15th International Mineral Processing Symposium ...

15th International Mineral Processing Symposium, Istanbul-Turkey, October 19-21, 2016

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EXTRACTION OF METALS FROM WASTE OF PRINTED CIRCUIT BOARDS (WPCB) IN SULPHATE AND CHLORIDE SOLUTIONS E. Y. Yazici, H. Deveci Hydromet B&PM Group, Div. of Mineral&Coal Processing, Dept. of Mining Eng., Karadeniz Technical University, 61080, Trabzon, Türkiye Extraction of base/precious metals (Cu/Fe/Ni/Pb/Sn/Au/Ag/Pd) from waste of printed circuit boards (WPCB) was investigated in sulphate and chloride solutions under oxidising conditions. WPCB sample contained 18.5% Cu, 86 g/t Au, 694 g/t Ag and 97 g/t Pd. Chloride leaching tests were conducted under the conditions of 1 M HCl, 47 g/L Cl-, 1% w/v solids at 80°C in the absence/presence of MnO2 (10 g/L) as the oxidant. The addition of MnO2 with a standard potential of +1.22 V greatly enhanced the leaching of metals with complete extraction of copper even at 15 min vs 14.7% Cu recovery without MnO2 over 120 min. Extraction of precious metals were also substantially improved from none to 71.8% Au and from ≤4.1% to ≥98.7% for Ag/Pd over 120 min. In sulphate media (1.2 M H2SO4, 5% w/v, 80°C) addition of NaNO2 (3 g/L) was tested in the presence of 2 L/min O2 to investigate its effect on leaching of Cu, Fe, Ni, Ag and Pd over 60 min. The addition of NaNO2 improved the leaching performance of copper from 28.2% to 67.0% over 60 min. However, no improvement was observed in the extraction of precious metals (i.e. Ag and Pd) under the conditions tested. The results showed that MnO2 could be suitably used as an oxidant in chloride media to improve extraction of base/precious metals from WPCB while in sulphate media NaNO2 can be used for selective dissolution of copper (and Fe/Ni) over precious metals (Ag and Pd). Key Words: WEEE, Metals, Leaching, Oxidant, MnO2, NaNO2, Sulphate, Chloride.

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INTRODUCTION Recycling/recovery of waste electrical and electronic equipments (WEEE) is of great importance since WEEE pose environmental threat when improperly disposed in landfills with municipal waste due to hazardous organic/inorganic material content (e.g. heavy metals, chlorinated/brominated flame retardants) (Babu et al., 2007; Widmer et al., 2005; Yazıcı et al., 2010). None the less, recovery of metals from WEEE is of economic interest due to its noticeably high content of base (particularly Cu) and precious metals (Au, Ag and Pd) (Akçil et al., 2009; Yazıcı and Deveci, 2009). Waste of printed circuit boards (WPCB) is the most important metal source in WEEE stream. Among various treatment routes suggested for WEEE (i.e. physical, pyrometallurgical and bio/hydrometallurgical), hydrometallurgical route has received great interest owing to its several advantages e.g. economically viable for low-grade waste (i.e. 0.1 M Cl-, which facilitates dissolution of metals (Muir, 2002; Senanayake and Muir, 2003). In this study, the influence of alternative oxidising agents (MnO2 and NaNO2) on the extraction of base (Cu/Fe/Ni/Pb/Sn) and precious metals (Au/Ag/Pd) from WPCB in chloride or sulphate media was investigated. The effect of manganese(IV) dioxide (010 g/L MnO2) was tested in chloride media under the conditions of 1 M HCl, 47 g/L Cl, 1% w/v solids at 80°C over 120 min. In sulphate media (1.2 M H2SO4, 5% w/v, 80°C, 2 L/min O2) addition of sodium nitrite (3 g/L NaNO2) was tested in its absence and presence over a period of 60 min. MATERIALS AND METHODS Waste of printed circuit boards (WPCB) with an amount of 250 kg was manually dismantled from end-of-life computers of various brands. After removal of board components (e.g. capacitors, resistors, transistors and cables) by hand tools, WPCB were size reduced to -250 m after a multi-stage crushing/grinding operation (Fig. 1). Details of the preparation process of WPCB can be found elsewhere (Yazici, 2012). Chemical analysis of WPCB (Table 1) was carried out by hot aqua-regia digestion followed by AAS finish using a Perkin Elmer AAnalyst 400 atomic absorption spectrometer.

