Waste Management xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate Andrea Alzate a,b,⇑, Maria Esperanza López a, Claudia Serna a a b
GIPIMME Research Group, Department of Materials Engineering, University of Antioquia, CL 67 53-108, Medellin, Colombia Ingeniería, Suministros y Montajes S.A.S, INSUMON S.A.S, CL 36 36-9, Medellín, Colombia
a r t i c l e
i n f o
Article history: Received 3 November 2015 Revised 28 January 2016 Accepted 30 January 2016 Available online xxxx Keywords: Gold recovery Waste electrical and electronic equipment (WEEE) Ammonium persulfate Response surface methodology Recycling
a b s t r a c t This paper presents a novel methodology to recover gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate ((NH4)2S2O8). Gold was recovered as a fine coating using substrate oxidation without shredding or grinding process. The WEEE sample was characterized giving values of Au: 1.05 g/kg, Fe: 86.00 g/kg, Ni: 73.64 g/kg, Cu: 26.65 g/kg. The effect of (NH4)2S2O8 concentration (0.22–1.10 M), oxygen (0.0–1.4 L/min) and L/S ratio (10–30 mL/g) on the main responses (substrate oxidation and Au recovery) was investigated implementing response surface methodology with numerical optimization. A quadratic model was developed and quantities greater than 98% of Au were recovered. The findings presented suggest that, optimized quantities of ammonium persulfate in aqueous highly oxygenated media could be used to extract superficial gold from WEEE. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Non polluting methodologies of valuable metals recovery from secondary sources have been a current topic in several studies because of their low environmental impact. Many of these studies have focused on recovering gold avoiding the hazardous implications of mineral extraction such as water pollution, deforestation and health repercussions. Nowadays, one of the main secondary sources to recover precious (Au, Ag, Pd) and base metals (Cu, Ni, Fe) is waste electrical and electronic equipment (WEEE) (Yazici and Deveci, 2014). The rapid growth in the manufacturing of technological devices has generated large quantities of WEEE. The annual WEEE global growth has been estimated at 8.8% (2004– 2011) and 17.6% (2011–2016) calculating for 2016 a global volume of 93.5 millions of tons (Akcil et al., 2015; Yu et al., 2014). These values expose the adequate final disposal of WEEE as one of the main challenges in WEEE management due to waste of valuable metals (Hadi et al., 2013) and the environmental problems associated with conventional disposal and recycling methods. Conventional disposal methods of WEEE such as landfill, incineration and hydrometallurgical recycling techniques generates soil and water pollutants and harmful substances that are released into
⇑ Corresponding author at: GIPIMME Research Group, University of Antioquia, CL 67 53-108, Medellin, Colombia. E-mail address: [email protected] (A. Alzate).
the air. On the first hand, landfill and incineration produce toxic substances that comprehend Hg, Pd, Cd, dioxins, furans and heavy metals vapors (Yazici and Deveci, 2013). However, developing countries still use landfill and incineration causing public health and environmental risks. Risks of landfill leachate production and air, soil, water or underwater pollution, make these disposal process unsuitable (Hadi et al., 2015a). On the other hand, hydrometallurgical recycling techniques with strong acids (HCl, HNO3, H2SO4/H2O2) or oxidative reagents (cyanide, thiourea, halide, nitrate and iodide) have been suggested by a variety of authors to process WEEE and reach the recovery of gold (Birloaga et al., 2014; Petter et al., 2014; Shibayama et al., 2013). Methods of gold recovery from WEEE with hydrometallurgical techniques focus on primary physical separation (shredding, grinding) and secondary leaching of total metallic fraction (Bas et al., 2014). The metallic fraction (precious and base metals) is obtained after shredding and grinding, which are recognized by dust pollution generation (Naseri Joda and Rashchi, 2012). In addition, grinding fraction is dissolved using strong acids or oxidative reagents (Zhang et al., 2012) and the resulting solution is separated through timeconsuming chemical process (cementation, solvent extraction, precipitation or coagulation) with the aim to recover the metal of interest from the solution (Syed, 2012). Hydrometallurgy extensively use cyanide, thiourea, halides and some strong acids. These agents are recognized by its toxic potential, low chemical stability and environmental impact (Tuncuk et al., 2012).
