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Thermal decomposition mechanism of low-content-fluorite Bayan Obo RE concentrate roasted with sodium carbonate and its consequent separation study
Journal Pre-proof Thermal decomposition mechanism of low-content-fluorite Bayan Obo RE concentrate roasted with sodium carbonate and its consequent separation study Dan Zou, Ji Chen, Jiashi Hu, Kai Li, Deqian Li PII:
S1002-0721(19)30345-X
DOI:
https://doi.org/10.1016/j.jre.2019.09.015
Reference:
JRE 621
To appear in:
Journal of Rare Earths
Received Date: 7 May 2019 Revised Date:
17 September 2019
Accepted Date: 27 September 2019
Please cite this article as: Zou D, Chen J, Hu J, Li K, Li D, Thermal decomposition mechanism of low-content-fluorite Bayan Obo RE concentrate roasted with sodium carbonate and its consequent separation study, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.09.015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Thermal decomposition mechanism of low-content-fluorite Bayan Obo RE concentrate roasted with sodium carbonate and its consequent separation study Dan Zoua, Ji Chena, *, Jiashi Huc, Kai Lia,b, Deqian Lia a
State Key Laboratory of Rare Earth Resources Utilization Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b. c
University of Science and Technology of China, Hefei 230026, China College of Chemistry and Chemical Engineering, Inner Mongolia University,
Hohhot 010021, China
Foundation item: Project supported by the National Basic Research Program of China (2012CBA1202), the National Natural Science Foundation of China (51174184), and the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-201-1). *Corresponding author. E-mail address: [email protected] (J. Chen). Tel.: +86 431 85262646
Abstract Thermal decomposition and phase transformation for the mixture of Bayan Obo rare earth concentrate (BORC) and sodium carbonate (Na2CO3) roasted at different temperature with weight ratio of 100: 20 are studied in detail in our study. The aim of our study is to reveal the nature of roasting reaction between BORC and Na2CO3 and thus providing a new method for processing BORC. The results indicate that BORC could be decomposed completely with Na2CO3 at around 600 oC after 3 h. During the calcination process, Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2 are formed after the decomposition of BORC with Na2CO3. In addition, the thermal decomposition mechanism is determined in the paper. Based on these facts, a clean technique processing BORC is developed in the paper. And a CeF3 powder, whose composition is measured and stability is also evaluated, is obtained for some potential application from the new technique. This research is of significance in terms of the Na2CO3-roasting BORC solid reaction study and sheds a light on a potential clean technique for BORC.
Keywords: Thermal decomposition mechanism, Bayan Obo RE concentrate, Na2CO3, technique, CeF3, Rare earths
1. Introduction With the rapid development of society, there is an increasing demand for rare earth elements (REEs), due to their particular physicochemical properties. REEs have a wide application in every walk of life, such as permanent magnets, electronics, superconductors, medical and nuclear industry [1,2]. Currently, over 90% of REE production was supplied by China, followed by United States, Australia and India. Bastnaesite (REFCO3), monazite (REPO4) and xenotime (YPO4) are the three main kinds of minerals containing REEs [3]. In China, Bayan Obo rare earth concentrate (BORC) was the largest light rare earth resources, in which REEs occur dominantly as bastnaesite and monazite [4]. The grade of REO (rare earth oxides) in BORC can reach as high as 65% due to the improvement of mineral processing technology. Among the total REEs in BORC, cerium (Ce) accounts for approximately 50%. Besides, there is 7%–8% fluorine (F) as well as 4%–6% phosphorus (P) in the concentrate. Preferential separation of Ce from other REEs could bring conveniences and advantages for the whole extraction process [5]. So far, the main treatment for BORC applied industrially has always been the concentrated sulfuric acid roasting method [6]. However, because of serious emission of “three wastes” and ever-increasing improvement of environmental protection requirements, new clean technology for processing BORC is needed to propose to replace concentrated sulfuric acid roasting method [7]. In recent years, many clean processes for BORC, such as two-step oxidation method, have been developed [8-10]. The main steps are: oxidation roasting of bastnaesite and sulfuric acid baking/alkaline attacking of monazite to decompose BORC, and then sulfuric acid leaching of roasting product to obtain a liquor containing Ce(IV)-F-P, extraction of liquor with Cyanex 923. A lot of researches have reported that Ce(IV) and F can form stable complex which could be extracted by different types of extractant [11-23]. Apart from that, Ce(IV) and P were reported to coexist in sulfuric media and could also be extracted by Cyanex 923 [24,25]. Two-step method is a clean process which can recover REEs as well as the associate resources F and P. In the meanwhile, it avoids from environmental pollution. Another important process for BORC was Na2CO3-roasting method, which was applied in industry as early as 1970s [26,27].
However, because of the appearance of melt in the process of roasting with Na2CO3 and thus causing “kiln-ringing”, the method couldn’t be applied in practical production. Studies have shown that the appearance of melt was the eutectic mixture of CaF2 and Na2CO3 formed at 555 oC [28]. As the ever-increasing improvement in mineral process technology, fluorite (CaF2) could be effectively separated and the content of fluorite in BORC was as low as less than 2%. Such low content of fluorite could significantly decrease amount of melt (Na2CO3-CaF2 eutectic mixture) in the roasting process. Moreover, with the improvement of calcined equipment to overcome the kiln-ringing, Na2CO3-roasting technique may be the potential and proper one for processing BORC nowadays. Consequently, it was necessary to study the solid state reactions during the roasting process. The thermal decomposition kinetics of BORC of high grade of REO roasted in air was studied in detail by our lab [29]. Qiao et al, studied the mechanism of roast reaction of BORC with high content of fluorite mixed with 15%–25% Na2CO3 [30,31]. There were some reports about the solid reaction of sintered monazite with Na2CO3 [32-35]. However, few have been reported for the roast mechanism of mixed RE mineral of bastnaesite and monazite with Na2CO3. In this paper, the thermal decomposition and phase transformation mechanism of BORC roasted with 20% Na2CO3 were studied. The reaction products were examined by TG-MS, X-ray diffraction, FT-IR and SEM-mapping methods to reveal the decomposition process. Besides, a clean technique about Na2CO3- roasting of BORC were developed. And the obtained CeF3 powder was characterized and estimated its stability. 2. Experimental 2.1. Materials Bayan Obo RE concentrate (BORC) was provided by Baotou Iron & Steel Rare-Earth Hi-Tech Co., Ltd. in China. The chemical compositions of raw material were measured and are listed in Table 1.Table 2 shows the BORC partition of REO in BORC and table 3 gives the main mineralogical phase components of raw materials. It can be seen that main mineral phases of BORC are bastnaesite and monazite. The sum of the two accounts for as much as 92.55%. Sodium carbonate (Na2CO3, purity>99.8%, molecular weight=105.99), hydrochloric acid (HCl, 36 wt%–38 wt%,
molecular weight=36.46), sulfuric acid (H2SO4, 95 wt%–98 wt%, molecular weight=98.08) and nitric acid (HNO3, 65 wt%–68 wt%, molecular weight=63.01) were purchased from Beijing Chemical Works. All chemicals and reagents used in this study were of analytical grade. Cyanex 923 was supplied by Cytec Canada Inc and diluted with n-heptane being purified. 2.2. Method and Analysis 2.2.1 Experimental (1) Sample preparation BORC was dried at 105 oC for 24 h and ground (~200 mesh) using a planetary ball mill (QM-3SP2). The ground BORC was blended completely with a certain amount of Na2CO3 (in the weight ratio of 100:20) in a mixer machine. The blended samples were put into a platinum crucible and then roasted in a muffle furnace (Shanghai Jinghong Instrument Plant, SXL-11008) at the predefined temperature for 3 h. The obtained samples were cooled down in a desiccator for the measurement and following experiments. (2) Water washing experiment About 10 g of roasted products at different temperatures were washed with 500 mL of deionized water in a reactor holding in a constant temperature heating magnetic agitator (DF-101S, GongYi, China) at 90 oC for 1 h. Then the reacted product was filtrated in time and the filter cake was washed with hot water of 90 oC until the pH value of filtrate was close to 7–8. The obtained water leaching liquor was used to analyze the concentration of F and P. (3) Nitric acid leaching experiment The obtained filter cake was leached with 11 mol/L nitric acid at 90–95 oC for 1 h. After filtration, washing the wet leach residue with 5% HNO3 solution for 3–4 times, then merging the filtrate and washing solution, which was used to analyze the REO content. (4) Extraction and stripping experiment The extraction condition is as follows: Organic phase: 30% Cyanex 923~heptane solution; aqueous phase: sulfuric acid liquor; phase ratio (Vo:Va): 2:1; contact time: 30 min; temperature: 298±1 K.
