TECHNICAL PAPER TP 2664

Removal of Phosphorous from Steel Produced by Melting SpongeIron in Induction Furnace

Swaren S. Bedarkar • Ram Singh

Received: 28 December 2011 / Accepted: 18 February 2013 / Published online: 22 March 2013

� Indian Institute of Metals 2013

Abstract In secondary route of steelmaking, production

through induction melting furnace contributes about 31 %

of India’s total steel production. The main raw materials

used are steel scrap, cast iron and sponge iron. In India,

majority of the induction furnaces are operated using acidic

lining of silica based ramming mass to produce structural

steel where basicity cannot be maintained to remove

phosphorous and sulphur. In the present work, efforts are

being made to generate the experimental data for refining

of steel in 750 kg induction furnace. The basicity is

maintained by addition of CaO and MgO in the form of

flux. The slag is made oxidizing in nature by addition of

sponge iron in the bath. Oxidation potential of the slag is

fulfilled by presence of FeO in sponge iron. In every single

heat, samples of slag and metal are collected. The degree of

dephosphorization obtained is as high as 82 %.

Keywords Induction furnace � Phosphorous �Sponge iron � Basicity

1 Introduction

Modern steelmaking has been divided into two categories

namely primary route of steelmaking and secondary route

of steelmaking. The steel produced using iron ore as a raw

material in its initial stage, is considered as primary route of

steelmaking. The process in which steel is produced using

scrap is known as secondary route of steelmaking. The main

furnaces that are used to produce steel through secondary

route are electric arc furnaces and induction melting furnaces

(IMF). In induction furnace category, coreless induction

furnaces are very popular. As per the report published by

Ministry of Steel, Government of India [1], total production

of crude steel of India for the year 2009–2010, is 64.87

million MT. Out of 64.87 million MT, 31 % steel was pro-

duced using induction furnaces. The main raw material for

induction furnaces is steel scrap, cast iron and sponge iron.

India is the only country where use of sponge iron contributes

a large share in annual crude steel production. The amount

of sponge iron in the charge mix varies from 0 to 90 %

depending on its availability and economics of production.

Majority of the steel produced through induction furnace

route is plain carbon steel and construction quality steel [2].

The main limitation in maintaining quality of con-

struction steel is controlling the quantum of phosphorus in

steel produced through induction furnace route. In this

route, the steel retains phosphorous in the range of 0.045 %

to as high as 0.09 % depending on the quality of raw

materials. The main source of phosphorous in induction

furnace is sponge iron and cast iron, the quality of which is

directly related to quality of iron ore.

The presence of phosphorous in steel is detrimental to its

quality and therefore required to be maintained below a

specified amount. The removal of phosphorous takes place

by oxidation. The product of the oxidation is held in

combination with basic constituents in the slag. The extent

of the removal is governed by equilibrium condition which

is characterized by the metal and slag compositions. Effect

of temperature is also important [3].

In other primary steelmaking furnaces, phosphorous is

removed using direct oxygen lancing in the bath. Refining

the steel with oxygen lancing in induction furnace is dif-

ficult as the furnace is operated under full volume condi-

tion. The next major limitation with induction furnace steel

production in India is that almost all the furnaces are

S. S. Bedarkar (&) � R. Singh

Electrotherm (India) Limited, Ahmedabad, Gujarat, India

e-mail: swarenbedarkar@gmail.com

123

Trans Indian Inst Met (June 2013) 66(3):207–211

DOI 10.1007/s12666-013-0244-z

operated with acidic or silica lining, in which it is difficult

to maintain the basicity of the slag. Magnesia based basic

lining and alumina based neutral lining have also been used

in induction furnaces. Basic linings are more popular in

foundry based induction furnaces with the heat size less

than 5T. Alumina based lining in induction furnaces have

been tried in a few furnaces. The main limitation with

alumina lining is its cost which is almost 10–15 times

higher compared to silica. Both the linings, magnesia based

and alumina based, may become popular on acceptance of

refining of steel in induction furnace in terms of phos-

phorous and sulphur removal. Without the regular use, it is

difficult to comment on their lining life and economics of

steel production. Thorough mixing of slag with liquid

metal bath is also critical in IMF. It is important to improve

the understanding of the process from metallurgical point

of view.

Despite few efforts [2, 4–6], not much data is available

where physical chemistry of slag-metal reactions and

measurements of thermochemical properties are discussed

for IMF steelmaking process. Thus, the main objective of

present work was to generate the experimental data useful

to industrial scale IMF steelmaking. Efforts are being made

to remove phosphorous from steel melted in coreless

induction furnace. The charge mixes used are cast iron,

sponge iron and steel scrap. Amount of sponge iron in the

charge mix is considered 50 %. Degree of dephosphor-

ization is discussed in terms of phosphorous distribution

ratio. In subsequent work, phosphate capacity and chemical

equilibria of dephosphorization reaction will be discussed.

