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Memorandum
To: Randy Buffington
Cc: Jeff Snyder
From: Bill Pennstrom
Date: January 14, 2014
Re: Hycroft Concentrate Treatment Test Work Study Update
Beginning in 2007, Allied Nevada has been examining options for treating concentrates derived
from flotation of sulfidic ores from the Hycroft Mine. The original focus was on traditional
oxidation methods currently employed in the industry, including pressure oxidation (POX),
roasting, and direct cyanidation. Test work on these processes concludes that each of these
options will work, with varying degrees of economic recovery. A mine plan was developed
using on-site POX to treat one-third of the rougher concentrate and sell the remaining
concentrate for processing in third party facilities.
In late 2012, the company began a review of other oxidation processes with a goal of
determining an economically viable on-site process in lieu of building a capital and operating
cost intensive autoclave in addition to relying on offsite sales. The ultimate goal of the project is
to produce a saleable gold/silver doré on site, as it is currently doing with its heap leach
operation.
The first program was focused on bio-oxidation (BIOX) testing, which was completed by SGS
Lakefield and SGS South Africa in collaboration with Gold Fields Ltd. Final results from the
BIOX testing received in early 2013 revealed a number of important characteristics regarding
the oxidation of Hycroft concentrates. Most importantly, the concentrate appears to oxidize
rapidly in the correct environment and requires lower levels of sulfide oxidation to achieve gold
and silver recoveries at or better than those achieved through the current mine plan.
With this knowledge in hand, first phase testing of concentrate at Hazen laboratories in Golden,
Colorado, began on a suite of commonly used oxidation methods including chlorination,
ambient pressure alkaline oxidation, fine-grind with intense cyanidation, and the Albion
oxidation process. Initial results using each of these methods had been positive and indicated
that an on-site option may be economically viable allowing for processing of 100% of the
rougher concentrates and production of doré on-site.
Testing indicates that processing rougher concentrate works as well as processing cleaner
concentrate. Flotation test work indicated as much as 10% gold recovery is lost in the cleaning
stages of flotation versus rougher flotation alone. Thus, a focus on treating rougher
Hycroft Concentrate Treatment Test Work Study Update
Page 2
concentrate, to maximize overall gold recovery and project economics, has become the focus
of the ongoing test work.
Based on the positive results from the first phase testing, a second phase of testing was
initiated in May of 2013 and focused on ambient pressure alkaline oxidation to determine the
optimal parameters to yield the best return for the project. Additional tests on high intensity
cyanidation, and using the Albion process with caustic as the pH modifier instead of lime, were
conducted as well.
The phase 2 test work program was nearing completion at the writing of this report. Sufficient
test work has been completed to provide an update of the initial findings. This memo provides a
brief review of the phase 2 findings and provides a path forward for conducting a pilot scale test
campaign, which is being initiated mid-January, to verify the phase 2 results and to provide
additional process parameters for incorporation into a prefeasibility level study (end of first
quarter of 2014) and eventual feasibility level study later in the third quarter of 2014.
We expect final Hazen reports for the first and second phase of oxidation test work in the first
quarter of 2014.
PREVIOUS OXIDATION TESTING
Autoclave Previous test work on POX had been conducted on pilot plant concentrates to determine
operating criteria. The results indicate that: 1) an operating temperature range of 383°F to
401°F; 2) 100 psi oxygen overpressure; and 3) 60 minutes residence time produces the
highest cyanide amenability for gold and silver recovery. The POX tests also indicate that the
concentrates may be prone to form jarosites that can inhibit silver recovery. The evidence for
jarosite formation is:
Color of the acidic autoclaved pulp is yellow on discharge and reddish brown when
conditioned with a lime boil.
Silver recovery is higher when the pulp is treated with a lime boil, a procedure which
subjects the hot pulp for several hours to alkaline conditions.
The gold and silver recoveries from rougher concentrate POX discharge material that has been
lime boiled and then leached with cyanide was in the mid-90s and 80s, respectively.
Roaster
Roast test work was conducted on the Brimstone concentrate, Bulk Sample B, from the pilot plant to determine optimum conditions for processing. The results indicate that the optimum roast temperatures are between 425 and 450°C. Results indicated that average recoveries of 89% Au and 74% Ag are achievable from the concentrates by varying the leach and roast conditions slightly for the majority of the concentrate produced.
Hycroft Concentrate Treatment Test Work Study Update
Page 3
Direct Cyanidation Direct cyanidation leach results of bulk samples taken from all zones on the deposit were
conducted early in the test program in 2010, yielding poor results as expected. Concentrate
was ground to a P80 of 325 mesh for this testing. Recoveries from Brimstone and Vortex, the
two largest components of the deposit, were in the mid-20% range for gold and 80% range for
silver, while other smaller components of the deposit yielded recoveries ranging from 45 to
50% for gold and 55 to 83% for silver. In general, all samples being tested are direct cyanide
leached for baseline comparison.
PHASE 1 TEST WORK PROGRAM and RESULTS
A sulfide concentrate treatment test program was initiated by Hazen Research under the
direction of Allied Nevada and by Bill Pennstrom of Pennstrom Consulting Inc. Rougher and
second cleaner Hycroft concentrates from past pilot plant test work were used for this phase of
testing. The Hazen test work focused on several methods for partially oxidizing the
concentrates, which would then be cyanide leached on site at Hycroft, allowing for a doré
product to be shipped from Hycroft directly to a precious metals refinery for final treatment and
sale.
Recent test work by KCA and Hazen focused on sulfide oxidation by either bio-oxidation or by
chlorine oxidation. Both programs were successful, with a strong relationship being observed
between the percentage of oxidation of the sulfide and the ability to cyanide leach the gold and
silver from the sulfide concentrates. The following chart shows the relationship of the degree of
sulfide oxidation and gold recovery from the BIOX test work on rougher concentrates.
Hycroft Concentrate Treatment Test Work Study Update
Page 4
The BIOX test work clearly indicated that complete sulfide oxidation is not required in order to
achieve gold recoveries above 80 percent in a downstream cyanide leach circuit. The data also
indicated that silver recovery tends to decrease with increasing sulfide oxidation at pH values
below 5.0, which is believed to be caused by jarosite formation at low pH, which locks up silver.
