2.1 Mineral samples preparation

The sample ore for this study was obtained from Tianli gold mine, China. Chemical analysis of the representative sample by AAS (atomic absorption spectroscopy) was conducted. The sample Ore was sieved to -0.075 mm before used for all leaching experiments. X-ray diffraction analysis of the sample showed pyrite (FeS₂) as the major phase and Chemical analyses also indicated that the sample contained arsenical pyrite or arsenopyrite (FeAsS). The particle size of the sample was 80% below 75 μm (Figure 1). X-ray fluorescence spectroscopic analysis of the sample was conducted and the result is shown in table 1.

Figure 1: particle size of the sample ore.

Table 1: Chemical analysis of the sample ore by X-ray fluorescence (XRF).

2.2 Harvest of culture medium

A mixed culture medium (HQ 2011) was used, the studies, which utilized ferrous ion or elemental sulphur as energy source. Cultivated by Northeastern University and mainly contain Acidithiobacillus ferrooxidans, Ferroplasma acidiphilum, Leptospirillum ferriphilum etc. The cells were incubated at 180 rpm and 37°C in sterile 9K basic salt medium containing different energy substrates [7] After incubation for three days, some portion of the cells was harvested by centrifugation at 10,000 rpm for 10 min.

2.3 Bioleaching experiments

The experiments were conducted in 500mL conical flasks containing 200mL of four different media, mix bacterial medium, mix centrifuged bacterial culture medium, in acidic medium and in a sterile medium. The pulp density was 3%. Sample of the ore mineral was added into media without Fe²⁺,[9] added with culture medium (inoculated amount of 10%), and pH was adjusted to 1.6 with H₂SO₄ and then placed into a constant-temperature incubator (175 r/p) for days.

2.4 Chemical analysis

Measurements of pH and potential were performed every day. Samples were aseptically withdrawn from the flasks every two to three days for chemical analysis. The samples were separated by centrifugation at 5000 rpm for 10 min. the dissolve Arsenic and total iron and were measured.

3.1 pH

At 37°C, 10% Inoculum, 175 r/p, 3% pulp density and 36 days the leaching time. The effect of pH on the sample ore bio-oxidation was studied. The results are presented in Figure 3. The pH of the centrifuge and mixed bacterial media during the dissolution of the sample showed almost similar characteristics. Within the first 3 day of the experiment, the percentage of the total Fe ion were 86% and 87 for the sample with mixed and centrifuge bacterial culture medium (Figure 4), this clearly indicates that the oxidation rate of the sample is higher at the initial stage, so the increase in pH is apparent. Furthermore, the increase in concentration of total iron (the Fe³⁺ ion) which partake in the oxidation process resulted in the subsequent decrease of pH (eq. (1)), [8] to nearly 1.1 on the 36^th day. On the other hand the percentage of the dissolved total Fe ion in acidic and pure sterile media were 16% and 15% respectively, indicating that the oxidation rate is lower in the initial stage. It is believed that the decrease in the pH resulted from the dissolution of pyrite being net acid releasing sulfide and arsenopyrite presents in the ore sample.

Figure 2: X-ray diffraction analyses of the sample ore.

Figure 3: pH of the sample ore under three different treatments.

Figure 4: the percentage of the dissolve total iron of the sample treated under three different media.

Fe + 14Fe 3+  + 8H 2 O→ 15Fe 2+  + 2SO 4 2-  + 16H + (1)

3.2 Oxidation potential

The variations in the redox potential (Eh) with time is given in Figure 5. It can be seen that after the first day, the redox potential of the samples with mix and centrifuge bacterial medium were increased reaching the values of about 640 and 465 mV after 3 day of the experiment. On the sixth day the redox potential of the sample with mix bacterial medium reached its peak value (680 mV) which indicates that microorganisms of the mix culture medium were more active at pH of about 1.5 to 1.6, whereas that of the centrifuge bacterial medium reached it maximum value (650 mV) on the 18^th day of the experiment. The higher dissolution of arsenic and total Fe could be attributed to the higher redox potential of the solution. On the other hand the oxidation potentials of pure sterile and acidic media were 310mV and 340mV indicating that the dissolution of the sulphide was much lower.

Figure 5: potential (Eh) of the samples under three different treatment.

3.3 Percentages of dissolved arsenic and total Fe

It was observed that the dissolution of the arsenic and total iron for the sample with mix and centrifuge bacterial medium increases with the gradual decrease in the pH, initially the dissolution of arsenic were 73% and 34% (Figure 6) and the dissolved total Fe (Figure 4) were 87% and 86% within the first three day of the experiment and continue to rise during the process reaching up 99% both while the dissolution of the arsenic for the sample in acidic and pure sterile medium were 28% and 18% whereas those of the total Fe were 16% and 15% within the first three day of the experiment which appear to be low at the initial pH. The oxidation potential of the sample with mix bacterial and centrifuge bacterial medium is low but continue to rise during the whole process, which indicates that the bacteria are difficult to survive at the pH of 2 or above though could be alive but less active [8].

Figure 6: the percentage of the dissolved arsenic of the sample treated three under different media.