Industrial electrochemistry, Chemistry tutorial

Introduction:

The relationship between chemistry and electricity has been for a very long time going via Alessandro Volta's discovery in the year 1793. He introduced that electricity could be produced by putting two dissimilar metals on opposite sides of a moistened paper. Nicholson and Carlisle in the year 1800 exhibited that an electric current could decompose water to oxygen and hydrogen. They in reality employed Volta's primitive battery as a source of electricity. This chemistry-electricity relationship has become so significant that the utilization of electricity as a means of bringing regarding chemical change has continued to play a central role in the growth of chemistry. In the chemical industries, temperatures more than 1500oC can only be accomplished on the commercial scale via making use of electricity. Chemical industries not only produce power via electrical motors, however they as well make use of electricity to give rise to elevated temperatures and to cause the chemical change. Chemical reactions which occur as a result of the impact of electrical energy are known as electrochemical reactions. This is the base of electrochemistry in the industry.

Electrochemistry:

This has been established that electricity causes the chemical changes to take place. We thus, state electrochemistry as the study of chemical reactions which occur at the interface of an electrode generally a semi-conductor and an ionic conductor, the electrolyte. Such reactions comprise electric charges moving between the electrodes and electrolyte. We can as well state it as a science which deals by the relation of electricity to chemical changes and by the inter-conversion of chemical and electrical energy. 

Over the years, electrochemical methods affected by direct electric current have been preferably utilized in the chemical industries as compare to the usual chemical methods. This is because of it following merits: 

a) Simplicity of the technological method.

b) Raw materials and energy are used entirely.

c) Some of the valuable products are obtained concurrently.

d) Highly pure products are made up.

Electrochemical processes, though, does encompass a demerits which is that there is a large consumption of the electric power.

Basic Electrochemical Terms:

For a clearer understanding of electrochemistry, we should be familiar with some of the terminologies employed and what they mean or represent.

1) Electrolysis: This signifies to the decomposition of a substance via an electric current. This in reality is the main stay of electrochemical industries. The father of electrochemistry - Micheal Faraday stipulated two laws from his studies to preside over electrolysis. These laws are defines thus:

a) The weights of substances made at an electrode throughout electrolysis are directly proportional to the quantity of electricity which passes via the electrolyte.

b) The weights of various substances made up by the passage of the similar quantity of electricity are proportional to the equivalent weight of each and every substance. 

Note:  The equivalent weight of a substance is stated as the molar mass, divided by the number of electrons needed to oxidize or reduce each and every unit of the substance. For illustration, one mole of V3+ corresponds to three equivalents of this species and will therefore need three (3) Faradays of charge to deposit it as metallic vanadium. One mole of electric charge, 1F = 96,500 coulombs.                                              

2) Electrode:  The electrode is an electrical conductor employed to make contact by a non-metallic part of a circuit. This is a conductor via which electricity enters or leaves an electrolyte. For a material to serve up as an electrode, it should encompass these significant needs: 

  • It must encompass high electrical conductivity.
  • It must be stable to corrosion throughout the passage of current or interruptions. 
  • It must give a high current effectiveness of the required product. 
  • It must have sufficient mechanical strength and processability throughout the manufacture of desired sizes and shapes.
  • It must be available and encompass reasonable cost.

3) Anode: It is the positively charged electrode and it attracts anions or electrons. Oxidation takes place at the anode.

4) Cathode: It is the negatively charged electrode that attracts positive charge or cations. The cathode is the site of reduction.

5) Electrolyte: The electrolyte is a substance which ionizes whenever dissolved in appropriate ionizing solvents like water. This comprises most soluble salts, acids and bases. A few gases, like HCl, under conditions of high temperature or low pressure can as well function as electrolytes.

Industrial applications of electrochemistry:

In this, we shall be considering the use or role of electrolysis in the industry. Electrolysis has been broadly used to produce substances like aluminum, sodium, fluorine and so on. Metals can be obtained from the aqueous solution of their salts or from melts via electrolysis. Though not all the electrolytic methods results in the metal deposition. Considering electrolysis of aqueous solutions, it can be with or without the metal deposition. Let us in brief illustrate such electrolytic methods.  

