Improving low-grade ore recovery with the use of efficient blasting techniques

Drilling and blasting operations have historically been viewed as a necessary 'evil' part of mining, whose costs should be minimised by whatever means possible. In the past, when operations depended primarily on crushing and milling for ore recovery, it has been difficult to accurately quantify the 'downstream' effects of blast performance. As a result, blast performance has been typically evaluated in terms of cost per tonne or kilograms of explosive per tonne. This assessment has led to blast designs that were inefficient and actually increased the overall cost of an operation due to oversized fragmentation. This 'culture' has to change to take full advantage of the application of heap leach technologies for low-grade deposits. Recent advances in heap leach/solvent extraction processes have lowered the economic cut-off grades to relatively low levels (0.1 per cent). In run of mine heap leaching the fragmentation produced by blasting is one of the major factors that controls the percentage of ore recovered. Small reductions in the avenge fragment size will provide substantial increases in recovery, even with relatively low-grade deposits. To produce smaller blast fragmentation the cost of drilling and blasting will generally increase. In many cases the existing drilling and blasting costs can be doubled to achieve an optimum fragmentation size that provides the best recovery and in turn the highest overall profitability. The challenge is to define the optimum blast design based on the relationship between drilling and blasting costs and overall recovery. This challenge can only be met by accurately quantifying blast performance in terms of cost and fragmentation. Once this relationship has been established, an economic model can be developed that takes into account total reserves, average grade and potential recovery based on fragment size. This model can then be used to guide the refinement of the blast designs. Before changing a design to 'improve' fragmentation it is important to define where the oversize is originating from. There are specific design modifications that should be evaluated to reduce oversize in each zone. Without the proper quantification of blast performance and blast design refinement it is unlikely that a mining operation can approach a truly optimised level of overall cost efficiency.