Petroleum Derived Carbons

30 Effect o f Carbon Reactivity o n the Operation o f Carbon Reductants in the Iron Blast Furnace—A

Downloaded by SUNY STONY BROOK on October 27, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch030

Comparative Study by Reflectance Microscopy ROBERT T. JOSEPH FMC Corp., Princeton, N.J. 08540 WILLIAM F. BERRY W. F. Berry Associates, Murrysville, Penn. 15668 The use of carbon to smelt iron from its oxide ores is prehistoric in nature and probably began with the observation that the residue from a fire on certain soils contained a hard, nonbreakable substance that could be fashioned into a weapon or tool. The carbon source was, most likely, charcoal, and because the initial process discovery remained traditional, charcoal remained the reductant used in the iron ore reaction system which can be described in its most simplistic form for hematitic iron as: 2Fe O + 3C -> 2Fe + 3CO 2

3

2

While this reaction might have been sufficient to describe what occurred in the production of iron up to the introduction of the blast furnace, the only common characteristic with present-day iron making is the use of carbon as a reductant. Even the nature of the carbon used across the centuries varied with the availability of the source and supply at any given point in time or geography. This change in the types of carbon used is a progression from wood charcoal, to reasonably hard coals, to coke produced by thermal decomposition of soft coals, to the production of a new type of reductant by synthetic agglomeration of carbon containing materials that cannot be agglomerated by thermal decomposition alone. The change in the physical and chemical characteristics of carbon with this progression resulted from the raw materials used and processing conditions required to make a blast furnace reductant out of that raw material. While the physical strength, shape and size are of prominent importance in specifying reductant quality for present-day coal cokes, the control by chemical characteristic has been largely implied as being the "reactivity" of the contained carbon. (1) Reactivity of carbon is defined, for the purposes of this paper, as the comparative rate at which solid-state carbon reacts chemically with any other atom under any given arbitrary set of 435

In Petroleum Derived Carbons; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


PETROLEUM DERIVED CARBONS

environmental c o n d i t i o n s . Since c o n d i t i o n s are a r b i t r a r y , a l l measurements o f carbon r e a c t i v i t y are r e l a t i v e and bear meaning o n l y t o r e s u l t s obtained through t h a t p r e s c r i b e d system f o r measuring r e a c t i v i t y . R e a c t i v i t y o f carbons has been measured i n the l a b o r a t o r y and i n a c t u a l o p e r a t i n g commercial equipment. The work d e s c r i b e d h e r e i n presents i n f o r m a t i o n from both sources. G e n e r a l l y speaking, i n making l a b o r a t o r y r e a c t i v i t y measurements, the environment o f the r e a c t i o n may be v a r i e d i n temperat u r e and the substance used t o r e a c t w i t h the carbon. The nature o f t h i s substance and the c o n d i t i o n s imposed on the t e s t are u s u a l l y s e l e c t e d t o match the a c t u a l o p e r a t i o n p r a c t i c e d as c l o s e l y as p o s s i b l e . Temperatures may be v a r i e d from 1000 t o 3000°F. The r e a c t a n t s are u s u a l l y the oxide o f any atom t h a t i s i n v o l v e d i n the a c t u a l o p e r a t i o n , i . e . oxides o f hydrogen, carbon, i r o n , e t c . The systems used u s u a l l y measure the amount o f gas evolved o r the weight o f carbon l o s t and, from such i n f o r m a t i o n , one can c a l c u l a t e a r e a c t i o n r a t e on some r e l a t i v e b a s i s . That i s , one carbon i s compared t o another o r t o some a r b i t r a r i l y s e l e c t e d standard carbon. The i n f o r m a t i o n c o l l e c t e d may be used i n formal k i n e t i c s t u d i e s , but from the standpoint o f p r a c t i c a l a p p l i c a t i o n , the data on r e a c t i v i t y r e p o r t e d here are not r e a d i l y s u i t a b l e f o r such an a n a l y s i s . In p r o d u c t i o n , the measurement o f the r e a c t i v i t y o f carbon i s c a r r i e d out d u r i n g p r o d u c t i o n i n p r o d u c t i o n or semi-production equipment. I n d i v i d u a l carbon behavior i s observed i n cupolas o r b l a s t furnaces, and the e f f e c t o f r e a c t i v i t y i s measured by comp a r i s o n a g a i n s t a b a s e l i n e p e r i o d i n terms o f changes i n y i e l d , mass balance, ease o f o p e r a t i o n and r a t e o f p r o d u c t i o n . Some r e a c t i v i t i e s f o r the carbons used i n the work d e s c r i b e d h e r e i n have been determined i n the l a b o r a t o r y by a s p e c i f i c standa r d i z e d , but unpublished, in-house method a t g i v e n temperatures, and r e s u l t s are l i s t e d i n Table I . TABLE I COMPARATIVE REACTIVITIES

