Abstract: Zinc borate is a kind of green and environmental-friendly inorganic flame retardant.zinc borate（4ZnO·B2O3·H2O） had been synthesized by hydrothermal synthesis method with an industrial by-product zinc mud as zinc source and borax as raw material.The prepared product was characterized by some analytical test methods，such as X-ray diffraction（XRD）， thermal gravimetric（TG-DTA），and scanning electron microscopy（SEM），and the optimum conditions were determined by investigating the factors affecting the hydrothermal synthesis.Results of experiments showed that zinc borate can be most favorably synthesized under such conditions as follows: the hydrothermal temperature was 140 ℃，the hydrothermal time was over 7 h，the amount-of-substance ratio of zinc to borax was 1∶1.0，and the mass ratio of solid to liquid was 1∶12.The XRD pattern of the resulting zinc borate powders was consistent with that of 4ZnO·B2O3·H2O，with dehydration temperature above 415 ℃.Addition of sodium stearate and polyethylene glycol in the reaction system，the hydrophobic zinc borate samples could be obtained.If 10% zinc borate was added in wood flour，the char yield at 400 ℃ was at 17%~18% higher than that of pure wood，and that indicated zinc borate had a certain degree of flame retardancy.

Key words: by-product zinc mud, zinc borate, flame retardancy, hydrothermal synthesis