This Preliminary Economic Assessment describes the processing plan for the El Cubo project, which will use the existing El Cubo Mill at a design rate of 750 tonnes per day to produce gold-silver concentrate from underground mine feed and surface stockpiles.
The El Cubo Mill was constructed as a conventional crushing, grinding, and flotation plant. It includes two-stage crushing, ball mill grinding, reagent storage, flotation, flotation concentrate filtration for product shipment, and tailings disposal. The plant will treat precious metals bearing ore using conventional methods. The mill will be operated at 750 tonnes per day to produce approximately 12 tonnes of gold-silver concentrate. That rate will be achieved by a combination of feed from the El Cubo underground mine and the El Pinguico above-ground and underground stockpiles.
Historical recoveries from the El Cubo Mine for ore processed through this mill averaged 87% for silver and 86.5% for gold. However, a 1,000-tonne bulk sample taken from the stockpile located above ground at the El Pinguico shaft and processed through a neighboring mill of approximately the same configuration averaged 60% for silver and 75% for gold. These results suggest a notable difference in recovery between the two feed sources.
Water supply for the El Cubo Mill will be from the existing underground workings, which currently make sufficient water for the plant requirements. Power supply will be from an existing 13 kV overhead transmission line. The El Cubo site is located at an elevation of approximately 2,200 meters above sea level, which was considered in equipment and motor design.
The plant design is based on a throughput rate of 750 tonnes per day or 34 tonnes per hour. Considering mine availability and plant maintenance downtime, the annual rate is estimated to be 273,750 tonnes per year. Mining rates have tentatively been set at 250 tonnes per day for initial operation and would ramp up to 750 tonnes per day during the first year of operation.
The El Cubo Mill has a minimum rate that it must be operated at to keep tanks and lines clear and operational. The crushing circuit is designed to process 100 tonnes per hour, which requires that the plant only be operated approximately 8 hours per day to provide the required tonnage to fill the ball mill feed bin for several days of operation. Likewise, the plant would only need to operate approximately 8 hours per day at 30 tonnes per hour to process the initial feed rate of 250 tonnes per day. Hence, the plant would likely campaign ore through the crushing and milling circuit once sufficient ore has been stockpiled by the mine.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Plant throughput rate | 750 | tonnes per day | Design rate |
| Plant throughput rate | 34 | tonnes per hour | Design rate |
| Annual throughput | 273,750 | tonnes per year | Considering mine availability and maintenance downtime |
| Concentrate production | 12 | tonnes per day | Gold-silver concentrate |
| Initial mining rate | 250 | tonnes per day | Tentatively set for initial operation |
| Crushing circuit rate | 100 | tonnes per hour | Design rate |
| Ball mill feed rate | 37.5 | tonnes per hour | Regulates single stage ball mill |
| Bond Work Index | 16-21 | kWh/t | |
| Ball Mill Feed (F80) | 16,000 | microns | |
| Product Size (P80) | 74 | microns | 200 mesh |
| Historical silver recovery | 87 | % | El Cubo Mine average |
| Historical gold recovery | 86.5 | % | El Cubo Mine average |
| Bulk sample silver recovery | 60 | % | El Pinguico stockpile sample |
| Bulk sample gold recovery | 75 | % | El Pinguico stockpile sample |
| Water supply | Not stated | Not applicable | Existing underground workings |
| Power supply | 13 | kV | Existing overhead transmission line |
| Elevation | 2,200 | meters | Above sea level |
| Product size | 14,000 | microns | Primary crusher product |
Overview
The El Cubo project involves reactivation of the existing El Cubo Mill to process ore from the El Cubo underground mine and stockpiles at El Pinguico. The processing facility uses conventional crushing, grinding, and flotation technology to produce a gold-silver concentrate. The operation is designed around a 750 tonne per day throughput rate, with initial operation at a lower rate expected during ramp-up.
Key Process Stages
The processing flowsheet consists of several integrated stages. Rail cars from the El Cubo underground mine discharge primary crushed ore into a 650-tonne rail dump storage hopper at a rate of 30 tonnes in 10 minutes. The rail cars may discharge along the entire length of the rail dump storage hopper. Trucks haul ore from El Pinguico and place material in a storage area that can contain up to 3,000 tonnes. Ore trucks from El Cubo will also place material in the same storage area.
Primary crushed ore is discharged from the rail dump hopper via a series of clam shell feeders onto a series of belts that carry the material to a vibrating screen. The crushing circuit processes ore at approximately 40 tonnes per hour for the primary crusher. Crusher product falls onto a conveyor that transports the material to the rail car dump hopper. This configuration limits the storage bin storage capacity if only the primary crusher is operated as a source of crushed ore feed.
Secondary crusher product is discharged onto the vibrating screen product conveyor and the combined stream is conveyed to a 650-tonne mill storage feed bin. The minus 5/8-inch material is reclaimed from the 650-tonne mill storage feed bin with a slot feeder and fed to a single stage ball mill at the rate of 37.5 tonnes per hour. The 12 foot diameter by 14 foot EGL ball mill operates in closed circuit with hydro-cyclones and grinds the material to a 200 mesh (75 micron) product size.
Cyclone overflow flows by gravity to a conditioning tank where flotation reagents are added. Conditioned slurry is fed to a 5-stage 30 cubic meter tank cell rougher flotation circuit. Flotation concentrate is filtered for product shipment. Tailings are thickened to approximately 55% solids and pumped to a conventional tailing pond for final disposal.
Additional Interesting Data and Summary
The El Cubo Mill was recently operated at 1,500 tonnes per day of ore to produce approximately 25 tonnes of gold-silver concentrate. Due to restrictions in the mine, mill feed was curtailed to approximately 700 tonnes per day at the end of 2019. This historical operating data provides context for the current design rate of 750 tonnes per day.
The crushing circuit is designed to process 100 tonnes per hour, which requires that the plant only be operated approximately 8 hours per day to provide the required tonnage to fill the ball mill feed bin for several days of operation. The intermittent nature of the crushing operation means that plant operators will need to campaign ore through the circuit once sufficient ore has been stockpiled.
The design parameters include a Bond Work Index range of 16-21 kWh/t, a ball mill feed size of 16,000 microns, and a product size of 74 microns. The primary crusher has a product size of 14,000 microns. These process design criteria show the ore characteristics expected from the combined feed sources.
Key Processes
- Two-stage crushing with vibrating screen
- Single-stage ball mill grinding in closed circuit with hydro-cyclones
- Reagent addition in conditioning tank
- 5-stage tank cell rougher flotation circuit
- Concentrate filtration for product shipment
- Tailings thickening to approximately 55% solids and disposal to conventional tailing pond
- Ore reclaim from 650-tonne mill storage feed bin with slot feeder
Source: Preliminary Economic Assessment – El Cubo, 2021.
Project website: Preliminary Economic Assessment – El Cubo
