Strength of iron-ore agglomerates during reduction

International Journal of Mineral Processing, 1 (1974) 193--213 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

STRENGTH OF IRON-ORE AGGLOMERATES DURING REDUCTION*

BIRGITTA HASSLER

Mineral Processing Division, Royal Institute of Technology, Stockholm, (Sweden) (Accepted for publication January 11, 1974)

ABSTRACT H~issler, B., 1974. Strength of iron-ore agglomerates during reduction. Int. J. Miner. Process., 1: 193--213. The paper deals with the reduction strength of iron-ore agglomerates, especially under the conditions in the blast furnace. As an introduction, the reduction zones, temperature zones and pressures exerted on the charge are covered. Special attention has been paid to the reasons for the often very large decrease in strength during heating and reduction. The plasticity of the wiistite is, in this case, of special interest. A number of testing methods for investigation of stresses and strains of the agglomerates during reduction are introduced. The test methods cover different procedures in the blast furnace -- isothermal as well as blast furnace simulating. A method of measuring the pressure strength during reduction has been worked out which comprises testing the agglomerates at high temperature during the reduction. It was originally developed in order to study the cold-bound balls during the reducing process. With the aid of this test method the necessary conditions were established for the development of perfect agglomerates according to the COBO method. A series of results from investigations made with these tests are given both for coldbonded COBO balls of various compositions and for commercial types of burnt pellets.

INTRODUCTION The term "reduction strength" means the mechanical strength of iron-ore a g g l o m e r a t e s d u r i n g r e d u c t i o n . T h e i r o n - o x i d e m a t e r i a l in t h e a g g l o m e r a t e s c o n s i s t s o f i r o n o r e - - h e m a t i t e a n d / o r m a g n e t i t e - - w h i c h in v a r i o u s f o r m s is r e d u c e d t o i r o n . T h e r e d u c t i o n u s u a l l y t a k e s p l a c e in a b l a s t f u r n a c e , y i e l d i n g m o l t e n pig i r o n as t h e e n d p r o d u c t . N o w a d a y s , h o w e v e r , a g r e a t d e a l o f i n t e r est is b e i n g s h o w n in d i r e c t r e d u c t i o n o f i r o n - o r e a g g l o m e r a t e s t o s p o n g e i r o n , so t h e s e m e t h o d s o u g h t also t o be c o n s i d e r e d in a n y s t u d y o f i r o n - o r e a g g l o m erates that lend themselves to such m e t h o d s of reduction. I r o n - o r e a g g l o m e r a t e s i n t e n d e d f o r b l a s t f u r n a c e r e d u c t i o n are: (1) S i n t e r e d p e l l e t s , w h i c h c o n s i s t p r i n c i p a l l y o f i r o n - o r e c o n c e n t r a t e s r o l l e d i n t o p e l l e t s a n d s i n t e r e d at a b o u t 1 , 2 5 0 ° C in an o x i d i s i n g a t m o s p h e r e ; t h e p e l l e t s are u s u a l l y 10 t o 12.5 m m in d i a m e t e r . * Paper presented at CIM Conference of Metallurgists, Quebec City, August 26--29, 1973.

194

(2) Sinters, consisting of iron-ore concentrates sintered with a binding agent in a pan or on a belt; the sintered cake is then broken up into pieces smaller than 50 ram. (3) Grangcold pellets, which are cement-bonded iron-ore pellets about 18 mm in diameter -- air-hardened sinter-pellet concentrates; this is a fairly new product developed at Gr~ingesberg, Sweden. (4) COBO pellets, which are autoclaved (cold-bonded) iron-ore pellets consisting of two-thirds coarse concentrate and one-third pellet concentrate, usually with lime and possibly slag as a binder. COBO pellets are a new, as yet unestablished product devised and developed by the Mineral Processing Division of the Royal Institute of Technology in Stockholm. The COBO process was presented in 1972 at the 11th Annual Conference of Metallurgists in Halifax THE BLAST FURNACE The most c o m m o n type of reduction furnace is without doubt the blast furnace. It is a shaft furnace whose design and working principle are well known. The blast furnace is charged with alternate layers of coke and iron-bearing material -- iron-ore agglomerates and sometimes also lump ore. Limestone is also dosed into the blast furnace to regulate the viscosity and composition of the slag arising from the reduction process. The blast-furnace shaft is a countercurrent reactor in which gas moves upward, giving off heat, while the material moves downward and is heated, reduced and melted in the process. Air preheated to 1,000°C or higher is blown in at the tuy~re level where the coke in the charge undergoes partial combustion, yielding a reduction gas rich in carbon monoxide. The gas is forced upward through the charge. The blast furnace can be divided into different reduction zones according to the temperature of the charge and the composition of the gas (Fig. 1 ): At the top is a drying zone where free water and water of crystallisation are driven off. Next comes a prereduction zone where hematite Fe203, is indirectly reduced to magnetite,. Fe304, with a lower degree of oxidation. Thermal decomposition of carbonates also takes place in this zone. After this there is a gas-reduction zone, where the principal reaction is indirect reduction of magnetite, Fe304, to wi]stite, " F e O " . The wi]stite is to some extent reduced to iron, and thermal decomposition of limestone and dolomite takes place. When the charge temperature exceeds 1,000°C we arrive at the direct reduction zone, where the wiistite is directly reduced to iron by the carbon in the coke. Any remaining magnetite in this zone is reduced direct to wi]stite. In the smelting zone the reduced iron melts and slag forms on its surface. Liquid slag is drawn off through the slag notch and liquid iron through the taphole.

