Recovery and Recycling of Tungsten by Alkaline Leaching of Scrap

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Recovery and Recycling of Tungsten by Alkaline Leaching of Scrap and Charged Amino Group Assisted Precipitation Takahiko Makino, Satoshi Nagai, Ferry Iskandar, Kikuo Okuyama, and Takashi Ogi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04689 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Sustainable Chemistry & Engineering

Recovery and Recycling of Tungsten by Alkaline Leaching of Scrap and Charged Amino Group Assisted Precipitation

Takahiko Makino†, Satoshi Nagai‡, Ferry Iskandar§, Kikuo Okuyama‡, Takashi Ogi‡, *

†

Cutting Tool R&D Division, Kyocera Corporation, Kagoshima Sendai Plant, 1810 Taki-cho,

Satsumasendai, Kagoshima 895-0292, Japan ‡

Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Hiroshima

739-8527, Japan §

Department of Physics, Institute of Technology Bandung, Ganesha 10, Bandung 40132, West

Java, Indonesia

*Corresponding author: Email: [email protected] Tel/Fax: +81-82-424-3765 Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, 739-8527, Japan

KEYWORDS: Recovery, Urban mining, Precipitant, Lysine, Polyoxometalate

ACS Paragon Plus Environment

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ABSTRACT Herein, we present a charged amino group assisted precipitation (CAAP) method for recovery and recycling of high-purity tungsten from a solution obtained by alkaline leaching of cemented carbide (WC-Co alloy) scrap. When lysine hydrochloride, which contains a charged amino group (NH3+), was added to an alkaline leaching solution containing tungsten (307 mmol L−1), sodium (1236 mmol L−1), and vanadium (1.7 mmol L−1), over 90% of the tungsten was rapidly recovered as a lysine–tungsten precipitate. The sodium in the precipitate was successfully removed by a simple washing process. Calcination was then performed to obtain 99.6% high-purity tungsten oxide, and reduction and carburization of the obtained tungsten oxide resulted in tungsten carbide. A pilot study using the CAAP method demonstrated a considerable decrease (60%) in the displacement (i.e., volume of chemicals used/waste created) compared with that required for the conventional ion-exchange method for tungsten recycling.


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INTRODUCTION Based on the principles of green chemistry and engineering, recycling of rare metals from waste products should increase to meet growing global demands and enable creation of a sustainable society.1-4 Tungsten (W) is one of the most important rare metals, and over 80% of W (62,000 t per year) is produced in China.5 This metal is classed as a critical raw material in Japan, the European Union, and the United States.6 It is widely used for machine-part cutting tools, mill rollers, electric light filaments, thermal shield plates, and photocatalysts.7-10 The main use of W is in cemented carbide (WC-Co alloy), which is indispensable for production of cutting tools used for metalworking in the manufacturing industry. There is a demand for technology to recover W from waste to provide a sustainable and stable W supply. In Japan in 2015, 1039 t of W was collected from urban mines (cemented carbide tools, 775 t; and catalysts and other scrap, 264 t).11 Many methods have been developed for recovery of soluble W by solvent extraction,12,13 electroplating,14 hydrometallurgy,15,16 ion exchange (IE),17,18 direct acid leaching,

19

and biosorption.20-22 The W recovery methods

currently used in industry are dry (zinc treatment

23

) and wet chemical treatment methods.24-26

The wet chemical treatment method is an indirect method used to recover W and involves oxidizing and roasting WC-Co alloy scrap to prepare scrap powder. Alkaline leaching is then performed by dissolving this powder in an aqueous sodium hydroxide solution with heating to obtain a sodium tungstate (Na2WO4) solution. An aqueous solution of ammonium tungstate is then obtained by precipitation using calcium chloride27-31 or with an IE resin.24-25 Finally, crystallization

is

used to

obtain

the intermediate ammonium

paratungstate (APT,

(NH4)10(H2W12O42)·4H2O), which is transformed into tungsten carbide (WC) by thermal decomposition, reduction, and carburization. However, several problems remain with the


