Executive Summary
Milling is commonly deployed in many industrial sectors for particle size reduction. In this project, we aim to develop a robust methodology to link material grindability with particle dynamics in a mill in order to provide an innovative step--‐change in mill fingerprinting and optimization. This involves characterizing the stressing events that prevail in a milling operation and establishing material grindability in the context of the stressing events. The material grindability will require a detailed study of the fundamental fracture and breakage mechanisms of individual particles under different loading regimes, and how they relate to the mechanical properties and the final size distribution. This will provide the fundamental scientific basis for developing appropriate grindability tests capable of analyzing particle breakage subjected to particle impact, compression, shear and abrasion etc. pertaining to a milling process, which in turn will provide the basis for an improved particle breakage model calibrated against defined grindability. The IFPRI Grindability Project commenced on January 2013 and is planned for 6 years. The centrifugal impact pin mill has been selected as the first mill to be studied for this project, in collaboration with Hosokawa Micron Ltd. In the first year, the work focused on developing a computer model and conducting the discrete element modelling of pin mill to examine the particle dynamics during the milling process. This provided a better understanding of the comminution process such as the local stressing events and spatial distribution of impacts etc. under different operating conditions. This report summarizes the work performed within the second year of the project. Two materials, zeolite and alumina, have been chosen as the test particles with semi--‐ brittle and brittle failure behaviour respectively. The compressive test of single particle coupled with X--‐ray microcomputed tomography (XìCT) was carried out to give a better understanding of particle breakage from the micro--‐scale standpoint. The 3D volume of particle was visualized and the 3D object was segmented to characterize the inherent crack in the particle by specifying thresholding value in Avizo Fire™. The spatial orientation of particle during projection was investigated and this problem was solved using a linear interpolation algorithm. By using 3D Digital Image Correlation (DIC), with a pair of images taken before and after an inherent crack propagates, full field displacement can be observed and the extent of crack development can be quantified to some extent. The presented results show that the combined use of XCT and DIC is an enhanced tool to study the breakage mechanism of individual particles under compression. Single impact test has been conducted to investigate the particle behaviour under dynamic impact. The breakage event was captured using a high--‐speed camera and the breakage pattern under high impact velocity was observed. The effect of impact velocity and incidence angle on particle breakage has been examined. The breakage rate increases with the increase of impact velocity, with chipping being the main breakage 2 type at low impact velocities whereas fragmentation dominates at high impact velocities. Whilst the normal velocity component plays an important role in particle breakage, one key finding is that the tangential component of the velocity becomes increasingly important as impact velocity increases. Preliminary results of pin mill test for both zeolite and alumina were presented and compared with the predictions from the most common particle breakage models in the literature. The model fitting with the impact test data suggests that current models is deficient in predicting the breakage behavior of particle under impact comminution. IFPRI has funded a further collaboration project between Edinburgh and Dr Colin Hare of the University of Leeds to supplement the characterisation of these test particles using the well established facilities at Leeds to provide further measurements deploying the impact and nano--‐indentation techniques. The results from the collaboration are reported separately. Over the next 12 months, the project will focus on: particle breakage model development, which will consider the measured mechanical properties and loading conditions. Then breakage comparison with the newly developed model will be made with the final set of milling experiments to be conducted. Further investigation will be done in terms of in--‐situ loading single particle compression under simultaneous X--‐ray ìCT. The numerical work will continue to compute particle dynamics in real--‐geometry pin mill based on the first year’s numerical work. The new understanding will form the basis for developing the grindability test(s) capable of characterising particle breakage subjected to the dominant loading events identified within a milling operation.