195 MATERIAL TEMPERATURE 25°C DRYING ZONE 350°C

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Fig.1. Blast-furnace process temperature zones. If we consider the t e m per a t ur e zones of the blast furnace, we find t hat heat exchange b etween the gas and the charge occurs i n t w o discrete zones (Fig.2). Between these two zones there is a thermally inactive zone where the gas and charge temperatures are virtually the same, a b o u t 1,000°C. This zone is also a chemical reserve zone where equilibrium prevails between the gas and the wfstite. The charge lies here in a stable arch which constitutes the lower part of the blast furnace. The low mechanical strength of the wfistite makes this zone the most critical as regards the risk of disintegration or compression of the iron-ore agglomerates. If this happens the pore openings are blocked, which greatly increases the resistance to the flow of gas. It is at this t e m p e r a t u r e and gas composition and at this level in the charge that the mechanical strength of the iron-bearing material becomes a m at t er of particular interest. There is one o t he r part of the blast furnace where mechanical strength may be an i m p o r t a n t factor; this is the pr er e duc t ion zone, where hematite is reduced to magnetite. R e d u c t i o n of hematite to magnetite involves restructuring of a hexagonal to a cubic lattice, which necessarily results in some dislocation of the structure. This is the reason for the disintegration which occurs below 700°C in hematitic materials such as hematite lump ore and in pellets, sinters and cold-bonded pellets made of hematite concentrates. Operating conditions in a blast furnace are c o n n e c t e d with permeability and r ed u ctio n in the shaft. Permeability to gas depends on the particle size distribution o f the charge and on the mechanical strength during reduct i on

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Fig.2. Blast-furnace temperature zones.

of both the coke and the iron-bearing material. Reduction depends on t h e reactivity of the coke and on the composition and reducibility of the ironbearing material. The shape, composition and reduction properties of the iromore agglomerates are thus among the factors that can be influer~ced and improved to obtain good operating conditions in the blast furnace. The stack of material in a shaft furnace is supported largely by the hard and generally resistant lumps of coke and by the gas pressure; the influence of the latter is greatest in the zone of high resistance to gas flow. If the ironbearing material has a high reduction strength, it is possible to cut down coke consumption to the quantity actually needed for reduction, while at the same time the m o v e m e n t of material proceeds more smoothly due to a lowering of the gas flow resistance. A less deformed charge can absorb a greater proportion of dust. A smoother process probably also helps to increase the rate of production. A further consequence of high agglomerate strength during reduction is that the quality of the coke used need not meet such stringent requirements and a cheaper coke charge is possible. Working with iron-ore agglomerates of high mechanical strength may also make it feasible to replace part of the expensive coke charge with injected liquid or gaseous fuels such as oil, natural gas or coke furnace gas. Injection of 100 kg of oil per tonne of pig iron could substitute for 120 kg of coke.

197 What then is the pressure on the charge in a blast furnace? It is greatest at the bottom of the reserve zone. Polthier (1967) estimates the charge pressure at the 15-m level at 0.6 kg/cm 2 with and 1.0 kg/cm 2 without gas flow. Polthier assumed a volumetric weight of 1.2 tonnes/m 3 for the charge (Fig.3). Others put the volumetric weight at about 1.7 tonnes/m% which would make the static pressure about 2.6 kg/cm 2 at the 15-m level.

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Fig.3. Calculated charge pressure in a blast furnace. (From Polthier, 1967.) If the tuy~re pressure temporarily drops, the stack of material in the shaft settles, whereupon the pressure on the lower portions of the charge rises abruptly. Actual charge pressures can thus be higher than the estimates just given, as has been proved by inspection of cooled blast furnaces where even pellets known to be of high mechanical strength were found squashed flat. PLASTICITY OF WlJSTITE How does it come about that iron-oxide material, which in the cold state may have a crushing strength as high as a couple of hundred kilograms force

198

per pellet, loses nearly all its strength during reduction at high temperature? As previously mentioned, wiJstite is plastic at the temperatures involved. The plasticity is due to the iron deficiency of wfistite in relation to the stoichiometric composition FeO. Vacancies in the lattice, and the resultant dislocations and migration of dislocations, give rise to a high plasticity of wiistite material, especially at elevated temperatures. Reduction strength in these conditions is totally dependent on the gangue or binder phases that link the iron-oxide grains. The composition, nature and distribution of these phases are thus crucial to the mechanical strength of the material at high temperatures. Ceramic phases with a high softening point, well distributed in the agglomerates, represent the ideal. In the process of agglomerating iron-ore concentrates it is possible to some extent to influence the composition and distribution of the binder phases. The possibilities are limited with down-draught sintering, greater with pelletising and greater still with cold binding by the COBO method. Mixing and grinding of concentrates and binders, possibly with additives, before pelletisation gives a very good distribution of gangue and binder in iron-ore pellets of the COBO type. The plasticity of the wbstite can, however, also be influenced in iron-ore agglomerates. If free lime, CaO, is still present after hardening, Ca 2÷ ions can diffuse into the wiJstite lattice during reduction and occupy the vacancies. The Ca 2÷ ions gradually fill o u t the lattice, stabilising it and blocking the dislocations. The wiJstite lattice is stiffened, and its plasticity is diminished. Lime-bound COBO pellets usually contain free lime after being hardened. Both microprobe analysis and X-ray scanning can be used to prove that diffusion of Ca 2÷ ions into the wiistite lattice actually occurs. The approximate Ca2+= 0.96,~,