Carbon

Source

Intended Use

Formcoke (Briquetted Iron B l a s t Furnace Carbon Reductant)

R e a c t i v i t y i n 10% Carbon Dioxide 1650°F 2400°F (Wt. % Loss Per Hour Reaction Time) 22

619

By-Product Oven Coke Iron B l a s t Furnace

0.6

79

Beehive Oven Coke

0.1

45

Foundry Cupola



30.

JOSEPH A N D BERRY

Operation of Carbon Reductants

437

T h i s information leaves no doubt t h a t the formcoke, a b r i q u e t t e d coal-carbon reductant, i s much more r e a c t i v e than comparable cokes from thermal decomposition i n by-product o r beehive ovens. Since the r e s u l t s from p r o d u c t i o n p r a c t i c e were obtained by using by-product oven coke and formcoke o f the same s i z e i n the same p r o d u c t i o n equipment, the comparison o f these o p e r a t i o n a l r e s u l t s i s v a l i d r e g a r d l e s s o f the source o f c o a l o r process v a r i a t i o n i n the methods o f p r o d u c t i o n . What i s being compared i s the e f f e c t o f r e a c t i v i t y values and not the cause o f those v a l u e s . However, because the formcoke's h i g h r e l a t i v e r a t e o f carbon consumption a t low r e a c t i o n temperature, 1650°F, i n carbon d i o x i d e and because the 10 percent c o n c e n t r a t i o n i s a reasonably good simu l a t i o n o f gas composition from the top t o the bottom o f the b l a s t furnace atmosphere, e x t r a p o l a t i o n o f t h i s formcoke c h a r a c t e r i s t i c to expected b l a s t furnace o p e r a t i o n r e s u l t e d i n the f o l l o w i n g assumptions o f probable behavior i n the b l a s t furnace: (2) (3) 1. The r e q u i r e d formcoke per ton o f hot metal produced would go up because o f excess carbon g a s i f i c a t i o n i n the top regions o f the furnace b e f o r e temperatures occur where i r o n o r e r e d u c t i o n could begin. 2. The p e l l e t (briquet) s t r e n g t h would be eroded because o f the i n t r a p e l l e t r e a c t i o n w i t h carbon d i o x i d e and water vapor which would d e s t r o y the bonds t h a t formed the p e l l e t and produce part i c u l a t e matter t h a t i s c a r r i e d out w i t h the gas stream; t h i s assumes b i n d e r coke t o be more r e a c t i v e than c o a l cokes. 3. The p e l l e t would absorb i r o n and s l a g a t a much g r e a t e r r a t e , because o f t h i s attack on i t s p a r t i c u l a t e micropore s t r u c t u r e , than would oven coke - which i s d e r i v e d from a coalesced molten c o a l and has l i t t l e o r no micropore volume - and would allow considerable p e n e t r a t i o n by the molten i r o n and s l a g . In the case o f s t u d i e s t o determine the e f f e c t o f s o l i d c a r bon r e a c t i v i t y on cupola o p e r a t i o n , i n the middle and l a t e 60's, considerable commercial s c a l e experiments were c a r r i e d out and r e p o r t e d i n the l i t e r a t u r e o f v a r i o u s meetings i n Great B r i t a i n and on the C o n t i n e n t . These e f f o r t s i n d i c a t e d t h a t r e a c t i v e coke, d i d not perform as w e l l as d i d the b a s e l i n e coke. Off-gas a n a l yses showed an i n c r e a s e i n CO/CO r a t i o i n d i c a t i n g l e s s heat r e l e a s e d i n the cupola which r e s u l t e d i n lower y i e l d s o f molten metal f o r foundry c a s t i n g . With r e s p e c t t o the chemistry o f subsequent i r o n , t h e r e appeared t o be i n s u f f i c i e n t carbon pickup t o produce gray i r o n which was a t t r i b u t e d t o the lower h o t metal temperature as a r e s u l t o f the carbon g a s i f i c a t i o n due t o higher reactivity. While t h e r e was no comparison t o formcokes o r t o h i g h l y r e a c t i v e formcokes, the c o n c l u s i o n based on the oven coke work was a p p l i e d t o formcokes and e x t r a p o l a t e d from cupola t o b l a s t furnace. 2



