The Dolores processing operation combines a three-stage crushed heap leach circuit with an integrated pulp agglomeration plant, both feeding a Merrill-Crowe recovery plant to produce doré.
Report context
The 2021 Technical Report for the Dolores Property, located in Chihuahua, Mexico, describes the recovery methods employed at the mine. The operation uses a conventional crushed heap leach circuit operated in parallel with a pulp agglomeration circuit, with a combined throughput of approximately 21,300 tpd. The report covers the processing design, operating history, and testwork data as of 2021.
Processing route
Heap leach crushing and conveyor stacking
Run-of-mine ore is trucked to the crushing plant and reduced to a particle size of P80 6.7 to 9.2 mm in a three-stage crushing circuit at an average rate of 15,860 tpd. The crushed ore is sampled at regular intervals by a sample tower, conveyed to the leach pads via an overland conveyor system, and placed using portable grasshopper conveyors and a radial stacking system. Approximately 5,440 tonnes of crushed ore is scalped from the heap leach circuit prior to stacking for agglomeration with filtered high grade from the pulp agglomeration circuit.
Pebble lime (CaO) is added to the crushed ore for pH control before conveyor stacking onto a multiple-lift, single-use leach pad. Sodium cyanide solution is prepared in the Merrill-Crowe plant, pumped to the leach pads, and applied using drip and sprayer systems. Pregnant leach solution flows by gravity to a pregnant solution pond and is pumped to the Merrill-Crowe plant.
Metal recoveries are a function of solution flow rates, cyanide concentration, and time. The metal leaching period can cover years, continuing as subsequent lifts are placed on the pads. Gravity collectors and/or vertical perforated steel pipes fitted with internal pumps transfer the pregnant leach solution from the bottom of the leach piles to the pregnant solution pond.
Pulp agglomeration
The pulp agglomeration circuit was constructed at Dolores and has been operating since commissioning in 2017. The plant operates in an integrated fashion with the heap leach processing facilities. High-grade ore is stockpiled separately on the ROM pad and reclaimed using front-end loaders at a rate of 5,440 tpd.
The high-grade feed is processed through a primary jaw crusher with discharge conveyed to a vibrating screen. Screen oversize passes through a secondary cone crusher in closed circuit to produce a rod mill feed with a P80 size of 12.8 mm. The screen undersize is stockpiled using a radial stacker and reclaimed by feeders and a reclaim tunnel conveyor system to feed the rod mill.
The rod mill discharge is pumped in closed circuit to a Vertimill/vibrating wet screen circuit with screen oversize recycled to the Vertimill. Screen undersize is pumped to a surge tank for feed to the pressure filters to produce a P80 0.425 mm ground product. Cyanide and lime are fed into the rod mill to initiate the metals recovery process. Pregnant solution from the heap leach is used as process water for the rod mill.
The milled slurry is pumped to a filter feed tank and then to one of three horizontal pressure filters equipped with vertical plates, air blowers, and a membrane squeeze to produce a filter cake with approximately 15% moisture. The recovered filtrate solution is pumped to a pregnant solution holding tank, where a pinned bed clarifier removes suspended solids before metal recovery.
The dewatered filter cake is combined with cement at around 19 kg of cement per tonne of filter cake along with coarse medium-grade ore in a mixer before being fed into a rotating agglomeration drum. Additional solution is sprayed onto the mixture to control moisture content and agglomerate ball formation. The agglomerate balls are conveyed to the heap leach pad feed conveyor and combined with crushed medium-grade heap ore.
Most make-up water for the pulp agglomeration process is sourced by diverting a portion of the pregnant solution flow from the heap leach pads. Additional cyanide solution comes from the sodium cyanide solution make-up plant in the Merrill-Crowe plant.
Merrill-Crowe recovery plant and refinery
The Merrill-Crowe recovery plant at Dolores recovers gold and silver from pregnant solution by zinc cementation. Pregnant solution at a nominal rate of 1,300 m³/h is clarified using pressure leaf type clarification filters which remove suspended solids to levels of less than 1 mg/L. Diatomaceous Earth is used to pre-coat the clarification filters and is metered into the pregnant solution.
The clarified pregnant solution is sent to a deaeration tower for oxygen removal, where liquid seal ring vacuum pumps maintain oxygen levels below 1 ppm. Deaerated solution is pumped to the precipitate filter presses, with ultra-fine zinc added at the pump suction to precipitate gold and silver. The barren solution discharging from the filter presses is returned to the leach system.
Precipitate from the filter presses is treated in a mercury retort to remove trace mercury and then smelted to produce doré bars containing between 6% and 9% gold and 91% to 94% silver, with generally less than 1% impurities.
Power and water
Water for the operations is sourced from wells, pit and underground dewatering activities, the nearby Tutuaca River, and the Chabacan Dam. The Chabacan dam reservoir has a capacity of 1.2 million m³. Permitted water usage from the Tutuaca River is 2.0 million m³ per annum at a maximum rate of 64 liters per second.
A 115 kV, 98 km power line connected to the Mexican national grid in 2016 supplies power to the mine. Backup power is available from six 1,800 kW Cummins and two 1,200 kW Caterpillar diesel generators. Total power requirements are estimated at 11.5 MW.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Combined throughput | ~21,300 tpd | Design |
| Heap leach feed rate | 15,860 tpd average | Design |
| Heap leach crush size | P80 6.7–9.2 mm | Design |
| Pulp agglomeration feed rate | 5,440 tpd | Design |
| Pulp agglomeration grind size | P80 0.425 mm (35 mesh) | Design |
| Rod mill feed size | P80 12.8 mm | Design |
| Filter cake moisture | ~15% | Design |
| Cement addition rate | ~19 kg/t filter cake | Design |
| Merrill-Crowe solution flow | 1,300 m³/h nominal | Design |
| Clarified solution solids | <1 mg/L | Design |
| Deaeration oxygen level | <1 ppm | Design |
| Doré gold content | 6%–9% | Actual (reported) |
| Doré silver content | 91%–94% | Actual (reported) |
| Power requirement | 11.5 MW | Design estimate |
| Chabacan Dam capacity | 1.2 million m³ | Design |
| Tutuaca River permitted usage | 2.0 million m³/year at max 64 L/s | Design |
Project website: https://panamericansilver.com/operations/gold-segment/dolores/
Technical qualifications
The report describes the current operation design with throughput, crush size, and process parameter values. Actual operational performance may vary based on ore characteristics, solution management, and other site-specific factors. The pulp agglomeration circuit has been operating since 2017, and its described performance represents the design basis at the time of the 2021 report. The doré composition range represents reported actual production values. Power and water supply figures are design estimates and do not constitute guaranteed long-term performance. The report does not include a tailings facility, as the operation uses a closed-circuit processing system.
Source: Technical Report for the Dolores Property, Chihuahua, Mexico (2021), Section 17 Recovery Methods and listed subsections.
