Lithium orthosilicate with halloysite as silicon source for high temperature CO₂ capture

Abstract

Lithium orthosilicate (Li₄SiO₄)-based sorbents were synthesized using a low cost and naturally available mineral resource (halloysite) as silicon source for high temperature CO₂ capture. Halloysite nanotubes (HNTs) were acid-treated by 6 M HCl for 6 h to leach diaspore-like sheet to produce Si source (H6), which showed the best extraction effects. Pure Li₄SiO₄ (P-LS) and H6-Li₄SiO₄ (H6-LS) were thermally analyzed under a CO₂ flux. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), nitrogen adsorption, and thermogravimetric (TG) analysis were used to examine the character of the phase, morphology, and CO₂ chemisorption properties of the samples. The Li₄SiO₄ sorbent was synthesized at 800 °C for 4 h (H6-LS800), and the metal impurities especially aluminum were doped into the structure of H6-LS800, resulting in the increased surface area. Thermal analysis indicated the maximum CO₂ adsorption capacity of H6-LS800 reached up to 34.45%, with the temperature range from 350 to 720 °C, which had a better CO₂ adsorption than P-LS. In addition, H6-LS800 sorbent exhibited excellent adsorption–desorption performance.

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1. Introduction

Carbon dioxide (CO₂) is a key anthropogenic greenhouse gas and the rapidly increasing concentration of CO₂ in the atmosphere has brought more and more problems like global warming and climate change over the last few decades.^1,2 CO₂ concentration in the atmosphere has monotonically increased from 310 to 390 ppm from 1960 to 2010.³ At present, carbon-based fossil fuels which provide most of the world's energy needs are the large stationary sources of CO₂ gas,⁴ and the temperature of these flue gases produced after the combustion of fossil fuels in turbine is usually in the range of 352–627 °C.⁵ Therefore, development of the material with high adsorption efficiency at high-temperature, cheap and stable CO₂ adsorption is urgently needed for carbon capture and (underground) storage. The high-temperature sorbents for CO₂ capture, such as hydrotalcite-like compounds,⁶ calcium-based sorbents,^7,8 and lithium-based sorbents,⁹ have been investigated in detail.

Recently, lithium-based CO₂ adsorbent materials, including Li₄TiO₄,¹⁰ Li₄SiO₄,¹¹ Li₅AlO₄,¹² and Li₆Zr₂O₇ ¹³ have been widely used in CO₂ adsorption with excellent performance in adsorption efficiency, and stability, especially lithium orthosilicate (Li₄SiO₄). Li₄SiO₄ sorbent has a better adsorption and faster adsorption rate for CO₂ than other lithium-based materials, for example Li₂ZrO₃,¹⁴ and has shown excellent adsorption–desorption performance.¹⁵ Li₄SiO₄ has been variously studied about the synthesis method, including the common solid-state,¹⁶ precipitation,¹⁷ impregnated suspension¹⁸ and sol–gel method.¹⁹ Solid-state is a conventional method to synthesize Li₄SiO₄ samples, which usually consists of solid mixtures of amorphous silica (SiO₂) and a lithium compound, heated in air for a period of time. Further to enhance the inherent reactivity of Li₄SiO₄ for sorption of CO₂, doping Li₄SiO₄ with sodium¹⁷ and other hetero-elements^20–22 should be used. Other scholars tried to improve the sequestration capacity, cyclic stability by changing the synthesis condition, sintering time, raw materials, particle size and so on.^23–26 According to the previous report, CO₂ was captured by this material through the following reaction,²⁶ generally, it can completely react in 750 °C for 6 h.

