then melted down to give a material of high purity (99.99 per cent zinc).

7. Refining of Iron:

In case of purification of iron, the iron-ammonium sulphate is used as electrolyte and energy consumption 1,000—1,600 kWh per tonne of refined iron.

The summary of the some of the important refining processes is given in Table 8.3.

Application # 3. Production of Chemicals:

There are various industrial applications of electrolysis in the production of chemicals such as production of caustic soda, chlorine, potassium permanganate, ammonium per sulphate, hydrogen and oxygen by electrolysis.

The most important processes of production caustic soda by electrolysis of brine and production of hydrogen and oxygen gases by electrolysis of water are explained below:

1. Production of Caustic Soda:

The oldest process is Diaphragm process. There are a number of variations, but all essentially consist of an anode compartment separated from a cathode compartment by a porous diaphragm which prevents the mechanical mixing of the two solutions. Chlorine is formed at the anode, and most of it is evolved as a gas, a small part going into solution. Sodium is discharged at the cathode and reacts with the hydroxyl ions to form sodium hydroxide and hydrogen gas is liberated at the cathode. Usually, the brine is fed into the anode compartment to resist the flow of hydroxyl ions towards the anode.

Another method of production of caustic soda by electrolytic process is mercury-cathode process. Mercury cells may be built in small (1,000 A) to very large sizes (3,000 A) or even as high as 50,000 A or more per unit. Purified saturated brine is fed to the cells and commonly circulated through them with a reduction in salt concentration. Brine is resaturated for another cycle. Each unit consists of two sections ; one which is closed and has a chlorine outlet, graphite anodes, and a mercury cathode; and a second, the denuder section, containing a mercury amalgam anode and iron cathodes.

Circulation of mercury between the two sections constitutes the electrical connection. The current efficiencies depend upon the concentration of the amalgam formed in the first section and the effectiveness of the circulation of the amalgam. Current densities in mercury cells are considerably higher than in diaphragm units, being of the order of 1.5 mA or more per square mm of anode surface.

Energy efficiencies are of the order of 50-60%. Anode consumption, when graphite anodes are employed, is of the order of 3-3.5 kg per tonne of chlorine. The voltage requirement is about 4 volts per cell.

Other productions that can be obtained from electrolysis of brine by interaction of the cathodic and the anodic products are sodium hydrochloride, bleaching powder and hydrogen when the solution is cold and sodium chlorate (NaClO₃) and hydrogen when the solution is hot.

2. Production of Hydrogen and Oxygen Gas by Electrolysis of Water:

Gases obtained by this process are of high purity and at a cheap cost because of the low energy consumption. The electrolyte consists of 15-20% solution of caustic soda or its equivalent caustic potash, and electrodes are of iron. Sulphuric acid is no longer used.

With caustic soda as electrolyte the chemical reactions are:

Cathode reaction 2Na + 2H₂O = 2NaOH + H₂

Anode reaction 20H = H₂O + ½O₂

Cathode reaction 2Na + 2H₂O = 2NaOH + H₂

Anode reaction 2OH = H₂O + ½O₂

Thus hydrogen and oxygen gases are liberated at cathode and anode respectively and water disappears while the quantity of caustic soda remains constant. It is, therefore, necessary to add water to the solution periodically.

The voltage requirement is 2-2.2 V per cell during operation and 2.3-2.5 V per cell during starting period. Energy consumption is about 6 kWh per cubic metre of hydrogen and 1/2 cubic metre of oxygen.

Various industrial applications of electrolysis in the production of chemicals are summarized is Table 8.4.

Application # 4. Electroplating:

Electroplating is an art of depositing a superior or a more noble metal on an inferior or a base metal by means of electrolysis of an aqueous solution of a suitable electrolyte. For example, metals like iron which are easily corroded by atmospheric air, moisture and carbon dioxide, are coated electrolytically with deposits of nickel or chromium which are more resistant to chemical attack. Picture frames and machinery parts are often chromium-plated to protect them from corrosion and at the same time to give them a good polish.

