Extraction of elemental arsenic and regeneration of calcium oxide from waste calcium arsenate produced from wastewater treatment

Minerals Engineering 134 (2019) 309–316

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Extraction of elemental arsenic and regeneration of calcium oxide from waste calcium arsenate produced from wastewater treatment Kang Yang, Wenqing Qin, Wei Liu

T

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School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, Hunan, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Arsenic Arsenic removal Arsenate Elemental arsenic Reductive roasting

Roasting waste calcium arsenate with charcoal powder was studied, serving the purpose of minimizing and/or reclaiming the waste by obtaining elemental arsenic of commercial value while regenerating recyclable CaO. Exploratory experiments were carried out to confirm the feasibility of the suggested process, in which elemental arsenic reaching the grade of 94.5% was obtained. Effects including roasting temperature, dosage of charcoal powder and roasting time were then investigated. The results showed that about 99.0% arsenic could be extracted by volatilization after the sample was roasted at 900 °C for 150 min with 14% charcoal powder, leaving the roasting residue, mainly consisting of CaO, containing arsenic as low as 0.42%. Thermodynamic calculations with respect to equilibrium compositions of the reaction system are diagramed and discussed. Attempts were made to determine and explain the mechanism by comprehensively analyzing the results of Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was found that As (V) in the waste calcium arsenate was reduced to As (III) before its conversion into the elementary state.

1. Introduction As a metalloid, third-row, group V element, arsenic has a long history of uses. It has once been extensively employed in the production of pesticides, herbicides and insecticides (Bothe and Brown (1999)), and nowadays, it is still indispensable for some industries, such as the production of lead car batteries, of anti-friction agent in bearings and of gallium-arsenide semi-conductors (Long et al., 2012). Despite the fact that the contribution arsenic has made for the development of the society should never be underestimated, the overuse of it has led to serious environmental problems, reports having shown that over 144 million people around the world have been affected by its contamination of drinking water (Clancy et al., 2013). Given its high toxicity, it is never too much to consider the situation as being extremely serious. In response to this, many countries have started to put restrictions on its use and encourage searching for the available substitute. Nevertheless, the production of arsenic waste is inevitable because it usually coexists with other wanted materials in nature (Leist et al., 2003) and thus will be anthropologically generated with processing these necessities. One of the most common sources should be the extractive process of non-ferrous metals in which arsenic as valueless element is often discarded (Hopkin, 1989; Yao et al., 2018). Typically, solid waste mainly containing calcium-arsenic phases is produced in a large scale because ⁎

it is one economical and frequently used means to precipitate the arsenic in the wastewater with lime (Nazari et al., 2017). However, the long-term stability of these precipitates is questionable for the atmospheric carbon dioxide and moisture can lead to carbonation, which makes the arsenic easy to be leached out (Robins and Tozawa, 1982; Robins and Jayaweera, 1992). As a result, further treatment is of necessity to stabilize the waste before its final disposal in order to prevent arsenic diffusing into the environment as efficient as possible. Much work has been done for this purpose. Calcination in air has been proposed as one effective method for immobilizing arsenic, during which process, calcium arsenate/arsenite precipitate generated from wastewater treatment can be converted into very insoluble Ca3(AsO4)2 (Nishimura and Tozawa, 1984). However, this phase was apparently attackable under natural acidic conditions and thus unready for land disposal. Re-dissolution of the arsenic-calcium wastes and precipitating arsenic into arsenical natroalunite was experimentally studied and the results were promising (Viñals et al., 2010). In addition, although not developed specifically for the treatment of arsenic-calcium waste, other results obtained by related researchers are also valuable to be referenced including: (1) synthesis of scorodite (Bluteau and Demopoulos, 2007; Caetano et al., 2009; Demopoulos et al., 1995; Filippou and Demopoulos, 1997; Fujita et al., 2008, 2009; Le Berre et al., 2008; Nazari et al., 2017; Paktunc and Bruggeman, 2010; Paktunc et al.,

Corresponding author. E-mail addresses: [email protected] (K. Yang), [email protected] (W. Liu).

https://doi.org/10.1016/j.mineng.2019.02.022 Received 13 November 2018; Received in revised form 20 January 2019; Accepted 5 February 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.

