2
would otherwise consume large quantities of acid during
pre-acidulation.
• Typically, operating temperature can vary between
these processes as well. Operating acidic POX at
a medium temperature (160°C) can lead to the
formation of elemental sulfur, which results in
agglomeration and subsequently reduces gold recov-
ery. Therefore, acidic POX requires a higher oper-
ating temperature to avoid these issues (Corriou &
Knkindai, 1981). In contrast, alkaline POX mini-
mizes the formation of elemental sulfur even at a
lower temperature, due to the lack of acid, which in
turn reduces the sulfur agglomeration and improves
gold recovery (Peters, 2010).
• Both acidic and alkaline POX have unique advan-
tages and limitations. Acidic POX is known for its
ability to liberate gold effectively but requires sig-
nificant acid consumption for high carbonate ores,
which lead to increased operating costs and increased
process complexity from additional steps such as
acidulation and neutralization. Alkaline POX, on
the other hand, reduces acid usage but often results
in the formation of an “armoured” product layer that
can inhibit further reactions. The armoured layer,
which is composed of insoluble precipitates, can act
as a physical barrier, reducing the efficiency of subse-
quent leaching steps (Song, 2023).
• Choosing between these two methods depends on
the specific characteristics of the ore and economic
factors, such as the availability of acid and neutral-
izing reagents, fuel type, the reagent and utility costs,
and the costs associated with off-gas treatment, heat
recovery, and materials of construction (MOC).
POX PROCESS SELECTION
The selection between acidic and alkaline POX processes
depends on several critical factors, primarily focusing on
economic considerations and ore characteristics, specifi-
cally the carbonate-to-sulfide sulfur (CO3/SS) ratio. This
ratio is used as a rule of thumb to determine the feasibility
of applying either acidic or alkaline POX for the treatment
of refractory gold ores.
Key Criteria for POX Selection
Historically, the carbonate-to-sulfide sulfur ratio has served
as a fundamental criterion. For ores containing substantial
amounts of acid-consuming carbonates (10% CO3) and
relatively low sulfide sulfur content (2%), the cost of acid
neutralization becomes prohibitive, making the alkaline
POX process more attractive. In such cases, the expense for
additional heat requirements and/or alkaline reagents to
operate the autoclave under neutral or alkaline conditions
is often lower compared to the cost of acid and neutralizing
reagents required for acidic oxidation (Marsden &House,
2006). This is the primary consideration that drives the
selection towards alkaline POX. However, alkaline POX is
suitable only for specific types of material due to its high
reagent consumption if the acid-consuming potential is not
significant enough.
The first alkaline POX process was applied commer-
cially at Mercur from 1988 into the 1990s. At Mercur,
oxidation was performed at 220°C and an oxygen partial
pressure of 140 to 180 kPa (Thomas &Williams, 2000).
The high carbonate content of the ore, combined with
relatively low sulfide content, made alkaline POX an eco-
nomically viable choice. According to Thomas &Williams
(2000), the CO3/SS ratio of the Mercur feed was greater
than 10, reflecting a high acid-consuming potential, which
effectively justified the use of alkaline POX, over acidic
POX. This example underscores the importance of ore char-
acteristics, such as high carbonate-to-sulfide sulfur ratios, in
determining the feasibility of alkaline POX and highlights
the need for a tailored approach to pre-treatment processes
based on economic and operational factors.
Another commercial application of alkaline POX is at
Barrick’s Goldstrike operation, which has been running an
alkaline POX circuit since 2014. In addition to naturally
occurring carbonate minerals, Goldstrike employs trona
(Na2CO3·NaHCO3·2H2O) as a reagent to prevent scaling
and encapsulation, which neutralizes acids generated by
pyrite oxidation and reduces scaling by enhancing sulfate
solubilization. These factors help to maintain the efficiency
of pyrite oxidation and improve overall process control,
making alkaline POX viable for high carbonate ores at
Goldstrike (Dani, 2013 &Song, 2023).
Economic and Operational Considerations
While the carbonate-to-sulfide sulfur ratio offers a useful
rule of thumb for selecting between acidic and alkaline
POX, it does not provide the complete picture. Several
additional economic and operational considerations must
be factored in to ensure that the selected approach opti-
mizes cost-effectiveness and operational performance:
• Gold Recovery: One of the significant drawbacks
of alkaline POX is its generally lower gold recovery
compared to acidic POX when treating the same ore
or concentrate. The accumulation of reaction prod-
ucts on reactive particle surfaces leads to armoring,
which inhibits further reactions and reduces the
would otherwise consume large quantities of acid during
pre-acidulation.
