Study on the Decomposition Process of Monazite Concentrate from
Coastal Placers in Vietnam and the Production of Commercial-
Grade Ce(CO₃)₂
ABSTRACT. Cerium(IV) carbonate (Ce(CO3)2) was recovered through an alkali decomposition
process of monazite ore, followed by selective precipitation. The characteristics of the recovered
product were analyzed using techniques such as Inductively Coupled Plasma Optical Emission
Spectroscopy (ICP-OES) and Differential Thermal Analysis (DTA). Experimental results
demonstrated a decomposition efficiency of 94.5% under optimal conditions - using a NaOH:REE
molar ratio of 0.6 and calcination at 500 0C for 1 hour. The subsequent precipitation of Ce(CO 3)2
achieved a maximum recovery yield of 51.7% at the same NaOH:REE ratio.
1. INTRODUCTION
Rare earth elements—often referred to as the “white gold” of the 21st century, or the
“vitamins” and “spices” of the economy—play an essential role amid the rapidly
increasing demand for high technology. Due to their unique physicochemical properties,
REEs can be applied across various fields, ranging from electronics and energy to
healthcare and national defense. According to a report published in early February 2025
by Investing News Network, based on the latest findings from the United States
Geological Survey (USGS) on rare earth elements, Vietnam possesses an estimated rare
earth reserve of approximately 3.5 million tons. In this context, research into rare earth
processing technologies has become an urgent necessity.[1-2]
Currently, there are several common rare earth processing methods in the world, such
as hydrometallurgical methods, electrolysis methods, ion exchange methods,
chromatography methods, and fractional crystallization methods. The common point of
these methods is that they all involve an extremely complex process consisting of
multiple stages, requiring advanced technology and skilled labor. On the other hand, rare
earth processing has significant negative environmental impacts, the most notable being
air, soil, and water pollution, which causes ecosystem imbalance. However, because it
brings substantial economic benefits, many countries have been and are implementing
solutions to minimize these impacts, aiming for a sustainable production process.[1-3]
Up to now, all rare earth ore processing technologies in Vietnam have mainly been
laboratory-scale and semi-industrial research projects, with almost no significant
applications at an industrial production scale. Based on semi-industrial scale experiments,
some studies on ore processing technology for the Nam Xe Nam, Đong Pao, and Yen Phu
mines, as well as some marine placer deposits and certain processing procedures, have
been proposed. Some opinions suggest that these research results could be applied to
production. However, currently, only some research results on monazite recovery from
marine placers have been implemented. Research in the fields of hydrometallurgical
refining and mining remains limited due to insufficient data and conditions to evaluate
capacity and economic efficiency.
The current rare earth market in Vietnam does not have any products that are oxides
of a single rare earth element; all are mostly in the form of mixed rare earth elements.
The separation of individual rare earth elements is not yet possible with Vietnam’s current
technology. According to research conducted by the Institute for Rare Radiation
Technology, the Institute of Chemistry, and the Vietnam Academy of Science and
Technology, Vietnam has successfully separated elements such as Ce, La, Nd, and Pr
from Vietnamese monazite ore using a combination of H₂SO₄ roasting and solvent
extraction. Some studies are currently in the pilot phase (semi-industrial scale testing) but
have not yet been fully industrialized. However, the process is still confined to the
laboratory scale and has not reached production-scale implementation.
Recently published studies are still incomplete regarding the influence of various
factors on the efficiency of individually separating rare earth elements due to their nearly
identical properties. The alkali treatment method for monazite concentrate, using a
mixing ratio of NaOH flakes/ore at 0.6:1, followed by heat treatment at 550°C for 60
minutes, and subsequent washing and filtration with 6M HCl solution at 90°C for 45
minutes, resulted in a total rare earth element separation efficiency of up to 92.5%. [2]
In this study, the influence of the mixing ratio of NaOH flakes/ore on the recovery
efficiency of cerium as a carbonate salt using the alkalization method was investigated.
2. RESEARCH METHODS
2.1. Materials
Monazite is a phosphate mineral containing rare earth elements and is one of the
primary sources of REEs. It is a common beach sand mineral that can primarily be found
in vein deposits and acidic magmatic ores. Monazite ore has a general formula of
(Ce,La,Th,Nd,Y)PO₄ or (REE)PO₄. Pure monazite contains approximately 20–30%
Ce₂O₃, 10–40% La₂O₃, and 4–12% ThO₂. [11] The monazite ore used in this study is
Bình Thuận monazite, which has a cerium content of about 22% in the mixture.
