Optical Sizing Analysis of Blasted Rock: Lessons Learned

Optical sizing of blasted rock has been used for about 15 years. Originally thought of as a specification type of measurement tool, it has over the years evolved into a process control tool. Because of the difficulty defining what the size of an irregular fragment is, it is difficult to reconcile different methods of measurement. Screening nominally measures intermediate diameter of particles, whereas optical methods typically measure a statistical average diameter. Because of the need for rapid nondisruptive measurements, optical systems measure particles in-situ while screening systems in essence handle each particle individually. These optical systems are faced with the need to calibrate for partially overlapped pieces, and fines that are too small to resolve in a single image or have fallen in and behind larger fragments. Given all these issues, optical sizing has enjoyed much greater success as a process control tool, evolved to measuring size distributions of material on conveyor belts, where issues of lighting variability, camera positioning, and sampling biases are easily controlled. As a process control tool it can be used to establish operating norms and to trigger alarms when the measured sizes move outside the measured norms. The typical application for such a system is to control feed sizes to processes such as crushing and grinding. Once the fragmentation is on a conveyor belt, it has usually passed through a primary crusher and thus is no longer completely indicative of the blasting process. In order to make measurements that reflect original blast sizes, but do not suffer from the issues associated with imaging muck piles, the images must of necessity be made between loading from the muck pile and dumping into the primary crusher. In practice, for many operations, this means that the best measurements can be made by imaging the rock while in transit between the muck pile and the primary crushing station. This includes surface and underground HD (Haul Dump) and LHD (Load Haul Dump) type vehicles which can be imaged as they dump, or prepare to dump, or as they pass through a gate or other restriction. Measurements of this type are much more complex. Systems need to use triggering systems to inform the software when to acquire the image, tracking systems to determine where the material has come from, and software algorithms to determine which part of the images contain rock to be measured and which contain extraneous information