THE CHLOR-ALKALI
INDUSTRY
Chlorine is manufactured by the electrolysis of
brine. Sodium hydroxide is produced at the
same time.
Three different methods are currently in use.
We only need to know the method that is
mainly used
Membrane cell electrolysis
The electrolysis cell is divided into two "rooms" by a cation
permeable membrane acting as an ion exchanger. Saturated
sodium (or potassium) chloride solution is passed through the
anode compartment, leaving at a lower concentration. Sodium
(or potassium) hydroxide solution is circulated through the
cathode compartment, exiting at a higher concentration. A
portion of the concentrated sodium hydroxide solution leaving
the cell is diverted as product, while the remainder is diluted
with deionized water and passed through the electrolyzer
again.
.
A membrane separates the cell into two halves.
The membrane only allows positive ions through.
It is called a positive ion preamble membrane.
Na+ ions through
Cl- and OH- ion NOT go through. (must mention
both negative ions in exam)
The concentrated NaCl solution is pumped into the
positive half-cell (anode).
Cl- is oxidised to Cl2 gas that is collected as it bubbles
off.
The positive (Na+) sodium ions move through the
membrane to the cathode half-cell.
This flow completes the circuit. The water is reduced in
this half-cell to form hydroxyl ions (OH-).
2H2O(l) + 2e-  H2(g) + 2OH-(aq)
The Na+ + OH- form a sodium hydroxide solution. The
solution is removed from the half-cell. The hydrogen byproduct is used in other industrial processes.
The chloralkali process is a redox reaction.
The redox reaction that takes place is:
2 NaCl(aq) + 2 H2O(l)  2 NaOH(aq) + Cl2(g) + 2H+
+ 2eand a further reaction takes place :
2 H+ + 2e-  H2 (g)
In the reatcion, the NaCl is oxidised to Cl2 and the
water is reduced to H+.
The uses for sodium hydroxide include:
•etching aluminium ( printing a design on
the metal);
•removing gases like SO2 and CO2 from
other industrial reactions, preventing their
release into the atmosphere;
•as a drain cleaner in homes (reacts with
fats to form soap that can then be washed
away); and
•for the making of soaps and detergents.
Products that are made from chlorine:
•PVC
•Sodium hypochlorite (NaOCl) is used as
bleach in the wood pulp and paper industry,
and is also used as a household bleach.
•Calcium hypochlorite (Ca(OCl)2) is used in
swimming pools to disinfect the swimming
pools. It is often called ‘chlorine’, even though it
is a compound of chlorine.
•Hydrochloric acid (HCl) is a strong acid that
can be used to clean metal in the iron and steel
industry. It is also used to steady the pH in
swimming pools.
Impact on environment and humans
•The factory producers hydrogen gas which is explosive and
can be dangerous as can cause death and destroy property
and infrastructure
•The Chlorine gas may leak into the environment that can
cause respiratory irritation and even death- used as mustard
gas in WW2
•NaOH (caustic soda) is highly corrosive and if leaked into
the groundwater- will contaminate it. Water cannot be used
for irrigation ( destroy plants) or drunk ( cause health
problems).