Column leach studies of a low-grade porphyry copper ore were made. The technique used involved alternating oxygen absorption and copper leach cycles. The leach solution was displaced from the column with oxygen. The volume of oxygen reacting with the ore was measured with a gas burette. The copper released as a result of exposure to oxygen was correlated with the oxygen uptake.
The Experimental Method
Reactions were carried out in plastic columns in which leach cycles were alternated with oxygen cycles. Samples of the leach solutions were taken periodically and the volume of oxygen reacted was measured for each cycle.
The leach columns were constructed from 6 ft. cast acrylic tubes having a 5 inch outside diameter and a 4.5 inch inside diameter. An acrylic lid was used to seal the columns, using a rubber O-ring as a gasket. This arrangement gave a satisfactory gas seal. The leach solution was drawn from the bottom of the columns, passed through a sampling chamber and returned to the top with a centrifugal pump.
The material leached in this research was a porphyry copper ore, hand selected from the Bingham Canyon Mine of the Kennecott Copper Corporation, Utah Division. The material was crushed and sized and three size fractions -4+6 mesh, -6+8 mesh, and -8 + 10 mesh were used in the leach studies. A representative sample of this ore was split for chemical and mineragraphic analysis.
All samples of ore used in this study were completely dried and then heated to 75° C for two hours prior to placement in the columns to be certain they were free of bacteria. The two samples used in the particle size study were heated at 100°C for 12 hours. The presence of these bacteria in the leach solutions is indicated by a drop in pH and an increase in the ferric-ferrous ratio. It was assumed the sterilization was complete since these conditions were not observed at any time during the study.
Experimental Results and Discussion
Fifty pounds of sized ore (-8 + 10 mesh) was placed in the column and was initially rinsed with a solution of pH 2.1. The pH of this solution reached 7.0 in the first few minutes after being placed on the ore.
After this initial rinse, the system was placed on oxygen. It was observed that the ore would not take up oxygen at this time. An attempt to leach the ore with a solution maintained at a pH of 2.1 yielded essentially no copper. The ore behaved as if the surfaces had been passivated by the pH excursion.
To remedy this situation, a solution of pH 1.0 was circulated in the column to neutralize the ore. The pH of this solution rose to 1.9 after 2 days, with no further acid additions.
After this treatment at pH 1.0-1.9, the ore was placed on oxygen. The solution was drained from the particles, leaving a thin aqueous layer on the surfaces. This procedure allows free access of oxygen to the mineral particles through pores and void spaces. The resultant oxygen uptake is shown in Figure 2 as the first oxygen cycle. After this first oxygen cycle, the ore was leached at pH 2.1 following which the column was drained and placed on oxygen again.
The sixth oxygen cycle was started after leaching at pH 2.0. The sudden increase in the rate at approximately 50 hours was caused by replacing the leach liquor, lowering the pH to 0.6, circulating for 10 minutes, and placing on oxygen again. Again the decrease in rate beyond 100 hours occurred as in the fifth cycle. The effect of pH on oxygen uptake is very apparent and was found to be reproducible.
At the end of the first oxygen cycle, a solution of pH 2.1 was introduced into the column and was circulated. Samples were removed periodically for analysis of copper and iron content. After the pH had risen above 2.6 the solution was drained and replaced with fresh solution of pH 2.1. Samples were again removed for analysis. This procedure was repeated several times. The sudden increases in copper extraction correspond to the initial leaching after fresh solution was added as described above. After 100 hours, concentrated sulfuric acid was added to maintain a pH of 2.1. It appears that if the pH is allowed to rise above approximately 2.5-2.6, the dissolution rate of the oxidized copper becomes extremely slow. Even though the pH excursions go well above pH 2.5, the rate of leaching is noticeably reduced in the pH 2.5-2.6 region.
The abrupt decrease in leaching rate when the pH of the leach solution rises to 2.5 to 2.6 is significant in that conditions giving rise to the precipitation of basic iron sulfates also stop the leaching of copper.
A sample of the ore was removed from the column after 7 leach cycles. Mineragraphic analysis showed that the bornite, originally present in the ore, had been altered to idaite, Cu5FeS6. Another sample was removed from this same column after 10 leach cycles. The sulfides were concentrated from this sample and an x-ray diffraction pattern of the concentrate was obtained.
A leach column was insulated and heated externally using a nichrome resistance heating element. The first oxygen cycle on the -4+6 mesh ore and was carried out at room temperature (21°C) . The cycle was started following a leach at pH 2.1. After 50 hours of exposure to oxygen, the ore was rinsed with distilled water until the solution on the ore reached a pH of 6.0. The rate of oxidation for this first cycle is little affected by the second rinse since the pores were filled with solution at a much lower pH. Extensive exposure to solution at a higher pH will passivate the column even for O2 pick-up. The second oxygen cycle was conducted at pH 1.3 and at 34°C. After 70 hours, the temperature was lowered to 21°C. A leach cycle was completed and the ore was again placed on oxygen at pH 1.3 and 21°C. This third oxygen cycle as illustrated demonstrated the large temperature dependence of the reaction. Raising the temperature of the system during the oxygen cycles greatly increases the rate of oxidation, and hence increases the total rate of copper extraction.
The quantity of iron present in the leach solutions varied greatly and was a function of the pH of the solution. Low pH solutions leached iron more rapidly.
At pH 2.0, the iron released paralleled the copper extraction except in the initial stages of leaching where the iron released was less than the copper released. This is consistent with the results expected for bornite dissolution. Sullivan has shown that in the leaching of bornite with acidified ferric sulfate, the copper is released initially with little dissolution of the iron. After a certain percentage of the copper has been removed, the iron is attacked and goes into solution.
In in-situ leaching difficulty may be encountered in draining the deposit sufficiently for effective ingress of oxygen due to the presence of ground water. Bubbling of oxygen through a solution-inundated deposit will have only limited effectiveness because diffusion paths will still be very long. Drainage of all but pellicular water is important to maximize oxidation. Similarly, counter-current or simultaneous leaching and oxidation can only be effective if the rate of leaching is very slow, such that the larger pores and cracks are not constantly filled with leach solution.