Pelletizing
of
Iron
Ores
Kurt Meyer
Kurt Meyer
PeIletizing of Iron Ores
W i t h 146 F i g u r e s
1980
Springer-Verlag Berlin Heidelberg NewYork
Verlag Stahleisen m.b.H. Düsseldorf
Professor Dr. Phil. Dr. Ing. Dipl.-Chem. Kurt Meyer
Peter-Böhler-Straße 22
6000 Frankfurt/M.
ISBN 3-540-1021.5-9 Springer-Verlag B e r l i n H e i d e l b e r g N e w Y o r k ISBN 0-387-10215-9 Springer-Verlag N e w Y o r k Heidelberg Berlin
ISBN 3-514-00246-0 Verlag Stahleisen m b H Düsseldorf
Library of Congress Cataloging in Publication Data: Meyer, Kurt, 1911-. Pelletizing of iron ores. Bibliography; p. Includes index. 1. Pelletizing (Ore-dressing). 2. Iron ores. I. Title.
TN 535.M47. 622'.341. 80-23891.
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Preface
After the Second World War, there was in many countries a great
back-log demand in nearly all branches of life, which resulted i. a. in a worldwide upswing in the iron and steel industry. Many d a m a g e d or destroyed production facilities had to be replaced, reconstructed and extended. This
was a good opportunity of revising the production concept of the iron and steel industry and introducing innovations where it was possible and pro-mising.
This renewal covered i. a. two important sectors. On the one hand, the dimensions of the production units and auxiliary equipment were extend-ed to such a degree that considerably higher capacities were achievextend-ed. An interesting example is the extension of the blast furnace volume and hearth diameters up to 15 m at a pig iron production of about 10,000 tons per day.
In addition, remarkable progress was m a d e by improving existing pro-cess parameters and introducing new technologies. Some of these innova-tions had already been formerly known, but not yet applied. A few exam-ples of such developments in connection with ore preparation and particu-larly with the agglomeration technology are given below:
1. Physical Preparation of Blast Furnace Burden by Crushing, Screening and Classification of Constituents
Already in the thirties, it was known to m a n y experts that it is advisable to classify the blast furnace burden before it is used 2). However, this idea
was not consistently realized until about 1950. F r o m this date onwards, raw ores, coke and other burden constituents were crushed, screened and supplied to the blast furnace in a narrow size range. Increasing amounts of fine ores emerged from this operation and required a strong extension of
sinter plant capacity, as shown in Figures 9 and 10, chapter 1. As a result of
these measures, by using a physically prepared b u r d e n with rising sinter portions, the gas permeability of the blast furnace b u r d e n greatly improv-ed the pig iron capacity increasimprov-ed and the coke consumption decreasimprov-ed.
2. Thermal Separation of Volatile Ballast Materials from Raw Materials
A f u r t h e r i m p r o v e m e n t of the blast f u r n a c e operation was achieved by removal of volatile ballast material, such as H2O or C O2, f r o m raw ores during sintering or pelletizing b e f o r e they enter the blast furnace. Such an ore p r e p a r a t i o n is of particular i m p o r t a n c e w h e n Minette or limonite ores have to b e treated. T h e r e are plants in which the blast f u r n a c e b u r d e n con-sists of 100% sinter.
3. Mechanical Beneficiation of Ores by Separation of Mineral Ballast Materials
By mechanical beneficiation, it is possible to r e m o v e a great portion of mineral ballast c o m p o n e n t s f r o m the iron ores b e f o r e they enter the blast furnace. But the concentrates so p r o d u c e d are very fine-grained, a n d ag-glomeration is thus necessary. This agag-glomeration is achieved by sintering and pelletizing. T h e pelletizing process was introduced on an industrial scale as a new agglomeration m e t h o d and an alternative to sintering.
4. Change of Chemical Composition of Ores During Agglomeration
T h e possibility of m o d i f y i n g the chemical composition of the ores by using corresponding additives during agglomeration leads to the desired change or i m p r o v e m e n t of metallurgical properties of the agglomerates.
F r o m this p r o c e d u r e resulted the p r o d u c t i o n of basic or even over-basic agglomerates, as already p r o p o s e d in 1938 2). T h e d e v e l o p m e n t of the ag-glomeration technology in the f o r m of sintering or pelletizing is closely connected with the p r o m o t i o n of the blast f u r n a c e technology.
Pellets o f f e r additional advantages d u e to their good transportability. With the introduction of pellets into the world market, the h o p e of m a n y metallurgists — to p r o d u c e steel f r o m ores by direct reduction and bypass the blast f u r n a c e — c a m e closer to realisation.
T h e increasing and successful efforts to i m p r o v e the direct reduction processes were one of the most i m p o r t a n t consequences of the newly deve-loped pelletizing technology.
Acknowledgement
T h e a u t h o r very m u c h thanks the m a n a g e m e n t of Lurgi C h e m i e und H ü t t e n -technik G m b H * for the permission to use test results and o t h e r relevant knowledge which h a d hitherto not been p u b l i s h e d as well as for providing the facilities r e q u i r e d for the p r e p a r a t i o n of this book.
H e also thanks his colleagues in F r a n k f u r t as well as those of the subsi-diary c o m p a n i e s in G r e a t Britain, C a n a d a , J a p a n , Sweden, T h e U n i t e d States, S o u t h A f r i c a and Australia for their assistance in c o m p i l i n g d a t a , in p e r f o r m i n g the necessary additional tests, in elaborating d r a w i n g s a n d dia-grams, in translating the text and in revising the translation.
F u r t h e r m o r e , the a u t h o r expresses his g r a t i t u d e to o t h e r sources for the kind disclosure of interesting a n d most recent i n f o r m a t i o n s p r i m a r i l y to the representatives of L K A B (Sweden), H o o g o v e n s I j m u i d e n ( N e t h e r -lands), H a n n a M i n i n g C o m p a n y , Cleveland ( O h i o ) , P i c k a n d s M a t h e r a n d C o m p a n y , Cleveland (Ohio), Studiengesellschaft f ü r E i s e n e r z a u f b e r e i t u n g (Federal R e p u b l i c of G e r m a n y ) , Verein Deutscher Eisenhüttenleute ( F e d eral R e p u b l i c of G e r m a n y ) , and Institut für E i s e n h ü t t e n k u n d e der R h e i -nisch-Westfälischen Technischen H o c h s c h u l e A a c h e n ( F e d e r a l R e p u b l i c of G e r m a n y ) .