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precipitation and IE methods. The precipitation method results in impurities, has many steps, generates large quantities of secondary waste, and has high energy consumption. The IE method uses large quantities of chemicals to desorb the W components. In addition to the high reagent and/or energy requirements of these methods, they also generate toxic sludge and other waste products.30 Recently, as an alternative method for W recovery, we have developed a facile and efficient W recovery method using charged amino group assisted precipitation (CAAP) 32 which allowed for efficient recovery of metal ions from a liquid with minimal environmental burden. Using the amino acid lysine as a precipitant, we showed that almost 100% of the W could be precipitated and recovered within 5 min from the Na2WO4·2H2O aqueous solution (model W solution). The positively charged amino group (NH3+) in a lysine acts as a counter cation to the W polyoxometalate in lysine -W solutions at pH values below 1.8. Therefore, the formation of various ionic W species (e.g., W6O192−, W7O222−, W3O102−, and HWO4−)33 can be promoted by electrostatic interactions between the positively charged lysine and negatively charged W oxoacid. Unlike inorganic precipitants,34 the organic moieties used in the CAAP method can be decomposed and removed by a relatively low temperature process (< 650 °C), which means that it is possible to obtain high-purity substances with low energy input. However, to date, the CAAP method has not been applied to a real scrap solution. As a continuation of our previous research, the present study reports on the application of the CAAP method to recycling of WC from scrap. Purification of the product (WO3), an evaluation of the mechanical properties of recycled WC, and a feasibility study of the CAAP process are investigated for the first time. Figure 1 shows the proposed W recovery processes using the CAAP method and with the conventional IE method. Considerable reductions in


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number of operational steps, energy use, and chemical use can be expected with our proposed process.

Figure 1. Comparison of the conventional and proposed tungsten carbide recycling processes.

EXPERIMENTAL Preparation of W solutions from scrap by alkaline leaching. A W solution was obtained by alkaline leaching of WC-Co alloy scraps. Generally, the mass fraction compositions of these scraps are WC (80%–95%), Co (6%–12%), TiC (0%–3%), ZrC (0%–3%), NbC (0%–4%), TaC (0%–5%), Cr3C2 (0%–2%), and VC (0%–1%). First, the WC-Co alloy scrap was placed into a rotary kiln furnace. WC-Co alloy scrap containing WC, TiC, NbC, TaC, ZrC, Cr3C2, and VC was oxidized by calcination at 800 °C. Oxidation of these compounds had been confirmed by our preliminary experiments. During rotation, the scrap pieces collided with each other and fragmented. The fragments were heated and oxidized at > 800 °C under atmospheric conditions in the rotary kiln furnace, producing WO3 and CoWO4 mixed powders. Next, unreacted substances were removed using a sieve (0.8 mm mesh). The separated unreacted substances and


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a fresh lot of scrap were placed back into the rotary kiln furnace. Generally, 30 kg of mixed powders were introduced into a tank for alkaline leaching. An aqueous NaOH solution (25% mass fraction) was added into the tank with a 4:1 molar ratio of NaOH to W. Then, deionized water was added to prepare a 500 mmol L−1 W solution. The total solution amount was generally 200 L. The prepared solution was heated to 100 °C and stirred for 7 h under atmospheric conditions. After the reaction, the solution was gradually cooled to room temperature and the W solution was separated from precipitates (e.g. Co(OH)2) by filtration. The obtained alkaline leaching solution was diluted by deionized water to adjust the W concentration. When we used the scraps containing WC (89.3 %) and VC (0.1 %) as a representative experiment, the leaching yield of W and V was 87% and 47%, respectively. The concentrations of metal ions in the alkaline leaching solution were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; ICPE-9820, Shimadzu Corporation, Kyoto, Japan). The solution contained high concentrations of W (307 mmol L−1) and Na (1236 mmol L−1), and a low concentration of V (1.7 mmol L−1). Co was not detected in the alkaline leaching solution because it formed insoluble compounds (e.g. Co(OH)2, CoWO4) in the alkaline leaching process, and these were removed by filtration. W recovery from the alkaline leaching solution via the CAAP method (beaker scale). A test solution was prepared by dissolving L-lysine hydrochloride (Ajinomoto Healthy Supply Co., Inc., Tokyo, Japan) in the alkaline leaching solution (pH 12). Then, hydrochloric acid (HCl, 1 × 104 mmol L−1; Kanto Chemical Co., Inc., Tokyo, Japan) and ultrapure water were added sequentially with mixing to increase the volume to 48 mL and precipitates formed instantaneously. The removal of metal ions was investigated using ICP-AES. Electrospray ionization-mass spectrometry (ESI-MS) was also used for the analysis of W species in the solution. The ICP-AES


