Characterization of DR Pellets for DRI Applications

Full text

Characterization of DR Pellets for DRI Applications
Bodil E. Monsen1, Edith S. Thomassen1, Irene Bragstad1, Eli Ringdalen1, Per H. Hoegaas1
1SINTEF Materials and Chemistry
Alfred Getz v 2b, Box 4760 Sluppen, N-7465 Trondheim, Norway
Phone: +47 98283919
Email: bodil.monsen@sintef.no
Keywords: DR Pellets, Strength, Abrasion Index, Tumble Index, Reduction, Reducibility, Metallization, Porosity
INTRODUCTION
In connection with a Norwegian research project at SINTEF we are looking into future possible usage of natural gas. When possible,
the natural gas should be used as a substitute for coal and coke in order to reduce fossil CO2 emissions. Using natural gas for iron
production will reduce fossil CO2 emissions considerably. It is well known that hematite iron ores are easily reduced by natural gas.
However, the requirements for DRI (Direct Reduced Iron) shaft processes as Midrex or HYL must be met. The required chemical and
physical properties of the raw materials are more or less dictated by the reaction vessel type. The use of highly reducible, strong, well-
sized, non-sticking and non-swelling materials is necessary to attain maximum productivity and performance of shaft processes.
The DRI market has been rapidly growing from a slow start in 1970 to 75 million tons in 20131. It can be classified in 3 market
segments, hot and cold DRI and hot briquetted iron, of which the two first constitutes around 92 %. The far most used technology is
shaft reduction processes as Midrex (around 63 %) and HYL/Energiron (around 15 %), while the remainder is mainly rotary furnace
based. The long-standing use of DRI has been as a charge material for electric arc furnace where DRI partly substitutes scrap.
In the present work DR pellets are characterized with respect to DRI shaft processes. DR pellets have been supplied from 4
commercial pellet producers. These pellets have been characterized with regard to compression strength, abrasion and tumble
resistance, porosity and reduction properties, to a large extent according to acknowledged ISO standards. However, we have
downsized the amounts needed and the equipment size in order to be more practical for research. The results indicate that the
downsized methods can be used for evaluating pellet quality and suitability for DRI shaft processes, or at least be a suitable tool for
research and development of new pellets.
PELLET SAMPLING
We kindly asked 4 different commercial pellet producers to supply us with commercial DR pellets for DRI applications. At the same
time we disclosed that the results would be published, but promised to keep the pellet source anonymous by naming the 4 companies
pellets by A, B, C, and D. In return the company will get the results in the same anonymous way, given the letter the company is
represented by. The response was positive and we received around 3 kg of pellets in the range 10.0 – 16.0 mm from each of the
industrial producers. The pellets were made from either hematite concentrates, magnetite concentrate, or from a mixture of both. The
pellets were dried at 105°C before use.
CHEMISTRY BY XRF AND WET CHEMICAL ANALYSES
The direct reduction shaft processes do not involve a slag phase, unlike the blast furnace. Gangue minerals present in the pellets will
remain in the DRI products, and not be transferred to a slag phase. The DRI will on the contrary be enriched on gangue since most of
the oxygen has been removed in gas-solid reactions. A high total iron content and a low gangue content are therefore of utmost
importance for DR pellets.
The total iron (Fe total) was measured by wet chemical analyses while the gangue oxides were detected by XRF and normalized to
100%, calculating Fe total as hematite. Table I gives a comparison of the chemistry of the 4 commercial DR pellet qualities. The
analyses are also compared to recommendations for good pellet qualities2. These pellets satisfy most of the recommendations. The
total iron content is higher than 67 %, being 67.9 % for B, C and D and 67.6 % for A. Both acid gangue (Al2O3 + SiO2) and basic
oxides (CaO + MgO) are much lower than 3 %, as recommended. A limited amount of basic oxides can be allowed if this can displace
purchased flux in steelmaking. Both C and D are basic pellets while A and B are partial basic.
Alkalis (Na and K) were not detected in A, B, and D and only 0.02 % K2O in C. Alkalis should be low since these promote swelling
and degradation during reduction.

