Molybdenite and chalcopyrite are two often-associated minerals. In Quebec, the Gaspe Copper Company, a subsidiary of the Noranda group, currently operates a molybdenite flotation circuit to recover a one percent trace of this mineral in the chalcopyrite concentrate. The low levels of molybdenite preclude the use of flotation to remove molybdenum directly from the ore while scavenging of chalcopyrite concentrate for molybdenite yields a concentrate of the latter of somewhat inadequate quality. Such a situation could possibly best be solved by hydrometallurgical means, if molybdenite could be leached selectively from the chalcopyrite concentrate.
In order to perform solubility measurements simultaneously with analyses of the solution for dissolved molybdenum and copper, an autoclave from Autoclave Engineer, PA, has been equipped with a liquid sampling system designed to operate at the pressure and temperature prevailing in the autoclave. A sampling cycle would remove and cooled down a 5 cc sample for chemical analysis and send a 0.2 cc sample to a chromatograph, following flash evaporation and water stripping on a drierite column.
Results and Discussion
In order to verify the accuracy of the present technique for solubility measurements, solubility data for oxygen, nitrogen, carbon monoxide have been determined as a function of pressure and temperature. The results for the solubility of oxygen are presented along with data from other workers. While results at low temperatures were numerous, little or no data were available at temperatures above 100°C. In the case of oxygen, data from Pray coupled with those of Zoss’ at very high pressure did permit an adequate regressional analysis.
To verify the hypothesis that oxygen transfer may limit to an extent : the rate of oxidation, oxidation of molybdenite can be followed with time. There are results for the following conditions: 180°C, 450 psi and molar ratio of 1.5. In spite of a decreasing surface area, the rate of dissolution remains constant up to the time where dissolution starts decreasing exponentially. As rate of dissolution appears to be independent of the amount of surface, the kinetics are probably not controlled by surface reactions but rather by mass transfer of one of the components Oxygen is most likely to be involved as was shown previously.
To find out whether molybdenite can be leached out of chalcopyrite concentrates containing 14% molybdenite, leaching experiments were conducted on 50 grams of chalcopyrite concentrate. The purpose of these tests were to find out how much oxygen and hydroxyl ions were consumed during leaching of the molybdenite assumed to be in the concentrate.
Leaching Tests with Ammonia
Rates of oxidation for molybdenite are seem to decrease by about 40% while those for chalcopyrite increase to a maximum corresponding to that obtained in KOH solutions, for the most stringent conditions. About 45% of the chalcopyrite will leach in one hour and this amount corresponds to complete consumption of hydroxyl ions as determined in the KOH leaching test, at 180°C, 450 psi of oxygen and a molar ratio of 1.5. This percentage of reaction most likely corresponds also to complete consumption of ammonium to form the complex with copper.
The substantial decrease in rate of oxydation of molybdenite in ammoniacal solution can probably be related to the low hydroxyl ion concentration resulting from a weak dissociation of ammonia at high temperature. It would thus appear possible to selectively leach chalcopyrite leaving behing molybdenite.