RECYCLING OF Cu AND Ni FROM WASTE SURFACE
RECYCLING OF Cu AND Ni FROM WASTE SURFACE-COATED PLASTIC USING AMMONIACAL LEACHING AND SOLVENT EXTRACTION J.-C. LEE*, M. JUN*, R. R. SRIVASTAVA**, M.-S. KIM* * Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Republic of Korea ** Research & Product Development, TAE-HYUNG Recycling, Gimcheon 740-872, Republic of Korea
SUMMARY: The present study deals with a hydrometallurgical recovery of metal values from the waste ABS plastics without degrading the basic properties of the polymer. A typical sample of waste ABS discarded from the waste electrical and electronic equipments, containing 4.1 wt.% Cu, 1.3 wt.% Ni, 0.03 wt.% Cr, and rest being the polymer substance, has been used. At first, the ammoniacal leaching dissolved Cu and Ni from the ABS surfaces as their ammine complexes, while remaining the chromium undissolved. The influence of parameters viz. ammonia concentration, type of buffering salts, temperature, time and agitation speed has been optimized. Different buffer media for the leaching of metals followed the extraction order: CO32− > Cl− > SO42−. The leaching in the carbonate media performed at 200g/L pulp density in 5.0 M total NH3 solution [with NH4OH:(NH4)2CO3 = 4:1] under the agitation speed 400 rpm and temperature 20 °C for 120 min yielded the optimal extraction of > 99% Cu and Ni. Subsequently, the solvent extraction technique has been employed to recover the metals from the ammoniacal leach liquor (containing 2.0 g/L Cu and 0.6 g/L Ni). A mixture of organic solvents, 0.25 M LIX 84I + 0.35 M TBP diluted in kerosene has shown the antagonistic effect for Ni, and provided the desired selectivity for Cu extraction (with > 92% efficiency in a single contact at O/A = 1:1) into the mixed adduct. In the latter step, Ni from the Cu depleted raffinate was extracted with 0.1 M LIX 84-I (diluted in kerosene), yielding > 99% efficiency. Both the loaded organic phases (the mixed LIX 84-I + TBP and LIX 84-I) were water washed and subjected to stripping with sulfuric acid separately to recover the enriched metal solutions back into the aqueous phase. More than 99.9% stripping efficiency for both Cu and Ni was achieved when 0.6 M and 0.12 M H2SO4, respectively were used. The developed process is simple and can easily be up-scaled for its commercial use.
1. INTRODUCTION The acrylonitrile butadiene styrene (ABS), a common thermoplastic polymer has a chemical formula (C8H8)x·(C4H6)y·(C3H3N)z. It is prepared by mixing the varying proportions of acrylonitrile, butadiene and styrene in the range of 15–35%, 5–30% and 40–60%, respectively (Swetham et al., 2017). The styrene makes in the plastic a shiny look with impervious surface, whereas, the rubbery substance butadiene provides resilience even at lower temperatures (Harper, 1975). The low cost, good machinability, and easy to fabricate, paint and glue are the factors making the ABS an ideal engineering plastic when the impact resistance, stiffness and strength are Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
desired (Prospector, 2016; http://www.emcoplastics.com/abs/). To provide a glossy look, enhancement in the hardness, and to ensure the constant electrical properties, the ABS surface is often metal coated. Generally, the metallic copper (Cu), nickel (Ni) and chromium (Cr) are used as the layered coating material onto the ABS surfaces. Some of its typical application includes support blocks and cases of households, automobiles, and structural components. The growing life-style, urbanization and better alternative to traditional wood and metal decorative components have risen the demands and uses of ABS. However, the decomposition of such plastic compounds are always a threat to environment, if disposed by landfill or, processed by heat treatment such as pyrolysis and combustion (Rutkowski and Levin, 1986). The heavy (hazardous) metals coated on the ABS surface can be leached to contaminate the soil and ground water near to the area of waste dump (Pathak et al., 2017). Therefore, the effecient recycling of ABS is challenging from environmental viewpoint, and the recovery of metals are vital for their possible re-use. Hence in this study, a recycling scheme for recovering the metals coated onto the surface of ABS has been investigated, without any degradation of the properties of the plastic (Jun, 2016). For this, ammonia was used as a suitable lixiviant to selectively dissolve Cu and Ni as amine species under the ambient conditions, while remaining the chromium undissolved. Subsquently, the solvent extraction was employed to separate and recover the metals from the ammoniacal leach liquor. To obtain the optimum yield in leaching and separation of metals, the effect of parameters like lixiviant medium and their conenctration as well as the ratio in leaching, time, temperature, whilst the separation of copper and nickel from the leach liquor was investigated by creating an antagonistic effect through mixing the TBP proportonately with LIX 84-I. Though the present stydy is limited to the recycling of surface-coated metals, but is primarily targeted for recycling the entire waste ABS including plastics, as a part of the sustainable process.
