چكيده به لاتين
Abstract:
Ilmenite, as the most significant source for titanium and its oxide extraction, is processed using sulfate, chloride, and smelting methods. These methods often fail to completely remove iron associated with titanium, complicating the production of white pigments. To improve iron removal, the pre-processing of ilmenite via carbothermic reduction, aimed at converting iron into soluble and fusible forms like metallic iron, is a widely adopted industrial solution. However, due to technical and environmental challenges associated with carbothermic reduction, replacing coke with hydrogen offers a novel approach with considerable advantages for subsequent separation processes. In this study, the reaction mechanism and statistical modeling of ilmenite reduction with pure hydrogen were investigated to separate metallic iron from ilmenite. Initially, Kahnuj ilmenite concentrate was characterized mineralogically, chemically, and structurally. Following pelletization, reduction experiments were conducted under two regimes: kinetic studies under the first regime and modeling using design of experiments under the second. Kinetic studies were performed in the temperature range of 500°C to 1100°C, achieving a maximum reduction degree of 60% for raw Kahnuj ilmenite pellets. Reduction proceeded topochemically, forming iron-rich outer layers that encapsulated unreacted ilmenite cores enriched in rutile. The layered iron-rutile structure acted as a barrier to further reduction. The controlling mechanism in the reduction of raw pellets was identified as hydrogen diffusion through the product layer in the temperature ranges of 500°C–800°C and 900°C–1100°C, with activation energies of 107 kJ/mol and 88.8 kJ/mol, respectively. To enhance reduction efficiency, pre-oxidized ilmenite pellets were subsequently reduced under the first regime. Compared to raw pellets, pre-oxidation altered the phase composition and microstructure, changed the controlling mechanism during reduction, and decreased the activation energy. Maximum reduction degrees of 85.8% and 73% were achieved for pre-oxidized pellets at 800°C and 1000°C, respectively. Reduction of pre-oxidized pellets at 800°C resulted in the formation of new ilmenite structures with a network of pores originating from needle-like rutile phases, hematite, and primary pseudorutile. This porous network facilitated hydrogen gas diffusion and shifted the reduction mechanism to chemical reaction at the interface, with an apparent activation energy of 35.5 kJ/mol for the temperature range of 900°C–1100°C. Pre-oxidation at 1000°C led to the formation of irregular rutile grains in a pseudobrookite matrix, with a chemical reaction mechanism controlling the reduction and an apparent activation energy of 48 kJ/mol. On average, in the 500°C–800°C temperature range, the apparent activation energy for pre-oxidized pellets at 800°C and 1000°C decreased by 60% and 46%, respectively, compared to raw pellets. The design of experiments was conducted using the response surface methodology and a central composite design, with reduction temperature, pre-oxidation temperature, and gas flow rate as variables, aiming to maximize weight loss and reduction degree. At a reduction temperature of 1050°C, pre-oxidation temperature of 880°C, and gas flow rate of 200 mL/min, the maximum reduction degree of 97.2% and weight loss of 15.1% were achieved, consistent with the predicted values of 98.4% and 15.3%, respectively.
