Caustic Soda Or NaOH (Castner Kellner’s Process)

Caustic Soda Or NaOH (Castner Kellner’s Process)
Introduction
Caustic Soda or Sodium Hydroxide is one of the most important chemicals of industrial use. Caustic Soda must be handled carefully because it is caustic to touch and causes painful burns.
Castner Kellner’s Process
Caustic Soda is manufacture by an electrolytic process. This process is carried out in an electrolytic cell known as Castner-Kellner’s cell and the process is called Castner-Kellner’s Process.
Construction Of Castner-Kellner’s Cell
The electrolytic solution is a 25% of NaCl solution. The anode consist of a number of titanium plates whereas cathode is a steam of flowing mercury.
Diagram Coming Soon
Working of Castner-Kellner’s Cell
Sodium chloride dissociates in water to give Na+ and Cl- ions are
2NaCl —-> 2Na+ + 2Cl-
The Cl- ions migrate towards titanium plates, it gains electron and convert into chlorine gas.
2Cl- —-> Cl2 + 2e- (Oxidation)
In castner-kellner’s process, H+ ions are not easily discharged due to high voltage of H+ ions, on the contrary Na+ ions are easily discharge over mercury surface. The sodium, thus liberated dissolves in mercury forming an amalgam.
2Na+ + 2e- —-> 2Na (Reduction)
Na + Hg —-> Na/Hg (Amalgum)’
The mercury containing dissolved sodium is sent to another chamber called Denuder where sodium reacts with water forming sodium hydroxide and hydrogen. Denuder is packed with graphite blocks as hydrogen is easily liberated over graphite surface.
2Na/Hg + 2H2O —-> 2NaOH + H2 + 2Hg
The mercury is recycled to dissolve more of sodium.
The solution, which flows out from denuder, is a NaOH solution, which is evaporated to dryness.

Sodium (Down’s Process)

Sodium (Down’s Process)
Introduction
On large scale, sodium is manufactured by electrolysis of fused sodium chloride. The process was given by scientist Dawn, therefore, it is called Dawn’s Process.
Construction of Down’s Cell
A special electrolytic cell known as Dawn’s cell is used for the electrolysis of sodium carbonate. The cell consists of a cylindrical shape iron basin lined inside by firebricks. The iron cathode is separated from anode by iron gauze diaphragm. Anode consists of a carbon rod, which is present between two iron cathodes. The electrolytic solution consists of a mixture of sodium chloride (NaCl) and calsium chloride (CaCl2). Calsium Chloride (CaCl2) is added to decrease the melting point of sodium chloride from 801ºC to 600ºC.
Working of Dawn’s Cell
When electric current is passed through molten NaCl, the sodium ion migrates towards cathode. It gains and electron and converts into molten sodium metal, which floats inside the cathode compartment. This molten sodium metal is allowed to pass through pipe ‘p’ which collects it in a vessel ‘V’ outside the cell

Sodium Carbonate (Ammonia-Solvay Process)

Sodium Carbonate (Ammonia-Solvay Process)
Introduction
Sodium Carbonate is an important compound. It is manufactured by Ammonia-Solvay Process.
Raw Meterials
The raw materials for the manufacture of sodium carbonate are
1. Brine
2. Ammonia, which is made by Haber’s Process
3. CO2, CaOH, which are obtained from limestone.
Ammonia-Solvay Process
The Ammonia-Solvay Process consist of the following steps.
Step I – Ammonation of Brine
In first step, ammonia gas is mixed with brine. This process is carried in Ammonation Tower. The ammonation tower consist of mushroom shaped buffels at short intervals. Brine is introduced from the top and ammonia is introduced from bottom. They both flow towards each other. Buffels control the flow of brine and ensure that they are mixed to the point of saturation with ammonia.
Step II – Carbonation of Ammonated Brine
In this step, ammonated brine is mixed with carbon dioxide brine is mixed with carbon dioxide in a tower called carbonating tower or Sonvai tower. Ammonated Brine is fed from the top where as carbon dioxide ascends from the bottom. When these two substances meet, the following chemical reactions takes place.
2NH3 + CO2 + H2O —-> (NH4)2CO3
(NH4)2CO3 + CO2 + H2O —-> 2NH4HCO3
2NH4HCO3 + NaCl —-> NaHCO3 + NH4Cl
Sodium bicarbonate is relatively insoluble, which is precipitated out from the solution by cooling the lower part of the tower. Sodium bicarbonate is separated from soluble ammonium chloride by vacuum filteration.
Step III – Production of Soda Ash
Sodium bicarbonate is heated in a long iron tube to obtain anhydrous sodium carbonate or Soda Ash.
This carbon dioxide is recycled to the solvay tower. This hydrated sodium carbonate is also called washing soda.
Recovery of Ammonia
Ammonia gas is recovered from the remaining solution by treating it with Calsium Hydroxide.
2NH4Cl + Ca(OH)2 —-> CaCl2 + 2H2O + NH3

