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Hydrometallurgy
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Recovery of gold and silver from spent mobile phones by means of acidothiourealeaching followed by adsorption using biosorbent prepared from persimmon tannin
Manju Gurung a, Birendra Babu Adhikari b, Hidetaka Kawakita a, Keisuke Ohto a,Katsutoshi Inoue a,⁎, Shafiq Alam b
a Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, 840-8502, Honjo-1, Saga, Japanb Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John's, NL, Canada A1B 3X5
Article history:Received 1 April 2012Received in revised form 2 October 2012Accepted 18 December 2012Available online 22 December 2012
Keywords:Spent mobile phonesPrecious metalsAcidothiourea leachingAdsorptionPersimmon extract
A new approach of recovering precious metals from printed circuit boards (PCBs) of mobile waste byacidothiourea leaching followed by selective adsorption on low-cost and environmentally benign biomasssorbent prepared from easily available agricultural waste is suggested. The influence of various parameterslike thiourea (TU) concentration, acid concentration, pulp density, temperature, and contact time werestudied for gold and silver leaching using a batchwise method. It was found that the PCB sample with small-er particle size distribution (53 to 75 μm) yielded higher amounts of gold and silver being leached into thesolution. The optimum conditions for the leaching of gold were found to be 0.5 M TU in 0.05 M H2SO4 at45 °C, while that of silver were 0.5 M TU in 0.01 M H2SO4 at 60 °C. Under the optimum leaching conditions,an average of 3.2 mg/g of gold and 6.8 mg/g of silver were extracted from incinerated sample of PCBs of mo-bile waste. Kinetic studies revealed that the complete leaching of silver was achieved in less than 2 h, whileit took approximately 6 h at ambient temperature in the case of gold. Presence of 0.01 M ferric ions facili-tated the gold dissolution rate and complete leaching was achieved within 2 h. Conventional copper ce-mentation for recovery of gold and silver from pregnant leached liquor was not so effective. Adsorptiverecovery of dissolved gold and silver using activated carbon as well as crosslinked persimmon tannin gelwas also examined. Low-cost persimmon tannin extract crosslinked with sulfuric acid was found to be apromising material for the complete recovery of gold and silver from the leached liquor. This adsorbentnot only adsorbed the dissolved precious metals selectively but also reduced the adsorbed cationic speciesof gold to the elemental gold.
As a result of rapid technological advancement in recent years,newer, cheaper and more advanced electronic gadgets are continu-ously replacing the old ones. This has not only increased the massconsumption but also the lifespan and lifecycle of electronic prod-ucts are rapidly changing due to individual's changing lifestyle andaspirations. Among such electronic devices, the use of mobile phoneshas rapidly grown in the last few years. Consequently, mobile phonesare causing a large problem because they are much frequently re-placed andnowconstitute the fastest growing component ofwaste elec-tric and electronic equipment (WEEE) (Robinson, 2009). In terms ofmaterials and components, WEEE is non-homogeneous and very com-plex (Hagelüken, 2007; Hagelüken and Meskers, 2009). Mobile phonesare similar in composition to other electronic devices and are consistedof plastics, ceramics, glass and metals (Osibanjo and Nnorom, 2008).
. Inoue).
l rights reserved.
Printed circuit board (PCB) is a key component of WEEE, which, as a re-source, contains variousmetals of interest (Park and Fray, 2009). PCBs ofWEEE can be considered as a significant secondary rawmaterial for pre-cious and special metals (Hagelüken, 2007). Consequently, in order toprevent the resource depletion, recycling of WEEE for recovery of valu-able metals is an important subject. From an economical point of view,recovery of preciousmetals fromWEEE is attractive because the contentof precious metals in waste PCBs is almost 10 times higher than those ofrich-content ores (Guo et al., 2008; He et al., 2006; Li et al., 2007).
In the past two decades, much attention has been devoted to thedevelopment of techniques for recycling of WEEE. In this context,the most active research on the recovery of metals from waste PCBsand electronic scraps has been focused on hydrometallurgical tech-niques which are reportedly more environment friendly, predictable,and easily controlled due to mild working conditions. In addition,hydrometallurgy leads to a higher recovery rate due to relative ease inleaching of product and the possibility of cascading (Cui and Zhang,2008; Kamberović et al., 2009; Kamberović et al., 2011; Liew, 2008;Quinet et al., 2005). For the recovery and recycling of metal valuesfrom WEEE, the complex materials such as PCBs are first subjected
85M. Gurung et al. / Hydrometallurgy 133 (2013) 84–93
to pyrometallurgical treatment in integrated copper or nickel smelterswhich are later subjected to hydrometallurgical process (HagelükenandMeskers, 2009; Tuncuk et al., 2012). However, there are only a lim-ited number of big integrated smelters in Japan which are also beingdiscouraged due to the severe environmental problem caused by off-gas emission in association with the problem of stable supply of metalrich ores suitable for smelting in the near future. As a consequence, inrecent years, attempts are being made for local treatment of E-wastein small as well as medium sized plants by environmentally benignprocess. Keeping in view the aforementioned adequacy, developmentof a purely hydrometallurgical route for the recovery of preciousmetal values from waste electronics is an urgent need.
The hydrometallurgical process consists of leaching and recoveringof metal values from the leached liquor. In general, leaching is the keystep of PCB recycling through hydrometallurgical route which involvesextraction of metal values into aqueous solution. A recent review de-scribes the currently used techniques for the extraction of metals fromE-waste by using either chemical or biological leaching (Pant et al.,2012). Though the process has some environmental consequences tobe considered, chemical leaching is rapid and efficient over biologicalleaching. In addition, quantitative leaching of metal values, alone bybiological leaching is not possible in most of the cases. Consequently,chemical leaching is the process that should be considered for effi-cient leaching of metals from E-wastes by the utilization of less toxicchemicals.