http://dx.doi.org/10.1016/j.wasman.2016.01.043 0956-053X/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Alzate, A., et al. Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.043
2
A. Alzate et al. / Waste Management xxx (2016) xxx–xxx
Nowadays, strategies on WEEE management have been adopted to mitigate health and environmental problems. The strategies contemplate the development of tools as Material Flow Analysis (MFA), Multi Criteria Analysis (MCA) and Life Cycle Assessment (LCA) (Kiddee et al., 2013). In develop countries, MFA and MCA have been successful applied to estimate the generation of WEEE and take environmental decisions to solve multi-criteria problems on disposal, while LCA has been used in several studies to evaluate the environmental impacts of WEEE (Wäger et al., 2011). Studies on LCA concluded that, compare with landfill or incineration, recycling techniques are more appropriate to manage WEEE (Kiddee et al., 2013). Nevertheless, any recycling process perform without environmental care may produce a highest impact on soil, air, water and humans. Developing suitable and optimal recycling methodologies capable of avoiding pollution, reducing reagents toxicity and time-consuming reactions has become one of the most important topics in WEEE management research. Hence, in recent years, some methodologies that attempt to use environmentally friendly agents and the optimization of gold and base metals recovery from WEEE have been suggested (Barbieri et al., 2010; Ha et al., 2014; Syed, 2006). For instance, alternative agents to extract non-leaching gold that include potassium persulfate (K2S2O8) (Syed, 2006) and cupric chloride (CuCl2) (Barbieri et al., 2010) were studied to recover gold from WEEE. These agents were used to oxidize and leach the metal substrate (Ni, Fe, and Cu) where gold was superficially associated as coating (Barbieri et al., 2010). The partial leaching of the substrate permitted gold recovery in a solid particulate state (Barbieri et al., 2010; Syed, 2006). Due to the elimination of gold leaching, it was possible to reduce reaction time avoiding purification stages and achieving the 98% in Au recovery with minimum formation of contaminant byproducts or total agent regeneration (Barbieri et al., 2010; Syed, 2006). Redox potential for the production of (SO2 4 ) ions in aqueous potassium persulfate was estimated in 2.01 V (Huang et al., 2002) and 0.48 V for the production of (CuCl2) ions in acid solution (Lundström et al., 2009). These estimations exposed the ability of K2S2O8 and CuCl2 to oxidize Ni, Fe and Cu and release gold from the substrate. Despites its great advantages, persulfate and cupric chloride oxidative systems to recover non-leaching gold from WEEE have not been extensively investigated. Selective persulfate oxidation proposed by Syed (2006) catalyzed with oxygen can be optimized in order to maximize the recovery of gold from WEEE reducing parameters like agent consumption and reaction time (Birloaga et al., 2013; Hadi et al., 2015b; Jordão et al., 2016). A way to optimize this process is to use response surface methodology (RSM), a series of statistical techniques. RSM has been adopted in several studies to find optimal region of operation into an experimental design space (Montgomery, 2013). This methodology was reported in WEEE management to determine the greater amount of gold leaching evaluating the incidence of thiosulfate, cooper and ammonia concentration (Ha et al., 2014). Besides, the extraction of Cu, Fe, Ni, Ag and Pd from waste printed circuit boards (WPCBs) was studied adopting RSM to establish the conditions that maximize metals extraction using H2SO4–CuSO4–NaCl solutions (Yazici and Deveci, 2013). In this study, an environmentally friendly methodology to extract non-leaching gold by partial substrate oxidation from WEEE using ammonium persulfate (NH4)2S2O8 was developed. The incidence of (NH4)2S2O8 concentration, oxygen and liquid/solid ratio on the recovery of gold was analyzed using response surface methodology with numerical optimization. The effects of (NH4)2S2O8 concentration (0.22–1.10 M), oxygen (0.0–1.4 L/min) and liquid/solid ratio (10–30 mL/g) over the gold recovery were studied in five different levels through a central composite design (CCD). (NH4)2S2O8 was selected over other environmentally friendly
