Johnson
Abstract
In achieving the necessary liberation for separation of valuable minerals from ores, metallurgists have traditionally selected from the competing options for size reduction devices by making decisions based on various types of costs. While these costs remain important in selection of devices, consideration of additional factors which incorporate the eco-efficiency of the devices provides a somewhat changed basis for selection.
This review examines the broad reasons for different technical efficiencies for various types of size reduction devices. In particular, size reduction devices with confined bed breakage, with stirred mills of various rotational speeds and therefore various energy intensities, and other mills such as the Hicom Nutating Mill and the Kelsey Fine Autogenous Grinding Mill are discussed in terms of their operating principles and their implications for the eco-efficiency of the device. The review also considers some additional attributes of each device which are relevant from the viewpoint of eco-efficiency.
From the viewpoint of eco-efficiency, compact size reduction devices (lowered energy consumption in manufacture) that have high throughputs and lowered energy usage per unit of feed are expected to be linked together to provide improved size reduction systems. The need for grinding media and an external classification system, both of which represent consumers of energy, for each device is discussed. It was observed that the useful ranges of some of the size reduction equipment are yet to be determined, such as the upper limits for the high intensity mills.
Some grinding systems can provide a given level of liberation at a coarser product sizing than for other systems, sometimes in conjunction with producing a desirable, more compact size distribution. Such outcomes may arise from improved matching of the size reduction mechanism(s) with the texture of the ore, or from a contribution from nonrandom breakage leading to one or more types of selective liberation. Alternative systems need to be compared from the viewpoint of the overall liberation level achieved for a given product sizing and the contributing mechanisms (if any) to selective liberation.
From the discussion, a number of the size reduction devices provided additional benefits during the size reduction step or downstream from the size reduction step. Examples are the absence of detrimental effects from media (inert or not required), the provision of shear in the size reduction step (potential surface cleaning of adhering particles and colloidal deposits), lowered resistance to breakage in downstream size reduction steps and some mechanical activation of the minerals of relevance in downstream steps. There was also flexibility with many of the devices for both wet and dry modes of operation.
Author
Professor N W Johnson