2 Experimental Set Up

Experiments were carried out in 750 kg/350 kW coreless

induction furnace of Electrotherm (India) Limited make.

The furnace was lined with basic ramming mass. The main

constituents of the lining material were MgO 70 %, SiO2

8 % and Cr2O3 8 %. Temperature was measured using

platinum–platinum rhodium thermocouple. The charge

mixes used were steel scrap, cast iron and sponge iron. The

proportion of charge mix was sponge iron 50 %, steel scrap

35 % and cast iron 15 %. The chemical composition of

the raw materials is mentioned in Table 1. The optical

emission spectrometer of make Shimadzu Analytical

(I) Private Limited, model OES-5500II, was used to

determine chemical composition of steel scrap, cast iron

and the processed steel. Sponge iron analysis was carried

out by wet chemical analysis, as per IS 10812:1992. Slag,

lime and dolomite compositions were determined by the

equipment based on X-ray fluorescence technique of make

Oxford, model—Lab-X3500. All the equipments were

calibrated prior to their use.

Figure 1 depicts the induction furnace used in the

experiments along with its dimensions. Initially, all cast

iron scrap was melted. A mixture of lime and dolomite

was used as a flux material. Required quantity of flux was

charged. A mixture of steel scrap and sponge iron was then

added in the furnace in five equal steps. Total processing

time is 210 min for one heat. Time to time samples of

metal and slag were collected. During processing, the

bath was stirred properly to allow maximum slag metal

reactions. Temperature was controlled to 1580 �C for

sampling. Required quantity of sponge iron was melted.

Table 1 Chemical composition of raw materials

Cast iron (%) Steel scrap (%) Sponge iron (%)

Metallic charge

C 3.60 C 0.20 C 0.14

S 0.03 S 0.05 S 0.03

P 0.095 P 0.05 P 0.07

Si 2.00 Si 0.25 Fe-T 88.24

Mn 0.45 Mn 0.50 Fe-M 76.24

SiO2 ? Al2O3 8.50

Lime (%) Dolomite (%)

Flux

CaO 82.00 CaO 52.00

MgO 3.00 MgO 33.00

SiO2 3.00 SiO2 3.00

Al2O3 0.50 Al2O3 0.50

S 0.10 S 0.10

37591

5460610

addition of sponge iron,cast iron,flux

induction coil

liquid bath slag

refractory

and steel scrapactual metal level

Fig. 1 Schematic of coreless induction furnace

208 Trans Indian Inst Met (June 2013) 66(3):207–211

123

Sampling of slag and liquid metal was done time to time

during complete heat.

3 Result and Discussion

In primary steel making, oxygen is used as an agent for

oxidation of dissolved impurities like C, Si, Mn and P. To

some extent, iron itself is also oxidized during the process.

The process is also known as oxygen steelmaking. On the

other hand, in induction furnace steel making, no direct

oxygen is introduced in the bath. The impurities are oxi-

dized by introducing FeO in the bath. The requirement of

FeO for slag formation and oxidation of various elements is

fulfilled by addition of sponge iron. Thus, presence of FeO

is important as it corresponds to oxygen potential of the

slag.

Removal of phosphorous takes place by oxidation. The

product, phosphorous pentoxide is being held by basic

constituents, like CaO, present in the slag. In steel making

processes basicity of the slag is maintained by addition of

calcined lime. The reaction may be given by,

P½ � þ 5=2 O2½ � þ 3=2 O�2� �

¼ PO�34

� �ð1Þ

The oxidation reactions for other elements may be

written as,

Si½ � þ 2 FeOð Þ ! SiO2ð Þ þ 2 Fe½ � ð2ÞMn½ � þ FeOð Þ ! MnOð Þ þ Fe½ � ð3ÞC½ � þ FeOð Þ ! COf g þ Fe½ � ð4Þ

In the present work, dephosphorization experiments have

been carried out as discussed in previous section. Total five

heats were processed using cast iron, steel and sponge iron.

During each individual heat, the samples of slag and liquid

metal have been collected at 1580 �C bath temperature.

Induction furnace provides precise control over input

power; hence required bath temperature can be maintained.

The results are given in Table 2. The results show chemical

analysis of metal and slag samples. From the data it can be

observed that degree of dephosphorization varies from

57 % to as high as 82 % for various heats.

The basicity for the process is mentioned in terms of

V-ratio (CaO/SiO2) in Table 2. It can be observed that in

the initial stage of any experiment, basicity is low and it

increases towards the end of the experiment. The reason

may be attributed to the decrease in silicon content as the

experiment progresses. Opening silicon in the bath is high

but with addition of sponge iron, silicon oxidizes as it

contains FeO. The slag is removed from the bath time to

time. Further gangue of sponge iron adds silica in the bath.

