(BAT) Reference Document for the Production of Chlor-alkali ...

2.6.11 Dealing with impurities
2.6.11.1 General description
Chapter 2
Chlorine gas from the electrolysis cells may contain impurities such as water, nitrogen
trichloride (NCl3), bromine (Br2), halogenated hydrocarbons (CXHYXZ) originating from
rubberised or plastic piping, carbon dioxide (CO2), oxygen (O2), nitrogen (N2) and hydrogen
(H2).
Nitrogen trichloride, bromine and halogenated hydrocarbons predominantly dissolve in the
liquid chlorine, whereas the non-condensable gases (CO2, O2, N2, H2) remain in the gas phase
and increase in concentration during chlorine liquefaction. Traces of sulphuric acid, ferric
sulphate, and/or ferric chloride and/or carbon tetrachloride might also be present in the liquid
gas phase after drying and liquefaction of chlorine.
Liquid chlorine of commercial quality has a purity of at least 99.5 wt-% with the following
specifications: water < 0.005 wt-%, solid residues < 0.02 wt-%, CO2 Q 0.5 wt-%, N2:
0.1 – 0.2 wt-% and O2: 0.1 – 0.2 wt-% [ 1, Ullmann's 2006 ].
Special attention should be paid to the following impurities The impurities described in the
following subsections are of particular concern:
2.6.11.2 Water
All metals are attacked by wet chlorine with the exception of titanium and tantalum. Titanium
can only be used in wet chlorine conditions; it spontaneously combusts in dry chlorine.
The presence or absence of water considerably alters the reactivity of chlorine towards the
equipment material (see Section 2.6.2).
2.6.11.3 Hydrogen
All three techniques technologies produce hydrogen which can form an explosive mixture (>4%
H2) in with chlorine or air. Light, friction and gas depressurisation may bring enough energy to
initiate the reaction at ambient temperature. The low explosive limit of hydrogen in pure
chlorine depends on temperature and very minimally on pressure, and equates to approximately
4 vol-% H2 at ambient temperatures. Most plants therefore keep the hydrogen concentration
below 4 vol-%, either by adding inert gases such as nitrogen or carbon dioxide or by reacting
the hydrogen to hydrochloric acid (see Section 2.6.8) [ 1, Ullmann's 2006 ], [ 10, Kirk-Othmer
2002 ]. The concentration of hydrogen in chlorine is usually measured continuously Chlorine
gas is analysed regularly to ensure the absence of an explosive mixture [ 3, Euro Chlor 2011 ].
2.6.11.4 Nitrogen trichloride
WORKING DRAFT IN PROGRESS
Nitrogen trichloride (NCl3) is formed during the electrolytic production of chlorine, due to side
reactions in the brine between the chlorine and various some nitrogen compounds, in the brine
solution originating from the raw materials (mainly ammonium and organic nitrogen
compounds). Molecular nitrogen and nitrates do not react with chlorine. Nitrogen may originate
from [ 201, Piersma 2001 ]:
salt: explosives used in rock salt mining, anti-caking agents (ferrocyanide), impurities
from transport vehicles;
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