438


These c o n c l u s i o n s , based on v a r i a t i o n i n r e a c t i v i t y o f oven cokes, n e c e s s i t a t e d proof t h a t the h i g h l y r e a c t i v e formcoke would not cause an adverse e f f e c t on b l a s t furnace o p e r a t i o n . With t h i s f a c t o r i n mind and the d r i v e t o use low-rank c o a l s t h a t could not be p a r t o f any oven charge, the B r i t i s h S t e e l Corporation embarked on a b l a s t furnace t e s t program t o determine whether o r not formcokes made from these low-rank c o a l s could be used as the carbon source f o r reducing i r o n . Three thousand tons o f b r i q u e t s , which were produced i n Southwest Wyoming from a c o a l t h a t does not melt and c o a l e s c e , were t e s t e d i n a week-long campaign i n a b l a s t furnace i n C a r d i f f , South Wales. (4) The r e s u l t s from t h i s t e s t show t h a t these p r e d i c t e d d e s c r i p t i o n s o f the behavior o f the r e a c t i v e formcoke b r i q u e t s i n the b l a s t furnace d i d not occur. In f a c t , the information c o l l e c t e d i n d i c a t e s these b r i q u e t s t o have performed t o the c o n t r a r y . The behavior o f the b r i q u e t s was compared t o the behavior of a high-grade furnace coke made from a c o a l t h a t melted and coalesced i n a by-product oven under c l o s e l y c o n t r o l l e d furnace c o n d i t i o n s . Some r e s u l t s from t h i s t e s t , p e r t i n e n t t o the t o p i c o f t h i s paper, are l i s t e d . TABLE I I ABBREVIATED RESULTS, COMPARATIVE OPERATION OF BRITISH STEEL CORPORATION NO. 3 BLAST FURNACE AT CARDIFF, WALES Type o f Coke

Furnace Coke

R e a c t i v i t y o f Coke a t 1650°F i n 10% C 0 (Wt. % l o s s per hour r e a c t i o n time)

Formcoke

2

S i z e o f Coke (Nominal

mm)

22

0.6

38 χ 38 χ 25

44

Carbon Rate (Kg o f carbon/metric ton o f iron)

479

471

Production Bate (Metric tons o f i r o n per day)

