pelletizing of iron ores - kurt meyer

Kurt Meyer

Pelletizing

of Iron Ores

Kurt Meyer

PeIletizing of Iron Ores

W i t h 146 F i g u r e s

1980Springer-Verlag Berlin Heidelberg NewYork Verlag Stahleisen m.b.H. Dsseldorf

Professor Dr. Phil. Dr. Ing. Dipl.-Chem. Kurt Meyer Peter-Bhler-Strae 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 Dsseldorf

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. T N 535.M47. 622'.341. 80-23891. This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under 54 of the G e r m a n Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. by Springer-Verlag Berlin, Heidelberg, and Verlag Stahleisen mbH, Dsseldorf, 1980 Printed in Germany. The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting, printing and bookbinding: Druckerei K. Triltsch, Wrzburg 2060/3020-543210

Preface

After the Second World War, there was in m a n y countries a great backlog d e m a n d in nearly all branches of life, which resulted i. a. in a production facilities h a d to be replaced, reconstructed and extended. This was a good opportunity of revising the p r o d u c t i o n concept of the iron and steel industry and introducing innovations where it was possible and promising. This renewal covered i. a. two i m p o r t a n t sectors. O n the one hand, the dimensions of the production units a n d auxiliary e q u i p m e n t were extended to such a degree that considerably higher capacities were achieved. A n interesting example is the extension of the blast furnace volume and hearth diameters u p to 15 m at a pig iron production of a b o u t 10,000 tons per day. In addition, remarkable progress was m a d e by improving existing process parameters and introducing new technologies. S o m e of these innovations had already been formerly known, but not yet applied. A few examples of such developments in connection with ore p r e p a r a t i o n and particularly with the agglomeration technology are given below:

worldwide

1. Physical Preparation of Blast Furnace Burden by Crushing, and Classification of Constituents

Screening

Already in the thirties, it was known to m a n y experts that it is advisable to classify the blast furnace b u r d e n b e f o r e it is used 2 ). However, this idea was not consistently realized until a b o u t 1950. F r o m this date onwards, raw ores, coke and other b u r d e n constituents were crushed, screened and supplied to the blast furnace in a narrow size range. Increasing amounts of fine ores emerged f r o m this operation and r e q u i r e d 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 p r e p a r e d b u r d e n with rising sinter portions, the gas permeability of the blast f u r n a c e b u r d e n greatly improved the pig iron capacity increased and the coke c o n s u m p t i o n decreased.

VI 2. Thermal Separation

Preface 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 o p e r a t i o n was a c h i e v e d b y r e m o v a l of volatile ballast m a t e r i a l , such as H 2 O or C O 2 , f r o m r a w ores d u r i n g sintering or pelletizing b e f o r e they enter t h e blast f u r n a c e . S u c h a n o r e p r e p a r a t i o n is of p a r t i c u l a r i m p o r t a n c e w h e n M i n e t t e or l i m o n i t e ores h a v e to b e treated. T h e r e are plants in w h i c h the blast f u r n a c e b u r d e n consists of 100% sinter.

3. Mechanical Beneficiation of Ores by of Mineral Ballast Materials

Separation

By m e c h a n i c a l b e n e f i c i a t i o n , it is possible to r e m o v e a great p o r t i o n of m i n e r a l 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 f u r n a c e . B u t t h e concentrates so p r o d u c e d are very f i n e - g r a i n e d , a n d agg l o m e r a t i o n is t h u s necessary. T h i s a g g l o m e r a t i o n is achieved by sintering and pelletizing. T h e pelletizing process was i n t r o d u c e d o n a n i n d u s t r i a l scale as a new a g g l o m e r a t i o n m e t h o d a n d a n 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 c o m p o s i t i o n of the ores b y using c o r r e s p o n d i n g additives d u r i n g a g g l o m e r a t i o n leads to the desired c h a n g e or i m p r o v e m e n t of metallurgical p r o p e r t i e s 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 a l r e a d y 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 agg l o m e r a t i o n 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 a d d i t i o n a l a d v a n t a g e s d u e to t h e i r good transportability. W i t h the i n t r o d u c t i o n of pellets into the world m a r k e t , 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 r e d u c t i o n a n d bypass the blast f u r n a c e c a m e closer to realisation. T h e increasing a n d successful efforts to i m p r o v e the direct r e d u c t i o n processes w e r e o n e of the m o s t i m p o r t a n t consequences of the newly developed pelletizing technology. Acknowledgement T h e a u t h o r very m u c h t h a n k s the m a n a g e m e n t of L u r g i C h e m i e u n d H t t e n technik G m b H * for t h e p e r m i s s i o n to use test results a n d o t h e r relevant k n o w l e d g e w h i c h h a d h i t h e r t o not b e e n p u b l i s h e d as well as for p r o v i d i n g t h e facilities r e q u i r e d for t h e p r e p a r a t i o n of this b o o k . * Frankfurt/Main, Federal Republic of Germany

Preface

VII

H e also t h a n k s his colleagues in F r a n k f u r t as well as those of the s u b s i 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 , S w e d e n , T h e U n i t e d States, S o u t h A f r i c a and A u s t r a l i a for t h e i r 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 t h e necessary a d d i t i o n a l tests, in e l a b o r a t i n g d r a w i n g s a n d d i a grams, in t r a n s l a t i n g t h e text a n d in revising t h e 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 f o r the kind disclosure of interesting a n d m o s t 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 ( S w e d e n ) , 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 , C l e v e l a n d ( O h i o ) , P i c k a n d s M a t h e r a n d Company, Cleveland (Ohio), Studiengesellschaft fr Eisenerzaufbereitung (Federal R e p u b l i c of G e r m a n y ) , Verein D e u t s c h e r Eisenhttenleute ( F e d eral R e p u b l i c of G e r m a n y ) , a n d Institut f r E i s e n h t t e n k u n d e d e r R h e i nisch-Westflischen T e c h n i s c h e n 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 ) . F r a n k f u r t / M a i n , S e p t e m b e r 1980 Kurt Meyer

Contents

Introduction 1 1.1 1.1.1 1.1.2 1.2 1.2.1 1.2.2 1.2.3 1.2.3.1 1.2.3.2 1.2.3.3 1.3 1.4 2 2.1 2.1.1 2.1.2 2.1.2.1 2.1.2.2 2.1.2.3 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.2.1 2.2.1.2.2 2.2.1.3 2.2.1.4 2.2.2 2.2.2.1 Definition and Development of Pelletizing Process . . Definition Differentiation Against O t h e r Iron O r e Agglomerates . Principal Process Steps for the P r o d u c t i o n of Pellets . D e v e l o p m e n t of Pelletizing Process . First Phase, Alternative to Sintering Second Phase, Pellets f r o m Concentrates T h i r d Phase, Pellets f r o m O r e s T h e Two-Stage G r a n u l a t i o n of the Sinter Mix . . . . Pellet Sintering Mixed Firing M e t h o d Pelletizing, a Contribution to Ore P r e p a r a t i o n . . . . Sites of Pelletizing Plants a n d Transportability of Pellets Fundamentals of Pelletizing Bonding Mechanisms for G r e e n Ball F o r m a t i o n I m p o r t a n t Bonding Factors Ball F o r m a t i o n Alternatives Compacting M e t h o d Green Ball F o r m a t i o n M e c h a n i s m of Ball F o r m a t i o n Induration of G r e e n Balls Drying of G r e e n Balls Drying Procedure of Individual Balls Drying of Pellets in a L a y e r Unidirectional Drying U p - D r a u g h t - D o w n - D r a u g h t Drying D r y Pellet Strength Shock Resistance Pellet Firing Bonding by Change of the Crystalline Structure

1 3 3 3 4 5 6 7 9 11 13 13 15 20 23 24 24 24 25 25 26 29 29 30 33 34 35 35 37 37 39

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X 2.2.2.1.1 2.2.2.1.2 2.2.2.2 2.2.3 3 3.1 3.1.1 3.1.1.1 3.1.1.1.1 3.1.1.1.2 3.1.1.1.3 3.1.1.1.4 3.1.2 3.1.3 3.1.4 3.1.4.1 3.1.4.2 3.1.4.3 3.1.4.4 3.1.4.5 3.2 3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.1.4 3.2.1.5 3.2.1.6 3.2.1.7 3.2.2 3.2.2.1 3.2.3 3.2.3.1 4 4.1 4.2 4.3 4.3.1

Contents 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 Magnetite Concentrate Crystal C h a n g e D u r i n g I n d u r a t i o n of H e m a t i t e Pellets T h e R e a c t i o n of S l a g - F o r m i n g C o m p o n e n t s C o o l i n g of I n d u r a t e d Pellets Raw Materials and Their Preparation for Pellet Production R a w Materials Iron-Bearing Materials N a t u r a l Iron Ores Magnetite Hematite W e a t h e r e d Ores Limonite Beneficiation Products S e c o n d a r y R a w Materials Binders and Additives Binders Additives Bentonite Lime Compounds Other Additives P r e p a r a t i o n of R a w M a t e r i a l s for Pelletizing . . . . Separation Washing Gravity Separation Flotation Magnetic Separation Magnetizing Roasting Electrostatic S e p a r a t i o n P r o p o r t i o n of D i f f e r e n t O r e s in Pellet P r o d u c t i o n . . Physical P r o p e r t i e s of Fine-grained Iron Ores . . . . Size D i s t r i b u t i o n - S p e c i f i c S u r f a c e A r e a Grinding Dewatering The Pelletizing Laboratory and its Tasks A p p l i c a t i o n R a n g e of L a b o r a t o r i e s T h e T a s k s of a L a b o r a t o r y R a w M a t e r i a l P r e p a r a t i o n a n d Pellet P r o d u c t i o n R a w Material Preparation

40 41 43 45

47 47 47 47 48 49 49 50 51 52 53 53 53 53 55 56 56 57 57 57 58 58 59 60 60 62 62 64 66 68 68 69 70 70

