Grindability Test Modelling, Measurement and Mill Fingerpriniting

Author Last Name: 
J. Y. Ooi, L.G. Wang, X.Z. Chen, C. Labra, J.F. Chen, J. Sun
Report Type: 
Research Area: 
Size Reduction
Publication Year: 
Publication Month: 
United Kingdom

Executive Summary
Milling is commonly deployed in many industrial sectors for intended particle size
reduction. In this project, we aim to develop new ideas and methodologies to link material
grindability with particle dynamics in a mill in order to provide an innovative step-change
in mill performance 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 measure
capable of analysing particle breakage subjected to particle impact, compression, and shear
etc. pertaining to a milling process, which in turn will provide the basis for an improved
particle breakage model calibrated against the defined grindability.
A hybrid of experimental, theoretical and numerical methods have been adopted to
elucidate the particle breakage mechanics. The material grindability was first investigated
by single particle loading experiments, including indentation tests and single particle
impact tests. It was found from the zeolite particle impact test that tangential component
velocity plays an increasingly important role in particle breakage with increasing impact
velocity. A new particle breakage model was proposed assuming that the subsurface lateral
crack accounts for the chipping mechanism. Considering the limitation of existing models
in predicting breakage under oblique impact and the significance of tangential component
velocity identified from experiment, the effect of impact angle is considered in developing
a new breakage model, which enables the contribution of the normal and tangential velocity
component to be rationalized. In particular, the contribution of tangential component velocity was incorporated in the new model using the mobilized dynamic friction. Milling
experiments were carried out using the UPZ100 impact pin mill to measure the
comminution characteristics of the test solids, which provides the basis for the validation
of numerical results. A Discrete Element Method (DEM) simulation of single particle
breakage subject to impact loading was conducted to evaluate the breakage propensity
which is then compared to experimental results. A recently developed new bonded contact
model by Brown et al. (2014) was utilized in which the bond contact is based on
Timoshenko beam theory considering axial, shear and bending behaviour of the bond.
The UPZ100 pin mill experiments were conducted to study the effect of rotary speed
and feed rate. DEM simulations were then performed to understand the fundamentals of
the particle dynamic and stressing conditions inside the pin mill. Furthermore, the impact
velocity distributions obtained through DEM simulations were utilized to inform the mill
operating dependent parameters in the Population Balance Model (PBM) at the process
scale. Variables in the PBM kernel were classified into particle materials dependent
parameters and mill operating dependent parameters. The remaining parameters of PBM,
i.e. particle material dependent parameters, were evaluated from the milling test at
12000RPM using a constraint optimisation technique. The DEM-PBM coupled multiscale
model was then used to predict the milling outcomes for the other three set of milling
experiments at different rotary speeds. A good agreement between the tests and the
predictions of product size distribution is achieved, which indicates a promising application
of the proposed DEM-PBM multiscale method for the optimization of milling devices.