Beneficiation of a goethitic rare earth bearing laterite ore through pyrometallurgical pre-treatment and magnetic separation

Faris, N 2019, Beneficiation of a goethitic rare earth bearing laterite ore through pyrometallurgical pre-treatment and magnetic separation, Doctor of Philosophy (PhD), Science, RMIT University.

Document type: Thesis
Collection: Theses

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Title Beneficiation of a goethitic rare earth bearing laterite ore through pyrometallurgical pre-treatment and magnetic separation
Author(s) Faris, N
Year 2019
Abstract The rare earth elements (REEs) are a group of elements consisting of the lanthanides (lanthanum to lutetium) plus scandium and yttrium; the last two typically being included in this group due to their common co-occurrence with the lanthanides in many ore deposits. This group of elements is of strategic importance due to their use in high-end technologies. In terms of mineral resources 95% of the total rare earths occur in three primary minerals; bastnaesite [(Ce, La)CO3F], monazite [(Ce, La)PO4] and xenotime (YPO4) which are hosted in a broad range of deposit types such as carbonatites, iron oxide-copper-gold-uranium (IOGC), hydrothermal, laterites, phosphorites and heavy mineral sands.

Currently there is significant interest in the exploitation of supergene rare earth bearing lateritic deposits derived from weathered carbonatites with several of these deposits located in Australia, Brazil, China, Russia and USA. However, the beneficiation and recovery of rare earth ores from these deposits, in which iron oxides in their various forms constitute the major gangue, is a challenge due to the mineralogical and textural complexities of mineral values and gangue alike; additional rare earth minerals and gangue such as hematite and goethite, exhibit similar physical and physico-chemical properties. Therefore, a number of these deposits have long defied conventional mineral beneficiation routes and are thus considered refractory. Importantly, in the case of the majority of these ores, the iron content is high enough (25-30%), that they can be classified as low-grade iron ore deposits in their own right. Therefore, past processes (both conventional and unconventional) that have been researched for the beneficiation of low grade, refractory iron ores may be potentially applicable towards beneficiation of iron-enriched rare earth ores. A survey of the literature indicated that pyrometallurgical processes which convert iron to a form more amenable to physical separation were suitable for the beneficiation of fine-grained iron ores and these processes had also been successfully adapted towards processing of iron-enriched rare earth ores, particularly those arising from the Bayan Obo Fe-REE-Nb deposit of Inner Mongolia, China.

In the present study, detailed characterisation of a lateritic rare earth ore was carried out to determine the rare earth and gangue mineralogy; the mode of occurrence and distribution of rare earths and gangue species in the ore; and how this may impact mineral beneficiation. Monazite was the primary rare earth bearing mineral identified whilst florencite was of secondary importance. Gangue minerals identified in order of decreasing abundance were goethite, fluorapatite and dolomite. The particle size distribution of the crushed ore showed that it consisted primarily of fine and ultra-fine particles below 38 μm with the rare earths and iron distributing to the finer size fractions. Microstructural characterisation of the rare earth minerals via QEMSCAN and SEM showed that they were fine-grained and occurred as polycrystalline aggregates or were finely disseminated in gangue. Based on the characterisation of the lateritic rare-earth ore, potential processing options were considered and magnetising roasting-magnetic separation was selected as a suitable process that merits further research towards beneficiating the rare-earths.   

Reduction roasting tests were carried out using sub-bituminous coal as a reductant to investigate the factors influencing goethite to magnetite conversion. The effect of temperature, time and reductant addition on magnetite conversion; and the optimal conditions which resulted in complete conversion of goethite to magnetite were determined through the use of satmagan and X-ray diffraction. Roasting temperature and coal addition were found to have the greatest influence on goethite to magnetite conversion. The relationship between these two variables was however inverse with higher temperatures requiring less coal for goethite reduction to magnetite. Despite the lower coal requirement at temperatures above 650°C, magnetite was not stable above this temperature and readily underwent further reduction to form wüstite which was undesirable. For this particular ore, optimal roasting conditions were determined to be a roasting temperature of 600 - 650°C with the addition 10 - 20 wt% coal in the mixture and a roasting time of 90 min. The primary rare earth mineral in the ore, monazite, was found to be thermally stable whilst florencite underwent thermal decomposition which resulted in an increase in monazite grade of the roasted ore product.

Based on the aforementioned optimal roasting conditions that were determined from static bed roasting tests, the feasibility of magnetically separating iron oxides from a reduction roasted lateritic rare earth ore was explored. The influence of magnetic field strength and feed particle size on magnetic separation of iron from the roasted ore was studied using a Davis Tube Tester and chemical and mineralogical analysis of the rougher magnetic concentrates and tails was evaluated. Application of a magnetic field strength at or above 0.2 T was successful in removing iron to the concentrate though this was accompanied by significant losses of rare earths to the magnetic fraction. Microstructural characterisation of the magnetic concentrates revealed that rare earth losses were due a combination of - insufficient liberation; presence of iron oxide coatings on mineral surfaces; and heteroflocculation. The findings of this study provided significant strategies towards optimising reduction roasting and magnetic separation in maximising iron removal and minimising rare earth losses, therefore proving the potential viability of reduction-roasting-magnetic separation process to beneficiate rare earths from a lateritic rare earth ore.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Science
Subjects Mineral Processing/Beneficiation
Industrial Chemistry
Keyword(s) Rare Earths
Reduction Roasting
Magnetic Separation
Mineral Processing
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Created: Fri, 25 Oct 2019, 09:25:59 EST by Adam Rivett
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