Selective oxidation of copper from liquid copper-silver alloys

Abstract

In this work, the oxygen refining of liquid copper-silver alloys with a borosilicate slag was studied. First, a comprehensive thermodynamic analysis was performed using the data available in the literature. The results indicate that since silver oxide is relatively unstable in silicate-based slags, then it should be thermodynamically feasible to oxidize copper from copper-silver alloys with a very low silver loss to the silicate slag. In actual practice, although relatively low copper levels can be achieved in the metal phase, the silver losses to the slag are excessive. Therefore, in the present work, both kinetic and equilibrium experiments were performed on a molten copper-silver alloy containing 12.68 mass pct silver in order to elucidate the mechanism of silver loss to the slag. The kinetic experimental results indicated that copper levels of less than 2 mass pct could be achieved with silver recoveries of about 95 pct after relatively short refining times of 15 minutes. In the equilibrium experiments, the copper contents of the metal were less than 1 mass pct, and these values were in good agreement with those which were calculated from the data of previous researchers. In order to explain the relatively high silver losses to the slag, a model was developed which is based on the transport of silver from the metal phase to the slag phase both in metallic form and as silver oxide in the copper oxide oxidation product. The copper and silver oxides and the metallic copper-silver alloy are all transported into the slag by the oxidizing gas bubbles. It is proposed that once in the slag, the silver oxide is unstable and decomposes into metallic silver which is not easily recovered in the metal phase. Also, the transfer of the copper-silver alloy into the slag, by the gas bubbles, promotes the slag-metal exchange reaction, which again results in the generation of silver particles in the slag.

A new interpretative approach to the chemistry of copper-alloy objects: source, recycling and technology

“…Bray & A.M. Pollard in certain of its metals depending on how many times it has been reheated, and to what temperature (e.g. Hampton et al 1965;Charles 1980;Pickles 1998;Beeley 2001: 497;Tanahashi et al 2005;Lee et al 2009).…”

Is a focus on ‘recycling’ useful? A wider look at metal mutability and the chemical character of copper alloys

“…These have long been a practical industrial concern in the modern day; for example, the Institute of Foundrymen set up a committee to track metal losses and chemical alteration in mid‐20th‐century factories (Hampton et al, 1965 , p. 225). There now exists a substantial literature on the general process of how remelting and casting copper alloys, even in slightly oxidising conditions, can alter the overall chemical character of an assemblage (Budd, 1991 ; Budd & Ottaway, 1991 ; Charles, 1980 ; Godfrey, 1996 ; McKerrell & Tylecote, 1972 ; Pickles, 1998 ; Sabatini, 2015 ). These include loss of vulnerable oxides of arsenic and antimony, and also segregation effects which would lead to later change through mechanical abrasion of enriched surfaces.…”

Beyond Provenance

“…A little earlier than this, in the 1930s, several groups of German-speaking researchers began large scale programs of chemical analyses on ancient metal artefacts. The first significant compilation of data was that of Helmut Otto (1910-1998 and Wilhelm Witter (1866, from the University of Halle, whose results were summarized in 1952 as "Handbuch der ältesten vorgeschichtlichen Metallurgie in Mitteleuropa". Their aim was to understand prehistoric metallurgy in Europe using chemical analysis, in contrast to the previous typological approach to metal artefacts.…”

“…The importance of such factors is shown by the fact that they are exploited by the modern copper industry in order to oxygen refine the raw copper produced by the smelter (Copper Development Association 2011), and also that such selective oxidations are now an important part of the metal recycling industry, e.g., recovering copper from mobile phones (Kaspar et al 2011). Consequently, there is an extensive literature on oxidation effects in molten metals that is useful for considering in the archaeological context: a copper alloy object will be depleted in certain of its metals depending on how many times it has been reheated, and to what temperature (e.g., Hampton et al 1965;Charles 1980;Pickles 1998;Beeley 2001: 497;Tanahashi et al 2005;Lee et al 2009). Unfortunately, however, the modern metal recycling industry is not interested in the same combinations of trace elements as we find in Bronze Age alloys, so a certain amount of experimental work has been necessary to understand these systems (Sabatini 2017).…”