Effect of mill type on the size reduction and phase transformation of gamma alumina

1. 3. Results 3.1. Particle size analysis using laser diffraction 3.1.1. γ-Al2O3 particles before and after dry milling in the JM Samples of γ-Al2O3 were fed through the JM for a total of 20 passes. Samples were collected for particle size analysis after 5, 10 and 15 passes. Fig. 3 shows the particle size distribution (PSD) and cumulative PSD of the particles. As shown in Table 1, there was a significant size reduction between the as-received (A-R) samples and the sample after 20 passes also shown by the measured characteristic sizes of the particles where the A-R sample had a d90 of 49.10 µm and the 20 pass sample had a d90 of 2.89 µm.

2. Conclusions Jet milling is a more suitable size reduction method for dry milling of γ-Al2O3 powders when compared with planetary ball milling and single ball milling, as it effectively reduces size with minimal effect on the morphology of the material. Dry planetary ball milling results in a phase change from γ to α-Al2O3 as observed by XRD patterns and octahedral crystal shapes in TEM. A significant loss of surface area from 136.6 m2/g to 82.6 m2/g is evident in the planetary ball mill samples, rendering the planetary ball mill as less suitable for size reduction of catalyst supports. The observation of transformation in planetary ball milled samples can be attributed to high shear stresses in the mill that result in shear nucleation and formation of α-Al2O3. Contamination of samples with ZrO2 also occurs during planetary ball milling and is observed to increase with time. The results favour a shear nucleation mechanism of phase transformation where the formation of α-Al2O3 occurs by slip on close packed oxygen planes and results in a change from the cubic close packed to hexagonally close packed structure.

3. Astract:The influence of stress modes and comminution conditions on the effectiveness of particle size reduction of a common catalyst support; γ-Alumina is examined through a comparative assessment of three different mill types. Air jet milling is found to be the most effective in reducing particle size from a d90 of 37 µm to 2.9 µm compared to planetary ball milling (30.2 µm) and single ball milling (10.5 µm). XRD and TEM studies confirm that the planetary ball mill causes phase transformation to the less desired α-Alumina resulting in a notable decrease in surface area from 136.6 m2/g to 82.5 m2/g as measured by the BET method. This is consistent with the large shear stresses under high shear rates prevailing in the planetary ball mill when compared to the other mill types. These observations are consistent with a shear-induced phase transformation mechanism brought about by slip on alternate close packed oxygen layers from a cubic close packed to a hexagonal close packed structure.

4. Introduction Milling is a widely used industrial operation common for cases where size reduction of particles is required (Reid et al., 2008). It can also be known as grinding and involves the size reduction of particles smaller than 10 mm. There is a vast range of mill types available commercially and the choice of mill is based on a variety of factors, such as properties of the material to be milled, e.g. failure mode, and the required product particle size (Angelo and Subramanian, 2008).

5. Materials and methodo The sample used for experiments is a commercially available γ-Al2O3 powder: a 99.99% pure γ-Al2O3 derived from synthetic boehmite. Single ball milling (SBM) was carried out using a Retsch MM200 vibratory single ball mill. An 11 ml stainless steel milling jar with a 12 mm diameter spherical stainless steel ball is used for single ball milling for durations of up to 1200 min at a milling frequency of 30 Hz. In single ball milling (Fig. 2a), short duration collisions dominate and the energy generated by the mill is determined by the chosen milling frequency. This can be shown by the contact force distribution reported by Kwan et al. (2005) where the impact forces are the most dominant (Kwan et al., 2005). Size reduction is mainly by impact in the SBM although shear and attrition is also present during milling.The sample used for experiments is a commercially available γ-Al2O3 powder: a 99.99% pure γ-Al2O3 derived from synthetic boehmite.Single ball milling (SBM) was carried out using a Retsch MM200 vibratory single ball mill. An 11 ml stainless steel milling jar with a 12 mm diameter spherical stainless steel ball is used for single ball milling for durations of up to 1200 min at a milling frequency of 30 Hz. In single ball milling (Fig. 2a), short duration collisions dominate and the energy generated by the mill is determined by the chosen milling frequency. This can be shown by the contact force distribution reported by Kwan et al. (2005) where the impact forces are the most dominant (Kwan et al., 2005). Size reduction is mainly by impact in the SBM although shear and attrition is also present during milling. jet milling (JM) was carried out using a Hosokawa Alpine 50AS spiral jet mill. The stress mode that effects size reduction in the air jet mill (Fig. 2b) is mainly impact, by way of particle–particle and particle–wall collisions. The collision energy is created by the high speed flow of compressed air (Neikov et al., 2009). Compressed air injection and grinding pressures of 6 bar and 4 bar have been used respectively and a maximum of 20 passes is used to achieve the desired size reduction. Planetary ball milling (PBM) was carried out using a Fritsch Pulverisette 7 planetary ball mill. The planetary ball mill (Fig. 2c), is a high energy mill (Angelo and Subramanian, 2008), where shearing and compression are more prevalent than high velocity collisions. Two 45 ml zirconia (ZrO2) milling jars have been used with zirconia grinding balls of 15 mm diameter and the ball to powder ratio used in the experiments was 10:1 by weight. A milling speed of 700 revolutions min−1 has been used for all planetary ball mill experiments with milling times ranging from 5 min to 300 min. Characterisation is carried out on the as-received (A-R) γ-Al2O3 sample and after milling on the SBM, PBM and JM samples using laser diffraction, SEM, BET nitrogen gas adsorption, X-ray diffraction and TEM. Laser diffraction, using a Malvern Mastersizer 2000, has been carried out for particle size analysis, using water as carrier medium. The samples were dispersed using in-built ultrasound and measured at an average obscuration of 12%. SEM analysis is carried out using a Carl Zeiss EVO MA15 scanning electron microscope at 20 kV in backscattered imaging mode. Carbon tabs were coated with powder samples and placed on SEM metal stubs. Sample stubs were sputter-coated with a conductive layer of gold before analysis to prevent charging. SEM quantitative analysis was carried out using Gatan Digital Micrograph Particle Analysis Software (Gatan, 2014). Average particle sizes were calculated from the derived size distributions of a minimum of 500 particles. The coefficient of variation was also derived by dividing the average particle size with the standard deviation of the size distributions.