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CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 1
Mysteries of Reduction Swelling
of Iron OresThis presentation will show the logic and experimental techniques which revealed the mechanism of Reduction Swelling of Iron Ores in a ground breaking research work done in Germany and created a sensation some years ago . Many of its results have been published, and it has been quoted extensively. But some important observations and new questions which came up with it remain a mystery and continue to challenge our imagination even today. They are topics for advanced research for developing new technologies of huge economic importance.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 2
What is Reduction Swelling ?
Some iron ores and pellets under certain reduction conditions swell abnormally 150% to 400% of original oxide volume .
This is known as Abnormal Swelling or Catastrophic Swelling .
The accepted limit in DR and Bf grade pellet is 20% ( ISO 4698 free swelling test method ). So , we can imagine what will happen with 150 %
or 400% swelling in DR and Blast Furnace Process !
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 3
Effect of Swelling on Permeability of the Charge
Before reduction-permeable Charge, easy gas flow
Reducing Gases
After reduction, swollen DRI blocks gas flow
ΔΡ
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 4
What happens in DR and Blast Furnace
chocking , clusters , sticking , hanging
BlockageAbove
Planum In
Midrex & HYLReactor
Ore/Pellet
Hanging above Cohesive Zone in Blast Furnace
Reducing Gas Inlet
850 / 950 C
Ore/Pellet/Sinter/Lime Stone/Coke
Hot Blast
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 5
Why Abnormal Swelling Happens ?
To answer this question, we must study the Phenomenonat it’s highest manifestation.
Accordingly 4 test series were designed.
Test Series 1: will identify ore that swells highest and conditions which favour it . We can then make the phenomenon manifest itself in its highest form.
Test Series 2: will find out during which step of reduction of Hematite to Iron , swelling happens?
Test Series 3: will find out inside structure of swollen DRI Test Series 4: will study ore surface during reduction
process : in-situ surface observations.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 6
Test Series 1 :Experimental SetupElectrical furnace heats the alumina tube which holds the pellet sample. One end of tube has quartz glass window and inlet for gases. Other end has thermocouple and exit for gases.
CO, CO2, H2, N2 from gas cylinders via Rotameter , set of valves and safety devices supply gases to alumina tube. Hartmann Brown URAS continuous CO2 gas analyzer plotted CO2 in exit gases indicating progress and end of reduction reaction.
Thermocouple touching the pellet measured isothermal reduction temperature. A camera was used to take images of pellet through the quartz glass window at close time intervals. After each test , diameter was read from images and volumes calculated . % Swelling = [(V2-V1)/V1]x 100.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 7
Experimental Procedure of Test Series 1
Pellets made from commercial iron ores and pure Bayer iron oxide were tested with CO and H2 reduction between 750 C to 1100 C isothermal reduction. CO2 plots of URAS continuous gas analyzer showed progress and end of reduction.At the end of each test, the sample is cooled In Nitrogen and % reduction determinedby wet chemical analysis.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 8
Results of Test Series 1
• Ore X showed Highest % Swelling
• Maximum Swelling was with CO reduction at 850 C
• Higher Temperature reduction gave lower % swelling
• At 1100 C, instead of swelling , DRI shrank due to sintering of iron
• Ore Y showed least swelling tendency
• Hydrogen reduction - no swelling
% Swelling plotted against Reduction Time for each pellet
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 9
Test Series 2
Now that we know how to make this phenomenon manifest itself in highest form , i.e. the highest swelling conditions, we can investigate the reason for swelling . This makes observations easier compared with a low swelling sample where everything is in smaller dimensions and so is difficult to observe accurately what’s going on inside .
The same logic is applied for studying other phenomenon- example : sun-spots are studied when solar activity peaks in the 11 year cycle!
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 10
Aim of Test Series 2: find out during which step of Hematite reduction to Iron , swelling happens.
We know that in practice, reduction proceeds Topochemically. That means , when CO molecules hit the pellet , reduction happens from surface to center of pellet . There are 3 transformations . 1. Hematite to Magnetite . 2.Magnetite to Wustite 3. Wustite to Iron .