This is similar to what was seen in the POX process, which would require a lime boil after
pressure oxidation to alleviate this issue.
With this information in mind a test program was put together to examine various methods of
sulfide oxidation on the Hycroft concentrates. The initial focus was on chlorine oxidation of the
concentrates, followed by tests including high intensity cyanide leaching, ambient pressure and
low pressure alkalkine oxidation, and Albion oxidation.
Initial results indicated that any method of oxidation would work on the concentrates provided a
minimum of 65 to 70% of the sulfide was oxidized. These results were in-line with the results of
the BIOX test work.
The following figure illustrates the results of all the Hazen oxidation test work performed to
date.
0
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120
0 20 40 60 80 100 120
Go
ld L
eac
h %
Sulfide Oxidation (%)
Au Leach vs Sulfide Oxidation All Oxidation Tests
Hycroft Concentrate Treatment Test Work Study Update
Page 5
The next graph shows the effects of sulfide oxidation on the cleaner concentrates only.
The following graph shows the effect of oxidation on rougher concentrate only.
0
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120
0 20 40 60 80 100 120
Go
ld L
eac
h %
Sulfide Oxidation (%)
Au Leach vs Sulfide Oxidation Cleaner Conc Only
0
10
20
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40
50
60
70
80
90
100
0 20 40 60 80 100
Go
ld L
eac
h %
Sulfide Oxidation (%)
Au Leach vs Sulfide Oxidation Rougher Conc Only
Hycroft Concentrate Treatment Test Work Study Update
Page 6
The results above show that the rougher concentrate and the cleaner concentrate react very
similarily to the degree of sulfide oxidation.
CHLORINE OXIDATION TEST WORK
The initial test work focused on using chlorine gas as the sulfide oxidant. Note that the majority
of the samples were not sufficiently oxidized to allow the majority of the gold to be leached
even though the slurrys were visually oxidized. Also note that the silver recoveries were
negatively impacted by the sulfide oxidation process. Looking further into the chemistry of the
process indicated that the formation of jarosites was likely occurring, which is believed to have
locked up the silver into an unleachable state, much like what occurs in a standard pressure
oxidation process at other operating POX facilities.
Although higher dosages of chlorine gas would have likely increased the amount of gold
available for leaching the tests were discontinued. Low silver recoveries and positive test work
from the other oxidiation work led to this decision.
0
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50
60
70
0 10 20 30 40 50 60
Go
ld L
eac
h %
Sulfide Oxidation (%)
Chlorine Oxidation Test Work Au Leach vs Sulfide Oxidation
Hycroft Concentrate Treatment Test Work Study Update
Page 7
PRESSURE ALKALINE OXIDATION TEST DATA
Scoping tests were conducted on using pressure oxidation at 100ºC and 40 psi oxygen
overpressure, as compared with previous POX test work completed using industry standard
temperatures (250ºC plus) and oxygen overpressures of 100 psi. This test was performed to
see if sulfide oxidation could be achieved at the lower limits for a POX circuit in an attempt to
lower costs and improve the economics of a POX circuit at Hycroft. Also unlike most POX
circuits, the pH was maintained above 5.0 to eliminate the formation of jarosites.
As shown in the following two graphs akaline POX will effectively oxidize the sulfides, allowing
for effective gold leaching, while also allowing for the effective leaching of silver. The results of
these tests were positive and this method remains as an option for the concentrate treatment at
Hycroft. However, the bottom limit for temperature and oxygen required for sulfide oxidation to
occur was not obtained as test work at yet lower temperatures and ambient pressure would
later prove.
0
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30
40
50
60
70
80
0 10 20 30 40 50 60
Silv
er
Leac
h %
Sulfide Oxidation (%)
Chlorine Oxidation Test Work Ag Leach vs Sulfide Oxidation
Hycroft Concentrate Treatment Test Work Study Update
Page 8
0
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60
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100
120
0 20 40 60 80 100
Go
ld L
eac
h %
Sulfide Oxidation (%)
Pressure Alkaline Oxidation Test Work Au Leach vs Sulfide Oxidation
0
20
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60
80
100
120
0 20 40 60 80 100
Silv
er
Leac
h %
Sulfide Oxidation (%)
Pressure Alkaline Oxidation Test Work Ag Leach vs Sulfide Oxidation
Hycroft Concentrate Treatment Test Work Study Update
Page 9
AMBIENT PRESSURE ALKALINE OXIDATION (“AAO”)
See also Appendix A: Pretreating Hycroft Concentrates by Atmospheric Alkaline Oxidation Using Trona – D. Gertenbach, Hazen Laboratories; and Appendix B: Ambient pressure, or low-pressure, oxidation is an oxidation process that has been used successfully for a number of years in the precious metals industry. The Albion process is a common form of ambient pressure oxidation, however, other less familiar versions of the process have been, or are currently in operation globally.
Low-pressure oxidative pretreatment has been applied at many operations for the treatment
of ores, for example, Homestake Lead (United States), Pamour Porcupine (Canada), and East
Dreifontein (South Africa); and for concentrates, for example, Agnico Eagle (Canada). (John
O. Marsden and C. Iain House, 2006)
Ambient pressure oxidation can be explained as:
Dissolved oxygen in solution under ambient conditions is capable of oxidizing some sulfide
minerals. This can be applied as a simple, low-cost, preaeration step before cyanide leaching
to oxidize and/or passivate the surfaces of some of the more reactive, reagent-consuming
sulfides such as pyrrhotite and marcasite. (John O. Marsden and C. Iain House, 2006)
The sulfide mineral in Hycroft ore is predominantly pyrite and marcasite, with the gold
associated on the rim of the sulfide crystal, not intimately associated within the sulfide crystal.
This was identified during the investigation of the biooxidation process on Hycroft concentrates
in 2012, noted earlier in this memo.
Low-pressure oxidative pretreatment, or preaeration, is most commonly applied prior to
cyanidation. Air or oxygen is sparged into agitatetd tanks, and sufficient retention time,
typically 4 to 24 hr, is provided to allow adequate oxidation and/or passivation of cyanide-
consuming minerals.
Air is a considerably cheaper source of dissolved oxygen than pure oxygen. However, in some
cases pure oxygen can substantially increase oxidation rates and improve the degree of
sulfide oxidation obtained with the corresponding further decrease in cyanide consumption.