The ultimate result of the process is decrease in silica in the

slag which increases basicity.

Borovsky [7] showed that phosphorous distribution ratio

i.e.(P2O5)/[P] or Up required higher basicities and FeO

content, lower SiO2 and Al2O3, and very low P2O5. How-

ever, FeO content in the slag is needed to be 15–35 % for

effective dephosphorization. Another very important con-

dition for dephosphorization is lowest possible temperature

[8]. In IMF operation, slag is removed from furnace top.

Many a times it is a manual process. Increase in basicity

leads to increase in slag volume. Large slag volume makes

IMF operation difficult. Moreover, upper surface of the

slag remains in contact with atmosphere which decreases

slag temperature. Hence the slag is continuously churned

by rod from the top. Considering these conditions, basicity

in the present experiments is kept in the range of 1.7–2.0.

Figure 2 depicts the removal of various elements from

liquid steel bath for heat No.1. For particular experiment,

total time is measured. Time while sampling is not mea-

sured. Efforts were made to divide total time in equal time

intervals while sampling. Hence in Fig. 2, the data is pre-

sented as sample number. It can be observed that, silicon is

eliminated in the early stage of slag metal reactions as its

oxide is most stable compared to the oxides of other ele-

ments present in the bath. Removal of silicon is followed

by Mn, C and P. During the process, change in sulphur

chemistry is also observed, but it can be attributed to the

change in concentration by addition of other metallic

charge in the bath such as steel scrap and sponge iron. The

favourable conditions for removal of sulphur from steel

bath are presence of reducing slag, high basicity and high

temperature [9]. In the present work slag is oxidizing in

nature as the minimum FeO content is about 16 %. Thus,

present work maintains conditions for phosphorous

removal.

Figure 3 depicts the effect of FeO content of slag on

phosphorous distribution ratio, Up. The values obtained in

the present work are in the range of 6–35. A slight increase

has been observed in phosphorous distribution ratio with

increase in FeO levels in slag. FeO content in the slag

varies from 15 to 33 %.

The same Up has been plotted as a function of %C in the

bath as shown in Fig. 4. The data confirms the increase in

phosphorous distribution ratio with decrease in carbon

level. It can be noted that for IMF steelmaking, phospho-

rous distribution ratio is about one order of magnitude less

compared to BOF steelmaking [10]. The reason may be

attributed to vigorous stirring in case of BOF.

For effective slag-metal reactions, the bath needs to be

continuously stirred. In induction furnace, the metal bath is

stirred by eddy currents. The slag, being lighter in weight,

always remains above the metal level. As IMFs are oper-

ated under full volume conditions, the slag layer comes in

contact with atmospheric air which cools it down and

temperature is decreased. To overcome this problem, the

Trans Indian Inst Met (June 2013) 66(3):207–211 209

123

slag layer is forcefully immersed in the liquid bath con-

tinuously. These operating conditions restrict the refining

capability of steel bath, which may give lower Up values.

With continuous experiments, more data related to phys-

ico-chemical properties of IMF steel making can be gen-

erated and the practice can be improved.

Table 2 Chemical composition of various samples and corresponding slag

Heat no. Sample no. %C %S %P %Si %Mn Degree of P

removal (%)

