International Journal of Mineral Processing 133 (2014) 73–82

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International Journal of Mineral Processing

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The fate of chromium impurities during acid sulphate digestion ofilmenite concentrates

Mark I. Pownceby ⁎CSIRO Mineral Resources Flagship, Bayview Avenue Clayton VIC 3168, Australia

⁎ Tel.: +61 3 95458820.E-mail address: Mark.Pownceby@csiro.au.

http://dx.doi.org/10.1016/j.minpro.2014.09.0160301-7516/Crown Copyright © 2014 Published by Elsevie

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 April 2014Accepted 30 September 2014Available online 7 October 2014

Keywords:ChromiumSpinelIlmeniteAcid sulphate digestionMurray Basin

Ilmenite concentrates from the Murray Basin region of southeastern Australia are contaminated with chromiumimpurities that must be removed for the ilmenite to become a satisfactory feedstock for the sulphate route to ti-tania pigment production. The chromia is present primarily as discrete, compositionally variable, chrome-richspinel grains with a smaller amount as intra-grain chromia distributed as coatings in fractures and pores ofweathered ilmenite grains. Characterisation of chromia deportment through a simulated acid sulphate digestionprocess showed a small but non-negligible solubility of the spinels. Most spinels were resistant to dissolutionwith the exception of those containing high Fe(Al,Cr)2O4 andmagnetite (Fe3O4) components. Intra grain chromiawas highly soluble. Processing to achieve lowbulk chromia using amagnetising roast proceduremust ensure thatwell crystallised rutile is not produced because of its insolubility in the sulphate process. It is also importantthat the roast conditions do not substantially increase the magnetite content of the spinels making them moresusceptible to dissolution. This work highlights the importance of characterising all spinel composition typeswithin ilmenite concentrates in addition to the level of intra-grain chromia associated with the ilmenite whenconsidering the suitability of Murray Basin primary ilmenites as a feedstock to sulphate route titania pigmentplants.

Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

1. Introduction

Australia is one of the largest producers of mineral sands in theworld with most of the current production coming from deposits inWestern Australia. The position of Australia as a continuing long-termsupplier of mineral sands, and their upgraded products, has beenreinforcedwith the discovery and development of extensivemineral re-serves in the Murray Basin, an area covering parts of South Australia,New South Wales and Victoria (Fig. 1). Deposits in the Murray Basinare of two general types, fine-grained (40–80 μm) offshore sheet-likedeposits, and coarse-grained (90–300 μm) beach facies strandlinedeposits (Roy et al., 2000). The latter deposits have size ranges compa-rable to Western Australian deposits and are the target for currentdevelopment in the Basin. Both deposit types have ilmenite as themajor heavy mineral component. Ilmenite, however, has the lowestcommercial value of the heavyminerals and to realise the full economicvalue of the deposits, processing of the ilmenite to amarketable productis essential.

Primary ilmenite concentrates (i.e. those containing 45–55 wt.%TiO2) produced from theMurray Basin deposits are potential feedstocksfor sulphate-route titania pigment plants. The requirements for suchfeedstocks (Harben, 2002), include; a high ferrous iron content to

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react with the sulphuric acid, a low rutile content since rutile is largelyinsoluble in sulphuric acid, low calcium and phosphorus which impedecrystallisation, and low levels of elements that could impart a colour tothe pigment. Chromium is a particular problem in this latter regard andthe acceptable chromia level for further processing of ilmenite is of theorder of ~0.1wt.% Cr2O3 or less (Beukes and vanNiekerk, 1999).MurrayBasin primary ilmenite concentrates generally meet the first threerequirements, but are limited in their suitability as sulphate-routefeedstocks because of elevated chromia contents, typically N0.5 wt.%Cr2O3 (Pownceby et al., 2003; Pownceby, 2005). Previousmineralogicalinvestigations have shown the chromia is present predominantlywithin discrete spinel grains so in theory it should be possible to makea clean separation of the spinels from the ilmenite using physicalseparation methods such as gravity and magnetic techniques. Inpractice, however, separation is difficult as the spinels have variablecompositions as a result of solid solution and/or weathering, therebyproviding correspondingly variable physical properties (Powncebyet al., 2001, 2003; Pownceby, 2005).

In this paper, the occurrence of chromia in both coarse- and fine-grained Murray Basin primary ilmenite concentrates is discussed, asare the effects of a laboratory-based, sulphate-route digestion proce-dure on the solubility of the contained chromia. Since the bulk of thechromia is known to be associated with the presence of individualchrome-bearing spinel grains, the aim is to characterise a series ofilmenite concentrates before treatment, followed by characterisation

MELBOURNE

Broken Hill

N

Boundary of Murray Basin

Mildura

0 160Kilometres

ADEL

AID

E

Coarse-grained strandline depositsFine-grained offshore deposits

Fig. 1. Map showing the location of the Murray Basin region in southeastern Australia.The boundary of the Murray Basin is indicated by the dashed line. The host unit for thecoarse- and fine-grained mineral sand deposits is the Loxton–Parilla sands (shadedregion).

FeT

Al

Cr

MgCr O2 4

MgAl O2 4

FeAl O2 4

FeCr O2 4

MgFe O2 4

(Fe O )3 4

(Al,Cr) O2.67 4

Mg

Fig. 2. Plot showing the range of spinel compositions measured in Murray Basin ilmeniteconcentrates (Pownceby, 2005).

74 M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

of the products and residues at various stages throughout the digestionprocess. Characterisation includes bulk chemistry determinations(solids and liquids) in associationwithquantitative electronmicroscopyof individual spinel grains within feedstocks and residues. Finally, theimplications of the results for the future processing of Murray Basinprimary ilmenite concentrates are discussed.

2. Murray Basin spinel compositional variation

Spinels contain two differing cations, or at least two different va-lences of the same cation, in the ratio 2:1. This gives the general formulaAB2O4. In general, the spinel types commonly associated with ilmeniteconcentrates are dominated by compositions containing the cationsCr, Mg, Fe2+, Fe3+, Al and Ti. The major cations substituting into the Asite are the divalent cations Mg and Fe2+ whereas substitution withinthe B site involves the cations Al, Cr, Fe3+ and Ti. Aluminium, iron andchromium are each trivalent. However, substitution of Ti4+ into the oc-tahedral B site may also occur (as in the case of ulvöspinel — Fe2TiO4)giving rise to a range of spinel solid solutions within the system (Fe2+,Mg)(Al,Cr,Fe3+)2O4–(Fe2+,Mg)2TiO4.