Chlorine (Castner-Kellner’s)

Chlorine (Castner-Kellner’s)
Introduction
Chlorine plays an important role in industries. Therefore to fulfill the demands, chlorine is manufactured on large scale. It is prepared by the electrolysis of aqueous solution sodium chloride, this process is known as Castner-Kellner’s Process and it is carried out in Castner-Kellner’s Cell.
Construction of Castner-Kellner’s Cell
The Castner-Kellner’s cell consist of a steel tank which contain Hg flows from right to left in the cell and is connected with the negative terminal of the battery therefore it acts as cathode. The cell is filled with saturated aqueous solution of NaCl, which also flows in the same direction as Hg. Some graphite rods are dipped into the solution of NaCl. These rods connected with positive terminal of the battery therefore act as anode.
Working Of Castner-Kellner’s Cell
When the electric current is passed through the cell. The sodium ion and chloride ion migrates towards their respective electrodes. Chlorine ion moves towards anode, loses its electron and converts into chlorine gas. The free chlorine gas comes out of the tube at the top of the cell. The sodium ion migrates towards cathode gains electrons and converts into sodium metal. The sodium is dissolved in Hg to form sodium amalgum which comes out with the flow of Hg from the cell.
NaCl <—-> Na+ + Cl-
Na+ + e- —-> Na0 (At Cathode)
2Cl —-> Cl2↑ + 2e (At anode)

Nitric Acid (Ostwald’s Process)

Nitric Acid (Ostwald’s Process)
Introduction
On industrial process three methods are employed for the manufacture of nitric acid. Ostwald’s Process uses Ammonia as raw material.
Ostwald’s Process
When a mixture of ammonia and air is passed through a catalytic chamber containing electrically heated Platinum Gauze, (NO) nitric oxide is formed. This nitric oxide combines with more oxygen to form nitrogen dioxide (NO2). This gas is then absorbed by water to form nitric acid. The plant used in Ostwald’s Process consist of following parts.
1. Catalytic Chamber
A mixture of pure and dry ammonia is passed through a catalytic chamber containing heated (800ºC) platinum, which serves as catalyst. Here ammonia is completely oxidized to nitric oxide.
2. Oxidation Tower
The nitric oxide is passed into oxidation tower where the temperature of gases falls to 200ºC – 250ºC. At this temperature nitric is oxidized to nitrogen dioxide.
2NO + O2 —-> 2NO2
3. Absorption Tower
Now the gases containing NO2 and oxygen are than sent to a series of absorption tower, which is filled with broken quartz and water is showered from top of the tower. Then NO2 reacts with H2O in the presence of oxygen to form HNO3.
4NO2 + 2H2O + O2 —-> $HNO3

Sulphuric Acid (Contact Process)

Sulphuric Acid (Contact Process)
Introduction
On industrial scale two methods are employed for the manufacture of sulphuric acid, the Lead Chamber Process and the Contact Process.
The Contact Process
When a mixture of pure and dry sulphur dioxide and air is passed over a heated catalyst such as Platinum Asbestos, then Sulphur dioxide is oxidized into sulphur trioxide. This gas gives sulphuric acid when treated with water.
The plant used in Contact in Contact Process is composed of following parts.
1. Sulphur and Pyrite Burner
In this part, SO2 is produced by either burning sulphur in air or roasting ores. Like ZnS, PbS, FeS2 in air.
S + O2 —-> SO2
4FeS2 + 11O2 —-> 2Fe2O3 + 8S2
2. Dust Chamber
The SO2 obtained from pyrite burner contains impurities such as unreacted coal, dust, moisture, arsenic compounds etc. These impurities are removed from the gas in dust chamber. In this chamber hot air is blown as a result the heavy particles are removed and unreacted sulphur convert into SO2. Now the gases are passed through a cooling pipe into water chamber.
3. Water Chamber
In this chamber a stream of water is showed on the gas to remove the dust particles. The SO2 gas is now proceed into another chamber, which is known as drying tower.
4. Drying Tower
In drying tower moist SO2 gas is mixed with concentrated H2SO4. As a result the gas become dry and dilute H2SO4 is sink at the bottom.
5. Oxidation Tower
Before oxidation the dry gas is allowed to pass through arsenic purifier containing ferric hydroxide here the arsenic compounds are absorbed. The purified gases are then passed through a strong beam of light to remove any remained dust particle.
The purified and dry SO2 gas is passed through the oxidation tower. This tower consist of iron pipes packed with plantanized asbestos coated with Vanadium Penta Oxide (V2O5), which act as catalyst. In this tower the temperature is kept constant at about 480ºC, under high pressure and high concentration of sulphur dioxide and oxygen, oxidation of SO2 takes place in a favourable condition to yield maximum amount of SO3.
2SO2 + O2 <—-> 2SO3 (45000 cal)
6. Absorption Tower
Sulphur trioxide formed in Oxidation Tower is passed up to Absorption tower, where it meets a stream of concentrated sulphuric acid and hence absorbed to form OLEUM. The maximum absorption of sulphur trioxide is 60% at which oleum has maximum density.
SO3 + H2SO4 —-> H2S2O7
Oleum can be diluted to obtain any desired concentration of sulphuric acid.
H2S2O7 + H2O —-> 2H2SO4
Sulphuric acid is obtained from this process is nearly 99.99% pure

