Characterization and distribution of chrome spinel grains in magnetically fractionated ilmenite concentrates

POWNCEBY, M.I., FISHER-WHITE, M.J., MACRAE, C.M., WILSON, N.C., and SPARROW, G.J. Characterization and distribution of chrome spinel grains in magnetically fractionated ilmenite concentrates. Heavy Minerals 2003, Johannesburg, South African Institute of Mining and Metallurgy, 2003. Characterization and distribution of chrome spinel grains in magnetically fractionated ilmenite concentrates M.I. POWNCEBY, M.J. FISHER-WHITE, C.M. MACRAE, N.C. WILSON, and G.J SPARROW CSIRO Minerals, Clayton South, Australia The distribution of chrome spinels in a magnetically fractionated ilmenite concentrate sourced from the Murray Basin was characterized by XRF assaying, quantitative electron microprobe analysis, grain mapping, automated mineral phase clustering, and modal analysis. At least 14 distinct chrome spinel types were present in the concentrate. There is considerable variation in the amount of each chrome spinel type present, although a high-mg, high-al phase makes up close to 45 per cent of the total spinel population. Magnetic fractionation results showed significant variation in the composition and relative abundance of spinels reporting to each fraction. Low magnetic field strength fractions (ie. high susceptibility grains) were dominated by chrome spinels that are mid-range in Al and Cr. These spinels have the highest magnetite, ulvospinel and Fe(Al,Cr) 2 O 4 components. In comparison, the high field strength fractions (i.e. low susceptibility grains) have the majority of the grains with high-al contents indicating a higher proportion of Mg(Al,Cr) 2 O 4 component. Spinels high in a (Al,Cr) 2.67 O 4 non-stoichiometric component are absent at low field strengths but tend to be concentrated in the high field strength, non-magnetic fractions. Intermediate field strength fractions tend to exhibit a broad spread in spinel compositional types. A mild roast treatment of the sample had no clear effect on the distribution of chrome spinels between different magnetic fractions, but it did cause a change in the magnetic properties of the accompanying ilmenite grains. The effect of more extreme roasting conditions on the magnetic properties of the chrome spinels is to be determined. Introduction The production of clean ilmenite concentrates from Murray Basin mineral sands deposits is hampered by the presence of chromium-containing minerals. The majority of the chromium occurs in separate chrome spinel grains, so in principle they can be separated from ilmenite grains by physical concentration procedures. However, in practice, the separation is difficult because the physical properties of the chrome spinel grains, such as specific gravity, conductivity and magnetic susceptibility, are similar to those of ilmenite grains. The situation is further complicated by: (a) the chrome spinels having a relatively wide range of compositions due to varying amounts of Mg, Al, Fe and Cr in solid solution, and (b) chemical weathering of both the ilmenite and chrome spinels in the deposits, which involves oxidation and leaching of iron. Both these factors modify the composition ranges of the minerals and result in a wide spread of their separation characteristics. In mineral sands deposits in which the chrome spinel distribution falls into chemically and magnetically tight populations, effective separation treatments have been developed. These usually involve roasting under various conditions to increase the magnetic susceptibility of Fe 2 O 3 - FeTiO 3 solid solutions, enabling the removal of chromiumbearing spinel from ilmenite concentrates through magnetic separation 1. In areas such as the Murray Basin region of Eastern Australia however, previous characterization testwork by CSIRO Minerals has demonstrated that the chrome spinel populations are highly variable and often exhibit a broad range of