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Influence of Water
Sinter Beds*
Condensation
on the Permeability of
By W. J. RANKIN** and P. W. ROLLER***
Synopsis
An experimental echniquehas been developed o investigate the e
ffect of
condensationof moisture on the permeability of beds of granulated sinter
feeds. The technique nvolves njecting atomized water into the air stream
of a laboratory permeameter at ambient temperature and measuring the
resulting change n pressure drop across the bottomportion of the bed where
the moisture accumulates. The technique provides a relatively simple
means of assessing the behaviour of sinter mixes towards condensation
during sintering without the need to perform a conventionalsinter test.
The technique was applied to a granulated Australian iron ore sinter
feed to investigate the change n permeability due to condensation rior to
collapse of the bed, the region of most practical interest. The results were
analysed using the Ergun equation for flow through packed beds on the
assumption that the condensedwater enters the granules, resulting in their
swelling with a consequent ecrease n voidage. These effectsand changes
in the granule shape actor, due to a combinationof a roundingeffectof the
added water and sagging due to weakening of bonds within granules, can
account or the observeddecrease n the permeability of the bed. However,
in the absenceof experimental confirmation, the alternative hypothesis hat
the decreased ermeability is due to a reduction in the bed void space due
to accumulation of condensed water in interstices between granules must
still be considered a possible explanation.
Key words: sintering; permeability in bed; water; granule; iron ore;
air flow.
I. Introduction
To prepare iron ore fines for sintering, the fines
are blended with coke, limestone and return sinter
fines and pre-agglomerated by mixing with water.
This has the effect of coating fine materials (the
adhering particles) onto coarser materials (the nucleus
particles) and raising the mean diameter of the sinter
feed by forming granules or quasi-particles. In turn,
this improves the permeability of beds of sinter feed
and increases the productivity of a sinter machine.
After the ignition of a bed of sinter feed, a narrow
combustion zone moves downwards through the bed.
The temperature of materials in the combustion zone
is raised to around 1 200 to 1 400C and sintering
occurs. Ahead of the zone, hot gas from above
dries and preheats the bed and evaporated moisture
is carried to lower regions in the bed where the gas
cools and moisture begins to condense when the dew
point temperature of the gas is reached. Condensa-
tion can continue until the raw mix zone reaches the
dew point temperature of the gas which is typically
in the range 55 to 65C. Condensation of water in
a bed of sinter feed can decrease the permeability of
the bed during sintering' and as a result the optimum
water level in a sinter feed is usually less (typically
10 to 20 % less) than that which gives maximum
pre-ignition permeability.
Wild and Dixon'~ measured moisture accumula-
tion in laboratory sinter tests for a variety of ores
and mix compositions and found that condensation
occurred only in the first 2 min of sintering and
thereafter the water content of the raw mix zone
remained constant. This was attributed to the fact
that the entire raw mix zone reached the dew point
temperature of the gas within 2 min. The increase
in moisture content in the condensation zone varied
between 0.9 and 1.3 % of water and the ratio of the
peak pressure differential across the bottom 115 mm
of the bed during sintering to that before ignition
varied from 1.3 to 3.2 due to condensation. The
explanation offered by Wild and Dixon'~ for this
phenomenon was that part of the condensed water
filled interstices between quasi-particles in the bed
and reduced the bed void fraction and, hence, the
permeability. However, no evidence was provided
to support the hypothesis. In a few experiments by
Wild and Dixon'~ the pressure ratio was greater than
20 but in these the incremental moisture content was
either very nearly zero or actually negative, indicat-
ing a decrease in moisture in that region. This was
attributed to a collapse of the bed and partial drainage
of water from the lower region.
