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Speight_Part II_C 11/7/01 3:08 PM Page 2.162

2.162 MANUFACTURE OF CHEMICALS
salt. Recycled salt solution is added. The precipitated impurities are
removed by settling and by filtration. The purified, saturated brine is then
fed to the cell, which typically is a rectangular box. It uses vertical anodes
(ruthenium dioxide with perhaps other rare metal oxides deposited on an
expanded titanium support). The cathode is perforated metal that supports
the asbestos diaphragm that has been vacuum deposited in a separate oper-
ation. The diaphragm serves to separate the anolyte (the feed brine) from
the catholyte (brine containing caustic soda). Chlorine is evolved at the
anode. It is collected under vacuum, washed with water to cool it, dried with
concentrated sulfuric acid, and further scrubbed, if necessary. It is then
compressed and sent to process as a gas or liquefied and sent to storage.
The membrane cell (Fig. 1) uses a cation exchange membrane in place of
an asbestos diaphragm. It permits the passage of sodium ions into the
catholyte but effectively excludes chloride ions. Thus the concept permits
the production of high-purity, high-concentration sodium hydroxide directly.
In the mercury cell process (Fig. 1), chlorine is liberated from a brine
solution at the anode. Collection and processing of the chlorine is similar
to the techniques employed when diaphragm cells are used. However, the
cathode is a flowing bed of mercury. When sodium is released by electrolysis
it is immediately amalgamated with the mercury. The mercury amalgam is
then decomposed in a separate cell to form sodium hydroxide and the mer-
cury is returned for reuse.
Other processes for making chlorine include sodium manufacture,
caustic potash manufacture, hydrogen chloride decomposition, the nitro-
syl chloride (NOCl) process, and a process where salt is treated with
nitric acid to form sodium nitrate and chlorine with nitrosyl chloride
(containing 4 to 10% nitrogen tetroxide) as a by-product. The nitrosyl
chloride vapor is placed in contact with oxygen to produce nitrogen
tetroxide and chlorine:
2NOCl + O → N O + C1
2 2 4 2
After liquefying and distilling the chlorine out, the nitrogen tetroxide is
absorbed in water to make nitric acid and nitrous acids, which are recycled:

N O + H O → HNO + HNO
2 4 2 3 2
The advantage of this process is that it produces chlorine but no caustic
soda. The demand for sodium nitrate regulates the amount of chlorine that
can be made in this way.
3NaCl + 4HNO → 3NaNO + C1 +NOCl + 2H O
3 3 2 2