agents due to the lack of research on its use and optimization in recovering gold from WEEE through substrate oxidation. In addition, the produced persulfate ions (S2O2 8 ) are not absorbed or bio accumulated in the soil after the process (Hernandez, 2005) and the generated by-products (sulfates) have not a negative effect on the environment (Syed, 2006). Comparing with potassium persulfate, sodium persulfate and cupric chloride, (NH4)2S2O8 has a greater leaching power to oxidize base metals than potassium and sodium persulfate (Babu et al., 2002) and is operationally safer and less toxic than CuCl2. The toxicity reduction responds to the absence of Cl2 (g) during the reaction. In brief, the system of methods applied was carried out without shredding or grinding stages which reduced secondary dust pollution, reaction time and contaminant by-products. This study aimed at maximizing gold recovery from WEEE with a novel methodology that includes the partial oxidation of metal substrate with ammonium persulfate in oxygenated media and the numerical optimization of the most significant parameters. 2. Experimental 2.1. Materials and reagents Intel Celeron–Pentium electronic processor scrap from end-oflife computers supplied by a local recycling company was the sample used (Fig. 1). A total amount of 50 processors were selected by reference, shape, weight, superficial distribution of gold and manufacturer to ensure sample homogeneity and statistical significance. Intel Celeron and Intel Pentium processors of 5 5 cm2 and an average weight of 8.97 g (Fig. 1) were used for chemical characterization, substrate oxidation and gold recovery tests without shredding or grinding stages. A sample of 35 g was used to determine the amount of gold and metal substrate (Fe, Ni, Cu) by chemical digestion using aqua-regia (Lee et al., 2011; Petter et al., 2014) followed by microwave plasma – atomic emission spectroscopy (MP-AES, AGILENT 4100) (Table 1). After chemical characterization, aqueous commercial grade ammonium persulfate (P98% (NH4)2S2O8) with a water solubility of 850 g/L at 25 °C (Hernandez, 2005) was the selected environmental reagent used to produce sulfate ions (SO 4 ), which partially oxidized the metal substrate breaking the Au–Cu–Ni–Fe bond and allowing gold to be extracted in its original non-leaching state.
Fig. 1. Processor scrap sample (ref. Intel Celeron–Intel Pentium).
An oxygen tank (99.9% O2) with flow control was used to deliver O2 and catalyze the oxidative reactions.
Please cite this article in press as: Alzate, A., et al. Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.043
3
A. Alzate et al. / Waste Management xxx (2016) xxx–xxx Table 1 Chemical composition of the processor scrap. Element
Fe
Ni
Cu
Ag
Au
Content (g/kg)
86.00
73.64
26.65
a) had not effect in the response. The calculated standard deviation for the model was 1.24%. The regression model that describes the substrate oxidation from processor scrap with (NH4)2S2O8 was established calculating the coefficients for each factor and developing a first order equation (Eq. (4)).
(FeSO4)+ and (NiSO4 (a)) (Medusa software, 2010). The equilibrium constant for this sulfates were established in (Log K(95°C) = 3.44) and (Log K(95°C) = 3.06) respectively (HSC Chemistry software, 2002). The coefficient of determination (R2) was estimated in 0.99 proving that the model explains the 99% of the variability in the response. Oxidation and leaching of the substrate were achieved by the diffusion of (SO2 4 ) and (O2) into the pines of the processors penetrating the solid sample from welding points to the core conformed by Fe–Ni–Cu (Fig. 2a). Oxidative dissolution of substrate took place breaking the Fe–Ni–Cu–Au bond without leaching of Au from the sample. Release of solid particulate gold was achieved after removing the sample from the reactor and applying a micro scale pressure washing (Fig. 2b).
Substrate oxidation ¼ 21:46 7:61A þ 5:63B þ 2:14C
A linear model for the release of solid gold from the sample as a response of the substrate oxidation was analyzed with the 23 factorial design. An empirical first order equation was developed (Eq. (5)).