1) Electrolysis of aqueous solutions with metal deposition: 

There are two significant processes of recovering metals from the aqueous solutions of their salts via electrolysis. The first process comprises in electrolysis of solutions obtained after leaching of the respective metal from ores or concentrates by the use of insoluble anodes. This process is applicable to like Zn, Cu, Pb, Cd, Mn, Cr and Fe.

The second process comprises in the electro-refining of metals. In this, the crude metal to be refined acts as anode and pure metal is deposited at the cathode. Throughout electrorefining of crude metal, noble metals like silver, gold and platinum are recovered as by-products.

2) Electrolysis of aqueous solutions without metal deposition:

This is mainly applicable to aqueous alkali solutions throughout electrolysis of aqueous alkali solutions. All through alkali metal chlorides, caustic alkalis and hydrogen are made at the cathode and chlorine gas is developed at the anode.

3) Electrolysis of Melts:

This method in utilized to produce substances that can't be produced via the electrolysis of aqueous solutions. Zirconium, Thorium, a few rare and rare earth metals can be obtained from melts via electrolysis. Aluminium as well is generated from a molten mixture of cryolite and alumina via electrolysis. There are several metals that can't be obtained at a solid cathode by electrolysis, although most of the metals are obtained at the solid cathode. In such a condition, the electrolysis of the melt by the liquid cathode is employed. An alloy of the metal of interest with the liquid cathode is employed as the cathode, and the metal is then distilled in vacuum from the liquid cathode encompassing a higher boiling point or the metal of the liquid cathode encompassing a lower boiling point is distilled off in vacuum.

Now, we have illustrated these electrolytic methods; let us observe some of the applications of industrial electrochemistry. Electrochemistry has been found helpful in the production of some heavy inorganic chemicals.

Production of Aluminum:

Aluminum was first obtained via heating aluminum chloride by a potassium mercury amalgam. Though, the whole world's production of aluminum is obtained via the electrolysis of a solution of alumina in fused cryolite (Na3AlF6) based on the discovery made via Hall and Heroult.

The preparation of aluminum comprises 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) that acts as a flux. The third step is the purification of aluminum made as an outcome of electrolytic reduction of pure bauxite. 

Step 1: Purification of Bauxite

Bauxite is often related by Fe2O3, SiO2 and TiO2. The impurities are eliminated by any of these three techniques namely: (a) Baeyer's method (b) Serpeck's method and (c) Hall's method. The Baeyer's method is the most popular therefore we will illustrate what it comprises.

=> The Baeyer's process: 

Bauxite mineral specifically that having surplus of iron oxide as impurity (that is, red bauxite) is first crushed in jaw crushers and them wet ground to 100 mesh. This is then mixed by concentrated solution of caustic soda, (41%) of specific gravity 1.45 in steam jacketed autoclave digesters and digested for around 2 hours under 4.5 atmospheric pressure at a temperature of about 150 to 160oC. As a result, aluminum oxide passes to solution as sodium aluminates and partially as colloidal alumina, whereas oxides of iron, titanium and silica remain unchanged.

Al2O3 + 2NaOH → 2NaAlO2 + H2O

The slurry is washed in a sequence of counter current thickeners and the impurities are eliminated via filtration by employing rotary filters. The filtrate having sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitated slurry is fed to the other set of counter current thickeners, where all the aluminum is eliminated.

NaAlO2 + 2H2O → Al(OH)3 + NaOH

The precipitate of Al (OH)3 is washed by water and then calcined in the tubular rotary kilns with fire bricks at around 1200 to 1300oC, whereby alumina is obtained. The resultant alumina that includes around 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 the multiple effect evaporator system and recycled to be employed again. 

2Al(OH)3 → Al2O3 + 3H2O

*The Serpecks method is used only whenever the bauxite mineral includes surplus of silica as impurity. 

Step 2: Electrolytic Reduction of Alumina

Electrolyzing pure alumina in a flux of molten cryolite and CaF2 results aluminum. Pure alumina is dissolved in fused cryolite and electrolyzed in electrolytic cells. Each and every cell is open at the top and first lined with fire bricks and then by gas carbon, coke or anthracite coal. This lining of carbon or coke is built in the form of a layer and acts as the cathode. The number of carbon rods made up of petroleum coke, joined to copper clamps and dipped in the fused electrolyte serves as the anodes.

