Abstract
Although phosphate rock has been considered as a potential new rare earth elements (REEs) resource, the recovery of REEs from phosphate rock is impeded by technical challenges and cost issues. This study investigated the effects of operation conditions on the leaching efficiencies of REEs and phosphorus from Zhijin phosphate ore, a large phosphate deposit in China. The leaching process overtime was also studied by chemical analysis, scanning electron microscope with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD). The results indicated that the REEs from Zhijin phosphate ore were mainly present in fluorapatite and dolomite, and REEs had similar trends of leaching efficiency to those of phosphorus and magnesium. Under the conditions of 25 wt% phosphoric acid concentration in the initial pulp, a weight ratio of liquid to solid of 12 mL/g, a temperature of 60°C, an agitation speed of 220 rpm, and leaching time of 120 min, REEs and phosphorus leaching efficiencies of 97.8% and 99.7% were obtained. Most parts of dissoluble substances were decomposed within 30 min. Chemical analysis, SEM-EDS, and XRD results indicated that leaching efficiencies of minerals in Zhijin phosphate ore increased following the order: quartz, aluminosilicate, pyrite, fluorapatite, dolomite, and calcite.
1 Introduction
Rare earth elements (REEs) have been considered as the critical materials in the world due to their specialized application in many modern technologies [1,2]. Due to a global supply shortage and strengthening demand of REEs, the exploration and processing of new sources (known as secondary sources) of REEs are necessary [3]. Phosphate rock is the most significant secondary REEs resource [4], and it contains 0.046 wt% REEs on average [5]. About 50 million tons of rare earths are stored in phosphate resources worldwide, nearly one hundred thousand of which are mined annually in the production of phosphate rock [6]. In phosphate rock, REEs occur primarily in the form of isomorphous substitution for Ca and deposit as the REE-francolite that can be easily released into leaching solution by mineral acids [7]. A small amount of REEs are present as particle inclusions such as monazite, xenotime, allanite, and carbonate in apatite, which result in difficulty in REEs dissolution during the leaching process [5].
Researchers have investigated the dissolving of REEs and phosphorus by HNO3 [8,9], HCl [5,10,11], H2SO4 [6,12,13], H3PO4 [14,15,16], and organic acids [17,18]. However, at present, phosphate rock is principally enriched by flotation process first, and then, the flotation concentrate is decomposed by H2SO4 process, producing phosphoric acid and phosphogypsum (an industrial waste produced by wet-process phosphoric acid). During the flotation process, 40% of the REEs enter the waste clay (phosphatic clay or slimes), 10% into sand tailings, and the remaining 50% into phosphate concentrate [6]. In addition, a substantial amount of phosphorous is lost in the flotation tailings. In the wet-process, 37.5% of the total REEs end up in phosphogypsum, and 12.5% enter phosphoric acid and fertilizer [6]. As a result, extra processes are required to recover REEs from these products. In the H3PO4 process, phosphate ore containing REEs and phosphorous is decomposed by H3PO4, forming Ca(H2PO4)2, REE(H2PO4)3, and insoluble substance [15]. After filtering, REEs are recovered by precipitation, crystallization, solvent extraction, and ion exchange [19,20,21].
In this paper, Zhijin phosphorite deposit in China was used to investigate the influence of operation conditions on leaching process, and the leaching residue over time was characterized to study the variation of different minerals in leaching process.
2 Materials and methods
2.1 Materials
The phosphate ore containing REEs was obtained from Guizhou Zhijin in China. It was crushed and wet ground to get samples for acid leaching and scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) tests, and further grinding was conducted to obtain samples for chemical analysis and inductively coupled plasma-optical emission spectrometry (ICP-OES) test. Chemical analysis (XRF) of the phosphate ore is shown in Table 1 and the partitioning of rare earth oxides conducted by ICP-OES test is displayed in Table 2. According to the data in Tables 1 and 2, this phosphate ore was a low-grade calcium siliceous phosphate with P2O5 content of 17.12%. The REEs content was 0.10 wt%, about two times of the average REEs content in general phosphate ores [5]. The proportion of light REEs was slightly higher than that of heavy REEs. La, Ce, Nd, and Y were the main REEs in the sample. The mineral composition of Zhijin phosphate ore was analyzed by Mineral Liberation Analyser and the results are shown in Table 3. Fluorapatite was the main phosphate mineral, and the content was 40.86%. Dolomite, quartz, aluminosilicate, calcite, and pyrite were the main gangue minerals. No independent rare earth minerals were found in this phosphate ore. The distribution of REEs was detected by SEM-EDS, and the main results are present in Figures 1 and 2. It shows that REEs were mainly found in fluorapatite and dolomite. Content of La, Ce, Nd, and Y in fluorapatite were 0%, 0.01%, 0%, and 0.04%, respectively, and the content of these elements in dolomite were 0.01%, 0.04%, 0.01%, and 0%, respectively. Analytical grade phosphoric acid (>85% H3PO4) produced by Tianjing Chemical Works was used. All other chemicals used were of analytical reagent grade. Distilled water was used in all the experiments.
