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

Chapter 2
which is usually reused for brine preparation (see Section 2.3.1). This high quality sodium
chloride can then be used to enrich depleted brine, sometimes it is used as a raw material for an
amalgam or membrane process. Sodium sulphate present in the cell liquor (0.12 – 0.65 wt-%)
also crystallises in the later stages of evaporation and may be isolated to avoid contamination of
the main portion of the recovered salt. The residual level of sodium chloride in sodium
hydroxide from diaphragm cells is approximately about 1 wt-% and sodium chlorate
approximately 0.1 wt-%. For this reason, it is unsuitable for certain end applications such as the
manufacture of rayon. The concentrations of salt and sodium chlorate in the caustic soda from
diaphragm cells can be reduced by extraction with anhydrous liquid ammonia extraction to
increase marketability, but at increased cost [ 1, Ullmann's 2006 ], [ 17, Dutch Ministry 1998 ].
In the case of the membrane cell technique, concentration of the caustic soda is normally
achieved in two or three stages using either plate or shell-and-tube evaporators. The number of
stages depends on factors such as plant size and the cost of steam. The caustic soda from
membrane cells is of high quality, although the caustic soda produced (usually around 33 wt-%
NaOH) needs to be concentrated concentration to 50 wt-% NaOH to be traded as a commodity
for some applications. The NaCl salt content of the membrane-cell caustic soda lies between
20 – 100 ppmw (in 100 % NaOH), but is on average slightly higher than mercury cell caustic
[ 1, Ullmann's 2006 ], [ 3, Euro Chlor 2011 ], [ 17, Dutch Ministry 1998 ](see Table 2.1).
In some plants the caustic soda is further concentrated to a 73 wt-% solution and to 100% as
solid caustic prills or flakes with a water content of < 0.5 – 1.5 wt-% using multi-effect
evaporators.
Some chlor-alkali production facilities can combine the caustic production process from
mercury and membrane cells in order to minimise energy costs (see Figure 2.8). It is possible to
feed 33% caustic from the membrane cells to the decomposer to produce 50% caustic without
the need for evaporation.
Storage and handling
Because of its highly reactive and corrosive properties, caustic soda may corrode containers and
handling equipment. Construction materials must be suited to the caustic soda handled and
stored.
Sodium hydroxide solutions require steam or electrical heating where temperatures can fall
below the upper freezing point. Depending on the concentration, the freezing point can be
higher than 0 °C; for example it is 5 °C for 32 wt-% NaOH and 12 °C for 50 wt-% NaOH.
Frozen pipelines present both safety and environmental risks when attempts are made to
unblock them [ 3, Euro Chlor 2011 ]. Safety measures are set out in Chapter 4.
Storage tanks may be lined in order to minimise iron contamination of the product or to avoid
prevent stress corrosion cracking of the tank. Tanks are usually included in procedures to
prevent overflow or spillage of caustic soda. Such procedures include containment and
mitigation.
WORKING DRAFT IN PROGRESS
It should be noted that Dissolved hydrogen gas can be released into the vapour space above the
liquid in storage tanks. Tanks are normally vented from the highest point. Testing for an
explosive mixture of hydrogen in and air normally precedes any maintenance activity in the
area.
60 December 2011 TB/EIPPCB/CAK_Draft_1