Spinel solid solutions may also contain some degree of non-stoichiometry associated with defects in the oxide structure. The mostlikely non-stoichiometry to occur in spinels associated with ilmeniteconcentrates is the defect spinel component (Fe3+,Al,Cr)2.67O4

(Pederson, 1978). This type of defect spinel is believed to occur in natu-ral systems as a result of the chemical weathering of the spinel by thesame mechanism that occurs in ilmenite alteration i.e. diffusion of ironand other divalent elements out of the spinel, with oxidation of theremaining iron to the trivalent state to maintain charge balance (Greyand Reid, 1976).

Pownceby (2005) previously characterised the range of spinel com-positional variation within Murray Basin ilmenite concentrates. Thesedata are shown in Fig. 2 plotted on a quaternary FeT–Mg–Cr–Al diagramwhich describes a compositional area enclosed by the seven spinel endmember components; Fe3O4, FeCr2O4, MgCr2O4, FeAl2O4, MgAl2O4,MgFe2O4 and (Al,Cr)2.67O4. Compositions which lie outside this areaare typically spinels that have either been extensively altered or leachedor represent more exotic spinel compositions such as those dominatedby Mn- and Zn-rich components. The bulk of the spinel data from theMurray Basin are Fe- and Cr-rich spinels such as chromite (FeCr2O4),however trends towards more MgAl-rich and MgCr-rich compositionsare evident. The spread of the data toward the FeT apex indicates spinelchemistries trending towardsmoreMgFe-rich ormore ferric-rich (mag-netite) compositions.

3. Experimental

3.1. Samples

Samples used for the digestion tests comprised two primaryilmenite concentrates (Ilmenites A and B) and one ilmenite concentrate(Ilmenite C) that had been subject to procedures to lower the total chro-mium content by removing some of the spinels through a low temper-ature roast treatment followed by magnetic separation to derive a lowchromia primary ilmenite concentrate. Details of the bulk chemistry,gangue mineralogy and processing conditions for each of the samplesare given below.

3.1.1. a) Primary unprocessed concentratesSamples A (54.2 wt.% TiO2; 0.435 wt.% Cr2O3) and B (53.5 wt.% TiO2;

0.551wt.% Cr2O3) were primary ilmenite concentrates that representedthe grainsize range of deposit types within the Basin. Ilmenite A was acoarse-grained strandline concentrate sourced from the western sectorof the Basin whilst Ilmenite B was typical of the fine-grained materialprevalent in the south-eastern sector of the Basin (Fig. 1). The sampleswere supplied from commercial operations and each represented theprimary ilmenite fraction only. In both samples, unweathered ilmenite

75M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

(50–60%), pseudorutile (30–40%) and minor rutile (b3–4%) were themajor titanate phases. Gangue minerals (b3–4% by weight in total)comprised spinels, aluminosilicates (mainly clay minerals filling pores,plus large, discrete tourmaline grains), quartz, zircon and monazite.

3.1.2. b) Processed concentrateSample C was initially a coarse-grained, high chromia containing

concentrate (1.27 wt.% Cr2O3) from the central part of the MurrayBasin, the mineralogy of which comprised relatively unweatheredprimary ilmenite (20%), highly weathered pseudorutile (70%) andminor secondary rutile (b5%). Gangueminerals (b5%) included; spinels,aluminosilicates, zircon and monazite. To prepare a low-Cr2O3 samplesuitable as a sulphate route feedstock, the bulk concentrate wasupgraded by magnetic separation at 1.5 kGauss to give a primaryilmenite feedstock containing 53.6 wt.% TiO2 and 0.28 wt.% Cr2O3. Thismaterial was then subjected to a low-temperature fluidised bed processtreatment to alter the magnetic properties of the ilmenite to allow sep-aration of the chrome spinel grains (Grey and Li, 2001; Fisher-Whiteet al., 2007). The roasting conditions involved heating the ilmenite con-centrate in a CO/N2 atmosphere at 625 °C for 60 min before quenchingunder a nitrogen gas stream. The sample was then magneticallyfractionated at 2.0 kGauss to remove the bulk of the remaining chromespinel contaminant grains. The final upgraded sample subject to diges-tion test work contained 55.9 wt.% TiO2 and 0.151 wt.% Cr2O3.

3.2. Digestion procedure

A CSIRO developed 100 g scale laboratory ilmenite digestion proce-dure was used to simulate the effect of a sulphate-route pigment plant(Grey et al., 1996). As in the pigment plant, the ilmenite concentratewas ground and sieved for the laboratory test work, and the −45 μmfraction used for the digestion tests. Digestion conditions were main-tained close to values used in pigment plants with the weight ratio ofacid to ore typically 1.6, and the acid strength 90–91% (on a wt/wtbasis). Comparisons of previous digestion results with the performanceof samples in a commercial sulphate route pigment plant have shownthat ilmenite samples that give a mass solubility N93% and chromiasolubilities of b0.05 wt.% in the laboratory test are usually satisfactoryas feedstocks.

The digestion apparatus comprised a 300 ml cylindrical glass vesselwith a hemispherical base, seated in a stainless steel heating bath filledwith silicone oil. A stainless steel stirrer with a 10 × 30 mm paddle,driven at 400 rpm, was located centrally in the digestion vessel. Glassthermometers were located in the oil bath and digestion vessel tomon-itor temperature.

For the digestions, a predetermined quantity of 98% H2SO4 waspreheated to 60 °C and stirring commenced. When the acid tempera-ture reached 60 °C, 100 g of the ground sample was added and mixedwith the acid for 20 min to ensure good wetting. About 10 ml of 10%H2SO4, sufficient to dilute the acid to the desired concentration, wasthen added to ‘set off’ the digestion reaction. This resulted in a rapidrise in the slurry temperature to about 100 °C as the rates of exothermicsulfation reactions became appreciable. The digestion then became au-togenous, and the temperature increased steadily to a maximum valuethat was typically 170–180 °C. During this time the slurry graduallythickened as various ferrous, ferric and titanium sulphates formed,and finally solidified, to the point where stirring could not be main-tained. The stirrer was then stopped and any slurry sticking to theupper wall of the digester (from slurry frothing) was scraped into thebulk. The digestion cakewas then ‘baked’ for ~1 hwith the temperaturemaintained at a constant value, usually the maximum temperatureencountered during digestion. At the end of the baking, the solid cakewas chipped out of the reaction vessel and weighed and stored in drynitrogen.