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Contents 4.3.2 4.3.3 4.3.4 4.4 4.4.1 4.4.2 4.4.2.1 4.4.2.1.1 4.4.2.1.2 4.4.2.1.3 4.4.2.1.4 4.4.3 4.4.4 4.5 4.5.1 4.5.2 4.5.2.1 4.5.2.2 4.5.2.3 4.5.2.4 4.5.3 4.6 4.6.1 4.6.1.1 4.6.1.2 4.6.1.3 4.6.2 4.6.2.1 4.6.2.1.1 4.6.2.1.2 4.6.2.1.2.1 4.6.2.1.2.2 4.6.2.1.2.3 4.6.2.1.2.4 4.6.2.1.2.5 4.6.2.1.3 4.6.2.1.3.1 4.6.2.1.3.2 4.6.2.1.3.3 4.6.2.1.3.4 4.6.2.1.4 Grinding. 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 Filtration 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 Mix Preparation G r e e n Ball F o r m a t i o n G r e e n Ball F o r m a t i o n a n d T e s t i n g M e t h o d s Pellet M o i s t u r e D e t e r m i n a t i o n C r u s h i n g Strength Drop Number D r o p Resistance Capacity Determination Bulk D e n s i t y G r e e n Ball I n d u r a t i o n F u r n a c e s f o r O r i e n t i n g Tests Stationary Pot G r a t e f o r P r i n c i p a l Tests P o t G r a t e w i t h Side W a l l s a n d H e a r t h L a y e r . . . . Pot G r a t e w i t h C o r r u g a t e d S i d e W a l l s C o n t r o l S c h e m e of Pot G r a t e Tests Movable Pot Grate Pilot Plants T h e P r o p e r t i e s 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 T h e Physical P r o p e r t i e s C r u s h i n g Strength T u m b l e r Resistance Mcroporosity 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 . Testing M e t h o d s for R e d u c t i o n M e c h a n i c a l Strength E x a m i n a t i o n of F i r e d Pellets f o r Blast F u r n a c e Operation 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) . 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) Swelling T e s t Reduction under Load T e s t ( R u L ) Other Testing Methods E x a m i n a t i o n of F i r e d Pellets f o r the D i r e c t R e d u c t i o n 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 ) . . . Swelling T e s t Sticking Test ( R M C ) D i r e c t R e d u c t i o n D i s i n t e g r a t i o n Stability T e s t ( D R D S ) Present State of Testing M e t h o d s

XI 73 75 76 77 77 77 79 79 80 82 82 82 83 83 83 84 85 85 86 86 89 89 89 90 90 91 91 93 93 93 93 94 94 94 95 96 96 96 96 96 98

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X 5 5.1 5.1.1 5.1.1.1 5.2 5.2.1 5.2.1.1 5.2.1.2 5.3 5.3.1 5.3.1.1 5.3.1.1.1 5.3.1.1.2 5.3.1.1.3 5.3.1.1.4 5.3.1.1.5 5.3.1.2 5.3.1.2.1 5.3.1.2.1.1 5.3.1.2.1.2 5.3.1.2.1.3 5.3.1.2.2 5.3.1.2.3 5.3.1.2.4 5.3.1.3 5.3.1.4 5.3.1.5 5.3.1.6 5.3.1.7 5.3.1.7.1 5.3.1.7.2

Contents Process Influencing Factors F a c t o r s Influencing Ball F o r m a t i o n G r a n u l o m e t r i c P r o p e r t i e s of R a w M a t e r i a l s G r a i n Size, Size D i s t r i b u t i o n a n d Specific S u r f a c e A r e a I n f l u e n c e 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 O p t i m u m Moisture Content O p t i m u m M o i s t u r e C o n t e n t a n d Specific S u r f a c e A r e a O p t i m u m M o i s t u r e C o n t e n t a n d S u r f a c e C o n d i t i o n of O r e Particles I n f l u e n c e of Binders a n d A d d i t i v e s F a c t o r s for l m p r o v i n g the M e c h a n i c a l P r o p e r t i e s . . . B e n t o n i t e as B i n d e r I n f l u e n c e of Bentonite o n G r e e n Pellet Strength a n d DropResistance I n f l u e n c e of Bentonite on D r y Pellet Strength . . . . I n f l u e n c e of Bentonite o n C r u s h i n g Strength a n d A b r a s i o n Resistance of F i r e d Pellets D i f f e r e n t Bentonite T y p e s I n f l u e n c e of Bentonite on t h e C h e m i c a l C o m p o s i t i o n of Pellets I n f l u e n c e of Alkaline Earth C o m p o u n d s T h e I n f l u e n c e of C a l c i u m O x i d e [ C a O ] a n d C a l c i u m Hydroxide [Ca(OH)2] I n f l u e n c e 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 I n f l u e n c e of [ C a ( O H ) 2 ] o n D r y Pellet Strength . . . . I n f l u e n c e of [ C a ( O H ) 2 ] o n C r u s h i n g Strength, T u m b l i n g R e s i s t a n c e a n d Porosity of I n d u r a t e d - P e l l e t s I n f l u e n c e of C a l c i u m C a r b o n a t e [ C a C O 3 ] a n d D o l o m i t e [ ( C a 1 M g ) C O 3 ] o n the Strength of I n d u r a t e d Pellets . . I n f l u e n c e of a M i x t u r e of D i f f e r e n t A d d i t i v e s o n Pellet Properties I n f l u e n c e of C a l c i u m C h l o r i d e [CaCl 2 ] on Pellet Properties I n f l u e n c e of Alkali C o m p o u n d s I n f l u e n c e of Ores with G o o d B o n d i n g P r o p e r t i e s . . . B e h a v i o u r of O r e M i x t u r e s I n f l u e n c e of Oxidized a n d P r e r e d u c e d R e t u r n F i n e s . . I n f l u e n c e of S p o n g e Iron o n Pellet P r o p e r t i e s I n f l u e n c e of S p o n g e Iron on G r e e n a n d D r y Pellet Strength I n f l u e n c e of S p o n g e Iron o n the Strength of I n d u r a t e d Pellets 99 99 100 100 105 105 106 107 109 110 110 110 112 112 113 114 115 116 116 118 121 121 125 126 127 127 128 131 132 133 133

Contents 5.3.1.8 5.3.1.9 5.3.1.10 5.3.1.11 5.4 5.4.1 5.4.1.1 5.4.1.1.1 5.4.1.1.2 5.4.1.2 5.4.1.3 5.4.2 5.4.2.1 5.4.3 5.4.3.1 5.4.3.2 5.4.3.3 5.4.3.4 5.4.4 5.4.5 5.4.6 5.4.6.1 5.4.6.2 6 6.1 6.1.1 6.1.2 6.1.2.1 6.1.2.2 6.1.2.3 6.1.2.3.1 6.1.2.3.2 6.1.2.3.3 6.1.2.4 I n f l u e n c e of Inplant F i n e s o n Pellet P r o p e r t i e s . . . . I n f l u e n c e of O r g a n i c B i n d e r s I n f l u e n c e of Coal A d d i t i o n Summarizing Considerations I n f l u e n c e of T h e r m a l T r e a t m e n t o n Pellet P r o p e r t i e s . F a c t o r s I n f l u e n c i n g G r e e n Ball D r y i n g T e m p e r a t u r e of D r y i n g G a s e s I n f l u e n c e 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 I n f l u e n c e of T e m p e r a t u r e o n S h o c k R e s i s t a n c e of Pellets During Drying I n f l u e n c e of Velocity of D r y i n g G a s F l o w o n D r y i n g Degree C h a n g e of Pellet Strength D u r i n g D r y i n g P r e h e a t i n g of D r i e d Pellets 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 P r e h e a t i n g of G r e e n Balls F i r i n g a n d Cooling of Pellets 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 Influence of F i r i n g T e m p e r a t u r e a n d Basic A d d i t i v e s o n t h e Strength of Pellets f r o m M a g n e t i t e O r e s F i r i n g of H e m a t i t e G r e e n Pellets 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 o n H e m a t i t e Pellet Q u a l i t y R e a c t i o n s of A d d i t i v e s w i t h I r o n O x i d e s a n d G a n g u e Constituents T h e r m a l D i s s o c i a t i o n of H e m a t i t e in Pellets S c h e m e of T h e r m a l T r e a t m e n t H e a t i n g P a t t e r n for Pellets f r o m M a g n e t i t e F i r i n g P a t t e r n f o r Pellets f r o m O t h e r O r e s Behaviour of Indurated Pellets During Reduction . . . C h a n g e of Pellet S t r u c t u r e D u r i n g R e d u c t i o n . . . . Reduction Mechanisms Structural C h a n g e D u r i n g R e d u c t i o n V o l u m e V a r i a t i o n b y Crystal C h a n g e Structural C h a n g e b y 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 O x i d e s a n d w i t h E a c h O t h e r I n f l u e n c e of A d d i t i v e s o n P e l l e t Swelling P r o p e r t i e s of A c i d Pellets w i t h a Basicity of Less t h a n 0.1 P r o p e r t i e s of Pellets w i t h a Basicity of 0.1 to 0.6 . . P r o p e r t i e s of Pellets w i t h a Basicity of H i g h e r t h a n 0.7 . I n f l u e n c e of G a n g u e B o n d s o n Pellet S t r u c t u r e at T e m p e r atures of 4 0 0 - 6 0 0 0 C U n d e r R e d u c i n g A t m o s p h e r e .

