4
the basis for assessing the overall economic feasibility and
comparing the CAPEX and OPEX of the two processes.
The net present value (NPV) analysis basis in Table 3
provides essential financial assumptions that serve as the
foundation for the economic evaluation of acidic versus
alkaline POX processes.
Base Case Summary
The base case for economic analysis is set with the following
parameters:
• Sulfide sulfur grade: 2.0%
• Carbonate-to-sulfide sulfur ratio: 5
• Recovery difference (acidic -alkaline POX) :
10.0%
• Gold price: $2,500/oz
• Gold grade: 5 g/t
• Fuel price: $1.0/L (propane)
In this analysis, both acidic and alkaline POX pro-
cesses were evaluated under identical feed rate, autoclave
conditions and other fixed parameters, as outlined in
Table 2. The economic impacts of alkaline POX—includ-
ing CAPEX, OPEX, and revenue—were assessed relative
to acidic POX, capturing the cost differences involved in
transitioning between the two methods. The net cash flow
difference (NCFD) represents the change in annual net rev-
enue or cash flow when shifting to alkaline POX, calculated
by deducting OPEX from revenue impacts. Finally, the net
present value (NPV) was determined based on the param-
eters set out in Table 3, and respective net present value
difference (NPVD) between acidic and alkaline POX was
obtained for comparison.
The base case analysis results are summarized in Table 4.
The cost drivers mentioned in Table 1 impact both CAPEX
and OPEX. The alkaline POX excludes the acidulation cir-
cuit and requires a relatively smaller neutralization system,
leading to reductions in capital costs. However, potential
increased autoclave vent flows due to in-situ CO2 genera-
tion necessitates a more extensive off-gas system, and the
higher steam requirements contribute to increased CAPEX.
Subsequently, the increased steam requirements and the
adjustment in reagent demands impact OPEX. Overall,
alkaline POX saw reduced sulfuric acid and lime consump-
tion, while increased fuel demand for steam generation.
The NPV for acidic POX was found to be more favor-
able compared to alkaline POX, showing the NPVD as a
negative value of $417.8 million.
Table 2. Economic analysis inputs for acidic and alkaline POX
Descriptions Unit Value
Fixed
Feed Properties
Solid Feed Flowrate t/h 200
Gold Grade g/t 10
AC Feed Temperature °C 150
Autoclave Conditions
AC Operating Temperature °C 220
AC Operating Pressure kPa (a) 3,200
Sulfide Oxidation (Acidic /
Alkaline)
%95% /
75%
Oxygen Utilization (Acidic /
Alkaline)
%65% /
50%
Oxygen to Sulfur Ratio t /t 2.75
Reagent Unit Cost
Cost of Sulfuric Acid (93%) USD /t 225
Cost of Limestone USD /t 35
Thermodynamic Properties
Heat of Sulfide Oxidation MJ /kmol 650
Cp of Feed Stream MJ /t /°C 3.0
CAPEX basis
Scale-up Coefficient -0.6
Total CAPEX for POX Circuit (@
SS grade of 2.0 wt%
M USD 200
%of Acidulation Circuit in Total
POX CAPEX
%8.0
%of Neutralization Circuit in
Total POX CAPEX
%8.0
%of Off-Gas Treatment Circuit in
Total POX CAPEX
%15.0
%of Boiler Circuit in Total POX
CAPEX
%0.5
%of Demin Circuit in Total POX
CAPEX
%0.5
Variable
Sulfide Sulfur (SS) Grade wt% 1–5
CO
3 /SS Ratio -0.1–10
Recovery Difference (Acidic–
Alkaline POX)
%0–20
Gold Price $/oz 2,000–
3,000
Gold Grade g/t 1–30
Fuel Price–Propane USD /L 0.5–1.5
Table 3. NPV Analysis Basis
Descriptions Unit Value
Life of Plant years 20
Discount Rate %10.0
O&M /Utilities Inflation %2.0
Gold Price inflation* %6.0
*The gold price inflation is based on an average rate over the
last 30 years (1994 to 2024).
the basis for assessing the overall economic feasibility and
comparing the CAPEX and OPEX of the two processes.