Equipment: Ball mill, centrifuge, vacuum pump, magnetic stirrer with heating, furnace,
XRD analyzer, DTA analyzer, ICP-OES analyzer.
Chemicals: NaOH flakes, HCl, KMnO₄, Na₂CO₃, distilled water.
2.2. Experimentation
Stage 1: Alkalization of Monazite Ore Grinding
Monazite Ore: The raw ore is initially divided into small portions, each weighing 30
grams. Each portion is ground using a ball mill for 10 minutes at a rotation speed of 600
rpm. After grinding, the sample is sieved, and particles with a size ≤ 75μm are selected.
Alkalization of Monazite Concentrate: NaOH flakes are thoroughly mixed with the
monazite concentrate powder at different ratios (0.6; 0.8; 1) using a ball mill. The
mixtures are then placed into individual nickel crucibles, each containing 40 grams, and
heated at 500°C for 60 minutes. After heating, the samples are cooled in ambient air to
room temperature.
The alkalization reaction is represented by equation (1):
RePO4 + 3NaOH → Re(OH)3↓ + Na3PO4 (1)
Phosphate Washing: Weigh 15 grams of the cooled roasted mixture and wash it
with water at 90°C at a ratio of 1:6. The mixture is stirred at 90°C for 70 minutes with a
stirring speed of 500 rpm. After stirring, the mixture is centrifuged for 10 minutes. The
solution is then tested using pH paper, and the process is stopped when the pH of the
solution is less than or equal to 9.
The preliminary processing procedure for the monazite concentrate is illustrated
in Diagram 1.
Figure 1: Monazite Ore Preliminary Processing Flowchart
Stage 2: Recovery of Ce(CO₃)₂
Dissolution of Rare Earth Hydroxide Precipitate Using 8M HCl: The dried
Re(OH)₃ precipitate is completely dissolved in 8M HCl at a mixing ratio where the
amount of HCl is six times the solid mass. The experiment is conducted for 45 minutes at
a temperature of 90°C. Afterward, the mixture is vacuum filtered, separating the solid
phase, and obtaining a liquid mixture containing chloride salts of rare earth elements.
The dissolution reaction is represented by equation (2):
3HC (2)
Re(OH)3 + → ReCl3 + 3H2O
l
Oxidation of Ce³⁺ to Ce⁴⁺ using KMnO₄: The rare earth chloride solution, after
being filtered to remove residues, undergoes an oxidation reaction with 1.25 grams of
KMnO₄ under heated conditions to oxidize Ce³⁺ in the solution to Ce⁴⁺.
The oxidation reaction is represented by equation (3):
5Ce3+ + 8H+ MnO4- → 5Ce4+ + Mn2+ + 4H2O (3)
Separation of Ce Element in the Form of Ce(CO₃)₂ Using Na₂CO₃: A certain
amount of Na₂CO₃ is added to the mixture. The pH level is continuously monitored until
it reaches 5. At this point, a white precipitate appears. This precipitate is Ce(CO₃)₂,
which is then filtered using a vacuum filtration system and dried to obtain the final
product.
The precipitation reaction is represented by equation (4):
Ce4+ + CO32- → Ce(CO3)2↓ (4)
The process of forming Ce(CO₃)₂ precipitate is summarized in Diagram 2.
Figure 2: Precipitation Process of Ce(CO₃)₂
3. RESULTS AND DISCUSSION
3.1. Composition of Binh Thuan Monazite Concentrate
Using the inductively coupled plasma optical emission spectrometry (ICP-OES)
method, the composition of rare earth elements in Bình Thuận monazite concentrate was
analyzed and listed in Table 1.
Table 1: Composition of Rare Earth Elements in Monazite Concentrate
Nguyên tố Hàm lượng (%) Nguyên tố Hàm lượng (%)
Sc < 0.00001 Tb 0.102
Y 0.711 Dy 0.304
La 10.182 Ho 0.057
Ce 22.342 Er 0.068
Pr 2.722 Tm 0.006
Nd 7.342 Yb 0.022
Sm 1.571 Lu 0.003
Eu Zr
0.035 0.011
Gd 0.733
The analysis results in Table 1 show that the Monazite sample has a very high REE
content, accounting for approximately 46% of the total sample mass. Notably, Ce has the
highest proportion (22.342%), followed by La (10.185%) and Nd (7.342%),
demonstrating significant potential for extraction and use as an input material in the
functional materials production chain. Since Ce has the highest concentration in the
sample, it has been selected as the target element for separation in this study.
