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

eduction of costs related to equipment and maintenance;
environmental legislation.
Chapter 4
Example plants
Techniques to reduce the levels of organic contaminants in the brine are used by many
membrane cell chlor-alkali plants.
Reference literature
{Please TWG provide information.}
4.3.7 Techniques to reduce generation of sulphuric acid waste
4.3.7.1 Overview
Concentrated sulphuric acid (92 – 98 wt-%) is used to dry chlorine. Up to 20 40 kg of acid is
consumed per tonne of chlorine produced (see Sections 2.6.5 and 3.3.3). The spent acid usually
becomes a waste product or one that requires reprocessing. The spent acid can be used to
control the pH in process and waste water streams or to destroy surplus hypochlorite if there is a
respective demand on-site. It can also be returned to an acid producer for reconcentration
reconcentrated (see Section 4.3.7.2) or sold as by-product to any user who can use accept this
quality of acid. Otherwise, the spent acid becomes waste [ 33, Euro Chlor 2011 ], [ 54, Euro
Chlor 2010 ].
4.3.7.2 On-site Reconcentration of spent sulphuric acid
Description
This technique consists in reconcentrating spent sulphuric acid on site or off site in closed-loop
evaporators under vacuum by indirect heating with steam.
Technical description
Sulphuric acid can also be reconcentrated on site in closed loop evaporators which reduces the
consumption to 0.1 kg of acid per tonne of chlorine produced. The spent sulphuric acid is
concentrated to 92 – 98 wt-% by indirect heating with steam. After preheating the spent
sulphuric acid with hot reconcentrated acid, the reconcentration is usually carried out in two
stages. The first evaporator is typically operated at approximately 80 mbar and concentrates the
acid to 90 wt-%, while the second is typically operated at 15 – 20 mbar to reach a final acid
concentration of 96 wt-%. The vapours from both evaporators pass through a scrubbing column
before being condensed in a condenser. The scrubbing column is necessary due to the steep
increase of the sulphuric acid content in the gas phase at liquid concentrations > 75 wt-%. The
condensate is used as working liquid for the vacuum generation via steam ejection before being
discharged. The product acid leaves the boiler and is cooled down [ 47, De Dietrich 2011 ].
Materials used must be highly corrosion-resistant to avoid corrosion problems, in particular if
the feed acid is polluted.
WORKING DRAFT IN PROGRESS
The reconcentration can be carried out on site or off site.
Achieved environmental benefits
The achieved environmental benefits of this technique include:
significant reduction of in sulphuric acid consumption;
reduction of sulphate emissions if spent acid would otherwise be neutralised and
discharged.
TB/EIPPCB/CAK_Draft_1 December 2011 251