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Fig. 1. Dismantled (left) and crushed (-8 mm) (right) WPCB (Yazici, 2012). Table 1. Metal contents of WPCB (Yazici, 2012). Cu

Fe

Ni

Al

Pb

Sn

Au

Ag

Pd

Metal % Content

18.5

2.05

0.43 1.33

g/t 2.66

4.91

86

694

97

Baffled-jacketed glass reactors with a nominal capacity of 750 mL were used for the leaching tests. Agitation was performed using overhead mechanical stirrers (IKA EUROST). These were equipped with PTFEcoated four-blade downward pumping 45° pitched blade impellers (diameter: 6.5 cm) operating at a rotation speed of 675 rpm. A water circulator (Polyscience) connected to reactors was used to control the leaching temperature at the desired level (i.e. 80±1°C). Top of the reactors were kept closed over the leaching period. Polyethylene (PE) spargers were used to introduce oxygen from an oxygen tank (99.9% O2) into the reactors, if required. Concentrated sulphuric acid (96% H2SO4) and sodium nitrite (99% NaNO2 in solid form) was used to prepare sulphate leach solutions while hydrochloric acid (37% HCl), sodium chloride (≥99% NaCl) and manganese(IV) dioxide (≥90% MnO2) was used in chloride based leaching tests. Solutions with a final volume of 500 mL were prepared 1234


in deionised-distilled water at the desired concentrations of leaching reagents. Leach solutions were heated to 80°C prior to addition of certain amount of WPCB sample. Pulp samples (34 mL) were removed at the predetermined intervals over the leaching period and centrifuged (4100 rpm for 5 min) to obtain clear solutions for analysis of Cu by atomic absorption spectrometer (AAS, Perkin Elmer AAnalyst 400). In addition, concentrations of the other metals (Fe/Ni/Pb/Sn/Au/Ag/Pd) in final solutions were also determined on the termination of the tests. pH and redox potentials (mV vs. Ag/AgCl) were also monitored at the sampling intervals. On the termination of the leaching test the solid residue was filtered and dried in an oven at 105°C for 3 h. prior to digestion in hot aqua regia for analysis of metals by AAS. The extent of leaching of a metal was calculated based on the metal dissolved into leach solution and remained in solid residue. RESULTS AND DISCUSSION Effect of MnO2 in chloride media Fig. 2a illustrates the effect of MnO2 on the rate and extent of extraction of copper from WPCB using a chloride solution (1 M HCl, 47 g/L Cl-, 80°C, 1% w/v solids) over a period of 120 min. The presence of MnO2 (10 g/L) appeared to lead to a substantial improvement in leaching performance of copper compared with negligible extraction of copper (i.e. ≤1.4%) without MnO2 over 30 min. A complete extraction of copper in the presence of 10 g/L MnO2 was achieved even at 15 min. (Fig. 2a). In the absence of MnO2 final extraction of copper was limited to 14.7% over the leaching period of 120 min (Fig. 2a).

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Fig. 2. Effect of MnO2 on leaching kinetics of copper (a) from WPCB and redox potential profiles (b) (1 M HCl, 47 g/L Cl-, 80°C, 1% w/v solids) The redox potential profiles (Fig. 2b) also supported high leaching performance of copper with MnO2 in that much higher potentials (i.e. 640-734 mV) were monitored in the presence of MnO2 when compared with its absence (i.e. 83-130 mV) over the leaching period. These high potentials was related with the MnO 2 itself and high Cu2+/Cu+ ratios provided through oxidation of Cu+ by MnO2 (Eqs. 4-5). Dissolution of metallic copper in the presence of MnO2 would occur via formation of chloro-complexes of cupric (CuCln2-n, 1≤n≤4) (Eq. 1) depending on the concentration of chloride (Herreros et al., 2005; Winand, 1991). Dissolved copper in cupric state 1236


(Cu2+) is then reduced to cuprous form in the form of cuprous chloride complexes (CuCln1-n, 1≤n≤4) and hence activate Cu2+/Cu+ redox couple (Eqs. 2-3). The presence of MnO2 would allow in-situ regeneration of Cu2+ through Eqs. 4-5. Cu0 + MnO2 + 4HCl → CuCl2 (aq)+ MnCl2 (aq)+ 2H2O

(1)

(G0(25°C)= ‒184 kJ and G0(80°C)= ‒185 kJ) Cu0 + Cu2+ + 4Cl- → 2CuCl2-

(2)

(G0(25°C)= ‒26 kJ and G0(80°C)= ‒33 kJ) Cu0 + Cu2+ + 6Cl- → 2CuCl32-

(3)

(G0(25°C)= ‒31 kJ and G0(80°C)= ‒31 kJ) 2CuCl2- + MnO2 + 4H+ → 2Cu2+ + MnCl2 (aq) + 2Cl- + 2H2O

(4)

(G0(25°C)= -73 kJ and G0(80°C)= -67 kJ) 2CuCl32- + MnO2 + 4H+ → 2Cu2+ + MnCl2 (aq) + 4Cl- + 2H2O

(5)

