- Chemical processing of minerals.
- Metallurgy and hydrometallurgical processes.
- Industrial electro-chemistry.
- Manufacture of some heavy inorganic chemicals.
- Cement and binding materials. Inorganic fertilizers.
- BASIC ELECTROCHEMISTRY TERMS
For a clearer understanding of electrochemistry, you must know some of the terminologies used and what they mean or stand for. You may have come across these terms in your study of chemistry but let’s just consider them as it applies to this course.
(i) Electrolysis: This refers to the decomposition of a substance by an electric current. This actually is the main stay of electrochemical industries. “The father of electrochemistry” Micheal Faraday stipulated two laws from his studies to govern electrolysis. These laws are stated thus:
(a) The weights of substances formed at an electrode during electrolysis are directly proportional to the quantity of electricity that passes through the electrolyte.
(b) The weights of different substances formed by the passage of the same quantity of electricity are proportional to the equivalent weight of each substance.
Note: The equivalent weight of a substance is defined as the molar mass, divided by the number of electrons required to oxidize or reduce each unit of the substance. For example, one mole of V3+ corresponds to three equivalents of this species and will thus require three (3) Faradays of charge to deposit it as metallic vanadium. One mole of electric charge, 1F = 96,500 coulombs.
The industrial application of electrolysis will be discussed later in this unit and the units to follow.
(ii) Electrode: An electrode is an electrical conductor used to make contact with a non-metallic part of a circuit. It is a conductor through which electricity enters or leaves an electrolyte. For a material to serve as an electrode, it must have these important requirements:
o It should have high electrical conductivity
o It should be stable to corrosion during passage of current or interruptions.
o It should provide a high current efficiency of the needed product.
o It should possess adequate mechanical strength and processability during the manufacture of desired shapes and sizes
o It should be available and have reasonable cost
(iii) Anode: This is the positively charged electrode and it attracts electrons or anions. Oxidation occurs at the anode.
(iv) Cathode: This is the negatively charged electrode which attracts cations or positive charge. The cathode is the site of reduction.
(v) Electrolyte: An electrolyte is a substance that ionizes when dissolved in suitable ionizing solvents such as water. This includes most soluble salts, acids and bases. Some gases, such as HCl, under conditions of high temperature or low pressure can also function as electrolytes.
Industrial applications of electrochemistry
Here we shall be considering the use/ role of electrolysis in the industry. Electrolysis has been used widely to produce substances such as aluminium, sodium, fluorine etc. Metals can be obtained from aqueous solution of their salts or from melts by electrolysis. However not all electrolytic processes results in metal deposition. Considering electrolysis of aqueous solutions, it can be with or without metal deposition. Let us briefly explain these electrolytic processes.
1. Electrolysis of aqueous solutions with metal deposition: There are two important methods of recovering metals from aqueous solutions of their salts by electrolysis. The first method consists in electrolysis of solutions obtained after leaching of the corresponding metal from ores or concentrates with the use of insoluble anodes. This method is applicable to metals such as Zn, Cu, Pb, Cd, Mn, Cr, and Fe. The second method consists in the electrorefining of metals. Here, the crude metal to be refined acts as anode and pure metal is deposited at the cathode. During electrorefining of crude metal, noble metals such as gold, silver and platinum are recovered as by-products.
2. Electrolysis of aqueous solutions without metal deposition: This is applicable to aqueous alkali solutions during electrolysis of aqueous alkali solutions. During the electrolysis of alkali metal chlorides, caustic alkalis and hydrogen are formed at the cathode, and chlorine gas is evolved at the anode.
3. Electrolysis of Melts: This process is used to produce substances which cannot be produced by the electrolysis of aqueous solutions. Zirconium, Thorium, some rare and rare earth metals can be obtained from melts by electrolysis. Aluminium also is produced from a molten mixture of cryolite and alumina by electrolysis. This will be discussed in detail later. There are certain metals which cannot be obtained at a solid cathode by electrolysis, though most metals are obtained at the solid cathode. In such a situation, the electrolysis of the melt with the liquid cathode is used. An alloy of the metal of interest with the liquid cathode is used as the cathode, and the metal is then distilled in vacuum from the liquid cathode having a higher boiling point or the metal of the liquid cathode having a lower boiling point is distilled off in vacuum.