Approximately half the digestion cake was crushed and sieved to−2 mm. A 50 g portion of the crushed cake was then leached for 4 h

at 60 °C in 80 ml of 10% H2SO4 with stirring. The leach solution wasfiltered and the residue washed three times with 20% acid, three timeswith distilled water and then dried overnight at 110 °C. The driedresidue was ashed at 400 °C and calcined at 800 °C. The ashed residuewas analysed by x-ray fluorescence spectroscopy (XRF) and the leachsolution, made up to 1 L, was analysed for Ti, Fe and Cr using ICP-AES.

3.3. Electron probe microanalyses

Characterisation of the spinels pre- and post-digestion was by theuse of quantitative electron probe microanalysis (EPMA). To locateindividual spinel grains in the pre-digested ilmenite concentrates,each sample block was automatically step-scanned on a grid of 640 ×640 points with 15 μm step size between points (total coverage9.6 mm × 9.6 mm or 92.16 mm2). At each grid point a 5 ms analysiswasmade for Cr X-rays using the Kα line. The Crmap showed the posi-tions of spinel grains as an array of bright spots, the coordinates ofwhich could be retrieved and stored. For the finer-grained concentratethe same size area was mapped, but the step size was reduced to8 μmwhilst for all post-digestion residues, the step size was further re-duced to 4 μm. In addition, the overall map area was decreased to61.44 mm2 for the digest residues as the number of spinels availablefor analysis was significantly larger than in the pre-digested samples.

Once the coordinates of individual spinel grains were stored,elemental analyses were obtained. For the quantitative analyses, themicroprobe was operated at 20 kV and 100 nA. The suite of elementsanalysed included Cr, Fe, Mg, Al, Mn, Si, Zn and Ti. Standards usedwere wollastonite (CaSiO3), spinel (MgAl2O4), MnFe alloy, hematite(Fe2O3), ZnS, rutile and chromium metal. Detection limits (2σ s.d.) forthe analysis conditions used were (in ppm); Cr 200, Fe 100, Mg 80, Al70, Ti 70, Zn 400, Si 100 and Mn 150. In this study, only analyses fromthe clear and optically homogeneous part of each chrome-spinel grainwere included in the dataset.

3.4. Data representation

Major compositional features of spinels are traditionally illustratedby plotting element ratios such as Al/(Al + Cr) against Mg/(Mg + Fe2+) or Fe3+/(Cr + Al + Fe3+) against Mg/(Mg + Fe2+).These plots include all the major elements concentrated in spinels en-abling any compositional trends to be revealed. To derive the Fe3+

from the total Fe measured, the common procedure is to recalculatethe ferric iron content based on the assumption of spinel stoichiometry.Stoichiometry-based calculation methods, however, result in the Fe2+:Fe3+ ratio being the major source of error in the data as this aspect ofthe mineral chemistry is the most likely to be affected by oxidationthrough either natural weathering processes (in the pre-digestedsamples) or during acid digestion. In this study therefore, major compo-sitional features of spinels are compared by plotting the element data asgraphs of Al/(Al + Cr) against Mg/(Mg+ FeT), where all iron values areshown as wt.% total iron. In addition to the element ratio plots, the dataare also displayed in a series of Cr versus Al, x–y scatter plots. Previousexperience in characterising spinels fromhigh-Cr ilmenite concentrateshas indicated that Al and Cr tend to be the least mobile of the cationscommonly found in spinels meaning that simple x–y plots can poten-tially supply reliable information regarding compositional variation(Pownceby et al., 2003).

When attempting to examine variations in spinel compositionsdetermined via microprobe analyses, it can be helpful to recast the mi-croprobe oxide data into individual spinel end-member components.For example, a spinel analysis containing only the oxides FeO, MgOand Cr2O3 may be considered to be composed of the end-membersFeCr2O4 and MgCr2O4 (the exact molar ratio of these components isdependent upon the relative amounts of FeO and MgO). Similarly, if aquantity of Al2O3 is also determined in the analysis, the end-membercomponents may be viewed as a combination of FeCr2O4, FeAl2O4,

Table 1Bulk chemical analyses (wt.%) of digestion feedstocks.

Oxide Sample

Ilmenite A —

coarse-grainedIlmenite B —

fine-grainedIlmenite C — upgradedcoarse-grained

TiO2 54.2 53.5 55.9Total Fe (as Fe2O3) 42.2 42.3 41.2FeOa 22.9 21.3 n.d.Cr2O3 0.435 0.551 0.151MgO 0.61 1.45 2.74MnO 2.35 1.53 0.81Al2O3 0.46 0.68 0.72SiO2 0.90 0.99 0.77

n.d. not determined.a Determined via titrimetric analysis.

76 M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

MgCr2O4, and MgAl2O4. In this type of procedure, however, it is notpossible to uniquely separate the Al and Cr into individual Fe or Mgspinel components because element mixing may occur on more thanone site (e.g. Fe2+ for Mg and Al for Cr). Therefore, for the purposes ofthe current investigation, in a spinel formula containing these fouroxides, the following calculated components were used Fe(Al,Cr)2O4

and Mg(Al,Cr)2O4, FeTi2O4, Fe3O4 and (Al,Cr)2.67O4 (the defect spinel).

4. Results

4.1. Sample characterisation

Bulk XRF chemical analyses for the three samples used in this workare given in Table 1. The ilmenites all show significant departuresfrom the stoichiometric composition (47.4 wt.% FeO, 52.6 wt.% TiO2)due to weathering, which involves hydration, oxidation of ferrous ironand leaching of iron. Impurities in the concentrates include; MgO,MnO, Al2O3, and SiO2 (typically up to 3.5–5.0 wt.% in total) whilstCr2O3 varies between 0.151 (processed Ilmenite C) and 0.551 wt.%

Table 2Electron microprobe analyses (wt.%) of ilmenite grains in samples A–D.