composition and alteration 2 3. Individual chrome spinel grain types can span the entire compositional spectrum from close to ideal chromite (FeCr 2 O 4 ) to almost pure MgAl 2 O 4. The effect of various roasting regimes on the magnetic separation characteristics of these complex spinels is largely unknown. In this paper we describe the application of electron microprobe (EMP) imaging techniques developed at CSIRO Minerals to quantify the mineralogy and chemistry of chrome spinels in an ilmenite concentrate sourced from the Murray Basin. Characterization of the concentrate was performed before and after a low temperature roast treatment and included techniques such as: magnetic fractionation, XRF assaying, grain mapping, quantitative EMP analysis of the chrome spinels, automated mineral phase clustering, and modal analysis. Spinel-group mineralogy All spinels contain two differing cations, or at least two different valence states of the same cation, in the ratio 2:1. This gives the general crystallographic formula AB 2 O 4 where the tetrahedrally coordinated sites are labelled A, and octahedrally coordinated sites, B. In general, the spinel types commonly associated with ilmenite concentrates are dominated by compositions containing the cations Mg, Fe 2+, Fe 3+, Mn (minor), Zn (minor), Ti, Al and Cr. The major cations substituting into the A site are the divalent cations Mg, Fe, Mn and Zn whereas substitution within the B site involves the cations Al, Cr, Fe and Ti. Aluminium, CHARACTERIZATION AND DISTRIBUTION OF CHROME SPINEL GRAINS 175

iron and chromium are present as trivalent cations however, substitution of Ti 4+ into the octahedral B site (as in the case of ulvospinel-fe 2 TiO 4 ), relies on a coupled substitution 2B 3+ =Ti 4+ +A 2+ mechanism, relative to the general AB 2 O 4 formula. This gives rise to a range of spinel solid solutions within the system (Fe 2+,Mg)(Al,Cr,Fe 3+ ) 2 O 4 - (Fe 2+,Mg) 2 TiO 4. Spinel solutions may also contain some degree of non-stoichiometry 4. Non-stoichiometry is associated with defects in the oxide structure resulting in the ratio of the elements in the ideal formula of the oxide becoming indefinite. The most likely to occur in spinels associated with ilmenite concentrates is the defect spinel component (Al,Cr) 2.67 O 4 which is believed to be the result of chemical weathering of the spinel by the same mechanism as occurs in ilmenite alteration 5 i.e. diffusion of iron and other divalent elements out of the spinel, with oxidation of the remaining iron to the trivalent state to maintain charge balance. In attempting to examine variations in spinel compositions determined via EMP analyses it is useful to recast the oxide data into hypothetical spinel end-member components. In the following discussion we have recalculated the EMP data using the following spinel species: ulvospinel (FeTi 2 O 4 ), magnetite (Fe 3 O 4 ), magnesium aluminochromate (Mg(Al,Cr) 2 O 4 ), iron aluminochromate (Fe(Al,Cr) 2 O 4 ), and (Al,Cr) 2.67 O 4. Experimental Sample preparation An ilmenite concentrate was prepared from a Murray Basin heavy mineral concentrate (HMC) by magnetically separating at a field strength of 11 kgauss. The sample was then further separated to give fractions in the ranges: 0 2 kgauss, 2 3 kgauss, 3 4 kgauss, 4 6 kgauss, 6 8 kgauss and 8-11 kgauss. At CSIRO Minerals, we have found that for certain Murray Basin deposits, the ilmenite concentrates have a ferrous-to-ferric iron ratio within the range for enhanced magnetic susceptibility 1, but that due to chemical weathering the ilmenite structure has largely been converted to weakly magnetic hydrated iron titanate weathering products. In these cases, the strong magnetic properties can be restored simply by heating at low temperatures (~600 C) in a neutral or close to neutral gas atmosphere 6. To examine the influence of roasting on the sample, a mild low temperature roast was conducted on the 0 11 