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

Chapter 4
4.3.6.1.2 Precipitation of sodium sulphate
Description
This technique consists in precipitating and recovering sodium sulphate from the brine
purification in the case of membrane cell plants and from the caustic soda evaporators in the
case of diaphragm cell plants.
Technical description
The sulphate concentration in the brine depends on the quality of the salt used and needs to be
carefully controlled to prevent damages to anodes and membranes (see Section 3.4.2.3.2 and
Table 2.4). This is usually achieved by purging a part of the brine (sometimes using
nanofiltration to produce a concentrated stream, see Section 4.3.6.2.2) or by precipitation as
barium sulphate. The latter technique requires the use of expensive barium salts which are
detrimental to membranes if not completely removed from the brine (see Section 4.3.6.2.2).
An alternative to these techniques is to precipitate sodium sulphate which is a saleable product
after purification. Sodium sulphate can precipitate in its anhydrous form Na2SO4, as decahydrate
Na2SO4 · 10 H2O, as mixed salt with NaCl or as mixed salt with NaCl and NaOH depending on
the temperature and composition of the solution [ 1, Ullmann's 2006 ], [ 205, Ullmann's 2000 ].
Precipitation of sodium sulphate can be achieved by cooling the brine stream [ 1, Ullmann's
2006 ]. A more energy-efficient process is to crystallise sodium sulphate from the concentrate of
a nanofiltration unit (see Section 4.3.6.2.2) [ 127, Eckert 2010 ]. At the AkzoNobel plant in
Bitterfeld (Germany), solution-mined brine is used as raw material for the membrane cells. The
depleted brine is reconcentrated in a four-stage evaporator. Sodium chloride precipitates in the
second and third stages and is reused for brine resaturation, while sodium sulphate precipitates
in the fourth stage and is subsequently purified and sold [ 69, Regierungspräsidium Dessau
1999 ].
In the diaphragm cell technique, pure sodium sulphate can be recovered from the cell liquor if
the caustic solution is concentrated in an evaporator [ 1, Ullmann's 2006 ].
Achieved environmental benefits
The achieved environmental benefits of this technique include:
reduction of sulphate emissions;
reduction of consumption of raw materials and energy (compared to other processes to
produce sodium sulphate).
Environmental performance and operational data
The AkzoNobel plant in Bitterfeld (Germany) precipitates sodium sulphate and reports an
annual average sulphate emission factor of 0.72 kg/t annual chlorine capacity.
Cross-media effects
Some raw materials and energy are consumed for the precipitation and purification of sodium
sulphate. Cooling of the brine stream to crystallise sodium sulphate with subsequent reheating
of the brine is energy-intensive. Brine resaturation with evaporators also consumes considerable
amounts of energy.
WORKING DRAFT IN PROGRESS
Technical considerations relevant to applicability
{Please TWG provide information.}
Economics
{Please TWG provide information.}
Driving force for implementation
The driving forces for implementation of this technique include:
232 December 2011 TB/EIPPCB/CAK_Draft_1