54 IE(I) Journal-MM

Characteristics of Foaming Slag in Smelting Reduction Processes

Dr S K Dutta, Member

R Sah, Non-member

Smelting reduction processes, without using coke, are alternative ironmaking technologies for production of hot metal. Gases, which are

generated due to reaction, cannot be removed from the reactor without foaming of the slag. Characteristics of foaming slag are important in the

smelting reduction processes. Main foaming parameters are foaming index and foam life.In this paper measurement of foaming index and

influence of additives on foam are discussed.

Keywords : Alternate ironmaking ; Non-coking coal ; Slag foam ; Foaming index ; Foam life

INTRODUCTION

Blast furnace (BF) ironmaking technology has dominated the world

scenario as most economic and widespread resource of iron used in

steelmaking. Till today, BFs have played a major role in achieving

high degree of gas utilisation. However, this dominancy of BF

technology has been facing problems due to shortage of metallurgical

coke and higher investment cost1. As shown in Table 1, the availability

of coking coal in India is limited (15.4% only of the total reserve).

while it has a huge reserve of non-coking coal2.

The need of an alternative ironmaking technology arises to

complement BF process in order to produce hot metal using non-

coking coal. Such processes are known as Smelting Reduction (SR)

Processes. The term smelting reduction is used to designate processes

for the production of hot metal without using metallurgical coke3.

Recently, the smelting reduction process, for production of liquid

iron, has received considerable attention due to its many advantages,

such as lower capital cost (due to absence of auxiliary units), high

production rate, and the diversity of charging materials4.

In most of the smelting reduction processes, coal and iron ore are

injected into an iron bath, the main reactions are the cracking of the

coal and the reduction of iron oxide in the slag phase by solid carbon

and carbon dissolved in metal. Therefore, a large amount of CO and

H2 gases are evolved when a high production rate is maintained. The

gases at the slag-metal or slag-carbon interface, as a result, form

bubbles and the volume of the slag increases extensively due to

foaming5, ie, the gases cannot come out from the reactor through

the slag phase without foaming. Slag foams are formed, when gas

bubbles entrapped in the slag can not readily coalesce and foam

Dr S K Dutta is with the Metallurgical Engineering Department, Faculty

of Technology and Engineering, M S University of Baroda, Vadodara

390 001; and R Sah is with the Mechanical Engineering Department,

Institute of Technology, Nirma University, Ahmedabad.

This paper was received on September 12, 2005. Written discussion on the

paper will be received until January 31, 2006.

comprises a system of tightly packed bubbles separated from one

another by thin films of liquid slag. In true foam, the liquid is

eliminated from the films separating the bubbles by drainage and

thus a high viscosity, by retarding the rate of drainage, tends to

stabilise the foam6. On the other hand, the energy requirement for

formation of foam increases with increasing surface tension, and

hence, low surface tensions are favourable for both formation and

durability of foam.

Foaming slag provides a large surface area and the chemical reactions

proceed more favourably. Slag foaming becomes the production rate

limiting step in the process. At a high production rate, the slag can

foam out of the reactor, which is somewhat similar to the slopping

phenomenon in oxygen steelmaking4. The foaming slag is also

important, because it is the medium for post combustion and heat

transfer, which is the key to an energy efficient process. Hence, foamed

slag plays an important role in heat transfer from the post combustion

flame to the bulk slag in the reactor. Therefore, slag foaming is

important in the smelting reduction process, and it is critical to

understand the fundamental features of slag foaming in the process.

FOAMING PARAMETERS

Foaming Index

Considerable research activities were concentrated toward

understanding the foaming behaviour of slag in the past decade, the

major contribution coming from Fruehan and co-workers4,5,7,8. They

measured the foaming behavior of different slags. To quantify the

foaming behaviour, Ito and Fruehan5 defined the foaming index

(Σ, s) of the slag as

=∑ / sgh V (1)

where h is the height (cm) of the foam at steady state when gas with

superficial velocity (sgV , cm /s) is passed through it.

The superficial gas velocity (sgV ) is defined as

= /sg gV Q A (2)

where gQ is a volumetric gas flow rate (cm3 /s) and A is cross-

sectional area (cm2) of the reactor.