Table I. Comparison of chemistry for 4 commercial pellet qualities.
B (2) = CaO/SiO2 B (4) = (CaO+MgO)/(SiO2+Al2O3)
Phosphorus should be as low as possible, preferably below 0.03 % which is the case for the pellet qualities A, C and D. P must be
removed in steelmaking. However, the precision of these XRF analyses is probably not too good for such small amounts.
TiO2 and MnO should also be low in order to reduce slag formation in the electric arc furnace. Sulfur should be below 0.008 % in DR
pellets to be used in the Midrex process because Sulfur can foul the reformer tubes.
CRUSHING STRENGTH
Mechanical strength is an important property for pellets to be used in direct reduction shaft processes. The size range and strength
determine the permeability, gas distribution, and reduction characteristics in such processes. The cold crushing strength is a measure
of the compressive load attained to cause breakage of a pellet, or the pellets ability to withstand size degradation. The cold crushing
strength was measured according to ISO 4700:20073, using a MTS 810 servo-hydraulic universal testing system with 10 tons capacity.
The pellets are compressed between 2 parallel steel plates at a speed of 10mm/min (ISO recommend the speed to be in the range 10-20
mm/min). The force and displacement are measured, logged and stored in a computer. The test is complete when the load has fallen to
50% or more of the maximum load recorded or the platen gap has been reduced to 50% of the initial test-piece diameter. The crushing
strength is the maximum load applied. Pellet size range was 10.0 - 12.5 mm in accordance with the ISO standard. The standard
recommends testing at least 60 pellets, but we have used 30 pellets in order to reduce the amount of work, which is the only deviation
from this ISO standard. The results are presented in Table II, giving the arithmetic mean and standard deviation of all measurements in
decanewtons per pellet (daN/p), and with one decimal, as recommended by the ISO standard.
Table II. Arithmetic mean and standard deviation of all crushing strength measurements in decanewton per pellet (daN/p)
Recommended
A
B
C
D
Crushing strength (daN/p)
(1 daN = 10 N = 1.02 kp )
>150
Good: 250
282.1 ± 81.4 236.2 ± 73.7 327.7 ± 126.6 309.2 ± 100.6
For the Midrex process the crushing strength as a minimum should be 150 daN, while over 250 daN is preferred. For the HYL process
a minimum of 200 daN is recommended. Hence, these 4 commercial pellet qualities have good strength!
The relative frequency in percentage of pellets within specified strength ranges should also be reported according to ISO 4700:2007,
as shown in Table III, classified at 50 daN intervals. For blast furnace pellets the average should be 250 daN at least, but not more than
10 % of the pellets should have a crushing strength lower than 200 daN, and maximum 5 % should have a crushing strength lower
than 150 daN. In the present work 7 % of A, 9 % of B but only 3 % of both C and D have a crushing strength lower than 150 daN.
There are some very strong pellets with crushing strength higher than 400 daN among pellet quality C (30 %) and D (21 %).
The results are perhaps better illustrated in Figures 1 and 2.
A B C DRecommended
Fe total (%) 67,58 67,93 67,87 67,90 > 67
SiO
2
(%) 1,54 1,27 0,84 0,99
CaO (%) 1,05 0,87 0,97 0,88
MgO (%) 0,09 0,14 0,64 0,58
Al
2
O
3
(%) 0,45 0,33 0,19 0,35
TiO
2
(%) 0,12 0,06 0,18 n.d. < 0,15
MnO (%) 0,08 0,10 0,07 0,12
P
2
O
5
(%) 0,06 0,12 0,07 n.d.
K
2
O (%) n.d. n.d. 0,02 n.d.
Na
2
O (%) n.d. n.d. n.d. n.d.
P (%) 0,027 0,051 0,029 n.d. < 0,03
SiO
2
+ Al
2
O
3
(%) 1,99 1,60 1,03 1,35 < 2-3
CaO + MgO (%) 1,13 1,01 1,62 1,46 < 3
B (2) 0,68 0,68 1,16 0,89
B (4) 0,57 0,63 1,57 1,09
LOI (%) 1000°C 0,03 0,17 0,10 0,04

