Extraction of Silver From Waste X-ray Films by Thiosulphate Leaching--IMPS2010-Libre

8/11/2019 Extraction of Silver From Waste X-ray Films by Thiosulphate Leaching--IMPS2010-Libre

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PROCEEDINGS

OF THE

XII th INTERNATIONAL MINERAL

PROCESSING SYMPOSIUM

6-8 OCTOBER 2010, CAPPADOCIA-NEV EH R, TURKEY

Edited by

zcan Y. GLSOY, . Levent ERGN, N.Metin CAN and lkay B.EL K

Hacettepe University, Department of Mining Engineering

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gg Dept. of Mining Engineering, Karadeniz Technical University, 61080, Trabzon, TurkeyAABBSSTTRRAACCTT:: In this study, the recovery of silver from waste X-ray films (0.7% Ag) byammoniacal copper thiosulphate leaching was studied. The experiments were designed usingTaguchi L 9 (33) experimental design. The effects of concentration of thiosulphate, (0.005-0.5M S2O3

2-), copper (0.005-0.5 M Cu 2+), and ammonia (0.005-1 M NH 3) on the extent of silver recovery from X-ray films were investigated at three levels. The experimental data were

analysed using a statistical software. The findings revealed that the concentration of ammonia(NH 3) and thiosulphate (S 2O3

2-) were statistically the most significant factors affecting therecovery of silver, while the effect of concentration of copper (Cu 2+) was found to beinsignificant over a reaction period of 0.5 h.IINNTTRROODDUUCCTTIIOONNSilver is often found with gold and sulphide minerals in the nature and can be extracted,largely as a by-product, from gold and base metal sulphide ores. Due to the rapid depletion of natural silver resources and high silver demand, the secondary silver resources have recently become ever important (Zhouxiang et al., 2008). Silver demand has increased about 5% inrecent years and 25% of silver demand is obtained by recycling of silver wastes including Owing to its high photosensitivity and electricalconductivity, silver is extensively used in photography and electrical industry. Approximately40-50% of the silver production is consumed for radiography and photographic films and papers (Bayram, 1997). Most of the X-ray (radiography) films (>94-98%) are used in medicalapplications (Khunprasert, 2008).

X-ray and photographic films may contain appreciable amounts of silver in the form of silver halide e.g. AgBr. X-ray films typically contain about 4.85 g/m 2 Ag (Kodak, 1998).During the photoprocessing silver halide is reduced to metallic silver on exposure to light (1)and the remaining silver halide is dissolved by thiosulphate into the waste solution (2) duringthe developing and fixing processes (Kodak, 1999).

0light AgAgX (1)

X)OS(AgOS2AgX 322232

(2)

It was claimed that during photo-processing ~60% of the silver remains on the film while~40% transfers to the solution (Khunprasert, 2008). Waste X-ray films may contain 0.7-2%

al., 2003). Despite the importance of photographic wastes as a secondary silver resource(Bayram, 1997), only a small portion of silver is recycled/recovered from these wastes (Syed etal., 2002). Recovery of silver from waste films/solutions is of importance not only for economic considerations, but also for environmental pollution. Silver is one of the most toxic

including X-ray films and processing solutions are unsafely disposed of, they may cause soil

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and water pollution. Silver from waste films can be recovered by hydrometallurgical (leaching)and pyrometallurgical (incineration) processes (Kunda, 1992). Incineration has severaldisadvantages including the generation of toxic gases and relatively low recovery of silver (99%)can be achieved by leaching (Garcia, 1986; Marinkovic, 2006, Khunprasert, 2008). Various

2003), nitric acid (Abdel-Aal and Farghaly, 2007; Khunprasert, 2008) and oxalic acid(Marinkovic, 2006) were investigated for the extraction of silver.