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2008), a stable compound of arsenic with the lowest solubility and dissolution rate among common arsenic minerals (Paktunc and Bruggeman, 2010); (2) stabilization/solidification (S/S) process involving different binders including cement, fly ashes, lime and polymeric materials (Akhter et al., 1997; Choi et al., 2009; Dutré and Vandecasteele, 1995a, Dutré and Vandecasteele, 1995b; Dutré and Vandecasteele, 1996; Singh and Pant, 2006; Vandecasteele et al., 2002; Yoon et al., 2010), and (3) vitrification (Jacobson and Mears, 1992; Zhao et al., 2016), means usually adopted to treat radioactive waste and in which arsenic is expected to be trapped by its incorporation into the thermally formed glass network. It can be seen that focuses of these studies have been mainly put on the immobilization of arsenic in the waste as it seems that improving arsenic stability is the only solution to related environmental problems. In response to this, here we try to offer another direction deserving exploring, that is, to convert the waste into elementary arsenic and recyclable compounds. On one hand, the formed elemental arsenic is of some commercial value and can be sold as commodity. On the other hand, reusing even partially the produced compounds can greatly reduce the quantity of the total waste to be disposed of. The worst case is that arsenic has to be taken as a waste rather than a commodity due to the market, but since the waste has already been minimized, it is much easier to be handled. Our previous study has shown that it is very efficient to extract elemental arsenic from waste sodium arsenate (Yang et al., 2018). However, reports are seldom available for the case of calcium arsenate. It can indeed find some studies about the reductive extraction of arsenic from different materials including industrial waste, but the emphases have been put on developing sophisticated process for the preparation of elemental arsenic that meets product standards(Fedorov and Churbanov, 2016; Fedorov et al., 2001; Potolokov et al., 2003; Smetanin et al., 2007; Smetanin et al., 2003; Turygin et al., 2008). Obviously, refining is too much for the purpose of waste minimization. In this study, the calcium-arsenic precipitate was roasted with charcoal powder. This route is proven to be very efficient for the decomposition of calcium-arsenic phases into elemental arsenic and calcium oxide, and as expected, the former was of relatively high purity and was ready to be sold or for further disposal, while the latter can be recycled at least partially into the process of treating the wastewater.

Table 1 The main composition of the sample (%). As

Ca

O

Fe

Sb

S

P

Si

Al

26.8

29.95

31.37

0.064

0.217

0.06

0.482

0.226

0.069

indicating the precipitate was mainly consisted of arsenic and calcium with a few impurities such as Fe, Sb, S, P, and so on. The contents of the charcoal powder employed in this study were also assayed, which contained 80.3% fixed carbon, 17.5% ash and 2.2% volatile matter. 2.2. Roasting procedure As shown in Fig. 1, the roasting procedure was conducted in a selfdesigned electric resistance furnace (Yang et al., 2018). Unless otherwise specified, for each test, 10 g of the raw material with certain amount of charcoal powder was manually mixed in a mortar using a pestle to ensure a well mixing. The mixture was transferred into a porcelain boat which had previously endured a roasting process for 2 h at 800 °C. The boat containing the sample was placed into the heating zone. Protecting gas N2 (99.5% purity) was introduced into the container at the velocity of 1.0 L/min and it was not until at least 5 min later was the heating started. The target temperature was achieved within 15 min. When the roasting was finished, the removable lid was immediately discharged and the boat was sequentially taken out to cool off under ambient conditions. After cooling to room temperature, the roasting residue was taken out from the boat and was sealed in a sample bag for further analysis. As for the volatile product, it was found to be condensed at different areas on the inner surface of the container, but was collected with care only from the internal surface of the removable lid. Since just a little amount of condensed phase was gathered in each run of these experiments, it was evenly compiled to a mixture. It should be mentioned that the temperature had fluctuated around the expected degree centigrade within ± 20 °C during the roasting process. 2.3. Sample analysis The content of fixed carbon in the charcoal powder was assayed by a Carbon/Sulfur Determinator (LECO CS844), particle size distribution analyzed using a Laser Particle Size Analyzer (LS-POP (6)). The arsenic content of the samples were obtained by Inductively Coupled PlasmaAtomic Emission Spectrometry (ICP-AES, IRIS Interpid II XSP). Crystal structure was characterized by X-ray diffraction (XRD, Bruker AXS D8 Advance) and the morphology observed by Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS, JSM-6360LV). In addition, X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher Scientific K-Alpha 1063 using AlKα X-ray as the excitation source.

