The essential grindWhat every metallurgist needs to read when it comes to comminution
What every metallurgist needs to read when it comes to comminution
By Robert E. McIvor
September 17, 2020
Courtesy of Robert E. McIvor
The published world on comminution is as perplexing as it is vast. A recent investigation, “The Truth About Population Balance Modelling of Ball Milling,” presented at the 2020 CMP revealed a history of misinformation that has been obscured by conceptual and mathematical complexity. However, there are a number of publications, identified over the length of my career, that every metallurgist should read to gain an appreciation of the basics of grinding and classification.
This is the definitive work on the relationship between ball milling circuit productivity and the circulating load ratio. Don’t let the words “crushing” or “screens” throw you off, this is about ball mill grinding (or whatever term you use for size reduction) and the classifier (whether screens or cyclones) interaction. That a high circulating load is essential for efficient circuit operation (incredibly sometimes still debated in some circles) is proven. The underlying reason is that the mill contents contains increasingly more “coarse” size material (plus product size, typically indicated by the circuit P80) as the mill residence time per pass is shortened with increasing circulating load. Besides pumping costs (in particular, replacement wear parts and associated downtime), the only constraints are flow rates so high that the mill overloads or grinding balls are carried out with the discharge slurry. Every operator should strive (by design, or re-design) for a high circulating load in their ball mill circuits, and this paper clearly shows why.
This is perhaps the only work ever conducted that provides the actual material contents of a continuously-operating, wet-grinding ball mill. These were measured by crash-stopping an operating pilot plant ball mill and collecting and analyzing the entire contents. Among key observations are the relative size distributions of mill feed, mill discharge and the mill contents, and the preferential retention of solids vs. water, as demonstrated by the relative percent solids of these streams. The assumption of equal solids and water retention used by mill “modellers” is simply destroyed. Like his 1925 work publication, Davis gives us a unique look into what is actually happening inside a ball mill.
This is the premier Bond Work Index equation application manual. Bond presents use of his breakthrough formula relating comminution circuit feed size (F80), circuit product size (P80) and the ore Work Index to industrial equipment energy usage. It covers crushing, rod and ball milling. The Bond Ball Mill Work Index remains today as the only tool used for ball mill scale-up design. For an (only nominally, not at all materially modified) update on its use for industrial mill sizing, see “Selection of Rod Mills, Ball Mills, Pebble Mills, and Regrind Mills,” Rowland, C. A., Jr., 1982, Chapter 23, Design and Installation of Comminution Circuits, SME of AIME. For the application of the Bond Work Index as a standard tool to measure size reduction circuit energy efficiency, see the Global Mining Standards and Guideline Group website.
Arterburn manages to unravel the extremely difficult and complex process of cyclone application, specifically for grinding circuits but broadly applicable in mineral processing, into a series of simple steps. It is impossible to describe just how masterfully this treatise dealt with such a difficult topic.
Through a series of pilot plant fully autogenous and semi-autogenous mill tests with different ball charge levels, Turner demonstrates how impact and abrasion can work synergistically on different ores to provide overall size reduction as efficiently as the multiple stages of crushing and grinding that the (S)AG mill replaces. To do so, he analyzed the entire mill contents under different conditions and associated grinding performances. The understanding of basic (S)AG milling mechanisms that this work provides was unprecedented, and remains unsurpassed today. Warning, it will make you ask how the industry has lost sight of the essential relationships that Turner demonstrated, and are not being used in current (S)AG mill design and operation.
Armed with the knowledge passed on by these important works, the new metallurgist, as well as the veteran, can more readily deal with the variety and complexity of the information and advice that they will encounter in their professional lives.
The majority of these papers are available on OneMine, which can be accessed by CIM National members through CIM.org.