Effect of oolite structure on direct reduction of oolitic iron ores

International Journal of Mineral Processing, 24 (1988) 55-71

55

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

E f f e c t o f O o l i t e S t r u c t u r e on D i r e c t R e d u c t i o n o f Oolitic Iron Ores Y. ZIMMELS, S. WEISSBERGER and I.J. LIN

Department of Civil Engineering, Technion HT, Haifa 32000 (Israel) (Received May 7, 1987; accepted after revision December 14, 1987)

ABSTRACT Zimmels, Y., Weissberger, S. and Lin, I.J., 1988. Effect of oolite structure on direct reduction of oolitic iron ores. Int. J. Miner. Process., 24: 55-71. The internal structure of iron-bearing oolites depends on their size. Smaller size involves a denser structure of concentric shells consisting of 1-2 ttm goethite particles as well as higher porosity of pores larger than 10 ttm. It is shown that the smaller shell spacing and higher porosity enhances the oolite amenability to direct reduction and also to growth of the metallic phase. Results of tests performed on oolites from different sources confirm these findings.

INTRODUCTION

Oolitic iron ores have been known for many years as a source of raw iron for the steel industry. This type of ore is currently exploited from several deposits around the world, for example in the U.S., France and in F.R. Germany. The amenability of iron oxide particles, which are embedded in the oolite matrix, to direct reduction bears on the technological feasibility and economic viability of the iron concentration and recovery processes. It is known that the oolites have the shape of ovals, i.e. ranging from spheres to elliptical bodies. The structure of a typical oolite consists of a core (mainly goethite) surrounded by 510/~m thick concentric shells having alternating dark and lighter brown colours (Rohrlich et al., 1980). Each shell comprises 1-2/lm polycrystalline flaky aggregates of goethite which are arranged parallel to each other as well as the shell contour (Rohrlich, 1974; Rohrlich et al., 1980). Typical oolites contain 49-54% Fe in the form of finely divided ferric oxides. Extraction of this iron by direct reduction requires that the iron be reduced to Fe °, and that the growth of the reduced phase be sufficiently large to make it amenable to magnetic separation. Since the original particle size of iron oxides is likely to be in the 1 - 2 / l m range, growth of one to two orders of magnitude 0301-7516/88/$03.50

© 1988 Elsevier Science Publishers B.V.

56

is required for low intensity magnetic separation to be efficient (Weissberger and Zimmels, 1983). This work sets to show that the oolite structure influences significantly its amenability to direct reduction and the subsequent magnetic separation. For this purpose several samples of oolitic iron ores which were obtained from different local and overseas sources were investigated, but most of the data was obtained using oolites from the Ramim deposit. This relatively lean ore contains 26-28% Fe occurring as goethite which form the typical concentric shell structure in 0.05-1 mm oolites. The results (Weissberger, 1984) show that oolite structure, as expressed by shell geometry and the distance between shells, influences the degree of reduction as well as the growth of the metallic phase, and hence the efficiency of the magnetic separation. In general it was found that compact shell structures enhance direct reduction and subsequent iron recovery whereas the reverse is expected with larger shell spacing. EXPERIMENTAL (WEISSBERGER, 1984)

Materials

The Ramim oolites were concentrated from the ore to virtually clean concentrates by dissolving carbonates (mainly C a C Q ) in acetic acid, by separation of clays using K-oxalate, and finally applying density separation in TBE (Tetra-Bromo-Ethane, p = 2.92 g/cm3). Chemical analysis of the oolites is given in Table I, and their size distribution in Table II. Other samples of oolites TABLEI Composition of Ramim, Wisconsin, Dogger fl and Lorraine oolites Components

Wisconsin

Dogger fl

Lorraine

Ramim

Fe2Q Si02 A1203 CaO P205 MgO MnO TiO2 S03 L.O.I.

