Grinding circuit breakthrough

The discovery that ball mill circuit performance is not controlled by a single “efficiency” but rather by two distinct and active efficiencies was first proposed by Robert McIvor in 1987 in his McGill University thesis. Now, validation of his theory by rigorous testing in industrial plants has paved the way for virtually every grinding circuit operator in the minerals industry to re-examine his plant’s performance.

“My thesis was initially rejected,” recalls McIvor, “but the disbelief expressed by the academic review only strengthened my resolve to bring this new concept to the industry, where it could be tested. Fortunately, practically minded engineers such as Ken Wood [then mill superintendent at Les Mines Selbaie in Joutel, Que.] and Bill Scheding [then process engineer at Kidd Creek Mines in Timmins, Ont.] sensed that this was something important.”

Wood and Scheding helped test the theory at Ohio-based Cleveland-Cliffs facilities over 10 years, and the concept has since become a practical tool for processing ore.

McIvor’s idea started with the realization that only the ball mill energy being applied to the targeted “coarse” particles was being used effectively. The “circuit classification system efficiency,” equal to the percentage of targeted coarse material (versus “fines”) inside the mill, was subsequently defined. McIvor says this efficiency is controlled by the pump and cyclone. An investigation in value analysis and engineering at McGill led to the further discovery of the “mill grinding efficiency,” equal to the ratio of the grinding rate of coarse particles in the mill per unit of energy applied to them, divided by the lab grindability of the ore. McIvor shows how this efficiency is controlled by factors such as the media sizing and mill per cent solids. The so-called “Functional Performance Equation” then describes overall circuit performance in terms of these two efficiencies and the key circuit inputs of the ore grindability and the mill power draw. The clear relationships between the two creates unprecedented understanding of overall circuit efficiency.

McIvor’s McGill thesis was accepted after a strongly argued re-submission. He says he owes his achievement to the ongoing support of Prof. James Finch, known for his background in industrial mineral-processing research.

“It has often been said that if an idea is not first received with skepticism, it probably doesn’t have much merit,” says McIvor. “So I guess this one qualifies as ‘worth a look.'”

Now, with millions in cost savings backing his theory, McIvor is re-presenting his formula to the mining industry. “Industrial Validation: the Functional Performance Equation” will be presented at the conference of Canadian Mineral Processors and the Society for Mining and Metallurgy, in Ottawa beginning Jan. 18. For more information, visit www.c-m-p.on.ca