Stripping experiment: 10 mL loaded organic phase was contacted with 10 mL 2% H2O2 aqueous solution, then kept shaking vigorously for 30 min to reach equilibrium. 2.2.2 Analysis The total REO content of solution was determined by standard oxalate gravimetric method. The concentration of Ce(IV) in aqueous solution was titrated with standard (NH4)2Fe(SO4)2 solution. After oxidizing the trivalent Ce into tetravalent with (NH4)S2O8, the concentration of the total Ce was determined with the same method. The contents of P and Th were determined by ICP-OES (Thermofisher Scientific iCAP 6300). The concentration of F was measured by fluorine ion selective electrode (PF-202-L). The concentration of H+ in the aqueous phase was titrated with standard NaOH solution using phenolphthalein as indicator. Thermo-gravimetric (TG-DTG) analyses were performed simultaneously by a thermal analyzer (STA 449F3, NETZSCH, Germany). 20 mg of sample were taken in corundum crucible with a blank crucible as a reference. The measurements were conducted from ambient temperature to 800 oC at a heating rate of 10 oC/min under an air flow of 20 mL/min. A quadrupole mass spectrometer (MS, QMS 403D, NETZSCH) coupled with the thermal analyzer was used for the analysis of escaping gases. The mass signals of the masses being 18 and 44, which should be H2O and CO2, were successively detected during the measurement. An X-ray diffraction (XRD) spectrometer (D8 ADVANCE, BRUKER) was adopted for the determination of the mineral phase compositions of the roasted samples. The scanning was performed within the range of 10 o–70 o with a 8 (o)/min scan speed. A FTIR spectroscopy (VERTEX 70, BRUKER) was used to record the Fourier transform infrared (FT-IR) spectra of the roasted samples. The spectrum of each sample was recorded within the range of 4000 to 400 cm–1. Scanning electron microscope (SEM) mapping images were obtained by SEM (S-4800, HITACHI) instrument. The REO leach rate (S), the removal rate (Rt) of F/P and TCeO/ΣREO referred in the text was calculated as follows： S=
× ×
%
× 100%
(1)
Rt =
cF , P × Vw m × wt %
×100%
TCeO / ∑ REO =
mCeO × 100% mREO
(2)
(3)
Where cREO is for the concentration of REO in nitric acid liquor (g/L), Va is for the volume of nitric acid liquor (L), m is for the weight of roasted product (g), wt% is for the mass fraction of REO in roasted product (%), cF,P is for the concentration of F or P in the water liqor (g/L), Vw is for the volume of water liquor (L), mCeO is for the mass of total cerium oxides in ores or solution (g) and mREO is for the mass of rare earth oxides in ores or solution (g).
3. Results and discussion 3.1 Roast mechanism 3.1.1 Thermal decomposition behavior TG-DTG/MS analyses were adopted to find out the thermal decomposition processes of BORC in the presence of Na2CO3 or absent from Na2CO3. And the results were shown in Fig. 1. As shown in the TG-DTG/MS curves of BORC (Fig .1(a)), a significant weight loss was observed within 400–550 oC. Accordingly, the endothermic peak and mass signal peak of m/z =44 at 465.2 oC appeared in MS curve. It could be resulted from the decomposition of bastnaesite of BORC and the release of CO2. Besides, a small weight loss appeared at 118.8 oC. At this temperature, the corresponding mass signal peak in MS curve was m/z =18. It could be explained by the removal of absorbed water of BORC. With regard to the mixture of BORC and Na2CO3, there are two weight loss behaviors being observed in TG curve within the temperature range of 400–700 oC (Fig 1b). Within 400–550 oC, it was the first weight loss stage. And two endothermic peaks appeared in the DTG curve at 100.8 and 453.8 oC. Consequently, one mass peak signal of m/z =18 at 100.8 oC and one mass peak signal of m/z = 44 at 453.8 oC were observed in MS curves. And hence, the weight loss happening within the temperature range 400–550 oC could be owed to the removal of absorbed water in BORC and the release of CO2 because of bastnaesite reacting with Na2CO3. As for the second weight
loss stage (550–800 oC), one endothermic peak appeared at DTG curve, which corresponded to the mass signal peak of m/z =44 at 658.1 oC in MS curve. It resulted from the further release of CO2 from the reaction of monazite with Na2CO3. By comparing the two processes of BORC with and without Na2CO3, it can be found that the initial decomposition temperature of bastnaesite in BORC mixed with Na2CO3 is lower than that of without Na2CO3, which indicated that the solid-state reaction between bastnaesite and Na2CO3 occurred. It was not a simple oxidation decomposition of bastnaesite in air for the first weight loss stage. 3.1.2. Phase transformation during the thermal treatment In order to investigate the phase transformation in calcination process, the XRD test of the obtained products of BORC and Na2CO3 roasted at different temperature was conducted and the patterns are given in Fig. 2. As shown in Fig. 2, it can be found that the main mineral phases of BORC were bastnaesite and monazite. After the mixture of BORC and Na2CO3 was roasted at 400 oC, the diffraction peaks of bastnaesite slightly decreased because of the release of CO2 from the decomposition of bastnaesite, which was consistent with the results of TG-DTG/MS analyses. When the mixture sample was roasted at 500 oC, it could be found that almost all the diffraction peaks of bastnaesite disappeared. After being roasted at 550 oC, the diffraction peaks of monazite disappeared, too. Further increasing the temperature from 550-800 oC, the diffraction patterns of roasted products nearly showed no difference from 550-800 oC. It was worthy to note that a new diffraction peak appeared at 38.8o, which was ascribed to NaF (JCPDS No. 36-1455). Besides, when the temperature rose to above 570 oC, the diffraction peak of Na3PO4 (JCPDS No. 31-1323) began to appear at 32.4o. Within the temperature range of 550-800 oC, the new diffraction peaks of roasted product appeared at 27.9o, 46.3 o, 55.1 o, 57.6 o, 67.8 o, 74.9 o and 77.1 o, respectively. These characteristic diffraction peaks can be ascribed to Ce0.5Nd0.5O1.75 (JCPDS No. 28-0267). Obviously, it could be found that bastnaesite began to decompose at 400 oC. At 500 oC, the diffraction peaks almost disappeared. When temperature was above 550 oC, the monazite reacted with Na2CO3 and diffraction peaks of monazite disappeared. From 570-800 oC, the BORC was completely decomposed by Na2CO3. The main phases of final roasted product were
Ce0.5Nd0.5O1.75, NaF, and Na3PO4. 3.1.3. Microstructure change during the thermal treatment-FTIR characterization XRD analyses results indicated that BORC was decomposed and transformed into Ce0.5Nd0.5O1.75, NaF, and Na3PO4 after reacting with Na2CO3. To reveal the microstructure change of BORC during the calcination, the FT-IR spectra of BORC caclined with Na2CO3 at different temperatures were recorded and are shown in Fig. 3. As shown in Fig. 3, it could be found that BORC exhibited the characteristic absorption of bastnaesite and monazite, respectively. The detailed information about their characteristic absorption was listed in Table 4. With regards to bastnaesite, there were four vibration absorptions for CO32– [36]. They were symmetric stretching vibration (ν1) at 1086.02 cm–1, out-of-plane bending vibration (ν2) at 878.57 and 867.15 cm–1, asymmetric stretching vibration (ν3) at 1444.70 cm–1 and in-plane bending vibration (ν4) at 727.94 cm–1. As shown in Fig. 3, with the increasing calcination temperature, the stretching vibration absorptions of bastnaesite were weakened gradually. This could be explained by the decomposition of bastnaesite. The result was consistent with those of TG/DTG-MS and XRD pattern analyses. After the mixture sample was roasted at 400 oC, the characteristic bands of bastnaesite were slightly decreased, indicating that the skeletal structure of bastnaesite was destroyed in a small degree. At 500 oC, the ν2 and ν4 vibration peaks of bastnaesite disappeared. When temperature was further increased to 570 oC, almost all the characteristic absorptions of CO32– in bastnaesite disappeared indicating that the structure of bastnaesite was further destroyed. As far as monazite is concerned, there are three kinds of vibration absorptions for PO43– [37]. There were δ1 vibration at 952.98 cm–1, δ3 vibration at 1047.78 cm–1 and ν4 vibration at 540.91 cm–1, 564.36 cm–1, 576.39 cm–1, 603.08 cm–1, of which δ1 and ν4 vibration bands were the important symbols to tell monazite from other phosphate minerals. As illustrated in Fig. 3, it could be found that there was little change about the characteristic absorptions of PO43– in monazite when temperature was below 550 o
