Brine Softening
(Calcium, Magnesium, Strontium, Barium)
Iodide, Silica and Alumina
Removal
In the production of chlorine and caustic soda, both the diaphragm and the mercury amalgam processes are increasingly being replaced by membrane technology that consumes less energy and is more environmentally benign.
The use of membranes for chlor-alkali electrolysis has introduced requirements
for much stricter purity control of the brine. The presence of impurities
such as Ca2+, Mg2+, Sr2+, Ba2+,
Al3+, SiO2, SO42-, and I-
can shorten the lifetime of the membranes or can damage the electrodes. This
results in a higher consumption of energy and higher membrane replacement
cost. The contaminants are brought into the system by salt, dilution water
and chemicals used in the process.
In general, the brine purification steps include: saturation, precipitation,
clarification, filtration, selective ion exchange, electrolysis and dechlorination.
If the salt is already of high purity, such as vacuum salt, a primary purification
with precipitation-filtration is not necessary and a secondary purification
with ion exchange only is sufficient. After dechlorination, the loop is closed
by sending the depleted brine back to saturation. Most membrane manufacturers
recommend that hardness be reduced to concentrations as low as 20-50 ppb.
Amberlite™ IRC747 gives hardness levels of consistently less than 5 ppb.
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If part of the circulating brine comes
from a mercury cell plant, then the mercury should be removed from the
brine by treatment with Ambersep™ GT74 prior to secondary treatment.
Fig. 1: Typical Chlor-Alkali brine circuit
Overall electrochemical
process: NaCl +
H2O
Both types can form stable complexes with alkaline earth ions with the following selectivity preference:
Ba2+< Sr2+< Ca2+< Mg2+
The difference between the two types lies in the different selectivities for the various alkaline earth elements. The AMP types have more pronounced affinity for Ca2+ and Mg2+ than the IDA types. On the other hand, the IDA types have more pronounced affinities for Sr2+ and Ba2+ than the AMP types. Consequently, the choice of the resin depends on the feed brine composition and on the criterion for the end-point of the loading cycle. If the end-point is based on Ca2+ and Mg2+ breakthrough then the AMP type of resin is recommended. If it is based on Sr2+ breakthrough, then the IDA type of resin gives a longer cycle.
Fig. 2: Typical brine ion exchange process merry go round system
Typical Operating conditions
Amberlite™ chelating resins have been optimized for very low hardness leakage and low regenerant consumption. Despite the fact that in the past few years the overall development of chelating resins has focused on higher and higher total capacity, Rohm and Haas has focused on achieving the lowest possible hardness leakage because this is the major cause of early membrane replacement and higher energy consumption. At the same time, we have focused on the lowest regenerant consumption possible to enable environmentally friendly processes. The result is a very high operating capacity and low operating costs for both Amberlite™ IRC747 and Amberlite™ IRC748.
Some chlor-alkali plants are progressively moving from mercury to membrane cell technology. Both processes may co-exist for many years and may have to share the same brine circuit feed loop. The brine circuit of a mercury cell process will contain mercury which is detrimental for membrane cell electrodes and which also irreversibly fouls Amberlite™ IRC747. The use of Ambersep™ GT74 in front of Amberlite™ IRC747 is an excellent solution to both problems.
Recommended Products
Amberlite™ chelating resins for hardness and
mercury removal are approved by all major licensors and are in operation
throughout the world since many years.
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NaOH + ½ H2 + ½ Cl2
