Influence of Impact Velocity on Fragmentation and the Energy Efficiency of Comminution
Mining operations during the last decade have continuously sought out lower operating costs as competition has intensified in the world mining industry. Comminution costs are a significant contribution at 30-40% of the total operating costs, while 3% of the world’s electrical energy and 1.3% of the United States is consumed by this process. Thus, every aspect of comminution justifies careful consideration in order to minimize all elements of cost and associated energy use.
Crushing and grinding are essential components of all mining and mineral processing operations to reduce the size of ore and rock, and subsequently, liberate the valuable mineral for beneficiation. However, comminution is very energy-intensive and consumes a major percentage of the total energy used during mineral recovery. In many cases, the efficiency of comminution is derived by measuring the ratio of energy ‘‘required’’ to energy used. From a fundamental viewpoint, it is the energy required to create new surfaces that should be considered.
Using this definition, comminution has been shown to be of the order of 1–2% efficient. Typical grinding efficiencies range from 1% to 2% with crushing efficiencies lying slightly higher at 3–4%. High-pressure rolls and roller crushers are reported to operate at levels as high as 7–8%, while blasting shows the highest efficiency of all processes ranging from 13% to 20%. Most of the energy input into comminution ends up as heat generated within the rock material, equipment, and water to eventually be dissipated into the surrounding atmosphere.
Surface area of fractured material is an important measurement to characterize energy use in comminution as well as surface reactions in down-stream processing steps. Conventional approaches to direct measurement of surface area consider the geometry of product and feed particles—both size and shape. What is often overlooked in such work is the influence of fractal geometry factors – surface roughness and the resolution used to measure roughness. Parameters that characterize surface variations include roughness and waviness, which are dimensional properties of the material. Roughness is defined as relatively finely spaced surface irregularities, of which the height, width, and direction establish a definite surface pattern. Waviness refers to a wavelike variation from a perfect surface to one with much wider spacing and higher amplitude changes than surface roughness.
This paper reports on studies conducted on strain rates achieved by various velocities of impacts that enhance energy efficiency and mineral liberation. The research focuses on understanding comminution fracture mechanics and on quantifying the distribution of energy with respect to generating new surface area.
In interpreting breakage energy phenomena, accurate measurements of surface roughness and surface area are essential. A novel approach to measure surface roughness and surface area based on a fractal analysis procedure was developed, using four drill core samples of volcanic rock from Northern Ontario. Changes in surface roughness of broken specimens under variable loading rates were studied using a laser probe to generate 3D topographical maps of the fracture surfaces.
The results indicated that surface roughness and hence, specific surface area, increase with increasing loading rate by several orders of magnitude as particle size decreases to ~1µm. Below this limit, surface roughness begins to diminish from particle–particle attrition. An apparatus to measure the quantitative parameters of impact at different velocities on aggregated rock samples is proposed. Experiments are being carried out at projectile velocities up to 500ms–1 utilizing a compressed-air device. The results suggest that the application of different loading rates or the use of various impact velocities and strain rates may provide an opportunity to achieve improved economies in rock breakage.
Link to full article:
S. Sadrai; J.A. Meech; M. Ghomshei; F. Sassani; D. Tromans. Nov, 2006.