It is known that many of the gold and silver deposits are often associated with sulfide minerals, specially pyrite. Some of the precious metals in such ores are often found as very finely disseminated particles inside the sulfide crystals. The encapsulation of the precious metals particles in this manner makes their extraction very difficult, as these metals become inaccessible to leaching solutions. In these cases the ores are known as refractory ores and much of the precious metals are lost in the tailings.
Several approaches have been attempted to improve the recovery of precious metals from refractory ores, but virtually none were attempted on auriferous sulfide tailings. The number of possible treatment processes for such ores are limited and expensive. To achieve a satisfactory recovery of the precious metals, it is necessary to first breakdown the sulfide crystals to liberate them, before applying any conventional treatment process. For example, roasting has been used in some cases as a pre-oxidation step before cyanidation. However, the stringent pollution control regulations and the fusion of silver which forms clinkers, make roasting undesirable. Grinding to very fine sizes helps the liberation of the precious metals, but still gives unsatisfactory recovery and leads to high chemicals consumption in extraction processes due to increased surface area. As the particle size of the precious metals inside the sulfide matrix can range from a few microns to submicrons, grinding becomes expensive and an unattractive approach.
An Alternative technique for treating such refractory ores and tailings is bioleaching followed by cyanidation or some other extraction method. The bioleaching process dissolves away the sulfide matrix, thus exposing the encapsulated precious metals to the leaching solution. Several investigators have advocated the use of thiobacillus ferrooxidans bacteria for oxidation of sulfide ores before leaching with cyanide solutions. The bioleaching process is an inexpensive alternative, but it is also a slow process. Fortunately it was found that adaptation of the microorganisms on ore substrates significantly increased the rate of bioleaching. In addition, Lawrence and co-workers found that continuous replacement of the solution during the bioleaching process increased the rate of sulfide bio-oxidation.
In this work a sample of high-pyrite tailings from Leadville gold ore was concentrated by froth flotation, and then bioleached with an adapted strain of thiobacillus ferrooxidans before the extraction of gold and silver by cyanidation.
Figure 1 shows the rate of pyrite oxidation as calculated from the dissolution of iron in leaching solution during the different periods of bioleaching. It also shows the change in the extent of pyrite bio-oxidation during the bioleaching tests. From the figure it is clear that there was no initial lag period. This might be due to the adaptation of bacteria on a substrate of the pyrite-tailings for 12 weeks before their use in bioleaching. The bioleaching of the pyrite was increased with treatment time, and it reached about 98% at the end of the test period of 28 days.
With respect to the bioleaching rate of pyrite, it started at a high rate (0.38%/hr) during the first 2 days, because all the conditions were suitable for the bacterial activity, i.e., the cultures were adapted on the same type of tailings, the iron content in the solution was low, the pH was around 2.0, in addition to abundant pyrite surfaces, and suitable aeration, agitation and temperature conditions. However, the rate of pyrite oxidation gradually decreased throughout the bioleaching test periods. The minimum rate of oxidation was 0.15%/hr after 27 day of leaching. The decline in the oxidation rate was due to the reduction in the available surface area and mass of residual pyrite, as the pyrite was progressively dissolved into solution. It is interesting to note that the oxidation (leaching) rate of pyrite changed in a step-wise fashion with bioleaching time. The reason for this behavior was not clear.
The change in the soluble iron concentration with bioleaching time is shown in Figure 2. The iron content in the leaching solution increased sharply with time and reached a maximum of 17.3 g/liter in the seventh day. Thereafter, the iron concentration declined gradually with leaching time and reached 2.9 g/l at the end of 28 days test period. The initial rise in iron concentration was understandable as the rate of iron dissolution was higher than rate of iron removal by the daily partial solution replacement. As the residual concentration of pyrite decreased with time, the leaching rate also declined which produced less soluble iron. In this case, the rate of iron dissolution became less than the rate of removal by the daily solution replacement thus resulting in the overall decrease in iron concentration with time. Interestingly, the maximum iron concentration (17.3 g/l) corresponded with 47% pyrite oxidation while the smallest iron concentration (2.9 g/l) corresponded with the highest pyrite oxidation of 98%.
- Bioleaching of gold and silver-bearing sulfide tailings before cyanidation markedly increases the extraction recovery of these precious metals from these tailings. Up to 95% and 98% recovery of gold and silver could be achieved when bioleaching was employed compared to 32% and 48% respectively when bioleaching was not employed.
- Bioleaching alone does not affect the dissolution recovery of gold and silver either from sulfides in the tailings or from metallic powders of these metals.
- The extraction recovery of gold and silver using bioleaching ahead of cyanidation is a function of pyrite (and other sulfides) bio-oxidation. The higher the degree of oxidation of pyrite, the higher is the extraction recovery of gold and silver. However, about 74% bio-oxidation of pyrite was enough to increase the cyanide extraction recovery of gold and silver to 89% and 95% respectively.
- Use of 12 weeks adapted bacteria virtually eliminates the lag period for bacterial activity.