All 3 stages happen simultaneously.
So the immediate question is, during which stage does swelling happen ? Is it in all 3 stages or in 2 or just in one stage of reduction?
This is explained in the next slide.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 11
Topochemical ReductionIf we stop reduction mid-way and look at the cross section- we see 4 layers: outer layer of Iron, next is Wustite, then Magnetite and a core of un-reduced Hematite. All 4 layers happen same time till whole pellet is completely reduced to Iron.
1 2 3
Fe
FeO
Fe2O3
Fe3O4
CO 850 C
So the Question is: Which reduction step is causing Swelling ? 1, 2 , 3 or all 3 steps.
Fe2O3 Fe3O4 FeO Fe
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Apply same Logic i.e. test at Maximum manifestation of the Phenomenon i.e. Maximum Swelling Condition.
Accordingly Sample X was reduced with 100% CO at 850 C by Step Reduction and % Swelling noted for each Step.
What is Step Reduction ? Reduce Hematite pellet fully to Magnetite right through, then Magnetite Pellet fully to Wustite and then Wustite Pellet to Iron in 3 distinct Steps.
Go from one step to the other only after the previous one is completed. Prevent overlapping of Steps
Test Series 2 is designed to answer this question: Experimental set-up is same as in Test Series 1.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 13
How to do Step-reduction ?
Step 1. Fe2O3 to Fe3O4
Step 2. Fe3O4 to FeO
Step 3. FeO to Fe
Bauer Glaessner Diagram
Fe2O3 Fe3O4 FeFeO
Bauer Glaessner Diagram shows equilibrium between Iron, Carbon and Oxygen . Read CO/CO2 ratios from it and make each step of reduction happen on command .
Pellet Pellet Pellet DRI
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 14
Procedure & Results of Step Reduction
Set up is same as Test Series1.Volume change of sample X was determined with Camera during each step of reduction. CO2 plot showed end of each reduction step.
Result: No swelling in first 2 steps.Abnormal Swelling happened when Wustite was reduced to Iron in the 3rd Step.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 15
Step-Reduction to see when does the bend in CO2 curve happen?
CO 100%CO 63 % / CO2 37 %
850 C
With Step-reduction, X was reduced straight to Wustit at 850 C with 63% CO/37%CO2 .
After flushing system including URAS with N2, 100% CO sent in to reduce Wustite to Iron.
Camera took images in both Steps.
CO2 plot showed End of each step .
Result: Bend happens in Step3 of Wustite reduction to Iron along with the Swelling phenomenon. No swelling and no bend till Wustite.
New Observation & more Questions:
Swelling happens from point 1 to 2 of the bend. Why ? What is happening there?
Hematite to Wustite Wustite to Iron
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 16
A New Observation :Bend in the CO2 Curve
It was mentioned earlier that gas analyzer plotted CO2 in exit gas showing progress and end of reduction.
CO2 Curve is in fact indicator of reduction rate . In swelling samples , the rate curve had a minima- maxima bend. It was highest when swelling was maximum at 850C, decreased with less swelling and at 1100 C, when there was no swelling, the bend was also absent. This is shown in next slide.
Sample X , 100% CO/700 to 1000 C
Multiple bends at 700 C are due to
2CO = CO2+C Boudouard reaction
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 17
Reduction Rate & Swelling v.s.Temperature
At 850 C when Swelling was highest ,the minima-maxima bend in CO2 curve is maximum .
It was lower with less swelling conditions.
At 1100 C when there was no swelling, the bend in CO2 curve was absent.
Question 1: Why does it happen?Question 2: When does it happen?Question 2 was answered with Step Reduction.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 18
Recap of Findings
•Ore X has Maximum Swelling tendency
•Maximum Swelling is with 100% CO at 850 C reduction.
•Ore Y has least swelling tendency
•H2 reduction does not cause abnormal swelling
•A minima-maxima bend happens in CO2 curve in swelling samples
• The min-max bend in CO2 curve is highest with maximum Swelling .
