Separation and Purification Technology 130 (2014) 151–159

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Separation and Purification Technology

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Treatment of manufacturing scrap TV boards by nitric acid leaching

http://dx.doi.org/10.1016/j.seppur.2014.04.0081383-5866/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +90 4623773681; fax: +90 4623257405.E-mail address: hdeveci@ktu.edu.tr (H. Deveci).

Ahmet Deniz Bas, Haci Deveci ⇑, Ersin Y. YaziciHydromet B&PM Group, Div. of Mineral & Coal Processing, Dept. of Mining Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 December 2013Received in revised form 2 April 2014Accepted 4 April 2014Available online 18 April 2014

Keywords:WEEERecyclingLeachingWaste treatmentCopper

The leachability tests for manufacturing scrap TV boards (STVB) have indicated the release of metalsbeyond the limit levels with potential problems for environmental pollution. Treatment of STVB is there-fore requisite for its safe disposal in landfills. The nitric acid leaching of STVB for the removal/recovery ofvaluable metals (Cu and Ag) was studied by adopting the Box–Behnken design. Statistical analysis of datahas revealed that the concentration of nitric acid is the most influential parameter affecting the leachingprocess. Effects of solids ratio and temperature on the rate and extent of the extraction of copper werealso proved to be statistically significant. However, the interaction effects of these parameters were foundto be insignificant. The leaching kinetics were consistent with the shrinking particle model under chem-ical control with an activation energy of 38.6 kJ/mol. High concentrations of nitric acid (2–5 M HNO3)were required to achieve high copper extractions (88.5–99.9%) at a pulp density of 6% w/v. The extractionof silver was enhanced from 14% to 68% with increasing the concentration of nitric acid from 1 to 5 M.These findings also demonstrate that copper may well be selectively extracted from STVB by adjustingthe concentration of nitric acid.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Rapid development of technology leads to the reduction inlifespan of most electrical and electronic equipments [1,2].Concomitantly, the generation of waste of electrical and electronicequipments (WEEE) has ever increased in recent years [3,4].Annual generation of WEEE is estimated to reach 40–70 mil-lion tonnes globally by 2015 [5]. WEEE contains hazardous organic(chlorinated/brominated flame retardants) and inorganic (Hg, Pb,etc.) materials. There is an ever growing concern for the disposalof ever increasing volume of WEEE in landfills since heavy metalsreleased from WEEE under the influence of atmospheric conditionscan lead to the pollution of soils, surface and underground waters[6–9]. The proper management of WEEE is a prerequisite for pre-vention of environmental pollution [6,10]. On the other hand, highcontent of precious (Au, Ag and Pd) and base metals (mainly Cu)renders WEEE a valuable secondary resource for these metals[11–13].

Metal values can be extracted from WEEE by utilizing physicalseparation, pyrometallurgical and bio-/hydrometallurgical meth-ods [10,14–16]. Physical separation methods are essentially usedto remove ferrous metals and non-metals prior to the extractionprocesses. Pyrometallurgical processes based on traditional

smelting and refining technologies are industrially exploited forthe extraction of metals from WEEE. However, the main detrac-tions to these processes include the requirement for high grade(particularly in Au) WEEE and costly off-gas treatment for mitigat-ing environmental pollution [11]. In recent years, the developmentof hydrometallurgical processes as potential alternatives havereceived great attention due to a number of attributes includingtheir suitability for small scale applications, reduced environmen-tal impact (i.e. no hazardous gas/dust emissions) and low capitalcosts [10,14,15].

