Kinetics of the Reactions in Carbon Composite Iron Ore Pellets under Various Pressures from Vacuum to 0.1 MPa

Abstract

In order to clarify the mechanism of the complicated reactions in carbon composite iron ore pellets, carbon composite iron ore pellets of 3.5 mm in diameter were heated in the five conditions; (a) in vacuum, (b) the closed system after evacuation, (c)-(e) the closed system of initially 0.001, 0.01 and 0.1 MPa nitrogen after evacuation. The weight loss was measured in the five conditions (a)-(e) by gravimetric method. The individual curves were composed of the fast first step and the relatively slow second step. The first step was analyzed as zero order reaction and the rate constant, k_s1, was obtained. The second step was analyzed by applying the rate equations for the diffusion control in the product shell of the shrinking core model for spherical iron ore particles. The applicability of the equations was pretty good. The rate con-stant, k_s2, was a function of the total pressure p_t and temperature T, which was expressed as k_s2=A(p_t)exp[−B(p_t)/T]. The apparent activation energy was ranged from 195 to 280 kJ/mol, which is near the activation energy for the diffusivity of carbon in γ-iron. The rate of the fractional weight loss seems to be in a mixed control regime of the direct reduction, indirect reduction and indirect gasification. The transition fractional weight loss from the first step to the second step was evaluated in the conditions (a) and (b). It decreased with increasing the total pressure until it reached nearly zero in the conditions (c), (d) and (e), where Eq. (11) was applicable in the whole range of F_t.