• Swelling happens from point 1 to 2 of CO2 plot (rate curve)
After gaining these new knowledge's, Test Series 3 began , where a Scanning Electron Microscope (SEM) was used to study the inner structure of swollen DRI.
Procedure: Produce highly swollen DRI in Test Series 1, cool in N2, take it out, split it carefully and study inner surface with Scanning Electron Microscope (SEM)
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 19
Results of SEM Surface Studies on DRI with % Swelling over 150%
We see Iron whiskers inside swollen DRI in many forms.
•Like Solar prominence
•Like Sea anemones
•Like Stalagmite
• Like Worms
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 20
Explanation for How whiskers cause Swelling
Consider 3 oxide particles inside a pellet.
After reduction , iron separates as whiskers and pushes particles from each other increasing diameter d1 of pellet to d2 of DRI.
DRI Swells abnormally(100 to 400%) , is highly porous, spongy, soft .
A Pellet is an agglomerate of thousands of individual oxide particles (-40μ) joined by diffusion and slag bonds. It has open and closed pores.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 21
SEM Images of DRI with No Abnormal Swelling
Higher temperature reduction show sintering of iron.
Hydrogen reduction show Iron separates parallel to geometric surface of original oxide pellet grains.
Both cases No Abnormal Swelling.
X /CO/1000 C X /H2/ 850 C
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 22
Test Series 4:Live Observation of surface during reduction of X and Y with CO and H2 at 750 to 1000 C with Hot Stage and Stereo Microscope
A Leitz Hot-Stage Type 850 was modified to pass CO/H2 through it.
A 5mm by 3 mm thick piece of ore was chipped from X and Y as samples. Surface of sample is unpolished , rough. Monocular-microscope has low depth of focus, will give blurred image.
So, a Stereo microscope was used to , observe surface changes , the maximum magnification possible was only 300 .
Normally a Hot-stage is used under vacuum to observe grain growth and micro-structure changes at high temperature using monocular-microscope . Sample must be polished . To do reduction inside Hot-stage , it was modified .
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 23
Set-up for Test Series 4 -live observation of Reduction
Facilities - CO and N2 bottles, rotameter, U-tube, valves, modified Hot stage and temperature recorder.
The piece of broken ore X,Y was placed at center of heating coil , heated in N2 stream to reduction temperature. With Step-reduction with CO , sample brought to Wustite. After a while 100% CO sent in to reduce Wustite to Fe . All the time surface was studied through the stereo-microscope.
Reduction was done 750 C to 1100 C
Hydrogen reduction was also studied.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 24
Observations during reduction X/ CO and H2/ 850 and 1000 CFollowing is a record of what was seen through the stereo microscope at magnification of 300. High speed filming was not available. Hence visual observation is recorded as described below.
Till the sample was brought to Wustite state at 850 C by step-reduction , there was no sign of iron formation . Then 100% CO was introduced., within 1 minute of CO hitting the surface , cracks appeared and soon bright metallic spots appeared on Wustite surface , which are the first nuclei of iron appearing on wustite surface. It was followed by fountain of whiskers springing from some nuclei in many shapes and sizes – eg. needles with side growths, beads, bands . All had bright reflecting metallic surface.
From some iron nuclei on Wustite surface , many whiskers sprouted vigorously.
Some growing whiskers were swaying, anchored on its base, and when one such swaying whisker touched another one growing from a different spot, one of it just disappeared from its base with no trace of where it went! It was possible to observe a sudden bend happening on growing whisker and rapid growth of fine thread like whiskers from the tip of a thicker whisker.
After about 10 minutes , the whole surface was covered with bright iron layer .
At 1000C reduction , a uniform layer of metallic iron covered the surface, no whiskers.
Hydrogen reduction : As soon as hydrogen hit sample, surface darkened suggesting temperature dropped (600 C), then regained colour means temperature came back to 850 C, then there was a pulsation and cracking of sample , spluttering and rapidly surface became highly reflective and bright . There were No whiskers . To see the surface in detail , the samples were frozen with Nitrogen and studied with SEM.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 25
SEM Images of samples from Hot Stage :Test Series 4
1. Wustite surface before 100% CO was sent in .
2.First iron nuclei and first whiskers appearing on wustite after 100% CO sent in.