Wajima et a1.2~ ampled materials below the com-
bustion zone at the fourth and seventh windbox on
a Dwight-Lloyd sinter machine having a total of 19
windboxes and found that the moisture content
increased from a value of 5.8 % in the sinter feed to
a maximum of about 7.5 and 8 %, respectively, in
the condensation zone. Measurements on samples
from sinter pot tests at 2.5 min after ignition gave
maximum moisture increases of about 1.2 and 1.8 %
for initial moisture contents of sinter beds of 4 and
6 %, respectively. An increase of 1 to 2 % of mois-
ture due to condensation appears to be typical of
most sinter feeds.'-4) Wajima et a1.2~ bserved experi-
mentally that as the amount of condensed water
reached a critical level at which the adhering forces
of grains in quasi-particles started to decrease, the
quasi-particles began to coalesce. This reduced the
bed void fraction and the resistance to gas flow
increased sharply at the start of breakdown of the
quasi-particles.
In this investigation, we developed a technique to
study the effect of condensation on the permeability
*
**
***
Manuscript received on September 27, 1986; accepted in the final form on November 14, 1986. 1987 ISIJ
Division of Mineral Engineering, CSIRO Australia, Clayton, Victoria 3168, Australia.
BHP Central Research Laboratories, Shortland, New South Wales, Australia.
(190)
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(191)
of beds of sinter feed which is quick and relatively
simple to perform and which does not involve the
firing of a sinter bed. The technique could be useful
for rapid assessment of the behaviour of an ore towards
condensation of water. Also, we have analysed the
results for one ore using an hypothesized mechanism
for the change in permeability due to condensation
in the stage prior to actual collapse of the bed.
II. Experiment
A porous Australian ore, consisting of polycrystal-
line hematite and bladed secondary hematite was
used in the experiments. This ore is referred to as
type B in previous investigations by the authors5-7)
though the sample used in these experiments was
from a different batch of the ore.
A batch of the ore was blended with 7 % coke
(
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and no preferential flow of air along the walls was
evident.
Pressure tappings were provided along the height
of the pot but in these tests only the two bottom most
tappings were used; the others were closed with
rubber stoppers. The pressure tappings were at
depths of 175 and 225 mm and the measured pressure
drop applied over a height of 50 mm toward the
bottom of the bed.
Condensation of water in the bed was simulated
by injecting atomized water into the air stream
entering the windbox. The atomizer was connected
to a compressed air line through a regulator and the
water inlet was connected to a burette so the amount
of water atomized could be monitored. The atmoizer
was of 10 ml/mm nominal capacity when operated at
a gauge pressure of 300 kPa. The air flow rate was
0.03 m3/min at STP and the spray angle was 55.
Under these conditions the maximum size of the
atomized water droplets was quoted by the manufac-
turer to be 2 m.
The procedure for the condensation experiments
was as follows. After forming the bed, pressure
readings at the bottom two tappings were taken at
a pre-set air flow rate. A fixed amount of water,
usually 50 ml, was then injected into the windbox
with the main airflow still on. The atomizer was
then turned off and pressure readings taken again
at the same air flow rate without water injection to
measure the new pressure readings due to the effect
of the condensed water. The atomizer was then
turned on again and a further fixed amount of water
was injected into the windbox; new pressure readings
were then taken. This procedure was repeated
several times to obtain pressure readings at different
condensed water contents for the same batch of
granulated sinter feed.
After the final reading the bottom portion of the
bed was removed and its final moisture content was
determined gravimetrically and expressed on a moist
basis:
We _ Mass loss of sample (105C) x 100M
ass of fresh sample (% )
..........................(2)
where fresh sample in this case refers to the sample
after condensation. The moisture contents of the
bed at the levels of moisture addition between the
initial and final values were determined by linear
interpolation between the measured initial and final
values according to the fraction of atomized water
added at each stage. The incremental increase of
the moisture content due to condensation was then
calculated:
Wr= Wt _ W~ (%)......................(3)
When the permeability of a moist bed of sinter mix
is measured using dry, compressed air some loss of
moisture by evaporation occurs. It has been our
experience that the change in permeability due to
evaporation is small and occurs only in the first few
minutes. In all cases in the present work the pres-
sure and flow readings were made during the first
minute of dry air injection and it is considered that
the error in the data due to evaporation effects is
very small.