kgauss concentrate using a 3 cm diameter fluid-bed reactor. Approximately 80 g of concentrate was roasted at 575 C for 1 hour using a gas mixture consisting of 75.5 per cent N2, 19 per cent CO 2, and 5.5 per cent H 2 O. At the end of the roast, the reactor was lifted from the furnace and allowed to cool with N 2 continuing to flow through the sample. The roasted concentrate was then separated into the identical pre-roast magnetic fractions described above. For the EMP testwork, grains from each fraction of the unroasted and roasted concentrates were dispersed in epoxy resin and mounted into 2.5 cm round blocks. The blocks were successively polished down to a final cutting size of 1µm and coated with a 25 nm layer of carbon before EMP analysis. Electron microprobe analyses EMP testwork used a JEOL 8900R Superprobe electron microprobe analyser equipped with one energy dispersive (EDS) and five wavelength dispersive (WDS) spectrometers. Two different methods were employed to examine the distribution of chrome spinels in the concentrates. The first involved acquiring a large area map of each fraction to give a semi-quantitative analysis of the composition of the various chrome spinel types present as well as an estimate of their abundance. The second method involved the location and quantitative analysis of all chrome spinels present in the fraction (i.e. those at the surface of the polished mount). Operating conditions for each of the EMP methods are described separately below. Large area maps Large area maps were obtained using a grid of analysis points 5000 x 5000 µm and the distribution of elements Fe, Ti, Mg, Cr, Al, Si, Ca, Zr, and Mn were measured by WDS. Standards used were Mg-Al spinel (MgAl 2 O 4 ), rutile, Cr 2 O 3, hematite, wollastonite (CaSiO 3 ), zircon (ZrSiO 4 ), apatite (Ca 5 (PO 4 ) 3 F), and Mn metal. Maps were obtained using an accelerating voltage of 20 kv, a beam current of 180 na, a step size of 5 µm (in X and Y), and counting times of 15 msec. per step. Each map took a minimum of 10.5 hours to acquire. To determine chrome spinel compositional types, the mapped areas were manipulated by the CSIRO Minerals developed software package CHIMAGE 7. CHIMAGE enables complete processing and interpretation of the data set off-line and elemental data can be displayed as histograms, scatter plots or ternary diagrams. Using these plotting capabilities, clusters corresponding to individual chrome spinel types can be readily identified. Note however, that the identification of discrete chrome spinel types becomes difficult when grains have undergone significant alteration due to weathering processes. This causes cluster boundaries to extend and become diffuse. To overcome this, an automated cluster recognition technique was used to define all chrome spinel types 8 9. In this technique, cluster centroids are randomly placed in the n- dimensional data set, and the data is assigned the closest centroid. The centroids are then moved to the mean of the data points assigned to them, and the data points are then reassigned to the closest centroid. This process is repeated until the centroid mean stops moving and the cluster is defined. Quantitative chrome spinel analyses Each sample block was step-scanned on a grid of 1600 x 1600 points with 10 µm between points (total coverage 256 mm 2 ). At each point a 5 msec. analysis was made for Cr X- rays using the Kα line. The Cr map showed the positions of chromium-rich grains as an array of bright spots, the coordinates of which could be retrieved and stored. Using this procedure, up to 1 0000 mineral grains could be scanned over a period of ~6 h. Once the coordinates of individual chrome spinel grains were stored, the elemental analyses proceeded automatically for each position. The microprobe was operated at 20 kv and 100 na and the elements analysed included: Ti, Fe, Mg, Al, Mn, Si, and Cr. Standards used were wollastonite (CaSiO 3 ), spinel (MgAl 2 O 4 ), MnFe alloy, hematite, rutile, and chromium metal. Results and discussion Characterization of the unroasted concentrate Mineralogy The mineralogy of the unroasted concentrate, determined 176 HEAVY MINERALS 2003