XIII 135 136 138 139 140 140" 141 141 142 142 143 145 145 147 148 150 151 152 153 154 155 155 157 159 162 163 165 165 169 170 174 174 176 176

XIV 6.1.2.5 6.2 6.3 7 7.1 7.1.1 7.1.2 7.2 7.2.1 7.2.2 7.3 7.3.1 7.3.2 7.4 8 8.1 8.2 8.2.1 8.2.2 8.2.2.1 8.2.2.1.1 8.2.2.1.2 8.2.2.2 8.2.2.2.1 8.2.2.2.2 8.2.2.3 8.2.3 8.2.3.1 8.2.3.1.1 8.2.3.1.2 8.2.3.2

Contents I n f l u e n c e of G a n g u e B o n d s at A b o u t 1 0 0 0 0 C o n Pellet Structure Under Reducing Atmosphere 178 B e h a v i o u r of I n d u r a t e d Pellets Consisting of M a g n e t i t e and Wstite During Reduction 178 Conclusions 180 Special Processes for Pellet Production Pellet H a r d e n i n g b y U s i n g Binders G r a n g c o l d Process C O B O a n d M T U Process C h l o r i d i z i n g Volatilization of N o n - F e r r o u s M e t a l O x i d e s a n d Pellet P r o d u c t i o n C h l o r i d i z i n g Volatilization a n d Pelletizing in the S h a f t Furnace C h l o r i d i z i n g Volatilization a n d Pellet P r o d u c t i o n in a Rotary Kiln R e c o v e r y of V a n a d i u m P e n t o x i d e f r o m V a n a d i u m B e a r i n g Iron Ores 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 Shaft Furnace . . . 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 D e a r s e n i f i c a t i o n and Pelletizing of I r o n Ores . . . . 181 181 181 182 183 184 186 188 189 190 190

Balling Equipment 192 H o m o g e n e i t y of C o m p o n e n t s for Pelletizing M i x t u r e s 192 D e m a n d s o n t h e M o d e of O p e r a t i o n of Balling U n i t s 193 Ball F o r m a t i o n a n d O p e r a t i o n of t h e Principal Pelletizing U n i t s Balling D r u m T h e P r i n c i p a l D r u m C o m p o n e n t P a r t s a n d the Balling Operation . . . T h e M a i n C o m p o n e n t Parts . . . O p e r a t i o n of the Balling D r u m I n f l u e n c i n g F a c t o r s for G r e e n Ball F o r m a t i o n in a D r u m D r u m Rotating Speed L e n g t h a n d Tilt A n g l e of D r u m Balling D r u m C a p a c i t y Balling Disc 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 . T h e M a i n C o m p o n e n t Parts O p e r a t i o n of the Balling D i s c I n f l u e n c i n g F a c t o r s of G r e e n Ball F o r m a t i o n o n a D i s c 196 196 196 197 198 198 199 201 202 202 202 203 204

Contents 8.2.3.2.1 8.2.3.2.2 8.2.3.2.3 8.2.3.2.4 8.2.3.3 8.2.4 8.2.5 8.2.6 8.2.6.1 8.2.6.2 8.2.6.3 8.2.6.4 8.3 8.3.1 8.3.2 8.3.3 9 9.1 9.1.1 9.1.2 9.1.2.1 9.1.2.2 9.1.2.3 9.1.2.4 9.1.3 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.2.6 9.3 9.3.1 9.3.2 9.3.3 9.3.4 9.3.5 9.3.5.1

XV

D i s c Slope a n d R i m H e i g h t 205 Disc Rotating Speed 206 R e s i d e n c e T i m e of M a t e r i a l in t h e D i s c 206 DiscDiameter 207 Balling D i s c C a p a c i t y 207 C o m p a r i s o n B e t w e e n Balling D r u m a n d Balling D i s c . . 208 C o m p a r i s o n of V i b r a t i n g a n d R o l l e r Screens 209 O t h e r Balling Systems 210 Balling C o n e 210 Mix Granulator 210 Vibrating Trough 211 Eccentrically M o v i n g U n i t 211 Handling and Feeding Devices 211 Roller C o n v e y o r 212 R o l l e r Screen 213 R o l l i n g Belt C o n v e y o r 213 H e a t Treatment Systems Shaft Furnace . . . Shaft Furnace Types Process Stages C h a r g i n g of G r e e n Balls to t h e F u r n a c e Drying, Preheating and Firing C o o l i n g of Pellets Heat Consumption Furnace Dimensions, Capacity and Market Position . The Grate-Kiln Combination T h e Travelling G r a t e a n d its F u n c t i o n s T h e R o t a r y K i l n a n d its F u n c t i o n s T h e Cooler Heat Consumption C u r r e n t State and M a r k e t P o s i t i o n of t h e Allis C h a l m e r s G r a t e - K i l n Process O t h e r G r a t e - K i l n Processes Travelling G r a t e Systems ' A p p l i c a t i o n 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 General Features U p - d r a u g h t I n d u r a t i o n Process f o r S p e c u l a r H e m a t i t e 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 L u r g i - D r a v o Travelling G r a t e Process I m p o r t a n t Process F e a t u r e s 215 216 217 218 218 219 221 221 221 223 224 225 226 226 227 228 229 230 231 232 233 235 235

XVI 9.3.5.2 9.3.5.2.1 9.3.5.2.2 9.3.5.2.3 9.4 9.4.1 9.4.2 9.4.3 9.5 9.5.1 9.5.2 9.5.3 9.5.4 10 10.1 10.2 10.2.1 10.2.2 10.2.3 10.2.4 10.2.5 10.3 11 11.1 11.2 11.3 11.4 11.5 12 13 13.1 13.1.1 13.1.1.1 13.1.1.2 13.1.1.3 13.1.1.4

Contents A p p l i c a t i o n of the Process o n an I n d u s t r i a l Scale . . 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 a n d the M o d e of O p e r a t i o n Firing Pattern and Heat Consumption C a p a c i t y , Flexibility a n d M a r k e t S i t u a t i o n O t h e r H e a t T r e a t m e n t Systems Circular Indurating Furnace " H e a t F a s t " Process " 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 C o m p a r i s o n of I m p o r t a n t Pelletizing Systems . . . . C h a n g e of O r e Basis P r o d u c t i o n F i g u r e s p e r U n i t of V a r i o u s I n d u r a t i n g Systems P r o p o r t i o n of V a r i o u s F i r i n g Systems in the W o r l d Pellet Production Cost C o m p a r i s o n . Plant Layout and Process Control Plant Layout . Process Control . . . . D i s t r i b u t i o n a n d P r o p o r t i o n i n g of M a t e r i a l F l o w . . . 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 . . . F o r m a t i o n a n d T r a n s p o r t of G r e e n Pellets G r e e n Pellet C h a r g i n g T h e r m a l T r e a t m e n t of G r e e n Balls D e v e l o p m e n t a n d T r e n d s of F u r t h e r C o n t r o l V a r i a n t s 236 237 238 239 241 242 242 243 243 244 244 245 246 247 247 247 249 249 249 249 250 250

Pellets in the Blast Furnace Burden 251 I n f l u e n c e of M e c h a n i c a l P r o p e r t i e s . . . 251 I n f l u e n c e of C h e m i c a l C o m p o s i t i o n 252 M e t h o d s of Pellet C h a r g i n g to the Blast F u r n a c e . . 253 C o m p a r i s o n of Pellets a n d Sinter 253 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 The Utilization of Pellets in Direct Reduction Plants S o m e Theoretical Considerations G r e e n Ball F o r m a t i o n B o n d s B e t w e e n W a t e r a n d F i n e - G r a i n e d Particles B o n d i n g b y L i q u i d Bridges B o n d i n g F o r c e s in T r a n s i t i o n R a n g e B o n d i n g F o r c e s in C a p i l l a r y R a n g e O v e r s a t u r a t i o n with W a t e r . 257 259 262 . . 263 263 264 264 265

Contents 13.1.2 13.1.3 13.1.4 13.1.4.1 13.1.4.2 13.1.4.3 13.2 13.2.1 13.2.2 13.2.2.1 13.2.2.1.1 13.2.2.1.2 13.2.2.1.3 13.2.2.2 13.2.2.2.1 13.2.2.2.2 13.2.3 13.2.4 13.2.4.1 13.2.4.2 13.2.5 13.2.5.1 13.2.5.2 13.3 13.3.1 13.3.2 I n f l u e n c e of G r a n u l o m e t r i c P r o p e r t i e s o n G r e e n Pellet Strength I n f l u e n c e of Rolling F o r c e s D u r i n g M o v e m e n t of G r e e n Pellets D e s i g n 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 D e s i g n a n d D i m e n s i o n s of Balling D i s c R o t a t i n g S p e e d a n d D i s c Slope Balling D r u m a n d its D e s i g n D a t a T h e r m a l T r e a t m e n t of G r e e n Pellets H e a t T r a n s f e r in a Pellet C h a r g e o n the T r a v e l l i n g G r a t e or in t h e S h a f t F u r n a c e H e a t T r a n s f e r of G r e e n Pellets b y C o n v e c t i o n a n d G a s Flow Gas Flow R e s i s t a n c e of a C h a r g e of U n i f o r m Pellet Size to G a s Flow R e s i s t a n c e of Pellet C h a r g 6 of D i f f e r e n t Pellet Size to Gas Flow R e s i s t a n c e of Pellet C h a r g e a n d T r a v e l l i n g G r a t e to G a s Flow Heat Transfer by Convection H e a t T r a n s f e r to the Pellet C h a r g e o n t h e T r a v e l l i n g Grate H e a t T r a n s f e r to the Pellet C h a r g e in t h e S h a f t F u r n a c e . H e a t T r a n s f e r to the Pellet C h a r g e in t h e R o t a r y K i l n . . H e a t T r a n s f e r by R a d i a t i o n . .. . . . Gas Radiation Kiln Lining Radiation Heat Conduction H e a t C o n d u c t i o n Inside t h e Pellet . . . . . H e a t C o n d u c t i o n b y P o i n t s of C o n t a c t G r e e n Ball D r y i n g D r y i n g of a n l n d i v i d u a l Pellet D r y i n g of a Pellet C h a r g e Final Remarks References Subject Index . : .

XVII

265 266 267 267 267 268 269 269 270 270 271 271 272 273 275 275 275 276 277 277 278 278 279 279 280 282 286 288 298

Introduction

In contrast to sintering, for which the down-draft sintering m e t h o d is only succesfully employed, pellets are today indurated according to three methods: in the shaft furnace, in grate-kilns a n d on travelling grates. T h e greatly varying properties of ores resulting f r o m their origin, genesis, shape, crystal f o r m and chemical composition are to be taken into account for ore p r e p a r a t i o n and pelletizing in order to p r o d u c e at any time pellets of u n i f o r m and good quality. N o w a d a y s , 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 b e based on generalized programmes. In each particular case, it is practically unavoidable to find out the o p t i m u m p a r a m e t e r s by the p e r f o r m a n c e 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: T h e most important development phases for pelletizing and the underlying causes are described in Chapter 1. T h e f u n d a m e n t a l s for successful green ball f o r m a t i o n 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 e q u i p p e d 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 a d e q u a t e tests for obtaining a u n i f o r m pellet quality. T h e s e considerations are the subject of Chapter 5. The decisive quality criterion is the b e h a v i o u r of pellets during reduction, Chapter 6. Experiments and considerations in connection with oxygen removal are described although despite m a n y efforts some question have not yet been fully clarified.

