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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2013, 20(7) : 16-24

Effects of CaO on Precipitation Morphology of Metallic Iron in Reduction of Iron Oxides Under CO Atmosphere

ZHAO Zhi-long1'2, TANG Hui-qing1, GUO Zhan-cheng1

( 1 . State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China; 2. Iron and Raw Material Handling Division, MCC Capital Engineering and Research Incorporation, Beijing 100176, China)

Abstract : Growth process of iron whiskers and mechanism of CaO influence on precipitation morphology of metallic iron at the gas-solid interfaces was studied. Analytical reagents of F e ( N 0 3 ) 3 and C a ( N 0 3 ) 2 aqueous solution were used to prepare sheet film sample of Fe 2 0 3 -CaO by thermal decomposition at high temperature. In-situ observation was con­ducted using a stereo optical microscope and a hot-stage. And reduction kinetics of samples was studied by thermo gravimetric ( T G ) method. Some samples after reduction were analyzed by using the scanning electron microscope ( S E M ) , energy dispersive spectrometer (EDS) and fourier transform infrared (FT-IR) spectrometer. Results indi­cate that during the reduction of iron oxides with CO, metallic iron is mostly precipitated as whisker and the precipi­tation behavior mainly depends on reduction rate. Doping CaO can significantly increase the reduction rate and effec­tively change the precipitation morphology of metallic iron after the reduction. When CaO doping concentration is less than i% (mass percent ) , CaO can promote whisker formation of reduced iron; as it reaches 6% (mass per­cent) , CaO inhibits iron whiskers growth; as it is more than 8% (mass percent) , no whiskers could be observed. Therefore, controlling the quantity of Ca2+ is effective to control the formation and growth of iron whiskers during gaseous reduction and thus eliminating ore grain sticking caused by intertexture of iron whiskers. Key words : stereo optical microscope; in-situ observation; gaseous direct reduction; iron whisker; sticking

Gaseous reduction is a new technology for iron and steel industry in recent years. Fluidized bed re­duction of iron ore fines has greatly boosted the pro­ductivity with the advantage that ore fines could be directly supplied. However, ore particles sticking in reduction results in fluidization stagnation and it could spread out over the whole fluidized bed quick-jy[i-2]_ Therefore, sticking of ore fines is a serious problem and hinders the industrialization of fluidized bed reduction technology.

Many studies indicated that ore particles stick­ing related closely to the precipitation morphology of fresh iron phase and that it is caused by inter-parti­cle physical contact by iron whiskers or by chemical action of the fresh iron [2_6]. In addition to reduction temperature, gas velocity, reduction gas type and

particle size, some components in iron ore (especially CaO) are considered to be a significant factor. Some researchers considered that CaO could promote the growth of iron whiskers from the study of CaO in­fluence on metallic iron precipitation in the pellet re­duction behavior^7-11-1. On the contrary, others con­sidered that CaO may have negative influence from the study on influence of CaO doped into ore parti­cles on the ore particle sticking during gaseous re­duction1-2'11-12-'. Comprehensively, the existing re­sults and methods cannot adequately describe effects of CaO on the precipitation of fresh metallic iron and therefore the mechanism of CaO influence on the gas-solid interface deserves some further investigations.

In this work, sheet-like Fe203 samples were prepared by simulation of the gas-solid reaction con-

Foundation Item: Item Sponsored by National Natural Science Foundation of China (50834007) ; National Basic Research Program of China (2012CB720401)

Biography:ZHAO Zhi-long(1984—), Male, Doctor; E-mail: zzll23222@126.com; Received Date: April 16, 2012 Corresponding Author : TANG Hui-qing(1970—), Male, Associate Professor; E-mail: hqtang@ustb.edu.cn

Issue 7 Effects of CaO on Precipitation Morphology of Metallic Iron in Reduction of Iron Oxides · 17 ·

dition on the surface of iron ore fines and micro-scale in-situ observations were then performed to s tudy effects of CaO on precipitation morphology and mi­crostructure development of fresh metallic iron dur­ing iron oxide reduction using CO. It is tried to dis­close the mechanism of CaO influence on precipitati­on morphology of metallic iron at the gas-solid inter­face. T h e research could provide some theoretical basis for unders tanding and resolving sticking caused by interlaced iron whiskers .

