IRON ORE CONFERENCE / PERTH, WA, 11 - 13 JULY 2011 281

INTRODUCTION

Australia’s iron ore industry is currently undergoing a major transformation. Reserves of traditional bedded iron deposits high-grade microplaty haematite Brockman-type ores are being depleted while the new ore types, the Martite-Goethite and the channel iron deposits tend to be lower in grade and sometimes require benefi ciation to remove or minimise contaminants the levels of contaminants such as alumina, silica and phosphorus (Dukino, England and Kneeshaw, 2000).

The phosphorus content of Australian iron ores is a particularly serious problem. There are huge deposits (~7 billion tonnes) of high phosphorus ore in Western Australia that are close to existing infrastructure. These deposits have acceptable iron grade, but the phosphorus levels (>0.1 per cent P) make them very diffi cult to sell because of the adverse effects of phosphorous on the quality of the steel (Dub, Dub and Makarycheva, 2006). Current specifi cations for phosphorus in iron ore requires ores averaging less than 0.07 - 0.08 per cent P (Cheng et al, 1999; Thorne et al, 2008), with penalties for every 0.001 per cent increase in P content above the acceptable limit (Cheng et al, 1999).

Previous work has shown that phosphorus occurs in Australian iron ores in a number of ways: as microscopic inclusions of the mineral hydroxyapatite Ca

5[PO

4]

3(OH)

(Morris, 1973), other secondary phosphates such as vivianite, Fe

32+[PO

4]

2.8H

2O, and wavellite, Al

3[PO

4]

2(OH,F)

3

(Ostwald, 1981), submicron, rare earth bearing phosphates (Graham, 1973; Ramanaidou et al, 2008), or within goethite (Dukino, England and Kneeshaw, 2000; Thorne et al, 2008). While previous work has shown there are a large number of potential sources for P in iron ore, the majority of P in iron ore, where the average levels are of the order of 0.1 per cent or more occurs not as discrete mineral phases but in association with goethite (Morris, 1985; Dukino and England, 1997; Ramanaidou et al, 2008).

Goethite is a naturally occurring iron oxy-hydroxide mineral and is found in association with several types of mineral deposits including iron, manganese, nickel and bauxite ores. The goethite forms during supergene metasomatic replacement of fi ne-grained chert and other gangue minerals in the ore (Morris, 1985; Dukino, England and Kneeshaw, 2000) causing the formation initially of ferrihydrite, FeO[OH]

3.

nH2O, which is subsequently dehydrated and recrystallised to

form goethite (Dukino, England and Kneeshaw, 2000). It may contain variable quantities of impurities such as P

2O

5, Al

2O

3,

MnO, CaO and SiO2, which sometimes reach a collective level

of fi ve per cent (Klein and Hurlbut, 1985). The association of phosphorus with goethite has been proven directly by Graham (1973) using electron microprobe analysis techniques and by Ward, Coles and Carr (1975) who observed that phosphorus contents were lower in drill cuttings that contained lower goethite content. Studies of different types of iron ore samples

1. Manager Microbeam Laboratory, CSIRO Process Science and Engineering, Bayview Avenue, Clayton Vic 3168. Email: Colin.macrae@csiro.au

2. Research Scientist, CSIRO Process Science and Engineering, Bayview Avenue, Clayton Vic 3168. Email: Nick.wilson@csiro.au

3. Principal Research Scientist, CSIRO Process Science and Engineering, Bayview Avenue, Clayton Vic 3168. Email: Mark.pownceby@csiro.au

4. Manager, Monash Centre for Electron Microscopy, Monash University, Clayton Campus, Vic 3800. Email:Peter.miller@mcem.monash.edu.au

The Occurrence of Phosphorus and Other Impurities in Australian Iron OresC M MacRae1, N C Wilson2, M I Pownceby3 and P R Miller4

ABSTRACT

The characterisation of high-P iron ores by fi eld emission gun electron probe microanalyser (FEG-EPMA) was carried out to determine elemental associations in iron oxides sensus lato and, using this knowledge, to speculate on possible P incorporation mechanisms in goethite. Quantitative data was collected from two goethite-rich bulk ores and results showed that in both samples goethite was the main repository for phosphorus. The P-rich goethite also contained elevated levels of both aluminium and silicon.