Thiosulphate (S2O32-) is an alternative leaching reagent for the recovery of silver/golddue to its relatively non-toxic nature (Ayata and Yildiran, 2006; Grge, 2006; Muir andAylmore, 2004). Thiosulphate leaching of silver/gold is conducted in ammoniacal solutions inthe presence of cupric copper (Cu2+) as catalyst (3) (Yen et al., 2001). Thiosulphate can formstrong silver-thiosulphate complexes (e.g. Ag(S2O3)23-, logK=13.5) in wide Eh-pH intervals(Muir and Aylmore, 2004). Ammonia (NH3) stabilises copper(II) by forming copper(II)tetraamine complex (Cu(NH3)42+, logK=12.5) and hence controls the decomposition of thiosulphate through the reduction of Cu(II) to Cu(I) (Muir and Aylmore, 2004). Ammoniacalcopper-thiosulphate reagent system can be used suitably in the leaching of metallic silver fromthe waste films. Although many researches investigated various reagents for leaching of silver from waste films, to the knowledge of the authors ammoniacal copper-thiosulphate leaching of photographic wastes has not been reported. Recently, Zhouxiang et al. (2008) used silver-freewaste fixing bath solution (which contains thiosulphate) in the leaching of silver from X-rayfilms. The overall reaction for the dissolution of silver in ammoniacal copper-thiosulphatesystem has been proposed as follows:

)aq(5332)aq(3)aq(

3232)aq(

243)aq(

232

0 )OS(Cu NH4)OS(Ag) NH(CuOS5Ag (3)

In this study, the recovery of silver from waste X-ray films by thiosulphate (S2O32-)leaching in the presence of copper (Cu2+) and ammonia (NH3) was studied. Taguchi L9 (33)experimental design was adopted for the study. The effects of concentrations of thiosulphate,copper and ammonia on the extraction of silver from X-ray films were investigated at threelevels. A statistical software was used to evaluate the experimental data for the contributionand significance of these factors.EEXXPPEERRIIMMEENNTTAALLMMaatteerriiaalla annddMMeetthhooddReagent grade sodium thiosulphate (Na 2S2O3.5H 2O) and copper sulphate (CuSO 4.5H 2O)were used in the study. Ammonia solution (NH 3, 25%) was used as the stock solution. Allsolutions were prepared using deionised-distilled water.

Waste X-ray films were cut into 0.5x0.5 cm 2 pieces and prepared as 5 g portions prior touse in the experiments. Silver content of waste films was determined to be 0.7% Ag by weight.Leaching tests were performed in 250-ml Erlenmeyer flasks. Leach solutions (100 ml) were prepared at the required strengths of thiosulphate (0.005-0.5 M S

2O

3

2), copper (0.005-0.5 MCu 2+), and ammonia (0.005-1 M NH 3) before the addition of the waste films (5 g). The flaskswere then placed in a temperature controlled (25C) orbital shaker operating at 180 rpm. Over the experimental period, the top of the flasks were kept covered. Samples were taken at certainintervals and residual silver was analysed. After the leaching period, film residues were filteredand dried. They were then digested in hot concentrated nitric acid (65% HNO 3) to determinetheir silver content. Silver analysis was performed by an atomic absorption spectrophotometer (Perkin Elmer AAnalyst 400). Replicate tests showed that relative standard deviation (RSD) of

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Taguchi design was adopted as a methodology for experimental design in this study (Roy,2001). The effects of initial concentration of thiosulphate (0.005-0.5 M S 2O3

2-), copper (0.005-0.05 M Cu 2+) and ammonia (0.005-1 M NH 3) on the extent of removal of silver from syntheticsolutions were investigated within Taguchi L 9 (3

3) orthogonal array (Tables 1 and 2). Minitab(2004) statistical software was used to perform ANOVA and evaluate the main effects of parameters.T

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(A) [S 2O32-], M 0.005 0.25 0.5

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RREESSUULLTTSSAANNDDDDIISSCCUUSSSSIIOONNRecovery of silver (%) for each run for different leaching periods (0.5-48 h) was presentedin Table 3. The extensive extraction of Ag occurred in all the runs except the first run.However, in the 7 th run, the silver extraction was most rapid i i ..ee .. ~93% at 0.5 h (Table 3). Meanvalues for the recovery of Ag (%) for parameters at each level were shown in Table 4. Deltavalues, which show the relative significance of the factors, were also calculated as thedifference between maximum and minimum mean values (Table 4). Concentration of NH 3appeared to be the most significant factor affecting the leaching process (Table 4).