Contents
Introduction 1 1 Definition and Development of Pelletizing Process . . 3
1.1 Definition 3 1.1.1 Differentiation Against Other Iron Ore Agglomerates . 3
1.1.2 Principal Process Steps for the Production of Pellets . 4
1.2 Development of Pelletizing Process . 5 1.2.1 First Phase, Alternative to Sintering 6 1.2.2 Second Phase, Pellets f r o m Concentrates 7
1.2.3 Third Phase, Pellets f r o m Ores 9 1.2.3.1 T h e Two-Stage Granulation of the Sinter Mix . . . . 11
1.2.3.2 Pellet Sintering 13 1.2.3.3 Mixed Firing Method 13 1.3 Pelletizing, a Contribution to Ore Preparation . . . . 15
1.4 Sites of Pelletizing Plants and Transportability of Pellets 20
2 Fundamentals of Pelletizing 23
2.1 Bonding Mechanisms for G r e e n Ball F o r m a t i o n . . . 24
2.1.1 Important Bonding Factors 24 2.1.2 Ball F o r m a t i o n Alternatives 24 2.1.2.1 Compacting Method 25 2.1.2.2 Green Ball Formation 25 2.1.2.3 Mechanism of Ball F o r m a t i o n . 26
2.2 Induration of Green Balls 29 2.2.1 Drying of G r e e n Balls 29 2.2.1.1 Drying Procedure of Individual Balls 30
2.2.1.2 Drying of Pellets in a Layer 33 2.2.1.2.1 Unidirectional Drying 34 2.2.1.2.2 U p - D r a u g h t - D o w n - D r a u g h t Drying 35
2.2.1.3 Dry Pellet Strength 35 2.2.1.4 Shock Resistance 37 2.2.2 Pellet Firing 37 2.2.2.1 Bonding by Change of the Crystalline Structure . . . 39
2.2.2.1.1 Crystal C h a n g e D u r i n g the I n d u r a t i o n of Pellets f r o m
M a g n e t i t e Concentrate 40 2.2.2.1.2 Crystal C h a n g e D u r i n g Induration of H e m a t i t e Pellets 41
2.2.2.2 T h e Reaction of S l a g - F o r m i n g C o m p o n e n t s 43
2.2.3 Cooling of I n d u r a t e d Pellets 45
3 Raw Materials and Their Preparation for Pellet
Production 47 3.1 R a w Materials 47 3.1.1 Iron-Bearing Materials 47 3.1.1.1 N a t u r a l Iron Ores 47 3.1.1.1.1 M a g n e t i t e 48 3.1.1.1.2 H e m a t i t e 49 3.1.1.1.3 W e a t h e r e d Ores 49 3.1.1.1.4 L i m o n i t e 50 3.1.2 Beneficiation Products 51 3.1.3 Secondary R a w Materials 52 3.1.4 Binders and Additives 53
3.1.4.1 Binders 53 3.1.4.2 Additives 53 3.1.4.3 Bentonite 53 3.1.4.4 L i m e C o m p o u n d s 55
3.1.4.5 O t h e r Additives 56 3.2 P r e p a r a t i o n of R a w Materials for Pelletizing . . . . 56
3.2.1 S e p a r a t i o n 57 3.2.1.1 W a s h i n g 57 3.2.1.2 G r a v i t y Separation 57 3.2.1.3 F l o t a t i o n 58 3.2.1.4 M a g n e t i c Separation 58 3.2.1.5 Magnetizing Roasting 59 3.2.1.6 Electrostatic Separation 60 3.2.1.7 P r o p o r t i o n of D i f f e r e n t Ores in Pellet Production . . 60
3.2.2 Physical Properties of Fine-grained Iron Ores . . . . 62
3.2.2.1 Size Distribution-Specific Surface A r e a 62
3.2.3 G r i n d i n g 64 3.2.3.1 D e w a t e r i n g 66
4 The Pelletizing Laboratory and its Tasks 68
4.1 A p p l i c a t i o n R a n g e of Laboratories 68
4.2 T h e Tasks of a L a b o r a t o r y 69 4.3 R a w Material P r e p a r a t i o n and Pellet Production . . . 70
4.3.2 G r i n d i n g . 73 4.3.3 G r i n d i n g E q u i p m e n t a n d G r i n d i n g Energy 75 4.3.4 Filtration 76 4.4 M i x P r e p a r a t i o n for Ball F o r m a t i o n 77 4.4.1 M i x P r e p a r a t i o n 77 4.4.2 G r e e n Ball F o r m a t i o n 77 4.4.2.1 G r e e n Ball F o r m a t i o n and Testing M e t h o d s 79
4.4.2.1.1 Pellet M o i s t u r e D e t e r m i n a t i o n 79 4.4.2.1.2 Crushing Strength 80 4.4.2.1.3 D r o p N u m b e r 82 4.4.2.1.4 D r o p Resistance 82 4.4.3 Capacity D e t e r m i n a t i o n 82 4.4.4 Bulk Density 83 4.5 G r e e n Ball I n d u r a t i o n 83
4.5.1 F u r n a c e s for Orienting Tests 83 4.5.2 Stationary Pot G r a t e for Principal Tests 84
4.5.2.1 Pot G r a t e w i t h Side Walls a n d H e a r t h L a y e r . . . . 85
4.5.2.2 Pot G r a t e w i t h C o r r u g a t e d Side Walls 85 4.5.2.3 Control S c h e m e of Pot G r a t e Tests 86
4.5.2.4 M o v a b l e P o t G r a t e 86 4.5.3 Pilot Plants 89 4.6 T h e Properties of I n d u r a t e d Pellets a n d T h e i r Testing M e t h o d s 89 4.6.1 T h e Physical Properties 89 4.6.1.1 Crushing Strength 90 4.6.1.2 T u m b l e r Resistance 90 4.6.1.3 M c r o p o r o s i t y 91 4.6.2 B e h a v i o u r of I n d u r a t e d Pellets D u r i n g R e d u c t i o n . . 91 4.6.2.1 Testing M e t h o d s for R e d u c t i o n 93 4.6.2.1.1 Mechanical Strength 93 4.6.2.1.2 E x a m i n a t i o n of Fired Pellets for Blast F u r n a c e
O p e r a t i o n 93 4.6.2.1.2.1 L o w - T e m p e r a t u r e D i s i n t e g r a t i o n Test (Static Test) . . 93
4.6.2.1.2.2 L o w - T e m p e r a t u r e D i s i n t e g r a t i o n Test ( D y n a m i c Test) . 94
4.6.2.1.2.3 Swelling Test 94 4.6.2.1.2.4 R e d u c t i o n u n d e r L o a d T e s t ( R u L ) 94
4.6.2.1.2.5 O t h e r Testing M e t h o d s 95 4.6.2.1.3 E x a m i n a t i o n of F i r e d Pellets for the D i r e c t R e d u c t i o n . 96
4.6.2.1.3.1 L o w - T e m p e r a t u r e D i s i n t e g r a t i o n Test ( D y n a m i c ) . . . 96
4.6.2.1.3.2 Swelling Test 96 4.6.2.1.3.3 Sticking Test ( R M C ) 96 4.6.2.1.3.4 Direct R e d u c t i o n D i s i n t e g r a t i o n Stability Test ( D R D S ) 96
5 Process Influencing Factors 99
5.1 Factors Influencing Ball F o r m a t i o n 99 5.1.1 G r a n u l o m e t r i c Properties of R a w Materials 100
5.1.1.1 G r a i n Size, Size Distribution a n d Specific S u r f a c e A r e a 100 5.2 Influence of W a t e r A d d i t i o n on G r e e n Ball F o r m a t i o n 105
5.2.1 O p t i m u m Moisture Content 105 5.2.1.1 O p t i m u m Moisture Content a n d Specific Surface A r e a 106
5.2.1.2 O p t i m u m Moisture Content and Surface C o n d i t i o n of
O r e Particles 107 5.3 Influence of Binders a n d Additives 109
5.3.1 F a c t o r s for l m p r o v i n g the Mechanical Properties . . . 110
5.3.1.1 Bentonite as Binder 110 5.3.1.1.1 Influence of Bentonite on G r e e n Pellet Strength and
D r o p R e s i s t a n c e 110 5.3.1.1.2 Influence of Bentonite on D r y Pellet Strength . . . . 112
5.3.1.1.3 Influence of Bentonite on Crushing Strength a n d
A b r a s i o n Resistance of F i r e d Pellets 112
5.3.1.1.4 D i f f e r e n t Bentonite Types 113 5.3.1.1.5 Influence of Bentonite on the C h e m i c a l C o m p o s i t i o n of
Pellets 114 5.3.1.2 Influence of Alkaline Earth C o m p o u n d s 115
5.3.1.2.1 T h e Influence of C a l c i u m O x i d e [CaO] and C a l c i u m
H y d r o x i d e [ C a ( O H )2] 116
5.3.1.2.1.1 Influence of C a l c i u m H y d r o x i d e [ C a ( O H )2] on G r e e n
Pellet Strength a n d D r o p Resistance 116 5.3.1.2.1.2 Influence of [Ca(OH)2] on Dry Pellet Strength . . . . 118
5.3.1.2.1.3 Influence of [ C a ( O H )2] o n Crushing Strength, T u m b l i n g
Resistance and Porosity of Indurated-Pellets 121 5.3.1.2.2 Influence of C a l c i u m C a r b o n a t e [CaCO3] and D o l o m i t e
[(Ca1Mg)CO3] on the Strength of I n d u r a t e d Pellets . . 121 5.3.1.2.3 Influence of a M i x t u r e of D i f f e r e n t Additives on Pellet
Properties 125 5.3.1.2.4 Influence of C a l c i u m Chloride [CaCl2] on Pellet
Properties 126 5.3.1.3 Influence of Alkali C o m p o u n d s 127
5.3.1.4 Influence of Ores with G o o d Bonding Properties . . . 127
5.3.1.5 B e h a v i o u r of O r e Mixtures 128 5.3.1.6 Influence of Oxidized and P r e r e d u c e d R e t u r n F i n e s . . 131
5.3.1.7 Influence of Sponge Iron on Pellet Properties 132 5.3.1.7.1 Influence of Sponge Iron on G r e e n a n d D r y Pellet
Strength 133 5.3.1.7.2 Influence of Sponge Iron on the Strength of I n d u r a t e d
5.3.1.8 Influence of Inplant F i n e s on Pellet P r o p e r t i e s . . . . 135
5.3.1.9 Influence of Organic Binders 136 5.3.1.10 Influence of Coal A d d i t i o n 138 5.3.1.11 S u m m a r i z i n g Considerations 139 5.4 Influence of T h e r m a l T r e a t m e n t o n Pellet Properties . 140
5.4.1 Factors Influencing G r e e n Ball D r y i n g 140" 5.4.1.1 T e m p e r a t u r e of Drying G a s e s 141 5.4.1.1.1 Influence of D r y i n g G a s T e m p e r a t u r e o n D r y i n g T i m e 141
5.4.1.1.2 Influence of T e m p e r a t u r e on S h o c k Resistance of Pellets
