Cupric chloride leaching (HCl–CuCl2–NaCl) of metals from waste printed circuit boards (WPCBs)

    Cupric Chloride Leaching (HCl-CuCl 2 -NaCl) of Metals from Waste Printed Circuit Boards (WPCBs) E.Y. Yazici, H. Deveci PII: DOI: Reference:

S0301-7516(14)00163-X doi: 10.1016/j.minpro.2014.10.012 MINPRO 2675

To appear in:

International Journal of Mineral Processing

Received date: Revised date: Accepted date:

24 February 2014 16 September 2014 27 October 2014

Please cite this article as: Yazici, E.Y., Deveci, H., Cupric Chloride Leaching (HClCuCl2 -NaCl) of Metals from Waste Printed Circuit Boards (WPCBs), International Journal of Mineral Processing (2014), doi: 10.1016/j.minpro.2014.10.012

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ACCEPTED MANUSCRIPT Cupric Chloride Leaching (HCl-CuCl2-NaCl) of Metals from Waste Printed Circuit Boards (WPCBs)

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E. Y. Yazici * and H. Deveci

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Div. of Mineral & Coal Processing, Dept. of Mining Eng., Karadeniz Technical University, 61080, Trabzon, Turkey * Corresponding author ([email protected]) Tel: +90 462 377 3681

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Fax: +90 462 325 7405

Abstract The leaching of metals from waste printed circuit boards (WPCBs) in cupric chloride

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solutions (HCl-CuCl2-NaCl) was studied. Effects of initial concentration of cupric (8-157 mM Cu2+), chloride (0.7-2.0 M NaCl), temperature (20-80°C), solids ratio (1-15 w/v) and air/oxygen (1 L/min) on the leaching of copper as well as other metals (i.e. Fe, Ni, Ag, Pd and

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Au) was investigated. The findings have shown that increasing the initial concentration of

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Cu2+ leads to a significant enhancement in the metal extractions. At Cu2+ concentrations of ≥79 mM, high extractions (i.e. ≥98%) for copper, nickel and iron were obtained over 120 min.

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An increase in the concentration of chloride (i.e. from 0.7 M to 2 M Cl-) decreased the extraction of copper by up to 40% at 60 min due to the decrease in the activity of free Cu2+. Rising the temperature from 20°C to 80°C resulted in a 1.38-fold increase in the copper

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extraction at 120 min. The activation energy (Ea) for leaching of copper was calculated to be ≈41 kJ/mol, indicative of a chemically controlled leaching reaction. Air/oxygen (1 L/min) was supplied for the regeneration of Cu2+, which improved the extraction of all the metals monitored. Increasing the solids ratio (1-15 w/v) adversely affected the leaching process with no copper extraction at 15 w/v. However, the extraction of metals (Cu, Ni, Ag and Pd) substantially ameliorated in the presence of oxygen. The current study have shown that copper, palladium, nickel and silver can be readily extracted from WPCBs in cupric chloride leaching where the extraction of gold is very limited.

Key words: Waste electrical and electronic equipments (WEEE), cupric chloride leaching, copper, precious metals, hydrometallurgy, recycling.

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ACCEPTED MANUSCRIPT 1. INTRODUCTION Owing to the diminution of ever increasing environmental concerns, recovery of metals from

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waste materials has gained particular attention in recent years. Treatment of waste electrical

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and electronic equipments (WEEE) with the continuous rise in their amount (e.g. an increase by ~11% between 2008 and 2014 is expected in the EU (Beck, 2012)), has become of great interest due to its pollution potential associated with its content of hazardous inorganic metals

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such as Hg, Pb and Cd, and organic substances such as chlorinated/brominated flame retardants (Widmer et al., 2005; McPherson, 2005; Yazıcı et al., 2010). On the other hand,

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WEEE offers a valuable resource with its significant copper, gold, silver and palladium content (Widmer et al., 2005; Deveci et al., 2010). Due to the environmental issues and

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economic potential, the European Union (EU) as well as many non-EU countries including

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Turkey have introduced directives to manage WEEE streams through collection and

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recycling/recovery processes (Widmer et al. 2005; Arcan 2012; EU 2012).

In recent years, hydrometallurgical treatment of WEEE has received significant attention as a

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viable alternative particularly for low-grade waste (i.e. <100 g/t Au) and for small scale applications (Deveci et al., 2010; Tuncuk et al., 2012). Various reagent systems including sulphate, chloride, nitrate, iodide, ammonia, cyanide, thiourea and thiosulphate medium have been tested for the recovery of metals from WPCBs (Cui and Zhang, 2008; Tuncuk et al., 2012; Birloaga et al., 2013; Yazıcı, 2012). As metals in WPCBs are present in native and/or alloy form, the development of oxidative leaching processes using an oxidant such as H2O2, O2 and Fe3+ is required. In chloride media, cupric (Cu2+) can also be used as an oxidant. Some researchers employed chloride media for the extraction of metals from WEEE in the presence of various oxidants including HNO3 (Quinet et al., 2005; Sheng and Etsell, 2007; Park and Fray, 2009; Luyima et al., 2011), H2O2 (Quinet et al., 2005) and electro-generated Cl2 (Kim et 2

ACCEPTED MANUSCRIPT al., 2010). However, to the authors' knowledge, chloride leaching of WPCBs in the presence of Cu2+ has received limited attention. Yazici and Deveci (2013) investigated the addition of

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chloride into acidic copper sulphate solution for the extraction of metals from WPCBs. In a

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recent study, Ping et al. (2009) studied the extraction of copper from WPCBs in a

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sulphate/chloride media (H2SO4-CuSO4-NaCl); but, they presented no data on the leaching behaviour of precious metals (i.e. Ag, Au and Pd) despite the appreciable content of precious

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metals of the sample used.

