Caustic soda can be produced along with chlorine and hydrogen through the electrolysis of brine. There are
several processes: ion-exchange membrane process, diaphragm process and mercury process. During the
post-war economic growth period in Japan, caustic soda was primarily produced by the mercury process.
The level of technology in Japan regarding the mercury process at that time was one of the highest in the
world, and the production of caustic soda using this process accounted for more than half of the mercury
consumption in Japan up to the mid-1970s (see Fig. 9
There used to be the assumption that there was no possibility of the occurrence of Minamata Disease in the
area around a caustic soda plant which only discharged inorganic mercury. Following a report of the third
occurrence of Minamata Disease caused by the discharge of mercury used for caustic soda production to
the Ariake Sea in 1973 (actually this report was later denied), some 1,200 fishing boats surrounded a
caustic soda plant in the Seto Inland Sea in June of the same year, causing the temporary shut down of the
plant.1 In response to such public anxiety, the national government decided to strictly enforce the use of a
closed system at caustic soda plants using the mercury process and to promote a conversion to the
diaphragm process.2 As a result of efforts by the Japan Soda Industry Association to promote this
conversion, the consumption of mercury per one ton of caustic soda produced fell from 113.9 g in 1973 to
2.3 g in 1979 (see Fig. 10). By 1986, the mercury process was completely withdrawn for the production of
caustic soda in Japan.3
During this conversion stage, the then MITI introduced a scheme for equal volume exchange between
caustic soda produced by the mercury process and caustic soda produced by the diaphragm process and a
system for price difference settlement in view of the higher production cost of the diaphragm process.
Under this system, the MITI paid out ¥3.87 billion for 975,000 tons of caustic soda, facilitating the
conversion to the diaphragm process.4 Because of the inferiority of the diaphragm process to the mercury
process in terms of energy consumption and product quality and because of the difficulty of completely