Plant alumina trihydrate (ATH, Al(OH)(3)) from Korea Alumina (KC) Ltd's Bayer alumina plant was used to synthesize boehmite, a new product used as fire retarding fillers or laminating materials. The hydrothermal treatment conditions were optimized from tests conducted with ATH-water slurries (30, 40 and 50 w/v%) at different temperatures (160-220 degrees C), reaction time (.10-80 min) and mean feed sizes D-50 (1.5-50 mu m). Well crystalline boehmite was formed at a temperature >200 degrees C after 60-minute treatment. The reaction pressure (1.0-2.0 MPa) has no effect on the process. The hexagonal crystal of the starting gibbsite changes to plate-like shape as boehmite is formed. After the treatment, the boehmite mean particle size (4-25 mu m) is generally smaller than the original gibbsite feed (8-50 mu m). For finer ATH feed (1.5-2.5 mu m) there is no change in particle size after the hydrothermal treatment. The highly crystalline boehmite product containing <0.05% Na2O and <0.01% Fe2O3 has higher purity and whiteness than the ATH feed. (C) 2012 Elsevier B.V. All rights reserved.