A novel process for recovery of rare earth and fluorine from bastnaesite concentrates. Part I: calcification roasting decomposition

2.1 Materials

Bastnaesite concentrates used as raw materials in the experiments were obtained from Mianning in Sichuan Province of China. The chemical compositions measured by X-ray fluorescence (XRF) are given in Table 1. A variety of rare earth elements exist in the ores, and TREO are approximately 70% among which lanthanum and cerium account for about 60%. The X-ray diffraction (XRD) pattern (Figure 2) reveals that the main mineral phases are bastnaesite (REFCO₃) with minor components of parisite (CaCe₂(CO₃)₃F₂), barite (BaSO₄) and fluorite (CaF₂) and less quartz (SiO₂), calcite (CaCO₃) and hematite (Fe₂O₃). The concentrates were obtained after desliming and dressing process, making a coarse granularity with particles of >38 μm accounting for more than 95%. Other reagents used in the experiments were all analytically pure and supplied by Sinopharm, China.

Figure 2:

X-ray diffraction pattern of bastnaesite concentrates.

2.2 Methods and analyses

The roasting experiments were performed in a box-type resistance furnace under air atmosphere. A sheathed K-type thermocouple was inserted into the furnace chamber to monitor the temperature in real time. Powder ore sample of 1 g was put into a nickel crucible and laid on the test point. The sample was roasted at 400~800°C for different times, ruled by CKW 3100 temperature controller. After roasting, the products were properly kept in a jar for test.

The decomposition rate was determined by roasting conditions (amount of calcium hydroxide, roasting temperature and time). They can be analyzed based on the variable valences of cerium. Cerium in raw ores is known as trivalent [Ce (III)]. In the heating process, part of the mineral can be decomposed and cerium can be oxidized into tetravalent [Ce (IV)], while that in the undecomposed mineral remains trivalent. As a result, the oxidation rate of cerium, defined as the following equation (1), can represent the decomposition ratio of ores to some extent. In the definition, Ce (IV) and TCe are, respectively, the content of tetravalent cerium and total cerium in roasted ore sample. They both can be determined by cerium sulfate method with ammonium ferrous sulfate solution as reducing agent and sodium diphenylamine sulfonate indicating final reduction point.

The elements contents in the ores were tested quantitatively through XRF and inductively coupled plasma spectrometry optical emission spectroscopy (ICP-OES) analyses. Laser particle size analyses were put into use to define the mineral particle size distribution. XRD analyses combined with scanning electron microscope and energy dispersive spectrometer (SEM-EDS) analyses made it possible to gain further insight into mineral morphology and the phase transformation in the decomposition reactions.

Elemental composition was mainly measured using a LEEMAN (China) Prodigy XP ICP-OES. Argon with a purity of 99.999% was used to form the plasma. Standard solutions for calibration were prepared by suitable dilution of the stock solutions (1000 μg/ml, Sinopharm, China). The dissolution of samples was conducted by using acid digestion in a high concentration of nitric acid. Plasma parameters were adjusted as follows: RF power 1.1 kW, nebulizer pressure 34 psi, coolant gas flow rate 18 l/min and sample uptake rate of 1.4 ml/min.

A PANalytical (Netherlands) X′ Pert Pro X-ray diffractometer with Cu Kα radiation was used. After fine grinding, the samples were loaded into a cell and flattened. Then they were put on the sample stage in the equipment. The scanning was performed with 2θ from 10° to 90°, and the speed was 4°/min with the step of 0.033°. The data processing was operated using HighScore Plus 2.0.

SEM-EDS analyses were conducted on a Zeiss (Germany) Ultra Plus SEM equipped with a X-Max 50 EDS. The accelerating voltage was set at 15 kV. The samples for analyses were prepared through a series of steps. Carbon conductive tape was first stuck on a cylindrical copper block, and the samples were sprinkled on the tape. Then the samples were gold coated using a sputter coater.

