Cerro Moro Gold – Silver Project — 2012 Technical Report

This 2012 technical report describes a proposed treatment plant design incorporating flotation, gravity recovery, intensive leaching, and Merrill Crowe processing for gold-silver mineralized rock.

Report context

This technical report, dated 2012 and titled “Cerro Moro Gold – Silver Project,” presents a preliminary economic assessment for the project. The report details a proposed treatment plant capable of processing 474,500 t/y of gold-silver mineralized rock from various deposits. The plant design incorporates both flotation and gravity recovery stages to maximise recovery from variable mineralogy, particularly of silver minerals. Due to high silver to gold ratios for the deposits (overall average ratio of 60:1 silver to gold), the Merrill Crowe (zinc cementation) process has been included instead of adsorption onto carbon for recovery of precious metals from leach solution. The plant design is intended to comply with requirements of the International Cyanide Management Code.

Processing route

Crushing circuit

A three stage closed circuit design has been selected for the crushing circuit. The circuit is designed for single shift crushing with a design throughput rate of 200 tonnes per hour. The primary crusher will be a single toggle jaw crusher with a 1,000 mm by 850 mm opening. An apron feeder will transfer feed stock from a 100 tonne capacity run of mine bin to a vibrating grizzly feeder that will scalp rock greater than 100 mm into the jaw crusher. Jaw crusher product and grizzly undersize will be transferred to the product screen feed conveyor by the primary crusher discharge conveyor. An in-line belt magnet will remove tramp metal at the head chute of the primary crusher discharge conveyor.

The secondary and tertiary crushers will be standard and shorthead cone crushers respectively that will operate in closed circuit with the product screen. The product screen will be a double deck banana screen 2.4 m wide by 7.3 m long with polyurethane panels on both decks having nominal cut sizes of 30 mm and 12 mm for the top and bottom decks respectively. Oversize from the top deck will be conveyed to the secondary crusher feed bin while oversize from the bottom deck will be conveyed to the tertiary crusher feed bin. The cone crushers will be 1,535 mm in size with an installed motor power of 355 kW and will operate with a 28 mm closed side setting for the secondary crusher and 10 mm for the tertiary crusher. The Metso Minerals Bruno crushing simulation package was used to determine equipment sizes and tonnage rates for the mass balance in the crushing plant.

Crushed product with a P100 of 12 mm and a P80 of 8.5 mm will be conveyed for storage in the crushed product bin. All transfer points will be enclosed and fitted with dust extraction hoods.

Crushed product storage and reclaim

A crushed product bin with a live capacity of 1,500 t will provide 24 hours of milling capacity and act as a buffer between the crushing and grinding circuits. A variable speed belt feeder will be used to remove crushed product from a slot in the base of the bin.

Grinding

The proposed grinding circuit design consists of a single stage overflow ball mill that will operate in closed circuit with hydrocyclones, flash flotation and gravity concentration machines. The treatment rate will be 59 t/h and the grinding circuit will operate 24 hours per day, seven days per week. The inclusion of both flash flotation and gravity recovery devices will maximise recovery of liberated precious metals minerals into a concentrate which will be reground to increase liberation and exposure of gold and silver.

The grinding mill will be a 4.88 m diameter (inside shell) by 4.88 m long (EGL) ball mill powered by a 2,000 kW motor. The mill will operate with a ball charge of 30% by volume and a top ball size of 80 mm. The cyclone cluster will consist of four 250 mm diameter hydrocyclones (three duty, one standby). Cyclone overflow will have a product particle size P80 of 75 µm.

Flash flotation and gravity concentration

A flash flotation machine and a centrifugal gravity separator will treat 80% of the cyclone underflow. Feed to the gravity circuit will be split from the cyclone underflow which will report to a screen to remove +3 mm particles. The flash flotation machine will be a SkimAir SK240 fitted with a dual outlet. The SK240 with a cell volume of 8 m³ will provide a residence time of 5 minutes. Tailing from the flash machine will report directly into a 760 mm diameter centrifugal gravity recovery machine. Tailing from the centrifugal concentrator will report to the feed of the ball mill. Concentrate from the gravity concentrator will combine with flash flotation concentrate and be directed to the concentrate regrind circuit.

Flotation

Cyclone overflow reports to a conventional flotation circuit consisting of a single roughing stage of three 10 m³ tank flotation cells. Cyclone overflow slurry will be screened on a trash screen to remove coarse particles, wood fibre or other trash before flotation. Flotation reagents used will be potassium amyl xanthate (collector) and Interfroth 50 (frother) while copper sulphate (activator) has also been allowed for in the flowsheet. Rougher flotation concentrate will be combined with flash flotation and gravity concentrates and directed to the concentrate regrind circuit. Rougher flotation tailings will report to the leach feed thickener hopper.

Gravity and flotation concentrate handling

Combined flash flotation, gravity and rougher flotation concentrates totalling approximately 4.1 t/h of solids will be thickened in the concentrate regrind feed thickener and then reground in an M500 IsaMill™ to a product size P80 of 30 µm, powered by a 200 kW motor. The concentrate regrind feed thickener will provide a feed density for the IsaMill™ of 50% solids. The use of ceramic media in the regrind mill will minimize the effect of iron on cyanide consumption and leach kinetics.

