Gatsuurt Mining & Milling
The mining of the Central Zone and the Main Zone will occur during two distinct phases. The initial phase involves the mining of the oxide and non-refractory transitional ore and the stockpiling of the small amounts of refractory sulphide ore. A typical mining program is planned with a 15 cubic metre shovel and a fleet of 90-tonne trucks to move the waste and ore. As Gatsuurt is approximately 55 kilometres (by road) from Boroo, the ores will be recovered from the Gatsuurt stockpiles and hauled to Boroo using road haul trucks with 40-tonne haulage capacity. Haulage of the oxide and non-refractory transitional ores to Boroo is planned to take place 24 hours per day and 7 days per week at a rate sufficient to ensure continuous processing of 5,100 tonnes of ore per day.
As of December 31, 2010, the initial infrastructure construction phase has been completed. The construction of the road between Gatsuurt and Boroo was completed in July 2010. The decision was made by the Company to suspend further construction work until such time as the Company has received the permitting and commission requirements for the Gatsuurt mine.
The second phase of mining is the pit development to expose and mine refractory sulphide ore at an average mill feed grade of 3.1 grams of gold per tonne. The mining rate and haulage rate will operate to support the 5,100 tonnes per day milling rate at the modified Boroo processing facility.
Once project approvals have been received, the Gatsuurt oxide and non-refractory transitional ores will be processed at the Boroo CIP facility. Any remaining low-grade Boroo ore will either remain in stockpile and be processed at the end of the project life, or, if the heap leach facility has received its operating license, be placed on the heap leach pad for processing. Approximately 18 months before completing mining and processing of the Gatsuurt oxide and transitional ores, construction will begin on the bio-oxidation plant, and associated infrastructure, in preparation of processing the Gatsuurt refractory ore
The processing of the Gatsuurt refractory ores at Boroo will require modifications and additions to the Boroo facility. As the refractory ores from Gatsuurt are harder, the instantaneous feed rate will be 5,100 tonnes per day (an average feed rate of 4,800 tonnes per day for 365 days per year) 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 16.7% sulphide-sulphur and 9% 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 initially and then to the tailings management facility.
Test work has assessed the recovery of gold in the gravity gold recovery circuit, the amenability of the oxide ores to cyanide leaching and the recovery of gold following the bio-oxidation process. An overall gold recovery of 87% is projected for the oxide via the mill leach circuit, 73% is projected for transitional material, and 87% total recovery projected for the refractory ores by the bio-oxidation treatment process. The process of treating the refractory ores has been tested and reported to Centerra by Biomin, the legal proprietor of BIOX®, (a bio-oxidation circuit), a proprietary and patented process.
The Gatsuurt ores will be crushed and ground at the Boroo mill. The slurries will be processed through a sulphide recovery flotation plant, which is part of the planned expansion of the Boroo plant. The concentrates, which will represent approximately 9% of the original mass and have a sulphide-sulphur level of 16.7%, will then be introduced to the bio-oxidation circuit.
With respect to the bio-oxidation plant, an engineering company has been engaged to provide engineering, procurement and construction management services, although such services have been temporarily suspended until the permitting and commissioning requirements are obtained for the Gatsuurt mine. Capital investments for phase 2 work are conditional on the receipt of the necessary permits and commissions for the Gatsuurt mine.