1 Experimental 1.1 Experimental set-up

In this study» in-situ observat ions were carried out using high tempera ture hot s tage and stereo op­tical microscope. Fig. 1 schematically shows the ex­perimental set-up. A thermal analyzer was used for the characterization of reduction r a t e , reduction de­gree , etc.

Control of PC

Quartz substrate

Gas inlet

High temperature hot stage Bottom light source

Fig. 1 Experimental set-up of in-situ observation

1. 2 Sample preparation Analytical reagents F e ( N 0 3 ) 3 · 9 H 2 0 , C a ( N 0 3 ) 2

and deionized water were used. Mixture solution of F e ( N 0 3 ) 3 and C a ( N 0 3 ) 2 was prepared in accordance with a predetermined ratio. A quartz slide with di­ameter of 6 m m and thickness of 1 m m was then placed on a refractory br ick , heated to 1073 K , and cooled to about 773 K under air a tmosphere . The re ­after the solution was sprayed rapidly on the quartz slide. After the solution evaporated, the sample was roasted at 1 273 K for 5 min to react completely and became compact. T h e n it was put in air and cooled to room temperature. In this study, six samples with CaO mass percent of 0—10% were prepared.

Mechanism of sample preparat ion is that a quick decomposition of F e ( N 0 3 ) 3 and C a ( N 0 3 ) 2 into Fe 2 0 3

and CaO takes place at high t empera tu re , and Fe2 0 3

and CaO are thus deposited on the quartz slide to form sheet like F e 2 0 3 - C a O sample. Fig. 2 shows the characteristics of a typical sheet F e 2 0 3 - C a O sample :

Fig. 2 Image of sheet Fe203-CaO under stereo optical microscope

long diameter is some 30 —150 μπι , short diameter is some 10—60 μπι , thickness is some 5 — 10 μπι and flatness is some 10.

1.3 Experimental procedure Exper iments were conducted using the experi­

mental appara tus shown in Fig. 1. A sample was placed in the high tempera ture hot s tage and the hot s tage was put under the stereo optical microscope. In the top and the bo t tom of hot s t age , high-bright­ness light sources were equipped. With the help of these light sources , color change of the sample can reflect s t ructura l evolution of iron oxides because of the high light t ransmit tance for the quartz slide and the sample. In addi t ion, g rowth process of iron whiskers can be monitored on line th rough their g rowth screenage with the help of the light source on the b o t t o m , due to the multi-directivity for the g rowth of iron whiskers .

The hot stage was heated at 60 K/min. When reac­hing the predetermined temperature, the reduction gas (mixture of CO and N2 ) was introduced. The reduc­tion process was observed using the on-line stereo optical microscope and was recorded one sheet every second. T h e in-situ observation lasted for 30 min. Some samples after in-situ observations were subjected to scanning electron microscope ( S E M ) , energy dis­persive spectrometer ( E D S ) and Fourier t ransform infrared spectrometer ( F T - I R ) analysis.

Under the same reduction conditions of temperature and CO concentration, an F e 2 0 3 - C a O sheet sample of 10 mg was used for T G analysis. Exper iment was done in a heating rate of 10 K/min and the pressure of 0. 1 MPa. T h e sample was heated to 1073 K in the protection of highly pure N 2 , and kept at 1073 K for 15 min , then reduced for 30 min in the reduction

• 18 · Journal of Iron and Steel Research, International Vol. 20

gas. Afterwards, the reduction gas was switched back to highly pure N2. After being kept at 1073 K for 15 min, the sample was cooled down to room tem­perature.