C. When the mixture sample was roasted at 550 oC, the characteristic absorptions
bands of monazite began to slightly weaken. At 570 oC, the characteristics bands of monazite disappeared, indicating that the structure of monazite was destroyed. The
spectra of roasted product within the temperature from 570–800 oC were almost identical. 3.1.4. Decomposition and phase transformation mechanism In order to further reveal the nature of thermal treatment process of BORC with Na2CO3, the F and P removal rates of water leaching liquor of roasted products at different temperatures were analyzed. The results were shown in Fig. 4. The washed filter cake was then leached with nitric acid. The REO leach rate of nitric acid solution was also calculated and the result was shown in Fig. 5. As indicated in Fig. 5, the REO leach rate increased with the increasing calcination temperature. At 400 oC, the REO leach rate was 26.81%. The REO leach rate rapidly increased to 90.21% at 500 oC. Within the temperature range from 550 to 600 oC, the growth in REO leach rate became slow. The REO leach rate reached as high as 98.43% at 570 oC, indicating that all the RE elements were almost leached out, that is to say, monazite and bastnaesite of BORC were nearly totally decomposed by Na2CO3 at 570 oC. The conclusion was consistent with the results of XRD and FT-IR. As indicated in Fig. 4, with the increasing calcination temperature, the removal rates of F and P increased. Especially, the F and P removal rates rapidly increased after 500 oC. After 600 oC, the F leach rate increased slowly. It was reported that rare earth oxyfluorites (REOF) reacted with Na2CO3 at about 560 oC and thus F was leached out. However, it should be appropriate 600 oC before the reaction finished. Around 600 oC, the amount of F removed generally equaled to the amount of F in bastnaesite in BORC. For P, it was hardly washed out before 500 oC. After 500 oC, the removal rate of P increased. This growth became rapid even after 600 oC. It suggested that monazite of BORC began to react with Na2CO3 at 600 oC. The removal rates of P were 3.45% at 500 oC, 19.33% at 600 oC and 44.62% at 800 oC, respectively. It was found that REO was nearly leached out after the calcination at 600 oC (Fig. 5), indicating the decomposition of monazite. However, the amount of removed P was far less than that of P in monazite. In XRD pattern of roasted product at 600 oC (Fig. 2), the characteristic diffraction peaks of Na3PO4 were also found. Based on these facts, it showed that monazite reacted with Na2CO3 to transform into other phase containing P apart from Na3PO4 after 600 oC. In order to further reveal the nature, the SEM
mapping was adopted for roasted product of BORC and Na2CO3 at 600 oC (Fig. 6). As seen from Fig. 6, there was the mixed phosphate phase, Na3RE(PO4)2 (marked as “+” in Fig. 6), appearing in roasted product at 600 oC, which was reported by Kizilyalli and Welch [33]. Na3RE(PO4)2, a rare earth double phosphate phase, is soluble in dilute acids but not soluble in water. The formation of intermediate phase Na3RE(PO4)2 helps explain why the removal rate of P in washing water is not high while the REO leach rate of roasted product of BORC mixed with Na2CO3 in nitric acid is close to 100%. However, the diffraction peaks of Na3RE(PO4)2 were not obvious and did not show in XRD patterns because of the weak intensity of diffraction peaks. Based on the above researches, it could be found that the optimum decomposition temperature of BORC with Na2CO3 was 570–600 oC. Within the temperature range, when the weight ratio of BORC and Na2CO3 was 100:20, the main reactions probably occurring in calcining process were as follows: >350 C REFCO3 → REOF(Ce 2 O3 F2 )+CO 2 ↑
(4)
>560 C 2REOF(Ce 2 O3 F2 )+Na 2 CO3 → Ce 0.5 Nd 0.5 O1.75 +2NaF+CO 2 ↑
(5)
o
o
> 570 C mREPO 4 +nNa 2CO3 → o
m- y RE 2O3 (CeO 2 )+xNa 3PO 4 +yNa 3RE(PO4 ) 2 +nCO 2 ↑ 2
(6)
3.2. Application study-Na2CO3 roasting technology for processing BORC A new technology for processing BORC reacted with Na2CO3 was developed in our paper. The flowsheet is shown in Fig. 7. The main steps are as follows: (i) Roasting of BORC with Na2CO3; (ii) Water washing of the roasted product with Na2CO3 before leaching with diluted HCl solution; (iii) Sulfuric acid leaching of the obtained residue after leached by hydrochloric acid; (iv) Solvent extraction of Ce(IV) from sulfuric acid liquor with Cyanex 923. 3.2.1 Effect of roasting temperature and time on REO leach rate Fig. 8 showed the photos of roasting products of BORC with Na2CO3 with mass
ratio of 100:20 at different temperatures for 3 h. It could be seen that the roasting product became sintered after 550 oC. As the further increasing in temperature, the roasting product was hard and lumped, resulting in the products being difficult to be dealt with in the following acid leaching step. Fig. 9 gave the results of effect of roasting temperature and time on REO leach rate. It can be seen that the REO leach rate increased with the increasing temperature and time. At 400 oC, REO leach rate increased slowly from 15.74% to 37.23% with the roasting time increasing from 0.5 to 4 h. Above 500 oC, REO leach rate increased obviously. And REO leach rate increased as the increasing roasting time. When the mixture of BORC and Na2CO3 was roasted at 570 and 600 oC for 3 h, the REO leach rate could reach as high as more than 98%. And therefore, the optimum roasting temperature and time for BORC were 570–600 oC and 3 h. The conclusion was in good agreement with the above results of thermal treatment of BORC roasted with Na2CO3. 3.2.2 Hydrochloric acid leaching and sulfuric acid leaching After BORC roasted with Na2CO3 (weight ratio of 100:20) at 600 oC for 3 h, the obtained roasted product was leached with 3 mol/L HCl solution at 45 oC for 3 h. After filtration, the hydrochloric acid leach residue was further leached with 2.5 mol/L H2SO4 solution at 85 oC for 0.5 h. Table 5 gives the composition of hydrochloric acid leach liquor and sulfuric acid leach liquor. In the following procedure, the H2SO4 leach liquor was extracted with Cyanex 923 to recover Ce(IV). The final H2SO4 leach residue rate was calculated to be 11.85% (about 11.85 g dry residue was finally obtained from processing 100 g roasted product). The yield of REO of the whole process was 96.78%. The results listed in Table 5 indicated that TCeO/ΣREO in the HCl leach liquor was less than 5% (3.99%) and the concentration of F was about 1 g/L while P was 11.47 g/L. However, in the H2SO4 leach liquor, the main composition was Ce(IV) and F. The content of them reached as high as 76.84 and 9.27 g/L, respectively. More importantly, it could be found that the concentration of P in H2SO4 leach liquor was as low as 0.22 g/L. Obviously, F and P were distributed in different solution, which supplied a potential way for the separation of F and P in BORC resources.