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LIME BOUND COBO PELLETS

199 level o f calcium in individual wi]stites can be determined with a microprobe. The face-centred cubic lattice of the w~stite can be examined and the edge length of unit cells det er m i ned by line broadening on a pow der diffraction camera. Diffusion of Ca 2÷ ions into the wi]stite lattice enlarges the unit cell -resulting in an increased edge length -- because the radius of the Ca 2÷ ion, 0.96 £, is greater than t hat of the ferrous ion, 0.83 £. Fig.4 shows th e lattice parameters of wi]stite in different autoclaved ironore agglomerates b o u n d with different binders. The determinations have been made on the basis of pictures taken with a Guinier-H~gg focusing camera. In a filler b o u n d with c e m ent -- fine concentrate and binder in proport i ons of two to one -- a m a x i m u m of about 4% Ca by weight was measured in the wiistite with the microprobe. Magnesia-bound filler has shown up to 20% Mg by weight. Full solubility in the solid state prevails in the wi]stite--MgO sy stem. The size of the Mg 2÷ ion -- radius 0.78 A -- is less than that of the ferrous ion, so th at there is some shrinkage of the lattice. The CaO, FeO and MgO phases all have the same face-centred cubic lattice structure. TESTING OF STRENGTH DURING REDUCTION Introduction R e d u c t i o n strength has in recent years be c om e the object of increasing interest in various parts of the world. There has been a growing awareness of its imp o r tan ce in b o t h blast furnace and direct reduction operations. There has also, as already mentioned, been a growth of interest in recent times in direct reduction; the feasibility of reducing iron oxide materials economically by such m et hods can be largely attributed to the availability of natural gas in ma ny countries. The direct reduction m et hods t hat are currently attracting the most a t t e nt i on are Midrex and Purofer, b o t h of which resemble blast-furnace r ed uct i on in that a shaft furnace is used and is charged with ironore pellets. The reducing agent in these furnaces is natural gas, modified in one way or another, and the iron oxide material is reduced to sponge iron. As the supporting coke stack of the blast furnace shaft is n o t present in the shaft of a direct r ed u ctio n furnace, these processes make even stiffer demands on the reduction strength of the iron-ore agglomerates. The term r ed u c t i on strength, however, can bear different interpretations. It is f r e q u e n t l y taken to mean the resistance of the iron oxide material to the disintegration that is liable to occur in the prereduct i on zone, where hematite may disintegrate at low t e m p e r a t u r e on conversion to magnetite. An alternative view is that there is little risk of disintegration in the prereduction zone because the rate of charging of blast furnaces is so high nowadays that the charge reaches a t e m p e r a t u r e of 800°C before any extensive reduction has time to occur. It is of more immediate interest to study the strength of the material in the lower p o r t i o n of the reserve zone -- the critical area -- before direct reduction and melting start.

200 Low-temperature disintegration is usually tested by measuring the resistance of the material to abrasion, either during reduction or after reduction in a static bed and cooling. Resistance to stress during reduction at high temperature deeper in the blast furnace shaft is measured in many different ways. At one time it was usual to reduce the material at high temperature, after which the sample was cooled and subjected to strength testing at room temperature. This still seems to be a fairly c o m m o n practice in both Germany and Eastern Europe, but especially in Japan. Neither wi]stite nor the binder phase, however, has the same properties at room temperature as at 1,000°C. It is obviously impossible to test the blast-furnace strength of a material at room temperature. Nowadays attempts are made to simulate blast-furnace conditions as closely as possible in the testing of reduction strength. The gas composition and t e m perature are adjusted according to what is known about conditions in tile blast furnace at the level in question. There are the isothermic tests in which the material is heated in an inert atmosphere to the test temperature, after which it is reduced to varying degrees. The material is then tested in the reduction furnace, either by measurement of its compressive strength and compression or by measurement of the pressure drop across a statically loaded bed, i.e. the permeability and compaction of the bed. In some places the resistance to abrasion of an isothermically reduced sample is measured in a manner similar to that used in the low-temperature disintegration test. There are also tests designed to simulate the whole reduction process in the blast furnace as far as the 1,000°C level. Special gas and temperature programmes for the entire sequence have been devised for these methods. One such furnace-simulating test also measures abrasion resistance during reduction. Another one measures compressive strength and resistance to deformation after different intervals of time during the reduction programme.