By a combination of in-situ observations and TG analysis, mechanism and influence of CaO on the particle morphology and microstructure evolution in gaseous reduction of iron oxide were analyzed.

2 Results and Discussion 2.1 Precipitation behavior of fresh metallic iron

Pure Fe203 sample was adopted to study the precipitation behavior of fresh metallic iron under different temperatures and CO concentrations. Opti­cal images in Fig. 3 (a) to Fig. 3 (o) show the effect of temperature on iron whisker growth. As temper-

Metallic iron is mainly precipitated as fibrous whisker or as little layered crystal (dense iron layer) in reduction using CO. Fresh metallic iron morphol­ogy is determined by reduction rate, which depends mainly on reduction temperature. In the reduction of wüstite with pure CO, the morphology of precipita­ted iron was related to gaseous reduction potential

atures increase, growth rate of iron whiskers signifi­cantly increases and their length also becomes much larger. At 973 K, iron whiskers grow slowly and slightly until 10 min; at 1073 K, iron whiskers grow fast, and grow noticeably at about 5 min, but nearly stop growing at some 30 min; at 1173 K, iron whiskers generate after some 3 min but grow fastest afterwards, and completely grow in only 10 min. Opti­cal images in Fig. 3 (a) to Fig. 3 (e) and Fig. 3 (p) to Fig. 3 (y) show effect of CO concentration on iron whisker growth. Number of iron whiskers and their length both increase with the increase of CO concen­tration. However, iron whisker formation time is almost fixed though CO concentration varies. Com­pared to temperature, CO concentration has small effects on iron whisker growth.

and temperature[11]. Considering effects of reduction temperature and at­

mosphere on iron whisker formation, its growth rate and the record-ability of online monitoring appara­tus , growing behaviors of iron whiskers were inves­tigated under reduction condition of 1073 K and 65% CO (volume percent) in this study.

( a ) , ( b ) , ( c ) , ( d ) , ( e ) 973K, yco = 65%; ( 0 ,(g) ,(h) ,(i) ,(j) 1073 K, ^ 0 = 65%, (k) ,(1) ,(m) ,(n) ,(o) 1173 K, yco = 65%! ( p ) , ( q ) , ( r ) , ( s ) , ( t ) 1073 K, çco = 50%; (u) , (v) ,(w) , (x) ,(y) 1073 K, yco = 80%i (a) ,(f) , (k) , (p) , (u) Orniti;

( b ) , ( g ) , ( l ) , ( q ) , ( v ) 5 mini (c) ,(h) ,(m) , (r) ,(w) 10 mini (d) , ( i ) , ( n ) , ( s ) , ( x ) 15 min, (e),(j) , ( o ) , ( t ) , ( y ) 30 min. Fig. 3 In-situ observations of reduction of Fe203 under conditions of different temperatures and CO doping concentrations

Issue 7 Effects of CaO on Precipitation Morphology of Metallic Iron in Reduction of Iron Oxides · 19 ·

On the basis of in-situ observations, it is assumed that iron oxide became dark in color at 4 min, and area of the sample gradually expanded and reached its maximum after 30 s; thereafter, the sample started to shrink and iron whiskers started to generate (Fig. 4). In Fig. 3 , the sample color during reduction changes due to the conversion of Fe2 0 3 -»■ Fe3 0 4 . Fe203 is trigonal system while Fe304 is tetragonal system. The sample undergoes a volume expansion in conversion of Fe2 0 3 -*· Fe3 0 4 . It conforms to the reduction law of iron oxide. In conversion of Fe304