To date, several chloride leaching processes including CLEAR, INTEC, CYMET and

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HydroCopper® employing cupric as an oxidant, have been tested at the lab/pilot scale for the treatment of copper concentrates (Dreisinger, 2006). HydroCopper® process (at pilot scale) is

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a more recent technology developed to leach copper from chalcopyrite concentrates in HClCuCl2-NaCl solutions (Lundström, 2009; Peacey et al., 2004). Chloride solutions offer several

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advantages over sulphate media such as higher solubility and activity of metals (Demopoulos, 1998; Muir, 2002; Senanayake and Muir, 2003). Other common advantages and

Table 1.

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disadvantages of cupric chloride leaching over other leaching systems are listed in Table 1.

In cupric chloride solutions, the dissolution of metallic copper takes place according to Eq. 1 where the role of chloride ion is to stabilise cuprous (Cu+) in the form of CuCln1-n (1≤n≤4) (Eq. 2) and hence, activate Cu2+/Cu+ couple. To prevent the precipitation of Cu+ in the form of CuCl (Ksp=1.86 x 10-7) (Eq. 3), a high level of chloride should be maintained in the leaching medium (Eqs. 4 and 5). Cupric copper may also present as chloro-complexes (CuCln2-n) in chloride solutions (Muir, 2002; Senanayake and Muir, 2003; Winand, 1991).

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ACCEPTED MANUSCRIPT Chloride salts in the form of NaCl, CaCl2 or MgCl2 could be added into the solution to

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increase the concentration of chloride (Jackson, 1986; Habashi, 2009; Muir, 2002).

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Cu0+ Cu2+ → 2Cu+

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Cu+ + nCl- → CuCln1-n

(1) (2)

(ΔG0(20°C)= -41 kJ/mol)

(3)

Cu0+ Cu2+ + 4Cl- → 2CuCl2-

(ΔG0(20°C)= -25 kJ/mol)

(4)

Cu0+ Cu2+ + 6Cl- → 2CuCl32-

(ΔG0(20°C)= -31 kJ/mol)

(5)

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Cu0+ Cu2+ + 2Cl- → 2CuCl(s)

Chloride leaching in highly concentrated solutions offers fast leaching kinetics under atmospheric conditions due to the increased proton activity and the formation of stable metal

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chloride complexes (Table 1). None the less, the increase in chloride concentration of the solution elevates the potential of Cu2+/Cu+ couple due to the increase in the stability constants

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of Cu+-Cl species over Cu2+-Cl species particularly at >0.1 M Cl- (Muir, 2002; Senanayake and Muir, 2003). Adversely, Fe3+ forms more stable complexes with chloride than that of

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Fe2+, which reduces the potential of Fe3+/Fe2+. This suggests that, particularly at high chloride concentrations of ≥~4.2 M, Cu2+ is a more potent oxidant than Fe3+ (Demopoulos, 1998). An increase in the concentration of chloride also leads to lower reduction potentials of base/precious metals (e.g. Cu, Au, Ag, Pd) excluding nickel (Winand, 1991) e.g. the potential of palladium (Pd2+/Pd0) decreases from 0.99 V (E0) to 0.62 V (for PdCl42-/Pd0 at aCl=1) (Puvvada et al., 2003). It can be inferred that, chloride solutions allow more readily extraction of metals even at milder oxidising conditions and Cu2+ as an oxidant become more effective than Fe3+ with the advantage of avoiding excess contamination of copper leach solutions by iron.

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ACCEPTED MANUSCRIPT In this study, leaching of copper as well as other metals (Fe, Ni, Ag, Pd and Au) from waste printed circuit boards (WPCBs) in HCl-CuCl2-NaCl media was studied. The effect of initial

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concentration of Cu2+ (8-157 mM), NaCl (0.7-2.0 M), temperature (20-80°C) and solids ratio

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was also tested in the absence and presence of 8 mM Cu2+.

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(1-15% w/v) on the extraction of metals was investigated. The effect of air/oxygen (1 L/min.)

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2. EXPERIMENTAL

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2.1. Preparation of WPCB Sample

WPCBs were collected from end-of-life computers of various brands. Following the manual

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separation of board components such as capacitors, resistors, transistors and cables, WPCBs

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were reduced in size within a two-stage crushing operation, first to -3.35 mm using a fourbladed rotary cutting shredder and, then, to -1 mm using a laboratory type rotary cutting mill

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(Thomas-Wiley Laboratory Mill Model 4). The shredded sample was further ground to -250 m using an ultra-centrifugal mill (Retsch ZM 200) prior to use in the leaching tests. Wet

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chemical analysis using hot aqua regia digestion was carried out to determine the metal content of the sample (Table 2). An atomic absorption spectrometer (AAS, Perkin Elmer AAnalyst 400) was used for the analysis of Cu, Fe, Ni, Ag, Pd and Au.

Table 2.

2.2. Leaching Tests In the leaching tests, flat-bottomed and baffled jacketed glass reactors with a nominal capacity of 750 mL were used. The reactors were connected to a water circulator (Polyscience) to maintain the desired temperature for leaching. Mechanical stirrers (IKA EURO-ST) equipped

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ACCEPTED MANUSCRIPT with four-blade, PTFE-coated, downward pumping 45° pitched blade impellers in 6.5 cm diameter were used for mixing the reactor contents. Rotational speed was set at 675 rpm (tip

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speed: 2.3 m/s). Top of the reactors were kept closed by plexiglass covers to avoid

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evaporation losses over the leaching period. Air or oxygen at the flowrate of 1 L/min was

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blown into the reactors using polyethylene (PE) spargers from an air pump or an oxygen cylinder (99.9% O2).

