Mechanism of growth of metallic phase in direct reduction of iron bearing oolites
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INTRODUCTION
D E P O S I T S of oolitic iron oxide minerals have been known for many years as a source of raw iron for the steel industry. Deposits of this type are currently exploited around the world, for example in the United States, France, and West Germany. The amenability of oolites to direct reduction processes is of interest both technologically and theoretically in the field of iron metallurgy. The reduction of iron oxides embedded in the oolite matrix is likely to involve two related mechanisms: the chain of reduction reactions proceeding down to a metallic phase and the growth of this phase into particles which are amenable to separation. In this context it is of interest to correlate the structure and composition of oolites with the different stages of reduction and growth of the metallic phase. Typical oolites occur as oval particles containing 49 to 54 pct Fe in the form of finely-divided ferric oxides. In this work oolites from different sources were tested, but most of the data was obtained using oolites from the "Ramim" deposit which occurs in the Galilee "pan handle" in northern Israel. This relatively lean ore contains 26 to 28 pct Fe occurring as goethite (Fe203" H20) which form the typical concentric shells structure in 0.05-1 mm oolites. The main gangue mineral associated with the goethite (81 pct) is kaolinite AlzO3" 2SIO2" 2H20 (8 to 12 pct). l It was shown elsewhere2 that direct reduction can be successfully applied to concentrates comprising "Ramim oolites", where following size reduction and magnetic separation of the reduced pellets, high quality concentrates (Fe > 90 pct), and good recoveries (R > 90 pct), were obtained. The direct reduction process, performed with coke, involved removal of oxygen from iron oxides leading to the growth of iron particles which were amenable to low intensity magnetic separation. Since the original particle size of goethite was in the 1 to 2/xm range, growth of one to two orders of magnitude in size (of the reduced iron particles) must have occurred for the magnetic separation to become efficient. For example, 10 to 30/xm metallic particles were obtained at 1000 ~ turning to a continuous phase at 1275 ~ S. WEISSBERGER, formerly with the Department of Civil Engineering, Mineral Engineering, Technion, Haifa, is with IMI Research Institute, Haifa Bay, Israel. Y. ZIMMELS and I. J. LIN are with the Department of Civil Engineering, Mineral Engineering, Technion, Haifa 32000, Israel. Manuscript submitted March 14, 1985. METALLURGICAL TRANSACTIONS B
Understanding of relationships between the direct reduction, growth of the metallic phase, structure of oolites, and parameters such as temperature, particle size, residence time, and type of reductant, may serve as a basis for control and optimization of the iron extraction process. This is also related to the identification of new compounds and fluxing agents which may participate in the reduction and growth of the metallic phase. To this end, use was made of techniques such as image analysis, X-ray diffraction
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