Effect of Alkalis on Reduction Behavior of Acid Iron

Ore Pellets*

By Taymour ELKASABGY**

Synopsis Under conditions relevant to the process of an iron blast furnace with alkali cycling, acid iron ore pellets were tested. The alkalis were intro-duced in three different ways: as part of the pellets gangue phase, as fine crystals of alkali carbonates near the pellets surface, and, as metal vapor mixed with the reducing gas.

During reduction alkalis flux the silicate binding bridges and react with

ferrous iron oxide forming low melting point slag which weakens the pellets and causes them to degrade. A mechanism of degradation will be given which relates the behavior of the pellets to the formation of this primary slag. Also, certain measures to control the reduction will be discussed.

I. Introduction

Alkalis are known to cycle and sometimes to ac-cumulate in the iron blast furnace.)-4) This phenom-enon is related to the counter current character of the furnace and the existence of two gradients of tempera-ture and oxygen potential between its top and the tuyeres level.5'6~ By sampling and direct observa-tion, during relining of the furnace refractories, it is known the alkali contents in the burden materials in-crease as they move down towards the melting zone in the blast furnace.3'5,6> The higher the alkalis in the raw materials the easier it is for them to circulate and accumulate and to build up to dangerously high levels. The adverse effects of alkalis are higher coke rate,

poor quality of hot metal and deterioration of the fur-nace lining. Also, they cause irregularities in the operation process and decrease the production. How-ever, the determinate effects of alkalis are more pro-nounced when the stability of the coke and/or the iron ore is relatively low.

In the present work it is intended to simulate to a certain extent reduction conditions relevant to the

process of an iron blast furnace with alkali cycling and to investigate the behavior of a typical acid iron ore

pellets before the formation of a strong metallic shell.

II. Behavior of Alkalis in the Blast Furnace

This paper is not designed to investigate in detail the stability of various alkali compounds and the mechanism of alkalis cycling in the iron blast furnace, they are given elsewhere.)-6~ The most important facts regarding the behavior of alkalis may be sum-marized as follows :

(1) Alkalis are known to cycle and sometimes to accumulate in the blast furnace.

(2) The movement of alkali compounds may be represented by two open loops. The first describes their movements as vapors (either metallic : Na and

* Received June 28, 1983. © 1984 ISIJ **

K, or in the form of cyanides KCN or NaCN), as fine droplets of cyanides and/or carbonates : Na2C03 and K2C03, or fine crystals of alkali cyanides, carbonates, or silicates, with the ascending gas stream. The sec-ond loop shows their movement down with the charge as condensed phases. When these two loops are com-bined they complete one cycle.

(3) At the lower parts of the blast furnace where: the temperature is higher, the reducing power of the gas is stronger, and the blast is almost free from alkalis, it is thermodynamically possible that alkalis, in the condensed phases are gasified to form vapours.

(4) Along the height of the furnace, as the gaseous stream moves upward, there is thermodynamic pos-sibilities of reactions which either produce or consume certain alkali compounds.

(5) Inside the iron ore pellets, the oxygen poten-tial is relatively much higher than in the bulk gas stream, due to iron oxide reduction. At the gas/iron oxide interface it is expected, based on thermody-namics, that wustite would oxidize the alkali metal vapors, cyanides, and, carbonates to form oxides,

provided silica as stabilizer is available. Complex alkali silicate compounds would result from this inter-action between reducing gases, iron oxide, alkalis and silica of the gangue phases. They have the highest chemical stability, among all the alkali compounds under blast furnace conditions. The only way to reduce alkali silicates is to get them down, with the charge, to the lower parts of the furnace to the higher temperature and reducing power zones.

(6) Based on the ternary phase diagrams, be-tween silica ferrous iron oxide and alkali oxides : al-kalis drastically reduce the melting point of silica and iron silicate gangues and slags.7'8)

From the above-mentioned points it may be con-cluded:

(1) The alkali contents of the charge can be much higher than originally in the raw materials.

(2) Alkalis influence both the iron oxides and the gangue phases, which are of silicate origin, in most of iron ore deposits.

(3) Alkalis may be introduced during reduction in the blast furnace and interact with silica and iron oxides to form low melting point slag.