Now, haven discussed these electrolytic processes, let us see some of the applications of industrial electrochemistry. Electrochemistry has been found useful in the production of certain heavy inorganic chemicals. We shall consider these in the next unit. So, we will concern ourselves with the production of some metals by electrolysis.
Production of Aluminium
Aluminium was first obtained by heating aluminium chloride with a potassium mercury amalgam. However, the entire world’s production of aluminium is obtained by the electrolysis of a solution of alumina in fused cryolite (Na3AlF6) based on the discovery made by Hall and Heroult.
The manufacture of aluminium consists of three steps. The first is the purification of bauxite. The second step is the electrolytic reduction of pure bauxite or alumina in a bath of fused cryolite (Na3AlF6) which acts as a flux. The third step is the purification of aluminium formed as a result of electrolytic reduction of pure bauxite.
Step 1: Purification of Bauxite
Bauxite is often associated with Fe2O3, SiO2 and TiO2. The impurities are removed by any of these three methods namely: (1) Baeyer’s process (2) Serpeck’s process and (3) Hall’s process. The Baeyer’s process is the most popular hence we will discuss what it involves.
The Baeyer’s process: Bauxite mineral particularly that containing excess of iron oxide as impurity (red bauxite) is first crushed in jaw crushers and then wet ground to 100 mesh. It is then mixed with concentrated solution of caustic soda, (41%) of specific gravity 1.45 in steam jacketed autoclave digesters and digested for about 2hours under 4.5 atmospheric pressure at a temperature of about 150-160 oC. As a result, aluminium oxide passes into solution as sodium aluminates and partly as colloidal alumina, while oxides of iron, titanium and silica remain unaffected.
Al2O3 + 2NaOH →2NaAlO2 + H2O
The slurry is washed in a series of counter current thickeners and the impurities are removed by filtration using rotary filters. The filtrate containing sodium aluminate is hydrolysed to precipitate aluminium hydroxide by cooling. The precipitated slurry is fed to another set of counter current thickeners, where all the aluminium is removed.
NaAlO2 + 2H2O →Al(OH)3 + NaOH
The precipitate of Al (OH)3 is washed with water and then calcined in tubular rotary kilns with fire bricks at about 1200 – 1300 °C, whereby alumina is obtained. The resulting alumina, which contains about 99.5 % Al2O3 is cooled and slipped to the reduction plant. The dilute caustic soda solution from the second set of thickeners is concentrated in a multiple effect evaporator system and recycled to be used again.
2Al(OH)3 →Al2O3 + 3H2O
*The Serpecks process is used only when the bauxite mineral contains excess of silica as impurity.
Step 2: Electrolytic. Reduction of Alumina
Electrolyzing pure alumina in a flux of molten cryolite and CaF2 yields aluminium. Pure alumina is dissolved in fused cryolite and electrolysed in electrolytic cells. Each cell is open at the top and first lined with fire bricks and then with gas carbon, coke or anthracite coal. This lining of carbon or coke is built in the form of a layer and acts as cathode. A number of carbon rods made of petroleum coke, attached to copper clamps and dipped in the fused electrolyte serves as the anodes.
The molten bath which contains 5-10 % Al2O3 in 90-95 % flux containing 64 % cryolite and 36 % CaF2 is made by putting solid ingredients of the flux in the cell and then melting the flux by strucking an arc between the lining and carbon rods as a result cryolite undergoes melting. The anodes are then raised and a calculated amount of pure alumina is spread over the frozen surface. Some coke is also thrown in to cover the surface of the electrolyte. The ensuing reaction is a combination of oxygen liberated from alumina, with the carbon of the anodes, which are consumed with the formation of CO and CO2. These gases are allowed to escape through the outlets and aluminium is deposited at the cathode along the bottom of the bath, from where it is tapped off.