Oxide Average all grains Average of 5 wt.% TiO2 intervals

40–45 45–50 50–55

Ilmenite A — coarse grainedTiO2 54.4 44.1 48.8 52.7Fe2O3 41.7 53.4 47.6 44.7Cr2O3 0.03 0.00 0.03 0.02MnO 2.54 0.68 2.08 2.87MgO 0.49 2.80 1.08 0.44Al2O3 0.16 0.23 0.20 0.09SiO2 0.23 0.01 0.32 0.14No. grains 200 1 19 120

Ilmenite B — fine grainedTiO2 53.4 43.6 48.5 52.4Fe2O3 42.2 52.1 47.4 44.2Cr2O3 0.04 0.01 0.04 0.04MnO 1.57 1.60 1.02 1.50MgO 1.62 3.25 2.29 1.78Al2O3 0.28 0.40 0.34 0.20SiO2 0.40 0.29 0.49 0.30No. grains 491 10 81 273

Ilmenite C — upgraded coarse graineda

TiO2 52.2 43.8 47.1 52.6Fe2O3 40.1 49.3 47.6 39.5Cr2O3 0.10 0.04 0.04 0.10MnO 0.85 0.67 0.61 0.90MgO 3.11 4.46 4.61 3.00Al2O3 0.55 0.45 0.33 0.42SiO2 0.32 0.03 0.03 0.35No. grains 659 68 292 61

a Data measured for −45 μm fraction — unground sample unavailable for examination.

(Ilmenite B). Magnesium and manganese impurities are typicallyassociated with the ilmenite grains as substitutional impurities whilstsilicon and aluminium are present as either quartz and/or aluminosili-cates (e.g. tourmaline, kaolinite) in the form of inclusions (minor),discrete grains or coatings. Some magnesium is also likely present inspinel grains as (Fe,Mg)Cr2O4 and (Fe,Mg)Al2O4 components. For themost part, chromium is largely concentrated within individualchrome-bearing spinel grains and a bulk assay value of 0.5 wt.% Cr2O3

is equivalent to about 1.5–2.0 wt.% of spinel contaminant grains.Quantitative EPMA data was collected on individual ilmenite grains

within the pre-ground concentrates. Point analyses were all within il-menite grains, so the results do not include contributions from gangue.Data are provided in Table 2 in the form of a calculated average ilmenitecomposition plus, for each sample, average data for TiO2 intervals of5 wt.% from 40–45% TiO2 to N75% TiO2. The former shows that the il-menite grains are largely free of chromia with average bulk Cr2O3 con-tents of 0.03 wt.% (Sample A), 0.04% (Ilmenite B), and 0.10% (IlmeniteC), confirming that the bulk of the chromia is concentrated within dis-crete spinel grains. The average ilmenite compositions given for eachsample thus represent the minimum Cr2O3 that could be achieved ifall chrome-containing spinels were able to be removed. An importantobservation from the ilmenite data at 5 wt.% TiO2 intervals is that asthe ilmenite grains become more weathered (i.e. higher in TiO2), thecontained Cr2O3 content increases. It is presumed that these higher as-says result from Cr-rich material that has been leached during theweathering processes and deposited, along with aluminosilicatealteration products, in fractures and micro- and nano-pores of thealtered ilmenite. This effect is much more pronounced in Ilmenite C,where 0.3–0.4 wt.% Cr2O3 was measured in the most altered ilmenitefractions. Chromium present in this form is likely to be highly solubleand therefore may have important implications when processing theores in acid sulphate solutions.

For Ilmenites A and B, the bulk average calculated from the EPMAdata approximates the measured bulk composition determined byXRF. The major points of difference are largely in the calculated Al2O3

55–60 60–65 65–70 70–75 N75

56.7 61.6 67.1 71.6 82.037.3 30.7 23.0 18.6 14.20.04 0.08 0.13 0.14 0.012.36 1.69 1.24 0.91 1.850.44 0.31 0.24 0.21 0.030.15 0.47 0.61 0.92 0.100.22 0.52 0.74 0.67 3.1

39 13 4 2 1

57.1 62.1 67.2 71.5 85.136.2 29.9 19.1 17.5 9.80.03 0.06 0.12 0.13 0.042.18 1.96 1.03 1.36 0.700.80 0.38 0.13 2.34 0.020.34 0.44 1.22 0.55 0.710.50 0.47 2.52 0.76 1.20

96 21 5 2 3

57.8 61.7 67.2 71.5 82.733.0 27.7 18.4 12.9 5.70.16 0.20 0.32 0.39 0.391.49 1.21 0.23 0.16 0.050.88 0.95 1.36 0.87 0.370.54 0.89 2.01 1.99 1.820.46 0.82 1.24 1.93 2.09

139 55 27 14 3

Table 3Summary of acid digestion results.

Parameter Sample

Ilmenite A —

coarse-grainedIlmenite B —

fine-grainedIlmenite C —

upgradedcoarse-grained

% Cr2O3 in feed 0.435 0.551 0.151Residue as % of ilmenite feed 5.44 5.94 9.52

XRF on residueTiO2 53.0 49.4 82.0Fe2O3 17.80 21.90 6.66Cr2O3 6.87 7.76 0.38

Results from residue and cake analyses% mass solubility 94.6 94.1 90.5% TiO2 solubility 94.7 94.5 86.2% Fe2O3 solubility 97.7 96.9 98.4% Cr2O3 solubility 14.0 16.4 75.9Soluble Cr2O3 (wt.%) 0.061 0.090 0.115

77M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

and SiO2 assays which are underestimated. This is due to theseelements/oxides being present in the bulk concentrate as separatequartz and aluminosilicate grains. In comparison, the average calculatedEPMA data for Ilmenite C underestimates the bulk TiO2 by about 3 wt.%TiO2. It is assumed that in these concentrates there is additional TiO2

present in the form of unseparated rutile particles or as fine-grainedsecondary rutile formed during weathering of the ilmenite or producedas a consequence of the roasting procedure.

4.2. Acid leach test results

A summary of the acid digestion results is given in Table 3. IlmenitesA and B both had high titania mass solubilities at N94% and wouldtherefore be expected to perform well in a sulphate-route pigmentplant. X-ray diffraction (XRD) analysis of the solid residues indicatedthat rutile was the major insoluble crystalline phase with minor insolu-ble phases including quartz, anatase, and spinels. For both samples,approximately 15% of the total chromia was attacked in the digestion.This corresponds to soluble chromia contents of 0.061 wt.% for IlmeniteA and 0.090 wt.% for Ilmenite B. In comparison, the amounts of solublechromia as measured directly on the leach solutions were 0.057 wt.%and 0.060 wt.%, respectively. The slight discrepancy between chromiasolubilities is believed due to incomplete dissolution of the spinelsduring preparation of fused beads for XRF analysis.

