Applications of MÃ¶ssbauer Spectroscopy in the Steel Industry

12 Applications of Mössbauer Spectroscopy in the

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 7, 2016 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch012

Steel Industry G. P. H U F F M A N

and F . E. H U G G I N S

U.S. Steel Corporation, Research Laboratory, Monroeville, P A 15146

Applications of

Mössbauer

spectroscopy in three principal

areas of interest to the steel industry are reviewed: (1) characterization and processing of iron ore; (2) investigations of coal and its derivatives; and (3) analysis of steel products. The ore section discusses the use of

Mössbauer

spectroscopy

for characterization of raw and processed iron ores, with emphasis on the direct-reduction process. Topics reviewed in the coal section include

Mössbauer

determinations of

pyritic sulfur and iron-phase mineralogies in coal, and in vestigations of mineral transformations during weathering, coking, and combustion.

In the steel section, traditional

topics such as the iron-carbon system are reviewed briefly, and more detailed discussions are given of recent work on internally nitrided high-strength low-alloy ( H S L A ) steels, and electron and x-ray re-emission

Mössbauer

studies of

surface and subsurface oxidation.

T r o n i s a n i m p o r t a n t , u s u a l l y d o m i n a n t , constituent o f most m a t e r i a l s o f interest t o t h e steel i n d u s t r y . I t is therefore n o t s u r p r i s i n g t h a t M o s s b a u e r s p e c t r o s c o p y finds m a n y a p p l i c a t i o n s o f b o t h a f u n d a m e n t a l a n d p r a c t i c a l n a t u r e i n steel research.

I n this c h a p t e r w e w i l l

summarize

s o m e o f these a p p l i c a t i o n s . T h r e e p r i n c i p a l areas w i l l b e c o v e r e d . applications

of Mossbauer

spectroscopy

T h e first section

describes

i n the characterization a n d

p r o c e s s i n g o f ores, w i t h emphasis o n studies r e l a t e d to t h e d i r e c t r e d u c t i o n of i r o n o r e . T h e s e c o n d section deals w i t h t h e p r i m a r y f u e l source o f t h e steel i n d u s t r y , c o a l , a n d i t s i m p o r t a n t d e r i v a t i v e , c o k e .

Several

p r o b l e m s o f c u r r e n t c o n c e r n i n t h e steel i n d u s t r y a r e d i s c u s s e d i n t h i s 0065-2393/81/0194-0265$09.25/0 © 1981 American Chemical Society Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

266

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

section, i n c l u d i n g M o s s b a u e r i n v e s t i g a t i o n s of o x i d i z e d or w e a t h e r e d c o a l , studies of m i n e r a l t r a n s f o r m a t i o n s t h a t o c c u r d u r i n g c o k i n g a n d t h e i r r e l a t i o n to c o k e q u a l i t y , a n d the p i v o t a l r o l e p l a y e d b y i r o n i n c o n t r o l l i n g ash metling and deposition d u r i n g high-temperature combustion. final

section deals w i t h M o s s b a u e r studies of steel p r o d u c t s .

The

Because

t h e r e is o b v i o u s l y a v e r y w i d e r a n g e of a p p l i c a t i o n s i n this area, this section focuses p r i m a r i l y o n r e l a t i v e l y r e c e n t investigations i n t h e area of surface a n a l y s i s , p r e c i p i t a t i o n s t r e n g t h e n i n g , a n d i n t e r n a l solute o x i d a


tion.

M o r e t r a d i t i o n a l areas, s u c h as studies of the i r o n - c a r b o n system

a n d q u a n t i t a t i v e p h a s e analysis of steels, are d i s c u s s e d o n l y b r i e f l y , b u t extensive references are g i v e n .

Characterization and Processing of Iron and Other Ores R a w a n d Beneficiated Ores.

I r o n ore m a y b e d e f i n e d as a n y n a t u

r a l l y o c c u r r i n g i r o n - b e a r i n g f o r m a t i o n t h a t contains i r o n i n sufficient q u a n t i t y a n d satisfactory f o r m to a l l o w its e c o n o m i c r e c o v e r y .

Therefore,

w h a t constitutes a s u i t a b l e i r o n o r e s i g n i f i c a n t l y d e p e n d s o n s u c h e c o n o m i c factors as the p r o x i m i t y of s t e e l - m a k i n g f a c i l i t i e s a n d t h e a v a i l a b i l i t y a n d cost of f u e l a n d t r a n s p o r t a t i o n . M o s t of the i r o n ore c u r r e n t l y m i n e d contains f r o m 25 to 5 0 %

iron.

C o n s e q u e n t l y , c o n c e n t r a t i o n of t h e i r o n - b e a r i n g phases, b y s u c h m e t h o d s as m a g n e t i c s e p a r a t i o n o r w a s h i n g a n d flotation, is u s u a l l y r e q u i r e d

(I).