via quantitative X-ray diffraction analysis, comprises 20 wt per cent ilmenite, 70 wt per cent pseudorutile (includes ~5 wt per cent of dense, hydrated pseudorutile), 7 wt per cent rutile and ~4 5 wt per cent gangue (mainly chrome spinel and tourmaline). The gangue content is an estimate only since the levels are too low to obtain a reliable analysis. Results from EMP mapping, however, indicate that the sample contains ~3.9 per cent chrome spinel. Assay results (Table I) show that the sample is relatively primary with a TiO 2 content at 55.5 wt per cent and a high proportion of chromium (1.27 wt per cent Cr 2 O 3 ). Not all of this chromium, however, is present in chrome spinel as quantitative EMP analyses obtained for the titanate minerals only (i.e. predominantly ilmenite and pseudorutile) reveal that the average Cr 2 O 3 is ~0.16 wt per cent within the titanate grains. This represents the lowest level of Cr 2 O 3 that could be attained assuming all chrome spinel gangue particles were removed from the concentrate. Magnetic fractionation of Cr 2 O 3 XRF assay results for the various bulk magnetic fractions are also provided in Table I. Results show that the Cr 2 O 3 content varies considerably with a minimum of 0.24 wt per cent in the 2 3 kg fraction and reaching a maximum of 1.82 wt per cent in the 4 6 kg fraction. These results suggest there are considerable changes in the abundance and composition of chrome spinel grains between fractions (notwithstanding the fact that the ilmenite grains themselves contain chromia). The effect of magnetic field strength on the distribution of the chrome spinel grains is discussed below. Distribution of chrome spinels Scatter plots of Al vs. Cr, Fe vs. Cr and Mg vs. Cr were used to identify the chrome spinel phase(s) within the bulk concentrate and each magnetic fraction (Figures 1 and 2). Previous experience in mapping high-cr concentrates has shown that Al and Cr tend to be the least mobile of the cations commonly found in spinels and they provide the most information regarding compositional variation 3. The Al vs. Cr scatter plot for the bulk concentrate is shown in Figure 1a. Some of the chrome spinel clusters are also preserved in the Mg vs. Cr or Fe vs. Cr scatter plots (Figures 1b and 1c) however, in general, the clusters in these plots are not as distinct. Auto clustering on the combined scatter plot data revealed at least 14 distinct chrome spinel types present. In Table II we have provided an estimated modal abundance for each of the clusters (from the map data), together with a corresponding typical EMP analysis. There is considerable variation in the amount of each chrome spinel type present, ranging from >20 per cent of the total chrome spinel population for each of clusters 13 and 14 compared to <2 per cent for each of the clusters 1, 2, 5 and 6. The high-al spinels (13 and 14) make up close to 45 per cent of the total spinel population. These are also rich in Mg indicative of a higher Mg(Al,Cr) 2 O 4 component, while the high-cr spinels (1 to 3) are, in general, low in total divalent ions (i.e. Fe 2+ and Mg), suggesting a non-stoichiometric spinel component. This is confirmed by an inspection of the calculated spinel end-member components also provided in Table II. Most of the clusters fall broadly along a trend line between high-al/low-cr and low-al/high-cr compositions. The clusters at high-al contents tend to be more defined, whilst those at mid- to low-al contents (e.g. clusters 1 to 3) are elongate mainly as a result of extensive variation in Cr. This is most likely due to other elements in the spinel structure, such as Fe, being more affected by weathering processes resulting in diffusion