High Concentrations Of Aluminum Sodium Carbonate Decomposition Products Of Na <sub> 2 </ Sub> O And Sio <sub> 2 </ Sub> Control

The majority of bauxite resource in China is characterized by medium & low-grade, therefore, Sintering process plays a special role in our country. Nowadays, some new technologies about production of alumina were come forth, but the problems of high energy consumption, low production capacity of equipment and deficiency in economic performance are still under solved, this is a great disadvantage to market competition for the alumina company in our country. In this case, the adoption of new technology for the purpose of reducing energy consumption is still a momentous challenge to the alumina production by Sintering process in China. Carbonization is one of the most important procedures in alumina production by Sintering process. Whether the control of production conditions is suitable or not will expose direct impact on the purity and granularity of alumina. Recently, Al₂O₃ concentration of green liquor can reach more than 180g/L with the breakthrough of the technical difficulties of clinker-leaching with high concentration and deep desilication, which affords the chance of putting carbonization of sodium aluminate solution with high concentration into practice. So it is great meaningful to study the technology of carbonization of sodium aluminate solution with high concentration for increasing the technology economic index of alumina production by sintering process.Based on the particular analysis about the relationship between caustic module of spent liquor and precipitation time during the process of carbonization of sodium aluminate solution with high concentration, the technology of controlling impurities (Na₂O and SiO₂) content in product during carbonization of sodium aluminate solution with high concentration (Al₂O₃ concentration 148～210g/L) without adding Al(OH)₃ seeds has been systematically investigated in this dissertation. Results are described as follows:1) During carbonization of sodium aluminate solution with high concentration, caustic module of spent liquor approximately takes on corrugation change with the change of precipitation time, this implies that the co-action between the reaction of the neutralization of OH^- by CO₂ and the reaction of NaAl(OH)₄ hydrolysis to precipitate Al(OH)₃ bring on the proceeding of carbonization process.2) The impurity of Na₂O in product mainly roots in the inter-crystalline soda formed in the process of Al(OH)₃ precipitation and soda in the form of Na₂O·Al₂O₃·2CO₂·nH₂O during carbonization of sodium aluminate solution with high concentration. The behavior of SiO₂ in the process of carbonization mainly depends on its equilibrium concentration in sodium aluminate solution and SiO₂ equilibrium concentration has a direct impact on the impurity of SiO₂ content in Al(OH)₃.3) The impurities of Na₂O and SiO₂ contents in Al(OH)₃ can be reduced by increasing carbonization temperature and silica module of green liquor during carbonization of sodium aluminate solution with high concentration. Meanwhile, choosing appropriate gas ventilating system is also beneficial to reduce the contents of Na₂O and SiO₂ in Al(OH)₃. Employing the gas ventilating system of combining rapid ventilation and slow ventilation can shorten the time of carbonization.4)The proper technical conditions of carbonization of sodium aluminate solution (Al₂O₃ concentration 170～180g/L, caustic moduleα_k 1.48) with high concentration without adding seeds are as follows: carbonization temperature is about 95℃, slow ventilation (CO₂ concentration < 30%, ventilating rate < 0.055 m³/h·L), silica module of original liquor for carbonization is more than 600, the carbonization ratio is about 93%. The product with good quality of Na₂O content < 0.3%, SiO₂ content < 0.03% from carbonization of sodium aluminate solution can be obtained under this conditions.