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Hydrochloric acid solution (37% HCl), reagent grade copper(II) chloride (CuCl2.2H2O) and

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sodium chloride (NaCl) were used as the chemicals. Initial concentration of HCl was set at 1 M in all the experiments. Leach solutions in a final volume of 500 mL were prepared at the required strength of 8-157 mM Cu2+ and 0.7-2.0 M NaCl using the stock solutions of 780 mM

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Cu2+ and 4.2 M NaCl. Deionised-distilled water was used for preparation of all the solutions. After temperature was set to the required level (20-80°C), the WPCB sample (5 g) was added

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to the leach solution to produce a solids density of 1% w/v, unless otherwise stated. During the leaching, 3-4 mL samples were taken at the predetermined intervals of 5, 15, 30, 60 and

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120 min. These samples were then centrifuged at 4100 rpm for 5 min and supernatants were analysed for copper by AAS. All the other metals were analysed only in the final sample (120 min) and the residues. pH and redox potentials (EAg/AgCl, mV) were also recorded from the supernatant solutions. On the completion of the leaching tests over a period of 120 min, the residues were separated on a Buchner filter using a vacuum pump, dried in an oven at 105°C for 3 h and subsequently, digested in hot aqua regia for analysis of metals by AAS. Metal extractions were calculated based on the metal content of the residues.

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ACCEPTED MANUSCRIPT 3. RESULTS AND DISCUSSION 3.1. Effect of initial Cu2+concentration

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Fig. 1a illustrates the effect of initial concentration of 8-157 mM Cu2+ on the leaching of

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copper as well as other metals under the experimental conditions of 1.3 M NaCl and 80°C. In

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the absence of Cu2+, only 14.7% extraction for copper occurred over the leaching period of 120 min (Fig. 1a). Increasing the initial concentration of Cu2+ from 8 to 31 mM, lead to a

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significant enhancement in the rate and extent of copper extraction from 33% to 100% over 120 min. For complete extraction of copper at 120 min, concentration of Cu2+ should be kept

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at ≥31 mM (Fig. 1a). In a previous study (Herreros et al., 2005), the enhancing effect of initial concentration of 350-820 mM Cu2+ on leaching of copper powders in a sulphate/chloride

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media (HCl-CuSO4-NaCl) was reported. Consistent with the findings (Fig. 1a), increasing the

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initial concentration of Cu2+ led to higher redox potentials (Fig. 1b) due to higher Cu2+/Cu+

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ratios, which resulted in higher extractions for copper. After a sharp decrease at 5 min, the

Fig. 1.

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redox potentials were almost stabilised during the following leaching period (Fig. 1b).

Table 3.

Relatively higher extractions for iron and nickel than that of copper (Fig. 1a, Table 3) were observed at ≤31 mM Cu2+ concentrations e.g. 49-82% for Ni and Fe cf. 33% Cu at 8 mM Cu2+ over 120 min. Almost complete extractions (≥98%) for iron and nickel were also readily achieved at Cu2+ concentrations of ≥79 mM over 120 min (Table 3).

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ACCEPTED MANUSCRIPT Extractions for precious metals, palladium and silver in particular, which were lower than those of Cu, Fe and Ni were also improved by the increase in the initial concentration of Cu2+

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(Table 3). It should be noted that, in contrast to gold (Eqs. 6-7), the dissolution of palladium

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and silver in cupric chloride solutions is thermodynamically favourable only at high

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concentrations of chloride (Eqs. 10-13). In this regard, high extractions for palladium and silver (i.e. 90% Pd and 99% Ag) were obtained; albeit, gold extraction was limited to only

(ΔG0(80°C) = +52 kJ/mol)

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 AuCl2- + CuCl2Au0 + Cu2+ + 4Cl- 

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14.4% at the maximum level of 157 mM Cu2+ (Table 3).

(6)

 AuCl4- + 3CuCl2- (ΔG0(80°C) = +695 kJ/mol) Au0 + 3Cu2+ + 10Cl- 

(7)

 PdCl+ + 2CuCl2Pd0 + 2Cu2+ + 5Cl- 

(8)

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(ΔG0(80°C) = +59 kJ/mol)

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 PdCl2 (aq) + 2CuCl2Pd0 + 2Cu2+ + 6Cl-   PdCl3- + 2CuCl2Pd0 + 2Cu2+ + 7Cl- 

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 PdCl42- + 2CuCl2Pd0 + 2Cu2+ + 8Cl- 

(ΔG0(80°C) = +79 kJ/mol) (ΔG0(80°C) = -14 kJ/mol) (ΔG0(80°C) = -11 kJ/mol)

(9) (10) (11)

 AgCl2- + CuCl2- (ΔG0(80°C) = -8.9 kJ/mol) Ag0 + Cu2+ + 4Cl- 

(12)

 AgCl32- + CuCl2Ag0 + Cu2+ + 5Cl- 

(13 )

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(ΔG0(80°C) = -4.7 kJ/mol)

3.2. Effect of NaCl concentration Fig. 2 shows that the increase in the concentration of chloride (0.7-2.0 M NaCl) slowed down the leaching of copper. To illustrate, a complete extraction of copper was achieved over 60 min at 0.7 M NaCl, compared with 60% extraction at 2 M NaCl over the same period. Notwithstanding this, the extraction for copper at ≥1.3 M NaCl proceeded to almost completion (≥98%) over an extended period of 120 min. (Fig. 2). Herreros et al. (2005) also

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ACCEPTED MANUSCRIPT reported the negative effect of increasing chloride level in the range of 0.56-1.43 M Cl- on the rate and extent of dissolution of metallic copper. They attributed this effect to the decrease in

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the activity of free Cu2+ within the medium. In this regard, thermodynamic calculations using

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Visual MINTEQ (2012) software corroborated that the activity of free Cu2+ decreased from

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12 to 7.1 mM with increasing the concentration of Cl- from 0.7 to 2.0 M under the experimental conditions ([HCl]: 1 M, [Cu2+]Total: 31 mM, 80°C). On the other hand, increasing the concentration of chloride leads to a decrease in the standard reduction

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potentials (Eo) of redox couples (Mez+/Me0) (Table 4) due to the formation of strong metal chloride complexes (Eq. 14). It can be inferred from Eqs. 14-16 that an increase in stability

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constant of a metal chloride complex (  n ) or in activity of chloride lowers the reduction o

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potentials (Yévenes, 2009). This suggests that the extraction of metals is possible under

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milder oxidising conditions (i.e. lower redox potentials) in chloride solutions (Puvvada et al., 2003). Therefore, chloride level is critically important for leaching of metals. It is pertinent to

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note that, increasing the concentration of Cl- induced no significant deviation (RSD: ≤4.9%) in the redox potentials. Although a large drop (by ~246 mV) in the redox potentials was

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observed after an initial period of 5 min, the potentials remained at the same level of ~236 mV till the end of the leaching process.

