This paper has attempted to demonstrate the technical superiority of thiosulfate over cyanide through laboratory testing. Table 3 provide the results of the two test runs, considering the variations in general leach circumstances. A conclusion has been reached by comparing the chemical properties of cyanide and thiosulfate lixiviants in order to illustrate the economic aspects of leaching.
I. First Run
Table 3
First Run Laboratory Results
Test-1
|
Thiosulfate (9 g/l)
|
|
|
|
Time
(hr.)
|
Au in solution
(g/t)
|
Percent Recovery
(%)
|
pH
|
Free TS
(ppm)
|
0
|
0.00
|
0.00
|
10.0
|
ND
|
2
|
0.32
|
25.64
|
11.18
|
ND
|
4
|
0.40
|
32.05
|
10.90
|
ND
|
8
|
0.52
|
41.67
|
10.62
|
ND
|
24
|
0.71
|
56.89
|
10.07
|
ND
|
30
|
0.74
|
59.29
|
9.79
|
ND
|
48
|
1.13
|
90.54
|
10.24
|
ND
|
Test-2
|
Sodium Cyanide (0.5 g/l)
|
|
|
|
Time
(hr.)
|
Au in solution
(g/t)
|
Percent Recovery
(%)
|
pH
|
Free CN−
(ppm)
|
0
|
0.00
|
0.00
|
10.0
|
500
|
2
|
0.26
|
20.83
|
10.26
|
460
|
4
|
0.35
|
28.04
|
10.24
|
450
|
8
|
0.51
|
40.87
|
9.97
|
350
|
24
|
0.63
|
50.48
|
9.86
|
300
|
30
|
0.72
|
57.69
|
10.65
|
285
|
48
|
0.77
|
61.70
|
10.24
|
200
|
For the two leaching agents mentioned above, comparable test circumstances were employed. Therefore, as Table 3 illustrates, thiosulfate exhibits more technical superiority over cyanide, producing a gold recovery in a solution of approximately 90.54% compared to cyanide's 61.70% after 48 hours of leaching. This suggests that in carbonaceous preg-robbing gold-bearing ores, thiosulfate has a far superior leaching recovery than cyanide.
II. Second Run
Table 4
Second Run Laboratory Results
Test-3
|
Thiosulfate (8 g/l)
|
|
|
|
Time
(hr.)
|
Au in solution
(g/t)
|
Percent Recovery
(%)
|
pH
|
Free TS
(ppm)
|
0
|
0.00
|
0.00
|
11.50
|
ND
|
2
|
0.41
|
32.85
|
11.67
|
ND
|
4
|
0.46
|
36.86
|
11.61
|
ND
|
8
|
0.52
|
41.67
|
11.44
|
ND
|
24
|
0.74
|
59.29
|
11.59
|
ND
|
30
|
0.78
|
62.50
|
11.51
|
ND
|
48
|
0.83
|
66.51
|
11.18
|
ND
|
Test-4
|
Sodium Cyanide (0.25 gm/lit)
|
|
|
|
Time
(hr.)
|
Au in solution
(gm/ton)
|
Percent Recovery
(%)
|
pH
|
Free CN−
(ppm)
|
0
|
0.00
|
0.00
|
11.92
|
250
|
2
|
0.22
|
17.63
|
11.69
|
150
|
4
|
0.30
|
24.04
|
11.60
|
35 + 150
|
8
|
0.37
|
29.65
|
11.42
|
150
|
24
|
0.33
|
26.44
|
11.72
|
100 + 150
|
30
|
0.45
|
36.06
|
11.35
|
200
|
48
|
0.48
|
38.46
|
11.21
|
150
|
The outcome of the fourth test as depicted in Table 4 demonstrates the preg-robbing impact of carbonaceous gold-bearing ores as leaching progresses and eliminates the free cyanide in the solution. Due to the nature of the ore, which absorbs free cyanide in solution to form other undesirable cyanide complexes, there is a significant loss of free cyanide in solution, as demonstrated by the cyanide leaching, which shows 35 ppm of free cyanide in solution at the beginning of the leaching process. Nonetheless, it is essential to note that for the gold dissolution reaction to occur, the solution must include at least 100–150 ppm of free cyanide. To make up for the lost cyanide in the solution, 0.15g/l of free cyanide was added to the leach solution on the fourth and twenty-fourth hours. This has increased the processing costs of the cyanidation leaching for carbonaceous gold-bearing ores and have a negative economic impact due to overdosing on cyanide dosage.