Table III. Relative frequency of the pellets A, B, C, and D in specified strength ranges.
Figure 1. Cold crushing strength of 4 commercial pellet qualities (A, B, C and D) compared to a typical good pellet.
Figure 2. Relative frequency of pellets within specified strength ranges, classified at 50 daN intervals (a) A and B, (b) C and D.
The precisions in the test results are 30 daN/pellet for A, 27 for B, 46 for C and 37 for D, calculated using the formula recommended
in the ISO 4700 standard. The precision would on an average improve by 10 daN/pellet by increasing the test portion from 30 to 60
pellets. Considering the large standard deviations in these measurements, an improvement of 10 daN/pellet is perhaps not worth the
additional work.
Strength Relative frequency (%)
range Percentage of pellets within specified strength range
(daN/p) A B CD
0-49 0 3 0 0
50-99 0 3 00
100-149 73 3 3
150-199 10 17 13 13
200-249 17 27 10 10
250-299 23 30 20 20
300-349 23 13 20 20
350-399 13 3 3 13
400-449 7 0 17 7
450-499 0 00 7
500-549 007 7
550-599 00 3 0
600-650 0 0 3 0

TUMBLE TEST
Increased amounts of fines will affect the performance of a shaft reduction furnace in a negative manner by decreasing bed
permeability, increasing pressure resistance, and restricting gas flow. Size reduction of pellets during handling can be a problem,
including generation of fines caused by pellet-pellet abrasion during transport. The tumble test ISO 32714 specifies a method to
provide a relative measure for evaluating the resistance of iron ores to size degradation by impact and abrasion, recommending 15 kg
pellet and 4 parallels, together 60 kg. Both the size of the drum and the required amount of material are huge. At SINTEF we have
downsized to 120 g pellets using a much smaller drum which is more convenient for research and development. A picture and sketch
of the drum used at SINTEF is shown in Figure 3. The same drum is also used for tumble tests of other materials at SINTEF. The
drum has 200 mm inner diameter while the ISO standard recommends 1 m, see Table IV for comparison of the test equipment and
conditions. The SINTEF test is longer and the rotation speed higher than in the ISO standard, but the height of the drops during
rotation is smaller. For easier comparison of different materials using a limited amount of pellets, the size range should be narrow.
Hence, SINTEF applies 50 % pellets in the range 10 – 12.5 mm and 50 % 12.5-16 mm, while the ISO-standard uses 6.3 - 40 mm. The
pellets are sieved at 0.5 mm and 6.3 mm after tumbling 30 minutes at 40 rpm. The results are reported as tumble index (wt% >6.3mm)
and abrasion index (wt%<0.5mm) in the same way as in the ISO-standard.
Figure 3. Tumble used in SINTEFs tumble test (a) Picture (b) Drawing
Table IV Comparison of ISO 3271 and SINTEFs tumble test conditions.
Sample
Number of
Sample
Tumble drum measures (mm)
Lifters
Tumble
Tumble
weight
parallels
Pellet diameter size
Diameter
Length
Lifters
No.
rate
time
ISO 3271
15 kg
4
6.3 – 40.0 mm
1000
500
500 . 50 . 5
2
25 rpm
8 min
SINTEF 120 g 1
50 % 10.0 - 12.5 mm +
50 % 12.5 - 16.0 mm
200 100 100 . 16 . 6 4 40 rpm 30 min
The results are shown in Figure 4. These 4 commercial pellet qualities have an abrasion index below 6 % and tumble index well above
92 %, which we would recommend for good pellets. For comparison, recommendations given using the ISO test is a tumble index of
minimum 92 % for DR pellets to be used in the Midrex process2. However, a tumble index of 95 % is preferred for the Midrex
process while the HYL process requires the same as a minimum. Less than 6 % fines < 0.5 mm is requested for the Midrex process,
but not more than 4 % is preferred. For the HYL process a maximum of 5 % fines less than 0.65 mm is requested. A minimum of
broken pellets between 0.5 and 6.3 mm is of course also preferred. Although the SINTEF and ISO tests are not directly comparable,
we find it reasonable that these 4 pellet qualities are not excluded using our recommendations.
Figure 4. Results of SINTEFs tumble test (a) Abrasion index (wt% < 0.5mm) (b) Tumble index (wt% >6.3mm).

