SCIENCE CHINA Technological Sciences •Progress of Projects Supported by NSFC

November 2012 Vol.55 No.11: 3029–3035 doi: 10.1007/s11431-012-5003-6

Effects of alkaline earth oxides on precipitation behavior of metallic iron under CO atmosphere ZHAO ZhiLong, TANG HuiQing*, ZHANG Ben & GUO ZhanCheng State Key Laboratory of Advanced Metallurgy, University of Science and Technology of Beijing, Beijing 100083, China Received April 12, 2012; accepted May 31, 2012; published online August 30, 2012

In gaseous reduction of iron ore fines, alkaline earth oxides have profound effects on the precipitation behavior of fresh metallic iron on the particle surface. In this work, in situ observation was performed to reveal the influence of alkaline earth oxides on the precipitation morphology and micro-structure variation of fresh metallic iron from microscopic level by simulation of the gas-solid reaction condition on the surface of ore particles. Experimental results indicate that doping MgO in the particle surface can inhibit the reduction of iron oxide and however doping CaO, SrO and BaO promote; all alkaline earth oxides tested in this study can change the precipitation morphology of fresh metallic iron; minimum doping mole fraction of one oxide to inhibit iron whiskers growth ( N AO ) is related to its cation radius ( rA2 ) and its extranuclear electronic layers( nA ), which can 2

be expressed as N AO  1.3  10 r 5

2 A 2

nA 2  .

gaseous reduction, CO atmosphere, in situ observation, alkaline earth oxides Citation:

Zhao Z L, Tang H Q, Zhang B, et al. Effects of alkaline earth oxides on precipitation behavior of metallic iron under CO atmosphere. Sci China Tech Sci, 2012, 55: 30293035, doi: 10.1007/s11431-012-5003-6

Precipitation morphology of fresh metallic iron on the ore particle surface includes three types. They are layered crystal (dense iron layer), porous crystal and fibrous crystal during gaseous reduction of iron ore fines [1–8]. Layered crystal is formed under temperature less than 750°C and it needs a long reduction time. Porous crystal is formed under temperature more than 850°C and in this case, a very high reduction degree could be reached with the formation of very active fresh metallic iron and a sharp increase of powder viscosity to cause grain sticking. Under the temperature near 800°C, a lot of fibrous whiskers are formed on the particle surface and their formation increases the viscosity of fines and leads to fluidization stagnation which quickly spreads out over the whole fluidized bed. Therefore, analyzing the precipitation morphology and micro-structure evolution of fresh metallic iron during the gaseous reduction *Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2012

of iron oxide fines is of great significance. Komatina et al. [2] considered that variation of the precipitation morphology of metallic iron were mainly attributed to diffusion conditions and its nucleation in the reduction. Zhao et al. [9] discovered that variation of the fresh iron morphology on the particle surface depended heavily on its precipitation rate in the reduction stage of FeO→Fe. Moreover some other researchers investigated effects of some alkaline earth oxides on the gaseous reduction behavior of ore fines [10–16]; however they cannot visually reflect the influences of these oxides on the reduction due to the lack of experimental facilities and methods and a systematic research on these effects can not be given. In the present work, the sheet samples were prepared by a simulation of the gas-solid reaction condition on the surface of iron ore fines and in situ observations were then performed to investigate effects of some alkaline earth oxides on the precipitation morphology and the micro-structech.scichina.com

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ture evolution of the fresh metallic iron in the reduction. Changes of mineral structure and reduction rate in the reduction were studied by thermogravimetric (TG) method. The work was focused on revealing the influence of the alkaline earth oxides on the precipitation micro-behavior of fresh metallic iron in views of migration of iron atoms and oxygen atoms in the ferrite, nucleation of fresh metallic iron and its growth mechanism.

