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

4.3.6.3.4 Catalytic decomposition reduction
Chapter 4
Description
This technique consists in converting free oxidants to chloride and oxygen by using
nickel-, iron- or cobalt-based catalysts. Catalytic decomposition can be carried out in fixed-bed
reactors or with a catalyst slurry.
Technical description
The catalytic decomposition occurs The decomposition of free oxidants to chloride and oxygen
can be catalysed by a nickel, iron or cobalt based catalyst, according to the following overall
reaction:
[M] n+
2 NaOCl V 2 NaCl + O2w
This decomposition reaction generally occurs in hypochlorite solutions, albeit too slow to
remove free oxidants in technical systems. The reaction is accelerated by metal catalysts, lower
pH values and higher temperatures (see Section 2.6.12.2). Depending on the conditions,
hypochlorite may instead decompose to chloride and chlorate (see Section 4.3.6.3.5).
Some systems operate with a catalyst slurry which uses solutions of nickel, iron or cobalt
compounds added to the waste water in stirred or agitated tanks [ 208, Johnson Matthey 2009 ].
Metal concentrations are typically 20 mg/l [ 2, Le Chlore 2002 ]. The high pH value of the
solution causes the metal ions to precipitate as their hydroxides, which are then removed and
regenerated. Alternatively, a fine dispersion of an insoluble metal compound can be used which
removes the need for the catalyst regeneration stage. is blended with the waste water stream to
promote the reaction, in a batch process. To avoid emissions of metals In both cases, the catalyst
must be allowed to settle before the supernatant water can be discharged, to avoid emissions of
heavy metals. The reaction time combined with the settling of the catalyst takes several days
[ 208, Johnson Matthey 2009 ]. The catalyst activity decreases from batch to batch, although it
is unclear whether this is due to deactivation of the catalyst or loss of metal [ 75, COM 2001 ].
Other systems operate with the catalyst on a fixed-bed reactor (Figure 4.7). The catalyst used is
a nickel oxide promoted with iron on an alumina support. The design of the reactor is modular.
The gravity-fed hypochlorite solution flows countercurrently with respect to the evolving
oxygen [ 206, Denye et al. 1995 ], [ 207, Stitt et al. 2001 ], [ 208, Johnson Matthey 2009 ]. , see
Figure 4.7, reducing the loss of metals and increasing the capacity (no settling required)
compared to slurry type catalytic systems. Also, the necessary concentration of catalyst is low
(20 ppm), [Le Chlore, 1996].
WORKING DRAFT IN PROGRESS
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