Cyanide leaching of gold and silver from their respective ores byusing alkaline solution of sodium cyanide has been practiced forover a century due to its simplicity and efficiency. However, theprocess uses toxic chemicals and has restricted disposal problems.In recent years, due to increased environmental pressure to ban orlimit the use of cyanide, there has been an increased interest to-wards alternative non-cyanide lixiviants such as halogens (Hilsonand Monhemius, 2006), thiocyanate (Kononova et al., 2007), thiosul-fate (Abbruzzese et al., 1995; Jeffery, 2001), thiourea (Chen et al.,1980; Deschênes and Ghali, 1988; Lacoste-Bouchet et al., 1998; Liand Miller, 2007; Murthy and Prasad, 1996; Murthy et al., 2003) etc.Some of these alternatives not only offer a safer and environmentallybenign method of leaching but they have also demonstrated the rapidleaching rates than the conventional cyanide one (Chen et al., 1980;Ficeriová et al., 2008; Groenewald, 1976). With regard to its verylow toxicity and rapid kinetics of gold and silver leaching, acidic thio-urea is the most promising among these alternative lixiviants andthiourea leaching of gold and silver from various wastes or concen-trates is getting increasing attention. In addition, since thiourea ispotentially capable of forming coordination bonds with gold and sil-ver through the lone pairs of electron on nitrogen and sulfur atoms,dissolution of base metals such as copper by thiourea is much lessthan that by cyanide solution (Chen et al., 1980; Lacoste-Bouchetet al., 1998). Accordingly, higher selectivity to precious metals overbase metals is the additional advantage of thiourea which helps it toachieve commercial application.
The competence of acidic thiourea for the leaching of gold and sil-ver from their ores compared to alkaline sodium cyanide solutionunder comparable conditions was examined by Eisle et al. (1981). Itwas found that while NaCN extracted more gold and silver at 2 g/lextractant levels (the commonly used level in cyanidation of preciousmetal ores) of both reagents, thiourea extracted more gold and silverat elevated concentrations. Evaluation of leaching parameters such asconcentrations of thiourea and acid, temperature, pH, pulp density,contact time, amount of oxidizing agent, acid pretreatment and phys-ical beneficiation of the ore, etc. for the leaching of gold and silverfrom their respective ores were conducted in a number of studies soas to optimize the metal dissolution and reduce the reagent con-sumption (Chen et al., 1980; Deschênes and Ghali, 1988; Ficeriováet al., 2008; Groenewald, 1976; Lacoste-Bouchet et al., 1998; Li andMiller, 2007; Murthy and Prasad, 1996; Murthy et al., 2003). Recently
Ha et al. (2010) evaluated the thiosulfate leaching of gold from scrapsand PCBs ofmobile phones.While 98% of gold was leached out from thescrap samples using a solution containing 20 mMCu, 0.12 M thiosulfateand 0.2 Mammonia, only 90% of goldwas leached fromPCBs containingan average of 0.12% gold content (Ha et al., 2010). Kamberović et al.(2009) investigated the acidic thiourea leaching of gold from the solidresidue of mechanically pretreated waste PCBs remained after leachingof copper with sulfuric acid. For laboratory scale experiment, optimumgold leaching was achieved by the consumption of 10 g dm−3 H2SO4,20 g dm−3 thiourea and 6 g dm−3 Fe3+ as oxidant. Although smallervolumes of leachant were used (S/L ratio=1/5), considerably higheramounts of sulfuric acid were consumed for stepwise leaching. In addi-tion, higher concentration of Fe3+ solution (0.1 M)was required for theleaching of gold. Consequently, a systematic study is necessary for opti-mization of leaching parameters so as to develop an eco-friendly andefficient process for the leaching and recycling of gold and silver fromPCBs of WEEE.
Recovery of metal values from leached liquor is another essentialstep in hydrometallurgical operations. Contrary to cyanide, thiocya-nate and thiosulfate leaching, thiourea forms cationic gold speciesbecause of which a variety of options such as cementation withsome base metals (Lee et al., 1997; Nguyen et al., 1997; Sulka andJaskula, 2002; Zhang et al., 1996), adsorption on activated carbon(Schmidt et al., 1993; Zhang et al., 2004), direct electrowinning (Juarezand Dutra, 2000), solvent extraction and ion exchange (Becker et al.,1983; Conradie et al., 1994; Nakahiro et al., 1992) etc. are available forsubsequent preconcentration and purification of leached liquor. The re-cent interest on thiourea leaching of gold and silver has probably beeninsisted mainly due to the cationic properties of the complex so thatthe most exciting applications appear in the field of ion exchange andadsorption. Utilizing this distinct possibility, activated carbon and cationexchange resins have been reported to exhibit excellent adsorption be-havior for gold–thiourea complex and are employed as commerciallyavailable sorbents to concentrate gold (Conradie et al., 1994; Nakahiroet al., 1992; Zhang et al., 2004). However, such commercially availablesorbents are relatively expensive in terms of cost effectiveness. Conse-quently, in recent years, interest has been shifted to sorption of preciousmetals from acidothiourea solution on biomass-based adsorbents.The rice husk carbonized at 573 K and Chlorella vulgaris heated at673 K, containing specific silanol [\Si(OH)] and oxygen containingfunctional groups [C=O, \COO\ etc.] of carbon, were used to ad-sorb gold–thiourea complex (Nakbanpote et al., 2000, 2002).
The aim of this work is to establish the optimum dissolution pa-rameters for the leaching of gold and silver from PCBs of mobilewaste with acidothiourea solution and to develop a more sustainable,eco-friendly alternative to preconcentrate and recover the dissolvedmetals by using biomass-based material as an active adsorbent forthe sorption of cationic species of gold and silver with thiourea. Thismanuscript reports a systematic study of each parameter associatedwith hydrometallurgical operation for dissolution of gold and silverpresent in PCBs of mobile waste and quantitative recovery of dissolvedmetal values by adsorption on the sorbent prepared from persimmontannin.With this, a new approach for the recovery and recycling of pre-cious metals from the PCBs of mobile waste by acidothiourea leachingfollowed by selective adsorption on low-cost and environment friendlybiomass sorbent prepared from easily available agricultural waste isproposed.
