Page 182 - CW E-Magazine (3-12-2024)

Special Report

rated brine solution floats on top of a The extent of oxidation of transition acidified before entering the cells, the
thin layer of mercury. The mercury is metals in NaOH solution is a function pH usually being 1-3 at temperatures up
the cathode, where sodium is produced of NaOH concentration and tempera- to 75-80°C. Materials must be chosen
and forms a sodium-mercury amal- ture, as will be shown later. Most of the to prevent corrosion under these condi-
gam with the mercury. The amalgam is corrosion problems observed in chlor- tions and must not contribute harmful
continuously drawn out of the cell and alkali operations are traceable to the impurities to the brine. To avoid dam-
reacted with water which decomposes above reactions. By properly counter- age by leakage currents, steel-based
the amalgam into sodium hydroxide, acting the reactants, many commonly pipelines are cathodically protected by
hydrogen and mercury. The mercury reported corrosion problems can be properly grounded target anodes.
is recycled into the electrolytic cell. avoided(5, 6, 7, 8).
Chlorine is produced at the anode and Heat exchangers in brine service
bubbles out of the cell. Mercury cells Material of construction in chlor- are usually titanium or copper-nickel
are being phased out due to concerns alkali operations alloys, and pumps are made of Type 316
about mercury poisoning from mercury The unit processes in a commercial SS, Ni-Resist titanium, rubber-lined
cell pollution(3, 4). chlor-alkali plant can be broadly classi- steel, or FRP. Storage tanks use many of
fied as follows:Brine preparation the same materials employed in piping
Irrespective of the cell techno- and purification; Electrolytic cells; systems, the choice reflecting process-
logy used for producing chlorine, the Cl processing & systems; Caustic ing conditions and the importance of
2
materials of construction employed systems; and H processing. avoiding contamination.
2
in a chlor-alkali plant encounter one
or more of:acid, oxygen, chlorine Brine preparation and purification Concrete flooring in the brine area
(gaseous or dissolved as Cl , HOCl, The various steps in the brine prepa- is liable to disintegration by brine, and
2
or OCl ), caustic, and hydrogen. All ration and purification processes involve it is protected by coating with materials
–
these species have the potential to wide ranges of temperature, salt concen- such as magnesium or zinc fluosilicate,
degrade the materials they are exposed tration, and pH. Each of these conditions epoxy or silicone penetrating sealers,
to.The cathodic reactions responsible affects the choice of materials of con- and drying oils. Concrete surfaces ex-
for the oxidation of metals are: (1) O struction. The simplest systems may be posed to mechanical wear are treated
2
reduction, (2) H+ ion reduction, and based on carbon steel with a heavy cor- with 3-6mm thick epoxy surfacing
(3) reduction of Cl , HOCl, or OCl , rosion allowance.The pH of raw brine materials. Sumps and trenches employ
–
2
as noted below. is generally 6.5 to 7.5. It can be pumped reinforced polyester or epoxy resin
through carbon steel pipelines, which systems(8, 9, 10, 11).
O + 4H+ + 4e 2H O (1) may be cathodically protected. Pipe
2
2
O + 2H O + 4e  4OH (2) bridges are protected from corrosion Electrolysis & electrolytic cells
–
2
2
2H +2eH (3) arising from oxygen reduction (Eqs. 1 Mercury cells are constructed with
+
2
HOCl + H + e  Cl + H O (4) and 2) by painting the outer surface or carbon steel, and the surfaces in contact
–
+
2
OCl + 2H + 2e Cl + H O (5) by cathodic protection. Underground with chlorine are rubber-lined.
–
+
–
2
Cl + 2 e  2 Cl - (6) lines may be wrapped or coveredwith
2
protective material or cathodically Diaphragm cells are also fabrica-
Note that the metal oxidation reac- protected. Alternatively, the piping ted with carbon steel bodies and FRP or
tion releases electrons by the reaction can be rubber-lined carbon steel, fibre- vinylester covers. The external carbon
M  M + e. reinforced polyester (FRP), vinylester, steel surfaces are usually treated with a
+
or a plastic such as PVC, PP, HDPE, high-performance catalysed polyamide
Iron is susceptible to oxidation by fluorinated ethylene/propylene (FEP), or or amine-cured painting system with
reaction (1) or (2), while Ni and stain- polytetrafluoroethylene(PTFE). Many 0.05-0.075-mm thick primer, 0.13-
less steels will corrode by reactions (3) of the same polymeric materials are 0.15-mm thick intermediate and a 0.13-
to (6). Another reaction of interest in used to line piping and equipment.The 0.15-mm thick finish coat, the thick-
chlor-alkali operations is oxidation of linings must be free of pores, or the nesses referring to the dry films.
Fe or Ni in alkaline media: brine will seep through and cause pit-
ting corrosion. The standard materials of construc-
Fe + 30H  HFeO - + H O + 2e (7) tion of membrane cells are titanium on
–
2
2
Ni + 30H  HNiO - + H O + 2e (8) Purified brine often is heated and the anode side and nickel on the cathode
–
2
2
182 Chemical Weekly December 3, 2024
Contents Index to Advertisers Index to Products Advertised