The result implies a faster leach recovery by thiosulfate, which also demonstrates a higher leaching recovery of 66.51% of thiosulfate over 26.44% cyanide leach in 24 hours of residence time. The results presented here solely demonstrate thiosulfate's technical advantage over cyanide. However, greater recovery may be obtained if conditions could be improved to a better state with a thorough study to be conducted by adjusting reagent concentrations and other leach conditions.
The result also shows a superior leaching recovery of 66.51% of thiosulfate over 26.44% cyanide leach in 24 hrs. of residence time, implying a faster leach recovery by thiosulfate. The figures here only indicate the technical superiority of thiosulfate over cyanide. But, if conditions could be optimized to a better state with rigorous research to be undertaken by varying reagent concentrations and other conditions for leach, a much better recovery and sound figures could be obtained. The preg-robbing effect, in which the leached gold in solution is stolen by the free carbon and other hydrocarbons from the carbonaceous ore, is further demonstrated by the lower percent recoveries and changes in recovery for cyanide in Test 4. This resulted in a significant drop of gold in the solution.
The other thing the authors noticed in the test results was the status of the solution's alkalinity for the best thiosulfate recovery. Test-1, conditioned at pH 10.0, produced a higher recovery of around 90.54%, while Test 3, conditioned at pH 11.5, produced a solution recovery of 62.5%. This study unequivocally demonstrated that the lower alkaline bound, or pH range of 9–10.5, yields a better and faster recovery of gold in thiosulfate solution. This pH range allows for the best possible recovery of gold in thiosulfate solutions.
Figure 1 below, displaying Residence time vs. Percent recovery clearly illustrates the discussions surrounding the aforementioned leaching results.
Finally, the gold left in the solid as residue after being fire assayed on finalizing the leaching process was examined in the laboratory, and the result is shown in Table 5:
Head grade = 1.25 g/t
Table 5
Gold grade in the solid residue
Test No.
|
Solid Grade (Residue, g/t)
|
Au in Solution after 48hrs. (g/t)
|
Head Grade
(g/t)
|
Test-1 (TS)
|
0.12
|
1.13
|
|
Test-2 (CN)
|
0.50
|
0.77
|
1.25
|
Test-3 (TS)
|
0.45
|
0.78
|
|
Test-4 (CN)
|
0.68
|
0.48
|
|
A. Economic result
This section compares the costs of cyanidation with thiosulfate leaching of gold ores. In addition to the primary operational expenses incurred during the conventional cyanidation process of detoxifying the cyanide from the tailing management facility, expenditures related to direct chemical consumption are also utilised for both lixiviants. Table 6 lists the costs along with the process for analyzing them.
The Legadembi metallurgy plant at MIDROC Gold Mine provided the following reagent cost, which is based on the current market pricing in Ethiopian Birr.
Table 6
Leaching Reagent Consumption and Costs
Chemical Name
|
Unit
|
Unit price (ETB)
|
Specific Consumption (Kg/t)
|
Amount (ETB/t)
|
Sodium cyanide
|
Kg
|
81.69
|
0.80
|
65.35
|
Sodium thiosulfate
|
Kg
|
16.60
|
9.00
|
149.40
|
Ammonia
|
Kg
|
9.00
|
13.60
|
122.40
|
Copper sulfate
|
Kg
|
44.00
|
0.80
|
35.20
|
Hypochlorite
|
Kg
|
79.64
|
0.02
|
1.59
|
Taking a particular case for MIDROC Legadembi mine, a facility which processes 1,100,000 tons of gold ore annually with the conventional cyanidation method to estimate operating costs; the detox plant uses a minimum of 20 tonnes of hypochlorite per annum for treating cyanide in tailing solution and taking an average power consumption charge of 0.60 ETB/KWh by EEPCO. Table 7 provides information about operating cost analysis for the detox process, with cost details depicted on Table 7a and Table 7b.