2. Experimental
2.1. Preparation of the sample of circuit board waste from mobile phonesand its characterization
Samples of PCBs of spent mobile phones were kindly supplied byShibata Industry Co. Ltd., Omuta, Japan, and sent to Shonan Factoryof Tanaka Kikinzoku Kogyo K.K., Hiratsuka, Japan, where they were
86 M. Gurung et al. / Hydrometallurgy 133 (2013) 84–93
mechanically crushed, and calcined at 750 °C for 6 h to burn offepoxy resin on PCBs. The calcined residue in powdered form wasemployed for the experimental work without further pretreatment.The powdered sample was comprised of various sizes rangingfrom b53, 53–75, 75–150, 150–300, and >300 μm, which was mea-sured by using different sieve sizes of a mechanical shaker (NonakaRikaki Co., Ltd.). The qualitative and quantitative analyses of themetal values present in the powdered sample were performed bytotal dissolution test using aquaregia at its boiling temperature. Thechemical composition of the sample was qualitatively determined byenergy dispersive X-ray (EDX) analysis using Rayny model EDX-800HS energy dispersive X-ray spectrometer (Shimadzu). The surfacemor-phology of the powdered PCB was observed by a digital microscope(VHX-1000, Keyence) as well as by a scanning electron microscope(S-3000N, Hitachi).
2.2. Leaching experiment
From the preliminary leaching test of various sizes of powderedsamples, the fraction with the smallest particle size was found toexhibit the highest quantity of dissolved metal contents. Thus, theparticle size of the powdered sample employed in the present exper-iment was below 75 μm. The varying volume of leached liquor wasmixed with the weighed amount of powdered sample (200 mg) ina conical flask, and the heterogeneous mixture was shaken in athermostated shaker at a specific temperature for given time periodto evaluate the effect of various parameters on the leaching reaction.The samples were then filtered and metal concentration in the filtratewas measured after proper dilution with distilled water.
The factors affecting the leaching reaction include concentrationof thiourea (TU) and sulfuric acid, pulp density, shaking time, temper-ature, presence or absence of external oxidant, etc. These factors werethoroughly examined. The volume of lixiviant was varied from 10 to70 ml for 0.2 g of the calcined sample of PCBs so that the pulp densityvaried from 0.00285 to 0.02 g/ml. The effects of TU concentration andpulp density were examined by varying the pulp density at variousconcentrations {0.03 to 1.0 M (M=mol dm−3)} of TU while keepingsulfuric acid concentration constant at 0.05 M and shaking the het-erogeneous mixture at 30 °C for 24 h. Likewise, the effects of sulfuricacid concentration and pulp density were examined by varying thepulp density at various concentrations of sulfuric acid ranging from0.01 to 1.0 M at constant TU concentration of 0.5 M and shaking at30 °C for 24 h. The effect of temperature was investigated at varioustemperatures ranging from 30 °C to 60 °C using the leachant with op-timum TU and sulfuric acid concentration for respective metals at thepulp density of 0.00285 g/ml, i.e., using 0.2 g PCB sample and 70 mllixiviant.
In all cases, the metal concentrations in the filtrate were measuredby inductively coupled plasma atomic emission spectrometer (ICP-AES,Shimadzu ICPS-8100) and/or atomic absorption spectrophotometer(AAS, Shimadzu 6800). The residue of the powdered sample after theleaching was also analyzed by means of the EDX analyzer to ensurethe complete dissolution of target metal values.
2.3. Preparation of adsorption gel from persimmon tannin extract
A sample of dry persimmon tannin powder (abbreviated as PTpowder hereafter) extracted from astringent persimmon was kindlydonated by Persimmon-Kaki Technology Development, Co. Ltd.,Jincheng, China. Adsorption gel was prepared from persimmontannin extract by treating it with concentrated sulfuric acid forcrosslinking between polymermatrices of tannin and/or polysaccha-rides existing in the extract together with tannin. Fifteen grams offine PT powder was mixed with 30 ml of concentrated sulfuric acidin a 200 ml three-necked round bottom flask and the slurry wasstirred for 24 h at 100 °C, in order to enhance the condensation
reaction for crosslinking. The acid treated black product was neutral-ized with sodium bicarbonate solution and filtered. The filter cakewas washed with distilled water and dried in a convection oven at70 °C for 24 h. Finally, the dried mass was crushed into a powder ofuniform particles up to 150 μm mesh size. The dried rigid mass ofcrosslinked persimmon tannin gel, abbreviated as CPT gel hereafter,was used for the adsorption experiments.
2.4. Recovery of gold and silver from acidothiourea leachate
After the leaching of gold and silver by using acidothiourea solution,various techniques such as precipitation by reduction with metalhydride, cementation using copper powder, and adsorptionwere testedfor their recovery from the leached liquor. In the cementation test, vary-ing weights of copper powder were mixed together with 10 ml of theleached liquor and the mixture was shaken at 30 °C for 12 h. In therecovery tests bymeans of adsorption, commercially available activatedcharcoal (granular size of 2–5 mm, prepared from peat and purchasedfrom Wako Pure Chemical Industries Ltd., Japan) and CPT gel wereemployed to adsorb gold–thiourea (Au–TU) and silver–thiourea(Ag–TU) complexes. The adsorption tests were carried out accordingto the conventional batchwise method. In a typical run, varyingweights of the adsorbent were mixed together with 10 ml of theleached liquor and the heterogeneous mixture was shaken at 30 °Cfor 24 h, after which the mixture was filtered and the concentrationsof metal ions before and after the adsorption were measured todetermine the extent of adsorption.
3. Results and discussion
3.1. Sample characterization
It was found that, while the particle size of one-fourth of the amountof the sample was below 75 μm,most of the particle sizes were distrib-uted above 300 μm. The results of particle size distribution of the cal-cined sample are summarized in SF1 (supporting information). A SEMphotomicrograph of the calcined PCBs of mobile waste (SF2) revealedthe extruded shape consisting of aggregated small particles with veryirregular shapes.