3.2.3 Extraction of Ce(IV) and the preparation of CeF3 powder The H2SO4 leach liquor was extracted with 30% Cyanex 923~heptane solution. Then the Ce(IV)-F-loaded Cyanex 923 was stripped with 2% H2O2 solution. The obtained precipitate was washed with water and absolute alcohol for 3 times. After dried at 60 oC for 24 h, the powder was used for characterization. The composition of powder was given in Table 6. It could be calculated that Ce/REE (mass ratio) of the powder was 99.89%, suggesting high purity (~99.9%) of Ce in the powder. In order to measure the stability of powder, the powder was roasted in air at different temperature from 30 oC to 700 oC. Fig. 10 gives their XRD patterns. It could be found that the main phase of powder was CeF3 (JCPDS No. 08-0045). Within the temperature range from 30 to 700 oC, the phase of powder remained unchanged. Further increasing the temperature to 900 oC (Fig. 11), a new phase CeO2 (JCPDS No. 75-0076) began to appear from 700 oC or more. The appearance of CeO2 may due to the decomposition reaction of Ce2(SO4)3 in air at higher temperature. From the above results, it could be seen that the obtained powder was very stable CeF3 particle with high purity of about 99.9%, which could be used for the raw material of scintillation crystal or polishing powder.
4. Conclusions Thermal decomposition and phase transformation for the mixture of BORC and sodium carbonate (Na2CO3) roasted at different temperature with weight ratio of 100: 20 are investigated in this work. TG/DTG-MS, XRD, FT-IR and SEM-mapping spectrometer was used to characterize the roasted products. It shows that BORC could be decomposed completely with Na2CO3 at around 600 oC for 3 h. During the calcination process, BORC, consisting of bastnaesite and monazite, is decomposed and transformed into Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2. The results of the washing experiment of F and P of roasted products further convince the existence of the intermediate phase Na3RE(PO4)2, which is soluble in diluted acid but not soluble in water. And the results of REO leaching experiment indicate that more than 98% of REO in the roasted product at 600 oC for 3 h could be almost leached out. The thermal decomposition mechanism is
determined. Based on our study, a clean technique-Na2CO3 roasting for processing low-content-fluorite BORC is developed in this paper. It sheds light on a potential application value for BORC. And a high-purity of CeF3 (~99.9%) powder is prepared and its composition and stability are measured and evaluated.
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Figure Captions: Fig. 1 TG-DTG/MS curves of BORC (a) and the mixture of BORC and Na2CO3 (b) Fig. 2 XRD patterns of BORC roasted with Na2CO3 at different temperatures Fig. 3 The FT-IR spectra of roasted products of BORC and Na2CO3 at different temperatures Fig. 4 The leach rates of F and P in water washing liquor Fig. 5 The REO leach rate of roasted product at different temperatures Fig. 6 SEM mapping images of BORC (1) and roasted product of BORC with Na2CO3 at 600 oC (2) Fig. 7 The flowsheet of processing BORC with Na2CO3 roasting method Fig. 8 The photos of roasting products at different temperature Fig. 9 Effect of roasting temperature and time on REO leach rate Fig. 10 Variable temperature XRD pattern of powder Fig. 11 XRD pattern of powder roasted at high temperature
Table 1 The chemical compositions of Bayan Obo RE concentrate Components w/wt%
REO 63.48
F 7.92
P 3.49
ThO2 0.18
CaO 5.42
TFeO 1.70
MnO 0.15
Table 2 The partition of REO in BORC Composition
Y2O3
La2O3
CeO2
w/wt%
0.21
27.91
50.96
Composition w/wt%
Pr6O11 Nd2O3 Sm2O3 Eu2O3 Gd2O3 4.76
14.37
1.01
0.19
0.39
Tb4O7 Dy2O3 Ho2O3
Er2O3 Tm2O3 Yb2O3 Lu2O3
Sc2O3
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
Table 3 The mineral phase components of Bayan Obo RE concentrate Mineral Phase
Bastnaesite Monazite Fergusonite Magnetite
Fluorite
Apatite
Barite
Content (wt%)
75.26
17.29
0.02
1.27
1.65
2.07
0.25
Mineral Phase
Dolomite
Calcite
Amphibole
Feldspar
Flogopite
Quartz
Others
Content (wt%)
0.32
0.30
0.10
0.05
0.12
0.19
1.12
Table 4 Infrared Vibration Peaks of Bastnaesite and Monazite (cm–1) PO43– Monazite
δ1
δ3
ν4
952.98
1047.78
540.91, 564.36, 576.39, 603.08 CO32–
Bastnaesite
ν1
ν2
ν3
ν4
1086.02
867.15, 878.57
1444.70
727.94
Table 5 The composition of HCl leach liquor and H2SO4 leach liquor (g/L) H+/M
Composition
ΣREO
Ce(IV)
Ce(t)
ThO2
F
P
HCl liquor
123.3
0
4.93
0.021
1.03
11.47
0.91
H2SO4 liquor
101.5
76.84
78.35
0.22
9.27
0.22
2.49
Table 6 The composition of powder / (mg/g) Elements
Ce
F
Th
SO42–
REEs
Content
711.8
188.7
0.13
41.54
712.58
Fig. 1 TG-DTG/MS curves of BORC (a) and the mixture of BORC and Na2CO3 (b)
Fig. 2 XRD patterns of BORC roasted with Na2CO3 at different temperatures
Fig. 3 The FT-IR spectra of roasted products of BORC and Na2CO3 at different temperatures
Fig. 4 The removal rates of F and P in water washing liquor
Fig. 5 The REO leach rate of roasted product at different temperatures
1
La
Ce
1
P
Ca
2
La
Ce
F
F
2
P
Na
Ca
Fig. 6 SEM mapping images of BORC (1) and roasted product of BORC with Na2CO3 at 600 oC (2) Bayan Obo RE concentrate
Na 2CO 3-roasting 600 oC/3 h
Hydrochloric acid leaching
Single RE product product (non-Ce) (non-Ce)
Sulfuric acid leaching
Solvent extraction Ce(IV) by Cyanex 923
Ce Ce product product
Fig. 7 The flowsheet of processing BORC with Na2CO3 roasting method
Fig. 8 Photos of roasting products at different temperature
Fig. 9 Effect of roasting temperature and time on REO leach rate
Fig. 10 Variable temperature XRD pattern of powder
Fig. 11 XRD pattern of powder roasted at high temperature
Table of contents Thermal decomposition mechanism of Bayan Obo RE concentrate with low-content fluorite calcined with sodium carbonate and its technology study were investigated in this paper. During the calcination process, BORC, consisting of bastnaesite and monazite, was decomposed and transformed into Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2.
Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
S1002-0721(19)30345-X
DOI:
https://doi.org/10.1016/j.jre.2019.09.015
Reference:
JRE 621
To appear in:
Journal of Rare Earths
Received Date: 7 May 2019 Revised Date:
17 September 2019
Accepted Date: 27 September 2019
Please cite this article as: Zou D, Chen J, Hu J, Li K, Li D, Thermal decomposition mechanism of low-content-fluorite Bayan Obo RE concentrate roasted with sodium carbonate and its consequent separation study, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.09.015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Thermal decomposition mechanism of low-content-fluorite Bayan Obo RE concentrate roasted with sodium carbonate and its consequent separation study Dan Zoua, Ji Chena, *, Jiashi Huc, Kai Lia,b, Deqian Lia a
State Key Laboratory of Rare Earth Resources Utilization Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b. c
University of Science and Technology of China, Hefei 230026, China College of Chemistry and Chemical Engineering, Inner Mongolia University,
Hohhot 010021, China
Foundation item: Project supported by the National Basic Research Program of China (2012CBA1202), the National Natural Science Foundation of China (51174184), and the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-201-1). *Corresponding author. E-mail address: [email protected] (J. Chen). Tel.: +86 431 85262646
Abstract Thermal decomposition and phase transformation for the mixture of Bayan Obo rare earth concentrate (BORC) and sodium carbonate (Na2CO3) roasted at different temperature with weight ratio of 100: 20 are studied in detail in our study. The aim of our study is to reveal the nature of roasting reaction between BORC and Na2CO3 and thus providing a new method for processing BORC. The results indicate that BORC could be decomposed completely with Na2CO3 at around 600 oC after 3 h. During the calcination process, Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2 are formed after the decomposition of BORC with Na2CO3. In addition, the thermal decomposition mechanism is determined in the paper. Based on these facts, a clean technique processing BORC is developed in the paper. And a CeF3 powder, whose composition is measured and stability is also evaluated, is obtained for some potential application from the new technique. This research is of significance in terms of the Na2CO3-roasting BORC solid reaction study and sheds a light on a potential clean technique for BORC.
Keywords: Thermal decomposition mechanism, Bayan Obo RE concentrate, Na2CO3, technique, CeF3, Rare earths
1. Introduction With the rapid development of society, there is an increasing demand for rare earth elements (REEs), due to their particular physicochemical properties. REEs have a wide application in every walk of life, such as permanent magnets, electronics, superconductors, medical and nuclear industry [1,2]. Currently, over 90% of REE production was supplied by China, followed by United States, Australia and India. Bastnaesite (REFCO3), monazite (REPO4) and xenotime (YPO4) are the three main kinds of minerals containing REEs [3]. In China, Bayan Obo rare earth concentrate (BORC) was the largest light rare earth resources, in which REEs occur dominantly as bastnaesite and monazite [4]. The grade of REO (rare earth oxides) in BORC can reach as high as 65% due to the improvement of mineral processing technology. Among the total REEs in BORC, cerium (Ce) accounts for approximately 50%. Besides, there is 7%–8% fluorine (F) as well as 4%–6% phosphorus (P) in the concentrate. Preferential separation of Ce from other REEs could bring conveniences and advantages for the whole extraction process [5]. So far, the main treatment for BORC applied industrially has always been the concentrated sulfuric acid roasting method [6]. However, because of serious emission of “three wastes” and ever-increasing improvement of environmental protection requirements, new clean technology for processing BORC is needed to propose to replace concentrated sulfuric acid roasting method [7]. In recent years, many clean processes for BORC, such as two-step oxidation method, have been developed [8-10]. The main steps are: oxidation roasting of bastnaesite and sulfuric acid baking/alkaline attacking of monazite to decompose BORC, and then sulfuric acid leaching of roasting product to obtain a liquor containing Ce(IV)-F-P, extraction of liquor with Cyanex 923. A lot of researches have reported that Ce(IV) and F can form stable complex which could be extracted by different types of extractant [11-23]. Apart from that, Ce(IV) and P were reported to coexist in sulfuric media and could also be extracted by Cyanex 923 [24,25]. Two-step method is a clean process which can recover REEs as well as the associate resources F and P. In the meanwhile, it avoids from environmental pollution. Another important process for BORC was Na2CO3-roasting method, which was applied in industry as early as 1970s [26,27].
However, because of the appearance of melt in the process of roasting with Na2CO3 and thus causing “kiln-ringing”, the method couldn’t be applied in practical production. Studies have shown that the appearance of melt was the eutectic mixture of CaF2 and Na2CO3 formed at 555 oC [28]. As the ever-increasing improvement in mineral process technology, fluorite (CaF2) could be effectively separated and the content of fluorite in BORC was as low as less than 2%. Such low content of fluorite could significantly decrease amount of melt (Na2CO3-CaF2 eutectic mixture) in the roasting process. Moreover, with the improvement of calcined equipment to overcome the kiln-ringing, Na2CO3-roasting technique may be the potential and proper one for processing BORC nowadays. Consequently, it was necessary to study the solid state reactions during the roasting process. The thermal decomposition kinetics of BORC of high grade of REO roasted in air was studied in detail by our lab [29]. Qiao et al, studied the mechanism of roast reaction of BORC with high content of fluorite mixed with 15%–25% Na2CO3 [30,31]. There were some reports about the solid reaction of sintered monazite with Na2CO3 [32-35]. However, few have been reported for the roast mechanism of mixed RE mineral of bastnaesite and monazite with Na2CO3. In this paper, the thermal decomposition and phase transformation mechanism of BORC roasted with 20% Na2CO3 were studied. The reaction products were examined by TG-MS, X-ray diffraction, FT-IR and SEM-mapping methods to reveal the decomposition process. Besides, a clean technique about Na2CO3- roasting of BORC were developed. And the obtained CeF3 powder was characterized and estimated its stability. 2. Experimental 2.1. Materials Bayan Obo RE concentrate (BORC) was provided by Baotou Iron & Steel Rare-Earth Hi-Tech Co., Ltd. in China. The chemical compositions of raw material were measured and are listed in Table 1.Table 2 shows the BORC partition of REO in BORC and table 3 gives the main mineralogical phase components of raw materials. It can be seen that main mineral phases of BORC are bastnaesite and monazite. The sum of the two accounts for as much as 92.55%. Sodium carbonate (Na2CO3, purity>99.8%, molecular weight=105.99), hydrochloric acid (HCl, 36 wt%–38 wt%,
molecular weight=36.46), sulfuric acid (H2SO4, 95 wt%–98 wt%, molecular weight=98.08) and nitric acid (HNO3, 65 wt%–68 wt%, molecular weight=63.01) were purchased from Beijing Chemical Works. All chemicals and reagents used in this study were of analytical grade. Cyanex 923 was supplied by Cytec Canada Inc and diluted with n-heptane being purified. 2.2. Method and Analysis 2.2.1 Experimental (1) Sample preparation BORC was dried at 105 oC for 24 h and ground (~200 mesh) using a planetary ball mill (QM-3SP2). The ground BORC was blended completely with a certain amount of Na2CO3 (in the weight ratio of 100:20) in a mixer machine. The blended samples were put into a platinum crucible and then roasted in a muffle furnace (Shanghai Jinghong Instrument Plant, SXL-11008) at the predefined temperature for 3 h. The obtained samples were cooled down in a desiccator for the measurement and following experiments. (2) Water washing experiment About 10 g of roasted products at different temperatures were washed with 500 mL of deionized water in a reactor holding in a constant temperature heating magnetic agitator (DF-101S, GongYi, China) at 90 oC for 1 h. Then the reacted product was filtrated in time and the filter cake was washed with hot water of 90 oC until the pH value of filtrate was close to 7–8. The obtained water leaching liquor was used to analyze the concentration of F and P. (3) Nitric acid leaching experiment The obtained filter cake was leached with 11 mol/L nitric acid at 90–95 oC for 1 h. After filtration, washing the wet leach residue with 5% HNO3 solution for 3–4 times, then merging the filtrate and washing solution, which was used to analyze the REO content. (4) Extraction and stripping experiment The extraction condition is as follows: Organic phase: 30% Cyanex 923~heptane solution; aqueous phase: sulfuric acid liquor; phase ratio (Vo:Va): 2:1; contact time: 30 min; temperature: 298±1 K.
