Direct Reduction Behaviors of Composite Binder Magnetite Pellets in Coal-based Grate-rotary Kiln Process

Abstract

Directly reduced iron is an imperative burden for EAF (electric arc furnace) to make good quality and special steel. As an alternative of direct reduction processes, an innovative coal-based direct reduction of composite binder magnetite pellets in grate-rotary kiln has been developed at Central South University and put into operation. A study of the direct reduction behaviors of composite binder magnetite pellets was carried out in a simulating coal-based grate-rotary kiln process in this paper. Preheated pellets made of magnetite concentrate and composite binder and fired oxide pellets containing bentonite as binder were directly reduced using non-coking coal as reductant in a tube furnace, and their reduction behaviors have been demonstrated by measuring reducibility, variations of compress strength, porosity, phases changes and micro structure under XRD, SEM and optical microscopy. It is shown that preheated pellets possess much better reducibility than fired oxide pellets. Superior reducibility of preheated pellets should be ascribed to their higher effective diffusivity due to higher porosity. The compressive strength of preheated pellets climbs quickly after reducing for 30 min and achieve a high value at the end of reduction, leading to smooth and intact metallized pellets whereas the compressive strength of metalized pellets from reducing of fired pellets is much lower, more cracks and fractures being formed. The preheated pellets possess self-curing function because of the multiple functions of composite binder, eliminate cracks and fractures and keep pellets intact during reducing, depressing reduction degradation and preventing accretion in rotary kiln.