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measurements were conducted as previously reported.32, 35-36 Details for the ICP-AES and ESIMS methods are given in the Supporting Information (SI-1). The experimental conditions were as follows: temperature, 25 °C; solution pH, 1.8; initial W concentration, 150–183 mmol L−1; and L-lysine hydrochloride concentration, 12.5–200 mmol L−1. Synthesis of WO3 from the precipitate. A solution containing the lysine-W precipitate was subjected to centrifugation at 7840 ×g for 5 min. The supernatant was discarded, and the precipitate was dried at 80 °C for 12 h to obtain a white solid. A sample of this solid (4 g) was purified by ultrasonication with 100 mL of ultrapure water for 5 min. This step was repeated between one and five times to obtain high-purity WO3. The WO3 precipitate was calcined at 750 °C for 4 h with a heating rate of 10 °C min−1 under atmospheric conditions. The gases generated in the calcination process were analyzed by thermogravimetric analysis-mass spectrometry with a thermogravimetric differential thermal analysis (TG-DTA) instrument (TG 8120, Rigaku, Chiba, Japan) and a mass spectrometer (M-QA-200-TS, Canon Anelva, Kanagawa, Japan). The powder sample obtained after heating was investigated using powder X-ray diffraction (XRD; D2-PHASER, Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation. Details for the TG-DTA and XRD measurements are given in the Supporting Information (SI-1). The purity of the WO3 was calculated from the ICP-AES results, as described in the Supporting Information (SI-2). In addition, the amount of Na in the WO3 was determined using Rietveld refinement. Reduction and carburization of WO3. High-purity WO3 particles were reduced at 750–890 °C in a hydrogen atmosphere for 3.6 h to obtain W. Then, an equimolar amount of carbon powder was mixed with the W, and the mixture was reacted under an inert atmosphere at 1350 °C for 4 h to synthesize the WC. The experimental conditions for reduction and carburization were


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determined based on previous studies.37-40 The crystal structure and particle morphology of the obtained powder sample were analyzed using XRD and scanning electron microscopy (SEM; VE-9800, Keyence, Osaka, Japan). Fabrication of cemented carbides (WC-Co alloy). Co powder (10% mass fraction) was added to the WC powder synthesized in the previous section as a binder and mixed, compression molded, and sintered under an inert atmosphere at 1400 °C for 1 h to produce WC-Co alloy. The hardness of the WC-Co alloy product was measured in triplicate using a Vickers hardness testing machine (AVK-C2, Mitutoyo Corporation, Kanagawa, Japan). Each loading test was conducted with a force of 196 N for 10 s. The details for the hardness measurements are given in the Supporting Information (SI-3).

RESULTS AND DISCUSSION Recovery of W from the alkaline leaching solution using lysine. The influence of the lysine concentration on the W concentration in the beaker scale test solution was investigated. From theoretical calculation of the degree of dissociation of amino groups (NH2 + H+ → NH3+), the NH3+ ratios of lysine at a solution pH of 2.0 were 99.999712% for α-NH2 (pKa = 9.06) and 99.999129% for the side chain (pKa = 10.54). As the lysine concentration increased, the recovery ratio of W also increased, and when the lysine concentration was 200 mmol L−1, 100% of the W in the alkaline leaching solution was recovered within 3 min (Figure 2(a)). The amount of W recovered per mole of lysine was not constant. With a lysine concentration of 12.5 or 25 mmol L−1, four moles of W were recovered per mole of lysine. However, by comparison, when the lysine concentration was increased above 25 mmol L−1, the amount of W recovered per mole of


