This section describes the recovery methods implemented in the design of the crushing and process facilities for the El Tigre Project Phase I and II.
Report context
The process plant design presented in this Pre-Feasibility Study, dated 2024, is based on a nominal 7,500 tpd (Years 1 to 3) and 15,000 tpd (Years 4 to 9) of ore with average grades of 0.40 g/t gold and 14.9 g/t silver. Testwork results presented in Section 13 of the report form the basis for flowsheet development and design criteria. The recovery methods are consistent with the crushing, pad loading, leaching, Merrill Crowe precipitation, refinery, and support processing methods presented in the 2023 PEA.
Processing route
Flowsheet overview
The process plant flowsheet design comprises three-stage conventional crushing, material handling of crushed product and loading onto lined heap pads. Solution ponds and a pumping system allow irrigation of loaded mineralized material and subsequent collection of the pregnant solution. The pregnant solution is pumped to the Merrill Crowe recovery facility for precipitation of the gold and silver in solution and filtered accordingly. The resultant precipitate will be smelted at an on-site refinery to produce doré bars for market. The barren solution from the recovery process is recirculated to the heap leach pond (barren) for cyanide addition and pumping to the heap leaching pad.
Revised recovery methods
Revised process recovery methods included in this PFS are as follows: the process plant facility is now located adjacent to the heap pond infrastructure to improve process operability and eliminate distant pumping and power demand. Water will be supplied by surface water wells. Selected equipment for crushing and recovery during Phase I has been upsized to lower Phase II direct capital requirements and related construction time. This includes the primary crusher system and material handling, discharge conveyors to the pad loading, and Merrill Crowe deaeration, vacuum and process pumps. The refinery has also been designed to treat Phase II metal production.
Primary crushing circuit
The proposed primary crushing circuit reduces the run-of-mine ore from a nominal top size of 600 mm to a product of P80 –3/8 in (9.0 mm) for conveyor loading to the heap leach pads. The front end has been upsized to handle Phase II design. The crushing circuit includes a ROM feed hopper complete with feeder and vibrating grizzly screen, a primary jaw crusher, associated conveyor belts, a belt scale and magnet, and a feed control system. The jaw crusher system processes a nominal 416 t/h (Phase I) and 833 t/h (Phase II post expansion) of oversized material. The selected 200 kW jaw crusher is Model # CJ815 (1,500 mm x 1,300 mm).
Primary crushed material stockpile and reclaim
The stockpile provides production surge capacity to ensure a steady rate to the secondary crushing circuit. Pan feeders discharge onto the secondary feed conveyor to feed crushed ore to the secondary screen unit. The feeders reclaim material from the stockpile and ensure a controlled feed rate to the secondary circuit.
Secondary and tertiary crushing circuit
Equipment in this area includes: secondary inclined linear double deck screen, secondary crusher with closed side setting of 44 mm, tertiary inclined screens operating in parallel, tertiary cone crushers with closed side setting of 9 mm (or less), and associated conveyors, belt feeders and discharge system for recirculation and discharge to the crushed ore stockpile. This equipment is also upsized to suit Phase II. The crushing circuit is located upstream of the heap leach pad facility and process plant and ponds. Crushed material is reclaimed via a series of feeders and grasshopper conveyors and stacking units to systemically load the crushed ore onto the lined pads.
Heap leach pad system and solution distribution
The heap leach pad areas cover 168,000 m² (Phase I) and 248,000 m² (Phase II) of LLDPE material. The stage pad areas are complete with all associated collection piping, geotextile, and supporting items.
Merrill Crowe (zinc precipitation) circuit
The loaded pregnant solution is pumped from the pond to the Merrill Crowe facility located within a housed building for both safety and security. Equipment includes: three clarification filters complete with pumps and samplers at 180 m²/unit, with an additional unit for Phase II; a 4 m diameter deaeration tower complete with packing, level control, and 45 Hp vacuum pump with separator; a zinc feeding system complete with lead nitrate feed tank; two precipitate press feed pumps complete with seal at 75 Hp; three precipitate filters at 180 m²/unit with recessed plates, feed and discharge piping, and precipitate trolley, with an additional unit for Phase II; a Diatomaceous Earth (PreCoat) skid with mix tank, agitators, and feed pumps; a secure refinery area with a 0.30 m³ induction bullion furnace complete with baghouse, power controls, scales and pots, sized for LOM service; and a Motor Control Center system. The barren leach solution discharges to the barren solution pond after NaCN and lime reagent addition and recirculation.