73.4 3.5 5.3 4.3 3.0 0.2 0.2 0.7 0.2 9.7 100.5

69.5 4.7 4.2 3.6 2.8 0.5 0.2 0.7 0.2 13.4 99.8

71.2 5.5 4.4 2.3 1.9 0.2 0.2 0.5 0.1 13.3 99.6

72.7 3.8 5.5 1.8 1.2 0.7 0.2 0.5 0.3 13.5 100.2

0.1 1.9

1.7

0.3 1.9

CO2 Moisture

0.5 1.8

57 TABLE II Particle size distribution of Ramim oolites Weight (%)

Average particle size of fraction (#m)

Size fraction(mesh)

0.1 4.9 40.6 14.8 11.9 7.7 10.0 10.0

920.0 630.5 358.5 273.5 230.0 179.5 111.5 59.0

-16+ 20 -20+ 40 -40+ 50 -50+ 60 - 60 + 70 - 70 + 100 - 100+200 - 200 + 325

tested in this work (see their chemical analysis in Table I) were received from Lorraine in France, Dogger fl in F.R. Germany and Wisconsin in U.S.A. It is seen t h a t oolites from different sources have approximately the same chemical composition with respect to Fe203 (69.5-73.4%) and gangue material (oxides and L.O.I.) contents. The fact t h a t most of the iron appears as goethite was found to be a common feature of all oolite samples. Low percentages of hematite were observed by X-ray diffraction in overseas oolites (i.e. the ones from Lorraine, Dogger fl and Wisconsin). The major gangue minerals were identified, as kaolinite, hydroxyapatite, quartz, anorthite (Dogger fl) and dolomite (Wisconsin).

Porosity measurements Porosity was measured using a 60,000 Psi Aminco mercury porosimeter. Pore size dictated the pressure range used, i.e. vacuum for large 12-170/lm pores, and high pressure (up to 60,000 Psi) for the ones ranging down to 30 ~,.

Scanning electron microscopy A J E O L J S M 35-C scanning electron microscope was used for backscattered electron imaging (BEI) analysis of oolite structure. This technique made it possible to discern between iron oxides and gangue minerals.

Procedure for direct reduction tests 5 g of 10-mm pellets comprising oolites and 2% bentonite (used as a binder) were placed in a covered 50-mm (top diameter) alumina crucible, under an 8g layer of anthracite ( - 100 # ). The crucibles were placed in an electric furnace and heated to a preset temperature, the residence time at this temperature

58 being 30 min. Upon completion of the reduction, the furnace was turned off, cooled down to 150 °-200 ° C, and the crucibles were taken out and placed in a desiccator for final cooling. RESULTS AND DISCUSSION (WEISSBERGER, 1984; WEISSBERGER ET AL., 1986)

