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

Chapter 2
should not be stronger than about 12 % by weight. Higher caustic concentrations can be used
providing adequate cooling is installed, but there is an increased risk of solids deposition and
blockage.
Tail gas from the chlorine liquefaction unit contains residual chlorine and carbon dioxide, which
are absorbed in the caustic, and hydrogen, which is diluted with air to less than 4% by volume to
avoid flammable mixtures.
Optimum design of scrubbing systems must include high reliability, automatic operation in
emergencies, and countercurrent flow of liquid and gas to achieve low exit concentrations.
{These paragraphs were updated and moved to Section 4.3.5.1.}
2.6.12.3 Use and treatment of the produced bleach
If sale is not possible, efficient decomposition of the sodium hypochlorite produced into sodium
chloride can be achieved using a nickel catalyst. The bleach (solution of sodium hypochlorite)
produced by the safety absorption unit usually has too low a concentration of active chlorine to
be sold, but can be used to feed the commercial hypochlorite production unit. In some cases,
this diluted bleach can be used directly on the site (e.g. for the treatment of cooling water) or
can be recycled to the brine system. If this is not possible, the bleach has to be destroyed. This
can be done by chemical reduction to chloride using agents such as sulphite, by thermal
decomposition to oxygen and chloride with or without a catalyst, by thermal decomposition to
chlorate and chloride at increased temperatures as well as by acidic decomposition with release
and recovery of the chlorine (see Section 4.3.6.3) [ 3, Euro Chlor 2011 ], [ 192, Euro Chlor
2011 ].
WORKING DRAFT IN PROGRESS
58 December 2011 TB/EIPPCB/CAK_Draft_1