Lecture Slides by Prof. Imitaz Ali at Ghulam Ishaq Khan Institute for Engineering Sciences and Technology (GIKI).

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Caustic soda
and chlorine
Industrial uses
Caustic Soda
• in the production of many useful organic chemicals
• inorganic chemicals like paints, glass and
ceramics,fuel cell production and cosmetics
• paper, pulp and cellulose industries
• food industry
• watertreatment(for the flocculationof heavy metals and
acidity control)
• soaps and detergentssectors
• textile sector (as a bleaching agent)
• mineraloils (preparationof greases and fuel additives)
and the synthesisof the synthetic fibre rayon
• in the process of refining aluminium from its ore bauxite
• synthesisof pharmaceuticalcompounds
• rubber recycling

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Industrial uses
Chlorine
• used for producing safe drinking water
• as a disinfectant in the form of the liquid hypo
• in the syntheses of synthetic rubber and PVC
(polyvinyl chloride)
• in agrochemicals and pharmaceuticals
• in bullet-proof vests, ultra-pure silicon chips
for solar panels and computer chips
• in solvents like chloroform and carbon
tetrachloride
• in dyestuffs, petroleum products, medicines,
antiseptics, insecticides, foodstuffs and paints
• in paper and pulp, explosives and pesticides
Production
• Sodium hydroxide (NaOH), lye or caustic soda is a
strong metallic base available in pellets, flakes,
granules, and as 50% saturated solution.
• NaOH and Cl2 are produced as co-products by the
electrolysis of brine.
• This yields sodium NaOHsolution, Cl2, and H2 in
the mass ratio 1: 0.88: 0.025 in accordance with
the following overall equation:
2 NaCl + 2H2O→2NaOH + Cl2 + H2

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Production
Various commercial cells have been developed
in order to keep the anode and cathode
products separate from one another.
– Diaphragm cell (Griesheim, Dow, Glanor, Hooker, HU
Monopolar, OxyTech)
– Mercury cathode cell (Castner – Kellner, Uhde, De Nora cell,
Olin – Mathieson, Solvay, Krebs Paris)
– Membrane cell (Asahi Kasei, Chlorine Engineers, Krupp Uhde,
EL-Tech, Ineos Chlor)

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Diaphragm cell process
• anode reaction
2Cl− → Cl2 + 2e−
• cathode reaction is
2H2O + 2e −→ H2 + 2OH−
Na+ + OH−  NaOH
Mercury cell process
• anode reaction
2Cl− → Cl2 + 2e−
Cl2 + H2O  H++ Cl- + HOCl
3Cl2 + 6OH−  ClO3
−+ 5 Cl−+3 H2O
• cathode reaction is
2H2O + 2e −→ H2 + 2OH−
Na+ + Hgx + e-  NaHgx
• decomposers
2 NaHg x + 2H2O  2NaOH + 2Hg x + H2

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Membrane cell process
• anode reaction
2Cl− → Cl2 + 2e−
Cl2 + H2O  H++ Cl- + HOCl
3Cl2+ 6OH−  ClO3
−+ 5 Cl−+3 H2O
• cathode reaction is
2H2O + 2e −→ H2 + 2OH−
Na+ + OH−  NaOH
styrene–divinylbenzene K/Na
Membrane characteristics
• High selectivity for the transport of sodium or
potassium ions
• Negligible transport of chloride, hypochlorite,
and chlorate ions
• Zero back-migration of hydroxide ions
• Low electrical resistance
• Good mechanical strength and properties
with long term stability

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Production
• The first step in all three processes is to purify the feed salt
brine.
• Brines contain many contaminants such as calcium,
magnesium, barium, and sulfate ions which are
detrimental to the electrolytic process.
• Removal of brine contaminants accounts for a significant
portion of overall chlor–alkali production cost, especially
for the membrane process since it requires a higher degree
of brine purity.
• Brines are treated with sodium carbonate to precipitate
calcium carbonate, followed by treatment with sodium
hydroxide to precipitate magnesium hydroxide.
Production
• Most trace metal impurities are also precipitated during the
process.
• The precipitates are allowed to settle in a clarifier where most of
the solids are removed as a mud.
• The brine is then filtered by sand filters and precoat polishing
filters (rotary vacuum filter).
• At this point the brine contains less that 4 ppm calcium and 0.5
ppm magnesium ions, which is satisfactory for the mercury and
diaphragm processes.
• The membrane process however requires additional ion exchange
to reach hardness levels below 20 ppb.
• The treated brine feed is also usually acidified with hydrochloric
acid to reduce oxygen and chlorate formation in the anolyte.
• HCl neutralizes residual hydroxide and carbonate in the brine and
prevents their reaction with chlorine formed at the anodes

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Production
• After treatment, the brine is sent to the electrochemical
cell.
• This liquor is concentrated to 50% NaOH in a series of
evaporative crystallizers.
• The crystallized NaCl is recovered and recycled.
• Sodium hydroxide remains in the liquid phase.
• Membrane cells produce 30–35% NaOH solutions which
are evaporated to 50% in a single evaporation step.
• Seventy three percent caustic containing very little salt is
made in the mercury cell decomposer.
• The product is filtered to remove entrained mercury and
further processing to meet final product specification on
concentration are done as needed.
Production
• The trend for new installations is to use membrane
cells since they give good performance with low energy
requirements.
• Environmental, health and safety issues have led to a
long term move away from mercury cells.
• The chlorine coproduct from electrolysis is dried in a
packed tower using concentrated sulfuric acid (96–98
wt%) to absorb water vapor.
• After drying, the chlorine is compressed.
• The hydrogen coproduct from electrolysis is relatively
pure and can be used for a wide variety of chemical
uses with only minimal additional processing.

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Diaphragm cell
• Advantage
– low purity rock salt, saving the need for purification
before electrolysis
• Disadvantage
– less efficient in terms of energy
– chlorine contains oxygen and must often be purified
by liquefaction and evaporation
– safety and disposal of asbestos diaphragms reach the
end of their life. The cell can, however,.

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Mercury cell
• Advantage
– NaOH obtained is highly pure.
– The process is very efficient.
– Possible reaction between NaOH and Cl2 is avoided as
NaOH is obtained in a separated chamber.
• Disadvantages
– High electricity consumption
– Environmental pollution due to escape of Hg vapours
Process flow diagram

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Production
• Owing to imbalances in the chlorine and
caustic markets, commercial interest in
production of sodium hydroxide by the lime
soda process:
Na2CO3 + CaO + H2O → 2NaOH + CaCO3