Li₄SiO₄ + CO₂ ⇔ Li₂SiO₃ + Li₂CO₃ (1)

Natural mineral, we can modify it by a variety of methods to obtain a high performance functional materials.^27–37 Halloysite with the aluminosilicate nanotubes internal surface composed of gibbsite-like array of Al–OH groups whereas the external surface consisting of Si–O–Si groups has been developed in many fields.^38–40 Halloysite clay nanotubes have been used for the controlled release of protective agents,⁴¹ for storage and controlled release of oxygen in aqueous media.⁴² Hybrid halloysite nanotubes (HNTs) were constructed as nanoarchitectures for enhancing the capture of oils from vapor and liquid phases,⁴³ the hydrophobically modified halloysite as reverse micelles for water-in-oil emulsion might be suitable for the industrial and biological applications.⁴⁴ Additionally, natural halloysite clay nanotubes have been applied as miniature vehicles for sustained release of drugs and proteins.⁴⁵ So natural halloysite clay has attracted specific research attention in material preparations, and gas sorption in recent years. Treatment of HNTs by organic or inorganic acids led to the leaching of the Al–OH groups from the structure,⁴⁶ hence, the silicon groups were separated from aluminum. The use of silicon groups as sources for the preparation of lithium orthosilicate and aluminum groups for other application could be an efficient route.

Herein, the aim of this work was to synthesize Li₄SiO₄-based sorbents from HNTs by solid-state reaction for high temperature CO₂ capture. Pure Li₄SiO₄ (P-LS) was also prepared for comparison purpose. The effect of sintering temperature and silicon source on phase composition and CO₂ adsorption properties were discussed. Additionally, the adsorption–desorption properties of Li₄SiO₄-based sorbents were investigated by thermogravimetric analysis.

2. Experimental

Halloysite nanotubes (HNTs) used in this study were taken from Hunan, China. All reagents were analytical grade and used without further purification. Based on a previous study on HNTs of our research group,⁴⁶ the HNTs samples (1.00 g) reacted with 250 mL of 6 M HCl solution at 100 °C in an oil bath. The reaction was carried out in a conical flask for 2 h, 4 h and 6 h with constant stirring, respectively. The suspension was then filtered, washed repeatedly with water until no acid was detected in the filtrate. Both treated HNTs were all dried at 80 °C overnight to produce a precursor, the as-obtained precursors were labelled as H2, H4, and H6 corresponding to different reaction times (2 h, 4 h and 6 h), respectively.

Li₄SiO₄-based sorbents from HNTs were synthesized by solid-state reaction of lithium carbonate (Li₂CO₃, AR) with acid-treated HNTs in a Li : Si mixture with molar ratio of 4 : 1. The powders were mixed with some anhydrous ethanol into the agate mortar, and continuously ground for 30 min, then dried at 80 °C in air to remove anhydrous ethanol. Finally, the powders were calcined at 800 °C for 4 h to produce Li₄SiO₄-based sorbents, which were labelled as Hx-LS800 (x = 2, 4 or 6). For comparison, pure Li₄SiO₄ (P-LS) was also prepared by following the same method described before from SiO₂ as silicon source.

The chemical composition of HNTs and sorbents were determined using X-ray fluorescence (XRF) spectrometer. The phases were identified by X-ray diffraction (XRD) analysis with a RIGAKU D/max-2550VBR + 18 kW powder diffractometer with Cu Kα-radiation (λ = 0.15418 Å). The data were collected from 5° to 80° of 2θ with a step of 0.02° s⁻¹. Compounds were identified conventionally by corresponding Joint Committee Powder Diffraction Standards (JCPDS) files. The morphology of the sample was observed with SEM (FEI Quanta-200). The surface of samples was coated with a layer of gold film to avoid electrical conductivity. The specific surface area, pore volume, and pore size distribution of the sorbents were measured by Quantachrome Instruments nitrogen (N₂) adsorption–desorption analyzer. The thermal stability and CO₂ absorption properties of the samples were tested by a thermogravimetric analyzer (STA449C Netzsch Germany) with N₂ as the protective gas and CO₂ as the purge gas. About 10 mg of the sorbent was placed into crucible and heated from room temperature to 800 °C at a heating rate of 5 °C min⁻¹ in 100 vol% CO₂ flow (60 mL min⁻¹). Furthermore, the CO₂ absorption isotherms of the sorbents were measured in 700 °C for 120 min. The sorption/desorption properties were analyzed in a fixed temperature (700 °C), during the test, the uptake conditions in a CO₂ atmosphere (60 min) and the desorption conditions in a N₂ atmosphere (60 min).

