23 Aug Improvement of Grinding Efficiency at Kevitsa
The Kevitsa open-pit mine in Finland is located approximately 142 kilometres north-northeast of Rovaniemi, the capital of Finnish Lapland. The mine hosts significant quantities of nickel-copper-cobalt-PGE (platinum group elements) ore and is one of the largest ever mineral discoveries in Finland.
The operation was commissioned in 2012, and consists of a partial secondary crushing circuit followed by an AG – Pebble grinding circuit with two partially closed-circuit AG mills and a single closed-circuit pebble mill.
The ore is variable and can range from average competency to extreme. As a result, the circuit can swing from being AG limited to pebble mill limited. When processing extremely competent ore, the circuit becomes AG mill limited, restricting throughput. This prompted the staff at Kevitsa to approach OMC to model the circuit and conduct simulations to confirm optimisation strategies to maximise throughput during these periods and to provide new insights and suggestions.
Prior to optimisation, the primary crushed product was partially secondary crushed, with the -100mm / +25mm secondary crushed prior to feeding the AG mill. The +100mm lump material and -25mm fines reported directly to the AG mill feed. The proportion of +100mm material in the primary crushed product was very high, at approximately 40% of the total primary crusher product. This material was building up in the AG mills, limiting throughput.
The classification efficiency for the AG mill cyclones was also observed to be very poor. These cyclones were operating at very high circulating loads in an attempt to improve the pebble extraction rate through the AG mill grates. However, this resulted in more than half of the product size material in the cyclone feed being recycled back to the AG mills. This consumed power and contributed to over-grinding.
Having the AG mills in partial closed circuit provided circuit flexibility to manage the split between the AG mills and the pebble mill, fully utilising available power. This strategy worked well on less competent ores when the circuit was pebble mill limited but not as well for extremely competent ore with the recycle further restricting AG mill capacity. In this case it was difficult to maintain sufficient flow to operate the AG mill cyclones efficiently and maintain sufficient flow to the pebble mill to prevent over-grinding.
Overall, during very competent ore campaigns, the circuit was found to be utilising approximately 10% more power than theoretically required based on the measured the ore properties.
The Optimisation Strategy
Given the issues identified, a two-pronged approach to optimisation was developed. This involved; reducing the amount of lump in the AG mill feed, and reducing the amount of over-grinding due to inefficient operation of the AG mill cyclones.
As part of the optimisation strategy, OMC modelled the impact of crushing a larger proportion of the lump material. This was achieved by increasing the aperture of the primary screen top-deck from 100mm to 150mm. This reduced the proportion of lump in the feed from 40% to approximately 25%.
OMC also recommended larger vortex finders and the use of less AG mill cyclones to reduce the circulating load and improve performance of the AG mill.
Progressively higher throughputs have been achieved at Kevitsa over time, even on hard ore types. Significant credit must be attributed to the staff at Kevitsa for their commitment to continual improvement, identifying issues, carrying out extensive testing (including 4 surveys) and implementation of optimisation strategies in the plant.
By increasing the top deck aperture on the primary screen, better control over the proportion of lump material reporting to the AG mills has been achieved. This has alleviated the AG mill limitation when treating extremely competent ore, increasing throughput rates.
A concerted effort by site to improve the classification efficiency of the AG mil cyclones has resulted in a reduction in the circulating load by approximately half and the proportion of product size material reporting to underflow has reduced from 60% to 25%.
Both the proportion of lump in the feed and improvements in the classification efficiency has contributed to significantly better breakage within the AG mills. This has resulted in higher scatting rates and a coarser AG mill discharge PSD. As a result, more of the AG mill discharge has been transferred to the pebble mill, reducing over-grinding and enabling a higher overall utilisation of power, maximising throughput.
The contribution of better breakage and more efficient classification has also improved the circuit efficiency. The circuit specific power consumption is now in line with theoretical predictions, a reduction in specific energy of approximately 10%.
To learn more about our Kevitsa project, or any of the other work that we’re doing, contact Orway Mineral Consultants today!