PROCESS DESIGN
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 ore properties. 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 100 mm to 150 mm. 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 four 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 %.
Upgrading VPA filters at Collahuasi
Faced with increased production demands, Compañía Minera Doña Inés de Collahuasi had the goal of increasing both the availability and performance of its vertical plate pressure filters( VPA) in Puerto Patache. In collaboration with Metso, Collahuasi developed an upgrade plan aimed at improving the performance of the equipment, as it was beginning to be outdated after more than 20 years of operation. With an upgrade plan based on using state-of-the-art technology, the desired objectives were met and resulted in a 15 % increase in filter availability.
Thanks to the collaborative relationship between Collahuasi and Metso, a joint diagnostic meeting was held to review the main needs and potential improvement to the filtrateconcentrate level, which was related to the requirements of its VPA vertical filters to increase their production capacity while also increasing the overall reliability of the system. Due to the high productivity requirements and over 20 years of use since its installation, the filters were showing a high degree of wear and misalignment, which reduced their availability due to the extra maintenance required by the filters and considerable unplanned downtime.
During the review stage of the overhaul, a joint decision was made to make improvements and upgrade the control systems, lubrication and structure of the equipment in order to achieve higher levels of availability. The challenge for Metso resulted in upgrading the control systems to the latest technology, achieving values of over 92 % availability and 100 % of design capability. This involved reinstalling and upgrading equipment in less than 20 days in order to minimise lost production. The engineering coordination, parts procurement and planning activities began six months earlier, in order to ensure the complete compatibility and successful outcome of this upgrade.
The extensive upgrade of the equipment
Changing the plates of the filter at Collahuasi
involved multiple steps. The Metso team first proposed improving the original design capacity by updating components and systems to produce some important benefits. For this, Metso focused on the main problems that were causing the structural misalignment of the filters. In order to solve the structural problems, the filter was completely disassembled and over 80 % of its parts were replaced with new parts using a nanocoating technology for corrosive environments. Using this approach would double the lifespan of the key parts and components and reduce potential downtime.
In order to achieve the optimal selection of fabric, a pilot filter was implemented at the worksite to take samples of concentrate. The samples were then used to help define the proper fabric for the filtering process given the specific mineral conditions that Collahuasi was processing. In many situations, these types of tests involve using Metso laboratories located in Brazil and Finland. However, with the pilot filter on site, it was possible to perform all the tests in situ with the client, improving the selection and making it easier and quicker to get the results.
Another technological improvement introduced was the implementation of OCS-4D,( an advanced process control solution). The solution assisted in determining the optimal control modes through analysing the interplay between the selection of fabrics and maintenance events.
Finally, the chute was also examined for possible improvement opportunities. As the VPA filters discharge the filtered material( cake) into
68 International Mining | JANUARY 2018