Low temperature disintegration As sinters and hematite lump ores, and also pellets, often disintegrate at temperatures of about 600°C in a weakly reducing environment corresponding to the prereduction zone of the blast furnace, a modified Linder test for low temperatures has come into use. It has been devised by LKAB in Sweden in collaboration with BISRA in the United Kingdom and is called the LTB test, which stands for Low Temperature Breakdown. The m e t h o d has been submitted by BISRA to the ISO Committee. The test measures the abrasion resistance of the material during reduction. It is performed in a rotating, horizontal reduction tube, a Linder tube equipped with lifters. The tube is installed in an electric furnace (Fig.5). A 500-g sample is heated to 600°C in a reducing atmosphere in the tube, which rotates the whole time. Test parameters are listed in the caption to Fig.5. The percentages by weight of material finer than 6.3, 3.15 and 0.5 m m after screening are taken

201

Fig.5. Low-temperature disintegration test. 1, flowmeters; 2, gas meter; 3, valve for gas analyses; 4, plain bearing; 5, sample; 6, barrel with lifters; 7, electric furnace; 8, thermocouple; 9, temperature control; 10, electrical motor; 11, temperature recorder; 12, gas outlet; 13, dust collector; 14, plain bearing with cooling device. Test conditions : Reduction tube: 130 mm ~ x 200 mm, four lifters 20 mm high. Test sample: 500 g in the fraction of 10--12.5 mm pellets, sinters or lump materials. Tumbling speed: 10 rpm. Temperature programme: 0--600°C for 45 min, 1 h at 600°C. Gas composition: 20% CO, 20% CO2, 60% N2. Gas flow: 15 N1/min. The test results are given in wt% -- 6.3-, -- 3.15- and -- 0.5-mm fraction after screening for 150 sec. Good blast furnace pellets ought to show degredation results/>80% + 6.3 mm and < 5% - - 0.5 mm. Good sinters ought to show degradation results/>60% + 6.3 mm and < 1 0 % - - 0.5 mm.

as a m e a s u r e o f r e d u c t i o n s t r e n g t h . G o o d b l a s t f u r n a c e p e l l e t s s h o u l d give m o r e t h a n 8 0 % g r e a t e r t h a n 6 . 3 m m a n d less t h a n 5% s m a l l e r t h a n 0 . 5 m m , w h i l e a g o o d s i n t e r s h o u l d give m o r e t h a n 600/0 g r e a t e r t h a n 6 . 3 m m a n d less t h a n 1 0 % s m a l l e r t h a n 0 . 5 m m . I t is n o w k n o w n t h a t h e a v y d i s i n t e g r a t i o n in t h e o r i g i n a l t e s t a c c o r d i n g t o L i n d e r is a l w a y s m a t c h e d b y h e a v y d i s i n t e g r a t i o n in t h e L T B t e s t . T h e L T B m e t h o d is m u c h s i m p l e r , q u i c k e r a n d c h e a p e r t h a n t h e L i n d e r method. In one application of the LTB method, a test of an overbasic sintered pellet sample showed extensive disintegration on reduction of hematite to magnetite (within the area of the LTB test), a fact which had not been revealed by other tests. This disintegration explained why these overbasic pellets with addition o f l i m e s t o n e h a d n o t g i v e n g o o d r e s u l t s in t h e b l a s t f u r n a c e .

202

High-temperature tests in simulated blast-furnace conditions Linder test This m e t h o d was originally devised b y L i n d e r ( 1 9 5 8 ) in S w e d e n . Its o b j e c t is t o d e t e r m i n e t h e s u s c e p t i b i l i t y o f i r o n - o r e a g g l o m e r a t e s t o d i s i n t e g r a t i o n d u r i n g r e d u c t i o n . T h e t e s t m e a s u r e s t h e a b r a s i o n resistance o f the m a t e r i a l during r e d u c t i o n . T h e s a m p l e m a t e r i a l is r e d u c e d a c c o r d i n g t o a gas a n d t e m p e r a t u r e p r o g r a m m e w h i c h in general s i m u l a t e s c o n d i t i o n s in a blast f u r n a c e . T h e m a x i m u m t e m p e r a t u r e is 1,000°C. T h e r e d u c t i o n t a k e s place in a r o t a t i n g h o r i z o n t a l t u b e p l a c e d in an electric f u r n a c e (Fig.6). N o w a d a y s a s o m e w h a t m o d i f i e d t e s t p r o g r a m m e is used c o m p a r e d t o t h e o n e originally p r o p o s e d b y Linder; T e s t p a r a m e t e r s are listed in t h e c a p t i o n t o Fig.6. T h e p e r c e n t a g e s b y w e i g h t o f m a t e r i a l finer t h a n 6, 3 and 1 m m in t h e s a m p l e a f t e r screening are t a k e n as a m e a s u r e o f r e d u c t i o n s t r e n g t h . TEMPERATORE CONTROL

