During the last few years we have been doing considerable experimenting on methods of precipitating copper. The first patent on precipitating from sulphate solutions by heating with SO2 under pressure was taken out by us (U. S. Pat. 723,949).
This reaction, CuSO4 + SO2 + 2H2O = Cu + 2H2SO4, is interesting, but there are a number of practical difficulties which will be met with in carrying it out on a scale of any size, the main ones being that it is not easy to get a sufficiently strong solution of SO2 to carry out the reaction, and that the design of an apparatus for heating large amounts of corrosive solutions under pressure is a considerable problem. If these can be overcome, I believe the process may be developed, but I agree with Dr. Ricketts’s opinion that electrolytic methods of precipitation are preferable.
Regarding our work at Douglas, we have felt that to say much about it was a little premature, as experiments are still in progress, but some notes on the preliminary experiments may be of some interest.
The general method which we adapted for the tests was sulphatizing roasting, leaching, and electrolysis from sulphate solutions, using depolarization.
The reason for the consideration of the sulphatizing roasting was that the materials which we have to treat contain in most cases large amounts of soluble alumina, and therefore, with a regenerative process only, we would be almost sure to have a deficiency of acid. With the sulphatizing roasting this deficiency of acid would be more or less made up.
The plant used for the preliminary experiments consisted of a 20-ft. six-hearth Wedge muffle furnace, several small leaching tanks, and an electrolytic installation designed to precipitate about 250 lb. of copper a day.
The objects of the roasting experiments were to determine the capacity of the furnace, the fuel consumption, and the percentage of extraction, both water soluble and acid soluble. The objects of the electrolytic experiments were to determine the power and the ampere efficiency, to determine whether the anodes which our preliminary experiments seemed to indicate would be satisfactory under working conditions, and also to determine the best methods of dissolving and applying the SO2 gas.
The results of our roasting experiments were as follows: We roasted three principal classes of material, in which the average insoluble copper in the calcines was 0.11, 0.16, and 0.41. The water-soluble copper showed 70 per cent, of the copper contents, 40 per cent., and 51 per cent., respectively, and the total extraction was 92.9, 88.0, and 70 per cent. One of the principal results of these experiments was to show us that for this class of roasting it was necessary to have more or less fine crushing as a preliminary.
The results of our preliminary electrolytic experiments were fairly satisfactory. We obtained a total amount of copper precipitated of something over 2 tons at an average figure for power of about 1.6 lb. of copper per kilowatt-hour. We produced acid per kilowatt-hour equivalent to about 5.1 lb. The ampere efficiency was low. This, however, was due to several reasons not directly connected with the process. We had a large number of short-circuits and other troubles, but we felt quite sure at the end of these preliminary experiments that we should get about 2 lb. of copper per kilowatt-hour. We had, however, a certain number of uncertainties and some anomalous results, so we started some further experiments on details of solution and other matters.
These further experiments resulted in the following general conclusions:
First, that it was not necessary to have such a large amount of SO2 in solution as we had previously thought, and that, in addition, much better results could be obtained by using SO2 together with iron, rather than either one alone. In other words, the function of the SO2 was not only, to act as a depolarizer, but also to act chemically in keeping the iron reduced. We also found there was a certain favorable effect from a high aluminum content. The temperature increase is favorable in the same way as in copper refining. With iron salts present there is a decreased ampere efficiency with increased temperature. This was counteracted by SO2 and by increased current density.
As a result of these further experiments, we laid down the conditions of high ferrous sulphate in solution, high alumina, a temperature of about 115° to 130° F., current density of about 15 amperes per square inch, and sufficient sulphur dioxide present to reduce the voltage, and maintained sufficiently to keep the iron in the ferrous condition, and under these conditions I expect it will be possible to get a yield of from 2 to 2.5 lb. of copper per kilowatt-hour.
As to “foul solutions,” which has heretofore seemed to be the principal bugbear in electrolysis, as long as the iron can be kept in the ferrous state, which can be done by SO2, it is fairly safe to say that, up to certain limits, better results will be obtained from so-called foul solutions than from pure copper sulphate solution.
We are now about to start a 75-ton plant, in which we expect to apply the information which we have gained to the treatment of one class of ore, and the result of the work which will be done in that plant will no doubt be published later.