Volta Grande Project — 2015 Technical Report

This feasibility-study process plant design for the Volta Grande gold project is based on metallurgical test work indicating free and finer gold grains in a non-refractory ore, with a two-phase expansion from 3.5 to 7 Mt/a using gravity concentration and a hybrid CIL/CIP cyanidation circuit.

Report context

This May 2015 NI 43-101 technical report presents a feasibility study for the Volta Grande Project owned by Belo Sun Mining Corp. The processing sections document the designed process plant, based on metallurgical test work conducted to date combined with industry best practices. The report describes a Phase 1 design capacity of 3.5 Mt/a with an expansion to 7 Mt/a in Year 3 (Phase 2).

Processing route

Process flow sheet selection

Geological evaluation and metallurgical testing of Volta Grande ores established that gold is present as free grains within quartz and also as finer grains within diorites. Test work indicated the presence of coarse gold and that the ore is generally non-refractory with respect to whole ore cyanidation. The standard processing route for this type of ore body is gravity concentration followed by cyanidation of the gravity tailings.

A hybrid carbon-in-leach / carbon-in-pulp (CIL / CIP) process consisting of leach tanks upstream of carbon-in-pulp tanks has been designed. This configuration aims to leverage the advantages offered by both the CIL (reduced capital cost) and the CIP (improved adsorption efficiency) configurations. The circuit is broadly referred to as a CIP circuit.

Crushing and coarse ore stockpile

A single C160 jaw crusher (or equivalent) was selected for both phases. When treating the initial coarse ROM size distribution, the crushing circuit is predicted to have a capacity of 4 to 4.5 Mt/a allowing a margin of capacity to treat Phase 1 production. To allow the crusher to accommodate the increased Phase 2 throughput, the ROM size distribution will be decreased. Modelling by Orica produced an optimized blast product distribution with a D100 of 650 mm and a D80 of 252 mm. When treating the fine distribution, the crusher is predicted to be capable of processing the expanded throughput of Phase 2. This prediction was confirmed by Metso. In Phase 2, the grinding circuit will require a crusher product with P80 <150 mm.

It is recommended that a plant trial should be undertaken in the first year of production to prove the capacity of the primary crusher. If the 7 Mt/a capacity is not achieved with a finer blast size, a second crusher of equal or smaller size will need to be installed. Layout space will be provided for this potential additional crusher.

Haul trucks will dump directly into the ROM bin. The ROM bin will be equipped with a static grizzly to prevent rocks >650 mm from entering. An apron feeder will extract rock from the ROM bin to feed the primary crusher via a vibrating grizzly. Undersize from the grizzly feeder will report directly to the crusher discharge conveyor and oversize will be crushed in the jaw crusher to a P80 of 150 mm.

The crusher discharge conveyor will deliver crushed ore to a 30,000 t coarse ore stockpile providing a buffer between mining and processing operations.

Grinding circuit

Grinding circuit modelling and mill sizing was completed by Orway Mineral Consultants (OMC). A single stage SAG mill was selected for the grinding duty of 3.5 Mt/a for Phase 1. The grinding circuit capacity will be expanded to 7 Mt/a with the addition of a pebble crusher and a secondary ball mill. The specific energy used for mill sizing was determined from the 85th percentile of the comminution test work results (Bond ball mill work index of 16.5 kWh/t and an A×b of 27.6), which were issued by SGS Chile in December 2013.

The SAG mill has been sized for a specific energy requirement of 25.7 kWh/t based on a F80 of 150 mm and a P80 of 75 µm, without the use of a pebble crusher. The SAG mill grate aperture size will be 20 mm in Phase 1 and will be increased to 70 mm in Phase 2. A pebble crusher (HP500 cone crusher or equivalent) will be added for the expansion to reduce the SAG mill specific energy to 12.9 kWh/t.

For Phase 2, the total required grinding specific energy is 25.9 kWh/t to grind 7 Mt/a from a F80 of 150 mm to a P80 of 75 µm. A secondary ball mill designed with a specific energy of 13.0 kWh/t will be added.

The SAG mill will be a 10.36 m diameter × 6.37 m effective grinding length (EGL) mill with a dual pinion drive with two 7 MW motors. The ball mill will be a 7.3 m diameter × 12.97 m EGL mill with the same motor configuration.

The cyclone cluster will consist of twelve ports with 4 × 660 mm cyclones in operation in Phase 1, increasing to seven plus two dedicated cyclones for feeding the gravity circuit in Phase 2. The cyclones will provide a target grind size of P80 75 µm.

Gravity circuit (Phase 2)

As the gravity circuit affects the leach / CIP circuit reaction time requirement more than recovery, the decision was made to defer the installation of the gravity circuit until Phase 2 to save on initial capital. To compensate, approximately 35 hours of total leaching residence time is provided in Phase 1 with 30 hours provided in Phase 2.