Chemical analysis of Zhijin phosphate ore (wt%)
| Oxides | P2O5 | CaO | MgO | SiO2 | Fe2O3 | Al2O3 | F | S | ΣREO | Others | LOI |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Content | 17.12 | 39.83 | 7.77 | 14.77 | 1.97 | 1.84 | 1.88 | 0.93 | 0.10 | 0.56 | 13.23 |
Partitioning of REO in Zhijin phosphate ore (×10−6)
| Oxides | La2O3 | CeO2 | Pr6O11 | Nd2O3 | Sm2O3 | Eu2O3 | Gd2O3 |
|---|---|---|---|---|---|---|---|
| Content | 196.84 | 139.63 | 33.15 | 139.45 | 25.79 | 6.61 | 31.82 |
| Oxides | Tb4O7 | Dy2O3 | Ho2O3 | Er2O3 | Tm2O3 | Yb2O3 | Lu2O3 |
|---|---|---|---|---|---|---|---|
| Content | 4.67 | 26.74 | 5.98 | 15.87 | 2.80 | 9.37 | 1.91 |
| Oxides | Y2O3 | Sc2O3 | ΣREO | ΣLREO | ΣHREO | δCe | δEu |
|---|---|---|---|---|---|---|---|
| Content | 366.90 | 9.78 | 1017.31 | 541.47 | 475.84 | 0.36 | 1.00 |
Mineral composition and content in Zhijin phosphate ore (wt%)
| Composition | Dolomite | Fluorapatite | Quartz | Wollastonite | Calcite | Pyrite |
|---|---|---|---|---|---|---|
| Content | 33.65 | 40.86 | 12.11 | 6.17 | 3.11 | 1.23 |
| Composition | Muscovite | Anorthite | Limonite | Others | Total |
|---|---|---|---|---|---|
| Content | 0.93 | 0.84 | 0.15 | 0.95 | 100 |
SEM-EDS image of a fluorapatite particle from which EDS elemental maps were obtained; (a) was a SEM image for fluorapatite particle; (b–i) were elemental maps for P, Y, Ca, Mg, O, La, Ce, and Nd, respectively.
SEM-EDS image of a dolomite particle from which EDS elemental maps were obtained; (a) was a SEM image for dolomite particle; (b–h) were elemental maps for Ca, Mg, P, Y, La, Ce, and Nd, respectively.
2.2 Leaching tests
For the acid leaching experiments, the acid solution with the desired concentration was prepared in a 250 mL flask and heated on a temperature-controlled water bath equipped with a magnetic stirrer that was able to maintain the temperature with an accuracy of ± 1°C. Then, a desired amount of phosphate ore (−0.074 mm = 83.8%) was added to the acid solution. A predetermined stirring speed was maintained for Teflon-coated stirring bar. After leaching for a desired time, the suspensions were filtered, dried, and weighed to get the samples for analyses. For each condition, at least three repeated tests were performed, and the average leaching efficiencies were reported in this work.
2.3 Chemical analysis
The concentration of various ions in the aqueous solutions was measured by an inductively coupled plasma-optical emission spectrometer (ICPS-7000 ver. 2 Shimadzu, Japan). Several solid samples were determined by ICP-OES after acid dissolution (combination of perchloric acid, hydrofluoric acid, and nitric acid). The analytical precision for ICP-OES in terms of the relative standard deviation of replicate analyses was less than 5%.
2.4 SEM-EDS measurements
Quanta 250 SEM (FEI, USA) was used to determine the attribution of REEs in Zhijin phosphate ore as well as the nature, morphology, and particle size variation before and after leaching. Before the SEM-EDS tests, the samples were sputter-coated with a layer of gold.
