Tepal Project Preliminary Economic Assessment — 2017 Technical Report

The Tepal Project preliminary economic assessment describes a proposed processing operation with separate sulphide and oxide plant designs, based on test work reported in Section 13 of the effective January 19, 2017 technical report.

Report context

This technical report, effective January 19, 2017, presents the preliminary economic assessment for the Tepal Project. The process design comprises two plants: a sulphide concentrator with a nominal capacity of 22,000 t/d and an oxide circuit with a nominal capacity of 5,500 t/d. The design relies on test work described in Section 13 of the report, with several design parameters requiring additional test work or engineering confirmation.

Processing route

The selected process consists of copper flotation of sulphides to produce a saleable concentrate, cyanide leaching of the first cleaner tailings and pyrite flotation concentrate, and cyanide leaching of oxide material in a carbon-in-leach (CIL) circuit. Both plants are designed to produce doré bars via a common adsorption, desorption and refining (ADR) plant.

Oxide plant design

Oxide feed is processed through the gyratory crusher one day out of five to provide 27,500 t to the crushed mineralized material stockpile, which has a 30,000 t capacity. The stockpile feeds secondary and tertiary crushing circuits at 5,500 t/d to produce a final product with 80% passing (P80) of 9.5 mm. The crusher product feeds one ball mill for further reduction to P80 of 143 µm. Cyclone overflow from the grinding circuit feeds the pre-leach thickener followed by the CIL circuit.

The CIL circuit consists of 10 tanks, each 10 to 10.5 m diameter by 12 m high, providing 24 hours of residence time. Each tank includes an agitator, carbon transfer pump and interstage screen. The average carbon concentration in the CIL circuit is expected to be approximately 25 g/L. Loaded carbon is transferred countercurrent to the slurry flow and collected at a rate of 3 t/d for transfer to the common ADR plant.

Lime slurry maintains protective alkalinity at a design pH of 10 in the first and second leach tanks. Cyanide is added to the circuit, and oxygen or air is sparged from the bottom of the leach tanks. CIL tailings flow by gravity onto a stationary safety screen to capture carbon particles before reporting to the cyanide destruction circuit.

Sulphide plant design

The sulphide concentrator processes 22,000 t/d through three stages of comminution. The primary gyratory crusher processes both sulphide and oxide ROM mineralized feed at 1,528 t/h. The crusher processes 27,500 t/d for four days to supply the sulphide stockpile with a five-day supply; on the fifth day, it processes oxide material.

The primary grinding circuit incorporates a SAG mill and ball mill at a processing rate of 996 t/h. The SAG mill feed size is P80 of 150 mm, with the ball mill product at P80 of 150 µm. The ball mill operates in closed circuit with cyclone clusters. Cyclone overflow feeds the copper rougher flotation circuit at approximately 35% solids by weight, with the target P80 of 150 µm. Cyclone underflow returns to the ball mill at approximately 70% solids, with a circulating load of approximately 300%.

Lime is added to the SAG mill feed belt to raise slurry pH to 10.5 to aid copper flotation. The SAG mill discharge, at 70% solids, passes over a screen; oversize pebbles are conveyed back to the SAG mill feed, with a pebble crusher included for harder Tizate material.

Copper flotation and regrind

The rougher flotation circuit consists of seven 100 m³ mechanically agitated cells with a total retention time of 16 minutes and a total mass pull of approximately 9.2%. Reagents include lime for pH control, 3418A and PAX as collectors, and methyl isobutyl carbinol as frother.

Copper rougher concentrate is pumped to the regrind circuit for reduction to a target P80 of 22 µm. Regrind cyclone underflow, containing approximately 70% of feed and at approximately 50% solids by weight, reports to the regrind mill feed. Cyclone overflow bypasses the regrind circuit.

Three stages of cleaner flotation produce the final copper concentrate. Third cleaner concentrate is the final product; third cleaner tailings flow to the second cleaner feed, second cleaner tailings flow to the first cleaner feed, and first cleaner tailings flow to the sulphide pre-leach thickener feed pump box.

Concentrate dewatering

The final copper concentrate is pumped to an 8 m diameter high rate thickener with flocculant addition. Underflow is thickened to 60% solids and pumped to an 8-hour concentrate stock tank. The pressure filter reduces moisture to approximately 8%. Filtered concentrate is loaded into trucks for transport to the nearest port.

