SHORT COMMUNICATION
Determination of the chlorine current eciency of mercury chlor-alkali cells
C.-P. CHEN, B. V. TILAK
Occidental Chemical Corporation, Technology Center, 2801 Long Road, Grand Island, New York 14072, USA
Received 15 February 1997; revised 22 May 1997
1. Introduction
Currently, about 14% of chlorine production in
North America and 65% in Western Europe is from
mercury cells which consume more energy than the
diaphragm or membrane chlor-alkali cells [1, 2]. It is,
therefore, essential that the energy consumption
expressed in terms of kWh ton
À1
of chlorine is
estimated to achieve optimal energy savings.
The purpose of this paper is to develop a chlorine
current eciency expression for the mercury cell op-
erations so that the energy consumption expressed in
kWh ton
À1
of chlorine can be reliably estimated.
There is no published current eciency expression for
mercury cell operations. Hence, a material balance
approach, similar to that employed for diaphragm
and membrane cell operations [3], was followed here.
2. Theory
2.1. Process chemistry
The primary electrochemical reactions in a mercury
chlor-alkali cell are as follows:
At the anode
Cl
À
3
1
2
Cl
2
e
À
1
At the cathode
Na
Hg e
À
3
NaHg2
Chlorine is evolved at the anode and an amalgam
with 0.25±0.5% sodium is formed at the mercury
cathode. The sodium amalgam produced is then fed
to a decomposer, where it reacts with water to form
sodium hydroxide and hydrogen gas as
NaHgH
2
O
3
NaOH
1
2
H
2
Hg 3
The recovered mercury is recycled to the electrolyser.
The caustic formed in this process is of high purity
and has very low salt content (typically 0.005 wt%).
The depleted brine from the electrolyser at a con-
centration of 21±22% is dechlorinated, resaturated
with salt, treated to remove impurities, and returned
to the electrolyser.
2.2. Derivation of chlorine current eciency expression
The chlorine current eciency, de®ned as the ratio of
the amount of chlorine leaving the electrolyser,
o
Cl
2
,
to the theoretically expected quantity of chlorine
produced, can be expressed as
g
Cl
2
o
Cl
2
a2
4
Acknowledging the practical diculties associated
with the direct measurement of the amount of chlo-
rine collected at the cell, an indirect approach is fol-
lowed to estimate it from the material balance (i.e.,
input+generation A loss = output) of the Cl or Cl
2
species across the anode compartment of the elec-
trolyser (see Fig. 1 for a schematic of the material
balance for a mercury cell). From the material bal-
ance of the Cl
2
species (Equations 5±8),
Input 3
f
NaClO
3
f
Cl
2
f
HOCl
f
NaOCl
5
Generation
e
Cl
2
a2 À 2
e
O
2
6
Loss 2
ch
O
2
7
Output 3
d
NaClO
3
d
Cl
2
d
HOCl
d
NaOCl
o
Cl
2
8
and noting that the total amount of O
2
Y
o
O
2
, collected
is from the electrochemical,
e
O
2
, and chemical reac-
tions,
ch
O
2
,as
o
O
2
e
O
2
ch
O
2
9
the chlorine current eciency can be deduced as
List of symbols
and superscripts refer to feed and depleted
brine, respectively.
source
species
concentration of species (noted in the
subscript) in the source stream (noted
in the superscript) (mol dm
À3
Faraday number 96X487 kA s mol
À1
load (kA)
source
species
molar ¯ow rate of species (noted in the
subscript) in the source stream (noted
in the superscript) mol s
À1
feed brine ¯ow rate dm
3
s
À1
depleted brine ¯ow dm
3
s
À1
f
AvXCl
2
total available chlorine in feed brine
f
Cl
2
f
HOCl
f
NaOCl
mol dm
À3
d
AvXCl
2
total available chlorine in depleted
brine
d
Cl
2
d
HOCl
d
NaOCl
mol dm
À3
%O
2
Nitrogen-free oxygen in the cell gas
= Measured value of % O
2
À
0X2658%N
2
f
Cl
2
and
d
Cl
2
refer to the soluble chlorine in feed and
depleted brine.
JOURNAL OF APPLIED ELECTROCHEMISTRY 27 (1997) 1300±1303
1300
0021-891X
Ó
1997 Chapman & Hall