| dc.description.abstract | Rare earth elements (REE) are considered as critical and non-substitutable metals
for electronics and green technology. A greater diversity of supply is needed and the
REE occur in a wide range of REE- and REE-bearing minerals within different ore
deposit types. The beneficiation processes for REE ores can vary widely based on
their mineralogy and texture. It is, therefore, essential to understand the
mineralogical characteristics when designing processing routes. Little research was
carried out on this topic until the last few years, apart from bastnäsite, monazite, and
xenotime, and most REE minerals in deposits currently under exploration are poorly
understood in terms of processing characteristics.
This geometallurgical study brings together the results of process mineralogy and
minerals processing to recover synchysite-(Ce) and apatite from the carbonatite at
Songwe Hill, Malawi. This deposit is unusual because it is a potential carbonatite
source of both LREE and HREE. Results from previous flowsheet development
studies on this deposit suggest that flotation is the most promising processing route
and therefore this study concentrated on testing this hypothesis. It sought to
understand the mineralogy better in order to predict processing response and carried
out a series of flotation experiments to improve the processing efficiency. It also
investigated the fundamental magnetic properties of the rare earth fluorcarbonate
minerals (including synchysite) and established for the first time that there is a
systematic variation in their properties that can be applied to minerals processing.
Eight samples of REE carbonatite drill core, crushed to 1700 μm, and a composite
sample ground to 53 μm and 38 μm were used throughout this research. Automated
mineralogy (QEMSCAN®) was applied to determine the mineralogical characteristics
of the ore deposit. This utilised a novel species identification protocol (SIP) for REE
minerals in carbonatites, which was validated by electron microscopy (SEM-EDS),
and electron probe microanalysis (EPMA).
The principal REE minerals at Songwe are the REE fluorcarbonates, synchysite-(Ce)
and also parisite-(Ce). These are challenging minerals for automated mineralogical
techniques owing to their chemical similarity and common occurrence either as
bladed (needle-like) crystals, which is the main textural type at Songwe Hill, or as
syntaxial intergrowths. However, using the SIP developed in this study, the
QEMSCAN® can distinguish between these minerals based on the Ca content and
can also recognise syntaxial intergrowths on a scale of about > 20 μm.
The Songwe Hill carbonatite hosts about 6 wt% to 10 wt% of REE- and REE-bearing
minerals. Apatite hosts the more valuable HREE in addition to P2O5, followed by
synchysite-(Ce)/parisite-(Ce) (mainly synchysite-(Ce)), and minor florencite-(Ce),
which host the LREE. These minerals are commonly associated with the
predominant gangue minerals, ankerite and calcite, and, to a lesser extent Fe-
Ox/CO3 and K-feldspar, strontianite and baryte.
Fundamental magnetic properties of pure REE fluorcarbonate single crystal minerals
using a vibrating sample magnetometer (VSM) were determined. The magnetic
susceptibility is highly dependent on the mineral composition. It is positive
(paramagnetic) for bastnäsite-(Ce) and gradually decreases as the amount of Ca
increases in parisite-(Ce), becoming negative (diamagnetic) for the Ca-rich member
of the series, röntgenite. Synchysite-(Ce) in this deposit was experimentally
determined by magnetic separation and behaved as a diamagnetic mineral. This can
be explained by the layered structure common to the REE fluorcarbonate series
minerals.
Selected laboratory scale mineral processing experiments including magnetic
separation and froth flotation were performed. Pre-concentration tests by magnetic
separation showed a recovery of 84% for P2O5, 80% for Y2O3, and 76% for Ce2O3 in
the non-magnetic product, with gangue minerals rejection of about 49% for ankerite
and 48% for Fe-Ox/CO3 to the magnetic product. Apatite and synchysite-(Ce) loss to
the magnetic product is mainly the result of their association with the paramagnetic
minerals i.e. ankerite and Fe-Ox/CO3 as indicated by automated mineralogy.
A spectrophotometer was utilised to measure the solubility of the organic chemical
reagents including fatty acids and lignin sulphonate in different alkaline solutions and
to determine the appropriate operating parameters for bench flotation tests. The
results indicated that the solubility of fatty acids increased with increasing the pH
value from 8.5 to 10.5, while the opposite was observed for lignin sulphonate.
35 bench-scale froth flotation tests under a wide range of chemical and operating
conditions including pH modifiers and dosages, soluble and insoluble collectors,
depressants, temperature, and conditioning time were performed. The results
demonstrated that fatty acids and lignin sulphonate are sensitive to changes in pH,
conditioning time, and temperature. These factors significantly affected flotation
efficiency. A recovery of 86% for P2O5 and 74% for both of Y2O3 and Ce2O3 with
TREO upgrading from 1.6 wt% to 3.8 wt% at a mass pull of 31% were achieved
under a constant pulp pH of 9.5, elevated temperature, and long conditioning time.
This study suggests that combining magnetic separation and froth flotation
techniques to pre-concentrate and upgrade the REE- and REE-bearing minerals,
should be considered further to minimise the cost of the chemical reagents used in
froth flotation and gangue leaching. | en_GB |