III. Results
The results are given in Table 2. For all the sinter
feeds, except the one granulated with 4.9 % water,
there is a significant increase in the pressure drop as
a result of accumulation of moisture in the bed. The
addition of an incremental amount of up to 2.6 %
of water to sinter feed granulated with 4.9 % of water
did not raise the pressure and it actually decreased
to 255 Pa before rising to the pre-condensation value
of 294 Pa. The variation of the ratio of pressure
drop after addition of water to that before condensa-
tion is shown in Fig. 2.
On adding water to a bed of sinter feed, a level
of water is reached at which the granules (or quasi-
particles) start to break down resulting in collapse
of the bed under the applied suction.lt2~ The void
fraction of the bed decreases and its resistance to gas
flow increases sharply at this stage; the ratio of the
Table 2. Experimental results.
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pressure drop after condensation to that for the pre-
ignited bed rises rapidly and Wild and Dixonl~
reported values of 5 for partially collapsed beds and
greater than 20 for severely collapsed beds over the
bottom 115 mm. If these values are scaled to a
50 mm portion of the condensation zone, the ratio
of the pressure drop before and after condensation is
2.2 for partially collapsed beds and greater than 8.7
for severely collapsed beds. From Fig. 2 it is appar-
ent that the present experiments were performed in
the range of condensation in which bed collapsed
was unlikely to occur, since in all but one experiment
the ratio of pressure drop before and after condensa-
tion was less than 2.2.
1 V. Discussion
During moisture atomization, the velocity of air
through the rotameter was sufficient to entrain the
water droplets and carry them into the bed. It is
important to emphasize that the pressure readings in
Table 2 were taken after the atomizer was turned
off with the air flow through the rotameter still on.
At the completion of an experiment the air flow
through the rotameter was turned off and the bed
was excavated. Two observations of significance
were noted during the latter operation : firstly, there
is always a relatively sharp interface between the wet
zone and the unaffected sinter mix above it and,
secondly, little or no drainage of water from the bed
occurs. The significance of these observations is that
the condensed water is part of the packed bed of
quasi-particles and is not free to move either upwards,
under the influence of the air flow, or downwards,
under the influence of gravity, in the absence of air
flow. The mechanisms by which the water is most
likely to be held in this manner are shown in Fig.
3. In Fig. 3(a) the condensed water is held in the
interstices between the quasi-particles by capillary
attraction while in Fig. 3(b) the condensed water
enters the adhering layer and becomes part of the
quasi-particles.
In the latter case the permeability of the bed is
amenable to analysis in terms of the Ergun equation
for flow through packed beds since the water is
hypothesized to be held tightly within the granules
and it is, therefore, a constituent of the granules.
In the former case, the presence of less tightly held
water in the interstices of the bed may make applica-
tion of the Ergun equation invalid since how this
water behaves during passage of air through the
bed is not known. The water, for example, may
spread and deform to varying extents according to
the flow rate and the amount of condensed water
Fig . 2.
Variation of the ratio of pressure drop
condensation over a zone of 50 mm near
after condensation to that before
the bottom of the permeameter.
Fig. 3. Mechanisms by which condensed water may be held
within a bed of quasi-particles.
(a) Within the interstices between quasi-particles
(b) Within the adhering layer of quasi-particles
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present while still occupying essentially the same
position in the bed. The Ergun equation in this case
can not adequately describe the pressure-flow relation
in the bed.
In an earlier study it was shown how the Ergun
equation (Eq. (4)) could be applied to beds of sinter
feed in the absence of condensation6~ :
4p
` 150r~yo(1E)2 1.75pvo(1-~)
dp~3 + dp~3
where, 4p : the pressure drop across the bed (Pa)
l: the bed height (m)
7 the viscosity (kg m-1 s-1)
p : the density (kg m-3) of gas
Vo: its superficial velocity (m s-1)
the void fraction of the bed
d~ : the effective diameter (m) of the granu-
lated feed.