POROSITY
Generally, highly porous pellets are expected to have a good reducibility while over-fired pellets will have low porosity and low
reducibility. The porosity was measured indirectly using a GeoPyc 1360 pycnometer. We measured 4 pellets, one by one, of each
pellet quality, 2 pellets in each of the size ranges 10.0 – 12.5 mm and 12.5 – 16.0 mm. The porosity of the two different size ranges
was found to be the same for each of the qualities A, B, C and D.
The pellet weight and the absolute density of the pellet were entered into the GeoPyc instrument. We used at first 5.2 g/cm3 as an
estimate for the absolute density, being close to the specific gravity for pure hematite. First the instrument measures the volume of
sand in the sample chamber. Thereafter, the pellet was covered in the sand and the new volume measured. The volume difference is
the pellet volume including open and closed pores (also called envelope volume). The unit reports the apparent density and the
corresponding total porosity (P1) of each pellet. While the apparent density is plainly the pellet weight divided by its volume, the
porosity is calculated using equation (1). The results are presented in Table V as average values with standard deviations (STDEV) as
provided by excel.
 (%) =  () ()
 ()100 (1)
 () = Absolute pellet density, the material density without open and closed pores
 () = Apparent material density measured with GeoPyc 1360
Table V Porosity and apparent density determined for 4 commercial pellet qualities.
A
B
C
D
Absolute density estimate for hematite pellets (ADE)
g/cm3
5.2
5.2
5.2
5.2
Apparent density measured, average and STDEV
g/cm3
3.50 ± 0.08
3.56 ± 0.15
3.87 ± 0.04
3.74 ± 0.06
P1: Total porosity, average and STDEV, based on ADE
(%)
32.7 ± 1.5
31.6 ± 2.9
25.6 ± 0.8
28.0 ± 1.0
Absolute density calculated from composition (ADC)
g/cm3
5.172
5.183
5.186
5.183
P2 Total porosity, average and STDEV, based on ADC
(%)
32.4 ± 1.5
31.4 ± 3.0
25.4 ± 0.8
27.8 ± 1.1
Later, a better value for the absolute density (ADC) was calculated from the composition of each pellet quality and the specific gravity
of each compound. The composition determined by chemical and XRF analyses, referring to Table I, was used, where total Fe was
calculated as Fe2O3. The share and specific gravity5 of each of the compounds Fe2O3 (5.24 g/cm3), SiO2 (2.65), CaO (3.30), MgO
(3.58), and Al2O3 (3.96) were multiplied, and the result normalized to 100%. This gives a slightly lower number for the absolute
density and an insignificant lower value for the porosity (0.2 - 0.3 % lower), as calculated by equation (1) and shown in Table V. The
porosity standard deviations for the 4 pellets are much higher (0.8 – 3.0 %).
The 4 commercial DR pellet qualities are all highly porous. A porosity of 20-30 % is usual for blast furnace (BF) and DR pellets6. The
porosity is also shown in Figure 5 where it is compared to the lower number, an acceptable porosity for BF pellets. The first and
obvious approach would be that the excellent porosity of A and B would be preferred, but emphasis on desired pellet properties differs
around the world. Model simulations of the influence of pellet porosity on the reduction rate during isothermal reduction at 900 °C
have shown that pellets with 28 % porosity are reduced at a higher initial reduction rate than pellets with 25 % porosity, in a reduction
gas consisting of H2 and CO (H2/CO = 1.6)7. A degree of reduction of 60% is already reached after 5 minutes for the higher porosity
pellet, while the lower needed 6.5 minutes. However, the degree of reduction approaches 92-94 % after 17 minutes for both.
Figure 5. Porosity of the 4 commercial DR pellet qualities compared to minimum porosity recommended.

