103303-8787 IJET-IJENS

International Journal of Engineering & Technology IJET-IJENS Vol:10 No:03 40

103303-8787 IJET-IJENS © June 2010 IJENS I J E N S

Molten Metal-Slag-Refractory Reactions

During Converting Process

Hady Efendy1, Mochamad Safarudin

1, Haeryip Sihombing.

2

1Fakulti Kejuruteraan Mekanikal, Universiti Teknikal Malaysia (UTeM) Malaka, Malaysia

2Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia (UTeM) Malaka, Malaysia

Correspondance : [email protected]

Abstract-- Magnesia–carbon (MgO-C) refractories are widely

used in converter because of their favorable properties such as low wetting by corrosive steelmaking slags chemical

compatibility with basic slags and better thermal properties.

The molten slag is in contact with the refractory during the

converting process, where temperatures >1450°C are common.

Local convection currents develop near the slag–refractory–nickel mette–air intersection that leads to small-scale

circulating flows that increase dissolution. In this report the

effects of dissolution of MgO-C refractory samples into nickel

matte and Fe2O3-S iO2-MgO slag were observation by optical

microscope and SEM/EDS. The experimental results show that the rate of dissolution of MgO-C refractory materials

increased with the temperature and immersion time. This

supports the assumption that the diffusion of magnesium

through the slag boundary layer formed around the refractory

samples would be the rate-determining step. The formation of a thin oxide layer at the interface is due the reaction between

magnesium vapor and the CO generated by the reaction MgO

and C in the refractory walls. The oxide inclusions formed in

the matte have been shown mainly to consist of MgO, Fe 2O3

and a mixture of them. The rate of corrosion increased with temperature and immersion time and decreased when the slag

was nearly saturated with MgO. The experimental results

confirm the assumption that the diffusion of magnesium oxide

through the slag phase boundary layer controls the corrosion

process. The corrosion mechanism seems to be the dissolution of elements in the refractory materials into the slag, followed

by penetration into the pores and grain boundaries. Finally,

grains are loosened from the refractory into the slag.

Index Term-- re-oxidation, converter, inclusions, refractory,

molten slag, corrosion rate, corrosion mechanisms.

I. INTRODUCTION

The converter removes the remaining silica, iron and

iron oxide, which are referred to collectively as slag from

the nickel matte product. This is achieved by heating the

molten matte and selectively oxidizing the iron by blowing

air through the molten liquid. The oxidation of the iron is

an exothermic reaction and release heat into the converter.

Silica flux is added which melts and together with the iron

oxide forms converter slag. The addition of scrap used to

help control the temperature of the converter content.

The converters are batch process. Furnace nickel matte

is put into the converter followed by a quantity of flux

material and scrap material from which nickel is to be

recovered. The converter are then blown (air is blow into

the molten slag-matte batch through the tuyere system). At

the completion of each blow the slag is poured out of the

converters into ladles for dumping. Then more furnace

matte, scrap and flux material is added, and the blowing

process is repeated. When the proportion of nickel in the

converter has risen to the required level, the final high

nickel converter slag is poured off and finally the converter

matte is poured off and sent to the granulation system for

finally processing and packaging.

The main chemical reactions in the converters are:

2FeS + 2O2 → 2FeO + SO2 (1)

FeO + xSiO2 → FeO (SiO2)x (2)

During this procedure a coating is formed on the

refractory material. This coating is made of slag coming

from pellet’s dust and impurities. As the thickness of the

reaction layer increase during time, the weight of the layer

involve a chipping off of big peaces of slag causing damage

to the refractory (peaces of brick crack and stay fixed to the

slag blocks). Due to this phenomenon the converter has to

be stopped every year for a maintenance period. The

converter is then cleaned and the damaged bricks are

replaced. The replacement of the bricks is an expensive

operation and it involves the complete stop of matte

production that induces a big overall cost. The composition

of converters matte is given in the following table I:

T ABLE I

THE COMPOSITION OF FURNACE NICKEL MATTE

Ni Co Fe

SiO2

MgO

S

0.173

0.027

18.7

45.5 22.5 0.22

Materials involved

This part will present the materials that have been used

in this study and give their main properties: Bricks, Slag,

SiO2 and Matte (The composition was given in table I).

Bricks

The Bricks used in the converters are made of MgO-C

refractory materials. Their composition is given in table II.

The main proprieties needed for refractory materials are

their high heat resistance, low thermal conductivity,

mechanical resistance, and thermal stresses resistance,

resistance to corrosion, resistance to erosion, liquid and gas

permeability [1].



T ABLE II

THE COMPOSITION OF MGO-C BRICK

MgO

Al2O3

CaO

SiO2

Cr2O3

Fe2O3

C

58.4 8.3 0.7 2.0 13.5 8.0 9.1

Slag

The slag, which forms on the walls of the converter, is

mainly constituted of disintegrated furnace matte, silica and

impurities. During the converting process this FeS is

transformed into FeO. FeO is one of the stable oxide forms

of iron that is finding in nature (the other one is magnetite).