(G0(25°C)= -69 kJ and G0(80°C)= -69 kJ) It is relevant to note that MnO2 as a strong oxidant (E0(MnO2/Mn2+): +1.22 V) (Lide, 2005) is dissolved in chloride solutions to produce MnCl2(aq) and chlorine (Cl2) (Eq. 6). However, this reaction (Eq. 6) is thermodynamically feasible at temperatures above ≈58°C (HSC-Chemistry, 2011) suggesting that Cl2 is likely to occur under the current test conditions (i.e. at 80°C). Chlorine produced from Eq. 6, as a powerful oxidant (i.e. +1.36 V), would also contribute to leaching of metals. MnO2 + 4HCl → MnCl2 (aq) + 2H2O + Cl2 (g)

(6)

(G0(25°C)= +9.0 kJ and G0(80°C)= ‒5.1 kJ) The dissolution of Fe, Ni, Pb and Sn over 120 min. with/without MnO 2 was presented in Fig. 3. The presence of MnO2 significantly enhanced the dissolution of Fe and Ni i.e. from 68.4% to 95.1% Fe and from 35.2% to 97.9% Ni over 120 min. Major portion of Pb (97.8%) and Sn (88.5%) already dissolved in the absence of MnO 2. Thereby, the 1237


addition of MnO2 induced no improvement for Pb and a limited increase (i.e. 9%) for Sn over 120 min (Fig. 3).The presence of MnO2 resulted in a substantial improvement for leaching of precious metals particularly for Ag and Pd (Fig. 4). Despite no dissolution of Au and Ag without MnO2, the addition of the oxidant allowed high extractions for Au (71.8%) and Ag (98.7%) over 120 min. Recovery of Pd was also enhanced from 4.1% to 100% by the addition of MnO 2 over the same period of 120 min (Fig. 4).

Fig. 3. Effect of MnO2 on extraction of Fe, Ni, Pb and Sn from WPCB (1 M HCl, 47 g/L Cl-, 80°C, 1% w/v solids, 120 min.)

Fig. 4. Effect of MnO2 on extraction of precious metals (Au, Ag and Pd) from WPCB (1 M HCl, 47 g/L Cl-, 80°C, 1% w/v solids, 120 min.) 1238


Geoffroy and Cardarelli (2005) investigated dissolution of gold foils in HCl solutions in the

presence

of

MnO2

at

varying

temperatures

up

to

100°C

under

atmospheric/pressure conditions. They showed that under atmospheric conditions temperature is the most important operating parameter since at temperatures below 80°C no significant dissolution of gold was observed. Yazici and Deveci (2015) tested the effect of air or oxygen (+1.23 V) as oxidants on extraction of metals and they found that leaching of silver and palladium from WPCB in HCl-NaCl solutions was greatly improved from ≤4.1% to ≥90.8% over 120 min. These researchers also noted that extraction of gold was also increased from none to ≤42.7% over the same period. McDonald et al. (1987) reported that introduction of air into cupric chloride solutions improved dissolution of metallic gold. They claimed that cupric ion essentially acts as a catalyst in oxidation of gold. In this regard, in chloride solutions in the presence of MnO2 dissolution of Au, Ag and Pd can be expressed in Eqs. 7-8, Eqs. 9-10 and Eqs. 11-12, respectively (HSC-Chemistry, 2011). Au0 + 1.5MnO2 + 7HCl → HAuCl4 (aq) + 1.5MnCl2 (aq) + 3H2O

(7)

(G0(25°C)= -94 kJ and G0(80°C)= -108 kJ) 2Au0 + MnO2 + 4HCl + 2Cl- → 2AuCl2- + MnCl2 (aq) + 2H2O

(8)

(G0(25°C)= -36 kJ and G0(80°C)= -51 kJ) 2Ag0 + MnO2 + 4HCl + 2Cl- → 2AgCl2- + MnCl2 (aq) + 2H2O

(9)

(G0(25°C)= -160 kJ and G0(80°C)= -173 kJ) 2Ag0 + MnO2 + 4HCl + 4Cl- → 2AgCl32- + MnCl2 (aq) + 2H2O

(10)

(G0(25°C)= -158 kJ and G0(80°C)= -165 kJ) Pd0 + MnO2 + 4HCl → PdCl2 (aq) + MnCl2 (aq) + 2H2O

(11)

(G0(25°C)= -77 kJ and G0(80°C)= -78 kJ) Pd0 + MnO2 + 4HCl + Cl- → PdCl3- + MnCl2 (aq) + 2H2O 1239

(12)