REDUCTION PROPERTIES
The use of highly reducible DR pellets is necessary to attain maximum productivity in direct reduction shaft processes. Usually this is
associated with porous pellets together with a well-sized bed that allows passage for the reducing gas and enables it to penetrate into
the pellets and react with the iron oxides, and the gaseous products of the reduction reactions to escape. The reduction gas can be
reformed natural gas, usually called syngas. Syngas is a mixture of the reducing species CO and H2 and the gaseous products CO2 and
H2O. The reduction reactions taking place are listed here, going from hematite via magnetite and wüstite to iron.
3 Fe2O3 + CO =2 Fe3O4 + CO2 (2)
3 Fe2O3 + H2 =2 Fe3O4 + H2O (3)
Fe3O4 + CO =3 FeO + CO2 (4)
Fe3O4 + H2 =3 FeO + H2O (5)
FeO + CO = Fe + CO2 (6)
FeO + H2 = Fe + H2O (7)
There does not seem to be universal agreement about the best methods for characterizing reduction properties as reducibility, degree
of reduction and metallization, nor for disintegration during reduction or for sticking tendencies which are determined in various
clustering tests. The following tests are all developed for blast furnace burdens and relevant blast furnace atmospheres; ISO 3271, ISO
13930, ISO 4698 and the reducibility in ISO 4695 (950 °C, 40/60 % CO/N2). The sticking tendency is important for shaft furnaces as
Midrex since the maximum furnace temperature is limited by the level at which the pellets begin to stick. However, the use of coatings
of lime, limestone or dolomite, etc., has eliminated sticking as a limiting factor, and increases in temperature have led to production
increases in the range 10-20 %2. The Linder test was designed primarily for low temperature degradation and reducibility, and is
similar to ISO 11257 with standard temperature 760°C8. ISO 11257 describes isothermal reduction for 5 hours in a gas consisting of
55 % H2, 36 % CO, 5 % CO2, and 4 % CH4 while the sample is tumbling in a rotating drum. Midrex gives guidelines for the Linder
test; 91 % metallization is acceptable, but 93 % preferred2, while 5% degradation (-3.36 mm) is accepted, but 2 % preferred. The hot
load test simulates the increasing load experienced by the pellets as they descend through a shaft furnace. It was designed for
clustering, but gives also the reducibility for a standard temperature of 816 °C. However, there are some concerns that the results from
lower temperature tests may not necessarily be relevant to higher temperature operation in many Midrex plants today.
The ISO 11258 standard9 specifies a method for reduction under conditions resembling those prevailing in shaft direct reduction
processes. This standard provides a relative measure for evaluating the extent and ease to which oxygen can be removed from iron
ores, and specifies the determination of reducibility, final degree of reduction and degree of metallization. ISO 11258 was chosen as it
seems to be adequate, it provides measures for requested properties, and suitable equipment is easily available. The test prescribes
isothermal reduction at 800 °C for 90 minutes in a mixture of reduction gas consisting of 45 % H2, 30 % CO, 15 % CO2 and 10 % N2.
The pellets were placed on a grid in a retort suspended in a balance, as illustrated in Figure 6. A furnace can easily be moved up
around the retort. The retort is made from a steel tube with double walls for preheating of the reduction gas. Inert gas is passed through
the sample bed while heated to test temperature. The temperature is measured with a thermocouple inside the sample, while the
furnace is controlled by another thermocouple. Both temperatures are recorded.
Figure 6. Retort with double walls for gas preheating. The sample pellets are placed on a grid.
The retort has an inner diameter of 48 mm, but the ISO 11258 standard recommends 75 mm. We have down-sized retort size, sample
size and gas flow rate accordingly. Pellet size was 50% 10.0-12.5 mm and 50% 12.5-16.0 mm, as recommended. The main differences
in experimental conditions are presented in Table VI.

