Gordon Chiu Building J01 Department of Chemical Engineering Maze Crescent University of Sydney NSW 2006 Australia

Intec Ltd Superior and Sustainable Metals Production

ASX Code: INL ABN 25 001 150 849

Telephone: +612-9351-6741

Facsimile: +612-9351-7180 Email: mail@intec.com.au

Website: www.intec.com.au

The Intec Gold Process (IGP)

1. Introduction

The Intec Gold Process (IGP) has been developed as a halide-based alternative for the recovery of gold from refractory sulphide deposits. The development of such deposits is generally by way of flotation of the ground ore to produce a concentrate, which is subsequently treated to oxidise sulphide minerals in a pre-treatment step, culminating in the extraction of the gold from the oxidation residue using cyanide.

Commercially available options for the pre-treatment step include roasting, pressure oxidation (POx) and biological oxidation (BiOx). The IGP differs from the hydrometallurgical POx and BiOx options in that a halide rather than sulphate medium is used. Gold is insoluble in sulphate, whereas halides, like cyanide, form strong complexes with gold to facilitate its dissolution and subsequent recovery by adsorption onto activated carbon.

Halides are weaker ligands than cyanide, requiring an acidic environment (pH <2) and higher solution temperature and potential (Eh) to achieve the same gold extraction efficiencies.

In the treatment of refractory sulphides, the use of the halide medium allows sulphide oxidation to be performed concurrently with gold dissolution. Once the gold-laden solution is separated from the oxidised mineral slurry, the dissolved gold can be recovered by adsorption onto activated carbon, which is subsequently eluted with cyanide for the ultimate recovery of gold metal by electrowinning.

The IGP therefore differs from current commercial practice where gold is extracted from the oxidation residue using cyanide, requiring a separate dedicated leach circuit with the costly requirement for residual cyanide destruction.

The IGP is put into context in terms of current hydrometallurgical practice in Table 1.

Leach Liquor Process Primary Secondary

Oxygen Source

Temperature (0C)

Pressure (Atm.)

Retention Time

IGP Chloride None air or O2 90-95 1 6-20 hrs BiOx Sulphate Cyanide * air 45-75 1 100-150 hrs POx Sulphate Cyanide * O2 >200 >30 1-2 hrs

* Via a conventional CIL/CIP treatment plant.

Table 1: IGP characteristics relative to competing refractory gold processing technologies

There are a number of factors that can render a gold-bearing ore refractory, as shown in Table 2.

Type Causes of Refractory Characteristics Liberation Physical locking in silicates, sulphides, carbon, etc. Occlusion Passivation due to formation of a chemical layer. Chemistry Formation of auriferous compounds e.g. gold tellurides and aurostibnite. Substitution Elemental replacement by gold in mineral lattice e.g. “solid solution” gold in pyritic ores. Adsorption Adsorption of dissolved gold by ‘active’ carbonaceous material in the ore pulp.

Table 2: Causes of refractory characteristics

The IGP has been developed specifically to treat concentrates produced from those refractory ores falling into the latter two categories of “substitution” and “adsorption”. The major proportion of the world’s gold reserves fall into these two categories, which are dominated by iron sulphides such as arsenopyrite and pyrite, occurring either separately or more commonly in combination. Further complication occurs when “active” carbon is also present.

2. IGP Process Chemistry and Flowsheet

The IGP Process flowsheet and chemistry are described below for the treatment of refractory gold concentrates containing the following mineral types:

• Arsenopyrite

• Arsenopyrite plus pyrite

• Arsenopyrite plus pyrite plus carbon.

Arsenopyrite

The presence of arsenic in refractory gold concentrates is chiefly in the form of arsenopyrite (FeAsS). Gold is typically “locked” in this arsenopyrite principally as a lattice-bound species, often referred to as solid solution, rather than as native gold and consequently requires the complete destruction of the arsenopyrite lattice for its liberation. Destruction of the arsenopyrite lattice in the IGP is achieved by chemical oxidation according to the following overall reaction:

FeAsS + 2O2 FeAsO4 +S (1)

The oxygen does not oxidise the arsenopyrite directly, but acts through several intermediate steps, as its solubility in the process liquor is exceedingly low.

The oxygen, supplied directly from air sparged into the leach at atmospheric pressure, is initially used to generate a soluble oxidant in the form of cupric ion (Cu2+) according to the following reaction:

2Cu+ + ½O2 + 2H+ 2Cu2+ + H2O (2)

This reaction takes place at the interface between the air bubbles and the process liquor. The cupric ion then oxidises the arsenopyrite according to the following reaction:

FeAsS + 7Cu2+ + 4H2O H3AsO4 + Fe2+ + S + 5H+ + 7Cu+ (3)

The ferrous and cuprous reaction products are subsequently oxidised by further air sparging according to reaction (2) and the following reaction:

Cu2+ + Fe2+ Cu+ + Fe3+ (4)

In the presence of ferric ion, the arsenic acid readily forms insoluble ferric arsenate according to the following reaction:

H3AsO4 + Fe3+ FeAsO4 + 3H+ (5)

Ferric arsenate formed in the high chloride electrolyte and under the operating conditions used in the IGP is typically crystalline and stable in the environment.