To understand the mechanism of P incorporation in goethite, high-P grains were located by coarse mapping of polished blocks with the FEG-EPMA. Analysis of element distribution maps collected on the P-rich grains showed that low-P regions within grains were typically associated with low-Al and high-Si, whereas high-P regions were associated with high-Al and low-Si. Based on follow-up quantitative analyses, a coupled substitution mechanism for phosphorus incorporation within goethite was proposed: 2Si4+ = 1P5+ + 1Al3+. Additional quantitative analyses on further examples indicated the mechanism was only valid for Si contents in goethite of <1 wt per cent. Preliminary transmission electron microscope (TEM) studies confi rmed the presence of P, Al and Si in goethite.

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have shown it is possible to categorise the goethite into a number of morphological and textural/microstructural types and evidence suggests that the phosphorus is mainly located in a texturally distinct goethite phase termed ochreous goethite (Dukino, England and Kneeshaw, 2000). Ochreous goethite is also typically associated with high levels of other impurity elements such as silicon and aluminium.

Some of the phosphorus in the ore can be removed if it reports predominantly to the fi ne size fractions after crushing and de-sliming, but iron units are typically lost in the separation process (Bensely and Rogers, 1987). Alternatively, it has been shown that the phosphorus content of the goethitic ore can be substantially reduced by heat treatment and subsequent leaching in acid or caustic solution with minimal loss of iron units (Cheng et al, 1999). This has been confi rmed more recently by Fisher-White, Lovel and Sparrow (2009). The method has also been demonstrated to be effective in reducing other impurities in the ore such as aluminium and silicon. This method of phosphorus and impurity level reduction, however, has yet to implemented on a commercial scale and currently in Australia, the majority of iron ore feedstock production is a blend of low-P (<0.05 per cent P) haematite-rich ores and more common high-P (>0.10 per cent P) goethite-rich ores.

There is, at present, an incomplete understanding of the mechanisms of incorporation of phosphorus and other impurities within Australian iron ores; however, such an understanding may be important in identifying the best technique for their removal. In an effort to better understand the occurrence of phosphorus and associated impurities within goethite-rich Hamersley iron ores, a detailed electron microprobe study was undertaken. This paper is divided into three parts: an assessment of the phosphorus (and other impurity element) deportment within iron ores; the association between phosphorus and other impurity element within the P-rich phase(s); and a summary of fi ndings of a study to determine a plausible mechanism for phosphorus incorporation in goethite.

METHODS

Two samples of P-rich iron ore (labelled as Bulk Ore 1 and Bulk Ore 2) were prepared by mounting the material in epoxy impregnated with carbon and curing in an oven overnight at 60ºC. After curing, the samples were sectioned to expose a fresh surface, polished fl at using successively fi ner, 6, 3, 1 and 0.5 μm diamond pastes and then fi nished with a 20 nm colloidal silica suspension polish. Finally, the samples were coated with a 10 nm fi lm of carbon prevent charge build up when the surface of the samples were probed by the electron beam.

The characterisation of high-P iron ores was carried out using a JEOL JXA-8500F fi eld emission gun electron probe microanalyser (FEG-EPMA) operating in either a mapping or quantitative analysis mode. The FEG-EPMA was equipped with fi ve wavelength dispersive (WD) spectrometers and two silicon drift detectors energy dispersive (ED) spectrometers and was capable of operating at low accelerating voltages, with moderate beam currents, resulting in high X-ray spatial resolution.

Quantitative analysis of the bulk oresSince previous work had indicated that goethite commonly contains signifi cant impurity levels (Klein and Hurlbut, 1985; Dukino, England and Kneeshaw, 2000), both samples were initially examined by quantitative analysis methods in order

to assess the distribution of the impurity elements P, Al, and Si within the goethite phase in the bulk ores. Approximately 200 (Bulk Ore 1) and 500 (Bulk Ore 2) randomly targeted goethite grains were analysed quantitatively by FEG-EPMA. There was no distinction made between goethite textural types in the samples. Quantitative analyses were performed in wavelength dispersive (WD) mode with the FEG-EPMA operated at an accelerating voltage of 15 kV and a beam current of 20 nA. At each analysis position, the electron beam was defocused to 2 μm and the following suite of elements measured: Na, P, Fe, Ca, Mg, Al, Mn and Si. Oxygen was calculated by difference, based on valence. The following standards used were: spinel (‘Magalox’, MgAl

2O

4), haematite (Fe

2O

3), albite (NaAlSi

3O

8),

pyroxmangite (MnSiO3), apatite (Ca

5[PO

4]

3(OH,F) and

wollastonite (CaSiO3).

