5.2.3. Mathieson Process
Sulfur dioxide is the reducing agent in the Mathieson process. The main overall reaction is as follows:
2 NaClO3 + H2SO4 + SO2 ¾® 2 ClO2 + 2 NaHSO4
Solutions of sodium chlorate and sulfuric acid are added continuously to a relatively large, cylindrical, lead-lined tank (primary reactor) from the
top. Sulfur dioxide, diluted with air, is introduced through gas diffusion plates at four points in the bottom of the tank. The reaction mass
overflows to a smaller secondary reactor of similar construction. The generated chlorine dioxide still contains some unreacted sulfur dioxide; it is
stripped from the reaction mass by air and then washed in a scrubber packed with Raschig rings. The scrubber is installed on top of the primary
generator, and the fresh sodium chlorate solution serves as a washing liquid on its way down to the primary generator. The mixture of chlorine
dioxide, air, and chlorine coming out of the scrubber goes to the absorption tower.
5.2.4. Solvay Process
The Solvay process uses methanol as reducing agent. The main overall reaction is as follows:
2 NaClO3 + CH3OH + H2SO4 ¾® 2 ClO2 + HCHO + Na2SO4 + 2 H2O
The reaction between sodium chlorate, methanol, and sulfuric acid takes place in two jacketed, lead-lined steel reactors. The chemicals are added to
the bottom of the first vessel and flow by gravity from one reactor to the other. Each vessel has its own supply of methanol and air. The two reactors
are operated at different temperatures and chlorate concentrations. Up to 70 % of the reaction is completed in the first reactor. Additional methanol
and up to 10 % of the total acid requirement is fed to the second reactor; the conversion of chlorate is then more than 95 %.
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