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3 0.005 0.05 1 69.5 80.5 85.8 89.5 99.94 0.25 0.005 0.5 89.1 89.4 97.6 96.7 99.85 0.25 0.025 1 89.2 94.6 102 96.8 1006 0.25 0.05 0.005 27.0 33.5 45.6 59.7 99.77 0.5 0.005 1 93.1 95.3 96.3 93.0 99.98 0.5 0.025 0.005 30.2 41.3 49.6 53.3 99.39 0.5 0.05 0.5 70.5 73.6 79.3 91.0 99.6

TTaabbllee4 4..MMeeaannv vaalluueesso off r reeccoovveerry yo off AAgga att0 0..55h hf f oorrp paarraammeetteerrssa atte eaacchhl leevveellaannddd deellttaas sttaattiissttiiccssf f oorrp paarraammeetteerrssPPaarraammeetteerrss RReeccoovveerry yo off AAgg( (%%) ) D Deellttaa((MMaaxx..--MMiinn..) ) R RaannkkLLeevveell1 1 L Leevveell2 2 L Leevveell3 3(A) [S 2O3 -], M 36.30 68.43 64.60 32.13 2(B) [Cu +], M 62.17 51.50 55.67 10.67 3(C) [NH 3], M 20.50 64.90 83.93 63.43 1

Statistical evaluation of the experimental data was performed by analysis of variance(ANOVA) for the data at 0.5 h. (Table 5). The significance level of parameters was determined based on P values. The P value indicates the probability that the test statistic will take on a

value that is at least as extreme as the observed value of the statistic when the null hypothesis(H 0 for the significance test, then the effect of parameter on the process is significant. In thisregard, the calculated P values (Table 5) revealed that effect of concentration of NH 3 in the

2O32- confidence level. The contributions of NH 3 and S 2O3

2- to the response i.e. silver extractionwere found to be 74.1% and 21.5%, respectively (Table 5). However, the contribution of copper in this range of concentration was minimal as ANOVA data suggested. Contributionsvalues also reflect the relative importance of each factor (Table 5).T

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(A) [S2O32-], M 2 1848.1 924.07 9.13 0.099 21.5(B) [Cu 2+], M 2 173.4 86.69 0.86 0.539 2.0(C) [NH 3], M 2 6357.4 3178.71 31.41 0.031 74.1Residual Error 2 202.4 101.19 2.4Total 8 8581.3 100

Figure 1 presents the effect of parameters on silver recovery (%) based on the mean valuesfrom Table 4. The increase in the concentration of NH 3 leads to an increase in silver recovery(Fig. 1). This could be ascribed to the stabilising role of NH 3 on Cu(II) resulting in the

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prevention of decompositon of S 2O32- and its lixiviant effect on silver (Muir and Aylmore,

2004). Increasing the concentration of S 2O32- from 0.25 M to 0.5 M i i .

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found to have limited effect on the recovery of Ag. In other words, the concentration of thiosulphate at 0.25 M was sufficient for high levels of the extraction of silver.

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Leaching kinetics is also of importance on the recovery of Ag (Fig. 3). ANOVA wasapplied for different leaching periods (0.5-48 h) and P values were presented in Table 6. It can

h, the effect of concentration of thiosulphate on the process became statistically insignificanteven at 90% confidence level (Table 6). The effect of the concentration of ammonia was alsofound to be statistically significant only at 0.5-3 h.

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extraction at 0.5 h (Table 1). The model was found to be statistically valid at 99.9% confidence 2) of 95.8%. A verification

experiment was performed to test the validity of the model at concentrations of S 2O32-, Cu2+

and NH 3 of 0.25 M, 0.05 M and 1 M, respectively (Fig. 4).

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CCOONNCCLLUUSSIIOONNSSThis study has shown that ammoniacal copper thiosulphate system can be readily used for the leaching of silver from waste X-ray films with almost complete recovery of silver. Theeffects of concentration of thiosulphate, ammonia and copper on the extent of recovery of silver from waste X-ray films were demonstrated using Taguchi L 9 (33) experimental design.The concentrations of NH 3 and S 2O3

2- are the most significant parameters as confirmed bystatistical evaluation of the experimental data. Concentration of Cu 2+ was shown to beinsignificant in the range tested.