3.2. Particle Size Distribution of Monazite Concentrate
The monazite concentrate is finely ground into powder using a ball mill and then
sieved to obtain experimental particles with a size of ≤ 75μm. The particle size
distribution is analyzed using the HORIBA LA-920 device and the laser scattering
method in a wet environment, with results presented in Figure 3.
Figure 3: Particle Size Distribution of Monazite Concentrate
The statistical parameters of particle size distribution in the sample are presented in Table
2.
Table 2: Particle Size Distribution Parameters (Cumulative Percentage)
Phần trăm tích Đường kính tương ứng (µm)
lũy
5% 0.2966
10% 0.3515
20% 0.6497
30% 1.4535
40% 2.3205
60% 7.4702
70% 10.7693
80% 14.9532
90% 21.6243
95% 28.4106
From the data in Table 2, it can be observed that the sample has a wide particle
size distribution, as indicated by the high Span index (4.56), which is suitable for natural
or heterogeneous material systems. The fine particle content (<1 µm) is relatively high,
which may affect suspension viscosity, dissolution rate, or reaction processes in various
treatments. The mean and median values (mean ~8.41 µm, median ~4.67 µm) indicate
that most particles fall within the 5–20 µm range, making them well-suited for
applications such as filtration, precipitation, or catalyst synthesis.
3.3. XRD Diagram of Monazite Concentrate
The results of the mineral composition survey of monazite ore are presented in Figure 4.
Figure 4: XRD Diagram of Monazite Concentrate
The diffraction pattern shows characteristic peaks at 2θ angles corresponding to
the crystal planes of the Ce₃(PO₄)₂ compound, in the monoclinic Monazite-Ce form
(JCPDS 01-088-9097). The main peaks are located at approximately 2θ ≈ 17.0°, 21.2°,
28.5°, 30.1°, 35.3°, and 47.1°, aligning well with the accepted reference database. No
significant presence of secondary phases such as oxides or sulfides of other metals is
observed, indicating a high phase purity of the sample.
The intensity of the diffraction peaks is sharp and well-distributed, demonstrating
a highly ordered crystal structure. The agreement between experimental data and
reference standards confirms the successful formation of the Monazite-Ce phase in the
studied material.
3.4. Investigation of the Effect of NaOH Flake/Ore Mixing Ratio on the
Recovery Efficiency of Total Rare Earth Hydroxides
The decomposition efficiency of REE in NaOH is shown in Figure 5.
Figure 5: Decomposition Efficiency of REE in NaOH by Ratio
With different NaOH:REE mixing ratios, the amount of recovered rare earth metals
varies. The experimental results in Figure 5 show that REE decomposition efficiency
gradually decreases as the NaOH:REE mixing ratio increases. At a ratio of 0.6, the
efficiency reaches its highest value (94.50%), but as the ratio increases, the efficiency
drops to 91.2%—a decrease of nearly 4% compared to the initial value. This suggests that
excessive NaOH does not enhance efficiency but may lead to side reactions or reduce the
effectiveness of the main reaction.
3.5. Recovery Efficiency of Ce(CO₃)₂ Corresponding to Different NaOH/REE
Mixing Ratios
The recovery efficiency of Ce is shown in Figure 6
Hiệu suất thu hồi Ce
55.00%
54.00%
53.00% 54.20%
52.00%
51.07%
51.00%
50.00%
49.00%
48.00%
48.27%
47.00%
46.00%
45.00%
0.6 0.8 1
Figure 6: Recovery
Efficiency of Ce(CO₃)₂ Corresponding to NaOH:REE Mixing Ratios of 0.6, 0.8, and
1.
The Ce(CO₃)₂ product is recovered through the precipitation reaction of ReCl₃
solution with Na₂CO₃, corresponding to different NaOH:REE reaction ratios, along with
pH monitoring under ambient conditions. The experimental results in Figure 6 show that
the reaction ratio has a significant impact on the recovery efficiency of the product.
Specifically, the recovery efficiency of CeCO₃ reaches 54.2% at a ratio of 0.6, gradually
decreasing to 51.7% (0.8) and 48.27% at a ratio of 1. This trend indicates that the 0.6
ratio is the optimal condition in the study for the precipitation and recovery of Ce(CO₃)₂.
3.6. DTA Results of Raw Monazite Ore
The DTA/TGA measurement results of the raw monazite ore are presented in Figure 7.