The molten bath that comprises 5 to 10% Al2O3 in 90 to 95% flux having 64% cryolite and 36% CaF2 is made up by putting solid ingredients of the flux in the cell and then melting the flux via strucking an arc between the lining and carbon rods as an outcome cryolite undergoes melting. The anodes are then increased and a computed amount of pure alumina is spread over the frozen surface. Some coke is as well thrown in to cover the surface of the electrolyte. The ensuing reaction is the combination of oxygen released from alumina, by the carbon of the anodes that are consumed by the formation of CO and CO2. Such gases are allowed to escape via the outlets and aluminum is deposited at the cathode all along the bottom of the bath, from where it is tapped off.

Step 3: Electrolytic refining or purification of Aluminium

This is done by Hoope's method. The molten aluminium from the electrolytic reduction cell is passed to refinery furnace which comprises of three fused layers of various specific gravities. The layers are:

  • The base layer of copper, aluminum and silicon that acts as anode.
  • The middle layer comprising of cryolite and barium fluoride and acts as the electrolyte.
  • The upper layer of pure molten aluminum that acts as the cathode.

Whenever an electric current is passed, aluminum from the middle layer passes to the top layer and an equivalent quantity from the base layer passes to the middle layer. The aluminium copper alloy on the base of the cell can be replenished by low purity alumina. The high purity (that is, 99.9% pure) aluminum floats to the top and is drained off under CO-CO2 atmosphere. 

Production of Magnesium:

Magnesium, a silvery white metal that gets dull simply on exposure to air is extracted or prepared by either of the given methods:

a) By electrolysis of the fused anhydrous carnalite, KCl, MgCl2.6 H2

b) By the electrolysis of fused anhydrous MgCl2 with fused CaCl2 and NaCl.

c) Carbothermal method (that is, reduction of MgO with carbon or calcium carbide)

d) Pidgeon or Silico Thermal method (that is, reduction of MgO by silicon).

From the above four methods, it is apparent that magnesium is either obtained from MgCl2 or MgO. The cheapest and the best process of preparing or manufacturing magnesium are by the electrolytic method. Magnesium is obtained via the electrolysis of fused MgCl2. The dehydrated magnesium chloride obtained from the sea water is fused by anhydrous CaCl2 and sodium chloride in such a manner that the mixture includes 25% MgCl2, 15% CaCl2 and 60% NaCl. The presence of sodium chloride reduces the melting point and raises the conductivity. The electrolysis in carried out at 710oC that is more than the melting point of magnesium (651oC).

Two kinds of cells have been employed in the manufacture of magnesium via the electrolysis of fused MgCl2. Let's illustrate these.

One of the cells is termed as Dow electrolytic cell. The cells are large rectangular ceramic covered steel tanks, 5 feet broad, 11 feet long at 6 feet deep and hold around 10 tons of fused magnesium chloride and salts. The internal portions 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 around 710oC by the electric current and by external heat supplied via gas fired outside furnaces. The molten magnesium discharged at the cathode rises up to the bath surface and tapped off from time to time. The chlorine released as a byproduct is separately reacted by hydrogen to form HCl. Magnesium obtained via this process is 99% pure. This is further refined via subliming at 600oC under a pressure of 1mmHg.

The other kind of cell is a close top smaller cell made up of steel. This comprises of a centrally placed graphite electrode surrounded via 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 by ceramic material. A large number of such cells are combined in series. As an outcome of the passage of electricity, electrolysis occurs and Cl2 is released at the anode. The magnesium metal floats on the surface of the bath and protected from oxidation via circulating coal gas. The chlorine gas is collected from the top and molten metal is as well collected from the top.

=> Manufacture of magnesium electrolysis of MgO:

Magnesium can as well be made commercially via the electrolysis of magnesium oxide obtained from the calcinations of the ore, magnesite.

MgCO3 → MgO + CO2

Magnesia (MgO) is dissolved in the mixture of fused fluorides of magnesium, barium and sodium in a steel tank at around 900 to 950oC. The steel tank acts as the electrolytic cell in which the cast iron cathodes project to the electrolyte from below and the carbon anodes are suspended from above. On passing the electric current, molten electrolyte makes a solid crust at the surface of the molten mass and the molten magnesium, being lighter, increases up and collects beneath the crust. The magnesium is therefore protected from being oxidized.

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