Effect Of CaO, FeO, MgO, SiO2 and Al2O3 Content of Slag on Dephosphorization of Steels

Effect Of CaO, FeO, MgO, SiO2 and
Al2O3 Content of Slag on
Dephosphorization of Steels
Presented By:-
KUMAR KARUNA NIDHI
ROLL NO. FF17M26
NATIONAL INSTITUTE OF FOUNDRY AND FORGE TECHNOLOGY
RANCHI
Guided By :-
Dr. Nandita Gupta

Effects of phosphorus on properties
of steels

Phosphorus has tendency to increase cold shortness (brittle
behavior) in steels.
Low phosphorus content steel is use in thin sheets for deep
drawn applications, automobile exteriors and pipelines for
transportation of natural gas and petroleum products.
P decreases ductility and hence increase the tendency of the
steel to produce crack during cold working.

 Phosphorus has atomic number 15 and it can give up all 5
electrons from its outermost shell to become P5+ or accept 3
electrons to become P3- to attain stable configuration.
 This means that phosphorus can be removed both under
oxidizing as well as reducing conditions.
 But removal of phosphorus under reducing conditions is not
practical since its removal is highly hazardous.
 Thus P removal is practised mostly under oxidizing
conditions(i.e. in Basic Oxygen Furnace).

2[P] + 5[O] (P2O5)
2[P] + 5[O] + 4(CaO) (4CaO. P2O5)
2[P] + 5(FeO) (P2O5).5[Fe]
[X] represents species dissolved in metal phase
(Y) represents species dissolved in the slag phase.

 A basic lining and addition of basic fluxes were essential for the removal of
phosphorus.
 The increases in the concentrations of Ca2+ ,Fe2+ and Mg2+ result in a
decrease of the activity coefficient of P2O5 whereas SiO2 has the opposite
effect.
 Highly basic slag is required to accept phosphorus from the metal to the
slag.
 A high oxygen potential must exist to force phosphorus from the metal to
slag.
 Dephosphorization reaction is exothermic and that the slag capacity for
dephosphorization decreases with increase in temperature.
 The stirring conditions and slag composition are two key variables to enable
optimum P removal.

 2P + 5O = P2O5 (liquid)
 At T more than 1382 degree K, Delta G(free energy) becomes
positive which results in decomposition of P2O5 to P and O.
Thus removal of P requires that P2O5 need to be rich in slag.

Effect of CaO
 CaO (basic oxide) are necessary slag constituents for the
dephosphorization of steel.
 But over saturating the slag with CaO does not seem to benefit the process
to any extent.
 Lime (CaO) and dolo (MgO) is added as fluxing agent in the converter
steelmaking process.
 This lime (CaO) fluxes the P2O5 by decreasing the activity of P2O5 by
forming a stable compound like 3CaO. P2O5 or 4Cao.P2O5 .
 The P distribution ratio increases with increasing content of CaO in the slag
and with decreasing temperature.

Effect of FeO
 FeO is necessary slag constituents for the dephosphorization of steel.
 It is very important to maintain FeO content in the slag as it increases the
oxidizing power of slag.
 Dephosphorization is enhanced by a decrease in temperature and increase
in FeO content and basicity of the slag.
 The P partition ratio initially increases with increasing FeO content but then
decreases after a certain level is reached.
 This level is a function of slag basicity and temperature.

Figure 1 : Effect of FeO variation by keeping MgO at 10(wt %) on degree or
percentage of dephosphorization. [3]

FeO Content in the Slag
 Phosphorous partition
is initially enhanced
with increasing (FeO)
in the range of 15% to
24%. After that over
24% it start decreasing.
Figure 2: Total Iron content in slag and
phosphorus distribustion. [4]

Effect of MgO
 Over saturating the slag with MgO does not seem to benefit the process.
 The activity coefficient of P2O5 is decreased by increasing MgO (basic oxide)
upto 5 wt%.
 The P partition appears to be lower with an increase of MgO content at
temperature 1600 deg C to 1650 deg C.
 MgO is added to minimize the chemical wear of refractory lining of
converter. However , it has negative impact on dephosphorization as it
increases melting point and viscosity of slag.

Figure 3 : Effect of MgO variation on degree or percentage of
dephosphorization. [3]

Figure 4: Slag (%MgO) content and phosphorus
distribution.[5]
 The slag (%MgO) content
is an important for control
the wear of the furnace,
increasing the viscosity of
the slag and to improve
their sticking and melting
properties.
 At lower (%MgO) content
the melting temperature
increases quickly.
 At higher (%MgO)-content
the precipitation of solid
MgO starts and the
viscosity increases with
negative impact on
Dephosphorization
efficiency.

Effect of SiO2
 The activity coefficient(the ratio of the chemical activity of any substance to
its molar concentration) of P2O5 is increased by silica (SiO2).
 The increases in the concentrations of Ca2+,Fe2+ and Mg2+ result in a
decrease of the activity coefficient of P2O5 whereas (SiO4)4- has the opposite
effect.

