Hydrometallurgy 121–124 (2012) 22–27

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Recovery of silver from X-ray film processing effluents by hydrogenperoxide treatment

A.D. Bas, E.Y. Yazici ⁎, H. DeveciDiv. of Mineral & Coal Processing, Dept. of Mining Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey

⁎ Corresponding author at: Karadeniz Teknik Univ. MaTurkey. Tel.: +90 462 377 4113; fax: +90 462 325 740

E-mail address: eyazici@ktu.edu.tr (E.Y.. Yazici).

0304-386X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.hydromet.2012.04.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 October 2011Received in revised form 1 March 2012Accepted 16 April 2012Available online 23 April 2012

Keywords:SilverWaste X-ray solutionsPrecipitationHydrogen peroxideEthylene glycol

In this study, recovery of silver from X-ray film processing effluents by precipitation was studied. Hydrogenperoxidewas used as the precipitating agent. A full factorial design (42) approachwas adopted for the study. Theresults have shown that precipitation process is highly exothermic in nature with the evolution of copiousamount of heat apparently owing to the concomitant oxidation of thiosulphate. The precipitation of silver byhydrogen peroxide is a fast reaction, which is almost complete within minutes. It is also an acid consumingreactionwith the tendency of pH to increase towards neutral/alkaline conditions. The concentration of hydrogenperoxide was proved to be statistically the most significant factor affecting the precipitation process. High silverrecoveries (≥95%) from thewaste solution (1.1 g/L Ag, 113 g/L S2O3

2−) were obtained only at high levels of H2O2

(≥37.6 g/L). Over the reaction period, a substantial increase in the concentration of sulphate was notedindicating the consumption of H2O2 mainly by the oxidation of thiosulphate. Increasing pHwas found to have abeneficial effect on the recovery of silver noticeably at low H2O2 concentrations. The addition of ethylene glycol(0.5–10 mL) enhanced the recovery of Ag (by 1.3–18.7%) presumably due to its stabilising effect on H2O2. SEM–

EDS and XRD analyses of the precipitates have revealed that silver is presentmainly as fine silver sulphide. Thesefindings demonstrate that the waste photoprocessing solutions can be suitably treated by hydrogen peroxide torecover silver and remove thiosulphate.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Owing to its prominent photosensitivity of silver halides, approx-imately 8.3% of silver is used in photography (GMSF, 2011). Whenexposed to light, silver-halides such as AgBr on photographic filmsare reduced to metallic silver (Eq. (1)). During the development andfixing of film, silver-halide crystals that are not exposed to light areremoved/leached by thiosulphate (Eq. (2)) from the film into theprocessing solution (Bober et al., 2006). Since photoprocessingsolutions are used over and over, the effluents may contain highlevels of silver. To illustrate, the effluents of X-ray film processingfacilities can reach a silver content of 1–12 g/L (Kodak, 1999a). Silveris one of the most toxic metals regulated by the regulatory bodies(Baş, 2009) and the film processing effluents are classified ashazardous waste since they may cause soil and water pollution, ifnot properly disposed of or treated (Kodak, 1996; USEPA, 1991).

AgX →light

Ag0 þ X X : Halides e:g: Br−; I−;Cl−ð Þ ð1Þ

den Muh. Bol. 61080, Trabzon,5.

l rights reserved.

AgXþ 2S2O2−3 →Ag S2O3ð Þ3−2 þ X−

: ð2Þ

World silver production was reported to be insufficient to meet thedemand, which steadily increases by ~2–2.5% per annum (Buttermanand Hilliard, 2005; GMSF, 2011). In this regard, recycling of silverassumes prime importance for the market. Photographic wastesincluding scrap films/papers and effluentswith their high silver contentoffer a significant resource potential for secondary silver (Buttermanand Hilliard, 2005; Nakiboğlu et al., 2003). There are over 1350 publicand private hospitals and healthcare centres in Turkey and, in 2009, atotal number of about 96 millions of medical X-ray filmswere recordedto be processed in all 683 public hospitals of the Turkish Ministry ofHealth (Yazici et al., 2011). It can be estimated from these data thatscrap films and X-ray film processing effluents discarded only in thesepublic hospitals carry approximately 69 tonnes of silver. Therefore,treatment of these effluents for reclamation of silver providessignificant economic as well as environmental benefits.