1 Experimental 1.1

Set-up

In situ observations were carried out using a high temperature hot stage (TS 1500 Linkam Co. UK) and a stereo optical microscope (SteREO Discovery.V20, Zeiss Co. Germany). Figure 1 schematically shows the experimental set-up. A thermal analyzer (STA PT 1600 Linseis Co. Germany) was used for TG analysis. 1.2

Sample preparation

Analytical reagents of Fe(NO3)3·9H2O, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, Sr(NO3)2, Ba(NO3)2 and deionized water were used. Mixture solution of Fe(NO3)3 and A(NO3)2 (A: Mg, Ca, Sr or Ba) was prepared in accordance with the predetermined ratio. the solution was then sprayed to a quartz slide (Φ=6 mm, h=1 mm, preheated for 10 min at 1000°C) at high temperature , and then the sample was cooled in the air to room temperature. Principle of sample preparation is that a quick decomposition of Fe(NO3)3 and A(NO3)2 into Fe2O3 and AO occurs at high temperatures, and AO is deposited on the quartz slide to form sheet sample of Fe2O3-AO. Reactions involved in sample preparation are  4Fe(NO3 )3  9H 2 O   2Fe 2 O3  12NO 2 (g)

3O 2 (g)  36H 2 O (g)

(1)

Figure 1 Experimental set-up of in situ observation for high temperature reduction. 1, Hot stage; 2, Stereo microscope; 3, Bottom light source; 4, Top light source; 5, Controller of hot stage; 6, Camera; 7, Control of PC; 8, Control interface of hot stage; 9, Cooling water; 10, Workaround of hot stage; 11, Gas inlet; 12, Gas outlet

 Ca(NO3 ) 2  4H 2 O   CaO  2NO 2 (g)

O 2 (g)  4H 2 O (g)

(2)

 Mg(NO3 ) 2  6H 2 O   MgO  2NO 2 (g)  O 2 (g)

6H 2 O (g)

(3)

 Sr(NO3 ) 2   SrO  2NO 2 (g)  O 2 (g)

(4)

 Ba(NO3 ) 2   BaO  2NO 2 (g)  O2 (g)

(5)

Figure 2 depicts the characteristics of a typical sheet Fe2O3-AO: length diameter D is some 30–150 µm; short diameter d some 10–60 µm and thickness h some 10–50 µm. 1.3

Experimental procedure

All Experiments were conducted using experimental apparatus shown in Figure 1. The quartz slide with Fe2O3-AO sample was placed in the high temperature hot stage and it was then heated at a rate of 60°C/min. When the predetermined temperature (800°C) was reached, a high-pure gas mixture of CO-CO2 (flow rate of CO was 40 mL/min and flow rate of CO2 was 10 mL/min) was introduced into the stage as reduction gas for 30 min. Thereafter the sample was cooled to the room temperature at a rate of 60°C/min. The whole reduction process was observed by the stereo optical microscope (1000x), and recorded at a rate of 1 sheet/s. The reduced sample was still subjected to SEM and EDS analysis. The Fe2O3-AO sheet sample of 10 mg was prepared for TG analysis under conditions of the same temperature and the same CO-CO2 ratio as those for in situ observation. The sample was heated to 800°C under the protection of high-pure N2 at 10°C/min and was kept at this constant temperature for 15 min. Then it was reduced for 30 min. Afterwards the reduction gas was switched back to high-pure N2 and the sample was cooled to the room temperature. By a combination of in situ observation and TG analysis, mechanism and influence of alkaline earth oxides on the fresh metallic iron morphology and its micro-structure evolution in the reduction were investigated.

Figure 2

Image of sheet sample under the stereo optical microscope.

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2 Results and discussion 2.1 Effects of alkaline earth oxides with the same concentration on the precipitation morphology of metallic iron Previous study on effects of MgO on the reduction of Fe2O3 showes that doping 2 mass% MgO in Fe2O3 can effectively change the precipitation morphology of metallic iron and completely eliminate the formation of iron whiskers as shown Figure 3(a). Hence, effects of different alkaline earth oxides with the same molar concentration (7.5 mole%) on metallic iron morphology during the reduction were studied in this section and Figure 3 gives the result of in situ observation. A wide comparison of these images obtained in in situ observation indicates that some iron whiskers formed in the sample of doping CaO while the growth of much more iron whiskers occurs in less than 5 min in the sample of doping SrO or BaO. Iron whiskers mainly generate in stage of FeO→Fe in the reduction of pure Fe2O3 sample and

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so it could be considered that doping MgO has a strong inhibition to the conversion of FeO→Fe and postpones the precipitation of metallic iron; doping CaO slightly restrains the nucleation of iron whiskers and however promotes their growth and accelerate the whole reaction; doping SrO or BaO is beneficial to reduction rate increase in the initial stage of metallic iron precipitation and promotes the formation of iron whiskers. SEM analysis of the reduced samples is given in Figure 4. It could be observed that fresh iron presents as layered crystal (dense iron layer), and no iron whisker is formed in the sample of doping MgO. Iron whiskers become a little denser in the sample of doping CaO than in the sample of doping MgO. Compared to the sample of doping CaO, number of iron whiskers slightly increase and they become dense in the sample of doping SrO. As the sample of doping BaO, the number of iron whiskers continuously increases and they grow even more densely. It still could be seen from the appearance of iron whiskers in Figure 4 that size of iron

Figure 3 In situ observation of effects of alkaline earth oxides with the same molar concentration on the precipitation morphology of metallic iron (7.5 mole%). (a) MgO; (b) CaO; (c) SrO; (c) BaO.