Reagent handling and storage
Fresh water pumped from two wells will be utilized for all reagent mixing within the process plant and for make-up water to the heap pads for evaporation and wetting fresh feed material. The process plant also includes a mixing area containment. Required reagents include: lime (hydrated) bulk for silo; sodium cyanide (NaCN) dry super sacs; Diatomaceous Earth (PreCoat) bagged; zinc dust canned; and refinery slagging reagents bagged.
Assay and metallurgical laboratory
A fully equipped stand-alone laboratory, located close to the main process facility, is equipped with the necessary analytics to provide all required data for the mining operation, main process facility, and environmental monitoring requirements. The laboratory will also provide on-time process monitoring, daily production reporting, blast hole sampling, and exploration samples.
Water supply
Well water will supply all facets of the Project including make-up water for process losses from evaporation, reagent mixing, and emergency water. Treated water will be supplied to a fully operational on-site camp facility. In addition to the well water supply, rain and run-off water during the rainy season will also be diverted and collected. Multiple high head pumps are installed at the water sources to pump water to the process plant’s freshwater tank.
Air supply and standby power
An air distribution system is included to supply required process air to the main Merrill Crowe plant facility, and instrument air is included for required instrumentation and controls. An emergency generator rated for 1,825 kW will supply electrical power for essential process operations during shutdown or system failure (process and solution pumps).
Testwork basis
Column testing indicated both higher gold and silver recovery (oxide and sulphide) at a 3/8 in. crush size. In the process design and financial model, these have been discounted by 3% for leaching in the field versus optimum conditions in the laboratory. Cyanide consumption is also discounted for the process design, operating costs, and financial model. Lime is not discounted from the column test data.
Key reported parameters
| Criteria | Units | Value | Basis |
|---|---|---|---|
| Ore Characteristics | |||
| Specific Gravity | g/cm³ | 2.6 | Design |
| Process Plant Feed | |||
| Phase I Years 1-3 | tpd | 7,500 | Design |
| Phase I Years 1-3 | tpy | 2,737,500 | Design |
| Phase II Years 4-9 | tpd | 15,000 | Design |
| Phase II Years 4-9 | tpy | 5,475,000 | Design |
| Crushing Availability | % | 75 | Design |
| Heap and Processing Plant Availability | % | 95 | Design |
| Crushing Product (to pad) P80 | inch (mm) | 3/8 (9.0) | Design |
| Head Grades (Life of Mine Average) | |||
| Gold | g/t | 0.40 | Design |
| Silver | g/t | 14.9 | Design |
| Metal Recoveries | |||
| Overall Gold Oxide Recovery – design | % | 83 | Testwork, discounted 3% |
| Overall Silver Oxide Recovery – design | % | 45 | Testwork, discounted 3% |
| Overall Gold Sulphide Recovery – design | % | 56 | Testwork, discounted 3% |
| Overall Silver Sulphide Recovery – design | % | 40 | Testwork, discounted 3% |
| Reagent Addition Rates – design | |||
| Cyanide | kg/t | 0.25 | Testwork, discounted |
| Lime | kg/t | 1.5 | Testwork, not discounted |
| Merrill Crowe Flow | |||
| Phase I – design | m³/h | 590 | Design |
| Phase II – design | m³/h | 1,180 | Design |
| Merrill Crowe Recovery – Au and Ag | % | 99 | Design |
Project website: https://silvertigermetals.com/el-tigre-district
Project website: https://silvertigermetals.com/
Technical qualifications
Column testing indicated both higher gold and silver recovery (oxide and sulphide) at a 3/8 in. crush size. In the process design and financial model, these have been discounted by 3% for leaching in the field versus optimum conditions in the laboratory. Cyanide consumption is also discounted for the process design, operating costs, and financial model. Lime is not discounted from the column test data. PFS testwork results indicate the presence of oxide, transition, sulphide domains as well as associated variable recovery.
Source: Silver Tiger Metals Inc., El Tigre Pre-Feasibility and Preliminary Economic Assessment, Report No. 481, Section 17.0 Recovery Methods and Process Plant, 2024.