T h e t y p e of b e n e f i c i a t i o n processes r e q u i r e d f o r a p a r t i c u l a r ore d e p e n d s o n its i r o n - p h a s e d i s t r i b u t i o n , w h i c h is r e a d i l y d e t e r m i n e d f r o m b a u e r spectroscopy.

Moss

S p e c t r a of s o m e t y p i c a l ores are s h o w n i n F i g u r e s

1 a n d 2. T h e a b s o r p t i o n p e a k s d e r i v e d f r o m m a g n e t i t e , h e m a t i t e , goethite, a n d siderite are i n d i c a t e d . A n ore t h a t has m a g n e t i t e as its p r i n c i p a l i r o n - b e a r i n g constituent c a n b e b e n e f i c i a t e d b y m a g n e t i c s e p a r a t i o n w i t h o u t p r i o r treatment. T h i s is c e r t a i n l y t h e case f o r t h e t a c o n i t e - m a g n e t i t e ore of F i g u r e 1 ( t o p s p e c t r u m ) .

( T a c o n i t e is the g e n e r a l n a m e g i v e n

to most ores n o w m i n e d i n the M i n n e s o t a ore fields. I t consists p r i n c i p a l l y of a m i x t u r e of s i l i c a a n d v a r i o u s i r o n o x i d e s . ) D i r e c t m a g n e t i c b e n e f i c i a t i o n w o u l d p r o b a b l y also b e s u i t a b l e f o r t h e I n d i a n ore s a m p l e of F i g u r e 1 ( b o t t o m ) , p r o v i d e d the h e m a t i t e c o m p o n e n t is i n t i m a t e l y associated w i t h t h e m a g n e t i t e , as is often t h e case. I n recent years, leaner, m o r e h i g h l y w e a t h e r e d ores, s u c h as the t a c o n i t e ore w i t h the s p e c t r u m s h o w n i n F i g u r e 2 ( t o p ) , h a v e b e c o m e i m p o r t a n t sources of i r o n . F o r s u c h ores, " m a g n e t i c r o a s t i n g " treatments a r e r e q u i r e d p r i o r to m a g n e t i c b e n e f i c i a t i o n .

S u c h treatments consist of

a p p r o x i m a t e l y 1 h i n a r e d u c i n g a t m o s p h e r e ( H - C O ) at 400° to 5 0 0 ° C . 2

T h i s converts essentially a l l t h e i r o n to m a g n e t i t e , as i l l u s t r a t e d b y the l o w e r s p e c t r u m of F i g u r e 2.

T h e m a g n e t i t e i n this i n s t a n c e is a n o n -

s t o i c h i o m e t r i c f o r m ( F e . a . 0 ) , as e v i d e n c e d b y t h e B / A site r a t i o of 1.14. 3

4

Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

12.

The

H U F F M A N A N D HUGGINS I

104

I

1

i

I

Steel

267

Industry

I I Magnetite

I

I

I

I

i—r

Siderite

100

96


92

2

84 Magnetite

~n

rr

Goethite 100

96

92

-12

-10

-L. -2

-8

0

-L. 2

10

12

V(MM/S)

Figure

1.

Mossbauer

Sinter Q u a l i t y .

spectra of a taconite-magnetite an Indian iron ore (bottom)

ore (top) and of

A f t e r b e n e f i c i a t i o n o f t h e o r e , t y p i c a l l y to m o r e

t h a n 9 0 % i r o n o x i d e , i t is u s u a l l y s u b j e c t e d to o n e of s e v e r a l a g g l o m e r a t i o n processes t o c o n v e r t i t t o a s u i t a b l e " b u r d e n " m a t e r i a l f o r t h e b l a s t f u r n a c e . S i n t e r i n g is p e r h a p s t h e m o s t w i d e l y u s e d a g g l o m e r a t i o n process (I).

T h e finely d i v i d e d , b e n e f i c i a t e d ore is i n t i m a t e l y m i x e d w i t h a finely

d i v i d e d f u e l s u c h as c o k e breeze.

F l u x (limestone, dolomite)

required

f o r p r o p e r b l a s t - f u r n a c e o p e r a t i o n i s also n o r m a l l y a d d e d t o t h e m i x t u r e , w h i c h is t h e n p a s s e d o n a c o n t i n u o u s l y m o v i n g b e d t h r o u g h a n i g n i t i o n furnace.

T h i s causes t h e f u e l t o b e c o m b u s t e d a n d t h e o r e p a r t i c l e s t o

b e s i n t e r e d together i n t o p o r o u s , coherent l u m p s c a l l e d " s i n t e r . " M o s s b a u e r spectra o f t w o sinters, o n e o f w h i c h g a v e g o o d p e r f o r m ance i n t h e b l a s t f u r n a c e , t h e o t h e r p o o r , a r e s h o w n i n F i g u r e 3. B o t h sinters e x h i b i t a b s o r p t i o n p e a k s f r o m h e m a t i t e , c a l c i u m - a n d m a g n e s i u m s u b s t i t u t e d m a g n e t i t e , a n d a set o f b r o a d p a r a m a g n e t i c p e a k s

derived

f r o m glass a n d c a l c i u m f e r r i t e . T h e s e d a t a i n d i c a t e t h a t t h e p o o r sinter contains a p p r o x i m a t e l y t w i c e as m u c h glassy m a t e r i a l as t h e g o o d sinter (28%

o f t h e i r o n i n p a r a m a g n e t i c phases f o r t h e p o o r sinter v s . 1 3 %

f o r t h e g o o d s i n t e r ) . T h i s r e s u l t is i n a c c o r d w i t h s c a n n i n g - e l e c t r o n a n d