Feg-Epma MappingTo identify the high-P containing grains, coarse mapping was initially performed using an accelerating voltage of 15 kV, a beam current of 100 nA and a step size (in x and y) of 2 μm. Typically a 2 × 2 mm region was mapped (ie 2000 × 2000 μm). The FEG-EPMA was set-up so that phosphorus Kα X-rays were measured on the most sensitive WD spectrometer providing a sensitivity below hundreds of parts per million (MacRae et al, 2010). Once the coarse phosphorus distribution maps were collected, they were then individually examined for candidate grains containing elevated levels of phosphorus. The coordinates of all high-P grains were recorded for subsequent high resolution mapping and microanalysis.

High resolution maps were collected using an accelerating voltage of 7 kV, a beam current of 70 nA, and step sizes of 100 nm. The resolution of the instrument operating at these conditions was verifi ed by calculating the interaction volume of phosphorus Kα X-rays in goethite using the Monte Carlo simulation software package, CASINO (Drouin et al, 2007). Results gave a calculated beam interaction volume of the order of ~150 nm meaning that features as small as these could be imaged and quantitatively analysed. For the high resolution maps, in addition to phosphorus, Al, Si, Fe and Ca were also mapped using WD spectrometers while any other elements that were not measured directly by WD spectrometry were measured using two energy-dispersive (ED) spectrometers. Measuring both ED and WD signals simultaneously ensured complete spectral information regarding the chemistry at each step interval in the map was obtained. This additional information can be important when trying the identify phases that contain elements not present in the main WD element map suite.

The resulting hyperspectral maps were examined using an in-house developed package known as CHIMAGE (Harrowfi eld, MacRae and Wilson, 1993; Pownceby, MacRae and Wilson, 2005). This software enables WD elemental data to be combined with region-of-interest elemental data extracted from the ED spectra data. These elements are calibrated against known standards, much in the same way as quantitative microanalysis is performed, and the results is k-ratio calibrated elemental levels (approximately equivalent to wt per cent). CHIMAGE allows the individual element data to be displayed either as single element distribution scatter plots or as combined element maps where data for three elements are combined on the one mapped region using a red/blue/green palette. Both techniques when applied to analysing the raw microprobe data make correlations between elements readily detectable, and lead to the identifi cation of individual mineral/chemical phases.

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THE OCCURRENCE OF PHOSPHORUS AND OTHER IMPURITIES IN AUSTRALIAN IRON ORES

283

High resolution quantitative analysisOnce high-P grains were identifi ed and mapped, quantitative microanalyses on were performed with an accelerating voltage of 7 kV, a beam current of 13 nA with P, Al, Si, Fe being measured by WD spectrometry and oxygen calculated by stoichiometry. The P, Al and Si were analysed using the Kα line, while Fe was analysed using the Lα line. Standards used were; berlinite (AlPO

4) for phosphorus, spinel (MgAl

2O

4) for

aluminium, wollastonite (CaSiO3) for silicon, and haematite

(Fe2O

3) for iron. The matrix correction procedure used was

used a PhiRhoZ method implemented in STRATA software (Pouchou and Pichoir, 1984a, 1984b; Pouchou and Thiot, 1996).

Transmission electron microscopyIn addition to the FEG-EPMA characterisation of the sample, preliminary transmission electron microscopy (TEM) analysis was conducted. This was done in an attempt to determine if phosphorus (or other impurities) was present as discrete, nanometre-sized inclusions or whether it was incorporated within the structure of the goethite. The TEM technique requires samples to be presented as electron transparent thin sections and these were prepared using a scanning electron microscope (SEM) equipped with a focused ion beam (FIB-SEM), model FEI Quanta 3D FEG. The FIB-SEM was equipped with a micromanipulator to allow sections to be removed and attached to a TEM holder. The subsequent TEM investigation used a Philips CM20 equipped with an energy dispersive X-ray spectrometer and was operated at 200 kV.