out of the oxide. The lack of scatter in clusters containing a high proportion of MgAl 2 O 4 component is a reflection of these spinels being more durable in natural systems 10. Chrome spinels containing significant levels of magnetite and ulvospinel components tend to plot in the middle of the Al versus Cr scatter plot (e.g. clusters 4, 5?, 8, 9). These spinels are high in total Fe, and noticeably low in Mg (see Figure 1c). Table I XRF results for the bulk unroasted concentrate (0 11 kg) and six magnetic fractions plus average EMP results for the bulk concentrate and the titanates only. All data expressed as wt% oxide Oxide XRF EMP Bulk conc. 0 2 kg 2 3 kg 3 4 kg 4-6 kg 6 8 kg 8 11 kg Bulk conc. Titanates 0 11 kg 0 11 kg 0 11 kg * TiO 2 55.5 52.6 53.0 56.0 58.2 55.0 47.9 56.64 60.76 Fe 2 O 3 31.3 39.1 41.6 36.5 28.6 23.6 14.1 29.83 30.88 MnO 0.91 0.966 0.728 0.840 1.07 0.937 0.476 0.842 0.754 Al 2 O 3 3.01 1.05 0.753 1.06 2.39 5.86 8.78 2.62 0.731 SiO 2 2.12 1.21 0.774 1.01 1.46 3.61 12.2 1.64 0.164 MgO 2.04 2.28 3.00 2.34 1.40 1.68 1.78 2.09 1.64 CaO 0.031 0.020 0.018 0.018 0.014 0.055 0.099 0.028 0.014 Cr 2 O 3 1.27 0.999 0.241 0.703 1.82 1.54 1.14 1.58 0.164 SO 3 0.026 0.020 0.015 0.021 0.025 0.028 0.019 0.036 0.039 P 2 O 5 0.239 0.061 0.045 0.077 0.188 0.546 0.723 0.167 0.091 ThO 2 0.034 <DL <DL 0.001 0.017 0.086 0.127 0.043 0.010 ZrO 2 0.178 0.602 0.159 0.156 0.106 0.150 3.74 n.d. n.d. Total 96.66 98.91 100.33 98.73 95.29 93.09 91.08 95.54 96.04 n=498 n=464 Wt% 100 4.2 18.7 16.9 35.0 15.8 9.4 - - * excluding 19 chrome spinel, 2 monazite, 1 staurolite, 10 tourmaline, and 2 zircon grains from the bulk concentrate EMP analysis DL = 10ppm (0.001 wt%) CHARACTERIZATION AND DISTRIBUTION OF CHROME SPINEL GRAINS 177

Figure 1. Scatter plots used to distinguish the various chrome spinels types present in the unroasted concentrate. Element data expressed as k-ratio values (~wt% element). Auto clustering calculations on the combined scatter plot data revealed at least 14 distinct chrome spinel types are present. These are indicated by the circled clusters. Note that not all clusters are present in each scatter plot. Representative compositions for each of the clusters are provided in Table II A visual inspection of the Al versus Cr scatter plots for each of the magnetic fractions shows progressive differences in the chrome spinel distribution (Figure 2). The low magnetic field strength plots (0 2 kg and 2 3 kg) are dominated by chrome spinels that are mid-range in both Al and Cr. Quantitative EMP data in Tables II and III show that the average compositions for these spinels tend to be rich in FeO and Cr 2 O 3 and low in MgO and Al 2 O 3. In contrast, the high field strength plots (6 8 kg and 8 11 kg) generally have the majority of data clustered at high-al contents. An important observation from these latter two plots is that spinels high in non-stoichiometric component (i.e. clusters 1 3 in Table II) are absent at low field strengths, but tend to be concentrated in the high field strength, non-magnetic fractions (Figures 2e and 2f). Intermediate field strength plots (3 4 kg and 4 6 kg) exhibit a broad spread in spinel compositional types. Average EMP data confirm these spinels are compositionally intermediate between the high and low field strength fractions. In order to further assess the compositional variation in chrome spinels between the different magnetic fractions, average calculated spinel end-member components from all chrome spinels present within each fraction are given in Table III and represented graphically in Figure 3. From this data the following observations are made: magnetite and ulvospinel components are highest in the low field strength fractions. Ulvospinel is consistently low in all fractions examined (<3 per cent), but in general, shows a systematic decrease toward the nonmagnetic fractions. Magnetite also exhibits a decreasing trend toward the non-magnetic fractions As indicated earlier, this fraction contains the largest amount of altered spinel and thus may be expected to also contain a highly oxidized γ-fe 2 O 3 spinel component the proportion of Fe(Al,Cr) 2 O 4 component decreases when going from the 0 2 kg to the 8 11 kg fractions. 178 HEAVY MINERALS 2003

