Controlling ore loss and dilution is critical to every mining operation. If you get it wrong, the result can mean tens of millions of dollars in lost revenue.
As a grade control geologist, you are tasked with maintaining the grade going to your mill. Dilution can be a major contributor to poor grade results, and blast movement is a significant cause of dilution and ore loss1.
BMT works with geologists from over 105 open pit operations around the world to accurately measure 3D blast movement so that the ore is sent to the mill and waste ends up in the waste pile.
The impact of blast movement on grade control.
Grade control geologists spend many hours modelling the distribution of minerals throughout a rock mass based on data from drilling and assaying a significant investment. After all this effort, mining engineers blast the in situ rock so it can be efficiently handled in the mill. In open pit mines the required explosive displaces ore boundaries from where the geologists originally defined them.
- Failure to accurately account for blast movement results in ore loss, dilution, or misclassification (low grade to high grade ore, sulfide to oxide, or other contaminates).
- The financial consequence of getting this wrong is significant and diminishes the geological QA/QC done in defining the ore body.
Cowal gold mine increased mill feed grade by 7% through blast movement monitoring
The main challenge for the geologists at Cowal is to maintain grades through the mill. Dilution and ore loss are the major factors in this and the Cowal team utilize blast monitoring equipment to eliminate them. Monitoring blast movement has improved grade and end-of-month reconciliation, and being a low-grade bulk tonnage mine, this has substantially increased revenue.
Studies at Cowal show a 7% increase in their mill head grade on a yearly basis.
What is blast movement?
So that solid rock can be excavated, explosives rapidly release energy to fragment the mineral resource. The explosive exerts a force equally in all directions, and the rocks with least resistance begin to move. Typically, movement is perpendicular to the blast initiation timing contours. The moving rocks in turn act on neighbouring rocks, which results in bulk movement of the rock mass.
- Upon detonation, each discrete element of the explosive exerts a force equally in all directions on the adjacent rock. The rocks with least resistance, that is rocks with less unblasted rock in front of them, begin to move. These rocks in turn act on neighbouring rocks, which results in bulk movement of the rock mass.
- The rock moves in the direction of least resistance. The material moves into the void created by the hole before it and the new material settles into the new location, making movement approximately perpendicular to the initiation timing contours.
- The rock at the top of the bench is not directly impacted by the explosive but instead is indirectly moved by collisions from other rocks from the drill hole behind. The further a rock is above the explosive column, the less energy it receives and therefore the less distance it will move.
- Greater movement is in the mid-to-lower level of the bench and movement reduces near the floor due to friction from the unbroken floor.
Blast Movement. Each hole goes off in sequence. Blasted material moves into the void created before it, and then settles into this new location.