Stripping experiment: 10 mL loaded organic phase was contacted with 10 mL 2% H2O2 aqueous solution, then kept shaking vigorously for 30 min to reach equilibrium. 2.2.2 Analysis The total REO content of solution was determined by standard oxalate gravimetric method. The concentration of Ce(IV) in aqueous solution was titrated with standard (NH4)2Fe(SO4)2 solution. After oxidizing the trivalent Ce into tetravalent with (NH4)S2O8, the concentration of the total Ce was determined with the same method. The contents of P and Th were determined by ICP-OES (Thermofisher Scientific iCAP 6300). The concentration of F was measured by fluorine ion selective electrode (PF-202-L). The concentration of H+ in the aqueous phase was titrated with standard NaOH solution using phenolphthalein as indicator. Thermo-gravimetric (TG-DTG) analyses were performed simultaneously by a thermal analyzer (STA 449F3, NETZSCH, Germany). 20 mg of sample were taken in corundum crucible with a blank crucible as a reference. The measurements were conducted from ambient temperature to 800 oC at a heating rate of 10 oC/min under an air flow of 20 mL/min. A quadrupole mass spectrometer (MS, QMS 403D, NETZSCH) coupled with the thermal analyzer was used for the analysis of escaping gases. The mass signals of the masses being 18 and 44, which should be H2O and CO2, were successively detected during the measurement. An X-ray diffraction (XRD) spectrometer (D8 ADVANCE, BRUKER) was adopted for the determination of the mineral phase compositions of the roasted samples. The scanning was performed within the range of 10 o–70 o with a 8 (o)/min scan speed. A FTIR spectroscopy (VERTEX 70, BRUKER) was used to record the Fourier transform infrared (FT-IR) spectra of the roasted samples. The spectrum of each sample was recorded within the range of 4000 to 400 cm–1. Scanning electron microscope (SEM) mapping images were obtained by SEM (S-4800, HITACHI) instrument. The REO leach rate (S), the removal rate (Rt) of F/P and TCeO/ΣREO referred in the text was calculated as follows： S=
× ×
%
× 100%
(1)
Rt =
cF , P × Vw m × wt %
×100%
TCeO / ∑ REO =
mCeO × 100% mREO
(2)
(3)
Where cREO is for the concentration of REO in nitric acid liquor (g/L), Va is for the volume of nitric acid liquor (L), m is for the weight of roasted product (g), wt% is for the mass fraction of REO in roasted product (%), cF,P is for the concentration of F or P in the water liqor (g/L), Vw is for the volume of water liquor (L), mCeO is for the mass of total cerium oxides in ores or solution (g) and mREO is for the mass of rare earth oxides in ores or solution (g).
3. Results and discussion 3.1 Roast mechanism 3.1.1 Thermal decomposition behavior TG-DTG/MS analyses were adopted to find out the thermal decomposition processes of BORC in the presence of Na2CO3 or absent from Na2CO3. And the results were shown in Fig. 1. As shown in the TG-DTG/MS curves of BORC (Fig .1(a)), a significant weight loss was observed within 400–550 oC. Accordingly, the endothermic peak and mass signal peak of m/z =44 at 465.2 oC appeared in MS curve. It could be resulted from the decomposition of bastnaesite of BORC and the release of CO2. Besides, a small weight loss appeared at 118.8 oC. At this temperature, the corresponding mass signal peak in MS curve was m/z =18. It could be explained by the removal of absorbed water of BORC. With regard to the mixture of BORC and Na2CO3, there are two weight loss behaviors being observed in TG curve within the temperature range of 400–700 oC (Fig 1b). Within 400–550 oC, it was the first weight loss stage. And two endothermic peaks appeared in the DTG curve at 100.8 and 453.8 oC. Consequently, one mass peak signal of m/z =18 at 100.8 oC and one mass peak signal of m/z = 44 at 453.8 oC were observed in MS curves. And hence, the weight loss happening within the temperature range 400–550 oC could be owed to the removal of absorbed water in BORC and the release of CO2 because of bastnaesite reacting with Na2CO3. As for the second weight
loss stage (550–800 oC), one endothermic peak appeared at DTG curve, which corresponded to the mass signal peak of m/z =44 at 658.1 oC in MS curve. It resulted from the further release of CO2 from the reaction of monazite with Na2CO3. By comparing the two processes of BORC with and without Na2CO3, it can be found that the initial decomposition temperature of bastnaesite in BORC mixed with Na2CO3 is lower than that of without Na2CO3, which indicated that the solid-state reaction between bastnaesite and Na2CO3 occurred. It was not a simple oxidation decomposition of bastnaesite in air for the first weight loss stage. 3.1.2. Phase transformation during the thermal treatment In order to investigate the phase transformation in calcination process, the XRD test of the obtained products of BORC and Na2CO3 roasted at different temperature was conducted and the patterns are given in Fig. 2. As shown in Fig. 2, it can be found that the main mineral phases of BORC were bastnaesite and monazite. After the mixture of BORC and Na2CO3 was roasted at 400 oC, the diffraction peaks of bastnaesite slightly decreased because of the release of CO2 from the decomposition of bastnaesite, which was consistent with the results of TG-DTG/MS analyses. When the mixture sample was roasted at 500 oC, it could be found that almost all the diffraction peaks of bastnaesite disappeared. After being roasted at 550 oC, the diffraction peaks of monazite disappeared, too. Further increasing the temperature from 550-800 oC, the diffraction patterns of roasted products nearly showed no difference from 550-800 oC. It was worthy to note that a new diffraction peak appeared at 38.8o, which was ascribed to NaF (JCPDS No. 36-1455). Besides, when the temperature rose to above 570 oC, the diffraction peak of Na3PO4 (JCPDS No. 31-1323) began to appear at 32.4o. Within the temperature range of 550-800 oC, the new diffraction peaks of roasted product appeared at 27.9o, 46.3 o, 55.1 o, 57.6 o, 67.8 o, 74.9 o and 77.1 o, respectively. These characteristic diffraction peaks can be ascribed to Ce0.5Nd0.5O1.75 (JCPDS No. 28-0267). Obviously, it could be found that bastnaesite began to decompose at 400 oC. At 500 oC, the diffraction peaks almost disappeared. When temperature was above 550 oC, the monazite reacted with Na2CO3 and diffraction peaks of monazite disappeared. From 570-800 oC, the BORC was completely decomposed by Na2CO3. The main phases of final roasted product were
Ce0.5Nd0.5O1.75, NaF, and Na3PO4. 3.1.3. Microstructure change during the thermal treatment-FTIR characterization XRD analyses results indicated that BORC was decomposed and transformed into Ce0.5Nd0.5O1.75, NaF, and Na3PO4 after reacting with Na2CO3. To reveal the microstructure change of BORC during the calcination, the FT-IR spectra of BORC caclined with Na2CO3 at different temperatures were recorded and are shown in Fig. 3. As shown in Fig. 3, it could be found that BORC exhibited the characteristic absorption of bastnaesite and monazite, respectively. The detailed information about their characteristic absorption was listed in Table 4. With regards to bastnaesite, there were four vibration absorptions for CO32– [36]. They were symmetric stretching vibration (ν1) at 1086.02 cm–1, out-of-plane bending vibration (ν2) at 878.57 and 867.15 cm–1, asymmetric stretching vibration (ν3) at 1444.70 cm–1 and in-plane bending vibration (ν4) at 727.94 cm–1. As shown in Fig. 3, with the increasing calcination temperature, the stretching vibration absorptions of bastnaesite were weakened gradually. This could be explained by the decomposition of bastnaesite. The result was consistent with those of TG/DTG-MS and XRD pattern analyses. After the mixture sample was roasted at 400 oC, the characteristic bands of bastnaesite were slightly decreased, indicating that the skeletal structure of bastnaesite was destroyed in a small degree. At 500 oC, the ν2 and ν4 vibration peaks of bastnaesite disappeared. When temperature was further increased to 570 oC, almost all the characteristic absorptions of CO32– in bastnaesite disappeared indicating that the structure of bastnaesite was further destroyed. As far as monazite is concerned, there are three kinds of vibration absorptions for PO43– [37]. There were δ1 vibration at 952.98 cm–1, δ3 vibration at 1047.78 cm–1 and ν4 vibration at 540.91 cm–1, 564.36 cm–1, 576.39 cm–1, 603.08 cm–1, of which δ1 and ν4 vibration bands were the important symbols to tell monazite from other phosphate minerals. As illustrated in Fig. 3, it could be found that there was little change about the characteristic absorptions of PO43– in monazite when temperature was below 550 o
