GRINDING BREAKTHROUGH – Two Efficiencies Better Than One

ISHPEMING, MINNESOTA - Major savings in grinding plant costs are on the horizon. The discovery that ball mill circu...
ISHPEMING, MINNESOTA - Major savings in grinding plant costs are on the horizon. The discovery that ball mill circuit performance is not controlled by a single "efficiency", but rather that there are two distinct and active efficiencies at work in the process, was first proposed by Rob McIvor in his McGill University thesis in 1987. Now, validation of this theory by rigorous testing carried out in industrial plants has paved the way for the re-examination of virtually every grinding circuit in the minerals industry.

"My thesis was initially rejected," McIvor was quoted as saying. "But the disbelief expressed by the academic review only strengthened my resolve to bring this new concept to industry where it could be tested. Fortunately, practical-minded engineers like Ken Wood (then mill superintendent at Les Mines Selbaie) and Bill Scheding (then process engineer at Kidd Creek Mines) sensed that this was something important. It was with their support, and others like them at Cleveland-Cliffs (where detailed testing has taken place over the last ten years), that this concept has now proven out to be a highly practical tool for the business of ore processing." Now, millions of dollars of proven cost savings later, McIvor is re-presenting his formula, along with plant case studies that show how it works, to the industry in general.

The concept started with the realization that only the ball mill energy being applied to the targeted "coarse" particles is being effectively used. The "circuit classification system efficiency", equal to the percentage of targeted coarse material (versus "fines") inside the mill, was thusly defined. Importantly, McIvor shows how this efficiency is controlled by pump and cyclone parameters. 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 percent solids. The "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 each of the two, different efficiencies and specific circuit design and operating variables that can be manipulated creates unprecedented understanding and opportunity for improving overall circuit efficiency.

"Industrial Validation of the Functional Performance Equation" is being presented at the Canadian Mineral Processors (in Ottawa, Jan. 18-20, and the Society for Mining and Metallurgy (in Salt Lake City, Feb. 28-Mar. 2, conferences in 2005. Further information is also available from Metcom Consulting, LLC (


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