Study On The Behavior Of Sulfur In Alumina Production With Series-Combination Process

The series process has higher alumina recovery, lower alkali consumption and cost, comparing with traditional sintering process, parallel process and mixed combining process. Due to the shortage of high-quality bauxite resources, the alumina manufacturing enterprises have to use low grade bauxite. A great part of low grade bauxite contains sulfur. Sulfur will enter liquor inevitably. If sulfur is not eliminated, the S2-in the system will accumulate and eventually increase the iron content in the liquor, which results in high iron oxide content in aluminum hydroxide and aluminum oxide. Hence, the elimination of sulfur has become an urgent issue confronted by enterprises. Therefore, it is necessary to study the behavior of sulfur and to eliminate its negative effect.In the first part, the form and transform of sulfur in the Bayer process were investigated. The results showed that sodium aluminate solution contained S2-、S2O32-、SO32-and SO42-. When their concentrations risen, the sulfur content of red mud increased. The dissolution rates of sulfur changed with its content in bauxite. S2O32-was not stable under digestion temperature, and produced S2-and SO32-through disproportionation reaction. The concentration of S2O32-did not change significantly in the dilution and desilication of slurry and separation of red mud process, which meaned that S2O32-remained stable at low temperature. The solid sulfur minerals would not be easy to dissolve further in the following process after digestion at elevated temperature. In the wet oxidation process, S2-、S2O32-and SO32-were not easy to be oxidized at low temperature.In addition, the behavior and transformation of sulfur in sintering process were studied. Research showed that most of sulfur entered the solution and only a few of sulfur entered the red mud during the digestion after oxidation sintering. Increasing the amount of reducing agent, raising temperature, prolonging sintering time could facilitate the reduction process of sodium sulfate to sulfide by coal. Sulfide content increased slightly with Ferric oxide. Through the simulation of the sintering process, the products formed of the reaction between sulfur and iron were studied. The results revealed that sodium sulfate was reduced to sulfide and formed NaFeS2by reacted with iron oxide. NaFeS2was extremely small black solid particle, converted into NaFeS2·2H2O in alkali solution, which was very similar to hydroxylation thiosulfate sodium ferrite found in there mud generated in the Bayer process.