Chapter 6 Investigation of biological thiourea (TU) gold leaching method using
6.3. Results and discussion
6.3.1. Optimization of bio-TU leaching condition
6.3.1.1. Pre-grown cells vs. harvested cells at various TU:Fe(II) ratios
3.0 2.5 2.0 1.5 1.0
pH
14 12
10 8
6 4
2 0
Time (days)
TU 1 mM, Ste TU 1 mM, Fv-Ap TU 10 mM, Ste TU 10 mM, Fv-Ap TU 20 mM, Ste TU 20 mM, Fv-Ap
800 700 600 500 400 300 200
Eh v s. SHE ( m V)
14 12
10 8
6 4
2 0
Time (days)
(a)
(b)
129 12
10 8 6 4 2 0
Fe con cent ration ( m M)
14 12
10 8
6 4
2 0
Time (days) TU 1 mM
12 10 8 6 4 2 0
Fe con cent ration ( m M)
14 12
10 8
6 4
2 0
Time (days) TU 10 mM
12 10 8 6 4 2 0
Fe con cent ration ( m M)
14 12
10 8
6 4
2 0
Time (days) TU 20 mM
(c.3) (c.2) (c.1)
130 30
25 20 15 10 5
TU co n cent ration ( m M)
014 12
10 8
6 4
2 0
Time (days)
0.025 0.020 0.015 0.010 0.005 0.000
Au con cent ration ( m M)
14 12
10 8
6 4
2 0
Time (days)
80 60 40 20 0
Au dis so lu tio n ( %)
107
2 4 6
108
2 4 6
109
Cell d en sity ( cells/ml)
14 12
10 8
6 4
2 0
Time (days)
(d)
(e)
(f)
131 Figure 6.1 Changes in (a) pH, (b) Eh, (c.1-3) Fe concentration 1, 10, 20 mM, (d) TU concentration, (e) Au concentration, and (f) cell density, respectively. For Fe concentration, solid and broken lines indicate total Fe and Fe(II). Solid and blank symbols indicate inoculated and sterile control, respectively.
PCB sample was added to the flask at day 6 when the most ferrous has been oxidized into ferric. pH in each point kept stable around 1.5-1.6. This implies that acid wash of sample prior to leaching affected to pH because the alkalinity of the sample had been removed.
Eh value at day 0 was around 400-500 mV. Eh kept increasing until day 6 for the inoculated flask because of the cell’s growth, suggested that cells oxidized ferrous into ferric. While sterile control, Eh was stable around 400. It was observed from Fe(II) and Fe total measurement that ferric was gradually dominant in the inoculated flask while ferrous was dominant in sterile control.
Gold concentration and recovery are shown in Figure 6.1e. Gold solubilization for all conditions was gradually increased as well as the increase of TU concentration (Equation 1.8). The experimental results also showed that the dissolution of gold in sterile control is higher than the inoculated condition. This can be related to TU concentration remains in solution. At day 6 (when PCB was added), for 1 mM TU concentration in sterile control, TU concentration remain in solution was 0.25 mM. It can be calculated that TU loss was TU initial – TU remains in solution. So, TU loss was 0.75 mM which was able to leach out 0.006 mM of Au. However, TU loss cannot be interpreted precisely as TU used for complexing with Au only, since TU is also decomposed. There are many reactions happened in the loss of TU in solution, i.e. for complexing with Au, oxidation to formamidine disulfide in acidic solution, oxidation
132 by ferric, hydrolysis into urea, and dissociation into cyanamide. Dissolution of metals in acidic TU solution depends on many factors such as the concentration of TU, the value of pH, temperature, and redox potential.
Increasing redox potential produces a rapid decrease in the dissolution rate of gold. This could be related to the degradation of TU itself and the adsorption of gold-TU complex to the elemental sulfur product of degradation of TU. It can be seen in sterile control of all conditions that TU concentration remained in solution is higher than inoculated one. It was also observed in Fe measurement that almost no ferric in the system which indicated no reaction with TU. This might be the reason that TU in sterile control seems to be stable.
Meanwhile, in inoculated condition, Eh value was higher than 600 mV, implying that ferric dominates in the leaching system, resulting in TU decomposition. It is believed that redox potential control is also needed to carry out gold leaching, in between 380-500 mV. Based on these experimental results, it can be assumed that the amount of ferrous added to the system was excess.
Gold recovery at day 6 for sterile control in TU 1, 10, 20 mM are 5%, 10%, and 7%, respectively. While for inoculated conditions are 2%, 6%, and 8%, respectively. The recovery in sterile control kept increased until day 14, while for inoculated condition started to decrease at day 7. The possible reason for this might be due to the increasing of Eh value which affected the stability of Au-TU complex, so it started to decompose.