Metals are present in native form and/or alloys in WEEE and,therefore, oxidizing conditions (e.g. Eq. (1) cf. Eq. (2)) are oftenrequired for leaching of base and precious metals, in particular[17]. A variety of leaching reagent systems appeared to be testedfor the extraction of target metals (e.g. Cu, Pd, Ag and Au) [15].Despite their extensive use, sulfuric and hydrochloric acids aloneare not effective and require the presence of a suitable oxidant(e.g. H2O2, Fe3+, Cl2, O2 or bacteria) [17–26]. On the other hand,nitric acid has the peculiarity of strong oxidizing acid, which ren-ders it a competent leaching agent Eq. (3) for the treatment ofWEEE. Therefore, it is more effective than sulfuric and hydrochloricacids for the extraction of metals from WEEE [27]. Although nitricacid is comparatively an expensive acid, it can be readily regener-ated from nitric oxides (NO and NO2) that form as the reactionproducts (e.g. Eq. (3)) [28,29]. Traditional downstream purificationand metal/acid recovery processes i.e. solvent extraction and

Table 1Chemical composition of the STVB sample used in this study.

Metal TV sample

Cu (%) 11.2Zn (%) 0.15Al (%) 0.30

152 A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159

electrowinning can be readily exploited for the treatment of metal-loaded nitric acid leach solutions [28]. Despite its high potential,nitric acid leaching of WEEE has received comparatively less inter-est with the scarcity of detailed studies.

Cu0 þ 2Hþ ! Cu2þ þH2ðgÞ DG0 ¼ þ65:50 kJ=mol ð1Þ

Ca (%) 0.40Co (%) 0.04Ni (%) 0.02Fe (g/ton) 43

Cu0 þ 1=2O2 þ 2Hþ ! Cu2þ þH2O DG0 ¼ �171:63 kJ=mol ð2Þ

Pb (g/ton) 126As (g/ton) 2.2Cd (g/ton) 157Cr (g/ton) 91Au (g/ton) 0.14Ag (g/ton) 48

3M0 þ 8HNO3 ! 3MðNO3Þ2 þ 4H2Oþ 2NOðgÞðM : Cu; Pb; Zn; Ag; etc:Þ ð3Þ

Nitric acid is extensively used in combination with hydrochloricacid in the form of aqua reqia for non-selective and aggressivedigestion of base and precious metals in WEEE [30–32]. When usedalone, nitric acid is capable of selectively extracting metals such asCu, Ni, Pb and Ag over metals such as Au and Sn [33–35]. Thesestudies have focused essentially on the leaching of high grade(i.e. precious metal rich) WEEE such as personal computers andmobile phones. On the other hand, treatment of ‘‘low grade’’ WEEE,e.g. manufacturing scrap TV boards (STVB) with its relatively lowbase/precious metal content was not much considered. STVB isrelatively poor in metals and essentially unsuitable for industrialsmelting. Enrichment of metal content of such low grade wasteby physical separation methods was reported to be possible onlyat unacceptably high metal losses [13]. Therefore, the developmentof a suitable management approach is required for the treatment ofsuch low grade WEEE prior to its disposal in landfills.

This study was developed to provide an overall managementscheme for STVB, which involves initial heat treatment of STVBto recover electronic components, and also reducing harmful met-als such as Sb, Pb and Tl to low levels and then, hydrometallurgicaltreatment (nitric acid leaching) for the recovery of valuable metals.The leachability tests (TCLP, SPLP and EN 12457-2) were initiallycarried out to assess the hazardous potential of STVB after heattreatment. Then, the removal/recovery of valuable metals, copperand silver, from STVB was investigated. Box–Behnken design wasused to study the effect and interaction of leaching parameters;nitric acid concentration (0.2–1 M), temperature (30–70 �C) andpulp density (2–10% w/v). Leaching kinetics was also examinedusing shrinking particle model. The findings of statisticallydesigned experiments were exploited to design further tests inan attempt to maximizing the extraction of copper and silver.

Fig. 1. Schematic presentation of experimental procedure adopted for the treat-ment of STVB.

