During the refining of steel by basic processes, a multitude of mineralogical
reactions take place in the refractories with which the furnace
is lined. The reactions are caused by an influx of material from
the slag and are a function of the composition of the slag and of the
refractory. The flow of material from the slag into the refractory resembles
the process of metasomatism encountered in nature.
The reactions between slags of different composition and magnesiteand
chromite-containing refractories could be studied systematically as
a result of a series of four experiments. During each of the experiments
four different refractories were subjected to the influence of a
slag having a specific composition for a considerable length of time.
The experiments were conducted in a rotating furnace at 1700°0~
Mineralogical reactions in the refractories used in these experiments
can be divided into two main categories, viz. reactions involving
the oxides and reactions involving the silicates.
The relevant oxides are periclase, the sesquioxides and spinel.
Periclase is found to Mndergo reactions like grain growth, resorption
and elongation normal to the hot face of the refractory. These
reactions are dependent on the composition of the refractory and of the
Reaction between periclase and sesquioxides yield spinel, provided
there is no excess of lime. Exsolution of bodies of spinel from periclase
is governed by the crystal-chemistry of both minerals.
At elevated temperature, spinel, and more specifically chromite,
acts as a source of sesquioxides in general, and of ferric oxide in particular.
The stability of chromite depends on the molecular ratio between
lime and silica of both the refractory and the slag. This ratio
determines the amount of sesquioxides present, but not in spinel or in
periclase, i.e. in the matrix of the refractory.
The postulation of the reactions involving the silicates is possible
because of the tendency of the refractory to reach equilibrium with
the composition of the slag.
The silicates present in acid slags are not in equilibrium with
magnesia in the refractory, and consequent reaction yields the minerals monticellite and merwinite. These minerals are not in equilibrium with
the bonding silicates of magnesite refractories which have high molecular
lime/silica ratios and also not with the bonding silicates of chromite-
containing refractories having ver.y low molecular lime/silica ratios.
Reaction between metasomatic and autochthonous silicates result
in a new set of silicates.
The silicates present in basic slags are in equilibrium with magnesia
of the refractories as well as with the bonding silicates of magnesite
refractories having high molecular lime/silica ratios. Consequently,
no reaction results. Reaction does, however, take place between
the silicates supplied by basic slags and the bonding silicates of chromite-
Ultrabasic slags react mildly with magnesite and drastically with
chrome-magnesite bricks. The reason is that in the former reaction,
only monticellite and merwinite are not in equilibrium with the composition
of the melt, but these minerals have a relatively high mobility,
and migrate de~per into the refractory. In chromite-containing refractories,
however, neither of the minerals chromite, monticellite or forsterite
are in equilibrium with the composition of the slag. Reaction
results in the formation of an association of minerals in the area of
contact between the slag and the refractory which differs completely from
the original minerals of the refractory.
The conclusion arrived at is that chromite has a beneficial influence
on refractories used in contact with less basic slags, because of the
textural superiority of chromite-containing refractories over magnesite
refractories. In contact with basic slags, however, metasomatism leads
to the formation of components which have low melting points and low viscosities.
During the refining of steel by basic processes, ultrabasic
slags are preferred and magnesite refractories having high molecular
lime/silica ratios are better suited for use in contact with the liquid