2

Introduction

Special processes for pellet i n d u r a t i o n a n d t h e possibilities of utilizing u n u s u a l r a w m a t e r i a l s are dealt with in Chapter 7. A survey of t h e m a i n e q u i p m e n t a n d f u r n a c e s used is given in Chapters 8 and 9. A t t h e s a m e time, these chapters s h o w the i m p o r t a n c e of the c o o p e r a t i o n of process engineers, m e c h a n i c a l engineers a n d 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 d a t a to o t h e r conditions s h o u l d be c o n s i d e r e d carefully to avoid m i s i n t e r p r e t a t i o n , 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 r e d u c t i o n plants is very essential for j u s t i f y i n g the a p p l i c a t i o n of the pelletizing process a n d its f u t u r e i m p o r t a n c e , Chapters 11 and 12. In o r d e r not to i n t e r r u p t the continuity of the v a r i o u s c h a p t e r s by theoretical 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 a n d e q u a t i o n s are c o m p i l e d in Chapter 13 a n d s i m u l t a n e o u s l y references are m a d e to relevant literature. This b o o k will n e i t h e r be a m a n u a l for the construction of i n d u s t r i a l 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 d i f f e r d u e to the great variety of r a w m a t e r i a l characteristics. T h e p u r p o s e of this b o o k is r a t h e r to investigate a n d to describe t h e possibilities a n d m e t h o d s to p r o d u c e pellets of good a n d u n i f o r m quality irrespective of t h e varying p r o p e r t i e s of r a w materials. T h e existing a n d ever increasing a m o u n t of literature was considered ins o f a r as it w a s u s e f u l f o r the c o n f i r m a t i o n a n d s u p p o r t 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 DefinitionPellets are balls p r o d u c e d f r o m 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 915 m m diameter high and even porosity of 2 5 - 3 0 % high iron content of m o r e than 63% iron practically no loss on ignition or volatiles uniform mineralogical composition in the f o r m of an easily reducible hematite or hematite-bearing c o m p o u n d s high and u n i f o r m mechanical strength low tendency to abrasion and good behaviour during transportation sufficient mechanical strength even at thermal stress under reducing atmosphere.

1.1.1 D i f f e r e n t a t i o n A g a i n s t O t h e r Iron O r e 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 u n d e r high mechanical pressure. These briquettes m a y undergo direct f u r t h e r treatment or thermal processing before their use. A l t h o u g h their metallurgical behaviour in melting or reduction furnaces is very good, the iron ore briquetting could not m a k e h e a d w a y since t h e processing costs are relatively high and, above all, the production capacity of briquetting units is limited when c o m p a r e d with the enormous quantities of fine ores or 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.

concentrates

4

I

Definition and Development of Pelletizing Process

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 a g g l o m e r a t i o n process is down-draught sintering. It d i f f e r s f r o m pelletizing by v a r i o u s characteristics, s u c h as: f e e d of c o a r s e r - g r a i n e d 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 u p of the g r a n u l a t e d m i x to slightly a b o v e the s o f t e n i n g temperature the final p r o d u c t consists of a spongy sinter cake, partly m o l t e n , w h i c h by crushing, g r i n d i n g a n d screening, is b r o u g h t to the necessary grain size of 5 - 3 0 o r 5 - 5 0 m m . Fig. 1 shows the d i f f e r e n t o u t e r s h a p e of the a g g l o m e r a t e s p r o d u c e d according to the t h r e e 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 f o r the P r o d u c t i o n o f 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. F i n e - g r a i n e d iron ores having a d e q u a t e size d i s t r i b u t i o n are rolled with the a d d i t i o n 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 f o r m a t i o n , it is also possible to use other a d d i t i v e s for i m p r o v i n g t h e properties of green a n d fired pellets, e.g. b e n t o n i t e , a n d for c h a n g i n g t h e metallurgical p r o p e r t i e s of t h e i n d u r a t e d pellets, e.g. l i m e s t o n e or d o lomite. In a second step the green pellets are dried a n d i n d u r a t e d to o b t a i n the typical f e a t u r e s of pellets. T h i s is achieved, in m o s t cases, by c a r e f u l heating u n d e r oxidizing a t m o s p h e r e to just below the s o f t e n i n g p o i n t of the

1.2 Development of Pelletizing Process

5

ores used. D u r i n g this heating, not only the crystalline s t r u c t u r e is c h a n g e d b u t also o t h e r b o n d s a p p e a r , such as reactions b e t w e e n s l a g - f o r m i n g constituents b o t h b e t w e e n each o t h e r a n d with i r o n 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 s t r u c t u r e s a n d o t h e r b o n d s as well as to avoid tension cracks. G r e e n pellets can also be i n d u r a t e d by h y d r a u l i c a l l y acting binders, e.g. cement or c a l c i u m h y d r o x i d e , possibly b y using s t e a m u n d e r h i g h pressure. H o w e v e r , s o m e p r o p e r t i e s of such pellets d i f f e r f r o m t h e r m a l l y i n d u r a t e d pellets (see C h a p . 7).

1.2 Development of Pelletizing ProcessIron ore agglomerates, be they b r i q u e t t e s , sinter or pellets, a r e not t h e 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 a n i n t e r m e d i a t e p r o d u c t on the way f r o m the ore m i n e to t h e blast f u r n a c e o r direct r e d u c t i o n plant. T h e sole p u r p o s e of a g g l o m e r a t e p r o d u c t i o n is to k e e p t h e cost p r i c e of pig iron or steel at the lowest level. F o r m a n y years until a b o u t the t u r n of the century, the iron ores charged to blast furnaces h a d been crushed a n d partly classified either at the m i n e or at t h e i r o n a n d steel works. In this case l u m p ores w e r e p r e f e r r e d a l t h o u g h small p o r t i o n s of fine ores could be tolerated. As a result t h e fines w h i c h were not utilised f o r m e d c o n t i n u o u s l y growing d u m p s with n o e c o n o m i c use. T h e y c o u l d 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 d e c r e a s e d t h e 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 i r r e g u l a r m a n n e r a n d d i s t u r b e d the blast f u r n a c e o p e r a t i o n . M o r e o v e r , a great p a r t of these fines was b l o w n o u t 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 d u s t q u a n t i t i e s r e p r e s e n t e d a considerable i r o n v a l u e which, like t h e 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 w i t h g r e a t iron reserves t h a n 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 d u s t d e p e n d s largely o n the ore type t r e a t e d . In t h e case of M i n e t t e or o t h e r ores with a h i g h loss of ignition, it is s u b s t a n t i a l l y g r e a t e r t h a n in the case of high-grade, dense ores with a small loss o n ignition. Possibilities w e r e e x a m i n e d a n d tests to a g g l o m e r a t e t h e f l u e dust b y sintering or briquetting a n d to recycle it to the blast f u r n a c e were started at a p p r o x i m a t e l y t h e t u r n of the c e n t u r y in v a r i o u s i n d u s t r i a l i s e d countries a l t h o u g h with d i f f e r i n g intensities. C o u n t r i e s w i t h i m p o r t a n t i r o n reserves were less interested in this a g g l o m e r a t i o n . T h e y c o n s i d e r e d sintering as a

6


"necessary evil"1sobrescrito).T h e situation was q u i t e d i f f e r e n t 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 c o n t i n u e d intensively a n d not only flue dust b u t also o t h e r i r o n - b e a r i n g s e c o n d a r y r a w materials s u c h 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 t h e crushing a n d screening of unclassified l u m p ores, was also gaining significance. At a b o u t t h e s a m e t i m e , s o m e researchers w e r e looking for an alternative process to sintering, especially in areas in w h i c h very fine ores o r concentrates were available. This was t h e b e g i n n i n g of the pelletizing process.

1 . 2 . 1 F i r s t P h a s e , A l t e r n a t i v e to S i n t e r i n g T h e d i f f e r e n t stages of d e v e l o p m e n t , progress a n d s p e e d of i n t r o d u c t i o n of the sintering process, especially if very fine i r o n ores were to b e t r e a t e d , led to considerations for i m p r o v i n g t h e process a n d finally for d e v e l o p i n g a n alternative to sintering, n a m e l y pelletizing. A b o v e all, countries such as S w e d e n or G e r m a n y 2 ) , w h i c h in the very early days h a d a l r e a d y b e e n c o m p e l l e d to give p a r t i c u l a r a t t e n t i o n 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 p o r t i o n s of such fines in the sinter mix, limits o n 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. C o n c e n t r a t e s were n o longer a d d e d to the sinter mix b u t were s e p a r a t e l y f o r m e d into balls with the a d d i t i o n of water a n d t h e n i n d u r a t e d by using b i n d e r s or in a t h e r m a l way. S u c h a p a t e n t h a d a l r e a d y b e e n g r a n t e d in 1912 u n d e r No. 35 124 to the S w e d e A. G . Andersson. U n fortunately, n o f u r t h e r details or metallurgical results w e r e p u b l i s h e d 3 ). A l m o s t s i m u l t a n e o u s l y , similar research a n d d e v e l o p m e n t w o r k was started in G e r m a n y . In 1913, a G e r m a n p a t e n t N o . 289606 was g r a n t e d to the inventor C. A. Brackelsberg. T h i s patent protects a process according to w h i c h ore fines were m i x e d with w a t e r or binders, f o r m e d into balls a n d i n d u r a t e d at low t e m p e r a t u r e s . N o c o n s e q u e n c e s resulted f r o m the S w e d i s h p a t e n t w h e r e a s Brackelsberg k n o w l e d g e t h a t t h e pellets ( r e f e r r e d to as "GEROELL" d e r i v a t e d f r o m rolling) 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 m a t e r i a l . In the course of this w o r k , a pilot p l a n t 5 ) with a capacity of 120 tons per d a y "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. T h i s plant was reconstructed in 1935, and a l r e a d y s h o w e d essential f e a t u r e s of the pelletizing process. T h i s pilot p l a n t was d i s m a n t l e d in 1937 to m a k e available the area r e q u i r e d for the construction of a large sinter plant.