3. Detail of one whisker
4. Cluster of whiskers with the tips like thin wire form
1
4 3
2
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 26
SEM surface images of samples from Hot Stage
Base of whiskers
Beads and dendrites on one whisker with a kink
Foot and tapering to needle with kinks at end
Thick whisker ending up in fine needle and sudden kink and continuation of needle
This is the first time in Ferrous Metallurgy a live observation of iron ore reduction was done . The record of events and images of iron formation on Wustite surface made a sensation in the research circle and triggered a number of research in this direction.
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 27
So ! What are the Mysteries ?
1. Why some ores swell others do not swell ?
2. Why iron separates as whisker ? What is the mechanism of whisker formation ?
3. What are the beads on the surface of the whisker ?
4. How do we explain the various shapes and sizes of whiskers ?
5. Why a whisker disappears when it came in contact with another whisker ?
7. Why there is minima / maxima in CO2 curve with abnormal swelling?
8. Why hydrogen reduction gives sudden pulsation ?
9. Why hydrogen reduction does not show abnormal swelling ?
10. Why whisker forms an arch as in solar prominence
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 28
Solar Prominence and Whisker Loops
Solar prominences are loops of plasma anchored to the solar surface . Magnetic fields are thought to be causing solar prominences. Exact reason is not known.
Why iron whiskers form loops anchored to Wustite surface? We don’t know. Can a strong magnetic field affect morphology of iron formation ? These are unsolved Mysteries !
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 29
1. Is abnormal swelling really bad? When outer iron layer is full of whiskers , it opens up, helps rapid gas-solid contact to unreduced layers - rate of reduction is faster.If iron layer is sintered and continuous , gas-solid contact is via cracks, open pores or by slow diffusion process through the compact iron layer –reduction rate is slow-more unreduced core.
2 . In DR , can we use 850 C instead of 950 C and save energy / t DRI ?3. Can we invent a process to take advantage of Abnormal Swelling phenomenon ?
QuestionsIn Science , when one question is solved , many New Questions come up with it.
Questions which surfaced in this research work are :
CO
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 30
4 . Highly swollen DRI is very reactive , it can be good catalyst for self-reforming inside DR reactor, in Haber-Bosch process for nitrogen extraction from air for artificial fertilizer . Can we develop better catalyst promoting iron formation in the form of whiskers?
5. Highly swollen DRI is soft and spongy and so easy to make good HBI . Can we commercially produce highly swollen DRI to produce highly compacted HBI briquetts with less energy.
6 .Whisker is believed to be single crystal free of dislocations, possess high strength. Can we grow long iron whiskers to make high strength composite materials ?
More Questions !
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 31
Answers to the Mysteries & Questions lie in Surface Chemistry
CO + FeO = Disordered F++ ions + electrons -------- Disordered Fe atoms ------- Super Saturation ----------
--------------Nucleation----- Ordered Fe atoms -------- grow to Iron whiskers
Gas-solid reaction is a process on solid surface at atomic/molecular level. The Nobel Prize in Chemistry for 2007 went to Professor Gerhard Ertl of Germany "for his studies of chemical processes on solid surfaces“ – one of his questions is why iron rusts. Our question is what happens during iron oxide reduction? His methods could answer the Mysteries of Reduction Swelling . Research in this field requires intelligent experimental techniques and advanced equipment to observe how atoms and molecules behave on ore surface. The results could have great economic importance –e.g. a better iron catalysts for Haber-Bosch process , better self-reforming DR process, high strength composite materials and may be more!
0.2mm
CVRD 6th DR Seminar 26-30/11/2007 Belo Horizonte 32
Ferrous Metallurgy Institute of RWTH-Aachen, Intzestr1, 51-Aachen, Germany
Author :Mukundan Ponthenkandath from Kerala State, India, based on his PhD Thesis at RWTH-Aachen, Germany, July 1971