268

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

o p t i c a l m i c r o s c o p y , w h i c h s h o w t h e p o o r sinter t o consist o f a b u n d a n t h e m a t i t e , m a g n e t i t e , a n d f e r r i t e crystals i n a glassy m a t r i x w h i c h

fills

essentially a l l o f t h e space b e t w e e n c r y s t a l l i t e s , w h i l e t h e g o o d sinter exhibits a m u c h more porous morphology.

T h e poor reducibility of the

g l a s s - r i c h sinter p r e s u m a b l y arises because o f its l o w p o r o s i t y . T h e r e has b e e n m u c h

Investigation of Direct-Reduction Processes.

interest i n r e c e n t years i n d i r e c t - r e d u c t i o n processes d u r i n g w h i c h b e n e ficiated

ores a r e r e d u c e d

to essentially a m e t a l l i c state.

Such highly


r e d u c e d m a t e r i a l c a n b e u s e d d i r e c t l y i n e l e c t r i c furnaces i n p l a c e o f m o r e expensive s c r a p , p r o v i d i n g metalfics r e l a t i v e l y free o f c o n t a m i n a n t s . A l t e r n a t i v e l y , i f i t i s u s e d as b u r d e n m a t e r i a l i n t h e b l a s t f u r n a c e , t h e h i g h state o f r e d u c t i o n p e r m i t s significant decreases i n c o k e usage. There are numerous direct-reduction techniques

but the

(1,2,3),

shaft-furnace r e d u c t i o n process i s p e r h a p s t h e most p o p u l a r . T h e b e n e ficiated

ore i s first m i x e d w i t h a v e r y s m a l l a m o u n t ( ~ V i %)

of a binder

s u c h as b e n t o n i t e , r o l l e d i n t o pellets, a n d i n d u r a t e d a t 1300° t o 1 3 5 0 ° C i n a i r . T h i s i n d u r a t i o n process converts a l l i r o n t o h e m a t i t e a n d p r o d u c e s sintering between t h e hematite particles. T h e i n d u r a t e d pellets are then

I

102

I

I

n i i

n

I I Hematite

I

Goethit Siderite

II

I

I

I

8

10

r II

94

K Z UI

o cr 100

96

92

88 h 84 -12

Figure 2.

6

12

Mossbauer spectra of a highly weathered taconite ore before (top) and after (bottom) "magnetic roasting*


12.

The

H U F F M A N A N D HUGGINS

|

i

I

i

i

269

Steel Industry

1

I

I

I

I

I

I

l

i 6

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i 10

1 12


Hematitt

0

I

I -10

-12

i -8

i -6

i -4

i -2

i 0

i 2

i 4

V(MM/S)

Figure

3.

Spectra of two sinter samples that exhibited good (top) and poor (bottom) performance in the blast furnace

loaded into the t o p of a large shaft-reduction furnace, illustrated i n F i g u r e 4. T h e r e d u c i n g gas is n o r m a l l y > 9 0 % H

2

+ C O (H :CO ~ 2

1

to 3 ) . T o t a l p e l l e t t r a n s i t t i m e t h r o u g h t h e f u r n a c e i s t y p i c a l l y o n t h e o r d e r of Vi t o 1 h . W o r k at U . S . Steel ( 4 ) has established that Mossbauer

spectroscopy

is t h e best m e t h o d a v a i l a b l e f o r m o m t o r i n g t h e i r o n - p h a s e d i s t r i b u t i o n o f d i r e c t - r e d u c e d pellets. T h i s is i l l u s t r a t e d i n F i g u r e 5, w h i c h shows s p e c t r a o b t a i n e d f r o m p e l l e t s r e d u c e d b y s e v e r a l s o m e w h a t different

methods.

T h e i r o n - p h a s e percentages d e t e r m i n e d f o r these t h r e e samples s h o w t h a t t h e i r t o t a l o x y g e n contents h a v e b e e n r e d u c e d b y 3 5 % , 8 5 % , a n d 1 0 0 % . ( F o r a l l s u c h results r e p o r t e d i n t h i s c h a p t e r , t h e p e a k areas h a v e b e e n


270

M O S S B A U E R SPECTROSCOPY A N D ITS C H E M I C A L A P P L I C A T I O N S

Hematite pellets


Off gas, used as fuel

Hot zone temperature 800-900°C

Reducing gas H ,CO, (C0 H 0) 2

2

Applications of MÃ¶ssbauer Spectroscopy in the Steel Industry

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