C. When the mixture sample was roasted at 550 oC, the characteristic absorptions
bands of monazite began to slightly weaken. At 570 oC, the characteristics bands of monazite disappeared, indicating that the structure of monazite was destroyed. The
spectra of roasted product within the temperature from 570–800 oC were almost identical. 3.1.4. Decomposition and phase transformation mechanism In order to further reveal the nature of thermal treatment process of BORC with Na2CO3, the F and P removal rates of water leaching liquor of roasted products at different temperatures were analyzed. The results were shown in Fig. 4. The washed filter cake was then leached with nitric acid. The REO leach rate of nitric acid solution was also calculated and the result was shown in Fig. 5. As indicated in Fig. 5, the REO leach rate increased with the increasing calcination temperature. At 400 oC, the REO leach rate was 26.81%. The REO leach rate rapidly increased to 90.21% at 500 oC. Within the temperature range from 550 to 600 oC, the growth in REO leach rate became slow. The REO leach rate reached as high as 98.43% at 570 oC, indicating that all the RE elements were almost leached out, that is to say, monazite and bastnaesite of BORC were nearly totally decomposed by Na2CO3 at 570 oC. The conclusion was consistent with the results of XRD and FT-IR. As indicated in Fig. 4, with the increasing calcination temperature, the removal rates of F and P increased. Especially, the F and P removal rates rapidly increased after 500 oC. After 600 oC, the F leach rate increased slowly. It was reported that rare earth oxyfluorites (REOF) reacted with Na2CO3 at about 560 oC and thus F was leached out. However, it should be appropriate 600 oC before the reaction finished. Around 600 oC, the amount of F removed generally equaled to the amount of F in bastnaesite in BORC. For P, it was hardly washed out before 500 oC. After 500 oC, the removal rate of P increased. This growth became rapid even after 600 oC. It suggested that monazite of BORC began to react with Na2CO3 at 600 oC. The removal rates of P were 3.45% at 500 oC, 19.33% at 600 oC and 44.62% at 800 oC, respectively. It was found that REO was nearly leached out after the calcination at 600 oC (Fig. 5), indicating the decomposition of monazite. However, the amount of removed P was far less than that of P in monazite. In XRD pattern of roasted product at 600 oC (Fig. 2), the characteristic diffraction peaks of Na3PO4 were also found. Based on these facts, it showed that monazite reacted with Na2CO3 to transform into other phase containing P apart from Na3PO4 after 600 oC. In order to further reveal the nature, the SEM
mapping was adopted for roasted product of BORC and Na2CO3 at 600 oC (Fig. 6). As seen from Fig. 6, there was the mixed phosphate phase, Na3RE(PO4)2 (marked as “+” in Fig. 6), appearing in roasted product at 600 oC, which was reported by Kizilyalli and Welch [33]. Na3RE(PO4)2, a rare earth double phosphate phase, is soluble in dilute acids but not soluble in water. The formation of intermediate phase Na3RE(PO4)2 helps explain why the removal rate of P in washing water is not high while the REO leach rate of roasted product of BORC mixed with Na2CO3 in nitric acid is close to 100%. However, the diffraction peaks of Na3RE(PO4)2 were not obvious and did not show in XRD patterns because of the weak intensity of diffraction peaks. Based on the above researches, it could be found that the optimum decomposition temperature of BORC with Na2CO3 was 570–600 oC. Within the temperature range, when the weight ratio of BORC and Na2CO3 was 100:20, the main reactions probably occurring in calcining process were as follows: >350 C REFCO3 → REOF(Ce 2 O3 F2 )+CO 2 ↑
(4)
>560 C 2REOF(Ce 2 O3 F2 )+Na 2 CO3 → Ce 0.5 Nd 0.5 O1.75 +2NaF+CO 2 ↑
(5)
o
o
> 570 C mREPO 4 +nNa 2CO3 → o
m- y RE 2O3 (CeO 2 )+xNa 3PO 4 +yNa 3RE(PO4 ) 2 +nCO 2 ↑ 2
(6)
3.2. Application study-Na2CO3 roasting technology for processing BORC A new technology for processing BORC reacted with Na2CO3 was developed in our paper. The flowsheet is shown in Fig. 7. The main steps are as follows: (i) Roasting of BORC with Na2CO3; (ii) Water washing of the roasted product with Na2CO3 before leaching with diluted HCl solution; (iii) Sulfuric acid leaching of the obtained residue after leached by hydrochloric acid; (iv) Solvent extraction of Ce(IV) from sulfuric acid liquor with Cyanex 923. 3.2.1 Effect of roasting temperature and time on REO leach rate Fig. 8 showed the photos of roasting products of BORC with Na2CO3 with mass
ratio of 100:20 at different temperatures for 3 h. It could be seen that the roasting product became sintered after 550 oC. As the further increasing in temperature, the roasting product was hard and lumped, resulting in the products being difficult to be dealt with in the following acid leaching step. Fig. 9 gave the results of effect of roasting temperature and time on REO leach rate. It can be seen that the REO leach rate increased with the increasing temperature and time. At 400 oC, REO leach rate increased slowly from 15.74% to 37.23% with the roasting time increasing from 0.5 to 4 h. Above 500 oC, REO leach rate increased obviously. And REO leach rate increased as the increasing roasting time. When the mixture of BORC and Na2CO3 was roasted at 570 and 600 oC for 3 h, the REO leach rate could reach as high as more than 98%. And therefore, the optimum roasting temperature and time for BORC were 570–600 oC and 3 h. The conclusion was in good agreement with the above results of thermal treatment of BORC roasted with Na2CO3. 3.2.2 Hydrochloric acid leaching and sulfuric acid leaching After BORC roasted with Na2CO3 (weight ratio of 100:20) at 600 oC for 3 h, the obtained roasted product was leached with 3 mol/L HCl solution at 45 oC for 3 h. After filtration, the hydrochloric acid leach residue was further leached with 2.5 mol/L H2SO4 solution at 85 oC for 0.5 h. Table 5 gives the composition of hydrochloric acid leach liquor and sulfuric acid leach liquor. In the following procedure, the H2SO4 leach liquor was extracted with Cyanex 923 to recover Ce(IV). The final H2SO4 leach residue rate was calculated to be 11.85% (about 11.85 g dry residue was finally obtained from processing 100 g roasted product). The yield of REO of the whole process was 96.78%. The results listed in Table 5 indicated that TCeO/ΣREO in the HCl leach liquor was less than 5% (3.99%) and the concentration of F was about 1 g/L while P was 11.47 g/L. However, in the H2SO4 leach liquor, the main composition was Ce(IV) and F. The content of them reached as high as 76.84 and 9.27 g/L, respectively. More importantly, it could be found that the concentration of P in H2SO4 leach liquor was as low as 0.22 g/L. Obviously, F and P were distributed in different solution, which supplied a potential way for the separation of F and P in BORC resources.
