Influencia del agua en la sinterizacion

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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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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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(196) Transactions ISIJ, Vol. 27, 1987

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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Influencia del agua en la sinterizacion