Table 7
Detoxification Operating Cost Analysis
Item No.
|
Major Items
|
Costs Per Annum (ETB)
|
1
|
Detox plant operating items (Details: Table 7a)
|
-
|
1.1
|
Spare part replacements
|
950,000.00
|
1.2
|
Running items and Utilities
|
446,266.80
|
2
|
Tailing dam operating items (Details: Table 7b)
|
-
|
2.1
|
Spare part replacements
|
2,610,000.00
|
2.2
|
Running items and Utilities
|
549,564.00
|
3
|
Supervisory fee for two Units (One person working for 8hrs/day with monthly salary of 10,000ETB)
|
120,000.00
|
|
Grand Total
|
4,621,080.80
|
|
Annual Budgeted Plant Processing Capacity
|
1.1 million tons
|
|
Unit Cost for Detoxification of Cyanide Tailings (ETB/t)
|
4.20
|
Table 7a
Detox Plant Operating Items
Item No.
|
Spare part Replacements
|
Costs per Annum (ETB)
|
1
|
Electrical parts (breakers, contactors, cables etc.)
|
200,000.00
|
2
|
Pump parts
|
300,000.00
|
3
|
Pipes and Valve replacements
|
250,000.00
|
4
|
Tank spare parts (gearbox and others)
|
200,000.00
|
|
Total
|
950,000.00
|
Item No.
|
Running items and Utilities
|
Costs per Annum (ETB)
|
1
|
Consumables for Cyanide titration & pH determination
|
54,750.00
|
2
|
Power consumption for:
|
|
2.1
|
Mixing tank agitator at 11KW capacity working for 24 hours per day
|
57,816.00
|
2.2
|
Transfer pump at 11KW capacity working for 24 hours per month
|
1,900.80
|
2.3
|
Dozing pump at 10KW capacity working for 20 hours per day
|
43,800.00
|
3
|
Man power (four operators with 6,000 ETB monthly salary each)
|
288,000.00
|
|
Total
|
446,266.80
|
Table 7b
Tailing Dam Operating Items
Item No.
|
Spare part Replacements
|
Costs per Annum (ETB)
|
1
|
Electrical parts (breakers, contactors, cables etc.)
|
100,000.00
|
2
|
Pump parts
|
1,200,000.00
|
3
|
Return pipe and fitting replacements
|
1,100,000.00
|
4
|
Hose and Valve parts
|
150,000.00
|
5
|
Bearing replacements
|
60,000.00
|
|
Total
|
2,610,000.00
|
Item No.
|
Running items and Utilities
|
Costs per Annum (ETB)
|
1
|
Consumables for Cyanide titration & pH determination
|
54,750.00
|
2
|
Power consumption for:
|
|
2.1
|
Two Return pumps at 11KW capacity working for 23 hours per day
|
110,814.00
|
3
|
Man power (Eight operators with 4,000 ETB monthly salary each)
|
384,000.00
|
|
Total
|
549,564.00
|
Pre-treatment of ores is mandatory for refractory ores before leaching with cyanide. Thus, roasting of ores as pre-treatment method has been adopted for this economic analysis. The roasting cost for carbonaceous ores here has been estimated with taking a particular case of Derba Cement’s clinker production kiln (Table 8), a major cement producing company in Ethiopia.
-
The Derba kiln uses around 735 KCAL of energy to produce 1 Kg of clinker
-
The cement plant mostly uses lignite coals as energy source with caloric value of around 2310 KCAL/Kg
-
Lignite coal types are sold with 938.40 ETB per tons at international market including 20% for overhead cost
Table 8
Pre-treatment (Roasting cost )
|
Economic output
|
Kiln coal requirement
|
0.32 Kg/Kg of clinker
|
300.80 ETB/t of clinker
|
Lignite unit price
|
0.94 ETB/Kg
|
|
The cement plant kiln for clinker production could here be taken as equivalent to a kiln used for roasting of refractory ores to produce pre-treated ores for cyanide leaching for estimating roasting cost; which resulted in 300.80 ETB/t of ore pre-treated.
In summary, the above cost analysis gives the following summarised costs for both lixiviants as shown in Table 9:
Table 9
Leaching Economics Result
Leaching Method
|
Total Unit Cost (ETB/t)
|
Cyanide Leaching
|
371.94
|
Thiosulfate Leaching
|
307.00
|
The result demonstrates that thiosulfate leaching of gold also benefits the economy. By improving the thiosulfate leach process for refractory ores, which would probably reduce the requirement of higher concentrations of reagents in the leach system, this economic benefit of thiosulfate might be further utilised. The increased cost of the conventional cyanidation process demonstrates why such types of refractory ores are not treated using this method. Thiosulfate recovery offers a considerable advantage over cyanide, as can be seen from the test results. This means that there will be an increase in revenue as the recovery is increased by utilising thiosulfate. Furthermore, quantifying the non-environmental benefits of thiosulfate vs cyanide in monetary terms would undoubtedly increase the thiosulfate leach system's economic advantage for carbonaceous ores.