%MgO %SiO2 %P2O5 %FeO %CaO %Al2O3 V-ratio

Heat-1 Sample-1 1.22 0.040 0.087 0.80 0.40 68.97

Sample-2 0.99 0.034 0.085 0.23 0.23

Sample-3 0.55 0.028 0.071 0.04 0.11 7.42 23.74 0.626 26.40 26.30 13.70 1.11

Sample-4 0.25 0.037 0.048 0.04 0.08 6.19 25.05 0.589 28.06 22.88 15.62 0.91

Sample-5 0.08 0.038 0.027 0.04 0.06 7.3 17.34 0.653 30.64 28.13 14.15 1.62

Heat-2 Sample-1 1.45 0.047 0.089 1.22 0.40 77.53

Sample-2 1.11 0.049 0.086 0.25 0.23 6.89 30.22 0.808 18.20 26.80 15.29 0.89

Sample-3 0.57 0.040 0.064 0.04 0.08 10.43 26.46 0.674 19.07 32.23 11.35 1.22

Sample-4 0.08 0.042 0.044 0.04 0.07 7.91 19.48 0.527 30.87 25.55 13.36 1.31

Sample-5 0.03 0.036 0.020 0.04 0.06 6.52 14.99 0.574 31.80 30.75 13.24 2.05

Heat-3 Sample-1 0.85 0.045 0.077 0.45 0.31 57.14

Sample-2 0.65 0.044 0.069 0.06 0.19 2.35 27.09 0.833 16.87 34.33 17.12 1.27

Sample-3 0.48 0.053 0.068 0.04 0.08 3.09 23.78 0.625 19.65 30.71 19.22 1.29

Sample-4 0.20 0.056 0.056 0.04 0.07 3.22 18.26 0.483 22.74 30.84 23.25 1.69

Sample-5 0.10 0.064 0.047 0.04 0.06 3.02 19.83 0.798 19.16 34.47 21.36 1.74

Sample-6 0.05 0.067 0.033 0.04 0.05 3.44 16.68 0.819 23.92 36.30 17.38 2.18

Heat-4 Sample-1 1.40 0.040 0.097 0.770 0.23 82.47

Sample-2 0.97 0.046 0.092 0.040 0.11 6.14 30.12 0.708 22.36 25.97 13.36 0.86

Sample-3 0.42 0.050 0.053 0.040 0.06 7.19 22.88 0.737 20.82 33.62 13.80 1.47

Sample-4 0.12 0.040 0.023 0.064 0.07 7.47 21.40 0.504 26.49 31.13 12.02 1.45

Sample-5 0.05 0.068 0.021 0.040 0.06 8.15 19.57 0.698 25.43 32.04 13.07 1.64

Sample-6 0.03 0.060 0.017 0.040 0.07 7.34 17.71 0.601 29.40 31.17 12.70 1.76

Heat-5 Sample-1 0.88 0.045 0.081 0.55 0.37 66.67

Sample-2 0.68 0.039 0.075 0.13 0.21 6.38 25.75 0.594 19.86 27.87 16.67 1.08

Sample-3 0.43 0.038 0.068 0.04 0.10 6.78 24.11 0.593 23.98 26.25 16.37 1.09

Sample-4 0.22 0.038 0.050 0.04 0.08 10.09 19.05 0.675 16.71 35.45 16.93 1.86

Sample-5 0.09 0.046 0.034 0.04 0.06 7.49 18.70 0.697 25.96 30.13 15.49 1.61

Sample-6 0.05 0.049 0.027 0.04 0.07 7.25 15.94 0.726 27.46 30.79 16.38 1.93

Fig. 2 Removal of various elements from IMF steel bath

Fig. 3 Change in phosphorous distribution ratio as a function of FeO

in slag

210 Trans Indian Inst Met (June 2013) 66(3):207–211

123

4 Conclusion

Though IMF steelmaking is not as popular as primary

steelmaking process, the metallurgical aspects of the pro-

cess are needed to be studied. Despite unavailability of

sufficient data of IMF steelmaking, authors have made

sincere efforts to study the process. In the subsequent work

authors will be discussing chemical equilibria and phos-

phate capacity for dephosphorization in IMF. Authors

also feel that development of melting practices with basic

or neutral linings are very important from the point of

removal of phosphorous or sulphur from the steel. Stirring

of slag and metal in IMF is also important.

Following points can be concluded from the present

work.

• A mixture of sponge iron, cast iron & steel scrap can be

successfully melted in the furnace lined with MgO

based lining.

• Phosphorous can be removed in IMF by maintaining

required basicity and FeO in the slag.

• In the present work, maximum 82.47 % dephosphor-

ization has been achieved.

• Basicity of the slag is maintained by addition of CaO

and MgO in IMF

• Requirement of FeO in slag can be adjusted by the FeO

present in sponge iron.

• Phosphorous distribution ratios obtained for the present

studies are almost one order of magnitude less compared

to BOF steel making.

References

1. Ministry of Steel, Government of India, Annual Report,

(2010–2011) 23.

2. Jiang T, Qiu G, Xu J, Zhu D, and Singh R, Direct Reduction ofComposite Binder Pellets and Use of DRI, Electrotherm (India)

Limited, Ahmedabad (2007) 238.

3. Deo B and Boom R, Fundamentals of Steelmaking Metallurgy,

Prentice Hall International, UK (1993) 73.

4. Dutta S K, Lele A B, and Pancholi N K, Trans Ind Inst Met, 57(2004) 467.

5. Arato T, Uchida T and Omori Y, ISIJ, 25 (1985) 326.

6. Gandhewar V R, Bansod S V and Borade A B, Int J Engg Tech, 3(2011) 277.

7. Borovsky T., Kijak J. and Domovec M., Acta Meta. Slovaca, 16(2010) 165.

8. Ghosh A and Chatterjee A, Ironmaking and Steelmaking, PHI

Learning Private Limited, New Delhi, India (2011) p 277.

9. Turkdogan E.T., Fundamentals of Steelmaking, The Institute of

Materials, London (1996) p 236.

10. Turkdogan E.T, Trans ISIJ, 24 (1984) p 591.

Fig. 4 Variation of phosphorous distribution ratio as a function of

C in steel bath, exclusively for IMF steelmaking

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