Caustic Soda Ghaziabad
Caustic Soda
Sodium hydroxide (caustic soda) is
a co-product from the manufacture of chlorine using a solution of the readily
available raw material, rock salt (sodium chloride). These factors
contribute to it being the cheapest and most widely used strong alkali.
Another major use of Caustic Soda
is in the manufacture of paper from wood. In the most used process, the Kraft
process, wood is treated with a solution containing a mixture of sodium sulfide
and Caustic Soda. Most of the unwanted material in the wood, such as the
lignins, dissolve in the liquor, leaving relatively pure cellulose which is
filtered off. It is this cellulose which after further purification forms the
basis of paper.Other uses include the production
of surfactants, soaps and bleaches, the latter being
usually produced by passing chlorine gas into a solution of Caustic Soda, which
generates a solution containing sodium chlorate(I) (sodium hypochlorite)Caustic Soda is used in making
organic chemicals, one of the most important being epoxypropane (propylene
oxide), used in turn to make polyurethanes. Propene is treated
with chlorine in water to make a mixture of 1-chloropropan-2-ol and
2-chloropropan-1-ol. Epoxypropane forms on addition of a solution of
either sodium hydroxide or calcium hydroxide.Manufacture of Caustic SodaCaustic Soda and chlorine are
manufactured together by the electrolysis of sodium chloride.Large deposits of sodium chloride
(rock salt) are found in many parts of the world. These deposits are
almost pure sodium chloride and are often several hundred metres deep (some are
up to 3000 metres deep) with seams of 30 m to 500 m thick . They
were evaporated from trapped seas in the Triassic period, 200 million years
ago. For example, in Europe, the seas produced deposits which stretch, although
not continuously, from Cheshire, Lancashire, Staffordshire and Cleveland in the
UK to Poland. They are also found throughout the US, particularly in
Louisiana and Texas.A small amount is mined as rock
salt, the majority is solution-mined by the controlled pumping of water at high
pressure into the salt seam. A proportion of the solution-mined brine
produced in this manner is evaporated to produce dry salt.Solar salt, produced by the
evaporation of sea water using solar heating, is also a source of sodium
chloride.Saturated brine, prior to the
electrolysis, is purified to precipitate calcium, magnesium and other
detrimental cations by addition of sodium carbonate, sodium hydroxide and other
reagents. The suspended solids are removed from the brine by settling and
filtration.There are three electrolytic
processes employed today. The concentration of caustic soda produced from
each of the processes varies:
- Membrane Cells: Caustic
soda is produced as a pure ca 30% (w/w) solution which is normally
concentrated by evaporation to a 50% (w/w) solution using steam under
pressure.
- Mercury Cells: Caustic soda is produced as a pure 50% (w/w) solution,
which is the concentration most commonly sold on the world market.
Some is concentrated by evaporation to 75% and then heated at
750-850 K to obtain solid sodium hydroxide.
- Diaphragm Cells: The caustic soda is produced as an impure solution
called 'diaphragm cell liquor' (DCL) with typical concentrations 10-12%
(w/w) sodium hydroxide and 15% (w/w) sodium chloride. In order to
produce the 50% (w/w) strength that is usually required, the DCL has to be
concentrated using evaporation units that are much larger and more complex
than those used on membrane cell plants. Large quantities of salt
are precipitated during this process, which is normally reused to produce
a saturated brine feed to the cells. An additional aspect of the
sodium hydroxide produced in the diaphragm cell is that the product has a
small amount (1%) of salt present as a contaminant, which may make the
material unsuitable for some purposes.
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