The superficial gas velocity is also correlated to the void function (α),

Table 1 Coal reserves in India2

Type of Coal Reserves, Mt Percentage of Total

Coking coal 28 031 15.4

Non-coking coal 148 284 81.3

Lignite 5 978 3.3

Total 182 293 100.0

Vol 86, October 2005 55

volumetric fraction of gas, and the actual gas velocity ( gV ,cm /s)

= αsg gV V (3)

The foam height (h) is expressed as a function of void function and

foam layer thickness (L , cm)

= αh L (4)

Finally, foaming index is expressed in terms of the foam layer and

actual gas velocity as=∑ / gL V (5)

From equation (5) it is clear that foaming index means the average

gas travelling time through the foamed layer. This equation is valid

when void function (α) is independent of foam height (h), ie, void

function can be assumed as constant. The foaming index was found

to be independent of reactor size for reactor diameter greater than

3 cm and depends only on the physical properties of the slag4.

Knowing the foaming index of the slag, the gas evolution rate, and

the reactor size, the foam height in any process can be calculated.

The foaming index means the foaming ability of the slag in the

foam caused by blowing gas. So, the foaming index has been

correlated as a function of the physical properties such as the density,

viscosity, and surface tension of the liquid slag. Zhang and Fruehan7

have demonstrated that the foaming index is also inversely

proportional to the gas bubble size. For dimensional analysis, Jiang

and Fruehan4 have assumed that the foaming index is a function of

all the variables and dimensional constants that may affect the

foaming index (Σ).

Therefore,

= µ σ ρ∑ f ( , , , )bd (6)

where µ, σ, and ρ are the viscosity (g /cm-s), surface tension (g /s2),

and density (g /cm3) of slag respectively, and db is the gas bubble

diameter (cm).

Foam Life

The foam volume is determined by the balance equation9

Rate of change of foam volume = {(rate of gas generation

or injection)-(rate of volume change due to bubble rupture)} (7)

The gas bubble rupture on the top layer of foam causes a decrease in

foam volume because of gas escape. Bubble rupture inside the foam

leads to bubble coalescence and, consequently, a change in the liquid

film thickness between the bubbles and their packing. Coalescence

of bubbles also leads to a decrease in foam volume. Besides, non-

uniform bubbles, which are produced by coalescence, make the foam

unstable. Hence, the bubble rupture rate can be assumed to be

proportional to the number of bubbles. Assume that the kinetics

of bubble ruptures follow first order rate equation.

Therefore,

Rate of volume change due to bubble rupture = bk NV (8)

where k, N and bV are the rate constant for bubble rupture (s-1),

total number of bubbles, and average volume of a gas bubble (cm3),

respectively.

The total volume of foam and the bubbles volume are can be related

by:

α = /bNV V (9)

where α is the average void fraction and V is the volume of foam

(cm3).

Using equations (8) and (9), equation (7) can be written as:

= − αd /dV t Q k V (10)

where Q is the rate of gas generation or injection (cm3/s). If foam

is produced in a reactor of uniform cross sectional area, the

equation (10) can be written as:

= − αd /d sgh t V k h (11)

Foaming index is defined by equation (1), so equation (11) becomes:

d /d ( / )h t h k h= − α∑ (12)

Therefore, at steady state, equation (12) can be written as:

= α∑ 1/( )k (13)

The bubble rupture takes place due to drainage of the liquid. The

average foam life (τ, s) is defined as10

τ = ∫0(1/ ) dV t V (14)

where V0 is an initial liquid volume (cm3) in foam.

Again,

−= −0 (1 e )ktV V (15)

Therefore,

−= 0d /d e ktV t kV (16)

Combining equations (14) and (16) can be written as:

−τ = =∫ e d 1/ktk t t k (17)

Foam life (τ) is the time (s) required to drain the liquid entrapped

between two consecutive layers of bubbles, the rate constant (k) for

bubble rupture is inversely proportional to foam life (τ). Again by

combining equations (13) and (17), can be written as:

τ = α∑ (18)

Equation (18) shows the relationship between foaming index and

foam life. For an ideal slag (ie, a slag of constant void fraction) the

foaming index is equal to the average foam life.

56 IE(I) Journal-MM

MEASUREMENT OF FOAMING INDEX

Fruehan and co-workers5,8 used a molybdenum disilicide

resistance furnace for experiment. Slag was taken in an alumina

crucible (41 mm intenal diameter and 300 mm height). Argon gas

was introduced into the molten slag through an alumina tube

(1.57 mm intenal diameter). When foam height reached a steady

state level, the foam-gas interface was detected by two

molybdenum wire probes (0.76 mm φ). They observed that the

foam height increases linearly with the increasing superficial gas

velocity (Figure 1). The foaming index is obtained from the slope

of the line shown in Figure 1. Similar experiments were also carried

out at different temperatures. As shown in Figure 2, the foaming

index decreases with increasing temperature because of a decrease

in viscosity and an increase in surface tension. Similar observations

were also made by Wu, et al 11.