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Fig.6. Schematic layout of Linder test apparatus. Conditions: Reduction tube: 130 mm ~ x 200 ram. Test sample: 500 g in the fraction of 10--12.5 mm pellets, 15--20 mm or 20--40 mm sinters and 10--15 mm lump materials. Tumbling speed: 30 rpm. Temperature programme: 0--700°C for 2 h and 700--1,000°C for 3 h. Gas composition programmes: 2% H2 + 58% N~ during the whole test period. 0--2 h 30/10 CO/CO2, 2--4 h 35/5 CO/COs, 4--5 h 38/2 CO/COs. Gas flow: 18--20 Nl/min. The result are given as wt% -- 6 mm, -- 3 mm and -- 0.5 mm. T h e L i n d e r t e s t is used o n pellets, sinters a n d l u m p ore. In t h e case o f pellets, at least, t h e view is held t h a t t h e m e t h o d does n o t p r o v i d e s u f f i c i e n t i n f o r m a t i o n a b o u t t h e p r o p e r t i e s o f t h e m a t e r i a l . T h e grading o f r e d u c t i o n s t r e n g t h in t h e t e s t i n g o f d i f f e r e n t qualities is u n s a t i s f a c t o r y . F o r sinters a n d l u m p ore, o n the o t h e r h a n d , t h e m e t h o d has b e e n c o n s i d e r e d v e r y valuable. A d i s a d v a n t a g e o f t h e t e s t is t h a t s o m e o f t h e fine m a t e r i a l t e n d s t o stick t o t h e walls o f t h e t u b e , while s o m e o f t h e d u s t a g g l o m e r a t e s during r o t a t i o n . T h e m e a s u r e d frac-

203

tions thus do not reflect the true composition of the material. The test takes a long time to perform and is fairly expensive. Reduction strength test according to Henderson and Seaton This m e t h o d , p r e s e n t e d b y H e n d e r s o n a n d S e a t o n ( 1 9 7 0 ) a n d o t h e r s in Britain, is i n t e n d e d t o m e a s u r e the resistance o f an i r o n - o r e a g g l o m e r a t e to d e f o r m a t i o n d u r i n g a r e d u c t i o n p r o g r a m m e . T h e t e s t m a t e r i a l is r e d u c e d in a f u r n a c e a c c o r d i n g t o a gas a n d t e m p e r a t u r e p r o g r a m m e w h i c h generally simulates t h e c o n d i t i o n s in a blast f u r n a c e (Fig.7). T w o s o m e w h a t d i f f e r e n t prog r a m m e s are a c t u a l l y used f o r r e d u c t i o n : p r o g r a m m e I simulates a blast furn a c e w i t h a l o w e r r a t e o f charging t h a n p r o g r a m m e 2. T h e H e n d e r s o n a n d S e a t o n t e s t is a p p l i e d to pellets, sinters and l u m p ore, see T a b l e I. A s q u a r e s p e c i m e n is c a r e f u l l y c u t o u t o f t h e s a m p l e pellet or l u m p o f ore (three s p e c i m e n s in t h e case o f sinters) and r e d u c e d a c c o r d i n g t o

GAS OUTLET ]

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EXTENSION BAR FROM LOADCEL L

Fig.7. Reduction strength test. (From Henderson and Seaton, 1970.) Test sample: 7 x 7 x 10 mm. Number: i cut from pellets and lump materials; 3 cut from sinters Temperature programmes: (1) 900°C for 3 h, 900°C for 3 h, 900°C--1,200°C for 2 h. (2) 900°C for 2 h, 900°C for 2 h. Gas composition programmes: 15% H during the whole test, (I) 13--11% CO2 26--28%CO for 6 h 6% CO2 34% CO for 1 h 0% CO2 40% CO for ] h rest N2 (2) 18--11% CO2 22--28%CO for 4 h 6% CO2 34% CO for 1 h 0% CO2 40% CO for 1 h Gas flow: 3 l/rain. Straining rate: 0.5 mm/min. The load is stated in kg/cm2 for 5 and 10% deformation or earlier fracture.

204 TABLE I Some examples of minimum strength (From Henderson and Seaton, 1970.) Sample

Programme No.

Minimum strength (kg/cm 2) at reduction rate (%)

Carol Lake pellets Malmberget pellets Grangcold D6 Kiruna B, lump ore Dannemora B, lump ore Basic sinter, self-fluxing

1 1 2 1 1 2

approx. approx. below approx.

7 3 2 8 26 1

48 88 45 74 69 66 and 82

one o f the programmes. The programme is interrupted and the specimen is compressed between flat tools during reduction, the load and d e f o r m a t i o n being recorded. The process is then repeated with a fresh specimen, this time with the compression test at a different poi nt in the reduction programme. The load is stated in kg/cm 2 at 5% and 10% deformation. On fracture, the fracture load is stated in kg/cm 2. The degrees of reduction of the samples are measured in a thermal balance. An obvious drawback to this m e t h o d is that only one specimen can be tested at a time. The specimens also require extensive preparation.

Isothermic high-temperature tests Reduction-under-load test according to Burghardt The Burghardt test is a r e duc t i on strength test that makes allowance for dust f o r m a t i o n during r e duc t i on and its charge-clogging effect as well as for softening o f the material. The m e t h o d has been devised by Burghardt and Grebe (1969) in Germany. The Burghardt test is isothermic and is generally p e r f o r m e d at 1,000 or 1,050°C. The reduction gas is a 40:60 m i xt ur e of CO and N2. The m e t h o d is characterised by reduction under a constant static load of 0.5 kg/cm 2 on the sample. The stated quality criteria are the pressure drop Ap across t he sample bed and the settling ABH of the sample at 80% reduction. As the divergence of Ap is large, the pressure drop is given in levels: 0 , 1 5 m m is classed as a good result and 15--20 m m as acceptable, whereas more than 50 m m is reckoned as p o o r for the material. The reducibility of the sample is also obtained, as the r ed u ctio n takes place in a tube suspended from a balance in an electric furnace. The apparatus is illustrated in Fig.8 and the test parameters are given in the caption to that figure. The m e t h o d is primarily intended for testing sintered pellets, but is also applied to lump ore.