805

873

As can be seen, d e s p i t e t h i s great d i f f e r e n c e i n r e a c t i v i t y of the coke carbon a t 1650°F, the formcoke r e q u i r e d per ton o f hot metal was a b i t l e s s . However, the y i e l d o f i r o n was g r e a t e r i n the p e r i o d when formcoke b r i q u e t s were the reductant. I t may be argued w i t h some v a l i d i t y t h a t the r e a c t i v i t i e s o f c o a l carbons tends t o merge as the r e a c t i o n temperature i n an o x i d i z i n g atmosphere i n c r e a s e s . An attempt to demonstrate t h i s by an in-house experiment i n a 10 percent CO /90 percent n i t r o g e n atmosphere was made. R e s u l t s are shown as a complete curve o f weight l o s s r a t e versus r e a c t i o n temperature, on an equal exposed surface b a s i s . F i g u r e 1 presents these r e s u l t s . 2




JOSEPH A N D BERRY

2700·Ρ

2400*F

I650»F TEMPERATURE

Figure 1.

Form-coke vs. oven-coke. Effect of temperature on re activity in 10% CO,.




440

Notice how the r a t e l o s s curves tend t o merge as the temper ature approaches 2700°F which i s about 300°F above the molten temperature o f reduced i r o n . In the r e g i o n o f 2400°F, consumption o f s o l i d carbon i n formcoke f a l l s o f f by a f a c t o r o f about two. T h i s i s a t t r i b u t e d t o a change i n r e a c t i o n mechanism from r a t e to d i f f u s i o n c o n t r o l . The p r a c t i c a l importance o f t h i s i s t h a t under furnace c o n d i t i o n s a t these e l e v a t e d temperatures, the o x i d i z i n g gases - carbon d i o x i d e and water vapor - do not penetrate the formcoke s t r u c t u r e and r e a c t i n t e r n a l l y . Therefore, i t can be expected t h a t the b r i q u e t w i l l hold i t s shape and s t r e n g t h throughout i t s h i s t o r y i n the furnace. While work i n Japan (2) has i n d i c a t e d - with oven cokes - h i g h e r r e a c t i v i t y r e s u l t s i n lower s t r e n g t h a t high temperatures under furnace c o n d i t i o n s (micro-strength), the data contained h e r e i n p r e d i c t otherwise. When coupled with the s l i g h t l y lower amount o f formcoke necessary per ton o f hot metal, i t would appear t h a t the formcoke i r o n ore chemistry must be d i f f e r e n t from oven coke i r o n ore chemistry; t h a t high r a t e r e d u c t i o n f i r o n ore by the formcoke used takes p l a c e a t a much lower temperature than when u s i n g oven cokes. Table I I I compares the r a t e o f reducing i r o n ores by t h i s type o f formcoke - made from a broad rank s e l e c t i o n o f c o a l s - w i t h "highgrade" oven coke. Q

TABLE I I I COMPARATIVE IRON ORE REDUCTION FORMCOKE VS. OVEN COKE Coal From Which Reductant Was Produced

Rank o f C o a l (ASTM)

Appalachain Coking (Produced i n By Coal Blend product Oven)

10% C 0 Reactivity %/Hr @ 1800°F 2

% Reduction 60 Minutes @ 1800°F

2.7

39.4

South A f r i c a

MVB

10.0

90.0

Kentucky

HVAB

24.6

90.8

I l l i n o i s No. 6

HVAB

22.3

93.8

Utah

Sub-bituminous A

24.3

89.0

Wyoming

Sub-bituminous Β

23.5

91.6

Texas

Lignite

57.2

94.2

In the American counterpart t o t h i s B r i t i s h e f f o r t , (5) 20,000 tons o f formcoke b r i q u e t s were charged, and the t e s t was considered s u c c e s s f u l . The t e s t was run f o r one month on Inland S t e e l ' s No. 5 furnace a t b e t t e r than 100,000 ACFM b l a s t r a t e .



30.