-»-FeO, the sample color does not change; however, its volume notably shrinks. Results of in-situ obser­vations indicate that the rate of Fe304-*-FeO conver­sion is faster than that of Fe2 03-»-Ρε3θ4 conversion

and that the rate of FeO-»-Fe coversion is the slo­west. Besides, during reduction, some parts of sheet samples crack, which is the formed Fe304

cracks in (001)H II ( 1 1 1 ) M due to the increase of in­ternal thermal stress[13]. From results of in-situ ob­servations and step-by-step sampling, it is concluded that iron whiskers do not generate in the stages of Fe2 03-*-Fe3 04 and Fe304-^-FeO but only in the stage of FeO-»-Fe in CO reduction of Fe203 . Fig. 5 shows that iron whiskers consist of metallic iron (Point 2 ) , and its growth substrate is FeO (Point 1). Thus, it is confirmed that in the conversion of FeO-*Fe, Fe at­oms reduced from FeO migrate on the sample sur­face and iron whiskers continuously form and grow owing to crystal orientation of metallic iron phase.

(a) 4 min; (b) 4. 5 min; (e) 5 min; (d) 10 min; (e) 30 min. Fig. 4 Sequential SEM images of Fe203-»Fe304-»FeO-*Fe conversion in reduction

1500

1000

500

(b) F

--O

Fe

L.^

e

Fe Λ

Point 1

(a) SEM image; (b) EDS analysis of Points 1 and 2 in (a). Fig. 5 Growth of iron whiskers in reduction of iron oxides at reduction time of 10 min

• 20 · Journal of Iron and Steel Research, International Vol. 20

2. 2 Effects of CaO on iron whisker growth Based on the above experimental results, sam­

ples with CaO of 2% to 10% (mass percent) were

Comparing the reduction of Fe203 before and after doping CaO, it is found that there are no sig­nificant difference for the time to form iron whiskers between with 2% (mass percent) CaO and without CaO, and under both conditions, iron whiskers be­gin to grow during 5 — 6 min. In the samples of do­ping 4% (mass percent) CaO to 6% (mass percent) CaO, the growth of iron whisker is delayed, which generates at some 10 min. Thus, in the course of Fez03 reduction after doping CaO, the nucleation of iron whiskers could be postponed with the increase of CaO.

The growth rate of iron whisker rises fast, and the whole growth period becomes short although on­set nucleation of whisker is slightly delayed after do­ping CaO. Results of in-situ observations show that there are no significant changes on the morphology of iron whiskers after reduction for 15 — 20 min.

Moreover, CaO has an obvious influence on the

reduced under 1073 K and 65% (volume percent) CO atmosphere. Results of in-situ observations are shown in Fig. 6.

number and size of iron whiskers. From Fig. 6, it is concluded that after reduction, number of iron whiskers increases when CaO doping ratio is less than 4% (mass percent) and decreases when it is more than 6% (mass percent). Iron whisker could not be observed within 30 min when CaO doping ra­tio is more than 8% (mass percent). Fig. 7 shows SEM images of samples with different CaO doping concentration after reduction for 30 min. From Fig. 7, It could be measured that iron whisker has a diameter of about 1 —1.2 μπι and a length of more than 10 μτη in the sample without CaO. In the sam­ple with 2% (mass percent) CaO, iron whisker is much slender and short, and has a diameter of about 0. 8 μπι and a length of about 3 — 5 μπι. In sample with more than 4% (mass percent) CaO, iron whisker becomes so tiny that iron whisker cannot be identified using the stereo optical microscope. Therefore doping CaO can inhibit the morphology of

( a ) , ( b ) , ( c ) , ( d ) , ( e ) tuco = 0; ( f ) , ( g ) , (h ) , ( i ) , ( j ) «;c,o = 2%i ( k ) , ( l ) , ( m ) , ( n ) , ( o ) w;caO = 4%i ( p ) , ( q ) , ( r ) , ( s ) , ( t ) ™c.o = 6%; (u) ,(v) ,(w) ,(x) , (y) ^ c o = 8%i (a) ,(f) ,(k) ,(p) ,(u) Omini

