Preparation method of ultralow-cutoff-point spherical silica powder for HDI

The invention discloses a preparation method of ultralow-cutoff-point spherical silica powder for HDI (hexamethylene diisocyanate). The method comprises the following steps: (1) selecting flame method spherical silica powder with D50 of 1-15 microns and D100 of 10-50 microns as a raw material; (2) mixing water with the flame-method spherical silica powder in proportion, and performing high-speed stirring and ultrasonic treatment to obtain slurry A; (3) settling the slurry A, and sucking out upper-layer slurry to obtain a product B; (4) centrifuging the product B, and dehydrating the centrifuged upper paste to obtain a product C; and (5) depolymerizing the product C to obtain the spherical silica powder of which D100 is less than or equal to 6 microns. The method is simple and easy to implement, common flame method micron-sized spherical silicon is used as a raw material, the steps of pulping, sedimentation, centrifugation and depolymerization are sequentially carried out, the spherical silicon micropowder wit