The action of the Cu2+/Cu+ couple is supplemented by the Fe3+/Fe2+ couple as a background concentration of iron is always present in the process liquor. The potential achievable under the influence of the Cu2+ and Fe3+ is in the region of 850mV (versus SHE) in the presence of oxygen. This potential is sufficient for the dissolution of gold, due to the stabilisation of the gold by the formation of a chloride complex according to the following reaction:

3Cu2+ + Au + 4Cl - AuCl4- + 3Cu+ (6)

The oxidation is carried out at a temperature of 90-95oC in an 8M-chloride electrolyte containing 20-40g/l Cu2+ ion.

Pyrite

The oxidation of pyrite (FeS2) in the IGP is achieved via the same series of intermediate reactions as employed for arsenopyrite oxidation according to the following overall reaction:

4FeS2 + 15O2 + 2H2O 8SO42- + 4Fe3+ + 4H+ (7)

It should be noted that the pyritic sulphur is oxidised all the way to sulphate in contrast to the arsenopyritic sulphur that is only oxidised to the elemental state.

Pyrite is more refractory than arsenopyrite, requiring a finer grind size to achieve acceptable reaction kinetics as explained in Section 3. However, individual pyrite samples exhibit variable reactivity that is thought to be influenced by arsenic substitution for a portion of the sulphur in the crystal lattice. Such pyrite is often termed arsenical pyrite and the higher the arsenic contamination the more the pyrite reactivity approaches that of true arsenopyrite with an As/S ratio of one. For particularly refractory examples of pyrite, a higher oxidation potential than is achievable with air may be needed. In these cases, pure oxygen may be required.

Intec Gold Process (IGP)

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The reaction proceeds through the Cu2+/Cu+ couple as for arsenopyrite at a temperature of 90-95oC in the same liquor used for arsenopyrite oxidation according to the following reaction:

FeS2 + 14Cu2+ + 8H2O 2SO42- + Fe2+ + 16H+ +

14Cu+ (8)

The Cu+ and Fe2+ are oxidised by air or oxygen (if required) sparging according to reactions 2 and 4 with the ferric and sulphate formed being precipitated as hematite and anhydrite by the addition of limestone to a pH of approximately 1-1.5 according to the following reactions:

2Fe3+ + 3H2O Fe2O3 + 6H+ (9)

2H+ + SO42- + CaCO3 CaSO4 + H2O + CO2 (10)

Limestone addition is controlled to maintain a stable solution and to precipitate excess iron.

Process Flowsheet

The IGP flowsheet for the treatment of mixed arsenopyrite/pyrite gold concentrates is presented in Figure 1.

By-Product(Zn, Pb, Ni, Cd, Mn,Mg)

SL

WaterLimestone

Leach Residue

Gold Dore

Gold Concentrate

Air

Activated Carbon Column

LimeLimestone

pH9Precipitation

Copper Recovery

SINGLE-STAGE LEACH

GOLD RECOVERY

Activated Carbon

IMPURITY BLEED

Copper RecoveryPlus pH9

Precipitation

SL

SL

Figure 1: IGP simplified flowsheet for the treatment of mixed arsenopyrite/pyrite gold concentrates

Arsenopyrite plus Pyrite plus Carbon (Double Refractory)

The impact of naturally occurring carbon in the processing of gold concentrates is largely a function of its grade and activity. At the lower range of carbon content, organic additives may be used to inhibit gold adsorption. In these instances, the oxidation of the arsenopyrite and pyrite is as described previously.

When the content of carbon becomes significant, the effectiveness of inhibition is greatly reduced as “preg-robbing” of the gold by the naturally occurring carbon increases. In this instance, the destruction of the carbon by roasting is the main treatment option that is practiced. This can be a relatively complex process, as gold extraction from the resulting calcine is affected by the roasting

conditions. Further, the optimal conditions for pyrite roasting differ from those for arsenopyrite, necessitating a two-stage roasting process.

The use of the IGP prior to roasting can selectively leach arsenic and sulphur to simplify subsequent roasting, which in this instance becomes a simpler single-stage process. Further, the removal of arsenic and sulphur reduces the duty for off-gas cleaning from roaster operations as As2O3 and SO2 are greatly reduced. The impact is thus one of significantly reduced capital and operating costs in the roasting step.