The upgraded Ilmenite C, gave a very different result from the others.Whilst its overall mass solubility was close to specification at 90.5%, themass percent of soluble chromia in the feed was extremely high at75.9%. This translates to 0.115 wt.% soluble chromia (0.095 wt.% from

Table 4Summary of quantitative EPMA results for spinels in feeds and digestion residues.

Sample Element (wt.%)

Ti Zn Al Cr Fe

Pre-groundA coarse 0.53 0.42 8.06 35.00 19.29B fine 0.80 0.80 6.90 37.23 17.89

−45 μm ground fractionA coarse 1.37 0.51 8.02 34.32 18.85B fine 1.33 0.62 7.55 36.05 17.65C coarse (ug) 1.56 0.46 9.69 28.50 22.80

After digestion (residue)A coarse 1.25 0.44 8.02 35.48 17.16B fine 1.03 0.62 7.62 36.37 16.85C coarse (ug) 1.97 0.59 9.32 29.75 20.96

ug = upgraded via roasting and magnetic separation.

the leach solution). The amount of soluble titania in this material wasalso noticeably less than the other concentrates indicating a higherinsoluble TiO2 content. This was confirmed by XRD analysis which indi-cated high levels of rutile in the leach residue.

4.3. Characterisation of spinels through the digestion process

A total of 2910 individual spinel analyses from each stage of the di-gestion process were collected. The total number of analyses consistedof 1437 measurements from the fine-grained concentrate (IlmeniteB) and 1473 measurements from the two coarse fractions (Ilmenites Aand C). Where possible, the samples were analysed at the followingstages: as received, after grinding to −45 μm, and then after digestion(residue).

4.3.1. Pre ground concentratesA summary of EPMA data for spinel grains from the pre-ground

Ilmenites A and B is given in Table 4 and Fig. 3. There are no data forIlmenite C as the un-ground sample was unavailable for examination.

The EPMA results showed there were no significant differencesbetween the types of spinels present in the two pre-ground samplesexamined. Variation in the number of grains examined for each samplereflected differences in average grainsize i.e. more spinel grains permeasured area were located in the fine-grained Ilmenite B sample.The data in Table 4 indicates that spinels in both concentrates werehigh in Cr and Fe and comparatively lower in Al and Fe. Inspection ofthe plots in Fig. 3, however, reveals the averaged data in Table 4masks what is essentially a similarly broad range in composition domi-nated by a strong trend ranging from Al-rich spinel compositions (up to20% Al) to Cr-rich compositions (up to 50% Cr). This variation in spinelcomposition is also reflected in the Al/(Al + Cr) versus Mg/(Mg +FeT) plots provided in Fig. 3. It is noted, however, the bulk of the datain these latter plots are clustered around generally low Al/(Al + Cr)and low Mg/(Mg + FeT) ratios indicating compositions dominated byFe- and Cr-rich spinel components and with only minor trends towardMgCr2O4 andMgAl2O4 compositions evident. In other Murray Basin de-posits, the compositions of the spinels tend to be richer in Al2O3, insome cases containing up to 50–60 wt.% Al2O3 (Pownceby et al.,2003). The lack of high-Al spinels in the current samples reflect thefact that high-Al spinels are generally non-magnetic (except if there isa significant Fe content) and are therefore absent from these primary il-menite concentrates i.e. the more Al-rich spinels would have been re-moved as part of the non-magnetic fraction (Pownceby and Bourne,2006).

The high Cr and Fe contents are echoed in the high proportion ofcalculated Fe(Al,Cr)2O4 component (~60–61 mol%) for the samples(Table 5).Whilst calculation of this component also implies some corre-lation between Fe and Al, analysis of additional x–y plots constructed for

Al/(Al + Cr) Mg/(Mg + FeT) Analyses

Mg

4.47 0.187 0.188 1874.49 0.156 0.200 473

4.54 0.189 0.194 1334.43 0.173 0.201 2884.44 0.253 0.163 43

4.98 0.225 0.225 9794.99 0.228 0.228 6764.77 0.185 0.185 131

Ilmenite A - coarse

0

4

8

12

16

20

0 10 20 30 40 50 60Cr (wt.%)

Al - Cr s ubst itution, mainly

- (Fe,Mg)CrO

3+

3+

24

MgAl O24

0

4

8

12

16

20

0 10 20 30 40 50 60Cr (wt.%)

0

0.2

0.4

0.6

0.8

0 0.1 0.2 0.3 0.4 0.5

Increasing

MgCr O24

Mg

/ (M

g +

Fe

)T

Al / (Al + Cr)

0

0.2

0.4

0.6

0.8

0 0.1 0.2 0.3 0.4 0.5M

g / (

Mg

+ Fe

) T

b)

d)

Al / (Al + Cr)

Ilmenite B - fine

Al (

wt.%

)

N = 187

Al (

wt.%

)

N = 473

Increas ing

MgA l O2 4

a)

c)

Fig. 3. Cr vs. Al and Al/(Al+ Cr) vs. Mg/(Mg+ FeT) plots showing the compositional variability in the chrome spinels containedwithin the starting ilmenite concentrates (unground). Thestarting material for Ilmenite C was unavailable for analysis. N = number of analyses.

78 M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

other element combinations indicate the amount of FeAl2O4 componentis likely to be small, with most Al associated with an MgAl2O4

component, as part of an MgAl2O4-MgCr2O4 series. This was confirmedby the 36–37 mol% Mg(Al,Cr)2O4 component calculated for bothconcentrates. The calculated spinel components also show that thenon-stoichiometric end-member is not present in the samples. Whilstthis is true for the averaged data in Table 4, analysis of single grainmeasurements indicates there are clearly individual grains within theconcentrates which can contain significant levels of (Al,Cr)2.67O4 com-ponent. These grains however, are relatively low in abundance.

4.3.2. −45 ground fractionsA summary of the EPMA results from the −45 μm ground fraction

are provided in Table 4. Comparison of the pre- and post-ground datafor Ilmenites A, B and D show that, as would be expected, grindinghas not had a significant effect on the average composition of thespinels. Similarly, the element distribution plots in Fig. 4 show the

Table 5Calculated spinel components (mol%) in feeds and residues.