The gravity circuit is designed with a 0.015% mass pull and a nominal concentrate production of 2.9 t/d. The circuit will consist of two parallel lines each equipped with a trash screen, centrifugal concentrator and gravity tails return pumping equipment. The design capacity of each gravity concentrator is 100% of the fresh feed or 438 t/h. Operation of the gravity concentrator will be semi-batch.

Gravity recovery test work results indicate 37% Au recovery for Ouro Verde, 34% for Grota Seca and 37% for saprolite, for a total of 35% Au.

Intensive leaching (Phase 2)

Intensive cyanidation will be performed daily on the gravity concentrate produced during the previous day. Gravity concentrate will accumulate in the concentrate storage cone over a 24 hour period and be discharged by gravity into the intensive cyanidation reaction vessel. Intensive cyanidation will use a concentrated solution of cyanide and liquid oxidant to maximize leach kinetics. These intensive leaching conditions will allow accelerated dissolution of coarse gold particles within a period of less than 24 hours.

Pre-leach thickening

The 28 m diameter high rate pre-leach thickener was sized using the Pocock Industrial dynamic thickening test results of 3.86 m³/h/m² for an ore blend of 90% bedrock (Ouro Verde plus Grota Seca) and 10% saprolite at a total throughput of 3.5 Mt/a.

In Phase 2, it is proposed to pass the thickener feed through dewatering cyclones to reduce the duty on the installed thickener. This concept requires further development. If this concept cannot be substantiated, other options include either installing a larger 40 m diameter thickener or implementing design and layout modifications to allow for the addition of a second 28 m diameter thickener.

Leach / CIP circuit

The metallurgical test work leaching kinetics were relatively slow, requiring approximately 32 hours to obtain acceptable recoveries. A total of three leach tanks and six adsorption tanks will be installed in Phase 1. The circuit is designed with a total leaching residence time of approximately 35 hours, with approximately 20 hours in the leach tanks and the remaining 15 hours in the CIP tanks. When the throughput is increased to 7 Mt/a in Phase 2, four additional leach tanks will be installed to maintain a total leach / CIP residence time greater than 30 hours.

Leach / CIP recovery test work results indicate 92% Au recovery for total ore in Phase 1 and 91% in Phase 2. Overall recovery (including gravity) is 92% in Phase 1 and 93% in Phase 2.

Elution and electrowinning

The elution circuit will be a batch AARL (Anglo American Research Laboratories) type circuit. Loaded carbon will be recovered on the loaded carbon recovery screen and directed to the acid wash column. Dilute HCl acid will be used to remove contaminants, such as calcium, from the loaded carbon.

In Phase 1, acid washing and elution processes will be performed in a single 10 t column. In Phase 2, a second column of equal size will be installed, with one column dedicated to acid washing and the other to elution.

One electrowinning cell will be installed in Phase 1 and a second parallel cell along with a second pregnant solution tank will be added in Phase 2. Direct current will be passed through stainless steel anodes and stainless steel wool mesh cathodes to deposit gold sludge onto the cathodes. Sludge will be periodically removed from the cathodes in-situ using a pressure washer.

Carbon regeneration (Phase 2)

Installation of the carbon dewatering screen, the regeneration kiln, the carbon quench tank and the carbon sizing screen will be deferred to the Phase 2 expansion. A horizontal rotating kiln will be employed for carbon regeneration at a rate of 600 kg/h.

During Phase 1, make-up carbon will be fed directly into the last active CIP tank. A consumption rate of 0.035 kg/t has been assumed to account for the absence of the regeneration kiln and some carbon fouling. The carbon consumption rate was then assumed to decrease to 0.025 kg/t in Phase 2. In Phase 1, any carbon fines in the fresh carbon will not be removed prior to entering the circuit, which may result in increased gold losses.

Cyanide destruction

The SGS 2014 (February and March) batch test work observed that a cyanide destruction residence time of 3.2 hours and an SO₂ addition of 4.5 kg/kg CN⁻ were required to destroy cyanide to less than 1 ppm CN WAD. A more typical 1 hour residence time and SO₂ addition of 4 kg/kg CN⁻ has been used for plant design, with the acknowledgement that further test work is required to validate these assumptions.

The SO₂ / air process will be employed to oxidize free and weak acid dissociable (WAD) cyanide species. During Phase 1, the cyanide destruction process will be performed in one tank. A second cyanide destruction tank will be installed in Phase 2 to maintain the 1 hour residence time.

Key reported parameters

Ore characterization (test work basis)

Technical qualifications

Specific limitations and risks identified in the report include:

Additional testing and engineering work is required for the cyanide destruction, Phase 2 pre-leach thickening and decant return purification areas.

Risks to capital increase include: additional cyanide detoxification residence time resulting in larger tank