The IGP in the case of a double-refractory concentrate consists of the five process steps of arsenic leach, pyrite leach, arsenic precipitation, carbon oxidation by roasting and gold leaching as shown in Figure 2.

Intec Gold Process (IGP)

4

SOx

Gold Concentrate

Thickener

Arsenopyrite Leach

Ferric Arsenate Residue

Bleed Circuit

ContaminantBy-Products

(Zn,Pb, Ni, Cd, Mn, Mg)

Roaster

PyriteLeach

Air

LS

FeAsO4

Precipitation

AirLimestone

As2O3

LS

Carbon Column

H2O

H2O

Air

Fuel

SecondaryGas Cleaning

Recycle Wash Water Limestone

CaSO4 + CaSO3

Gold Leach

Air

Leach Residue

Gold Dore

Carbon Column

Lime

Limestone

PrimaryGas Cleaning

LS

Figure 2: IGP flow diagram for double-refractory gold concentrate.

The double-refractory concentrate is firstly fed to an arsenic leach, where arsenopyrite is oxidised according to equation 3, using the high-potential cupric solution generated during the arsenic precipitation step. Temperature for reaction is typically 90-95oC and retention time is 3 to 4 hours. When reaction is complete, the slurry is separated in a thickener with the arsenic-bearing thickener overflow sent to gold recovery and arsenic precipitation as described below and the underflow sent to the pyrite leach.

Pyrite oxidation then proceeds according to equation 8 using a portion of the liquor from arsenic precipitation, however the acid generated from sulphate formation is not neutralized as described in equation 10, but utilized to maintain all leached iron in soluble form. The high oxidation potential of the liquor has the added benefit of oxidising residual arsenopyrite with the high acidity maintaining the arsenate formed in solution.

Retention time in the leach is a function of the pyrite grade and reactivity and ranges from 6 to 20 hours. At the completion of the pyrite leach, the solids are largely depleted of iron, arsenic and to a lesser extent sulphur (arsenopyritic sulphur is only oxidised to the elemental state and is not further oxidised during the pyrite leach). The pyrite-leach slurry is sent directly to pressure filtration, with the cupric/ferric solution directed to gold recovery and

arsenic precipitation. The solids are washed and sent to the subsequent roasting operation.

The liquor from the two leach operations are combined and passed through carbon columns to recover any gold from solution that has not been “preg-robbed” by the active carbon in the concentrate prior to passing to arsenic precipitation where air is sparged to precipitate arsenic and regenerate the cupric solution for return to the arsenic leach according to equations 2, 4 and 5. Excess acid is neutralized with limestone according to equation 10.

Limestone is added only after air addition is complete in order to maintain a high iron background during iron arsenate formation, which further enhances its environmental stability. The precipitated solids are separated by pressure filtration where they are washed prior to discharge. The filtrate represents the regenerated solution for leaching operations.

The calcine produced during the roasting operations for carbon oxidation is now suitable for the leaching of gold using the same regenerated leach liquor used for both the arsenopyrite and pyrite leaches as described in Section 2. The leached gold is recovered from the liquor on activated carbon after leach residue filtration and washing, with the barren liquor sent to the arsenic precipitation circuit.

Intec Gold Process (IGP)

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3. Concentrate Grind Size

Concentrates are typically received in the size range of 80% passing 70-100 microns. Tests have indicated that reaction kinetics are significantly enhanced when concentrates are reground to a finer size that is dependent on the characteristics of each individual concentrate.

Where arsenopyrite is the sole gold-bearing mineral, a size of 80% passing 30-40 microns has proven adequate to achieve good gold extraction and an acceptable leach retention time.

Where gold is locked in pyrite, the grind size will principally depend on the reactivity of the pyrite, which as previously explained, can vary greatly. For a highly active pyrite, the grind employed for arsenopyrite is used, but more refractory pyrite examples require finer grinding. This may extend to an ultra-fine grind in the most refractory cases.

4. Environmental Advantages of the IGP

In addition to optimising project economics, the gold industry’s worldwide focus on the development of improved processing techniques for refractory gold deposits has also been driven by the need for more environmentally acceptable processing routes.

Conventional processes treat the entire concentrate feedstock with cyanide such that the final residue (after gold extraction) is contaminated with the cyanide leachant. Thus the residues suffer from numerous inherent risks – both real and perceived – associated with cyanide usage and disposal and some projects have been vetoed for this reason. Cyanide is invariably disposed of into open-air tailings dams from whence it is lost, either ideally by natural decay or by unwanted leakage. Alternatively, it is expensively destroyed by chemical oxidants such as hypochlorites. On the other hand, the IGP leach residue contains only minor residual chlorides and is essentially the same as the residue from the Intec Copper demonstration plant that passed under EPA test procedures in New South Wales as being suitable for disposal in a landfill.