3. Results and discussion

The composition HNTs and acid-treated HNTs samples (H2, H4, and H6) were given in Table 1. HNTs mainly consisted of silica-like sheet (SiO₂) and diaspore-like sheet (Al₂O₃), treatment of HNTs by acid let the Al₂O₃ leach from the structure. And the leaching degree of aluminate increased with increasing the acid-treated time. When HNTs were acid-treated for 6 h, the silicon content rose to 96.12%, which indicated that the diaspore-like sheet was almost all leached and the structure of halloysite was a silica-like sheet. The halloysite acid-treated for 6 h (H6) could be used as the silica source to synthesize lithium orthosilicate. One thing to note is the metal impurities, such as Al, Zn, Ca, and Mg, still existed in the sample of H6. The XRD analysis showed that the data of HNTs fitted well with the characteristic data of halloysite (Fig. 1), the (001) diffraction peaks at 2θ = 12.0° corresponded to basal spacing of 0.73 nm, which means this sample is halloysite-7 Å.⁴⁷ After acid treatment of 6 h, these diffraction peaks of halloysite disappeared, and only presented a broad peak at 2θ of 22°, which demonstrated the amorphous nature of the silica. The full width at half-maximum (FWHM) of H6 was wider than that of HNTs, indicating that the obtained amorphous silica had a more highly disordered structure.

Table 1 Chemical components of halloysite after acid treated for different times

Samples	SiO2	Al2O3	ZnO	CaO	MgO	K2O
HNTs	54.85	43.859	0.972	0.218	0.06	0.03
H2	69.27	29.66	0.779	0.201	0.05	0.04
H4	89.24	9.82	0.71	0.16	0.04	0.03
H6	95.43	3.76	0.61	0.12	0.06	0.02
H6-LS800	96.12	3.17	0.57	0.09	0.05	0.05

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Fig. 1 XRD patterns of HNTs, acid-treated H6, P-LS and Li₄SiO₄-based samples.

Fig. 1 shows the comparison of XRD patterns of P-LS and those of the Li₄SiO₄-based samples synthesized by solid-state method between Li₂CO₃ and H6 from different conversion temperature. P-LS and H6-LS800 patterns fitted very well to JCPDS file of 37-1472, which represents the same compound. While some Li₂CO₃ phases were observed in the sample of H6-LS650, and the purities of Li₄SiO₄ samples increased with the increase of calcination temperature. When the calcination temperature reached up to 800 °C, the pure Li₄SiO₄ phase could be successfully synthesized.

In order to analyze the influence of calcination temperature and raw material on the CO₂ capture properties of Li₄SiO₄, preliminary sorption runs performed on P-LS and H6-LS (650–800). Fig. 2 presented the dynamic TG analysis of P-Li₄SiO₄ and Li₄SiO₄-based sorbents into a CO₂ flux. The thermal behavior analysis in Fig. 2 clearly showed that the CO₂ absorption capacities increased with the increase of the purity of Li₄SiO₄ in the synthesized samples. The samples synthesized at 650–800 °C exhibited absorption capacities. According to the results, H6-LS800 synthesized at 800 °C for 4 h showed the highest CO₂ chemisorption. CO₂ sorption capacities of H6-LS800 showed a little improvement compared with P-LS. Fig. 2b provided the TG-DSC curves of H6-LS800 which exhibited high CO₂ chemisorption. The 34.45 wt% increment of weight change occurred between 400 and 720 °C, corresponding to an exothermic peak (704 °C) present in the DSC curve, indicated the samples reacted with CO₂ and released heat meanwhile. The weight of samples began to reduce when the temperature increased up to about 720 °C. The weight loss of 8.2 wt% occurred between 720 and 800 °C, which corresponded to an endothermic peak in the DSC curve. The desorption process needs to absorb heat and the chemical reaction towards the opposite direction (eqn (1)).

Fig. 2 (a) TG analysis of as-synthesized Li₄SiO₄-based sorbents and (b) TG-DSC curves of H6-LS800 sample.

