BACKGROUND: Biomining is one of the few cost-effective technologies that can be applied to low-grade ores and also mining tailings. In the current work, the bio-oxidation of a gold sulfide flotation tailing at high pulp densities was investigated and information concerning the high cyanide consumption commonly reported in bio-oxidative processes is discussed in order to promote less use of this chemical reagent during cyanidation. RESULTS: Various reactors were compared alongside the performance of strains of At. ferrooxidans and S. thermosulfidooxidans. Shaking flask tests performed using 5% (w/v) solids revealed that the mesophiles presented a faster adaptation to the solids and resulted in better tailings oxidation as compared to the moderate thermophile. Rolling bottle (pilot-scale rotating drum) experiments, carried out afterwards, enabled the adaptation of the At. ferrooxidans strain to 20% (w/v) solids. This adapted strain was thus utilized in a 10 L stirred tank reactor (pilot-scale stirred tank) experiment at the same pulp density. The concentration of the ferrous iron used in the bio-oxidation step had a detrimental effect on the cyanide consumption during cyanidation of the bio-oxidized material. The cyanide consumption varied from 6.9 to 13.7 kg t −1 as the initial Fe 2+ concentration increased from 0.0 to 10.0 g L −1 Fe 2+ , respectively, in the shaking flasks. CONCLUSIONS: Higher pulp densities (20% (w/v) solids) resulted in a lower cyanide consumption, which was around 7.0 kg t −1 in the experiments carried out in both rolling bottle and stirred tank reactors when 5.0 g L −1 Fe 2+ was used during bio-oxidation. This cyanide consumption was associated with a gold extraction of around 95%.