Ito and Fruehan5 found that the foaming index was independent

of reactor diameter (>3.2 cm) and wall effects were small. Foaming

index decreased with increasing basicity (B = CaO / SiO2) upto a

maximum (B=1.2 to 1.22) and then increased (Figure 3) at 1673K

due to presence of second phase particles (CaO or 2CaO. SiO2). The

Figure 1 The relation between foam height and gas flow rate for a 48 %

CaO - 32 % SiO2 -10 % Al2O3 - 10 % FeO slag at 1873 K

Foam

ing h

eig

ht, c

m

Superficial gas velocity, cm/s

1.50

1.25

1.00

0.75

0.50

0.25

0.000.00 0.50 1.00 1.50 2.00 2.50

Form

ing in

dex

Temperature, K

1.50

1.25

1.00

0.75

0.50

1700 1750 1800 1850 1900

Figure 2 Effect of temperature on foaming index for a slag containing

48% CaO, 32% SiO2, 10% Al2O3, and 10% FeO

CaO-SiO2-FeO-Al

2O

3

FeO=30%, Al2O

3=3%-5%

20

10

5

2

1

0 0.5 1.0 1.5 2.0

1573 K

1673 K

CaO/(SiO2+Al

2O

3)

Σ,s

Figure 3 Relation between foaming index ΣΣΣΣΣ and the basicity ratio of the

slag at 1573K and 1673 K

surface tension increased and viscosity decreased with increasing CaO

in slag. Therefore, low surface tension and high viscosity stabilised

the slag foam. On the other hand, foaming index increased with

increasing basicity, when basicity was greater than the liquidus

composition. This was because solid particles such as 2CaO. SiO2and CaO precipitated at higher CaO content and the particles

significantly increased the foam stability. Therefore, the precipitation

of second phase particles had a larger effect than the increase in surface

tension and decrease in viscosity on foam stability. Table 2 shows the

laboratory experimental data of foaming index for smelting reduction

slag at 1773 K.

Table 2 Foaming Index for Smelting Reduction Slag at 1773 K

Basicity Slag Addition to Foaming Reference

(CaO/SiO2) Composition the Slag, % Index (Σ,Σ,Σ,Σ,Σ,s)

0.50 30% CaO 10% CaF2

2.000

60% SiO2

1.00 - 5% FeO 1.400

7.5% FeO 1.200

10% FeO 0.900

12.5% FeO 0.800

15% FeO 0.750

1.25 - 0% FeO 0.600

3% FeO 1.300

5% FeO 0.900

7.5% FeO 0.800 Jiang and

10% FeO 0.800 Fruehan4

15% FeO 0.700

1.50 45% CaO 0% FeO 2.900

30% SiO2

1% FeO 2.000

10% MgO 3% FeO 1.600

15% Al2O

36% FeO 1.300

9% FeO 1.200

1.10 37.2% CaO -

33.8% SiO2

0.387

18% MgO

11% Al2O

3

1.60 41.2% CaO -

25.8% SiO2

1.073

8% MgO

15% Al2O

3Wu, et al 11

2.00 48.7% CaO

24.3% SiO2

20% CaF2

0.681

7% MgO

2.60 47.7% CaO

18.3% SiO2

20% CaF2

1.170

9% MgO

10% Al2O

3

Vol 86, October 2005 57

Hara and Ogino 13 also studied the effect of surface active components

on foaming of slag. They found that vigorous foaming appears

when the slag contains components, which stabilize the bubble,

especially surface active components such as SiO2, P2O5, and CaF2.

Figure 4 Iron ore pellet in foaming slag

Foam

Pellet

Dense-slag

Jiang and Fruehan 4 have conducted slag forming measurements in

terms of the foaming index on reduction smelting slags (CaO - SiO2- FeO, CaO - SiO2 - MgO - Al2O3 - FeO) at 1773 K and found that

the slag foam stability decreases with increasing FeO (>2 %) content

and basicity. For the slag system (CaO - SiO2 - FeO), no stable foam

was observed at very low FeO content (< 2%). As percent of FeO

increases, the slag foaming index goes through a maximum and

then decreases. Foams formed from gases, resulting from chemical

reactions on metal surfaces, have significantly smaller bubbles and

more stability.

