DEYSEL, K. Leucoxene study: a liberation analysis (MLA) investigation. The 6th International Heavy Conference ‘Back to Basics’, The Southern African Institute of Mining and Metallurgy, 2007.

Leucoxene study: a mineral liberation analysis (MLA) investigation

K. DEYSEL Richards Bay Minerals, Natal, South Africa

Determination of mineral composition is of significant importance for the mineral sands industry as this information is required for orebody evaluation, the control of mineral processing plants and the determination of product quality. Mineral processing is made more difficult by the presence of pseudorutile, leucoxene, altered- and particle inclusions. The paper investigates the properties of such elements from a single product stream using an MLA approach.

Introduction a lesser extent the unconsolidated sands of the Berea and Richards Bay Minerals, situated just north of Richards Bay, Port Durnford Formations, are important sources of is currently mining unconsolidated aeolian deposits in a recycled ilmenite (Hammerbeck, 1976). coastal dune cordon. Economic heavy minerals recovered The primary source of ilmenite is from rocks belonging to include ilmenite, and zircon, of which ilmenite is the the basement rocks of the Kaapvaal Craton and the Natal abundant mineral present and is the largest global Metamorphic Province, as well as Karoo and post-Karoo resource of (Hugo and Cornell, 1991). Ilmenite has volcanics (Hammerbeck, 1976; Fockema, 1986; Hugo, a wide range of chemical compositions because Mg2+, Mn2+ 1990). Studies done by Hugo (1990 and 1993) have and Fe3+ can substitute for Fe2+ in the ilmenite structure indicated that rutile is solely derived from the Natal (Hugo, 1993). The mineral may also contain small Metamorphic Province. Zircon is thought to have primarily quantities of Cr, Zn, Cu, Al, Si and Ca. High levels of these been derived from the Natal Metamorphic Province and the elements lower the quality of an ilmenite concentrate Kaapvaal Craton (Fockema, 1986). Work done by Whitmore et al. (2002) suggested three distinct zircon because they decrease the grade of TiO2 and represent unwanted impurities in products. The mineral also forms a populations including Natal Metamorphic Province, large variety of intergrowths with other iron-titanium 650–500 Ma Pan-African belt, and zircons from a unknown oxides as a result of exsolution, oxidation or hydrothermal provenance. processes. Ilmenite ‘locked’ with these phases has different chemical compositions and physical properties to Nomenclature homogeneous ilmenite (Hugo, 1993). The alteration process No standard nomenclature exists for the ilmenite alteration of ilmenite in the deposit involves the removal of iron from products, and the following nomenclature will be used: the crystal lattice, resulting in a TiO2 enriched product • Ilmenite—homogeneous, hexagonal-trigonal minerals, (Bailey et al., 1956; Temple, 1966). The alteration process unaltered ilmenite grains with a composition close to does not only affect the overall grade, but changes the the theoretical formula (FeTiO ) magnetic susceptibility (Temple, 1966; Frost et al., 1986) 3 • Pseudorutile—a deformed hexagonal mineral formed and the density of the mineral (Temple, 1966), thereby by the alteration of ilmenite, whose composition affecting the mineral behaviour in a specific plant approximates Fe2Ti3O9 (Deer, Howie, and Zussman, environment. Studies have indicated that SiO2 and Al2O3 impurity levels in the grains increase with increasing 1992) alteration and therefore have a direct bearing on the quality • Leucoxene—an industrial term used for the alteration and recoverability of ilmenite (Frost et al., 1986; Hugo and products of all titanium-bearing minerals. These Cornell, 1991). This paper will focus on the properties of leucoxene species will contain very fine to fine leucoxene species present in a single RBM product stream intergrowths of pseudorutile or rutile with quartz and using an MLA approach. other silicates, which could include clays (such as illite or kaolinite, and at times possibly smectite). A whole Provenance of coastal heavy minerals host of minerals can be deposited in the ‘open’ structure of pseudorutile/secondary rutile The abundance and distribution of heavy minerals is related • Rutile—optically homogeneous mineral, with to the geology, physiography and coastal dynamics of the composition of essentially pure TiO (Deer, Howie, and region. These factors have resulted in the detritus from 2 Zussman, 1992). various rock types being transported by rivers, dispersed along the coast by currents and concentrated on beaches and dunes by wave action and aeolian processes The alteration process respectively (Ware, 2003). The mineralogy of coastal heavy Most of the ilmenite in Holocene dunes along the east coast minerals indicates that the provenance is mainly from rocks of South Africa occurs as homogeneous, unaltered grains, of the Karoo Igneous Province and the KwaZulu-Natal but evidence of three alteration mechanisms has been basement. The sandstones of the Vryheid Formation, and to studied (Hugo, 1988):

