Preparation Of Nearly Spherical Superfine Aluminum Hydroxide By Seeded Precipitation From Sodium Aluminate Solution

Superfine aluminum hydroxide is extensively used as green non-halogen flame-retardant,and is also acted as the precursor of high-quality alumina.Plate-like superfine aluminum hydroxide has been extensively produced by seeded precipitation from the saturated sodium aluminate solution.To meet the development of flame-retardant and high performance of alumina,the nearly spherical aluminum hydroxide with well crystallization is required.Different from the tranditional method of adopting the expensive aluminum-bearing salts and aluminum alkoxide,the saturated sodium aluminate solution was used to precipitate the nearly spherical superfine aluminum hydroxide.Preparation of superfine seed,morphological evolution of gibbsite,and growth of various crystal surfaces of gibbsite were studied in detail.The conclusions are listed as follows:（1）The effects of inducer,temperature,aging duration,sodium bicarbonate dosage and addition of PEG2000 on seed properties were studied.Adding Na HCO₃and H₂O₂produced columnar seed with average size less than 1μm in the sodium aluminate solution of 50 g/L Na₂O_k.Decreasing temperature reduced the particle size and increased content of gibbsite in the seed with adding Na HCO₃solution.Aging treatment benefited to the crystallization of gibbsite grain with perfect morphology,and also inhibited particle agglomeration.When the concentration of Na HCO₃solution was 80 g/L and the volume ratio(V_{Na HCO3}:V_{Sodium aluminate})was 0.5,the particle size of seed with the perfect plate-like morphology was becoming superfine.The addition of PEG2000improved the thickness of seed and restrained the particle agglomeration.（2）The effects of temperature,seed dosage and organic additives on the morphology,particle size and precipitation rate of gibbsite were investigated.Increasing temperature benefited to the growth of crystal surface and decreased particle size.Increasing seed dosage reduced particles size,inhibited particle agglomeration and raised the precipitation rate.Elevating temperature and increasing seed dosage both promoted the growth（101）and（112）face of gibbsite,precipitating the compressed spherical gibbsite with the superfine particles and well crystallization.Additives A and B had minor effect on the morphology of plate-like gibbsite.Additives C and D favored the growth of（101）and（112）faces of gibbsite in the form of compressed spherical grain.However,additive E accelerated the growth of（101）and（110）faces and restrained the growth of（001）face,generating a nearly spherical gibbsite.（3）A novel method to prepare nearly spherical superfine aluminum hydroxide was proposed.High temperature and additive E produced initially a hexagonal prism with well crystallized（001）,（100）and（110）faces of gibbsite.Prolonging precipitation time favored the growth of（101）and（112）faces of gibbsite,but reduced（001）,（100）and（110）faces of gibbsite.The nearly spherical gibbsite with 36 m L/100 g oil adsorption values,6.48%acid dissolution rate was finally received after precipitation at 80°C for 48 h.In addition,the gibbsite particle with the nearly spherical grain was becoming more uniform and superfine under condition of decreasing temperature from 80°C to 60°C.This circumstance suggests that the nearly spherical gibbsite,rather than the traditional plate-like gibbsite,can be precipitated from the saturated sodium aluminate solution.