2. Experimental 2.1. Materials The calcium arsenate residue used in this study was prepared by treating sodium arsenate solution with calcium oxide. The sodium arsenate was by-product of recovering antimony from arsenic-alkaline residue, which was a typical waste produced during antimony metallurgy in China (Li and Liang, 2010; Qiu et al., 2005; Shao et al., 2012; Yang et al., 2018). The preparation procedures were described as follow: firstly, the solution containing 10 g/L arsenic was obtained using the sodium arsenate; secondly, the pH value of the solution was adjusted to 8 ∼ 9 by hydrochloric acid (A.R. grade) because previous study had shown that much more calcium oxide was needed for the precipitation of arsenic in a solution with high alkalinity; and finally, the lime slurry (20 wt%) was added into the prepared solution (the molar ratio of calcium against arsenic was set as 2:1). The reaction system was stirred with a velocity of 120 r/min for 6 h at room temperature (about 28 °C), and the precipitates were then separated from the aqueous phase via vacuum filtration. The obtained cake was washed with enough water, making sure that no soluble salts (such as NaCl and the unreacted arsenates) were left, and sequentially it was dried at 105 °C until all the moisture was excluded. The sample was subjected to X-ray Fluorescence (XRF) to analyze its general elements, and chemical analyses were conducted for the accurate assays of arsenic and calcium. The results are shown in Table 1,

3. Results and discussion 3.1. Characterization of the precipitate As shown in Fig. 2a, the particles size, distributing from 0.2 μm to larger than 100 μm, was concentrated around 11.07 μm (Fig. 2a). The values of d50 and d90 were 10.191 μm and 52.388 μm, respectively. Obviously, the fineness of the calcium arsenate was favorable for its contact with the charcoal powder and thus beneficial for the reaction. Fig. 2b shows the XRD pattern of the residue. XRD reveals crystal structures of materials by comparing the obtained d values with those of the standard substances. It can be seen from Fig. 2b that although there were some peaks found from 10° to 60°, it was still hard to identify the d values as these peaks were comprised by irregular 310

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Fig. 1. Experimental setup for the thermal treatment.

fluctuations even on the top. However, it was found that some peaks ranging from 10° to 45° had included the lines of Ca5(AsO4)3OH (PDF #33-0265). Despite of the absence of a perfect match and as to the fact that several calcium hydroxyarsenates are known to form., it still suggests the phase was possibly presented with poorly crystallized structure. Other arsenic species were failed to be detected by XRD, which must be attributed to their amorphous structures. Fig. 2c shows the XPS wide scan spectrum of the residue. XPS is a nondestructive surface analysis method for solid materials, and it provides information regarding the chemical composition of the very surface (0.2–0.5 nm) of the sample (Wang and Mulligan, 2008). The existence of arsenic could be confirmed by the most intense As XPS peak As 2p3 at 1326 eV, which was further be supported by peaks of As 3p3, As 3p1 and As 3d at 144 eV, 148 and 45 eV, respectively, and peaks at

268, 232, 208 eV could be assigned to As Auger lines (Groenewold et al., 2013). In addition to the As signature, significant peaks corresponding to O 1s, C 1s, Ca 2s, Ca 2p and Ca 3p were observed. Other elements which were found by XRF could not be detected by XPS due likely to their low contents. As has already been reported by many researchers, the binding energy of As 3d is indicative of the oxidation states of arsenic, with a value of 44 eV and 45 eV for As(III) and As(V), respectively (Bang et al., 2005; Groenewold et al., 2013; Wang and Mulligan, 2009). Further high-resolution scans of As 3d is shown in Fig. 2d, which shows a binding energy of 44.98 eV. Although the binding energy of the sample seems close to that of As (V), the peak shape is asymmetrical, which suggests the possible presence of multiple arsenic species. However, phase analyses of the sodium arsenate employed in this study (which

Fig. 2. Characterization of the precipitate (a. particle size distribution; b. XRD pattern; c. XPS wide scan spectrum; d. XPS spectra of As 3d). 311