2. Experimental

2.1. Scrap TV boards sample

STVB used in the current study was kindly provided by VestelElectronics Company (Manisa, Turkey). In the plant, STVB withcomponents is heat treated to remove solder and most board com-ponents, which allows the removal of hazardous heavy metals tolow levels (Table 1) and reuse of these components recovered.The heat-treated scrap (80 kg) was crushed to �3.35 mm using arotary shredder and then ground to �250 lm using a Ultra Centrif-ugal Mill (Retsch ZM 200) in a two-stage size reduction operation(Fig. 1). The crushed samples were used in the leachability tests.The ground sample (�250 lm) was split into representative sub-samples (250 g each) for use in the leaching tests and chemicalanalysis [36]. STVB was determined to contain only copper(11.2% Cu) and silver (48 g/ton) in appreciable quantities (Table 1).A sample of STVB was also examined under SEM–EDX (figure notshown) where copper as the abundant phase was identified to bein metallic form.

2.2. Leaching tests

Baffled glass reactors (inner dia.: 6.5 cm) with a 250-mL nomi-nal capacity were used in the leaching tests. The reactors were puton a multi magnetic stirrer (Thermo Scientific Variomag) placed ina water bath (Cole-Palmer). PTFE-coated magnetic bars (crossshaped, dia.: 3 cm) were used for mixing the reactor contents at350 rpm. The top of the reactors were kept covered with lids overthe leaching period.

A nitric acid solution (65% HNO3) was used to prepare leachingsolutions at different acid strengths in a final volume of 200 mL.Deionised-distilled water was used in the preparation of solutions.

A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159 153

Once the leaching solution reached the desired temperature, arequired amount of STVB sample was added into the reactors. Sam-ples were taken at the predetermined intervals over the leachingperiod of 120 min and then, centrifuged at 4100 rpm for 5 min toobtain clear supernatants for the analysis of metals (Cu and Ag).On the termination of the experiments, the leaching residues wereseparated by filtration and then, dried at 105 �C for 6 h prior to thedigestion in hot aqua regia for the analysis of metals using anatomic absorption spectrometer (Perkin Elmer AAnalyst 400).Metal extractions were calculated based on the results of the resi-due analysis.

2.3. Experimental design

Response surface methodology (RSM) proved to be a powerfuland effective tool for the assessment of effects of several indepen-dent parameters (variables) on a dependent parameter (i.e.response). A Box–Behnken design (BBD), which is one of the com-monly used designs for RSM, has three levels i.e. low (�1), center(0) and high (+1) per factor with equally spaced intervals betweenthe levels. It is an effective design particularly for estimation of thefirst- and second-order response.

In this study, effects of concentration of HNO3 (0.2–1 M), solidsratio (2–10% w/v) and temperature (30–70 �C) on the extraction ofcopper were investigated through a BBD with a total 15 runs.Experimental design matrix with coded and corresponding actuallevels of parameters is shown in Table 2. Based on the findingsobtained from BBD runs, additional leaching tests were also under-taken at higher concentrations of HNO3 (2–5 M) to improve theleaching recoveries for copper and silver.

In the experimental design (Table 2), actual (uncoded) levelsand the relationship between the coded and actual values wereobtained by Eqs. (4) and (5), respectively [37]:

Xcenter ¼Xhigh � Xlow

2

� �ð4Þ

xcoded ¼Xactual � Xcenter

Xcenter � Xmin

� �ð5Þ

where xcoded is coded value; Xactual, Xcenter and Xmin are correspond-ing actual value, actual value in the center and minimum (low)actual value, respectively. Total number of experiments (N) in aBBD is calculated by the formula i.e. N = n2 + n + cp, where n and cp

are the number of parameters and replicates in the central point,respectively [37,38].

Table 2Experimental design matrix with coded and actual levels of parameters.