Most of the experimental work which had been carried out concerned almost exclusively with the in-fluence of alkalis on the iron oxides.9-11> It is well known that high grades iron ore pellets, which con-tain alkalis, swell abnormally due to iron whiskers

Horas Investments (Kingston), Kingston, Ontario K7L 3C7, Canada. On leave from Department of Metallurgy & Materials Science, The Catholic University of Rio de Janeiro, Rio de Janeiro, CEP 22453, Brazil.

612 ) Research Article

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formation, the practical solution is to decrease the al-kali content, and/or, the alkali to silica ratio in the used ore.9'10~ On testing ordinarily acid pellets, which contains about 5 wt% silicate gangue phases, it was found that they swell and crack without evidence of iron whiskers formation.12,13)

In designing the experiments alkalis were intro-duced in three different ways corresponding to an iron blast furnace with alkalis cycling:

(1) As part of the gangue phases : representing pellets originally of high alkali content.

(2) As alkali carbonate fine crystals near the pel-lets outer surface : representing pellets with precipi-tated alkali compounds, as a result of condensation and chemical reactions involving alkali vapors in the blast furnace stack. Alkali cyanides cannot be used for safety reasons; however, they are assumed to be-have much like alkali carbonates.5,s~

(3) As alkali metal vapors mixed with the reduc-ing gas : representing the conditions of reduction by a blast furnace gas contains alkali vapors.

III. Experimental Work

1. Sample and Its Preparation

Most of the specimens used were commercial iron ore pellets which had about 5 wt% silicate gangues. The same iron ore concentrates, which was used in manufacturing those pellets was used to prepare pel-let specimens doped with alkalis. The chemical com-position of the pellets and ore, as received from the plant, is given in Table 1. It represents magnetite iron ore deposit, and, hematite pellets, respectively, To examine the sole effect of alkalis on the iron oxides, the silicate gangue phases was removed from an iron ore concentrate sample, by washing in warm concen-trated hydrofluoric acid solution, followed by distilled water, several times. The treated sample was dried and fired in air at 1200 °C for about one day, to remove the traces of the fluoric acid and to decompose the ferrous and ferric fluorides. After this treatment the sample had about 0.025 wt% SiO2, according to chemical analysis.

The specimens used in the reduction and swelling tests can be grouped in the following way:

(1) Commercial acid iron ore pellets: were used to test the sole effect of acid gangue phases, also were dipped in alkali carbonate aqueous solutions followed by drying at 150 °C to result in specimens with a layer near their outer surface which contained fine alkali carbonate crystals, called impregnated, and, were re-duced with gas mixture contained alkali metal va-pors.

(2) Modified pellets: same like in (1) but were pre-reduced to magnetite or wustite at a temperature of 900 °C or pre-reduced followed by oxidizing in air at 800 °C. This treatment was done to modify their structure and made the pellets' body more open so that the alkali carbonate can be distributed more uni-formly when they were dipped in its solution.

(3) Laboratory pellets made of iron ore concen-trate : they were made from the same iron ore con-centrate; however, alkali carbonates were introduced

prior to pelletizing and firing in air for 2 hr called doped pellets.

(4) Laboratory pellets and briquettes of high pu-rity iron oxides : the pellets were made of chemical reagent hematite (with a purity of 99.8 % Fe2O3) but no alkalis were introduced before pelletizing and, fir-ing in air, then were dipped in alkali carbonate solu-tions followed by drying at 150 °C to result in high

purity pellets with a layer near their outer surface, which contained alkalis. The briquettes were made from the iron ore sample after gangue removal, and mixing with alkali carbonate, followed by briquetting and firing in air for 2 hr.

2. Experimental Apparatus and Procedures

The experimental apparatus consisted, mainly, of a Marshall resistance heated tubular furnace, which contained a movable quartz tube of 40 mm inside di-ameter. The components of the reducing gas mix-ture : CO, CO2 and N2 were supplied by separate cylinders, with flow rates monitored by individual flow meters, and, mixed before entering the reaction tube. The specimens were placed in the middle zone of the tube on top of ceramic or platinum boats, during the reduction tests. They were moved together with the quartz tube to the hot zone of the furnace, and left under N0 flow for about 10 min until the isother-mal temperature level was reached, which was check-ed using a separate thermocouple placed just above the specimens. Then, the reducing gas mixture was allowed into the tube and the reduction time was counted using an automatic stopwatch. Once the re-duction is over, the gas mixture was switched off, and

purified N2 was allowed to flow, while the partially reduced pellets were taken together with the tube to the cooler end of the Marshall furnace.