Figure 4 SEM images after the reduction for 30 min in the case of doping alkaline earth oxides with the same mole concentration. (a) MgO; (b) CaO; (c) SrO; (c) BaO.

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whisker reduces gradually with the order of doping CaO, SrO or BaO in the sample. Since the number of iron whiskers increases with the increase of molecular weight of the doped oxide, it is indicated that alkaline earth oxides with a fixed mole concentration cannot change the precipitation morphology of fresh metallic iron but with increase of the molecular weight of the doped oxide, its effect of on the reduction of iron oxide varies from inhibition to promotion. 2.2 Effects of alkaline earth oxides with different concentrations on the precipitation morphology of metallic iron Experimental results have indicated that alkaline earth oxides with a fixed concentration can not effectively change the precipitation morphology of metallic iron. Doping SrO or BaO with the same concentration has an excellent pro-

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motion effect for the formation of iron whiskers. Effects of alkaline earth oxides with different concentrations on the precipitation morphology transformations of metallic iron were studied in this section. Images of in situ observation in Figure 5 and SEM analysis in Figure 6 indicate that metallic iron after the reduction precipitates as layered crystal (dense iron layer) in the sample of doping 2 mass% MgO, which could effectively inhibit the formation and growth of iron whiskers. To achieve a similar effect, the needed quantity of CaO was increased to 8 mass%. As for the samples of doping SrO and BaO, most of reduced iron presents as layered crystal (dense iron layer) and few iron whiskers form when the doping quantities of SrO and BaO reach more than 25 mass% and more than 50 mass% respectively. Therefore doping any alkaline earth oxide can influence the formation and the growth of iron whiskers and effectively change the precipitation morphology of fresh metallic

Figure 5 In situ observation for effects of alkaline earth oxides with different concentrations on the precipitation of metallic iron after the reduction (mass%). (a) 2 mass% MgO; (b) 8 mas%CaO; (c) 25 mass%SrO; (d) 50 mass%BaO.

Figure 6 SEM images after the reduction for 30 min in the case of doping alkaline earth oxides with different concentrations. (a) 2 mass%MgO; (b) 8 mass%CaO; (c) 25 mass%SrO; (d) 50 mass%BaO.

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iron. The difference between them to achieve these effects is the needed doping quantity. For a doping oxide, the minimum needed quantity increases with the increase of its atomic number. It is therefore conjectured that the minimum needed quantity of an alkaline earth oxide to completely inhibit the formation of iron whiskers depends on its properties and the micro structure of prepared sample. 2.3 Effects of alkaline earth oxides on the reduction kinetics of iron oxide In situ observations (Figures 3 and 5) show that doping any alkaline earth oxide could change the precipitation morphology of fresh metallic iron during the reduction and could effectively inhibit the formation of iron whiskers. Growth of iron whiskers is related to the reduction rate [9] and thus the effects of doping alkaline earth oxides with the same mole concentration on gas-solid reduction kinetics were investigated here. TG curves of several samples in Figure 7 show that alkaline earth oxides notably influence the reduction behavior of iron oxide. In Figure 7, the time was recorded from the introduction of reduction gas and peaks at the instant 0 were due to gas switch in TG runs. Weight loss occurs at some 1 min. From a view of thermodynamics, weight loss of iron oxide in reduction is mainly a process of oxygen removal. The weight loss ratio is 3.3% in the complete conversion of Fe2O3→Fe3O4, 10% in the complete conversion of Fe2O3→ Fe3O4→FeO and 30% in the complete conversion of Fe2O3 →Fe. A combination of weight loss curves in Figure 7 and the phase transformation of iron oxide reduction indicate that doping CaO, SrO or BaO has little effect on the conversion of Fe2O3→Fe3O4→FeO and only has effect on the conversion of FeO→Fe. One exception is that doping MgO can inhibit the reduction in the late stage of Fe3O4→FeO. Conversion of FeO→Fe occurs at 2.2 min in the reduction of pure Fe2O3. In the counterpart in situ observation, iron whiskers begin to nucleate and grow at this instant. Weight loss rate of the sample of doping MgO obviously decreases from 3.1 min; however the reduction rate increases in the sample of doping of CaO, SrO or BaO with the same mole concentration and obvious turning-points on these curves appear at some 6 min, where the reduction rate suddenly drops. Most FeO has been reduced to Fe while the remaining FeO continues to be reduced but the reduction rate becomes very slow because the reduction of FeO needs to overcome a high activation energy. From the TG curves in Figure 7, it could be seen that the reduction proceeds very slowly at 30 min and that doping MgO can decrease the reduction degree of Fe2O3 while CaO, SrO and BaO with the same mole concentration can accelerate the reduction and increase the reduction degree. According to the weight loss estimation at reduction time of 30 min, the reduction degree of iron oxide reaches 87% for