Exp. no HNO3 (A) Solids ratio (B) Tem

Design matrix (coded values)

1 �1 �1 02 +1 �1 03 �1 +1 04 +1 +1 05 �1 0 �16 +1 0 �17 �1 0 +18 +1 0 +19 0 �1 �1

10 0 +1 �111 0 �1 +112 0 +1 +113 0 0 014 0 0 015 0 0 0

Based on the experimental data obtained, a second orderregression model Eq. (6) was constructed for the estimation of anunknown response (Y) within the experimental range ofconditions:

Y ¼ b0 þ b1x1 þ b2x2 þ b3x3 þ b11x21 þ b22x2

2 þ b33x23 þ b12x1x2

þ b13x1x3 þ b23x2x3 ð6Þ

where Y is the predicted response, b0 model constant; x1, x2, x3 andx4 symbols for independent variables; b1, b2, b3 and b4 are linearcoefficients; b12, b13 and b23 are interaction coefficients and b11,b22 and b33 are the quadratic coefficients [37,38].

To evaluate the statistical significance of the model and param-eters, the analysis of variance (ANOVA) was performed whereP-values were used for the significance test. Simply, a P-value of<0.05 suggested the rejection of the null hypothesis i.e. the signif-icance of a parameter/term at a confidence level of 95% (a = 0.05)[37,38]. Design Expert� [39] software was used to perform statisti-cal evaluation of the experimental data and generation of responsesurface plots.

2.4. Hazardous potential of STVB: Leachability tests

Leachability tests including TCLP (Toxicity Characteristic Leach-ing Procedure), SPLP (Synthetic Precipitation Leaching Procedure)and EN 12457-2 were carried out to evaluate hazardous character-istics of STVB by simulating different environmental scenario. TCLPand SPLP tests were defined by U.S. Environmental ProtectionAgency [40] while EN 12457-2 by European Union [41]. Thecrushed samples of finer than 3.35 mm were directly used in thesetests. In TCLP tests, the suitable extraction solution was deter-mined by the standard procedure based on the sample pH [40].Accordingly, the extraction fluid 1 (5.7 mL glacial acetic acid and64.3 mL 1 N NaOH in 1-L reagent water at pH 4.93 ± 0.05) wasselected. The extraction fluid for SPLP tests was prepared using amixture of H2SO4 and HNO3 (60/40 ratio by wt.) to adjust a finalpH of 4.20 ± 0.05 in distilled water [40]. In EN 12457-2 test, dis-tilled water was utilized as the extraction fluid. All the leachabilitytests were performed in polypropylene bottles in a 200-mL ofextraction fluid. A required amount of STVB sample was added tomaintain a liquid-to-solid ratio of 20:1 for TCLP/SPLP and 10:1for EN 12457-2. The bottles were agitated on a tumbler rotatingat 30 rpm for 18 h for TCLP/SPLP tests and 24 h for EN 12457-2 test.Leachates were then collected using 0.8-lm cellulose nitratefilters. Following the measurement of pH, a required volume ofconcentrated HNO3 was added to acidify (<pH 2) a 50-mL aliquot

p. (C) HNO3

(A) (M)Solids ratio(B) (% w/v)

Temp.(C) (�C)

0.2 2 501 2 500.2 10 501 10 500.2 6 301 6 300.2 6 701 6 700.6 2 300.6 10 300.6 2 700.6 10 700.6 6 500.6 6 500.6 6 50

154 A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159

of the leachate. ICP-AES (Spectro-Genesis) was used for analysis ofmetals. All the tests were performed in duplicate and the mean val-ues were presented.

3. Results and discussion

3.1. Leachability of metals from STVB

The results of the leachability tests (Table 3) suggest that STVBcan be classified as non-hazardous waste since the concentrationsof metals, which are of interest in TCLP test, are lower than the reg-ulatory limits (Table 3). However, TCLP test only considers eightelements excluding heavy metals such as copper. Therefore, othertests (i.e., SPLP and EN 12457-2) were also used to further assessthe leachability characteristics of metals. Although SPLP test isnot a regulatory test like TCLP, is commonly used to detect theleachability of metals by simulating rain water under atmosphericconditions [40]. EN 12457-2 is a regulatory batch test for determi-nation of leachability of inorganic contaminants by simulatingactual environmental conditions of a landfill [42,43]. In Table 4,the concentration of metals of the leachates obtained from SPLPand EN 12457-2 tests, and the water quality standards (Types I–IV) according to Turkish regulations for water pollution control[44] and European regulations [41] are presented. The results of

Table 3The leachability of metals from STVBs in TCLP, SPLP and EN 12457-2 tests.