Fig. 3 shows the CO₂ sorption isotherms of H6-LS800 and P-LS at 700 °C for 120 min, the CO₂ capacity increased with the time. H6-LS800 reached to a maximum adsorption capacity 34.75 wt% after 2 h, while that of P-LS was 30.49 wt%. Then the adsorption kinetics of CO₂ on H6-LS800 and P-LS was studied by considering double exponential model:

y = A exp^−k₁x + B exp^−k₂x + C (2)

where y is the weight percentage of CO₂ absorbed, x is time, A, B and C are the pre-exponential factors. k₁, k₂ is the chemisorption constant and diffusion constant, respectively. As Li₄SiO₄ took place by two different processes, the chemisorption and the lithium diffusion. Lithium located on the surface of Li₄SiO₄ particles first reacted with CO₂, and produced a Li₂CO₃ external shield (k₁), then lithium and oxygen diffusion (k₂) which went across external shield to react with CO₂ controlled the chemisorption kinetics, when the external layer was completely formed. Table 2 shows the kinetic parameters obtained from the isotherms of the two samples, H6-LS800 more fits the double exponential model. k₁ of H6-LS800 is 1 order of magnitude higher than k₂, implying that the lithium diffusion is the limiting step during the adsorption process. However, k₁ and k₂ values of P-LS are high than those of H6-LS800, which may be due to the different materials preparation. P-LS only took 5–8 min to reach 95% of maximum adsorbed amount in Fig. 3, faster than H6-LS800. In such fast time, the Li₂CO₃ external shield was formed too quickly, which led to the less of lithium and oxygen diffusion.

Fig. 3 Isotherms of CO₂ sorption on P-LS, H6-LS800 and corresponding fit double exponential model.

Table 2 Kinetic parameters obtained from the isotherms at 700 °C fitted to a double exponential model

Sample	k1	k2	R^2
H6-LS800	0.214	0.036	0.990
P-LS	0.620	0.621	0.878

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A crucial issue for an adsorbent is its chemical stability during the regeneration process. From the experimental results presented above, the most promising sorbent among those compared is the sample H6-LS800, so the regeneration properties and the thermal stability after 10 cycles of this sample was assessed. The test was carried out at a fixed temperature (700 °C) in a Netzsch TG analyzer, the uptake conditions in a CO₂ atmosphere (60 min) and the desorption conditions in a N₂ atmosphere (60 min) during the test. The adsorption–desorption performance of H6-LS800 indicated that the adsorption capacity increased firstly from 28.74 wt% to 31.99 wt% after the 3rd cycle (Fig. 4). Then the capacity decreased for several cycles and to 28.9 wt% after 10th cycle. The CO₂ chemisorption capacity varied in a small area range with the increase of cycle number. The stability of this sorbent showed excellent cyclic adsorption stability, indicating that the sorbent could have interesting potential as a kind of materials for high temperature CO₂ capture.

Fig. 4 CO₂ chemisorption capacities of H6-LS800 as a function of the cycle number during the multiple sorption–desorption cycles.

As was previously mentioned, Li₄SiO₄ is a promising CO₂ absorbent material. Moreover, H6-LS800 sorbents from halloysite showed an enhancing CO₂ absorption compared with P-LS. In order to elucidate this enhancement effects, the CO₂ adsorption behavior of H6-LS800 was discussed. After Li₄SiO₄-based sorbent was characterized by XRF and XRD, the metal impurities, such as Al, Zn et al. still existed in the H6-LS800 sorbents (Table 1), which did not modify the XRD patterns of the Li₄SiO₄-based phase (Fig. 1), but XRD patterns showed some profile changes compared with P-LS, (200) and (002) crystal planes both moved a little to the left (Fig. 5), which indicated that the crystalline structure has been slightly expanded. Ortiz-Landeros et al.²⁰ synthesized the lithium orthosilicate solid solutions: Li_4+x(Si_1−xAl_x)O₄ and Li_4−x(Si_1−xV_x)O₄, and noted that both aluminum and vanadium ions occupied silicon sites into the Li₄SiO₄ lattice and the dissolution of aluminum was compensated by Li⁺ insertion, the dissolution of vanadium led to the lithium vacancies formation, the doping Li₄SiO₄ with aluminum and vanadium caused the expanded crystalline structures and the shift of the diffraction peaks. Such results were also observed in the H6-LS800, Al³⁺ (0.039 nm) and Zn (0.135 nm) possessed larger ionic radii than Si⁴⁺ (0.026 nm), which was contributed to the cell expansion. The levels of Ca, Mg and K content were low; therefore there were other phases in the H6-LS800.