(John O. Marsden and C. Iain House, 2006)
During the investigation of ambient pressure oxidation on Hycroft concentrate, air, pure oxygen
and various mixtures of the two were assessed for effectiveness. This test work indicated that
a blend of 20% air and 80% oxygen appears to achieve the desired oxidation.
The acid generated reacts with available alkali metal salts in the ore to precipitate gypsum or
other sulfate species. Alternatively, if the reactive sulfide content is high or in the absence of
neutralizing salts, a suitable material such as limestone, lime, dolomite, or sodium carbonate
may be added to neutralize acid as it is formed.
Hycroft Concentrate Treatment Test Work Study Update
Page 10
Oxidation is usually performed in the pH range of 8 to 11, although the pH does not appear to
be critical. (John O. Marsden and C. Iain House, 2006)
Certain variables in the process may differ among the operating plants, however the
technology is the same. Hazen investigated a number of materials for acid neutralization,
including those noted above. Trona is a mineral within the sodium carbonate family that
usually contains 70-97% of a complex salt of sodium carbonate (Na2CO3) and sodium
bicarbonate (NaHCO3) in a hydrated crystal form known as sodium sesquicarbonate (Na2C03 •
NaHC03 • 2H20). Trona was found to be as effective or more effective than the other acid
neutralizing agents, with the added benefit of being readily available (Green River, WY) and
relatively inexpensive. Currently, Trona is used in various process streams as an acid reducer
and/or neutralizer, including Newmont’s Carlin Roasters in Nevada (Wood, DeSomber, &
Marshall, 2001). Lime is added at the end of the oxidation phase to adjust the pH to an
appropriate level for cyanidation to reduce cyanide consumption.
AMBIENT PRESSURE ALKALINE OXIDATION TEST DATA (“AAO”)
To further understand the lower limits of temperature on the ability to oxidize the sulfide in an
alkaline slurry, ambient pressure test work was performed on temperatures ranging between
40ºC and 75ºC. The following two graphs show the relationship of sulfide oxidation percentage
on gold and silver cyanidation recovery at ambient pressure.
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ld L
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Sulfide Oxidation (%)
Alkaline Ambient Pressure Test Work Au Leach vs Sulfide Oxidation
Hycroft Concentrate Treatment Test Work Study Update
Page 11
As the graphs clearly indicate, sulfide oxidation can be performed at ambient pressure and
lower temperatures, allowing effective gold and silver leaching. Test work quickly focused on
obtaining the minimum temperature at which the oxidation reaction would occur. Tests were
performed at 75ºC, 60ºC, and 40ºC while maintaining a pH of 11.25 with NaOH (also known as
sodium hydroxide or caustic soda) and using oxygen as the oxidant. The following two graphs
show the effect of oxidation time versus temperature for the rougher and cleaner concentrates.
Sulfide oxidation of at least 60% must be achieved to obtain acceptable gold leach recoveries.
0
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60
80
100
120
0 20 40 60 80 100 120
Silv
er
Leac
h %
Sulfide Oxidation (%)
Alkaline Ambient Pressure Test Work Ag Leach vs Sulfide Oxidation
Hycroft Concentrate Treatment Test Work Study Update
Page 12
0
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80
100
120
0 20 40 60 80
Sulf
ide
Oxi
dat
ion
%
Time, hours
Alkaline Ambient Pressure Test Work Time vs Sulfide Oxidation @ 75C & 60C
Rougher Concentrate @ 44 microns
Temperature 75C Rougher
Temperature 60C Rougher
0
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0 10 20 30 40 50 60
Sulf
ide
Oxi
dat
ion
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Time, hours
Alkaline Ambient Pressure Test Work Time vs Sulfide Oxidation @ 75C & 60C
Rougher Concentrate @ 150 microns
Temperature 75C Rougher
Temperature 60C Rougher
Hycroft Concentrate Treatment Test Work Study Update
Page 13
Optimum results to date for rougher concentrate occurred at an oxidation time of 24 hours, a
reaction temperature of 75ºC, and a particle size of 150 microns, giving a sulfide oxidation
percentage of 60% and recoveries of 85% gold and 82% silver. In comparison, the optimum
cleaner concentrate results to date occurred at an oxidation time of 24 hours at 75ºC reaction
temperature, giving a sulfide oxidation percentage of 71% and resulting recoveries of 88% gold
and 97% silver.
This test work gave very promising results using caustic as the pH modifier. However, due to
the high cost of caustic, other pH modifiers were tested. A full screen of test work was
performed using lime (calcium oxide), sodium carbonate, sodium bicarbonate, potassium
carbonate, sodium silicate, limestone, and sodium sesquicarbonate (Trona) (Na2CO3 • NaHCO3
• 2H2O). All of the listed reagents resulted in acceptable sulfide oxidation percentages except
for the lime and limestone. The following table, Table 1, provides some of the data obtained
from using the various modifiers.
0
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40
60
80
100
120
0 20 40 60 80
Sulf
ide
Oxi
dat
ion
%
Time, hours
Alkaline Ambient Pressure Test Work Time vs Sulfide Oxidation @ 75C & 60C
Cleaner Concentrate @ 44 microns
Temperature 75C Cleaner
Temperature 60C Cleaner
Hycroft Concentrate Treatment Test Work Study Update
Page 14
Test work using lime showed no oxidation. It was later discovered that lime was creating an
impervious oxide coating on the surface of the pyrite, which impeded the oxidation process. A
literature search on lime and sulfide oxidation turned up several papers that discussed this
phenomena in numerous test programs. Lime was therefore removed from the reagent
candidate list for future tests.
Limestone was also found to be ineffective for oxidizing the sulfides. It is believed that the
limestone had some of the same issues as lime, i.e. sulfide coating formation, and using
limestone to achieve a pH of higher than 5.5 was not possible. Limestone was therefore also
dropped from the reagent candidate list going forward.
In order to narrow the scope for future test work, a preliminary reagent cost evaluation versus
reagent oxidation reactivity was made using current pricing for the reagents tested. This
economic evaluation quickly pointed to Trona as being the best economic choice as ground
Trona pricing was $135 per tonne delivered to Hycroft, and later found to be $110 per tonne
delivered for crushed Trona.