Internal structure of the Ramim oolites BEI analysis of polished sections, prepared from raw oolites, showed a characteristic internal structure which is size-dependent. In - 1 6 + 20 # oolites, shell width and shell spacing were found in the 3-4 pm and 2-3 pm ranges (see Fig. 1 ), respectively. These data were obtained for shells observed half the distance between the core and the rim of the oolite. Single goethite crystals were also identified and one which is ca 1.5 pm in size, and having a typical wedge shape, is encircled in Fig. 2. Note that the 3-4 pm wide shells are made of the 1-2 pm goethite particles. In - 20 + 40 # oolites (see Fig. 3 ) shell width and shell spacing decrease to ~ 3 #m and ~ 2 pm, respectively. Concentric shell geometry and a 2-/xm shell spacing are approximately maintained in - 40 + 50 # oolites (see Figs. 4 and 5, respectively). Higher magnification reveals that the internal structure of - 4 0 + 5 0 # oolites varies from the rim to the core. BEI analysis of other - 40 + 50 # oolites showed variation of structure within the oolites. 5-10 #m wide shells which are maintained at the rim (Fig. 6) turn into a network of shell fragments ( ~ 2/2m wide ) when the oolite core is approached (see Fig. 7 ). The dark area, i.e. the core, in the centre of the oolite (Fig. 7) contains gangue minerals. SEI scanning (Fig. 8) was also performed on the same area as shown in Fig. 7. This technique provides data on the morphology of the polished surface. The dark areas which can be seen in Fig. 8 are believed to be holes formed due to the removal of softer gangue minerals in the polishing process. In yet smaller oolites the network of shell fragments expands from the core towards the rim. This is clearly seen by comparing Figs. 9 and 10, which were taken on polished sections of - 7 0 + 100 # oolites, and Figs. 4-7. No concentric shells could be identified in - 1 0 0 + 325 # oolites which comprised only shell fragments up to the oolite rim (see Figs. 11 and 12) in which the interfragment spacing d was estimated at d ~<1 pm. Since size reduction of oolites does not change their shell spacing, it is clear that shell spacing of crushed oolites is larger compared with shell (or shell fragment) spacing in uncrushed oolites when both the crushed and the intact oolites have the same size. Thus in a mixture of - 100 + 325 # oolite particles those which are originally of this size will have ~ 1 ttm spacing of shell fragments (see Fig. 12) whereas those originating from - 2 0 + 6 0 # oolites have ~ 2/xm shell spacing (see Fig. 13). Fig. 14 depicts this observation schematically. As shown later the dependence of internal structure of oolites on their

59

lOpm Fig. 1. BEI micrograph of Ramim oolites, - 16 + 20 # , X 760.

1,urn i-..---i Fig. 2. BEI micrograph of Ramim oolites, - 16 + 20 # , X 5470.

lO~um i

Fig. 3. BEI micrograph of Ramim oolites, - 20 + 40 # , X 760.

I

60

=-

100 ,um

Fig. 4. B E I micrograph of R a m i m oolites, - 40 + 50 # , × 150.

1,urn

Fig. 5. B E I micrograph of R a m i m oolites, - 40 + 50 # , X 3040.

lO,Um I--------4

Fig. 6. B E I micrograph of R a m i m oolites, - 40 + 50 # , X 760.

61

10,urn I

Fig. 7. BEI micrograph of Ramim oolites, - 40 + 50 # , X 760.

10,urn Fig. 8. SEI micrograph of Ramim oolites, - 40 + 50 # , X 760.

lOpm i,--.-i

Fig. 9. BEI micrograph of Ramim oolites, - 70 + 100 # , X 4b(.

I

62

lOjum I,

Fig. 10. BEI micrograph of R a m i m oolites, - 70 + 100 # , X 760.

lO,um Fig. 11. B E I micrograph of R a m i m oolites, - 1 0 0 + 320 # , X 760.

Fig. 12. BEI micrograph of R a m i m oolites, - 100 + 325 # , X 3340.

j

63

!

JII

Fig. 13. BEI micrograph of Ramim ground oolites, - 100 + 325 #, × 4560.

INTACT

OOLITE

CRUSHED

OOLITE

I N T A C T OOLITE OF THE SAME SIZE AS

THE CRUSHED ONE

Fig. 14. Comparisonbetween shell spacing of crushed and intact oolite having the same size. size has an important role in determining their amenability to direct reduction and to concentration of the iron. The size-dependence of the oolite structure was also discovered by porosity measurements which are described below.

