Phu Kham, Laos: Ready for the advent of harder ores
Metso assisted Phu BIA Mining Limited to improve their throughput forecasting and prepare for future needs well in advance. They are ready to face changes in ore type without compromising on productivity.
Authors: Duncan Bennett, Principal Metallurgist, PanAust Limited & Alan Tordoir Drill & Blast Superintendent, Phu Kham Operations, Phu Bia Mining Limited & Peter Walker, General Manager, Technical Services, PanAust Limited & Walter Valery, PhD, Senior Vice President, Global, Metso Process Technology & Innovation & David La Rosa, Manager, Mining Technology, Metso Process Technology & Innovation & Alex Jankovic, PhD, General Manager, Technology & Innovation, Metso Process Technology & Innovation
An open-pit mine with complex geology
Phu Kham is located approximately 100 km northeast of the Laos capital Vientiane. The geology of the deposit is highly variable due to weathering, alteration, faulting and folding. The rock mass strength and degree of weathering vary considerably across the deposit with extremely hard rock found in the deeper levels.
The operation comprises a large conventional open-pit mine feeding ore to a process plant consisting of crushing, grinding and flotation to recover copper and precious metals. The current capacity is 19.5
Mtpa. The concentrate contains 22 to 25 % copper, 7 grams per tonne (g/t) gold and 60 g/t silver.
The Phu Kham copper-gold deposit in Laos is an extremely heterogeneous orebody. The deposit has complex and variable mineralogical, geological and geotechnical properties, which affect plant throughput and metallurgical performance. To complicate things further, the operation is expecting harder ores as mining progresses deeper into the pit, which have the potential to limit throughput and poses a risk to long-term profitability.
To evaluate how to maintain the target throughputs – and profit – over the Life-of-Mine (LOM), the owner of the mine, Phu Bia Mining Limited, embarked on a throughput forecasting and optimisation project with the help of Metso’s Process Technology and Innovation team in 2012. The goal was to identify opportunities to increase throughput when treating hard ores, develop a throughput forecasting model, and determine if and when secondary crushing or other process changes would be required to maintain the target throughput over the LOM.
The project involved ore characterisation, detailed audits of blasting and comminution practices linked with ore characterisation data using Metso’s SmartTagTM ore tracking, and development of site-specific models for blasting and comminution processes. Integrating these models resulted in an optimisation tool for the overall operation, and for throughput forecasting.
A “Cookbook” for effective blasting
The optimisation process began with ore characterisation to define domains within the orebody that will behave similarly throughout the blasting and comminution processes. Ore within a domain will produce similar Run-Of-Mine (ROM) fragmentation for a given blast design. Improved plant throughput can be achieved by manipulating ROM fragmentation.
One of the main objectives was to develop strategies to maximise mill throughput to maintain LOM operational targets even when treating harder ores. As expected, the blast modelling and simulations that followed indicated that tightening the blast pattern to increase the powder factor resulted in a significant increase in the fines generated in the blast. Reduction of stemming length also generated more fines and reduced the top size of the rock due to the increased explosive energy at the stemming horizon. These simulations indicated the potential to increase throughput by increasing the fines and reducing the top size of the ROM fragmentation by optimising the blasting parameters.
Simulations were conducted for each of the ore domains defined at Phu Kham, and the blast design was optimised for each of the ore domains. This resulted in a “cookbook” which provides a “recipe” (i.e. an optimised blast design) for each ore domain. Blasting according to this cookbook provides a more consistent and optimised feed size distribution to the downstream processes, increasing throughput, process stability and efficiency. Following the cookbook also avoids excessive blasting in softer ore domains, thus reducing energy consumption and costs, and preventing the excessive production of ultrafines that can be detrimental to some downstream processes.
An integrated approach for further increases in throughput
The ROM size distributions generated in the blasting simulation study were used as inputs to the comminution models. This allowed changes to blasting practices and comminution circuit operation to be evaluated with respect to the entire operation. This integrated approach was used to determine effective operating strategies to increase throughput when processing harder ores.
