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Recirculating fluidized bed process for the roasting of molybdenite concentrates Wilkomirsky, Igor A. E.

Abstract

The development of a new, recirculating fluidized bed process for the roasting of molybdenite concentrates has been successfully completed in a bench scale pilot plant. The process employs high-efficiency cyclones with a novel pneumatic injection system for continuously recirculating the calcines and feeding the molybdenite concentrates into the reactor. The fluidized bed consists of a mixture of calcines and coarse sand with a wide size range. The latter provides smooth fluidization behaviour and an attrition effect to prevent calcine particles from agglomerating. The fine calcines (-325 mesh) are continuously elutriated from the bed and recycled to the reactor, while the coarser sand particles remain in the bed. A rotary mechanical scraper inside the fluidized bed prevents build-up of material along the reactor walls. The reactor design was based on studies of fluidization characteristics, gas and solid mixing, and particle stratification using a two-dimension fluidized bed operated at room temperature. The performance of the process on molybdenite concentrates from four different sources was evaluated. Concentrates containing less than 0.04% calcium were roasted to molybdenum trioxide with sulphur levels below 0.15%, which is suitable for metallurgical uses. An economic comparison showed that the fluidized bed process is able to compete favourably with the existing multiple hearth process. The fluidized bed process has an output that is 30 to 50 times larger per total area of furnace than the multiple hearth roaster (3 to 5 times larger per unit floor area), and a capital cost that is lower by 50%. In addition,potentially lower operating costs and higher levels of SO₂ in the off gases may be realized by the fluidized bed process. Tests showed that direct slurry feeding of MoS₂ concentrates is feasible. This would eliminate the need for filtration and drying steps prior to roasting. Batch kinetic studies in the fluidized bed, and observations using hot stage and scanning electron microscopes indicate that the transformation of MoS₂ to MoO₃ is a complex process which involves an initial fast oxidation step followed by a slower second stage. The first step appears to be controlled by the rate of the chemical reaction which is first order with respect to oxygen concentration and strongly temperature dependent. In the slow second stage it is possible that solid state diffusion is the rate limiting process. The volatilization and subsequent condensation of MoO₃ seems to play an important role during the transformation. Continuous operation in a 12.5 cm diameter reactor showed that the main variables that control the final residual sulphur in the calcines are the average residence time of molybdenite particles in the fluidized bed, and the calcium content of the concentrates. Residence times in excess of 20 hours are required to achieve sulphur levels below 0.15%. The optimum temperature range for roasting was found to be very narrow, 520-550°C; in this range the final composition is not strongly dependent on temperature.

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