Figure 7 TG analysis for the reduction of Fe2O3 after doping the alkaline earth oxides. (1) Fe2O3→Fe3O4; (2) Fe3O4→FeO; (3) FeO→Fe.

the sample of doping MgO, 73% for the sample of doping CaO, 93% for the sample of doping SrO, and 97% for the sample of doping BaO. A comprehensive analysis on results of TG analysis and in situ observation show that all alkaline earth oxides can inhibit the growth of iron whiskers though their mechanisms vary. MgO can inhibit the nucleation and growth of iron whiskers through restraining the conversion of FeO→Fe and the precipitation of metallic iron; CaO, SrO and BaO can eliminate the growth of iron whiskers through promoting the conversion of FeO→Fe and the precipitation of metallic iron. 2.4 Mechanisms of effects of alkaline earth oxides on the precipitation morphology transition of metallic iron From the results of in situ observations and TG analysis, it is concluded that doping any alkaline earth oxide has an obvious effect on the precipitation of metallic iron. Samples of doping alkaline earth oxides were then subjected to EDS analysis. Here the sample doped with MgO is taken as an example (Figure 8). Figure 8 shows that a uniform distribution of alkaline earth element changes the distribution of element Fe and reduces its distribution density. Such a pattern of element distribution is one of the influencing factors on iron whisker growth. XRD analysis was also conducted to all samples doped with alkaline earth oxides to understand the states of elements in them. Results are given in Figure 9. XRD patterns of different samples are satisfied to those of ferrates of Fe2O3 and the corresponding alkaline earth oxides respectively. XRD pattern of simple oxide as AO is not found in the samples. Schenck et al. [17] found mutual dissolution between MgO and wustite below 1371°C in the investigation for phase equilibrium of MgO-FeO. Fukuyama et al. [18], Fossdal et al. [19] and Goto et al. [20] proved that AxFeyOz could be formed under different temperatures by studying systems of CaO-Fe2O3, SrO-Fe2O3 and BaO-Fe2O3. Thus, it is demonstrated that, ionic compound of ferrate is

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Figure 8 SEM image for the sheet oxide iron with 2 mass% MgO and surface scanning image of elements.

growth of iron crystal to avoid formation and growth of iron whiskers. Moreover, doping alkaline earth oxides to form compounds as MgFe2O4, CaFe2O4, SrFe12O19 and BaFe12O19 decreases the total weight loss to some extents. Experimental results show that the minimum amount of doped oxide to inhibit the growth of iron whiskers depends on its molecular weight. The above analysis indicates that the quantity of doped oxide is related to the cation characteristics of the alkaline earth oxide. Comparison of the cation characteristics of alkaline earth oxides in Table 1 reveals that the minimum amount of doped oxide is related to its ion radius ( rA2 ) and its extranuclear electronic layers ( nA2 ).

Figure 9

XRD patterns of Fe2O3 after doping alkaline earth elements.

