Contact: Ian R Firth. +1 775 784 4185
Introduction
One of the most frequently encountered requirements of mining operations in which rock is broken using bench blasting techniques, is the need to produce a well fragmented muckpile which will "dig" easily. "Diggability" is a function of various aspects of the blast design, notably timing and powder factor. A "hard digging" muckpile will cause poor productivity in the loading fleet, excessive machine wear and will adversely effect areas as diverse as mining plan conformance and mill through-put. The most common approach to the provision of a muckpile which is "easy" to dig is the combination of long inter row delay and relatively high powder factor, promoting extensive lateral movement and vertical heave within the rock mass.
In direct contradiction of this approach is the desire to minimize dilution of ore zones and reduce mixing of material types within the muckpile. In practice this is achieved by the use of "choke" blasting techniques. While choke blasting is accepted as being successful in this regard, there currently exists no method of quantifying the degree of movement and mixing experienced within a given muckpile, and relating this mixing to other aspects of the blast such as degree of fragmentation and grade recovery.
The Mackay School of Mines, Department of Mining Engineering, has now completed four investigations regarding the categorization and measurement of blast movement at precious metal mines in Nevada. The focus of the work has been the development of a methodology which will allow the operator to characterize blast movement for a given blast design and in a given geologic setting, prior to the beginning of the loading cycle. This would allow more accurate placement of relevant material boundaries, with the concomitant result of improved value recovery and reduced unit cost. Techniques trialed have included marker bags (Zhang 1994) and magnetometry (Harris 1997). While these studies have demonstrated considerable success in terms of their respective research objectives, neither system has been developed to a level at which it will provide the necessary information in a manner suitable for use on a regular basis within the operational constraints of a mine site.
Numerical modeling of bench blasting may be considered the key to a fundamental understanding of the blast movement and fragmentation process. To date no code is commercially available which is capable of realistically modeling the complex rock mass and gas flow dynamics required to accurately predict blast movement and fragmentation. The Orica Explosives group have developed the Discrete Motion Code, (DMC), which is capable of the full sequence of blast movement modeling, but this code is proprietary and its use is limited to Orica personnel. Other significant models include Sabrex (Jorgensen and Chung 1987), also a proprietary product written by Orica explosives and 3x3 Pro, a commercial model written by the JKMRC. These models tend to allow only very limited understanding of the mechanics of the blast and are by no means accurate. The combination of physical measurement and numerical modeling when integrated with a mine planning software suite, would enable accurate digging limits to be set in the field as a routine aspect of the daily mining plan.