2.5 X-ray diffraction (XRD) analysis
The sample phase and crystallinity were analyzed using a Bruker D8 Advanced X-ray diffractometer with a one-dimensional detector and a Cu Kα irradiation source (λ = 1.5406 Å) at 40 kV, 40 mA, and room temperature. The Zhijin phosphate ore leached for different time was rinsed, dried, and ground for XRD measurement. The samples were scanned in reflection mode in the 2θ range from 10° to 80° with a step size of 0.02° and a collection time of 2.5 s/step.
3 Results and discussion
3.1 Effect of initial phosphoric acid concentration in pulp
Since H+ concentration in the pulp is closely related to mineral digestion speed, it exerts a critical influence on REEs and P2O5 leaching efficiencies [12]. Figure 3 shows the leaching efficiency of phosphate and REEs as a function of phosphoric acid concentration in initial leaching pulp. Since La, Ce, Nd, and Y were the main REEs in the phosphate sample, their leaching efficiencies were also plotted. The leaching efficiencies of REEs and P2O5 increased sharply with increasing concentration of phosphoric acid, reaching a plateau at concentration of 25% with more than 96% and 97% leaching efficiency of REEs and P2O5, respectively. With further increase of phosphoric acid concentration, the leaching efficiencies of REEs and P2O5 were kept virtually constant. In the phosphoric acid concentration range studied, the increased trends of REEs and P2O5 leaching efficiency were almost the same, which is consistent with the fact that in phosphate ore, REEs present majorly in the form of isomorphous substitution for Ca and deposit as the REE phosphate [22]. Below phosphoric acid concentration of 25%, the H+ available for phosphate digestion increased with the increase of phosphoric acid concentration, leading to a dramatic rise in leaching efficiency [15]. However, above phosphoric acid concentration of 25%, the H+ for phosphate digestion was saturated and excessive H+ was not able to participate in the reaction, so further increase of phosphoric acid concentration has little effect on the leaching efficiencies. Hence, phosphoric acid concentration of 25% was used in the following experiments.
Leaching efficiencies of REE and P2O5 as a function of phosphoric acid concentration in initial leaching pulp.
3.2 Effect of ratio of liquid to solid
The effect of liquid to solid ratio on REEs and P2O5 leaching efficiency was tested in the range of 3 to 15 mL/g, and the results are shown in Figure 4. The REEs and P2O5 leaching efficiency increased drastically first as the ratio rose until it reached a platform, about 96% and 97% leaching efficiencies of REEs and P2O5, respectively, at the ratio of liquid to solid of 12 mL/g. Further increase of liquid to solid ratio had little effect on the leaching efficiency. As the liquid to solid ratio increased, the viscosity of the leaching pulp decreased, and thus, the diffusion speed of ions on phosphate surface increased, resulting in accelerated acidolysis rate [12]. In addition, the concentration of various insoluble substances produced was reduced with the increase of liquid to solid ratio, making it difficult to adhere to the surface of minerals, and thus, reducing its negative influence on the decomposition of minerals. In order to get higher leaching efficiencies, liquid to solid ratio of 12 mL/g was utilized in the following experiments.
Leaching efficiencies as a function of ratio of liquid to solid.
3.3 Effect of temperature
Figure 5 presents the leaching efficiencies of REEs and P2O5 as a function of temperature. The leaching efficiencies increased with the increase of temperature, reached the maximum at 60°C, and dropped with further increase of leaching temperature. Increasing temperature will lead to two competing effects on the leaching efficiency. Equations 1–3 were endothermic and occurred simultaneously in the whole temperature range studied. Equations 1 and 2 describe the dissolution of REEs and phosphorous in phosphoric acid solutions, whereas Eq. 3 is the process forming the REEPO4 precipitates. At temperature below 60°C, the viscosity of the pulp reduced with the increase of temperature, which is beneficial to the diffusion of H+, leading to the increase of leaching efficiency. In addition, the reaction of H+ with fluorapatite and dolomite (Eq. 1 and 2) is endothermic and plays a major role. Thus, increasing the temperature accelerated the attacking of H+ on fluorapatite and dolomite at temperature below 60°C, and therefore, the leaching efficiencies of REEs and phosphorous increased. However, the solubility of REEs in mixture solution of Ca(H2PO4)2–H3PO4 decreased with temperature increasing and insoluble REE phosphate was formed, especially at high temperature [5,6]. Formation of REEPO4 precipitates (Eq. 3) played an increasingly important role with the increase of temperature. Therefore, the leaching efficiencies of REEs and phosphorous reduced at temperature above 60°C. Thus, 60°C was set in the following leaching tests.