Sample Spinel component (mol%)

Mg(Al,Cr)2O4 Fe(Al,Cr)2O4 Mt Usp

Pre-groundA coarse 36.47 59.93 1.51 2.19B fine 37.07 60.53 1.0 3.34

−45 μm ground fractionA coarse 37.06 58.03 – 5.67B fine 36.48 60.88 – 5.56C coarse (ug) 35.73 53.05 4.45 6.76

After digestion (residue)A coarse 40.44 56.38 – 5.17B fine 40.75 56.78 – 4.28C coarse (ug) 39.46 50.49 1.98 8.07

ug = upgraded via roasting and magnetic separation.Mt = magnetite, Fe3O4.Usp = ulvöspinel, Fe2TiO4.

data have almost identical ranges in Al and Cr and Al/(Al + Cr) versusMg/(Mg + FeT) to those measured in the pre-ground concentrates.Calculated spinel components in Table 4 also confirm the comparablechemistry of spinels in both the pre-ground and −45 μm groundfractions.

The −45 μm fraction data for Ilmenite C show a significant differ-ence in the chemistry of the spinels compared to those present in theunprocessed samples. In particular, the spinels in the upgraded sampleare much higher in total Fe and lower in Cr (Table 4). Average Al levelsare also slightly elevated in Ilmenite C and there ismore scatter in the Alversus Cr datawith a trend toward spinels that are low in both Cr andAl.This generally indicates the presence of spinels with increased Fe3O4

magnetite component whilst the Al/(Al + Cr) against Mg/(Mg + FeT)data show a lack of spinels with a recognisable MgCr2O4 component.The calculated spinel components for Ilmenite C indicate that the pro-portion of Mg(Al,Cr)2O4 is similar to Ilmenites A and B but the Fe(Al,Cr)2O4 content is significantly reduced (Table 5). The decrease in thiscomponent is balanced by a concomitant increase in calculated Fe3O4

component, consistent with this material representing the most mag-netic fraction of the upgraded sample.

Besides the different chemistry of the spinels in Ilmenite C, a furthernoticeable contrastwith the unprocessed sampleswas the total numberof spinels located for analysis. The sample was mapped on threeseparate occasions in order to generate a total of only 43 quantitativedata points. The low numbers of spinels analysed in Ilmenite C wasdue to the roast treatment successfully removing many of the lessmagnetic spinels.

4.3.3. After digestionFor all concentrates, spinel analyses from the digested samples are

provided in Fig. 5. A cursory examination of the scatter plots of pre-digested and digested data suggests little change in spinel compositionshas occurred. However, comparison of the calculated Al/(Al + Cr) andMg/(Mg + FeT) values listed in Table 4 indicate the major effects ofthe digestion procedure have been to increase both ratios, for all threesamples, relative to the pre-digested samples. This indicates a decreasein the amount of Cr- and Fe-rich spinels remaining after digestion.

0

5

10

15

20

25

0 10 20 30 40 50 60Cr (wt.%)

0

5

10

15

20

25

0 10 20 30 40 50 60Cr (wt.%)

0.0

0.2

0.4

0.6

0.8

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.2

0.4

0.6

0.8

0.0 0.1 0.2 0.3 0.4 0.5

Ilmenite A - coarse

0

5

10

15

20

25

0 10 20 30 40 50 60

N = 09N = 13

Ilmenite C - upgraded, coarse

N = 21

0.0

0.2

0.4

0.6

0.8

0.0 0.1 0 .2 0.3 0.4 0.5Cr (wt.%)

No Mg Cr Ocomponent

2 4

Mg

/ (M

g +

Fe

)T

Mg

/ (M

g +

Fe

) TM

g / (

Mg

+ Fe

)T

b)

d)

f)

Al (

wt.%

)

N = 133

Al (

wt.%

)

N = 288

Al / (Al + Cr)

Al / (Al + Cr)

Ilmenite B - fine

Al (

wt.

%)

Al / (Al + Cr)

Increas ing

Fe O34

a)

c)

e)

Fig. 4. Cr vs. Al and Al/(Al + Cr) vs. Mg/(Mg + FeT) plots showing the compositional variability in the chrome spinels contained within the ground −45 μm ilmenite concentrates. N =number of analyses.

79M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

Comparison of the spinel molar component data in Table 5 revealsunambiguous changes in the chemistry of the spinels reporting to thedigestion residue. The results are also presented visually in Fig. 6 in aplot showing the percent change, after digestion, of the four calculatedspinel components. For Ilmenites A and B, both concentrates exhibit in-creases in the amounts of ulvöspinel, Mg-rich spinel component(Mg(Al,Cr)2O4), and corresponding reductions in the Fe-rich magnetiteand Fe(Al,Cr)2O4 components. The depletion in Fe-rich spinel compo-nents indicates these phases are more susceptible to attack by the acidand hence spinels of these compositions are likely to be the mostsoluble. The trend from Ilmenite C paralleled that of the unprocessedsamples although the changes were not as great. The biggest changein Ilmenite C was a large decrease in the amount of magnetite spinelcomponent indicating a higher solubility of Fe3O4 during digestion.

4.3.4. Textural examinationAs well as chemical, textural analysis of spinels within the pre- and

post-digested samples was conducted using scanning electron micros-copy (SEM) to examine if there were any obvious textural differencesin the spinels. Representative back-scattered electron (BSE) imagesfrom these samples are shown in Fig. 7.

The pre-digested sample exhibited a large array of textural types(Fig. 7, grains 1–4). The variation in textures between individual spinelgrains was closely related to composition with many of the textures

observed having developed in direct response to mechanical and natu-ral chemical leaching effects experienced during either transport or postdeposition. For example, compositions which contained relatively largeamounts ofMgOandAl2O3 typically appear as chemically homogeneousgrains with evidence for micro-cracking and alteration largely confinedto the outermost rim regions (grains 1 and 2). The textures of thesegrains were consistent with the spinels being resistant to low-gradealteration and mechanical breakdown, thus retaining their physical in-tegrity and key compositional information. In comparison, spinel grainscontaining more Fe-, Cr-rich compositions showed evidence for havingundergone alteration giving rise to textures characterised by fracturingand cracking, porosity and compositional inhomogeneity (grains 3 and4). These grains appeared more susceptible to chemical weatheringoriginating mainly at cracks, rims and pores resulting in leaching of Feand Mg (e.g. see Pownceby, 2005). The grainsize of the high Mg andAl grains was larger than the more Fe- and Cr-rich spinels indicative oftheir more refractory nature.