Reagent Consumption
Test ID #
P80 PSA,
μm
Oxidant
Gas Time, h Reagent
Reagent
Consumption
(dry basis), lb/st
Sulfide
Oxidation, %
Gold
Extraction, %
Silver
Extraction, %
3738-16 15 O2 24 50% NaOH 491 95.2 95.5 99+
3738-27 15 O2 24 15% limestone slurry 220 18.9 10.6 54.3
3738-23 15 O2 24 K2CO3 2,537 58.8 86.1 96
3738-34 15 O2 24 Na2SiO3 1,921 81.1 77.8 86.7
3738-35 15 O2 9.5 Na2CO3 737 58.7 81.9 93.4
3738-44 15 O2 17.3
Na2CO3 • NaHCO3 • 2H2O
Trona 487 64.2 82.4 93
Oxidation Target Conditions Results
Table 1 - Results from Selected Tests Using Different pH Modifiers
Hycroft Concentrate Treatment Test Work Study Update
Page 15
Phase 2 - AAO Test Work Using Trona as the pH Modifier
AAO test work proceeded, based on the results from the oxidant screening tests evaluation.
The decision was also made to proceed with rougher concentration testing only, since the
concentrate was now planned to be processed on site at Hycroft, and because the flotation test
work showed that approximately 10% gold recovery was lost in the cleaner flotation steps.
Since the concentrate was being processed onsite it made economic sense to push gold
recovery and being much less concerned with the grade of the concentrate being processed
downstream.
Test work on rougher concentrate at various oxidation times, from 8 hours to 24 hours, has
shown encouraging results. It is apparent that by varying temperature, grind size, pH and
oxygen levels there is an opportunity to have a lower oxidation time than the 24 hours. The
following figures and discussion illustrate the effects of varying reaction parameters in the AAO
process.
Gold and Silver Extraction versus Sulfide Oxidation Using Trona as the pH Modifier
Numerous tests have been performed using Trona solely as the pH modifier. Tests indicate
that the Trona works as well as or better than most of the other reagents tested. Research
suggests that the sodium carbonate and bicarbonate reaction occurring at the surface of the
pyrite grain has a spalling effect, causing the pyrite surface to be cleaned as it is oxidized,
enhancing the reaction kinetics for sulfide oxidation.
The following figures present the test work data when using Trona only as the pH modifier.
The first two figures illustrate the relationship between sulfide oxidation and gold or silver
recovery. Not surprisingly, the data falls into a familiar pattern as has been observed will all of
the pH modifiers tested to date. Gold extraction reaches 80% at roughly 60% sulfide oxidation.
Silver recovery also increases with sulfide oxidation, which was not observed when using low
pH sulfide oxidation techniques like pressure oxidation or bio-oxidation.
Hycroft Concentrate Treatment Test Work Study Update
Page 16
Test work has shown that Trona consumption is directly related to the amount of sulfide
oxidation achieved, as is shown in the following graph.
Hycroft Concentrate Treatment Test Work Study Update
Page 17
Lastly, gold extraction can be tied to Trona consumption as is depicted from the test data
displayed in the following graph.
Sulfide Oxidation versus Trona Addition
Test work using Trona as the pH modifier began with varying the amounts of Trona used in
order to get a relationship between Trona addition and sulfide oxidation. The following figure
illustrates the effect of varying Trona addition on sulfide oxidation.
Hycroft Concentrate Treatment Test Work Study Update
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The following picture shows the oxidized sulfide slurry using 100, 200, and 300 kg/t Trona
additions.
Hycroft Concentrate Treatment Test Work Study Update
Page 19
Although the reaction time is influenced by the amount of Trona added, adding Trona at about
250 kg/t of rougher concentrate is sufficient to obtain a sulfide oxidation percentage of greater
than 60% given sufficient reaction time.
The following figure shows the amount of Trona consumed to reach varying rates of sulfide
oxidation.
As the previous figure illustrates, the Trona consumption is not greatly influenced by the Trona
addition, even at an addition rate of 700kg/t concentrate. Trona consumption remains fairly
constant at about 250 kg/t concentrate at a sulfide oxidation percentage of 60%. In other
words, Trona consumption is driven almost solely by the degree of sulfide oxidization or the
amount of sulfide oxidized.
Other test parameters have been investigated to aid in evaluating the effect of other
parameters on sulfide oxidation kinetics. These parameters include:
Oxidation temperature;
Concentrate particle size;
Hycroft Concentrate Treatment Test Work Study Update
Page 20
Oxygen concentration; and
Oxidation slurry percent solids.
The following discusses the test work results for varying these parameters.
Oxidation Temperature
The effect on the reaction kinetics of the temperature maintained during oxidation has been
tested. The data indicates that higher reaction temperatures improve reaction kinetics and
increase the rate of the oxidation reaction. The following figure shows the test results for three
different temperatures.
Since it is believed that the oxidation reaction will be exothermic, maintaining higher
temperatures should not be an issue. The upper limit on reaction temperature is believed to be
around 90ºC, due to difficulties in maintaining oxygen levels within the slurry at higher
temperatures.
Concentrate Particle Size
Test work has been performed to obtain an understanding of the effect of the sulfide
concentrate particle size on oxidation reaction kinetics. The effect on sulfide oxidation at four
different grind sizes and at two different percent solids was tested. The following figure shows
the effect that particle size has on oxidation kinetics. As expected, finer sulfide concentrate grid
sizes give improved oxidation kinetics.
Hycroft Concentrate Treatment Test Work Study Update
Page 21
Oxygen Concentration
The effect on sulfide oxidation reaction kinetics from varying the oxygen concentration of the
gas being introduced into the slurry was tested. Since it is difficult to model oxygen
consumption in bench scale tests, oxygen consumptions were not taken. Instead, the gas
stream bubbled into the slurry was changed by changing the ratio of air to oxygen in the gas
stream.
Hycroft Concentrate Treatment Test Work Study Update
Page 22
Oxidation Slurry Percent Solids
The percent solids of the oxidation slurry were tested to get a view of the effect of percent
solids on reaction kinetics. Slurry density has an obvious impact on tank and thickener sizing.
However, the test results indicate that lower percent solids improve reaction kinetics to some
degree. The following figure shows the effect on sulfide oxidation at varying percent solids.