Figure 7: DTA/TGA Diagram of Raw Monazite Ore
The thermal analysis diagram presented is the result obtained from the combined
measurement of Differential Scanning Calorimetry (DSC) and Thermogravimetric
Analysis (TGA). The diagram evaluates the decomposition potential of Monazite mineral
using an alkaline agent (NaOH) to recover rare earth elements.
In the range of 30–150°C, a slight mass reduction is observed, likely due to
surface water desorption or adsorbed gas molecules; the DSC curve does not show a
distinct thermal signal. From 150–250°C, a significant mass reduction occurs along with
a weak exothermic effect, possibly related to the decomposition of organic compounds or
volatile impurities. In the 250–500°C range, the mass continues to decrease at a slower
rate, while the exothermic thermal signal becomes more pronounced, reflecting the
partial breakdown of phosphate structures or complex minerals characteristic of
Monazite. Between 500–800°C, the sample mass remains relatively stable, indicating the
high thermal stability of the remaining mineral structure. However, the DSC signal still
exhibits variations, particularly in the 800–950°C range, where a strong exothermic
phenomenon occurs, likely reflecting solid-phase transitions or internal crystal lattice
restructuring.
The TGA-DSC data suggest that the temperature range of 200–600°C is suitable
for ensuring effective mineral structure decomposition, as simultaneous mass reduction
and thermal variations occur, reflecting controlled mineral breakdown processes. Among
these, the 300–500°C range is considered optimal for alkaline decomposition reactions,
as significant transformations occur without inducing complex phase restructuring at
higher temperatures. Selecting this temperature range will enhance reaction efficiency
while minimizing the formation of insoluble by-products, thereby improving rare earth
element recovery in ore processing. Consequently, this study selects 500°C as the
condition for ore decomposition.
3.7. DTA Results of the Product
The DTA/TGA measurement results of the product are presented in Figure 8.
Figure 8: DTA/TGA Diagram of the Product
The thermal analysis diagram presents results obtained from the combined
measurement of Thermogravimetric Analysis (TGA) and Differential Scanning
Calorimetry (DSC). The red curve represents the percentage of remaining mass as a
function of temperature (TGA spectrum), while the black curve illustrates the
corresponding heat flow (DSC spectrum). The objective of this analysis is to determine
the characteristic temperature range for the decomposition of Ce(CO₃)₂.
From Figure 8, in the temperature range of 30–200°C, a slight mass loss (~2–3%)
is observed, likely due to the evaporation of adsorbed surface water. Between 200–
500°C, a significant mass reduction (~15–20%) occurs along with a distinct exothermic
signal in the DSC spectrum, indicating the thermal decomposition of Ce(CO₃)₂ into
CeO₂ with the release of CO₂. Beyond 500°C, the rate of mass loss gradually decreases,
and the sample mass stabilizes around 650°C, reflecting the complete formation and high
thermal stability of CeO₂. The thermal peak observed in the 750–800°C range is believed
to signify the completion of the decomposition reaction.
Thus, the TGA-DSC results clearly identify the optimal temperature range for the
complete transformation of Ce(CO₃)₂ into CeO₂. This serves as a crucial basis for
establishing thermal conditions in the processing, purification, or reutilization of cerium-
containing compounds, ensuring high-purity products that meet the requirements for
applications in analysis, catalysis, or rare-earth material technologies.
3.8. Discussion
The analysis results indicate that the rare earth elements in the concentrate primarily
exist in the form of light rare earths, with Ce being the most abundant (characteristic of
monazite-Ce mineral composition).
During conventional ball milling, the ore particles become smaller but remain discrete.
When ball milling with alkali, the ore particles also decrease in size but tend to aggregate
into clusters due to the chemical reaction between the alkali and monazite.
The results shown in Figure 5 indicate that the rare earth decomposition efficiency is
relatively high, reaching 94.5%. However, when the same mass is used but the NaOH
flake content in the mixing ratio is increased, the recovery efficiency decreases slightly.
The ability to recover Ce in carbonate form, as shown in Figure 6, demonstrates a
significant successful recovery rate of 51.7%.
However, the experimental results show inconsistencies between the two reaction
stages, which may be due to incomplete oxidation of Ce³⁺ or suboptimal pH conditions.
4. CONCLUSION
The study successfully developed a process for recovering Ce from monazite ore
using alkaline treatment combined with selective precipitation. The NaOH:REE mixing
ratio of 0.6 at 500°C for 1 hour resulted in the highest recovery efficiency of Ce(CO₃)₂.
The findings open new directions for efficient rare earth mining and processing in
Vietnam. Additionally, this process can be further developed to extract other rare earth
elements such as Nd, La, and Pr…
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