| dc.description.abstract | Monazite is a chemically inert, rare earth phosphate mineral, which is difficult to
process using conventional chemical digestive techniques. Monazite contains
important commercial sources of thorium and lanthanides. The monetary value of
monazite stems from the light rare earth metals (Ce, La, Pr, Nd and Y), thorium and
uranium contained within its crystal structure. Conventional chemical processing of
monazite requires the use of harsh chemicals in a highly complicated, corrosive,
laborious and expensive process which can cause severe environmental damage as
has been demonstrated in China. South Africa plans to beneficiate monazite as part
of its mineral beneficiation strategy. Doing so competitively would require a cost
effective and environmentally friendlier process. A new process that involves feeding
monazite into a plasma reactor to alter the crystal structure is being investigated. If
successful, this new process has the potential to make monazite chemically reactive
and recovery of rare earth oxides susceptible to less harsh chemical methodologies.
To confirm the chemical decomposition of monazite in the presence of carbon,
thermodynamic calculations were used. Monazite can be decomposed in the presence
of carbon into the rare earth oxides at a temperature of between 1200 and 1400 °C
with a monazite-carbon ratio of 2:5. Thorium- and uranium carbide can be formed in
the same plasma process assuming the temperature is above 2170 °C. The rare earth
oxides, thorium- and uranium carbides are desired products as they are more
susceptible to leaching with aqueous mineral acids. Monazite, in the absence of
carbon, theoretically decomposes into the oxides above the melting point of the rare
earth phosphates. Using current thermodynamic data, the decomposition temperature
of monazite in the absence of carbon remains unconfirmed. It was determined that the
energy cost of decomposing monazite on its own would be higher than when monazite
and carbon are heated together to decompose the monazite.
Theoretical calculations of the reaction between monazite and the selected rare earth
oxides with ammonium bifluoride were conducted. It was determined that ammonium
bifluoride can be used as a viable alternative for the fluorination of monazite and the
rare earth oxides. The fluorinated rare earth mixture can then be separated using
various methods.
It is hypothesised that by placing monazite in a high temperature plasma, its chemical
reactivity could be increased. To evaluate this theory, monazite was placed in a DC
direct arc batch reactor. When the high temperature plasma heat was not applied
directly for the correct length of time, the monazite has a minor increase in chemical
reactivity. By increasing the reaction time (heating period) the monazite melts and theresulting molten monazite becomes more inert to chemical and physical attack. The
high temperature plasma heat must be applied directly onto the monazite and a
correct reaction time is a requirement to ensure the correct conversion of the crystal
structure of the monazite.
The proper treatment of monazite in a plasma is evident using microscopic and
chemical analysis. When treated correctly with a plasma, the original monazite
structure is converted into a more chemically reactive phase that permits the removal
of 30.49 % of the rare earth elements, which is 21 times more effective than from
untreated monazite, 16.89 % of the thorium and 42.70 % of the uranium using 32 %
HCl at 80 °C for 1 h. Visual analysis of the Plasma Treated Monazite (PTM) which was
leached confirmed that not all of the monazite was decomposed during plasma
treatment which results in not all of the REE, thorium and uranium being leached. The
extraction of rare earths from treatment of monazite may be improved by optimizing
the carbon to monazite ratio in an inflight plasma (temperature above 1400 °C).
In this study the effect of the plasma interaction on the monazite crystal structure to
ensure increased extractability of rare earths from generated PTM with different
mineral acids were evaluated. Theoretical calculations were initially conducted on the
leaching of the rare earth phosphates and oxides along with thorium- and uranium
carbide with the mineral acids. This indicated that PTM can be leached more easily at
low temperatures as PTM is chemically more reactive than monazite. Using the
conventional digestion processes on PTM, higher quantities of the REE were leached;
however the same chemical and radioactive waste would still be present as what is
found when monazite is treated. The direct digestion of PTM with 32 % HCl at 80 ºC
for 1 h extracted the highest quantities of the REE, thorium and uranium into the aqueous mineral acids and the extraction is higher than the conventional digestion
process when used on monazite.
The overall conclusion of the study is that the plasma treatment of monazite increases
its chemical reactivity. This process can now be used to develop a more efficient and
economical process than the comparable conventional chemical digestion methods
currently employed to digest monazite. This new process will use an in-flight plasma
with a monazite-carbon mixture followed by leaching of the plasma product with on
aqueous mineral acid such as HCl. | en_US |