Intensive leaching

The intensive leaching circuit consists of five agitated leach tanks, each with a capacity of 50 m³ of slurry, resulting in a total leach retention time of 24 hours. Leaching will be conducted at 32% solids using a solution of 1% sodium cyanide and 0.2% sodium hydroxide. Oxygen will be sparged down the agitator shaft of each leach tank. The leached residue will be thickened and filtered to remove soluble gold and silver. The leach residue thickener is a 5 m diameter high rate thickener capable of achieving an underflow density of 50% solids. Thickened leach residue will be further dewatered by a 1.5 m wide by 15 m long vacuum belt filter with a filtration area of 17 m². Resulting pregnant solution will be pumped into the Merrill Crowe circuit with pregnant solution from the main leaching circuit. The washed filter cake will be repulped and pumped back into the main leach feed circuit.

Leaching and CCD

The main leaching circuit design includes a leach feed thickening stage. A 9 m diameter high rate thickener was selected based on a settling flux rate of 1.0 t/m²h determined from test work to achieve an underflow density of 40% solids. The leaching circuit design has been based on a leach feed density of 38% solids, an initial leach feed pH of 10.5 and maintenance of a free sodium cyanide level of 1,000 ppm throughout the leach train via staged addition of cyanide. The leaching circuit will provide a residence time of 48 hours. Five stages of leaching have been incorporated into the design to minimize short circuiting.

Leach tail will flow to the first of five CCD thickeners used to separate and recover the solution phase carrying dissolved precious metals from the residue solids. Pregnant solution will be removed from the first CCD thickener. Barren solution from the Merrill Crowe circuit will be added to the fifth (last) CCD thickener as wash solution. A five stage CCD circuit has been selected to achieve a wash efficiency of 99% due to difficulty filtering the slurry, which has a high inherent fines content due to presence of illite clay. CCD thickeners will be 9 m diameter high rate thickeners.

Merrill Crowe

The Merrill Crowe circuit has been designed on the basis of information from existing operations and laboratory test work. It will be a vendor supplied package with a volumetric capacity of 190 to 380 m³/h. It will be designed to handle pregnant tenors between 2 to 6 ppm gold and 52 to 207 ppm silver. Barren solution from the Merrill Crowe circuit will be recycled to the back of the CCD circuit as wash water and also used to provide flocculant dilution water in the CCD circuit. Excess cyanide containing water will be directed to the cyanide destruction circuit.

Reagents

Process additives specified for operation include Interfroth 50, potassium amyl xanthate, copper sulphate, hydrated lime, sodium cyanide, sodium hydroxide, oxygen, sodium metabisulphite, zinc powder, diatomaceous earth, sulphuric acid, flocculant, liquefied petroleum gas, and smelting fluxes.

Cyanide destruction

Underflow from the last CCD thickener will be combined with excess barren solution and pumped at a pulp density of 38% solids to the cyanide destruction circuit. Cyanide destruction will be based on the Inco cyanide destruction process. The circuit will consist of two 250 m³ capacity agitated cyanide destruction tanks with a combined nominal residence time of 4 hours. Sodium metabisulphite, copper sulphate and lime will be added to the first cyanide destruction tank. The WAD cyanide level can be reduced to less than 1.0 ppm using this circuit if required.

Tailings thickening

Tailings from cyanide destruction will be pumped to the tailings thickener for thickening to 55% solids prior to disposal in the tailings storage facility. The tailings thickener will be a 9 m diameter high rate thickener.

Water services

Raw water will be produced from desalination plants on the coast and pumped to site, discharging into a raw water pond (nominal capacity 3,000 m³). The process water pond (nominal capacity 3,000 m³) will be fed from the raw water pond overflow. Tailings return water will be pumped to a settling pond to remove suspended solids. Overflows from the leach feed thickener and tailings thickener will also be directed to the settling pond, which will overflow into the process water pond. Given the presence of both flotation and leaching circuits, the design philosophy has been to ensure that non-cyanide and cyanide containing process water are effectively segregated.

Air services

A set of three wet screw air compressors will generate plant air stored in a plant air receiver with 5 m³ capacity. Instrument air will be supplied from a filtered and dried air stream directed to a 1.0 m³ capacity receiver. Flotation air will be supplied by two dedicated blowers in duty-standby configuration.

Electrical

Power will be supplied to the site by the local supply authority at 11,000 volts. A 13,000 V switchboard will be installed in the wet plant switchroom. Transformer feeders will supply a 1,500 kVA 13,000 V/415 volt step-down transformer for the Wet Plant 415 V Motor Control Centre and a 1,000 kVA 13,000/415 volt transformer for the Crushing and Screening 415 V MCC. The process plant control system will be a PLC based system with HMI utilising standard personal computers running Citect software.

Key reported parameters

Technical qualifications

The report describes the proposed leach circuit and design criteria; no unreported operating values are inferred.