A variety of recovery methods for silver from photoprocessingeffluents appear to be available. Electrolysis, metallic replacementand chemical precipitation have received the most attention to date(KODAK, 1999a; USEPA, 1991). Electrolysis is capable of producingsilver with high purity by suitable control of operating conditions.However, it is used suitably only for silver-rich effluents and unableto reduce the silver levels below 100 mg/L with the requirement for

Table 1Factors and their levels adopted for the experimental design.

Parameters Levels

1 2 3 4

(A) H2O2 (g/L) 5.8 22.4 37.6 51.6(B) pH 4.2a 5 6 7

a Original pH of the solution (no addition of NaOH).

23A.D.. Bas et al. / Hydrometallurgy 121–124 (2012) 22–27

further treatment by ion exchange or metallic replacement togenerate environmentally acceptable effluents (i.e. b5 mg/L Ag)(KODAK, 1999a; Yazici et al., 2011). Metallic replacement based onthe use of more active metals such as Fe, Al, Zn and Cu than silver is aneffective method for the recovery of silver (Aktas, 2008; Bober et al.,2006; Kırmızıkan et al., 2006). However, it introduces metalimpurities (e.g. Fe2+, Al3+, Zn2+, Cu2+) to the effluent and silversludge produced is not pure and needs costly refinement processes(KODAK, 1999a). Several chemicals including sodium sulphide(Na2S), sodium dithionate (Na2S2O4), potassium borohydride(KBH4) and 2,4,6-trimercapto-s-triazine (TMT; C3N3S33−) have beenused as precipitating agents to recover silver from waste photo-processing solutions (Blais et al., 2008; KODAK, 1999b; Rivera et al.,2007; Yazici et al., 2011; Zhouxiang et al., 2008). Silver can be readilyrecovered from the waste solutions by sulphide precipitation leadingto the effluent silver levels as low as 0.1–1 mg/L. However, carefulcontrol of precipitation process and sulphide dosing are essential toprevent the release of noxious hydrogen sulphide gas (H2S) (KODAK,1999a). Despite its relatively high cost and fineness of precipitateswith potential filtering problems, TMT appears to be a promisingagent for the recovery of silver since it is effective with a low labourcost, easy control of operation and relatively low toxicity (Bober et al.,2006; Yazici et al., 2011).

Hydrogen peroxide with oxidising and reducing properties undersuitable conditions is often regarded as a green chemical with nohazardous by products since it decomposes only into oxygen and water(Eq. (3)) (FMC, 2002; Yazıcı and Deveci, 2010). Reduction of silver ionto metal by hydrogen peroxide appears to be thermodynamicallyfeasible (Eq. (4)). Furthermore, inorganic compounds e.g. thiosulphateand sulphite, and organic compounds e.g. formaldehyde and hydroqui-none, which are abundantly present in the photoprocessing effluents(Yazici et al., 2011), can be readily destroyed by hydrogen peroxide (e.g.Eqs. (5), (6)) (Jones, 1999; US Peroxide, 2011). These environmentaland technical attributesmake hydrogen peroxide a potential alternativefor the treatment of photoprocessing effluents.

H2O2→H2Oþ 1=2O2 gð Þ ð3Þ

2Agþ þ H2O2→2Ag0 þ 2 Hþ þ O2 gð Þ ΔG293 ¼ −20:3 kJ=molð Þ ð4Þ

2S2O32− þH2O2 þ 2 Hþ→S4O6

2−

þ 2H2O ΔG293 ¼ −342:7 kJ=mol;pH 4–8ð Þ ð5Þ

S2O32− þ 4H2O2 þ 2OH−→SO4

2−

þ 5H2O ΔG293 ¼ −1307 kJ=mol; > pH 8ð Þ: ð6Þ

In this study, the treatment of X-ray film processing effluents byhydrogen peroxide to recover silver was investigated. Effect ofconcentration of hydrogen peroxide (5.8–51.6 g/L H2O2) and pH(4.2–7.0) on the rate and extent of the recovery of silver were studiedwithin a full factorial design (42). Furthermore, the influence of theaddition of ethylene glycol on silver recovery was also examined. Silverprecipitates were characterised by chemical analysis, SEM–EDS andXRD to identify the nature of precipitates and provide an insight into theprecipitation process.

