A simple, rapid, and environmentally friendly method for selectively

Jul 19, 2018 - Demand for tantalum (Ta) is dramatically increasing and Ta recovery from waste products is essential to attempt to avoid social conflic...
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Letter Cite This: ACS Sustainable Chem. Eng. 2018, 6, 9585−9590

Simple, Rapid, and Environmentally Friendly Method for Selectively Recovering Tantalum by Guanidine-Assisted Precipitation Takashi Ogi,*,† Hayato Horiuchi,† Takahiko Makino,‡ Aditya Farhan Arif,† and Kikuo Okuyama† †

Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Hiroshima 739-8527, Japan Cutting Tool R&D Division, Kyocera Corporation, Kagoshima Sendai Plant, 1810 Taki-cho, Satsumasendai, Kagoshima 895-0292, Japan

‡

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S Supporting Information *

ABSTRACT: Demand for tantalum (Ta) is dramatically increasing and Ta recovery from waste products is essential to attempt to avoid social conflicts caused by escalating Ta mining. Here, we developed a rapid and facile guanidineassisted precipitation method for recovering Ta from an aqueous solution containing Ta, tungsten (W), and silicon (Si). The guanidine showed high selectivity for removal of Ta over W or Si, and the highest 99.96% of the Ta was recovered as a precipitate in 3 min. The calcined precipitate was very pure crystalline Ta2O5. The method performed well at room temperature without requiring pH adjustment. The proposed method can be regarded as environmentally benign because it minimizes energy consumption, produces little liquid waste, and enables sustainable supply of useful resources. KEYWORDS: Rare metals, Polyoxometalate, Tantalum oxide, Resource recovery, Waste products

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achieved using ion exchange processes,20 supercritical water,21 ionic liquids,22,23 and alkaline compounds24 to form Ta precipitates. Ta is present as a polyoxometalate in basic media, and can be precipitated by medium modification (e.g., addition of an alkali or ionic liquid). The target product is usually Ta2O5 because it is used in most electronic device applications, but many attempts to precipitate Ta have produced Ta complexes as the final products.23,25 These complexes contain an anionic metal that was used as the precipitating agent, and removal of this metal is a serious problem in terms of the purity of the Ta product. Therefore, it is necessary to develop a Ta precipitation method that produces Ta2O5 with high purity. We recently developed a new approach for recovering and recycling rare metals that gives very pure products. In this method, an amino acid is used as a precipitant. When lysine is added to an aqueous solution containing tungsten (W), the W ions instantly form a white precipitate and the W is completely removed from the solution.26,27 We found that precipitation is caused by electrostatic interactions between the amino group and the polyoxometalate. This method could be used to recover Ta from solution because Ta also forms polyoxometalates in aqueous solutions. To date, we have not previously applied this green recovery method to Ta recycling. In the present study, to work toward a method for Ta recovery, we developed a simple, selective, and rapid

INTRODUCTION Supply strategies for rare metals are of concern to both academics and governments. 1−6 Tantalum (Ta) is an important rare metal that is essential in many products, such as in electronic devices (e.g., in capacitor used in smart phones and electric vehicles), high-strength low-alloy steel, cemented carbide tools, optical lenses, catalysts, and surface-acousticwave filters.7,8 Demand for Ta capacitor and the price of Ta have increased with demand for next-generation devices in society. Furthermore, there are serious political problems and social conflicts in Ta mining areas, including the use of child labor in Ta mining areas, over 60% of which are located in Rwanda and the Democratic Republic of the Congo.9 According to a report prepared by the European Union,10 Ta is one of 14 critical elements because of these problems. The limited reserves of Ta and social problems surrounding Ta mining have led to development of methods for recovering Ta from waste products. Interest in cost-effective and environmentally benign techniques for recovering Ta from waste products has been growing for several years. The first method developed to recover Ta from waste solutions, Marignac’s process, is an energy-efficient hydrometallurgical method that uses hydrogen fluoride to form a Ta complex, which is then solvent extracted.11 Hydrometallurgical processes are not as well-developed as dry processes such as pyrometallurgy, chloride metallurgy, oxidative metallurgy, and solid-phase separation methods.12−19 Improvements in hydrometallurgical processes have focused on process simplification, replacement of hydrogen fluoride with less toxic chemicals, and increasing the product purity. These improvements have been © 2018 American Chemical Society