LEUCOXENE STUDY: A MINERAL LIBERATION ANALYSIS (MLA) INVESTIGATION 167 • Type I—the gradual weathering of ilmenite to distribution of altered grains is expected if the alteration leucoxene via hydrated ilmenite and pseudorutile, process occurred before final deposition (Hugo and Cornell, occurring in a groundwater environment 1991). However, if alteration occurred after deposition, • Type II—the direct weathering of ilmenite to leucoxene altered grains may be concentrated in areas of the deposit in sediments above the water table and where conditions are most conducive to alteration. Various • Type III—the alteration of ilmenite to hematite plus ilmenite alteration products are undesirable as they rutile in source rocks. contribute to the impurity levels and ultimately affect the For the purpose of this study, the focus will be on Type I local grade and volume of recoverable material. As and Type II alteration mechanisms. illustrated, strongly magnetic ilmenites have significantly higher TiO2 content, whereas magnetic leucoxenes tend to Type I alteration have relatively higher iron content, while non-magnetic leucoxenes have relatively higher SiO2 content. The alteration, as described by various authors, begins as It is found that the RBM deposit contains a varied and irregular patches of hydrated ilmenite along the grain petrographically complex suite of altered ilmenite grains. boundaries and weakened areas within the grain, or as The Type I alteration process appears to be the dominant orientated stringers along the basal cleavage planes of the mechanism of alteration although all three types of ilmenite (Hugo and Cornell, 1991). Various types of alteration mechanisms are evident. This, together with other analyses have indicated that the altered areas within the petrographic and mineralogical data indicates that some ilmenite have variable TiO2 contents and that the increasing ilmenite alteration occurred prior to deposition and that TiO2 content corresponds with a decrease in iron content. dune reworking has blended the alteration products (Hugo Increases in the Al2O3 and SiO2 content are also noted with and Cornell, 1991). the alteration process (Hugo and Cornell, 1991). Further studies (Merret, 1998) have indicated that The second stage of alteration is also marked by the ilmenite alteration increases with increasing dune depth and development of leucoxene. Leucoxene in most instances that several dune horizons within the orebody also affect develops from hydrated ilmenite or pseudorutile but may the ilmenite alteration throughout the orebody. Because also replace the ilmenite. This alteration type may be alteration types do not show statistically uniform explained by the two-stage model of Grey and Reid (1975), distributions within the deposit, it is believed that most of where the stage involves the oxidation of all the ferrous the alteration occurs in situ. As the alteration process iron and the leaching of one third of the ferric iron from the proceeds to more advanced stages, Si, Al, Mg, Mn, Ca, K, ilmenite lattice by electrochemical corrosion and is P and Na are found. These impurities are present in larger considered to occur in a mildly acidic groundwater situation quantities and have an effect on the separation process. (Hugo and Cornell, 1991):

[1] The effect of alteration Numerous studies including MLA analysis (Temple, 1966; Frost et al., 1986; Hugo and Cornell, 1991) have shown that The second stage of alteration beyond pseudorutile occurs the magnetic susceptibility of ilmenite decreases with via a dissolution reprecipitation process whereby both the increasing alteration, and findings indicate the following: iron and titanium are dissolved but the iron is leached while • The ilmenite-pseudorutile grains are slightly less the titanium is redeposited. This leads to the formation of magnetic than unaltered ilmenite and will in most cases rutile in beach deposits and is believed to occur in the near- report to the magnetic concentrate fraction surface regions of a deposit (Grey and Reid, 1975): • Altered ilmenites containing leucoxene or TiO2 [2] pseudomorphs have a large range of magnetic susceptibility and may report to either the magnetic In accordance with [1], where hydrated ilmenite and concentrate or the magnetic middlings fraction pseudorutile develop from ilmenite in the Zululand • Leucoxenes have a large range of magnetic deposits, these phases appear to be readily displaced by susceptibility extending from that of ilmenite to non- leucoxene (Hugo and Cornell, 1991). This suggests that magnetic rutile, and that most leucoxenes report to the pseudorutile is unstable in the deposit and alters readily to magnetic middlings and the non-magnetic fractions. leucoxene, according to process [2]. Studies by Hugo (1988) have indicated that three groups of leucoxene exist, namely: Type II alteration • Magnetic leucoxenes (mags), which have a similar Altered ilmenite and ilmenite are observed altering directly magnetic susceptibility to ilmenite to leucoxene in this type of alteration (Hugo and Cornell, • Magnetic middling leucoxene (mids), which have a 1991). The alteration occurs from the grain boundaries magnetic susceptibility between that of ilmenite and along weaknesses or, in some cases, the leucoxene may rutile grow as replacement fronts across grains. The boundary • Non-magnetic leucoxenes (NM), which have a similar between the ilmenite and leucoxene may consist of porous magnetic susceptibility to rutile. areas of microcrystalline leucoxene, and some of these pores indicate silica and aluminium contents (Hugo and This last fraction is divided into the following Cornell, 1991). This style of ilmenite alteration to subdividions: leucoxene or rutile may be expressed as: • NM leucoxenes having a specific gravity less than 3.6 3 [3] g/cm • NM leucoxenes having a specific gravity between 3.6 The paragenesis of ilmenite alteration is significant as the and 4.2 g/cm3 site of alteration will affect the distribution and proportions • NM leucoxenes having a specific gravity greater than of various types of alteration in the deposit. A fairly random 4.2 g/cm3.

