Experimental Investigation on Reduction Kinetics of Stainless Steel-making slag in Iron bath Smelting Reduction

ZHANG Bo1, NIU Shuai1, CHEN Wenbin1, LI Wencai1, CHEN Fatao1, LI Tao1, LIANG lisheng2, HONG Xin1a

1Shanghai Key Laboratory of Modern Metallurgy & Materials Processing,

Shanghai University, Shanghai China

2Baosteel co., Ltd, Shanghai China

aRoom 417 Building Rixin Shanghai University NO.149 Yanchang Road Shanghai China,

E-mail: xhong@mail.shu.edu.cn

Key Words: stainless steel-making slag, smelting reduction, reduction rate, iron bath

Abstract: Reduction kinetics of stainless steel slag in iron bath smelting reduction was studied at

the temperature of 1500℃ ~ 1650℃. It was concluded that the reduction process consisted of two

parts. That is to say smelting reduction was controlled by stainless steel slag melting initially and by

interface reaction later. In order to increase smelting reaction rate, the melting point of slag should

be decreased at the first stage and adjust the liquidity of slag at later stage. Smelting reaction rate

will be accelerated by means of optimize the slag content. The optimal reduction result that all most

all of the chromium in slag been recovered was obtained in temperature was 1500℃, basicity of

slag was 1.0~1.2, the value of Al2O3+MgO was 25%.

1. Introduction

The stainless steel industry in China has developed rapidly in the 21st century[1]

. Except small

amount slag been recovered and reused, stacking and landfilling were adopted as the main treatment

for most of the slag. On the other hand, surface and underground water will be polluted by the

leaching solutions of chromium slag.

According to former studies[4][5]

, stainless steel-making slag could be reduced quickly by

controlling the conditions of temperature and basicity, using carbon as reducing agent in iron bath.

The experimental results showed that temperature and basicity had a critical influence on reduction

rate of Cr2O3. In the appropriate experimental conditions, the residual chromium content in the final

slag was reduced to 0.05% and the value of recovery rate of chromium reached 97%.

2. experimental equipment and material

As shown in Fig.1, experimental equipment consist of induction furnace, heating element,

water cooling system, bottom stirring element and gas flux controller etc. The furnace was

designed for work at high temperature and production of 10 kg grade.

Fig.1 Experimental apparatus

Advanced Materials Research Vol. 721 (2013) pp 164-168Online available since 2013/Jul/31 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.721.164

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Stainless steel-making slag generated from electric arc furnace was used as experimental

material. High purity nitrogen was used as experimental bottom blowing gas.

The basicity of slag which was melted was 1.0~1.6. The temperature of the reaction was

1500~1650℃. Nitrogen was blown into the furnace from bottom all the time. 10kg Fe contain

approximately 4% [C] was added into the furnace. After iron smelted, 1kg slag was added into the

furnace. Slag sample and iron sample were collected every 2 minutes. Composition of the slag was

shown in table 1.

Tab.1 Composition analysis of stainless steel-making slag

material Cr% Fe% Al% Ca% Si% Mg%

content 3.58 3.52 3.77 54.67 18.60 11.90

As shown in fig.2, the main elements in the slag were Ca, Si and Mg. Chromium existed

mainly as the structure of Cr2O3 and FeCr2O4.

Fig.2 XRD diffraction patterns of slag samples

As shown in table 2, parameters were designed for the experiments.

Tab.2 Parameters of different experiments

No. basicity Al2O3/% (Cr)/% [Cr]/% Bottom blow

flux /L/h

1(A) 1.2 10% 5% 0% 750

2 1.2 10% 10% 0% 750

3 1.2 10% 5% 5% 750

4(D) 1.2 10% 5% 10% 750

5 1.2 10% 5% 0% 750

6 2.0 10% 5% 0% 750

7(C) 1.0 10% 5% 0% 750

8 1.4 10% 5% 0% 750

9(E) 1.6 10% 5% 0% 750

10 1.8 10% 5% 0% 750

11(B) 1.2 15% 5% 0% 750

12 1.2 20% 5% 0% 750

13 1.2 25% 5% 0% 750

3. Experimental result analysis

As shown in fig.3, residual chromium in sample A and B was decreased to 0.25% after 10

minutes. And the value could reach 0.18% in sample C. But residual chromium in sample E was

much higher than other samples, mainly because the basicity of sample E was higher than other

Advanced Materials Research Vol. 721 165

experiments which influenced the smelting process of slag greatly. Smelting point of the slag

increased with the increase of basicity. At the same time, a transitory saturated state appeared when

smelting rate higher than the reduction rate.

0 2 4 6 8 10 12 14 16 18 20 22

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Resi

du

al

ch

rom

ium

/wt%

Time/min

A

B

C

D

E

Fig3. Relationship between residual chromium and time

Reduction of stainless steel-making slag was controlled with melting of slag, transmission of

chromium oxides in the slag, transmission of [C] in liquid iron, boundary chemical reaction,

effusion of generated carbon monoxide and transmission of [Cr] in liquid iron[6]

.