2. Experimental

2.1. Effluent sample and reagents

A sample of X-ray film processing effluent obtained from FarabiHospital (Karadeniz Technical University, Trabzon, Turkey) was used inthis study. The effluent sample was characterised to contain 1.1 g/L Ag,17 g/L SO4

2− and 113 g/L S2O32− at pH 4.2. Reagent grade sodium

hydroxide (NaOH) and hydrogen peroxide (H2O2, 35% w/w) were used

to prepare test solutions using deionised-distilled water. Ethyleneglycol (C2H6O2, ≥99%) was also tested to stabilise hydrogen peroxide.

2.2. Precipitation tests and analytical methods

In the current study, the experiments were designed by using afull factorial design (42) (Montgomery, 2001) to investigate theeffects of concentration of hydrogen peroxide (5.8–51.6 g/L H2O2)and pH (4.2–7) on the recovery of silver. The range of concentrationof hydrogen peroxide was determined by the preliminary tests andtheoretical calculations based on silver and thiosulphate content ofthe effluent sample. Factors and their levels are shown in Table 1.Furthermore, the addition of ethylene glycol (0.5–10 mL) on therecovery of silver was also investigated at pH 4.2 and 22.4 g/L H2O2.

Precipitation tests were carried out in 50-mL Erlenmeyer flasks. pHof the waste solution was, if required, adjusted using 4 M NaOH beforethe addition of hydrogen peroxide (35% w/w). The flasks were thenplaced on a reciprocal shaker operating at 140 min−1. Due to theexothermic nature of the reactions, hydrogen peroxide was added at apredetermined rate of 0.5 mL per 1.5 min unless otherwise stated. Overthe reaction period, 5-mL aliquots were removed at preset intervals andfiltered through 0.45 μm cellulose nitrate filters. These samples werethen used for the analysis of residual silver (Ag) and sulphate (SO4

2−).Silver was analysed using an atomic absorption spectrophotometer

(AAS; PerkinElmer AAnalyst 400). Thiosulphate content (S2O32−) of the

effluent was determined by iodometric titration (Jeffery et al., 1989)while sulphate (SO4

2−) in samples was monitored colorimetrically usinga filter photometer (Palintest 5000) at a wavelength of 520 nm. Due tothe interference by the intermediate sulphur compounds and the res-idual H2O2 the concentration of thiosulphatewas notmonitored over thereaction period. The statistical analysis of the experimental data based onANOVA was performed using Minitab statistical software (2004).

2.3. Characterisation of silver precipitates

A waste solution with a high silver content (4.5 g/L) was used toobtain sufficient amount of precipitate for chemical and mineralogicalanalysis. Precipitates were collected via filtration (0.45 μm, cellulosenitrate filter) and washed twice with deionised-distilled water prior todrying at 105 °C for 6 h. Dried precipitates were fixed on conductivecarbon tabs and examined under a Scanning ElectronMicroscope (SEM)(Zeiss EVO LS10) coupled with an Energy Dispersive Spectrometry(EDS) unit. X-ray diffraction (XRD) analyses of the precipitates werecarried out using a Rikagu D/max-IIIC X-ray diffractometer, operatingwith Cu–Kα1 radiation source (λ=1.54059 Å) at 40 kV and 30 mA. Thesample was scanned over a 2θ range of 5–80° with a 0.005° step size.Chemical analysis of the precipitate sample was also undertaken by hotaqua-regia digestion followed by the spectrophotometric finish.

3. Results and discussion

3.1. Kinetics of silver precipitation

Kinetics of precipitation of silver by hydrogen peroxide (34 g/L)was initially determined from the as-received photoprocessing wastesolution (pH 4.2). Fig. 1 illustrates that it is a fast reaction as the

Fig. 2. Temperature profiles at different rates of H2O2 addition (volume of wastesolution: 50 mL).

Table 2Recovery of silver from the waste solution under different conditions of pH and

24 A.D.. Bas et al. / Hydrometallurgy 121–124 (2012) 22–27

precipitation of 77% Ag already occurred within 5 min under theseconditions. Silver recovery remained at these levels over an extendedperiod of 60 min. with the indication of the completion of thereaction. Formation of sulphate through the oxidation of thiosulphatewas also monitored (Fig. 1). A substantial increase in the sulphateconcentration from 17.4 g/L to 71.1 g/L was recorded over thereaction period of 60 min. This suggests that hydrogen peroxide ismainly consumed via the oxidation of thiosulphate present in thewaste solution. During the treatment, pH tended to increase with afinal pH of 5.22, which is consistent with Eq. (5) (Jones, 1999).

