Process for extracting sialic acid from fermentation liquor and application of process

The invention discloses a process for extracting sialic acid from fermentation liquor, which comprises the following steps: S1, carrying out solid-liquid separation on the fermentation liquor containing polysialic acid obtained by a microbial fermentation method, and then adding ferroferric oxide magnetic powder into the obtained liquid phase; and adding ethanol, and separating out the precipitate after the crystal substance is precipitated, thereby obtaining the polysialic acid. And S2, adding hydrochloric acid into the aqueous solution of polysialic acid for hydrolysis, and then adding ferroferric oxide for neutralization to obtain hydrolysate. And S3, decolorizing the hydrolysate, concentrating, adding ethanol, separating precipitate, washing, and drying to obtain sialic acid. And S4, heating the liquid phase from which the precipitate is separated to separate out ethanol, then adding sodium acetate trihydrate and ethylene glycol into the residual liquid phase for hydrothermal reaction, and separating out