Lecture Notes for undergraduate students of Metallurgy and Materials Engineering.

1. Refining Slags
Steel Making Slag
“Make a slag & slag makes steel”
By
Dr. Muhammad Ali Siddiqui
Assistant Professor
Metallurgical Engineering Department
NED University of Engineering and Technology, Pakistan

2. What is Slag? (Structure/ composition of Slag)
• “Molten oxide by product of smelting and refining is known
as slag”  lighter than hot metal (molten steel)
• SiO2, P2O5, B2O5, etc are called acid oxide. Tetrahedral
• CaO, MgO, MnO, FeO, Na2O, K2O etc are called basic oxide. 
octahedral
• The opposite chemical nature results in the formation of
complex phase (silicate or phosphate)
• Therefore, “molten oxide containing two opposite chemical
nature of oxide (acid oxide + basic oxide) is called slag.
• Where (R ) represents monovalent or divalent catains Fe2+,
Mn2+, K+, Na+ etc  maintain aqueous acid-base
nuetralization.
Solution of oxides and sulphides

3. • Basic oxides are ionic solids
• Acid oxides are covalent solids.
• If the ionic character is predominates (be in
the majority) over the covalent character of
the bond, Upon melting , these dissociates;
• Short range order exist in the melt and no
complex ions are formed and ions behaves as
fairly free mobile ions, which can further take
part in refining. Ca+2, O-2 network breakers
Free oxygen
simple anions

4. • If the covalent character is predominates over
the ionic character of the bond, upon melting,
the tendency of complex ion formation
increases.
• Because, acid oxides have dominant covalent
bonding and do not dissociate into a simple ions
, but it dissociate in a complex manner as:
• This indicates, there is no activity of oxygen ions
in the melt.
Complex Oxides = Network Formers

5. • The majority of the steel production comes from the
basic process because the structure of the slag is
reasonably well known.(…may contain the following
ions depending upon the chemical composition)
• Simple cations = Fe2+, Ca2+, Mg2+, Mn2+, na+, K+, etc
• Simple anions = O2-, S2-, etc
• Complex anions = SiO4
4-, PO4
3-, AlO3
3, FeO3
2-, etc

6. • Acid Slag: In acid steel making (in acid Bessemer or
acid open hearth process) the slag is silica saturated
has practically no Basicity, this slag acts only as a sink
for oxide products it does not take part during
refining, it may be very viscous or dry so that it can be
readily separated from the steel melt.
• Basic Slag: in basic steel making processes, the slag is
thin and wet it takes part during refining, it increases
mass transport during refining and thereby increase
reaction rate. Thin slag has good thermal conductivity.
This slag is specially required for efficient refining of
non-metallic impurities, the slag acts as a thermal
barrier and the rate of heat transfer to the metal bath
is inversely proportional to the thickness of the slag
layer.

7. Role of Slag
• Act as sink for impurities.
• Control oxidizing and reducing power. (with the
help of FeO content)
• Higher FeO  Slag is Oxidizing (De-C, De-P)
• Lower FeO  Slag is Reducing (De-S)
• Act as a thermal barrier – to prevent heat transfer
• Prevents the passage of unwanted gases to the
steel from atmosphere (N2 and H2)
• Protect the steel from re-oxidation
• EAF –slag prevents the radiation of heat may be
to walls or roof from the electrode.

8. Refining Slag Properties
• The main slag properties of interest are;
 Its ability to retain oxide product. (Basicity)
 Foaming Slag (basic oxygen steel making)
 Oxidizing potential (oxidizing/reducing power)
• The following other properties are also
important from the rate of reaction point of
view.(Slag-metal, slag-gas and slag-refractory
reaction).
• Viscosity, Thermal conductivity and Surface
tension etc

9. • It is generally expressed as the ratio of basic to
acid oxide, expressed as
– Basicity = ∑%(all basic oxides) / ∑%(all acid oxides)
– V Ratio = (wt % CaO) / ( wt % SiO2)
– Modified V Ratio = %CaO / ∑%(SiO2 + AL2O3 + P2O5)
• In practice slag with much higher V ratio are
required to effectively eliminate phosphorous
and sulphur during refining.
• Slag basicity between 1.2 to 2.5 is also
facilitating the foaming slag also provides good
properties for desulfurization..
1. Basicity: Slag ability to retain oxide product.
Basic slag V >1
Acid slag V < 1

11. 2. Foaming slag
• Slag foaming is obtained by injecting oxygen
into the liquid steel, where mainly iron is
oxidized according to the reaction:
O2 + 2 Fe = 2 (FeO)
• Carbon powder is then injected
simultaneously into the slag phase where iron
oxide is reduced.
(FeO) + C = Fe + CO (g)
• The resulting CO gas is a critical component in
order to obtain a foaming slag.

