The July 2013 update to the Goldfield feasibility study describes three-stage crushing to minus one inch, 25-foot heap lifts, a 90-day single-stage leach, five carbon columns and 85% gold recovery for about 76,800 oz of gold a year.
Goldfield is a gold and silver property in Nevada, USA. The NI 43-101 Technical Report, an update to the feasibility study on the Goldfield property, has an effective date of June 17, 2013 and a signature date of July 25, 2013. It was prepared for International Minerals Corporation. The process flowsheet selected comprises three-stage crushing and screening, heap leaching, carbon adsorption, stripping and regeneration, then electrowinning and smelting to make doré bars of gold and silver for shipment to third-party refiners. Crushing and pad loading will operate on day shift only, 12 hours a day, seven days a week, while leaching and gold recovery will operate on two shifts, 24 hours a day, seven days a week.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Crushing rate | 8,250 | dry T/d | One 12-hour shift, 7 days per week |
| Final crushed product | 100% passing 1 | in | Report text; table lists 0.5 in |
| Primary crusher availability | 80 | % | Design |
| Secondary and tertiary availability | 85 | % | Design |
| Design feed grade | 0.032 / 0.146 | oz/T Au / oz/T Ag | Design |
| Load-out dump truck | 50 | ton | Filled in one minute |
| Leach pad area | About 4 | million sq ft | Built in phases |
| Liner | 80 | mil HDPE | Over low permeability soil |
| Lift height | 25 | ft | Ultimate height 200 ft |
| Operating slope | 2.5:1 | Not applicable | Catch benches |
| Primary leach cycle | 90 | days | 0.005 gpm/ft2 |
| Solution ratio | 2.5 | tons per ton of ore | NaCN solution |
| Cyanide consumption | 1.05 | lb/T | Feed |
| Lime consumption | 2.3 | lb/T | Feed |
| Gold recovery | 85 | % | Metallurgical |
| Silver recovery | 5 | % | Metallurgical |
| Nominal gold production | 210 / 76,787 | oz per day / oz per year | Design |
| Raw water requirement | About 425 | gpm | From perimeter dewatering wells |
Overview
Ore from the pits is dumped directly into the primary crusher feed hopper. A three-stage crushing and screening plant makes leach feed, conveyed to an uncovered conical load-out pile next to the leach pad. Pebble lime is added at a set rate on the load-out feed conveyor, and dump trucks carry the crushed ore to the pad. Solution is applied by buried drippers to the heap, collected at the toe and pumped to a carbon adsorption plant, with gold and silver then eluted, electrowon and smelted on site.
Key Process Stages
Crushing and load-out
The tertiary screen undersize, the final product, is conveyed to the load-out pile. A remotely operated clamshell dump gate reclaims crushed ore onto a high-speed load-out conveyor designed to fill a 50-ton dump truck in one minute.
Leach pad and heap leaching
SRK of Elko and Reno, Nevada designed the leach pads, ponds, pumps and solution piping. The pad will be built in phases to cover about 4 million square feet. It meets State of Nevada Division of Environment requirements with a base of low permeability soil under an 80-mil HDPE geomembrane covered with crushed over-liner, which contains the solution collection piping. Ore is placed in 25 ft lifts to an ultimate height of 200 ft, and heap faces are contoured with catch benches to an overall operating slope of 2.5:1. Because of low rainfall and high evaporation, the water balance matters. Solutions are applied by drippers, buried to limit evaporation and winter freezing. Test work indicates the ore leaches rapidly, so a single-stage system delivers 2.5 tons of sodium cyanide solution per ton of ore, about 90 days at 0.005 gpm per square foot.
Solution handling
Leach effluent is collected in a common launder at the toe of the heap and flows by gravity to the pregnant solution pond, then is pumped to the carbon adsorption plant. Cyanide is added to barren solution in the barren solution tank before return to leaching. The barren pond doubles as the event containment pond and retains pad runoff until it can be returned to the circuit in place of raw water. Both ponds are double-lined with intermediate leak detection, fenced and covered with netting or bird balls for wildfowl protection.
Carbon adsorption, elution and refining
Carbon adsorption is a single train of five up-flow columns. Loaded carbon is acid washed with dilute hydrochloric acid, rinsed and stripped in a pressure stripping system, and spent acid wash and acidified rinse are neutralized with caustic soda and sent to the barren pond. Gold and silver are electrowon from loaded eluate onto stainless steel wool cathodes, rinsed off and collected as sludge, which is dried and smelted on site. Stripped carbon is reactivated in a gas-fired rotary kiln at 1,200 °F. The stripping, regeneration, cathode, sludge handling and smelting circuits have mercury vapor capture equipment. The high-security refinery sits next to the process building.
Additional Interesting Data and Summary
Raw water of about 425 gpm comes from dewatering wells installed around the perimeter of the ultimate pit, and a distribution system supplies drilling, dust control, process make-up and cooling. The report notes that test results suggest average gold recoveries will be slightly higher than first projected, and that the slight increase in production can easily be handled by the current process design.
Bulk handling systems will receive, store, mix and distribute sodium hydroxide, sodium cyanide, hydrochloric acid, anti-scalant, activated carbon and propane.
The process design was developed from experience at similar projects together with the metallurgical test results in the report. Carbon adsorption is sized at 25 gpm per square foot, carbon loading is designed at 100 oz per ton of gold and silver combined, and carbon stripping capacity is 2.5 tons a day. Nominal silver production is 60 oz a day, or 21,982 oz a year.
Key Processes
- Three-stage crushing and screening
- Lime addition on the load-out conveyor
- Truck placement on a lined multi-lift heap leach pad
- Buried dripper cyanide leaching
- Five-column carbon adsorption
- Acid washing, pressure stripping and electrowinning
- Rotary kiln regeneration and mercury capture
- On-site smelting to doré
Source: NI 43-101 Technical Report, Update to Feasibility Study on the Goldfield Property, Nevada, USA, July 25, 2013.