7:62AB þ 3:43AC 0:53BC þ 2:42ABC
ð4Þ
Eq. (4) shows that B and C were the most effective linear factors, while A was the less effective linear factor. Positive sign of B and C indicates that the increment of the oxygen and L/S ratio positively influenced the oxidation and leaching of the substrate. This could be explained by the preferential formation of soluble oxides include CuFeO2 (Medusa software, 2010) with an equilibrium constant greater than zero (Log K(95°C) = 4.45) (HSC Chemistry software, 2002). The negative sign of A indicates a negative linear influence in the response. However, the interaction between [(NH4)2S2O8] – L/S ratio (AC) has a positive influence. This influence demonstrates that (NH4)2S2O8 needs a careful balance between water added and quantity of scrap to enable the speciation of 2 (SO2 4 ) and to oxidize the substrate. The (SO4 ) ions leached the metallic nickel and iron by formation of soluble sulfates includes
3.2. Recovery of gold – first order model
Au Recovery ¼ 89:61 þ 3:30A þ 3:63B þ 4:61C 0:67AB þ 1:30AC 1:92BC þ 1:37ABC
ð5Þ
All the linear factors and the interaction AC and ABC had a positive influence on the Au recovery. The calculated standard deviation of the data was 5.52%. The linear model equation sets out to obtain high values of Au recovery through coefficients of determination close to 100%. However, the estimated coefficient of determination was (R2 = 0.72) and the adjusted determination was (Adj-R2 = 0.54). With these results the model explains the 72% of the variability. The low values of correlation suggested possible quadratic effects. Hence, the model was adjusted to the curvature with the aim to optimize the Au recovery. 3.3. Recovery of gold – second order model
Table 5 Analysis of variance (ANOVA) for substrate oxidation. Source
Sum of squares
Degree of freedom
Mean squares
p-value
Model A – [(NH4)2S2O8] B – Oxygen C – L/S Ratio AB AC BC ABC Residual Pure error Corrected total
2719.61 925.38 507.83 73.53 926.59 188.24 4.35 93.70 16.79 12.55 2736.41
7 1 1 1 1 1 1 1 11 10 18
388.52 925.38 507.83 73.53 926.59 188.24 4.35 93.70 1.53 1.26
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate Andrea Alzate a,b,⇑, Maria Esperanza López a, Claudia Serna a a b
GIPIMME Research Group, Department of Materials Engineering, University of Antioquia, CL 67 53-108, Medellin, Colombia Ingeniería, Suministros y Montajes S.A.S, INSUMON S.A.S, CL 36 36-9, Medellín, Colombia
a r t i c l e
i n f o
Article history: Received 3 November 2015 Revised 28 January 2016 Accepted 30 January 2016 Available online xxxx Keywords: Gold recovery Waste electrical and electronic equipment (WEEE) Ammonium persulfate Response surface methodology Recycling
a b s t r a c t This paper presents a novel methodology to recover gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate ((NH4)2S2O8). Gold was recovered as a fine coating using substrate oxidation without shredding or grinding process. The WEEE sample was characterized giving values of Au: 1.05 g/kg, Fe: 86.00 g/kg, Ni: 73.64 g/kg, Cu: 26.65 g/kg. The effect of (NH4)2S2O8 concentration (0.22–1.10 M), oxygen (0.0–1.4 L/min) and L/S ratio (10–30 mL/g) on the main responses (substrate oxidation and Au recovery) was investigated implementing response surface methodology with numerical optimization. A quadratic model was developed and quantities greater than 98% of Au were recovered. The findings presented suggest that, optimized quantities of ammonium persulfate in aqueous highly oxygenated media could be used to extract superficial gold from WEEE. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Non polluting methodologies of valuable metals recovery from secondary sources have been a current topic in several studies because of their low environmental impact. Many of these studies have focused on recovering gold avoiding the hazardous implications of mineral extraction such as water pollution, deforestation and health repercussions. Nowadays, one of the main secondary sources to recover precious (Au, Ag, Pd) and base metals (Cu, Ni, Fe) is waste electrical and electronic equipment (WEEE) (Yazici and Deveci, 2014). The rapid growth in the manufacturing of technological devices has generated large quantities of WEEE. The annual WEEE global growth has been estimated at 8.8% (2004– 2011) and 17.6% (2011–2016) calculating for 2016 a global volume of 93.5 millions of tons (Akcil et al., 2015; Yu et al., 2014). These values expose the adequate final disposal of WEEE as one of the main challenges in WEEE management due to waste of valuable metals (Hadi et al., 2013) and the environmental problems associated with conventional disposal and recycling methods. Conventional disposal methods of WEEE such as landfill, incineration and hydrometallurgical recycling techniques generates soil and water pollutants and harmful substances that are released into
⇑ Corresponding author at: GIPIMME Research Group, University of Antioquia, CL 67 53-108, Medellin, Colombia. E-mail address: [email protected] (A. Alzate).