Step 3: Electrolytic refining or purification of Aluminium
This is carried out by Hoope’s process. The molten aluminium from the electrolytic reduction cell is carried to refinery furnace which consists of three fused layers of different specific gravities. The layers are:
o The bottom layer of copper, aluminium and silicon which acts as anode.
o The middle layer consisting of cryolite and barium fluoride and acts as the electrolyte
o The upper layer of pure molten aluminium which acts as the cathode.
When an electric current is passed, aluminium from the middle layer passes into the top layer and an equivalent amount from the base layer passes into the middle layer. The aluminium copper alloy on the bottom of the cell can be replenished with low purity alumina. The high purity (99.9% pure) aluminium floats to the top and is drained off under CO-CO2 atmosphere.
Production of Magnesium
Magnesium, a silvery white metal which gets dull easily on exposure to air is extracted or produced by either of the following processes:
i. The electrolysis of fused anhydrous carnallite, KCl, MgCl2.6 H2O
ii. The electrolysis of fused anhydrous MgCl2 containing fused CaCl2 and NaCl.
iii. Carbothermal process (Reduction of MgO by carbon or calcium carbide)
iv. Pidgeon or Silico Thermal process (Reduction of MgO with silicon).
From these four processes, it is evident that magnesium is either obtained from MgCl2 or MgO. The cheapest and the best method of manufacturing magnesium is by the electrolytic process. Magnesium is obtained by the electrolysis of fused MgCl2. The dehydrated magnesium chloride obtained from sea water is fused with anhydrous CaCl2 and sodium chloride so that the mixture contains 25 % MgCl2, 15 % CaCl2 and 60 % NaCl. The presence of sodium chloride decreases the melting point and increases the conductivity. The electrolysis in carried out at 710 °C, which is greater than the melting point of magnesium (651oC).
Two types of cells have been used in the manufacture of magnesium by the electrolysis of fused MgCl2. Let’s describe these.
One of the cell is known as Dow electrolytic cell. The cells are large rectangular ceramic covered steel tanks, 5 feet wide, 11 feet long at 6 feet deep and hold about 10 tonnes of fused magnesium chloride and salts. The internal parts of the cell act as the cathode and graphite rods suspended vertically in the top of the cell act as anodes. The temperature of the cell is maintained at about 710 oC by the electric current and by external heat supplied by gas fired outside furnaces. The molten magnesium liberated at the cathode rises up to the bath surface and tapped off from time to time. The chlorine liberated as a by-product is separately reacted with hydrogen to form HCl. Magnesium obtained by this method is 99 % pure. It is further refined by subliming at 600 °C under a pressure of 1mmHg.
The other type of cell is a close top smaller cell made of steel. It consists of a centrally placed graphite electrode surrounded by a perforated porcelain tube. The inner surface of the steel cell acts as the cathode and vertical graphite rod as the anode. The bottom and lower sides of the cell are lined with ceramic material. A large number of such cells are joined in series. As a result of the passage of electricity, electrolysis takes place and Cl2 is liberated at the anode. The magnesium metal floats on the surface of the bath and protected from oxidation by circulating coal gas. The chlorine gas is collected from the top and molten metal is also collected from the top.
Manufacture of magnesium by electrolysis of MgO
Magnesium can also be prepared commercially by the electrolysis of magnesium oxide obtained from the calcination of the ore, magnesite.
MgCO3 →MgO + CO2.
Magnesia (MgO) is dissolved in a mixture of fused fluorides of magnesium, barium and sodium in a steel tank at about 900 – 950 °C. The steel tank acts as the electrolytic cell in which cast iron cathodes project into the electrolyte from below and the carbon anodes are suspended from above. On passing an electric current, molten electrolyte forms a solid crust at the surface of the molten mass and the molten magnesium, being lighter, rises up and collects below the crust. The magnesium is thus protected from being oxidized.