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elemental arsenic from calcium arsenate phase by roasting with charcoal was attempted and verified. The sample (20 g of mixture with 12% C) was roasted for 2 h at 900 °C. The experiment was repeated three times in order to collect enough condensed product for analysis, which is characterized in Fig. 4. The particle size of the collected sample was extremely fine (much less than 10 μm), and semi-quantitative assay of EDS shows that it was consisted mainly of arsenic (97.01 wt%), while a trace amount of impurities including S, Pb, Cl, K Sb and Ca were also found (Fig. 4a). Further quantitative analysis was carried out and it confirmed that the sample was rich in arsenic, which reached a grade of 94.5%. This basically indicated elemental arsenic was formed. The morphology of the product can be more clearly seen in Fig. 4b. Grains with amorphous shape were observed and probably due to the high surface energy, agglomeration of these particles was a predominant tendency. Some crystal-like particles were found as emphasized with the arrows in Fig. 4b. Fig. 4c shows XRD pattern of the product, suggesting the presence of amorphous phases in the sample, which is in consistence with observation made by SEM. Meanwhile, some sharp peaks were exhibited, fitting the pattern of As2O3. There were no other substances detected by XRD. The presence of the arsenic oxide must be caused by oxidation which happened when the removable lid of the reactor was discharged immediately after the reaction time was reached. Although the XRD failed to reveal the presence of elemental arsenic, evidence could be found in XPS spectra. As shown in Fig. 4d, characteristic peaks for arsenic including As 3d, As 3p, As 3s, As 3p3 and its Auger lines were clearly demonstrated and the image had perfectly matched the reported pattern for elemental arsenic (Moulder et al., 1992). Moreover, in Fig. 4e, where the high-resolution of the peak As 3d was presented, it could read the binding energies of 41.6 and 42.3 eV. Again, the values are in agreement with those reported by Moulder et al. (1992) for As 3d5 and As 3d3 as in its elementary state, respectively. This is the soundest evidence for the formation of elemental arsenic. As for the small peak found at 45.2 eV, it could be attributed to As2O3 generated by surface oxidation of the particles. Apparently, it is proved that the direct reduction of calcium arsenate into elemental arsenic by roasting with charcoal powder is feasible.

was also the subject of our previous one) has indicated that arsenic exists as As (V) in Na3AsO4·12H2O (or Na3AsO4·0.25NaOH·12H2O) (Yang et al., 2018). Meanwhile, it was unlikely that the steps of preparing calcium arsenate precipitates (described in Section 2.1) could reduce As (V) to As (III). Therefore, the asymmetrical peak shape should be resulted from the presence of different arsenic phases with As (V). This is one reasonable deduction as it has been reported that protonation can lead to some shifts in As 3d binding energies (Wang and Mulligan, 2008). It is thus probable that the precipitate consists of various calcium-arsenic phases with different extent of protonation. As an example demonstrated in Fig. 2d, using the software XPSPEAK41, the peak could be fitted with two components with binding energies of 45.15 eV (Phase I) and 43.9 eV (Phase II). The mole fractions show that 74.69% Phase I and 25.31% Phase II are distributed on the surface and near-surface of the precipitate. 3.2. Thermodynamic consideration Using Outotec's Chemical Reaction and Equilibrium Software HSC Chemistry (Version 6.0), the equilibrium compositions of the system generated by roasting calcium arsenate (Ca3(AsO4)2) with elementary carbon (C) can be calculated (Fig. 3). It helps to estimate the feasibility of the proposed route and to know more details thermodynamically. Using this calculation approach will give different results depending on which compounds one designated in the set up, but it does demonstrate the possibility that carbon roasting is a viable approach. From Fig. 3, it is obvious that reduction of As (V) to As0 is thermodynamically viable. Fig. 3a shows the effects of temperature on the equilibrium compositions of the roasting system. It seems that Ca (AsO2)2 and As0 are simultaneously produced during the roasting, and with increasing temperature, gaseous species including As4 (g) and As2 (g) are produced. Meanwhile, CaCO3 is formed, and it is then totally transformed into CaO at temperature higher than 900 °C. Fig. 3b demonstrates effects resulted from varying the amounts of the reductant charcoal powder. It can be seen that Ca3(AsO4)2 is decreased with increasing C and completely disappeared at C = 1 kmol. There are no elemental arsenic produced at C amount lower than 1 kmol, so the decrease of Ca3(AsO4)2 can be accounted by the formation of Ca (AsO2)2, which starts to decrease with increasing charcoal powder and disappears when molar ratio of C against Ca3(AsO4)2 reaches 4:1. In general, Fig. 3 seems to indicate that, with enough C and at temperature < 700 °C, Ca3(AsO4)2 will decompose into Ca(AsO2)2 and solid As, and at temperature ≥900 °C, it will directly be decomposed into gaseous arsenic phases.

