Milling is a ubiquitous process in the glass industry for both feed and recycled materials. A good example of materials of interest is silica, which is available in natural quartzite rock as well as fused silica. Such materials are hard, tough and stiff and require considerable energy for their size reduction. So it is of a great interest to establish the most efficient size reduction method for both amorphous and crystalline structures of such materials. Three sets of silicate materials were chosen for this study: set 1: fused silica (SiO2) and quartzite; set 2: amorphous lithium silicate and crystalline lithium silicate (Li2Si2O5); set 3: amorphous sodium aluminium silicate and crystalline sodium aluminium silicate (NaAl(SiO4)).
The test materials properties were characterised for their physical, chemical and mechanical properties. The milling behaviour was studied using a Retsch single ball mill. This was chosen as its dynamics has previously been extensively investigated. The milling results were analysed using a first order rate equation. The milling rate constant (Kp) and grinding limit size (Dl) were determined as a function of humidity. It was found that humidity had no effect on the milling rate. The Rosin-Rammler distribution was found to fit best for the mono mode cumulative particle size distribution.
The test materials were cut and prepared in the cube form with a linear dimension of about 7.5 mm. The process of breakage was closely observed. At the start of the milling process, the cubes underwent chipping at the edges and corners. As the process continued, micro-cracks began to propagate through the body of the cube due to fatigue. This resulted in rapid fragmentation and disintegration of the cube, quickly reaching the grinding limit size. For the quartzite and sodium aluminium silicate materials crack propagation took place through inter-crystalline boundaries. After this stage individual grains started to reduce in size until they reached the grinding limit size. In the case of crystalline lithium silicate the spherulitic microstructure causes adhesion of fines to the milling media. The crushed particles do not detach easily from the mother particle and form a thick coating on its surfaces, thereby protecting it. Also the fine powder covers the surfaces of the capsule and stainless steel ball. A new method of milling should be explored, which reduces the adhesion of fines to surfaces.
The energy consumption in the shaking ball milling calculated with EDEM simulation. It was found that the input power can be increased with the size of stainless steel ball. At 30 Hz, 11.97 mm steel ball and 1 g of cube samples are calculated the input power as 1.89 W.
The mechanical properties of hardness and toughness were characterised by various methods and the breakability index (H/(Kc^2 )) was calculated. It was found that the milling rate correlated well with the breakability index for all the tested materials. The breakability index provides a methodology whereby the milling performance, i.e. milling rate and energy utilization can be predicted from the characterisation of mechanical properties.
