Chemical Engineering June 2021 - 24

Technology Profile
Production of Sodium Hydroxide from Brine
By Intratec Solutions
S
odium hydroxide (NaOH) is a
white, translucent crystalline
solid that is widely used in
the chemical process industries
(CPI). NaOH is often referred to
as caustic soda, due to its corrosive
action on many substances: it decomposes
proteins at room temperatures
and may cause chemical burns
to human bodies. Although it does
not occur in nature, NaOH has been
manufactured at large scale for many
years from readily obtainable raw materials
and is used in numerous industrial
processes (Figure 1).
Sodium hydroxide is mainly used in
pulp and paper manufacturing, alumina
extraction from bauxite in aluminum
production, as well as in the
textiles industry and drinking water
production. NaOH is also an important
compound in the manufacture
of soaps and detergents, in waste
gases scrubbing, saponification and
etherification and esterfication reactions;
as well as in basic catalysis.
Pure NaOH has a high affinity for
water and may form hydrates depending
on the concentration. Since
some hydrates have melting points
greater than 0°C, insulation or heating
during storage and transportation
considerations may be required.
Additional care is required because
in the presence of moisture, NaOH
readily reacts with atmospheric carbon
dioxide to form sodium carbonate.
Shipping NaOH can be
accomplished in steel drums for
concentrations up to 50%. For higher
concentrations, pure nickel drums
are required due to corrosion effects.
Sodium chloride,
electricity
Electrolysis
(membrane)
Chlorine, hydrogen
Sodium chloride,
electricity
Electrolysis
(mercury)
Chlorine, hydrogen
Sodium hydroxide
(NaOH, 50%)
Electrolysis
(diaphragm)
Chlorine, hydrogen
Lime-soda
process
Calcium carbonate
FIGURE 1. Several processes involving the electrolysis of salt brine, along with one using lime soda and sodium
carbonate, can be used to produce sodium hydroxide
NaOH production process
The production of NaOH from sodium
chloride (NaCl) comprises three
major sections: (1) brine purification;
(2) electrolysis; and (3) product recovery
(Figure 2).
Brine purification. Initially, recycled
depleted brine is mixed with water
and re-saturated with fresh NaCl.
Heavy metal ions (for example, Ca2+
and Mg2+) present in the brine would
be harmful to the membranes. So the
brine is treated with precipitants in
such a way that the metals precipitate,
forming a sludge, which is removed
by settling in a clarifier. Subsequently,
the clarified solution is filtered
and then purified by ion-exchange
resins to remove residual hardness
and achieve acceptable levels.
Electrolysis. The ultrapure brine and
electricity are the main inputs to the
electrolysis area. The brine is fed into
the anolyte compartments of the
cells, separated from the catholyte by
cation-exchange membranes. Chlorine
gas is generated at the anodes
and sodium ions migrate through the
membranes into the catholytes. The
depleted brine from the anode compartments
is dechlorinated downstream
and then returned to the brine
saturation stage. On the catholyte
side, water is electrolyzed, generating
hydrogen gas and hydroxyl ions. The
membranes prevent the migration of
such hydroxyl ions into the anolytes
in such a way that hydroxyl ions combine
with the sodium ions, forming
caustic soda. The addition of demineralized
water keeps the catholytes
concentration at the desired level.
Products recovery. Hydrogen from
the electrolysis process is compressed
and directed to consumers.
The caustic soda solution is concentrated
to a saturated 50 wt.% NaOH
solution - the commercialized form.
The chlorine gas produced is sent to
a drying system, consisting of drying
towers, where concentrated sulfuric
acid circulates as a dehydrating
agent. The dry chlorine gas is compressed
and then liquefied before being
send to storage vessels.
n
Edited by Scott Jenkins
Editor's note: Content for this column was originally developed
by Intratec Solutions LLC (Houston; www.intratec.us) and is edited
by Chemical Engineering. The analyses presented are based
on publicly available and non-confidential information. The content
represents the opinions of Intratec only.
More information about the methodology
for preparing the analyses can be
found, along with terms of use, at www.
intratec.us/che.
1. Brine saturation
2. Precipitation
3. Filtration
4. Ion exchange
5. Electrolysis
6. Dechlorination
7. H2 compression
8. NaOH evaporation
9. Cl2 drying
10. Cl2 liquefaction
11. Steam boiler
12 Cooling tower
13. Refrigeration unit
FIGURE 2. The diagram shown here depicts the industrial production process for NaOH
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JUNE2021
CW Cooling water
ST Steam
RF Refrigerant
23
Lime, sodium
carbonate
Sodium chloride,
electricity

http://www.intratec.us http://www.intratec.us/che http://WWW.CHEMENGONLINE.COM

Chemical Engineering June 2021

Table of Contents for the Digital Edition of Chemical Engineering June 2021