Sometimes, electroplating is done, with a view to repair worn-out parts of machinery. In such cases the suitable material is deposited electrolytically on the effected parts of the machinery.

Electroplating is also done occasionally for ornamentation and decoration purposes. For example, several articles made of copper or its alloys, such as table-wares, decoration pieces, are coated with silver or gold.

The electrolytic deposits are crystalline in nature. The crystals must be very fine in order to get firm, coherent and uniform deposits. For this purpose, suitable electrolytes should be used in the electrolytic bath and current density used should have an appropriate value. The temperature should also be maintained at a proper level.

By experiments, certain optimum values of current density and temperature have been worked out for each electrolyte. For example, for Na₃ [Cu(CN)₄] bath, the optimum current density lies between 32-65 A/m² and optimum temperature is 45°C. The optimum conditions for [Ag(CN)₂] bath, are 42 A/m² of current density and 20°C as the temperature. The articles to be coated with the noble metals should be in as high a state of purity as possible.

These conditions are briefly discussed below:

1. Preparation for Plating:

The preparation of an object for plating may involve any or all of the following operations:

i. Removal of oil, grease, or other organic material.

ii. Removal of rust, scale, oxides, or other inorganic coatings adhering to the metal.

iii. Mechanical preparation of the surface of the metal to receive the deposited metal, by polishing, buffing etc.

For the first, soaps, hot alkali solutions, or organic solvents such as gasoline or carbon tetrachloride are used, for the second, various acids, alkali and salt solutions, mechanical abrasion, and electrolytic cleaning, and for the third mechanical abrasion and polishing are used.

ii. Cleaning Methods:

In case the object to be electroplated is not cleaned, polished and degreased, the deposit formed may not be well adherent to the base metal and is likely to peel off. For smooth, bright and strong deposit, the surface upon which a layer of a noble metal is required, should be thoroughly cleaned first mechanically by grinding or scratching (against a rough surface) or sandblasting and then chemically by treatment with hot alkalis (if the surface is greasy) or with dilute acids (if the surface has oxide films) or with organic solvent.

iii. Electrolytic Bath:

The electrolyte used in the electrolytic bath depends upon the nature of the metal to be deposited.

For copper plating two types of electrolytic baths are used. In acid type bath, solution is made of 150-200 gm of copper sulphate and 25-35 gm of sulphuric acid per 1,000 cc of solution. Current density used is 150-400 A/m² and temperature of 25 to 50°C. Deposit obtained is thick and rough requiring polishing.

In cyanide bath solution is made of 25 gm of copper cyanide, 28 gm of sodium cyanide, 6 gm of sodium carbonate and 6 gm of sodium bisulphate per 1,000 cc of solution. Current density used is 50-150 A/m² and the temperature required is 25-40°C. It provides thin and smooth deposits. Copper anodes are used in both of the baths.

For silver plating solution consisting of 24 gm of silver cyanide, 24 gm of potassium carbonate and 36 gm of potassium cyanide per 1,000 cc is used. The required current density and temperature are 50-150 Aim’ and temperature of 20-35°C respectively.

For gold plating solution used consists of 18 gm of potassium gold cyanide, 12 gm of potassium cyanide, 6 gm of potassium sulphate and 12 gm of caustic potash per 1,000 cc Anode employed is of stainless steel. Current density of 50— 150 A/m² and temperature of 50-70°C are used.

For chromium plating solution most commonly used consists of 180-300 gm of chromic acid and 2-3 gm of sulphuric acid per 1,000 cc. Current density employed is 1,500-2,500 A/m² and the working temperature is 35-50°C. Current density used is higher for hard chromium plating than for decorative plating. Anodes are of antimonial lead. Vats used for chromium plating are of steel with lead lining. Chromic acid is added in the solution when required. Arrangements for removal of the fumes are also to be provided.

For nickel plating the solution consists of 180-240 gm of nickel sulphate, 36 gm of nickel chloride and 24 gm of boric acid per 1,000 cc. The current density used is 100-200 A/m². The working temperature is 25-40°C. Anode is of pure nickel.