D u r i n g D r y i n g 142 5.4.1.2 Influence of Velocity of D r y i n g G a s F l o w o n D r y i n g D e g r e e 142 5.4.1.3 C h a n g e of Pellet Strength D u r i n g D r y i n g 143 5.4.2 Preheating of D r i e d Pellets 145 5.4.2.1 C h a n g e of W e i g h t a n d Strength D u r i n g D r y i n g a n d Preheating of G r e e n Balls 145 5.4.3 F i r i n g and Cooling of Pellets 147 5.4.3.1 H e a t - H a r d e n i n g of M a g n e t i t e G r e e n Pellets 148
5.4.3.2 Influence of F i r i n g T e m p e r a t u r e a n d Basic Additives on
the Strength of Pellets f r o m M a g n e t i t e Ores 150
5.4.3.3 Firing of H e m a t i t e G r e e n Pellets 151 5.4.3.4 Influence of T e m p e r a t u r e a n d Basic A d d i t i v e s on
H e m a t i t e Pellet Quality 152 5.4.4 Reactions of Additives with Iron Oxides a n d G a n g u e
Constituents 153 5.4.5 T h e r m a l Dissociation of H e m a t i t e in Pellets 154
5.4.6 Scheme of T h e r m a l T r e a t m e n t 155 5.4.6.1 H e a t i n g Pattern for Pellets f r o m M a g n e t i t e 155
5.4.6.2 F i r i n g Pattern for Pellets f r o m O t h e r Ores 157
6 Behaviour of Indurated Pellets During Reduction . . . 159
6.1 C h a n g e of Pellet Structure D u r i n g R e d u c t i o n . . . . 162
6.1.1 R e d u c t i o n M e c h a n i s m s 163 6.1.2 Structural C h a n g e D u r i n g R e d u c t i o n 165
6.1.2.1 V o l u m e Variation by Crystal C h a n g e 165 6.1.2.2 Structural C h a n g e by R e a c t i o n of G a n g u e C o m p o n e n t s
with Iron Oxides and with E a c h O t h e r 169 6.1.2.3 Influence of Additives o n P e l l e t Swelling 170 6.1.2.3.1 Properties of Acid Pellets with a Basicity of Less t h a n 0.1 174
6.1.2.3.2 Properties of Pellets with a Basicity of 0.1 to 0.6 . . 174 6.1.2.3.3 Properties of Pellets w i t h a Basicity of H i g h e r t h a n 0.7 . 176 6.1.2.4 Influence of G a n g u e B o n d s on Pellet Structure at T e m p e r
6.1.2.5 Influence of G a n g u e Bonds at A b o u t 1 0 0 00C on Pellet
Structure U n d e r R e d u c i n g A t m o s p h e r e 178 6.2 B e h a v i o u r of I n d u r a t e d Pellets Consisting of Magnetite
a n d Wüstite D u r i n g R e d u c t i o n 178
6.3 Conclusions 180
7 Special Processes for Pellet Production 181
7.1 Pellet H a r d e n i n g by Using Binders 181
7.1.1 G r a n g c o l d Process 181 7.1.2 C O B O and M T U Process 182 7.2 Chloridizing Volatilization of N o n - F e r r o u s Metal Oxides
and Pellet P r o d u c t i o n 183 7.2.1 Chloridizing Volatilization and Pelletizing in the Shaft
F u r n a c e 184 7.2.2 Chloridizing Volatilization and Pellet P r o d u c t i o n in a
R o t a r y Kiln 186 7.3 Recovery of V a n a d i u m Pentoxide f r o m V a n a d i u m
-Bearing Iron Ores 188 7.3.1 T h e r m a l T r e a t m e n t of V a n a d i u m - B e a r i n g Pellets in a
S h a f t F u r n a c e . . . 189 7.3.2 T h e r m a l T r e a t m e n t of V a n a d i u m - B e a r i n g Pellets
A c c o r d i n g to the G r a t e - K i l n Process 190 7.4 D e a r s e n i f i c a t i o n and Pelletizing of Iron Ores . . . . 190
8 Balling Equipment 192
8.1 H o m o g e n e i t y of C o m p o n e n t s for Pelletizing Mixtures 192 8.2 D e m a n d s on the M o d e of O p e r a t i o n of Balling U n i t s 193
8.2.1 Ball F o r m a t i o n a n d O p e r a t i o n of the Principal Pelletizing Units . 194 8.2.2 Balling D r u m 196
8.2.2.1 T h e Principal D r u m C o m p o n e n t Parts and the Balling
O p e r a t i o n . . . 196 8.2.2.1.1 T h e M a i n C o m p o n e n t Parts . . . 196
8.2.2.1.2 O p e r a t i o n of the Balling D r u m 197 8.2.2.2 Influencing Factors for G r e e n Ball F o r m a t i o n in a D r u m 198
8.2.2.2.1 D r u m R o t a t i n g Speed 198 8.2.2.2.2 L e n g t h and Tilt Angle of D r u m 199
8.2.2.3 Balling D r u m Capacity 201
8.2.3 Balling Disc 202 8.2.3.1 T h e Principal C o m p o n e n t Parts a n d D i s c O p e r a t i o n . 202
8.2.3.1.1 T h e M a i n C o m p o n e n t Parts 202 8.2.3.1.2 O p e r a t i o n of the Balling Disc 203 8.2.3.2 Influencing Factors of G r e e n Ball F o r m a t i o n on a Disc 204
8.2.3.2.1 D i s c Slope a n d R i m H e i g h t 205 8.2.3.2.2 D i s c R o t a t i n g Speed 206 8.2.3.2.3 Residence T i m e of M a t e r i a l in the D i s c 206
8.2.3.2.4 D i s c D i a m e t e r 207 8.2.3.3 Balling D i s c C a p a c i t y 207 8.2.4 C o m p a r i s o n Between Balling D r u m a n d Balling D i s c . . 208
8.2.5 C o m p a r i s o n of Vibrating a n d Roller Screens 209
8.2.6 O t h e r Balling Systems 210 8.2.6.1 Balling C o n e 210 8.2.6.2 M i x G r a n u l a t o r 210 8.2.6.3 V i b r a t i n g T r o u g h 211 8.2.6.4 Eccentrically M o v i n g U n i t 211 8.3 H a n d l i n g a n d F e e d i n g Devices 211 8.3.1 Roller C o n v e y o r 212 8.3.2 Roller Screen 213 8.3.3 Rolling Belt Conveyor 213
9 Heat Treatment Systems 215
9.1 S h a f t F u r n a c e . . . 216 9.1.1 Shaft F u r n a c e Types 217 9.1.2 Process Stages 218 9.1.2.1 C h a r g i n g of G r e e n Balls to the F u r n a c e 218 9.1.2.2 D r y i n g , P r e h e a t i n g a n d F i r i n g 219 9.1.2.3 Cooling of Pellets 221 9.1.2.4 H e a t C o n s u m p t i o n 221 9.1.3 F u r n a c e Dimensions, C a p a c i t y a n d M a r k e t Position . 221 9.2 T h e G r a t e - K i l n C o m b i n a t i o n 223 9.2.1 T h e Travelling G r a t e and its F u n c t i o n s 224
9.2.2 T h e R o t a r y K i l n and its F u n c t i o n s 225
9.2.3 T h e Cooler 226 9.2.4 H e a t C o n s u m p t i o n 226 9.2.5 C u r r e n t State and M a r k e t Position of the Allis C h a l m e r s
G r a t e - K i l n Process 227 9.2.6 O t h e r G r a t e - K i l n Processes 228 9.3 Travelling G r a t e Systems ' 229 9.3.1 Application of Travelling G r a t e s to T h e r m a l T r e a t m e n t of Pellets 230 9.3.2 G e n e r a l F e a t u r e s 231 9.3.3 U p - d r a u g h t I n d u r a t i o n Process for S p e c u l a r H e m a t i t e 232 9.3.4 Travelling G r a t e Process A c c o r d i n g to A r t h u r G . M c K e e and C o m p a n y 233 9.3.5 L u r g i - D r a v o Travelling G r a t e Process 235 9.3.5.1 I m p o r t a n t Process F e a t u r e s 235
9.3.5.2 A p p l i c a t i o n of the Process on an Industrial Scale . . 236 9.3.5.2.1 C h a r g i n g of G r e e n Balls to I n d u r a t i o n G r a t e and the
M o d e of O p e r a t i o n • 237
9.3.5.2.2 F i r i n g Pattern and H e a t C o n s u m p t i o n 238 9.3.5.2.3 Capacity, Flexibility and M a r k e t Situation 239
9.4 O t h e r H e a t T r e a t m e n t Systems 241 9.4.1 Circular I n d u r a t i n g F u r n a c e 242 9.4.2 " H e a t F a s t " Process 242 9.4.3 " A n n u l a r F u r n a c e " of H u n t i n g t o n - H e b e r l e i n 243 9.5 C o m p a r i s o n of I m p o r t a n t Pelletizing Systems . . . . 243 9.5.1 C h a n g e of O r e Basis 244 9.5.2 P r o d u c t i o n Figures per U n i t of V a r i o u s I n d u r a t i n g Systems 244 9.5.3 P r o p o r t i o n of Various F i r i n g Systems in the W o r l d Pellet
P r o d u c t i o n 245 9.5.4 Cost C o m p a r i s o n . 246
10 Plant Layout and Process Control 247
10.1 Plant L a y o u t . 247 10.2 Process Control . . . . 247
10.2.1 D i s t r i b u t i o n a n d Proportioning of Material F l o w . . . 249 10.2.2 P r o p o r t i o n i n g of Solid C o m p o n e n t s a n d W a t e r . . . 249
10.2.3 F o r m a t i o n and Transport of G r e e n Pellets 249
10.2.4 G r e e n Pellet Charging 249 10.2.5 T h e r m a l T r e a t m e n t of G r e e n Balls 250
10.3 D e v e l o p m e n t and Trends of F u r t h e r Control Variants 250
11 Pellets in the Blast Furnace Burden 251
11.1 Influence of Mechanical Properties . . . 251
11.2 Influence of C h e m i c a l C o m p o s i t i o n 252 11.3 M e t h o d s of Pellet Charging to the Blast F u r n a c e . . 253
11.4 C o m p a r i s o n of Pellets a n d Sinter 253 11.5 Pellet P r o p o r t i o n in the Blast F u r n a c e B u r d e n . . . . 255
12 The Utilization of Pellets in Direct Reduction Plants . 257
13 Some Theoretical Considerations 259
13.1 G r e e n Ball F o r m a t i o n 262 13.1.1 Bonds Between W a t e r and F i n e - G r a i n e d Particles . . 263
13.1.1.1 B o n d i n g by L i q u i d Bridges 263 13.1.1.2 B o n d i n g Forces in Transition R a n g e 264
13.1.1.3 B o n d i n g Forces in Capillary R a n g e 264
13.1.2 I n f l u e n c e of G r a n u l o m e t r i c Properties o n G r e e n Pellet