133 Experiment 2. The use of pre-grown cells for bio-TU leaching at various TU: Fe(II) ratio.
3.0 2.5 2.0 1.5 1.0
pH
14 12
10 8
6 4
2 0
Time (days)
TU/Fe(II)=10/1, Ste TU/Fe(II)=10/1, Fv-Ap TU/Fe(II)=20/1, Ste TU/Fe(II)=20/1, Fv-Ap TU/Fe(II)=40/1, Ste TU/Fe(II)=40/1, Fv-Ap
800 700 600 500 400
Eh vs. SHE (mV)
14 12
10 8
6 4
2 0
Time (days) 1.2
1.0 0.8 0.6 0.4 0.2 0.0
Fe concentration (mM)
14 12
10 8
6 4
2 0
Time (days) (a)
(b)
(c)
134 Figure 6.2 Changes in (a) pH, (b) Eh, (c) Fe concentration, (d) TU concentration, and (e) Au concentration, respectively. For Fe concentration, solid and broken lines indicate total Fe and Fe(II). Solid and blank symbols indicate inoculated and sterile control, respectively.
Figure 6.2e shows that higher ratio of TU: Fe(II) resulting in higher gold dissolution.
This implies that higher availability of TU enhanced gold leaching. The optimum gold dissolution achieved on the first day after adding PCB, but then start to decompose at
10 8 6 4 2 0
TU co n cent ration ( m M)
14 12
10 8
6 4
2 0
Time (days)
0.025 0.020 0.015 0.010 0.005 0.000
Au con cent ration ( m M)
14 12
10 8
6 4
2 0
Time (days)
80 60 40 20 0
Au dis so lu tio n ( %)
(d)
(e)
135 the second day. The process of leaching is fast enough, probably due to the limited amount of gold contained in the sample. Therefore, to observe the iron recycle by cells, sufficient amount of gold should be available and cell density should be higher.
Experiment 3. The use of pre-grown cells for bio-TU leaching at various TU: Fe(II) ratio with shorter leaching period.
Considering the rapid Au leaching as shown previously, this experiment was repeated as a similar condition set up with shorter leaching period.
3.0
2.5 2.0 1.5 1.0
pH
8 6
4 2
0
Time (days)
TU/Fe(II)=10/1, Ste TU/Fe(II)=10/1, FvAp TU/Fe(II)=20/1, Ste TU/Fe(II)=20/1, FvAp TU/Fe(II)=40/1, Ste TU/Fe(II)=40/1, FvAp
(a)
136
800 700 600 500 400 300
Eh (m V)
8 6
4 2
0
Time (days)
1.2 1.0 0.8 0.6 0.4 0.2 0.0
Fe con cent ration ( m M)
8 6
4 2
0
Time (days)
10 8 6 4 2 0
TU co n cent ration ( m M)
48 36
24 12
0
Time (hours)
(b)
(c)
(d)
137 Figure 6.3 Changes in (a) pH, (b) Eh, (c) Fe concentration, (d) TU concentration, and (e) Au concentration, respectively. For Fe concentration, solid and broken lines indicate total Fe and Fe(II). Solid and blank symbols indicate inoculated and sterile control, respectively.
PCB was added into the flask at day 6 when all ferrous has been oxidized into ferric.
Eh kept increasing until day 6 for FvAp up to 700 mV due to the cell’s ability to oxidize ferrous to ferric, as confirmed in Figure 6.3c that ferric was gradually dominant in inoculated while ferrous was dominant in sterile control.
Since Au leaching has been observed in the shorter period, 48 hours since PCB was added, Au dissolution between sterile and inoculated was distinguishable. Au leaching occurred within 1 day. Au recoveries were 40-55% for inoculated and 30-40% for sterile. As shown in figure 4.7e, Au dissolutions in inoculated were approximately 30% higher than sterile control. The maximum Au recovery was 55% reached by 10/1 ratio. Due to this result, it can be concluded that TU/Fe(II) ratio of 10/1 gives the optimum Au dissolution. The purpose of using a different ratio of TU/Fe(II) was to generate different Eh to determine/ optimize Au leaching.
0.025 0.020 0.015 0.010 0.005 0.000
Au concentration (mM)
50 40
30 20
10 0
Time (hours)
100 80 60 40 20 0
Au dissolution (%)
(e)
138 Improvement for Au recovery is required by changing some limiting factors, such as TU concentration, Fe(II) concentration, or cell density. Impurities from other metal such as Cu has been minimized by acid wash prior to Au leaching. However, this pre-treatment was unable to annihilate Cu. This might affect the system because Cu is easier to bond with TU than Au.
