Table Of ContentThe
EQUILIBRIUM CONSTANT
of the reaction between
LIQUID IRON
and
HYDROGEN SULPHIDE«
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T H E S I S
submitted to the
UNIVERSITY OF GLASGOW
HUGH SKELLY, B.Se.,
for the
DEGREE QF DOCTOR OF PHILOSOPHY -
CONTENTS.
Page
INTRODUCTION....................................................................................... 1
PRINCIPLE OF METHOD............................................................ 12
APPARATUS.............................................................................................. 14
PRELIMINARY INVESTIGATIONS............................................................26
EXPERIMENTAL METHOD ............................................................... 33
EXPERIMENTAL RESULTS .................................................. . 44
DISCUSSION OF RESULTS......................................................................59
CONCLUSION..................................................................................................66
ACKNOWLEDGMENTS........................................................................................69
REFERENCES ......................................................................70
1.
INTRODUCTION,
Balanced Reactions and D esulphurisation in the Basic
Open-Hearth Furnace.
The reactions which bring about the removal of im purities
from the bath o f metal in the basic, open-hearth furnace have been
established in a q u alitativ e manner. The following equations re
present the principal reactions involved in the p u rificatio n of
the molten m etal:
C +■ FeO = Fe t CO - 36 cals .--------------------(1)
Mn ■+• FeO = Fe -h MnO + 26 c a ls .--------------------(2)
Si *f* 2FeO = 2Fe +■ Si02 + 65 c a ls .--------------------(3)
2P +* 5FeO - 5Fe-h P205 +• 37 c a ls ----------------------(4)
Besides these fundamental reactions, secondary reactions such as
the follow ing may occur:
5C •+• P205 = SCO +• 2P - 220 c a ls .--------------------(5)
The d ifferen t constituents of the slag also Interact and produce
compounds which may have a great influence on the process of puri
ficatio n . Thus the follow ing reaction is possible:
4CaO -h P205 = 4CaO .P2O5- ----------------------------------(6)
The form ation o f tri-calcium phosphate is also possible, and the
production of the te tra - or tri-calcium phosphate w ill probably
depend on the amount of available lime in the slag. The removal of
the im purities consists then essen tially in the oxidation of tne
carbon, manganese, silico n , and phosphorus by means of iron oxide.
I t is probable th at the ferrous oxide also plays an important
part in the removal of sulphur from the bath of molten m etal. The
chief desulphurising agents used are manganese and lime, and th e ir
actions may be represented by the equations:
FeS +- GaO = FeO h- CaS — — — ---------— .- ...- ( 7)
FeS + Mn = MnS Fe—-------------------------------------------------(8)
Some of the MnS formed by reaction (8) rise's into the slag where i t
is oxidised as follow s:
2MnS -h 202 = 2MnO -h 2302------------------------------------------------(9)
The greater the amount of lime and the sm aller the amount of fe r
rous oxide, the fu rth er w ill reaction (7) tend to go to completion
towards the rig h t. In other words, the larger the ratio CaO/FeO,
the more sulphur w ill be transferred from the stee l to the slag.
Lime also combines with P2O5 to form 4Ca0 .P205, and the more lime
there is the greater w ill be the amount of calcium phosphate pro
duced. The P2O5 resu lts from the oxidation of phosphorus by FeO,
(equation (4) ) , and i f there is an excess of lime in the slag,
then the P2O5 w ill quickly combine with the lime to form 4CaO.Po05.
This w ill hasten the oxidation of the phosphorus and so the FeO
content of the Slag w ill f a ll. As the FeO also oxidises the man
ganese in the bath (equation (2)), then the amount of manganese
in the bath w ill increase due to the decrease in the FeO content
of the slag. Thus, it may be said th at the effect of adding ex
cess lime to the slag is to Increase the amount of manganese in
the bath. This manganese w ill then proceed to desulphurise as
shown in equations (8) and (9). The transference of sulphur from
1 ' * *'
metal to slag thus depends on the ratio CaO/(FeO+-MhO) ^
" 3.' '
! '
..
J /V The active desulphurising agent then seems to be manganese'
7 /
'
which is present eith er as a d irect addition or through the re
duction of MnO from the slag. The ferrous o'xide also plays an
im portant part in the removal of sulphur, because the lower the
FeO content of the slag, the less manganese w ill be oxidised and
so there w ill be more of the la tte r available for desulphurisation.
Moreover, when FeO is added to the bath it probably reacts with
the MnS th u s:
FeO +• MnS = FeS + MnO-------------;------------------------(10)
The FeS w ill remain dissolved in the m etal, but the MnO w ill go to
the slag and so the manganese content of the metal w ill decrease,
with a consequent decrease in the process of desulphurisation.