Mathematical analysis of the obtained experimental data indicates that the relationship of minimum mole fraction of the doped oxide ( N AO ), which can change the precipitation morphology of metallic iron, its cation radius ( rA2 ) and its

formed when AO is doped into Fe2O3 and the formation of new structure is another reason for influencing the precipitation morphology of metallic iron. Taking MgFe2O4 as an example. In its spinel structure, O2- is arranged in face-centred cubic style and Mg2+ and Fe3+ are respectively filled in the framework of O2, which contains 64 tetrahedral interstitial and 32 octahedral interstitial. rMg2 ,the effective radius of Mg2+ (Shannon radius [21])

extranuclear electronic layers ( nA2 ) can be expressed as N AO 2 A 2

r

rSr2 is 113 pm and rBa 2 is 135 pm. Mg 2+

(6)

nA2 could be deduced in accordance with eq. (6): N AO  1.3  10 5 rA22 nA 2 .

has a smaller

radius than Fe has and so it can enter [FeO6] to form solid solution with the lattice structure of Fe2O3 crystal being remained and thus a small doping quantity of MgO can effectively change the precipitation morphology of metallic iron and inhibit the formation of iron whiskers. However, radius of Ca2+, Sr2+ or Ba2+ is larger than that of Fe2+ and any one of them would destroy the existing [FeO6] lattice structure though they can be mutually soluble with ferrite and promote the reduction. Because Alkaline earth elements have the same charge capability, the atom with a comparatively large superficial area has less charge per unit area and a weaker adsorption of atoms Fe and O. Thus, it is only when enough doping quantity is reached that the doped alkaline earth oxide can restrain the migration and oriented

 Constant.

Thus, the following relationship between N AO and rA2 ,

is 65 pm, rCa 2 is 99 pm, rFe2 is 76 pm, rFe3 is 64 pm, 2+

nA2

(7)

From the above analysis, the mathematical relationship between the quantity of doped oxide and the precipitation morphology evolution of metallic iron can be obtained. Table 2 gives the results of comparison between results Table 1 Cation characteristics of alkaline earth oxides Element (A)

Ion radius (pm)

Mg2+

65

Extranuclear Molecular weight electronic layers n of oxide MAO 2 40

Ca2+

99

3

56

Sr2+

113

4

104

Ba2+

135

5

153

Fe2+

76





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Table 2 Comparing the quantities of doping oxides by formula and experiment Mineral oxide AxOy MgO

Quantity for calculation NAO

WAO (%)

Quantity for experiment W′AO (%)

0.078

2.062

~2

CaO

0.221

9.018

~8

SrO

0.332

24.417

~25

BaO

0.530

49.102

~50

of in situ observation (W′AO%) and results of calculation using eq. (7) (WAO%). The present empirical formula can well describe the relationship between the quantity of doped oxide and the precipitation morphology of metallic iron. Therefore, the mineral structure of iron ore and the characteristics of alkaline earth oxides are the main factors on the precipitation morphology of fresh metallic iron. The minimum quantity of doped oxide to change the precipitation morphology of metallic iron depends on its cation characteristics. The present study could be applied to control the surface morphology of ore particles after the reduction and to inhibit nucleation and growth of iron whiskers. Different processes can be developed on the basis on the content of mineral oxides in the ore fines.

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1.3  105 rA22 nA2 . This work was supported by the National Natural Science Foundation of China and Baosteel (Grant No. 50834007) and the National Basic Research Program of China (973 Program) (Grant No. 2012CB720401). 1

2

3 4 5

6

7

8

9

3 Conclusions 10

Doping alkaline earth oxides have obvious effects on the precipitation of metallic iron. Doping MgO can strongly inhibit the formation and growth of iron whiskers; doping CaO can postpone slightly the formation of iron whiskers, and afterwards, promote their growth; doping SrO or BaO can promote the nucleation and growth of iron whiskers at the initial stage of the reduction. MgO can only affect the lenth of iron whisker; however, CaO, SrO and BaO have a refining effect on whiskers size. Doping alkaline earth oxides can obviously influence the reduction. Doping MgO can decrease the reduction rate in the late stage of Fe3O4→FeO and inhibit the formation of iron whiskers by restraining the reduction; doping CaO, SrO and BaO can increase the reduction rate in the conversion of FeO→Fe, and eliminate the formation of iron whiskers by promoting the reduction. Doping alkaline earth oxides can change the precipitation morphology of metallic iron in the reduction. The precipitation morphology of metallic iron depends on the quantity of alkaline earth oxides. The minimum mole fraction of doped oxide ( N AO ) which can inhibit the growth of iron whiskers

15

is related to its cation radius ( rA2 ) and its extranuclear

21

electronic layers ( nA2 ), which is expressed as N AO 

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11 12

13

14

16 17

18 19

20

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