DEYSEL, K. Leucoxene study: a mineral liberation analysis (MLA) investigation. The 6th International Heavy Minerals 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-ilmenite 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, rutile and zircon, of which ilmenite is the the basement rocks of the Kaapvaal Craton and the Natal abundant ore mineral present and is the largest global Metamorphic Province, as well as Karoo and post-Karoo resource of titanium (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