Fig. 5 Zoom of the XRD patterns of P-LS and H6-LS800 samples.

A Rietveld refinement analysis was further performed in order to corroborate the XRD results above. The lattice parameters and cell volume obtained showed that the refinements should be considered as acceptable according to the R_wp values in Table 3. The cell parameters of H6-LS800, a, b, c and β is 5.29927, 6.1125, 5.1556 Å and 90.29 deg, respectively, as well as the cell volume (167.06 ų), show a slight increase compared with P-LS, which indicated the impurities (Al³⁺, Na⁺) doping into the Li₄SiO₄ structure could expand the crystal lattices. Then the morphology of the samples were obtained by SEM (Fig. 6), showing the SEM images of P-LS and H6-LS800 samples, (A) and (B) represent the SEM images of Li₄SiO₄ obtained by H6, SiO₂ as the silicon source, respectively. The H6-LS800 sample presented a particle size average of 50 μm (Fig. 6a). These particles seemed to be the highly sintered agglomerates; this kind of morphology corresponded well with the high temperature used during the thermal treatment. Some holes were observed on their surface from inserted figure. The P-LS sample presented more dense particles (Fig. 6b), with a particle size of around 60 μm, having a polyhedral shape, and the surface became smoother than H6-LS800, there's no any other holes appeared on it. The simple changes on morphology were probably associated with the existence of metal impurities; actually these metals were not completely leached from HNTs. The metal impurities especially aluminate,⁴⁸ were reported to inhibit the growing of the particles, and the material seemed to have a higher surface area of H6-LS800. To further understand the effect of these metal impurities, the morphologies of H6-LS800 and P-LS after CO₂ sorption were given in Fig. 6c and d. Compared with the SEM image of the initial samples, the particles appear more loose and smaller, the surface of H6-LS800 sorbent presents porous structure, which is beneficial to the CO₂ diffusion into the interior of sorbent. Wang et al. also studied the effect of potassium and sodium content on the absorption capacity,⁵ and found that the surface of Li₄SiO₄-based sample presented loose and porous structure after 15 cycles sorption/desorption processes, which could ensure the good regenerability properties.

Table 3 Rietveld refinement data obtained from P-LS and H6-LS800

Samples	Cell parameters (Å)			β (deg)	Cell volume (Å)^3	Rwp
	a	b	c
Ref	5.298	6.102	5.150	90.25	166.5
P-LS	5.2975	6.1018	5.1504	90.05	166.48	13.15
H6-LS800	5.2993	6.1125	5.1556	90.29	167.06	12.46

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Fig. 6 SEM image of as-synthesized (a) H6-LS800, (b) P-LS sorbents, and the corresponding the SEM image of (c) H6-LS800, (d) P-LS after CO₂ sorption. The rectangle inset shows magnification of corresponding samples.

SEM images showed that the surface of H6-LS800 presented porous structure, indicating that H6-LS800 had more pores and higher surface area, the existence of metal in the H6-LS800 could be the factor for this phenomenon. Fig. 7 compared the pore size distributions and BET surface area of P-LS and H6-LS800. There a peak was located in the range of 0–20 nm. These two samples all showed a low surface area due to the highly sintered in 800 °C. However, the surfaces of two sorbents are 5.029 m² g⁻¹ and 4.563 m² g⁻¹, respectively. From the curves of pore size distributions, H6-LS800 also had a small increase compared with P-LS, therefore, the metal in H6-LS800 indeed exerted a certain extent effect on pore size and BET surface area.

Fig. 7 Pore size distributions of P-LS and H6-LS800.

