Non-isothermal reduction kinetics of low-grade iron ore-coal mini-pellet in a low-temperature rotary kiln process

Abstract

The kinetics of non-isothermal reduction of low-grade iron ore/coal mini-pellet reduced at a low-temperature rotary kiln was investigated. Meanwhile, a 3-D computational fluid dynamics (CFD)-based simulation for heat transfer between flue gas and pellet in rotary kiln has been performed in this paper, which reveals the dependence of heat-transfer rate on diameter of pellet during its reduction at a rotary kiln. XRD, SEM-EDX, and Chemical dissolution analysis were applied for further understanding of the mechanisms. The results demonstrated that the non-isothermal reduction process was controlled by chemical reaction mechanism (C2) at the initial stage (5 % ≤ α ≤ 33 %) and another chemical reaction mechanism (C1) was the rate determining step at the major stage (33 % ≤ α ≤ 87 %). The estimated activation energies of the two stages were 15.1, 178.9 kJ·mol−1, respectively. The rate of heat transfer improved significantly by minishing dimension of pellets according to results of a 3-D CFD simulation. A rapidly rising temperature in the core of the sphere facilitated a rapid integral reduction of mini-pellets, thus the temperature (≤1250 K) needed is significantly decreased and the reduction time (90–120 min) is greatly shortened compared with that (T = 1400–1600 K, t = 150–360 min) of traditional pyrometallurgical coal-based direct reduction processes.