168 HEAVY MINERALS 2007 Detailed microprobe analysis (Hugo and Cornell, 1991) The focus of the study was to investigate the mineral and MLA analysis have indicated a significant increase in species that constitute the leucoxene group and to the SiO2 and Al2O3 content of the ilmenite alteration phases investigate these species individually focusing specifically (referred to as siliceous leucoxenes) and agrees with on modal mineralogy that reflects the chemistry of the findings by Frost et al. (1986). It is therefore evident that sample. Figure 3 shows the X-ray spectra for some altered ilmenite and leucoxenes will contribute to the silica leucoxene species and confirms the presence of varying and aluminium impurity levels in the product streams. proportions of aluminium and silicon in titanium-rich leucoxenes. Sampling and analytical techniques At RBM the leucoxene mineral group consists of three mineral sub-species based on the difference in chemical The study material consisted of a representative sample composition (see Figure 4 and Table II). taken from one of RBM’s product streams (rutile special Rutile leucoxene was the most abundant leucoxene grade product stream consisting of mostly rutile). The species present in the three study samples, i.e. with 12.25% sample was split (based on mass) into four fractions using a in the NM, 13.12% in the mids and 18.83% in the mags high intensity magnetic separator (HIMS/ IRM), resulting in samples: L1 (very non-magnetic material, mostly consisting of rutile), L2 (the remainder of the non- Modal abundance magnetic material), L3 (middling materials associated with 100% this product) and L4 (the more magnetic material associated with this product). The sample consisting of the very non- 80% magnetic material (L1) was not used for the purpose of this 60% investigation as the sample contained virtually no leucoxene species. The main focus for this study was to 40% identify ilmenite alteration species (specifically leucoxenes) 20% associated with very non-magnetic product streams looking specifically at sample fractions L2, L3 and L4. These 0% L2_NM L3 _MIDS L3_MAGS sample fractions were analysed by MLA. Study material The mineral liberation analyser (MLA) combines an automated scanning electron microscope (SEM), multiple Figure 1. Mineral abundance of study species energy dispersive X-ray detectors with state-of-the-art automated quantitative mineralogy software developed by JKMRC/ FEI. With the MLA ore particle sections can be Table I analysed to better understand, optimize and predict mineral Mineral abundance of fractions used for the study processing circuit performance. Information about the liberation distribution of the minerals is vital in determining Mineral L2—Nm L3—MIDS L4—Mags whether inefficient separation is the result of the presence of unliberated particles or as a result of poor mechanical or Ilmenite 0.01 0.01 0.11 separator performance. Geometallurgical and ore Leucoxene 12.94 13.85 22.30 Rutile 76.72 75.34 56.59 characterization information such as mineral association Zircon 7.30 7.73 11.44 data and grain size distribution are essential to assist with 1.13 1.04 3.16 the optimization of plant feed quality by avoiding Epidote 0.00 0.00 0.13 metallurgically poor feed stock or by facilitating effective Quartz 0.90 0.90 0.85 ore blending. The quantitative chemical analysis of the Feldspar 0.02 0.03 0.03 altered grains was performed using the MLA. Clays 0.34 0.36 0.73 Phos—Apatite 0.00 0.02 0.00 Phos—Xenotime 0.00 0.00 0.01 Discussion Phos—Monazite 0.00 0.00 0.07 The three study samples were analysed on the MLA and FeOxide 0.30 0.36 3.41 their images quantified. Leucoxene, rutile and zircon were Others 0.32 0.35 1.09 the main mineral species identified. Leucoxene is Total 100.00 100.00 100.00 prominent in the non-magnetic fraction (13%), the middlings fraction (14%) and the magnetic fraction (22%) as shown in Figure 1. The full quantitative assessment of the three sample fractions is shown in Table I. The particle density distribution indicates the presence of particles of various densities, as shown in Figure 2. Most of the particles from the different samples fractions have a density of around 4.3 g/cm3, i.e. 68%, 67.8% and 48.3% respectively to the NM, mids and mags fractions. It appears that most of particles (48.3%) in the Mags fraction reported Percentage to a density of 4.3 g/cm3, while 20.9% reported to the 3.8 g/cm3 and 10.3% to the 4.7 g/cm3 respectively. This could be because 22% of the leucoxene species reported to the mags fraction and resulted in the broader range of densities of these particles. Leucoxene species have a much broader spectrum of densities as the result of their chemical Density (wt%) composition cased by alteration. Figure 2. Particle density distribution

