A bulk sorting trial of copper ore using a magnetic resonance analyser

Abstract

Sensor-based bulk ore sorting (BOS) is a technique that may lead to uplift of processing grades by excluding waste as well as an increase in usable resource by sorting material designated as mineralised waste. A magnetic resonance analyser (MRA) was installed on the primary crushed ore feed to the sulphide plant at the Kansanshi copper mine in northwestern Zambia to trial BOS. The conveyor belt operated at 2800tph and carried ore of which greater than 85% by weight of the copper-bearing minerals are chalcopyrite. The MRA was therefore tuned for chalcopyrite's zero static field magnetic resonance. The analyser achieved a 3σ detection limit of 0.045 wt% copper as chalcopyrite over a 4 s measurement time and standard belt loading, with lower detection thresholds possible if the measurement time increased. A trial of ore diversion was then carried out at the site. Pods of ore measured as less than 0.2 wt% copper as chalcopyrite by the MRA were diverted by a dead box diverter system to form a low-grade stockpile of approximately 10,000 tonnes. A cavity in the mill feed stockpile was arranged and the rejected material fed back through the crusher, past the MRA and directly through the milling circuit to the flotation feed where a Vezin sampler was used to take a sample every 5 min. This sample was then tested for copper grade by the site metallurgical laboratory. The MRA measured a grade of 0.16 wt% copper as chalcopyrite over the test time period and the Vezin samples for the last hour of the trial reported approximately 0.2–0.3 wt% copper despite some mixing with unsorted ore being inevitable. This is as clear a demonstration of bulk ore sorting (BOS) as practically achieveable in the production environment at Kansanshi and, to the author's knowledge, the first demonstration of BOS on a copper ore stream of at least this throughput.