JOSEPH A N D BERRY


441

During each o f these t e s t s , samples o f the carbon reductants t h a t were charged, were taken; samples o f each reductant were recovered from the coke t h a t had been through the e n t i r e furnace and escaped out the tap hole w i t h the molten metal w i t h each c a s t to remove hot i r o n from the furnace. These samples were examined by r e f l e c t a n c e microscopy using a method d e s c r i b e d i n the l i t e r ature (6) a f t e r which were developed the ASTM Standards D-2796, D-2797, D-2798 and D-2799-72. E s s e n t i a l l y , the method c o n s i s t s o f embedding a sample o f the coke i n some s u i t a b l e p l a s t i c , p o l i s h i n g and embedded sample t o an o p t i c a l f l a t , observing the sample, which i s i l l u m i n a t e d by a l i g h t source o f 548 nanometers wave l e n g t h and r e c o r d i n g the mean maximum r e f l e c t a n c e , γο, a t d i f f e r e n t stage angles. During t h i s a n a l y t i c a l procedure some 1,000 photomicrographs, a t d i f f e r ent m a g n i f i c a t i o n s were taken and analyzed. Of a l l the photo micrographs s t u d i e d , P l a t e s I through XVII were s e l e c t e d as b e s t i l l u s t r a t i n g the general conclusions drawn from t h i s work and presented here: I.

Formcoke B r i q u e t s Charged t o the Furnace.

1. These formcoke b r i q u e t s , i n a l l cases, appear i n d i v i d u a l l y as a uniform s t r u c t u r e throughout. The carbon from the binder source amounts t o about 5 percent o f the carbon content o f the b r i q u e t and i s e q u i v a l e n t t o approximately 15 percent b i n d e r i n the o r i g i n a l u n d e v o l a t i l i z e d specimen. 2. The formcoke b r i q u e t s , i n a l l cases, are strong w i t h a t o t a l i n t e r n a l p o r o s i t y o f approximately 40 t o 45 p e r c e n t . The b r i q u e t s are r e l a t i v e l y dense i n appearance and r e s u l t from an apparently well-mixed, well-bonded uniform mass. The pore volume i s i n a micropore s t r u c t u r e and not the r e s u l t o f l a r g e , i n t e r connected h o l e s . T h i s i s the b a s i s f o r the b r i q u e t s t r e n g t h which t r a n s l a t e s t o an a b i l i t y t o support 4,000 pounds p e r square i n c h a t furnace o p e r a t i n g temperature. 3. As a r e s u l t o f t h i s u n i f o r m i t y , the r e a c t i v i t y o f the e n t i r e b r i q u e t i s equal as the b r i q u e t enters the furnace, and any changes i n r e a c t i v i t y as the b r i q u e t passes through the furnace can be expected t o be uniform. 4. The i n t e r n a l s t r u c t u r e appears f r e e from cracks and flaws.


442

II.


P a r t i a l l y Consumed Formcoke B r i q u e t s Recovered From the Top o f the Molten Iron C a s t .


1. The recovered formcoke b r i q u e t s r e t a i n u n i f o r m i t y , s t r e n g t h , and have i n c r e a s e d i n m i c r o p o r o s i t y by o n l y 4 p e r c e n t . 2. The o x i d a t i o n o f the b r i q u e t carbon i s s t r i c t l y topochemical with p e n e t r a t i o n not exceeding 1 mm uniformly, even with specimens t h a t have been 90 p e r c e n t consumed. The topochemical r e a c t i o n process a p p l i e s t o a l l r e a c t i o n s i n c l u d i n g the a l k a l i a t t a c k on a n i s o t r o p i c carbon t o form a l k a l i - c a r b i d e s . The unreacted p o r t i o n o f the b r i q u e t appears about the same i n s t r u c t u r e as the b r i q u e t s b e f o r e charging t o the furnace. III.

By-product Oven Furnace Coke Charged t o the Furnace.

