Inorganic Chemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 12

produce about 44 million metric tons of the 260 million metric tons produced
worldwide [23]. Salt is obtained by underground mining of deposits. Mining
is done by traditional dry mining and by solution mining. Solution mining
involves injection of water into bedded or domal salt and then recovery of
the brine [24]. Salt is also recovered from seawater and other saline waters
by solar evaporation. In the United States, solar salt is a minor method of
production. Salt for highway deicing is the major use but this is followed
closely by consumption by the chemical industry, where the major use is as a
feedstock for chloralkali plants. Salt is also used in agriculture, food, and in
water treatment.
Chloralkali is a term that is used for the coproduction of chlorine (Cl 2 )and
caustic. Caustic, sometimes called caustic soda, is sodium hydroxide (NaOH).
They are coproduced from salt and water by the following reaction.

2 NaCl + 2 H 2 O 2 NaOH + Cl 2 + H 2

The reaction is an electrochemical reaction. At the anode, chloride is oxi-
dized to form chlorine; at the cathode, water is reduced to form hydrogen and
hydroxide.

2 Cl − Cl 2 + 2 e −

Anode

2 H 2 O + 2 e − H 2 + 2 OH −

Cathode

2 Na + + 2 Cl − + 2 H 2 O 2 Na + + 2 OH − + Cl 2 + H 2

Overall

The process consumes a large quantity of electricity and energy costs are
an important consideration for profitability. From the balanced equation, two
moles of sodium chloride produce 2 moles of sodium hydroxide and 1 mole
of chlorine gas. Don't confuse the molar ratio with the mass ratio. By using
atomic masses, we can say that 117 g (2 moles

× 58.5 g/mole) sodium chloride

produces 80 g (2 moles × 40 g/mole) sodium hydroxide and 71 g chlorine

(1 moles 71 g/mole). Therefore, the mass ratio of caustic to chlorine is 80

to 71. The mass ratio is true for all mass units. A production of 80 pounds
of caustic coproduces 71 pounds of chlorine. A plant consuming 117 tons of
salt coproduces 80 tons of sodium hydroxide and 71 tons of chlorine. One of
these is not produced without coproducing the other. This can be a problem in
the marketplace when there is a demand for caustic but not chlorine or when
there is a demand for chlorine but not caustic.
There are three major processes used for chloralkali production. They are
mercury, diaphragm, and membrane and differ in the type of cell used. In
the mercury cell process, the cathode is a mercury film. This process was

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