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lysine decreased. These results imply that the CAAP is not a simple reaction. Lysine plays an important role in promotion of the condensation and precipitation of W oxoacids.32. Analysis of the changes in the concentrations of the metal ions in the alkaline leaching solution (Table S1 in SI-4) showed that the initial test solution contained a large amount of Na from the leaching process, and this Na was present in sodium salts of ionic W oxoacids. The molar ratio of Na to W in the leaching solution could be 4:1 because 4 mol of NaOH was used for every 1 mol of W in the alkaline reaction. ESI-MS analysis confirmed the presence of ions such as NaW6O192− (Figure S1 in SI-5). As shown in the Table S1, addition of lysine led to precipitation and recovery of the W (recovery ratio = 92.2%), but at the same time, the sample was contaminated with some Na impurities. Although the Na recovery ratio was 7.6% with a lysine concentration of 100 mmol L−1, the high Na concentration in the alkaline leaching solution meant that a large amount of Na remained in the lysine-W precipitate. Moreover, around 21.0% of the V was recovered, which was in agreement with the results of a previous study.21 W-V heteropolyacids, such as [W3V3O19]5−, form and cause the simultaneous precipitation of W and V when both elements are present in a solution. However, because the concentration of V in the alkaline leaching solution was as low as 0.79 mmol L−1, the concentration of V in the white precipitate was only 7.38 mmol kg−1. Analysis of samples 30 min after the start of the experiment clearly showed that the sample with lysine contained a white precipitate (Figure 2(b)). In addition, the solvent was yellow, both with and without lysine, and we presumed this color was derived from the V contained in the solution. To confirm this, we prepared a V solution prepared by dissolving V2O5 powder in NaOH, and it was also yellow (Figure S2 in SI-6).


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Figure 2. (a) Time-dependent changes in the liquid phase W concentration for each concentration of added lysine (initial W concentration 145 mmol L−1, solution pH 1.8, temperature 25 °C, lysine concentration 12.5–200 mmol L−1). The error bars are for experimental replicates. (b) Photograph of the test solutions with and without lysine at 30 min after the start of the experiment. The lysine concentration was 96 mmol L−1. The pH of the control sample (without lysine) was adjusted to 1.8.

Synthesis of WO3 from the precipitate. Calcination was performed to recover the W from the lysine-W precipitate obtained in the previous step. First, thermogravimetric analysis-mass spectrometry was used to analyze the gas that evolved from the precipitate on heating and to determine the calcination conditions. The mass of the sample decreased in three steps (Figure S3 in SI-7), with an overall mass loss of 16.6% by 800 °C. Most of the water contained in the white precipitate evaporated during the first stage between 100 °C and 200 °C, water and ammonia were generated during the second stage between 250 °C and 400 °C, and lysine decomposed to generate carbon dioxide during the third stage between 500 °C and 700 °C. Based on these results, the calcination of the lysine-W precipitate was performed at 750 °C. By comparison, the


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conventional process for obtaining WO3 powder from APT uses heating at 700-750 °C for 3 h under atmospheric conditions.41 XRD patterns were recorded for the powders obtained after calcination of the lysine-W precipitate in air at 750 °C for 4 h (Figure 3(a)). When the precipitate was not washed, many peaks in addition to those of WO3 were detected, and analysis of these peaks with the Rietveld refinement revealed that Na5W14O44 was abundant (Figure 3(b)). Therefore, the lysine-W precipitate was washed to remove Na impurities. The signals derived from the Na species decreased as the number of washing steps increased. Na was not detected in the powder after four or more washing steps, which indicated the Na impurities in the WO3 powders were removed. The powders obtained after calcination were different colors (Figure 3(c)). The powder samples that were not washed were dark green, with the color attributed to the presence of Na5W14O44.42 As the number of washing steps increased, the color changed to the characteristic canary yellow of WO3.43,44 With five washing steps for purification, calcination of the white precipitate gave WO3 with very high purity (99.6%). The concentration of W in this WO3 sample was approximately 30 times the concentration of W in the initial alkaline leaching solution.