Received: May 27, 2018 Revised: July 2, 2018 Published: July 19, 2018 9585

DOI: 10.1021/acssuschemeng.8b02440 ACS Sustainable Chem. Eng. 2018, 6, 9585−9590

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ACS Sustainable Chemistry & Engineering

that the guanidine concentration is the most important factor affecting Ta recovery.28 Therefore, we investigated the effect of the guanidine concentration on Ta recovery from solutions containing different guanidine concentrations but with the same initial pH (13.2) and Ta concentration (9.5 mM). The pH did not change after guanidine was added. The Ta recovery was defined as the change in the dissolved Ta ion concentration. Changes in the Ta concentration over time were investigated with different concentrations of guanidine (Figure 1a). In the absence of guanidine, the Ta ion

hydrometallurgical process for recovering Ta using guanidine as the precipitating agent. A Ta−guanidine complex precipitated within 3 min of the guanidine being added to the solution, and almost 100% of the Ta in the original solution was recovered. The precipitate was purified, and the final product was very pure Ta2O5. The method did not require pH adjustment and performed satisfactorily at room temperature. Guanidine selectively precipitated Ta over silicon (Si) and W. This method could be developed further to recover Ta from liquid waste from plants that produce electronic equipment.

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EXPERIMENTAL SECTION

Materials. Guanidine carbonate ((CN3H5)2H2CO3) was purchased from Kanto Chemical Co. (Tokyo, Japan). Tantalum oxide (Ta2O5) was purchased from Sigma-Aldrich (St Louis, MO, USA). Potassium hydroxide (KOH) was purchased from Wako Pure Chemical Industries (Osaka, Japan). Silicon dioxide (SiO2) was purchased from Fuso Chemical (Kyoto, Japan). Sodium tungstate (VI) dihydrate (Na2WO4·2H2O) was purchased from Kanto Chemical Co. All chemicals were used as received. Preparation of Metal Solutions. A Ta solution was prepared by melting 1.0 g of Ta2O5 and 5.0 g of KOH in a 35 mL zirconium crucible (Metal Technology Co., Albany, OR, USA) under an alcohol burner, and then dissolving the product in ultrapure water. A Si solution was prepared by melting 0.5 g of SiO2 and 2.5 g of KOH in a crucible under an alcohol burner, and then dissolving the product in ultrapure water. A W solution was prepared by dissolving Na2WO4· 2H2O in ultrapure water. The metal ion concentrations in the solutions were determined using an inductively coupled plasma atomic emission spectrometer (ICPE; ICPE-9820, Shimadzu, Kyoto, Japan). Ta Recovery Tests Using Guanidine. Single-component test solutions were prepared by mixing ultrapure water, guanidine carbonate, and the stock Ta solution. The total volume of each test solution was 18 mL. The concentration of Ta ions remaining in solution after a test was measured by ICPE. After an experiment had started, test samples were collected using a syringe at specific time intervals. Each sample was passed through a 0.22-μm filter (Membrane Solutions Co., Albany, WA, USA) and analyzed by ICPE. Solutions containing Ta and Si were prepared by mixing ultrapure water, guanidine carbonate, the stock Ta solution, and the stock Si solution. The same method was used to prepare test solutions containing Ta and W but using the stock W solution instead of the stock Si solution. Again, the total volume of each test solution was 18 mL. Experiments were performed at temperatures between 0 and 40 °C. The initial Ta concentration in each mixed solution was between 1 and 50 mM, and the guanidine concentration in the mixed solution was between 10 and 50 mM. The white precipitate in a test solution was collected after centrifuging at 3500g for 20 min. The precipitate was dried at 100 °C for 12 h at a pressure of 0.01 MPa. The main functional groups involved in binding Ta and guanidine were identified by analyzing a dry precipitate sample, guanidine carbonate, and Ta2O5 using a Spectrum One System Fourier transform infrared spectrometry (FTIR; PerkinElmer, Waltham, MA, USA). The dry precipitate sample was calcined in a furnace in air. The optimum calcining temperature was determined by analyzing a precipitate sample using a thermogravimetric analyzer (TGA-60, Shimadzu) with a heating rate of 10 °C/min in dry air. The crystal structure of the Ta2O5 powder obtained by calcining the precipitate was examined by powder X-ray diffraction (XRD) using a D2 Phaser instrument (Bruker, Billerica, MA, USA) with Cu Kα radiation. Carbon amount in the precipitates was measured by LECO C-600.

