Phu Kham, Laos: Ready for the advent of harder ores

Open-pit mine with complex geological conditions

The Phu Kham open-pit copper and gold mine is located about 100 kilometers northeast of Vientiane, the capital of Laos. Due to weathering, alteration, fracture, folds and other reasons, the geological conditions of the deposit are very changeable. The geological composition is a complex heterogeneous mineralized layer of gold-copper network veins and skarn mineralization. The contact between weathering and groundwater level has formed a soft leaching zone and an overlying transition zone with secondary copper mineralization dominated by epigenetic chalcocite and clay-rich coal gangue. The rock body strength and weathering degree of the entire deposit vary greatly, and hard rocks are found in the deep layers of the ore body.

The mine has a large number of conventional open-pit mine feed equipment, as well as process equipment consisting of crushing, grinding and flotation processes to recover copper and precious metals. The current annual output is 19.5 metric tons. The crushing section is a single-stage crushing consisting of a gyratory crusher. The grinding circuit includes one semi-automatic mill and two parallel ball mills, one for each closed circuit and equipped with a cyclone. The next flotation circuit includes rough selection, regrind and several selection stages. The product is 22-25% copper and gold concentrate, 7g/t gold and 60g/t silver.

The Phu Kham copper-gold deposit in Laos is an extremely heterogeneous ore body. Mineral deposits have complex and changeable mineralogical, geological and geotechnical properties, which will affect mine output and metallurgical properties. As the mining depth increases, the situation will become more complicated. Therefore, potential capacity constraints will bring risks to long-term profitability.

In 2012, with the help of Metso's process technology and innovation team, Phu BIA Mining Co., Ltd. began to implement capacity forecasting and optimization projects to evaluate how to maintain target production and profit within the mine's lifecycle (LOM). The goal is to find ways to increase production while processing hard ore, develop production forecasting models, and determine if and when changes to the crushing and other processes are needed to maintain the target production during the mine’s life.

The project involves ore characterization; using Metso's SmartTag™ ore tracking system to evaluate in detail the blasting and crushing operations related to ore characteristic data; and to develop specific field models of the blasting and crushing process. Integrate these models to obtain optimization tools for the entire production process and capacity prediction.

The "recipe" for effective blasting

The optimization process starts with qualitative ore to determine the same mine area in the entire blasting and crushing process. With a given blasting design, similar raw ore (ROM) fragments are formed in the ore in the domain. By controlling the fragments of the original ore, the mine can be increased in production.

One of the main goals is to develop strategies to maximize mill output to maintain the target output during the mine's lifetime (LOM) while processing hard ores. As expected, the blasting model and simulation show that dense blasting holes and increasing the effective coefficient of explosives can greatly increase the amount of fine particles produced by blasting. In addition, reducing the length of the blasthole sealing mud can also produce more blasting fine particles and reduce the maximum particle size of the ore. The reason is to increase the blasting energy of the blasthole sealing mud position. These simulations show that by optimizing the blasting parameters, the amount of fine powder can be increased and the maximum particle size of the raw ore fragments can be reduced, thereby increasing the output.

Simulations and specific blasting designs have been carried out for each mining area, so as to form an optimized blasting design "good recipe" for each mining area. Use these "good recipes" to achieve a more consistent and optimized downstream process feedstock particle size distribution, and improve output, process stability and efficiency. Following these methods can also avoid excessive blasting of soft minerals, thereby reducing energy consumption and costs, and preventing the generation of a large number of ultra-fine particles, so as not to adversely affect certain 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 SmartTag^TM and GeoMetso™

To link the process performance with ore characterisation and blasting outcomes, ore is tracked from the mine through the process using SmartTag^TM 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 SmartTag^TM 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 optimize 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 specialized tools such as SmartTag^TM and GeoMetso^TM 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.