Compressive strength testing during reduction A m e t h o d has been devised (Thaning, 1971), and improved at the Mineral Processing Division of the Royal Institute of T e c h n o l o g y in S t o c k h o l m for

205

EJ

o g)

/_/

Lk

Fig.8. Reduction-under-load test. (After Burghardt, 1970.) 1, balance; 2, compressed air to pressure cylinder; 3, temperature recorder; 4, gas outlet; 5, gas inlet; 6, A1203-pellets; 7, sample; 8, recorder for pressure drop and weight loss. Conditions: Reduction tube: 125 mm diameter. Test sample: 1,200 g in the fraction of 10--12.5 ram. pellets or lump materials. Load: 0.5 kg/cm 2. Test temperature: 1,000°C or 1,050°C. Gas composition: 40% CO, 60% N~. Gas flow: 5 N mS/h. The results are recorded as the pressure drop over the test sample bed expressed in mm water gauge at 80% degree of reduction and the shrinkage of the test sample bed. Good blast furnace pellets should show a pressure drop of not more than about 15 mm w.g. testing the compressive s t r e n g t h o f iron-ore agglomerates during i s o t h e r m i c reduction. The principle o f this r e d u c t i o n strength test is t h a t the compressive strengths o f individual agglomerates are m e a s u r e d w h e n t h e y are at their w e a k e s t in the r e d u c t i o n process. This stage is reached, as previously m e n t i o n e d , w h e n the i r o n - o x i d e material in the agglomerates has progressed t o the wiistite stage, as r e d u c t i o n s t r e n g t h is crucially i n f l u e n c e d by the plastic n a t u r e o f wi]stite. The agglomerates are r e d u c e d i s o t h e r m i c a l l y and c o m p r e s s e d at r e d u c t i o n

206

temperature between flat tools, the load and compression being recorded. The agglomerates are usually in pellet form, though briquettes can also be Used. The procedure of measuring individual agglomerates means that the agglomerates in the selected sample should be as uniform as possible in order to obtain a small divergence in the readings. Eight pellets or briquettes are usually tested at a time.

CONNECTION WITH WRITER

Fig.9. Apparatus for determination of reduction strength. 1, reduction furnace; 2, thermo couple; 3, connection with regulator; 4, gas inlet; 5, gas preheating; 6, power transmission; 7, movable pressure rod; 8, test samples; 9, compressed air for ejector; 10, ejector for gas outlet; 11, carousel feeder; 12, counter pressure rod with loading cell; 13, device for prevention of sticking; 14, furnace stand; 15, position transmitter.

207 Since several agglomerates can be c o m p r e s s e d in t h e same r e d u c t i o n test, the a p p a r a t u s is also suitable f o r progressive m e a s u r e m e n t s o f s t r e n g t h in conditions o f rising t e m p e r a t u r e . If desired, this can be d o n e in c o m b i n a t i o n with p r o g r a m m e - c o n t r o l l e d r e d u c t i o n gas c o m p o s i t i o n . T h e r e d u c t i o n strength test apparatus (Fig.9) consists o f a vertical t u b u l a r f u r n a c e with i n t e r i o r e q u i p m e n t comprising a carousel feeder, a vertical plunger, a pressure plate, an anti-adhesion guard and a gas p r e h e a t e r c h a m b e r . This last is filled with ceramic e l e m e n t s t o h e a t the gas. A m e c h a n i c a l loading unit is used t o generate the test pressure; it is c o n t i n u o u s l y adjustable. The n o r m a l rate o f plunger m o v e m e n t is 5 m m / m i n . A load cell is used t o measure the load and a p o s i t i o n i n d i c a t o r to measure the compression. T h e load and def o r m a t i o n curve is p l o t t e d on an X--Y c o o r d i n a t e r e c o r d e r t o which the load cell and p o s i t i o n i n d i c a t o r are c o n n e c t e d . T h e iron-ore agglomerates are n o r m a l l y t e s t e d in a r e d u c t i o n gas consisting o f 35% CO, 5% CO2 and 60% N2 b y v o l u m e at 1,000°C. Heating is d o n e in a n i t r o g e n a t m o s p h e r e , a f t e r which the agglomerates are r e d u c e d isothermically for a certain length o f time. A c o u p l e o f the agglomerates are t h e n c o m p r e s s e d one at a t i m e -- still at r e d u c t i o n t e m p e r a t u r e -- while the loading and compression s e q u e n c e is r e c o r d e d . A f t e r f u r t h e r r e d u c t i o n the n e x t pair o f agglomerates is t e s t e d one at a time. T h e r e d u c t i o n and c o m p r e s s i o n p r o c e d u r e is r e p e a t e d until all the 8 agglomerates have b e e n tested. All t h e agglomerates can o f course be t e s t e d one at a t i m e a f t e r t h e same p e r i o d of r e d u c t i o n . T e s t pressure is applied b y t h e vertical p l u n g e r and c o n t i n u e s until f r a c t u r e or -- if o n l y d e f o r m a t i o n o c c u r s -- until the limit o f d e f o r m a t i o n has b e e n passed and the compression a m o u n t s to at least 25% o f the original height o f the agglomerate. The loading and c o m p r e s s i o n curve for each test is r e c o r d e d o n millimetre graph paper. T h e f r a c t u r e load P--B in kg and the c o m p r e s s i o n A l in m m at TABLE II Compression strength during reduction -- test conditions Reduction furnace: 105 mm diam x 320 mm Pellet size and number: 10--12 mm diam; 8 samples 12--25 mm diam; 5 samples Test temperature: 1,000°C Gas composition: 35% CO, 5% CO2, 60% N: Gas flow: 800 1/h Straining rate: 5 ram/rain The stress is noted in kg for collapse and for 10 and 25% deformation or earlier fracture