2. EXPERIMENTAL 2.1 Materials The waste ABS plastics obtained from a local vendor (Dongwon Co., South Korea) was used in this study. The ABS was first crushed to smaller size (see Figure 1a) to facilitate mixing and suspending the particles by stirring during the leaching experiments. The analysis by the Scanning Electron Microscope (SEM, JSM-6380, JEOL) shown in Figure 1b clearly reveals the presence of a 3-layer coating of metals. The analysis by an electron dispersive X-ray spectroscopy indicates the presence of chromium on the upper layer, nickel at middle and copper onto the bottom layer with ABS. Further, the metal composition of the samples was analyzed by their dissolution in aqua regia solution and analyzing the aliquat using the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Model: ICAP6000 series, Thermo Scientific). The analysis of metals (in weight percentage) was obtained as 0.03% Cr, 1.3% Ni, and 4.1% Cu with balance being the ABS plastics. The ammonia solution and ammonium salts (chloride, sulfate and carbonate) were supplied by Junsei Chemical Co. Ltd. (Japan). The organic extractant, LIX 84-I (purity > 96%) was supplied by BASF (Germany); whereas, tri-butyl phosphate (TBP, purity > 98%) used for antagonistic effect on nickel extraction and kerosene used as diluent besides NaOH and H2SO4 were supplied by Junsei Chemical Co. (Japan). Solution of both NaOH and H2SO4 were prepared in deionized water for the pH adjustment during extraction and for stripping of the loaded metals in organic, respectively. All the reagents were used without further purification.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. (a) Waste ABS sample used in this study, (b) SEM-EDX images of the cross section of a 3-layer coating on ABS. 2.2 Methods 2.2.1 Leaching The leaching experiments were carried out using a Pyrex reactor (500 mL) fitted over a water bath, with a provision of a mechanical agitation. A pre-determined amount of ABS while maintaining the desired pulp density (PD, usually 100 g/L) was charged into the ammoniacal solution under stirring condition (400 rpm) at ambient temperature (~20 °C). The ammoniacal solution was prepared by mixing ammonium salt into ammonia solution at a desired proportion and also by considering the total ammonia concentration. All the leaching experiments were performed with a constant air feed rate of 1.0 L/min. After completion of leaching, the leach liquor was collected after a solid-liquid separation and the metal concentration in the leach liquor was analyzed by ICP-AES. The leaching efficiency of metals was calculated as follows: %Leaching = {(Mi – Mf)/Mi} x 100
(1)
where Mi and Mf are the metal mass in the initial feed sample and in the leach liquor (after leaching at a given time), respectively. It is noteworthy to mention that the chromium coated onto ABS surface was found to be undissloved, which can be separated from metal-depleted ABS by simply a rinsing and sieving after the filtration of leach liquor. Results on the chromium have not been included in the discussion of this study. 2.2.2 Solvent extraction All the solvent extraction experiments were performed at room temperature (298±2K) by contacting a fixed volume (25 mL) of aqueous phase to the same volume of organic phase into a 60 mL separating funnel for 5 minutes. After the phase separation, the metal contents in the aqueous phase was analyzed after the proper dilution with 10 wt.% of HCl. The extractability of metal ions was calculated as follows: %Extraction = {Morg/(Morg + Maq)} x 100
(2)
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
where Morg and Maq are metal concentrations in the organic and aqueous phase, respectively.
3. RESULTS AND DISCUSSION 3.1 Studies on leaching 3.1.1 Effect of ammoniacal media The components of the leaching medium, ammonia solution significantly influences the buffering effect and complexation with the metal ions. Hence, the leaching study was performed with three different salt (chloride, sulfate and carbonate) of ammoniacal solution at a fixed ammonia concentration of 5.0 M (total) with the contribution ratio of solution-to-salt (S:S) contents of 4:1. The other parameters of 100 g/L pulp density, 20 °C temperature and 400 rpm agitation speed were also maintained invaribly. Results presented in Figures 2a and 2b clearly depict the influence of ammoniacal medium on the leaching efficiency of copper and nickel, exhibiting the extraction order as: CO32− > Cl− > SO42−. As can be seen from the figures, the leaching efficiency of both metals reached maximum (> 99%) after 180 min using the cabonate medium. But using the Cl− and SO42− media, the corresponding efficiency for copper was < 60% and 80%, and for nickel it was < 40% and < 80% after a 180 min of leaching. The prolonged leaching (> 180 min) for copper however, showed an increaseing trend with Cl− and SO42− media, whereas nickel was unaffected. Because of the high leaching efficiency obtained with the carbonate system, further leaching studies were carried out in NH4OH-(NH4)2CO3 solutions.
