The University of Johannesburg (UJ) School of Mechanical and Industrial Engineering’s MinPET three-dimensional (3D) diamond-in-rock detection system is now nearly ready for scaling up to commercial scale. The system detects diamonds enclosed in kimberlite ores, after mining and during processing.
“We have essentially completed the technology development,” explains Professor Simon Connell, the project physicist and a member of the school’s staff. “It has been proven at the laboratory test scale. We’ve done computer simulations of the scale-up and these are delivering very encouraging results. The simulations indicate that scaling up will pose no problems. Indeed, some aspects of the process could become easier with the scale-up.” The simulations were done assuming a throughput of 500 t/hour of ore, which is generally the throughput used with existing processes.
“Now we need to build a fully operational system at a mine,” he states. “It would be both the pilot plant and the first full-scale commercial plant – a commercial pilot plant, so to speak.” The project team is close to signing a partnership agreement with a venture capitalist, which would see the latter become a co-developer. The intellectual property developed by the school through this project will be vested in the UJ.
Currently, to get diamonds out of kimberlite involves crushing a considerable quantity of rock, which can damage diamonds (for example, causing them to split along their cleavage planes), greatly reducing their value. “The mines are reluctant to reveal this, but the value lost from breaking diamonds in crushing is considerable,” he notes. To reduce this loss, many mines now employ X-ray-based systems to detect diamonds enclosed in the ore, and these systems have become increasingly sophisticated. “But they still have only a limited ability to detect enclosed diamonds. The hope is that they are liberated – freed from the surrounding rock – by the crushing. Even so, about 30% of the enclosed diamonds are not liberated. Geologists can tell if a diamond is split as a result of mining or crushing, known as fresh breakage, or whether this happened through natural processes in the past, called old breakage. Miners are convinced that little breakage is caused by blasting during mining and that most is caused by crushing.”
It is the reduction in the value of diamonds caused by fresh breakage that would justify the installation of MinPET systems at mines. MinPET uses positron emission tomography (PET) technology (hence its name) – basically the same technology used in hospital PET and PET/computed tomography scanners. The physics that makes MinPET work is, in principle, quite simple: natural carbon (and diamonds are, of course, an incredibly stable form of carbon), which mainly comprises Carbon-12, is converted by a photon secondary beam derived from an electron accelerator into Carbon-11 by ejecting one neutron from the carbon nucleus. Carbon-11 is unstable (radioactive) and decays by releasing a positron (an antimatter particle) which then annihilates with an electron, in the process creating characteristic pairs of “back-to-back” photons, which are then imaged by opposing detectors. Carbon-11 has a short half-life indeed, just 20 minutes, so the rock is not radioactive for long and there is no health risk to workers.
“The idea is that the radioactive emissions from the Carbon-11 show that there is carbon within the rock and it also allows the creation of a 3D carbon concentration map of the rock,” says Connell. With kimberlite ores, a carbon concentration within the rock pretty much has to be a diamond. The operational process involves coarse crushing of the ore, which minimises the danger of any damage to diamonds within the rock. The crushed ore is then passed through an “activation” stage, creating the Carbon-11. The activated material is then passed between PET detectors (which are basically the same as medical PET detectors, but modified for use in a mining environment). The system then indicates which pieces of rock contain diamonds. The actual retrieval of the pieces of ore containing diamonds can then be done by any one of a number of currently available technologies and processes.
“The 3D mapping makes a huge difference in finding diamonds,” highlights Dr Martin Cook, who is responsible for the commercialisation of MinPET. “It gives a lot more information about the diamond and the rock around it.”
“For our technology demonstration, the rocks were activated at the ASTRID2 particle accelerator at Aarhus, in Denmark, and we used portable detectors we brought from South Africa,” he adds. “This was because there are no electron accelerators in South Africa that are powerful enough to activate the rock. For the future commercial operations, we ideally need electron accelerators with higher energy than those used in medicine but lower energy than in dedicated research accelerators.”
The tests started off by using a 12.5 ct diamond as the target. Then the size of the target diamond was significantly reduced, to 2.9 ct. “It worked just as well,” Cook reports. “Then we tested the system’s effectiveness with calcite, which contains a lot of background PET emitters. But we found we could still image a diamond enclosed in calcite quite nicely.”