 MeCln (z-n) + nH2O (Me: Metal) Me(H2O)nz+ + nCl- 

 Me0 + nClMeCln (z-n) + ze- 

o EMeCl zn / Me  EMe  2,303 z / Me n

o  n

aMeClnzn aMez aCln 

(14)

(15)

o n RT  n aCl   log  zF  aMeClnzn 

Fig. 2 9

(16)

ACCEPTED MANUSCRIPT Iron and nickel were almost completely extracted (i.e. ≥95.2%) even at 0.7 M NaCl over 120 min (Table 3). Increasing the concentration of NaCl from 0.7 to 2.0 M had no apparent

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influence on the extraction of silver while the extraction of palladium significantly improved

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from 9% to 43% (Table 3). The enhancement in the palladium extraction can be attributed to

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the fact that the increase in the Cl- allows the formation of more stable palladium chloride complexes compared to that of other metals (Smith et al., 2004). No dissolution of gold occurred under these experimental conditions (Table 3). It can be inferred from these findings

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WPCBs leaving Pd and Au in the residue.

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that, at low concentration of chloride, Cu, Fe, Ni and Ag can be selectively extracted from

It should be also noted that precipitation of metal chlorides (e.g. CuCl, AgCl, AuCl3, PdCl2)

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(Eq. 3 and Eqs. 17-19) occurs at low chloride concentrations (Muir, 2002; Senanayake and Muir, 2003). In this regard,

speciation calculations using MEDUSA (2010) and Visual

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MINTEQ (2012) confirmed that no precipitation of these metals would occur even at the lowest free chloride concentration (i.e. [Cl-]:0.7 M) apparently due to the sufficiently high

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chloride to metal ratio maintained.

 AgCl (s) Ag+ + Cl- 

(ΔG0(20°C)= -56 kJ/mol)

(17)

 AuCl3 (s) (ΔG0(20°C)= -91 kJ/mol) Au3+ + 3Cl- 

(18)

 PdCl2 (s) Pd2+ + 2Cl- 

(19)

(ΔG0(20°C)= -40 kJ/mol)

3.3. Effect of temperature

Fig. 3 illustrates the effect of temperature (20-80°C) on the extraction of copper. A significant improvement in the rate by 83-fold and extent by 46% of copper extraction was observed with increasing the temperature from 20 to 80°C (Fig. 3). The activation energy (Ea) was calculated

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ACCEPTED MANUSCRIPT by Arrhenius equation to be ≈41 kJ/mol indicating that the dissolution process is chemically controlled (Robertson et al., 2005). At all the temperatures tested, the redox potentials (RSD:

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≤4.4%) decreased from ~501 mV to ~280 mV after 5 min and remained at these levels.

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The temperature had a limited influence on the extractions of iron and nickel over 120 min. (Table 5) as 93% Fe and 87% Ni were already achieved at 20°C. However, the increase in the temperature enhanced the extraction of silver from 80% (20°C) to 98% (80°C) over 120 min

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(Table 5). Palladium extractions were comparatively lower than those of silver i.e. ≤68% Pd

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vs. ≤98% Ag (Table 5). Gold remained in the residues under these conditions. Fig. 3

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3.4. Effect of air and oxygen

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Table 5.

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In chloride leaching, cupric (Cu2+) should be continuously supplied or rather regenerated to maintain highly oxidising conditions suitable for leaching of metals since it is consumed through Eqs. 3-5 as the leaching progresses. In this regard, air or oxygen (1 L/min) was

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introduced into the leaching media as the oxidants for in-situ regeneration of Cu2+ to achieve high metal extractions. With the aid of air/oxygen, a high ratio of Cu2+/Cu+ (i.e. high redox potential) can be maintained through the oxidation of Cu+ to Cu2+ (Eq. 20). In the presence of air/oxygen, Fe3+/Fe2+ couple would also contribute, to some extent, to the leaching process.

 4Cu2+ + 2H2O 4Cu+ + O2 (aq) + 4H+ 

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(ΔG0(80°C)= -99.8 kJ/mol)

(20)

ACCEPTED MANUSCRIPT It is relevant to note that in the presence of air/oxygen at
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1976):

 12CuCl2 (aq) + 6H2O 12CuCl + 12HCl + 3O2 (aq) 

(21)

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(ΔG0(80°C)= -58.1 kJ/mol)

Since the above reaction is acid consuming, pH tends to rise. If pH reaches above 1 this leads

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to the rapid oxidation of ferrous chloride (Eq. 22):

(aq)

(ΔG0(80°C)= -69.2 kJ/mol)

(22)

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 2FeO.OH + 4FeCl3 6FeCl2 (aq) + 3/2 O2 (aq)+ H2O 

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At >pH 1 ferric is unstable and precipitates as goethite and/or oxide, which generates acid (Eqs. 23 and 24). The occurrence of acid consuming (Eq. 21) and generating reactions (Eqs.

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23 and 24) in cupric chloride solutions leads to the stabilisation of pH.

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 FeO.OH + 12HCl FeCl3 (aq) + 8H2O 

 Fe(OH)3 + HCl FeCl3 (aq) + 3H2O 

(ΔG0(80°C)= -72.1 kJ/mol)

(ΔG0(80°C)= -46.2 kJ/mol)

(23)

(24)

In cupric chloride system, oxidation of cuprous chloride to cupric chloride is promoted by the increase in acidity resulting from precipitation of goethite precipitation (Eq. 25). pH should be kept at below 3 to minimise formation of copper oxychloride (Demarthe et al., 1976).