Although the reactions which bring about the removal of im
p u rities from the molten metal in the basic open-hearth process
have been q u alitativ ely established, much work has yet to be
carried out on the physical chemical ch aracteristics of these re
actions. A wider and more accurate knowledge of the equilibrium
constants of the balanced reactions is desirable, together with
the effect on the reactions of various controlling facto rs, such
as tem perature and the con stitu tio n of the slag.
Me Cancel has pointed out th at d ifferen t reactions which are
in te rre la te d have a ch aracteristic property which can be used to
derive values for the equilibrium constants. The follow ing ex
ample is an illu s tra tio n of MG Cance's technique. Consider the
three equations:
4.
FeO +- CO s Fe +■ COg----------------------------------- ( n )
2C02 - 2CO + 02---------------------------------— 12)
2Fe0 » 2Fe +• Og------------------ (13)
I f equation (11) is m ultiplied by two and added algebraically
to (12), then equation (13) w ill resu lt. A sim ilar relationship
w ill hold fo r the equilibrium constants of these reactions and
th erefo re;
Kia - Kfx.K12
where Kpq, % 2 5 an<^ %3 are respective equilibrium constants.
Thus i f K-j^ and K1P are known i t is possible to derive a value for
the equilibrium constant of the reaction representing the d isso ci
ation o f ferrous oxide. This method cannot be used when the pow
ers of the equilibrium constants on both sides of the equation are
equal, because the equation then becomes an id en tity and is unsol-
vable.
The reaction between ferrous oxide and carbon is one of the
most im portant in steel-m aking. I t is the only one of the primary
reactions (1) to (4) which is endothermic and some investigators
have stated th at i t is confined to the higher tem peratures of
steel-m aking. The equilibrium constant of th is reaction may be'
w ritten:
K., s Fe.Pco
FeO.C
where Pco ‘the pressure of the carbon monoxide and is taken as
a measure of its concentration. The concentration of iron in the
ste e l bath w ill be l i t t l e affected by the course of the reaction
and i t may be taken as constant. As the pressure of the carbon
5.
monoxide w ill be lim ited to one atmosphere - above th is pressure
the excess gas w ill escape from the bath - then i t may also be
taken as being constant, and thus the follow ing is obtained:
PeO.C
Vacher and Hamilton obtained a value of O-Oll at 1620°C for th is
constant using weight percentages. The equilibrium stage of th is
reaction is influenced by the solution pressure of carbon monoxide
and the d isso ciatio n pressure of ferrous oxide. Both of these are
very low, and so the reaction proceeds u n til the carbon concen
tra tio n is very sm all, as long as there is su fficien t iron oxide
availab le. The oxidation of the carbon by ferrous oxide has been
discussed from the point of view of the bath only, but i t is also
necessary to. consider the part played by the slag in th is reaction.
The ferrous oxide is dissolved in both the slag and the molten
metal) and any v ariatio n in the iron oxide content of the slag w ill
be communicated to the m etal. The p a rtitio n coefficient of the
ferrous oxide between metal and slag may be represented by Lp@o
and for p ractical purposes i t can be taken as a constant depend
ing only on tem perature.
The equilibrium constant of reaction (2) may be w ritten asi
(MnO) OPeJ
(FeO) [Mo]
where LMnQ is the p a rtitio n co efficient of MnO between metal and
slag, and ( ) indicates the concentration of constituents in the
slag, and [ ] the concentration of constituents in the m etal.
The concentration of the iron may again be taken as constant and
6.
thus
Kq sc It'MhO (MnO)
Lpe0 (FeO) [Mu]
This equation indicates th at the concentration of manganese in the
metal is dependent on the ratio of the MnO to the FeO in the slag.
(3)
Mc Gance, by p lo ttin g many p ractical re su lts, has shown th at th is
theory is supported by experim ental evidence. Korber and O elser^
give the follow ing equation as representing the variation of the
constant with tem perature;
log K2 = 6234 _ 1 ‘ 026
nMr.»
In basic open-hearth furnace practice it is found th at s i l i
con is the f ir s t element to be oxidised and th is fact supports the
thermochemical data. The equilibrium constant of the reaction
(equation (3}) by which silico n is removed Is as follow s;
K3 = Fe2 . Si02
Si . FeO2
The s ilic a formed is probably insoluble in the steel bath, andi ts
concentration in the stee l may be taken as constant, asm ay also
the concentration of the iron. Thus we have;
- Si . FeO2 << R,
0)
M° Cance deduced th eo retic al values for th is constant and obtained
a value of 0*56 . 10~4 for a t 1600°C on a percentage weight
b asis. The best experim ental resu lts are those of Knrber and
Oelserfp whose value for Kg at 1600°G is 7*1 . IQ”4 . Their equation
for the v ariatio n of the constant tem perature is as follow s:
log K3 r -32 ?360 + 14-153
T-
