Iron and Steel Division - Kinetics of the Hydrogen Reduction of a Low-Grade Siliceous Iron Oxide Ore

A kinetic study of the hydrogen reduction of taconite from the Wisconsin Gogebic range was made over the temperature range from 500° to 1000°C on eleven size fractions from 4 to 150 mesh. Two stages of reduction were observed, a rapid initial reaction followed by a slower reaction presumably taking place at an internal receding FexO-Fe interface. Calculated mean activation energies for the two reactions were 7800 and 14,600 cal per mole respectively. In recent years there has been an increased interest in the direct reduction of iron oxide ores to metallic iron by means of gaseous reducing agents such as hydrogen and carbon monoxide.1,2 These investigations have been motivated by the desire to develop a process that will by-pass the blast furnace in the production of steel. The majority of these processes have been developed to produce metallic iron by direct reduction from high-grade iron oxide ores.3"5 Very little work has been done on the low-grade siliceous iron oxide ores.6"8 The work reported here is directed principally towards the fundamental aspect of the reduction reactions of siliceous iron oxide ores, since the practical aspects of the gaseous reductant process were discussed previously. The ore used in the investigation was obtained originally from the United States Bureau of Mines10 trench sample of the Yale member of the Wisconsin Gogebic range iron formation. The Yale taconite has an average analysis of 28.2 pct Fe and approximately 50 pct SiO2. Chemically, these nonmagnetic taconites contain on the average about 53 pct silica and 30 pct Fe. An average analysis calculated fron the USBM10 figures for a composite of the five members (Norrie, Plymouth, Pence, Pabst, and ale) of the iron formation is given in Table I. Mineralogically, based on previous X-ray diffraction and other studies, the principal minerals are hematite (Fe2O3), geothite (Fe2O3.H2O), and quartz (SiO2). Minor amounts of iron containing silicates, siderite (FeCO3) and magnetite (FeO. Fe2O3) are present.10,11 The iron minerals and the quartz are finely disseminated requiring extremely fine grinding for liberation. For the experimental work, the ore sample was screened into a number of size fractions, the analyses and densities of which are given in Table II. EXPERIMENTAL PROCEDURE The reduction experiments were carried out in a 24 in. by 2 in. ID nichrome-wound vertical-tube furnace mounted on a stand. A 1 1/2 in. OD zirconium-oxide combustion tube was used inside the furnace to contain the atmosphere. To the top of this combustion tube was joined a pyrex ground-glass joint to which was attached a glass bulb with a small (1.0 mm) opening at the center, and with two outlets controlled with a stopcock on each side. A pyrex-glass joint with connections to three stopcock joints for introducing hydrogen, nitrogen, and mixtures of hydrogen and nitrogen gas was joined to the tapered bottom of the combustion tube. The sized ore fractions were contained in a 1 in. diameter by 1 1/2 in. 150-mesh nickel screen basket. The basket was suspended in the furnace from a balance by means of a supporting nickel wire which hung freely inside the combustion tube. The furnace temperature was controlled by a Leeds and Northrup Speedomax Type G controller, connected to a chromel-alumel thermocouple placed