Figure 2. Effect of ammonium salts as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (5.0 M NH3 as NH4OH:ammonium salt = 4:1, agitation speed 400 rpm, temp. 20 °C, pulp density 100 g/L). 3.1.2 Effect of total ammonia concentration The formation of metal ammine complexes in ammoniacal buffer solution also depends on the total concentration of ammonia (Mishra et al., 2011; Srivastava et al. 2013). Hence, the effect of varying total ammonia concentration in the range 3.0 to 6.0 M on leaching efficiency of copper and nickel was examined at a constant S:S ration of 4:1. The other parameters of 100 g/L pulp density, 20 °C temperature and 400 rpm agitation speed were also maintained invaribly in these experiments. Results presented in Figures 3a and 3b clearly depict the influence of
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ammonia concentration on the complexation efficiency with copper and nickel. It can be seen that 3.0 M ammonia was insufficient (< 50%) to leach nickel and also required a long 400 min to efficiently leach the copper. Total ammonia ≥ 5.0 M was found to be sufficient to leach > 99% copper and nickel in the NH4OH-(NH4)2CO3 solution within a 150 min of leaching, and hence this was maintined in further experiments. The requirement of higher ammonia concentration in the case of nickel leaching can be ascribed to the difference in stoichiometric ammonia needed to form the stable ammine complexes as the following equations: Cu + 1/2(O2) + 2NH3 + 2NH4+ + H2O Ni + 1/2(O2) + 4NH3 + 2NH4+ + H2O
Cu(H2O)2(NH3)42+ Ni(NH3)62+ + H2O
(3) (4)
Figure 3. Effect of total ammonia concentration as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (NH4OH:(NH4)2CO3 = 4:1, agitation speed 400 rpm, temp. 20 °C, pulp density 100 g/L). 3.1.3 Effect of solution-to-salt ratio The effect of NH4OH-(NH4)2CO3 ratio (S:S) in ammoniacal solution for leaching the copper and nickel from ABS was investigated by varying the S:S from 2:1 to 8:1, without altering the concentration of total ammonia (5.0 M) in the solution. Other leaching parameters of 100 g/L pulp density, 20 °C temperature and 400 rpm were also maintained as in previous set of experiments. Results for copper and nickel leaching with respect to time are presented in Figures 4a and 4b, respectively. These results clearly reveal that the metals leaching efficiency as well as the leaching rate in ammoniacal solution were least with S:S = 2:1 and required 240 min to attain the equilibrium (with > 99% efficiency), which could be achieved in 180 min of leaching by S:S = 4:1 and 6:1. Interestingly a further increase in S:S to 8:1, the maximum leaching efficiencies of metals were found to decrease to around 94% copper and 75% nickel even after a duration of 240 min. The analysis of the results indicates that S:S of 4:1 is optimum and can be maintained in next set of experiments.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4. Effect of NH4OH:(NH4)2CO3 in total ammonia concentration as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (5.0 M total NH3, agitation speed 400 rpm, temp. 20 °C, pulp density 100 g/L). 3.1.4 Effect of pulp density In order to utilize the maximum capacity of lixiviant and getting more metal contents in the leach liquor, the effect of solid-to-liquid ratio (pulp density) was examined in the range 100 to 300 g/L. The leaching experiments were carried out under the conditions viz., S:S ratio for NH4OH-(NH4)2CO3 = 4:1, total ammonia concentration = 5.0 M, temperature = 20 °C, and agitation speed 400 rpm. Results shown in Figures 5a and 5b reveal the increase in leaching efficincy with respect to time when the pulp density was increasing from 100 to 200 g/L, and then decreased with further increasing the pulp density particulary for nickel.
Figure 5. Effect of pulp density as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (5.0 M total NH3 as NH4OH:(NH4)2CO3 = 4:1, agitation speed 400 rpm, temp. 20 °C). The steep rise in leaching behaviour of copper can be ascribed to the oxidation behaviour of Cu2+ which could increase the redox potential of ammoniacal solution thus resulting in enhanced rate of reaction by the self-catalytic activity of Cu2+ (Jun et al., 2016a and 2016b; Mishra et al. 2011).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.2 Studies on solvent extraction Leaching performed in ammoniacal carbonate solution under the optimized condition : 5.0 M total NH3 concentration with S:S = 4:1; pulp density 200 g/L; agitation speed 400 rpm; temperature 20 °C; and time 120 min yielded leach liquor of 2.0 g/L Cu2+ and 0.6 g/L Ni2+. This solution was used for the separation and recovery of metals by employing the solvent extraction technique. The use of commercially available oxime based organic solvents (mainly of LIX 84 series) is well established for extracting both nickel and copper at once from the ammoniacal solution (Jun et al., 2016b; Kumar et al., 1991; Pandey et al., 1989). The usual separation could be achieved by the selective stripping of these metals as a function of acid concentration. In contrast to above reports, we have examined the selective extraction of metals by mixing a neutral extractant, TBP with LIX 84-I in the present study. The batch study using LIX 84-I for extracting the copper and nickel reflects the applicability of 0.25 M extractant which was found to be sufficient to extract both metals with > 99% efficiency at an O/A = 1. Hence, this concentration of LIX 84-I was kept invariably to mix TBP into the organic phase. 3.2.1 Selective extraction of copper by TBP addition with LIX 84-I The effect of TBP addition with a requisite concecentration of LIX 84-I (0.25 M) onto the extraction efficiency and selectivity of metals was examined while varying the amount of TBP in the range 0−0.5 M into the organic phase diluted in kerosene. For equilibrating the organic phase with leach liquor, the parameters like O/A = 1, contact time 5 min, contact temperature 20 °C were maintained. Experimental results presented in Figure 6a clearly demonstrate that LIX 84-I only (without TBP addiition) could extract > 99% Cu2+ and Ni2+ from the ammoniacal leach liquor. However, introducing only a 0.05 M TBP into the organic phase caused to decline the extraction of nickel to ~50%, which further lowered down to ~30% with 0.1 M TBP addition.
