Understanding blast movement to optimise grade control practices at the Ahafo Gold Mine in Ghana

Abstract

Drilling and blasting are required in most open pit mining operations to fragment and loosen the in situ rock mass. The mischaracterisation of grade boundaries as a result of blast movement can have a major impact on ore loss and dilution, and can lead to significant financial losses in open pit mines. Ore loss takes place when valuable mineral is sent to waste dumps, and ore dilution occurs when waste material is mischaracterised as ore and sent for processing. Misclassification of ore also takes place when low-grade ore is processed through the mill instead of high-grade ore. Therefore, significant increases in a mining operation’s profit can be realised if blast movement is accurately measured and accounted for in grade control practices. Recent research conducted at The University of Queensland, Australia has developed innovative tools and techniques to measure and model blast movement. These tools and techniques can further advance the understanding of blast movement dynamics and optimise mine site grade control practices. This research project uses these tools and techniques to demonstrate the importance of field measurements and modelling blast movement, and to develop effective and practical grade control solutions to reduce ore loss and dilution. A comprehensive blast monitoring program, conducted at the Ahafo gold mine in Ghana between 2010 and 2011, revealed that uncontrollable and inconsistent blast movement takes place due to uneven free faces, irregular drill patterns, poor stemming practices and excessive confinement along the centre line regions of a blast. It is estimated that, at Ahafo, not adjusting grade boundaries for blast movement can result in dilution and misclassification of ore of approximately 3 to 5 per cent and 3 to 8 per cent, respectively. The potential economic impact of this, for the three baseline blasts monitored, is estimated at US$0.5 million of dilution and US$2.5 million of opportunity cost due to misclassification. Two site-specific strategies implemented to reduce blast-induced ore loss and dilution are: 1) designs to promote controlled blast movement along the strike of the ore body 2) procedures to adjust grade boundaries to account for blast movement. Designs to promote controlled blast movement involve optimising initiation timing, minimising edge effects, and reducing cratering. In order to account for blast movement, a movement template approach is implemented, and grade boundaries are adjusted by applying the most common horizontal movement vector in the most common direction measured during the trials. Experience at the mine suggests that the template approach is a practical and reliable method of accounting for blast movement, and in reducing blast-induced ore loss and dilution. Operational data from the Ahafo mine shows a better reconciliation between mine-to-mill grade and a reduction in diluted tonnes since the implementation of the grade optimisation strategies. The magnitude and direction of blast movement is site-specific and depends on a number of parameters, including blast design, blast geometry and geology. The inherent variability associated with many of these input parameters makes stochastic approaches more appropriate to modelling blast movement. As part of this research project, a stochastic model is developed and data from the Ahafo gold mine is used to demonstrate its effectiveness in predicting grade boundary displacement. The stochastic model builds on the template method to incorporate the measured variability in blast movement. This approach is shown to be a quick and effective method for grade boundary adjustment and can alleviate the need to continually measure blast movement for every blast.