Strength 265 13.1.3 - Influence of Rolling Forces D u r i n g M o v e m e n t of G r e e n
Pellets 266 13.1.4 Design a n d O p e r a t i o n of Balling D i s c a n d Balling D r u m 267
13.1.4.1 D e s i g n and D i m e n s i o n s of Balling D i s c 267
13.1.4.2 R o t a t i n g Speed a n d D i s c Slope 267 13.1.4.3 Balling D r u m and its D e s i g n D a t a 268 13.2 T h e r m a l T r e a t m e n t of G r e e n Pellets 269 13.2.1 H e a t T r a n s f e r in a Pellet C h a r g e on the Travelling G r a t e
or in the Shaft F u r n a c e 269 13.2.2 H e a t T r a n s f e r of G r e e n Pellets by C o n v e c t i o n a n d G a s
F l o w 270 13.2.2.1 G a s F l o w 270 13.2.2.1.1 Resistance of a C h a r g e of U n i f o r m Pellet Size to G a s
F l o w 271 13.2.2.1.2 Resistance of Pellet C h a r g 6 of D i f f e r e n t Pellet Size to
G a s F l o w 271 13.2.2.1.3 Resistance of Pellet C h a r g e and Travelling G r a t e to G a s
F l o w 272 13.2.2.2 H e a t T r a n s f e r by Convection 273
13.2.2.2.1 H e a t T r a n s f e r to the Pellet C h a r g e o n the Travelling
G r a t e 275 13.2.2.2.2 H e a t T r a n s f e r to the Pellet C h a r g e in the S h a f t F u r n a c e . 275
13.2.3 H e a t T r a n s f e r to the Pellet C h a r g e in the R o t a r y K i l n . . 275 13.2.4 H e a t T r a n s f e r by R a d i a t i o n . .. . . . 276
13.2.4.1 G a s R a d i a t i o n 277 13.2.4.2 K i l n Lining R a d i a t i o n 277 13.2.5 H e a t C o n d u c t i o n 278 13.2.5.1 H e a t C o n d u c t i o n Inside the Pellet . . . 278
13.2.5.2 H e a t C o n d u c t i o n by Points of C o n t a c t 279 13.3 G r e e n Ball Drying 279 13.3.1 D r y i n g of an lndividual Pellet 280 13.3.2 D r y i n g of a Pellet C h a r g e 282 Final Remarks 286 References : 288 Subject Index . . 298
Introduction
In contrast to sintering, for which the down-draft sintering method is only succesfully employed, pellets are today indurated according to three methods: in the shaft furnace, in grate-kilns and on travelling grates.
The greatly varying properties of ores resulting f r o m their origin, genesis, shape, crystal form and chemical composition are to be taken into account for ore preparation and pelletizing in order to produce at any time pellets of uniform and good quality. Nowadays, measures are known, by which the differences in the ore properties can be compensated. However, the corresponding parameters have to be variable and selected according to the nature of ores involved. In practice, this means that the design of new plants or the conversion of existing ones to other ore types cannot be based on generalized programmes. In each particular case, it is practically unavoidable to find out the o p t i m u m parameters by the performance of tests. This is a decisive factor for the concept and composition of this book, which virtually relies on experimental knowledge and comprises the following chapters:
— The most important development phases for pelletizing and the under-lying causes are described in Chapter 1.
— The fundamentals for successful green ball formation and induration are described in Chapter 2.
— Chapter 3 deals with the utilizable ores and additives as well as their
preparation for processing into pellets.
— In Chapter 4 adequately equipped laboratories and the testing of pellet quality according to different test standards are described.
— The efficiency and kind of the individual influencing factors are to be found out in adequate tests for obtaining a uniform pellet quality. These considerations are the subject of Chapter 5.
— The decisive quality criterion is the behaviour of pellets during reduc-tion, Chapter 6. Experiments and considerations in connection with oxy-gen removal are described although despite many efforts some question have not yet been fully clarified.
— Special processes for pellet i n d u r a t i o n a n d the possibilities of utilizing unusual raw materials are dealt with in Chapter 7.
— A survey of the m a i n e q u i p m e n t and furnaces used is given in Chapters
8 and 9. A t the s a m e time, these chapters show the i m p o r t a n c e of the
cooperation of process engineers, mechanical engineers and designers for ensuring a successful process application.
— F u r t h e r m o r e , s o m e c o m m e n t s on plant design and a u t o m a t i c process control are m a d e . A direct conversion of the collected data to other con-ditions should be considered carefully to avoid misinterpretation,
Chapter 10.
— T h e b e h a v i o u r of pellets in the blast f u r n a c e or in direct reduction plants is very essential for justifying the application of the pelletizing process a n d its f u t u r e importance, Chapters 11 and 12.
— In order not to i n t e r r u p t the continuity of the various chapters by theo-retical or m a t h e m a t i c a l considerations, s o m e interesting f o r m u l a e and equations are c o m p i l e d in Chapter 13 and simultaneously references are m a d e to relevant literature.
This b o o k will neither be a m a n u a l for the construction of industrial pelletizing plants nor gives it precise instructions for pellet p r o d u c t i o n since the u n d e r l y i n g conditions considerably differ d u e to the great variety of raw material characteristics.
T h e p u r p o s e of this b o o k is rather to investigate and to describe the possibilities a n d m e t h o d s to p r o d u c e pellets of good and u n i f o r m quality irrespective of the varying properties of raw materials.
T h e existing a n d ever increasing a m o u n t of literature was considered in-sofar as it was useful for the c o n f i r m a t i o n and support of relevant theories, p a r a m e t e r s a n d correlations.
1 Definition and Development of Pelletizing
Process
1.1 Definition
Pellets are balls produced from concentrates and natural iron ores of different mineralogical and chemical composition with some remarkable properties such as:
— uniform size distribution within a m a i n range of 9—15 m m diameter — high and even porosity of 2 5 - 3 0 %
— high iron content of more than 63% iron — practically no loss on ignition or volatiles
— uniform mineralogical composition in the form of an easily reducible hematite or hematite-bearing compounds
— high and uniform mechanical strength
— low tendency to abrasion and good behaviour during transportation — sufficient mechanical strength even at thermal stress under reducing
atmosphere.
1.1.1 Differentation Against O t h e r Iron Ore A g g l o m e r a t e s
T h e simplest and earliest process for agglomerating fine-grained raw materials is briquetting. Fine-grained iron ores, for example, are pressed into briquettes with the addition of some water or another binder under high mechanical pressure. These briquettes m a y undergo direct further treatment or thermal processing before their use. Although their metallur-gical behaviour in melting or reduction furnaces is very good, the iron ore briquetting could not m a k e headway since the processing costs are relati-vely high and, above all, the production capacity of briquetting units is
li-mited when compared with the enormous quantities of fine ores or concentrates to be agglomerated. The briquetting process is still utilized to agglomerate small quantities of
dust or other circulating materials. This process has, of late, acquired growing importance for briquetting of fine-grained sponge iron.
Fig. 1. Comparison of Briquettes, Pellets and Sinter
T h e second a n d presently m o s t i m p o r t a n t agglomeration process is
down-draught sintering. It differs f r o m pelletizing by various
character-istics, such as:
— feed of coarser-grained ore particles u p to a d i a m e t e r of 8 m m — coke breeze as m a i n energy source
— heating up of the granulated m i x to slightly above the softening t e m p e r a t u r e
— the final p r o d u c t consists of a spongy sinter cake, partly molten, which by crushing, grinding and screening, is b r o u g h t to the necessary grain size of 5 - 3 0 or 5 - 5 0 m m . Fig. 1 shows the different outer s h a p e of the agglomerates p r o d u c e d according to the three processes.