Preliminary tests indicated that the reactions involved in thehydrogen peroxide treatment of the waste solution are highlyexothermic in nature (e.g. ΔH293=−74.1 kcal/mol for Eq. (5)) withthe generation of copious amount of heat. Decomposition rate ofhydrogen peroxide was reported to increase rapidly with increasingtemperature (Yazıcı and Deveci, 2010) resulting in excessively highconsumption of hydrogen peroxide. Therefore, the tests were per-formed to monitor the evolution of temperature at different rates ofaddition of hydrogenperoxide (Fig. 2). It can bededuced fromFig. 2 thatdosing of hydrogen peroxide is required to control the temperature.Accordingly, an addition rate of 0.5 mL H2O2 per 1.5 min was selectedfor the precipitation tests.

3.2. Effect of concentration of hydrogen peroxide and pH

A full factorial design approachwas adopted to evaluate the effect ofinitial concentration of hydrogen peroxide (5.8–51.6 g/L H2O2) and pH(4.2–7) on the precipitation of silver. The results are presented inTable 2. Recovery of silver was found to depend strongly on theconcentration of H2O2. High silver recoveries (≥95%) were achieved atH2O2 concentrations of≥37.6 g/L,which is considerably higher than thestoichiometric requirement for the recovery of silver (Eq. (4)) appar-ently due to the concurrent oxidation of thiosulphate. An increase in pHwas observed to improve the precipitation of silver, which was evidentparticularly at low concentrations of H2O2 (Table 2). To illustrate, therecovery of silver was enhanced by 34% with increasing the initial pHfrom4.2 to 7 at a H2O2 of 5.8 g/Lwhile the corresponding increase in thesilver recovery was 21% and only b2% at 22.4 and ≥37.6 g/L H2O2,respectively. pH was noted to deviate from the initially set valuestowards neutral/alkaline region (Table 2).

The formation of sulphate due to the oxidation of thiosulphatewas also monitored during the precipitation tests (Fig. 3). Theconcentration of sulphate in solution was determined to dependessentially on the concentration of H2O2 with no marked effect of pH.The oxidation of thiosulphate into sulphate (Eqs. (5), (7)–(9)) wasreported to proceed through the formation of intermediates such astetrathionate (S4O6

2−) (Eq. (5)), trithionate (S3O62−) (Eq. (7)) and

sulphite (SO32−) (Eq. (8)) (Solvay Interax, 2001). Although

Fig. 1. Kinetics of the precipitation of silver from waste X-ray solutions (34 g/L H2O2,pH 4.2).

tetrathionate is the primary reaction product at low concentrationsof H2O2, the formation of the intermediates and sulphate increaseswith increasing the concentration of H2O2 (Fig. 3). The presence ofmetals catalyses the conversion of thiosulphate by hydrogen peroxideinto sulphate (Jones, 1999; US peroxide, 2011).

S4O2−6 þ 3H2O2→S3O

2−6 þ SO2−

4 þ 2H2Oþ 2 Hþ ð7Þ

S3O62− þ H2O2 þH2O→3SO3

2− þ 4 Hþ ð8Þ

SO2−3 þ H2O2→SO2−

4 þH2O: ð9Þ

Statistical assessment of the results was carried out by the analysisof variance (ANOVA) (Table 3). P values were determined for theparameters tested. The P value shows the probability that the teststatistic will take on a value that is at least as extreme as the observedvalue of the statistic when the null hypothesis (H0) holds true(Montgomery, 2001). In this respect, the calculated P values (Table 3)confirmed that the effect of concentration of H2O2 in the range testedwas statistically highly significant even at 99.9% (α=0.001) confi-dence level while pH was not a significant factor under theseconditions. Statistical analysis of the data also indicated that thecontributions of H2O2 concentration and pH to the response i.e. silver

hydrogen peroxide concentration (addition rate: 0.5 mL H2O2 per 1.5 min; precipita-tion time: 45 min).