Figure 6. (a) effect of TBP addition with a 0.25 M LIX 84-I in keroene, on the extraction of copper and nickel from the ammoniacal leach liquor (2.0 g/L Cu2+, 0.62 g/L Ni2+ ; O/A = 1, time 5 min); (b) effect of LIX 84-I concentration on the extraction of nickel from Cu-depleted solution (0.62 g/L Ni2+, O/A = 1, time 5 min). As can be seen 0.35 M TBP mixed with 0.25 M LIX 84-I could entirely supress the nickel extraction. A minor decline during the extraction of copper (~90%) was however, found to be very selective for the two metals, leaving total nickel in the raffinate. Therefore after a two-stage extraction of copper, the Ni2+-bearing aqueous solution (raffinate) was contacted with only LIX
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
84-I of concentrations 0.01 M to 0.1 M. As can be seen from Figure 6b, the extraction of nickel significantly increased from ~20% to 96.5% using 0.01 M and 0.06 M LIX 84-I, respectively in the organic solution. Thereafter, it slowly reaches to the maxium extraction 99.3% with 0.1 M LIX 84-I. The metal extracted loaded organics were investigated for stripping by contacting of metals with sulfuric acid solutions of different concentrations at unit phase ratio for 5 min. Stripping results as a function of H2SO4 concetration are shown in Figures 7a and 7b for copper and nickel, respectively. Figure 7a shows the stripping of 40.5% copper with 0.1 M H2SO4 which reached to approximately 70% with twice the acid concentration and could finally yield > 99% stripping efficiency when a 0.6 M H2SO4 solution was contacted with the Cu2+-loaded organic. Figure 7b reveals the stripping efficiency of nickel to almost 97% with 0.02 M to 0.10 M H2SO4, that it reached to >99% with 0.12 M acid solution. Thus obtained stripped solution of copper and nickel sulfate can be used for electrowinning for recovery as metal cathodes, after a proper enrichemnt of metal concentration in the aqueous solutions.
Figure 7. Effect of sulfuric acid concentration on (a) Cu stripping from 1.85 g/L Cu2+-loaded organic and (b) Ni stripping from 0.61 g/L Ni2+-loaded organic (O/A = 1 for 10 min).
4. CONCLUSIONS The present study proposes an efficient recycling scheme for the waste ABS plastics by ammoniacal leaching of surface-coated metals on the ABS. Use of carbonate salt in the ammoniacal solution yielded the maximum leaching of >99% metals compared to less than 60% leaching with chloride and 99%) Cu and Ni was achieved under the conditions: NH4OH:(NH4)2CO3 solution 4:1, total NH3 concentration 5.0 M, agitation speed 400 rpm, temp. 20 °C and time 120 min, yielding 2.0 g/L Cu2+ and 0.6 g/L Ni2+ in the leach liquor. In subsquent step of metal separation, solvent extraction using a mixture of LIX 84-I and TBP showed an antagonistic effect for nickel extraction with increasing concentration of TBP in the organic phase. With 0.35 M TBP mixed with 0.25 M LIX 84-I in kerosene the extraction of nickel was completely suppressed. From the Cu-depleted raffinate 99.3% Ni2+ could be extracted with 0.1 M LIX 84-I. Almost 99.9% Cu and Ni both were stripped with 0.6 M and 0.12 M H2SO4 solution from the respective metal loaded organics at unit phase ratios. Chromium can also be separated as the undissolved particles by rinsing and sieving of leaching
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residue obained after the filteration of leach liquor. The developed hydrometallurgical recycling of metals from ABS is attractive as it did not deterioate the very properties of the native plastic for recycling of ABS in the next step, unlike the leaching process based on mineral acid.
AKNOWLEDGEMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20165020101170).