(G0(25°C)= -139 kJ and G0(80°C)= -142 kJ) Despite high enhancing effect of MnO2 on leaching performance of base/precious metals (Figs. 2-4) the main disadvantage of this oxidant is the contamination of leach solution by manganese which requires additional solution purification steps in downstream processing of pregnant leach solutions for metals recovery. Effect of NaNO2 in acidic sulphate media The effect of presence of NaNO2 (3 g/L) on extraction of metals from WPCB was tested in acidic sulphate solutions under the conditions of 1.2 M H2SO4, 80°C, 5% w/v solids and 2 L/min O2 over the period of 60 min (Fig. 5). The addition of NaNO2 increased the leaching kinetics of copper by ≈2-fold. Dissolution of copper was improved from 28.2% to 67.0% over 60 min (Fig. 5). Dissolution reactions for copper in H2SO4-NaNO2 system with/without O2 are shown in Eqs. 13-14. Cu0 + 2NaNO2(aq) + 2H2SO4 → CuSO4(aq) + Na2SO4(aq) + 2H2O + 2NO(g)

(13)

(G0(25°C)= -264 kJ and G0(80°C)= -260 kJ) 4Cu0 + 4NaNO2(aq) + 6H2SO4 + O2 (g)→4CuSO4(aq) +2 Na2SO4(aq) +6H2O +4NO(g) (14) (G0(25°C)= -982 kJ and G0(80°C)= -939 kJ) In acidic solutions sodium nitrite (NaNO2) dissociates into nitrogen compounds (HNO2, N2O4, NO, NO2−, N2O3, HNO3 and NO3−) in a series of reactions in which nitrous acid (HNO2) is the most reactive form (Gok and Anderson, 2013). Standard potentials for nitrous acid (HNO2) are +0.86 (HNO2/H2N2O2), +0.98 V (HNO2/NO) and +1.30 (HNO2/N2O) (Lide, 2005). In situ regeneration of NOx gases produced during acidic sulphate leaching (H2SO4-NaNO2) could be an important advantage (Gok and Anderson, 2013).

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Fig. 5. Extraction kinetics of copper from WPCB in the absence/presence of NaNO2 (1.2 M H2SO4, 80°C, 5% w/v solids, 2 L/min O2) Extraction of Fe, Ni and precious metals (Ag and Pd) was presented in Fig. 6. For Fe and Ni, even in the absence of NaNO2 almost complete recovery (≥97.9% over 60 min.) was noted (Fig. 6). The addition of NaNO2 had negligible effect on extraction of precious metals in that recoveries were remained at ≤0.7% Ag and ≤7.1% Pd over 60 min (Fig. 6). Thermodynamic calculations indicated that dissolution reactions for silver (Eq. 15) and palladium (Eq. 16) in the presence of NaNO 2 are favourable (HSCChemistry, 2011) suggesting that more oxidising conditions (i.e. high presence of NaNO2) is required for high extraction of Ag and Pd. These findings also implied that under suitable conditions base metals (i.e. Cu, Fe and Ni) can be selectively extracted over precious metals (i.e. Ag and Pd). 2Ag0 + 2NaNO2 (aq) + 2H2SO4 → Ag2SO4 (aq) + Na2SO4 (aq) + 2H2O + 2NO (g)

(15)

(G0(25°C)= -176 kJ and G0(80°C)= -181 kJ) Pd0 + 2NaNO2 (aq) + 2H2SO4 → PdSO4 (aq) + Na2SO4 (aq) + 2H2O + 2NO (g) (G0(25°C)= -154 kJ and G0(80°C)= -149 kJ)

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(16)


Fig. 6. Extraction of Fe, Ni and precious metals (Ag and Pd) from WPCB in the absence/presence of NaNO2 (1.2 M H2SO4, 80°C, 5% w/v solids, 2 L/min O2, 60 min.)

CONCLUSION The influence of MnO2 (in chloride system) and NaNO2 (in sulphate system) as the oxidants was tested on extraction of base/precious metals from waste of printed circuit boards (WPCB). In chloride solutions (1 M HCl, 47 g/L Cl-, 80°C, 1% w/v solids) the presence of MnO2 (10 g/L) was tested over a period of 120 min. It was found that addition of MnO2 greatly improved the leaching of copper from 14.7% to 99.8% over 120 min. as well as precious metals (Ag, Ag and Pd) with an increase by up to 71.898.7% over the same period. Sodium nitrite (3 g/L NaNO 2) was utilised in sulphate based leaching system (1.2 M H2SO4, 80°C, 5% w/v solids and 2 L/min O2). The presence of NaNO2 led to a significant improvement in extraction of copper from 28.2% to 67% over 60 min. while no significant leaching of precious metals (Ag and Pd) was observed i.e. ≤7.1% Ag/Pd even in the presence of NaNO 2. The findings showed that MnO2 can be suitably used as an oxidant in chloride leaching of WPCB to improve leaching performance of base/precious metals. The results also suggested that in sulphate 1242


system NaNO2 can be utilised for selective extraction of copper (and other base metals) over precious metals. ACKNOWLEDGEMENTS 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 support. REFERENCES

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