Pressure Alkaline Oxidation Test Work Au Leach vs Sulfide Oxidation

38 Pages·2014·1.43 MB·English

by  Preferred Customer

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
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.

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Hycroft Concentrate Treatment Test Work Study Update
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 P of 325 mesh for this testing. Recoveries from Brimstone and Vortex, the
80
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.

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Hycroft Concentrate Treatment Test Work Study Update
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.
Au Leach vs Sulfide Oxidation
All Oxidation Tests
120
100
80
%
h
c
a
e 60
L
d
lo
G
40
20
0
0 20 40 60 80 100 120
Sulfide Oxidation (%)

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Hycroft Concentrate Treatment Test Work Study Update
The next graph shows the effects of sulfide oxidation on the cleaner concentrates only.
Au Leach vs Sulfide Oxidation
Cleaner Conc Only
120
100
80
%
h
c
a
e 60
L
d
lo
G
40
20
0
0 20 40 60 80 100 120
Sulfide Oxidation (%)

The following graph shows the effect of oxidation on rougher concentrate only.
Au Leach vs Sulfide Oxidation
Rougher Conc Only
100
90
80
70
%
h 60
c
a
e 50
L
d
lo 40
G
30
20
10
0
0 20 40 60 80 100
Sulfide Oxidation (%)

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Hycroft Concentrate Treatment Test Work Study Update
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.
Chlorine Oxidation Test Work
Au Leach vs Sulfide Oxidation
70
60
50
%
h
c 40
a
e
L
d 30
lo
G
20
10
0
0 10 20 30 40 50 60
Sulfide Oxidation (%)

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Hycroft Concentrate Treatment Test Work Study Update
Chlorine Oxidation Test Work
Ag Leach vs Sulfide Oxidation
80
70
60
%
h 50
c
a
e 40
L
r
ev 30
liS
20
10
0
0 10 20 30 40 50 60
Sulfide Oxidation (%)

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.

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Hycroft Concentrate Treatment Test Work Study Update
Pressure Alkaline Oxidation Test Work
Au Leach vs Sulfide Oxidation
120
100
% 80
h
c
a
e 60
L
d
lo
G 40
20
0
0 20 40 60 80 100
Sulfide Oxidation (%)

Pressure Alkaline Oxidation Test Work
Ag Leach vs Sulfide Oxidation
120
100
% 80
h
c
a
e 60
L
r
e
v
liS 40
20
0
0 20 40 60 80 100
Sulfide Oxidation (%)

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Hycroft Concentrate Treatment Test Work Study Update
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.
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Hycroft Concentrate Treatment Test Work Study Update
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 (Na CO ) and sodium
2 3
bicarbonate (NaHCO ) in a hydrated crystal form known as sodium sesquicarbonate (Na C0 •
3 2 3
NaHC0 • 2H 0). Trona was found to be as effective or more effective than the other acid
3 2
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.

Alkaline Ambient Pressure Test Work
Au Leach vs Sulfide Oxidation
120
100
% 80
h
c
a
e 60
L
d
lo
G 40
20
0
0 20 40 60 80 100 120
Sulfide Oxidation (%)

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