Study On Preparation Of High Whiteness Aluminum Trihydroxide Powder From High Concentration Sodium Aluminate Solution By Seeded Precipitation

The super-fine aluminum trihydroxide (ATH) acts as an important inorganic material with high value, large profit and extensive use compared with metallurgical grade ATH. The super-fine ATH is usually prepared from low concentration sodium aluminate solution by seeded precipitation or carbonation, which is high production cost and low output. Compared with foreign countries’, the quality of domestic supper-fine ATH has great gap in particle size distribution(PSD), morphology and the impurity contents. Therefore, the preparation of high whiteness super-fine ATH was studied systematically by seeded precipitation from high concentration sodium aluminate solution in Bayer process with diasporic bauxite based on PSD, SEM and automatic colorimeter. And then, the technology was determined. The results were summarized as follows.(1) The effects of temperature, types of seed, composition of sodium aluminate solution and impurities, etc. on seeded precipitation ratio and morphology, PSD, and purity of the super-fine ATH were studied. The results show that the particle size of seed and products decreases with seed prepared at the low temperature. Seed aging benefits the uniformity of particle size and regularity of morphology whereas ATH appears in less uniformity and poor in morphology in addition of the fresh seed. Purified solution and low concentration of sodium aluminate solution increase the seeded precipitation ratio and benefit regular morphology of ATH and its purity. Sodium carbonate, sodium sulfate or silica in solution has negative effect on seeded precipitation ratio and silica makes particle size of ATH fine.(2) The super-fine ATH powder has been prepared by following steps:adding super-fine seed to purified sodium aluminate solution, temperature45～60℃, time40～60h, pH of filtrate<10. The super-fine ATH exists in regular flake with average particle size about1～2μm, whiteness over98%, content of Fe2O3and SiO2in super-fine ATH below0.03%and content of Na2O below0.3%. (3) The existing form of Na2O and its variation of content in seed and the super-fine ATH during the seeded precipitation were also investigated. The results show that Na2O in the products rarely exists in the form of sodium hydrate aluminosilicate(DSP). Reducing the volume of seed initiator or increasing the temperature during seeded preparation stage reduces the Na2O content in seed. The Na2O content in ATH decreases slowly in the seeded precipitation process. It is efficient to reduce alkali content in ATH by increasing the temperature to control the decomposition rate in early stage of seeded precipitation process.