At first glance the figure implies that lower percent solids significantly enhance reaction
kinetics. However, oxygen availability is likely contributing to the improved kinetics at the lower
percent solids, as each sulfide particle has more oxygen available to it in the lower density
case. Also, Trona solubility becomes impacted at the higher percent solids, and can therefore
be negatively impacting the reaction kinetics at the higher density. Additional tests will be
performed at 25% solids to confirm these findings.
Simple Project Economic Evaluation of AAO Process
During the Phase 2 test work campaign, a review of the project economics was performed to
provide an indication of economic viability for the project. Also, during this same time frame, M3
Engineers were engaged to begin an engineering review of the AAO process envisioned for
Hycroft. M3 was tasked with performing a simple economic evaluation using the results from
the Hazen test work for the sulfide oxidation process only. The boundary limits for this
evaluation was at receipt of the flotation rougher concentrate, through sulfide oxidation and
cyanide leaching of the oxidation products, including cyanide destruct. The parameters used by
M3 were the following:
Hycroft Concentrate Treatment Test Work Study Update
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Trona as the pH modifier at a consumption rate of 400 lb/st (200 kg/mt) of concentrate
Oxygen consumption calculated based on sulfide oxidation demand
Labor, Power, Cyanide, Flocculent, etc, based on a process flow sheet developed by M3
Following is the conceptual flow sheet developed by M3.
Hycroft Concentrate Treatment Test Work Study Update
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Page 25
Based on the above assumptions and parameters, M3 developed the following cost table.
The cost model presented above indicates costs for the oxidation and leaching portion of the
flowsheet. This will be incorporated into the costs for the milling process as part of the updated
prefeasibility study. It is expected that there will be some offsetting decreases to the overall
operating cost for the elimination of certain processes such as two stages of cleaner flotation,
cleaner concentrate thickening, filtration, and handling, etc. These cost offsets are not
reflected in the above cost table and will be better identified in the updated prefeasibility,
expected to be completed at the end of the first quarter of 2014.
Ore Processed (stpd) 130,000
Mass Pull 13.8%
Concentrate (stpd) 17,940
Sulfide Sulfur Oxidation 60%
Trona Consumption 200 kg/mt
Gold Recovery 80%
Labor Staff $/hr $/day $/ton Labor Staff $/hr $/day $/ton
Operator 4 $33.75 $1,620 Operator 2 $33.75 $810
Helper 4 $29.70 $1,426 Helper 2 $29.70 $713
Mechanic 2 $29.70 $713 Mechanic 1 $29.70 $356
Mechanic Helper 2 $29.70 $713 Mechanic Helper 1 $29.70 $356
Total Labor $4,471 0.00 Total Labor $2,236 0.000
Power kWh/day $/kWh $/day Power kWh/day $/kWh $/day
Conveyor 4,600 $0.061 $281 Conveyor $0.061
Agitators 2,520 $154 Agitators 2,835 $173
Blowers 705 $43 Blowers 616.88 $38
Pumps 2,837 $173 Pumps 1,419 $87
Thickener 150 $9 Thickener 600 $37
Total Power 10,812 $660 0.01 Total Power 5,470 $334 0.003
mt/day $/t $/day t/day $/t $/day
Oxygen 2,240 $25.00 $56,009 0.43 Oxygen - $60.00 $0 0.000
Reagents mt/day $/mt $/day Reagents t/day $/t $/day
Trona 3,255 $110.00 $358,051 2.75 Trona - $139.00 $0 0.000
kg/t ore $/kg $/day kg/t ore $/kg $/day
Cyanide 0.50 $3.00 $24,413
Lime 2.00 $0.18 $5,859
Metabisulfite 0.40 $0.71 $4,622
Flocculant 0.02 $3.90 $0 0.00 Flocculant 0.02 $3.90 $5,597 0.043
Total Reagents $358,051 2.75 Total Reagents $40,491 0.311
Maintenance equipment cost % factor $/day Maintenance equipment cost % factor $/day
Parts $20,000,000 5.0% $2,740 0.02 Parts $10,000,000 5.0% $1,370 0.011
$/t ore $/day $/t ore $/day
Supplies & Services $0.020 $2,600 Supplies & Services $0.010 $1,300
Regrind Cost $0.230 $29,900 0.23 0.000
Total Oxidation Leach $454,431 3.50 Total Cyanide Leach $45,730 0.352
Oxidation Leach Cyanide Leach
Allied Nevada Gold Corporation
Hycroft Mine
Hycroft Concentrate Treatment Test Work Study Update
Page 26
The primary driver for the sulfide oxidation costs is the Trona at a consumption of 200 kg/mt of
concentrate. However, of all of the pH modifiers tested, Trona is by far the least expensive on a
unit cost basis, and it has been found to be one of the most effective pH modifiers for this
process. Oxygen is the second most expensive contributor to the process. The last parameter
to be investigated was whether or not heat would need to be added to the process. Heating
slurries would be expensive if it were required. To this end, M3 was tasked with developing a
heat balance around the process. This heat balance follows below.
Ore Processed (stpd) 130,000
Mass Pull 13.8%
Concentrate (mtpd) 16,275
Sulfide Sulfur Content 10%
MTons Sulfde sulfur 1,627.5
Sulfide Sulfur Oxidation 60%
Oxygen Required 2.3 t/t S
3,743 tons
Target Oxidation 60% 2,246 tons O2 Required
Power Cost 0.061 $/kWh
Reagents
Oxygen 60 $/tonne (includes power)
AAO O2 Requirement:Air Oxygen
Air to Oxygen Ratio 20% 80%
Constituent O2 Content 21% 95%
Resulting O2 Content 80%
Tons of Air/O2 Mixture Required (tonnes per day) 2,800
Quantity Required per Day of Each (tonnes per day) 560 2,240
Oxygen Consumption Calculations
Hycroft Concentrate Treatment Test Work Study Update
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The costs developed by M3 indicate that this process is cost effective and meets economic
requirements, warranting a pilot plant scale study.
INPUT SPECIES (1)
Formula
Temper.
