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Impact-induced rock fragmentation is relevant for many fields of science and technology. Impact mills have been applied in mineral, coal, cement and chemical industries for a long time. The literature shows that substantial effort has been expended in understanding the impact mill performance in relation to machine configuration and operational conditions through experimental work and mathematical modelling. However, due to lack of detailed knowledge on velocity and energy distributions of collision inside a milling chamber, the mechanisms are still not clear [1].
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Bond testing has been in use since the late 1920s; laboratories and operations around world used the procedure as a component of comminution circuit design as well as evaluation of plant performance. In spite of such long-standing use, the topic of accuracy and precision of Bond work index determinations recurs with great frequency [2].10799
Single particle tests to determine the comminution behaviour of ore can be separated into twin pendulum device (Fig. 1) and drop weight apparatus (Fig. 2) based tests [3], [4], [5], [6] and [7].
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Schuhmann [8] reported that the size reduction events could themselves involve varying energy input, varying feed particle size and varying size distribution. Flavel and Rimmer [9] have reviewed single-particle breakage on the pendulum device, and stated that the pendulum device can be suitable for obtaining the relation between the breakage product distribution and comminution energy.
Twin pendulum test relies on the particle being broken between an input pendulum released from a known height and a rebound pendulum. Twin pendulum, however, has some important limitations, particularly regarding its low flexibility and reproducibility, long test duration, and the poor accuracy in estimating the comminution energy as a result of secondary motion of the rebound pendulum [4], [5], [6] and [7].
The drop weight test differs, in that the particles are placed on a hard surface and struck by a falling weight. Both tests have been extensively used in the field of comminution. In recent years, however, the drop weight apparatus are being replaced by the twin pendulum. The standard drop weight device is fitted with a 20 kg mass, which can be extended to 50 kg. The effective range of drop heights is from 0.05 to 1.0 m, which represents a wide energy range from 0.01 to 50 kW h/t (based 10–50 mm particles). Following sample preparation the mean mass of each set of particles to be broken is calculated. The results from the drop weight tests provide an energy/input size/product size relationship. This relationship is analysed using a set of curves to describe the size distribution produced from breakage events of increasing size reduction or energy input [3], [4], [5], [10] and [11].
In the drop weight test, a known mass falls through a given height onto a single particle providing an event that allows characterisation of the ore under impact breakage. Although, the drop weight test has advantages in terms of statistical reliability and the potential use of the data from the analysis, it has a number of disadvantages, including necessity a special apparatus, tiring and particularly the length of time taken to carry out a test. For each drop weight test, 15 samples are tested in five size fractions at three levels of energy input [10], [11], [12] and [13].
Narayanan [14] was used a novel procedure for estimation of breakage distribution functions of ores from t-family of curves. In this method, the product size distribution can be represented by a family of curves using marker points on the size distribution defined as the percentage passing (t) at a fraction of the parent particle size. Thus, t2 is the percentage passing an aperture of half the size of the parent particle size, t4 is one quarter and t10 is one-tenth of parent particle size. Narayanan and Whiten [15] have proposed empirical equations for relating the reference curve data t10 with the impact energy.