| dc.description.abstract | Gas-Solid Reactions in Metallurgical Processes
Gas-solid reactions are of current interest to process metallurgists because of their growing importance in industrial processes. In the present work, investigations were carried out on gas-solid reactions of the noncatalytic type, and two types of systems were selected for study:
Roasting Type
In this category, the roasting of MoS? in air was examined in the temperature range of 935-1050°C. In this range, the roasting of MoS? is characterized by rapid evaporation of MoO?, formed as a result of the reaction. Experiments were conducted in a vertical tube furnace, and the weight and temperature changes of the sample were measured simultaneously.
Analysis of the experimental data indicates agreement with a mixed control mechanism, where both the chemical reaction and the transport of O? through the boundary layer play equally important roles in the overall kinetics. Furthermore, the reacting sample was found to be at a higher temperature than its surroundings throughout the reaction period. The exothermic behavior of the reaction has been analyzed on the basis of heat transfer concepts, assuming steady-state conditions.
Reduction Type
The reduction of NiO in H? was studied using dense sintered spherical pellets in hydrogen and hydrogen-nitrogen mixtures within the temperature range of 400-800°C. Specific surface area measurements of the unreduced and reduced samples were carried out using the BET technique. The average pore diameter in the reduced Ni metal was found to be small enough for Knudsen diffusion to be significant. A theoretical treatment revealed that a substantial gradient of total pressure can exist within the reacted shell during the reduction process.
The kinetic data obtained are consistent with a transport-controlled process, assuming a pressure gradient exists in the reduced shell. The value of porosity-to-tortuosity ratio, calculated from the kinetic data based on this model, was found to be in good agreement with corresponding values measured independently using a diffusion cell. Similarly, the heat transfer coefficients calculated from the temperature changes of the sample were in fair agreement with independently measured values.
One of the key findings in this work is the possibility of a total pressure gradient occurring in the reacted shell when the diffusivities of the reactant and product gases differ. A detailed theoretical analysis of the pressure build-up inside a reacting pellet during gas-solid reactions has been carried out and is presented. The analysis incorporates time and positional dependence of diffusivities of the reactant and product gases. The influence of relevant dimensionless variables-such as the ratios of total diffusivity of reactant and product gases, the ratio of the square roots of their molecular weights, the equilibrium constant of the reaction, the viscous flow parameter, the porosity-to-tortuosity ratio of the product layer, and the Biot modulus-on the pressure build-up has been discussed.
For completeness, studies on Mo single-pellet reduction have been extended to include investigations in a fluidized bed. A mathematical model of the shallow-bed batch fluidization process for calculating solid conversion with time is presented. The kinetic data obtained experimentally using the fluidized bed are analyzed based on the proposed model. | |