REFERENCES Swetham T., Reddy K.M.M., Huggi A. and Kumar M.N. (2017). A critical review on of 3D printing materials and details of materials used in FDM. IJSRSET, vol. 3, 353–361. Harper C.A. (1975). Handbook ISBN 0070266816
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http://www.emcoplastics.com/abs/ [accessed date 10 April 2017]. Prospector, 2016. https://plastics.ulprospector.com/generics/1/c/t/acrylonitrile-butadienestyrene-abs-properties-processing Rutkowski J.V. and Levin, B.C. (1986). Acrylonitrile-butadiene-styrene coploymers (ABS): pyrolysis and combustion products and their toxicity- a review of the literature. Fire and Materials, vol. 10, 93–105. Pathak P. (2017). An assessment of strontium sorption onto bentonite buffer material in waste repository. Environmental Science and Pollution Research, vol. 24, 8825–8836. Jun M. (2016). Recovery of Cu and Ni from waste surface-coated plastic using ammoniacal leaching and solvent extraction. Master Thesis, Resources Recycling, University of Science and Technology, 2016. Mishra D., Srivastava R.R., Sahu K.K., Singh T.B. and Jana R.K. (2011). Leaching of roastreduced manganese nodules in NH3-(NH4)2CO3 medium. Hydrometallurgy, vol. 109, 215–220. Srivastava R.R., Mishra A.N. and Singh V.K. (2013). Co-precipitation study of nickel and cobalt from ammoniacal leach liquor of laterite ore. Journal of Metallurgy and Materials Science, vol. 55, 67–72. Jun M., Srivastava R.R., Lee J.-c. and Jeong J. (2016a). Leaching of copper and other metal impurities from a Si-sludge using waste copper nitrate solution. Journal of the Korean Instiute of Resources Recycling, vol. 25, 11–19. Jun M., Srivastava R.R., Jeong J., Lee J.-c. and Kim M.s. (2016b). Simple recycling of copper by the synergistic exploitation of industrial wastes: a step towards sustainability. Green Chemistry, vol. 18, 3823–3834. Kumar V., Pandey B.D. and Bagchi D. (1991). Application of LIX 84 for separation of copper, nickel and cobalt in ammoniacal leaching of ocean nodules. Materials Transactions JIM, vol. 32, 157–163. Pandey B.D., Kumar V., Bagchi D. and Akerkar D.D. (1989). Extraction of nickel and copper from the ammoniacal leach solution of sea nodules by LIX 64N. Industrial Eng. Chem. Res., vol. 28, 1664–1669.
SUMMARY: The present study deals with a hydrometallurgical recovery of metal values from the waste ABS plastics without degrading the basic properties of the polymer. A typical sample of waste ABS discarded from the waste electrical and electronic equipments, containing 4.1 wt.% Cu, 1.3 wt.% Ni, 0.03 wt.% Cr, and rest being the polymer substance, has been used. At first, the ammoniacal leaching dissolved Cu and Ni from the ABS surfaces as their ammine complexes, while remaining the chromium undissolved. The influence of parameters viz. ammonia concentration, type of buffering salts, temperature, time and agitation speed has been optimized. Different buffer media for the leaching of metals followed the extraction order: CO32− > Cl− > SO42−. The leaching in the carbonate media performed at 200g/L pulp density in 5.0 M total NH3 solution [with NH4OH:(NH4)2CO3 = 4:1] under the agitation speed 400 rpm and temperature 20 °C for 120 min yielded the optimal extraction of > 99% Cu and Ni. Subsequently, the solvent extraction technique has been employed to recover the metals from the ammoniacal leach liquor (containing 2.0 g/L Cu and 0.6 g/L Ni). A mixture of organic solvents, 0.25 M LIX 84I + 0.35 M TBP diluted in kerosene has shown the antagonistic effect for Ni, and provided the desired selectivity for Cu extraction (with > 92% efficiency in a single contact at O/A = 1:1) into the mixed adduct. In the latter step, Ni from the Cu depleted raffinate was extracted with 0.1 M LIX 84-I (diluted in kerosene), yielding > 99% efficiency. Both the loaded organic phases (the mixed LIX 84-I + TBP and LIX 84-I) were water washed and subjected to stripping with sulfuric acid separately to recover the enriched metal solutions back into the aqueous phase. More than 99.9% stripping efficiency for both Cu and Ni was achieved when 0.6 M and 0.12 M H2SO4, respectively were used. The developed process is simple and can easily be up-scaled for its commercial use.
1. INTRODUCTION The acrylonitrile butadiene styrene (ABS), a common thermoplastic polymer has a chemical formula (C8H8)x·(C4H6)y·(C3H3N)z. It is prepared by mixing the varying proportions of acrylonitrile, butadiene and styrene in the range of 15–35%, 5–30% and 40–60%, respectively (Swetham et al., 2017). The styrene makes in the plastic a shiny look with impervious surface, whereas, the rubbery substance butadiene provides resilience even at lower temperatures (Harper, 1975). The low cost, good machinability, and easy to fabricate, paint and glue are the factors making the ABS an ideal engineering plastic when the impact resistance, stiffness and strength are Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
desired (Prospector, 2016; http://www.emcoplastics.com/abs/). To provide a glossy look, enhancement in the hardness, and to ensure the constant electrical properties, the ABS surface is often metal coated. Generally, the metallic copper (Cu), nickel (Ni) and chromium (Cr) are used as the layered coating material onto the ABS surfaces. Some of its typical application includes support blocks and cases of households, automobiles, and structural components. The growing life-style, urbanization and better alternative to traditional wood and metal decorative components have risen the demands and uses of ABS. However, the decomposition of such plastic compounds are always a threat to environment, if disposed by landfill or, processed by heat treatment such as pyrolysis and combustion (Rutkowski and Levin, 1986). The heavy (hazardous) metals coated on the ABS surface can be leached to contaminate the soil and ground water near to the area of waste dump (Pathak et al., 2017). Therefore, the effecient recycling of ABS is challenging from environmental viewpoint, and the recovery of metals are vital for their possible re-use. Hence in this study, a recycling scheme for recovering the metals coated onto the surface of ABS has been investigated, without any degradation of the properties of the plastic (Jun, 2016). For this, ammonia was used as a suitable lixiviant to selectively dissolve Cu and Ni as amine species under the ambient conditions, while remaining the chromium undissolved. Subsquently, the solvent extraction was employed to separate and recover the metals from the ammoniacal leach liquor. To obtain the optimum yield in leaching and separation of metals, the effect of parameters like lixiviant medium and their conenctration as well as the ratio in leaching, time, temperature, whilst the separation of copper and nickel from the leach liquor was investigated by creating an antagonistic effect through mixing the TBP proportonately with LIX 84-I. Though the present stydy is limited to the recycling of surface-coated metals, but is primarily targeted for recycling the entire waste ABS including plastics, as a part of the sustainable process.