3.4. Roasting procedure 3.4.1. Effects of temperature The effects of temperature on arsenic volatilization are shown in Fig. 5. It can be seen that arsenic extraction depends greatly on the temperature, and increasing the temperature is favorable for arsenic volatilization. Only 13.35% was removed at 600 °C, while more than 80% was extracted when the temperature was risen to 900 °C. As already known, elemental arsenic could not be efficiently volatilized

3.3. Exploratory experiment In the exploratory experiments, the feasibility of directly extracting

Fig. 3. Equilibrium compositions of Ca3(AsO4)2 + C system (a. Equilibrium compositions at 500–1200 °C for 1 kmol Ca3(AsO4)2 + 5 kmol C system; b. Equilibrium compositions at 900 °C with 1 kmol Ca3(AsO4)2 while varying the C amount). 312

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Fig. 4. Characterization of the collected volatilized product.

extent to which the carbon is oxidized. It can be calculated that 8% of charcoal powder is enough for the reaction supposing carbon is only converted into CO2. However, as can be seen from Fig. 3, CO is also one of the products of the roasting process theoretically, the formation of which has actually halved the quantity of electrons carbon can offer. Therefore, it is explainable that only about 74% arsenic was removed with 8% reductant while it hardly reached 95% when the dosage of the charcoal powder was doubled.

unless the temperature reaches its sublimation point at 616 °C (Haynes, 2015). Therefore, even if the calcium arsenate had been converted into elemental arsenic, not much arsenic could be separated at 600 °C. Besides, the increase of arsenic extraction was unsatisfactory at 700 °C, which actually indicated the conversion of As (V) into As0 was inefficient. Accordingly, for a better arsenic extraction, the temperature has to be increased to higher than 800 °C. This is basically consistent with calculation shown in Fig. 3, where As4 (g) becomes obviously observable above 750 °C.

3.4.3. Effects of roasting time The effects of roasting time arsenic volatilization are shown in Fig. 7. It was found that the reaction occurred fast as more than 85% arsenic was removed within the initial 30 min, and it kept increasing with time. However, the uptrend ceased after the sample was roasted for 90 min, where arsenic extraction almost reached 99% and further prolonging the time had brought little effect. Specifically, arsenic volatilization was increased from 98.53% at 90 min to 99.18% at 150 min.

3.4.2. Effects of charcoal powder dosage The effects of charcoal powder dosage on arsenic volatilization are shown in Fig. 6. It is obvious that dosage of the reductant plays a very important role in the arsenic extraction. The more the charcoal powder, the more arsenic is extracted. In the system, carbon in the charcoal powder is obviously the electron donator for the reduction of arsenic. Ideally, the amount of elemental arsenic produced depends on the 313

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Fig. 5. Effects of roasting temperature on arsenic volatilization (dosage of charcoal powder: 10%; roasting time: 60 min).

Fig. 8. XRD patterns of the roasting residues (A. sample roasted at 600 °C with 10% for 60 min; B. sample roasted at 700 °C with 10% for 60 min; C. sample roasted at 800 °C with 10% for 60 min; D. sample roasted at 900 °C with 10% for 60 min; E. sample roasted at 900 °C with 14% for 150 min).