In addition to demonstrating the benefits of the recommended blast design changes, the results indicated that further increases in throughput could be achieved by reducing the primary crusher closed side setting (CSS). The changes to blast designs increase the amount of fine material in the SAG feed and reduce the top size of the ROM size distribution, while reducing crusher gap primarily affects the amount of coarse particles in the SAG feed. Therefore, these are complementary strategies for increasing throughput.
Managing ore types with SmartTagTM and GeoMetso™
To link the process performance with ore characterisation and blasting outcomes, ore is tracked from the mine through the process using SmartTagTM ore tracking. At Phu Kham, SmartTag™ antennas are installed under the crusher product and SAG mill feed conveyors. As the SmartTags™ and associated ore pass the antennas in the process plant, the system automatically records the time and tag ID, thus the source of the ore being processed at any given time is known. During the project at Phu Kham, this ensured that ore from blasting trials was being fed to the concentrator during the plant audits, and allowed correlations to be established between ore origin and process performance.
A GeoMetso™ system is also being implemented at Phu Kham. It uses the SmartTagTM ore tracking technology to continuously collect plant performance data and automatically update the predictive models and block model in real time. This eliminates the need for further expensive ore characterisation tests, and improves the accuracy and predictive abilities of the geometallurgical models that were developed. This allows optimisation of the overall operation for each ore type to maximise production and minimise costs. With a better understanding of the impact of ore types on performance, long term mine planning can be improved and capital equipment purchases can be predicted well in advance of their requirement. In the short term, the processing plant receives advance notice of the ore type/s about to be processed and adjustments can be made to operating conditions to optimise plant performance.
Planning for the future
One of the primary objectives of the project was to develop a throughput forecasting model based on geometallurgical modelling for long term planning and optimisation. This was achieved using the integrated site-specific models for blasting and comminution in combination with the Phu Kham LOM plan.
The LOM ore delivery plan provided by Phu Kham was used along with the geotechnical block model to determine the ore characteristics for each period. The structure and strength increase as the pit gets deeper, before sharply dropping off again with shallow cut-backs until the end of the mine life. The integrated models were applied to this data to generate a throughput forecast, and indicate if or when harder ore would prevent throughput targets from being met.
Is secondary crushing or pebble crushing needed?
A further objective of the project was to evaluate whether or not a secondary crushing circuit or other process changes would be required to maintain throughput over the LOM. The throughput forecast results indicate that the annual throughput target can be achieved until 2018. However, in 2019, when the hardest and blockiest material is scheduled to be delivered, the annual throughput is predicted to be significantly less.
A secondary crushing circuit represents a significant capital investment. Therefore, firstly, the changes to blasting practices and comminution circuit operation identified earlier in the project (increasing blasthole diameter, tightening the blast pattern to increase powder factor and reducing the primary crusher CSS) were investigated to determine whether these could sufficiently increase throughput. Combining these changes could recover most of the throughput drop (increasing throughput by more than 8 %), but may not be sufficient to maintain the target throughput for 2019. However, according to the mine plan and throughput forecast, the hard and blocky ore and lower than target throughput will only occur during 2019. The structure and strength drop off again after that until the end of the mine life. Therefore, the expenditure for a secondary crushing or pebble circuit is not justified, particularly if the other recommended changes are implemented to reduce the drop in throughput caused by the ore characteristics.
We find the path together
By working together, Metso has assisted Phu Kham to develop a plan for the future; to maximise throughput and cope with the harder ores expected in the future. Using expert knowledge and specialised tools such as SmartTagTM and GeoMetsoTM ensures informed decisions are made on the path forward; maximising profitability and preventing unwarranted expenditure.
This article was originally published in the proceedings of the 12th AusIMM Mill Operators’ Conference 2014. The authors acknowledge Phu Bia Mining Limited and PanAust Limited for allowing this article to be published.