To simplify the comparison of the results, most of the reduction test were carried out under the same standard conditions for 1 hr using a gas mixture of CO, CO2, and, N2 at a total flow rate of 500 cc (STP)/ min (the individual flow rates were : N2 300, CO 160, and CO2 40 cc (STP)/min, respectively), at a temperature of 900 °C, which corresponds to that of

Table 1. Chemical analysis of iron ore concentrate and commercial pellets.

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the thermal and chemical reserves zone of some blast furnaces.14)

Weighing and volume measurement by a mercury displacement were carried out before and after the reduction experiments to obtain the extents of reduc-

tion and swelling. The degree of reduction was cal-

culated based on the initial oxygen content with the iron oxides, from chemical analysis, and, the amount

of oxygen taken during reduction. The swelling in-dex is defined as the percentage increase of the ap-

parent volume of the specimen after reduction. On some occasions, due to the effect of alkalis, cracks and

fissures were developed during reduction. The ap-

parent volumes of the reduced specimens measured by mercury displacement did not include that of the fis-sures with a width larger than about 10 to 12 ,u, due

to the effect of the mercury surface tension. Accord-ingly, the given swelling index may be smaller than

that if we include the fissures' volume. Unfortunate-ly, it was rather difficult to measure the volume of

those macro-fissures and cracks; hence the results will

be presented without adding their volume. It was found that their size and distribution is a function of the amount and distribution of alkali in the partially

reduced pellets which contained silicate ,gangues. For the reduction tests performed with reducing

gas contained alkali vapors, they were generated by passing the gas mixture over alkali carbonate melt, which was placed in a separate platinum crucible. I t

was located prior to where the specimen was posi-tioned in the upstream in the direction of the gas flow.

The partial pressure of the alklai metal vapor was measured indirectly, by weighing the alkali carbo-

nate's crucible before and after gasification, and as-suming the rate of generating the alklai vapors was

constant during the test. To protect the quartz tube

from the alkali vapors, a mild steel tube, of about 38 mm outside diameter, and 35 mm inside, was used.

It was placed inside the quartz tube and carried the tested materials, during the reduction experiments.

A scanning electron microscope SEM, and, an elec-tron probe microanalyzer were used to examine the

topography of the grains, and, the formation and movement of the liquid slag during reduction.

Iv. Experimental Results

As received commercial acid pellets, which had a

porosity of about 25 %, swelled normally (about 18 %) after the standard reduction test. SEM ob-servation indicated no iron whiskers formation, also, the pellets did not show a tendency to form macro-cracks or fissures during reduction. Of course, care was exercised not to choose pellets with defects orr ir-regularities, which may arise during their industrial making, and, effect the results, for any of the tests in this work.

To study the effect of gangue, the pellets made from chemical reagent hematite were tested. Those of a porosity of about 20 % disintegrated during re-duction, but more compact specimens of porosity less than 10 % swelled normally, after the standard reduc-tion test.

1. The Sole Effect of Alkalis

The results of the standard reduction tests are given in Table 2. The two groups of specimens represents high purity hematite samples which were almost free from the silicate gangue phases. However, the meth-od of introducing the alkalis was not the same in each group. Alkalis did not seem to effect the reduction behavior of the high purity impregnated pellets. Neither the swelling index nor the degree of reduction were in-fluence, in a noticeable way, by the alkalis, which were introduced after making the pellets, without sub-sequent firing.

The alkali doped briquettes, which had a porosity comparable to the high purity pellets (about 10 %), behaved in a different way. They swell abnormally and SEM examination indicated the formation of iron whiskers. The swelling index and the degree of re-duction were also influenced by the firing tempera-ture, prior to reduction, and increased with elevating the induration temperature from 1 200 to 1 300 °C, under otherwise identical conditions.

It was more appropriate to use the same material

(the sample of iron ore concentrate, which was treated for gangue removal) in making both types of the high

purity specimens. However, the amount of the mate-

Table 2. Standard reduction of gangue free specimens.