It got its name from a Greek word meaning blood-like,

because of its red color. The slag is then constituted of FeO,

impurities coming from the brick, and SiO2 introduced by

the blowing. Its composition is given in the following table

III:

T ABLE III

THE COMPOSITION OF T HE CONVERTER SLAG

Ni Co Fe SiO2 MgO

0.48 0.22 51.9 25.8 3.4

We can see in this chemical analys is that the main

impurities in the Slag after FeO are SiO2 and MgO. SiO2 is

forming a glassy phase that bounds the dust together and

permits the slag to enter into the pores of the refractory

materials and be fixed.

II. SAMPLE SOURCE AND PREPARATION

Magnesia-graphite Converter slag bricks were recovered

from brick piles after lining tear-out. The compositions of

these bricks were MgO-C and slag coated samples were

specifically chosen to increase the odds of retaining slag line

reaction products. A typical pos t-mortem MgO-C brick

specimen from a converter slag line was about 20 cm long

and included a 10 to 25 mm slag coating. Magnesia grains

were generally translucent and clear white at the slag-brick

interfaces and at the bottom (cold face) of the bricks and

blackened in the interior of the bricks. The cold zones of the

bricks were also loosely held and disintegrated easily. The

bricks were sectioned perpendicular to the hot face, vacuum

impregnated with a low viscosity resin and cured at 70oC.

Both polished and polished thin sections were made from

the impregnated specimens. The final polishing was

completed with a vibrating polisher with 1 to 0.5 micron

diamond paste and lapping oil.

The papers explained the mechanism responsible for the

penetration of slag into the brick and the different

parameters that may influence the degradation of the

refractory. Using SEM and optical microscopy the

influence of slag penetration on surface brick has been

studied and the resistance of the bricks after using in the

converting process. Than concluded that a combination

between diffusion and infiltration of the slag was

responsible of the slag attack.

III. OBSERVATION AND RESULTS

Infiltration of iron slag into refractory bricks

Because pores exist in a refractory, liquids penetrate into

refractory through the open pore in contact with liquid. The

mechanism of the penetration differs a little according to the

lining orientation of the refractory or the pressure applying

on it, but the main driving force of penetration is the s uction

of liquid like molten slag due to capillarity. Fig. 1 showing

a depth penetration of slag into refractory brick.

Wet ability between a refractory and a liquid like molten

slag is an important factor influencing penetration. Also,

surface energies of refractory materials, surface tension of

molten slag, as well as interfacial energy between the solid

and liquid are factors related to penetration. Basic

refractories such as MgO which are easily penetrated may

be impregnated with tar or pitch to fill the pores, so that

residual carbon in the pores prevents wetting by molten slag.

Fig. 1. Depth Penetration Slag into Refractory Brick

Slag coating

In contact with slag (Fig. 2), a dense spinell layer is

formed in a first stage. This layer becomes enriched very

quickly in MgO on the refractory side. It is completely

transformed into magnesia after several minutes. Then the

thickness does not increase in time and remains about 150

mm. The decarburized zone has a much smaller thickness

than for the non-deoxidized grade and it does not form a

continuous layer. Metal infiltrations in the refractory have

been observed, and they increase with time. After

application in steel making process, a composition gradient

of the inclusionary cluster is observed with magnesia on the

refractory side.

Fig. 2. Photograpf of The Slag- MgO-C Refractory Interface

At the working surface, the refractory is eroded by

molten FeO, while the back of the refractory is decarburized

by air. When the working surface reached the carburized

part, the corrosion rate abruptly increases due to spalling.

The slag-refractory interface is characterized by presence of

metallic Fe beads and crystallization spinel (MgAl2O4)

crystal [2]. Metallic iron beads at the interface are always



associated with graphite in the sample and often from

oxidized magnetite and hematite rims. Such association

indicates reduction of the FeO component of the slag by

graphite to from metallic Fe and CO gas at the hot face of

refractory. The following reaction describes the observed

behavior [3],

FeO(l) + C(s) => Fe(s) + CO(g) (3)

Representative microstructures of the refractory-slag

interface for a post-mortem MgO-C are shown in Fig. 2.

Confirmation of dense layer formation on the surface of the

refractories was achieved by observation of the cross section

of the specimen after converter process (Fig. 3 and Fig. 4).

Fig. 3. Cross Section View of The MgO-C Refractory Interface

Fig. 4. Cross Section View Detail of The MgO-C Refractory Interface

Interface

The slag-brick interface is characterized by the presence

of metallic Fe beads and crystallization of euhedral spinel

(MgO.Al2O3) crystals. Metallic iron beads at the interface

are always associated with graphite in the brick and often

form oxidized magnetite and hematite rims. Such

association indicates reduction of the FeO component of the

slag by graphite to form metallic Fe and CO gas at the hot

face of the brick. The following reaction describes the

observed behavior [3],

FeO(l) (in slag) + C(s) => Fe(s) + CO(g) (4)

Blocky and euhedral spinel crystals at the interface

exhibit and form an irregular and often discontinuous chain-

like structure.

