Silver leaching with the nitrite–copper novel system: A kinetic study

Highlights

• Nitrite-copper is a totally new and promising silver leaching system.

• NO₂–Cu^2 + system is capable of complexing silver metal without thiosulfate.

• Nitrite ions act as a complexing as well as oxidizing agent.

• Leaching is controlled by chemical reaction of lixiviant at the Ag surface.

• About 96% silver can be recovered at 55 °C and 6 h.

Abstract

This work presents a novel leaching system involving NO₂–Cu to oxidize and complex silver without the use of cyanide, ammonia or thiosulfate. It is possible to oxidize metallic silver through the reduction reaction of the nitrite ions, the cupric-nitrite complexes seem to be responsible for increasing the silver leaching kinetics; the nitrite ions act as a complexing agent for the silver ions in the solution. The effect of cupric ions concentration (0.05 M, 0.1 M and 0.2 M) and nitrite ions concentration (1.0 M, 1.35 M and 1.5 M) on the dissolution kinetics of silver was evaluated. Based on the results the best leaching conditions were worked out as: 1.5 M nitrite ions and 0.2 M copper ions at a natural pH of 5.0. This leaching solution dissolves 80% of silver within 6 h. The effect of temperature on the dissolution kinetics of silver was studied at the room temperature till 30, 45, 50 and 55 °C, showing a progressive increase in silver recovery from 7.69% to 10.38%, 28.62%, 44.7% and 53.93%, respectively in 30 min. The kinetic analysis revealed that silver leaching depends on the temperature and rate controlling step corresponds to the silver dissolution through the chemical reaction. The apparent activation energy of the reaction is found to be 50.92 kJ/mol, along with the enthalpy and entropy of activation values of 48.34 kJ/mol and − 139.05 J/mol K, respectively. The kinetic model corresponding to the silver leaching can be represented by the expression: 1 − (1 − x)^1/3 = k₀ exp. (− 50.92/RT) t. The analysis of solid residue obtained after leaching at pH 5 by XRD and SEM with elemental mapping showed the formation of a copper-hydroxysulfate species, brochantite, formed in the bulk solution and not on the silver particle surface. The leaching results of silver by the NO₂–Cu system were also ratified by the species distribution diagrams.

Introduction

Cyanidation is the process traditionally employed worldwide for the extraction of silver and gold from different sources. Metallic silver can be found in different wastes. For example in the so-called urban ores, it is of environmental and economic importance to leach metallic silver from printed circuit boards (PCB's), electronics, spent catalysts and fabrics containing silver nanoparticles (Ficeriová et al., 2008; Manju Gurung et al., 2013, Jadhav and Hocheng, 2012, Pasricha et al., 2012). Presently the mining and recycling industry has been involved in solving new technological and environmental challenges (Aylmore and Muir, 2001, Ahern et al., 2006). Some of the main alternatives to replace the cyanide from the leaching systems includes the use of thiourea (Jing-ying et al., 2012) and thiosulfate as the lixiviant (Abbruzzese et al., 1995, Tanriverdi et al., 2000, Aylmore and Muir, 2001, Breuer and Jeffrey, 2000, Breuer and Jeffrey, 2002, Wan and LeVier, 2003, Zhang and Nicol, 2003, Grosse et al., 2003, Ji et al., 2003, Senanayake, 2005, Gudkov et al., 2010a, Gudkov et al., 2010b, Gudkov et al., 2010c, Puente-Siller et al., 2014, Alvarado-Macías et al., 2015). The leaching of metallic silver has been studied by different researchers using alternative leaching systems in the presence of thiosulfate or ozone (Puente-Siller et al., 2013, Puente-Siller et al., 2014, Alvarado-Macías et al., 2015, Rivera et al., 2015, Viñals et al., 2005). The thiosulfate system seems to be the most promising for the extraction of precious metals, because of its capability to increase the metal dissolution rate from 18 to 20 times (Aylmore and Muir, 2001). However, one of the main disadvantages of the thiosulfate system relates to its oxidative degradation to tetrathionates (Aylmore and Muir, 2001). Therefore, in order to reduce such degradation, researches are being carried out using different additives such as citrate, EDTA and amino acids (Puente-Siller et al., 2013, Puente-Siller et al., 2014, Feng and Van Denventer, 2010, Feng and Van Denventer, 2011). These investigations have shown the possibility of decreasing the oxidative degradation of the thiosulfate ions by lowering the redox potential of the system. However, the drawback is yet to be overcome.

Recently, Alvarado-Macías et al. (2015) examined an alternate process for silver leaching involving S₂O₃-NO₂-Cu which avoids the use of cyanide or ammonia in the thiosulfate process and minimizes the environmental impact. However, the formation of a Cu-S coating on the surface of silver particles (Alvarado-Macías et al., 2015) due to the degradation of thiosulfate has retarded the leaching (Aylmore and Muir, 2001) due to the hindered contact between the lixiviant and unreacted silver. In addition, the Cu-S catalyzes the oxidative decomposition of thiosulfate in solution by air resulting in low silver extraction (Chanda and Rempel, 1985, Puente-Siller et al., 2014).

In view of the above, the current researches have aptly focused on exploring a new system to dissolve the precious metals from different resources without the use of cyanide, thiosulfate or thiourea. Mention may be made of a recent study in which Oraby and Eksteen (2015) used glycine-peroxide for the gold and silver leaching. The gold leaching rate of 0.322 μmol/m².s was achieved using a solution of 0.5 M glycine and 1% peroxide at pH 11 in 48 h. In fact, this result was better as compared to the gold leaching rate obtained after six days with thiosulfate–EDTA or thiosulfate–oxalate systems in the presence of thiourea. Other systems that do not use cyanide for the leaching of silver/precious metals are based on the nitric acid (Holloway et al., 2004, Pan-Pan et al., 2014), the nitrogen species catalyzed (NSC) (Anderson, 1995, Anderson and Nordwick, 1996, Anderson et al., 1996a, Anderson et al., 1996b, Anderson, 2003a, Anderson, 2003b), and the bisulfide (Hunter et al., 1998) in the conventional leaching operations. However, the use of nitrite ions as oxidant and complexing agent for silver has not been investigated systematically until the present work.

In this work, use of a novel leaching system involving NO₂–Cu^2 + solution has been investigated to understand the dissolution behavior of silver and establish the most suitable conditions to accelerate the leaching kinetics, with the aim of using it in future, this information would be valuable to dissolve metallic silver contained in different wastes (mining industry or urban mines). The research is based on the premise that nitrite ions can oxidize the metallic silver. In fact, nitrite reduction can produce ammonia and ammonium ions as previously reported by Alvarado-Macías et al. (2015) according to the reactions (1 and 2) depending on the pH of the solution. The formation of ammonia and ammonium ions may cause oxidative dissolution of the precious metals as a result of the redox process.

Thus at pH > 9 ammonia is predominantly generated as Eq. (1):

NO₂⁻ + 5H₂O + 6e⁻ → NH₃ + 7OH⁻

Whereas, at pH < 9 ammonium ions are predominantly formed (2):

8H⁺ + 6e⁻ + NO₂⁻ → NH₄⁺ + 2H₂O