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Figure 3. Powders obtained after washing and calcination of the white lysine-W precipitate. (a) Powder X-ray diffraction patterns, (b) Rietveld refinement, and (c) appearance after calcination. Calcination conditions: 750 °C, 4 h, and 10 °C min−1 under atmospheric conditions. Reduction and carburization of WO3 and preparation of a WC-Co alloy. The XRD patterns (Figure 4(a)) and SEM images (Figure 4(b)) recorded of the black WC powder were obtained after reduction and carburization of WO3. For comparison, we also prepared a control powder by reduction and carburization of WO3 from oxidation of commercially available WC. The XRD


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results confirmed that the WO3 obtained from the W solution leached from the scrap was sufficiently reduced and carbonized (Figure 4(a)). The SEM images showed that there were no major differences between the two WC samples for the shapes or sizes of the particles. Figure 4(c) and (d) show the recycled WC-Co alloy prepared from scrap. The results showed no difference between the structures of the two cemented carbide samples. The hardness of WC-Co alloy is very important for practical applications. Consequently, we measured the hardness for each product in triplicate using the Vickers hardness test. The average hardness values for the recycled and control products were 13.54 GPa (individual measurements 13.28, 13.38, and 13.97) and 13.53 GPa (individual measurements 13.33, 13.69, and 13.58), respectively. These results show the recycled WC-Co product is comparable to the commercial product.


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Figure 4. Preparation of cemented carbide (WC-Co alloy) from W recovered by the CAAP method. For comparison, WC (control) was also prepared from WO3 obtained by oxidation of commercially available WC. (a) X-ray diffraction patterns of the WC powders, (b) scanning electron microscopy (SEM) images and photographs of the WC powders, (c) photographs of WC-Co alloy products, and (d) structural SEM photographs. Reduction of WO3 was conducted at 750–890 °C in a hydrogen atmosphere for 3.6 h. Carburization of W was conducted under an inert atmosphere at 1350 °C for 4 h.

Comparison of the proposed and conventional processes. WC recycling was carried out in a pilot plant as described Figure 5. The detailed chemical reaction equations of the whole recovery process are given in the Supporting Information (SI-8). For a typical one batch process, 50 kg of scrap was calcined at 950 °C for over 15 h in a rotary furnace to obtain a calcined powder. The calcined powder was then reacted with an aqueous alkaline solution to prepare 200 L of an alkaline leaching solution. The yield of W leaching for the alkaline leaching process was about 87%. The W concentration was kept constant by addition of deionized water, then 12 kg of lysine hydrochloride was added and the granules were dissolved by stirring. Next, 115 L of aqueous HCl (15% mass fraction) was added with stirring, and the pH was adjusted to 1.8–2.0 to give a white precipitate. The yield of W recovered for the precipitation process was > 90%. The white precipitate was separated from the solution with a filter press, and deionized water was passed through the filter press to remove Na impurities. About 5–10% of the W was lost in the multiple washing steps. However, this washing liquid could be reused in the precipitation process for the W concentration adjustment. The white precipitate was recovered, dried at 50 °C,


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and calcined to yield WO3. Finally, reduction, carburization, and alloying were performed to give a WC-Co alloy product. The overall W recovery from the scrap was 70-80%. Next, we compared the chemical costs and volumes of displacement (effluent) created for the proposed method and the conventional IE method. Because the operating conditions for the entire process have not been optimized yet, we focused on the precipitation/adsorption step in this paper. All data for the IE method were obtained from an early study.45 The prices of lysine and the IE resin were compared as shown in Table 1, assuming that the resin would be reused 240 times (i.e. for about 2 years). The total cost of the chemicals required for the CAAP method was about 1.4 times that for the IE method. In the IE method, the target product, (NH4)2WO4, is extracted into a solution and then heated and crystallized before being pulverized into APT. By contrast, the proposed method does not use heating because the target product can be precipitated and recovered as a powder. Therefore, the CAAP method is competitive with the conventional method when the total energy requirements are considered. In addition, we calculated the displacement for the proposed process and the IE process to obtain 1 t of recycled WC (SI-9). Here, we defined the displacement as the total volume of chemicals used in the precipitation/IE process. The displacement for the CAAP method was about 40% of that for the IE method (Table 1). This is because the IE method must be performed at low concentrations to avoid precipitation of (NH4)2WO4 in the IE resin and because many reagents are required to regenerate the IE resin. Another advantage of our method compared with the conventional method is that it is simpler. The proposed process has one less step to get from the (NH4)2WO4 solution to APT, and one less step to get from APT to WO3, which reduces the required heating energy. Therefore, the CAAP method could be carried out in smaller plants than the conventional method. In addition, the WO3 particle size can be controlled by adjusting the precipitation conditions, such