The effective diameter is related to the mean granule
diameter (d) by the equation:
dp=~bxd ...........................(5)
where ~b is the granule shape factor. The applica-
tion of the Ergun equation has been made possible by
the development of techniques to measure the mean
granule diameter5~ and void fraction7~ of beds of
sinter feed. According to the Ergun equation, at
constant flow rate, temperature and gas composition,
dp/l depends on s, d and ~b nd thus the increase in
dp/l due to condensation for the mechanism in Fig.
3(b) must be due to changes in one or more of these.
In the mechanism shown in Fig. 3(b) the condensed
water is assumed to enter the adhering layers of
granules resulting in their swelling but without any
collapse of the adhering layers; since the volume of
the bed remains constant during condensation, there
is a decrease in bed void fraction.
The initial mean diameter of the granules is known
experimentally. The diameter of a granule after
condensation is given by:
V li3
=(H) (m)....................(6)
V is the volume of a granule after it has absorbed con-
densed water and is given by:
V=V+4V (m3) .....................(7)
where, V the initial volume of a granule
4 V: the volume of condensed water which
enters a granule.
For Eq. (7) to be valid all internal porosity in the
granules must be filled with water prior to condensa-
tion. Previous work7~has shown that this is probably
true for this particular ore.
From a mass balance on water for one granule,
W WgxM+Wx 100 ) ............ ()% 8r M+W
where, M: the mass of a moist granule before con-
densation
*1Calculated using Eqs. 6)
, (7), (9)(11).
*2 Mean particle diameter of granulated sinter feed from
Table 1.
*3 Calculatedusing Eq
. (12).
*4Value from Eq. (4) using values of ~( obtainedpreviousy7~
W: the mass of condensed water.
The volume of condensed water per granule is, there-
fore, given by:
W~xM 3
100-Wt) x pw
where, pw the density of water.
The mass of a granule before condensation is given
by:
M = pgranule Vo (kg) .....................(10)
100
where, pgranuie 100
- W W (kg m-3) .........(11)
+ 9
po pw
po is the density of the dry sinter feed and a value of
4.08 g/cm3 applies for the mix used in this study.7)
Table 3.
Changes in
void fraction
in Fig. 3 (b))
mean granule
according to
diameter and
the mechanism
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The derivation of Eq. (11) was given in a previous
paper7~ and applies in the situation where all internal
porosity of the quasi-particles is filled with water,
which appears to be the case for the present ore
type,7)
By applying Eqs. (11), (10), (9), (7) and (6) in
order, the mean diameters of the granules after con-
densation were calculated for the tests in Table 2.
The results are presented in Table 3 and reveal that
the mean diameter of a granule changes appreciably
during condensation. These values could not be
checked independently, however, since because of
their added moisture the granules were too weak to
remove from the permeameter for sizing using the
liquid nitrogen technique.
The overall volume of the beds of sinter mix did
not increase during condensation and the void frac-
tion after condensation, therefore, is given by:
= 1- V (1-E) .....................(12)
The values of s0, the void fraction of the bed prior to
condensation, were calculated using Eq. (4) and the
data of Table 2 at W~= 0. The values obtained are
presented in Table 3. Values of ~, the void fraction
after condensation, obtained using Eq. (12) are given
in Table 3 also and, as expected, they indicate that
the void fraction of the beds decreased during con-
densation. As was the case for the granule diameters
after condensation, these values could not be con-
firmed independently by the kerosene displacement
method as the granules could not be removed from
the permeameter without destroying their structure.
