This report describes the existing processing facility, historical operating performance, and plant design criteria for the Zaruma gold project as of 2014.
Report context
This technical report, dated September 2014, documents the processing methods and recovery systems for the Zaruma Gold Project. The information presented covers the plant as currently configured and operating at that time, with supporting historical production data from 2011 through 2013. Design criteria are provided separately from actual operating data. The report was prepared for Dynasty Metals & Mining Inc.
Processing route
Crushing Circuit
Run-of-mine material is delivered by 20 t tipper trucks to a ROM pad measuring 75 m x 75 m, where material is stockpiled in three areas designated for high, medium, and low grade ore. A front end loader feeds material into a steel feed hopper with a capacity of 41 m³, which feeds at a rate of approximately 67 t/h. The primary jaw crusher is a 48" x 36" unit rated at approximately 104 t/h capacity, crushing material to minus 4 inches.
An 800 mm wide conveyor transports primary crushed material to a double deck vibrating screen measuring 4.8 m x 1.5 m. Material greater than 2 inches returns to the jaw crusher. Material between 1 inch and 2 inches passes through an electromagnet for scrap iron removal and is fed to a 4¼' cone crusher rated at 90 tph. Material smaller than 1 inch reports to a fine material stockpile silo for delivery to the ball mill circuit via stockpile reclaim feeders. The crushed ore stockpile capacity is 5000 t.
Milling Circuit
Fine crushed material is reclaimed from the silo via five reciprocating feeders, with only one used at a time. Reclaimed material feeds into the mill feed chute together with lime, cyanide, and water to achieve a pulp density of 72% solids, including hydrocyclone underflow. Two ball mills in parallel, each 12' x 14' Marcy brand with 2 x 750 kW power, operate with an estimated milling retention time of approximately 20 minutes. Approximately 73% of gold dissolution is estimated to take place in the mill and gravity leaching circuit.
Mill discharge is pumped to a gravity circuit protected by a screen. Screen oversize is combined with gravity tails and pumped to a cluster of 16 Krebs hydrocyclones (seven used at current throughput). Underflow from the screen reports to a Falcon centrifugal gravity concentrator. After gravity concentration, gravity tailings are pumped to the hydrocyclone nest. Gravity concentrate is treated with intensive cyanidation in six tanks, with the leachate returned to the general circuit for recovery with activated carbon. Hydrocyclone overflow at a nominal grind size of 106 µm feeds the first leaching tank at an average flow of 1500 m³/d and density of 1.32 t/m³.
Leaching Circuit
The leaching circuit comprises four tanks in series, each 7.5 m diameter and 10 m high with a capacity of 384 m³ pulp per tank. This equates to 9.2 hours of leaching time per tank, for a total leach time of 36 hours operating at 31.4 t/h. Air injection from three compressors and mechanical agitation support the cyanidation process. An additional 8% leaching is estimated to take place in this circuit at a pH of 10.5.
Carbon-in-Pulp Circuit
The CIP circuit comprises nine tanks, each 3.5 m diameter and 6 m high with a volume of 51 m³. Each tank provides a pulp residence time of 1.22 hours, for a total circuit time of 11 hours. Pulp travels continuously from the first tank to the last, by which time approximately 95% of gold is in solution and adsorbed onto activated carbon. Carbon moves counter-current to pulp flow via an air-lift system, with intertank screens preventing carbon passage between tanks. Gold loading of approximately 7.5 kg Au per tonne of carbon is reached before carbon is transferred via air lift to a loaded carbon vibrating screen with water sprays.
Carbon Desorption and Electrowinning
Loaded carbon is washed over a vibrating screen and fed to a 4 t stainless steel holding tank before transfer to the desorption circuit. Two stainless steel desorption towers with double-layer thermal insulation have a capacity of 5 t each. Desorption takes place at temperatures between 90°C and 98°C at pH 12, achieved by adding caustic soda at 100 kg per batch. Water flow required is 1.25 L/s. Pregnant solution passes through electrowinning cells with stainless steel cathodes operating at 500 A to 1200 A DC and 2.5 V to 4 V. Two cells each produce approximately 45 kg of precipitate over 48 hours, collected and smelted for gold doré production.
