Method for producing compound

A purification apparatus that purifies crystals, the purification apparatus comprising:

• a hydraulic wash column that includes a discharging port for a crystal-containing circulation slurry and a return port for a circulation liquid containing a melt of discharged crystals;
• a pipe that feeds a crystal-containing slurry to the hydraulic wash column;
• a filter that filters the crystal-containing slurry in the hydraulic wash column;
• a pipe that is connected to the filter and discharges a mother liquor;
• a unit that melts crystals in a circulation slurry discharged through the discharging port;
• a mechanism that returns a portion of a circulation liquid containing a melt obtained in the unit that melts crystals to the hydraulic wash column so that at least a portion of the returned circulation liquid serves as a washing liquid for crystals;
• a mechanism that controls an amount of the circulation liquid to be returned, wherein the return port is provided at the bottom of the hydraulic wash column so that the circulation liquid is returned upward, and
• wherein a thermal conductivity of the filter is different from a thermal conductivity of the pipe that is connected to the filter and discharges a mother liquor.