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rials, left after the treatment, was not enough to make the necessary number of specimens.

2. The Combined Effect of Acid Gangue and Alkalis

In Fig. 1, there is two groups of laboratory pellets, which are represented by two curves. Those made without adding alkali carbonate prior to pelletizing and firing seemed to have better resistance to swelling which was increased by elevating the firing and in-duration temperature in air, prior to reduction. On the other hand, alkali doped pellets which were fired at 1 200 and 1 300 °C, seemed to swell much more, also, they developed hairline cracks. With the pellets fired at 1 100 °C, the alkali doped specimens had slightly lower swelling index; however, both types showed a tendency of cracking and sometimes split into pieces, after reduction. Figure 2 illustrates the effect of alkalis on increasing the swelling index and the degree of reduction of alkali doped pellets which was fired and indurate in air at 1 200 °C before reduc-tion. The extent of swelling and reduction increased with the alkalis.

The results of the standard reduction test using im-

pregnated commercial pellets are given in Fig. 3. To illustrate that the observed swelling and crack forma-tion were independent of the step of metallization, which may cause abnormal swelling due to iron whiskers formation (as in the case of alkali doped

briquettes in Section IV. 1), impregnated pellets with almost the same amount of alkalis were examined and reduced with reducing gas mixtures with different re-ducing power, but under otherwise similar conditions. The results are given in Fig. 4. The swelling index seemed to be independent of reducing power, as far as the gas mixture was able to reduce the hematite to magnetite or wustite.

The experimental results given in Fig. 5, shows the

Fig . 1. Effect of firing and alkalis on the swelling of 1

tory pellets.

abora-

Fig . 2. Swelling and reduction of acid pellets

alkalis prior to standard reduction.

doped with

Fig. 3. Swelling and reduction of commercial pellets which

were dipped in alkali carbonate solution followed by

drying and standard reduction.

Fig. 4. Alkali impregnated commercial pellets reduced with

gas mixtures of different reducing power which shows the swelling is almost independent of the

metallization of wustite and the formation of iron.

Fig. 5. Swelling and reduction

duced with gas mixture

of commercial pellets, re-

containing alkali vapors.

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effect of alkali metal vapors. The extent of swelling and reduction increased with increasing the partial

pressure of the potassium with the reducing gas mix-ture, under similar conditions. The Marshall furnace used to carry the tests had a three zone windings to adjust the temperature profile, which were employed so that the gasification zone (where the alkali vapors were generated) can be adjusted to have a tempera-ture level which may be equal or higher than that of the standard reduction of 900 °C. The three gasifica-tion temperatures, which corresponded to the three

partial pressures of potassium given in Fig. 5, were 900, 1000, and, 1 100 °C, respectively.

The partially reduced pellets, in the three men-tioned cases, were examined by the SEM, and, there was no evidence that the influence of alkalis on the reduction behavior of commercial pellets is directly related to iron whiskers formation. Photograph 1 is given to compare the appearance of the specimens under SEM after standard reduction, they represent commercial pellets as well as high purity specimens.

Photographs 1(A) and (B) were taken after stan-dard reduction of alkali impregnated commercial pellet (with 1.32 wt% K2O, before reduction). They represent two different regions of the partially reduced specimen. Photograph 1(A) represents an area on the outer surface zone where one of the hairline cracks started, while Photo. 1(B) represents another interior zone where the same observed crack was about to end. The pellet's grains near or on the surface were bigger in size and it could be observed, from their shape and

topography, that they were split, most probably due to the movement of slag, which was seen to concen-trate in certain areas and formed small pockets within the grains. In the second interior zone, the grains were much smaller, with rounded edges, and, almost with a continuous layer of slag around them, in a pat-tern which may indicate that in this region the slag

phase and the iron oxide particles were agglomerated together in a manner which made it more difficult for the interior grains to be reduced further.

Photographs 1(C) and (D) represent the appear-ance of the partially reduced high purity specimens under SEM (Table 2). The examination showed no indication of liquid slag formation. Photograph 1(C) represents a typical view of partially reduced alkali impregnated pellets, there was no strong evidence of iron whiskers formation, while Photo. 1(D) shows the only case in this work where iron whiskers were ob-served under SEM with alkali doped briquettes.