Aluminium (Extraction)

Aluminium (Extraction)
Extraction of Aluminium
Aluminium does not occur free in nature, but it widely distributed in combine state. Aluminium is mainly extracted from Bauxite ore (Al2O3.nH2O). The extraction of Aluminium from Bauxite proceeds through following two steps.
1. Purification of Bauxite to Alumina.
2. Electrolysis of Pure Bauxite (Alumina)
1. Purification of Bauxite
Bauxite contains iron oxide (Fe2O3) and Silica (SiO2) as chief impurites. These impurities must be removed from Bauxite, because they make the aluminium brittle and liable to corrosion. Bauxite ore may be purified by the following methods.
a. Hall’s Method
This method is used for the purification of Bauxite containing Fe2O3 and SiO2 as impurities. The finely divided Bauxite is fused with sodium carbonate (Na2CO3), as a result sodium aluminate is formed while the impurities are left unaffected.
Al2O3.nH2O + Na2CO3 —-> 2NaAlO2 + CO2 + nH2O
The fused mass is rapidly extracted with water leaving behind both the impurites Sodium aluminate is heated up to 50ºC – 60ºC in the presence of CO2. As a result while glatinous precipitates of aluminium hydroxide are formed.
The precipitates of Al(OH)3 are separated, washed to remove Na2CO3, dried and ignited about 1500ºC to get pure bauxite (alumina)
b. Bayer’s Method
This method is used for the purification of Bauxite containing excess of Fe2O3 as impurity. The finely divided Bauxite is treated with concentrated solution of Sodium Hydroxide. As a result soluble sodium Aluminate is formed, while the impurities are removed by filtering the solution.
Al2O3.nH2O + NaOH —-> 2NaAlO2 + 2(n)H2O
Soluble sodium Aluminate is treated with excess water to form white gatinous precipitates of Aluminium Hydroxide.
2NaAlO2 + 4H2O —-> 2Al(OH)3 + 2NaOH
The precipitates of Al(OH)3 are separated, washed to remove NaOH, dried and ignited about 1500ºC to get pure bauxite (alumina)
c. Serpek’s Method
This method is used for the impurities of Bauxite ore containing excess of SiO2 as impurity. The finely divided Bauxite is mixed with carbon and heated up to 1800ºC in the current of Nitrogen. As a result Aluminium Nitride is formed.
2. Electrolysis of Pure Bauxite (Alumina)
The electrolysis of pure bauxite is carried out in a steel tank lined with carbon (graphite). The carbon lining serve as cathode. The anode consist of carbon rods hanging in the molten mass.
In fused state pure alumina is bad conductor of electricity and its melting point is about 2050ºC. So flourspar and cryolite is added in alumina to increase the fluidity of the melt and lower the melting point respectively. When electric current is passed through this mixture, the aluminium is obtained at cathode in liquid state. It sink to bottom from where it drawn, periodically through the tapping hole.
Aluminium produced by this method is 98% and contains traces of Fe, Si and Al2O3 etc.
Refining of Aluminium
The molten aluminium so obtained contains 2% impurities which is further purified by electrolysis in hoop’s cell.
Construction
The Hoop’s cell consist of an iron box lined with carbon (graphite), the carbon lining served as anode. This iron box consist of three layers, the upper most layer is of pure aluminium the middle layer molten flourides (AlF3, BaF2 and NaF) act as electrolyte, where as the lower layer consist of impure aluminium. The cathode consist of carbon rods hanging in pure aluminium.
Working
When electric current is passed through the impure aluminium goes into the middle layer as all leaving impurities and the pure aluminium is deposited at the top of molten flourides at cathode. The aluminium layer grows and drawn off time to time from tapping hole. The refined Aluminium so obtained is 99.99% pure.