The 2018 Preliminary Economic Assessment for the Shafter silver project in Presidio County, Texas, describes a whole-ore cyanide leach facility designed to treat 600 tonnes per day of mineralized material over a 3.6-year mine life.
The proposed Shafter processing facility will use whole-ore cyanide leaching to extract silver from mineralized material, with metal recovery achieved using counter current decantation (CCD) and the Merrill Crowe method. The process design is based on a nominal feed rate of 600 tonnes per day, or 210,000 tonnes per year, over a mine life of 3.6 years. The plant is designed for 350 operating days per year, with crushing operations running 12 hours per day at 75 percent availability, while the mill and leach circuits operate 24 hours per day at 90 percent availability.
The comminution circuit consists of two stages of crushing: a primary jaw crusher and a secondary cone crusher, with the objective of reducing run-of-mine material to 80 percent passing 1.0 inch. Crushed material is conveyed to a crushed mill feed stockpile and reclaimed by three pan feeders, with pebble lime added by screw feeder during transfer to the grinding circuit. A single ball mill reduces the material to 80 percent passing 504 microns, with the mill operating in closed circuit with cyclones for classification. Dilution to the design 45 percent solids is achieved primarily by filtrate returned from the tailings filters, and thickener overflow reports to the process water tank for distribution to the grinding circuit and as wash water for the tailings filter.
Leaching is conducted in a train of leach tanks designed for 72 hours of retention, with a cyanide ion concentration of 2,000 ppm in the first tank, decreasing to approximately 100 ppm in the final tank through consumption. The leach slurry exits the final tank and reports to the CCD circuit, which uses four thickeners to wash the slurry at an anticipated wash efficiency of 99.6 percent. Final pregnant solution carrying approximately 99.1 percent of the solubilized silver is expected to have a design tenor of 200 ppm silver. The cleaned residue from the CCD circuit undergoes a final wash at the tailings plate and frame filters, where barren fresh water is combined with the underflow from CCD number four, reducing the tailings cake moisture to 15 percent. The final washed tenor of the moisture in the tailings cake will be around 11 ppm silver. Tailings are conveyed to a load-out area and hauled to a lined, dry-stacked storage facility, with the note that tailings handling was not included in the scope of the estimate.
Silver is recovered from the pregnant solution through a Merrill Crowe process, which includes deaeration of the solution before zinc is introduced. A zinc eductor adds low-solids zinc slurry at 300 ppm to the deaerated pregnant solution, with inline mixers providing contact for the cementation reaction, precipitating silver as a solid. The precipitate is collected on two filter presses, each designed to accommodate 24 hours of precipitate, then dried, smelted, and poured into silver doré for shipment to a refiner.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Annual production | 210,000 | tpy | Nominal |
| Daily production | 600 | tpd | Nominal |
| Mine life | 3.6 | years | Not stated |
| Mine life | 749,000 | tons | Not stated |
| Silver feed grade | 10.3 | Oz Ag/ton | Testwork/history |
| Silver production | 6,156 | Oz Ag/day | Calculation |
| Silver production | 2,152,000 | Oz Ag/year | Calculation |
| Leach extraction | 85.7 | % | Testwork/history |
| Overall Ag recovery | 85.4 | % | Not stated |
| Recovered silver | 5,256 | Oz Ag/day | Calculation |
| Recovered silver | 1,837,300 | Oz Ag/year | Calculation |
| Silver to tails | 900 | Oz Ag/day | Calculation |
| Tails Ag grade | 1.50 | Oz Ag/t | Testwork/history |
| Leach retention | 72 | hours | Design |
| Crush size | 80 | % passing | 1.0 inch |
| Grind size | 80 | % passing | 504 microns |
| CCD wash efficiency | 99.6 | % | Anticipated |
| Pregnant solution tenor | 200 | ppm Ag | Design |
| Cyanide concentration, first tank | 2,000 | ppm | Not stated |
| Cyanide concentration, final tank | 100 | ppm | Anticipated |
| Zinc concentration | 300 | ppm | Low solids zinc slurry |
| Tails cake moisture | 15 | % | Final press |
| Specific gravity | 2.77 | Not stated | Testwork |
Overview
The Shafter Project is a proposed silver mining operation located in Presidio County, Texas, USA, owned by Aurcana Corporation. The 2018 Preliminary Economic Assessment outlines a processing facility designed to treat 600 tonnes per day of mineralized material with a mine life of 3.6 years. The process flowsheet incorporates two stages of crushing, single-stage ball mill grinding, whole-ore cyanide leaching, counter current decantation for solids-liquid separation, and Merrill Crowe precipitation for silver recovery. The project includes a retort for mercury removal and a gas-fired furnace for smelting.
Key Process Stages
The processing plant is divided into several distinct unit operations. Comminution begins with a primary jaw crusher and secondary cone crusher reducing run-of-mine material to 80 percent passing 1.0 inch. The crushed ore is then ground in a single ball mill in closed circuit with cyclones to achieve a final product size of 80 percent passing 504 microns, with pebble lime added during the transfer to the grinding area. Grinding circuit dilution is provided primarily by filtrate returned from the tailings filters, with water distributed from the process water tank.
Leaching occurs in a train of existing leach tanks designed for 72 hours of retention to achieve a silver extraction of 85.7 percent. The cyanide ion concentration begins at 2,000 ppm in the first tank and decreases through consumption to approximately 100 ppm in the final tank. The slurry exits the leach circuit to a CCD circuit consisting of four thickeners, achieving a wash efficiency of 99.6 percent. Final pregnant solution is expected to have a design tenor of 200 ppm silver, carrying approximately 99.1 percent of the solubilized silver.
The Merrill Crowe recovery circuit begins with deaeration of the pregnant solution, followed by the introduction of zinc slurry at 300 ppm through an eductor. Inline mixers ensure adequate contact for the cementation reaction, and two precipitate filters, each designed to handle 24 hours of precipitate, collect the silver solids. A final wash at the tailings filters captures remaining leached silver, with barren fresh water combined with CCD underflow to provide final dilution before the filter press reduces the tailings cake moisture to 15 percent.
Additional Interesting Data and Summary
The process design criteria include a crushing hourly rate of 66.7 tonnes per hour, based on 9 operating hours per day, and a mill hourly rate of 25 tonnes per hour based on 24-hour operation. The mill feed crushed stockpile provides surge capacity for the facility. The pre-leach thickener overflow reports to the process water tank for use in the grinding circuit and as wash water for the tailings filter. The process water system distributes water from the thickener overflow back to the grinding circuit for dilution at the mill and cyclone feed tank, and for final leach residue wash at the tailings filters. Final washed tenor of the moisture in tailings cake will be around 11 ppm silver.
Key Processes
- Whole-ore cyanide leaching for silver extraction
- Counter current decantation (CCD) with four thickeners for solids-liquid separation
- Merrill Crowe zinc precipitation for silver recovery
- Two-stage crushing (jaw and cone crusher) to 80 percent passing 1.0 inch
- Single ball mill in closed circuit with cyclones for grinding to 80 percent passing 504 microns
- 72-hour leach retention with cyanide concentration control from 2,000 ppm to 100 ppm
- Final residue washing at plate and frame tailings filters
Source: Shafter Project , 2018 Technical Report, 2018.
Project website: Shafter Project, 2018 Technical Report
