Document Type : Research Article
Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, I.R. IRAN
Sulfide ions in the solution of the Bayer process can accelerate the corrosion of the equipment and increase the impurity of the final product. In the current investigation, sulfide ions concentration of the aluminate solution during the Bayer process was determined using an indirect and inexpensive method. The method did not require any advanced apparatus, which made it suitable for sulfide concentrations over the range of 0.001-1 g/L. To investigate the source of sulfide ions in the aluminate solution, the chemical composition and crystalline structure of the bauxite used to produce alumina were characterized via XRF, XRD, and SEM analyses. The results demonstrated that the main source of sulfide ions was pyrite in bauxite. The advantages and disadvantages of sulfide removal method by nitrate from aluminate solution were investigated. Thermodynamically, the spontaneity of different half-reactions during the reduction of NO3- and oxidation of S2- was studied. Finally, a technique was proposed for the removal of sulfide ions in the aluminate solution by adding nitrate. Moreover, the effect of nitrate concentration on lowering of sulfide ions concentration was evaluated in practical conditions of the bauxite digestion during the Bayer process. The results demonstrated that in conditions of bauxite digestion (at 270 °C, 52 bar, and 60 minutes) by adding 2.5 g/L nitrate ions, the majority of sulfide ions (more than 96%) were eliminated and their undesirable effects were prevented.