Pyrite flotation and regrind

Copper rougher tailings feed six 100 m³ mechanically agitated pyrite rougher flotation cells providing 12 minutes of retention time and approximately 6.6% mass pull to concentrate. Pyrite concentrate is pumped to the regrind circuit for size reduction to P80 of 23 µm. Pyrite flotation tailings report to the final tailings pump box.

Sulphide CIL leaching

Pyrite concentrate combined with first copper cleaner tailings is thickened to approximately 45% solids. The CIL circuit for this feed consists of eight tanks, each 7 m diameter by 8 m high, providing eight hours of residence time. The tanks are the same design as the oxide leach tanks.

Loaded carbon from the sulphide leach circuit is sent to the common ADR plant. Sulphide CIL tailings are pumped to the cyanide destruction circuit where they are combined with oxide CIL tailings.

ADR plant

The ADR plant processes 3 t of carbon per day from each of the oxide and sulphide circuits, with an overall capacity of 6 t/d. Loaded carbon is treated in an acid wash with 3% hydrochloric acid to remove calcium deposits, magnesium, sodium salts, silica and fine iron particles. Organic foulants are removed after elution by thermal reactivation.

Carbon stripping uses a solution containing approximately 1% sodium hydroxide and 0.1% sodium cyanide at 140°C and 450 kPa. The strip column is carbon steel. An electric boiler serves as the primary solution heater, with a heat recovery heat exchanger preheating incoming solution.

Carbon regeneration uses an electric fired horizontal kiln with residual heat dryer. The kiln discharge is quenched and returned to the circuit. New carbon is added to compensate for attrition losses.

Gold electrowinning produces gold sludge washed off steel cathodes by high-pressure water. The sludge is filtered, dried, mixed with fluxes and smelted in an electric direct-fire induction furnace to produce gold doré, which is stored in a vault.

Cyanide destruction

Cyanide destruction of CIL tailings thickener underflow uses three mechanically agitated tanks, each with a capacity of 620 m³. The SO₂/Air process is used with process air sparged near the bottom of two 9 m diameter by 10 m high tanks for two hours. Lime slurry maintains a pH of 8.0 to 9.0, and copper sulphate is added as a catalyst. Sodium metabisulphite is dosed as the SO₂ source. The system is designed to reduce total cyanide concentration to less than 5 ppm CN WAD. No test work has been completed for cyanide destruction, and this is recommended for the next engineering stage.

Tailings

Combined oxide and sulphide leach tailings from the cyanide destruction circuit and pyrite rougher tailings report to the final tailings pump box and are pumped to the tailings pond. Water from the tailings pond is pumped back to the process water tank for reuse.

Water supply and process control

Process water is supplied primarily from tailings pond reclaim water, with the balance from fresh water tank and thickener overflow. The process communication system is based on an ethernet fibre-optic network, with radio-based communication to in-pit pump stations and the reclaim barge. A firewall router connects process and business communication systems.

Reagents

Reagents include lime, Aero 3418, MIBC, PAX, flocculants, caustic soda, sodium cyanide, sodium metabisulphite, copper sulphate and hydrochloric acid. Each reagent has its own preparation system with bulk handling, mixing and storage tanks. Dry lime is added to the SAG mill feed belt with seven days of storage. MIBC and HCL are supplied in 1 t totes.

Key reported parameters

Technical qualifications

The report identifies several limitations on the design basis. The pre-leach thickener settling rate of 0.75 t/h/m² is vendor-recommended with no test work available; the report notes "No test work available" for this parameter. The report recommends additional test work in the next engineering stage to confirm the Bond ball mill work index for the oxide circuit. The report also recommends additional test work to determine settling and filtration rates for copper concentrate. No test work has been completed for cyanide destruction, and the report recommends confirmation of design parameters in the next engineering stage. Additional test work is recommended to confirm carbon loading and design parameters for the ADR plant.

The SAG mill and ball mills were sized based on drop weight index and Bond ball mill work index for the Tepal North and South deposits and the Tizate deposit. Flotation cells were sized based on estimated slurry flow rates and retention times from laboratory tests, with typical scale-up factors applied to laboratory retention times.

Source: Tepal Project Preliminary Economic Assessment , 2017 Technical Report, effective January 19, 2017, Sections 17.1 through 17.3.13, 17.4, including Table 17.1 and Table 17.2.