Pore size distribution Fig. 15 depicts pore size distribution of different size fractions of original, i.e. uncrushed oolites. It shows results for the - 2 0 + 4 0 # , -40+70#, - 7 0 + 100 # and - 1 0 0 + 325 # size fractions. Fig. 16 shows pore size distribution obtained for oolites (originating from - 20 + 60 # ) which were ground to - 1 0 0 + 3 2 5 # . Data of pore size distribution expressed in terms of total porosity PT and total porosity of pores larger t h a n 10 pm Pwlo, versus oolite size are given in Table III. Fig. 15 and Table III show t h a t the total porosity of intact oolites increases as their size decreases. For example, total porosity increases from 26.8% to 55.1% when one selects the - 1 0 0 + 3 2 5 # fraction instead of the - 20 + 40 # size fraction. The observed pore size distributions are bimodal with the first being around 0.02 ttm pores and the second larger one is for pores larger t h a n 10 ttm. The first mode varies only slightly (in the 8-

64 180

180

120

120

I

I

I

pd

pd 60

60

a)

-r'0 0.001

0,01

0.I

I D,

180

I0

0.01

0.I

I0

I00

I0

I00

D,#m

M,m •

,

,

0 0.001

I00

180

,

120

120

pd

pd

60

60

0.001

0.01

0. I

I

I0 D,

I00

0,001

0,01

#m

0. I

I D, M.m

Fig. 15. Pore size distribution of original oolites, a ) - 20 + 40 # ; b ) - 40 + 70 # ; c ) - 70 + 100 # ; d) -100+325#. 180

[

,

~

I

r

120 pd 60

o 0,001

0.01

0. I

I

I0

I00

D, [Lm

Fig. 16. Pore size distribution of ground oolites,

-- 100 + 325 # .

13% t o t a l p o r o s i t y r a n g e ) with oolite size. H o w e v e r , the s e c o n d m o d e is m o r e sensitive to oolite size. Fig. 17 shows a plot of PTlO versus Dp where Dp is average particle size of a size fraction. In t h e coarse size range 179.5 ~
65 TABLE III Data of pore size distribution as a function of the oolites size Type of oolites

Oolites particle size (mesh)

Average particle size of fraction, Dp (~m)

PT (%)

PT10 (%)

intact intact intact intact ground

- 2 0 + 40 - 4 0 + 70 - 7 0 + 100 - 100+325 - 100+325

630.5 315 179.5 96.5 96.5

26.8 43.2 48.8 55.1 48.7

5.0 24.9 32.5 33.3 16.9

35 30

25 o

V- 20 15 fC

I 0

I00

1

I

I

I

200

300

400

500

I 600

700

Dp, ~m Fig. 17. PTloas function of Dp. larger t h a n 1 0 / l m from 70/~m to 20/~m upon decreasing the fraction size from - 20 + 40 # down to - 100 + 325 # . Although the difference in total porosity between - 1 0 0 + 3 2 5 # crushed (48.7%) and intact oolites (55.1%) is small (see Fig. 16 and Table III), the difference in PTlO is substantial. Here values of Pwlo = 16.9% and Pwlo----33.3% were obtained for the - 100 + 325 # crushed and intact oolites, respectively. F u r t h e r m o r e the value of Pwlo for the crushed oolites is close to the one corresponding to - 20 + 60 # intact oolites from which the - 100 + 325 # fraction was obtained. This substantiates the importance of information concerning the origin of the oolite particle in addition to its current size. Finally the pore size distribution of ground oolites indicates t h a t size reduction increases the total porosity and the st andard deviation of pore size. In the following evidence for the effect of oolite structure and pore size distribution on results of direct reduction is presented.

Direct reduction of oolites In direct reduction experiments (Weissberger, 1984) results of percent metallization were determined at different reduction temperatures. P e r c e n t me-

66

tallization is defined as the weight ratio of iron which was reduced to metallic state to the total iron. Fig. 18 shows a plot of percent metallization versus temperature for - 100 + 325 # crushed and intact oolites. Intact (original) oolites yield higher metallization throughout the 900 °-1250 oC temperature range. Below 900 oC only a low percentage of metallization was observed for both cases while for temperature higher than 1200 °C it exceeded 90%. Setting the temperature to 1100°C yielded 90% metallization in intact oolites but only 75% in the crushed ones. The results of metallization versus reduction temperature can be correlated to the internal structure of the oolites with respect to the shell pattern and pores which are larger than 10/~m. The denser spacing of shell fragments, and higher content of pores larger than 10/zm, which characterize intact oolites (as compared to crushed ones ), enhance diffusion of CO (towards the reaction sites) and of reduced Fe within its surrounding matrix. The result is enhanced reduction and growth of the metallic phase. This is supported by comparing the degree of percent metallization versus temperature for two sizes of intact oolites which are summarized in Table IV. For example, at 1050 ° C metalliza-