208 fracture can be read off direct from the curves. In the case of plastic deformation, the plastic deformation limit load P--S and the loadings at 10% and 25% compression are read off in kg for an assessment of the plasticity of the agglomerate. The test parameters of the method are given in Table II. The temperature and gas composition are Chosen to simulate conditions at the end of the blastfurnace reserve zone as closely as possible. SOME R E S U L T S O F C O M P R E S S I V E S T R E N G T H T E S T S

The following figures illustrate some reduction-strength graphs obtained from tests of compressive reduction strength as just described. The reduction temperature in all cases is 1,000°C. Some of the graphs show average values from compressing a batch of eight samples, with the limits of variation indicated, and some show curves from test compression after different periods of reduction. Fig.10 shows reduction strength curves for some Swedish commercial acid sintered pellets made from low-gangue magnetite concentrates at various LKAB plants during 1972. All the different types of pellets have diameters of 10--12.6 mm and cold compressive strengths in excess of 200 kg per pellet. The reduction strength of these pellets, measured as compressive strength according to the Division's method, is, as the graphs show, very low. The KPB pellets, t o d a y regarded as good blast furnace pellets, can take a load of about 6 kg before they begin to flatten, while the others have even lower reduction strengths. Load kg CO

/J

3O 2O ~o

/

1/11

/

~

fl

Ooi2~ MPB1 ~

/

0

1 2 3 MPB3 ~

0

1 2 3 SPB

0

1 2 KPB

3 Cornpression, rnm

Fig.10. R e d u c t i o n s t r e n g t h curves o f s o m e c o m m e r c i a l s i n t e r e d pellets f r o m L K A B , S w e d e n , 1972. Average curves w i t h limits of variation. 11.5 m m diam. pellets; r e d u c t i o n t e m p e r a t u r e 1,000°C; r e d u c t i o n t i m e 60 min. Pellets t y p e MPB1 a n d MPB3 are n o longer in p r o d u c t i o n .

It may be mentioned that sintered pellets of quality MPB are an excellent material for direct reduction in Midrex plants. For purposes of direct reduction, therefore, the reduction strength should be tested in the conditions prevailing in direct-reduction furnaces. In an a t t e m p t to improve the reduction strength of sintered pellets, LKAB has made a test pellet with an overbasic composition, with addition of dolomite and

209

which stands the LTB test. F i g . l l shows the reduction-strength curves for this quality of pellet. The strength has been raised compared to the pellets previously mentioned, but the curves show above all that the softening properties have been substantially improved. The pellets do not flatten as soon as they are subjected to increased loading. Load kg

30min

60

Z,5min

,0rain

90rain

120min

5O 4O 302O 10

0 0

0

0

0 '

,mm

Fig.11. Reduction strength curves of sintered experimental pellets from SKAB, Sweden, 1972. Average curves from 3 determinations on each type. Dolomite pellets, 12--13 mm

diam. anomal pellets; reduction temperature 1,000°C.

Fig.12 shows reduction strength curves for overbasic, cement:bonded Grangcold pellets. The strength during reduction is better than 10 kg, but the softening properties are not of the best -- the pellets flatten quickly under increased loading. The cold compressive strength of these 14.5-mm pellets has been measured at about 130 kg. LOAD kg 'Reduction lime 30rain / / 50l

////

4oF / / 3o t . / . . , / y / / /

Reduction time 45rain

20 ~ j / / 100[/f 0 1 2 3 4

0 1. 2 3 4

Compression,ram

Fig.12. Reduction strength curves of grangcold pellets from Gr/inges, Sweden, 1972. Average curves with variation limits. GPI. 11.5 mm diam. pellets; reduction temperature 1,000°C.

210 Lime-bonded autoclaved COBO pellets generally show much higher reduction strengths and better softening properties than the commercial types of pellets. Thus in Fig.13 we see the reduction strength curves for magnetite and hematite pellets produced in the laboratory. The magnetite pellets contain high-grade magnetite concentrate from Malmberget with some quartz in the gangue. The hematite pellets contain quartz-bearing hematite concentrate from Risberget. The cold compressive strength of an l l . 5 - m m pellet of this type is about 80 kg. LOQ( kg 70 6O 5O 40 3O

• /

/i

////////

///

i/~///

/

/

2O 10 0

r

i

i

1234 01234 Compression, mm Magnetite pellets Hematite pellets

Fig.13. Reduction strength curves of laboratory produced lime-bounded COBO-pellets. Average curves with variation limits. 11.5 mm diam. pellets; reduction temperature 1,000°C; reduction time 30 min.