Table II Representative EMP analyses of the 14 chrome spinel clusters in the unroasted concentrate. All data expressed as wt% oxide Oxide Chrome spinel cluster 1 2 3 4 5 6 7 8 9 10 11 12 13 14 MgO 8.41 9.48 7.74 3.32 14.01 9.09 11.36 8.51 13.02 3.41 14.11 19.50 21.04 22.35 FeO* 10.08 16.02 20.52 43.65 41.36 18.76 17.69 29.78 30.47 31.41 26.86 14.25 14.51 10.55 TiO 2 0.48 0.13 0.13 1.03 3.23 0.17 0.30 0.60 3.27 0.47 0.94 0.66 0.15 0.19 Al 2 O 3 1.25 7.35 7.56 7.75 10.29 13.79 21.50 20.68 24.87 28.20 27.71 41.63 54.12 58.17 Cr 2 O 3 71.94 67.24 63.74 39.86 29.95 58.40 50.51 39.71 29.65 32.83 32.53 25.24 11.36 10.34 MnO 6.76 0.47 0.45 1.13 0.86 0.59 0.16 0.21 0.38 0.06 0.31 0.05 0.33 0.21 Total 98.91 100.7 100.1 96.74 99.69 100.8 101.5 99.50 101.6 96.39 102.5 101.3 101.5 101.8 Element ratio 2.78 2.25 2.07 0.98 0.65 2.14 2.06 1.48 1.18 1.89 1.34 1.68 1.67 1.82 Abundance 1.0 2.0 3.3 8.5 1.6 1.0 11.1 3.6 14.7 4.8 2.4 1.7 20.5 23.9 Spinel Components Mg(Al,Cr) 2 O 4 45.6 46.8 39.1 17.9 n.d. 43.9 51.9 40.6 58.2 16.8 61.5 78.9 80.8 84.0 Fe(Al,Cr) 2 O 4 30.7 44.4 58.2 55.7 n.d. 50.9 45.3 48.6 20.9 80.9 23.8 14.8 13.0 12.8 Fe 2 TiO 4 1.3 0.3 0.3 2.8 n.d. 0.4 0.7 1.4 7.4 1.2 2.1 1.3 0.3 0.4 Fe 3 O 4 0 0 0 23.6 n.d. 0 0 9.4 13.6 1.2 12.6 5.0 5.9 2.9 (Al,Cr) 2.67 O 4 22.4 8.4 2.4 0 n.d. 4.8 2.1 0 0 0 0 0 0 0 * Microprobe analysis for Fe expressed as FeO Σ(Cr+Al) / Σ(Fe+Mg). Samples with Σ > 2.0 contain a non-stoichiometric spinel component while those with Σ < 2.0 contain a magnetite component Modal abundance of chrome spinels only (i.e. excluding titanates) measured in area% Table III Average compositions (wt%) of the chrome spinels and calculated spinel compositions (mole%) in the unroasted concentrate Oxide Bulk (0-11 kg) Fraction 0-2 kg 2-3 kg 3-4 kg 4-6 kg 6-8 kg 8-11 kg MgO 14.16 6.03 9.25 10.18 11.53 15.26 16.04 FeO* 18.86 36.28 26.93 23.60 20.60 16.38 15.85 TiO 2 0.38 1.06 1.11 0.90 0.70 0.54 0.38 Al 2 O 3 32.42 12.15 20.03 20.69 23.73 32.67 37.32 Cr 2 O 3 33.40 42.06 42.17 44.25 43.32 33.57 29.17 MnO 0.21 0.62 0.40 0.31 0.28 0.19 0.21 Total 99.43 98.18 99.89 99.92 100.15 98.55 99.11 #analyses 63 76 31 143 373 178 105 Element ratio 1.75 1.21 1.57 1.70 1.81 1.78 1.80 Spinel Components Mg(Al,Cr) 2 O 4 62.2 30.7 43.9 47.9 53.0 67.0 68.6 Fe(Al,Cr) 2 O 4 33.1 50.7 46.9 45.9 43.0 28.8 27.6 Fe 2 TiO 4 0.9 2.7 2.7 2.1 1.6 1.2 1.1 Fe 3 O 4 3.9 15.9 6.5 4.1 2.3 3.1 2.7 (Al,Cr) 2.67 O 4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 * Microprobe analyses for Fe expressed as FeO In comparison, the proportion of Mg(Al,Cr) 2 O 4 component shows the reverse trend the calculated spinel components imply that the nonstoichiometric end-member is not present in any of the fractions. While this is true for the average data as shown in Table III, there are clearly individual grains within the concentrate which are extremely high in (Al,Cr) 2.67 O 4 (e.g. clusters 1 and 2 in Table II). These grains, however, are low in total abundance, even in the most non-magnetic fraction, which is dominated by grains high in Mg(Al,Cr) 2 O 4 component (Table III). Comparison of results with the roasted concentrate Experimental data for the roasted concentrate are summarized in Tables IV and V. Assay results on individual magnetic fractions indicate an increase of 9.6 wt per cent in the amount of material now reporting to the 0 2 kg fraction compared to the pre-roast sample. This is balanced by a decrease in material within the range 2 6 kg and an average increase of ~2 wt per cent in the range 6 11 kg. Chromia contents are reduced by approximately onehalf in the 0 2 kg fraction and are increased relative to the unroasted concentrate in the 6 11 kg fractions. The changes in chromia content with roasting is consistent with the roast conditions changing the magnetic properties of the ilmenite grains, allowing more of the ilmenite to be collected at lower field strengths. CHARACTERIZATION AND DISTRIBUTION OF CHROME SPINEL GRAINS 179