°C
Amount
kmol
Amount
kg
Amount
Nm³
Latent H
Mcal
Total H
Mcal
FeS2 35.000 1.559 187.081 0.037 0.23 -63.69
Na2CO3 25.000 0.943 100.000 0.039 0.00 -254.99
NaHCO3 25.000 1.190 100.000 0.046 0.00 -270.51
O2(g) 25.000 4.313 138.000 96.662 0.00 0.00
N2(g) 25.000 4.891 137.000 109.614 0.00 0.00
SiO2 35.000 13.530 812.919 0.313 1.46 -2943.94
H2O 35.000 103.088 1857.143 2.025 18.55 -7023.88
OUTPUT SPECIES (1)
Formula
Temper.
°C
Amount
kmol
Amount
kg
Amount
Nm³
Latent H
Mcal
Total H
Mcal
FeO*OH 75.000 0.935 83.078 0.020 0.86 -124.28
Na(+a) 75.000 3.076 70.715 0.000 1.71 -174.95
SO4(-2a) 75.000 1.871 179.707 0.000 -4.98 -411.69
SiO2 75.000 13.530 812.919 0.313 7.65 -2937.76
H2O 75.000 103.088 1857.143 2.025 92.91 -6949.52
N2(g) 75.000 4.891 137.000 109.614 1.70 1.70
CO2(g) 75.000 2.133 93.873 47.808 0.98 -199.63
FeS2 75.000 0.624 74.833 0.015 0.48 -25.09
kmol kg Nm³ Mcal Mcal
BALANCE: 0.633 -22.876 -48.943 81.07 -264.20
MATERIAL BALANCE
ELEMENT Input Output Balance Input Output Balance
kmol kmol kmol kg kg kg
C 2.134 2.133 -0.001 25.630 25.619 -0.011
Fe 1.559 1.559 -0.001 87.090 87.053 -0.037
H 207.366 207.110 -0.255 209.004 208.746 -0.257
N 9.781 9.781 0.000 137.000 137.000 0.000
Na 3.077 3.076 -0.001 70.748 70.717 -0.032
O 145.174 143.766 -1.408 2322.693 2300.170 -22.523
S 3.119 3.118 -0.001 99.991 99.974 -0.017
Si 13.530 13.530 0.000 379.987 379.987 0.000
e- 0.000 0.666 0.666 0.000 0.000 0.000
Temperature of Products = 203.148 °C When Heat Balance = 0
AAO Heat Balance Using Trona
Hycroft Concentrate Treatment Test Work Study Update
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PILOT SCALE TEST WORK PROGRAM
The next phase of the test program on the Hycroft concentrates will include a pilot scale test
work program to be performed at Hazen. The pilot plant will be run to confirm the results from
the bench scale work on what is believed to be the most economic operating parameters, in a
larger scale, continuous, closed loop circuit that more closely models an actual operating plant.
Information gathered on the effects of recirculating solutions on oxidation performance and
reagent consumption will also be gathered to determine any positive or negative issues that
arise in a closed loop circuit. We would anticipate that issues not previously encountered in the
bench scale test work will occur. The pilot plant results will be used to fine tune the design of
the Hycroft processing facility and specifically the Metsim model, which is then used to
establish the material balances and circulating loads.
The pilot plant will be run using the three primary sulfide domains that will be mined at Hycroft.
Each ore type will be run separately in order to determine any operating differences between
the three sulfide domains. The pilot plant will begin with crushing, grinding, and sulfide rougher
flotation. The rougher concentrate will be reground to a predetermined particle size and
subjected to a closed loop AAO circuit. Monitoring of recirculated solutions will be a critical task
during the oxidation phase of the pilot plant test work. Oxidation products will then be subjected
to cyanide leaching. A block flow diagram illustrating the pilot plant process is provided on the
following page.
The pilot plant is scheduled to begin in late January with the pilot runs being complete by the
end of March. The pilot plant results will be used by M3 to develop a prefeasibility study level
document.
Hycroft Concentrate Treatment Test Work Study Update
Page 29
Hycroft Ore
Flotation
Grinding Circuit
86.2 % of solids to tails
13.8 % of solids
Regrind circuit
Continuous AAO
Cyanide leach
Merrill-Crowe
S/L separation, Vacuum filtration
Liquor recyle
S/L separation
Batch Settle and Decant
(50 – 55 % Solids)
Batch Settle and Decant
(50 % Solids)
Flocculant
Makeup Water
Trona Makeup
Over Flow
Under Flow
Over Flow
Under Flow
L
S
Sodium Sulfate Bleed
Hycroft Concentrate Treatment Test Work Study Update
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Next Steps
The plan for going forward will first focus on running a pilot plant scale test to confirm bench
scale work performed to date.
Pilot Plant Test Work
The pilot plant work will be performed on individual material domains established by the ANV
and Hycroft geology groups. Previous work was performed on a master composite that was put
together based on Hycroft mine plans and the amount of each domain that was planned to go
through the sulfide treatment circuit. This pilot run will treat each domain sample separately,
such that any differences in performance and, therefore, operating parameters can be
examined by domain. The results by domain can be used to establish gold and silver
recoveries, as well as operating parameters and costs, by domain for incorporation into mine
development plans and financial models. A confirmation pilot test will also be run on the master
concentrate composite that was used for the bench scale oxidation tests.
The pilot plant will also be run in closed circuit to establish the effects of recirculating solutions.
The effects of two of the primary reaction products, sulfates and sodium, building up in the
process circuit, will be much better understood. A Metsim model of the pilot plant will also be
used to predict the circulating loads of the various constituents.
As results are obtained from the pilot tests, changes to test parameters will be made, and a
second pilot plant run will be performed. Pilot plant results will dictate if additional runs will be
required. The goal of the pilot plant is to establish operating parameters and ranges for the
eventual Hycroft oxidation facility and to establish what, if any, limiting factors exist for the
oxidation of pyrite using this chemical scheme.
Sulfide Oxidation Pre-feasibility Level Study
The bench and pilot plant test results will be incorporated into a pre-feasibility level study. The
study will provide a first look at the layout of the oxidation facility, will provide flow sheets and
material balances for sizing equipment, and provide capital and operating costs for performing
a financial analysis of the oxidation project. The study is currently scheduled for completion
towards the end of the first quarter or early second quarter of 2014. Should a positive financial
outcome be the result of the pre-feasibility study, the sulfide oxidation facility will be
incorporated into a feasibility study on the entire Hycroft mine and processing facilities.
