SINGAPORE: Sodium hydroxide, also known as caustic soda or lye, is an inorganic compound with the chemical formula NaOH. It is a white solid, and is a highly caustic metallic base and alkali salt. It is available in pellets, flakes, granules, and as prepared solutions at a number of different concentrations.
Sodium hydroxide is industrially produced as a 50 per cent solution by variations of the electrolytic chloralkali process. Chlorine gas is also produced in this process. Solid sodium hydroxide is obtained from this solution by the evaporation of water.
Various methods of preparation of sodium hydroxide are -
Castner - Kellener Process
Principle - In castner-kellner method NaOH is prepared by the electrolysis of aqueous solution of NaCl (Brine).
Castner-kellner cell - It is a rectangular tank of steel. Inside of tank is lined with ‘ebonite.’ Anode is made of titanium. Flowing layer of mercury (Hg) at the bottom of tank serves as cathode.
Ionization of NaCl - 2NaCl e- 2Na+ + 2Cl-
When electric current is passed through brine, +ve and -ve ions migrate towards their respective electrodes. Na+ ions are discharged at mercury cathode. The sodium deposited at mercury forms Sodium Amalgam. Chlorine produced at the anode is removed from the top of the cell.
Reaction at cathode
2Na+ +2 e- à 2Na
(Na forms amalgam)
Na + Hg à Na/Hg
Na+ ions are discharged in preference to H+ ions due to high over voltage.
Reaction at anode
2Cl- à Cl2 + 2e-
Formation of NaOH
Amalgam moves to another chamber called ‘denuder,’ where it is treated with water to produce NaOH which is in liquid state. Solid NaOH is obtained by the evaporation of this solution.
2Na/Hg + 2H2O à 2NaOH + H2 + 2Hg
NaOH obtained is highly pure and the process is very effeicient.
Nelson Diaphragm Cell
Principle: Electrolyte used in this process is aqueous NaCl (Brine).
Procedure: Porous diaphragm of asbestos or metal oxide with polymer separates anode and cathode compartments. Diaphragm prevents hydroxide ions entering anode compartment and prevents chloride ions entering cathode compartment. Saturated brine enters anode compartment where chlorine gas is produced.
Anode (positive electrode): carbon (graphite) or titanium coated with Ru-Ti oxide.
Cathode (negative electrode): steel mesh
Reaction at anode(oxidation):
2Cl- (aq) àCl2(g) + 2e
Cathode reaction (reduction):
2H2O (l) + 2e à H2(g) + 2OH-(aq)
Na+ migrates across diaphragm to cathode compartment combining with OH- to form NaOH.
Overall cell reaction (showing Na+ spectator ions):
2H2O (l) + 2Cl- (aq) + 2Na+ (aq) -----> 2Na+(aq) + 2OH-(aq) + H2(g) + Cl2(g)
Product contains sodium chloride and sodium hydroxide. NaOH(s) can be crystallised out.
Loewig’s process for caustic soda preparation depends on the formation of sodium ferrate (Na2Fe2O4), which is then decomposed with water. The soda liquors are mixed with ferric oxide, and the mass evaporated to dryness and calcined at a bright red heat, usually in a revolving furnace. By the calcination, a reaction between the sodium carbonate and the iron oxide is brought about, carbon dioxide escaping and sodium ferrate remaining in the furnace. The mass is washed with cold water until all soluble matter is removed; then water at 900 C is run over the sodium ferrate, by which it is decomposed, caustic soda formed, and iron oxide regenerated; the last is returned to the calcining process. The ferric oxide used is a natural iron ore, very clean and free from silica or other impurities; that made by calcining a precipitated ferric hydroxide is not well adapted to the process, as it gives a product difficult to lixiviate.
White liquor oxidation process
In the white liquor oxidation process, the nitrogen sulfide (Na2S) in the white liquor is oxidized by air to polysulfides, which are used in the digestion process. The digestion process is where cellulose and semi-cellulose (polysaccharide) are separated from the chips which serve as the wood material for making pulp. White liquor being a chemical such as Na2S, NaOH etc.
This air-oxidation process is situated in between the caustification process and the digestion process. The white liquor that is to be air-oxidized is taken from the line that connects the caustification to the digestion process, and the polysulfides generated are channeled back into process. This means that no modification to the existing system is necessary. The air-oxidation process is comprised mainly of a white-liquor filtration device and an oxidation reactor. The upper-current type is used in order for the filter to efficiently remove the SS, the major components being CaCO3 and other substances
generated during the caustification process. The oxidation reactor is filled with oxidation catalysts, then air supplied via a blower keeps the oxidation reaction going.
The NaOH generated along with the polysulfides in the reaction is used effectively in the digestion process, which also serves to reduce the load placed on the caustification process.
In Carmichael’s apparatus, an asbestos diaphragm, impregnated with Portland cement, is used. The diaphragm rests horizontally on the cathode at the bottom of the cell; above it is a bell to collect the hydrogen given off. On anode is a grating of copper rods, covered with hard rubber, through which platinum points project into the brine. This anode is suspended in the top of the cell, and the chlorine set free is thus only momentarily in contact with the liquid. The salt solution is fed into the cell at the top, in a rapid stream of drops while the mixture of caustic soda and salt flows continuously from the bottom. The supply of brine is so regulated that the caustic formed at the cathode is drawn off before it has time to diffuse through the liquid. The solution drawn from the cell contains about 20 per cent of caustic soda, and about 75 per cent of the salt is decomposed. The reaction is carried on at a temperature of about 80° C in the top of the cell near the anode, while the region around the cathode is kept as cool as possible.
Being removed from the immediate action of the chlorine, the diaphragms are very durable.
The process uses Lunge’s apparatus. The cathode, of iron wire gauze is placed in a slanting position. On it rests the diaphragm, consisting of two parts, a sheet of parchment paper and it double sheet of asbestos cemented together by blood albumin, coagulated and hardened by treatment with potassium bichromate. An earthenware bell enclosed the anode, which was made of lead, carrying carbon rods dipping into the salt solution. Caustic soda is formed in the solution outside the bell, and owing to the inclined position of the cathode, the hydrogen was expected to escape readily, thus preventing polarization. But it proved in practice that the earthenware bells were disintegrated by the caustic soda solution, while the hydrogen set free. The diaphragms are rapidly destroyed, lasting only from 24 to 48 hours. The anodes are consumed more slowly, lasting about six weeks. The process yields a solution of caustic containing 10 per cent NaOH.
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