1. The oven coke charged appears strong and uniform w i t h a p o r o s i t y o f about 30 p e r c e n t . T h i s p o r o s i t y i s developed i n the macropore s t r u c t u r e and r e s u l t s i n the v e r y small s u r f a c e area measured i n most oven cokes, about 2 t o 3 sq. meters per gram. 2. There appears t o be a reasonably small amount o f a n i s o t r o p i c carbon, but what amount i s p r e s e n t appears to be nonuniformly d i s t r i b u t e d . 3. There i s a b a s i c crack system t h a t appears through the coke which does not appear t o e f f e c t the c o l d s t r e n g t h o f the coke. IV.

P a r t i a l l y Consumed By-product Oven Coke Recovered From the Top o f the Molten Iron C a s t .

1. The s t r u c t u r e and p o r o s i t y o f the oven coke allowed a complete i n t e r n a l r e a c t i o n w i t h a l l i n t e r n a l s u r f a c e areas o f the coke. T h i s should be compared t o the s t r i c t l y topochemical r e a c t i o n s d i s p l a y e d by the formcoke samples exposed t o the same process. 2. T h i s extensive i n t e r n a l r e a c t i o n has weakened the remaining coke s t r u c t u r e t o the p o i n t where the burden support necessary i n the b l a s t furnace i s impared. 3. There i s deep and extensive p e n e t r a t i o n o f the oven coke by i r o n , s l a g , and a l k a l i - c a r b i d e s i n t o the i n t e r n a l coke structure. 4. A l k a l i - c a r b i d e d e p o s i t i o n i s apparent throughout the e n t i r e s t r u c t u r e o f the oven coke, and the d e p o s i t i o n o f the a l k a l i e s appears t o have a strong a f f i n i t y t o the a n i s o t r o p i c carbon forms o f the coke. 5. P e n e t r a t i o n o f vapor o r l i q u i d phase i r o n , appears t o cause a strong carbon r e a c t i o n around the area o f p e n e t r a t i o n l e a d i n g t o g r a p h i t e formation. 6. P e n e t r a t i o n o f s l a g i n t o the coke's s t r u c t u r e seems t o be u n r e l a t e d t o any carbon r e a c t i o n w i t h i n the coke. The s l a g appears t o s e a l o f f the coke p o r o s i t y .


30.

JOSEPH A N D BERRY


443


As best as can be determined by the analyses o f the photo micrographs, w i t h formcoke b r i q u e t s the chemistry w i t h i n the b l a s t furnace i s s t r i c t l y surface topochemical wherein the o r i g i n a l formcoke s t r e n g t h p r o p e r t i e s a r e r e t a i n e d over the whole r e a c t i o n p e r i o d . The comparable furnace chemistry f o r by-product oven coke i n d i c a t e s nonuniform but deep p e n e t r a t i o n i n t o the center o f the coke p i e c e t h a t tends t o destroy the o r i g i n a l coke s t r u c t u r e , reduce the s t r e n g t h , and permit almost s a t u r a t i o n o f coke by s l a g and i r o n which s e a l o f f pores and slow down o r stop the r e a c t i o n .

LITERATURE CITED 1. Thompson, R. R., Mantione, A. F., Aikman, R. P., Blast Furnace Steel Plant, "Improvement of Coke Uniformity Through Measurement of Coke Reactivity", (March, 1971). 2. Nippon Steel Corporation/FMC Corporation, Private Communication, "NSC's View as to the Correlation Between the Coke Strength and Reactivity", (May, 1971). 3. Price, J . D., Proceedings of American Institute of Mechanical Engineers, "Coke Combustibility: A Neglected Characteristic", (February, 1959). 4. Holgate, J. Κ., Pinchbeck, P. Η., Journal of the Iron and Steel Institute, "Use of Formed Coke: BSC Experience 1971/ 1972", (August, 1973), pp 547-566. 5. Laursen, F. W., Engineering and Mining Journal, "News Release", (February, 1975), 176, (2). 6. Gray, R. J., Schapiro, Ν., Blast Furnace and Steel Plant, "Relation of Coke Structure to Reactivity", (April, 1963) pp 273280.


Petroleum Derived Carbons

Recommend Documents