( b ) , ( g ) , ( l ) , ( q ) , ( v ) 5 mini (c) , (h) , (m) , (r) , (w) 10 min, (d) ,(i) ,(n) ,(s) ,(x) 15 mini (e) ,(j) ,(o) , ( t ) ,(y) 30 min. Fig. 6 In-situ observations for influences of CaO doping concentration on precipitation morphology of metallic iron

Issue 7 Effects of CaO on Precipitation Morphology of Metallic Iron in Reduction of Iron Oxides · 21 ·

(a) wc.o = 0! (b) wco = 2%i (c) wco = 4%; (d) iuco = 6%i (e) zt/co = 8%. Fig. 7 SEM images of samples with different CaO doping concentrations after reduction for 30 min

iron whisker and make it fine.

2. 3 Effects of CaO on kinetics of F e 2 0 3 reduction In-situ observat ions in the above section show

that the precipitation morphology of metallic iron could be changed by doping CaO into the sample. In accordance wi th the above sect ion, the precipitation style of the fresh metallic iron is related to reduction rate. T h u s , effects of CaO on the reduction kinetics of the gas/sol id phase were studied here. T h e sheet sample is separated from the quartz s l ide, and then sample of some 10 mg was used in each T G test . T G resul ts are shown in Fig. 8.

In each T G r u n , t ime was recorded from the in­troduction of reduction gas and peak at the instant 0 was due to gas swi tch . M a s s loss occured at some 1 min. From a view of the rmodynamics , mass loss of

0

-5

-10

| - 1 5

-20

-25

-30

-

-

I 1

1

V 2

1 3

^ » » — ==*=; ί * " " · " —

*-

■Fe-ìOa • WCaO=2% A wcfco~4% ▼ H'caO-8%

10 15 Time/min

20 25 30

1—Fe203—Fe304i 2—Fe304—FeOi 3-FeO^Fe. Fig. 8 TG results reflecting effects of CaO on Fe203 reduction

iron oxide in reduction is mainly a process of oxygen removal. T G curves and the s t ruc ture of iron oxides are correspondingly analyzed. T h e mass loss rate is 3. 3 % in the complete conversion of F e 2 0 3 - » - F e 3 0 4 , 1 0 % in the complete conversion of Fe 30 4-»*FeO and 3 0 % in the complete conversion of FeO-*-Fe. The re ­fore, the conversion of Fe203—*-Fe304 takes place at 1 min , and this process lasts for 30 s. In this case, doping CaO has no effects on the reduction rate. T h e sample for T G reduction loses mass slightly earlier than that of the result of in-situ observation. T h a t is due to the different reactor construction of gas circuit.

T G curves present inflection points and the de­crease of slopes at 1. 5 min owning to the conversion of Fe 3 0 4 -»"FeO, which is endothermic react ion, and such stage lasts for 40 — 50 s.

T G curves exhibit differences from one another at 2. 2 min , and F e O ^ - F e conversion proceeds since that point. Mass loss is obviously accelerated in the CaO doped samples. In in-situ observation countpar-t s in above sect ions , iron whiskers begin to nucleate and grow in this stage. Reduction rate increases quickly wi th the increase of CaO in te rm of T G curves. T G curves of the samples wi th and wi thout CaO all become flat at about 6 min. At this mo­ment , most of iron oxide in sample is reduced to me­tallic iron while the remaining FeO continues to be reduced but the reduction rate is very slow. T h e g rowth of iron whiskers nearly s t o p s , and the newly produced iron precipitates as layered crystal (dense iron layer ) .

• 22 · Journal of Iron and Steel Research, International Vol. 20

In addit ion, the reduction rate obviously increa­ses after doping CaO. It is indicated that CaO has posi­tive effects on reduction. This result is represented as reduced period from nucleation to full g rowth for iron whiskers in the case of in-situ observation.