3.2. Qualitative analysis of metals present in calcined PCB sample
The results of the quantitative analysis of various metals in thesample obtained after total dissolution of the as received samplewith different lixiviants is presented in Table 1, which indicates thatthe scrap of PCBs is a complex material consisting of a variety of con-stituents in different proportions. The major elements present in thescrap were found to be copper, nickel, iron, lead and aluminumalong with some gold and silver. The qualitative assay of the freshsample by EDX (SF3) also supports the results of the quantitativeanalysis of metal values done by dissolution test and measured byICP-AES. The predominant elements present in PCBs, as identified byEDX, are Cu, Ni, Pb, and Fe. From the data presented in Table 1 andEDX analysis of the sample, it is clear that Cu is the main elementpresent in PCBs together with Pb, Fe, and Ni, contributing to the sig-nificant proportion of the metallic constituent of the scrap. Eventhough the precious metals constitute a relatively small proportionof the scrap, it is obvious that PCBs of spent mobile phones are out-standing secondary sources of those highly valued metals.
3.3. Optimization of leaching parameters
3.3.1. Effect of particle sizeThe effect of particle size on the amount of gold and silver leached
was examined by using the calcined PCBs of different particle sizes(>300, 150–300, 75–150 and 53–75 μm), keeping all other variables
Table 1Quantitative analysis of metals (mg/g) present in PCBs leached with different lixiviantsand measured quantitatively by ICP-AES.
a 1.0 g powdered sample was mixed with 50 ml of lixiviant and the mixture wasboiled for 30 min. Then, the mixture was allowed to cool to room temperature andcontinued shaking for overnight.
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Fig. 1. Effect of TU concentration and pulp density on gold (a) and silver (b) leachingfrom the calcined PCB sample. Acid=0.05 M H2SO4, contact time=24 h at 30 °C,and shaking speed=150 rpm.
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such as the TU (0.5 M) and acid concentration (0.05 M), agitationspeed (500 rpm), and temperature (30 °C) constant for 24 h of shak-ing. The results are presented in Table 2, from which it is evident thatthe fraction with the smallest particle size yielded the highest quanti-ty of dissolved gold and silver. This is most likely due to the high sur-face area of the smallest particle size fraction and subsequently ahigher proportion of metals exposed to the leached liquor. Thus, forfurther experiments, the sample of particle size less than 53–75 μmwas employed.
3.3.2. Effect of TU concentrationFig. 1(a) shows the results of the effect of TU concentration at
varying pulp density on the leaching of gold at a constant acid con-centration. It is apparent from this figure that the leaching of gold in-creases with decreasing pulp density (i.e. increasing L/S ratio, theratio of volume of lixiviant to the amount of powdered PCB sampleused) and TU concentration from 0.03 up to 0.5 M. The effect of TUconcentration on gold dissolution is not so efficacious when it ishigher than 0.5 M. At lower TU concentration, the gold extraction re-actions are sluggish and the recovery does not exceed 50%, even athigher L/S ratio. It is important to note that the action of TU on goldis effective only at proper TU concentration, where dissolution ofgold proceeds smoothly and the maximum recovery is achieved. Tak-ing account of the cost-effective consumption, 0.5 M TU at pulp den-sity of 0.00285 g/ml is recommended for the leaching of gold fromPCBs of mobile waste.
Fig. 1(b) shows the effect of TU concentration and L/S ratio onleaching of silver from powdered PCBs of waste mobile phones. Similarto the gold extraction, the silver extraction from the present samplealso increased with increasing TU concentration up to 0.5 M and there-after remained constant at higher concentration. The amount of silverleached was found to be maximum at TU concentration of 0.5 M in0.05 M acid solution and pulp density of 0.00285 g/ml. Although silverleachingwas almost quantitativewith 1.0 MTU solution at pulp densityof 0.005 g/ml, the use of concentrated thiourea solution hindered the
Table 2Results of the effect of particle size on the dissolution of PCBs of mobile waste sample.
leaching process due to precipitation. In addition, keeping in view thelow consumption of reagent, using 0.5 M TU solution with pulp densityof 0.00285 g/ml is more frugal than using 1.0 M TU solution with pulpdensity of 0.005 g/ml.
3.3.3. Effect of sulfuric acid concentrationFig. 2(a) shows the relationship between the amount of gold
leached from the calcined sample and pulp density over the rangeof 0.00285–0.020 g/ml at varying sulfuric acid concentrations. It isclear from this result that there is no beneficial effect of increasingacid concentration beyond 0.05 M for the dissolution of gold. Also,as it is evident from this figure, the gold leaching was increased nearlylinearly with decreasing pulp density and quantitative leaching wasachieved at the pulp density of 0.00285 g/ml, regardless of the sulfu-ric acid concentration. Taking the consumption of acid into account,0.05 M sulfuric acid at the pulp density of 0.00285 g/ml seems to beeffective for the leaching of gold.
Fig. 2(b) shows the effects of sulfuric acid concentration and pulpdensity on the dissolution of silver. It was found that a concentrationof 0.01 M was adequate to achieve a high dissolution of silver even athigher pulp density. Concentration higher than 0.01 M was not effec-tive for silver dissolution. This result can be explained by the fact thatlow concentration of sulfuric acid activates the lone-pair electrons onthiourea to facilitate the complexation with silver. On the contrary,high acid concentration protonates the thiourea and impedes thecomplexation, resulting in the low extraction (Biswas et al., 2010).
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Fig. 2. Effect of sulfuric acid concentration and pulp density on gold (a) and silver(b) leaching from the calcined PCB sample. TU concentration=0.5 M, temperature=30 °C, contact time=24 h, and shaking speed=150 rpm.
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Fig. 3. Effect of shaking speed and contact time on gold leaching from the calcined PCBsample. Temperature=30 °C and leachant=0.5 M TU in 0.05 M sulfuric acid.