RESULTS AND DISCUSSION

Quantitative analysis of the bulk oresResults from the FEG-EPMA quantitative analyses are summarised in Table 1 which also includes data from bulk XRF analyses for comparison. For Bulk Ore 1, the results indicate the impurities Al and Si are present in goethite at signifi cantly lower levels than measured in the bulk. This suggests an additional source(s) for Al and Si impurities in the ore which likely include phases such as aluminosilicates and quartz. In comparison, goethite appears to be the main repository for phosphorus containing just over double the amount of P than measured for the bulk sample. For Bulk Ore 2, Al and Si impurities within the bulk and within the goethite are almost identical implying low levels of gangue accessory phases in the ore. As for Bulk Ore 1, goethite was confi rmed as the main P-bearing phase, containing over three times the amount of P measured in the bulk.

The quantitative EPMA data have are presented in Figure 1 as a series of x-y scatter plots to show trends between Al and P,

Si and P, and Al and Si. For both ores there appears to be a trend of increasing P with increasing Al. This trend is more obvious in the data for Bulk Ore 2. Close inspection of the data, however, suggests the possibility of two distinct Al/P trends with one group characterised by much high Al and lower P contents (typically <0.5 per cent) and the other group having high P and lower Al (typically <2.0 per cent). Further work is required to verify any trend between Al and P although we note that previous workers (eg Ramanaidou et al, 2008; Mohapatra et al, 2008) have also noted a relationship between Al and P in goethite. Scatter plots showing Si versus P reveal that for both ores, high P levels correspond to low Si levels and vice versa. Plots for Al versus Si do not indicate any clear correlation.

FEG-EPMA mapping and high resolution quantitative analysis A large area, low-resolution map from Bulk Ore 1 map shows a wide range of grain sizes and textures (Figure 2). The Fe, Al, and P elemental map shows that phosphorus exists as discrete P-rich grains which are either apatite or rare earth (RE) containing phosphates (circled grains). The region highlighted at the top left of Figure 2 shows phosphorus enrichment around the edges of a goethite grain. A magnifi ed, backscattered electron (BSE) image of this grain is provided in Figure 3, which shows two distinct morphologies within the grain: a dense upper layer which appears to be associated with elevated phosphorus; and a lower layer which shows signifi cantly higher porosity and less phosphorus.

The resultant high resolution map collected over this grain, shown in Figure 4, indicates a denser, phosphorus-enriched zone along the top of the grain possibly indicating late stage precipitation. In comparison, the more porous central parts of the grain have signifi cantly less phosphorus but contain elevated Si levels. This low phosphorus core is interspersed with high Al and P inclusions (yellow grains in Figure 4), possibly fi lling voids within the original structure.

In order to better understand the mechanism of phosphorus and other impurity element incorporation within the goethite, a quantitative microanalysis line traverse was taken to show the variation in Al and Si levels with P levels. The elemental and spatial relationships between Al versus P and Si versus P can be seen in the accompanying elemental scatter plots in Figure 4. The results show that low-P regions are associated with low-Al and high-Si, whereas the high-P regions are associated with high-Al and low-Si. These relationships are also borne out by the elemental scatter plots from the high resolution map data (data not shown here).

On the basis of ionic size considerations, the pentavalent P5+ cation (0.38Å) is considerably smaller than the Fe3+ ionic

Sample Element (wt %) analysis

Na P Fe Ca Mg Al Mn Si Total

Bulk Ore 1 0.043 0.298 56.69 0.022 0.051 0.701 0.061 0.903 85.22 261

XRF (Bulk) n.d.† 0.146 62.1 0.021 b.d.‡ 1.20 0.068 1.47

Bulk Ore 2 0.011 0.340 57.16 0.006 0.074 0.909 0.226 1.31 87.45 532

XRF (Bulk) n.d. 0.100 63.6 b.d. b.d. 0.951 0.081 1.23

† n.d. = not determined

‡ b.d. = below detection limit

TABLE 1

Summary table showing average EPMA data, excluding oxygen, as well as bulk XRF results for the two bulk ores examined in this study.