Element TCLP SPLP

Leachate (ppb) Regulatory limita (ppb) Leachate (ppb) Type Ib (ppb

Ag n.d. 5000 n.d.Al 53.0 0.3As n.d. 5000 9.08 20Ba 400 100,000 64.7 1000Cd 1.55 1000 n.d. 3Cr n.d. 5000 n.d. 20Cu 177 20Co n.d. 10Fe 4.56 300Hg n.d. 200 n.d. 0.1Ni n.d. 20Pb 631 5000 n.d. 10Se n.d. 1000 n.d. 10

pHFinal 4.87 7.10

n.d. denotes ‘‘not detected’’.a EPA (1997).b High quality water.c Medium quality water.d Highly polluted water (Ministry of Environment and Forestry, 2004).e Limit values for granular waste acceptable at landfills for hazardous waste (EC, 200

Table 4Box–Behnken design with experimental conditions and corresponding leaching recoveries

Exp. no HNO3 (A) (M) Solids ratio (B) (% w/v) Temp. (

1 0.2 2 502 1 2 503 0.2 10 504 1 10 505 0.2 6 306 1 6 307 0.2 6 708 1 6 709 0.6 2 30

10 0.6 10 3011 0.6 2 7012 0.6 10 7013–15 0.6 6 50

SPLP test indicated relatively high leachability of copper and alu-minum (Table 3). In EN 12457-2 test, the release of copper wasdetermined to be �2.3-fold higher than the regulatory limitapplied for hazardous waste acceptable at landfills (Table 3). Theseresults justify the need for development of a suitable treatmentprocess for STVB to remove/recover hazardous, but, valuable met-als prior to landfilling.

3.2. Nitric acid leaching of STVB

3.2.1. Effects of parameters on leaching of copperA total of 15 tests within a Box–Behnken design were per-

formed. The experimental conditions and the corresponding datafor the leaching kinetics and the extraction of copper were pre-sented in Table 4. Fig. 2 also illustrates typical leaching profilesfor the time-dependent extraction of copper under the influenceof leaching parameters. The leaching kinetics was evaluated andfound to be consistent with shrinking particle model Eq. (7):

1� ð1� RÞ1=3 ¼ kt ð7Þ

where R, k and t are the leached fraction (0 6 R 6 1), rate constant(min�1) and time (min), respectively. The initial rate constants (k)were determined by linear-regression analysis with consistentlyhigh correlation coefficients (Table 4).

EN 12457-2

) Type IIc (ppb) Type IVd (ppb) Leachate (ppb) Regulatory limite (ppb)

n.d.0.3 >1 29.950 >100 n.d. 252000 >2000 19.9 3005 >10 n.d. 550 >200 n.d. 7050 >200 230 10020 >200 n.d.1000 >5000 9.880.5 >2 n.d. 250 >200 n.d. 4020 >50 8.39 5010 >20 n.d. 7

7.59

3).

for copper (%) over 120 min.

C) (�C) Extraction of Cu (%) Rate constant

t = 120 min k � 1000 (1/min) R2

29.3 0.92 0.9982.4 4.27 0.916.1 0.29 0.9943.2 2.70 0.997.1 0.39 0.9919.1 0.85 0.9926.6 1.66 0.9947.3 3.02 0.9918.1 0.95 0.968.1 0.58 0.9956.3 2.80 1.0014.7 0.73 0.9836.4 ± 0.7 1.47 ± 0.15

Fig. 2. Time-dependent extraction of copper from the STVB under the influence of (a) nitric acid concentration (2% w/v and 50 �C), (b) temperature (0.6 M HNO3 and 2% w/v)and (c) solids ratio (0.6 M HNO3 and 70 �C).