Figure 1. (a) Dissolved Ta ion concentrations plotted against time after addition of guanidine at different concentrations. The initial Ta concentration was 10 mM in all cases, the temperature was 20 °C, and the solution pH was 13.2. (b) Digital photographs of the samples 30 min after guanidine addition.

concentration remained stable and no precipitate was observed after 30 min (Figure 1b). When the guanidine concentration was 10 mM, the Ta ion concentration decreased but the change was not statistically significant and no precipitate was observed. The Ta ion concentration decreased markedly when the guanidine concentration was 15 mM or higher. The Ta ion concentration decreased most rapidly in the first 3 min after the guanidine was added, implying that Ta was recovered quickly. The Ta recovery achieved in 3 min was 100% when the guanidine concentration was 40 or 50 mM. As the guanidine concentration increased, the final Ta ion concentration decreased and a larger quantity of white precipitate formed (Figure 1a,b), indicating that the decrease in the Ta ion concentration was directly related to the Ta recovery. Similar results were found in our previous study of W recovery.26 The Ta formed a stable hexatantalate anion [Ta6O19]8− in a highly basic solution, similar to what was found for W. This suggests that [Ta6O19]8− is highly deprotonated.29,30 Guanidine is very basic (pKa of 13.6) and was present as [H 2 NC(NH 2 ) 2 ] + . 28,31 The acidity of [Ta6O19]8− and basicity of [H2NC(NH2)2]+ will mean that [Ta6O19]8− and [H2NC(NH2)2]+ mainly interact electrostati-

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RESULTS AND DISCUSSION The results of previous studies on the possible interactions between guanidine carbonate and oxyanions have indicated 9586

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found in the precipitate spectra. These changes strongly indicated that cleavage of the guanidine carbonate group was followed by formation of a bond between [Ta6O19]8− and the guanidine amino group. The peaks at 700 cm−1, assigned to Ta−O−Ta, were stronger in the precipitate spectra than in the reference spectra, implying that stable Ta oxides formed in the precipitates.31,32 A dry precipitate sample was subjected to XRD to investigate the crystal structure. The XRD results (Figure 3a) contained weak and broad peaks between 20° and 40° that were difficult to assign to particular compounds. However, the FTIR spectra clearly indicated that both Ta and guanidine were present. Because it was important to obtain a pure Taoxide product, a Ta−guanidine precipitate sample and guanidine carbonate were subjected to thermogravimetric analysis to determine if calcination removed guanidine. The mass loss curves (Figure 3b) showed marked mass losses for both samples as the temperature increased. The mass remained constant once the temperature reached 900 °C. The calcined to raw Ta−guanidine precipitate ratio was 2.4% and slightly higher than the ratio for guanidine carbonate (2.0%), indicating that calcination removed most of the guanidine. To investigate this further, dry Ta−guanidine precipitate was calcined at 900 °C for 4 h and then subjected to XRD analysis. The XRD results for the calcined precipitate (Figure 3a) indicated that very pure crystalline Ta2O5 was formed and confirmed that Ta2O5 was successfully recovered. From an analysis to the residue, we revealed the carbon residues in the samples before and after calcination of the Ta−guanidine precipitates are 4.41 and 0.061 wt %, respectively. The effects of several parameters on the recovery ratio were investigated to attempt to improve the recovery of Ta2O5. The effect of the guanidine/Ta concentration ratio on the Ta recovery ratio was evaluated using initial Ta concentrations of 1, 10, and 50 mM (Figure 4a). No Ta was recovered when the initial Ta concentration was 1 mM up to a guanidine/Ta concentration ratio of 5. However, 77.6% of the Ta could be

cally. Theoretical calculations for dissociation of the amino group through reaction 1: HN=C(NH 2)2 + H+ → [H 2N=C(NH 2)2 ]+

