Refining of Precious Metal Bearing Materials from Secondary Sources-Methanesulfonic Acid Leaching of Raw Silver Granules as a Promising Approach towards a Green Way of Silver Refining

Abstract

The state-of-the-art technology of raw silver refining in a silver nitrate-based electrorefining process (Moebius-electrolysis) is accompanied by several disadvantages, both from a technological and from an ecological point of view. In addition, increasing concentrations of critical impurities from secondary sources, like palladium, in raw silver are a further challenge for the future of silver refining. Thus, there is strong motivation for the development of an adequate, alternative process of raw silver refining to substitute the existing Moebius-electrolysis. Due to its less environmentally toxic character and the high aqueous solubility of its silver salt, methanesulfonic acid (MSA) is a possible base chemical for the design of an efficient refining method based on leaching of raw silver followed by electrowinning, with less ecological and technological complications. In this paper the results of some fundamental investigations on the leaching of raw silver granules, containing approx. 94% silver, with methanesulfonic acid and hydrogen peroxide as an oxidation agent are presented. Agitation leaching experiments were conducted on a laboratory scale and the effects of the solid concentration, the hydrogen peroxide dosage and the temperature as leaching parameters were studied. The obtained results indicate that silver leaching yields of more than 90% are achievable with leaching at elevated temperatures of 65 °C or 80 °C, solid concentrations of 500 g/L and at a stoichiometric H2O2:Ag-ratio of 3:1. Increased solid concentrations greater than 500 g/L and elevated temperatures of 65 °C or 80 °C additionally improved the selectivity of the process regarding the leaching of Pd.

Methanesulfonic acid (MSA) in clean processes and applications: a tutorial review

“…In other words: there are not enough free water molecules left to fully saturate the first coordination sphere of the metal cations by hydration. This reduction of solubility in the presence of MSA has been reported for silver( i ), 50 copper( ii ), 42 and zinc( ii ) methanesulfonates. 42 This decrease in solubility in aqueous solutions is a factor that must be taken into account when preparing electrolytes for electrodeposition.…”

“…As part of the development of a new MSA-based process for the refining of crude silver recycled from secondary resources, the dissolution characteristics of silver metal granules in MSA were studied. 138,139 While MSA alone did not dissolve the granules, the addition of hydrogen peroxide as an oxidizing agent resulted in dissolution. Silver granules, containing approximately 94% of silver along small impurities of gold and PGMs, could effectively be leached by a mixture of MSA and H 2 O 2 .…”

“…A correlation was noted between the solubility of silver( i ) methanesulfonate and the concentration of free methanesulfonic acid (MSA) following the leaching process. 139 …”

Rare Erath Elements: Their Geological Sources and the Potential of Organosulfonic Acids For Rare Earth Elements Leaching from Coal Ash and Process Optimization

“…In light of non RREs extraction, the potential of MSA has been explored through research. In this regard, in the recovery of lead and silver from zing leaching residue, MSA has been found to have a proven hydrometallurgical potential (Rodriguez, Onghena, & Binnemans, 2019) as has its potential for the leaching of silver granules (Hopf, Weigelt, Bombach, Stelter, & Charitos, 2021). The detailed chemistry of mtal leaching by MSA is described by the following equations (Feng et al, 2015):…”