Summary of the various processes is given in Table 8.5.

Application # 5. Deposition of Alloys:

It is possible to deposit alloys also provided the electrode potentials of the constituent metals are not much different. Electroplating of brass and bronze are example of it. For deposition of alloys the anode is made of an alloy to be deposited and the electrolyte consists of a mixture of electrolytes which would have been employed for separate deposition of the metals consumed. For brass plating solution of double cyanides of zinc and potassium and of copper and potassium is used as electrolyte and current density of 25-40 amperes/m² is used.

Application # 6. Electroforming:

Reproduction of objects by electrodeposition on some sort of a mould or form is known as electroforming. This is another application of electrodeposition.

In the reproduction of coins, medals, engravings and the like, a mould is first made by impressing the object, say, in wax. The surface of the wax, which bears exact impressions of the object, is coated by powdered graphite in order to make it conducting. The mould is then dipped in an electroforming cell as a cathode. After obtaining coating of desired thickness, the article is removed and the wax core is melted out of the metal shell.

This process is employed in the manufacture of gramophone records, in which the original recording is done on a record made of wax composition. This mould is then coated with powdered gold to make it conducting and dipped into a blue vitriol electrolyte as the cathode. The solution is kept saturated by using a copper anode. The plating of copper on to the wax record produces a negative master plate which is used to stamp a large number of shellac discs.

Production of seamless tubes is another application of electroforming. Ferric chloride solution is used as an electrolyte, cast iron as anodes and revolving mandrel as cathode for producing iron tubes. For producing copper tubes copper sulphate acid solution is used as an electrolyte and cast copper is made as anode. A thin coating of material, which renders easy withdrawal of tube of material deposited, is given to the mandrel.

Application # 7. Electrotyping:

This is a special application of electroforming and it is used to reproduce printer’s type, engraving and medals etc. The process is same as for electroforming.

Application # 8. Electrofacing:

It is a process of coating of metallic surface with a harder metal by electro-deposition in order to increase its durability.

Application # 9. Electrometallisation:

It is a process of depositing metal on conducting base for protection and decoration. Non-conductive base is made conductive by a coating of graphite employed as cathode.

Application # 10. Electrodeposition of Rubber:

Rubber latex, as obtained from the tree, consists of very fine colloidal particles of rubber suspended in water. Like other colloidal solutions, particles of rubber are negatively charged. On electrolysis of the solution the rubber particles migrate towards the anode and deposit on it. Current density of roughly 100 A/m² is used.

Application # 11. Anodizing:

Anodizing is a process of anodic oxidation in which a thin uniform passive (tough thin, uniform and impermeable) film is produced artificially by the passage of electric current. Passive film is formed to protect the base metal from further corrosion. In the process of anodizing electric current of 10 to 30 amperes per square metre is passed through an electrolyte of 3% solution of chromic acid for a period of half an hour to one hour, the article to be anodized is thoroughly cleaned and made anode and carbon rod is made cathode.

Aluminium and magnesium have capacity of producing such passive films and, therefore, anodizing is applicable to them. In case of aluminium great feature of these coatings is their submicroscopic porosity. This makes the passive films to take dyes which are absorbed and affect the entire firm length.

Application # 12. Electropolishing:

Closely allied to bright dipping is electropolishing, which utilises anodic treatment in specially formulated electrolytes to bring up a polished surface on such metals as stainless steel. Electropolishing is also useful as a tool in preparing metallic surfaces for microscopic examination. Both bright dipping and electropolishing depend on the more rapid eating away by the solution of micro-projections on the metal, so that smoother surface results.

Electropolishing has one advantage over buffing. Unlike buffing electropolished surfaces are free from cold worked surface cemented layer embedding abrasive particles of mop in the metal.

The process of polishing, in principle, consists of making the work as anode in a suitable electrolytic solution. This develops insoluble compounds that are broken down by more anodic action on the hills than on valleys of surface. Thus smooth surface is produced.