LEUCOXENE STUDY: A MINERAL LIBERATION ANALYSIS (MLA) INVESTIGATION 169 The BSE images (Figures 6–11) illustrate that a significant amount of ilmenite alteration products (especially leucoxene) are present within the RBM product stream and that various ilmenite alteration stages are present. These mineral species will have a significant effect on product quality as well as on the overall mineral processing as the behaviour of these mineral species is difficult to determine. 50 D

Figure 3. ED spectra of a leucoxene grain indicating the presence of Al, Si, K

Sample species

Figure 5. Comparison of the mineral grain size distribution among the different leucoxene species Percentage

Wt% CK 18.91 OK 22.63 AlK 02.87 Study Samples TaM 10.90 TiK 40.44 FeK 04.12 Figure 4. Leucoxene species abundance in the study samples

Table II Comparative Assay analysis for the sample fractions

Element L2—Nm L3—Mids L4—Mags Figure 6. BSE image of a pseudorutile leucoxene grain from sample L2 NM fraction illustrating the nature of complex Al 0.55 0.59 0.93 alteration Ca 0.29 0.28 0.85 Fe 1.71 1.83 5.22 Mg 0.08 0.09 0.13 O 39.41 39.38 38.32 Si 2.63 2.73 4.17 Ti 51.54 51.07 44.10 Elem. Wt% CK 22.70 Zr 3.55 3.76 5.57 OK 23.80 AlK 01.01 SiK 01.47 TiK 42.51 fraction (as illustrated in Figure 4). Pseudorutile leucoxene FeK 08.51 reported mostly to the mags fraction, whereas the siliceous leucoxene reported relatively evenly to all the sample fractions. It appears that the siliceous leucoxene in the NM and mids fractions is much finer in comparison with the mags fraction. This might suggest that they are possibly inclusions rather than fully liberated particles, whereas all the species reporting to the mags fraction appear to be fully liberated because of their similar grain size distribution Figure 7. BSE image of a pseudorutile leucoxene grain inclusion pattern. within a zircon grain from sample L2 NM fraction

170 HEAVY MINERALS 2007 Figure 8. MLA classified image of a highly altered grain from sample L2 NM fraction Figure 11. BSE image of a highly altered (siliceous) leucoxene grain, porous leucoxene consisting of thin, prismatic microcrystals resembling rutile, taken from sample L4 mags fraction

Conclusion The main focus of the study was to identify ilmenite alteration species, specifically leucoxenes, which are associated with very non-magnetic product streams. Leucoxene is prominent in the non-magnetic fraction (13%), the middlings fraction (14%) and the magnetic fraction (22%) of the non-magnetic product stream as the magnetic susceptibility decreases with increasing alteration. At RBM the leucoxene mineral group consists of three mineral sub-species, namely: siliceous leucoxene, rutile leucoxene and pseudorutile leucoxene. The mineral sub- divisions are based on differences in chemical composition caused by alteration processes. Rutile leucoxene is the most abundant leucoxene species within the study material with 12.25% in the L2 NM, 13.12% in the L3 mids and 18.83% in the L4 mags sample. The alteration process alters the Figure 9. BSE image of stages in the alteration process in a single chemical composition of the mineral species, thereby grain from sample L2 NM fraction affecting the particles’ magnetic susceptibility as well as the density. Most of the leucoxene sub-species in the sample fractions have a density of between 3.6 and 4.3 g/cm3 as indicated by previous studies. Observations have shown that as density of the minerals increases, a general increase in TiO2 and a corresponding decrease in SiO2 occur. The mineral grain size also indicates that the sizing of the individual particles influences the mineral separation. The SEM analysis also revealed that SiO2 in the siliceous leucoxenes occurs as coatings and mostly as infillings in cavities or pores or as distinct highly siliceous areas in inhomogeneous leucoxene grains.

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