Golden Dawn's May 2016 updated preliminary economic assessment for the Greenwood Gold Project in British Columbia would reuse an idled concentrator to treat Lexington-Grenoble and Golden Crown ore into a gold gravity concentrate and a copper-gold flotation concentrate.
The Greenwood Gold Project lies near Greenwood in south central British Columbia, Canada, and combines the Lexington-Grenoble and Golden Crown deposits. P&E Mining Consultants prepared the Updated Preliminary Economic Assessment for Golden Dawn Minerals Inc. and Huakan International Mining Inc. The report has an effective date of March 24, 2016 and a signing date of May 20, 2016. Mill feed would go to the existing Greenwood Mill and concentrator, which is on care and maintenance, 17 km from the Lexington-Grenoble mine and 2 km from Golden Crown. Construction of the plant began in 2007, and it operated from April to December 2008 before shutting down for reasons reported as low metal prices and lower than expected head grades from mining dilution.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Lexington-Grenoble mill feed | 356,000 | t | Life of mines plan |
| Golden Crown mill feed | 191,000 | t | Life of mines plan |
| Lexington-Grenoble head grade | 5.48 | g/t Au | 0.90% Cu |
| Golden Crown head grade | 8.67 | g/t Au | 0.48% Cu |
| Initial throughput | 212 | t/d | 8.3 t/h average |
| Expanded throughput | 424 | t/d | After one year |
| Plant availability | 93 | % | Not applicable |
| Bond ball mill work index | 12.5 | kWh/t | Not applicable |
| Primary grind | 105 | µm P80 | Ball mill cyclone overflow |
| Cleaner flotation grind | 74 | µm P80 | After regrind |
| Gravity gold recovery | 38.5 | % | Tabled |
| Gold to flotation concentrate | 42.0 | % | Not applicable |
| Combined gold recovery | 80.4 | % | Gravity plus flotation |
| Copper recovery | 85.6 | % | To flotation concentrate |
| Concentrate grades | 97 g/t Au / 23% Cu | Not applicable | Float concentrate |
Overview
The plant incorporates conventional mineral processing to produce a gold gravity concentrate, and a copper-gold flotation concentrate. Ore is crushed in a jaw and cone circuit, ground in a ball mill with cyclones, and part of the cyclone underflow goes to a Knelson concentrator. Cyclone overflow goes to rougher flotation with three stages of cleaning. The final concentrate is thickened, filtered and bagged for sale. Lexington-Grenoble recoveries are 86% for gold and 87.5% for copper, based on laboratory work and earlier operating data. Golden Crown has recoveries of 70% for gold and 82% for copper, and the report calls its test work limited and variable.
Key Process Stages
Crushing
Mine material of minus 460 mm arrives in 40 t loads from a contractor and is dumped on coarse mill feed stockpiles. A front-end loader feeds a receiving hopper through a 460 mm grizzly, and a portable rock breaker handles oversize. A belt feeder takes ore to a 100 hp, 24 inch by 36 inch jaw crusher, which reduces it to a nominal 75 mm. A 250 hp, 4.5 ft standard head cone crusher then works in closed circuit with a 40 hp double deck vibrating screen. The minus 9.5 mm undersize goes to an outdoor fine ore stockpile on a concrete pad, with two slots feeding the ball mill conveyor and a second pair reserved for expansion. The crushing equipment was bought new from China as a mobile system fixed to a steel frame on a concrete pad, and it is not under cover. It was originally sized for more than double the planned throughput.
Grinding and gravity
Fine feed goes to a 200 hp, 8 ft by 6 ft Koppers conical ball mill in closed circuit with a hydrocyclone at a 300% design circulating load. Cyclone overflow is aimed at 80% passing 90 µm and goes to flotation. Underflow reports to a 10 mesh vibrating screen, with oversize back to the mill and undersize to a Knelson CD-20 centrifugal concentrator. Backflush times are set by hand according to head grade. The gravity concentrate is cleaned about once a day on a Gemini shaking table. The site has no furnace, so tabled concentrate would be stored in a safe and shipped elsewhere for smelting to doré. A 4 ft by 8 ft, 50 hp regrind ball mill, in closed circuit with a hydrocyclone, grinds rougher concentrate to 74 µm P80 ahead of cleaning.
Flotation
The flotation circuit has a bulk rougher concentrate cleaned in the regrind mill before three stages of cleaning. Methyl isobutyl carbinol (MIBC) is the frother, and potassium amyl xanthate (PAX) and Aerophine A3418 are the collectors. Lime depresses pyrite and sets pH, mainly in the cleaning circuit. Lime arrives in two tonne bulk bags and is slaked beside the circuit. The Denver cells are four-cell banks for roughing, rougher scavenging, first cleaning and first cleaner scavenging (each cell 5 ft by 5 ft by 4 ft deep), plus 4 ft by 4.5 ft by 3.5 ft banks for second and third cleaning. Tailings go by a 10 hp centrifugal pump to a membrane-lined storage pond located several tens of metres below the plant.
Concentrate handling
Third cleaner concentrate is thickened in one of two 20 ft thickeners, with overflow recycled to the process water tank. Underflow goes to a stock tank and then to a Larox PF3 pressure filter, which cuts moisture to about 10 wt.%. Aerodri 104 can be used as a filtering agent. Concentrate is discharged to two conveyors and bagged by hand, and each bag is sampled before it goes to covered storage to wait for transport.
Additional Interesting Data and Summary
The mill was cleaned out and shut down properly in 2008, but some equipment needs refurbishment and electrical and instrumentation items may need repair. Remaining equipment is mostly refurbished and housed in an insulated steel-frame mill building. Site facilities include an administration trailer, a sample room in a cargo container, a tensioned fabric structure for storing bagged concentrate and an assay lab with an atomic absorption spectrophotometer and fire assay furnace.
The expansion to 424 t/d requires a second primary grinding mill and changes to the rougher flotation circuit. Much of the crushing, regrind, scavenger and cleaner flotation and dewatering equipment was designed to handle it. Part of the Golden Crown gold reports with iron sulphides, which are depressed during cleaning to protect copper grade. The report treats that gold as lost, while noting it as an opportunity to lift gold yield, and recommends more test work.
Reagent use is 25 g/t of MIBC, 50 g/t each of A3418 and PAX, 20 g/t of flocculant and 0.70 kg/t of lime. Grinding media is 50 mm alloy steel balls at 1.0 kg per tonne of mill feed.
Key Processes
- Jaw and cone crushing in closed circuit with a double deck screen
- Ball milling with cyclone classification
- Knelson gravity concentration and shaking table cleaning
- Bulk rougher flotation with three stages of cleaning
- Regrinding of rougher concentrate
- Thickening and pressure filtration of concentrate
Source: Updated Preliminary Economic Assessment on the Greenwood Gold Project, Greenwood, British Columbia, Canada, May 20, 2016.