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C M MACRAE et al

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0.0

1.0

2.0

3.0

4.0

5.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

P (wt.%)

Al (

wt.%

)

0.0

1.0

2.0

3.0

4.0

5.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

P (wt.%)

Si (w

t.%)

0.0

1.0

2.0

3.0

4.0

5.0

0.0 1.0 2.0 3.0 4.0 5.0

Si (wt.%)

Al (

wt.%

)

0.0

1.0

2.0

3.0

4.0

5.0

0.0 0.5 1.0 1.5 2.0

P (wt.%)

Al (

wt.%

)

0.0

1.0

2.0

3.0

4.0

5.0

0.0 0.5 1.0 1.5 2.0

P (wt.%)Si

(wt.%

)

0.0

1.0

2.0

3.0

4.0

5.0

0.0 1.0 2.0 3.0 4.0 5.0

Si (wt.%)

Al (

wt.%

)

Bulk Ore 1 Bulk Ore 2

FIG 1 - Quantitative EPMA scatter plot data for the elements P, Al and Si in goethite for the two bulk ore samples.

Fe

Al

P

78 0

6.4 0.0

1.6 0.1

1 mm

FIG 2 - Low resolution FEG-EPMA map for Bulk Ore 1. This sighting map reveals a number of high phosphorus grains (circled), which are apatite or RE phosphates, and a separate grain highlighted in the rectangular region which appears to have elevated phosphorus levels.

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THE OCCURRENCE OF PHOSPHORUS AND OTHER IMPURITIES IN AUSTRALIAN IRON ORES

285

FIG 3 - Backscattered electron image of the phosphorus-rich grain outlined in Figure 2 showing a number of distinct growth zones. The dense upper layer appears to be associated with elevated phosphorus levels.

y = -1.1008x + 1.3409R2 = 0.6519

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

P wt%

Si w

t%

P versus Al

y = 0.8566x + 0.1972R2 = 0.8425

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

P (wt%)

Al (

wt%

)

Fe

Al

P

75 37

2.10.3

1.90.3

50 μm

FIG 4 - High resolution FEG-EPMA map of the goethite grain highlighted in Figures 2 and 3. The phosphorus enriched outer zone (top of map), shows evidence of a late stage precipitation, with elevated Al levels in the material below. A low phosphorus core has interspersed high Al and P inclusions. A quantitative microanalysis line

traverse was taken to quantitatively show the variation in Al and Si levels with P levels. The elemental and spatial relationships between Al versus P and Si versus. P can be seen in the elemental scatter plots.

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radius (Shannon, 1976) and would be expected to easily replace the iron in the goethite structure. However, charge considerations act to prevent extensive replacement of Fe and for that reason, previous workers have suggested that phosphorus does not substitute into goethite to any great extent and, if present, is probably in an occluded form, most likely as a discrete phase such as strengite (FePO

4.2H

2O), or as

an adsorbed phosphate species on growing surfaces or within micropores (Ostwald, 1981; Quin et al, 1988; Galvez, Barron and Torrent, 1999; Thorne et al, 2008: Dukino, England and Kneeshaw, 2000). We believe, however, the preliminary EPMA results from this study support an alternative phosphorus mechanism, that of a coupled substitution reaction mechanism according to:

2Si4+ = 1P5+ + 1Al3+ (1)

In this reaction mechanism, the presence of the trivalent Al and quadrivalent Si are both vital in stabilising P within goethite. Equation 1 may also explain the positive correlation between Al and P in goethite and also the observation that P-containing goethites tend to have elevated levels of Si.

To check the possibility of the coupled substitution, in Figure 5 we have plotted all the EPMA from the traverse in Figure 4 in a plot of Al+P versus Si and Al+Si versus 2Si for the high-P data only. For the regions where high P and high Al levels were measured, the slope of the graph is approximately 0.5 when Al+P are plotted against Si (and 1.0 when plotted against 2Si as shown in the insert) which supports the mechanism outlined in Equation 1. However, the results in Figure 5 also suggest that the coupled substitution is only valid for low-Si containing goethite. As the silicon concentration in the goethite increases, the total Al+P decreases until the silicon level in the goethite reaches greater than about 1.0 wt per cent. Thereafter, both Al and P remain constant at ~0.4 wt per cent (combined).

