MINERALS TO ELEMENTS
EXTRACTION OF HALOGENS
Introduction

Chlorine and bromine occur in nature in the form of negative ions (Cl- / Br-)

Conversion of these negative ions into the atoms of the element concerned involves
OXIDATION (i.e. loss of electrons).
2 Cl-(aq)  Cl2(g) + 2e
2 Br-(aq)  Br2(l) + 2e

Chloride ions are smaller than bromide ions and it is therefore harder to remove the
electron. Electrolysis is needed.

Chlorine gas itself can be used as an OXIDISING AGENT – to remove electrons from
bromide ions.
Extraction of chlorine

Chlorine is manufactured by electrolysis of aqueous sodium chloride (brine)

Aqueous solutions of sodium chloride contain FOUR ions. Two of these come from the
water. The ions are: Na+ ClH+
OH-

When sodium chloride solution is electrolysed, the positive ions (cations) move
towards the negative electrode (cathode).
At the cathode, positive ions gain electrons. In other words they are reduced.
Hydrogen ions are more easily reduced than sodium ions and hydrogen gas is
produced. The following reaction takes place:
2 H+(aq) + 2e  H2(g)

When sodium chloride solution is electrolysed, the negative ions (anions) move
towards the positive electrode (anode).
At the cathode, positive ions lose electrons. In other words they are oxidised.
Chloride ions are more easily oxidised than hydroxide ions and chlorine gas is
produced. The following reaction takes place:
2 Cl- (aq) - 2e  Cl2(g)

The solution that remains contains sodium ions and hydroxide ions. Sodium hydroxide
can be extracted from this solution.

The main practical difficulty that arises when this process is carried out on a large
scale is that some of the products will react with one another. Hydrogen and chlorine
react explosively to form hydrogen chloride and chlorine reacts with sodium
hydroxide, forming sodium chlorate(I).

Several solutions have been offered to this problem. Examples include the mercury
cell, the diaphragm cell and the membrane cell. Activity M 1.5 looks at the last of
these options.
Minerals to Elements / Class Notes 1
Page 1
Extraction of bromine

The waters of the Dead Sea contain a fairly high concentration of bromide ions (5.2 grams per
dm3). This makes it relatively easy to extract the bromine.

Ocean waters contain a much lower concentration of bromide ions (0.07 grams per dm 3) and
the process of extraction is more complex.

The principles underlying the process are summarised below. This explanation should be
studied in conjunction with the diagram on the Activity Sheet for M 1.2.
STEP
Sea water is acidified using
dilute sulphuric acid
Chlorine gas is passed through
acidified seawater.
Air is blown through this
solution and bromine vapour is
produced. The rest of the
seawater is returned to the
ocean.
Bromine vapour is treated with
sulphur dioxide gas and the
resulting hydrogen bromide is
dissolved in a spray of water
to give solution containing 13%
by mass of hydrobromic acid.
PRINCIPLE(S)
Both chlorine and bromine react
with alkaline solutions. This would
hamper the desired reaction.
Chlorine oxidises bromide ions
giving very dilute aqueous
bromine.
Bromine is relatively volatile (low
boiling point) and vaporises easily.
The bromine/air mixture is too
dilute to be economic.
Bromine is reduced by sulphur
dioxide giving hydrogen bromide.
The hydrobromic acid passes
to the steaming out tower
where it meets a stream of
chlorine gas. This produces
hot vapour (steam/ chlorine/
bromine) and an aqueous
solution containing acids.
Chlorine oxidises bromide ions.
The aqueous bromine vaporises
because the mixture is hot.
The hot vapour is cooled.
Bromine and water condense.
Chlorine can be recirculated
Bromine and water have much
higher boiling points than
chlorine.
Bromine and water pass into a
separator where the bromine
sinks to the bottom
Bromine is much more dense
than water. The liquids mix
slightly so the bromine is damp.
The bromine layer is dried
using concentrated sulphuric
acid.
Concentrated sulphuric acid is
highly hygroscopic. It therefore
removes the water from bromine.
EQUATION(S)
Cl2(g) + 2 Br-(aq) 
Br2(aq) + 2 Cl-
Br2(aq)  Br2(g)
SO2 + 2 H2O + Br2 
H2SO4 + 2 HBr
Cl2(g) + 2 Br-(aq) 
Br2(aq) + 2
ClBr2(aq)  Br2(g)
Minerals to Elements / Class Notes 1
Br2(g)  Br2(l)
H2O(g)  H2O(l)
H2SO4(l) + aq  H2SO4(aq)
Page 2