Leaching efficiencies as a function of leaching temperature.
3.4 Effect of agitation speed
Another parameter on which the extent of dissolution of phosphate ore in acidic media depends is the speed of agitation. During these experiments, the temperature was kept at 60°C, and H3PO4 of 25% was used. As shown in Figure 6, the leaching efficiencies increased significantly with the increase of stirring speed, reaching a plateau at 220 rpm. At speed above 220 rpm, this effect became less remarkable. At speed below 220 rpm, the increase of the agitation enhanced the diffusion of H+ to the surface of phosphate and dolomite [14], and the leached ions were able to transfer rapidly from the surface of phosphate and dolomite to acid solution; as a result, the leaching efficiencies were promoted. Therefore, agitation speed of 220 rpm was performed in the following leaching tests.
Leaching efficiencies as a function of agitation speed.
3.5 Effect of leaching time
Leaching time is an important kinetic parameter for leaching processes of minerals since it is the judgment basis of reaction equilibrium. Therefore, the effect of leaching time on leaching efficiencies of REEs and P2O5 was investigated in 25% phosphoric acid with stirring speed 220 rpm, L/S = 12:1 mL/g at 60°C. As shown in Figure 7, the most part of dissoluble REEs and P2O5 were decomposed in the first half an hour. The increasing trend of leaching efficiencies became mild after leaching for 30 min. This demonstrates that further prolonging leaching time would not bring significant improvement of leaching efficiency. From the point of view of shortening leaching time and optimizing leaching efficiency, 120 min was considered as the proper leaching time. Thus, the optimum leaching conditions for Zhijin phosphate ore using phosphoric acid were listed as follows: phosphoric acid concentration in the initial leaching pulp of 25–35%, temperature of 60°C, ratio of liquid to solid of 12–14 mL/g, agitation speed of 220 rpm, and leaching time of more than 120 min. With phosphoric acid concentration of 25%, temperature of 60°C, ratio of liquid to solid of 12 mL/g, agitation speed of 220 rpm, and leaching time of 120 min, chemical composition of the leachate is (g/L): REO = 0.086, CaO = 32.32, MgO = 6.41, Fe2O3 = 0.79, Al2O3 = 0.40, F = 1.43, S = 0.29. Solvent extraction, ion exchange, and precipitation have been used to recover REEs from pregnant leaching solutions obtained from acid leaching [23]. In the 1960s, ion exchange was the only practical method to recover REEs from leachate [23]. Nowadays, solvent extraction is the most widely used approach [23]. However, large amount of waste is generated during the processes and toxic chemicals are often used [24]. It has been reported that solubility of REEs at 90°C in phosphate acid solution is very low [3]. The most promising method to recover REEs and phosphorous simultaneously is to form precipitate of rare earth phosphates at this temperature, resulting in separation of REEs and other elements. After that, recovery of phosphoric acid and production of gypsum are achieved by adopting concentrated sulfuric acid at ambient temperature and the phosphoric acid can be recycled to decompose crude phosphate rock [3,25].
Leaching efficiencies as a function of leaching time.
3.6 Analysis of leaching residue for different leaching time
In order to study the leaching process, chemical analysis of leaching residue after different leaching time was conducted and the results are presented in Table 4. The content of P2O5, CaO, and MgO declined gradually, indicating that fluorapatite, calcite, and dolomite were dissolving with the prolonging of time. The content of F decreased at the first stage and increased slightly at the second stage. At the first stage, dissolving of fluorapatite was the main factor, thus the content of F decreased. At the second stage, the content of F was mainly governed by the formation of F-related precipitation, leading to slight increase of F in the leaching residue. The content of SiO2, Fe2O3, Al2O3, and SO3 increased gradually, indicating that the minerals containing Si, Fe, Al, and S were not as soluble as those of fluorapatite, calcite, and dolomite in phosphoric acid solution.
