There are certain grades which have a strict limitation on
max-Si permitted, typically 0.03% (wt), so it’s necessary to avoid silicon
reversion from slag (that happens when we “OVER” kill slag[FeO] with Al).
Normally de-O precedes de-S. Once
SUFFICIENT de-O is achieved, de-S should be initiated WITHOUT delay & deep
de-O should be avoided. In order to de-O, we add Al in ONLY-Al killed grades. Once
added, this aluminium reduces FeO(unstable, weak oxide) first. THEN…., it looks
for other oxides to reduce, when this heavily-reducing atmosphere is maintained
unbalanced. Once free oxygen and
FeO are killed, now Al looks at silica and MnO, and starts reducing them-
unless CaO (lime) is added to raise the basicity.
Once lime is added, reaction-collisions (Al + SiO2 =>Al2O3+
Siˇ) decrease, leading to lesser Si reversion. aluminium is now surrounded by
more CaO molecules than SiO2 molecules, so Al now attacks CaO and develops
Ca-ions. These Ca-ions attack FeS and START de-S reaction. So instead of [Al + SiO2
=>Al2O3+Siˇ), [Al + CaO => Ca+ + Al2O3
] reaction dominates, enabling better de-S.
So when it comes to Al-killed grade, we need to control slag
very carefully. De-O of only FeO & MnO should
be permitted. De-O of silica should NOT
be allowed to happen.
That means…in Si-killed grades, slag can be maintained fluidic FOR PROLONGED DURATION, while
in Al-killed grades longer durations
of reducing atmosphere+fludic slag returns
Si into metal. Hence CaO addition at the RIGHT time* in RIGHT quantity$
is very important. Excess fluidity is not permitted for longer duration.
RIGHT time*- because late
addition, would have already resulted in Si reversion into metal.
RIGHT time*- because early
addition, will impede de-O RATE.
RIGHT quantity$ - because too much, will reduce de-S RATE due to reduced reaction-collisions.
RIGHT quantity$ - because too less, will permit Si-reversion & will reduce de-S as slag becomes
easily S-saturated.
We can NOT add lime in excess quantity (basicity should not go
beyond 3.2). Excess lime will make slag very thick/dry/stiff, so de-S RATE will
be heavily reduced due to reduced reaction-collisions.
This thick/dry/stiff slag may lead to
arc-flare as arc gets deflected, which may reduce ladle-lining-life, apart from
reduced de-S rate.
And, thin/fluidic slag will NOT bury
the arc, leads to energy waste, arc flare may also happen. ULC/IF grades will
pickup too much carbon from electrode. May lead to electrode spalling.
So we have to walk a thin-line
between too-thin slag and too-thick slag in only-Al-killed, de-S grades, where
Si reversion is NOT permitted (Si < 0.03 wt%).
In Si-killed grades de-S becomes
easier, since Si-reversion is permitted; delayed CaO addition can be done in
too many multiple batches of very small quantities.This permits slower
thickening of slag, thereby expediting de-S through increased
reaction-collisions of fluid slag molecules.
Finally, one more point I shall add.
It is not Al or Si that decides the basicity but the oxides of them. Both oxides
are acidic slag formers, making slag more fluidic.
About amphoteric nature of alumina in basic-steel-making practices,
I have no clear idea; though I may venture to say, aumina acts as an acidic
slag in basic steel making conditions.