Thermodynamic Equilibrium Calculations to Simulate the Metallurgy of Fahlores
For the calculations various compositions containing copper, arsenic, and sulphur in stoichiometric ratio were determined (the EKVICALC software searched all compounds containing these elements), a temperature was selected, and the equilibrium composition was calculated for increasing oxygen levels. This procedure simulates the reactions during roasting and smelting of the copper ores.
We have to keep in mind that these equilibrium calculations are thermodynamic. During practical smelting processes, the chemical reactions are overlaid by the reaction kinetics. For example, if one compound is evaporating during roasting, it is lost for the metallurgical process, but in our calculations the elements remain in the system and the concentrations are constant.
Fig. 2 shows the raw data for a calculation in logarithmic scale, and it is rather complex. For a better understanding, we changed to the decimal scale and used the relevant sector for presentation and discussion.
Behaviour of As during Smelting Arsenical Cu Ores
In Fig. 3a the simulation of the whole process is shown, starting from pure sulphide and adding oxygen till CuO is formed. To simulate the chemical reactions of roasting, carbon was added to the system. The calculations confirm the already known fact that sulphides cannot be reduced by carbon. As can be seen in Fig. 3b, at first the carbon reacts with oxygen to CO and, in the next step, CO reacts to CO2. During this process step, some COS can be formed additionally. When the carbon has been totally consumed, SO2 formation starts. In practice the used charcoal for roasting is necessary to increase the temperature to a value that oxygen can react with the sulphidic ore. For the roasting reaction itself, an adequacy of oxygen has to be available, which can be carried out in roasting beds but not in shaft furnaces.
At 800 °C roasting starts with the reaction of As2S3 to liquid As2S2 and gaseous SO2. In the next step, the As2S2 reacts with oxygen forming gaseous As4O6 (Fig. 3c). Cu2S reaction with O starts after the total conversion of As2S2 into As4O6. For the metallurgical process, this means that As can evaporate completely during roasting, even at 800 °C.
Since there is no Cu excess in relation to the Cu3As in these calculations, the formation of Cu3As begins with the decomposition of Cu2S, whereby metallic Cu would arise without the presence of As. In practice, the As could already have evaporated as As4O6 by this point in time, which means that, at the point in time when Cu is formed, As is no longer available for the reaction to form Cu3As.
With a further increase of oxygen, Cu3As reacts at first to Cu2O followed by CuO formation. As forms As4O6, which can evaporate.
Another calculation was performed at 1100 °C (Fig. 4). In contrast to 800 °C, the As2S3 directly reacts to gaseous As2 and As4, respectively. At higher O contents, Cu3As is formed and decomposes to Cu2O and As4O6.
In metallurgical practice fahlore smelting means that, during roasting at lower temperatures, As evaporates as As2O6 and at higher temperatures as As2–As4 mixtures. The Cu3As formation starts not before Cu2S is decomposed. Thus As evaporates partially on the roasting bed, and, in the following shaft furnace process, the remaining As can react to Cu3As. Cu3As is a very stable intermetallic phase and remains in the alloy until Cu2O formation takes place.
Behaviour of Sb during Smelting Fahlors
The simultaneous presence of As and Sb was calculated at 1100 °C (Fig. 5a). In this case As2S3 reacts at first with oxygen and volatile As2 and As4 (Fig. 5b). The Cu3As formation starts when metallic Cu is available for the reaction. The gap between the As2S3 decomposition and Cu3As formation is caused by the reaction of Sb2S3 to liquid Sb3O3 (Fig. 5c). On the one hand, before metallic Cu is present Cu3As is formed. On the other hand, the metallic Cu is oxidized before Cu3As reacts.
The summary for the chemical reactions in Cu-As-Sb-S-containing ores is:
As2S3 reacts to gaseous As2 and As4, which evaporate during roasting.
Sb2S3 reacts to liquid Sb3O3, which will remain in the roasted ore.
If metallic Cu is present, Cu3As is immediately formed.
After Cu3As formation, metallic Cu is formed from Cu2S.
Metallic Cu reacts with O at first to Cu2O.
Finally, the Cu3As is oxidized, and Cu2O, CuO, and As4O6 are formed.