INFLUENCE OF ADDITIVES ON FOAM

Coke can reduce slag foaming in steelmaking processes. It was reportedthat top injection of coke was very effective in controlling excessfoaming during smelting reduction of iron chrome ore. The use ofcarbonaceous particles in controlling foaming had been experimentedon 1 t smelting reduction furnace at Sakai Works, Nippon SteelCorporation, Osaka-fu, Japan as pilot scale bath-smeltingexperiments12. Zhang and Fruehan 7 observed that the foam heightwas found to decrease significantly with the increase of the ratio ofthe carbonaceous particles to that of the slag. X-ray examinationsshowed that small gas bubbles ruptured and spread over the surfaceof a coke particle present in the slag. Then several spread bubblescoalesced and evolved from the top of the coke particle as a singlelarger bubble. Wettable particles showed a completely differentbehaviour when interacting with the foaming slag. The X-ray imagesof an iron ore pellet in the foamed slag is shown in Figure 4 7. Theiron ore pellet is totally immersed in the foaming slag. The foamgrew and passed the pellet without any gas bubbles being rupturedor coalesced, even when some of the mechanical movement wasapplied to the pellet. The wettability of the solid particle with theliquid slag plays a key role in slag foaming.

Ito and Fruehan 5 studied the effect of P2O5, sulphur, MgO, andCaF2 on foaming of slag. Potassium (K), phosphorous (P), andsulphur (S) are surface active components, which lower the surfacetension of the slag. P2O5 slightly increases foaming index (Σ) whereasmarginally decreases foaming index indicating surface tension alonedoes not determine slag foamability. CaF2 decreased foaming indexby lowering the viscosity of the slag. Large addition of CaF2significantly decreases the foam stability by increasing CaO solubilityand consequently dissolving some of the second phase particles.MgO increases foaming index probably because it increases theamount of solid particles in the slag.

Zhang and Fruehan 7 found that the anti-foam effect of coke or coalchar particles was primarily contributed by the non-wetting natureof the carbonaceous materials with the liquid slag. Wu, et al 11 alsoinvestigated foaming behaviour of slag with addition of additivessuch as coal, coke, graphite and CaO. The effect of different coke sizeon the foam behaviour of slag at 1773 K is shown in Figure 5. Thefoam height increases with fine powder coke (76 µm and 105 µm)and decreases with grain coke (1 mm and 3 mm). Effect of numberof particle and size of coke on foaming index for laboratoryexperimental data is shown in Table 3 7. In the smelting reductionprocess of the thick slag layer, it is very important to keep slag heightstable without abnormal slag foaming. Adding carbonaceous materialcan control the slag foaming.

Table 3 Effect of Number of Particle and Size of Coke on Foaming

Index at 0.5 Slag Basicity7

Diameter of Coke Number of Particles Foaming Index

Particle, mm (Σ, s)

3 0 1.65

2 1.10

6 0 1.80

1 0.90

4 0.62

8 0 3.60

1 1.30

5 0.51

10 0 2.00

2 0.75

4 0.47

Added 76 µm coke

Added 110 µm coke

No additive

Added 1 µm coke

Added 3 µm coke

Fo

am

he

igh

t, ∆

h, cm

Flow rate,V, cm-s-1

6

5

4

3

2

1

00 1 2 3 4

Figure 5 Effect of various coke sizes on form behavior of sample slag at

1773 K

58 IE(I) Journal-MM

The mechanism of the stabilization of the foam is considered to be

a surface tension driven flow, namely, the Marangoni effect. This

effect also plays an important role in the suppression of foaming by

coke addition.

SUMMARY

The slag foaming is an important factor for the smelting reduction

process. To quantify the foaming behavior, the foaming index (S) of

the slag is measured. The foaming index means the foaming ability

of the slag in the foam caused by injecting of gas. For an ideal slag (ie,

a slag of constant void fraction) the foaming index is equal to the

average foam life. The control of the foaming index is required for

steady state operation in the smelting reduction process.

Foaming index decreases with increasing basicity up to a maximum

and then increases due to presence of second phase particles (CaO or

2CaO.SiO2). The slag foam stability decreases with increasing FeO

content and basicity. The foaming index decreases with increasing

temperature because of a decrease in viscosity and an increase in

surface tension.

The anti-foam effect of coke or coal char particles was primarily

contributed by the non-wetting nature of the carbonaceous materials

with the liquid slag. Adding carbonaceous material can control the

slag foaming. Wettability between the particle and the slag is the

key factor in determining the ability of the particle to control

foaming of slag.

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