3.1 The influences of roasting conditions on bastnaesite decomposition

The influences of roasting conditions on cerium oxidation rate (η) were studied by several groups of single-factor experiments. The three factors including calcium hydroxide addition, roasting temperature and time were analyzed respectively with the other two remaining unchanged. As can be seen from Figure 3A, calcium hydroxide can effectively promote mineral decomposition. Cerium oxidation rate markedly increased from 55.88% to 69.82% when the amount of calcium hydroxide rose to 10%. After this, the change trend of oxidation rate slowed down with the addition of calcium hydroxide further increased. The rate even reduced when the amount exceeded 15%. This may be due to the excess calcium hydroxide that wrapped the minerals and restrained the oxygenation of cerium. The calculation according to chemical formula of calcium fluoride (CaF₂) showed that the additional amount of 15.89% was essential to ensure that all fluorine in the minerals can turn into calcium fluoride. In conclusion, the amount of calcium hydroxide can be 20%. Figure 3B proved that temperature had the greatest impact on bastnaesite decomposition. The oxidation rate of cerium increased rapidly when temperature rose from 400°C to 550°C. Then the growth range diminished, and 650°C can be the appropriate temperature. After roasting for some time, cerium oxidation rates in the ores were measured. Figure 3C indicated that roasting time was not a decisive factor. It did not take a long time for bastnaesite decomposition and cerium oxidation. Nevertheless, some time of heat preservation was still required for crystal growth, and 60 min should be guaranteed. Overall, the bastnaesite decomposition process should be operated at 650°C for 60 min with 20% calcium hydroxide. Under this condition, a higher oxidation rate of 88.46% was achieved. Sulfuric acid solution of 6 mol/l was used to leach roasted ore at 60~80°C, and the leaching rate of rare earth can reach 98.84%. The difference between cerium oxidation rate and leaching rate was attributed to the existence of Ce₂O₃.

Figure 3:

The influences of roasting conditions on bastnaesite decomposition. Source of error bars: standard deviation of data. (A) Roasted at 500°C for 60 min; (B) roasted for 60 min with 20% Ca(OH)₂; (C) roasted at 500°C with 20% Ca(OH)₂.

3.2 Comparison in the absence and presence of calcium hydroxide

3.2.2 Major mineral phase transformation

Figure 4 shows the XRD patterns of S2 roasted at 650°C and 750°C. The mineral (REFCO₃) could be largely decomposed at 650°C while REOF still existed. Meanwhile, REO and barite (BaSO₄) became major phases. When the temperature reached 750°C, bastnaesite concentrates were almost completely decomposed into REO, and fluorite (CaF₂) emerged in the XRD pattern (Figure 4B). The XRD pattern of S1 roasted at 650°C is shown in Figure 5. In contrast to S2, rare earth ores in S1 were completely decomposed at 650°C. In Figures 4B and 5, CaF₂ was easily identified, and the crystal form and intensity were both different with that in raw ores. That meant new CaF₂ was generated during roasting. It can be concluded that proper amount of calcium hydroxide can promote the decomposition of bastnaesite and lower reaction temperature. Due to the addition of calcium hydroxide, calcium fluoride became prominent minerals. At the same time, a large amount of calcium oxide existed in the roasted ores. A range of reactions may happen as equations (2)–(6) in the calcification roasting process.

Figure 4:

X-ray diffraction pattern of S2 roasted at 650°C and 750°C.

Figure 5:

X-ray diffraction pattern of S1 roasted at 650°C.