Given the unavoidable existence of experimental errors, these results should be considered as being remained at the same level. 3.5. Mechanism discussion 3.5.1. Analyses of XRD patterns of the roasting residues Fig. 8 shows XRD patterns of different samples obtained under different roasting parameters. Unlike the pattern in Fig. 2 showing broad peaks of arsenic compounds, sharp characteristic peaks of Ca5(AsO4)3OH are observed for sample A and sample B. The peaks became less intensive when the temperature was increased to higher than 800 °C (sample C and sample D). Obviously, this suggests higher temperature is favorable for the decomposition of Ca5(AsO4)3OH. Correspondingly, it could be noted that the peaks of CaCO3, which were small at 600 °C and thus were ignored, became gradually obvious at temperature ranging from 700–800 °C (sample B and sample C). This means the reduction of Ca5(AsO4)3OH is responsible for the growth on CaCO3. When the temperature was further raised to 900 °C, peak intensity of CaCO3 was decreased (sample D). Apparently, this could be accounted by the more efficient decomposition of CaCO3 at 900 °C. On the other hand, signal for CaO was not visible until the temperature reached 700 °C (sample B), and it was enhanced with increasing temperature. Further, it was found that CaO was the only detectable phase after the sample was roasted for 150 min with 14% charcoal powder at 900 °C (sample E). Arsenic content in sample E was assayed to be 0.42%.

Fig. 6. Effects of charcoal powder dosage on arsenic volatilization (temperature: 900 °C; roasting time: 60 min).

3.5.2. Analyses of XPS spectra of the roasting residues In order to define the details of the roasting process, some samples were subjected to analysis by XPS. As shown in Fig. 9a, patterns of As 3d varies depending on the temperature. In comparison to the sample unroasted (Fig. 2d), no obvious change was found at 600 °C, and the binding energy still was about 45 eV (sample A). Next, the peak ridge toward the direction of lower energy seemed to be lifted at 700 °C, evidently leading to an asymmetrical peak shape (sample B). When the temperature was increased to 800 °C, a bulge emerged at 43.6 eV, indicating a new peak was generated (sample C). Finally, the peak at 45 eV had disappeared completely and the newly-formed one at 43.6 eV had grown (sample D). Since the shifting of binding energy for As 3d represents the reduction of As (V) to As (III) (Bang et al., 2005; Groenewold et al., 2013; Wang and Mulligan, 2009), it thus indicates that As (V) was first reduced to As

Fig. 7. Effects of roasting time on arsenic volatilization (temperature: 900 °C; dosage of charcoal powder: 14%).

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Fig. 9. XPS narrow scan of (a) As 3d and (b) C 1s for the roasting residues (A. sample roasted at 600 °C with 10% for 60 min; B. sample roasted at 700 °C with 10% for 60 min; C. sample roasted at 800 °C with 10% for 60 min; D. sample roasted at 900 °C with 10% for 60 min).

(III) before As0 was ultimately formed. On the other hand, one unexpected observation was made in the case of the binding energy of C 1s. As can be seen from Fig. 9b, peaks around 283 eV were found at 600 °C and 700 °C, which disappeared when the temperature was increased to higher than 800 °C. As reported (Moulder et al., 1992), carbide was responsible for these peaks. The reason for its formation is unclear, but it apparently complicates the explanation on mechanism. Further research is thus necessary for a more sophisticatedly comprehension on this roasting process. 4. Conclusion The present paper describes the direct extraction of elemental arsenic from waste calcium arsenate by roasting with charcoal powder. Based on the results and discussion above, conclusions have been drawn as follows: (1) about 99.0% arsenic was directly extracted in the form of elemental arsenic when the waste calcium arsenate was roasted at 900 °C for 150 min with 14% charcoal powder; (2) the roasting residue under optimized conditions was mainly consisted of CaO, containing arsenic as low as 0.42%; (3) due to restrictions originated from experimental procedures, the obtained elemental arsenic only reached a grade of 94.5% in this study; (4) reduction of arsenic during the roasting should obey the route: As (V) → As(III) → As0. Accordingly, it is feasible to extract efficiently elemental arsenic and regenerate CaO from the waste calcium arsenate produced from precipitating the arsenic in the wastewater using CaO. The obtained crude arsenic can be sold or for further safe disposal while the regenerated CaO can be recycled into the treatment of the wastewater. Acknowledgement The authors gratefully acknowledge the financial support by Provincial Science and Technology Leader (Innovation team of interface chemistry of efficient and clean utilization of complex mineral resources) (Project No. 2016RS2016); Co-Innovation Centre for Clean and Efficient Utilization of Strategic Metal Mineral Resources and the 315

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