 6FeO.OH + 12CuCl2 (aq) 6FeCl2 (aq)+ 12CuCl + 9/2O2 (aq)+ 3H2O 

(ΔG0(80°C)= -202 kJ/mol)

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(25)

ACCEPTED MANUSCRIPT The effect of air/oxygen on the extraction of copper was tested in the absence (Fig. 4a) and initial presence of Cu2+ (8 mM) (Fig. 4b). The presence of 8 mM Cu2+ was observed to

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increase the rate and extent of copper extraction, particularly at the initial stages, apparently

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due to the initial availability of Cu2+ within the medium (Fig. 4a-b). To illustrate, in the

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presence of oxygen 55% and 83% copper extractions over 15 min were obtained with and without the initial 8 mM Cu2+, respectively. The contribution of oxygen to the rate of copper extraction was inherently higher than that of air, apparently in the absence of Cu2+ e.g. 4.4-

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fold and 1.9-fold faster kinetics in the absence and presence of Cu2+, respectively (Fig. 4a-b). It was shown that almost complete extraction of copper can be achieved over 120 min with

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air/oxygen supply even in the absence of Cu2+. Consistent with these findings in Fig. 4, Ping

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et al. (2009) reported the positive contribution of air on the extraction of copper from WPCBs.

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Fig. 4.

Air/oxygen supply also improved the extent of Fe, Ni, Ag, Au and Pd extractions, which were

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even higher in the presence of initially 8 mM Cu2+ (Table 5). Gold extraction was determined to be 44% at the most aggressive conditions tested i.e. in the presence of oxygen and Cu2+ (Table 5). High extractions for silver (98%) and palladium (90%) can be readily achieved with oxygen supply even in the absence of Cu2+. The redox potential profiles (Fig. 5) were consistent with the corresponding leaching data in that the introduction of air and oxygen, in particular, allowed the maintenance of higher redox potentials apparently due to higher Cu2+/Cu+ ratios. It is pertinent to note that, the oxidation of gold, palladium and silver by dissolved oxygen is also thermodynamically favourable (Eqs. 27-35) consistent with these findings. It can be inferred that introduction of air or preferably oxygen is required for achieving high extractions in cupric chloride leaching of metals.

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(26)

 4AuCl2- + 2H2O (ΔG0(80°C) = -18 kJ/mol) 4Au0 + O2 (aq) + 4H+ + 8Cl- 

(27)

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 4AuCl4- + 6H2O (ΔG0(80°C) = +483 kJ/mol) 4Au0 + 3O2 (aq) + 12H+ + 16Cl- 

(28)

 2PdCl+ + 2H2O 2Pd0 + O2 (aq) + 4H+ + 2Cl- 

(29)

(ΔG0(80°C) = -62 kJ/mol)

(ΔG0(80°C) = -127 kJ/mol)

 2PdCl42- + 2H2O 2Pd0 + O2 (aq) + 4H+ + 8Cl- 

(ΔG0(80°C) = -152 kJ/mol)

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 2PdCl3- + 2H2O 2Pd0 + O2 (aq) + 4H+ + 6Cl- 

(ΔG0(80°C) = -82 kJ/mol)

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 2PdCl2 (aq)+ 2H2O 2Pd0 + O2 (aq) + 4H+ + 4Cl- 

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 4AuCl32- + 2H2O (ΔG0(80°C) = -16 kJ/mol) 4Au0 + O2 (aq) + 4H+ + 12Cl- 

(ΔG0(80°C) = -79 kJ/mol)

 4AgCl2- + 2H2O 4Ag0 + O2 (aq) + 4H+ + 8Cl- 

(30) (31) (32)

(33)

(ΔG0(80°C) = -75 kJ/mol)

(34)

 4AgCl32- + 2H2O 4Ag0 + O2 (aq) + 4H+ + 16Cl- 

(ΔG0(80°C) = -63 kJ/mol)

(35)

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Fig. 5

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 4AgCl32- + 2H2O 4Ag0 + O2 (aq) + 4H+ + 12Cl- 

3.3. Effect of solids ratio

The effect of solids ratio (1-15% w/v) on the metal extractions is illustrated in Fig. 6. These tests were carried out under the experimental conditions of 157 mM Cu2+, 1.3 M NaCl and 80°C. Increasing solids ratio from 1% to 15% w/v had a detrimental effect on metal extractions, particularly for copper i.e. 99% extraction at 1% w/v compared with no extraction at 15 w/v over the same period of 120 min (Fig. 6). Iron and nickel extractions were also decreased from 99% (1% w/v) to 80% and 28% (15% w/v), respectively. The release of silver and palladium was negligible (i.e.  1.8%) at 15 w/v. The adverse effect of solids concentration was also reported by Ping et al. (2009). The researchers investigated the

14

ACCEPTED MANUSCRIPT extraction of copper from WPCBs in H2SO4-CuSO4-NaCl media and showed that a 3-fold increase in solids ratio from 1:30 to 1:10 g/mL significantly prolonged the period for

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complete extraction.

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Fig. 6

In order to improve the metal extractions at 15 w/v solids ratio, oxygen at 1 L/min was

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introduced into the medium sustaining stronger oxidising conditions e.g. 436 mV with O2 c.f. 82 mV with no O2 at 120 min. A significant improvement in the extraction of copper and

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nickel was observed in the presence of oxygen i.e. from zero to 90% and from 28% to 97% for copper and nickel, respectively (Fig. 6). Extraction of silver and palladium were also

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enhanced with oxygen supply, i.e. from ≤1.8% to 64% and 29%, respectively, whilst only

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4.1% of gold was extracted even in the presence of oxygen (Fig. 6).

In contrast to the other metals, iron extraction in the presence of oxygen was remarkably low with only 1.3% (Fig. 6) compared with 80% in the absence of oxygen supply. This can be

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attributed to the increase in pH to 3.10 on the termination of the experiment apparently due to the acid consuming nature of the metal dissolution reactions in the presence of oxygen (Eqs. 20-21 and 26-35). Iron would be present in its oxidised form (i.e. Fe3+) under oxygen supply and hence, may precipitate in the form of jarosite, hydroxide and/or goethite at ≥pH 1.5-2.5 and ≥80°C (Gupta and Mukherjee, 1990).

