Memorandum - Allied Nevada€¦ · · 2014-06-06Memorandum To: Randy Buffington Cc: Jeff Snyder ... Sulfide Oxidation (%) Pressure Alkaline Oxidation Test Work Au Leach vs Sulfide

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.


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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.


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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.

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Sulfide Oxidation (%)

Au Leach vs Sulfide Oxidation All Oxidation Tests


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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.

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Au Leach vs Sulfide Oxidation Cleaner Conc Only

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Au Leach vs Sulfide Oxidation Rougher Conc Only


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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.

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Chlorine Oxidation Test Work Au Leach vs Sulfide Oxidation


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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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Chlorine Oxidation Test Work Ag Leach vs Sulfide Oxidation


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Pressure Alkaline Oxidation Test Work Au Leach vs Sulfide Oxidation

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Pressure Alkaline Oxidation Test Work Ag Leach vs Sulfide Oxidation


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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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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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Alkaline Ambient Pressure Test Work Au Leach vs Sulfide Oxidation


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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.

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Alkaline Ambient Pressure Test Work Ag Leach vs Sulfide Oxidation


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Alkaline Ambient Pressure Test Work Time vs Sulfide Oxidation @ 75C & 60C

Rougher Concentrate @ 44 microns

Temperature 75C Rougher


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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.

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Temperature 60C Cleaner


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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


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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.


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Test work has shown that Trona consumption is directly related to the amount of sulfide

oxidation achieved, as is shown in the following graph.


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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.


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The following picture shows the oxidized sulfide slurry using 100, 200, and 300 kg/t Trona

additions.


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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;


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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.


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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.


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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:


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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.


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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


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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


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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


Page 28

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.


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


Page 30

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


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


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 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.

Documents

Memorandum - Allied Nevada€¦ · · 2014-06-06Memorandum To: Randy Buffington Cc: Jeff Snyder ... Sulfide Oxidation (%) Pressure Alkaline Oxidation Test Work Au Leach vs Sulfide