Boo ~. eo _N 60 w

40

2o_~ o~ 900

I I I 9~o

,ooo

I l ,o5o

I I .oo

l [ HSO

1

I

,ZOO

TEMPERATURE

I ~ZSO ,

°C

Fig. 18. Metal]ization versus temperature of - 100 + 325 # oolites.

T A B L E IV E f f e c t o f r e d u c t i o n t e m p e r a t u r e o n m e t a l l i z a t i o n o f - 40-t- 70 # a n d - 100 + 325 # oolites Metallization % at temperatures:

Particle size (mesh)

1200 o C

1100 °C

1050 °C

1000 ° C

950 °C

90.4 95.3

77.1 91.1

55.3 75.6

37.4 40.3

8.5 13.1

-40+ 70 - 100+325

67

l~m N

Fig. 19. B E I micrograph of Lorraine oolites, - 40 + 50 # , X 3040.

1pro

Fig. 20. B E I micrograph of Dogger fl oolites, - 40 + 50 # , × 3040.

ltJm Fig. 21. B E I micrograph of Wisconsin oolites, - 40 + 50 # , X 3040.

68

100 ,um

Fig. 22. BEI micrograph of Dogger fl oolites, - 40 + 50 # , × 150.

100 p m

Fig. 23. BEI micrograph of Lorraine oolites, - 4 0 + 50 # , × 150. TABLE V The values of PT and PTlO (in % ) of the world oolites PTIO

PT

Ore

30.2 35.0 29.8 24.9

45.4 53.0 45.0 43.2

Lorrain Wisconsin Dogger// Ramim

t i o n o f t h e - 200 + 325 # f r a c t i o n w a s 2 0 % h i g h e r t h a n t h e o n e o b t a i n e d f o r t h e - 40 + 70 # f r a c t i o n . H e r e s i m i l a r t o t h e r e s u l t s s h o w n i n F i g . 18, m e t a l l i -

69 I00

90

80

70

--

I

~. 6O Z 0 w IN

50

,-I ._1

~ 4o 30

o --RAMIM x --LORRAINE

20

• --WISCONSIN o--DOGGER

o

I 950

i ~ooo

I

I

I

~o5o

Hoo

,,5o

TEMPERATURE,

B

] ~2oo

"C

Fig. 24. Percent metallization versus reduction temperature of -40 + 70 # oolites from different sources. zation becomes higher and less dependent on oolite size at higher reduction temperatures (Weissberger et al., 1986). Oolites from other sources BEI analysis of polished sections which were prepared from - 40 + 50 # raw oolites showed a clear difference in shell spacing between the Lorraine (Fig. 19) and the Dogger fl (Fig. 20) and Wisconsin (Fig. 21) samples. Lorraine oolites are characterized by 1-/zm shell spacing in contrast with a value of 2 pm observed for oolites pertaining to the other samples. Fig. 22 shows clearly the well defined concentric shell structure of Dogger fl oolites. A cross-section in - 40 + 50 # Lorraine oolites shows that they consist of smaller oolites (characterized by smaller shell spacing) which are interconnected by goethite grains (see Fig. 23 ). This is a basic structural difference that makes the Loraine oolites unique with respect to their response to direct reduction and magnetic

