Process and apparatus for making ultra-pure DMSO

A process for purifying dimethylsulfoxide by melt crystallization, comprising the steps of:

• a. providing a liquid phase comprising greater than about 90% by weight of liquid dimethylsulfoxide, and one or more impurities,
• b. agitating and cooling the liquid phase to approach a temperature of about 18.5° C. or lower, to form a mixture of solid dimethylsulfoxide and a liquid residue, the liquid residue comprising liquid dimethylsulfoxide and a substantial portion of the impurities,
• c. substantially separating the solid dimethylsulfoxide from the liquid residue, and
• d. melting the solid dimethylsulfoxide to provide purified dimethylsulfoxide, wherein:
• i. the liquid residue is not recycled to the extent it results in treatment of the liquid residue by step b,
• ii. the purified dimethylsulfoxide is not subsequently subjected to steps a, b, and c, and
• iii. the ultraviolet absorbance of the purified dimethylsulfoxide at 275 nanometers is less than 0.20, and the ratio of the ultraviolet absorbance at 285 nanometers to the absorbance at 275 nanometers is less than 0.65, and the ratio of the ultraviolet absorbance at 295 nanometers to the absorbance at 275 nanometers is less than 0.45, wherein the ultraviolet absorbance spectrum of the purified dimethylsulfoxide is measured as a liquid between 270 and 350 nanometers in a 1 centimeter cell at a temperature of less 20° C. using water as a blank.