Opportunities
The pilot plant test work will include sulfide flotation and concentrate oxidation, and cyanide
leaching of the oxidized concentrate. Opportunities to the currently planned flotation circuit that
will be tested in the pilot plant test work include:
Rougher flotation using a higher weight pull. Since the concentrate is now planned to be
processed on site and not sold to an offsite processing facility, a lower grade
Hycroft Concentrate Treatment Test Work Study Update
Page 31
concentrate can likely be financially tolerated. Typically, higher weight pull to the
concentrate gives higher sulfide, gold, and silver recoveries.
Cyanide leaching of flotation tails. Since there will be a cyanide circuit to treat the
oxidized sulfides, a cyanide circuit may become financially feasible for treating the
flotation tailings. The current thought would be to take the oxidized sulfide cyanide
leach tails and mix them with the flotation tails, thus making use of the cyanide
remaining in the oxidized sulfide cyanide leach tails to leach the flotation tails. A test will
be performed during the pilot plant test work to investigate the plausibility of this
concept.
Incorporating all facilities at Hycroft. Any excess solutions from the sulfide oxidation
facility could possibly be used in the existing Hycroft heap leach facility. This would be
especially true of any solutions that contain cyanide, gold, or silver. Making use of the
entire Hycroft facility to maximize solution management capabilities and the recovery of
values from the sulfide plant should be investigated.
Trona costs. Discussions have been initiated with the primary Trona supplier at Green
River, WY, Natronx Technologies LLC. High level meetings are being planned with
Natronx to discuss product quantities and qualities, and costs. Should the sulfide
oxidation using Trona prove to be the route going forward, Trona costs and supply will be crucial to the success of the process. Hycroft will become a major consumer of Trona, which will put them into a strong negotiating position.
As the pilot plant and oxidation project move forward additional opportunities will likely develop.
Bibliography John O. Marsden and C. Iain House. (2006). The Chemistry of Gold Extraction (Second Edition). Englewood, CO:
Society for Mining, Metallurgy and Exploration.
Wood, M. D., DeSomber, R. K., & Marshall, D. L. (2001). Patent No. 6270555. United States.
Page 32
APPENDIX A
Pretreating Hycroft Concentrates by Atmospheric Alkaline Oxidation Using Trona
Dennis Gertenbach
Sr. Vice President
Hazen Research, Inc.
Page 33
Pretreating Hycroft Concentrates by Atmospheric Alkaline Oxidation Using Trona
As with many refractory gold deposits, much of the gold in Hycroft flotation concentrates is
associated with sulfide minerals and is not leached directly in cyanide solutions. To liberate the
gold within the sulfide minerals, allowing recovery in subsequent cyanide leaching, the sulfide
minerals must be oxidized in a pretreatment step. The gold-bearing sulfides in the Hycroft deposit
are mostly pyrite and marcasite. Marcasite is more readily oxidized at milder conditions and
concentrates containing marcasite are amenable to less aggressive oxidizing pretreatment options.
A number of oxidation pretreatment technologies were evaluated for the Hycroft concentrates and
all showed that the amount of gold recovered during the subsequent cyanide leach was
proportional to the extent of sulfide oxidation. The same gold recoveries were achieved for a given
sulfide oxidation, regardless of the pretreatment technology used to oxidize the sulfide minerals.
This allowed the selection of the appropriate pretreatment technology to be made on the basis of
capital and operating costs.
All oxidation pretreatment technologies (including roasting, pressure oxidation, and biological
leaching), oxidize sulfides to acid. Unless there is a market for this acid, it must be neutralized for
disposal. This makes the cost of the neutralizing agent a critical consideration in selecting a
pretreatment technology for sulfide concentrates.
Based on the reactive sulfide minerals in the Hycroft concentrate, the insensitivity of gold recovery
to the pretreatment oxidation process, and the cost of the neutralizing agent, atmospheric alkaline
oxidation (AAO) was selected. AAO operates at a pH between 9 and 12, using oxygen or oxygen-
enriched air as the oxidant. Neutralizing agents considered for AAO included lime, sodium
hydroxide, sodium carbonate, and trona. Although lime is a relatively inexpensive neutralizing
agent, it cannot be used for AAO because the sulfide particles become coated with a calcium
precipitate and sulfide oxidation stops. Sodium hydroxide works well for this application;
however, the cost is too high for an economically viable process. Sodium carbonate is even more
effective than sodium hydroxide for AAO, due to what is referred in the literature as the “carbonate
effect”. Carbonate in solution keeps the sulfide surfaces clean during oxidation, improving the
oxidation rate compared to other neutralizing agents.
Trona, a mixture of sodium carbonate and sodium bicarbonate, is an inexpensive source of
carbonate for AAO. The Hycroft gold mine is uniquely situated close to the Green River, Wyoming,
trona deposits, the largest trona source in the world. Also, the AAO process does not need a high
purity carbonate source, so run-of-mine trona can be used.
AAO processes have been used commercially. East Driefontein in South Africa used AAO to oxidize
minor amounts of pyrrhotite before cyanidation to reduce cyanide consumption. Lime was used to
Page 34
control the pH at 10.5-11, as they were interested in passivating the sulfide surfaces, not oxidizing
a large fraction of the sulfide minerals in the ore. The Homestake Mine in South Dakota also used
an alkaline pretreatment to passivate and oxidize sulfides. Similarly, Joutel in Quebec used lime
and oxygen-enriched air on flotation concentrates. Jerritt Canyon used chlorine as the oxidant
under alkaline conditions for 16 years. In this operation, sodium carbonate was used to control
pH. Pine Creek in Australia also oxidized reactive sulfides at alkaline conditions, using hydrogen
peroxide as the oxidant.
For the Hycroft concentrate, AAO using trona and either oxygen or oxygen-enriched air provides
the necessary sulfide oxidation to achieve the desired gold recovery at a lower cost. Trona is a
relatively inexpensive neutralizing agent and has the added benefit of enhancing sulfide oxidation
because of the carbonate effect. Oxygen (or oxygen-enriched air) is a less expensive oxidant than
either chlorine or hydrogen peroxide that has been used elsewhere.
Dennis Gertenbach
Sr. Vice President
Hazen Research, Inc.