Fig.14 shows reduction-strength curves for laboratory-made COBO pellets bonded with lime and silica dust. These autoclaved pellets are designed primarily for good transport characteristics. Both the magnetite and hematite concentrates are low-gangue grades from Malmberget. The cold compressive strength of both types is more than 200 kg per l l . 5 - m m pellet. The curves show that the hematite pellets are much stronger than the magnetite pellets. The sharply marked maximum strength on the curves indicates that the pellets undergo extensive fissuring at a given loading. Similar pellets bonded with lime only, without silica dust, do not display this heavy fissuring. Fig.15 shows reduction-strength curves for laboratory-made autoclaved COBO pellets bonded with kaldo furnace slag and lime. The coarse concentrate consists of Gr~ingesberg export dust ore with a high phosphorus grade. The fine concentrate consists of magnetite with hematite from B15tberget. This is a high-phosphorus type of pellet intended to be produced immediately prior to blast furnace charging, and resistance to abrasion in transport is thus of secondary importance. The cold compressive strength, for example, is only a little over 40 kg per 11.5-mm pellet, whereas the reduction strength at its lowest is about 20 kg per pellet. Fig.16 shows reduction-strength curves for laboratory-made autoclaved COBO pellets bonded with lime, magnesia and reactive alumina. The ore material consists of high-grade Malmberget concentrates of magnetitic and hematitic type. The composition of the binder has been chosen with a view to direct

211

Lo,2d REDUCTIONTIME30ram kg

45ram

90mm

6Omen

HEMAI~GENEPlEETSpE LELL~E/

l /

50 40 30 20~01 2 3 4 5

1 2 3 L 5 0

2 3 4 5

!

~

~

'

!

~Comp .........

Fig.14. Reduction strength curves of laboratory produced COBO pellets bonded with lime and silicon fume. Upper curves representing hematite pellets and lower curves magnetite pellets. 11.5 mm diam. pellets; reduction temperature 1,000°C.

Load kg 6350

Reduction

time 30rain

y

45rain

60mm

90mrn

40 30 20 10 o

I

2

~

~

I

~

~

~ Compression, ' "-

mm

Fig.15. Reduction strength curves of laboratory produced magnetite pellets of COBO type bonded with Kaldo slag and lime. 11.5 mm diam. pellets; reduction temperature 1,000°C.

212

Load

42°/° degree of reduction 30rain reduction time

"

// /

60-

56°/° 45min

,ll

42°/° degree of reduc 30rain reduction t ' ~ e "

/

65°/° 50min

//

54°/° 45rain

II

75 °& 75min

//.,

64°/~ 60rain

U /

90m,n

MAGNETIT

50-

40-

30-

20-

10-

0 2

3

z~

0

1

2

3

4

0

1

2

3

a

0

1

2

3

4 Compression, mm

Fig.16. Reduction strength of laboratory produced COBO pellets bonded with lime, magnesia and alumina. Upper curves represent hamatite pellets and lower curves magnetite pellets. 11.5 mm diam. pellets; reduction temperature 1,000°C. reduction followed by smelting in an electric steel furnace. The direct-reduction pellets have been tested here at a reduction temperature and in a gas mixture approximating to blast-furnace conditions. These pellets are likewise intended to be produced immediately prior to the reduction process, with transport strength as a minor consideration. They have cold compressive strengths of 55 to 60 kg per l l . 5 - m m pellet, while the minimum reduction strengths are more than 40 kg for hematite pellets and about 20 kg for magnetite pellets. The various graphs show t h a t autoclaved hematite pellets stand up better to reduction conditions despite the fact that they necessarily swell rather more than the corresponding magnetite pellets. Cold compressive strength, as previously mentioned, has no correlation whatsoever with reduction strength. Autoclaved pellets generally have better reduction strength and softening properties than sintered pellets. The composition of the binder influences the tendency of autoclaved COBO pellets to fissure and soften -- the steeper the curve, the better the softening properties. ACKNOWLEDGEMENT The results in this paper are mainly based on investigations within the project "Agglomeration of low low-gangue iron ore concentrates" carried out with economic support from the Swedish Board for Technical Development and the Hesselman Foundation.

213

REFERENCES Burghardt, O. and Grebe, K., 1969. Untersuchung des mechanischen Verhaltens von Eisenerz und Pellets unter isothermen Reduktionsbedingungen. Stahl und Eisen, 89: 561--573. Henderson, T.A. and Seaton, P.T., 1970. The resistance to compression at elevated temperatures of iron blast furnace charge constituents when exposed to programmed reducing conditions. Proc. o f Int. Conf. on the Science and Technology o f Iron and Steel, Tokyo,

Sept. 7--11. Linder, R., 1958. Programme-controlled reduction test for blast furnace burdens. J. Iron and Steel Inst., 189: 233--243. Polthier, K., 1967. Der Druck der Beschiekung in Hochofen. Stahl und Eisen, 87(16): 960--962. Thaning, G., 1971. Mineralogical studies of binding phases and reduction properties of cold-bound iron ore pellets. Jernkontorets Ann., 155: 4 7 - - 9 2 .