To test our hypothesis of a coupled substitution mechanism on another sample, a second high-P was examined using the FEG-EPMA. In this example, we examined an area where a Fe-oxide (haematite) grain was surrounded by goethite. The resultant high resolution map is shown in Figure 6. In contrast to our observations in Figure 4, the elemental maps in Figure 6 show at least two different impurity element associations within the goethite. The fi rst causes phosphorus (pink) enrichment along the haematite-goethite grain boundary-interface, while the second involves Al incorporation (green-yellow) within the goethitic region. The phosphorus appears to have penetrated the haematite, with high phosphorus levels being involved in the initial incorporation/conversion of the Fe-oxide into goethite followed by the two distinct high-P and high-Al forms of goethite. The Al element distribution map shown in Figure 6 indicates Al is also associated with P incorporation in the P-rich goethite, however, it appears to be at signifi cantly lower levels than in the Al-rich form of goethite (which has a more patchy distribution).

To quantify the elemental chemistry, a series of quantitative microanalyses were performed along a line traversing from the Fe-oxide through the phosphorus-rich interface and into the goethite region. A scatter plot of Al+P versus Si shows three distinct associations, one for each of; Goethite, Fe-oxide and replaced Fe-oxide (Figure 7). The Fe-oxide does not favour incorporation of Al and/or P at all, however, Si is easily accommodated at low levels (up to 1.2 wt per cent). In comparison, within the replaced Fe-oxide there is a clear association between Al+P and Si, with results suggesting that as the Si content of the replaced Fe-oxide increases, the amount of Al and P able to be incorporated into the structure reduces. Although we have termed this phase ‘replaced Fe-oxide’ we note that compositionally it is a form of goethite. The reduction of Al+P with increasing Si was confi rmed by analyses extended into the surrounding goethite phase. In this region, high levels of Si (between 0.8 - 1.8 wt per cent) appear to prohibit Al+P substitution beyond a certain value

FIG 5 - Plot of Al+P versus Si from the EPMA line traverse shown in Figure 4. Data at Si <1.0 wt per cent are from the high-P, high-Al region of Fig 4, while data at Si >1.0 wt per cent are from the low-P, low-Al region. The inset graph shows the high-P, high-Al data plotted as Al+P versus 2Si (in atom percentage).

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287

Al

1.3

0.2 10 μm

Si 3.6

1.1 10 μm

Fe

Al

P

10 μm FIG 6 - Elemental maps from Bulk Ore 2 showing the infi ltration of Al, Si and P into Fe-oxide. Goethite is represented by the pink/mauve material interstitial to the

haematite (blue). The areas containing elevated aluminium (green regions), are a goethite phase containing up to several weight percent aluminium. The line on the right hand Fe/Al/P map shows the position of the quantitative line traverse.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Si

Al +

P

Fe-oxide

GoethiteReplaced Fe-oxide

FIG 7 - Scatter plot of Al plus P versus Si quantitative microanalyses performed on a line traverse from haematite through the phosphorus-rich interface and into the goethite region from the area shown in Figure 5. The scatter plot shows three distinct associations: Goethite, Fe-oxide and Replaced Fe-oxide.

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(~0.3 - 0.4 wt per cent combined). These results support our previous hypothesis that at low Si contents, Al+P are stabilised in goethite, however, beyond a certain substitution level (typically >1.0 wt per cent Si) the goethite cannot accommodate any more Al or P within its structure.

Transmission electron microscopyThe FEG-EPMA analyses offered tantalising glimpses of the possible mechanism(s) of phosphorus incorporation in goethite. Results, however, were not conclusive as the chemical correlations may also be explained by the presence of sub 100 nm inclusions of P-, Al- and Si-rich phases. If present, these would be below the ~150 nm resolution of the FEG-EPMA technique but when probed, would appear to be present as solid solution components. To determine conclusively the mechanism of P, Al and Si incorporation requires examination of goethite-rich regions that are known to contain these impurities, via transmission electron microscopy (TEM). The TEM is capable of providing chemical and structural information, down to atomic levels, provided the impurity-rich goethite can be successfully located, prepared as a thin fi lm, and then analysed. Each of these steps provides a considerable challenge.