A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159 155

The copper extraction data obtained at 120 min were used inthe analysis of data. The relative standard deviation (RSD) of theextraction data was calculated to be 61.80%. The statistical analy-sis of data for the effect of leaching parameters on the extraction ofcopper was performed based on the regression model Eq. (8)derived:

R ¼ 36:40þ 15:36A� 14:25Bþ 11:56C þ 2:29A2 þ 1:56B2

� 13:66C2 � 4ABþ 2:18AC � 7:9BC ð8Þ

Table 5Analysis of variance (ANOVA) of the regression model derived from the data.

Source Degree of freedom Sum of squares Adjusted sum

Modela 9 5666 5666Linear 3 4582 4582Square 3 752 752Interaction 3 333 333Residual error 5 430 430

Total 14 6096

a R2 = 0.93.

Table 6Estimated regression coefficients and statistical significance test of the improved model te

Source Sum of squares df

Modela 5556.937 5A – HNO3 1888.051 1B – Solids ratio 1624.5 1C – Temperature 1069.531 1BC 249.64 1C2 725.2146 1Residual 539.1563 9

Total 6096.09 14

a R2 = 0.91; Adj R2 = 0.86; Pred R2 = 0.76; Adequate Precision = 14.40.

The analysis of variance (ANOVA) (Table 5) confirmed that themodel Eq. (8) was statistically significant at a confidence level of95% (a = 0.05). The linear (main) effects with a contribution of75.2% were detected to be statistically important at a = 0.05(Table 5) while square (quadratic) and interaction effects werefound to be insignificant at the same confidence interval.

The regression coefficients of the model Eq. (8) were estimated.However, the statistical analysis of the model terms revealed thatall the linear (main) terms and only square (quadratic) term of

of squares Adjusted mean square P-value Contribution (%)

630 0.021 93.01527 0.004 75.2

251 0.140 12.3111 0.374 5.5

86 7.0

100

rms.

Mean square F value P-value Prob > F

1111.387 18.55211 0.00021888.051 31.51677 0.00031624.5 27.11737 0.00061069.531 17.85342 0.0022249.64 4.167178 0.0716725.2146 12.10583 0.006959.90625

156 A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159

temperature (C2) were significant at a = 0.05. Therefore, thoseinsignificant terms with P values greater than 0.10 were eliminatedto improve the model Eq. (9) (Table 6). The model P-value impliesthat there is only a 0.02% chance that a ‘‘Model F-Value’’ this large

Fig. 3. Surface plots showing the simultaneous effects of dual parameters on the extractilevel), (a) concentration of HNO3 and solids ratio (AB), (b) concentration of HNO3 and te

could occur due to noise. The predicted R2 reasonably agrees withthe adjusted R2 as they are required to be within 0.20 of each other.‘‘Adequate Precision’’, which measures the ‘‘signal to noise ratio’’and is desired to be greater than 4, indicates an adequate signal

on of copper (% Cu at 120 min) from STVB (the third parameter is held at the centermperature (AC) and (c) solids ratio and temperature (BC).

Fig. 4. The Arrhenius plot for the leaching of copper based on the rate constantsobtained from shrinking core model.

Fig. 5. Effect of increasing the concentration of nitric acid (1–5 M HNO3) on theextraction of copper at 6% w/v STVB and 70 �C.

80

A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159 157

(Table 6). The absolute values of the regression coefficients(Table 6) suggest that the concentration of HNO3 is the most signif-icant parameter affecting the leaching of copper. These data alsostatistically corroborates the adverse effect of solids ratio on theextraction of copper.