Exp. no. H2O2 (g/L) pH Ag recovery (%) Final pH

1 5.8 4.2a 22.7 5.562 5.8 5 35.2 7.773 5.8 6 53.4 8.304 5.8 7 79.1 8.285 22.4 4.2a 63.4 7.086 22.4 5 71.5 7.737 22.4 6 75.4 7.928 22.4 7 84.5 8.209 37.6 4.2a 94.5 7.0010 37.6 5 95.9 7.6411 37.6 6 96.1 7.8712 37.6 7 96.5 8.0413 51.6 4.2a 100 5.2214 51.6 5 99.9 6.1515 51.6 6 99.0 6.5216 51.6 7 99.6 8.27

a Original pH of the solution. No addition of NaOH.

Fig. 3. Initial and final concentrations of sulphate in solution at different concentrationsof hydrogen peroxide (as the mean of data obtained at different pHs tested with errorbars showing±standard deviation).

Fig. 4. Effect of concentration of hydrogen peroxide (a) and pH (b) at four levels.

25A.D.. Bas et al. / Hydrometallurgy 121–124 (2012) 22–27

recovery were 77.3% and 10.1%, respectively (Table 3). Contributionvalues also reflect the relative importance of each parameter tested.

Fig. 4 illustrates the main effects plots based on the mean valuesfor the concentration of H2O2 and pH showing the silver recovery ateach level of these factors as if they are independent. This plotconfirms the positive effect of increasing the concentration of H2O2

and pH in the range tested. The surface plot of silver recovery (%)versus the levels of H2O2 concentration and pH was also presented inFig. 5 to depict the interaction effects of these parameters on theresponse. Accordingly, the effect of pH on the precipitation of silverwas discernible only at low levels (1 and 2) of H2O2 (i.e. 5.8–22.4 g/L)while the most significant enhancement in the recovery of silver wasachieved by increasing H2O2 concentration from 5.8 g/L to 37.6 g/L atall levels of pH tested.

Despite its great potential with technical and environmentalbenefits, the utilisation of hydrogen peroxide in the treatment ofwaste photographic solutions has appeared to receive limited interestwith no detailed data being available. Rabah et al. (1989) investigatedthe acid and alkaline treatment of spent colour-photography solutionsto obtain a silver sludge followed by its thermal treatment (at 980 °C) toproduce silver metal. They also tested the addition of H2O2 (74 mL of30% H2O2 by volume per litre of waste solution) in a single experimentand did not provide data for silver recovery (though it was assumed tobe 89% in their cost analysis). Based on the yield of silver sludge, theseinvestigators also provided a cost analysis and claimed that the acidtreatment by a mixture of sulphuric and nitric acids was more effectivethan H2O2 and alkaline treatments. However, it appeared that they didnot consider the factors such as neutralisation of the acidic effluents andthe formation of hazardousNOx gases in the acid treatment. In an earlierpatent, Daignault et al. (1982) proposed the treatment of wastephotographic solutions with a mixture of peroxide and ozone todestroy the complexing agents (EDTA, NTA and thiosulphate) therebyrecovering/removing the heavy metals present. They also demonstrat-ed that 91% of silver could be recovered with the addition of 10–20%H2O2 (using 70% H2O2 solution) by volume of the waste solution at pH

Table 3Results of analysis of variance (ANOVA) for the effect of hydrogen peroxideconcentration and pH.

Source ofdeviation

Degree offreedom

Sum ofsquares

Meansquares

F value P value Contribution(%)

(A) H2O2 (g/L) 3 6971.8 2323.9 18.41 0.000 77.3(B) pH 3 907.1 302.4 2.39 0.136 10.1Residual error 9 1136.2 126.2 12.6Total 15 9015.1 100

4.5 followed by increasing pH to 9.5 by the addition of NaOH. They alsoshowed that further treatment of the effluents with ozone and thenNa2S were required to achieve high levels (≥98%) of recovery/removalof Ag, Cd, Fe and Pb.

3.3. Effect of addition of ethylene glycol

Hydrogen peroxide is relatively an expensive reagent and hasinherently low stability in that its catalytic decomposition occurs in the

Fig. 5. Surface plot of silver recovery as a function of levels of H2O2 concentration and pH.