• Typically, operating temperature can vary between
these processes as well. Operating acidic POX at
a medium temperature (160°C) can lead to the
formation of elemental sulfur, which results in
agglomeration and subsequently reduces gold recov-
ery. Therefore, acidic POX requires a higher oper-
ating temperature to avoid these issues (Corriou &
Knkindai, 1981). In contrast, alkaline POX mini-
mizes the formation of elemental sulfur even at a
lower temperature, due to the lack of acid, which in
turn reduces the sulfur agglomeration and improves
gold recovery (Peters, 2010).
• Both acidic and alkaline POX have unique advan-
tages and limitations. Acidic POX is known for its
ability to liberate gold effectively but requires sig-
nificant acid consumption for high carbonate ores,
which lead to increased operating costs and increased
process complexity from additional steps such as
acidulation and neutralization. Alkaline POX, on
the other hand, reduces acid usage but often results
in the formation of an “armoured” product layer that
can inhibit further reactions. The armoured layer,
which is composed of insoluble precipitates, can act
as a physical barrier, reducing the efficiency of subse-
quent leaching steps (Song, 2023).
• Choosing between these two methods depends on
the specific characteristics of the ore and economic
factors, such as the availability of acid and neutral-
izing reagents, fuel type, the reagent and utility costs,
and the costs associated with off-gas treatment, heat
recovery, and materials of construction (MOC).
POX PROCESS SELECTION
The selection between acidic and alkaline POX processes
depends on several critical factors, primarily focusing on
economic considerations and ore characteristics, specifi-
cally the carbonate-to-sulfide sulfur (CO3/SS) ratio. This
ratio is used as a rule of thumb to determine the feasibility
of applying either acidic or alkaline POX for the treatment
of refractory gold ores.
Key Criteria for POX Selection
Historically, the carbonate-to-sulfide sulfur ratio has served
as a fundamental criterion. For ores containing substantial
amounts of acid-consuming carbonates (10% CO3) and
relatively low sulfide sulfur content (2%), the cost of acid
neutralization becomes prohibitive, making the alkaline
POX process more attractive. In such cases, the expense for
additional heat requirements and/or alkaline reagents to
operate the autoclave under neutral or alkaline conditions
is often lower compared to the cost of acid and neutralizing
reagents required for acidic oxidation (Marsden &House,
2006). This is the primary consideration that drives the
selection towards alkaline POX. However, alkaline POX is
suitable only for specific types of material due to its high
reagent consumption if the acid-consuming potential is not
significant enough.
The first alkaline POX process was applied commer-
cially at Mercur from 1988 into the 1990s. At Mercur,
oxidation was performed at 220°C and an oxygen partial
pressure of 140 to 180 kPa (Thomas &Williams, 2000).
The high carbonate content of the ore, combined with
relatively low sulfide content, made alkaline POX an eco-
nomically viable choice. According to Thomas &Williams
(2000), the CO3/SS ratio of the Mercur feed was greater
than 10, reflecting a high acid-consuming potential, which
effectively justified the use of alkaline POX, over acidic
POX. This example underscores the importance of ore char-
acteristics, such as high carbonate-to-sulfide sulfur ratios, in
determining the feasibility of alkaline POX and highlights
the need for a tailored approach to pre-treatment processes
based on economic and operational factors.
Another commercial application of alkaline POX is at
Barrick’s Goldstrike operation, which has been running an
alkaline POX circuit since 2014. In addition to naturally
occurring carbonate minerals, Goldstrike employs trona
(Na2CO3·NaHCO3·2H2O) as a reagent to prevent scaling
and encapsulation, which neutralizes acids generated by
pyrite oxidation and reduces scaling by enhancing sulfate
solubilization. These factors help to maintain the efficiency
of pyrite oxidation and improve overall process control,
making alkaline POX viable for high carbonate ores at
Goldstrike (Dani, 2013 &Song, 2023).
Economic and Operational Considerations
While the carbonate-to-sulfide sulfur ratio offers a useful
rule of thumb for selecting between acidic and alkaline
POX, it does not provide the complete picture. Several
additional economic and operational considerations must
be factored in to ensure that the selected approach opti-
mizes cost-effectiveness and operational performance:
• Gold Recovery: One of the significant drawbacks
of alkaline POX is its generally lower gold recovery
compared to acidic POX when treating the same ore
or concentrate. The accumulation of reaction prod-
ucts on reactive particle surfaces leads to armoring,
which inhibits further reactions and reduces the