Method for preparing magnesium-based flame retardant from Grignard wastewater

The invention discloses a method for preparing a magnesium-based flame retardant from Grignard wastewater, wherein the method comprises the steps: (1) adding an aluminum-containing compound into the Grignard wastewater, sufficiently stirring to react for 0.5-3 h, lowering the acidity of the wastewater solution to make the pH value reach 3-4, and filtering to obtain a filtrate and an insoluble substance for the next batch of reaction; (2) adding soluble aluminum salt into the filtrate obtained in the step (1), stirring and dissolving to obtain a mixed solution A of magnesium-aluminum bimetallicsalt for later use; (3) making an alkaline compound into a mixed solution B for later use; and (4) rapidly mixing and co-precipitating the mixed solution A and the mixed solution B to prepare magnesium-aluminum composite hydroxide slurry, and then crystallizing, filtering, washing and drying to obtain the magnesium-based flame retardant. According to the method, magnesium resources in the Grignard wastewater are fully ut