The report details the gold recovery methods for the SKO Project, encompassing an existing 1.2 Mtpa facility and a proposed 2.5 Mtpa plant design based on testwork and historical operating data.
Report context
This technical report, dated 30 March 2012, addresses the gold recovery methods for the SKO Expansion Project (SKOEP). The existing 1.2 Mtpa Jubilee processing facility, commissioned in 1987, currently processes production from SKO mines and the Alacer-owned (49%) Frog’s Leg mine. A new 2.5 Mtpa processing facility is being considered as an integral part of SKOEP, with Alacer having commissioned GRES in 2011 to evaluate plant design options and develop associated capital and operating costs.
Processing route
Existing 1.2 Mtpa Jubilee Processing Facility
The Jubilee processing facility process flow sheet comprises primary jaw crushing, secondary and tertiary cone crushing in closed circuit with screens, primary ball milling, closed circuit secondary ball milling, cyanide leaching, carbon-in-pulp (CIP) adsorption, split Anglo-American Research Laboratories (AARL) elution, electrowinning, smelting and tailings disposal. Ore from various sources is separately stockpiled on the ROM pad and reclaimed by front-end loader to achieve the required daily blend specification, specifically on the basis of hardness.
A gravity circuit treats a portion of the secondary ball mill cyclone underflow. Concentrate from a 30-inch Knelson concentrator is treated in an Acacia leach reactor, with leach solution reporting to the eluate tanks in the secure gold room for electrowinning. Solid tailings are returned to the mill discharge stream. The gravity circuit recovers approximately 30 to 40% of the overall gold recovered by the Jubilee processing facility.
Leaching takes place in three mechanically agitated 600 m³ tanks, with a target cyanide concentration of 200 to 250 ppm. The pH is kept relatively low at approximately 9.1 due to the hypersaline water used in the process, which contains 60,000 to 90,000 ppm total dissolved solids (TDS). Adsorption is performed in eight 375 m³ mechanically agitated tanks with activated carbon advanced counter-current to the slurry flow by recessed impeller pumps. The adsorption tanks are split into two parallel trains. Carbon is loaded to a gold concentration of up to approximately 2,500 to 3,500 g/t.
Gold is eluted from the carbon by a conventional split AARL process, electrowon and smelted to produce Doré bullion. The elution circuit operates six days per week. Gold electrowinning and smelting processes are performed inside a secure gold room following conventional practice in the Western Australian gold industry.
Unthickened tailings are currently pumped to the previously worked out Samphire open pit. SKO currently holds active TSF licence approvals for Samphire and additional TSF capacity in the historical Jubilee TSF.
Proposed 2.5 Mtpa Processing Facility (SKOEP)
The new 2.5 Mtpa processing facility will be constructed on the plant site previously occupied by the New Celebration treatment plant, which was deconstructed and cleared in 2008/9. This site is central to the expanded SKO pits and new TSFs. The process design and equipment selection has been based on the feed ore being 100% HBJ porphyry ore, with the plant designed for a feed grade of 2.5 g/t inclusive of Frog’s Leg ore.
Crushing, Milling, Gravity and Classification
The new plant will comprise a three-stage crushing circuit utilising a double toggle jaw crusher, a scalping screen and a single secondary cone crusher operating in open circuit. The secondary crusher product and scalping screen undersize will combine and report to a double deck product screen. Oversize from the product screen will be conveyed to the tertiary crusher surge bin, with each bin equipped with a vibratory pan feeder to meter ore delivery to each of two tertiary cone crushers. Each tertiary crusher (Metso HP5 or equivalent) would have a 450 kW motor installed and operate with a closed side setting of 13 mm to produce a product having 80% passing 9 mm.
Product screen undersize material will be stored in a fine ore bin equipped with an emergency load-out door to facilitate creation of an emergency mill feed stockpile.
The grinding circuit design utilised measured values for Bond rod work indices and Bond ball work indices from testwork. The single stage ball mill, selected to be 6.10 m diameter by 9.0 m long, will operate as an overflow mill with a ball charge volume of 34% and at 75% critical speed, fitted with a 6,000 kW drive motor and reduction gearbox. The ball mill will operate in closed circuit with cyclone classification and a gravity circuit. A bleed stream of cyclone underflow will be delivered to the gravity screen, with screen undersize feeding a 40-inch Knelson concentrator (QS40 model). The concentrator can be configured to use the same rotating assembly as the XD40 Knelson but utilise a different bowl.