The IGP therefore has a major advantage over all other refractory gold processes in that it is unnecessary to include a secondary cyanide leach to recover gold from pre-treated materials; rather the gold is recovered directly onto conventional carbon from the primary leach solution. Retention time for gold adsorption is 10-15 minutes, which is similar to conventional practice for cyanide systems.

Gold loading onto the carbon is typically 2-5% w/w due to the relatively high gold concentrations in such solutions (typically 10-100mg/l), which are a consequence of the typical high gold grade of the concentrate. The gold is recovered by conventional elution or by burning of the carbon, with the choice depending on the economics of the individual situation.

The IGP’s ability to dispose of by-products such as arsenic and sulphur in a responsible manner is of similar environmental importance. Roasting creates toxic arsenic trioxide (As2O3), which must be carefully stored and eventually disposed of at considerable expense. In contrast, the IGP creates crystalline ferric arsenate (FeAsO4.2H2O) in its most stable form, similar to the naturally occurring mineral scorodite.

The presence of impurities in the feed concentrate (such as Cd, Mn, Mg, etc.) has no detrimental effect on either the leaching or arsenic precipitation operations. Nevertheless, a method for the management of impurities is required. This is achieved via precipitation from a bleed of the regenerated cupric solution with the purified brine returned to the process. It is important to note that the IGP does not generate any liquid effluents or gaseous emissions and that all impurities are produced as solid by-products.

Limestone is added to the bleed to adjust the pH to 3.5, precipitating residual iron and copper, which are removed by filtration and recycled to the leach. Impurities, such as Cd, Mn and Mg, are then removed via slaked lime addition at pH 9 to form insoluble oxides that are recovered by filtration for disposal.

In oxidising arsenopyrite and pyrite, the IGP produces elemental sulphur and anhydrite respectively. These are stable residues compared with the sulphate residues produced by pressure oxidation and the even less stable sulphate residues produced by bacterial oxidation.

5. Materials of Construction

The IGP unit operations are virtually the same as those of the Intec Copper Process that has been operated at a 1tpd demonstration-plant scale for approximately 1 year. The same materials of construction as were proven in that plant will be utilized for the IGP.

These materials include conventional fibreglass-reinforced plastics (FRP) for tanks and piping and conventional plastics for pumps.

Intec Gold Process (IGP)

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Mixers are made from titanium to withstand the corrosive environment as is standard practice in pressure oxidation processes.

Many years of operating experience have been gained by the nickel industry in the production of nickel metal from nickel sulphide matte in a very similar chemical matrix, which significantly reduces the level of risk associated with the IGP.

6. Economic Advantages of the IGP Intec has commissioned a comparative cost analysis comparing the economics of the IGP against BiOx and Pox based on first quarter 2004 costs. Roasting was not included in the comparative cost analysis for the reasons outlined earlier.

J R Goode and Associates, an internationally respected gold metallurgical consulting company based in Toronto, Canada, undertook the process costing for BiOx and POx. H.G. Engineering (also of Toronto) undertook the process costing for the IGP. Both analyses assumed a North American location and a concentrate feed material that was an average based on analysis of 19 different refractory gold concentrates (gold grade of 59 g/t and 20.2% sulphur). The battery limits for the study included the oxidation and gold recovery circuits but excluded flotation and concentrate regrinding. The degree of regrinding needed for each of the processing routes will differ depending on the mineralogy of each concentrate type. The IGP oxidation circuit assumes the use of air as the source of oxygen.

The results of the comparative cost analysis are presented in Figures 3 and 4 for a gold concentrate throughput rate of 50,000tpa.

0

2

4

6

8

10

12

14

16

18

20

IGP BiOx POx

US$

mil

lion

Figure 3: Comparative capital expenditure for

50,000tpa of gold concentrate

0

10

20

30

40

50

60

70

80

90

100

IGP BiOx POx

US$

per

ton

ne

of c

once

ntr

ate

Figure 4: Comparative operating costs for

50,000tpa of gold concentrate

As shown above, the IGP has a significant cost advantage over both BiOx and POx in the processing of refractory gold concentrates.

7. Conclusion

The IGP represents a new approach to the recovery of gold from refractory sulphide deposits that offers significant benefits in reduced capital and operating costs.

Significant environmental benefits also flow from the elimination of cyanide that in some countries is becoming more and more difficult to permit.

Intec’s successful conclusion to the laboratory-scale development of the IGP has led to the design of a pilot-scale plant in Sydney, which will be constructed and commissioned during the second half of 2003.

Enquiries should be directed to:

Mr A John Moyes Technical Director Intec Ltd Gordon Chiu Building J01 Department of Chemical Engineering Maze Crescent University of Sydney NSW 2006 Australia

Telephone: +612-9351-6741 Facsimile: +612-9351-7180 Email: john@intec.com.au Website: www.intec.com.au