The net present value (NPV) analysis basis in Table 3
provides essential financial assumptions that serve as the
foundation for the economic evaluation of acidic versus
alkaline POX processes.
Base Case Summary
The base case for economic analysis is set with the following
parameters:
• Sulfide sulfur grade: 2.0%
• Carbonate-to-sulfide sulfur ratio: 5
• Recovery difference (acidic -alkaline POX) :
10.0%
• Gold price: $2,500/oz
• Gold grade: 5 g/t
• Fuel price: $1.0/L (propane)
In this analysis, both acidic and alkaline POX pro-
cesses were evaluated under identical feed rate, autoclave
conditions and other fixed parameters, as outlined in
Table 2. The economic impacts of alkaline POX—includ-
ing CAPEX, OPEX, and revenue—were assessed relative
to acidic POX, capturing the cost differences involved in
transitioning between the two methods. The net cash flow
difference (NCFD) represents the change in annual net rev-
enue or cash flow when shifting to alkaline POX, calculated
by deducting OPEX from revenue impacts. Finally, the net
present value (NPV) was determined based on the param-
eters set out in Table 3, and respective net present value
difference (NPVD) between acidic and alkaline POX was
obtained for comparison.
The base case analysis results are summarized in Table 4.
The cost drivers mentioned in Table 1 impact both CAPEX
and OPEX. The alkaline POX excludes the acidulation cir-
cuit and requires a relatively smaller neutralization system,
leading to reductions in capital costs. However, potential
increased autoclave vent flows due to in-situ CO2 genera-
tion necessitates a more extensive off-gas system, and the
higher steam requirements contribute to increased CAPEX.
Subsequently, the increased steam requirements and the
adjustment in reagent demands impact OPEX. Overall,
alkaline POX saw reduced sulfuric acid and lime consump-
tion, while increased fuel demand for steam generation.
The NPV for acidic POX was found to be more favor-
able compared to alkaline POX, showing the NPVD as a
negative value of $417.8 million.
Table 2. Economic analysis inputs for acidic and alkaline POX
Descriptions Unit Value
Fixed
Feed Properties
Solid Feed Flowrate t/h 200
Gold Grade g/t 10
AC Feed Temperature °C 150
Autoclave Conditions
AC Operating Temperature °C 220
AC Operating Pressure kPa (a) 3,200
Sulfide Oxidation (Acidic /
Alkaline)
%95% /
75%
Oxygen Utilization (Acidic /
Alkaline)
%65% /
50%
Oxygen to Sulfur Ratio t /t 2.75
Reagent Unit Cost
Cost of Sulfuric Acid (93%) USD /t 225
Cost of Limestone USD /t 35
Thermodynamic Properties
Heat of Sulfide Oxidation MJ /kmol 650
Cp of Feed Stream MJ /t /°C 3.0
CAPEX basis
Scale-up Coefficient -0.6
Total CAPEX for POX Circuit (@
SS grade of 2.0 wt%
M USD 200
%of Acidulation Circuit in Total
POX CAPEX
%8.0
%of Neutralization Circuit in
Total POX CAPEX
%8.0
%of Off-Gas Treatment Circuit in
Total POX CAPEX
%15.0
%of Boiler Circuit in Total POX
CAPEX
%0.5
%of Demin Circuit in Total POX
CAPEX
%0.5
Variable
Sulfide Sulfur (SS) Grade wt% 1–5
CO
3 /SS Ratio -0.1–10
Recovery Difference (Acidic–
Alkaline POX)
%0–20
Gold Price $/oz 2,000–
3,000
Gold Grade g/t 1–30
Fuel Price–Propane USD /L 0.5–1.5
Table 3. NPV Analysis Basis
Descriptions Unit Value
Life of Plant years 20
Discount Rate %10.0
O&M /Utilities Inflation %2.0
Gold Price inflation* %6.0
*The gold price inflation is based on an average rate over the
last 30 years (1994 to 2024).