Modeling the development of strength in pellets
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I.
INTRODUCTION
THE production of pellets of iron ore remains a high tonnage metallurgical operation, although production in 1983 at 160 million tonnes was only 60 pet of installed capacity.l An adequate fired strength of pellets is the primary objective of the process and is often used as an index to aid control of pellet plants. As reviewed by Wynnyckyj and Batterham, 2 mathematical modeling has been used extensively to optimize the performance of various pelletizing processes, in particular to help minimize the energy consumption. 3 Given the close but complex relation between fuel used and the resulting fired strength of pellets, it is important that mathematical models used for describing plant behavior should include an adequate description of the development of strength. This paper discusses the mechanisms of strength development during the firing of pellets and presents a formalism to describe strength development. The method presented is suitable for use in mathematical models of plant performance and adequately fits experimental results generated under laboratory conditions where the time-temperature profiles and the resulting fired properties of the pellets were known.
II.
REVIEW OF PREVIOUS WORK
A. Mechanisms of Strength Development While much of the literature on strength development shows how gross operating variables affect the final product, there have been significant attempts to describe the strengthening mechanism during calcining. For unfluxed pellets, strength development is associated with sintering reactions accompanied by an increase in pellet density and a decrease in volume. The mechanisms are the same as in powder compaction and have been well researched. With fluxed pellets, liquid phases appear during calcining and the increase in density can occur much more rapidly. Regrouping of particles can lead to large reductions in interfacial area and shrinkage. Liquid phases can also aid development of chemical reactions and solution-precipitation which further increase strengthening. For pellets, Malysheva and Chernyshev4 show that for both the fluxed and unfluxed cases, the fine (0.01 mm) R. J. BATTERHAM is Chief, CSIRO Division of Mineral Engineering, P.O. Box 312, Clayton, Victoria 3168, Australia. Manuscript submitted April 22, 1985. METALLURGICALTRANSACTIONS B
size fraction remains inert while the fine haematite particles recrystallize in unfluxed pellets, or, in fluxed pellets, the ferrite binder recrystallizes. At higher temperatures, for unfluxed pellets, the sintering continues with dissociation and melting of the contained minerals while for fluxed mixes, the ferrites melt and react with the other minerals. The broad behavior has been reported in detail for various ores and fluxes. 5,6,7 B. Formalisms to Describe Shrinkage Given the range and complexity of the mechanisms of strength development, it is not surprising that no single, cogent theory yet exists to describe numerically the development of strength. The densification literature on powder compacts has provided several f
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