Causes of Powder Adhesion to the Wall and Sedimentation Caking in Planetary Ball Mill Jars

For planetary ball mills, we usually classify the ground particle size distribution into the following grades: millimeter to micron (3 mm–10 μm), submicron (10 μm–3 μm), and nanometer (3 μm–1 nm). Generally, the grinding process from millimeter to micron proceeds very smoothly; using zirconia beads of 15 mm, 10 mm, and 5 mm can reduce the particle size rapidly. However, grinding from the micron to the submicron and then to the nanometer scale presents certain difficulties, and problems such as powder adhering tightly to the inner wall of the ball mill jar or caking at the bottom tend to occur. The reasons are as follows:
1.Ball mill jars with low wall smoothness are prone to powder adhesion.

2. For example, materials with low melting points may undergo cold welding due to excessive temperature during grinding.
3. If the powder itself has a high water content, the powder at this particle size range will clump together under the influence of water molecules. Additionally, static electricity generated by the powder can cause it to adhere to the grinding balls and the inner wall of the jar.

In light of the above factors, approximately 50% of the material may stop being refined further. It is necessary to adjust the grinding scheme promptly for subsequent experiments. Measures include replacing the jar with a material featuring higher surface smoothness, drying the sample to remove moisture, reducing the operating temperature of the ball mill, or using grinding media with smaller particle sizes to assist in powder grinding. Overall, dry grinding can reduce most materials to the 3 mm–10 μm range. Wet grinding offers more favorable conditions and can usually achieve nanoscale particle sizes. Nevertheless, for powder processing, dry grinding is preferred whenever possible, as minimizing subsequent treatment processes is a common choice in numerous experiments.

Solutions

1. It is recommended to sieve the powder at this stage and switch to polyurethane jars for ultra-fine grinding. The main reason for adopting this two-step grinding approach is that when ceramic powder reaches the 10 μm mark, it enters a critical point where agglomeration becomes prominent. Jars lacking high surface smoothness will inevitably suffer from powder adhesion. Even though zirconia jars already have a relatively high smoothness, the issue of powder adhesion still persists. Polyurethane jars, which possess the highest surface smoothness among all jar materials, play an irreplaceable role in this scenario.
2. Meanwhile, reduce the size of the zirconia grinding balls to 1–3 mm. Grinding with this scheme can reduce the powder to approximately 3 μm. Further refinement beyond this point offers limited progress, mainly due to the persistent problem of powder adhesion. Additionally, achieving finer particle sizes would require the use of even smaller microbeads, and the subsequent separation of these beads from the powder becomes another key challenge. Only when the surface activity of the ceramic powder is sufficiently high can dry grinding be continued to achieve the nanometer scale.
3.