Abstract:
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.