The two regions of the partially reduced impreg-nated pellets which are represented by Photos. 1(A) and (B) were examined further by the X-ray unit of the SEM. In both cases it was observed that the slag areas were high in Si and K. Photograph 2 gives the X-ray mapping for K, Si, and Fe. The only differ-ence between the two regions was in the distribution of the slag which was more uniformly distributed be-tween the grains in the interior zone.

In order to understand better the mechanism of the slag formation and its movement the following two experiments were carried out using commercial pel-

Photo. 1. Some SEM photographs of partially reduced specimens after standard reduction.

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lets: (1) A cylindrical specimen of about 1 g was made

by cutting and shaping a commercial pellet. It was

placed on top of about 0.1 g potassium carbonate, in the upright position. The specimen was then reduced using the standard reduction procedures.

(2) A commercial pellet was left, immersed in molten potassium carbonate, under flowing N2 for 1 hr at 900 °C.

The two specimens were examined using an elec-tron probe microanalyzer (with electron beam of about 30 pm in diameter), and the distributions of K, Si, and Fe were plotted. The results given in Fig. 6 indicate that although the pellet was completely im-mersed in molten potassium carbonate under flowing N2 for 1 hr there was relatively poor penetration. Near the surface the silica was moved inward with the

potassium to a distance of about 200 dim (region AB in Fig. 6). Beyond this region where silica was de-

preciated there was another region BC, of about 900 im where Si and K, appeared to be in phase. Fur-ther inward, the penetration of potassium nearly stop-

ped, and larger peaks of Si, representing the silicate gangue phases was observed in the pellets interior. The reproducibility of these results was checked in three different locations which showed almost the same

profiles for K, Si, and Fe. It was confirmed that the potassium carbonate was in its molten state by its ap-pearance in the platinum crucible after cooling the specimen to room temperature.

In the experiment with the specimen which was left to be reduced on top of potassium carbonate melt, all the potassium carbonate was taken up by the speci-

Transactions ISIJ, Vol. 24, 1984 (617)

men's body, as shown in Fig. 7, which also illustrates that the distributions of K and Si were almost identi-

cal ` and uniformly dispersed throughout the speci-men's body from the bottom to the top. No peaks of

Si could be observed, which may give an indication that almost all the gangue phases were reacted with

potassium to form the slag. Also, it seems that the movement of the slag occurs against the gravity force,

most probably due to capillary force, which results

from the liquid slag surface tension. Modified commercial pellets, which were restruc-

tured by prereduction or prereduction followed by

oxidation at 800 °C and were relatively more porous than as received pellets, showed a weaker tendency to

swell when reduced using the standard reduction pro-cedures, as given in Table 3.

To demonstrate that the observed swelling and cracking did not occur during the subsequent cooling,

after reduction. An alkali impregnated commercial

pellet was photographed, while was : heated, reduced, and, during cooling. The results are given in Photo. 3 which shows clearly that the swelling and cracking

occurred during the standard reduction test, and re-mained during cooling or heating under N2 flow.

V. Discussion

After firing and induration the iron ore pellets owe their strength to the formation of hematite, and,

gangue bridges which find the iron oxide grains to-

gether. During reduction once the hematite bridges are transferred to magnetite the structure as a whole

is relaxed and the pellets swell. Normal swelling occurs almost to all kinds of hematite pellets and

Photo. 2. SEM photographs and X-ray mapping of Fe, Si, and K after

shows clearly that the slag areas are high in both Si and K.

reduction of alkali impregnated pellets. It

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compacts, which contains reasonable amount of gan-

gue phases.15-17) The experimental results given in Fig. 1 with lab-

oratory pellets made without alkali addition, showed

that the swelling index decreased with elevating the

firing temperature. On the other hand, high purity

pellets which had structure and overall porosity closer to the acid pellets tended to disintegrate as ex-

Fig. 6. Distribution of K, Si, and Fe near the pellet surface after heated under N2 no reduction) but was immersed totally in molten K,C03 during testing.

flow (300 CC STP/min and

Fig . 7. Distribution of K, Si, and Fe across the center line after standard

which was placed on the top of K2C03 melt during reduction.

reduction of acid pellet specimen

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pected.15,16) It may be claimed that high purity specimens, which were prepared without alkali addi-tion, and were more consolidated with a lower poros-ity (less than 10 %) swelled almost normally, because there was still a hard core of unreduced hematite with-in the specimens after reduction for 1 hr.

