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

2.5 Brine supply
2.5.1 Sources, qualities and storage of salt Salt unloading and
storage
Chapter 2
The brine used in the mercury cell and membrane cells processes is normally obtained by
dissolving saturated with solid salt in depleted brine, although some installations use
solution-mined brine on a once-through basis (i.e. no brine recirculation). The brine supply for
diaphragm cells is always used on a once-through basis, most commonly using solution-mined
brine, although the salt recovered from the caustic evaporators may be recycled into the brine
supply (see Figure 2.1).
Brine is generally produced by the dissolving of fresh salt in water or depleted brine from
mercury and membrane processes. The basic raw material is usually solid salt: rock salt
obtained by mechanical mining; solar salt produced by solar evaporation of sea water or brine;
or vacuum-evaporated salt from purifying and evaporating solution-mined brine. Other sources
include salt recovered from the caustic evaporators of the diaphragm cell technique and
solution-mined brine obtained by forcing water or weak brine into a salt deposit to dissolve the
material and carry it back to the surface [ 1, Ullmann's 2006 ], [ 10, Kirk-Othmer 2002 ].
Another source is used in Spain where 70 % of the sodium chloride used in chlor-alkali plants is
obtained by purification of NaCl-containing wastes incurred from the mining of potash (KCl)
including wastes from historic landfills [ 27, ANE 2010 ]. In specific cases it is also possible to
recycle salt-containing waste water from other production processes [ 33, Euro Chlor 2011 ].
In EU-27 and EFTA countries in 2011, mercury cell plants mostly use vacuum salt and rock
salt, diaphragm cell plants use solution-mined brine and membrane cell plants mostly use
vacuum salt and to a lesser extent solution-mined brine and rock salt. Solar salt is only used by a
few plants in Southern Europe and salt from potash mining wastes by plants in Spain. Some
plants use a combination of salt types [ 57, EIPPCB 2011 ].
The compositions from different sources vary widely but the main impurity in nearly all salts is
some form of calcium sulphate (Table 2.3). Solar salt is usually purer than rock salt, at least
after the common operation of washing. However, it is more susceptible to caking and
mechanical degradation. Rock salt usually contains more calcium sulphate and in proportion
less magnesium. The higher ratio of calcium to magnesium improves the precipitation of
magnesium hydroxide due to the formation of hybrid particles. Many chlor-alkali plants operate
with salt that has already undergone some purification. For example, most solar salts are washed
to remove occluded liquor and surface impurities and vacuum salt is recrystallised from brine
after most of the impurities have been removed by chemical treatment [ 10, Kirk-Othmer
2002 ].
Table 2.3: Typical compositions of various types of salt
Component Rock salt
Washed solar
salt
Vacuum salt
NaCl 93 – 99 % 99 % 99.95 %
SO4 2- 0.2 – 1 % 0.2 % 0.04 %
Ca 2+ 0.05 – 0.4 % 0.04 % 0.0012 %
Mg 2+ 0.01 – 0.1 % 0.01 % 0.0001 %
Source: [ 10, Kirk-Othmer 2002 ]
WORKING DRAFT IN PROGRESS
Generally the salt is stored in a sealed area equipped with a roof to prevent it from blowing off
site. Protective systems are installed to prevent contamination, in particular in case of rain or if
the storage area is located near surface water or groundwater. Because of its high purity the
vacuum salt in particular needs to be protected. Anti-caking agents, mostly ferrocyanides, are
TB/EIPPCB/CAK_Draft_1 December 2011 39