It has to be emphasized that H6-LS800 sorbent captured CO₂ more efficiently than P-LS. The thermograms of P-LS and H6-LS800 presented similar behavior, P-LS samples were very stable at temperature lower than 450 °C, then began to absorb CO₂, finishing around 700 °C with 30.6% maximum absorption, the capability was well agreement with the previously reported. However, H6-LS800, had the aluminum and other mental impurities, showed a significantly improved CO₂ absorption of 34.45% CO₂ adsorption capacity. The reaction between Li₄SiO₄ and CO₂ was proven to occur at the outer surface of the crystal grain, the ion diffusion of Li⁺ and O²⁻ contributed to react with CO₂ and to form lithium carbonate. Thus, defects in the crystalline Li₄SiO₄ could improve its reactivity.⁴⁹ On the one hand, the addition of aluminum and other metal impurities from HNTs formed defects in H6-LS800 samples, and improved ion mobility. On the other hand, the existence of aluminum inhibited the growing of the particles, and the material tended to have a higher surface area. Therefore, the CO₂ chemisorption of H6-LS800 was enhanced compared with P-LS. Because aluminum ions could occupy silicon sites into the Li₄SiO₄ lattice to form Li₄SiO₄ solid solutions containing aluminum (Li_4+x(Si_1−xAl_x)O₄),²⁰ so H6-LS800 sorbents captured CO₂ with a reaction as following:

Li_4+x(Si_1−xAl_x)O₄ + (1 + x)CO₂ → (1 + x)Li₂CO₃ + (1 − x)Li₂SiO₃ + xLiAlO₂ (3)

Table 4 showed the comparison of the CO₂ capture properties of the as-synthesized H6-LS800 sorbent with other Li₄SiO₄-based sorbents in literature, CO₂ chemisorption capacity and temperature ranges were calculated in the process of the sorbents heated from room temperature to 800 °C in 100 vol% CO₂ flow. The top half of Table 4, from RHA-Li₄SiO₄ to IDS-Li₄SiO₄ sample, showed all kinds of sorbents with different silicon sources, such as rice husk and diatomite. The bottom half indicated some researches on Li₄SiO₄-based sorbents which were doped with sodium, vanadium and aluminum ions in order to enhance the CO₂ adsorb capacity. Compared with diatomite and rice husk as silicon sources, H6-LS800 sample showed an obvious advantage, additionally, H6-LS800 was synthesized by one-step method, which could simplify the doping step of the metal ions.

Table 4 Comparison of CO₂ capture properties of as-synthesized samples with the reported materials

Samples	Raw materials	Synthesis method	CO2 chemisorption capacity in 100% CO2 (wt%)	CO2 chemisorption temperature ranges (°C)	Reference
RHA-Li4SiO4	Rice husk, Li2CO3	Solid-state	32.4	450–710	5
Diatomite-Li4SiO4	Diatomite, Li2CO3	Solid-state	30.32	480–620	11
IDS-Li4SiO4	Diatomite, LiNO3, NH3·H2O	Impregnation precipitation	29.89	450–680	15
Li4−xNaxSiO4	TEOS, Li2CO3, Na2CO3	Co-precipitation	19.3	450–680	17
Li4−x(Si1−xVx)O4	SiO2, LiOH, V2O	Solid state	4.1	450–680	20
Li4+x(Si1−xAlx)O4	SiO2, LiOH, α-Al2O3	Solid state	17	380–770	20
H6-LS800	Halloysite, Li2CO3	Solid state	34.45	400–720	This work

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4. Conclusions

An economical and environmentally friendly Li₄SiO₄-based sorbent was successfully prepared via solid-state method at 800 °C for 4 h by using natural mineral as silicon sources (H6). The metal impurities especially aluminum from H6 were demonstrated to dope into the structure of Li₄SiO₄, and the pore volume and surface area of the sorbent were increased. The presence of metals in the sorbent produced an increase of CO₂ capacity than pure Li₄SiO₄. The maximum CO₂ adsorption capacity of the sorbent reached up to 34.45%, and the temperature range of CO₂ absorption was 350–720 °C. Compared with pure Li₄SiO₄ prepared by SiO₂ as silica source, Li₄SiO₄-based sorbent not only reduced the cost of synthesis, but also improved the capability of CO₂ adsorption. Meanwhile the sorbent exhibited fast kinetics and good stability on ten cycles of CO₂ adsorption/desorption performance, indicating a preferential potential material for the higher CO₂ quantity and the wider CO₂ chemisorption temperature ranges.

Acknowledgements

This work was supported by the National Science Fund for Distinguished Young Scholars (51225403), the National Natural Science Foundation of China (41572036), the State Key Laboratory of Powder Metallurgy, Central South University (2015-19) and the Hunan Provincial Co-Innovation Centre for Clean and Efficient Utilization of Strategic Metal Mineral Resources (2014-405).

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