A t reduction for 30 min , conversion of Fe 2 0 3 -» -F e 3 0 4 is faster than tha t of F e 3 0 4 - * - F e O , and con­version of FeO-*-Fe t ransforms most slowly. During FeO-»-Fe conversion, new s t ruc ture may form be­cause of doped CaO and could influence the reduction rate. Based on the T G resu l t s , as CaO doping con­centration increases , reduction rate progressively in­creases , and reduction degree progressively increases as well. According to the total mass loss est imation of T G t e s t , the reduction degree of the sample wi th­out CaO is 8 2 % , tha t of the sample of doping 4 %

(mass percent ) CaO increases to 9 3 % , and that of the sample of doping 8% ( m a s s percent ) CaO rea­ches 9 7 % .

2. 4 Mechanism analysis of effects of doping CaO on precipitation morphology transition of metallic iron after reduction

By investigating effects of CaO on the Fe2 0 3 re­duction at 1073 K , it is found tha t metallic iron is mainly precipitated as the fibrous whisker . Doping adequate amount of CaO into F e 2 0 3 can avoid the formation of iron whiskers . This is because disper­sive Ca2 + breaks the continuity s t ruc ture of Fe2 0 3 , which notably changes the distr ibution of element Fe on the sample surface and reduces its density pro­file. Fig. 9 shows the density profiles of elements Fe

(a) Fe2C>3 s (b) Density profile of Ca; (c) Density profile of Fe. Fig. 9 SEM images of Fe203 sheet sample with 8% (mass percent) CaO

and Ca of the sheet sample with 8% (mass percent) CaO. It is conjectured tha t the distr ibution of ele­ment Fe is an important factor tha t influences the transformation of precipitation morphology of metal­lic iron on the sample surface.

H S C thermodynamic calculation chemical soft­ware was used to simulate the preparat ion condition in the at tendance of 8% (mass percent) CaO at high temperatures . Solid phases included F e 2 0 3 , C a O , F e O , CaO · F e 2 0 3 , 2CaO · F e 2 O s , C a F e 3 0 5 and C a F e 5 0 7 etc in this reaction system. Howeve r , the calculation indicated main products were 89. 5 % F e 2 0 3 and 9. 3 % CaO · F e 2 0 3 at 1273 K , and others were negligible, such as 0. 7 % 2CaO · F e 2 O s , 0. 3 % CaO and so on.

Results of FT-IR analysis of the sample with 8% (mass percent) CaO and a simple mixture wi th CaO and F e 2 0 3 are given in Fig. 10. By compar ing, it is found tha t the H 2 0 peaks of 1474 and 3 396 c m - 1 in the sample are absen t , and the sharp peaks of C a - 0 and F e - 0 in 1110 and 780 c m - 1 disappear, in especial,

400 800 1600 2400 3200 4000 v/cirT

1—Fe20ä + CaOi 2—Sample with 8% (mass percent) CaO. Fig. 10 FT-IR analysis results of two different samples

the flat peak of F e - 0 in 400 — 600 c m - 1 changes into the sharp peaks of 556 and 479 c m - 1 , which are the vibration absorpt ion peaks of [ F e 0 6 ] in the spinel structure1-14-1. There fore , it can be seen tha t samples are not the physical mix tures of M g O and F e 2 0 3 but the compound of Ca^Fe^O*. Similarly, the doped CaO could enter into F e 2 0 3 lattice forming solid so-

Issue 7 Effects of CaO on Precipitation Morphology of Metallic Iron in Reduction of Iron Oxides · 23 ·

lutionat 1073 KC15]. Fig. 6 shows that when CaO doping ratio is 2%

(mass percent) , iron whiskers begin to form around 5 min for reduction. Thus, when doping 2% (mass percent) CaO into the sample, Ca2+ has little effect on iron whisker formation during reduction because of a small amount of CaO. Atom Fe after the reduc­tion still could migrate on the surface and link each other to achieve crystal orientation. With the in­crease of CaO doping ratio, Ca2+ gradually prohibit migration of Fe atoms. When the mass percent of CaO reaches 4 % — 6% , iron whiskers generate at