1 . 1 . 2 P r i n c i p a l P r o c e s s S t e p s for the P r o d u c t i o n of P e l l e t s
T h e first stage is the f o r m a t i o n of green balls. Fine-grained iron ores having a d e q u a t e size distribution are rolled with the addition of a wetting liquid, usually water, in suitable devices such as d r u m s or discs. In this way, wet balls are f o r m e d , the so-called green pellets. D u r i n g the ball formation, it is also possible to use other additives for i m p r o v i n g the properties of green a n d fired pellets, e.g. bentonite, and for changing the metallurgical properties of the i n d u r a t e d pellets, e.g. limestone or do-lomite.
In a second step the green pellets are dried a n d i n d u r a t e d to obtain the typical features of pellets. This is achieved, in most cases, by careful heating u n d e r oxidizing a t m o s p h e r e to just below the softening point of the
ores used. D u r i n g this heating, not only the crystalline structure is c h a n g e d but also other bonds a p p e a r , such as reactions b e t w e e n slag-forming con-stituents — both between each other a n d with iron oxides.
T h e hot pellets are carefully cooled in o r d e r to m a i n t a i n as far as possible the resulting crystalline structures a n d other b o n d s as well as to avoid tension cracks.
Green pellets can also be i n d u r a t e d by hydraulically acting binders, e.g. cement or calcium hydroxide, possibly by using s t e a m u n d e r high pressure. However, s o m e properties of such pellets d i f f e r f r o m thermally indurated pellets (see C h a p . 7).
1.2 Development of Pelletizing Process
Iron ore agglomerates, be they briquettes, sinter or pellets, are not the final products. T h e y are f o r m e d f r o m such f i n e - g r a i n e d iron ores which, in this physical shape, cannot be utilized and serve as an intermediate product on the way f r o m the ore m i n e to the blast f u r n a c e or direct r e d u c t i o n plant. T h e sole p u r p o s e of agglomerate p r o d u c t i o n is to k e e p the cost price of pig iron or steel at the lowest level. F o r m a n y years until a b o u t the turn of the century, the iron ores charged to blast furnaces h a d been crushed and partly classified either at the m i n e or at the iron a n d steel works. In this case l u m p ores were p r e f e r r e d a l t h o u g h small portions of fine ores could be tolerated.
As a result the fines which were not utilised f o r m e d continuously grow-ing d u m p s with no economic use. T h e y could only be e m p l o y e d to a limited extent in the blast f u r n a c e since they decreased the gas p e r m e -ability of the blast f u r n a c e b u r d e n in an irregular m a n n e r a n d disturbed the blast f u r n a c e operation.
Moreover, a great p a r t of these fines was blown out of the blast f u r n a c e and h a d to be recovered as flue dust. T h e s e dust quantities represented a considerable iron value which, like the u n u s e d fine ore d u m p s , was lost; this was of lesser i m p o r t a n c e in countries with great iron reserves than in those with small iron reserves. T h e a m o u n t of the a c c u m u l a t i n g dust depends largely on the ore type treated. In the case of M i n e t t e or other ores with a high loss of ignition, it is substantially greater t h a n in the case of high-grade, dense ores with a small loss on ignition.
Possibilities were e x a m i n e d a n d tests to a g g l o m e r a t e the flue dust by
sintering or briquetting a n d to recycle it to the blast f u r n a c e were started at
approximately the turn of the century in various industrialised countries although with d i f f e r i n g intensities. Countries with i m p o r t a n t iron reserves were less interested in this agglomeration. T h e y considered sintering as a
"necessary evil" 1sobrescrito). T h e situation was quite different in countries with small ore reserves. H e r e the d e v e l o p m e n t of t h e sinter process continued intensively a n d not only flue dust b u t also o t h e r iron-bearing secondary raw materials such as pyrite cinders, mill scale or red m u d were of great interest. T h e sintering of fines, o b t a i n e d d u r i n g the crushing and screening of unclassified l u m p ores, was also gaining significance.
At a b o u t the s a m e time, s o m e researchers were looking for an alterna-tive process to sintering, especially in areas in which very fine ores or con-centrates were available. This was the beginning of the pelletizing process.
1 . 2 . 1 First P h a s e , Alternative to S i n t e r i n g
T h e different stages of development, progress and speed of introduction of the sintering process, especially if very fine iron ores were to b e treated, led to considerations for i m p r o v i n g the process a n d finally for developing a n alternative to sintering, namely pelletizing.
A b o v e all, countries such as S w e d e n or G e r m a n y2) , which in the very early days h a d already b e e n compelled to give particular attention to sintering, h a d to solve the p r o b l e m of processing increasing a m o u n t s of very fine concentrates. U p o n using m a j o r portions of such fines in the sinter mix, limits on the specific p r o d u c t i o n capacity of sinter plants b e c a m e evident. T h i s b r o u g h t a b o u t , first in Sweden, the d e v e l o p m e n t of pelletizing. Concentrates were no longer a d d e d to the sinter mix but were separately f o r m e d into balls with the addition of water and then i n d u r a t e d by using binders or in a thermal way. Such a p a t e n t h a d already been granted in 1912 under No. 35 124 to the Swede A. G . Andersson. U n -fortunately, n o f u r t h e r details or metallurgical results were p u b l i s h e d 3). A l m o s t simultaneously, similar research a n d d e v e l o p m e n t work was started in G e r m a n y . In 1913, a G e r m a n patent N o . 289606 was granted to the inventor C. A. Brackelsberg. This patent protects a process according to which ore fines were mixed with water or binders, f o r m e d into balls and indurated at low temperatures.
N o consequences resulted f r o m the Swedish patent whereas Brackelsberg continued his w o r k4) . O n e of the most interesting test results was the knowledge t h a t the pellets (referred to as "GEROELL" derivated f r o m
roll-ing) could b e m o r e quickly r e d u c e d than l u m p ore or sinter m a d e of the s a m e raw material. In the course of this work, a pilot p l a n t5) with a capacity of 120 tons per day "GEROELL" was built in 1926 for K r u p p at the R h e i n h a u s e n steel plant. This plant was reconstructed in 1935, and already showed essential features of the pelletizing process. This pilot plant was dismantled in 1937 to m a k e available the area r e q u i r e d for the con-struction of a large sinter plant.
In this way, the first d e v e l o p m e n t p h a s e c a m e to a n a b r u p t end. T h e pelletizing was forgotten. Sintering spread as the only i m p o r t a n t agglom-eration process t h r o u g h o u t the entire world. T h e pelletizing k n o w - h o w was practically lost until the way was p a v e d for the second phase, especially in the U S A a n d again in Sweden.
1 . 2 . 2 S e c o n d P h a s e , P e l l e t s f r o m C o n c e n t r a t e s
T h e second p h a s e was initiated by the p r o b l e m of securing the ore supply f r o m the L a k e S u p e r i o r region, especially f r o m the M e s a b i R a n g e . F r o m there, m a n y iron a n d steel works in the U n i t e d States h a d h i t h e r t o been supplied with high-grade l u m p ores a n d coarse-grained concentrates with an iron content of 50% and m o r e , d e m a n d i n g n o f u r t h e r treatment. D u r i n g and at the end of the Second W o r l d W a r , the reserves of such high-grade ores were on the decline so t h a t other sources had to b e opened up. O n e of the richest deposits in the M e s a b i R a n g e contained large ore reserves, the well-known " t a c o n i t e s " which h a v e a. low iron content, a b o u t 30% total iron, almost exclusively in the f o r m of magnetite. These taconites are mechanically very h a r d . T o liberate the magnetite, very finely disseminated t h r o u g h the ore, a very f i n e grinding was necessary which, after m a g n e t i t e separation, yielded concentrates with more t h a n 85% fines m i n u s 325 m e s h (0.044 m m ) .
A typical analysis of such a concentrate, h a v i n g b e e n treated in the plant of Reserve M i n i n g C o m p . , Silver Bay M i n n e s o t a , is s h o w n in T a b l e 16) .
Table 1. Chemical composition and grain structure of magnetite concentrate in pellet plant of Reserve Mining Company 8)
Chemical analysis, dry Structure
% mesh wt-% Cumu-lative wt-% Fe 65.5 + 100 0.1 0.1 SiO2 7.8 + 150 0.6 0.7 Al2O3 0.5 + 200 1.5 2.2 CaO 0.5 + 270 4.0 6.2 MgO 0.6 + 325 3.5 9.7 Mn 0.25 + 400 9.0 18.7 P 0.032 + 500 8.4 27.1 S 0.003 - 5 0 0 72.9 100.0 TiO2 0.10 Fe2+ 21.75 Specific
Moisture, 10.00 Surface Area
T h e high fine ore portion of a b o v e 96% m i n u s 325 mesh is too fine-grained for efficient sintering d u e to the low permeability of the sinter mix.