(1)

2+

indicated that the NH concentration would be 71.5% of the guanidine concentration at pH 13.2. This implies that enough cations would be present to bind with the hexametalate anions to form complex molecules, which would precipitate. The precipitate was subjected to FTIR analysis to investigate the bond between guanidine and [Ta6O19]8−. The Ta2O5 powder and guanidine carbonate were analyzed for reference. The peaks at 1550 cm−1, assigned to −NH groups, in the spectra of the Ta precipitates (Figure 2) were less intense and

Figure 2. Fourier transform infrared spectra of Ta2O5 powder, Ta− guanidine precipitates, and guanidine carbonate.

broader than in the reference spectra. The peaks at 1390 and 890 cm−1 in the reference spectra, assigned to −CO2, were not

Figure 3. (a) X-ray diffractometry results for the precipitates before and after calcination, Ta2O5, and guanidine carbonate, and digital photographs of the samples. (b) Mass loss curves for dry Ta−guanidine precipitate and guanidine carbonate at a heating rate of 10 °C min−1 in dry air. 9587

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Figure 5. (a) Concentrations of Si and Ta after addition of 50 mM guanidine to a test solution containing Si and Ta at pH 13.6. (b) Concentrations of W and Ta after addition of 50 mM guanidine to a test solution containing W and Ta at pH 13.2.

Figure 4. (a) Ta recovery ratios obtained after 30 min using different initial guanidine/Ta concentration ratios (1.0 mM at pH 1.7, 10.0 mM at pH 13.5, and 50.0 mM at pH 14.3). The temperature was kept at 20 °C. (b) Ta recovery ratios found at 30 min using different temperatures and initial guanidine concentrations of 20 and 30 mM. The initial solution pH was 1.0, and the initial Ta concentration was 100 mM.

concentrations were much higher than the Ta concentration. The Si concentration was 16 mM and the W concentration was 28 mM. The Ta, Si, and W ion concentrations were measured at different times after guanidine addition (Figure 5). With an initial solution pH of 13.6 and addition of 50 mM guanidine, the Ta ion concentration decreased markedly and the Ta recovery after 3 min was 99.5% (Figure 5a). The Si ion concentration remained constant. In this case, Ta would be present as hexametalate anions, but the dissolved SiO2 could form stable SiO42− or SiO−.33,34 These silicate ions are oppositely charged to [H2NC(NH2)2]+ but have very low ionic activities, especially in the presence of other minerals, and their ionic activities are independent of the pH.34 Therefore, Ta hexametalate is more likely than silicate to bind to guanidine. Guanidine also removed Ta in preference to W (Figure 5b). After 50 mM guanidine was added, the Ta ion concentration decreased markedly but the W ion concentration remained constant. The Ta recovery after 3 min was 99.5%, which was the same as in the Ta−Si system. W can form stable hexametalate ions, [W6O19]8−, in the presence of other metal ions.35 The formation of such polyanions will be preceded by the formation of HWO4− ions.26 However, the formation of these ions would not be promoted at pH 13.2, which was the pH used in this experiment. Therefore, no W precipitated. The ability of guanidine to selectively recover Ta in the presence of Si and W indicates that the method described here could be used to recover Ta from real wastewater containing Ta. Finally, we conducted a cost analysis for this method using the cost for guanidine per mass of Ta recovered. Our results (Figure 4a) showed that approximately 1.3 kg of guanidine was needed to recover 1 kg of Ta with an initial Ta concentration of 10 mM. Industrial-grade guanidine from Alibaba.com is priced at US $3/kg, which gives a guanidine cost of around US$4 per kilogram of recovered Ta for our method. A comparison of this cost with that of virgin Ta (US$500/kg from Alibaba.com) shows that the guanidine cost is 0.8% that of virgin Ta (cost of guanidine in US$/cost of virgin Ta in US$). Even though this cost evaluation does not take into consideration other factors (e.g., the cost for energy input and required equipment), the cost for guanidine is still much lower than that for virgin Ta. It furthermore indicates that this method is feasible even if the guanidine is not recovered after calcination.