I t seems the silicate gangue bridges between the iron oxide grains can bear with the stresses which may arise during magnetite to hematite transformation. Of course their effect in helping the pellets to resist degradation depends on their relative amount and distribution inside the pellets as well as the firing conditions. The presence of about 5 wt% gangues is said to be sufficient for ordinary ore pellets.l5>

The results with high purity specimens, which was almost free from silica, suggest high temperature firing for sufficient time is needed to cause iron whiskers formation as illustrated in Table 2 and Photos. 1(C) and (D). It must be pointed out, the effect of high temperature firing on iron whiskers formation is op-posite compare to the case when lime was added to high purity iron oxides.18 It may be assumed, the decomposition of alkali ferrites, which may form dur-ing induration and firing, and the incorporation of alkalis, as solid solution in wustite, in irregular pat-tern, is the reason for iron whiskers formation.

The experiments carried out to simulate the influ-

ence of alkali on the reduction behavior of acid pellets

showed, clearly, that the formation of iron whiskers is not the cause of their degradation. Visual observa-

tion, and, SEM examination of the partially reduced specimens, showed the pellets to swell, and to form

macrocracks and fissures as result of alkalis attack. The results given in Figs. 1 to 3, and 5 suggest that

the extent of swelling and reduction to increase with

alkalis. Also, Fig. 4 supports the idea that the ob-served swelling is independent of the metallization

step. The correlation between the existence of liquid slag high in both alkalis and silica and the formation

of cracks is supported by SEM and electron probe microanalyzer examinations.

The first step of reduction from hematite to mag-netite seems to be necessary for the movement and

progress of the alkali contained slag inside the pellets. This important piece of information can be seen if the results in Figs. 6 and 7 are compared. Reduction

and liquid formation are necessary conditions, how-ever, not sufficient. Uneven distribution of alkali liquid slag inside the pellets is needed. The results

given in Table 3, with alkali impregnated modified pellets, indicated no big change in swelling and crack-ing, when alkalis were relatively more uniformly dis-

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Table 3. Standard reduction of modified pellets.

Photo. 3. Photographs

us, prior to

duction and

taking during heating,

standard reduction test.

is not very sensitive to

reduction, and cooling of commercial pellet, impregnated with alka-

It is clear the swelling and cracking occur during early stage of re-

the cooling and heating procedures under N2.

(620) Transactions ISIJ, Vol. 24, 1984

tributed. During drying of green wares in ceramic industry different shrinkage rates, due to uneven moisture con-tent between the surface, and the interior parts can cause cracking. The case under investigation may be close but in the reverse direction. During reduction liquid alkali silicate slag is formed near the surface, and moves inward most probably due to capillary forces. However, at any moment its distribution is not uniform. The uneven distribution can be under-stood in the following way : the fine crystals of alkali carbonates were concentrated near the surface when reduction started (alkali impregnated pellets) ; when they were added prior to pelletizing and firing, alkalis were more or less distributed uniformly, however, re-duction started near the surface; therefore, the dis-tribution of liquid slag was also not uniform (alkali doped pellets) ; with reduction using alkali contained

gas, both the reduction and the slagging processes oc-curred at the gas/solid contacts near the surface, in a way very similar to the case with impregnated pellets. The slagging process near the surface started the cracks

(Photo. 1(A)), while its movement, and, in later stage, its aggregation with the iron oxide grains, facilitated their propagation (Photo. 1(B)).