R ¼ 38:60þ 15:36A� 14:25Bþ 11:56C � 13:66C2 � 7:9BC ð9Þ

Simultaneous effect of dual parameters on the extraction ofcopper can be easily observed by the response surface plots(Fig. 3). Fig. 3a and b illustrates that an increase in the concentra-tion of HNO3 significantly enhanced the dissolution of copper at alllevels of solids ratio (Fig. 3a) and temperature (Fig. 3b). The leach-ing of copper was significantly improved by 53% with increasingthe concentration of HNO3 from 0.2 to 1 M at a solids ratio of 2%w/v (Table 4). In compliance with the current findings (Fig. 3aand b), some researchers [33,35,45,46] reported the positive effectof initial concentration of HNO3 on the recovery of copper fromWPCBs by nitric acid solutions. Demir et al. [47] also investigateddissolution of metallic copper (�355 + 300 lm) in nitric acid solu-tions and reported that increasing the HNO3 concentration from10% to 40% w/w lead to a 1.3-fold improvement in the dissolutionof copper over a reaction period of 30 min.

High solids ratio exerted an adverse effect on the dissolution ofcopper (Fig. 3a and c), apparently due to the increased reagent (i.e.HNO3) consumption e.g. 82.4% Cu recovery 2% w/v (Exp. 2) cf.43.2% Cu (Exp. 4) at 10% w/v (Table 4). Long Le et al. [35] studiedthe extraction of copper from pre-concentrated (air separation)WPCBs (49.3%, �1000 + 63 lm) in nitric acid solutions. They foundthat increasing solids ratio particularly at P12% w/v exerted a det-rimental effect on the metal extraction. Demir et al. [47] alsoreported a �3-fold decrease the dissolution of metallic copper withincreasing the solids ratio from 0.25 g to 2 g per 100 mL of leachingsolutions.

Fig. 3b and c illustrates the positive effect of temperature on theextraction of copper as also noted in the previous studies[33,35,45,46]. The activation energy (Ea) from the Arrhenius equa-tion was calculated to be 38.6 kJ/mol, indicating that the dissolu-tion of copper is a chemically controlled process (Fig. 4). In thisregard, Demir et al. [47] also reported that the dissolution of metal-lic copper in HNO3 solutions is controlled by chemical surface reac-tions with activation energy of 47.5 kJ/mol.

Experimental data were also analyzed for the extraction kinet-ics using the initial rate constants (k) as the response (Table 4). Therelative magnitude of the effect of parameters was found to be inthe decreasing order of HNO3 concentration, solids ratio and tem-perature (data not shown). The increase in HNO3 concentrationand temperature positively contributes to the extraction kineticswhile solids ratio had a detrimental effect.

0

20

40

60

Ext

ract

ion

of A

g (%

)

1 M HNO3 2 M HNO3 3 M HNO 5 M HNO33

Fig. 6. Effect of the concentration of nitric acid (1–5 M) on the extraction of silverover 120 min at 6% w/v solids and 70 �C.

3.2.2. Effect of high concentrations of HNO3

The analysis of data from the Box–Behnken design of experi-ments has suggested that even higher extractions for copper maywell be achieved at higher concentrations of HNO3. Therefore,further tests were designed at 6% w/v solids ratio, 70 �C and theconcentrations of HNO3 even higher than those tested in theBox–Behnken design (i.e. 1–5 M HNO3). Increasing HNO3 concen-tration from 1 to 5 M significantly improved the rate and extentof copper extraction i.e. a 4.6-fold improvement in leaching kinet-ics and 52.4% increase (i.e. from 47.3% to 99.7%) in copper extrac-tion at 120 min (Fig. 5). Fig. 5 also shows that almost completeextraction (P99%) can be achieved at HNO3 concentrations ofP3 M over 120 min.

Silver is readily soluble in nitric acid solutions Eq. (10) [48]. Inaddition to copper, the extraction of silver from STVB (Fig. 6) wasalso determined in these tests. The extraction of silver increased

from 14.4% to 68.2% with increasing the concentration of HNO3

concentration from 1 to 5 M.