Fig. 6. Effect of the addition of ethylene glycol on the recovery of Ag from the as-received waste solution (pH 4.2) at a H2O2 concentration of 22.4 g/L.

26 A.D.. Bas et al. / Hydrometallurgy 121–124 (2012) 22–27

presence of metal ions and solids, and at high temperatures and pHs(Yazıcı and Deveci, 2010). The severe detraction to hydrogen peroxidetreatment is therefore its high consumption. Rabah et al. (1989) foundthat H2O2 treatment had the highest reagent cost compared with acidand alkaline treatments. In this study, the effect of the addition ofethylene glycol was examined to reduce the consumption of hydrogenperoxide per silver recovery. Fig. 6 illustrates a 1.3 to 18.7%improvement in the recovery of silver with increasing the addition ofethylene glycol from 0.5 to 10 mL. This improvement in the silverrecovery can be attributed to the stabilising effect of ethylene glycol onhydrogen peroxide apparently mitigating its decomposition during theprecipitation process. Mahajan et al. (2007) also reported the stabilisingeffect of ethylene glycol for hydrogen peroxide during the leaching ofchalcopyrite at elevated temperatures. They demonstrated that theaddition of ethylene glycol significantly slowed down the decomposi-tion of hydrogen peroxide i.e. the complete loss of H2O2 even after 2 hcompared with only 25% loss (after 4 h) in the presence of 8 mL/Lethylene glycol.

3.4. Characterisation of silver precipitates

Chemical and mineralogical characterisations of silver precipitateswere performed to provide an insight into the precipitation process.Silver content of the precipitate was determined to be 65.1%. SEM

Fig. 7. SEM image of the silver precipitate with EDS p

studies showed that the silver precipitate, which was finely grained,was composed of silver and sulphur as the elemental phases present(Fig. 7). Fig. 7 also illustrates a typical EDS profile where the chemicalcomposition of the precipitate was determined to be 86.5% Ag and13.5% S, which is analogous to silver sulphide (Ag2S; 87.1% Ag). X-raydiffraction pattern of the precipitate sample confirmed the presenceof silver sulphide, metallic silver and elemental sulphur with theformer being the most abundant silver phase (Fig. 8). These findingssuggest that silver is precipitated from the waste solution mainly inthe form of silver sulphide (Ag2S). A chemical simulation and reactionsoftware (HSC Chemistry, 2011) with extensive thermochemical database was exploited to identify the thermodynamically feasiblereactions (Eqs. (10)–(14)) for the precipitation of silver from such awaste solution.

2Ag S2O3ð Þ3−2 þH2O2 þ 2 Hþ→2Ag0 þ 2S4O2−6

þ 2H2O ΔG293 ¼ −96:2 kcal=molð Þ ð10Þ

6Ag S2O3ð Þ23− þ 13H2O2 þ 6 Hþ→3Ag2Sþ 5S4O62− þ SO4

2−

þ 16H2Oþ 6O2 ΔG293 ¼ −492:8 kcal=molð Þ ð11Þ

4Ag S2O3ð Þ23− þ 4H2O2 þ 4 Hþ→2Ag2Sþ S4O62− þ 3SO4

2−

þ 7 S0 þ 6H2Oþ 4O2 ΔG293 ¼ −48:4 kcal=molð Þ ð12Þ

6S2O32− þ 6H2O2→S2− þ 2S4O6

2− þ 3SO42− þ 6H2O

ΔG293 ¼ −415:0 kcal=molð Þð13Þ

2Ag S2O3ð Þ23− þ S2−→Ag2Sþ 4S2O32−

ΔG293 ¼ −43:8 kcal=molð Þ:ð14Þ

Rabah et al. (1989) proposed that the oxidising reagents e.g. HNO3

under acidic conditions attack thiosulphate leading to the formationof sulphate, elemental sulphur or polysulphates. Furthermore, theseinvestigators mooted that, in addition to these sulphur species,hydrogen sulphide may also form during the acid and peroxidetreatment, and reacts with the liberated silver to yield insoluble silversulphide. This was consistent with their XRD analysis of the silversludge in which silver sulphide (Ag2S) and halide (AgBr) are the mainphases identified. It may be relevant to note that, in the current study,the treatment of the silver precipitate by hot concentrated nitric acidresulted in a fine residue, which was also examined under SEM–EDSand determined to be AgCl (not shown). Silver halides (e.g. AgCl and

0

5

10

15

20

S

S

Ag

Ag

2 4 6

rofile indicating the presence of silver sulphide.