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The authors would like to express their sincere thanks and appreciation to the ResearchFoundation of Karadeniz Technical University for the financial support (Project No:2006.112.008.1).RREEFFEERREENNCCEESSAyata, S., and Yildiran, H., 2006. Silver extraction from argentite by thiosulphate leaching, JFS,Vol 29, pp. 16-31.

Thesis, Karadeniz Technical University, Trabzon, Turkey, (in Turkish).

Abdel-Aal, E.A., and Farghaly, F.E., 2007. Preparation of silver powders in micron size from

used photographic films via leachingcementation technique, Powder Technology, Vol. 178, No. 1, pp. 51-55.

Garcia, R.M., 1986. The recovery of silver from photographic film: a study of the leachingreaction with cyanide solution for industrial use, Hydrometallurgy, February, Vol. 16, No. 1, pp. 395-500.

Grge, D., 2006. Extraction of gold from Kaletas gold ore by thiosulphate leaching, B.Sc.Thesis, Karadeniz Technical University, Trabzon, Turkey (in Turkish).

Gldan, G., 2005. Recovery of silver from waste X-ray films and solutions, B.Sc. Thesis,Karadeniz Technical University, Trabzon, Turkey (in Turkish).

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Khunprasert, P., Grisdanurak, N., Thaveesri, J., Danutra, V., Puttitavorn, W., 2008. Radiographicfilm waste management in Thailand and cleaner technology for silver leaching, Journal of Cleaner Production, January, Vol 16, No 1, pp. 28-36.

Kodak, 1998. Sources of silver in photographic processing facilities, Eastman Kodak Company ,Publication No: J-210, 8 pp.

Kodak, 1999. Recovering silver from photographic processing solutions, Eastman Kodak Company, Publication No: J-215, 11 pp.

Kunda, W., 1995. Photographic fixer regeneration, United States Patent.Montgomery, D.C., 2001. Design and Analysis of Experiments, John Wiley&Sons Inc., New

York 684 pp.Marinkovic, J., Korac, M., Kamberovic, Z., Matic, L., 2006. Recycling of silver from exposed X-

ray films, Acta Metallurgica Slovaca, Vol. 12, pp. 262-268.Minitab, 2004. Minitab Inc., Statistical Software, Release 14.12.0Muir, D.M., and Aylmore, M.G., 2004. Thiosulphate as an alternative to cyanide for gold

processing issues and impedients, Mineral Processing and Extractive Metallurgy, April, Vol.113, No. 1, pp. 2-12.

photographic films with NaOH stripping, Turk J. Chem., Vol. 27, pp. 127-133.Roy, R.K., 2001. Design of Experiments Using The Taguchi Approach: 16 Steps to Product and

Process Improvement, Wiley-Interscience, New York, 560 pp.Shankar, S., More, S.V., Laxman R.S., 2010. Recovery of silver from waste X-ray film by

alkaline protease from conidiobolus coronatus, Kathmandu University Journal of Science-Engineering and Technology, March, Vol. 6, No. 1, pp. 60-69.

Syed, S., Suresha, S., Sharma, L.M., Syed, A.A., 2002. Clean technology for the recovery of silver from processed radiographic films, Hydrometallurgy, January, Vol. 63, No. 1, pp. 277-280.

Yen, W.T., Deschenes, G., Aylmore, M., 2001. Thiosulphate leaching as an alternative to

cyanidation: A Review of the Latest Developments, 33rd Annual Operators Conference of the Canadian Mineral Processors, January, 2001, Ottawa, Ontario.

Zhouxiang, H., Jianying, W., Ma, Z., Jifan, 2008. A method to recover silver from waste x-rayfilms with spent fixing bath, Hydrometallurgy, February, Vol. 92, No.1, pp. 14-151.

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Documents

Extraction of Silver From Waste X-ray Films by Thiosulphate Leaching--IMPS2010-Libre