A column flotation system was designed to conduct release analyses of a very finely disseminated, stratiform, copper ore from the Denali Copper Prospect, Alaska. Chalcopyrite is finely disseminated in a carbonate matrix and pyrite is disseminated in both the chalcopyrite and gangue, requiring grinding to 20 microns for adequate liberation.
The ore is almost entirely chalcopyrite and pyrite hosted in a carbonate matrix with about equal amounts of each sulfide mineral. The pyrite is disseminated within the gangue, and in the chalcopyrite grains. These disseminated pyrite grains are as small as 1 micron. The ore is of quite high grade. The sample used in this study averaged 8.6% copper.
The flotation column was constructed of glass tubing and was 25 mm (1 inch) inside diameter by 2.4 m (8 feet) high. A feed slurry was fed to the column at the 1.27 m level by a variable speed peristaltic pump and a tailings slurry was removed from the base of the column by another variable speed peristaltic pump.
Mineral laden froth overflowed the top of the column into a 100 mm diameter collection launder, which directed the froth flow to sample pans. Wash water was added to the top of the column through a spray nozzle (12 mm outside diameter), which extended 38 mm into the top of the column. The nozzle acted as a froth crowd in addition to supplying a spray mist of wash water at rates from 15-20 ml/minute. The wash water also contained MIBC at 100 ppm.
The resulting release analysis products were filtered, dried, weighed and prepared for elemental analyses. Copper analysis was by atomic absorption spectroscopy. Iron, calcium and magnesium analyses were by ICP spectroscopy.
The association of pyrite and chalcopyrite may be summarized as follows:
- Although chalcopyrite varies in grain size, pyrite occurs primarily as framboids, 1 to 5 microns in size and occasionally as large as 15 microns .
- Large chalcopyrite grains occur ranging up to 100 microns in size with little or no pyrite inclusions. Pyrite however is found at the edge of these grains ).
- Large chalcopyrite grains occur up to 100 microns in size, which contain pyrite 1 micron or smaller in size. There is no possibility of liberating this pyrite, but chalcopyrite of good purity can still be liberated at a relatively coarse size.
A convenient gradient scale may be constructed by drawing a vertical line at 100 on the abscissa, and scaling it linearly from 0% at the base to 100% at the top. Lines’ joining points of a release curve to the origin pass through this scale at their metal grade.
The simple mineralogy permitted their calculation from an analysis of acid soluble Cu, Fe, Ca and Mg in the fractions. All copper was calculated as chalcopyrite. Iron not accounted for by chalcopyrite stoichiometry was expressed as pyrite. Calcium and magnesium were converted to carbonate. The remaining minerals consist of quartz and clay minerals. These calculations provided very accurate mineral distribution analyses and were found to be very useful in evaluating flotation results for rejection efficiency as well as mineral liberation. The conclusion is that pyrite is only partially liberatable and liberation is improved at finer grinds.
Column flotation release analysis shows potential for characterizing the liberation and flotation properties of fine grained sulfide ores. The results obtained in this study indicate good reproducibility and the ability to differentiate release characteristics between finely and very finely ground ore. In the case of the Denali copper ore, release analyses indicate a 20% copper grade is achievable at a 75% copper recovery from 99% passing 20 micron flotation feed grading 8.5% copper.