the air. On the first hand, landfill and incineration produce toxic substances that comprehend Hg, Pd, Cd, dioxins, furans and heavy metals vapors (Yazici and Deveci, 2013). However, developing countries still use landfill and incineration causing public health and environmental risks. Risks of landfill leachate production and air, soil, water or underwater pollution, make these disposal process unsuitable (Hadi et al., 2015a). On the other hand, hydrometallurgical recycling techniques with strong acids (HCl, HNO3, H2SO4/H2O2) or oxidative reagents (cyanide, thiourea, halide, nitrate and iodide) have been suggested by a variety of authors to process WEEE and reach the recovery of gold (Birloaga et al., 2014; Petter et al., 2014; Shibayama et al., 2013). Methods of gold recovery from WEEE with hydrometallurgical techniques focus on primary physical separation (shredding, grinding) and secondary leaching of total metallic fraction (Bas et al., 2014). The metallic fraction (precious and base metals) is obtained after shredding and grinding, which are recognized by dust pollution generation (Naseri Joda and Rashchi, 2012). In addition, grinding fraction is dissolved using strong acids or oxidative reagents (Zhang et al., 2012) and the resulting solution is separated through timeconsuming chemical process (cementation, solvent extraction, precipitation or coagulation) with the aim to recover the metal of interest from the solution (Syed, 2012). Hydrometallurgy extensively use cyanide, thiourea, halides and some strong acids. These agents are recognized by its toxic potential, low chemical stability and environmental impact (Tuncuk et al., 2012).
http://dx.doi.org/10.1016/j.wasman.2016.01.043 0956-053X/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Alzate, A., et al. Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.043
2
A. Alzate et al. / Waste Management xxx (2016) xxx–xxx
Nowadays, strategies on WEEE management have been adopted to mitigate health and environmental problems. The strategies contemplate the development of tools as Material Flow Analysis (MFA), Multi Criteria Analysis (MCA) and Life Cycle Assessment (LCA) (Kiddee et al., 2013). In develop countries, MFA and MCA have been successful applied to estimate the generation of WEEE and take environmental decisions to solve multi-criteria problems on disposal, while LCA has been used in several studies to evaluate the environmental impacts of WEEE (Wäger et al., 2011). Studies on LCA concluded that, compare with landfill or incineration, recycling techniques are more appropriate to manage WEEE (Kiddee et al., 2013). Nevertheless, any recycling process perform without environmental care may produce a highest impact on soil, air, water and humans. Developing suitable and optimal recycling methodologies capable of avoiding pollution, reducing reagents toxicity and time-consuming reactions has become one of the most important topics in WEEE management research. Hence, in recent years, some methodologies that attempt to use environmentally friendly agents and the optimization of gold and base metals recovery from WEEE have been suggested (Barbieri et al., 2010; Ha et al., 2014; Syed, 2006). For instance, alternative agents to extract non-leaching gold that include potassium persulfate (K2S2O8) (Syed, 2006) and cupric chloride (CuCl2) (Barbieri et al., 2010) were studied to recover gold from WEEE. These agents were used to oxidize and leach the metal substrate (Ni, Fe, and Cu) where gold was superficially associated as coating (Barbieri et al., 2010). The partial leaching of the substrate permitted gold recovery in a solid particulate state (Barbieri et al., 2010; Syed, 2006). Due to the elimination of gold leaching, it was possible to reduce reaction time avoiding purification stages and achieving the 98% in Au recovery with minimum formation of contaminant byproducts or total agent regeneration (Barbieri et al., 2010; Syed, 2006). Redox potential for the production of (SO2 4 ) ions in aqueous potassium persulfate was estimated in 2.01 V (Huang et al., 2002) and 0.48 V for the production of (CuCl2) ions in acid solution (Lundström et al., 2009). These