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3.3.4. Effect of agitation speedTo investigate the influence of agitation speed on the leaching rate
of gold, the dissolution test was carried out by varying the agitationspeed between 150 and 500 rpm using 0.5 M TU in 0.05 M sulfuricacid as leaching agent at 0.00285 g/ml pulp density. Fig. 3 showsthe effect of agitation speed on the amount of gold extracted. The re-sults indicate that the leaching rate is independent of the agitationspeed at higher than 150 rpm, which suggests that the shakingspeed of 150 rpm is sufficient to provide adequate particle suspen-sion. Therefore, in the subsequent experiments, the agitation speedwas kept constant at 150 rpm to ensure effective particle suspension.The recovery of silver was also almost independent of agitation speed(data not shown).
3.3.5. Effect of the oxidant used and contact timeGold and silver dissolution in acidothiourea solution is known to
be an electrochemical reaction catalyzed by the presence of ferricion (Chen et al., 1980; Li and Miller, 2007). As shown in the followingreactions, the role of ferric ion in the complexation process is to facil-itate the oxidation of metallic gold and silver to aurous (Au+) andargentous (Ag+) ions, respectively.
Auþ 2CS NH2ð Þ2 þ Fe3þ→Au CS NH2ð Þ2� �þ
2 þ Fe2þ ð1Þ
Ag þ 3CS NH2ð Þ2 þ Fe3þ→Ag CS NH2ð Þ2� �þ
3 þ Fe2þ: ð2Þ
Reports have shown that ferric sulfate can best speed up theleaching reaction and TU in the presence of ferric ions exhibits a po-tential improvement over the use of TU alone for gold leaching(Chen et al., 1980). Huyhua et al. (1989) found that the leaching ofgold with TU in the presence of ferric sulfate as an oxidant was upto four times faster than TU alone in air. The dissolution of gold andsilver is dependent on the oxidant concentration because, if thisvalue is too high, an irreversible decomposition reaction of thioureaoccurs producing elemental sulfur and cyanamide, which rendersthe gold and silver surface passive and thus decreases the leaching ca-pacity (Farinha et al., 1992; Ubaldini et al., 1998).
Fig. 4(a) shows the effect of the presence of ferric ion on theleaching efficiency of acidothiourea leachate towards gold as a func-tion of time. It can be seen that ferric sulfate can greatly enhancethe leaching rate of gold. In the absence of ferric ion, the extraction in-creased sluggishly with time at the beginning of the leaching and itattained the equilibrium within 6 h. Thus, 6 h shaking is consideredas optimum shaking time to ensure complete leaching of gold fromcalcined PCB sample in the absence of external oxidant. It is notewor-thy that the rate of gold dissolution in the presence of ferric ions wasrapid during the first hour of reaction and the extraction reached thesteady state within 2 h. Additionally, it was found that the dissolutionrate of gold was enhanced significantly by adding smaller amount offerric sulfate. However, when the ferric ion concentration was higherthan 0.01 M, gold recovery was lower (data not shown). Accordingly,0.01 M ferric ion concentration in acidothiourea solution was foundto be the optimum concentration for the leaching of gold in our ex-perimental conditions. Table 1 indicates that the PCBs of mobilewaste contain appreciable amounts of iron. Thus, it is reasonablethat the iron present in the waste serves as a partial supply of ferricion oxidant for TU–gold interaction and only a small additionalamount of ferric ion gives better results of gold dissolution.
Fig. 4(b) shows the percentage of leaching of silver as a function oftime in the presence as well as the absence of ferric ions withacidothiourea solution. It can be seen that there was no beneficial ef-fect of ferric ions on the leaching of silver. Rather, the dissolution ef-ficiency was slightly decreased. These experimental results suggestthat the proper amount of ferric ion oxidant, necessary for the oxida-tion of silver, is supplied by the feed material itself. The difference inleaching behavior of gold and silver in the presence of an oxidizingagent is attributable to the respective ORP values of these metals.Since gold has lower oxidation potential values than silver, it seemsreasonable that the presence of external oxidizing agent enhancesthe gold dissolution.
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Fig. 4. Effect of shaking time in the presence or absence of ferric ion on gold (a) and sil-ver (b) leaching from calcined PCB sample. Leachant=0.5 M TU in 0.05 M H2SO4 forgold and 0.5 M TU in 0.01 M H2SO4 for silver, pulp density=0.00285 g/ml, tempera-ture=30 °C, shaking speed=150 rpm, and added ferric ion concentration=0.01 M.
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(b)
Fig. 5. Effect of temperature on gold (a) and silver (b) leaching from the calcined PCBsample. Leachant=0.5 M TU in 0.05 M H2SO4 in the presence of 0.01 M ferric ions forgold and 0.5 M TU in 0.01 M H2SO4 for silver, pulp density=0.00285 g/ml, and shak-ing speed=150 rpm.
89M. Gurung et al. / Hydrometallurgy 133 (2013) 84–93
3.3.6. Effect of temperatureThe effects of temperature on the extraction rate of gold and silver
are presented in Fig. 5(a) and (b), respectively. Leaching of gold wasexamined in the presence of 0.01 M ferric ions while that of silverwas investigated in the absence of ferric ions, as no beneficial effectwas observed for the leaching of silver in the presence of ferric ionsas mentioned earlier. The results indicate that the extent of gold dis-solution slightly increased with increasing the temperature from30 °C up to 45 °C but decreased with further increasing the tempera-ture to 60 °C. It may be considered that increasing the temperatureactivates the reactant species to promote the reaction, while it wassuppressed at further elevated temperature. Since the gold dissolu-tion was examined in the presence of external oxidant, this may beattributable to the rapid decomposition of TU, resulting in the forma-tion of colloidal sulfur (Groenewald, 1976) and retardation of the re-action through surface passivation. Thus, under our experimentalconditions, 45 °C appears to be the optimum temperature for theleaching of gold. Nevertheless, the results support the fact that sub-stantial dissolution of gold from PCB samples can be achieved withacidothiourea solution in the presence of ferric ions even at ambienttemperature. However, as it is obvious from Fig. 5(b), increasing thetemperature up to 60 °C slightly enhanced the silver dissolutionwith acidothiourea solution in the earlier hours of contact.