In the spinel structure of Ca^Fe.,0* with the vertex of O 2 - , because Ca2+ (99 pm [ l t ]) has a larger radius comparing with Fe2+ (76 pm) , it is prone to destroy the structure of Fe-O, and thus to promote the diffusion of O 2 - . Therefore, the diffusion resist­ance of O 2 - is much larger than that of Fe2+ under the condition of a small doping quantity of CaO, and doped Ca2+ has an inhibition effect of iron whisker nucleation but promotes iron whisker growth. In the case of a large quantity of CaO, the diffusion resist­ance of Fe2+ is much larger than that of O 2 - , and the doped CaO has a better boost to the FeO reduc­tion, and reduction degree rises to 97% from 82%.

In conclusion, in the reduction of iron oxides with CO, the precipitation morphology of metallic iron can be controlled with the quantity of doped CaO. Therefore, particular technology for the fluidized-bed reduction can be developed in accordance with the content of CaO and its existing form in iron ore in practice. Moreover, iron ore surface properties can be modified by doping CaO into its surface to avoid sticking in the gaseous reduction. In fact, it is proved that this method is feasible by the fluidized

some 10 min. Diameter of iron whisker becomes smal­ler. This is because homodisperse of Ca2+ in FeO re­strains local diffusion of Fe atoms. When CaO doping ratio reaches 6% (mass percent), Ca2+ begins to in­hibit the quantity of iron whiskers. Because the mi­gration of Fe atoms is restricted by doped Ca2+ and iron whiskers decrease significantly, and therefore the precipitation morphology of metallic iron turns into bead (particle) but not iron whisker (Fig. 11). As CaO doping ratio increases to above 8% (mass percent) , Ca2+ completely inhibits the migration of Fe atoms and iron whiskers entirely disappear.

bed in laboratory scale tests.

3 Conclusions 1) At 1073 K, in CO reduction of Fe 2 0 3 , re­

duced iron is mostly precipitated as whiskers, and a small quantity of Fe is precipitated as layered crystal (dense iron layer). Moreover, such precipitation ways concern with reduction rate, which depends predominantly on the temperature.

2) Under CO atmosphere, iron whiskers are formed and grow continuously in the stage of FeO-»-Fe conversion. With an increase of temperature, number and length of iron whiskers increase respec­tively, however, their diameters remain about 1 μΐη.

3 ) Doped CaO has obvious effects on the growth of iron whiskers. When CaO doping ratio is less than i%, Ca2+ can promote the growth of iron whiskers, and when it reaches 6 %, Ca2+ can inhibit the growth of iron whiskers. Moreover, CaO can change the length of iron whisker and make it fine.

4) Doped CaO can effectively influence the re­duction of iron oxides. In Fez03->*Fe30,i-»*FeO con­version, Ca2+ has little effects on the reduction. In

(b)

O " Fe

A Ca Fe . . F e

Point 1

EfkeV

(a) SEM image; (b) EDS analysis of Points 1 and 2 in (a). Fig. 11 Formation of iron whiskers and distribution of CaO in sample with 6% (mass percent) CaO

• 24 · J o u r n a l of I r o n a n d S tee l R e s e a r c h , I n t e r n a t i o n a l Vo l . 20

the conversion of FeO-*-Fe, doped Ca2+ has a re­markable promotion on the reduction by breaking the lattice structure of FeO and facilitating the diffu­sion of O 2 - .

5) Doped CaO can effectively change the precip­itation behavior of metallic iron after the reduction of iron oxides. Dispersive Ca2+ can change the dis­tribution of atoms of Fe and O, and effectively block the oriented migration of Fe2+ , which can enable the nucleation of iron whiskers to be properly delayed. With the increase of CaO, metallic iron transforms gradually into layered crystal ( dense iron layer ) from the whisker. Iron whiskers completely disap­pear until CaO doping ratio is 8% (mass percent).

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