2. EXPERIMENTAL 2.1 Materials The waste ABS plastics obtained from a local vendor (Dongwon Co., South Korea) was used in this study. The ABS was first crushed to smaller size (see Figure 1a) to facilitate mixing and suspending the particles by stirring during the leaching experiments. The analysis by the Scanning Electron Microscope (SEM, JSM-6380, JEOL) shown in Figure 1b clearly reveals the presence of a 3-layer coating of metals. The analysis by an electron dispersive X-ray spectroscopy indicates the presence of chromium on the upper layer, nickel at middle and copper onto the bottom layer with ABS. Further, the metal composition of the samples was analyzed by their dissolution in aqua regia solution and analyzing the aliquat using the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Model: ICAP6000 series, Thermo Scientific). The analysis of metals (in weight percentage) was obtained as 0.03% Cr, 1.3% Ni, and 4.1% Cu with balance being the ABS plastics. The ammonia solution and ammonium salts (chloride, sulfate and carbonate) were supplied by Junsei Chemical Co. Ltd. (Japan). The organic extractant, LIX 84-I (purity > 96%) was supplied by BASF (Germany); whereas, tri-butyl phosphate (TBP, purity > 98%) used for antagonistic effect on nickel extraction and kerosene used as diluent besides NaOH and H2SO4 were supplied by Junsei Chemical Co. (Japan). Solution of both NaOH and H2SO4 were prepared in deionized water for the pH adjustment during extraction and for stripping of the loaded metals in organic, respectively. All the reagents were used without further purification.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. (a) Waste ABS sample used in this study, (b) SEM-EDX images of the cross section of a 3-layer coating on ABS. 2.2 Methods 2.2.1 Leaching The leaching experiments were carried out using a Pyrex reactor (500 mL) fitted over a water bath, with a provision of a mechanical agitation. A pre-determined amount of ABS while maintaining the desired pulp density (PD, usually 100 g/L) was charged into the ammoniacal solution under stirring condition (400 rpm) at ambient temperature (~20 °C). The ammoniacal solution was prepared by mixing ammonium salt into ammonia solution at a desired proportion and also by considering the total ammonia concentration. All the leaching experiments were performed with a constant air feed rate of 1.0 L/min. After completion of leaching, the leach liquor was collected after a solid-liquid separation and the metal concentration in the leach liquor was analyzed by ICP-AES. The leaching efficiency of metals was calculated as follows: %Leaching = {(Mi – Mf)/Mi} x 100
(1)
where Mi and Mf are the metal mass in the initial feed sample and in the leach liquor (after leaching at a given time), respectively. It is noteworthy to mention that the chromium coated onto ABS surface was found to be undissloved, which can be separated from metal-depleted ABS by simply a rinsing and sieving after the filtration of leach liquor. Results on the chromium have not been included in the discussion of this study. 2.2.2 Solvent extraction All the solvent extraction experiments were performed at room temperature (298±2K) by contacting a fixed volume (25 mL) of aqueous phase to the same volume of organic phase into a 60 mL separating funnel for 5 minutes. After the phase separation, the metal contents in the aqueous phase was analyzed after the proper dilution with 10 wt.% of HCl. The extractability of metal ions was calculated as follows: %Extraction = {Morg/(Morg + Maq)} x 100
(2)
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where Morg and Maq are metal concentrations in the organic and aqueous phase, respectively.
3. RESULTS AND DISCUSSION 3.1 Studies on leaching 3.1.1 Effect of ammoniacal media The components of the leaching medium, ammonia solution significantly influences the buffering effect and complexation with the metal ions. Hence, the leaching study was performed with three different salt (chloride, sulfate and carbonate) of ammoniacal solution at a fixed ammonia concentration of 5.0 M (total) with the contribution ratio of solution-to-salt (S:S) contents of 4:1. The other parameters of 100 g/L pulp density, 20 °C temperature and 400 rpm agitation speed were also maintained invaribly. Results presented in Figures 2a and 2b clearly depict the influence of ammoniacal medium on the leaching efficiency of copper and nickel, exhibiting the extraction order as: CO32− > Cl− > SO42−. As can be seen from the figures, the leaching efficiency of both metals reached maximum (> 99%) after 180 min using the cabonate medium. But using the Cl− and SO42− media, the corresponding efficiency for copper was < 60% and 80%, and for nickel it was < 40% and < 80% after a 180 min of leaching. The prolonged leaching (> 180 min) for copper however, showed an increaseing trend with Cl− and SO42− media, whereas nickel was unaffected. Because of the high leaching efficiency obtained with the carbonate system, further leaching studies were carried out in NH4OH-(NH4)2CO3 solutions.
