Sanbrado Gold Project — Feasibility Study

The Sanbrado Gold Project feasibility study describes a conventional free-milling gold processing route with a SABC grinding circuit, gravity recovery, and carbon-in-leach (CIL) facilities for a 2.0 Mtpa open pit and underground operation in Burkina Faso.

Report context

This article is based on the NI 43-101 Technical Report: Open Pit and Underground Feasibility Study, Sanbrado Gold Project, Burkina Faso, prepared for West African Resources Limited. The feasibility study was completed to a bankable feasibility level and incorporates metallurgical test work programmes conducted from 2012 to 2018. The process plant design is based on the results of this test work and applies a conventional flowsheet suited to free milling non-refractory gold ores.

Processing route

Ore types and feed blending

Four major ore types were assessed to identify variation in plant performance and operating conditions: Fresh (FRS), Weakly Oxidised (WOX), Moderately Oxidised (MOX) and Strongly Oxidised (SOX). Processing characteristics such as grind size and reagent conditions are similar for each ore type, and ore types can be blended to optimise mining and plant throughput.

The proposed feed blend comprises 14.6% SOX, 32.4% MOX, 11.1% WOX and 41.9% FRS as the design basis. The average feed composition to the plant over life of mine will comprise 58.9% oxide and 41.9% fresh/transitional material. Ore will be stockpiled by source, weathering and grade type, and blended to optimise mill throughput, feed grade, and metallurgical recovery. Mill feed blending will occur by feeding different ore types stockpiled at the ROM pad into the crusher circuit.

Comminution test work and design

Comminution test work was completed using diamond core including a range of rock types and composites. A full suite of comminution work included JK Parameter determination for SAG milling, conventional Rod and Ball Mill Work Index testing, Levin tests on SOX material, Crushing Work Index, Abrasion Index and UCS testing.

Test work data confirms the ore is very soft in the SOX/MOX zone with Bond Work Indices ranging from 4 kWh/t to nominally 8 to 9 kWh/t and A·b of 150. The FRS/WOX zone is medium hard to very hard with a Bond Work Index of nominally 22 kWh/t and A·b values of the order of 33. The crushing work index varies between 4.1 kWh/t for SOX/MOX and 10.8 kWh/t for FRS/WOX. Abrasion Index results range from 0.098 to 0.351, signifying material that is non-abrasive to slightly abrasive.

The comminution circuit selected is a SABC (semi-autogenous, ball, crush) circuit fed with primary crushed ore. The SAG mill is 6.7 metres diameter by 3.8 metres effective grinding length with an installed motor power of 3 megawatts. The ball mill is 5 metres diameter by 8.8 metres effective grinding length with an installed motor power of 3.4 megawatts. Both mills will have variable speed drives to allow adjustment for changes in ore characteristics. The SAG mill will operate with a duty ball charge of 10% with 125 mm balls at a maximum 13% ball charge. The ball mill will operate with a duty ball charge of 30% with 50 mm balls at a maximum of 35% ball charge.

The grinding circuit is designed for a nominal throughput of 250 tph with a design grind product size of 80% passing 90 microns. The circuit will be capable of producing a P₈₀ product transfer size of 75 microns when beneficial for recovery on certain blends.

Primary crushing and stockpile

The ROM bin will be fed blended ore from the ROM stockpiles using a front end loader. A 1,500 mm wide apron feeder will direct feed into the primary jaw crusher with a feed opening of 1,200 x 1,400 millimetres, accepting nominal minus 600 millimetre rocks. Crusher jaw plates will be adjusted to a 150 millimetre closed side setting. Crushed product will be delivered to a 15,000 tonne total capacity conical stockpile, sized to provide up to a maximum 12 hours live feed capacity at the design throughput of 250 tph. The total stockpile capacity is equivalent to 60 hours of feed at design throughput.