Figure 2. Al versus Cr scatter plots showing the differences in the chrome spinel populations for each of the magnetic fractions in the unroasted concentrate. Element data expressed as k-ratio values (~wt% element) An inspection of the Al versus Cr plots for the roasted sample shown in Figure 4, indicate they are almost identical to those obtained for the unroasted concentrate, although there are small changes in the distribution of some phases in some magnetic fractions. Similarly, a plot of calculated spinel end-member components (Figure 3b) shows similar trends for all components to that in the unroasted fractions. As some of the clusters only correspond to a small number of chrome spinel grains it is difficult to make a confident statement about the effect of roasting on the distribution of chrome spinels in this sample. Further work using more extreme roasting conditions is to be carried out to determine whether the small changes in the distribution of the chrome spinels observed in the mild roast can be seen more clearly. Summary The mineralogy and chemistry of chrome spinels in an ilmenite concentrate sourced from the Murray Basin have been examined. Characterization of the concentrate was performed before and after a low temperature roast treatment and included techniques such as: magnetic fractionation, XRF assaying, grain mapping, quantitative EMP analysis of the chrome spinels, automated mineral phase clustering, and modal analysis. In the pre-roast sample, auto clustering on the map data revealed at least 14 distinct chrome spinel types are present. There is considerable variation in the amount of each chrome spinel type present, although a high-mg, high-al phase makes up close to 45 per cent of the total spinel 180 HEAVY MINERALS 2003

Figure 3. Graphs showing the variation in calculated spinel components for each of the magnetic fractions. Data were calculated using the mean chrome spinel composition determined for each fraction in the unroasted and roasted concentrates (Tables III and V respectively) Table IV XRF results for the roasted bulk concentrate (0-11 kg) and six magnetic fractions. All data expressed as wt% oxide Oxide XRF Bulk conc.0-11 kg 0 2 kg 2 3 kg 3 4 kg 4 6 kg 6 8 kg 8 11 kg TiO 2 55.5 53.8 54.5 56.6 58.8 56.4 51.2 Fe 2 O 3 30.6 41.4 40.3 34.5 28.6 25.1 16.5 MnO 0.902 0.743 0.721 0.886 1.16 1.02 0.610 Al 2 O 3 3.05 0.750 0.832 1.21 2.63 5.27 7.49 SiO 2 2.77 0.812 0.818 1.15 1.55 3.27 10.3 MgO 1.96 2.88 2.97 2.13 1.32 1.40 1.59 CaO 0.031 0.019 0.020 0.017 0.018 0.042 0.084 Cr 2 O 3 1.21 0.419 0.348 0.942 2.00 1.46 1.13 SO 3 0.014 0.006 0.011 0.018 0.020 0.020 0.014 P 2 O 5 0.265 0.048 0.054 0.099 0.213 0.453 0.710 ThO 2 0.034 <DL <DL 0.004 0.018 0.070 0.122 ZrO 2 0.601 0.180 0.165 0.224 0.107 0.122 3.37 Total 96.94 101.06 100.74 97.78 96.44 94.63 93.12 Wt% 100 16.0 15.3 9.6 30.5 17.7 11.0 * +9.6-3.4-7.3-4.5 +1.9 +1.6 * = difference (in wt%) between unroasted and roasted samples DL = 10ppm (0.001 wt%) CHARACTERIZATION AND DISTRIBUTION OF CHROME SPINEL GRAINS 181

Oxide Table V Microprobe analyses of chrome spinel grains (wt%) and calculated spinel compositions (mole%) in the roasted concentrate Fraction 0 2 kg 2 3 kg 3 4 kg 4 6 kg 6 8 kg 8 11 kg MgO 6.68 9.25 9.80 11.85 14.31 16.34 FeO* 34.07 27.12 24.03 19.96 17.25 17.34 TiO2 1.01 1.46 1.27 0.90 0.46 0.77 Al2O3 13.14 19.24 20.50 24.26 29.40 35.69 Cr2O3 43.37 42.11 43.89 42.53 38.92 29.63 MnO 0.56 0.29 0.34 0.27 0.18 0.15 Total 98.83 99.47 99.83 99.77 100.51 99.91 #analyses 55 73 196 325 52 26 Element ratio 1.29 1.54 1.70 1.81 1.83 1.69 Spinel Components Mg(Al,Cr) 2 O 4 33.6 44.2 46.4 54.5 63.0 69.6 Fe(Al,Cr) 2 O 4 50.3 45.6 47.0 41.5 33.7 24.0 Fe 2 TiO 4 2.6 3.5 3.0 2.1 1.0 1.6 Fe3O4 13.5 6.7 3.6 2.0 2.3 4.7 (Al,Cr) 2. 67 O 0.0 0.0 0.0 0.0 0.0 0.0 * Microprobe analyses for Fe expressed as FeO Σ(Cr+Al) / Σ(Fe+Mg) Figure 4. Al versus Cr scatter plots showing the differences in the chrome spinel populations for each of the magnetic fractions in the roasted concentrate. Element data expressed as k-ratio values (~wt% element) 182 HEAVY MINERALS 2003

