Page 172 - CW E-Magazine (23-4-2024)

Special Report

Development of the modern chlor-alkali industry –

a perspective

n the chlor-alkali process electri- mistakenly believing that it also con- A.K. DAS
city is passed through an aqueous tained oxygen. Sir Humphrey Davy (1778- Retd. Industrial Adviser
Isolution of sodium chloride (NaCl), 1829) proved in 1810 that it was a distinct Department of Chemicals & Petro-
which decomposes to form sodium element and later named it chlorine (from chemicals, Government of India
hydroxide (NaOH) and chlorine, besides Greek ‘chlor’ means yellowish green). Email: amiyakantidas2010@gmail.com
hydrogen. It is so called because the
products formed in this process are The fi rst production of chlorine Three electrolytic processes have
chlorine and sodium hydroxide (alkali). by electrolysis of brine is attributed to been developed over the years, which use
There are three key players in the pro- William Cruikshank, a Scottish military different methods to keep the chlorine
cess: salt, water and electricity. When surgeon and chemist (1740-1811) in produced at the anode separated from
calcium chloride or potassium chloride 1800. In 1833, Faraday formulated the NaOH and hydrogen produced at the
is used, instead of sodium chloride, laws governing electrolysis of aqueous cathode. In historical order these cells
calcium hydroxide [Ca(OH) ] or solutions. In 1851, a patent was gran- are Diaphragm Cells, Mercury or
2
potassium hydroxide (KOH) are ted to Cook and Watt and in 1853 to ‘Amalgam Cells’ and Membrane Cells.
produced, instead of NaOH. Stanley for the production of chlorine
from brine. In 1869, Zenobe Gramme Diaphragm Cells
History of making and use invented the Gramme machine mak- Diaphragm Cells use a simple and
NaOH was being used long before it ing electrolysis easy. After inven- economical brine system and require
was chemically identifi ed. The ancient tions that enabled the supply of high less electrical energy than Mercury
Egyptians and Babylonians are credited electrical currents, basic electrolysis Cells. A primary disadvantage of
with preparation of a weak solution of methods were invented. But success- Diaphragm Cells is the low concentra-
the alkali by percolating water through ful implementation to commercially tion of caustic soda solution produced,
vegetable ashes and using the solution produce chlorine took many years. In which requires several concentration
for washing. A parallel story goes that the US, the fi rst plant to produce chlo- operations to achieve the purity needed
towards the end of the BC era, Romans rine electrically for making bleach was for industrial use. The Diaphragm Cells
discovered a lye formed when rainwater established at Rumford Falls, Maine, are also known to be a source of pollu-
washed volcanic ash, and used this for in 1882. The plant used a cell equipped tion from asbestos fi bres, the primary
cleansing with animal fat (from animal with an asbestos diaphragm developed material of the diaphragm. Because
sacrifi ces). The Sumerians (approxi- by Ernest A Lesueur from Canada. of these shortcomings Mercury Cells
mately 3000 BC) also used a slurry of began to compete in early 20th cen-
ashes and water to remove grease from Basics of process tury after Castner patented the Rocking
raw wool and cloth, so that it could The primary process of chlor-alkali Mercury Cell method in England, while
be dyed. They referred to the ashes as industry involves passing electricity in Austria, Kelner patented a Mercury
‘al qualy’ – the origin of the modern through NaCl solution. The chemical Cell of similar design in 1892.
chemistry term ‘alkali’. Modern analy- reaction that takes place is:
tical methods have shown that these Mercury Cells
ashes have high NaOH content. 2NaCl(aq) + 2H O = 2NaOH(aq) + Mercury Cells produce much purer
2
Cl (g) + H (g). and more concentrated caustic soda,
2
2
Nicolas Le Blanc (1742-1806), a which can be used without further treat-
French chemist, prepared NaOH solu- The chlorine produced is collected ment in most cases. However, mercury
tion by reacting soda ash and slaked at the anode, while the hydrogen and has extremely serious ecological
lime. Carl Wilhelm Scheele (1742- hydroxide ions (as NaOH) are collected impacts when dispersed from chemical
1786), a Swedish-German chemist, at the cathode. If the primary products process effl uents, and can even enter
obtained a green-yellowish gas by from salt electrolysis come in contact the food chain. As a consequence,
reacting hydrochloric acid (HCl) with after formation, they can react with Mercury Cells are being phased out
the mineral pyrolusite (MnO ) in 1774. each other to form oxygenated com- under the global Minamata Convention.
2
But he failed to realise his discovery, pounds of chlorine. More recently, non-asbestos diaphragms,

172 Chemical Weekly April 23, 2024

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