Table of Contents
The melting of cathode copper, usually containing 99.98+ per cent. Cu, would appear to be a simple matter. Owing to the well known affinity of copper for sulphur, however, so much sulphur is absorbed by the copper during the operation that a long and expensive refining process is required to remove it, and the final product is inferior in purity and lower in electrical conductivity than the original cathode copper.
The furnace operation, as now conducted, includes the following stages:
- Charging the cathodes into the furnace.
- Melting the cathodes.
- Oxidizing period—saturating the copper with cuprous sub-oxide and bringing the metal to the condition known as “set copper;” this is now done by blowing air into the molten bath of metal.
- Reduction period, otherwise known as “poling,” whereby the oxygen previously added is removed by covering the molten bath completely with coke or charcoal and introducing large poles of green wood; this reduces the cuprous sub-oxide, bringing the copper to the “tough pitch” stage ready for casting.
- Casting Period.
Many experiments have been made to obviate the long periods of oxidation and subsequent reduction, with no success. While the copper first, cast will be good, as indicated by the set surface of the bars, it is impossible to hold it to the “tough pitch” condition, and long before the whole charge has been poured the copper will become “overpoled,”’ entailing numerous difficulties.
Sulphur has always been blamed by the furnace men as the reason why this refining process is necessary, yet the literature on the subject does not sufficiently emphasize how small a proportion of sulphur will ruin copper for practical purposes.
The sulphur absorbed by copper during the melting period comes from two sources: (1) the sulphate left on the cathode sheets in the tank houses; (2) the fuel. The percentage of sulphates on the cathodes is variable, and depends on the care taken in washing the cathodes after removing them from the electrolytic tanks. Crystallized copper sulphate can often be seen on the top edges, particularly around the loops of the cathodes, and even well washed cathodes will show from 0.002 to 0.004 per cent, of sulphur. In the furnace, the copper sulphate becomes dehydrated and considerable quantities of it are blown out of the furnace, most of this being caught in the flues of the waste-heat boilers; samples of flue dust collected beyond these boilers analyzed 16.5 per cent, of water-soluble sulphates. The fuel whether soft coal or oil, always contains an appreciable amount of sulphur, which is likely to be absorbed by the copper, as the products of combustion pass over the charge.
Melting under Coke Cover
In order to study the behavior of sulphur and oxygen during the refining of cathode copper, samples were taken at ½-hr. intervals from
a 500,000-lb. charge of copper. Results are compiled in Table 1 and shown graphically in Fig. 1.
Analysis of the final product gave 99.93 per cent. Cu, and the average conductivity of the samples was 100.
Referring to Fig. 1, during the oxidizing period, as expected, the oxygen percentage steadily increases while the sulphur continuously decreases, until the reversible reaction.
Cu2S + 2Cu2O ⇔ 6Cu + SO2
has been completed so far as the cuprous sulphide in the copper is concerned. Further, no sulphur is absorbed, that is, the reaction does not
become reversible until the charge has been brought up to nearly the “tough pitch” stage, when it again takes up sulphur.
This residual sulphur, amounting to only 15 lb. in the whole charge, can come from two sources only: the fuel and the coke used to cover the charge. A good grade of 72-hr. coke, analyzing 0.83 per cent. S, was used, and as 1200 lb. was required, this introduced 10 lb. of sulphur into the charge.
Melting under Charcoal Cover
This test was duplicated on another furnace charge of the same weight, using charcoal instead of coke to cover the charge. This charcoal contained only 0.03 per cent. S. The results are compiled in Table 2 and shown graphically in Fig. 2.
The analysis of the final product gave 99.966 per cent. Cu, and the average conductivity of the samples was 100.5.
Referring to Fig. 2, the behavior of oxygen and sulphur during refining follows closely that of the former charge; but owing to the absence of
sulphur in the charcoal used to cover the charge during the poling and casting periods, the absorption of sulphur by the copper was much less.
The comparison of the two charges, given in Table 3, shows not only a lower sulphur content for the charge refined under charcoal, but also lower oxygen contents, and a better quality of copper.
It is becoming better understood that the “surface set,” or the pitch of copper, is directly controlled by the sulphur and perhaps certain reducing gases, such as carbon monoxide and hydrogen, rather than by oxygen; and that sulphur raises the “set” or overpoles the copper, thus necessitating a certain amount of oxygen, which depresses the “set”, to counterbalance the action of the sulphur.
In these two charges, the “pitch” of the copper was the same, yet owing to the lower sulphur in product No. 2, its oxygen contents is also much lower than that of No. 1, affording great improvement in the quality of the copper. The refiner did not know how much sulphur was present in his final product; he simply worked the copper to the desired pitch, and the sulphur present in each charge determined the amount of oxygen necessary to give the proper “surface set” on the copper.
Oxygen and Sulphur in the Melting of Copper Cathodes
Samples taken at regular intervals during the melting and refining of copper cathodes show:
- The total elimination of sulphur during the “oxidation” period and before poling.
- The re-absorption of sulphur by the charge when the percentage of cuprous oxide is low enough to allow the reversible reaction
6Cu + SO2 ⇔ Cu2S + 2Cu2O
to take place. - The advisability and importance of using a low-sulphur coke or charcoal to cover the molten bath during the poling and casting periods.