estimations exposed the ability of K2S2O8 and CuCl2 to oxidize Ni, Fe and Cu and release gold from the substrate. Despites its great advantages, persulfate and cupric chloride oxidative systems to recover non-leaching gold from WEEE have not been extensively investigated. Selective persulfate oxidation proposed by Syed (2006) catalyzed with oxygen can be optimized in order to maximize the recovery of gold from WEEE reducing parameters like agent consumption and reaction time (Birloaga et al., 2013; Hadi et al., 2015b; Jordão et al., 2016). A way to optimize this process is to use response surface methodology (RSM), a series of statistical techniques. RSM has been adopted in several studies to find optimal region of operation into an experimental design space (Montgomery, 2013). This methodology was reported in WEEE management to determine the greater amount of gold leaching evaluating the incidence of thiosulfate, cooper and ammonia concentration (Ha et al., 2014). Besides, the extraction of Cu, Fe, Ni, Ag and Pd from waste printed circuit boards (WPCBs) was studied adopting RSM to establish the conditions that maximize metals extraction using H2SO4–CuSO4–NaCl solutions (Yazici and Deveci, 2013). In this study, an environmentally friendly methodology to extract non-leaching gold by partial substrate oxidation from WEEE using ammonium persulfate (NH4)2S2O8 was developed. The incidence of (NH4)2S2O8 concentration, oxygen and liquid/solid ratio on the recovery of gold was analyzed using response surface methodology with numerical optimization. The effects of (NH4)2S2O8 concentration (0.22–1.10 M), oxygen (0.0–1.4 L/min) and liquid/solid ratio (10–30 mL/g) over the gold recovery were studied in five different levels through a central composite design (CCD). (NH4)2S2O8 was selected over other environmentally friendly
agents due to the lack of research on its use and optimization in recovering gold from WEEE through substrate oxidation. In addition, the produced persulfate ions (S2O2 8 ) are not absorbed or bio accumulated in the soil after the process (Hernandez, 2005) and the generated by-products (sulfates) have not a negative effect on the environment (Syed, 2006). Comparing with potassium persulfate, sodium persulfate and cupric chloride, (NH4)2S2O8 has a greater leaching power to oxidize base metals than potassium and sodium persulfate (Babu et al., 2002) and is operationally safer and less toxic than CuCl2. The toxicity reduction responds to the absence of Cl2 (g) during the reaction. In brief, the system of methods applied was carried out without shredding or grinding stages which reduced secondary dust pollution, reaction time and contaminant by-products. This study aimed at maximizing gold recovery from WEEE with a novel methodology that includes the partial oxidation of metal substrate with ammonium persulfate in oxygenated media and the numerical optimization of the most significant parameters. 2. Experimental 2.1. Materials and reagents Intel Celeron–Pentium electronic processor scrap from end-oflife computers supplied by a local recycling company was the sample used (Fig. 1). A total amount of 50 processors were selected by reference, shape, weight, superficial distribution of gold and manufacturer to ensure sample homogeneity and statistical significance. Intel Celeron and Intel Pentium processors of 5 5 cm2 and an average weight of 8.97 g (Fig. 1) were used for chemical characterization, substrate oxidation and gold recovery tests without shredding or grinding stages. A sample of 35 g was used to determine the amount of gold and metal substrate (Fe, Ni, Cu) by chemical digestion using aqua-regia (Lee et al., 2011; Petter et al., 2014) followed by microwave plasma – atomic emission spectroscopy (MP-AES, AGILENT 4100) (Table 1). After chemical characterization, aqueous commercial grade ammonium persulfate (P98% (NH4)2S2O8) with a water solubility of 850 g/L at 25 °C (Hernandez, 2005) was the selected environmental reagent used to produce sulfate ions (SO 4 ), which partially oxidized the metal substrate breaking the Au–Cu–Ni–Fe bond and allowing gold to be extracted in its original non-leaching state.