3.3.7. Optimum leaching parameters and reagent consumptionFrom the aforementioned findings, optimum leaching parameters
for the dissolution of gold and silver from calcined sample of PCBs ofspent mobile phones with acidothiourea solution can be summarized
as follows. For the leaching of gold, 0.5 M TU in 0.05 M H2SO4 at 45 °Cand 150 rpm shaking speed for 6 h seems to be the best suited condi-tion. Presence of ferric ions as external oxidizing agent facilitated thegold dissolution and shortened the dissolution time from 6 h to 2 h.At the pulp density of 0.00285 g/ml, the amount of reagent consump-tion was 13.32 g TU, 1.71 g sulfuric acid and 1.4 g ferric sulfate (0.2 gFe3+ ions). With this consumption of reagent, an average of 3.2 mg ofgold was extracted from unit gram of the calcined sample. For theleaching of silver, 0.5 M TU in 0.01 M H2SO4 at 30 °C and 150 rpmshaking speed for 2 h seems to be the best suited condition. Consider-ing nearly quantitative leaching of silver even at higher pulp densityof 0.02 g/ml, the amount of reagent consumption was 1.9 g TU and0.5 g sulfuric acid per gram of the sample leached. With this con-sumption of reagent, an average of 6.8 mg silver was extracted fromunit gram of calcined sample. Since the sample used in this studywas not homogeneous, the amount of metal extracted was slightlydependent on various conditions. Nevertheless, it is evident that,under the optimum leaching conditions, nearly 3.2 mg gold and6.8 mg silver can be extracted from one gram of calcined sample.From the comparison of gold leaching by alkaline thiosulfate solutionfrom waste PCBs reported previously (Ha et al., 2010) with the pres-ent results, it is worth mentioning that acidothiourea is more effec-tive than the alkaline thiosulfate solution for the leaching of goldfrom waste PCBs. While 90% of gold was leached from waste PCBscontaining an average of 0.12% Au, suggesting the extraction of1.08 mg gold per gram of PCB sample with thiosulfate leaching
90 M. Gurung et al. / Hydrometallurgy 133 (2013) 84–93
(Ha et al., 2010), acidothiourea extracts nearly three times higheramount of gold from waste PCBs we had examined.
3.4. Composition of acidothiourea leached residue
After the leaching of gold and silver under their optimum leachingconditions, the residue was dried and again leached with boiling aquaregia and the concentration of different elements extracted in aquaregia from the leached residue was analyzed quantitatively to under-stand the composition of acidothiourea leached residue. The quanti-tative analysis of different elements present in the acidothioureaextract of the fresh sample and aqua regia extract of acidothiourealeached calcined PCB is presented in Table 3. It can be understoodfrom the data presented in Table 3 that gold and silver present inwaste PCBs were almost quantitatively leached with acidothioureasolution. Along with gold and silver, acidothiourea solution leachedsignificant amount of copper and some amount of nickel as well. Re-covery of acidothiourea leached copper is, however, beyond the inter-est of this research.
The morphological changes, which occurred in PCB sample afterleaching of metal values, were also observed in SEM image of thesample after leaching the metal values (SF4). It is seen from the com-parison of SEM images presented in SF2 with that in SF4 that the sur-face roughness was decreased after leaching. The absence of peakscorresponding to gold and silver in the EDX spectrum of the calcinedPCB sample after leaching of gold (SF5) and silver (SF6) at their opti-mal leaching condition also indicates that the gold and silver presentin the calcined sample were leached out (almost) quantitatively.
3.5. Recovery of gold and silver by means of cementation from theleached liquor
It has been reported that Cu, Pb, Zn, Ni, and Fe have potentiality toform the metal thiourea complexes, Cu[CS(NH2)2]42+, Pb[CS(NH2)2]42+,Zn[CS(NH2)2]22+, Ni[CS(NH2)]42+, and Fe[CS(NH2)2]23+, respectively(Bombiez et al., 2004; Eaton and Zaw, 1975; Hiskey, 1981). However,as acidothiourea solution is less sensitive to base metals, only 12% Zn,15% Fe, 24% Ni, and 34% Pb (as compared to the content of total dissolu-tionwith aquaregia) were leached alongwith gold and silver. It is desir-able that the leached gold and silver should be selectively recoveredfrom acidothiourea solutions containing the base metals. Reduction–precipitation of gold by treating with metal hydride (Awadalla andRitcey, 1991), electrowinning, and cementation are three proposed
Table 3Quantitative analysis of metals extracted from the calcined PCBs with different lixiviants.
Element Amount extracted from calcined PCB sample (mg/g)a
a Slight differences in the total amount of metal extracted is due to inhomogeneity ofthe matrix used.
b Optimum dissolution condition of gold.c The residue obtained after leachingof gold under optimumconditionwas subjected to
aquaregia leaching.d Optimum dissolution condition of silver.e The residue obtained after leaching of silver under optimum condition was subjected
to aquaregia leaching.
methods for gold recovery that are available elsewhere in the litera-tures. The most popular reductant is sodium borohydride (Awadallaand Ritcey, 1991; Awadalla et al., 1989) and it has been employed toreduce gold and silver from leached liquors on a commercial scale.However, in our case, reduction with sodium borohydride did not ap-pear to be effective in gold and silver recovery from acidothiourea solu-tion since it precipitated the base metals along with gold and silver(data not shown). In addition, due to the toxicity of boron in the envi-ronment, the use of such boron compounds is not recommended.