population. Magnetic fractionation results showed significant variation in the composition and relative abundance of spinels reporting to each fraction. Low magnetic field strength fractions were dominated by chrome spinels that are mid-range in Al and Cr. These spinels have the highest magnetite, ulvospinel and Fe(Al,Cr) 2 O 4 components. In comparison, the high field strength fractions have the majority of data clustered at high-al contents indicating a higher proportion of Mg(Al,Cr) 2 O 4 component. Spinels high in the (Al,Cr) 2.67 O 4 non-stoichiometric component are absent at low field strengths, but tend to be concentrated in the high field strength, non-magnetic fractions. Intermediate field strength fractions tend to exhibit a broad spread in spinel compositional types. Results showed that a mild roast treatment of the sample had no clear effect on the magnetic properties and hence the distribution of chrome spinels in the sample, but it did cause a change in the distribution of the accompanying ilmenite grains. It is proposed to use more extreme roasting conditions to see if any clear differences can be seen. Acknowlegements The authors wish to acknowledge the assistance of colleagues Ken McDonald (roasting testwork), Christina Li (magnetic separations), Cameron Davidson (EMP sample preparation), and Keri Constanti-Carey (data presentation). Ian Grey is also thanked for reviewing the manuscript. References 1. NELL, J., and DEN HOED, P. Separation of chromium oxides from ilmenite by roasting and increasing the magnetic susceptibility of Fe 2 O 3 - FeTiO 3 (ilmenite) solid solutions. Heavy Minerals 1997, Johannesburg, The South African Institute of Mining and Metallurgy, 1997. pp 75 78. 2. GREY, I.E., MACRAE, C.M., and NICHOLSON, T. Alteration of ilmenite in the Murray Basin implications for processing. Murray Basin Mineral Sands Extended Abstracts, Australian Institute of Geoscientists Bulletin, No. 26, 1999. pp 129-134. 3. POWNCEBY, M.I, MACRAE, C.M., and WILSON, N.C. Electron microprobe mapping A diagnostic tool for ilmenite characterisation. International Heavy Minerals 2001, Fremantle, The Australasian Institution of Mining and Metallurgy, 2001. pp. 69 74. 4. PEDERSON, K. Non-stoichiometric magnesian spinels in shale xenoliths from a native iron-bearing andesite at Asuk, Disko, central west Greenland. Contrib. Mineral. Petrol., vol. 67, 1978. pp. 331 340. 5. GREY, I.E. and REID, A.F. The structure of pseudorutile and its role in the natural alteration of ilmenite. Am. Mineral., vol. 60, 1976. pp. 898 906. 6. GREY, I.E. and LI, C. Low temperature roasting of ilmenite phase chemistry and applications. Proceedings AUSIMM, vol. 306(2), 2001. pp. 35 42. 7. HARROWFIELD, I.R., MACRAE, C.M., and WILSON, N.C. Chemical imaging in electron microprobes. Proceedings of the 27th annual MAS meeting 1993, New York, Microbeam Analysis Society, 1993. pp. 547 548. 8. WILSON, N.C., HARROWFIELD, I.R., MACRAE, C.M., and SCARLETT, N.V. New approaches in feature and phase identification in X-ray mapping. Proceedings of 16th Australian Conference on Electron Microscopy, 2000. pp. 46. 9. RICHARDS, J.A. Remote Sensing Digital Image Analysis: An Introduction. Springer-Verlag, New York, NY, 1986. 10. LUMPKIN, G.R. Crystal chemistry and durability of the spinel structure type in natural systems. Progr. Nucl. Energy, vol. 38, 2001. pp. 447 454. CHARACTERIZATION AND DISTRIBUTION OF CHROME SPINEL GRAINS 183

184 HEAVY MINERALS 2003