3Ag0 þ 4HNO3 ! 3AgNO3 þ NOðgÞ

þ 2H2O DG0ð20 �CÞ ¼ �51 kJ=mol

� �ð10Þ

158 A.D. Bas et al. / Separation and Purification Technology 130 (2014) 151–159

Naseri Joda and Rashchi [46] also observed the beneficial effectof HNO3 concentration on the leaching of silver from WPCBs.Despite the almost complete extraction of copper at 5 M HNO3, sil-ver extraction was limited to 68.2%. This was apparently due to thehigh reduction potential of silver (0.80 V) compared with that ofcopper (0.34 V) suggesting that even higher HNO3 concentrations(or the maintenance of high redox potential) are required for com-plete extraction of silver. It can be also inferred that selectiveextraction of copper over silver can be achieved by controllingthe concentration of nitric acid. In this regard, Kinoshita et al.[34] proposed a two-stage leaching process for a sample of non-mounted wiring boards, based on the selective leaching of nickelin dilute HNO3 (0.1 M) followed by the second stage leaching ofcopper in more concentrated HNO3. They did not present data forsilver. Furthermore, the limited extraction of silver could be alsoattributed to the precipitation of silver as AgCl (Ksp = 1.77 � 10�10)and AgBr (Ksp = 5.35 � 10�13) only if the release of Cl�/Br� occurredfrom STVB during the leaching process. Further detailed studies arerequired to confirm this.

The overall scheme for management of STVB adopted withinthis study offers environmental and economic benefits. Heat treat-ment for the recovery of electronic components for re-use is appar-ently the best approach considering the cost and complexity oftreatment via recycling and manufacturing of these components.Furthermore, it allows the extensive removal of hazardous andimpurity metals such as Sb, Pb, Tl and Fe to low levels (Table 1).This has also potential benefits for leaching with reduced acid con-sumption and for downstream recovery processes with the likelyelimination of costly solution purification stage. The current find-ings indicated that the concentration of nitric acid is the mostimportant parameter for the leaching of copper and silver. Nitricacid consumption would probably determine the economics ofleaching process. In this regard, regeneration of nitric acid fromthe reaction products (NO/NO2) and enrichment of metal contentwith an effective method of separation would improve the eco-nomics of overall management scheme. The former would haveenvironmental benefits allowing the control of hazardous NO/NO2 fumes.

4. Conclusions

In this study, the leachability of metals from manufacturingscrap TV circuit boards (STVB) was studied to show the pollutionpotential of such waste in landfills. Development of a treatmentprocess for WEEE such as STVB based on nitric acid leaching wasinvestigated to eliminate hazardous components and recover valu-able metals before landfilling. In nitric acid leaching tests, theeffects of nitric acid concentration, solids ratio and temperatureon the nitric acid leaching of copper from a low grade manufactur-ing scrap TV circuit boards (STVB) were demonstrated using theBox–Behnken design. The kinetics of copper extraction appearedto be consistent the shrinking particle model. The concentrationof nitric acid was identified to be the most significant parameterfor the rate and extent of copper extraction. The extraction of cop-per is enhanced with increasing the temperature from 20 to 70 �Cwhilst it is adversely affected by increasing solids ratio. A nitricacid concentration of P2–3 M was required to achieve high copperextractions (88.5–99.9%) at 6% w/v solids ratio and 70 �C. Theleaching of copper is a chemically controlled reaction withactivation energy of 36.8 kJ/mol. The extraction of silver was deter-mined to be controlled closely by the concentration of nitric acide.g. 14–68% Ag extractions at 1–5 M HNO3. These findings suggestthat copper and silver could be readily/effectively recovered bynitric acid leaching from such scrap TV boards. The recovery ofthese valuable metals from STVB is also of prime importance from

environmental view point considering the high leachability of cop-per, in particular. Based on these findings, a management schemeinvolving heat treatment for recovery of components followed bynitric acid leaching for recovery/removal of hazardous, but, valu-able metals can be proposed for safe disposal of STVB.

Acknowledgements

The authors would like to express their sincere thanks toResearch Foundation of Karadeniz Technical University (ProjectCodes: 889 and 8647) for their support and to Vestel Electronics(Turkey) for kindly providing scrap TV boards.

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