This report summarizes the processing and recovery methods for the Mount Hamilton Gold and Silver Project, as presented in a 2014 feasibility study technical report.
Report context
This 2014 Technical Report for the Mt. Hamilton Gold and Silver Project presents the feasibility study findings, including the proposed processing methods and plant designs. The report details the plan for gold recovery via a multi-lift heap leach operation with a carbon ADR (Adsorption, Desorption, and Refining) plant.
Processing route
Proposed Processing Methods
Recovery of gold from the Mt. Hamilton Project will be accomplished by a multi-lift heap leach with a carbon ADR plant. The dedicated heap leach pad, process ponds, ADR plant and ancillary facilities were designed to accommodate a leachable reserve of approximately 22.5 Mt of crushed ore from the Seligman and Centennial open pits. The 22.5 Mt is the physical limitation for leach pad construction on the private property.
Crushing and Conveying
Run-of-Mine (RoM) ore will be primary crushed near the southwest rim of the Centennial open pit. Ores will be crushed in two stages to 90% passing ¾ inch size. Primary crushing will occur on a pad at an elevation of 8,650 ft. A 415 ft, steel lined, 60 inch diameter vertical raise will transport the crushed ore to a feeder and conveyor belt. A 4,425 ft long conveyor belt will transport the ore at a grade of approximately -15% to a stockpile near the secondary crusher.
Primary crushing will involve a vibrating grizzly feeder with 4 inch openings, with oversize fed to a 36 inch x 50 inch Lippman jaw crusher with a closed side setting of 4 inches. The secondary crushing plant consists of a 6 ft x 20 ft two-deck screen and a Terex MVP 550 cone crusher. Material greater than ¾ inch will be crushed, and the screen undersize and crusher product will be combined to produce the 90% passing ¾ inch feed to the heap leach pad.
Heap Leach Pad
The proposed heap leach pad is designed to be constructed in four phases, each consisting of five cells for a total of 20 cells. The leach pad and associated facilities will cover an area of about 135 ac. The stacked ore height will reach a maximum of approximately 210 ft above the prepared base.
The pad liner system will be a compacted 12 inch-thick low-permeability soil layer overlain by a single geosynthetic liner. The primary liner will be a double-textured 80 mil high density polyethylene (HDPE) geomembrane, with a 60 mil HDPE secondary liner. A leak collection and recovery system (LCRS) will be installed to monitor and detect leaks.
Solution Application and Recovery
Solution applied to the stacked ore will be distributed from the barren solution tank by pumps. The pregnant leach solution flow collected from the base of the leach pad is estimated at 3,000 gpm. The leach application rate will be 0.004 gpm/ft².
The pregnant solution collection system will consist of a network of collection pipes (4 inch, 12 inch, and 24 inch diameters) transporting solution to the process ponds. Three solution ponds will be constructed: a pregnant solution pond, a barren solution pond, and an event pond. Each pond will be double-lined and equipped with a leak collection and recovery system.
ADR Plant
The ADR plant will consist of five 12 ft diameter carbon columns, a 4.5 t strip and acid wash system, electrolytic cells, a mercury retort and mercury controls, and an induction smelting furnace. The final product will be a doré bar.
The pregnant solution will flow through a cascading series of carbon columns. Activated carbon will be transferred countercurrent to the solution flow. Loaded carbon will be acid washed, stripped in a pressure vessel, and regenerated in a kiln. The stripped solution will be pumped to electrolytic cells for doré recovery. The sludge from the cells will be filtered, retorted, and smelted into doré bars.
A sampling system will be installed on the belt conveyor to collect a sample for the assay laboratory.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Leachable reserve | 22.5 | Mt | Design |
| Crushed ore bulk density | 110 | lb/ft³ | Design |
| Ore lift height | 30 | ft | Design |
| Solution application rate | 0.004 | gpm/ft² | Design |
| Ore leach cycle | 210 | days | Design |
| Ore leach area | 4.43 | Mft² | Design |
| Solution pumping rate | 3,240 | gpm | Design |
| HLP maximum height | 210 | ft | Design |
| Initial lift factor of safety (static) | 1.74 | – | Testwork |
| Final surface factor of safety (static) | 1.30 | – | Testwork |
| Initial lift factor of safety (pseudostatic) | 1.10 | – | Testwork |
| Final surface factor of safety (pseudostatic) | 1.05 | – | Testwork |
| Pregnant solution flow rate | 3,000 | gpm | Design |
| Peak water make-up requirement | 500 | gpm | Design |
| Water storage tank height | 20 | ft | Design |
| Water storage tank diameter | 80 | ft | Design |
| ADR plant feed rate | 3,000 | gpm | Design |
| Carbon column diameter | 12 | ft | Design |
| Major reagent: Sodium Cyanide | 0.6 | lb/t | Testwork |
| Strip circuit capacity | 4.5 | t/d | Design |
| Regeneration kiln capacity | 2.64 | t/d | Design |
Project website: https://www.makominingcorp.com/projects/usa/advanced-development/mt-hamilton/
Technical qualifications
The report notes several limitations: The 22.5 Mt leachable reserve is a physical limitation for leach pad construction on the property. The maximum design height of 210 ft is within the tested maximum stacking height of 220 ft without agglomeration, as confirmed by large column height/percolation tests performed in 2011. Reagent amounts were determined during metallurgical test work performed in 2011. The stability analyses report that all factors of safety under static and pseudostatic conditions are higher than the required minimum, indicating stability for the proposed design. The ADR plant will have all the mercury controls installed as currently required by the State of Nevada. A 2013 storm event was considered in pond design criteria.
Source: Mt. Hamilton Gold and Silver Project , 2014 Technical Report, October 2014.