Figure 2. Effect of ammonium salts as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (5.0 M NH3 as NH4OH:ammonium salt = 4:1, agitation speed 400 rpm, temp. 20 °C, pulp density 100 g/L). 3.1.2 Effect of total ammonia concentration The formation of metal ammine complexes in ammoniacal buffer solution also depends on the total concentration of ammonia (Mishra et al., 2011; Srivastava et al. 2013). Hence, the effect of varying total ammonia concentration in the range 3.0 to 6.0 M on leaching efficiency of copper and nickel was examined at a constant S:S ration of 4:1. The other parameters of 100 g/L pulp density, 20 °C temperature and 400 rpm agitation speed were also maintained invaribly in these experiments. Results presented in Figures 3a and 3b clearly depict the influence of
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ammonia concentration on the complexation efficiency with copper and nickel. It can be seen that 3.0 M ammonia was insufficient (< 50%) to leach nickel and also required a long 400 min to efficiently leach the copper. Total ammonia ≥ 5.0 M was found to be sufficient to leach > 99% copper and nickel in the NH4OH-(NH4)2CO3 solution within a 150 min of leaching, and hence this was maintined in further experiments. The requirement of higher ammonia concentration in the case of nickel leaching can be ascribed to the difference in stoichiometric ammonia needed to form the stable ammine complexes as the following equations: Cu + 1/2(O2) + 2NH3 + 2NH4+ + H2O Ni + 1/2(O2) + 4NH3 + 2NH4+ + H2O
Cu(H2O)2(NH3)42+ Ni(NH3)62+ + H2O
(3) (4)
Figure 3. Effect of total ammonia concentration as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (NH4OH:(NH4)2CO3 = 4:1, agitation speed 400 rpm, temp. 20 °C, pulp density 100 g/L). 3.1.3 Effect of solution-to-salt ratio The effect of NH4OH-(NH4)2CO3 ratio (S:S) in ammoniacal solution for leaching the copper and nickel from ABS was investigated by varying the S:S from 2:1 to 8:1, without altering the concentration of total ammonia (5.0 M) in the solution. Other leaching parameters of 100 g/L pulp density, 20 °C temperature and 400 rpm were also maintained as in previous set of experiments. Results for copper and nickel leaching with respect to time are presented in Figures 4a and 4b, respectively. These results clearly reveal that the metals leaching efficiency as well as the leaching rate in ammoniacal solution were least with S:S = 2:1 and required 240 min to attain the equilibrium (with > 99% efficiency), which could be achieved in 180 min of leaching by S:S = 4:1 and 6:1. Interestingly a further increase in S:S to 8:1, the maximum leaching efficiencies of metals were found to decrease to around 94% copper and 75% nickel even after a duration of 240 min. The analysis of the results indicates that S:S of 4:1 is optimum and can be maintained in next set of experiments.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4. Effect of NH4OH:(NH4)2CO3 in total ammonia concentration as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (5.0 M total NH3, agitation speed 400 rpm, temp. 20 °C, pulp density 100 g/L). 3.1.4 Effect of pulp density In order to utilize the maximum capacity of lixiviant and getting more metal contents in the leach liquor, the effect of solid-to-liquid ratio (pulp density) was examined in the range 100 to 300 g/L. The leaching experiments were carried out under the conditions viz., S:S ratio for NH4OH-(NH4)2CO3 = 4:1, total ammonia concentration = 5.0 M, temperature = 20 °C, and agitation speed 400 rpm. Results shown in Figures 5a and 5b reveal the increase in leaching efficincy with respect to time when the pulp density was increasing from 100 to 200 g/L, and then decreased with further increasing the pulp density particulary for nickel.