Analysis of spinel grains in the digested samples showed that grainswith high levels of Al (~20 wt.%) and Mg (~15 wt.%) exhibited less ev-idence for attack by acid, retaining reasonably homogeneous internalcompositions (Fig. 7, grains 5 and 6). Where attack of these grains wasevident, it was primarily along individual grain boundaries resulting inan etched-like texture (grain 5). Increasing Fe and Cr in the spinel to~20 wt.% and 35–40 wt.%, respectively, resulted in grains that were

0

5

10

15

20

25

0 10 20 30 40 50 60

05

101520253035

0 10 20 30 40 5 0 60

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

0.0 0 .1 0 .2 0 .3 0.4 0.5 0.6

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Cr (wt.%)

N = 382N = 294

Ilmenite A - coarse

0

5

10

15

20

25

0 10 20 30 40 50 60Cr (wt.%)

N = 362N = 617

N = 50N = 81

Ilmenite C - upgraded, coarse

0.0

0 .2

0 .4

0 .6

0 .8

1 .0

Cr (wt.%)

High MgAl Oc ompone nt

2 4

Mg

/ (M

g +

Fe

)T

Mg

/ (M

g +

Fe)

TM

g / (

Mg

+ Fe

) T f)

0 0.2

Al / (Al + Cr)

Al (

wt.%

)A

l (w

t.%)

Ilmenite B - fine

Al / (Al + Cr)

Al / (Al + Cr)

Al (

wt.%

)

High MgAl Ocomponent

2 4

a)

c)

e)

b)

d)

0.80.4 0.6

Fig. 5. Cr vs. Al and Al/(Al + Cr) vs. Mg/(Mg + FeT) plots showing the compositional variability in the chrome spinels contained within the digested ilmenite concentrate residues. N =number of analyses.

80 M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

characterised by a trellis or lattice texture (grains 7 and 8). These grainsalso retained sharp boundaries suggestingminimal attack by the acid—

in fact the BSE images indicate the bright part of the trellis texturerepresent an exsolved, but un-reacted Fe-rich phase. Spinel grains ofinitially high Fe (~28 wt.%) and Cr (~33 wt.%) were texturally differentto those with average Fe or with high Mg and Al, by virtue that theyappeared to be more altered/dissolved (grain 9). These grains had acharacteristically rounded texture and evidence for acid dissolution/

Fe(AlCr) O2 4 Fe O3 4

Spinel component (mole%)Usp

Ilmenite A-coarse

Ilmenite B-fine

Ilmenite C-coarse, upgraded

)noitsegidretfa(egnah

C%

Mg(AlCr) O2 4

6.0

4.0

2.0

0.0

-2.0

-4.0

-6.0

Fig. 6. Percentage change in calculated spinel molar components after digestion in acid.Data for Ilmenite C are relative to the −45 μm ground fraction. Mt = magnetite, Fe3O4;Usp = ulvöspinel, Fe2TiO4.

attack at the rims and along boundaries within the grain which becameseverely depleted in Fe as well as Cr and Mg. The more homogeneousparts of these grains still retained the originally high Fe contents indicat-ing incomplete dissolution/attack by the acid.

The lack of significant attack on the spinels confirms that overall,spinels are considerably more resistant to dissolution in sulphuricacid than ilmenite with the most resistant to dissolution being spinelsthat are enriched in Al and Mg (as an Mg(Al,Cr)2O4 component). Incomparison, spinels with higher Fe contents, tending towards higherFe(Al,Cr)2O4 and magnetite components, appear more soluble.

5. Discussion

Amer and Ibrahim (1996), Vardar et al. (1994) and Geveci et al.(2002) all examined spinel solubility in sulphuric acid and concludedthat high solubilities of FeCr2O4-rich spinel could be achieved at hightemperatures and high acid concentrations (e.g. 140–240 °C and 75–90% byweight H2SO4). In contrast, other studies have shown that spinelis highly resistant to dissolution in highly acidicmedia (Lumpkin, 2001).The differences appear to be related to the composition of the spinelwith spinels having compositions closer to the MgAl2O4 end membercomponent being much more durable/resistant whereas Fe-richcompositions such as chromite and magnetite are more susceptibleto oxidation of divalent Fe cations and leaching. The suite of spinelspresent in Murray Basin ilmenite concentrates is considerably morebroader in their compositional range compared to the chromites used

21

54

7 9

6

3

8

Fig. 7. Back-scattered electron images showing differences in texture between pre- and post-digested spinel grains. Images 1–4 are pre-digested grains from Ilmenite Cwhilst images 5–9are post-digested grains with images 5–8 from Ilmenite A and image 9 from Ilmenite C. The scale bar in images 1–4 is 100 μm and 20 μm in images 5–9.

81M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

in the solubility studies of Vardar et al. (1994) and Geveci et al. (2002)which contained less Fe and more Mg and Al in their structure. Theyare therefore likely to be less soluble in acid solutions.

The digestion data for Ilmenites A and B, representing coarse andfine concentrates respectively, confirm both had high titania solubilities(N94%) combined with low chromia solubilities of 0.061% and 0.090%,respectively. The concentrates would likely be acceptable in a commer-cial pigment plant where specifications typically require that solublechromia should be less than 0.10 wt.% Cr2O3 in the final product andpreferably closer to 0.05 wt.% Cr2O3. Although the data indicate low sol-ubility of spinels, the amount of soluble chromia in the two concentratesis greater than is accounted for by the amount of chromia that is presentwithin the ilmenite grains (average of 0.03 and 0.04 wt.% Cr2O3 forIlmenites A and B, respectively). This indicates that at least some ofthe spinels present in both concentrates are partially dissolved in theacid. Based on the characterisation data, Fe(Al,Cr)-rich spinels contrib-ute most to the chromia solubility with this component decreasing byabout 4 mol% relative to the spinel content in the pre-digested sample.Themagnetite component also decreased by 1–1.5mol% in the digestedsamples consistentwith Fe-rich spinel compositions beingmore solublein highly acidic solutions.

Ilmenite C exhibited a lower titania solubility (~91%) than IlmenitesA and B which was due to a higher amount of insoluble rutile. Thesample initially contained the lowest bulk chromia content, howeverthe digestion results indicated that the chromia in the sample was in avery digestible form with chromia solubility of 0.115%. The levels oflow titania combined with high chromia solubility would makethe upgraded Ilmenite C feedstock unsuitable for use in a pigmentplant. The high chromia solubility of Ilmenite C can be attributed tothe following factors:

– Ilmenite C has a different spinel population than present in the twounprocessed samples. In particular, the spinels in the upgraded

sample are higher in total Fe and lower in Cr indicative of increasedFe3O4 magnetite component and consistent with partial reductionduring roasting. As indicated previously, Fe-rich spinels composi-tions have a higher solubility in acid sulphate solutions.