The values of d and r during condensation were
applied in the Ergun equation to find the remaining
unknown; viz., the shape factor (~b). The values
obtained are shown graphically in Fig. 4 and indicate
a complex variation with both W and W
For the mechanism in Fig. 3(b) to be feasible a
plausible explanation for the calculated variation of
shape factor is necessary. At 4.9 % granulation
water, ~b increases during condensation; i.e., the
granules assume a more spherical shape. This could
happen as a result of the rounding effect due to sur-
face tension of water in the granules. The rounding
could occur without rearrangement of grains within
granules by water filling surface pores and forming
a smooth film on the granules. At the other extreme,
at 8.0 % granulation water, addition of condensed
water resulted in a decrease in c. This could happen
if the condensed water in the granules reduced the
strength of bonds between the grains sufficiently to
allow sagging of the granules without actual collapse.
At levels in between, the proportion of the effect of
rounding may decrease relative to the effect of sagging
as the amount of granulation water is increased. The
trends in ~b, therefore, are not unrealistic and the
mechanism is at least feasible. The mechanism is
supported further by the fact that it accommodates
the observed decrease in ap/l during condensation at
4.9 % granulation water. Unfortunately, there is
no quantitative way of predicting the variation of c
Fig. 4. The variation of shape factor during condensation
all water is absorbed into the granules.
according to the Ergun equation assuming
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due to condensation which would permit a direct
comparison with the variation calculated using the
Ergun equation.
The foregoing analysis does not prove that the
mechanisms shown in Fig. 3(b) is responsible for the
decreased permeability of the sinter bed due to con-
densation. It merely shows that the mechanism is
feasible. The alternative mechanism shown in Fig.
3(a) cannot, therefore, be excluded as the cause; it
is possible that both mechanisms contribute to the
decrease in permeability.
V. Conclusions
(1) A relatively simple experimental technique
has been developed to investigate the effect of con-
densation of moisture on the permeability of granu-
lated sinter feeds. The method may prove useful
for quick assessment of the effect of condensation on
the permeability of unfired sinter beds. The attrac-
tiveness of the method is that the test is done at
ambient temperatures and without igniting the bed.
(2) The results obtained on a granulated Austra-
lian iron ore sinter feed showed a continuous increase
in 4b/1 due to condensation prior to bed collapse.
The effect was greater the greater was the amount of
water used for granulation.
(3) The decrease in permeability due to conden-
sation occurs probably as a result of a reduction in
the void space of the bed due to accumulation of
condensed water in interstices between granules, as
hypothesized by Wild and Dixon,i~ or by absorption
of the condensed water into the granules resulting in
their swelling with a consequent increase in mean
granule diameter and decrease in the bed void
fraction.
(4) The latter mechanism is amenable to analysis
using the Ergun equation but the former is not. An
analysis revealed that the latter mechanism is con-
sistent with the experimental results provided the
shape factor of the granules undergoes a continuous
change during condensation. A mechanism has been
proposed by which this may happen without actual
collapse of the granules. Experimental evidence is
lacking at this stage to confirm the analysis and both
mechanisms remain as possible explanations for the
decrease in permeability.
Acknowledgement
This paper is published by permission of the Broken
Hill Proprietary Company Limited.
REFERENCES
1) R. Wild and K. G. Dixon Agglomeration, d. by W.A.
Knepper, IntersciencePublishers,New York, (1962),565.
2) M. Wajima, Y. Hosotani,J. Shibata, H. Soma and K.
Tashiro Yetsu-to-Hagane,8 (1982),1719.
3) A. A. Sigov: Izvest.Vyshikh cheb. avedenii hernaya et.,
8 (1958),7.
4) V. G. Kotovand V. A. Shurkhal: Steel n the USSR,3
(1973),800.
5) W. J. Rankin, P. W. Roller and R. J. Batterham: The
Joint Symposium f ISIJ and AIMM, ISIJ, Tokyo, 1983),
13.
6) W. J. Rankin,P. W. Rollerand R. J. Batterham: Minerals
and Metallurgicalrocessing, (1984),53.
7) W.J. Rankin and P. W. Roller: Trans. SIJ, 25 (1985),
1016.
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