It is also pertinent to note that, white precipitates were observed after cooling of the pregnant leach solution obtained at 15% w/v solids. After filtering and drying for 3 h at 105°C, SEMEDS analysis of the precipitates was performed. These needle-shape precipitates were determined to be PbCl2 crystals (Fig. 7). It is well known that, solubility of lead-chloride

15

ACCEPTED MANUSCRIPT complexes (PbCln2-n, n=1-4) is strongly dependent on the temperature i.e. at low temperatures lead precipitates as PbCl2 (Ksp=1.66 x 10-5) (Habashi, 1999; Jackson, 1986).

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Fig. 7

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4. CONCLUSIONS

Cupric chloride leaching (HCl-CuCl2-NaCl) of metals from waste printed circuit boards was

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reported. Increasing the initial concentration of Cu2+ to ≥79 mM substantially improved the extraction of metals with almost complete extractions (≥98%) for copper, silver, nickel and

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iron, and 68-90% for palladium (at 80°C) over a leaching period of 120 min. Gold extraction was limited to only 14.4% even at 157 mM Cu2+. Increasing the addition of chloride (>0.7 M

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NaCl) adversely affected the kinetics of copper extraction i.e. complete extraction at 0.7 M

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NaCl c.f. 61% at 2.0 M NaCl over 60 min. This was attributed to the decrease in the activity

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of free Cu2+. The extraction of palladium improved from 9 to 43% with increasing the concentration of chloride from 0.7 M to 2 M NaCl presumably due to the formation of more stable palladium chloride complexes. Increasing the temperature from 20 to 80°C was found

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to remarkably enhance the leaching rate and extent of copper i.e. a 46% increase in copper extraction over 30 min. Furthermore, the positive effect of temperature (20-80°C) on the extraction of silver and palladium was also observed by 18% and 35% improvements, respectively. An activation energy (Ea) of ≈41 kJ/mol indicated that leaching of copper is a chemically controlled reaction. The introduction of air/oxygen significantly enhanced the metal extraction with complete extraction of copper over 120 min even in the absence of initial Cu2+ (8 mM). Increasing the solids ratio from 1% to 15 w/v resulted in a severe reduction (e.g. from 99% to none for copper) in the extraction of metals. It can be inferred from the current study that the supply of air or preferably oxygen is required for the in-situ

16

ACCEPTED MANUSCRIPT (re)generation of Cu2+ to maintain high oxidising conditions and achieve high metal extractions particularly at high solids ratios.

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Acknowledgements

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The authors would like to express their sincere thanks and appreciations to The Scientific and Technological Research Council of Turkey (TUBITAK) (Project no: 109M111) and Research Foundation of Karadeniz Technical University (Project No's: 889 and 8647) for their financial

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support and to Res. Assistant Ahmet Deniz Baş (KTU) for his help during the preparation of

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the samples.

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REFERENCES

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Arbiter, N., Fletcher, A.W., 1994. Copper hydrometallurgy - evolution and milestones. Mining Engineering 46, 118–123.

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Arcan, M.Ş., 2012. Directive of waste electrical and electronic equipments. Waste Management Symposium, 24-26 April, Antalya, Turkey (in Turkish). Beck, M., 2012. Mixed feelings about revised WEEE directive. Recycling International, March, 30–35.

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Birloaga, I., De Michelis, I., Ferella, F., Buzatu, M., Vegliò, F., 2013. Study on the influence of various factors in the hydrometallurgical processing of waste printed circuit boards for copper and gold recovery. Waste Manag. 33, 4, 935-941. Cui, J., Zhang, L., 2008. Metallurgical recovery of metals from electronic waste: A review. J. Hazard. Mater. 158, 228–256. Demarthe, J.M., Gandon, L., Georgeaux, A., 1976. A new hydrometallurgical process for copper, in: Extractive Metallurgy of Copper. TMS (The Minerals, Metals & Materials Society), pp. 825–848. Demopoulos, G.P., 1998. Aqueous processing and its role in the production of inorganic materials and environmental protection. Can. Metall. Q. 37, 1–18. Deveci, H., Yazıcı, E.Y., Aydın, U., Akçil, A.U., 2010. Extraction of copper from scrap TV boards by sulphuric acid leaching under oxidising conditions. in: Going Green-CARE INNOVATION. Vienna, Austria (2010), Paper no: 045. Dreisinger, D., 2006. Copper leaching from primary sulfides: Options for biological and chemical extraction of copper. Hydrometallurgy 83, 10–20.

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ACCEPTED MANUSCRIPT EU, 2012. Directive 2012/19/EU of the European Parliament and of the Council of 8 June 2011 on Waste Electrical and Electronic Equipment (WEEE) (recasted), Official Journal L 197, 24/07/2012.

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Gupta, C.K., Mukherjee, T.K., 1990. Hydrometallurgy in Extraction Processes, Vol. II, CRC Press, Boston.

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Habashi, F., 2009. Researches on Copper: History and Metallurgy, Métallurgie Extractive, Québec.

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Han, K.N., 2002. Fundamentals of Aqueous Metallurgy. Society for Mining, Metallurgy, and Exploration (SME). Herreros, O., Quiroz, R., Restovic, A., Viñals, J., 2005. Dissolution kinetics of metallic copper with CuSO4-NaCl-HCl, Hydrometallurgy 77, 183–190.Jackson, E., 1986. Hydrometallurgical Extraction and Reclamation, Ellis Horwood Ltd., UK.

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Kim, E., Kim, M., Lee, J., Jeong, J., Pandey, B.D., 2010. Leaching behavior of copper using electro-generated chlorine in hydrochloric acid solution, Hydrometallurgy 100, 95– 102.

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Lundström, M., 2009. Chalcopyrite Dissolution in Cupric Chloride Solutions. PhD thesis, Helsinki University of Technology, Finland.

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Luyima, A., Shi, H., Zhang, L., 2011. Leaching studies for metals recovery from waste printed wiring boards. JOM 63, 38–41.