– a significant proportion of the chromia in Ilmenite C is presentwithin the ilmenite grains (~0.10%). This chromia is derivedfrom post-depositional alteration within the deposit leading todissolution of chromia from the associated spinels and deposition/precipitation in fractures and pores of the ilmenite. Whilst themagnetising roast treatment succeeded in removing many of thechrome-bearing spinels, chromia associated with ilmenite makesup 66% of the total chromia in the sample. The chromia in thisform is easily digested and contributes significantly to the amountof soluble chromia.

Characterisation and digestion results for the three different ilmen-ite concentrates indicate that it is a combination of a) the intrinsictextural and compositional properties of the gangue spinel grains,together with b) the amount of soluble chromia associated with theilmenite grains that determines the overall fate of chromia duringsulphate digestion. Spinel grains containing a higher proportion ofFe-rich components are more susceptible to chromia leaching duringdigestion. As well, any chromia associated with the ilmenite grainsas fine-grained coatings on pores in fractures will contribute to highchromia solubilities.

5.1. Implications for processing

The acid sulphate digestion data indicate a small but important andnon-negligible solubility of spinels within typical Murray Basin primaryilmenite concentrates. In particular, spinels with high Fe contents corre-sponding to elevated Fe3O4 or Fe(Al,Cr)2O4 components, are the mostsusceptible to dissolution under conditions used in acid pigment plants.

82 M.I. Pownceby / International Journal of Mineral Processing 133 (2014) 73–82

In terms of the ability to effectively utilise these resources it is thereforeimportant that the total amount of spinels present in the concentrates isminimised in order to avoid unacceptable high levels of soluble chromiain the digest product. Assuming the current results reflect the typicalspinel populations within Murray Basin primary ilmenite concentrates,bulk Cr2O3 levels of around 0.4–0.6 wt.% (e.g. Ilmenites A and B) arecapable of being processed in acid pigment plants, leading to b0.1%soluble Cr2O3 in the product. This assumes however that a) rutile levelsin the concentrates are low enabling a high TiO2 solubility of N93%, and,b) the bulk of the chromia is contained within separate spinel grainsand not associated with impurities in the ilmenite. It is therefore crucialthat before digestion the amount of rutile in the concentrates, eitherun-separated primary grains or secondary rutile after weathering, andthe amount and type of spinel grains are both fully characterised andunderstood. As well, the amount of chromia directly associated withthe ilmenite grains needs to be determined as chromia present inweathered products in pores and fractures will be highly solubleunder acid digestion conditions.

Primary Murray Basin ilmenite concentrates that have N1 wt.% bulkchromia will require additional processing to minimise the amount ofsoluble chromia. These can be removed by applying a magnetisingroast treatment that selectively enhances themagnetism of the ilmenitegrains, followed by magnetic separation (e.g. Bergeron and Prest, 1974;Merritt and Cranswick, 1994; Nell and den Hoed, 1997; Reaveley andScanlon, 2001; Grey et al., 2003). However it is important that theroast conditions do not produce well crystallised rutile as one of theroast products because of its relative insolubility in the sulphate routeacid digestion. It is also important that the roast conditions do notsubstantially increase the magnetite component of the spinels as thesewill have a higher solubility in sulphuric acid. Spinel grains most likelyto be affected by the reduced oxygen roast conditions are those thatare already Fe-rich, which are less durable and already show evidencefor having undergone partial weathering and oxidation.

Fisher-White et al. (2007) have demonstrated at a laboratory scalethat a magnetising roast at low temperatures (b650 °C) in slightlyreducing fluidising gases (simulating fully combusted natural gas) isan effective roast treatment for mildly weathered ilmenite concentratesresulting in b0.1 wt.% Cr2O3 levels. The low temperatures are necessaryto ensure any rutile formed during the roast has a small particle size(b10 nm) and high surface area making it appreciably soluble in strongsulphuric acid. They also demonstrated that ilmenite concentrates fromthe central Murray Basin region (e.g. Ilmenite C) pose particularproblems because of the high levels of intra-grain chromia (0.2–0.3 wt.% Cr2O3). Even if the discrete spinel contaminant grains arecompletely removed from the concentrates, the residual level of intra-grain chromia may make them unsuitable as sulphate route feedstocks.

6. Conclusions

Ilmenite concentrates from the Murray Basin contain high chromialevels thatmake them unsuitable as feedstocks to sulphate route titaniapigment plants. Themajority of the chromia is present as discrete grainsof chrome spinels which exhibit a broad spectrum of compositionsranging from Al-rich spinel compositions (up to 20% Al) to Cr-richcompositions (up to 50% Cr). More weathered ilmenite grains inthe concentrates can also contain significant amounts of intra-grainchromia (0.2–0.3wt.% Cr2O3) present as coatings in fractures and pores.

Most spinels in Murray Basin ilmenite concentrates are highly resis-tant to sulphate acid dissolution however magnetite and Fe(Al,Cr)-richspinels contribute most to the chromia solubility. As well, any intra-grain chromia associated with the ilmenite grains adds to the overallchromia solubility.

Any additional processing to achieve low bulk chromia (e.g. mag-netising roasting) must ensure well crystallised rutile is not producedbecause of its relative insolubility in the sulphate route acid digestion.It is also important that the roast conditions do not substantially

increase the magnetite content of the spinels making them moresusceptible to dissolution in sulphuric acid.

This work has highlighted the importance of characterising therange of spinel composition types within the concentrates in additionto the level of intra-grain chromia associated with the ilmenite whenconsidering the suitability of Murray Basin primary ilmenites as a feed-stock to sulphate route titania pigment plants.

Acknowledgements

This work is based on research that has been on-going at CSIRO since2003 and could not have proceededwithout the provision of samples bycompanies that are involved, orwere previously involved, in theMurrayBasin mineral-sands industry. I would like to acknowledge theassistance provided by my CSIRO colleagues Michael Fisher-White(magnetic separations and sample preparation), Luda Malishev andCameron Davidson (SEM and EPMA sample preparation), Colin MacRaeand Nick Wilson (microprobe setup), Marshall Lanyon (sample diges-tions) and Steve Peacock (XRF analyses). Graham Sparrow and WarrenBruckard (both CSIRO) are thanked for providing thorough and timelyreviews.

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