The Microstructure of the Pig Iron Nuggets

Abstract

The pig iron nugget process (referred to as the Iron Technology Mark 3, or ITmk3, process by Kobe Steel) was developed as an alternative to the traditional blast furnace process. Throughout this process self reducing-fluxing dried greenballs are reduced and smelted in to nuggets of metal. The objective of this research was to produce pig iron nuggets at laboratory scale, then characterize and compare them with the blast furnace pig iron. Pig iron nuggets were characterized utilizing apparent density measurements, optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, and bulk chemical analysis. It was determined that pig iron nuggets had high apparent density (6.7-7 g/cm 3 ); had a high iron content (95-97 %); and exhibited microstructures similar to white cast iron, which is essentially the same as pig iron from a blast furnace.KEY WORDS: pig iron nuggets; iron smelting; self reducing-fluxing dried greenballs. 53© 2007 ISIJ Table 1. The comparison of some of the basic process properties of the blast furnace and pig iron nugget process.where they are reheated in the blast furnace. The pig iron nugget process aims to save energy by making it possible to produce pig iron from self reducing-fluxing dried greenballs with just one heating step. Self reducing-fluxing dried greenballs will be heat treated directly without any induration or sintering steps. This will enable the elimination of the energy used by heating, cooling and reheating of the pellets and sinters.iii. Reduced Residence Time in the Reactor For the pig iron nugget process the kinetics of the reduction and carburization reactions are enhanced by agglomeration of an iron ore source, flux, and reducing-carburizing agent together. The close contact of the reacting materials and availability of a large number of reacting sites enhances the solid-solid reduction and slag forming reactions. Also, the internal gas generation, and small diffusion distances in the sample enhances the solid-gas reduction reactions, reducing gas regeneration, slag forming and carburization reactions. [10][11][12][13][14][15] iv. Impurity (Gangue) Separation with Formation of Fusible Slag For the blast furnace process and pig iron nugget process, the impurities are separated from the metal by the formation of slag. Flux materials such as limestone, dolomite, and magnesia are added to lower fusion temperatures to enable the slag formation. The slag can only be separated from the metal when metal and slag are in liquid state. This requires metal (pure iron fusion temperature of 1 535°C) and slag (slag fusion temperature around 1 200°C) to be heated above their fusion temperatures. The blast furnace process operates at temperatures above the fusion temperature of iron and produces liquid metal (pig iron) and slag. On the other hand, throughout the pig iron nugget process, it is intended to have carburization of pure iron, lowering its fusion temperature (this effect takes place until the eutectic carbon composition is reached). This should allow the pi...

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