recovered when the guanidine/Ta concentration ratio was increased to 10, as shown in Figure S.1 (Supporting Information). The recovery further increased to 99.6% using a guanidine/Ta concentration ratio of 20. Lower guanidine concentration was needed for the recovery to start when the initial Ta concentration was 10 or 50 mM. In general, the recovery increased as the guanidine/Ta concentration ratio increased. Using the same guanidine/Ta concentration ratio, Ta recovery was proportional to the initial Ta concentration. For example, using the guanidine/Ta concentration ratio of 2, the recovery of Ta reached 82.9% when the initial Ta concentration was 50 mM, whereas only 60% of Ta was recovered when the initial Ta concentration was 10 mM. The minimum guanidine/Ta concentration ratio required to achieve a Ta recovery of ∼99% was 4 for the initial Ta concentrations of 10 and 50 mM, with the highest recovery being 99.95%. This trend indicated that the concentration of either component, i.e., tantalum or guanidine, should be higher than the other for the precipitation to occur as it affected the solubility of the complex. That is to say that the solubility of the Ta−guanidine complex determined the success of recovering Ta. As mentioned above, precipitation of the Ta−guanidine complex was strongly related to the solubility of the complex, and the temperature was expected to greatly affect Ta recovery. We investigated the Ta recovery ratio as a function of temperature using initial guanidine concentrations of 20 and 30 mM (Figure 4b). The Ta recovery ratio increased as the temperature decreased. A Ta recovery of almost 100% was achieved at 0 °C using 30 mM guanidine. Generally, increasing the temperature increases the solubility of a substance. For the Ta−guanidine system, increasing the solubility of the Ta− guanidine complex disfavors precipitation of the complex. In other words, decreasing the temperature will increase the Ta recovery ratio. Ta is rarely present alone in waste solutions from plants that produce electronic equipment and process metals, and these liquid wastes normally also contain Si and W. Therefore, we evaluated the selectivity of guanidine recovery of Ta from solutions containing Si and W (Figure 5). Guanidine was added to two samples: one containing Si and Ta, and the other W and Ta. The initial Ta concentration was 9 mM in both samples. To ensure the results were robust, the initial Si and W

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CONCLUSIONS Adding guanidine to an aqueous solution of Ta resulted in Ta precipitation within 3 min without pH or temperature 9588