Cementation is one of the most effective and economical tech-niques widely employed in hydrometallurgical processes of solutionstreams, including the recovery of precious metal values from preg-nant leached liquors. The advantages of the cementation process in-clude its relative simplicity, ease of control and its ability to recovervaluable metals. For gold and silver cementation, there are two simul-taneous reactions. The cationic gold and silver in the complexes Au[SC(NH2)2]2+ and Ag[SC(NH2)2]3+ are reduced to elemental gold andsilver, while the oxidation of less noble metals such as Cu, Fe, Zn,and Al, supplies the necessary electrons for the reduction reaction.
A number of research works have been conducted on cementationof noble metals (Lee et al., 1997; Sulka and Jaskula, 2002; Zhang et al.,1996). Fe, Zn, and Cu have been studied as the reducing metals forgold and silver recovery from acidothiourea solution. Since the standardreduction potential of Zn (−0.76 V) and Fe (−0.44 V) are lower thanthat of Ni (−0.26 V), Pb (−0.13 V), Cu (+0.34 V), Ag (+0.80 V), andAu (+1.50 V), Zn and Fe were considered unsuitable for the cementa-tion process as they would reduce and precipitate some base metalsalong with gold and silver. In fact, Pb, Cu, and Ni were also precipitatedwhen the zinc cementation process was attempted (data not shown).Copper appeared more desirable because it is relatively inexpensiveand has lower reduction potential than those of gold and silver buthigher reduction potential than that of other basemetals. Consequently,copper is expected to selectively reduce gold and silver over basemetalsfrom the leached liquor for the present cementation process. The resultsof cementation test for the recovery of gold and silver using copperpowder are shown in Fig. 6. It is clear from this figure that complete re-covery of silver was achieved by copper cementation at pulp density of0.004 g/ml, while nearly 80% gold was precipitated from acidic Au–TUcomplex by copper cementation process even at lower pulp density(the ratio of copper powder to the volume of leached liquor). The differ-ence in oxidation reduction potential (ORP) values of gold (+1.50 V),silver (+0.80 V), and copper (+0.34 V) certainly plays a major roleto achieve this feat. In this cementation process, copper powder reducessilver(I) and gold(I) to their respective elemental form according to thefollowing reactions.
2Au SC NH2ð Þ2� �þ
2 þ Cu→Cu2þ þ 2Auþ 4SC NH2ð Þ2 ð3Þ
2Ag SC NH2ð Þ2� �þ
3 þ Cu→Cu2þ þ 2Agþ 6SC NH2ð Þ2: ð4Þ
Although the recovery of gold was worthy of consideration atlower pulp density (more than 80% recovery was achieved even atpulp density of 0.001 g/ml), the gold recovery was not increasedwith increasing pulp density up to 0.01 g/ml. It required much higheramounts of copper for the quantitative precipitation of gold. Hence, itis necessary to investigate a more economically viable method for thepreconcentration and recovery of leached gold from the pregnantleached liquor.
3.6. Adsorptive preconcentration and recovery of gold and silver fromleached solutions
The possibility of adsorptive recovery of dissolved gold and silverwas investigated also by using CPT gel and activated charcoal as adsor-bents and the results are presented in Fig. 7(a) and (b), respectively. The
0
20
40
60
80
100
0.001 0.003 0.005 0.007 0.009
% R
ecov
ery
Pulp density (g/ml)
Au
Ag
Fe
Ni
Cu
Zn
Fig. 6. Recovery of gold and silver by means of cementation with copper powder.Liquid=10 ml of gold and silver containing acidothiourea leached solution, shakingspeed=150 rpm, and time=10 h at 30 °C.
0
20
40
60
80
100
0 0.01 0.02 0.03 0.04
% R
ecov
ery
Pulp density (g/ml)
Au
Ag
Fe
Zn
Ni
Cu
0
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0 0.005 0.01 0.015 0.02 0.025 0.03
% R
ecov
ery
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Au
Ag
Cu
Zn
Ni
Fe
(a)
(b)
Fig. 7. Adsorption of gold and silver from acidothiourea leachate by using (a) crosslinkedpersimmon tannin gel (CPT gel) and (b) activated charcoal. Solution=10 ml of gold andsilver containing acidothiourea leached liquor, shaking time=24 h, temperature=30 °C,and shaking speed=150 rpm.
0
100
200
300
400
500
20 30 40 50 60 70 80
2 (degree)
Inte
nsit
y
(b)
(a)
Fig. 8. (a) Digital micrograph and (b) XRD spectrum of CPT gel taken after adsorptionof gold from the leached liquor.
91M. Gurung et al. / Hydrometallurgy 133 (2013) 84–93
results indicate that the percentage of gold and silver adsorptionincreased with the increase in adsorbent dose of both adsorbents.Although the adsorption behavior for gold appears nearly the samefor both adsorbents, CPT gel was found to be more effective than thecommercial activated charcoal for silver adsorption. Quantitative ad-sorption of silver on CPT gel was achieved at pulp density (the ratio ofadsorbent amount to the volume of leached liquor) of 0.015 g/ml andthat of gold was achieved at pulp density of 0.04 g/ml. Interestingly,CPT gel exhibited much less adsorption for base metals, suggesting thepossibility ofmore selective recovery of gold and silver from the leachedliquor.
As reported by Matsuo and Ito (1978), the surface of persimmontannin consists of a number of ionizable phenolic hydroxyl groups,carbonyl groups, and ethereal oxygen atoms, which provide electronpairs to coordinate cationic species of gold and silver. Accordingly, theadsorption is inferred to occur by the combination of coordinationand cation exchange reactions with ionizable groups on the surfaceof the adsorbent. In the case of gold adsorption on CPT gel, it wasalso found that the adsorbed species of gold was reduced to elementalgold by the subsequent reduction reaction by adjacent phenolicgroups functioning as reducing functional groups (Gurung et al.,2011). As shown in Fig. 8(a), the digital micrograph image of theCPT gel taken after the adsorption of gold from the leached liquor,fine gold particles were distinctly observed on the surface of theadsorbent, confirming that the adsorbed species of gold was subse-quently reduced to the elemental gold. Reduction of adsorbed goldto metallic form is also evidenced by the X-ray diffraction (XRD) spec-trum of CPT gel taken after adsorption (Fig. 8(b)). The existence ofsharp peaks on the XRD spectrum at 2θ values of 22.8, 44.08, 64.42,and 77.56 confirms the reduction of gold(I) of gold–TU complex toelemental gold.