c o n t i n u e d his w o r k


7

In this way, t h e 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 s p r e a d as t h e only i m p o r t a n t a g g l o m 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 w a y was p a v e d for t h e second p h a s e , especially in t h e U S A a n d again in S w e d e n . 1.2.2 Second Phase, Pellets from Concentrates T h e second p h a s e was initiated b y the p r o b l e m of securing t h e 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 w o r k s in t h e U n i t e d States h a d h i t h e r t o been s u p p l i e d with h i g h - g r a d e l u m p ores a n d c o a r s e - g r a i n e d c o n c e n t r a t e s with a n i r o n content of 50% a n d m o r e , d e m a n d i n g n o f u r t h e r t r e a t m e n t . D u r i n g a n d at the end of the S e c o n d W o r l d W a r , t h e reserves of such h i g h - g r a d e ores were o n the decline so t h a t o t h e r sources h a d to b e o p e n e d up. O n e of the richest deposits in t h e M e s a b i R a n g e c o n t a i n e d large ore reserves, the w e l l - k n o w n " t a c o n i t e s " w h i c h h a v e a. low i r o n content, a b o u t 30% total iron, a l m o s t exclusively in t h e f o r m of m a g n e t i t e . These taconites are m e c h a n i c a l l y very h a r d . T o l i b e r a t e t h e m a g n e t i t e , very finely d i s s e m i n a t e d t h r o u g h the ore, a very f i n e g r i n d i n g was necessary which, a f t e r m a g n e t i t e s e p a r a t i o n , y i e l d e d 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 t r e a t e d in the p l a n t 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 1 6 ) . Table 1. Chemical composition and grain structure of magnetite concentrate in Chemical analysis, dry%

pellet

plan

Structure mesh wt-% Cumulative wt-% 0.1 0.7 2.2 6.2 9.7 18.7 27.1 100.0

Fe SiO2 Al2O3 CaO MgO Mn P S TiO 2 Fe 2+ Moisture, % H 2 O of Pellet Feed

65.5 7.8 0.5 0.5 0.6 0.25 0.032 0.003 0.10 21.75 10.00

+ 100 + 150 + 200 + 270 + 325 + 400 + 500 -500

0.1 0.6 1.5 4.0 3.5 9.0 8.4 72.9

Specific Surface Area 1700 cm 2 /g

8


T h e high fine o r e p o r t i o n of a b o v e 96% m i n u s 325 m e s h is t o o fine-grained for efficient sintering d u e to t h e low p e r m e a b i l i t y 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 f o r taconite concentrates was started u n d e r g u i d a n c e of a n d at the MINESEXPERIMENTAL 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 J e r n k o n toret (Swedish iron and steel institute) in S t o c k h o l m f o u n d e d a c o m m i t t e e in 1946 to a p p l y concentrates 7 ), a historical curiosity in r e m e m b r a n c e of the p a t e n t s p e c i f i c a t i o n by Andersson. T h e work of t h e Swedish researchers, u n d e r the direction of M a g n u s Tiegerschild, 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 r e d u c t i o n 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 s u i t a b l e for direct reduction and it was a decisive i m p u l s e for the f u r t h e r d e v e l o p m e n t of direct r e d u c t i o n processes. A t first, pellet p r o d u c t i o n a n d f u r t h e r d e v e l o p m e n t of c o r r e s p o n d i n g processes were limited to the two a b o v e regions w h e r e particularly f a v o u r a b l e c o n d i t i o n s 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 a n interesting d e v e l o p m e n t , o t h e r alternative processes for the agg 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 p u b l i c a t i o n 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 ), t h e 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 d e c i d e d to d e v e l o p f u r t h e r , on a large scale, a n o t h e r a g g l o m e r a t i o n m e t h o d , the so-called " n o d u l i z i n g p r o c e s s " for t h e p r o d u c t i o n of n o d u l e s f r o m fine-grained taconite c o n c e n t r a t e s in a d u f f coal fired rotary k i l n 1 0 ) . At the s a m e site a sintering plant was built a l t h o u g h n o r m a l l y such plants h a d h i t h e r t o b e e n erected n e a r t h e blast furnaces. A f t e r a p e r i o d of initial troubles, b o t h plants w e r e s t a r t e d u p a n d o p e r a t e d . H o w e v e r , a f t e r a few years, they were s h u t down. T h e p r o d u c t i o n of pellets w h o s e qualities were increasingly gaining recognition a d v a n c e d so r a p i d l y t h a t f r o m t h e n o n w a r d s 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 t h a t the original goal of also o p e n i n g u p deposits with a low iron content was fully achieved. A r o u n d 1955, the second p h a s e of the pelletizing process d e v e l o p m e n t was t e r m i n a t e d with the s t a r t - u p of the two large pelletizing plants of R e s e r v e M i n i n g Co. a n d Erie M i n i n g Co. with a n a n n u a l capacity of 12 million tons. T h e successful d e v e l o p m e n t of t h e d i f f e r e n t pelletizing processes in the M e s a b i R a n g e is the result of intensive c o o p e r a t i o n between big mining companies, e.g. Erie and R e s e r v e M i n i n g Co. with science, the U n i v e r s i t y of M i n n e s o t a as well as i m p o r t a n t engineering com-


9

panies, contractors a n d suppliers. T h e latter c o u l d partly use m a c h i n e r y , e q u i p m e n t a n d structures a l r e a d y k n o w n a n d a p p r o v e d . T w o f i r i n g systems f o r t h e i n d u r a t i o n of green pellets were e m p l o y e d o n a large-scale. Erie M i n i n g used the s h a f t f u r n a c e11sobrescrito),a n d R e s e r v e 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 d u r i n g this p e r i o d . W i t h o u t t h e i n t e n t i o n 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 R a n g e , d e v e l o p m e n t of t h e pelletizing process w o u l d p r o b a b l y not h a v e b e e n started at this location.

1.2.3 Third P h a s e , Pellets from O r e s T h e close correlations b e t w e e n sintering a n d pelletizing b e c a m e explicit d u r i n g b o t h the first a n d second d e v e l o p m e n t phases. Since there was n o c o m p u l s i o n at that t i m e to d e v e l o p f u r t h e r the i n g e n i o u s k n o w l e d g e of Andersson a n d Brackelsberg, the first p r o m i s i n g steps to pelletizing w e r e soon forgotten. In the second p h a s e of d e v e l o p m e n t t h e sinter process was exclusively c o n f r o n t e d w i t h ever-increasing q u a n t i t i e s of very f i n e - g r a i n e d magnetite concentrates, so that t h e p r o d u c t i o n c a p a c i t y was closely limited. T h e e n o r m o u s q u a n t i t i e s of concentrates a n d the necessity of f i n d i n g a s o l u t i o n to the p r o b l e m gave the i m p e t u s to d e v e l o p pelletizing u p to a process a p p l i c a b l e to industrial o p e r a t i o n . A t t h a t t i m e , n e i t h e r in t h e U S A n o r in S w e d e n was t h e r e a n e e d for pelletizing o t h e r ores t h a n concentrates. T h e third p h a s e started f r o m q u i t e a n o t h e r situation. Various newly d e v e l o p e d steps f o r i m p r o v i n g the sinter process w e r e 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 t h e basis f o r a new pelletizing process v a r i a n t for iron ores a n d mixtures. T h i s process was d e v e l o p e d in parallel to the k n o w l e d g e derived f r o m the pelletizing of concentrates. T h e sintering of i r o n 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 v a r i o u s countries according to the availability of ore reserves. C o n s e q u e n t l y , the readiness to give this process a n a d e q u a t e p o s i t i o n w i t h i n t h e c o n t e x t of pig i r o n production in conjunction with the o p e r a t i o n of the blast f u r n a c e d i f f e r e d greatly. T h i s was a p p a r e n t at the e n d of t h e F i r s t W o r l d W a r as well as at the beginning of a n d 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 i r o n f r o m l o w - g r a d e f i n e - g r a i n e d ores resulted in i n t e n s i f i e d d e v e l o p m e n t w o r k in t h e sintering field 12 ). T h i s d e v e l o p m e n t w o r k r e f e r r e d 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 m o s t recent d e v e l o p m e n t progress in sintering is d e s c r i b e d in a b o o k b y F. C a p p e l and H . B. W e n d e b o r n 2 ) . A g a i n a f t e r the S e c o n d W o r l d W a r in a varied r a w m a t e r i a l s i t u a t i o n h i g h e s t p r o d u c t i o n rates of the sinter

10


Fig. 2. Influence of particle size on sinter productivity

m a c h i n e at m i n i m u m f u e l c o n s u m p t i o n for the p r o d u c t i o n of a g g l o m e r a t e were the c o n d i t i o n s to be fulfilled. F l o t a t i o n p y r i t e cinders a n d S w e d i s h concentrates constituted growing portions of t h e ore mix. T h e g r e a t e r 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 / m 2 a r e a in 24 hours) as is clearly d e m o n s t r a t e d 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 c o n s i d e r a b l e o u t p u t decrease was observed 13 ). F r o m this resulted the d e m a n d to k e e p the fines p o r t i o n 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 p o r t i o n 0.125 m m should not exceed 10% 2 ). T h e fine ores at present available o n the world m a r k e t generally contain a m u c h h i g h e r percentage of fines. A c c o r d i n g to experience, a c o r r e s p o n d i n g d e c r e a s e of o u t p u t of sinter plants in o p e r a t i o n t h r o u g h o u t t h e world w o u l d b e unavoidable. F o r t h e owners of such sinter plants s u p p l i e d with ores f r o m m i n e s all over t h e world a n d not f r o m their o w n mines, this w o u l d m e a n h i g h e r p r i m e costs for sinter a n d 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 a b o v e 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 h a d to be compensated for. T h e s e p a r a t i o n of fines f r o m the sinter mix could be theoretically conceivable ( b u t d i f f i c u l t to realise) as is d e m o n s t r a t e d b y t h e test described below 14 ). A sinter m i x consisting of 11 d i f f e r e n t ore c o m p o n e n t s with a grain size p o r t i o n of a b o u t 40% 0.5 m m was sintered at a capacity of X t o n s / m 2 p e r d a y a f t e r c a r e f u l p r e p a r a t i o n . T h e s a m e ore was screened at 0.5 m m a n d only the coarser f r a c t i o n 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 c o m p e n s a t i n g the r e d u c e d capacity to be expected w o u l d be the e n l a r g e m e n t of the suction a r e a of the sinter strand. H o w -