A r o u n d 1943 intensive d e v e l o p m e n t of the pelletizing process for taconite concentrates was started under g u i d a n c e of and at the MINES
EXPERIMENTAL STATION OF THE UNIVERSITY OF MINNESOTA.
W h e n this d e v e l o p m e n t work b e c a m e k n o w n in Sweden, the Jernkon-toret (Swedish iron and steel institute) in Stockholm f o u n d e d a c o m m i t t e e in 1946 to apply concentrates 7), a historical curiosity in r e m e m b r a n c e of the patent specification by Andersson.
T h e work of the Swedish researchers, u n d e r the direction of M a g n u s Tiegerschiöld, soon led to the construction of several small industrial plants. T h e pellets p r o d u c e d in these plants were, inter alia, used for direct reduction of iron ores according to the W i b e r g process with r e m a r k a b l e success 8). T h i s was a first indication that pellets were particularly suitable for direct reduction and it was a decisive impulse for the f u r t h e r d e v e l o p m e n t of direct reduction processes. A t first, pellet p r o d u c t i o n and further d e v e l o p m e n t of corresponding processes were limited to the two above regions where particularly f a v o u r a b l e conditions prevailed. T h e concentrates p r o d u c e d there were so fine-grained that they could be
formed into green pellets without further grinding. Even if pelletizing
initiated an interesting development, other alternative processes for the ag-g l o m e r a t i o n of taconite concentrates were also tested. At almost the s a m e t i m e that the first m a j o r publication on the d e v e l o p m e n t progress of the new pelletizing process was m a d e in 1944 9), the Oliver M i n i n g Division of U. S. Steel C o r p o r a t i o n in Extaca, Virginia, M i n n e s o t a decided to develop further, on a large scale, another a g g l o m e r a t i o n m e t h o d , the so-called "nodulizing process" for the p r o d u c t i o n of nodules f r o m fine-grained taconite concentrates in a d u f f coal fired rotary kiln1 0). At the same site a sintering plant was built a l t h o u g h normally such plants h a d hitherto b e e n erected near the blast furnaces. A f t e r a period of initial troubles, b o t h plants were started u p and operated. However, a f t e r a few years, they were shut down. T h e p r o d u c t i o n of pellets — whose qualities were increasingly gaining recognition — advanced so rapidly that f r o m then onwards pelletizing plants were exclusively built. In these plants, the ever-growing quantities of concentrates p r o d u c e d could be processed successfully so that the original goal of also opening u p deposits with a low iron content was fully achieved. A r o u n d 1955, the second phase of the pelletizing process development was terminated with the start-up of the two large pelletizing plants of Reserve M i n i n g Co. and Erie Mining Co. with an annual capacity of 12 million tons. T h e successful d e v e l o p m e n t of the different pelletizing processes in the M e s a b i R a n g e is the result of intensive cooperation be-tween big mining companies, e.g. Erie and Reserve Mining Co. with
com-panies, contractors and suppliers. T h e latter could partly use machinery,
e q u i p m e n t a n d structures already k n o w n and a p p r o v e d . T w o firing systems for the i n d u r a t i o n of green pellets were e m p l o y e d on a large-scale. Erie M i n i n g used the s h a f t f u r n a c e 11sobrescrito), and Reserve M i n i n g a m o d i f i e d sintering m a c h i n e 6). In Sweden, the s h a f t f u r n a c e was exclusively used during this period.
W i t h o u t the intention or c o m p u l s i o n of securing the f u t u r e ore s u p p l y f r o m the M e s a b i Range, d e v e l o p m e n t of the pelletizing process would probably not h a v e b e e n started at this location.
1.2.3 Third P h a s e , P e l l e t s f r o m O r e s
T h e close correlations between sintering and pelletizing b e c a m e explicit during both the first and second d e v e l o p m e n t phases. Since there was no compulsion at that time to develop f u r t h e r the ingenious knowledge of Andersson a n d Brackelsberg, the first p r o m i s i n g steps to pelletizing were soon forgotten.
In the second phase of d e v e l o p m e n t the sinter process was exclusively confronted with ever-increasing quantities of very fine-grained magnetite
concentrates, so that the p r o d u c t i o n capacity was closely limited. T h e
enormous quantities of concentrates a n d the necessity of f i n d i n g a solution to the p r o b l e m gave the i m p e t u s to develop pelletizing u p to a process applicable to industrial operation. A t that time, neither in the U S A nor in Sweden was there a need for pelletizing o t h e r ores t h a n concentrates. T h e third phase started f r o m quite a n o t h e r situation.
Various newly developed steps f o r i m p r o v i n g the sinter process were m o d i f i e d a n d succesfully c o m b i n e d to f o r m the basis for a new pelletizing process variant for iron ores and mixtures. T h i s process was developed in parallel to the knowledge derived f r o m the pelletizing of concentrates. T h e sintering of iron ores h a d a very d i f f e r e n t i m p o r t a n c e in various countries according to the availability of ore reserves. Consequently, the readiness to give this process an a d e q u a t e position w i t h i n the context of pig iron production in conjunction with the operation of the blast furnace differed greatly. This was a p p a r e n t at the end of the F i r s t W o r l d W a r as well as at the beginning of and d u r i n g the Second W o r l d W a r .
T h e necessity also to p r o d u c e pig iron f r o m low-grade fine-grained ores resulted in intensified d e v e l o p m e n t w o r k in the sintering field 12). T h i s development work referred b o t h to process p a r a m e t e r s a n d constructional i m p r o v e m e n t s of the necessary m a c h i n e r y a n d e q u i p m e n t . T h e most recent d e v e l o p m e n t progress in sintering is described in a book b y F. Cappel and H. B. W e n d e b o r n2) . A g a i n after the Second W o r l d W a r in a varied raw material situation highest p r o d u c t i o n rates of the sinter
m a c h i n e at m i n i m u m fuel c o n s u m p t i o n for the p r o d u c t i o n of agglomerate were the conditions to be fulfilled. Flotation pyrite cinders a n d Swedish concentrates constituted growing portions of the ore mix. T h e greater fineness resulted in a r e d u c e d specific o u t p u t (in tons s i n t e r / m2 area in 24 hours) as is clearly demonstrated in Fig. 2. W i t h a rising p r o p o r t i o n of fines —0.2 m m a considerable o u t p u t decrease was observed 13).
F r o m this resulted the d e m a n d to keep the fines portion a p p r o x i m a t e l y —0.2 m m as low as possible. F o r example, in G e r m a n y it was expected that the portion —0.125 m m should not exceed 10%2). T h e fine ores at present available on the world m a r k e t generally contain a m u c h higher percentage of fines. According to experience, a corresponding decrease of output of sinter plants in operation t h r o u g h o u t the world would b e un-avoidable. F o r t h e owners of such sinter plants supplied with ores f r o m mines all over the world and not f r o m their own mines, this would m e a n higher p r i m e costs for sinter and thus for pig iron.
T h e designers a n d suppliers of such sinter plants were also c o n f r o n t e d with the above p r o b l e m . It was obvious that this u n a v o i d a b l e deficiency of
fine ores had to be compensated for. T h e separation of fines from the sinter
mix could be theoretically conceivable (but difficult to realise) as is demonstrated by the test described below 14).
A sinter mix consisting of 11 different ore c o m p o n e n t s with a grain size portion of a b o u t 40% —0.5 m m was sintered at a capacity of X t o n s / m2 per day after careful preparation. T h e same ore was screened at 0.5 m m a n d only the coarser fraction was sintered. In this case, the capacity rose by a b o u t 35% c o m p a r e d to the first test.
A second m e t h o d of compensating the reduced capacity to be expected would be the e n l a r g e m e n t of the suction area of the sinter strand. H o w
ever, a sinter strand with a suction area designed for specific ores would not be sufficiently flexible to compensate for the capacity fluctuations resulting f r o m varying ore mixtures and the correspondingly d i f f e r e n t fines portions. N o r would the separation of the very fine particles or the extension of the suction area b e considered suitable m e a s u r e s to solve this p r o b l e m .
1.2.3.1 The Two-Stage Granulation of the Sinter M i x
Finally, the introduction of a new process stage b r o u g h t the expect-ed solution. A f t e r the detrimental effect that excessive fines portion has on the sinter o u t p u t b e c a m e known, the sinter mix p e r m e a b i l i t y was im-proved in intensive laboratory tests by f u r t h e r t r e a t m e n t of the pre-pared sinter mix. In these tests the p r e p a r e d mix was conveyed to a second mixer in which it was additionally rolled. This process v a r i a n t is known as two-stage granulation (rerolling) a n d now f o r m s p a r t of m o s t of m o d e r n sinter plants in o p e r a t i o n t h r o u g h o u t the world. By reroll-ing, the very fine particles a d h e r e to coarser particles. T h e ore m i x now contains small balls a n d ore particles with a size distribution of about 0 . 5 - 8 m m . Micro-pellets are f o r m e d as can be seen f r o m the comparison s h o w n in Fig. 3. A n o r m a l sinter mix a n d a sinter m i x subjected to this after-treatment show distinct differences in the size distribution a n d shape.
