Effect of mill type on the size reduction and phase transformation of gamma aluminaDoor José Javier Salgado
1. 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, short duration collisions dominate and the energy generated by the mill is determined by the chosen milling frequency.
2. Materials and methods
3. The sample used for experiments is a commercially available γ-Al2O3 powder: a 99.99% pure γ-Al2O3 derived from synthetic boehmite.
4. The stresses may affect product attributes in different and often ‘unexpected’ ways through mechanochemical activation, so an understanding of the mill function on the product characteristics is highly desirable for optimising product functionality.
5. Results
6. 1. Particle size analysis using laser diffraction
7. 1.1. γ-Al2O3 particles before and after dry milling in the JM
8. 1.2. γ-Al2O3 particles before and after dry milling in the SBM
9. 1.3. γ-Al2O3 particles before and after dry milling in the PBM
10. 1.4. Comparison of dry milling using different mills
11. Discussion of particle size and particle morphology results
12. Further analysis was carried out by a comparison of particle or crystallite sizes for A-R, JM (20 passes), PBM (300 min) and SBM (1200 min), derived from BET, TEM and XRD
14. 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)
15. 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.
16. 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.
17. Milling is a widely used industrial operation common for cases where size reduction of particles is required. 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. failure mode, and the required product particle size.
18. Ball mills, vibratory mills, rod mills and jet mills can be used to achieve particles less than 1 mm in diameter but for ultrafine dry milling. particles (d90=<10 µm), vibratory ball milling, planetary ball milling and air jet milling are commonly used methods. In these mills particle size is reduced by impact, shear, attrition or compression or a combination of them.