Table VI Comparison of differences between SINTEF experimental conditions and ISO standard 11258, pellet size being equal.
Sample
Heating
Hold 10 min at 800°C
Reduction at 800 °C for 90 min
Cooling
(mo)
Gas flow
Gas flow
Gas composition
Flow rate
Gas flow
ISO 11258
500 g
25 l/min N2
50 l/min N2
30% CO, 45% H2, 15% CO2, 10% N2
50 l/min
25 l/min N2
SINTEF
100 g
5 l/min Ar
10 l/min Ar
30% CO, 45% H2, 15% CO2, 10% Ar
10 l/min
5 l/min Ar
The reduction gas is switched to inert gas automatically and the furnace lowered after 90 minutes isothermal reduction at 800 °C. The
sample mass was measured on a separate balance both before (mo) and after (m2) each experiment. An example of the recorded
temperatures and weight loss is shown in Figure 7.
Figure 7. Example of recorded temperatures and weight loss for pellet quality A. The balance is set at zero at start (tare weight).
A reduction curve (Rt) can be calculated according to equation (8) from the weight loss as a function of reduction time. Both % FeO
and % Fetotal are expressed as percentage of mass of the sample prior to the test.
=󰇡.%
.% +()
.%󰇢  100 (8)
t = Reduction time (min) at 800 °C
mo = The sample mass (g) prior to the test
m1 = mt=0 = The sample mass (g) immediately before the reduction is started
mt = The sample mass (g) after reduction time t
The first part of the equation is small compared to the second part because % FeO is small. A SATMAGAN instrument, designed to
measure magnetite in iron ore concentrates, was used to determine the magnetic part of the samples (powder). The results were
reported as 2.25 % magnetic iron in A, only 0.42 % in B, 1.18 % in C and 1.59 % in D. One third of the iron in magnetite is Fe2+
(magnetite can be written as FeO.Fe2O3). Based on this % FeO was calculated to 0.96 % in A, 0.18 % in B, 0.51 % in C and 0.60 % in
D. The impact of the first part of the equation on the degree of reduction is nearly negligible, Rt increases with 0.37 % for A, only 0.07
% for B, 0.19 % for C, and 0.23 % for D. The final degree of reduction (R90) after 90 minutes was calculated from equation (8). The
averages of two parallel tests for each pellet quality are presented in Figure 8, and compared to our recommendation for R90.
Figure 8. Final degree of reduction after 90 minutes (R90) for 4 commercial pellet qualities, and our R90 recommendation.

The total weight loss from the difference in sample mass before and after the experiments (mo - m2), measured outside the furnace on
another balance, is believed to be more accurate than the total weight loss measured during reduction (m1 – mt=90). The differences
were typically ± (0.2 – 0.4) g. The reduction curves were normalized accordingly, and presented in Figure 9.
Figure 9. Degree of reduction curves for the 4 different industrial DR pellet qualities, 2 parallels for each quality.
According to ISO 11258 the reducibility index for 40 % reduction can be calculated from equation (9) based on read off from the
reduction curves, as illustrated in Figure 9. Likewise, reducibility index for 90 % reduction can be calculated from equation (10), also
based on read off. The curve had to be extrapolated if 95 % reduction had not been reached. The results are shown in Table VII.

 =.
 (9)

 =.
 (10)
t30, t60, t80, t95: The time (min) to reach 30, 60, 80, and 95 % degree of reduction.
The degree of metallization is defined in equation (11). If the content of FeO is small and below 2 % before reduction (in our case it is
< 1 %), the degree of metallization (MR) can be calculated from R90 using equation (12) according to ISO 11258. The chemical
analyses of the products can then be avoided. The results are shown in Table VII, but also illustrated in Figure 10.
= % 
%  100 (11)
= 1.43   43 (12)