T h e rerolling a p p a r a t u s , e.g. a d r u m , is e q u i p p e d in a d i f f e r e n t way to a normal mixing d r u m . It is practically identical to a balling d r u m .
T h e rolling effect a n d the d i f f e r e n t sintering time for s o m e high-grade iron ores with good sintering properties are s h o w n in s o m e tests described
Fig. 4. Influence of re-rolling of a Venezuelan ore feed on sinter plant capacity
below which were carried out with and without portions of fine-grained flue dust1 5).
Venezuelan fine ores (mix A) are mixed with coke breeze, return fines and water a n d are sintered at a p r o d u c t i o n rate of 48 t / m2/ 2 4 h. D u r i n g rerolling, only a very slight o u t p u t increase is observed. By the a d d i t i o n of 10% flue dust (mix B), the o u t p u t drops to a b o u t 38 tons. By rerolling over 5 minutes the o u t p u t is re-increased to the previous value (see Fig. 4). A substantially lower specific o u t p u t ( a b o u t 22 t / m2/ 2 4 h) is obtained with another ore mix having a high p r o p o r t i o n of grains - 0 . 2 m m . If this m i x is rerolled b e f o r e sintering, the o u t p u t can be gradually raised to m o r e t h a n 30 tons, which represents an increase of almost 38% (Fig. 5).
Fig. 5. Influence of re-rolling of a sinter mix (magnetite concentrate, flue dust,
By this simple process variant, which is at present applied t h r o u g h o u t the world, it is possible to i m p r o v e the sinter mix p e r m e a b i l i t y and thus the sinter process in such a way that t o d a y portions of m o r e t h a n 10% fines - 0 . 2 m m can b e used. T h e result of this i m p r o v e d sintering m e t h o d was that in industrialised countries p u r c h a s i n g a high percentage of extraneous ores (Japan, G r e a t Britain, G e r m a n y ) , there was b u t little incentive to in-troduce the pelletizing process.
Nevertheless, the p r o d u c t i o n of micro-pellets was the first step towards pelletizing in these countries. T h i s m e t h o d of p r o d u c i n g micro-pellets, virtually d e v e l o p e d in G e r m a n y , was, in one particular case, a d o p t e d to
100% flotation pyrite cinders without the expected success.
1.2.3.2 Pellet Sintering
T h e sinter process was then m o d i f i e d in such a way that not the total mix but only the fine raw material was f o r m e d into balls of 3 - 6 m m d i a m e t e r which were m i x e d with return fines and coke breeze a n d sintered. T h i s process has b e c o m e k n o w n as pellet sintering and is an in-termediate process between n o r m a l sintering and pelletizing. T h e final product is a sinter cake in which the pellet structure of the raw mix can still be recognised in the finished sinter p r o d u c t . Such a pelletizing plant was operated for several years by C O M I N C O in Trail, C a n a d a . Similar results were o b t a i n e d in an u p - d r a u g h t pelletizing plant erected by Cleveland Cliffs Iron C o m p a n y n e a r Ishpeming, Michigan, called " E a g l e Mill" p l a n t1 6) . In this plant the total heat r e q u i r e d was s u p p l i e d in a solid state. T h e final p r o d u c t obtained consisted largely of sinter l u m p s and not, as expected, of pellets. However, n e i t h e r of the process variants was further developed. T h e s e plants are no longer in operation.
In m a n y countries w h i c h exclusively used the sinter process for iron ore agglomeration, a n o t h e r p r o b l e m arose, namely to supply the necessary solid, lean fuel, m a i n l y coke breeze. T h e r e was a scarcity of this fuel in certain countries because d u e to the considerably increasing n u m b e r of sinter plants being built, the necessary a m o u n t s of coke breeze, chiefly originating f r o m coke screening, were no longer sufficient to m e e t the requirements. It was necessary to crush high-grade, expensive metallurgi-cal coke for sintering. T h e search for c h e a p r e p l a c e m e n t fuel led to an increased use of blast f u r n a c e gas f r o m which a n o t h e r process variant of the normal d o w n - d r a u g h t sintering, n a m e l y the mixed firing method, was developed.
1.2.3.3 Mixed Firing Method
Normally, t h e coke breeze on the surface of the sinter b e d is ignited by the c o m b u s t i o n of flue gas in an ignition h o o d . T h e ignition h o o d length
Fig. 6. Influence of mixed firing on sintering of iron ores
and the d u r a t i o n of the influence of the hot c o m b u s t i o n gases is
accord-ingly limited. T h e heat a m o u n t developing inside the ignition h o o d is insignificant c o m p a r e d with the solid fuel a m o u n t contained in the sinter bed. W h e n adopting the mixed firing m e t h o d , the ignition h o o d is
substantially extended in order that a higher gas volume can be b u r n e d and greater heat quantities can be sucked into the sinter bed. W i t h such extended ignition hoods it is possible to decrease the coke c o n s u m p t i o n remarkably.
F l u e gas can be replaced by other heat sources, such as hot air of a b o u t 8 0 00C , oil or natural gas, according to their availability. C o k e breeze cannot be replaced by hot gas to an unlimited extent, without i m p a i r i n g the plant o u t p u t and sinter strength, as shown in Fig. 6. A n a d v a n t a g e of this m e t h o d is the better reducibility with increasing portion of hot gas1 7).
T h e mixed firing m e t h o d is n o w a d a y s o f t e n a d o p t e d particularly in countries w h e r e coke breeze is expensive. O n c e it h a d b e e n possible to p r o d u c e micro-pellets f r o m fine-grained m i x t u r e s by rolling of balls of 3—6 m m for pellet sintering, it was not very difficult to p r o d u c e green pellets of a greater d i a m e t e r and of a u n i f o r m , close size r a n g e ( 9 - 1 5 m m ) . However, it was not yet possible to i n d u r a t e these pellets exclusively with coke breeze or by a d o p t i n g the m i x e d firing m e t h o d in o r d e r to m e e t the pertinent requirements. A consistent f u r t h e r d e v e l o p m e n t of this firing technology by the exclusive use of gas or oil leads to the application of t h e pelletizing process. This firing technology applicable to the firing of green pellets f r o m iron ores was a d o p t e d inter alia to c e r a m i c mass, glass constituents, b u r n i n g of limestone 18).
T h e three d e v e l o p m e n t phases of pelletizing are s u m m a r i z e d below:
First Phase — Alternative to Sintering: 1910—1927. Besides the sintering
process u n d e r development, an alternative b e c o m e s a p p a r e n t , to agglom-erate very fine-grained ores by pelletizing w i t h o u t any consequences en-suing.
Second Phase - Pellets f r o m Concentrates: 1 9 4 0 - 1 9 5 5 . P r o d u c t i o n of very
fine-grained concentrates increasing considerably in s o m e regions, such as in the M e s a b i R a n g e / U S A , compels d e v e l o p m e n t of the pelletizing process as an alternative to sintering. T h e state of d e v e l o p m e n t at t h a t time did not yet allow for the use of h i g h p o r t i o n s of fines in the ore mix.
Third Phase - Pellets f r o m Ores: 1948 u p to the present date. In various
countries the sinter process is f u r t h e r d e v e l o p e d intensively to a d a p t it to the varying supply of ores of d i f f e r e n t fineness. As a result of this development work, the application range of the pelletizing process was also extended, b e y o n d the use of concentrates as only c o m p o n e n t s , to other ores and, moreover, with considerable success to ore mixtures, see item 9.3.5.
1.3 Pelletizing, a Contribution to Ore Preparation
F o r economic reasons, the treatment of iron ores in the blast f u r n a c e or in direct reduction plants is nowadays no longer possible w i t h o u t intensive
ore preparation. Even if individual process stages involve h i g h p r i m e costs,
these are accepted, provided that the total p r o d u c t i o n costs of pig iron or sponge iron can, in this way, b e k e p t at lowest level. T h e p u r p o s e of iron
ore p r e p a r a t i o n is the qualitative i m p r o v e m e n t of d i f f e r e n t features of raw materials:
(a) Mechanical crushing, grinding, screening, classification.
(b) Physical separation of various mineral constituents for the elimination of g a n g u e f r o m the lean ores and p r e p a r a t i o n of concentrates with high iron content.
(c) T h e r m a l or chemical treatment for the elimination of volatile energy-Consuming constituents such as H2O , C O2, SO4, S or conversion of hematite to magnetite.
(d) Metallurgical change by basic additives which decrease the energy c o n s u m p t i o n of the succeeding process stages.
In all four steps pelletizing plays a significant role, such as:
— Agglomeration of the finest ore particles or concentrates [(a) and (b)] — Volatilisation of components, such as H2O, CO2, SO4, S [(c)]
— Changing to the chemical composition by basic additives.