Example of Sb in a Slag Inclusion
A stray find slag from Prigglitz-Gasteil was investigated to describe its nature and to find out the process step where it comes from. From its appearance this slag shows a rough mainly dark brown surface and some spots in a beige colour (Fig. 6a). This indicates that the sample is a mixture of quartz (beige) and slag (dark brown) from an earlier production step of copper smelting [34, 35]. This mixture occurs probably when molten slag flows out of a furnace or crucible and is mixed with sand because no reaction zone on the quartz surface is visible.
Metallographic investigations confirm that quartz grains are surrounded by slag (Fig. 6b). The quartz shows many cracks, which can be explained by a contact of cold quartz with molten slag (Fig. 6b,c). The slag microstructure shown in Fig. 6d reveals large fayalite (Fe2SiO4) dendrites which primarily crystallized. In the interdendritic areas, a glass phase is present, containing all elements which cannot crystallize with fayalite, e.g. Al, Ca . Due to its complex composition, the glass phase is amorphous. As can be seen in the SEM-BSE image (Fig. 6d), there are many bright fine spots in the glass phase. Bright spots in BSE indicate the presence of massive atoms like Cu which are available metallic or as compounds. In the present case, the spots were not analyzed, but we can assume that they are not converted chalcopyrite or copper.
Additionally a spherical inclusion with 1 mm in diameter was observed in the slag containing various Cu compounds (Fig. 6e). The different levels of grey in the SEM-BSE image show that this inclusion is an intense mixture of different compounds and by EDX measurements the main elements Cu, Fe, S and O were observed (Fig. 6f). Surprisingly, small amounts of Sb were observed locally. To localize Sb in the microstructure an EDX mapping was executed (Fig. 7).
From the mapping of the particle shown in Fig. 6e,f we can summarize:
This particle is a partially roasted chalcopyrite ore because it contains S as well as O.
The bright regions consist of chalcopyrite with a grain size smaller than 20 µm surrounded by a grey phase containing O. In these grey areas, the chalcopyrite was already transformed into oxides. Large oxygen enriched cracks in the particle were observed additionally.
In oxide containing regions, mainly in the cracks, Sb is enriched. This confirms the thermodynamic calculations that Sb‑S compounds are firstly roasted and that Sb forms Sb oxides, which remain in the slag.
Example of a Copper Ingot Produced from Fahlore
We were able to investigate an ancient copper ingot with 4 wt.% As, 0.4 wt.% Sb, and 2 wt.%S (Fig. 8a; [14, 15]). We assume that this ingot is the final product of copper smelting before it is used for casting artefacts.
The setting of the ingot can be divided into the sections bulk-bottom and top-surface. The most important difference is the access of oxygen during ingot production.
At the bulk and bottom of the ingot, there is nearly no oxygen present and no oxides were observed consequently.
Almost impurity-free copper dendrites were firstly built during solidification of the ingot (Fig. 8b). The interdendritic areas are enriched with As, Sb, and S (Fig. 8c). In addition, some carbon was observed, which probably was introduced by stirring the alloy with a branch. In some regions, spherical Cu2S or Cu2As inclusions are detected. This indicates that during roasting some S and Sb remained in the ore and, during smelting in a charcoal heated shaft furnace, both elements cannot be eliminated and remain in the copper ingot (Fig. 8d).
On the top side of the ingot, oxygen access is easier resulting in the presence of oxides and small holes (Fig. 8e,f). Again, copper-rich dendrites had been formed surrounded by interdendritic areas containing the alloying elements (Fig. 8e). For detailed information about the element distribution, a SEM-EDX mapping was performed (Fig. 9). It can be seen that O, As, and Sb are enriched in some regions. Sulphur is concentrated in spherical spots, but there is no clear correlation with another element except Cu, suggesting the presence of Cu2S. Less but similar As-containing particles were observed corresponding to the Cu3As phase.
Moreover, the SEM-EDX mapping discloses a clear correlation between Sb, O, and Ni (Fig. 9). As it has been shown by the thermodynamic calculations, Sb can easily form a liquid oxide which remains in the Cu melt. When Ni is present, the Sb oxide reacts with Ni and Cu to Cu3Ni2SbO6, which corresponds with the mineral delafossite . In the thermodynamic calculations, this compound was not found because, on the one hand, Ni was not included to the calculations and, on the other hand, no thermodynamic data for delafossite are available.