11

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 o r e m i x t u r e s a n d the c o r r e s p o n d i n g l y d i f f e r e n t fines portions. N o r would the s e p a r a t i o n of the very fine particles or t h e extension of the suction a r e a b e c o n s i d e r e d s u i t a b l e 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, t h e i n t r o d u c t i o n of a n e w process stage b r o u g h t t h e expected solution. A f t e r the d e t r i m e n t a l effect t h a t excessive fines p o r t i o n h a s on the sinter o u t p u t b e c a m e k n o w n , t h e sinter m i x p e r m e a b i l i t y was improved in intensive l a b o r a t o r y tests b y f u r t h e r t r e a t m e n t of the prepared sinter mix. In these tests t h e p r e p a r e d m i x was c o n v e y e d to a second m i x e r in w h i c h it was a d d i t i o n a l l y rolled. T h i s process v a r i a n t is k n o w n as two-stage granulation (rerolling) a n d n o w 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 t h e world. By rerolling, the very f i n e particles a d h e r e to coarser particles. T h e ore m i x now contains small balls a n d o r e particles with a size d i s t r i b u t i o n of a b o u t 0 . 5 - 8 m m . Micro-pellets are f o r m e d as can be seen f r o m the c o m p a r i s o n s h o w n in Fig. 3. A n o r m a l sinter m i x a n d a sinter m i x subjected to this a f t e r - t r e a t m e n t s h o w distinct d i f f e r e n c e s in the size distribution a n d s h a p e . 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 m i x i n g 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 t i m e f o r s o m e h i g h - g r a d e iron ores w i t h good sintering p r o p e r t i e s are s h o w n in s o m e tests d e s c r i b e d

Fig. 3. Piles of sinter mix with and without re-rolling

12


Fig. 4. Influence of re-rolling of a Venezuelan ore feed on sinter plant capacity

below w h i c h w e r e carried out with a n d w i t h o u t portions of f i n e - g r a i n e d flue d u s t 1 5 ) . V e n e z u e l a n f i n e ores ( m i x A) a r e m i x e d with coke breeze, return fines a n d w a t e r a n d are sintered at a p r o d u c t i o n rate of 48 t / m 2 / 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 ( m i x B), the o u t p u t d r o p s to a b o u t 38 tons. By rerolling over 5 minutes the o u t p u t is re-increased to t h e p r e v i o u s value (see Fig. 4). A substantially l o w e r specific o u t p u t ( a b o u t 22 t / m 2 / 2 4 h) is o b t a i n e d with a n o t h e r ore m i x h a v i n g 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, t h e o u t p u t can be g r a d u a l l y raised to m o r e t h a n 30 tons, w h i c h represents an increase of almost 38% (Fig. 5).

Fig. 5. Influence of re-rolling of a sinter mix (magnetite concentrate, flue dust, pyrite cinders) on sinter plant capacity


13

By this s i m p l e process variant, w h i c h is at p r e s e n t a p p l i e d 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 t h a t t o d a y p o r t i o n s of m o r e t h a n 10% fines - 0 . 2 m m c a n 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 p e r c e n t a g e of e x t r a n e o u s ores (Japan, G r e a t Britain, G e r m a n y ) , t h e r e was b u t little incentive to introduce the pelletizing process. Nevertheless, the p r o d u c t i o n of m i c r o - p e l l e t s was t h e first step t o w a r d s 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 o n e p a r t i c u l a r case, a d o p t e d to 100% flotation pyrite cinders w i t h o u t 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 t h a t not the total m i x but only the fine r a w m a t e r i a l w a s f o r m e d into balls of 3 - 6 m m d i a m e t e r w h i c h were m i x e d w i t h r e t u r n fines a n d coke breeze a n d sintered. T h i s process h a s b e c o m e k n o w n as pellet sintering a n d is a n int e r m e d i a t e process b e t w e e n n o r m a l sintering a n d pelletizing. T h e final p r o d u c t is a sinter c a k e in w h i c h the pellet s t r u c t u r e of the raw mix c a n still be recognised in the finished sinter p r o d u c t . S u c h a pelletizing p l a n t was o p e r a t e d for several years by C O M I N C O in Trail, C a n a d a . S i m i l a r results were o b t a i n e d in an u p - d r a u g h t pelletizing p l a n t erected b y Cleveland Cliffs Iron C o m p a n y n e a r I s h p e m i n g , M i c h i g a n , called " E a g l e Mill" p l a n t 1 6 ) . In this p l a n t the total h e a t 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 o b t a i n e d consisted largely of sinter l u m p s a n d not, as expected, of pellets. H o w e v e r , n e i t h e r of t h e process variants was f u r t h e r d e v e l o p e d . T h e s e plants are n o longer in o p e r a t i o n . In m a n y countries w h i c h exclusively used the sinter process for i r o n ore a g g l o m e r a t i o n , a n o t h e r p r o b l e m arose, n a m e l y to s u p p l y the necessary solid, lean fuel, m a i n l y coke breeze. T h e r e was a scarcity of this f u e l in certain countries b e c a u s e d u e to t h e c o n s i d e r a b l y increasing n u m b e r of sinter plants b e i n g built, t h e necessary a m o u n t s of c o k e breeze, c h i e f l y originating f r o m coke screening, w e r e n o longer s u f f i c i e n t to m e e t the r e q u i r e m e n t s . It was necessary to c r u s h h i g h - g r a d e , e x p e n s i v e m e t a l l u r g i 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 a n increased use of blast f u r n a c e gas f r o m w h i c h a n o t h e r process variant of the n o r m a l 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 N o r m a l l y , t h e coke b r e e z e o n the s u r f a c e of the sinter b e d is ignited by the c o m b u s t i o n of f l u e gas in an i g n i t i o n h o o d . T h e ignition h o o d length

14


Fig. 6. Influence of mixed firing on sintering of iron ores

and the d u r a t i o n of the i n f l u e n c e of the h o t c o m b u s t i o n gases is accordingly limited. T h e heat a m o u n t d e v e l o p i n g inside the ignition h o o d is bed. W h e n a d o p t i n g the m i x e d firing m e t h o d , the ignition h o o d is substantially extended in o r d e r t h a t a h i g h e r gas v o l u m e can be b u r n e d a n d greater h e a t quantities c a n be sucked i n t o the sinter bed. W i t h such e x t e n d e d ignition h o o d s it is possible to d e c r e a s e the coke c o n s u m p t i o n remarkably. F l u e gas can be replaced by o t h e r h e a t sources, such as hot air of a b o u t 8 0 0 0 C , oil or natural gas, according to t h e i r availability. C o k e b r e e z e c a n n o t be r e p l a c e d by hot gas to a n u n l i m i t e d extent, w i t h o u t i m p a i r i n g the plant o u t p u t and sinter strength, as s h o w n in Fig. 6. A n a d v a n t a g e of this m e t h o d is t h e better reducibility with increasing p o r t i o n of h o t g a s 1 7 ) .

insignificant

1.3 Pelletizing, a Contribution to Ore Preparation

15

T h e m i x e d 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 p a r t i c u l a r l y 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 36 m m for pellet sintering, it was not very d i f f i c u l t to p r o d u c e green pellets of a g r e a t e r d i a m e t e r a n d 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 t h e s e pellets exclusively w i t h 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 r e q u i r e m e n t s . A consistent f u r t h e r d e v e l o p m e n t of this f i r i n g technology by the exclusive use of gas o r oil l e a d s to the a p p l i c a t i o n of t h e pelletizing process. T h i s firing technology a p p l i c a b l e to the firing of g r e e n 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 l i m e s t o n e 18 ). T h e three d e v e l o p m e n t p h a s e s of pelletizing are s u m m a r i z e d below: First Phase A l t e r n a t i v e to Sintering: 19101927. Besides t h e sintering process u n d e r d e v e l o p m e n t , a n alternative b e c o m e s a p p a r e n t , to a g g l o m erate very fine-grained ores by pelletizing w i t h o u t any c o n s e q u e n c e s ensuing. Second Phase - Pellets f r o m C o n c e n t r a t e s : 1 9 4 0 - 1 9 5 5 . P r o d u c t i o n of very fine-grained concentrates increasing c o n s i d e r a b l y in s o m e regions, such as in the M e s a b i R a n g e / U S A , c o m p e l s d e v e l o p m e n t of t h e pelletizing process as a n alternative to sintering. T h e state of d e v e l o p m e n t at t h a t t i m e did not yet allow for the use of h i g h p o r t i o n s of fines in t h e ore mix. Third Phase - Pellets f r o m Ores: 1948 u p to t h e p r e s e n t d a t e . In v a r i o u s countries t h e 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 s u p p l y of ores of d i f f e r e n t fineness. A s a result of this d e v e l o p m e n t work, the a p p l i c a t i o n r a n g e of t h e pelletizing process was also e x t e n d e d , b e y o n d the use of c o n c e n t r a t e s as only c o m p o n e n t s , to o t h e r ores a n d , m o r e o v e r , with c o n s i d e r a b l e success to o r e mixtures, see i t e m 9.3.5.

1.3 Pelletizing, a Contribution to Ore PreparationF o r e c o n o m i c reasons, the t r e a t m e n t of i r o n ores in t h e blast f u r n a c e or in direct r e d u c t i o n plants is nowadays n o longer possible w i t h o u t intensive ore preparation. Even if i n d i v i d u a l process stages involve h i g h p r i m e costs, these are accepted, p r o v i d e d t h a t t h e total p r o d u c t i o n costs of pig iron or s p o n g e 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 i r o n

16


o r e p r e p a r a t i o n is the q u a l i t a t i v e i m p r o v e m e n t of d i f f e r e n t f e a t u r e s of r a w materials: (a) M e c h a n i c a l crushing, grinding, screening, classification. (b) Physical s e p a r a t i o n of various m i n e r a l constituents for the elimination of g a n g u e f r o m the lean ores a n d p r e p a r a t i o n of concentrates with high i r o n content. (c) T h e r m a l or chemical t r e a t m e n t for the elimination of volatile energyConsuming constituents such as H 2 O , C O 2 , S O 4 , S or conversion of h e m a t i t e to magnetite. (d) Metallurgical c h a n g e by basic additives w h i c h decrease the energy c o n s u m p t i o n of the succeeding process stages. In all f o u r 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 p r e s e n t - d a y iron o r e p r e p a r a t i o n is s h o w n in Fig. 7. Since the significance of "physical mixing" is recognised t h r o u g h o u t the world and a c c e p t e d , even iron-rich ores are c r u s h e d 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. 3050 m m , for direct r e d u c t i o n to a p p r o x . 2 0 - 3 0 m m . T h e e l i m i n a t e d 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. L o w - g r a d e ores with an iron content of less t h a n 50% are u p g r a d e d , the solid g a n g u e constituents s e p a r a t e d a n d in so

