Patent ID: 6860941

Claim:
A method for manufacturing a high-purity potassium fluorotantalate crystal or a high-purity potassium fluoroniobate crystal comprising the steps of dissolving for producing a saturated solution using a hydrofluoric acid solution and any one of raw potassium fluorotantalate, raw potassium fluoroniobate and raw potassium oxyfluoroniobate, recrystallizing for obtaining granular recrystallized potassium fluorotantalate or recrystallized potassium fluoroniobate by cooling said saturated solution with agitation in a recrystallization bath, filtering said granular recrystallized potassium fluorotantalate or recrystallized potassium fluoroniobate from a solution subsequent to the completion of the recrystallizing step, and drying said filtered granular recrystallized potassium fluorotantalate or recrystallized potassium fluoroniobate, wherein the recrystallizing step comprises a first cooling process of cooling the saturated solution with a temperature of 60° C. to 90° C. obtained in the dissolving step until the solution temperature of said saturated solution becomes a temperature of the range of 35 to 50° C., and a second cooling process of cooling the solution from the end of said first cooling process to said solution temperature becoming a temperature of 10 to 20° C., wherein a cooling speed in the first cooling process is higher than that of the second cooling process, and wherein a difference in the cooling speed between the first and second cooling processes is 1° C./hour or higher.