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Minerals Engineering xxx (2005) xxx–xxx This article is also available online at: www.elsevier.com/locate/mineng
Silver recovery from synthetic photographic and medical X-ray process eﬄuents using activated carbon K.G. Adani, R.W. Barley *, R.D. Pascoe
Camborne School of Mines, School of Geography, Archaeology and Earth Resources, University of Exeter, Tremough Campus, Treliever Road, Penryn, Cornwall TR10 9EZ, United Kingdom Received 29 April 2005; accepted 24 May 2005
Abstract Adsorption of silver from synthetic photographic and spent ﬁx solutions on granulated activated carbon in a batch process has been investigated. The synthetic solutions prepared had similar properties to medical X-ray and photographic process eﬄuents. Sodium and ammonium thiosulfates are the major lixiviants used in the dissolution of silver halides present in photographic and medical X-ray ﬁlms. The resultant solutions contain substantial amounts of recoverable silver complexes in the form of thiosulfates. The as received carbons were alkaline in nature when contacted with deionised water. These carbons showed very low silver recoveries. However, when these carbons were pre-treated with 0.5 mol/dm3 sulphuric and nitric acids at 25 °C, it had resulted in significant silver recoveries of 98.5% and 95% from sodium and ammonium thiosulfate solutions, respectively. These results were achieved in a narrow range of pH between 3 and 4. Batch atmospheric stripping of adsorbed silver by 2.0 mol/dm3 HNO3 at 85 °C was also investigated. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Hydrometallurgy; Waste processing; Activated carbon; Silver
1. Introduction Reade discovered dissolution of silver halides in photographic ﬁxing process using ammonium thiosulfate (ATS) and sodium thiosulfate solutions (STS) as ﬁxing baths in 1837 (Eaton, 1965). Conventional ﬁxing of AgX in photographic and medical X-ray ﬁlms has been extensively used in the photo-industry for more than two centuries. The dissolution of silver halides by thiosulfates and thiocyanates are usually rapid and do not need any catalytic inﬂuence; hence they are highly soluble in these lixiviants and are considered to be thermo-
dynamically stable at slightly alkaline pH. The simpliﬁed mechanisms for dissolution of AgCl in thiosulfate solutions are proposed in Eqs. (1) and (2). AgClðsÞ þ 2ðNH4 Þ2 S2 O3ðaqÞ ðNH4 Þ3 ½AgðS2 O3 Þ2 ðaqÞ þ NH4 ClðaqÞ AgClðsÞ þ 2Na2 S2 O3ðaqÞ Na3 ½AgðS2 O3 Þ2 ðaqÞ þ NaClðaqÞ ð2Þ ð1Þ
Corresponding author. Tel.: +44 1326 371825; fax: +44 1326 371859. E-mail addresses: firstname.lastname@example.org (K.G. Adani), email@example.com, firstname.lastname@example.org (R.W. Barley), R.D.email@example.com (R.D. Pascoe). 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.05.021
ATS and STS are commonly used in ﬁxing baths that contain a number of other chemicals like acids, preservatives, hardeners and buﬀers, which perform various functions in the photographic ﬁxing process. This results in spent ﬁx solutions having a complex silver composition and thus this preliminary test work was performed on synthetic solutions. AgCl and AgBr were dissolved in ATS and STS to produce synthetic spent ﬁx solutions
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2 K.G. Adani et al. / Minerals Engineering xxx (2005) xxx–xxx Table 1 AAS results of initial metal ion concentrations in the Ôas receivedÕ medical X-ray and photographic eﬄuents Metal ion +
(SFS) as eﬄuents with similar silver concentrations. These synthetic solutions were used in the initial experiments. The use of carbon to selectively adsorb silver is based on the gold–silver cyanide complex adsorption principles as in carbon in pulp and leach circuits in mining operations. Carbon adsorptions for Au/Ag complexes were observed in the order of high to low aﬃnity as; Au halide > AuðCNÞ2 > Au½CSðNH2 Þ2 > Au2 ðS2 O3 Þ2 as reported by Gallagher et al. (1990) and Nicol et al. (1987)....
References: Adani, K.G., 2003. Silver adsorption from photographic and medical X-ray process eﬄuents by activated carbon. MSc. Thesis. Camborne School of Mines, University of Exeter, Cornwall, UK. Bailey, R., 1987. The characteristics of activated carbon adsorption. The South African IMM, vol. 1. Chamber of Mines of South Africa, Johannesburg. pp. 383–393. Barbosa, O., Monhemius, A.J., 1989. Thermochemistry of Thiocyanate Systems for Leaching Gold and Silver Ores. Royal School of Mines, London. Precious Metals Õ89. MMMS, 1988. pp. 307–339. Davidson, R.J., 1974. The properties of activated carbon adsorption. J. South African IMM, 67. Eaton, G.M., 1965. The Chemistry of Colour and Black-and-White Photography. Morgan and Morgan Inc. Publishers, New York. Gallagher, N.P., Hendrix, J.L., Milosavljevic, B.E., Nelson, H.J., Solujic, L., 1990. Aﬃnity of activated carbon towards some gold(I) complexes. J. Hydrometall. www.sciencedirect.com . Genik, S., Barezowsky, R.M., Weir, S., 1985. Gordon mines, European Patent Nr. ac(5)718, and Patent No. 1772929 (Eur), September, pp. 27.
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