Fig. 8. XRD pattern of the silver precipitate showing the presence of metallic silver,silver sulphide and elemental sulphur.

27A.D.. Bas et al. / Hydrometallurgy 121–124 (2012) 22–27

AgBr) would form provided that thiosulphate was extensivelydecomposed at sufficiently high concentrations of hydrogen peroxide.

The reagent cost based on the data (i.e. 37.6 g/L H2O2, 95% Agrecovery) obtained in the current studywas estimated to be ~$63/m3 ofthe effluent corresponding to ~$61/kg of silver recovered at an effluentconcentration of 1.1 g/L Ag and a H2O2 (50%w/w) price of $911/m3. It ispertinent to note that the effluent sample used in the current study isrelatively lean in silver content and the reagent costwill be considerablyreduced with an increase in the silver content of the effluent. In thecurrent study, ethylene glycol was used as a stabiliser to mitigate thecatalytic decomposition of H2O2 and an improvement in the recovery ofsilver at the same level of H2O2 was achieved. However, thisimprovement will not compromise its use due to its addition at highconcentrations (i.e. 20% v/v), which prohibitively increases (e.g. by upto 8-fold) the reagent costs for the treatment process. Further treatmentof the silver precipitate obtained in the peroxide process is also requiredto produce metallic silver. In this regard, Rabah et al. (1989) proposedthe thermal treatment of the silver sludge containing silver as sulphideand halide at 980 °C to yield metallic silver with a purity of 99.8%.

4. Conclusions

This study has demonstrated the treatment of the waste X-ray filmprocessing solutions by hydrogen peroxide for the recovery of silver.Kinetics tests have shown that the precipitation of silver from thewaste solution is a rapid process, but, highly exothermic in characterwith the generation of large amount of heat presumably due to theside reactions i.e. the concomitant oxidation of thiosulphate. Dosedaddition of hydrogen peroxide was found to be required to controlthe temperature. A full factorial design (42) for the factors, H2O2

concentration and pH was developed for the experiments. Theconcentration of hydrogen peroxide (5.8–51.6 g/L H2O2) was identi-fied to be the most significant parameter affecting the extent of silverrecovery as verified by the statistical analysis of data. Increasing pH(4.2–7) appeared to improve the recovery of silver discernibly at lowlevels of H2O2. The addition of ethylene glycol (0.5–10 mL) wasshown to enhance the silver recovery apparently due to its stabilisingeffect on hydrogen peroxide. Characterisation studies have revealedthat silver is precipitated as fine grains predominantly in the form ofsilver sulphide. It can be inferred from this study that hydrogenperoxide as a green chemical is potentially a suitable reagent for the

treatment of X-ray photoprocessing effluents allowing the recovery ofsilver as well as the removal of thiosulphate and possibly otherconstituents present.

Acknowledgement

The authors would like to express their sincere thanks andappreciations to the Research Foundation of Karadeniz TechnicalUniversity for the financial support (Project no: 2006.112.008.1) andto Mr. Fatih Erdemir (Dept. of Metallurgical & Materials Eng., KTU) forSEM–EDS analysis.

References

Aktas, S., 2008. Silver recovery from silver-rich photographic processing solutions bycopper. Can. Metall. Q. 47 (1), 37–43.

Baş, D., 2009. Recovery of silver from waste X-ray film solutions by precipitation. BScThesis, Karadeniz Technical University, Trabzon, Turkey, 54 p. (in Turkish).

Blais, J.F., Djedidi, Z., Cheikh, R.B., Tyagi, R.D., Mercier, G., 2008. Metals precipitation fromeffluents: review. Pract. Period. Hazard. Toxic Radioact.WasteManage. 12 (3), 135–149.

Bober, T.W., Vacco, D., Dagon, T.J., Fowler, H.E., 2006. Treatment of photographicwastes. In: Wang, L.K., Hung, Y.-T., Lo, H.H., Yapijakis, C. (Eds.), HazardousIndustrial Waste Treatment. CRC Press, pp. 361–408.

