6 Methods of Preparation of Caustic Soda

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6 Methods of Preparation of Caustic Soda

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Introduction

•  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.

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Castner - Kellener Process
Principle - In castner-kellner method NaOH is
prepared by the electrolysis of aqueous solution
of NaCl (Brine).
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• 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.

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• 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.

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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.

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Nelson Diaphragm Cell
Anode (positive electrode) carbon (graphite) or
titanium coated with Ru-Ti oxide. Cathode
(negative electrode) steel mesh
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• 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) -----gt 2Na(aq)
  2OH-(aq) H2(g) Cl2(g)
• Product contains sodium chloride and sodium
  hydroxide. NaOH(s) can be crystallised out.

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Loewigs process

• Loewigs 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.

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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.

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• 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.

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Carmichael method

• In Carmichaels 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.

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LeSueurs process

• The process uses Lunges 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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