Figure 5. Effect of pulp density as a function of time on (a) copper and (b) nickel leaching from the ABS in ammoniacal solution (5.0 M total NH3 as NH4OH:(NH4)2CO3 = 4:1, agitation speed 400 rpm, temp. 20 °C). The steep rise in leaching behaviour of copper can be ascribed to the oxidation behaviour of Cu2+ which could increase the redox potential of ammoniacal solution thus resulting in enhanced rate of reaction by the self-catalytic activity of Cu2+ (Jun et al., 2016a and 2016b; Mishra et al. 2011).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.2 Studies on solvent extraction Leaching performed in ammoniacal carbonate solution under the optimized condition : 5.0 M total NH3 concentration with S:S = 4:1; pulp density 200 g/L; agitation speed 400 rpm; temperature 20 °C; and time 120 min yielded leach liquor of 2.0 g/L Cu2+ and 0.6 g/L Ni2+. This solution was used for the separation and recovery of metals by employing the solvent extraction technique. The use of commercially available oxime based organic solvents (mainly of LIX 84 series) is well established for extracting both nickel and copper at once from the ammoniacal solution (Jun et al., 2016b; Kumar et al., 1991; Pandey et al., 1989). The usual separation could be achieved by the selective stripping of these metals as a function of acid concentration. In contrast to above reports, we have examined the selective extraction of metals by mixing a neutral extractant, TBP with LIX 84-I in the present study. The batch study using LIX 84-I for extracting the copper and nickel reflects the applicability of 0.25 M extractant which was found to be sufficient to extract both metals with > 99% efficiency at an O/A = 1. Hence, this concentration of LIX 84-I was kept invariably to mix TBP into the organic phase. 3.2.1 Selective extraction of copper by TBP addition with LIX 84-I The effect of TBP addition with a requisite concecentration of LIX 84-I (0.25 M) onto the extraction efficiency and selectivity of metals was examined while varying the amount of TBP in the range 0−0.5 M into the organic phase diluted in kerosene. For equilibrating the organic phase with leach liquor, the parameters like O/A = 1, contact time 5 min, contact temperature 20 °C were maintained. Experimental results presented in Figure 6a clearly demonstrate that LIX 84-I only (without TBP addiition) could extract > 99% Cu2+ and Ni2+ from the ammoniacal leach liquor. However, introducing only a 0.05 M TBP into the organic phase caused to decline the extraction of nickel to ~50%, which further lowered down to ~30% with 0.1 M TBP addition.
Figure 6. (a) effect of TBP addition with a 0.25 M LIX 84-I in keroene, on the extraction of copper and nickel from the ammoniacal leach liquor (2.0 g/L Cu2+, 0.62 g/L Ni2+ ; O/A = 1, time 5 min); (b) effect of LIX 84-I concentration on the extraction of nickel from Cu-depleted solution (0.62 g/L Ni2+, O/A = 1, time 5 min). As can be seen 0.35 M TBP mixed with 0.25 M LIX 84-I could entirely supress the nickel extraction. A minor decline during the extraction of copper (~90%) was however, found to be very selective for the two metals, leaving total nickel in the raffinate. Therefore after a two-stage extraction of copper, the Ni2+-bearing aqueous solution (raffinate) was contacted with only LIX
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
84-I of concentrations 0.01 M to 0.1 M. As can be seen from Figure 6b, the extraction of nickel significantly increased from ~20% to 96.5% using 0.01 M and 0.06 M LIX 84-I, respectively in the organic solution. Thereafter, it slowly reaches to the maxium extraction 99.3% with 0.1 M LIX 84-I. The metal extracted loaded organics were investigated for stripping by contacting of metals with sulfuric acid solutions of different concentrations at unit phase ratio for 5 min. Stripping results as a function of H2SO4 concetration are shown in Figures 7a and 7b for copper and nickel, respectively. Figure 7a shows the stripping of 40.5% copper with 0.1 M H2SO4 which reached to approximately 70% with twice the acid concentration and could finally yield > 99% stripping efficiency when a 0.6 M H2SO4 solution was contacted with the Cu2+-loaded organic. Figure 7b reveals the stripping efficiency of nickel to almost 97% with 0.02 M to 0.10 M H2SO4, that it reached to >99% with 0.12 M acid solution. Thus obtained stripped solution of copper and nickel sulfate can be used for electrowinning for recovery as metal cathodes, after a proper enrichemnt of metal concentration in the aqueous solutions.
Figure 7. Effect of sulfuric acid concentration on (a) Cu stripping from 1.85 g/L Cu2+-loaded organic and (b) Ni stripping from 0.61 g/L Ni2+-loaded organic (O/A = 1 for 10 min).
4. CONCLUSIONS The present study proposes an efficient recycling scheme for the waste ABS plastics by ammoniacal leaching of surface-coated metals on the ABS. Use of carbonate salt in the ammoniacal solution yielded the maximum leaching of >99% metals compared to less than 60% leaching with chloride and 99%) Cu and Ni was achieved under the conditions: NH4OH:(NH4)2CO3 solution 4:1, total NH3 concentration 5.0 M, agitation speed 400 rpm, temp. 20 °C and time 120 min, yielding 2.0 g/L Cu2+ and 0.6 g/L Ni2+ in the leach liquor. In subsquent step of metal separation, solvent extraction using a mixture of LIX 84-I and TBP showed an antagonistic effect for nickel extraction with increasing concentration of TBP in the organic phase. With 0.35 M TBP mixed with 0.25 M LIX 84-I in kerosene the extraction of nickel was completely suppressed. From the Cu-depleted raffinate 99.3% Ni2+ could be extracted with 0.1 M LIX 84-I. Almost 99.9% Cu and Ni both were stripped with 0.6 M and 0.12 M H2SO4 solution from the respective metal loaded organics at unit phase ratios. Chromium can also be separated as the undissolved particles by rinsing and sieving of leaching
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
residue obained after the filteration of leach liquor. The developed hydrometallurgical recycling of metals from ABS is attractive as it did not deterioate the very properties of the native plastic for recycling of ABS in the next step, unlike the leaching process based on mineral acid.
AKNOWLEDGEMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20165020101170).
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