Improvement of current leaching technology was found necessary when processing ores which are more difficult to leach because they generate highly viscous, air-rejective pulps. Cyanidation of these pulps has to run at low oxygen concentrations if the conventional aeration technique with compressed air is used. Since the dissolution rate of precious metals mainly depends on the concentration of dissolved oxygen in the pulp the kinetics of the gold extraction become slow in such cases, which in turn often leads to a significant decrease in the overall gold output within the conventional leaching time of up to 48 hours.
The most important disadvantage of the conventional leaching process is the inefficient transfer of the gaseous oxygen in the liquid phase. Therefore, even violent aeration results only in low concentration of dissolved oxygen in the pulp.
An alternative to solve this problem is the introduction of oxygen in a dissolved form. e.g. via the decomposition of hydrogen peroxide. Already 75 years ago, it was found in batch experiments that hydrogen peroxide increases the gold extraction rate, but the results were not convincing enough: It was also found that hydrogen peroxide attacks the cyanide, a process which was developed over the last 10 years by Degussa to destroy cyanide in gold mine effluents. However, as a result of recent research work carried out in Degussa’s mining laboratories, it could be shown that hydrogen peroxide can be used as an oxygen supplier in leaching of ores without any detrimental effect on cyanide. In addition, a control system has been developed to carry out the H2O2 dosage automatically and to perform the cyanidation at the most economical consumption rates. Even if hydrogen peroxide is more expensive than compressed air as oxygen supplier in cyanidation, the higher reagent costs are easily compensated for by the higher gold output.
Degussa’s cyanidation process is now being tested in a pilot plant and in a full-scale-plant. Figure 1 shows the pilot plant used for the small-scale testwork.
Precious metals like gold and silver are normally extracted by converting them into soluble cyanocomplexes. Since the ores usually contain the gold and sometimes the silver in its elementary form, an oxidant is needed to bring about the dissolution of the precious metals. Normally, the oxygen resulting from aeration of the pulp is used for this purpose in the mining industry.
(1) 4 Au° + 8 CN- + O2 + 2 H2O → 4 [Au(CN)2]- + 4 OH-
The oxidation of precious metals with dissolved oxygen is known to be the rate-determining step for the dissolution of gold and silver during cyanidation,
(2) 4 Au° + O2 + 2 H2O → 4 Au+ + 4 OH-
simultaneously followed by a fast formation of the gold cyanocomplex.
(3) Au+ + 2 CN- → [Au(CN)2]-
The dissolution rate of the precious metals is thus closely related to the concentration of dissolved oxygen in the pulp.
Extensive measurements of the oxygen level in various gold leaching plants have shown that the oxygen concentration is often significantly below the saturation limit (about 8 ppm). This results in poor extraction of precious metals. It is caused by the inefficient transfer of the gaseous oxygen through the phase barrier to the liquid phase. The dissolution rate drops and the cyanide-accessible gold and silver can not be extracted completely within the conventional leaching time of 48 hours.
Hydrogen peroxide has a strong effect on the oxygen concentration of the pulp. The chemistry of this phenomenon is not complicated: H2O2 is a strong oxidant. However, when added to the pulp, it is catalytically decomposed on contact with the ore to yield oxygen and water.
(4) H2O2 → H2O · ½ O2
The in-situ formed oxygen is completely dissolved in the liquid phase, leading to a homogenous distribution in the pulp. In this way the active oxygen of the H2O2 acts in its entirety as an oxidant, which is more effective than the aeration technique. For example, the H2O2-technique allows the oxygen saturation limit of the pulp (about 8 ppm) to be surpassed easily, if this is required.
The second effect which hydrogen peroxide can exert on the pulp is the oxidation of the cyanide to cyanate:
(5) NaCN + H2O2 → NaOCN + H2O
Therefore, one aim of Degussa’s cyanidation process was to avoid this ‘detoxification’ reaction. The investigations in this field have shown that only one of the above two competitive reactions is favoured, depending on both the concentration of the hydrogen peroxide dosed to the system and the concentration of hydrogen peroxide in the leach. Hydrogen peroxide may only be used in cyanidation provided that it is diluted to a concentration of 0.5-5% H2O2 by weight before it is added, since the decomposition reaction is much faster than the detoxification reaction under these conditions.
To guarantee the economy of the cyanidation process, Degussa has developed an automatic control of the H2O2 dosage. The principle of this automatic control is represented in Figure 2. The oxygen concentration is measured continuously with a special oxygen electrode in a small side stream which is separated of the pulp. Diluted hydrogen peroxide is dosed accordingly and the rate of dosage is controlled by the measurement of the oxygen concentration. The required O2-level can be achieved by fixing a setpoint corresponding to a certain O2-concentration in the pulp.
An equivalent amount of diluted hydrogen peroxide is added simultaneously to the leaching tank via a control valve or a dosing pump. The opening of the valve or the operation of the dosing pump is controlled by a signal obtained by multiplying the signal from the control unit by a second signal obtained from the pulp feed flow meter. The multiplication is carried out continuously by a microprocessor installed in the H2O2 pumping and dosing plant.
The automatic control of Degussa’s cyanidation process enables a plant to run at a constant oxygen level, because changes in the COD or BOD are compensated immediately. In this way the dosage of H2O2 can be reduced to a fraction of the amount that would be necessary, if the dosage has to be controlled manually.
O2-Profile during cyanidation
To compare the efficiency and economy of the different oxidants used in cyanidation, we investigated the oxygen profiles which were obtained for H2O2, pure oxygen and violent aeration with compressed air. The experiments were carried out with a typical air-rejective pulp, which is processed at a South African gold mine, on the basis of comparable costs (same chemical costs for H2O2 and O2 or mechanical costs for aeration, respectively).
Figure 3 illustrates the oxygen profiles in the pulp using the different oxidants in a batch test. A maximum O2- level of 12 ppm is reached after 14 min using H2O2, while a maximum level of 8 ppm is obtained only after 3 h with pure oxygen. With violent aeration, which was significantly more intensive in this test than used at mine-site, an O2-level of only 4.3 ppm was obtained after 1 h.
Even though the active oxygen of H2O2 is more expensive than that of O2 or air on a weight to weight basis, much less H2O2 is required to yield a similar effect. Table 1 shows that, at comparable costs, 64 times the amount of active oxygen is needed when pure oxygen is used. More than 800 times the amount of active oxygen is required using compressed air instead of H2O2. Besides this, a lower oxygen level is reached in the pulp in both cases.
This illustrates that the active oxygen in H2O2 reacts much more selectively which is demonstrated by a much lower consumption at comparable costs. Furthermore, H2O2 leads to s faster increase of the Cr concentration to a higher level than it is possible with pure oxygen. It also presents no problem to surpass the air saturation limit of a pulp by more than 100% using H2O2 as an oxygen supplier. Compressed air is significantly less effective.