Testing commercial acid pellets in the laboratory showed they swell and crack due to alkalis, the im-

portant question is whether it happens inside the blast furnace. If alkali contents are high in the ore, which is of ordinary composition, at the moment when fer-rous oxide becomes available, cracking and swelling are expected. Also, when the first stages of reduction are taking place using blast furnace gas containing alkalis : ferrous oxide and alkali oxides are formed simultaneously at the gas/solid interface creating fa-vorable conditions for cracking and swelling. How-ever, these two cases are not quite normal. It is rea-sonable to assume the conversion of hematite to magnetite, for the locations near the pellets surface, is completed before they arrive to the higher alkali zones in the blast furnace. Swelling and cracking are not expected to cause serious problems if the alkalis are introduced after prereduction as illustrated in Table 3. However, since the pellets contain relatively large

quantities of liquid phase due to the formation of alkali iron silicate slag, and are subject to compression load, they are expected to soften during reduction inside the blast furnace.

In most of the experiments carried out, the alkalis to silica ratio was varied; however, the initial gangue was kept the same. Figure 8 is given to illustrate the case when the silica content of the ore is modulated. It was calculated based on equilibrium between wus-tite, silica, and potassium oxide at 900 °C. Of course the formation of liquid phase and its relative amount depend on kinetic factors; however, the informations

given in Fig. 8 suggest the amount of liquid slag to be sensitive to the absolute amount of the acid gangues. Therefore acid pellets subject to the same level of alkali cycling are expected to have better resistance when the relative amount of gangues exceeds certain limit. The silica rich gangues may have a dual effect.

Besides its effect on liquid slag formation, the remain-ing silicate binding bridges, which are formed by ade-

quate firing, are roughly proportional to the silica content of the pellets and would be responsible for

absorbing the stresses which cause degradation. The result of this work showed that alkalis increase

the reducibility of acid pellets. It may be attributed

to their influence on swelling, and crack formation,

which enhance the mass transfer of the reducing gas between the iron oxide grains. Also, it had been sug-

gested alkalis and lime to help the metallization step, due to their effect on the nucleation and growth of

iron, when they are incorporated, as solid solutions, with wustite.10,11,18-20) The second claim may not

have a strong effect in our case, since SEM, and, elec-tron probe microanalyzer examinations showed the

alkali to be more concentrated with the slag. How-ever, a small amount of alkali, which was not detected,

may exist with the iron oxides and effect the metal-lization process.

VI. Conclusions

As a general mechanism formation of iron whiskers cannot adequately be accepted to explain all the causes of degradation of iron ore pellets due to cir-

culation and accumulation of alkalis in the blast fur-

nace. It seems the alkalis react with the silica rich

gangue and ferrous iron oxide to form primary liquid slag, at relatively lower temperature levels. Its for-

mation and movement between the iron oxide grains cause the binding phase to disintegrate during reduc-

tion.

When acid pellets were tested in the laboratory, and the alkalis were introduced in three ways relevant to alkalis' cycling in the blast furnace, they showed a

tendency to swell and crack due to formation of un-evenly distributed liquid slag coupled by hematite to

Fig. 8. Formation of alkali iron

based on the equilibrium

Fe0 in presence of iron.

silicate slag

between Si0

at 900 °C,

2, K20, and

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magnetite phase transformation. Formation of liquid

alkali iron silicates lowers the pellets resistance and

the phase transformation is likely to create stresses which may act as a driving force for cracking and swelling.

Inside the blast furnace the reduction process is accompanied by compression load. Therefore, the

formation of liquid alkali iron silicate slgas is expected to weaken the pellets. However, they are not likely

to swell, and crack because the first reduction steps to magnetite usually takes place in the upper parts of the furnace stack, where the extent of alkalis is a mini-mum. Softening may be the real cause of degrada-

tion when relatively large quantity of liquids are formed due to alkalis. Of course, the pellets resist-

ance is a function of both the alkalis and gangue quan-

tities, under similar reduction conditions. Minimi-zation of alkalis, which are cycling in the blast furnace, is always desirable. Also, there is a minimum amount

of gangue contents to ensure smooth operation and optimum behavior, which in turns is a function of

alkalis to silica ratio during reduction.

Acknowledgements

The author thanks Prof. W-K Lu, Stelco Professor,

Department of Metallurgy and Materials Science,

McMaster University, Hamilton, Ontario, Canada, for his useful suggestions and discussion.

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5) K. P. May and W-K Lu: AIME Ironmaking Proc, 37

(1978), 582. 6) T. ElKasabgy : Metallurgia ABM, 36 (1980), 228. 7) J. M. Steiler: "A Thermodynamic Study of Liquid K20-

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Research Article