In this study, transmission of [C] and [Cr] in liquid iron were not the control sections for

abundant molten pool and intensive stirring of bottom blowing. Same as the effusion of generated

carbon monoxide was not the control section. It could conclude that the reduction of chromium in

slag was controlled with smelting and boundary reaction at the first step. For the second step,

transmission of chromium oxides in the slag would be the control section with the decrease of

concentration of chromium oxides in the slag.

Reduction rate of stainless steel-making slag could be expressed with Eq.1.

2 3

2 3 (1)

Cr O n

Cr O

dwkw

dt=−

2 3Cr Odw

dt—Reduction rate of stainless steel-making slag, n—reaction order, k—reaction rate constant

As shown in fig.4, the zero point means original values of ln([Cr]+(Cr)). Reaction rate was

very quickly at the initial reaction stage. But several minutes later, reaction rate decreased obviously

because of slag could not melt for a lot of heat absorbed with the reduction. This kind of situation

was obviously exist in high chromium content of sample D and high basicity of sample E.

0 2 4 6 8 10 12 14 16 18 20 22

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

ln(( (( w

Cr

2O

3)) ))

Time/min

A

B

C

D

E

Fig.4 Residual chromium in slag as a function of reaction time

166 Recent Advancement on Material Science and Manufacturing Technologies

Taken sample A as example, smelting reduction process consisted of two parts as shown in

fig.5. The regression formulas of two parts could get as shown in Eq. 2 and 3.

0 2 4 6 8 10 12

0

1

2

3

4

5

Ln

(wC

r2O

3)

time / min

I

II

I

II

Fig.5 Linear fit of sample A

2 3 2 3

2 3 2 3

2 3

2 3 2 3

0

0.976

0

: ln( / ) 0.976

0.976

Cr O Cr O

t

Cr O Cr O

Cr O

Cr O Cr O

w w t

w w e

dwkw w

dt

i

Ⅰ

−

− =

=

− = =

2 3 2 3

2 3 2 3

2 3

2 3 2 3

0

0.13

0

(2)

: ln( / ) 1.630 0.13

1.630

0.13

Cr O Cr O

t

Cr O Cr O

Cr O

Cr O Cr O

w w t

w w e

dwkw w

dt

i

Ⅱ

−

− = +

=

− = = (3)

The constant reaction rates were 0.976 and 0.13 of I and II part separately. Constant reaction

rate of the first part was obviously higher than the later part mainly because the concentration of

chromium was higher. On the other hand, temperature decreased and slag crust formed would

influence the reaction rate greatly during the II part of reaction. Constant reaction rates of all

samples could be seen in Tab.3.

Tab.3 Constant reaction rates of all samples

samples Canstant reaction rates k / -

I (reaction control) II (mix control)

A 0.976 0.130

B 0.945 0.123

C 0.888 0.109

D 0.638 0.003

E 0.823 0.009

It could be concluded that the reaction rate was controlled with boundary reaction during I part

and mix control during II part. Smelting reduction kinetic conditions was not good enough when the

content of chromium in slag higher than 10% and basicity higher than 1.6. But smelting process of

slag would be enhanced with basicity and content of Al2O3 increased suitable. That was to say the

reaction rate of smelt reduction was determined with formation of liquid slag during the first part of

reaction. And the formation of liquid slag was determined with slag system and reaction

temperature.

Advanced Materials Research Vol. 721 167

4. Conclusions

The possibility of smelt reduction using stainless steel-making slag was proved through 10kg

grade induction furnace.

1) Kinetics of smelt reduction was greatly improved with ideal reaction temperature, slag

system and intensive stirring. Reduction rate of chromium was higher than 90% in 15 min.

2) The system of slag and ideal reaction temperature were the important factors for the

reduction reaction. According to the study, the optimal reduction reaction rate could get at the

condition of lowest residual chromium in slag with the reaction temperature was 1500℃, basicity

was 1.0~1.2, the value of Al2O3+MgO was 25%.

5. Acknowledgements

The authors acknowledge the financial support of the National Natural Science Foundation of

China (Grant No. 50634040) and Chinese National Key Technology R&D Program (Grant No.

2006BAE03A12).

6. References

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bulletin[N]. Vol.1,2011:28-29.

[2] HU Yong, QUAN Xuejun, WANG Wanneng, The Treatment of Pollution of Chromium Dregs

and the Present Situation of Its Use. Journal of Chongqing Institute of Technology[J]. Vol.18, No.5,

2004: 42-44

[3] Ken-ichiro. Effect of slag properties in reduction rate of chromium oxide in Cr2O3 containing

slag by carbon in Steel [J]. etsu to Hagane. Vol.88, No.12, 2002: 25-28.

[4] LIU Zhipeng, MAO Jiajun, LI Qiuju, HONG Xin, Experiment on Reduction of Chromium

Oxide in Stainless Steel Slag. SHANGHAI METALS[J]. 2009, 31(6):19-21.

[5] MAO Jiajun, CHEN Wenbin, WANG Qiang, HONG Xin, Smelting Reduction of Slag from

Stainless Steelmaking and the Toxicity Leaching Test, SHANGHAI METALS[J]. 2011, 33(4):

44-47.

[6] Stach M KKazuno M KKatayama H G. Smelting reduction behavior of synthetic chromite in

molten slag. Trans IS IJ, 1988, 28-29

168 Recent Advancement on Material Science and Manufacturing Technologies

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