Mining & Milling
Mining operations at Kumtor are carried out using conventional open-pit mining methods. The Central deposit is mined in a large open pit where total material mined in 2010 was approximately 116 million tonnes, or 318,000 tonnes per day. The overall waste to ore ratio from the open pits in 2010 was 19.2 to 1.
The top mining elevation in the Central deposit’s current ultimate pit design is at about 4,460 metres, and the very deepest part of the final pit excavation will be at 3,618 metres in the southwest part of the deposit. The crushing plant to which ore is delivered is at about 4,050 metres and ore transport was thus downhill for the upper portion of the orebody, and will have a maximum uphill haul of 432 metres for the lower portion. The ore haulage distance from the Sarytor and Southwest deposits, scheduled to be mined starting in 2015, will be 7.8 kilometres.
Waste disposal continues to be placed in the lower parts of the Davidov valley. The waste is not considered acid generating because of its high carbonate content and the capacity to neutralize any acids that may form.
Mining is based on eight-metre benches while the smaller Sarytor and Southwest deposits will be mined on nominal four-metre benches for better mining selectivity of the smaller ore zones. Blast holes are drilled using eleven rotary-percussion drill rigs for waste material and two DTH (down the hole) Hammers for ore blocks. Charging the holes is undertaken by special bulk explosives trucks delivering either ammonium nitrate with fuel oil (ANFO), or the use of emulsion explosives for wet holes.
The main loading fleet operating at the end of 2011 includes sixteen hydraulic excavators (some configured as shovels and others in a backhoe configuration) and three front-end loaders. Typically, the shovels are used for production and the loaders for ore blending, cleanup and support during shovel maintenance.
Due to the substantial increase in waste tonnages forecast to be mined in the years 2011 to 2015, the planned initial mining of the more distant Sarytor and Southwest deposit, and the required replacement of certain shovels and haul trucks, the mining fleet will be increased in the next few years in order to meet mine production schedule of the new life-of-mine plan.
Grade control in the pit is based on the sampling of blast hole cuttings, the grade and metallurgical character of which are determined at the metallurgical laboratories in the mill building. This information is entered into the ore grade control model, based upon which the various ore blocks are staked in the field for digging. The ore is then delivered to the crusher or the appropriate stockpile depending on the daily blending requirements. Kumtor has an active and dynamic blending program in close contact with the mill that adjusts the ore blend as required to maximize the gold recovery.
The current Kumtor plant flowsheet reflects the fine-grained nature of the gold and its intimate association with pyrite and consists of crushing, grinding, pyrite flotation and double re-grinding of the flotation concentrate. Two separate carbon-in-leach (“CIL”) circuits recover the gold from the re-ground concentrate and from the flotation tails, with final gold recovery accomplished by electrowinning and refining. The mill was originally designed with a capacity to process 4.8 million tonnes of ore per year, but the actual mill throughput in 2011 was approximately 5.7 million tonnes per year.
The ore to be milled is managed through a number of stockpiles that receive ore of different metallurgical character and of different grade ranges and thus allow blending of the mill feed. A gyratory crusher reduces the ore to minus 15-20 centimetres. The ore is then fed to a coarse ore stockpile from which it is reclaimed for grinding, first to a semi-autogenous (“SAG”) mill and then to a ball mill, which together reduce the grain size to 80% passing 140 microns. A bulk sulphide concentrate representing 7% to 11% of the original mill feed is then produced with a grade of 30 to 50 grams of gold per tonne and a gold recovery of 87% to 92% into the concentrate.
The flotation concentrate is re-ground to approximately 90% passing 20 microns. After thickening to 60% solids, it is once more re-ground to 95% to 98% passing 20 microns in an ultra-fine grinding (“ISA”) mill, re-pulped to 45% solids, pre-aerated for 40 hours and leached for 80 hours in the CIL circuit consisting of four agitated tanks in series. The ISA mill was commissioned in October 2005 and provides additional incremental liberation of the fine refractory gold (2-5 microns) enclosed in pyrite.
The flotation tailings with an average grade of 0.45 gram of gold per tonne are thickened to 50% solids and subjected to cyanidation for ten hours in a CIL circuit similar to the circuit used for the sulphide concentrate. The carbon in both CIL circuits is moved forward counter-current to the slurry flow, and the loaded carbon from the first flotation tailings CIL tank is pumped to the third concentrate CIL tank to continue loading. Loaded carbon from the first concentrate CIL tank is pumped to the gold recovery plant. The loaded carbon is stripped and the gold subsequently recovered by electro-winning. Gold recovery in the CIL circuits is generally 90% for the sulphide concentrate and 25-30% for the flotation tailings.
Concentrate CIL tailings and flotation CIL tailings are combined and discharged by gravity to the tailings disposal area through a slurry pipeline system.