IV. DISCUSSION

The wear mechanism of refractory materials by slag is

complex phenomenon. The experimental results indicate

that apart from chemical attack of the slag an the MgO-C

refractory brick, penetration of the slag cause serious direct

loss of the MgO-C refractory brick. The dissolution rate of

MgO-C refractory brick depends upon the some factor, such

as temperature converter process and viscosity slag. The

investigation of the sample after converter process show,

that there has been a formation of inclusion on the molten

metal. The inclusions found in the molten metal were

examined using SEM/EDX. The result showed that the

inclusions contained MgO. The formation of spinel is

practically a very significant aspect of the reaction between

MgO-C refractory and slag. As the magnesium gas diffuses

into the slag the following reaction is taking place:

MgO.Al2O3(s) => Mg (s) + 2Al (s) + 4O (5)

As the initial alloys did not contain any magnesium, the

presence of MgO in the inclusions should indicate a result

from the contamination by the refractory/slag reaction.

The reaction MgO (s) + C (s) ==> Mg (g) + CO (g)

proceeds to the right at higher temperatures and Mg(g)

diffuses toward the free surface of the sample where it

encounter a higher PO2[4]. Thereafter, magnesium is

oxidized to MgO, were it condenses and forms a MgO layer.

At the same time the CO (g) formed during MgO (s)

reduction by carbon will diffuses to the interface where it

will react with the molten slag forming MgO according to

the following reaction [5]:

Mg (s) + CO (g) ===> MgO (s) + C (s) (6)

The reaction occurs immediately after the reactive CO

gas come into contact with the surface of the slag. As a

result, a thin oxide film at MgO is formed at the interface.

The formation of a surface layer will inhibit any further

oxidation by CO, by retarding the diffusion of carbon and

oxygen a cross the layer. The dissolution process in the

refractory material is supported by optical microscope and

SEM investigations of the samples. The slag penetrated the

refractory material in pores and crack. It is possible to

observe that the slag phase has a concentration gradient at

the boundary layer between slag/refractory. The corrosion

of oxide often occurs not by dissolution or evaporation of

the oxide, but by the penetration of the solid by some all the

elements from the fluid slag [6]. The liquid phase may be

pulled into the open porosity of the solid by capillary forces,

and species from the fluid will diffuse both down the grain

boundaries and into the bulk of the solid.

The higher wetting angle makes it more difficult for the

slags penetrate pores and crack in the refractory. This is not

the only think that affects the infiltrating depth. The

infiltrating depth is also affected by the temperature gradient

in the brick. The temperature gradients will cause the

viscosity to increase and then the infiltration depth will

decrease.

V. CONCLUSIONS

During the production of nickel matte the degree of

oxide inclusions partly depends on the reaction of the melt

Interface

Refractory

MgO-C

Slag



with the converter lining and the pouring system. The

refractory material may be eroded by the molten steel and

slag as well as corroded through chemical reactions with the

slag and molten steel and the deoxidation products. In this

report of dissolution of MgO-C refractory into CaO-Al2O3-

SiO2-MgO slag were examined after converting process.

The results show that the infiltration of slag into MgO-C

refractory and dissolution of MgO-C refractory on the

molten slag. This supports the assumption that the diffusion

of magnesium through the slag boundary layer formed

around the refractory samples would be the rate-determining

step. The formation of a thin oxide layer at the interface is

due the reaction between magnesium vapor and the CO

generated by the reaction MgO and C in the refractory

walls. The oxide inclusions formed in the steel have been

shown mainly to consist of MgO, Al2O3 and a mixture of

them.

REFERENCES

[1] N. P. Cheremisinoff, Handbook of Ceramics and Composites, CRC Press,

1990, ISBN 0824780051 [2] Chen Y., Brooks G., Nightingale S.,”Slag Line Dissolution of MgO

Refractory”, Canadian Metalurgical, Vol.44, pp.323 -330, 2005 [3] Camelli S., Labadie M.”Analysisi of Wear Mechanis of MgO-C Slag Line

Bricks For Steel Ladle”, International Feuerfst -Kolloqium, Instituto Argentono de Siderurgia, San Nicolas, Argentina, 2006

[4] Watanabe A., Takahashi H., and Nakatami F., “Mechanism of Dense Magnesia Layer Formation near Surface of Magnesia-Carbon Brick,”

J.Am.Ceram.Soc.69, pp 213-214, 1986. [5] Poirier J., Thillou B., Guiban M.A., and G. Provost ,” Mechanism and

Countermeasures of Alumina Clogging in Submerged Nozzles, “ 78th Steelmaking Conf. Proc., Nasville, USA, Vol.78, pp 451-456, 1995

[6] Cooper A.R.,” Kinetic of Refractory Corrosion,” Ceram.Eng. and Sci. Proc., No.2, pp 1063-1086, 1982

103303-8787 IJET-IJENS

Documents

Transcript of 103303-8787 IJET-IJENS