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McPherson, D.T., 2005. Hazard Assessment of E-Waste from Desktop Computers. PhD thesis, Jackson State University, USA.

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MEDUSA, 2010. Software for Chemical Equilibrium Diagrams, 32 bit version. Royal Institute of Technology, Sweden. Muir, D.M., 2002. Basic principles of chloride hydrometallurgy, in: Chloride Metallurgy 2002, 32nd Annual Hydrometallurgy Meeting. pp. 759–777.

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Park, Y.J., Fray, D.J., 2009. Recovery of high purity precious metals from printed circuit boards. J. Hazard. Mater. 164, 1152–1158. Peacey, J., Guo, X.-J., Robles, E., 2004. Copper hydrometallurgy - current status, preliminary economics, future direction and positioning versus smelting. Trans. of Nonferrous Met. Soc. of China 14, 560–568. Ping, Z., ZeYun, F., Lie, L., Qiang, L., GuangRen, Q., Ming, Z., 2009. Enhancement of leaching copper by electro-oxidation from metal powders of waste printed circuit board. J. Hazard. Mater. 166, 746–750. Puvvada, G.V.K., Sridhar, R., Lakshmanan, V.I., 2003. Chloride metallurgy: PGM recovery and titanium dioxide production, JOM 55, 38–41. Quinet, P., Proost, J., Van Lierde, A., 2005. Recovery of precious metals from electronic scrap by hydrometallurgical processing routes. Miner. Metall. Process. 22, 17–22. Robertson, S., Jeffrey, M., Zhang, H., Ho, E., 2005. An introductory electrochemical approach to studying hydrometallurgical reactions, Metall. Mater. Trans. B 36, 313– 325. Senanayake, G., Muir, D.M., 2003. Chloride processing of metal sulphides: review of fundamentals and applications, in: C. Young, A. Alfantazi, C. Anderson, A. James, D. Dreisinger, and B. Harris (Eds). Proceedings of the 5th International Symposium 18

ACCEPTED MANUSCRIPT Honoring Professor Ian M. Ritchie, Hydrometallurgy 2003. Pennyslvania, USA: TMS (The Minerals, Metals & Materials Society), 517–531. Sheng, P.P., Etsell, T.H., 2007. Recovery of gold from computer circuit board scrap using aqua regia. Waste Manag. Res. 25, 380–383.

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Smith, R.M., Martell, A.E., Motekaitis, R.J., 2004. NIST Standard Reference Database 46. Critically Selected Constants of Metal Complexes Database, US National Institute of Standards and Technology (NIST), Gaithersburg.

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Tuncuk, A. Stazi, V., Akcil, A., Yazici, E.Y., Deveci, H., 2012. Aqueous metal recovery techniques from e-scrap: hydrometallurgy in recycling. Miner. Eng. 25, 28–37. Visual MINTEQ, 2012. Software for Chemical Equilibrium Models, Jon Petter Gustafsson, KTH, Department of Land and Water Resources Engineering.

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Widmer, R., Oswald-Krapf, H., Sinha-Khetriwal, D., Schnellmann, M., Böni, H., 2005. Global perspectives on e-waste. Environ. Impact Assess. 25, 436–458. Winand, R., 1991. Chloride hydrometallurgy. Hydrometallurgy 27, 285–316.

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Yazıcı, E.Y., Deveci, H., Alp, I., Akcil, A.U., Yazıcı, R., 2010. Characterisation of computer printed circuit boards for hazardous properties and beneficiation studies, in: XXV. Int. Mineral Processing Congress (IMPC 2010), Brisbane, Australia, 4009–4015.

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Yazici, E.Y., Deveci, H., 2013. Extraction of Metals from Waste Printed Circuit Boards (WPCBs) in H2SO4-CuSO4-NaCl Solutions, Hydrometallurgy 139, 30-38.

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Yazıcı, E.Y., 2012. Recovery of Metals from Electronic Wastes using Physical Separation and Hydrometallurgical Methods. PhD thesis, Karadeniz Technical University, Trabzon, Turkiye (in Turkish).

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Yévenes, L., 2009. The Kinetics of the Dissolution of Chalcopyrite in Chloride Media, PhD Thesis, Murdoch University, Australia.

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ACCEPTED MANUSCRIPT LIST of FIGURES Fig. 1. (a) Effect of initial concentration of Cu2+ (8-157 mM) on the leaching of Cu, (b)

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Redox profiles ([HCl]: 1 M, [NaCl]: 1.3 M, 80°C, %1 w/v solids).

Fig. 2. Effect of concentration of chloride (0.7-2.0 M NaCl) on the leaching of copper ([HCl]:

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1 M, [Cu2+]: 31 mM, 80°C, %1 w/v solids).

Fig. 3. Effect of temperature on the extraction of Cu ([HCl]: 1 M, [NaCl]: 1.3 M, [Cu2+]: 79

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mM, %1 w/v solids).

Fig. 4. The effect of air and oxygen (1 L/min) on the leaching of copper in the (a) absence and

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(b) presence of 8 mM Cu2+ ([HCl]: 1 M, [NaCl]: 1.3 M, 80°C, %1 w/v solids). Fig. 5. Redox potential profiles with and without air and oxygen (1 L/min) in the presence of

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8 mM Cu2+ ([HCl]: 1 M, [NaCl]: 1.3 M, 80°C, 1% w/v solids).

Fig. 6. The effect of solids ratio on the extraction of copper and other metals over 120 min

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([HCl]: 1 M, [Cu2+]: 157 mM, [NaCl]: 1.3 M, 80°C).

Fig. 7. (a) SEM photo and (b) EDS profile of PbCl2 crystals filtered from the pregnant leach

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solution after cooling (leach conditions: [HCl]: 1 M, [Cu2+]: 157 mM, [NaCl]: 1.3 M, 80°C, 15% w/v solids).