Grinding and classification circuit

Primary crushed ore will be fed via mill feed conveyor to the SAG mill. Mill slurry will discharge to the mill discharge vibrating screen which separates pebbles from the discharge. Oversize material will be directed either to a scats bunker or to the pebble crusher circuit. The pebble crusher will operate at 75 tph design rate with a close setting of 11 to 14 millimetres, and product will return to the mill feed conveyor.

The slurry in the mill discharge hopper will be pumped to an 18-pack classifying cyclone cluster with 13 operating cyclones and five standby. Cyclone overflow will report to the leaching circuit with a P₈₀ of 90 micron, while coarse underflow will be fed back to the ball mill.

Gravity recovery

The gravity scalping screen underflow will feed two centrifugal gravity concentrators acting as gravity roughers. The concentrators will remove 173 kg/h of concentrate into a rougher concentrate storage tank, which is part of the intensive leach reactor. Tailings from the gravity concentrators will be fed back into the ball mill discharge hopper. Oversized product from the scalping screens will be sent back to the ball mill for further crushing.

The intensive leach reactor is a batch process, intensively leaching the gravity concentrate to dissolve contained gold. Pregnant liquor will be pumped to the electrowinning module tank for electrowinning in a dedicated cell. Solution from the electrowinning cell will be recirculated back to the electrowinning module tank for further leaching. The gold sludge collected from the gravity circuit electrowinning cell will be smelted separately, allowing for separate metallurgical accounting of the gravity circuit.

CIL circuit

The CIL circuit will consist of a leach feed thickener followed by seven adsorption tanks. The leach feed thickener will be used to remove excess water, with thickener overflow fed to a process water tank. The thickener underflow will be pumped by leach feed pumps in a duty/standby arrangement to a distribution box directing slurry to either the first or second CIL tank.

The CIL train will comprise seven tanks of nominal dimensions 11 metre diameter and 12.5 metres high, each with a nominal capacity of 1,200 m³. This provides a slurry residence time of 24 hours at 50% solids by weight. Each tank will be fitted with recessed impeller type carbon transfer pumps. Oxygen sparging will be installed in the first three tanks through hollow shaft agitators, with nozzles for oxygen injection through the sides of the tanks.

A vibrating carbon safety screen will be located adjacent to tank 6, collecting any carbon escaping from tank 7 or tank 6 in the event tank 7 is offline. The screen undersize will be gravity fed to a tailings hopper, then pumped to the tailings storage facility.

Elution, electrowinning and smelting

The acid wash and rinse cycles will be performed in a 10 m³ capacity rubber lined acid wash hopper. After rinsing, carbon will be discharged into the elution column with a volumetric capacity of 10 m³, capable of holding four tonnes of carbon.

The strip solution will be injected with sodium hydroxide and sodium cyanide and preheated to 130°C by an in-line elution heater. After approximately one bed-volume of caustic solution pre-soaks the carbon, five bed volumes of hot rinse water will be passed through the column, followed by one bed volume of cold rinse water. Elution of gold from carbon is expected to take about 6 hours, with pregnant solution collected in one of two pregnant solution tanks feeding the electrowinning cells.

The first 3.5 bed volumes of pre-soak and hot rinse water will be returned via eluate filters to the pregnant solution tanks through a recovery heat exchanger. The last 3.5 bed volumes of hot and cold rinse water will be directed to the intermediate solution tank supplying feed water to the first half of the next strip.

At the completion of the elution cycle, barren carbon will be pumped from the elution column to the regeneration kiln carbon feed hopper located above the regeneration kiln, which sits above CIL tank 6. Carbon will be either regenerated in the kiln or discharged directly into CIL tank 6 depending on carbon activity level. The rotary kiln feed chute will drain residual and interstitial water prior to carbon entering the kiln. Kiln off-gases will dry the carbon before entry, and regenerated carbon will discharge back into CIL tank 7.

Gold sludge from the elution circuit electrowinning cell cathodes will be washed in the cathode wash box, transferred to the calcine oven for oxidation, then direct smelted in an LPG fired smelting furnace to produce final gold doré bars.