12. Does foaming slag suitable for removal of Phosphorus?

13. 3. Viscosity ( ) of the slag
• Property of liquid (molten metal) which
controls the movement of from one place to
another.
• Viscosity control the fluidity. (movement of
the slag layer)  should be fluid enough that
can be removed easily from the hot metal
surface.
• Viscosity depends on composition,
Temperature and solid percentage.

14. • If more viscous = less fluid = difficult to remove.
Like in Acidic slag  highly viscous, dry and thick.
• Viscosity Temperature
Composition
Basic oxide = Network Breaker = CaO, MgO, MnO, FeO, Na2O, K2O
Acid oxide = Network Former = SiO2, P2O5, B2O5
•Increase Lime
(CaO ), increase
the viscosity.
•Fluorspar (CaF2)
= fluidized the
slag
Addition of basic oxide decreases the

15. • Solid fraction:
Viscosity
Solid fraction
• If in between 5 – 10 %, then increase by
114 – 130 %. (Drastic Effect in the increase in
the viscosity)

16. 3. The oxidation / reduction potential
• The Capability of slag either to transfer the
oxygen into the steel OR to transfer the
oxygen from the steel to the slag. (Why we
are telling the FeO Content is important here
?)
• The oxidizing / reducing power is
thermodynamically equal to the partial
pressure of oxygen in equilibrium with the
slag.
Remember = FeO Content will be controlling this potential, How?

17. • There are many oxides present in slag phase
(such as MnO, SiO2, etc) but the iron oxide
(FeO) has the highest equilibrium partial
pressure of oxygen (po2).
• This means that the po2 value in equilibrium
with the FeO in the slag shall automatically be
in equilibrium with the rest of the oxides.

18. • If poo2 = equilibrium partial pressure of oxygen
in contact with pure iron oxide (i.e., iron oxide
held in an Fe container) the equilibrium partial
pressure of oxygen of a slag will be given by:
• Since the value of poo2 is very high, the oxidizing
power of a slag is, therefore, conventionally
expressed as the activity of iron oxide in slag.

19. slag Hot metal
Equilibrium constant
Oxidizing Potential (oxidizing slag):
Reducing Potential (Reducing Slag):
Backward reaction

20. • The Basicity and oxidizing power of slag are
independent properties.
• In practice Basicity is generally related to the
lime (CaO) concentration and oxidizing power
is related to the FeO content.

21. The approx. FeO and V-Ratios of the slags used in steel
making practices are shown below.

22. Selected Ternary and Quaternary
Oxide Systems
• Most steelmaking slags consist primarily of CaO,
MgO, SiO2 and FeO.
• In low-phosphorus steelmakingpractices, the
total concentration of these oxides in liquid slags
is in the range 88 to 92%.
• Therefore, the simplest type of steelmaking slag
to be considered is the quaternary systemCaO–
MgO–SiO2–FeO.
• First let us consider the ternary system CaO-SiO2–
FeO; the liquidus isotherms of this system is
shown in Fig.01

23. • Fig: 01Liquidus isotherms of CaO–SiO2–FeO system.

24. • The isothermal section of the composition
diagram in Fig. 02 shows the phase
equilibrium at 1600°C.
Four two-phase regions, shown
by dotted lines, the melt is
saturated withSiO2, 2CaO·SiO2,
3CaO·SiO2 or CaO;
two three-phase regions
(2CaO·SiO2 + 3CaO·SiO2 + liquid)
and (3CaO·SiO2 + CaO + liquid);
and one liquid phase region.

25. • Magnesia is another important ingredient of
steelmaking slags, which are invariably
saturated with MgO to minimize slag attack
on the magnesia refractory lining of the
furnace.
• The effect of MgO on the solubility of calcium
silicates and calcium oxide is shown in Fig 03
for the system (CaO + MgO)–SiO2–FeO, in
equilibrium with liquid iron at 1600°C.

26. Fig. 03 Effect of MgO (wt.%) on the solubility isotherms at
1600°C in the system (CaO + MgO)–SiO2–FeO in equilibrium with
liquid iron.
Broken-line curve delineates the
region of saturation of molten slag
with solid calcium (magnesium)
silicates and solid magnesio-
wustite (MgO–FeO solid solution).

27. References
1. Modern Steel Making Handbook R. H.
Tupkary, V. R. Tupkary,
2. The Making Shaping and Treating of Steel,
Steel Making Refining Volume, AISE steel
Foundation.
3. Electric Arc Furnace Simulation Guide,
www.steeluniversity.org