Tailings System
Pulp from the last CIP tank flows to a tailings thickener with total capacity of 346 m³. The thickener operates in closed circuit with excess solution returned to the grinding circuit by two 110 gpm centrifugal pumps. Two tailings storage facilities are available. The current facility is to be replaced with a second TSF having a capacity of approximately 2.5 Mt of tailings. The plant has no cyanide detoxification system; cyanide destruction occurs naturally within the tailings facility.
Water Treatment and Supply
Two water ponds with geomembrane-coated bases provide treatment. Pond 1 measures 60 m long x 40 m wide x 6 m deep, connected via a goose neck to Pond 2 measuring 40 m long x 40 m wide x 6 m deep. Treatment limits suspended material through sedimentation and eliminates organic pollution through aerobic oxidation. Water from neighbouring creeks is also deposited into these ponds, treated, and used exclusively for industrial purposes.
Water from Pond 2 is used in two ways: some is recycled after filtration and chlorination for grinding, milling, leaching, laboratory, dining room, and office areas; most is pumped to four streams: to leach tanks including coolant use; to mill pumps and cleaning; to mills then leaching then thickener; and to the crushing area for crusher cooling, ultimately discharged to local creeks at 20°C. Some water is also used for reforestation irrigation. Approximately 50% of processing water is recovered and re-used.
Energy, Water, and Process Materials
The processing facility has 7.5 MW of electrical power available. The site is fed from the regional grid into a 5 MW at 69,000 kV/4,200 V and 2.5 MW at 69,000 kV/13,800 V switch-yard. Most plant equipment operates at 440 V.
The plant uses conventional consumables for a CIP gold processing facility: lime, sodium cyanide, grinding media, sodium hydroxide, and oxygen. Consumables are sourced through internationally located procurement consultants. The supply line has been in operation for over 4 years.
Gravity Concentration
The Falcon centrifugal concentrator processes gravity feed protected by a screen. Gravity concentrate undergoes intensive cyanidation in six mechanical agitated tanks with air injection, each with 28 m³ live volume.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Plant design capacity | 1,100 tpd | Design |
| Operating throughput (2013) | 700–800 tpd | Actual |
| Operating throughput (2013 annual) | ~180,000 t | Actual |
| Feed gold grade (2013 average) | 6.4 g/t Au | Actual |
| Feed silver grade (2013 average) | 30.2 g/t Ag | Actual |
| Gold recovery (2013) | 91.2% | Actual |
| Silver recovery (2013) | 43.1% | Actual |
| Gold recovery (2011–2013 total) | 85.6% | Actual (weighted average 2013, arithmetic 2011–2012) |
| Silver recovery (2011–2013 total) | 38.5% | Actual (weighted average 2013, arithmetic 2011–2012) |
| ROM feed size (F80) | 254 mm | Design |
| Crushed product size (P80) | 19 mm | Design |
| Grind size (P80) | 208 µm | Design |
| Hydrocyclone overflow nominal grind size | 106 µm | Design |
| Ball mill power (each) | 2 x 750 kW | Design |
| Milling retention time | ~20 min | Design estimate |
| Leach tank residence per tank | 6.45 hrs | Design |
| Leach total residence | 25.78 hrs | Design |
| Leach total time (operating) | 36 hrs | Operating |
| CIP residence per tank | 0.97 hrs | Design |
| CIP total residence | 8.74 hrs | Design |
| CIP total time (operating) | 11 hrs | Operating |
| Gold loading on carbon | ~7.5 kg Au/t carbon | Operating |
| Tailings storage facility capacity (second) | 2.5 Mt | Design |
| Electrical power available | 7.5 MW | Design |
| Water recovery | ~50% | Operating |
Project website: https://www.metallaroyalty.com/royalties/development/zaruma/
Technical qualifications
The report states that the processing plant was operating at 64% of its installed capacity at the time of reporting, with the shortfall attributed to underground mine production rather than plant capability. The metallurgical recovery data for 2011 and 2012 are arithmetic averages, while 2013 recovery is a weighted average. The report notes that the processing facility does not have a cyanide detoxification system, with cyanide destruction occurring naturally within the tailings facility. Design criteria are presented separately from actual operating conditions.
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Source: Dynasty Metals & Mining Inc., Zaruma Gold Project Technical Report, September 2014, Section 17.0 Recovery Methods.
