Boroo Mining and Milling
The Boroo deposit was mined using conventional open pit mining methods. The Boroo oxide mineral reserve has been exhausted. Subject to the resolution of the status of the Gatsuurt project, the Boroo project could be re-activated to mine the remaining sulphide reserves for potential processing either through the existing CIL plant, in the event of a negative resolution on Gatsuurt, or through a newly constructed bio-oxidation facility, in the event of favourable resolution. The remaining life-of-mine strip ratio is expected to be 7.4 to 1. The mine historically operated two 12-hour shifts each day with four crews working a seven-day rotation at the site.
Mining was done with bench heights of five metres, with ore mined on half-benches for improved grade control in the flat lying ore. Three to four benches were typically under development at any given time. Blast hole drilling was carried out with two rotary-percussion drill rigs. Bulk explosives trucks blended ammonium nitrate with fuel oil or emulsion for wet holes as each hole was loaded. The principal mining equipment included two hydraulic excavators and ten 50-tonne haul trucks, and the waste rock mined was deposited on waste dumps immediately adjacent to the individual pits. Additional mining equipment included two front-end loaders for supplementary loading, ore handling and blending, two tracked dozers and one wheel dozer for the maintenance of waste dumps and benches and two graders for the maintenance of the roads and bench floors. Grade control was achieved by sampling of the blast hole cuttings. The blast hole assay data was determined at a laboratory in Ulaanbaatar and combined into an ore control model. The model was used to determine the boundaries for the various ore, stockpile and waste categories and to estimate the monthly pit production. Boundaries between material types were surveyed and digging supervised by grade control staff to ensure that ore and waste rock were separated correctly. Ore in seven blasthole samples was randomly selected for leach testing.
Following the cessation of mining at the end of November 2010 and the subsequent workforce reductions, Boroo has a total of approximately 300 employees (excluding long-term contractors).
The Boroo flowsheet for ore processing is a standard layout that consists of crushing, grinding, gravity concentration, cyanide leaching and gold recovery in a CIL circuit. The mill was designed with a capacity to process 1.8 million tonnes of ore per year but the actual mill throughput is currently approximately 2.1 million tonnes per year or approximately 6,840 tonnes per day.
Mill recovery is dependent on the head grade and mineralization of the ore feed material. Recovery has steadily decreased as oxide and transitional ores have been depleted. When processing sulphide or fresh ore, mill recovery is typically in the range of 40% to 70%. A significant portion of the recovery has historically been achieved by gravity separation and it is expected that this would continue for the remaining ore at Boroo and will continue for the Gatsuurt ores.
A jaw crusher reduces the ore to 100% minus 20 centimetres. The crushed ore is fed directly to a semi-autogeneous (“SAG”) mill (8.5-metre in diameter) or to a temporary coarse ore stockpile from which it can be reclaimed during crusher maintenance. Cyclones divide the ore into two streams, with the cyclone underflow reporting to the ball mill and the cyclone overflow reporting to the carbon in pulp (“CIP”) circuit. About 20% of the total cyclone underflow reports to the gravity circuit, which consists of two 750-millimetre Knelson concentrators followed by an Acacia reactor where the gravity-recovered gold is leached in high cyanide solution.
The cyclone overflow is thickened prior to the leaching circuit that consists of two pre-leach tanks where oxygen is injected, followed by six stage CIP tanks. Gold in solution from the leaching circuit is recovered on the carbon in the CIP circuit. The recovered gold is subsequently stripped from the carbon and again put in solution to be recovered by electrowinning, followed by smelting and the production of a doré bar. The tailings after processing of the ore are detoxified to meet a target cyanide level of one part per million using an air-sulphur-dioxide process. Heavy metals are removed by treatment with ferric sulphate. The tailings are discharged by gravity to the permanent tailings management facility five kilometres down gradient from the process plant.
Gatsuurt ores are expected to be processed at Boroo. The processing of the refractory ores from Gatsuurt at Boroo will require modifications and additions to the Boroo facility. As these refractory ores have been tested as being harder, the instantaneous feed rate is expected to be 5,100 tonnes per day at a grind of 80% passing 75 microns. Following the grinding circuit, a flotation circuit composed of a rougher and scavenger circuit will be added to recover the sulphides and produce a concentrate with 14% sulphide-sulphur and 8% to 10% of the original mass. The flotation concentrate will then report to the bio-oxidation circuit. Following oxidation of the sulphides and neutralization of the slurry, the oxidized slurry will then be leached with cyanide for 24 hours and the gold will be recovered on carbon in the Kemix carbon cells. The slurry reports to the cyanide destruction circuit and to the tailings management facility.
In June 2008, Centerra completed construction of and commissioned a three million tonne per year heap leach facility at Boroo for a capital investment of $26 million. A feasibility study was completed in September 2006 to support the project.
The heap leach project provides for strategic growth by creating an opportunity to process nearby low-grade deposits using heap leach technology.
TetraTech of Denver, Colorado provided the heap pad design while Ausenco Ltd. of Brisbane, Australia provided the process design and procurement assistance for the equipment and supplies. The lining of the pregnant solution pond and the emergency overflow pond were completed during the summer of 2007. The lining of the pad with linear low-density polyethylene was divided into two phases. Phase 1 was completed in 2007 and 169,900 square metres of pad was lined, while 118,000 square metres of Phase 2 pad was completed in 2008. The project was mechanically and electrically completed in March 2008; however, solution application commenced in June 2008 once all preliminary permitting was completed.
The heap leach operated under a temporary permit until October 2008, which was extended by the Government of Mongolia for six months. In April 2009, heap leach operations were suspended with the expiry of the temporary permit. When the permanent permit for the heap leach operations has been received, heap leach operations will resume the processing of the existing heap leap stockpile.
All remaining ore from the Boroo project and Gatsuurt project (when commissioned) is expected to be processed at the Boroo processing facilities. Modifications are also planned for the Boroo mill to enable concentrate flotation and bio-oxidation circuits for processing refractory ores.
For the remainder of Boroo’s mine life, gold production is expected to come from three sources:
- processing of the stockpiled oxide and transitional ores, the majority of which is low grade ore that was originally intended for the heap leach;
- heap leach processing provided the permanent permits are received; and
- processing of the refractory sulfide ore reserve whether through a bio-oxidation facility, in the event that the Gatsuurt project is commissioned, or through the existing CIP processing facility, in the event that Gatsuurt is not commissioned.