Copper concentrate quality improvements at Myra Falls
NVI Mining’s Myra Falls operation is a 40 year-old underground mining operation feeding a 4,000 metric tonne per day concentrator. The current mill, built in 1985, was designed to process a large massive sulphide ore body. This ore body, known as the “HW,” contains about 50 per cent pyrite. Minerals of value are chalcopyrite and sphalerite, with precious metals occurring in all mineral types. The flotation process used is sequential, first for copper and then zinc, and employs roughing, scavenging, concentrate regrinding and then cleaning of mechanical and column cells.
By 1997, the mining of a new ore body, known as Battle-Gap, had commenced. Mineralogically complex, Battle-Gap contains about 15 per cent pyrite and copper minerals, including primary chalcopyrite, secondary bornite and chalcocite and tennantite, combined with increased galena and sphalerite. Flotation performance generally decreased with increasing amounts of Battle-Gap ore types. The varying amount of secondary copper minerals resulted in inconsistent copper recoveries and variable copper grades in concentrate. A significant secondary effect was dilution of the copper concentrate with both galena and sphalerite. The combined lead and sphalerite grades became a concentrate marketing challenge, incurring large smelter penalty charges.
On-site plant and laboratory testwork confirmed that if the proportion of the Battle-Gap ore exceeded 10 per cent, then plant performance deteriorated. This greatly restricted the mining plan.
The objective became to develop a new flotation chemistry/flowsheet solution for Battle Gap ore types. On site, plant circuit surveys were completed that, combined with ethylenediaminetetraacetic acid (EDTA) metal ion extractions, gave good insight into why the sphalerite was being recovered into the copper products. This insight was confirmed by a TOF-LIMS study, which showed lead ions activating the sphalerite.
Off site, a composite of Battle-Gap ore was used to test varying flotation chemistries and flowsheets. The flotation chemistry and flowsheets of copper-lead bulk, followed by copper and lead split and sequential zinc flotation, versus a full sequential flowsheet of copper, then lead, then zinc flotation, were developed and tested. The overall evaluation was in favour of the fully sequential chemistry and flowsheet.
The new sequential chemistry included the addition of a novel lead-ion depressant within grinding, new copper collectors, a new lead activation stage followed by flotation with lead collector and, finally, zinc activation-flotation. The use of these new collectors increased flotation kinetics, allowing the plant flotation residence times to be roughly halved.
To confirm success, the new copper chemistry was introduced to the operation first; it immediately improved copper concentrate marketability. Following a series of staged mechanical flotation and flowsheet changes, the full sequential flowsheet was implemented. As part of this new flowsheet, all the cleaning circuits were changed to three stages of mechanical cleaning. Post commissioning, plant results improved to their predicated values, especially copper concentrate quality. A second TOF-LIMS study targeting the sphalerite contained within the copper concentrate demonstrated that little sphalerite showed lead-ion activation.
The use of a lead-specific depressant within the grinding stages, combined with a more typical copper collector combination, has resulted in the reduction of both lead and zinc contents of copper concentrate by some 40 per cent.
The use of the improved flotation reagent schemes and additional conditioning time improved flotation rates, allowing about a 50 per cent reduction in required plant residence times.
The novel plant lead depressant, dextrin:sodium mono-phosphate (80:20), is effective in copper flotation by two mechanisms: – Dextrin is used as the galena depressant, and – The sodium phosphate complexes the lead ions, preventing sphalerite activation.