Effect of Al2O3
Figure 5 : Effect of Al2O3 variation on degree or percentage of
dephosphorization. [3].

 In the CaO-SiO2-MgO-FeOx-
Al2O3 slag system, alumina
increases the liquid phase
region as it works as a
network breaker,
diminishing the viscosity of
the slags.
 For the lowest basicity
analysed, the LP
changing slightly from 1 to
5 and having the highest
value reached when Al2O3
equal to 7%.
 A small amount of
Al2O3 can promote the CaO
dissolution into the slags. Figur 6: Behaviour of LP with the Al2O3 content for
different basicities , T=1600°C .[7]

Effect of slag basicity
Figure 7 : Effect of slag basicity on degree or percentage of dephosphorization.[3]

Effect of Process Temperature on
dephosphorization of steel
Figure 7 : Variation of phosphorus control with tapping temperature[4]

Necessity of Dephosphorization of Steel
 Steels having low content of P are necessary for applications
where high ductility is needed, such as thin sheets, deep drawn
steel, and pipelines etc.
 In the earlier days, P control was not considered a big
challenge in steel production since iron ores with low P
contents were readily and cheaply available.
 However, in the recent past, because of high iron ore prices,
lower priced iron ores from sources which normally have higher
P content are being used and this has made P control an
important activity during the steelmaking.

Development in dephosphorization
of steel
 Research on the removal of phosphorus during steelmaking started with the
Bessemer and open hearth processes, during the late years of the 19th century.
 In 1872, study the reaction of phosphorus during the Bessemer process of
steelmaking. Use of a basic lining, based on lime and magnesia, for the removal of
phosphorus from high-phosphorus pig irons.
 In 1930, the concept of two-stage blowing were adopted, where silicon was first
oxidised in a vessel with acidic lining and the desiliconised metal was then transferred
to a second vessel with a basic lining for oxidation of phosphorus and carbon.
 In 1940, Larsen gives the idea about duration of holding molten metal in liquid form
and temperature for dephosphorization.
 Colclough carried out a series of experiments on the oxidation of phosphorus,
carbon and manganese from the hot metal bath in a basic open- hearth furnace and
concluded that P2O5 in the slag existed in the form of tetra- calcium phosphate
(4CaO.P2O5)

In 1940’s Balajiva et al. clearly showed that the partition of phosphorus between slag
and steel was enhanced by the presence of CaO and FeO in the slag and decreased with
increasing temperature.
RECENT DEVELOPMENT
 Nippon Steel Corporation’s Nagoya Works developed the Linz and Donawitz (LD)
converter-optimized refning process (LD-ORP), whereby Si and P were sequentially
removed from hot metal in one converter.
 The new process was put into commercial practice, and is now also used at Yawata
and Kimitsu Works for producing ultra-low-P steels.
 Nippon Steel Corporation developed another method of hotmetal pretreatment
using converters: the multirefining converter (MURC) process, whereby hot metal
is dephosphorized and decarburized sequentially in the same converter vessel with
deslagging in between. This process does not require vessel change after
dephosphorization, and thus allows steel refning at minimum heat loss because of
hot recycling of the slag formed during decarburization for the following charge,
internally called “inverse-slag-ﬂow refning.

1. Shukla, A.K.; Deo, B.: Department of Materials and Metallurgical Engineering, Indian Institute of
Technology Kanpur, “Mathematical modeling of Phosphorus prediction in BOF steelmaking, a
fundamental approach to produce low Phosphorus steels and ensure direct tap practice”
2. Bloom, T.A.; Fosnacht, D.R.; Haezebrouck, D.M.: Iron and Steelmaker, Vol 17 (1990), Page 35-41.
3. Yang X, Duan J, Shi C, Zhang M, Zhang Y, Wang J. A thermodynamic model of
phosphorus distribution ratio between CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al2O3-P2O5
slags and molten steel during a top-bottom combined blown converter steelmaking
process based on the ion and molecule coexistence theory. Metallurgical and Materials
Transactions B. 2011;42B:738-70.
4. Somnath Basu ; “Studies on dephosphorisation during steelmaking” School of Industrial
Engineering and Management ,Royal Institute of Technology ,Stockholm .
5. J. R. Stubbles: AISE Steel Technology, vol. 76 (12), 1999, pp. 44-50.
6. Almeida R, Vieira D, Bielefeldt W, Vilela A. MgO saturation analisys of CaO-SiO2-FeOMgO-Al2O3 slag
system. Materials Research [Internet]. 2018 [cited in 2018 june 06].
7. Ouchi K, Nakasuga T, Kimura S, Semura K. Effect of Al2O3 addition to CaO-SiO2-FeO
slags on dephopshorization behavior of hot metal. In: AISTech 2017 Proceedings.
Nashville, Tenn., USA; 2017.
REFERENCES

Effect Of CaO, FeO, MgO, SiO2 and Al2O3 Content of Slag on Dephosphorization of Steels

Effect Of CaO, FeO, MgO, SiO2 and Al2O3 Content of Slag on Dephosphorization of Steels

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