Chemical analysis of leaching residue after different leaching time (wt%)
| Leaching time (min) | P2O5 | SiO2 | CaO | MgO | Fe2O3 | Al2O3 | F | SO3 |
|---|---|---|---|---|---|---|---|---|
| 0 | 17.12 | 14.77 | 39.83 | 7.77 | 1.97 | 1.84 | 1.88 | 0.93 |
| 30 | 8.16 | 61.59 | 11.06 | 1.18 | 5.18 | 6.52 | 0.931 | 2.41 |
| 60 | 5.77 | 66.33 | 7.56 | 1.19 | 5.57 | 7.03 | 0.917 | 2.63 |
| 120 | 2.86 | 72.15 | 3.93 | 1.21 | 6.05 | 7.12 | 0.799 | 2.89 |
| 240 | 1.37 | 75.10 | 2.04 | 1.23 | 6.15 | 7.21 | 0.939 | 2.90 |
Leaching efficiencies of various elements at different leaching time were calculated/detected and the results were plotted in Figure 8. The leaching efficiencies increased sharply at the first half an hour and changed mildly with the prolonging of time. The leaching efficiencies almost increased following the order: Si, Al, S, Fe, F, REE, P, Ca, Mg.
Leaching efficiencies of various elements as a function of leaching time.
Figure 9 presents SEM images of the leaching residue before and after leaching for 120 min. The fluorapatite particle size markedly reduced; pyrite and quartz particle size moderately decreased, while dolomite and calcite particles were not detected after leaching for 120 min. In addition, the peak intensity of dolomite, fluorapatite, and calcite decreased with the prolonging of leaching time (Figure 10), suggesting the dissolving of these minerals. The peak intensity of quartz, pyrite, and muscovite increased with the increase of leaching time, indicating that these minerals were not as soluble as dolomite, fluorapatite, and calcite (Figure 10). What is more, calcite dissolves faster than dolomite [26,27]. Combining these results with Figure 8 and Table 3, it might be concluded that leaching difficulties of minerals decreased following the order: quartz, aluminosilicate, pyrite, fluorapatite, dolomite, and calcite.
SEM images of phosphate ore before (a) and after (b) leaching for 120 min.
XRD spectrum of phosphate ore after leaching for different time.
The leaching efficiencies of P and F were virtually identical after leaching for 30 min; thereafter, leaching efficiency of F was lower than that of P. In addition, the leaching efficiency of Si declined after 30 min; it might be concluded that insoluble fluorosilicate was formed after 30 min of leaching time. However, the content of fluorosilicate might be too low to be detected by XRD. The leaching efficiencies of Fe and S were almost the same, indicating that part of pyrite was dissolved in the acid hydrolysis solution. The leaching efficiency of Al was higher than that of Si, which could be attributed to two reasons. First, the solubility of quarts was lower than that of aluminosilicate. Secondly, aluminum oxide octahedron was not as stable as silicon oxide tetrahedron, thus Al was easier to be dissolved from aluminosilicate than Si [28].
4 Conclusion
REEs were mainly present in fluorapatite and dolomite in Zhijin phosphate ore. The optimum leaching conditions for Zhijin phosphate ore using phosphoric acid were as follows: phosphoric acid concentration in the initial leaching pulp of 25–35%, temperature of 60°C, ratio of liquid to solid of 12–14 mL/g, agitation speed of 220 rpm, and leaching time of more than 120 min. The REEs and phosphorus in the Zhijin phosphate ore could be leached out efficiently, reaching efficiencies of 97.8% and 99.7%, respectively. REE leaching efficiency showed similar trends to those of phosphorus.
In Zhijin phosphate ore, leaching efficiencies of minerals by phosphoric acid increased following the order: quartz, aluminosilicate, pyrite, fluorapatite, dolomite, and calcite. The most part of dissoluble REEs, phosphorous, and other dissolvable elements were decomposed in the first half an hour. Insoluble fluorosilicate might be formed during the leaching of Zhijin phosphate ore by phosphoric acid.
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Funding information: This research was funded by Hubei Tailings (Slag) Resource Utilization Engineering Technology Research Center Project, grant number No. 2019ZYYD070, China Geological Survey Project, grant number No. DD20190626, Scientific Research Foundation of Wuhan Institute of Technology (K202064), and Open Foundation of State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control (HB201912).
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Author contributions: Conceptualization: D. H. and Z. L.; data curation: Z. L., H. Z., and H. L.; formal analysis: J. D. and H. Z.; funding acquisition: D. H.; investigation: Z. X.; methodology: Z. L. and Z. X.; project administration: D. H.; resources: D. H.; supervision: D. H.; validation: J. D. and D. H.; visualization: Z. L. and H. L.; writing – original draft: Z. X.; writing – review and editing: Z. L. All authors have read and agreed to the published version of the manuscript.
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Conflict of interest: Authors state no conflict of interest.
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