Fig. 7. Alternatives for iron ore preparation


17

Fig. 8. Size distribution of pellets

doing the iron content in the c o n c e n t r a t e i n c r e a s e d . A c c o r d i n g to its fineness the c o n c e n t r a t e c a n 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 p r o p o r t i o n s concentrates c a n also b e i n c o r p o r a t e d 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 p r o p e r t i e s , high iron content t o g e t h e r w i t h u n i f o r m size d i s t r i b u tion and close size r a n g e pellets are a n i m p o r t a n t c h a r g e constituent f o r blast f u r n a c e s a n d direct r e d u c t i o n plants. T h e size d i s t r i b u t i o n , s h o w n in Fig. 8, as a n a v e r a g e of t h r e e c o m m e r c i a l s a m p l e s 1 9 ) is r e m a r k a b l e . T h e c o n d i t i o n s are very strict: 8 0 - 9 0 % of t h e pellets s h o u l d h a v e a d i a m e t e r of 9 - 1 5 m m , t h e m a j o r p a r t of w h i c h a d i a m e t e r of 912 m m . T h e c h a r g e s h o u l d , if possible, c o n t a i n n e i t h e r fines 5 m m nor pellets of a d i a m e t e r e x c e e d i n g 25 m m . T h i s is also a n i m p o r t a n t size range for use with o t h e r 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 f u r n a c e . T h e f e e d s h o u l d consists of grains 5 to m a x . 50 m m , p r e f e r a b l y 30 m m ; Fig. 9 s c h e m a t i c a l l y s h o w s the various o p e r a tions l e a d i n g to this d e s i r e d size distribution: c r u s h i n g of coarse ores a n d sinter cake, screening a n d s e p a r a t i o n i n t o 3 fractions. T h e oversize is recycled to t h e g r i n d i n g plant, the product of 5-50 mm or 30 mm goes into the blast furnace a n d t h e u n d e r s i z e is c o n v e y e d to a g g l o m e r a t i o n , e i t h e r sintering or pelletizing plant. T h e undersize also includes concentrates, pyrite cinders, mill scale, flue d u s t a n d possibly additives. Pellets, c r u s h e d sinter a n d classified raw ore constitute, e i t h e r a l o n e or in the f o r m of mixtures, the blast f u r n a c e b u r d e n .

18


Fig. 9. Scheme of preparation and size distribution of blast furnace feed

Besides sinter, still the m o s t i m p o r t a n t t y p e of a g g l o m e r a t e s f o r 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 g r o w i n g i m p o r t a n c e f o r p i g i r o n a n d sponge iron p r o d u c t i o n . In Fig. 10 the develo p m e n t of p i g i r o n p r o d u c t i o n f r o m 1960 to 1978 is c o m p a r e d with sinter a n d pellet p r o d u c t i o n . F r o m 1974 the curves f o r sinter a n d pig i r o n decline, t h e latter c a u s e d b y e c o n o m i c reasons. T h e pellet curve r e m a i n s steep u p to a n e s t i m a t e d average pellet p o r t i o n 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 f e e d m a terial for direct reduction processes is i n c l u d e d b u t the p o r t i o n is r e m a r k a bly h i g h e r by u p to 100%. In s h a f t f u r n a c e s efforts are m a d e to replace a p a r t of the pellets by c h e a p e r , carefully selected and screened l u m p ores. A m i x t u r e of 5575% pellets a n d 4525% l u m p ores h a s recently p r o v e d to b e suitable f o r o p t i m u m productivity, as s h o w n in Fig. 11.


19

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 shaft furnace capacity

20


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 a p p r o p r i a t e processes is directly connected with t h e b e n e f i c i a t i o n of lowg r a d e iron ores - originally chiefly m a g n e t i t e s to p r o d u c e (very fine-grained) In the first place, it was necessary to convert these concentrates into agglomerates r e a d y f o r the blast f u r n a c e . At t h e s a m e t i m e the q u e s t i o n arose r e g a r d i n g t h e o p t i m u m site of such a pelletizing plant. T h e very with long a n d cold winters because they freeze d u r i n g this t i m e a n d can neither be l o a d e d nor transported. T h e s e p r o b l e m s were solved by erecting the pelletizing p l a n t in the close vicinity of the b e n e f i c i a t i o n p l a n t a n d thus practically at the ore mine. Pellets are resistant to the influences of storage and winter conditions. T h e y w i t h s t a n d long t r a n s p o r t a t i o n routes with several t r a n s - s h i p m e n t s better t h a n l u m p ores o r even sinter. D u r i n g the first years, pellets were p r o d u c e d , m a i n l y in the U S A , to supply blast f u r n a c e s w i t h i n a controlled m a r k e t . T h e i r t r a n s p o r t a b i l i t y was accepted b u t n o special controls were instituted. O n l y a f t e r m a j o r pellet q u a n t i t i e s h a d a p p e a r e d o n the world m a r k e t a n d h a d r e a c h e d o t h e r blast furnaces, was greater i m p o r t a n c e attached to the b e h a v i o u r of pellets d u r i n g transport. T h u s , the p r e p a r a t i o n of a large-scale test (1963), r u n o n a b o u t 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 f u r n a c e of t h e 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. t h e study of the a b r a s i o n b e h a v i o u r , c o m p r e s s i o n strength a n d variation of pellet size 2 0 ). F r o m P o r t Silver Bay to the works p o r t s i t u a t e d on the river R h i n e , t h e pellets were subjected to six trans-shipments, a w a t e r t r a n s p o r t a n d a n 18 m o n t h storage period at the p o r t a r e a of A m s t e r d a m d u r i n g two winters. T h e fines p o r t i o n 6 m m , c o n t a i n e d in t h e pellets, was used as a reference value. This fines p o r t i o n was at L a k e S u p e r i o r 8.0% a n d increased d u r i n g t r a n s p o r t a t i o n via the P o r t 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, a n d a f t e r screening b e f o r e t h e blast f u r n a c e to a b o u t 18%. T h e p o r t i o n of 1015 m m d i a m e t e r varied f r o m 85% to 82% a n d , a f t e r screening, rose again to 88% at the blast f u r n a c e . In this way, p r o o f was f u r n i s h e d that pellets c a n be t r a n s p o r t e d satisfactorily. T h i s large-scale test successfully carried out in a n i n d e p e n d e n t steel plant with high pellet p r o p o r t i o n s in the blast f u r n a c e b u r d e n - b e c a m e k n o w n publicly a n d aroused increased interest in these n e w a g g l o m e r a t e s also at those works w h e r e pellets h a d h i t h e r t o not b e e n used at all.

concentrates.

fine-grained,

1.4 Sites of Pelletizing Plants and Transportability of Pellets Table 2. Countries leading in iron ore production and export 113 ) Countries 106 t/year 1966 Production Africa Liberia Rep. South Africa America Brazil* Venezuela Canada* USA Asia India Australia Europe France Great Britain Sweden USSR * Leading Exporters 17.0 6.8 23.3 17.9 36.6 91.6 26.8 11.0 55.7 13.9 28.0 160.2 Export 16.6 3.1 11.8 17.0 31.2 9.9 13.4 2.0 18.2-

21

1976 Production 35.0 15.7 70.0 23.0 56.0 81.2 42.6 93.1 45.5 4.6 30.5 239.0 Export 20.8 5.0 47.3 15.6 44.5 2.9 24.0 81.1 15.8

22.3 26.1

22.0 43.1

Pellet p r o d u c t i o n was initially always linked with a b e n e f i c i a t i o n plant, usually located at the m i n e w h e r e s u f f i c i e n t q u a n t i t i e s of very f i n e - g r a i n e d concentrates were available w i t h o u t a n y i n t e r m e d i a t e t r e a t m e n t . T h e increasing w o r l d - w i d e d e m a n d for h i g h - g r a d e i r o n ores led to the discovery of n e w deposits with i r o n ores of d i f f e r e n t m i n e r a l o g i c a l a n d chemical c o m p o s i t i o n . T h e i r quality c o u l d also be i m p r o v e d by b e n e f i c i a tion as in the case of M a r c o n a in P e r u . 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 f o r pellets so t h a t their e x p l o i t a t i o n could only be secured by e x p o r t a t i o n . S o m e countries h a v e a d a p t e d themselves p a r t i c u l a r l y to this s i t u a t i o n so that a c o n s i d e r a b l e c h a n g e of ore s u p p l i e r s h a s o c c u r r e d d u r i n g t h e last ten years, as is s h o w n in T a b l e 2 f o r the years 1 9 6 6 - 1 9 7 5 . Firstly, it tpy or 50% c o m p a r e d to 1966. In s o m e instances t h e r e were even m o r e i m p o r t a n t c h a n g e s as, f o r e x a m p l e , in A u s t r a l i a a n d Brazil. A f u r t h e r r e m a r k a b l e p h e n o m e n o n is the p r o d u c t i o n d e c r e a s e of l o w - g r a d e ore in F r a n c e ( m i n e t t e ) a n d in G r e a t B r i t a i n ( h o m e ores). T h e s e countries n o w i m p o r t the c o r r e s p o n d i n g iron units in t h e f o r m of h i g h - g r a d e ores. O t h e r countries, s u c h as J a p a n a n d 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 t h e i r r e q u i r e m e n t s a l m o s t exclusively by i m p o r t s .

indicates

22

1 Definition and Development of Pelletizing Process Table 3. Screen analysis of exported fine ores 2 )

Screen Undersize in mm -11.2 - 8 - 5.6 - 4 - 2.8 - 2 - 1.4 - 1 - 0.71 - 0.5 - 0.355 - 0.25 - 0.18 - 0.125 - 0.09 - 0.063 - 0.045

Liberia (Bomi Hill)

Mano River washed

Kiruna B Hamersley CVRD CVRD weight % (cumuCommon Sinter lative) Fines Feed B 1.0 11 24.5 35 43.5 49 54.5 60.5 65 69.5 75 79 84.5 87.5 90.5 92.5 21.5 34 42 47 51.5 54 55.5 57.5 59.5 61 62.6 64.5 67 70 75