Initial TEM investigations on thick electron transparent foils prepared from the part of the sample shown in Figure 6 have confi rmed the presence of elevated P, Al and Si as measured by EDS at the interface between haematite and goethite crystals. The thickness of this layer was approximately 20 nm and confi rms that these three elements are associated with the incorporation of phosphorus within goethite (Figure 8). The TEM probe was then moved approximately 50 nm further into the goethite and only Al and P were observed by EDS. This suggests that while silicon is a common impurity in haematite its role in goethite may be less important and that sometimes it may only play a role in the substitution of Al and P into the goethite structure. Based on these preliminary TEM results, however, the mechanism of replacement is not clear although we propose that an interfacial structure rich in Al, Si and P initially forms allowing the incorporation of Al and P into goethite. Further TEM studies are planned to probe the structure of the interfacial layer between haematite and goethite.

SUMMARY

The characterisation of high-P iron ores by fi eld emission gun electron probe microanalyser (FEG-EPMA) was carried out to determine elemental associations and, using this knowledge,

to speculate on possible P incorporation mechanisms. Quantitative data was collected from two goethite-rich bulk ores and results showed that in both samples, goethite was the main repository for phosphorus. The P-rich goethite also contained elevated levels of both aluminium and silicon.

To understand the mechanism of P incorporation in goethite, high-P grains were successfully located by FEG-EPMA mapping techniques. Element distribution maps within the mapped regions showed that low-P regions were typically associated with low-Al and high-Si, whereas high-P regions were associated with high-Al and low-Si. A coupled substitution mechanism for phosphorus incorporation within goethite was proposed: 2Si4+ = 1P5+ + 1Al3+. This was in contrast to the previously proposed mechanism of incorporation via adsorption of phosphate anions on the growing surfaces of the goethite (eg Dukino and England, 1997). Additional quantitative analyses on further examples indicated the mechanism was only valid for Si contents in goethite of <1 wt per cent. Preliminary transmission electron microscope (TEM) studies confi rmed the presence of P, Al and Si in goethite.

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blast furnace, BHP internal report (cited in Dukino and England, 1997).

Cheng, C Y, Misra, V N, Clough, J and Mun, R, 1999. Dephosphorisation of Western Australian iron ore by hydrometallurgical process, Minerals Engineering, 12:1083-1092.

Drouin, D, Couture, A R, Joly, D, Tastet, X, Aimez, V and Gauvin, R, 2007. CASINO V2.4.2 – A fast and easy-to-use modeling tool for scanning electron microscopy and microanalysis users, Scanning, 29:92-101.

Dub, V S, Dub, A V and Makarycheva, E V, 2006. Role of impurity and process elements in the formation of structure and properties of structural steels, Metal Science and Heat Treatment, 48:279-286.

Dukino, R D and England, B M, 1997. Phosphorus in Hamersley Range iron ores, in Proceedings Ironmaking Resources and Reserves Estimation, pp 197-202, 25 - 26 September, Perth.

Dukino, R D, England, B M and Kneeshaw, M, 2000. Phosphorus distribution in BIF-derived iron ores of Hamersley Province, Western Australia, Transactions of the Institutions of Mining and Metallurgy, Applied Earth Science, 108:B168-B176.

Fisher-White, M J, Lovel, R R and Sparrow, G J, 2009. Phosphorus removal from iron ore with a low temperature heat treatment, in Proceedings Iron Ore 2009 Conference, pp 249-254 (The Australasian Institute of Mining and Metallurgy: Melbourne).

O

Fe

P

Fe

Cu

Cu

CuAl Si

Fe

(A) (B)

FIG 8 - (A) TEM electron image, 200kV, showing haematite, (B) a bright interface and goethite. ED spectrum showing the presence of Al, Si and P at the interface. The Cu is from support grid.

IRON ORE CONFERENCE / PERTH, WA, 11 - 13 JULY 2011

THE OCCURRENCE OF PHOSPHORUS AND OTHER IMPURITIES IN AUSTRALIAN IRON ORES

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