production of chlorine and caustic soda

1. Production of Chlorine and
Caustic soda
Prepared by:
Bikash Kumar Mahato
Assistant Professor
IOE, Pulchowk Campus

2. Physical Properties
Chlorine
• Mol. Wt. 70.9
• M.P. -101.60C
• B.P. -34.60C
• Liquefaction point: 150C at
5.7atm
• Critical temperature and
pressure : 1460C &
93.5atm
Caustic soda
• Mol. Wt. 40.00
• B.P. 13900C
• M.P. 3180C
• Very soluble in water with
high exothermic heat of
solution.
• Grades: Available in solid form
of flakes, granules, sticks,
lumps, pellets and aq. Solution.

3. Production Methods
1. Electrolytic process producing Cl2, NaOH and H2 as co-
products (accounts for 80% production)
1. diaphragm electrolytic cell)- uses saturated NaCl solution and
produces 10-12% NaOH which must be concentrated; being
replaced by membrane cells
2. mercury electrolytic cell-uses saturated NaCl solution with solid
salt makeup, produces 70 caustic solutions directly
2. Chlorine processes without co-products
1. HCl-air oxidation with Fe2O3 catalyst
2. HCl-air-Cl2 oxychlroination processes, eg, production of ethylene
dichloride from ethylene
3. HNO3-NaCl-air process
3. NaOH process with no Cl2 co-product (lime-soda process)
1. Na2CO3-Ca(OH)2 no further investments allocated as process not
competitive

4. Electrolytic process

5. Raw Materials
• Major raw material:
– Depending on the type of electrolysis cell used,
purified solution of 10-15% NaCl or solid salt
• Minor quantities:
– Na2CO3 and NaOH for salt purification
– H2SO4 for Cl2 drying

7. Electrolysis of aq. Salt sol by mercury,
diaphragm and membrane process.

8. Process description
• A combination of the diaphragm and mercury cell
processes shown in flowchart and same discussed here
• Brine solution flows through pipelines to a storage
reservoir and then through a brine treatment system
• Caustic soda, soda ash and/or barium carbonate removes
calcium, magnesium and iron salts which would clog up
diaphragms
• This purified, saturated brine (25-28% NaCl) is heated
and electrolyzed in a diaphragm cell
• The cell, operating at 45-55% decomposition efficiency,
discharges a 10 -12% solution of NaOH with about an
equal concentration of NaCl
• Multiple effect evaporation concentrates the cell liquor to
50% NaOH solution.

9. Conti.
• Precipitated salt is separated, centrifuged, washed
and then slurried with treated brine
• Salt separator overflow is 50% caustic soda product
containing 2% NaCl and 0.1-0.5% NaCIO(sodium
hypochloride) on a dry basis
• This commercial caustic grade can be
– Evaporated to produce saturated 73% NaOH liquor or
– Fused to flake, granular or stick caustic
• Purified grade can be produced by a combination
treatment of
– CaCO3 to remove colloidal Fe and
– Liquid NH3 counter-current extraction to take out
chloride and chlorate impurities

10. Membrane process
• In a membrane cell, a cation exchange membrane separates
the anolyte and catholyte
• Brine is fed into the anode compartment where
– Chlorine gas is created and
– Sodium ion and associated water of hydration migrate through the
membrane into the catholyte
• Unlike the diaphragm cell process, cation exchange membrane
prevents the migration of chloride ions into the catholyte
• Depleted brine is discharged from the anolyte to maintain a
minimum NaCl concentration
• Water is electrolyzed at cathode and strong caustic (32-35%)
is produced either by
– Controlling water addition rate directly to catholyte or
– Recirculating caustic to which water has been added

11. Conti.
• There is some back-migration of hydroxyl ions into the
anolyte, which results in the loss of current efficiency
• Membrane is the most critical component of this cell process
– Thus current efficiency, cell voltage and hence energy
consumption, are greatly dependent on its quality
• Membrane in a chlor-alkali electrolyzer is exposed to chlorine
on one side and strong caustic on the other
• Thus requirements for membrane separators are as follows:
– Durability under the conditions of chlor-alkali electrolysis It should
have good mechanical properties and long term stability for
practical use
– High selectivity for transport of sodium or potassium ions
– Negligible transport of chloride, hydrochloride and chlorate ions
– Zero back-migration of hydroxide ion
– Low electrical resistance

12. Conti.
• For a new plant, membrane process is the first
choice because
– Both the capital and operating costs are the least
– Advantage of membrane process increases as the
energy prices increase
• Mercury and diaphragm plants are being
converted to or replaced by membrane process
because of
– Environmental regulations as well as cost
considerations

13. Mercury Process
Advantage
• Pure 50% NaOH solution
(without evaporation)
• Pure chlorine gas
Disadvantage
• Higher voltage than with the
diaphragm process
• Thus 10-15% higher
electrical energy
consumption
• More stringent brine
purification requirements
• Stringent mercury
contamination avoidance
measures required

14. Diaphragm Process
Advantage
• Utilization of less pure brine
• Lower voltage than in the
mercury process
Disadvantage
• NaOH produced is both
dilute and chloride-
contaminated thus
evaporation required for
purification.
• Chlorine gas contains
oxygen
• Rigorous measures required
to avoid asbestos emission

15. Membrane Process
Advantage
• Pure NaOH solution
• Electrical energy
consumption only about
77% of that of the mercury
process
• No mercury or asbestos
used
Disadvantage
• NaOH content only ~33%
by weight
• Chlorine gas contains
oxygen
• Very high purity brine
required
• High cost and short lifetime
of the membranes

16. Uses
Chlorine
• Pulp and paper
• PVC
• Organic & Inorganic
chemicals
• Pesticides and insecticides
• Water treatment
• Pharmaceuticals
• Rayon grade wood pulp
Caustic soda
• Textile industry
• Paper and pulp
• Soaps and detergents
• Dye
• Pharmaceuticals
• Petrochemicals
• Organic & Inorganic
chemicals

17. Major engineering problems
• choice of cell design
– All three cell processes are in use in India for production of
caustic soda
– Dominance in capacity and production is in the order of
mercury cell membrane cell and diaphragm cell
– Membrane cell process is the most modern, energy efficient
and pollution free
– Whereas conversion cost of existing mercury cells and
diaphragm cells has become prohibitive
• This is due to high cost of capital equipment, high technology
transfer fees, compounded by an import duty on capital equipment
• This has impeded the progress of adopting the membrane cell
technology at a faster rate.

18. Conti.
• corrosion
– Handling wet chlorine requires stoneware, plastic,
glass or rubber-lined equipment
– After drying with H₂SO₄, chlorine can be handled in
mild steel or iron
– Brine solutions are also very corrosive at elevated
temperatures
– Evaporators for cell liquor must be nickel-lined to
avoid iron contamination
– Remainder of the plant can be constructed of cast iron,
steel and wood
– Mercury cells are also made with rubber-lined steel

19. Conti.
• Economics
– Plant location
– Caustic-chlorine hydrogen balance
– Lime-soda process
– Choice of electrolytic cell
– Power cost
• ASSIGNMENT
– Write down the comparison between Diaphragm,
Mercury and Membrane process for the production of
Chlorine And Sodium hydroxide.