T h e entire c o m p l e x of present-day iron ore p r e p a r a t i o n is shown in Fig. 7.
Since the significance of "physical mixing" is recognised throughout the world and accepted, even iron-rich ores are crushed to a m a x i m u m l u m p size, for the blast f u r n a c e to approx. 30—50 m m , for direct reduction to approx. 2 0 - 3 0 m m . T h e eliminated fine ore c a n either b e sintered or a f t e r f u r t h e r fine g r i n d i n g pelletized. Low-grade ores with an iron content of less t h a n 50% are u p g r a d e d , the solid gangue constituents s e p a r a t e d and in so
Fig. 8. Size distribution of pellets
doing the iron content in the concentrate increased. According to its fineness the concentrate can be pelletized i m m e d i a t e l y or following f u r t h e r grinding. In certain proportions concentrates c a n also b e incorporated in sinter mixes. D u r i n g t h e r m a l t r e a t m e n t the s e p a r a t i o n of volatile c o m -ponents takes place in both pelletizing a n d sintering. Because of their d e f i n e d properties, high iron content together with u n i f o r m size distribu-tion and close size range pellets are an i m p o r t a n t c h a r g e constituent for blast furnaces a n d direct reduction plants.
T h e size distribution, shown in Fig. 8, as an average of three c o m m e r c i a l samples1 9) is r e m a r k a b l e . T h e conditions are very strict: 8 0 - 9 0 % of the pellets should have a d i a m e t e r of 9 - 1 5 m m , the m a j o r part of which a d i a m e t e r of 9—12 m m . T h e charge should, if possible, contain neither fines —5 m m nor pellets of a d i a m e t e r exceeding 25 m m . T h i s is also a n important size range for use with other i r o n - b e a r i n g c o m p o n e n t s in, f o r example, the blast furnace. T h e feed should consists of grains 5 to max. 50 m m , p r e f e r a b l y 30 m m ; Fig. 9 schematically shows the various o p e r a -tions leading to this desired size distribution: crushing of coarse ores a n d sinter cake, screening and separation into 3 fractions. T h e oversize is recycled to the grinding plant, the product of 5-50 mm or 30 mm goes into
the blast furnace and the undersize is conveyed to agglomeration, either
sintering or pelletizing plant. T h e undersize also includes concentrates, pyrite cinders, mill scale, flue dust a n d possibly additives. Pellets, crushed sinter and classified raw ore constitute, either alone or in the f o r m of mixtures, the blast f u r n a c e b u r d e n .
Fig. 9. Scheme of preparation and size distribution of blast furnace feed
Besides sinter, still the most i m p o r t a n t type of agglomerates for blast f u r n a c e b u r d e n s , pellet portions are, c o m p a r e d to l u m p ores, of growing i m p o r t a n c e for pig iron a n d sponge iron production. In Fig. 10 the devel-o p m e n t devel-of p i g irdevel-on p r devel-o d u c t i devel-o n f r devel-o m 1960 tdevel-o 1978 is c devel-o m p a r e d with sinter and pellet production. F r o m 1974 the curves for sinter and pig iron de-cline, the latter caused by economic reasons. T h e pellet curve r e m a i n s steep u p to an estimated average pellet portion of 25% in the blast furnace
burden.
In the pellet p r o d u c t i o n curve, Fig. 10, the a m o u n t of pellets as feed m a terial for direct reduction processes is included b u t the portion is r e m a r k a -bly higher by u p to 100%. In shaft furnaces efforts are m a d e to replace a part of the pellets by cheaper, carefully selected and screened l u m p ores. A m i x t u r e of 55—75% pellets and 45—25% l u m p ores has recently p r o v e d to b e suitable for o p t i m u m productivity, as shown in Fig. 11.
Fig. 10. World production of pig iron, sinter and pellets
Fig. 11. Influence of a mixture of pellets and sized lump ores on direct reduction
1.4 Sites of Pelletizing Plants and Transportability
of Pellets
As already m e n t i o n e d above, pellet p r o d u c t i o n a n d the d e v e l o p m e n t of appropriate processes is directly connected with the beneficiation of
low-grade iron ores - originally chiefly magnetites to p r o d u c e (very fine-grained) concentrates. In the first place, it was necessary to convert these concentrates into
ag-glomerates r e a d y for the blast f u r n a c e . At the s a m e time the question
arose regarding the o p t i m u m site of such a pelletizing plant. T h e very fine-grained, wet concentrates are unsuitable for storage, especially in regions with long a n d cold winters because they freeze during this time a n d can
neither be loaded nor transported. T h e s e p r o b l e m s were solved by erecting the pelletizing plant in the close vicinity of the beneficiation plant a n d thus practically at the ore mine. Pellets are resistant to the influences of storage and winter conditions. T h e y withstand long transportation routes with several trans-shipments better t h a n l u m p ores or even sinter.
D u r i n g the first years, pellets were p r o d u c e d , mainly in the U S A , to supply blast furnaces within a controlled m a r k e t . T h e i r transportability was accepted but no special controls were instituted. Only after m a j o r pellet quantities h a d a p p e a r e d on the world m a r k e t and h a d reached other blast furnaces, was greater importance attached to the behaviour of pellets during transport. Thus, the p r e p a r a t i o n of a large-scale test (1963), r u n on about 150,000 tons pellets f r o m the Reserve M i n i n g C o m p a n y in a blast furnace of the F e d e r a l R e p u b l i c of G e r m a n y , also included, e.g. the study of the abrasion b e h a v i o u r , compression strength and variation of pellet size2 0). F r o m Port Silver Bay to the works p o r t situated on the river Rhine, the pellets were subjected to six trans-shipments, a water transport and an 18 m o n t h storage period at the port area of A m s t e r d a m d u r i n g two winters. T h e fines portion —6 m m , contained in the pellets, was used as a reference value.
This fines portion was at L a k e Superior 8.0% and increased d u r i n g transportation via the Port of A m s t e r d a m to t h e D u i s b u r g - H u c k i n g e n Works, and a f t e r screening before the blast f u r n a c e to a b o u t 18%. T h e portion of 10—15 m m d i a m e t e r varied f r o m 85% to 82% and, after screening, rose again to 88% at the blast furnace. In this way, proof was furnished that pellets can be transported satisfactorily. This large-scale test — successfully carried out in an i n d e p e n d e n t steel plant with high pellet proportions in the blast furnace b u r d e n - b e c a m e known publicly a n d aroused increased interest in these new agglomerates also at those works where pellets h a d hitherto not been used at all.
Table 2. Countries leading in iron ore production and export113)
Countries 106 t/year
Countries
1966 1976
Countries
Production Export Production Export Africa
Liberiaª 17.0 16.6 35.0 20.8
Rep. South Africa 6.8 3.1 15.7 5.0
America Brazil* 23.3 11.8 70.0 47.3 Venezuela ª 17.9 17.0 23.0 15.6 Canada* 36.6 31.2 56.0 44.5 USA 91.6 9.9 81.2 2.9 Asia Indiaª 26.8 13.4 42.6 24.0 Australiaª 11.0 2.0 93.1 81.1 Europe France ª 55.7 18.2 45.5 15.8 Great Britain 13.9 - 4.6 — Swedenª 28.0 22.3 30.5 22.0 USSRª 160.2 26.1 239.0 43.1 * Leading Exporters
Pellet p r o d u c t i o n was initially always linked with a beneficiation plant, usually located at the m i n e where sufficient quantities of very fine-grained concentrates were available without any i n t e r m e d i a t e treatment.
T h e increasing world-wide d e m a n d for h i g h - g r a d e iron ores led to the discovery of new deposits with iron ores of d i f f e r e n t mineralogical and chemical composition. T h e i r quality could also be i m p r o v e d by beneficia-tion as in the case of M a r c o n a in Peru. H o w e v e r , m a n y of these deposits were located in countries with little d o m e s t i c d e m a n d for pellets so that their exploitation could only be secured by exportation.
Some countries have a d a p t e d themselves particularly to this situation so that a considerable change of ore suppliers has occurred d u r i n g the last
ten years, as is shown in T a b l e 2 for the years 1 9 6 6 - 1 9 7 5 . Firstly, it indicates the increase in p r o d u c t i o n within a ten-year p e r i o d by 170 million tpy or 50% c o m p a r e d to 1966. In s o m e instances there were even m o r e
important changes as, for example, in Australia a n d Brazil. A f u r t h e r remarkable p h e n o m e n o n is the p r o d u c t i o n decrease of low-grade ore in F r a n c e (minette) and in G r e a t Britain ( h o m e ores). T h e s e countries now import the c o r r e s p o n d i n g iron units in the f o r m of h i g h - g r a d e ores. O t h e r countries, such as J a p a n and the F e d e r a l R e p u b l i c of G e r m a n y as well as nearly all o t h e r E. E. C. countries m e e t their r e q u i r e m e n t s almost ex-clusively by imports.