Quantifying variability of ore breakage by impact – implications for SAG mill performance

The standard JK drop weight test breaks single particles at five size fractions each at different energy levels by dropping a given weight from a certain height. The progenies of all particles at a given energy level are sieved and the degree of breakage of all those particles at that energy level is presented as t% (percent passing 1/10 of original size). This method assumes that all of the particles in a single size fraction have the same mass and experience a similar degree of breakage and calculates the average breakage characteristics of an ore domain from impact loading. Hence, this standard approach does not capture the breakage variability that exists among ore particles. To investigate the breakage variability of ore, the drop weight testing method has been extended where the energy applied and the degree of breakage for each particle within the sample are measured separately. Therefore, the degree of breakage for the ore domain is expressed as an envelope of t% vs energy curves. This approach provides some insight into the intrinsic variability of the response to impact loading within an ore domain. Several rock types were tested by the extended DWT testing approach and the implications of ore breakage variability for a SAG mill treating these materials are simulated for a standard SAB circuit using the JKMRC grinding models

The extended drop weight testing approach – what it reveals

The heterogeneous nature of orebodies introduces large uncertainties into all quantitative evaluations, process design and process predictions. Measuring the extent of the variability of ore competence will allow the design process to account for variation in process performance through a quantitative knowledge of its uncertainties related to ore hardness. The conventional JKMRC drop-weight test (JKDWT) establishes the relationship between input energy (Ecs) and product fineness (t) from which the breakage potential parameter A × b can be estimated, by combining the broken progeny of groups of particles. A new method, the Extended Drop Weight Test (ExDWT), has been developed which is applied to individual particles and is therefore capable of capturing breakage heterogeneity at high resolution. This paper explores a number of features of the new method, based on breakage tests on individual particles from several different rock types. The results showed that more accurate descriptions of particle size resulted in higher (softer) A × b values which suggests that the standard method may have been over-estimating rock competence. Regular-shaped cores broken diametrally were found to have higher (softer) A × b values than axially broken cores and irregular shaped particles. These tests also suggested that the true ore intrinsic heterogeneity is the main source of breakage variability measured by the ExDWT. The mean A × b values determined by the ExDWT showed no statistical difference to those determined by the standard JKDWT method, but the standard deviation of the estimate was much lower. The results have demonstrated the potential of the new method for capturing the inherent heterogeneity of individual ore particles. Such information could be used to populate multi-component models of comminution