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(a)

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no Cu(II)

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8 mM Cu(II)

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31 mM Cu(II) 79 mM Cu(II) 157 mM Cu(II)

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Extraction of Cu (%)

100

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20

0 40

60 80 Time in minutes

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20

100

120

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0

350

8 mM Cu(II) 31 mM Cu(II)

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E (mV)

450

no Cu(II)

(b)

550

79 mM Cu(II) 157 mM Cu(II)

250 150 50

0

20

40

60 80 Time in minutes

Fig. 1.

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100

120

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80

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60 40 20 0 20

60 80 Time in minutes

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Fig. 2.

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Extraction of Cu (%)

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0.7 M NaCl 1.3 M NaCl 2.0 M NaCl

100

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(a)

20°C

50°C

80°C

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80

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60 40

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Extraction of Cu (%)

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20 0 0

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Time in minutes

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Fig. 3.

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(a) No air/oxygen

80

Oxygen (1 L/min)

60

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Air (1 L/min)

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Extraction of Cu (%)

100

40

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20

0

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0 20

40

60

80

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(b)

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80

40

No air/oxygen Air (1 L/min)

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Extraction of Cu (%)

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60

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Time in minutes

Oxygen (1 L/min)

20 0 0

20

40

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Time in minutes Fig. 4.

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80

100

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ACCEPTED MANUSCRIPT 600

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400 300

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E (mV)

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200

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100 0 20

40 60 80 Time in minutes

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0

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Fig. 5.

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No air/oxygen Air (1 L/min) Oxygen (1 L/min) 100

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Cu Au

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Ni Ag

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80 60

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Extraction (%)

100

Fe Pd

40

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20

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0 %1 solids

%15 solids

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Fig. 6.

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%15 solids+Oxygen

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(a)

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cps/eV

88

Pb

(b)

Pb

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77 66 55

Cl

Cl

44 33

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Cl Cl

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0

O

Pb

0 1

1

2

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3

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5 keV

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keV Fig. 7. 27

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ACCEPTED MANUSCRIPT LIST of TABLES

Table 1. Advantages and disadvantages of cupric chloride leaching (Arbiter and Fletcher,

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1994; Dreisinger, 2006; Peacey et al., 2004).

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Table 2. Metal content of the WPCB sample.

Table 3. Effect of initial concentrations of Cu2+ and NaCl on the leaching of Au, Ag, Pd, Fe and Ni ( Base conditions: 1 M HCl, 1.3 M NaCl, 31 mM Cu2+, 80°C, %1 w/v solids,

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120 min)

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Table 4. Reduction potential of some metal chloride complexes (Han, 2002; Puvvada et al., 2003).

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Table 5. Effect of temperature and air/O2 supply on the leaching of Au, Ag, Pd, Fe and Ni

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(Base conditions: 1 M HCl, 1.3 M NaCl, 80°C, %1 w/v solids, 120 min)

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ACCEPTED MANUSCRIPT Table 1. Advantages

Disadvantages

 Fast kinetics at low temperature and

 Due to excessive corrosion, inert materials should be used in construction  Unconventional

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 High solubility and activity of base/precious

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atmospheric pressure

metals

and

difficult

electrowinning of copper  Poor quality of copper product that needs

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 Low toxicity  No iron contamination

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further electrorefining

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ACCEPTED MANUSCRIPT Table 2. Fe

Ni

Al

Pb

Sn

Au

% 0.43

1.33

2.66

4.91

86

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2.05

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18.5

Ag

Pd

g/t

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Assay

Cu

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Metal

97

ACCEPTED MANUSCRIPT Table 3. Initial Concentration of Cu2+

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157 mM 14.4 98.5 89.6 98.9 98.9

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126 mM 11.9 98.5 82.0 98.9 98.7

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79 mM 0.8 97.9 67.7 98.4 98.6

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31 mM 3.9 35.3 4.2 92.0 80.5

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8 mM 2.3 25.5 8.8 85.9 49.2

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Au Ag Pd Fe Ni

0 mM 0 4.1 0 68.4 35.2

NaCl Added 0.7 M 0 91.5 8.6 98.4 95.6

2M 0 89.1 42.5 97.0 95.2

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Table 4.

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1.27 1.08 0.85 0.74 0.74 0.55

Activity of Cl1.0 5.0 0.166 0.042 0.480 0.397 1.15 1.07 1.00 0.945 0.73 0.65 0.68 0.64 0.62 0.54 0.431 0.348

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0.1 0.343

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Pd2+/Pd: 0.92 Rh3+/Rh: 0.76

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Paladium Rhodium

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Platinum

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Gold

Standard Potential (V) Cu2+/Cu: 0.34 Ag+/Ag: 0.80 Au+/Au: 1.70 Au3+/Au: 1.50 Pt2+/Pt: 1.19

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Copper Silver

Reaction couple CuCl3-/Cu0 AgCl2-/Ag0 AuCl2-/Au0 AuCl4-/Au0 PtCl42-/Pt0 PtCl62-/Pt0 PdCl42-/Pt0 RhCl63-/Rh0

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Metal

50 -0.094 0.279 0.95 0.87 0.53 0.58 0.42 0.23

ACCEPTED MANUSCRIPT Table 5. Effect of Air/O2 Supply (1 L/min)

Temperature (79 mM Cu2+)

8 mM Cu2+

Air

O2

0 4.1 0 7.9 35.2

8.5 97.7 78.4 99.3 99.3

42.7 98.4 90.9 98.7 98.9

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No

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80°C 0 97.9 67.7 98.4 98.6

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50°C 0 91.6 52.2 95.2 97.2

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Au Ag Pd Fe Ni

20°C 0 80.1 33.1 93.2 87.3

No Cu2+ No

Air

O2

2.3 25.5 8.8 85.9 63.7

43.5 98.1 85.1 98.9 97.9

43.7 98.2 89.5 99.1 97.6

ACCEPTED MANUSCRIPT Highlights  Cupric chloride leaching allows high metal extractions

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 Cupric (Cu(II)) is an effective oxidant in chloride leaching

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 Increasing initial Cu(II) concentration and temperature improves leaching of metals  Excessive addition of NaCl impedes the kinetics of copper extraction

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 Air/oxygen can be used for in-situ regeneration of Cu(II)

34