Nimba

15.5 22 29 35.5 42 49 56.5 64 72 80.5 89.5

13.5 24 35 44 53.5 61 68 75 80.5 85 88.5

9 18.5 27 35 41.5 47 51.5 55 58.5 63 67.5 73.5 80

3 10 19 26.5 32 36 40.5 45 49 52 57 63 74 85.5

8 16 24 31 35.5 39 42.5 46 50.5 55 68 73.5 80.5 84.5

T h e fine ores to be s h i p p e d f r o m p o r t to c u s t o m e r f r e q u e n t l y c o n t a i n a very high a n d d i f f e r e n t p o r t i o n of fines as is a p p a r e n t f r o m s o m e e x a m p l e s given in T a b l e 3. T r a n s p o r t a t i o n difficulties as a result of d u s t arising d u r i n g loading, t r a n s - s h i p m e n t a n d u n l o a d i n g of such ores as well as difficulties d u r i n g sintering led to the p r o d u c t i o n of pellets at the p o r t a n d to the e x p o r t a t i o n of pellets instead of fine ores. T h i s was the beginning of the construction of new pelletizing plants in s h i p p i n g ports, e.g. C V R D , T u b a r a o / B r a z i l , H a m e r s l e y , P o r t D a m p i e r / A u s t r a l i a , M a r c o n a , San concentrates s u p p l i e d f r o m various mines; they can be d e s i g n a t e d as pelletizing plants located at the port. A f t e r pelletizing technology h a d b e e n sufficiently d e v e l o p e d , b l e n d e d ore pellets consisting of varying ore types were also p r o d u c e d 21 ). A s a result of this it b e c a m e possible to build pelletizing plants in direct connection with the blast f u r n a c e . Only this latter c o m b i n a t i o n w o u l d h a v e the a d v a n t a g e of being a b l e to pelletize very f i n e - g r a i n e d ores a n d concentrates as well as waste oxides a n d thus release the sinter plant. A typical e x a m p l e of such a c o m b i n a t i o n is the pelletizing p l a n t of t h e K o n i n k l i j k e N e d e r l a n d s c h e H o o g o v e n s en S t a a l f a b r i e k e n N . V. in I j m u i d e n / N e t h e r l a n d s 22 ), where, besides the pellet plant (3.5 million tpy) t h r e e sinter plants, altogether h a v i n g the s a m e capacity, are in o p e r a t i o n . F u r t h e r plants are o p e r a t i n g at the J a p a n e s e Iron & Steel W o r k s K a w a s a k i - C h i b a , K o b e - N a k a h a m a a n d K o b e - K a k o gawa. This t h i r d t y p e of p l a n t can b e d e s i g n a t e d as pelletizing plant located near the blast furnace.

Nicolas/Peru,

Wab

2 Fundamentals of Pelletizing

Pellets differ f r o m l u m p ore and, to a certain extent, also f r o m sinter by several properties which are p r e d e t e r m i n e d and definable. Despite the great variety of raw materials used, the pellets p r o d u c e d must have the same properties which are j u d g e d in accordance with internationally accepted standards. D u e to their importance and great variety, the properties d e m a n d e d f r o m pellets are specified u n d e r item 1.1. According to today's technology, nearly all iron ores with a correspondingly high iron content can be pelletized: magnetite, hematite, limonite and their concentrates as well as purposely prepared mixtures, waste oxides, pyrite cinders or pertinent by-products f r o m other industries. To obtain the required properties and taking t h e great variety of raw materials into account, a d e q u a t e p r o d u c t i o n m e t h o d s are to be adopted which are described and discussed below: Three process stages are - Stage 1: R a w material - Stage 2: F o r m a t i o n of - Stage 3: Induration of involved to produce the pellet f r o m raw material: preparation green balls green balls.

Successful pellet production calls for an o p t i m u m efficiency and harmony between all three process steps with the preceding stage highly influencing the subsequent one. An error m a d e in the preceding stage can only be corrected to a limited extent in the subsequent process stages. Even during induration, no first-class pellet can be p r o d u c e d f r o m a defective green ball. T h e purpose of the green ball f o r m a t i o n is to obtain balls of the desired size range and having a mechanical strength which enables them to be safely transported f r o m the balling e q u i p m e n t to the induration facilities. D u r i n g the f o r m a t i o n of green balls f r o m solid fine-grained particles, many different forces co-act, which are designated as bonding mechanisms.

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2 Fundamentals of Pelletizing

2.1 Bonding Mechanisms for Green Ball FormationAt a n early stage, b o t h e x p e r i m e n t a l a n d theoretical considerations were m a d e with r e g a r d to the b o n d i n g m e c h a n i s m s u n d e r l y i n g the green ball f o r m a t i o n . S o m e of the theories d e v e l o p e d in this c o n n e c t i o n were a l r e a d y k n o w n f r o m o t h e r processes, e.g. those of the fertilizer industry, f o o d - s t u f f s industry, p h a r m a c e u t i c a l and r e f r a c t o r y industries. S i m i l a r b o n d i n g m e c h anisms also play a great p a r t in the g r a n u l a t i o n a n d c r u m b l i n g of the sinter mix. T h e r a p i d progress of pelletizing technology was achieved particularly by a l m o s t simultaneous e x p e r i m e n t a l a n d theoretical progress in contrast to o t h e r process d e v e l o p m e n t s , e.g. in the blast f u r n a c e technology.

2.1.1 Important Bonding Factors T h e decisive factors for the green ball f o r m a t i o n a n d green properties can b e d i v i d e d into the following g r o u p s 23 ): ball

(a) physical forces, such as van der Waals', m a g n e t i c or electrostatic forces (b) surface-dependent factors, such as particle size, particle size d i s t r i b u tion, particle s h a p e a n d crystalline structure (c) material-dependent factors, such as wettability, a b s o r p t i v e capacity d u e to p o r o u s structure, availability of swelling c o m p o n e n t s , c h e m i c a l p r o p e r t i e s in p r i m a r y ores or b y - p r o d u c t s a f t e r p r e v i o u s t r e a t m e n t (d) capillary forces and surface tension d u r i n g t h e a d d i t i o n of l i q u i d binders, s u c h as w a t e r or others. S o m e of these factors, mainly the raw m a t e r i a l - d e p e n d e n t factors are not variable. H o w e v e r , they influence the green ball f o r m a t i o n to a great extent. O t h e r forces also acting on the green ball p r o p e r t i e s are variable. By utilizing such forces, the raw materials to be pelletized can be a d a p t e d to the relevant r e q u i r e m e n t s . V a r i a b l e factors are, for e x a m p l e the q u a n t i t y of wetting agent a d d e d , the particle fineness and shape, the balling e q u i p m e n t used for green ball f o r m a t i o n , the forces arising in such e q u i p m e n t as well as the m o v e m e n t of raw materials in these units.

2.1.2 Ball Formation Alternatives G r e e n balls c a n be f o r m e d in d i f f e r e n t ways. In each p a r t i c u l a r case, the various b o n d i n g m e c h a n i s m s act at d i f f e r e n t intensity.

2.1 Bonding Mechanisms for Green Ball Formation 2.1.2.1 Compacting Method

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T h e solids are pressed into b r i q u e t t e s only by the a p p l i c a t i o n of h i g h mechanical forces. S h a r p - e d g e d b r i q u e t t e s m a y be r o u n d e d off b y rerolling. T h e i n d i v i d u a l grains are held t o g e t h e r b y a d h e s i v e a n d cohesive forces which, in turn, d e t e r m i n e the m e c h a n i c a l strength of the b r i q u e t t e s . T h e solid particles, m i x e d either with or w i t h o u t b i n d e r s are p a c k e d r a n d o m l y w i t h o u t any relative m o v e m e n t . T h e location of the v a r i o u s grain faces to each o t h e r is at r a n d o m it does n o t h a v e to b e t h e o p t i m u m l o c a t i o n for the f o r m a t i o n of the greatest a d h e s i v e forces. T h e c o m p a c t i n g t e c h n i q u e - well k n o w n f o r b r i q u e t t i n g a n d t a b l e t c o m pressing is n o w a d a y s a p p l i e d to o r e p r e p a r a t i o n only in specific cases. 2.1.2.2 Green Ball Formation Besides the solid phase, a liquid p h a s e is, in each case, r e q u i r e d f o r green ball f o r m a t i o n . T h e i n t e r f a c e forces arising h a v e a cohesive effect o n the solid particles, liquid a n d air. T h e s e i n t e r f a c e forces consist, on the one h a n d , of t h e s u r f a c e tension of the b i n d e r , usually w a t e r , a n d o n t h e other, of capillary forces d e v e l o p i n g in the l i q u i d b r i d g e s b e t w e e n t h e ind i v i d u a l o r e particle faces, the surfaces of w h i c h are of concave s h a p e . U n d e r these conditions, a certain tensile strength occurs. T h e forces resulting f r o m the s u r f a c e tension f o r m a concave l i q u i d s u r f a c e w h e r e b y compression strength b e c o m e s active. F o r the f o r m a t i o n of green balls f r o m solid particles a n d l i q u i d , t h e r e are two possibilities: T h e solid particles are w i t h o u t any active relative movement. They are mixed in s u i t a b l e e q u i p m e n t , so-called m i x g r a n u l a t o r s , by t h e r o t a t i o n of the vessel a n d the m i x i n g devices a r r a n g e d inside w i t h o u t h a v i n g to p e r f o r m their own rolling m o v e m e n t as in t h e case of a d r u m . A t the s a m e time, they are p u s h e d together by a p u s h i n g m o v e m e n t and slight m e c h a n i c a l forces a n d are c o m p a c t e d . T h e o r e particles are b r o u g h t to such a f a v o u r a b l e position to e a c h o t h e r t h a t the a g g l o m e r a t e f o r m s a tight packing. C a p i l l a r y a n d s u r f a c e forces co-act with a d h e s i v e f o r c e s 2 4 ) . T h e liquid b r i d g e s a n d surfaces b e t w e e n the i n d i v i d u a l o r e particles f o r m automatically. In most cases the relevant b a l l i n g facilities o p e r a t e intermittently. which are n o w a d a y s processed in pelletizing plants. H o w e v e r , in m a n y industries with l o w e r t h r o u g h p u t , t h e y are used with great success. In the s e c o n d case, t h e ball f o r m a t i o n is m a i n l y a c h i e v e d by rolling of s o l i d s / l i q u i d m i x in well-known b a l l i n g units, s u c h as d r u m s o r discs. T h e green ball f o r m a t i o n is similar to snow-ball g r o w t h by layering. T h e l i q u i d surface is f r e e l y m o v a b l e w h i c h is a d v a n t a g e o u s f

pelletizing of iron ores - kurt meyer

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