3.2.3 Extraction of Ce(IV) and the preparation of CeF3 powder The H2SO4 leach liquor was extracted with 30% Cyanex 923~heptane solution. Then the Ce(IV)-F-loaded Cyanex 923 was stripped with 2% H2O2 solution. The obtained precipitate was washed with water and absolute alcohol for 3 times. After dried at 60 oC for 24 h, the powder was used for characterization. The composition of powder was given in Table 6. It could be calculated that Ce/REE (mass ratio) of the powder was 99.89%, suggesting high purity (~99.9%) of Ce in the powder. In order to measure the stability of powder, the powder was roasted in air at different temperature from 30 oC to 700 oC. Fig. 10 gives their XRD patterns. It could be found that the main phase of powder was CeF3 (JCPDS No. 08-0045). Within the temperature range from 30 to 700 oC, the phase of powder remained unchanged. Further increasing the temperature to 900 oC (Fig. 11), a new phase CeO2 (JCPDS No. 75-0076) began to appear from 700 oC or more. The appearance of CeO2 may due to the decomposition reaction of Ce2(SO4)3 in air at higher temperature. From the above results, it could be seen that the obtained powder was very stable CeF3 particle with high purity of about 99.9%, which could be used for the raw material of scintillation crystal or polishing powder.
4. Conclusions Thermal decomposition and phase transformation for the mixture of BORC and sodium carbonate (Na2CO3) roasted at different temperature with weight ratio of 100: 20 are investigated in this work. TG/DTG-MS, XRD, FT-IR and SEM-mapping spectrometer was used to characterize the roasted products. It shows that BORC could be decomposed completely with Na2CO3 at around 600 oC for 3 h. During the calcination process, BORC, consisting of bastnaesite and monazite, is decomposed and transformed into Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2. The results of the washing experiment of F and P of roasted products further convince the existence of the intermediate phase Na3RE(PO4)2, which is soluble in diluted acid but not soluble in water. And the results of REO leaching experiment indicate that more than 98% of REO in the roasted product at 600 oC for 3 h could be almost leached out. The thermal decomposition mechanism is
determined. Based on our study, a clean technique-Na2CO3 roasting for processing low-content-fluorite BORC is developed in this paper. It sheds light on a potential application value for BORC. And a high-purity of CeF3 (~99.9%) powder is prepared and its composition and stability are measured and evaluated.
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Figure Captions: Fig. 1 TG-DTG/MS curves of BORC (a) and the mixture of BORC and Na2CO3 (b) Fig. 2 XRD patterns of BORC roasted with Na2CO3 at different temperatures Fig. 3 The FT-IR spectra of roasted products of BORC and Na2CO3 at different temperatures Fig. 4 The leach rates of F and P in water washing liquor Fig. 5 The REO leach rate of roasted product at different temperatures Fig. 6 SEM mapping images of BORC (1) and roasted product of BORC with Na2CO3 at 600 oC (2) Fig. 7 The flowsheet of processing BORC with Na2CO3 roasting method Fig. 8 The photos of roasting products at different temperature Fig. 9 Effect of roasting temperature and time on REO leach rate Fig. 10 Variable temperature XRD pattern of powder Fig. 11 XRD pattern of powder roasted at high temperature
Table 1 The chemical compositions of Bayan Obo RE concentrate Components w/wt%
REO 63.48
F 7.92
P 3.49
ThO2 0.18
CaO 5.42
TFeO 1.70
MnO 0.15
Table 2 The partition of REO in BORC Composition
Y2O3
La2O3
CeO2
w/wt%
0.21
27.91
50.96
Composition w/wt%
Pr6O11 Nd2O3 Sm2O3 Eu2O3 Gd2O3 4.76
14.37
1.01
0.19
0.39
Tb4O7 Dy2O3 Ho2O3
Er2O3 Tm2O3 Yb2O3 Lu2O3
Sc2O3
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
Table 3 The mineral phase components of Bayan Obo RE concentrate Mineral Phase
Bastnaesite Monazite Fergusonite Magnetite
Fluorite
Apatite
Barite
Content (wt%)
75.26
17.29
0.02
1.27
1.65
2.07
0.25
Mineral Phase
Dolomite
Calcite
Amphibole
Feldspar
Flogopite
Quartz
Others
Content (wt%)
0.32
0.30
0.10
0.05
0.12
0.19
1.12
Table 4 Infrared Vibration Peaks of Bastnaesite and Monazite (cm–1) PO43– Monazite
δ1
δ3
ν4
952.98
1047.78
540.91, 564.36, 576.39, 603.08 CO32–
Bastnaesite
ν1
ν2
ν3
ν4
1086.02
867.15, 878.57
1444.70
727.94
Table 5 The composition of HCl leach liquor and H2SO4 leach liquor (g/L) H+/M
Composition
ΣREO
Ce(IV)
Ce(t)
ThO2
F
P
HCl liquor
123.3
0
4.93
0.021
1.03
11.47
0.91
H2SO4 liquor
101.5
76.84
78.35
0.22
9.27
0.22
2.49
Table 6 The composition of powder / (mg/g) Elements
Ce
F
Th
SO42–
REEs
Content
711.8
188.7
0.13
41.54
712.58
Fig. 1 TG-DTG/MS curves of BORC (a) and the mixture of BORC and Na2CO3 (b)
Fig. 2 XRD patterns of BORC roasted with Na2CO3 at different temperatures
Fig. 3 The FT-IR spectra of roasted products of BORC and Na2CO3 at different temperatures
Fig. 4 The removal rates of F and P in water washing liquor
Fig. 5 The REO leach rate of roasted product at different temperatures
1
La
Ce
1
P
Ca
2
La
Ce
F
F
2
P
Na
Ca
Fig. 6 SEM mapping images of BORC (1) and roasted product of BORC with Na2CO3 at 600 oC (2) Bayan Obo RE concentrate
Na 2CO 3-roasting 600 oC/3 h
Hydrochloric acid leaching
Single RE product product (non-Ce) (non-Ce)
Sulfuric acid leaching
Solvent extraction Ce(IV) by Cyanex 923
Ce Ce product product
Fig. 7 The flowsheet of processing BORC with Na2CO3 roasting method
Fig. 8 Photos of roasting products at different temperature
Fig. 9 Effect of roasting temperature and time on REO leach rate
Fig. 10 Variable temperature XRD pattern of powder
Fig. 11 XRD pattern of powder roasted at high temperature
Table of contents Thermal decomposition mechanism of Bayan Obo RE concentrate with low-content fluorite calcined with sodium carbonate and its technology study were investigated in this paper. During the calcination process, BORC, consisting of bastnaesite and monazite, was decomposed and transformed into Ce0.5Nd0.5O1.75, NaF, Na3PO4, and a rare earth double phosphate phase Na3RE(PO4)2.
Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.