6 Methods of Preparation of Caustic Soda - PowerPoint PPT Presentation

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6 Methods of Preparation of Caustic Soda

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 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. – PowerPoint PPT presentation

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Title: 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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