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as the concentration and addition rate of lysine. Even though we still have to compare the total energy and process costs, we expect that our proposed CAAP method could be developed further as an attractive alternative for W recovery with the aim of reducing the environmental burden of metal recovery.

Figure 5. Proposed WC recycling process flow diagram for the CAAP method at a pilot plant and the yield for each process


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Table 1. Comparison of the performance of the CAAP method with that of the IE method for production of 1 t of recycled tungsten carbide adsorbent

CAAP method (Lysine)

Ion-exchange resin (SA-10A)

chemicals*a)

chemical amount needed

price of chemicals Price of sum of price displacement*c) needed for chemicals*b) 1 ton WC recovery needed

[-]

[kg]

[US$/kg]

[US$]

15% HCl

4880

0.06

283.4

Lysine

490

0.43

210.7

NH4Cl

735

0.11

80.9

25% NH4OH

400

0.30

120.0

12%NaClO4

192

0.27

51.8

NaOH

24

0.21

4.9

SA-10A

8573

2.78

99.3*C)

[US$]

[L]

494

29278

357

71272

a

The chemical requirements for the IE resin are based on data from an earlier study.45 bThese are reference values. The prices of all materials are based on those on Alibaba.com. cIt was assumed that the IE resin (SA10A) was reused 240 times (i.e., for about 2 years). *c) Here, we defined the displacement as the total volume of chemicals used in the process.

CONCLUSION In a beaker-scale test, over 90% of the W in an alkaline leaching solution obtained from WC-Co alloy scrap was recovered by precipitation with lysine (CAAP method). High-purity (99.6%) WO3 was synthesized from the lysine–W precipitate after washing and calcining. The WO3 was reduced and carbonized to WC, which is a constituent of WC-Co alloy tools. Finally, we performed a series of recycling processes at a pilot plant and produced recycled WC-Co alloy products. Our proposed W recycling process is lower environmental burden than the conventional IE method and promising for sustainable chemistry and engineering.


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Supporting Information Additional details for the methods, changes in the concentrations of metal ions contained in the alkaline leach solution over time, ESI-MS analyses of the lysine–W solution, color of a V ion solution, TG-MS measurements of the lysine–W precipitate, chemical reaction equations of the whole recovery process, and calculation of chemical displacement for CAAP and the conventional ion-exchange method.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by JSPS(JAPAN SOCIETY FOR THE PROMOTION OF SCIENCE) KAKENHI (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). The authors thank Y. Sakamoto, T. Kondo, and H. Horiuchi for help with experiments and measurements. We also thank Sohei Okazaki for help with Rietveld analysis. We thank Gabrielle David, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

REFERENCES (1) Anastas, P. T.; Allen, D. T., Twenty-Five Years of Green Chemistry and Green Engineering: The End of the Beginning. ACS Sustainable Chemistry and Engineering 2016, 4 (11), 58205820. (2) Robinson, B. H., E-waste: an assessment of global production and environmental impacts. Science of the total environment 2009, 408, (2), 183-191.


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Table of Contents Graphic (For Table of Contents Use Only)

Synopsis A sustainable tungsten carbide recycling process using charged amino group assisted precipitation (CAAP method) is highlighted.


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Recovery and Recycling of Tungsten by Alkaline Leaching of Scrap

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