Engineering Research On The Biooxidation Process Under Oxygen Enrichment Conditions

Bioleaching technology has made some progress in industrial applications currently. However, some properties, such as the slow bacterial growth rate and long oxidation cycle, which severely limited its application in large-scale industrial production.During the biooxidation process, oxygen was required as an electron acceptor. Therefore, adequate supply of oxygen (i.e., oxygen enrichment conditions) was critical for the rapid and efficient microbial oxidation reaction. Moreover, it is well known that the CO2 content in the air was very low (0.03% v/v). When the supply amount of intake air for the biooxidation process was controlled by the dissolved oxygen, CO2 that act as a carbon source would be in short supply, which would lead to the slow rates of cell growth and ore oxidation. Adding more CO2 would be in positive effect.on improving bioleaching bacteria biomass and accelerate the oxidation of ore under oxygen enrichment conditions. Fe2+ in the pulp could provide energy for iron oxidizing bacteria and would help to improve their biological oxidation activity, while the demand of Fe2+ by biooxidaton strain under oxygen enrichment conditions had not been reported publicly.A mixed bacteria which consists mainly of L. ferriphilum and S. thermosulfidooxidans were applied to pretreat the refractory gold concentrate in 1.5L laboratory stirred tank reactors. The effect of dissolved oxygen level (DO), CO2 content and initial Fe2+ concentration on the efficiency of biological oxidation processes were studied. It was focused on exploring the effects of CO2 and Fe2+ concentration on bacteria growth, oxidative activity, ore oxidation efficiency and leaching kinetics under oxygen enrichment conditions. Meanwhile, the microbial community structure were also explored under these conditions using high-throughput sequencing technology. Accordingly, the relationship among the oxidation efficiency, microbial community structure and gas supply were established.The results showed that the oxidation efficiency will firstly increased and then decreased when improve the dissolved oxygen levels.When DO levels exceed 3.75ppm, the efficiency of the bio-oxidation would be reduced by raising the level of dissolved oxygen. It was found that radical oxygen species (ROS) content increased significantly with the DO levels. In addition, microbial community structure under different DO levels had also been significantly affected. Therefore, it seems that more ROS which produced under high DO level would damage the cell and reduce the oxidative activity. With the increase of dissolved oxygen levels, microbial community structure changed. The proportion of L. ferriphilum increased first and then decreased, while S. thermosulfidooxidans had the opposite trend. Although the microbial community structure was optimum at 7.51 ppm dissolved oxygen concentration:the two main leaching bacteria both accounted for a large proportion, the inhibition of bacterial oxidation activity by high dissolved oxygen concentration led to poor oxidation results.Under the oxygen-rich (3.75 ppm) condition, it was suggested that high levels of carbon dioxide in the intake air could increase cell biomass, the oxidation of ore rates. But the biooxidation rate achieved maximum when the CO2 concentration in the intake was 5%(v/v). Although the amounts of bacteria were higher at 10% (v/v) CO2 concentration. Microbial community structure changed significantly at different CO2 concentration.When the intake air was normal air (0.03% CO2), the dominant species of bacteria were L. ferriphilum, accounting for 78.29% of the total number of bacteria, and S. thermosulfidooxidans, accounting for 13.39%. Under 5% CO2 condition, the dominant strains were S. thermosulfidooxidans, accounting for 60.82% of the total number of bacteria, and L. ferriphilum, accounting for 32.96%. While under conditions of 10% CO2, the dominant strain was S. thermosulfidooxidans, accounting for 91.92%of the total number of bacteria. Any other bacteria strain under 5% CO2 condition accounted for less than 2% of the total number of bacteria. The microbial community structure at 5% CO2 concentration was conducive to promote biological oxidation under this CO2 concentration condition.With the increase of initial Fe2+ concentration under oxygen enrichment conditions, the activity of the microbial group was inhibited at first and enhanced at later stage. While the cell growth was inhibited all the time, which ultimately led to the decrease of biooxidation efficiency.