Page 35
APPENDIX B
Atmospheric Alkaline Oxidation of Hycroft Concentrates
Art Ibrado – M3 Engineering & Technology
Dennis Gertenbach – Hazen Research, Inc.
Page 36
Atmospheric Alkaline Oxidation of Hycroft Concentrates
D. Gertenbach1 and A. Ibrado2 1Hazen Research
2M3 Engineering & Technology
Much of the gold in Hycroft ores is associated with sulfide minerals and is not leached directly
in cyanide solutions. The sulfide minerals must be broken down by oxidation to expose the gold
within the sulfide matrix to the leaching solution. The oxidation process may be applied to
whole ores or to flotation concentrates. The advantage of treating concentrates is the lower
tonnage both in the oxidation stage and the following leaching stage, and the ability to oxidize
at an elevated temperature without the need for external heat. However, the recovery of sulfide
sulfur, and more importantly of gold and silver into the concentrate, must be high, which is the
case for Hycroft.
The gold-bearing sulfides in the Hycroft deposit are mostly pyrite and marcasite. Marcasite is
more readily oxidized at milder conditions and concentrates containing marcasite are amenable
to less aggressive oxidizing pretreatment options. Laboratory results show that pyrites in the
concentrate are also extensively oxidized.
A number of oxidation pretreatment technologies were evaluated for the Hycroft concentrates
and the results indicated that the amount of gold recovered during the subsequent cyanide
leach was proportional to the extent of sulfide oxidation. The same gold recoveries were
achieved for a given sulfide oxidation, regardless of the pretreatment technology used to
oxidize the sulfide minerals. This allowed the selection of the appropriate pretreatment
technology to be made on the basis of capital and operating costs.
Atmospheric Alkaline Oxidation (AAO)
All oxidation pretreatment technologies (including AAO, roasting, pressure oxidation, and
biological leaching), oxidize sulfides to produce acid. Unless there is a market for this acid, it
must be neutralized for disposal. This makes the cost of the neutralizing agent a critical
consideration in selecting a pretreatment technology for sulfide concentrates.
AAO was chosen to treat Hycroft concentrates because of the high reactivity of the sulfide
minerals in the Hycroft concentrate, the insensitivity of gold recovery to the pretreatment
oxidation process, and the cost of the neutralizing agent (trona).
The AAO process is not new. Its chemistry is well studied and it has been used commercially
for a variety of purposes. The main component is an oxidizer in the form of oxygen in air,
Page 37
oxygen-enriched air, or pure oxygen, hydrogen peroxide, chlorine gas and others, applied at
atmospheric pressures. The other component is a base to neutralize the acid produced by the
oxidation process. This may come from lime, limestone, soda ash, sodium bicarbonate, trona
(a natural mineral composed of sodium carbonate and bicarbonate), and sodium hydroxide
(caustic).
Although lime is a relatively inexpensive neutralizing agent, it cannot be used for AAO because
the sulfide particles become coated with a calcium precipitate and sulfide oxidation stops.
Sodium hydroxide works well for this application; however, the cost is too high for an
economically viable process. Sodium carbonate is even more effective than sodium hydroxide
for AAO, due to what is referred in the literature as the “carbonate effect”. Carbonate in
solution keeps the sulfide surfaces clean during oxidation, improving the oxidation rate
compared to other neutralizing agents.
Some examples of commercial application of atmospheric alkaline oxidation are as follows:
East Driefontein in South Africa used AAO to oxidize minor amounts of pyrrhotite before
cyanidation to reduce cyanide consumption. Lime was used to control the pH at 10.5-
11, as they were interested in passivating the sulfide surfaces, not oxidizing a large
fraction of the sulfide minerals in the ore.
The Homestake Mine in South Dakota also used an alkaline pretreatment to passivate
and oxidize sulfides.
Joutel in Quebec used lime and oxygen-enriched air on flotation concentrates.
Jerritt Canyon in Nevada used chlorine as the oxidant under alkaline conditions for 16
years. In this operation, sodium carbonate was used to control pH.
Pine Creek in Australia also oxidized reactive sulfides at alkaline conditions, using
hydrogen peroxide as the oxidant.
Las Lagunas Project, PanTerra Gold in the Dominican Republic oxidizes sulfides with
oxygen and neutralizes the acid mostly with limestone.
The last example listed above (Las Lagunas) uses the Albion process. This is very similar to
Hycroft’s AAO process because it uses oxygen as the oxidizer and a soda ash-limestone
combination (mostly limestone) to neutralize. The pH is slightly acidic because limestone’s
neutralizing power is limited to pH 5 or 5.5. The use of pure oxygen and the ultra-fine grinding
required makes this process very expensive, both from a capital cost and operating cost
perspective. A study conducted by M3 estimates the capital cost for the Albion process at
Hycroft at about $1 billion dollars and an operating cost of about $10/ton of ore, which are
Page 38
significantly higher than the preliminary capital and operating costs of under $100 million and
under $4.00/ton, respectively, for AAO.
Atmospheric Alkaline Oxidation of Hycroft Concentrates
Atmospheric alkaline oxidation of Hycroft concentrates has been extensively tested in batch
using both caustic and trona. As mentioned earlier, trona is the preferred neutralizer because it
is cheaper the caustic and lime. This process offer several technical and economic advantages
over the other processes and chemical combinations. These are:
The high reactivity of Hycroft concentrates requires only mild oxidation conditions
No need for high-purity oxygen – lower power cost to produce
Lower oxygen requirement due to the use of oxygen-enriched air
Unit cost of low-pressure oxygen is about a third of the high-pressure oxygen needed
for POX
Mild pH conditions allow for the use of mild steel for majority of the materials of
construction
The reaction is thermally self-supporting
Trona, a mixture of sodium carbonate and sodium bicarbonate, is an inexpensive
neutralizer. The Hycroft gold mine is uniquely situated close to the Green River,
Wyoming, where the largest trona deposit in the world is located.
AAO process does not need a high purity carbonate source, so run-of-mine trona can
be used.
Trona provides the cleaning effect (carbonate effect) during oxidation/neutralization.
Overall, alkaline oxidation at atmospheric pressure with neutralization by trona looks to be the
best process for Hycroft because of the unique combination of the ore mineralogy and
proximity of the trona deposit.