Leach and Adsorption
The proposed leach circuit design is a hybrid CIL comprising two 2,250 m³ mechanically agitated leach tanks lined with ultra high build (UHB) epoxy coating. Sodium cyanide solution will be metered into the slurry at the feed point to commence leaching. Oxygen produced on site using a pressure swing adsorption (PSA) plant will be delivered under pressure down the leach tank agitator shafts as an aid to leaching kinetics.
The adsorption circuit will comprise six tanks having an operating volume of 1,100 m³ per tank, each fitted with a single mechanically wiped intertank screen. The carbon inventory within the CIL circuit will be approximately 76 tonnes. The carbon advance rate to maintain soluble losses below 0.02 ppm is 4.3 tpd; to maintain soluble losses below 0.01 ppm the CAR is predicted to be 5.2 tpd.
Elution and Gold Recovery
The elution system will be designed to operate as a Zadra pressure leach process with a 5-tonne capacity column fabricated from SAF2205 to serve the dual purpose of acid wash and elution. Cycle time is expected to be 16 hours for elution and 24 hours overall including acid wash and rinse. The electrowinning circuit will utilise stainless steel cathodes and high pressure water blasting to recover precious metal sludge. The Acacia reactor for intensive cyanidation of gravity concentrates will be housed within the gold room.
Tailings Disposal
Tailings will be deposited in the current licenced TSF being used at SKO, which will last until mid-way through 2015. Additional TSF locations will be required after 2015, with potential new in-pit tailings disposal locations to be assessed including the completed Celebration, Pernatty and Triumph open pits. An alternative TSF option using thickened tailings is also being investigated but has not been included in the reserve estimation.
Key reported parameters
| Parameter | Units | Design / New Plant | Historical (Jubilee) | Testwork Basis |
|---|---|---|---|---|
| Annual milling capacity | tpy | 2,500,000 | 1,200,000 | , |
| Head grade (design) | Au g/t | 2.5–2.6 | , | 100% HBJ porphyry: 1.3 g/t; includes Frog’s Leg |
| Gold recovery | % | 90–93 | 89.3–91.1 (F2006, C2009) | , |
| Crushing circuit | type | 3-stage | 3-stage | , |
| Ball mill power (installed) | MW | 5.6 | , | , |
| Milling rate | tph | 304 | , | , |
| Target grind | P80 µm | 106 | , | 14.7 kWh/t Bond Ball work index |
| Specific mill power (feed) | kWh/t | 16.1 | , | , |
| Gravity gold circuit | type | Knelson 40" + intensive leach | Knelson 30" + Acacia leach | , |
| Leach + CIP residence | hours | 24 | , | , |
| Leach tank volume | m³ | 2 × 2,250 | 3 × 600 | , |
| Adsorption tank volume | m³ | 6 × 1,100 | 8 × 375 | , |
| Process water TDS | ppm | , | 60,000–90,000 | , |
| Average load (new plant) | MW | 8 | 4 (existing) | , |
| Power supply design | MW | 10.5 (max) | , | , |
| Grinding balls consumption | kg/t | , | 0.56–0.89 (F2007, C2009) | , |
| Cyanide consumption | kg/t | , | 0.62–0.96 (F2007, C2009) | , |
| Lime consumption | kg/t | , | 3.05–5.17 (F2007, C2009) | , |
| Power consumption | kWh/t | , | 21.9–26.6 (F2007, C2009) | , |
| Bond Rod work index | kWh/t | , | , | 20.6 |
| Bond Ball work index | kWh/t | , | , | 14.7 |
| Ball mill pinion power (calc.) | kW | 5,200 | , | , |
Project website: https://www.newswire.ca/news-releases/alacer-gold-announces-fourth-quarter-mine-production-and-2012-guidance-509480841.html
Technical qualifications
The process design and equipment selection for the new 2.5 Mtpa facility has been based on the feed ore being 100% HBJ porphyry ore. The existing 1.2 Mtpa Jubilee processing facility is considered undersized for the project moving forward. The alternative TSF option using thickened tailings is currently being investigated by Alacer through testwork and has not been included in the reserve estimation. Historical performance data from the Jubilee facility is presented for typical annual examples in F2006 and C2009 periods. The new plant capacity takes into account processing of Alacer-owned (49%) ore feed from the Frog’s Leg mine.
Source: ALACER GOLD CORPORATION SKO Technical Report, Document No. 129510300$REP$R0001$00, 30 March 2012, Section 17.0 Recovery Methods.
