Accretion formation on the refractory lining during the melting of ferrosilicon

T.M Nemavhola,T Coetsee,A.M Garbers-Craig

doi:10.17159/2411-9717/1140/2021

Abstract

SYNOPSIS Accretions often form in furnaces when slag and charge materials attach to the refractory wall and build up over time. Accretion formation is usually unwanted because it reduces the working volume of the reactor and hinders material flow through the reactor. However, in some instances a thin, stable accretion layer may be desirable to protect the underlying refractory material. In order to prevent and/or manage accretion formation, it is important to understand the underlying principles of this phenomenon in the particular reactor. Excessive accretion formation hampered production at the Exxaro FerroAlloys ferrosilicon melting and atomization plant. This plant uses induction furnaces in which a 15% silicon-iron alloy is produced by batch smelting a mixture of ferrosilicon of 75%Si grade and low-carbon steel. The molten ferrosilicon alloy is then gas-atomized to a powdered product for use as a dense medium in mineral processing plants. The objective of this study was to investigate the effect of different impurity levels in the ferrosilicon feed material on the extent of accretion formation as well as the effect on the accretion properties, which influence the ease of accretion removal upon furnace shut-down. Refractory and accretion samples were collected after a furnace shut-down and characterized using X-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy. It was concluded that the trace elements in the FeSi-75 feed material (Al, Ca, Mn) were mostly responsible for accretion formation, but that rust on the low-carbon steel and oxidation of the steel contributed to accretion attachment to the lining. The total contaminant content, calcium to aluminium ratio in the FeSi-75 feed material, and thereby the liquid to solids ratio in the accretion at temperature determine the strength of attachment as well as growth of the accretion. Keywords: Accretion, build-up, slag, refractory lining, ferrosilicon.

Highlights

Atomized ferrosilicon is produced by melting high-grade crude FeSi-75 (75% Si and 25% Fe) and diluting it with low-carbon steel scrap in an induction furnace until the silicon content of the melt is 15% and the iron content 85%
Fe3Si occurred in all accretions, with metallic iron present in the accretion of Feed 1 and Fe5Si3 in the hot face of the Feed 2 accretion
Accretion build-up during the production of FeSi-15 was investigated by varying the impurity aluminium and calcium contents in the FeSi-75 feed material

Summary

Introduction

Atomized ferrosilicon is produced by melting high-grade crude FeSi-75 (75% Si and 25% Fe) and diluting it with low-carbon steel scrap in an induction furnace until the silicon content of the melt is 15% and the iron content 85%. One of the operational difficulties that can be experienced when producing FeSi-15 using an induction furnace is accretion build-up This is a phenomenon whereby slag, metallic oxides, or refractory oxide material (often with metal entrainment) adhere onto the refractory surface during the production cycle, thereby forming accretions on the refractory lining of the furnace (Figure 1, Williams and Naro, 2007). When the furnace is ready for de-slagging (1520–1540°C) a coagulant is added to the charge, which results in the formation of a slag layer on top of the alloy This slag is removed from the alloy using a metal spoon as it has adverse effects on tapping and atomization due to its rapid cooling rate, which causes runner build-ups and tundish blockages. The hot face of the refractory lining can reach a maximum temperature of 1600°C

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