Fig. 1. Processor scrap sample (ref. Intel Celeron–Intel Pentium).
An oxygen tank (99.9% O2) with flow control was used to deliver O2 and catalyze the oxidative reactions.
Please cite this article in press as: Alzate, A., et al. Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.043
3
A. Alzate et al. / Waste Management xxx (2016) xxx–xxx Table 1 Chemical composition of the processor scrap. Element
Fe
Ni
Cu
Ag
Au
Content (g/kg)
86.00
73.64
26.65
a) had not effect in the response. The calculated standard deviation for the model was 1.24%. The regression model that describes the substrate oxidation from processor scrap with (NH4)2S2O8 was established calculating the coefficients for each factor and developing a first order equation (Eq. (4)).
(FeSO4)+ and (NiSO4 (a)) (Medusa software, 2010). The equilibrium constant for this sulfates were established in (Log K(95°C) = 3.44) and (Log K(95°C) = 3.06) respectively (HSC Chemistry software, 2002). The coefficient of determination (R2) was estimated in 0.99 proving that the model explains the 99% of the variability in the response. Oxidation and leaching of the substrate were achieved by the diffusion of (SO2 4 ) and (O2) into the pines of the processors penetrating the solid sample from welding points to the core conformed by Fe–Ni–Cu (Fig. 2a). Oxidative dissolution of substrate took place breaking the Fe–Ni–Cu–Au bond without leaching of Au from the sample. Release of solid particulate gold was achieved after removing the sample from the reactor and applying a micro scale pressure washing (Fig. 2b).
Substrate oxidation ¼ 21:46 7:61A þ 5:63B þ 2:14C
A linear model for the release of solid gold from the sample as a response of the substrate oxidation was analyzed with the 23 factorial design. An empirical first order equation was developed (Eq. (5)).
7:62AB þ 3:43AC 0:53BC þ 2:42ABC
ð4Þ
Eq. (4) shows that B and C were the most effective linear factors, while A was the less effective linear factor. Positive sign of B and C indicates that the increment of the oxygen and L/S ratio positively influenced the oxidation and leaching of the substrate. This could be explained by the preferential formation of soluble oxides include CuFeO2 (Medusa software, 2010) with an equilibrium constant greater than zero (Log K(95°C) = 4.45) (HSC Chemistry software, 2002). The negative sign of A indicates a negative linear influence in the response. However, the interaction between [(NH4)2S2O8] – L/S ratio (AC) has a positive influence. This influence demonstrates that (NH4)2S2O8 needs a careful balance between water added and quantity of scrap to enable the speciation of 2 (SO2 4 ) and to oxidize the substrate. The (SO4 ) ions leached the metallic nickel and iron by formation of soluble sulfates includes
3.2. Recovery of gold – first order model
Au Recovery ¼ 89:61 þ 3:30A þ 3:63B þ 4:61C 0:67AB þ 1:30AC 1:92BC þ 1:37ABC
ð5Þ
All the linear factors and the interaction AC and ABC had a positive influence on the Au recovery. The calculated standard deviation of the data was 5.52%. The linear model equation sets out to obtain high values of Au recovery through coefficients of determination close to 100%. However, the estimated coefficient of determination was (R2 = 0.72) and the adjusted determination was (Adj-R2 = 0.54). With these results the model explains the 72% of the variability. The low values of correlation suggested possible quadratic effects. Hence, the model was adjusted to the curvature with the aim to optimize the Au recovery. 3.3. Recovery of gold – second order model
Table 5 Analysis of variance (ANOVA) for substrate oxidation. Source
Sum of squares
Degree of freedom
Mean squares
p-value
Model A – [(NH4)2S2O8] B – Oxygen C – L/S Ratio AB AC BC ABC Residual Pure error Corrected total
2719.61 925.38 507.83 73.53 926.59 188.24 4.35 93.70 16.79 12.55 2736.41
7 1 1 1 1 1 1 1 11 10 18
388.52 925.38 507.83 73.53 926.59 188.24 4.35 93.70 1.53 1.26
