Study On The New Process For Preparation Of High Purity α-Al ₂ 3 Powders

High-purity alumina podwer with the purity above 99.99% plays important roles in our materil industry. It is the key raw material for ceramic substrate of high-quality integrated circuit, tricolor fluorescent powder, plasma display material (PDP powder), phosphor with long persistence, substrate of the light-emitting diodes, and so on. The total usage amount of the high-purity alumina podwer is highest among the powder materials.In this thesis, a new process for the manufacture of high-purity alumina podwer is introduced. The raw material of this process is the leftover material from the aluminium production of Yunnan Aluminium Stock Company. The application of this new process will do much contributions to the the aluminium industry, such as broadenning the downstream industry and enhancing the additional value. This process is consisted of alkali dissolution with sodium hydroxide, silicon removal with calcium oxide at ordinary pressure, carbonating decomposition, removal of calcium and magnesium with hydrochloric acid, calcinations for crystal transformation and impurity further removal by acid pickling with ultrasonic wave. After these six workstages, the alumina podwer obtained has a high purity of above 99.99%. The contents of each impurity elements are all below 0.001%, achieving the application request of the substrate of the light-emitting diodes.Main research contents and results of this thesis are as follow:(1) Sodium aluminate solution is prepared from industrial aluminum which is dissolved by sodium hydroxide, and the technological condition is optimized. The influences of alkali concentration, solution temperature, time of dissolution and solid-to-liquid ratio on the dissolution performance are investigated. The dissolution rate increases with the elevation of the alkali concentration, firstly.And when the alkali concentration is above 112.4 g/L, the dissolution rate is highest and can’t be further enhanced. When the alkali concentration is 112.4 g/L, the solution temperature is 50～55℃, the time of dissolution is 5 h and the solid-to-liquid ratio is 1:8, aluminium can be all dissolved, and the aluminium and silicon concentration in the sodium aluminate solution is 40.034 g/L and 44.52 mg/L respectively.(2) Silicon in the sodium aluminate solution is removed by the addition of calcium oxide. The quantity of calcium oxide, the reaction temperature and the reaction time have significant influence on the result of Silicon removal. When the quantity of calcium oxide is 8 g/L, the reaction temperature is 80-85 oC and the reaction time is 2h, the desilication rate is as high as 98.52%, and the silicon content of the sodium aluminate solution can decrease from 44.2 mg/L to 0.66 mg/L.(3) Aluminum hydroxide is precipitated from the sodium aluminate solution by carbonating decomposition, then purified by acid pickling.The effects of CO2 concentration, the rate of CO2 flow, pH value at the end point of carbonating decomposition on the decomposition performance and the concentration of the impurities of calcium, sodium and magnesium are investigated. When the CO2 concentration is 5%, the rate of CO2 flow is 25 L/h, the reaction temperature is 55 oC, the reaction time is 15 min (start timing when the precipitate appears), the result of carbonating decomposition is best. After been washed by hydrochloric acid for 45 min with the concentration of 30 g/L and liquid-solid ratio of 15:1, the aluminum hydroxide obtained has a relatively high purity of 99.682%, andthe concentration of Na、Ca and Mg are 0.21%,0.0013% and 0.0014%respectively, the concentrations of Si、Fe、Cu and Ti are all below 0.001%.(4) High-temperature calcination is applied to aluminum hydroxide precipitate to prepare α-Al2O3. The TG-DSC test result of hydroxide precipitate shows that the aluminum hydroxide will lose its crystal water at 291 ℃, decompose at 504 ℃, and convert into α-Al2O3 at 1400 ℃. According to the result of TG-DSC, the calcination process is preliminarily projected and the process parameters are optimized, including calcination temperature, calcination time and temperature rate. When the temperature rate is 10 K/min, calcination time is 60 min, and calcination temperature is 1400 ℃, the crystallization performance is satisfactory. Boric acid is applied as additional agent during the calcination process to remove sodium in the α-Al2O3 . Boric acid can react with sodium and the sodium metaborate formed can volatilize at 1275-1450 ℃. By adding 1%boric acid, the sodium content in the α-Al2O3 can be decreased from 0.011%to 0.001%.(5) Ultrasonic wave is applied to the acid pickling process for strengthening the removal of the impurities of sodium and calcium. The effects of hydrochloric acid concentration, solid-to-liquid ratio and ultrasonic wave power on the picking result are investigated. When the hydrochloric acid concentration is 50 g/L, solid-to-liquid ratio is 20:1, picking temperature is 60 oC, picking time is 45 min, ultrasonic wave power is 80-90%, the purity of (α-Al2O3 is higher than 99.99%, and the concentrations of Si、Fe、Ca、Na、Ng、Ti、B are all below 0.001%. SEM test results show that the α-Al2O3 particles appear as flat bricks. The surface of the particles is smooth, but its shape and size is not uniform. The thickness of the particles is about 1 μm, and the size of the surface is about 1-3 pm.