Table VII. Reduction properties for 4 commercial pellet qualities. Average values and deviations between 2 tests for each quality.
Recommended
A
B
C
D
Degree of reduction after 90 min (R90)
>92 %
93.4 ± 1.8
94.7 ± 0.6
92.4 ± 0.4
94.7 ± 0.5
Reducibility dR/dtR=40
>1.2 %/min
1.5 ± 0.10
1.7 ± 0.06
1.6 ± 0.02
1.6 ± 0.02
Reducibility dR/dtR=90
-
0.35 ± 0.02
0.35 ± 0.00
0.31 ± 0.01
0.38 ± 0.03
Metallization after 90 min
>89 %
90.5 ± 2.6
92.4 ± 0.8
89.1 ± 0.5
92.4 ± 0.8
Figure 10. Degree of metallization after 90 minutes reduction for 4 commercial pellet qualities, and our MR recommendations
Table VII presents the reduction properties as determined according to ISO 11258 at 800 °C, with some exceptions from the standard
that already have been mentioned. Our recommendations here are based on the fact that these 4 commercial pellet qualities are all
world class pellets which should satisfy all minimum requirements.
The results from lower temperature tests may not necessarily be relevant to the higher temperature operation in many Midrex plants
today. Model simulations of the influence of temperature have shown that the reduction rate and final degree of reduction are
increasing with temperature7. If the temperature is raised from 850 to 1000 °C the final degree of reduction after 17 minutes will be
increased from around 95 % to 97.5 %, for pellets with 27 % porosity in a reduction gas consisting of H2 and CO (H2/CO = 1.6).
Normally 90-95 % of the iron in DRI is in the metallic form. An increase of only 0.7 % will bring the lowest R90 value up to 93.1 %
and the metallization beyond 90 %. Temperatures are expected to be higher than 800°C in todays Midrex shaft furnaces.
The reducibility index at 40 % degree of reduction (dR/dtR=40) is usually > 1.2 %/min for fluxed world class BF pellets while the
minimum requirement is 0.8 %/min2. Although not comparable (ISO 4695 reducibility test for BF pellets is carried out at 950 °C in
40/60 % CO/N2), the index for DR pellets should not be any lower, considering that the reduction curves levels off. Hence, we
propose that the reducibility index should be at least 1.2 %/min. The reduction curves give more information than the reducibility
index at 90 % reduction, since the curve will have to be extrapolated to calculate this index in case 95 % reduction not was reached.
SUMMARY
A summary of important physical and chemical test results are presented in Table VIII for four commercial DR pellet qualities. The
results and recommendations will hopefully help evaluating pellet quality for DRI shaft processes.
Table VIII. Summary of physical and chemical test results for 4 commercial DR pellet qualities, together with recommendations.
Recommended
A
B
C
D
Fe total (%)
> 67 %
67.6
67.9
67.9
67.9
Crushing strength
Good: 250 daN/p
>150 daN/p
282 ± 81 236 ± 74 328 ± 127 309 ± 101
Abrasion index (wt%< 0.5 mm)
<6 %
5.8
5.6
4.7
4.5
Tumble index (wt% > 6.3 mm)
>92 %
94.2
94.4
95.3
94.9
Porosity
>20 %
32.4 ± 1.5
31.4 ± 3.0
25.4 ± 0.8
27.8 ± 1.1
Degree of reduction after 90 min (R90)
>92 %
93.4 ± 1.8
94.7 ± 0.6
92.4 ± 0.4
94.7 ± 0.5
Reducibility dR/dtR=40
>1.2 %/min
1.5 ± 0.10
1.7 ± 0.06
1.6 ± 0.02
1.6 ± 0.02
Metallization after 90 min
>89 %
90.5 ± 2.6
92.4 ± 0.8
89.1 ± 0.5
92.4 ± 0.8

CONCLUSIONS
DR pellets for DRI applications have been supplied from 4 commercial pellet producers. These pellets have been characterized with
regard to compression strength, abrasion and tumble resistance, porosity and reduction properties, to a large extent according to
acknowledged ISO standards. However, we have downsized the amounts needed and the equipment size in order to be more practical
for research. The results indicate that the downsized methods can be used for evaluating pellet quality and suitability for DRI shaft
processes, or at least be a suitable tool for research and development of new pellets.
REFERENCES
1. World Steel Dynamics (WSD), 2013 World Direct Reduction Statistics, Midrex Technologies, North Carolina, USA,
www.midrex.com, 2nd quarter 2014.
2. David H. Wakelin, The Making, Shaping and Treating of Steel, The AISE Steel Foundation, Pittsburg, USA, 11th edition,
1999, pp 627, pp 651, pp 653-654, pp 657, pp 660.
3. ISO 4700:2007 (E), Iron ore pellets for blast furnace and direct reduction feedstocks – Determination of the crushing
strength, 2007.
4. ISO 3271: 2007 (E), Iron ores for blast furnace and direct reduction feedstocks – Determination of the tumble and abrasion
indices, 2007.
5. Geoffry C. Maitland and Gordon G. Slinn, Ceramists Handbook, Podmore and Sons Ltd., Stoke-on-Trent, England, 1973.
6. R. C. Gupta, Theory and Laboratory Experiments in Ferrous Metallurgy, Asoke K. Ghosh, 2010, pp 17.
7. Reza Beheshti, John M. Bustnes and Ragnhild E. Aune, Modeling and Simulation of Isothermal Reduction of a Single
Hematite Pellet in Gas Mixtures of H2 and CO, TMS (The Minerals, Metals & Materials Society), 2014, pp 495-502.
8. Jeffrey Myers, Improvements in Laboratory Testing of DR Oxides at Midres Technologies, Inc., 2nd International Symposium
on Iron Ore Sao Luis, Brazil, 2008.
9. ISO 11258:2007 (E), Iron ores for shaft direct-reduction feedstocks – Determination of the reducibility index, final degree of
reduction and degree of metallization, 2007.