70 separation. Porosity measurements given in Table V showed that the Wisconsin sample had 5-10% pores larger than 10 ]lm in excess of the other samples. Direct reduction tests were performed on the - 4 0 + 70# size fraction of each sample. Fig. 24 shows results of percent metallization versus reduction temperature of - 40 + 70 # oolites from Ramim, Lorraine, Wisconsin and Dogger ft. Oolites from Lorraine proved to be the most amenable to direct reduction. Their superiority can be related to their internal structure, i.e. concentric shell spaced at 1 ttm as compared to a 2-~m spacing of the other oolite samples. The higher metallization of Wisconsin oolites, which was observed at the lower ( T < 1000°C) temperature range (see Fig. 24), can be related to the 510% excess of pores larger than 10 ]lm (PTlo). Upon increasing the temperature above 1000 ° C a decrease in characteristic parameters of porosity (i.e. total porosity, median, and average pore size ) was observed elsewhere (Weissberger, 1984; Weissberger et al., 1986) (not shown here). Thus above 1000°C the 510% excess of porosity of the Wisconsin sample was eliminated. This is in accordance with the changes (Fig. 24) in metallization of the Wisconsin sample observed around 1000 ° C. CONCLUSIONS ( 1 ) The internal structure of the Ramim oolites varies with their size. A well defined concentric shell structure typical of large oolites is gradually replaced by a net pattern, comprising shell fragments, as oolite size gets smaller. The transition occurs in the oolite interior first, with the shell structure maintained in the rim. The disordered pattern expands upon further decrease in the oolite size, until the ordered shell structure is completely eliminated. (2) Smaller oolites have denser structure of shells or shell fragments. 16 + 20 # oolites are characterized by 2-3 jim shell spacing. - 100 + 325 # oolites exhibit spacings between shell fragments which are 1 ~tm or less. (3) Shell spacing can be correlated to porosity of the oolites. Denser shell spacing and larger total porosity of pores larger than 10 ttm, occur simultaneously in smaller oolites. (4) Dense shell spacing and larger porosity enhance, separately or together, permeability of reducing gases, efficiency of reduction, growth of the metallic phase and finally amenability to separation by magnetic or other beneficiation means. (5) Oolites from Ramim and overseas sources exhibit similar internal structure as well as chemical compositions, but differ in porosity and spacings of goethite particles. (6) Higher metallization was obtained from Lorraine oolites which were characterized by concentric shell twice as densely spaced as compared to oolites from Ramim, Wisconsin, and Dogger ft. (7) Identification of the structure of concentric shells of oolites has practical -

71 i m p l i c a t i o n regarding e x t r a c t i o n of iron b y direct reduction. Since higher growth o f t h e m e t a l l i c p h a s e is e x p e c t e d in cases of d e n s e r s t r u c t u r e s t h e l a t t e r determ i n e s t h e e x p e c t e d yield of iron, e v e n in cases o f lean ores. T h u s , if oolites of a d e n s e s t r u c t u r e exist in a lean ore, its e x p l o i t a t i o n d e p e n d s on t h e efficiency of oolite c o n c e n t r a t i o n r a t h e r t h a n on t h e direct r e d u c t i o n of oolites.

REFERENCES Rohrlich, V., 1974. Microstructure and microchemistry of iron oolites. Miner. Deposita, 9: 133142. Rohrlich, V., Metzer, A. and Zohar, E., 1980. Potential iron ores in the Lower Cretaceous of Israel and their origin. Israel J. Earth Sci., 29: 73-80. Weissberger, S., 1984. Mechanism of Growth of Metallic Phase in Direct Reduction of Iron Bearing Oolites. D. Sc. Thesis, Technion, Haifa, 180 pp. (in Hebrew, with English abstract). Weissberger, S. and Zimmels, Y., 1983. Studies on concentration and direct reduction of the ramim iron ore. Int. J. Miner. Process., 11: 115-130. Weissberger, S., Zimmels, Y. and Lin, I.J., 1986. Mechanism of growth of metallic phase in direct reduction of iron bearing oolites. Metall. Trans. B., 17B: 433-442.