In a coagulation test for the recovery of silver from Ag–TU com-plex using a coagulant prepared by dissolving persimmon tanninextract at pH 13.8, Biswas et al. (2010) observed the deposition ofmetallic silver at the surface of precipitation cake and postulatedthe reduction of Ag(I) to its metallic form by phenolic groups of per-simmon tannin. Since the peaks for elemental gold and silver appearat the same 2θ values (Singh et al., 2011), the sharp peaks observed inXRD spectrum (Fig. 8(b)) are attributable to that of elemental gold aswell as silver. Consequently, it is not unreasonable to propose the re-duction of Ag(I) to elemental silver with the phenolic groups presentin persimmon tannin. Nevertheless, since the reductive phenomenoninvolves many parameters and is a complex process, detailed investi-gation is necessary in order to fully understand the mechanism.
The adsorption of Au–TU and Ag–TU complex by the activatedcharcoal is ascribed to, in addition to high porosity and high surface
92 M. Gurung et al. / Hydrometallurgy 133 (2013) 84–93
area, the presence of many functional groups on the surface of acti-vated charcoal which are responsible for the sorption of metallic spe-cies (Deschênes, 1987; Lee et al., 1997).
3.7. Overview of the whole work
A complete flow-sheet recommended for the recovery of gold andsilver from printed circuit board of outmoded mobile phones, throughthe combination of leaching followed by sorption is depicted in Fig. 9.
4. Conclusions
The results of our work on optimization of the acidothiourealeaching condition for lixiviation of gold and silver from PCB sample,and the subsequent recovery of dissolved metal values led to thefollowing conclusions.
(a) Acidic solution of thiourea can be used as an effective alterna-tive for cyanide leaching of gold.
(b) The elemental gold present in scraps of PCB sample was effec-tively leached with 0.5 M thiourea solution in 0.05 M sulfuricacid. The gold dissolution process was enhanced three timesin the presence of ferric ions but higher ferric concentrationretarded the dissolution process. Temperature also played acritical role for the leaching of gold with acidothiourea solu-tion. Gold dissolution was found to be higher at temperaturesslightly higher than the ambient temperature. Thus, the opti-mum conditions for the leaching of gold were found to be0.5 M TU in 0.05 M H2SO4 at pulp density (weight of calcinedPCBs/volume of lixiviant) of 0.00285 at 45 °C for 6 h. Theleaching time could be reduced to 2 h when 0.01 M ferricions were added. In terms of reagent consumption, nearly3.17 mg of gold was extracted from 1 g of calcined sample atthe expense of 13.32 g TU, 1.71 g sulfuric acid and 1.4 g ferricsulfate.
(c) Effective leaching of silver was achieved using 0.5 M thioureain 0.01 M sulfuric acid solution. However, the presence offerric ions as oxidizing agents had no beneficial effect forleaching. Silver leaching was found to occur very rapidly and
Fig. 9. Recommended flow sheet for the leaching of precious metal values from PCBs ofmobile waste and recovery by adsorption method.
leached out quantitatively within 2 h of the reaction. Thus, theoptimum leaching conditions for the leaching of silver werefound to be 0.5 M TU in 0.01 M H2SO4 at 60 °C K for less than4 h. Nearly 6.78 mg silver was extracted from 1 g of the calcinedsample with the reagent consumption of 1.9 g TU and 0.5 gsulfuric acid.
(d) Although cementation was found to be effective for silver recov-ery, a larger amount of copper powder was required for goldrecovery. On the other hand, some basemetals were also precip-itatedwhen zinc cementationwas attempted for recovering goldand silver from the leached liquor. Consequently, cementation isnot applicable for the precipitation and recovery of gold fromAu–TU complex solution.
(e) The extracted gold and silver from the leached liquor wereadsorbed almost quantitatively on CPT gel. Furthermore, theadsorbed gold and silver were reduced to the metallic formwhich is the additional advantage of the present adsorptionprocess for preconcentration and separation of gold and silverfrom the above mentioned leached liquor. Since biomass canbe incinerated very easily, the adsorbed metal can be recov-ered by simple incineration of the metal loaded CPT gel.
(f) From the comparison of leaching efficiency of acidothioureawithalkaline thiosulfate, acidothiourea is more effective than alkalinethiosulfate for the leaching of gold from waste PCBs.
In conclusion, we have optimized several parameters in benchscale experiments for the effective leaching of gold and silver fromPCBs of spent mobile phones, and have devised an eco-friendlybiosorption method for the recovery of dissolved gold and silver.Though the present work suggests a laboratory scale of purely hy-drometallurgical path for the processing of precious metals presentin PCBs and replace the precious metal refining task with environ-mentally benign chemicals and low cost materials, considerable ef-forts are, however, necessary to replace the conventional process. Itis also necessary to devise a system for pilot study to analyze the eco-nomic feasibility and return on investment for obtained processingconditions for small and medium sized enterprises. In such cases,the presented hydrometallurgical route will allow for the recoveryof gold and silver from waste PCBs with great environmental andeconomical potentials in small plants and medium sized enterprises.
Acknowledgments
The present work was financially supported by a grant-in-aid forscientific research about establishing a sound material-cycle society(K2131) from theMinistry of Environment of the JapaneseGovernment.The authors are also indebted to Persimmon-Kaki TechnologyDevelopment Co., Ltd, Jincheng, China and Shibata Industry Co. Ltd.,Omuta, Japan, as well as Tanaka Kikinzoku Kogyo (TKK), Tokyo, Japanfor the kind supplies of the sample of crude persimmon extract powderand that of circuit board of spent mobile phones and its pretreatmentfor the present study, respectively.
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