Butterman,W.C., Hilliard, H.E., 2005. Silver. U.S. Geological Survey (USGS), Reston, Virginia.HSC Chemistry, 2011. Chemical Reaction and Equilibrium Software, v. 7.18. Outotec

Research Oy.Daignault, L.G: Schiller, E.E., 1982. Removal of complexed heavy metals from waste

effluents, United States Patent, Patent no: 4332687.FMC, 2002. Hydrogen peroxide technical bulletin. FMC Corporation, Pennsylvania. 30 pp.GMSF, 2011. World Silver Survey: A Summary of the Report. The Silver Institute. 11 pp.Jeffery, G., Bassett, J., Mendham, J., Denney, R., 1989. Vogel's Textbook of Quantitative

Chemical Analysis, fifth ed. John Wiley & Sons Inc., New York.Jones, C.W., 1999. Applications of hydrogen peroxide and derivatives. RSC Clean

Technology Monographs. The Royal Society of Chemistry, Cambridge, UK. 282 pp.Kırmızıkan, E., Güldan, G., Yazıcı, E.Y., Alp, İ., Deveci, H., Duran, C., Celep, O., 2006.

Recovery of silver from waste photographic solutions by cementation. In: Demir, C.,Yilmaz, A.O. (Eds.), Doğu Karadeniz Bölgesi Maden Kaynaklarının DeğerlendirilmesiSempozyumu, 14–16 Sept., Trabzon, Turkey, pp. 309–311 (in Turkish).

KODAK, 1996. The Regulation of Silver in Photographic Processing Facilities. Publicationno: J-214 Eastman Kodak Company. Available at: http://www.kodak.com (Retrievaldate: July 2003).

KODAK, 1999a. Recovering Silver from Photographic Processing Solutions. Publicationno: J-215 Eastman Kodak Company. Available at: http://www.kodak.com (Retrievaldate: July 2003).

KODAK, 1999b. The Technology of Silver Recovery for Photographic Processing Facilities.Publication no: J-212 Eastman Kodak Company. Available at: http://www.kodak.com(Retrieval date: July 2003).

Mahajan, V., Misra, M., Zhong, K., Fuerstenau, M.C., 2007. Enhanced leaching of copperfrom chalcopyrite in hydrogen peroxide–glycol system. Miner. Eng. 20 (7), 670–674.

Minitab, 2004. Statistical software. Evaluation Version 14.12.0. Minitab Inc., USA.Montgomery, D.C., 2001. Design and Analysis of Experiments, fifth ed. John Wiley &

Sons Inc., New York.Nakiboğlu, N., Toscalı, D., Nişli, G., 2003. A novel silver recovery method from waste

photographic films with NaOH stripping. Turk. J. Chem. 27, 127–133.US Peroxide, 2011. Reduced sulfur compound treatment with hydrogen peroxide

Avaliable at: www.h2o2.com (Retrival date: Oct. 2011).Rabah, M.A., El Barawy, K.A., Aly, F.H., 1989. Silver recovery from spent colour-

photography solutions. Int. J. Miner. Process. 26, 17–27.Rivera, A., Roca, M., Cruells, F., Patiño, E. Salinas, 2007. Study of silver precipitation in

thiosulfate solutions using sodium dithionite. Application to an industrial effluent.Hydrometallurgy 89, 89–98.

Solvay Interax, 2001. Hydrogen peroxide for controlling reduced sulfur compoundsAvailable at: http://www.solvayinterox.com.au/MSDS/Contolling%20reduced%20suphur%20species.pdf (Retrieval date: July 2011).

USEPA, 1991. Guides to Pollution Prevention—The Photo-processing Industry. EPA/625/7-91/012. United States Environmental Protection Agency, Office of Researchand Development, Washington.

Yazıcı, E.Y., Deveci, H., 2010. Factors affecting decomposition of hydrogen peroxide. XII.International Mineral Processing Symposium (IMPS), 6–8 October, Kapadokya,Turkey, pp. 609–616.

Yazici, E.Y., Deveci, H., Yazici, R., 2011. Recovery of silver from X-ray film processingeffluents using trimercapto-s-triazine (TMT). Sep. Sci. Technol. 46 (14), 2231–2238.

Zhouxiang, H., Jianying, W., Ma, Z., Jifan, H., 2008. A method to recover silver fromwaste X-ray films with spent fixing bath. Hydrometallurgy 92 (3–4), 148–151.