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2013; Eicke, S., Hahn, M., Eds.; GDMB: Clausthal-Zellerfeld, 2013; pp 1069−1084, DOI: 10.5937/vojtehg62-4809. (9) Mineral commodity summaries 2017; U.S. Geological Survey; U.S. Government Publishing Office: Washington, D.C., 2017; DOI: 10.3133/70180197. (10) European Commission. Report forecasts shortages of 14 critical mineral raw materials 2010, http://europa.eu/rapid/press-release_ MEMO-10-263_en.htm?locale=en, (accessed July 2, 2018). (11) Marignac; Blomstrand; Deville, H.; Troost, L.; Hermann, R. Tantalsäure, niobsäure,(ilmensäure) und titansäure. Anal. Bioanal. Chem. 1866, 5 (1), 384−389. (12) Mineta, K.; Okabe, T. H. Development of a recycling process for tantalum from capacitor scraps. J. Phys. Chem. Solids 2005, 66 (2− 4), 318−321. (13) Nonaka, R.; Funayama, H.; Sugawara, K. Release behavior of tantalum and niobium from refractory metal scrap during chlorination. Kagaku Kogaku Ronbunshu 2009, 35 (4), 403−410. (14) Kikuchi, R.; Yamamoto, T.; Nakamoto, M. Preliminary information of laboratorial tantalum recovery and considerations for a potential solution for conflict mineral and wildlife conservation. Environment and Natural Resources Research 2013, 4 (1), 31−38. (15) Niu, B.; Chen, Z.; Xu, Z. Recovery of Valuable Materials from Waste Tantalum Capacitors by Vacuum Pyrolysis Combined with Mechanical−Physical Separation. ACS Sustainable Chem. Eng. 2017, 5 (3), 2639−2647. (16) Niu, B.; Chen, Z.; Xu, Z. Method for Recycling Tantalum from Waste Tantalum Capacitors by Chloride Metallurgy. ACS Sustainable Chem. Eng. 2017, 5 (2), 1376−1381. (17) Niu, B.; Chen, Z.; Xu, Z. An integrated and environmentalfriendly technology for recovering valuable materials from waste tantalum capacitors. J. Cleaner Prod. 2017, 166, 512−518. (18) Niu, B.; Chen, Z.; Xu, Z. Application of pyrolysis to recycling organics from waste tantalum capacitors. J. Hazard. Mater. 2017, 335, 39−46. (19) Ueberschaar, M.; Dariusch Jalalpoor, D.; Korf, N.; Rotter, V. S. Potentials and barriers for tantalum recovery from waste electric and electronic equipment. J. Ind. Ecol. 2017, 21 (3), 700−714. (20) Nete, M.; Purcell, W.; Nel, J. Non-fluoride dissolution of tantalum and niobium oxides and their separation using ion exchange. Hydrometallurgy 2017, 173, 192−198. (21) Niu, B.; Chen, Z.; Xu, Z. Recovery of Tantalum from Waste Tantalum Capacitors by Supercritical Water Treatment. ACS Sustainable Chem. Eng. 2017, 5 (5), 4421−4428. (22) Spitczok von Brisinski, L.; Goldmann, D.; Endres, F. Recovery of metals from tantalum capacitors with ionic liquids. Chem. Ing. Tech. 2014, 86 (1−2), 196−199. (23) Turgis, R.; Arrachart, G.; Michel, S.; Legeai, S.; Lejeune, M.; Draye, M.; Pellet-Rostaing, S. Ketone functionalized task specific ionic liquids for selective tantalum extraction. Sep. Purif. Technol. 2018, 196 (8), 174−182. (24) Wang, X.; Zheng, S.; Xu, H.; Zhang, Y. Leaching of niobium and tantalum from a low-grade ore using a KOH roast−water leach system. Hydrometallurgy 2009, 98 (3−4), 219−223. (25) Deblonde, G. J.-P.; Chagnes, A.; Weigel, V.; Cote, G. Direct precipitation of niobium and tantalum from alkaline solutions using calcium-bearing reagents. Hydrometallurgy 2016, 165, 345−350. (26) Ogi, T.; Makino, T.; Nagai, S.; Stark, W. J.; Iskandar, F.; Okuyama, K. Facile and efficient removal of tungsten anions using lysine-promoted precipitation for recycling high-purity tungsten. ACS Sustainable Chem. Eng. 2017, 5 (4), 3141−3147. (27) Makino, T.; Nagai, S.; Iskandar, F.; Okuyama, K.; Ogi, T. Recovery and Recycling of Tungsten by Alkaline Leaching of Scrap and Charged Amino Group Assisted Precipitation. ACS Sustainable Chem. Eng. 2018, 6 (3), 4246−4252. (28) Springs, B.; Haake, P. Equilibrium constants for association of guanidinium and ammonium ions with oxyanions: The effect of changing basicity of the oxyanion. Bioorg. Chem. 1977, 6 (2), 181− 190.

adjustment. This facile method will make it possible to selectively recover Ta from multicomponent solutions containing Si and W ions. Calcining the guanidine−Ta precipitate at 900 °C for 4 h gave very pure crystalline Ta2O5. We are currently investigating the application of this method to industrial wastewater containing Ta.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02440.

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Ta recovery ratios after 30 min studied at an initial Ta concentration of 1 mM (PDF)

AUTHOR INFORMATION

Corresponding Author

*Takashi Ogi. E-mail: [email protected]. Tel/Fax: +8182-424-3765. ORCID

Takashi Ogi: 0000-0003-3982-857X Aditya Farhan Arif: 0000-0003-4219-4395 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science KAKENHI fund (Grant Number 26709061) and the Steel Foundation for Environmental Protection Technology. This work was partly supported by the Center for Functional Nano Oxide at Hiroshima University (Japan). We thank Gareth Thomas, PhD, and Gabrielle David, PhD, from Edanz Group (www.edanzediting.com/ac) for editing drafts of this paper.

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REFERENCES

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