A new approach to evaluate the particle size distribution from rock drilling: double peak characteristic analysis

The characteristics of the particle size distribution (PSD) are an important reflection of energy consumption during rock drilling and drilling efficiency. A new approach incorporating both fine and coarse fractions of the PSD to supplement the typical approach of merely utilizing D50 (50% passing diameter) to index drilling resistance and efficiency is reported in this paper. A series of drilling experiments are conducted on limestone with variable rotation rate and with measured response of both penetration rate and PSD. The PSD organizes into a bimodal distribution with two distinct peaks corresponding to fine (Peak I) and coarse (Peak II) fractions. The mean size of Peak I is mainly conditioned by the rock property and the contact angle of the drilling tool. Peak II results from the effects of grinding and comminution and is conditioned by operational drilling parameters of rotation speed and the penetration rate of the bit. With an increase in bit penetration rate, the mean particle size of Peak II increases and its volume percentage decreases. With an increase in rotation rate, the mean particle size of Peak II initially decreases before slightly increasing at higher rates, while sympathetically, its volume percentage initially increases before decreasing. Such mechanistic analyses applied to define the resulting form of the PSD suggest operational changes that may be used to control the resulting PSD and optimize drilling.