(2)Ca(OH)2(s)=CaO(s)+H2O(g) (2)

(3)RECO3F(s)=REOF(s)+CO2(g) (3)

(4)2REOF(s)+CaO(s)=RE2O3(s)+CaF2(s) (4)

(5)CaO(s)+CO2(g)=CaCO3(s) (5)

(6)CaCO3(s)=CaO(s)+CO2(g) (6)

3.3 Particle size analyses of calcification roasted ores

In order to obtain major phase composition distribution in different particle sizes, bastnaesite concentrates (S2) and roasted ores of S1 at 650°C were griddled and measured by XRF and ICP. Mineral particle size distributions are first listed in Table 3. Particles of >48 μm accounted for more than 98% (weight distribution) in bastnaesite concentrates, while that in roasted ore dropped to 87.16% as a consequence of the use of fine-particle calcium hydroxide.

Table 4 shows the main chemical composition of minerals in different sizes. Rare earth elements and fluorine had a higher proportion in large particles of concentrates, while the content of barium increased with the shrink of particle sizes. The distribution of calcium was uniform. In contrast, the contents of calcium and fluorine were higher in small particles of roasted ores, and barium proportion changed just a little. Rare earth elements had the similar variation rule as that in the concentrates.

Elemental distribution in multi-granularity roasted ores, reported in Table 5, can be calculated from Tables 3b and 4b. About 97.32% of the rare earths and 92.84% of the barium in the ores were concentrated in large particles of >48 μm, in which a little more than 50% of calcium and fluorine gathered. Combined with previous XRD patterns, REO and barium sulfate (BaSO₄) can be determined mainly in large particles, while calcium fluoride (CaF₂) distributed evenly in different size particles. As a result, it is practicable to separate the roasted ore with 48 μm as boundaries and study them, respectively.

3.4 Morphology and phase composition of calcification roasted ores

To observe surface morphology of the main phases, SEM-EDS analysis results of bastnaesite concentrates and roasted ores of S1 at 650°C are presented in Figures 6 and 7 and Table 6. As shown in Figure 6A, particle sizes were relatively uniform with larger grains occupying the majority. This agreed with the particle size distribution results mentioned in section 3.3. Bastnaesite and parisite were easily detected due to the large amount. Some REO or oxyfluorides (REOF) also arose along with calcium fluoride, which indicated that rare earth carbonates can be partly oxidized in nature.

Figure 6:

SEM image of bastnaesite concentrates.

Changes in granularity and phase composition were brought about by calcification roasting. More fine particles appeared in the SEM images of roasted ores. A number of REO, calcium oxides, calcium fluorides and calcium carbonates took the place of bastnaesite and parisite. Seen from Figure 7A, pure REO existed in roasted ores. Due to the emission of carbon dioxide generated during roasting, the surface was loose and uneven. Calcium oxide and carbonate were embedded in the ambient calcium fluoride, which displayed a kind of fluffy cotton morphology. A few of the REO attached to calcium fluoride can also be found in the field.

Figure 7:

SEM image of calcification roasted ore.

(A, B, C) >48 μm; (D, E, F) <48 μm.

Seen from the results above, original ores and calcium oxide particles should be regarded as the reaction cores. In the roasting process, fluorine escaped and diffused from rare earth ores and met the surrounding calcium oxides. Then calcium fluorides were generated in the interface resulting in the phenomena in Figure 7B and C. The calcium fluorides with loose structure like calcium oxide can be identified as new reaction products. It was easy for most of them to fall off from REO particles, beneficial to separation. A lot of calcium oxide and fluoride with some REO and barium sulfate concentrated in particles of <48 μg without calcium hydroxide. In roasted ores, calcium fluoride distribution had little to do with particle sizes, while REO concentrated in the particles of >48 μm. The generated calcium fluoride showed completely different fluffy cotton morphology, and most dropped from REO. It is workable to separate REO and calcium fluorides through beneficiation. Through calcification roasting decomposition, fluorine in bastnaesite concentrates can be recovered in the form of calcium fluoride. This is an environmentally friendly way to increase the economical value. Next, thermodynamics and dynamics analyses are necessary to study the reaction mechanism. Clearer mineral-embedded characteristics and surface properties are required to serve the following beneficiation process.