Tailings disposal

Underflow from the carbon safety screen will gravitate to a tailings hopper, discharged by duty/standby tailings pumps to the tailings storage facility. The tailings pipeline will be a 315 mm OD HDPE pipe installed above ground, with buried sections at road crossings. Leaks will be detected by comparison between two flow meters, one at the plant and one at the tailings storage facility. The tailings and decant return pipelines will be laid in a fully bunded and lined trench between the process plant and the TSF.

Reagents

Lime: Quicklime will be delivered to site in shipping containers with 20 tonnes of quicklime in one tonne bulk bags. The lime handling system includes a bag unloading system, pneumatic conveying, a 100 tonne lime silo, and a rotary valve delivering lime onto the mill feed conveyor. A volume equivalent to 5.2 days use is stored per batch.

Sodium hydroxide (caustic soda): Caustic soda will be delivered in 25 kilogram bags and mixed with raw water in a skid mounted caustic mixing system to create a 25% w/w solution. The mixing system comprises a bag splitter, 1 m³ mixing tank and agitator. A caustic dosing pump will deliver solution to the elution circuit. A volume equivalent to 1.7 days use is mixed and stored per batch.

Cyanide: Cyanide will be delivered in 20 tonne shipping containers in one tonne bulk bags, mixed with raw water to create a 30.5% w/w solution. The mixing system includes a hoist, bag splitter, 10 m³ mixing tank and agitator. The mixed solution will be transferred by pump to a separate 22 m³ storage tank, with duty/standby recirculating pumps circulating solution through a plant ring main. A dosing pump will deliver cyanide to the elution circuit. The storage tank holds a buffer volume of nominally 3.6 days.

Hydrochloric acid: Concentrated hydrochloric acid (32% w/w) will be delivered in 1,185 kilogram intermediate bulk containers, with a dosing pump transferring acid to the carbon acid wash hopper. The concrete containment bund around the acid preparation area complies with dangerous goods statutory requirements and is protected with coating to prevent acid damage.

Process control systems

The plant control system will comprise Process Logic Controllers (PLCs) beneath a Supervisory Control and Data Acquisition (SCADA) network layer. GE Fanuc RX3i PLCs and Citect SCADA are proposed. The PCS will provide plant status monitoring, fault annunciation and logging, drive diagnostics, and trending for all analogue process parameters, powered by UPS equipment providing thirty minutes supply after total power failure.

SCADA terminals will be installed in the CIL Control Room, Crusher Control Room, Desorption Control Panel, and Electrical Supervisor's Office. The SCADA system will be configured so that only Wet Plant drives can be controlled from the Main Control Room, only Crusher Drives from the Crusher Control Room, and only the Desorption Sequence from the Desorption Control Panel. Password protected user accounts will limit access to control functions by operator level.

Two SCADA terminals will exist in the Main Control Room providing redundancy so that if one terminal fails, the Wet Plant can still be operated from the other terminal. The Desorption terminal will be installed in a stand-alone metal cabinet with perspex window, located within the desorption area beside the Electrowinning Rectifiers.

Electrical reticulation

Power will be accepted at the plant main substation housing the plant main 11 kilovolt distribution board. Power distribution within the plant area will be at 11 kilovolts and 415 volts. Substation buildings will be demountable/transportable type, fully air-conditioned to maintain internal air temperature at 25°C maximum. Equipment will be designed for continuous operation at rated output with substation ambient temperature of 40°C maximum.

Substations will be provided for the crushing area, wet plant, SAG and ball mills, plant buildings, and mining contractor. All transformers will be pad mounted with compound fencing and underground earthing. High voltage switchgear will be supplied for the SAG mill and ball mill so that drive isolations can be performed under site maintenance control without requiring power station operator involvement.

Key reported parameters

Technical qualifications

The source describes a proposed plant design. Operating performance and final construction configuration are not stated unless explicitly reported.