Grinding Media: The Next Generation
As grinding requirements become finer and more specialized, and other issues such as material contamination become more crucial to ceramic manufacturing operations, it is increasingly important to evaluate the type of grinding media used in your mill. Using an inferior grade of grinding media can result in poor grinding, clogged screens, damaged separator rings, excessive mill and component wear, and mechanical seal failure. Additionally, many of today’s newer high-energy grinding mill configurations demand a better grade of grinding media.
Recent advances in the quality, roundness and uniformity of ceramic grinding media have resulted in profound improvements in the entire “media mill process.” However, the ceramic manufacturer must be aware of what qualities comprise this new generation of ceramic grinding beads.
Evaluating Grinding MediaTo evaluate grinding media, the following characteristics of the grinding bead must be analyzed:
- Chemical composition
- Manufacturing and forming process
- Surface condition
- Size range or gradation
Size is also important. A media charge with a wide size range is very inefficient because it causes a disruption of the desired flow pattern inside the mill.
“Next generation” beads feature a solid composition that eliminates hollow beads and surface satellites. They also feature a consistent microstructure, which provides improved hardness and toughness for long media life, resulting in less downtime. And they are much higher in density than other beads, generally carrying a specific gravity of 4.0 or higher. This increased density provides greater grinding efficiency in discrete pass or recirculation grinding, reduced processing times, higher throughputs and the ability to process higher viscosity formulations.
These new beads also feature excellent roundness, which increases efficiency and maximizes shear inside the milling device, and smooth bead surfaces, which reduce mill component wear and are more easily washed between formulations. Additionally, all boast a uniform narrow size distribution, which increases efficiency and results in more narrow particle size distributions in the finished product, as well as reduced media wear.
Benefits of High Quality Grinding MediaHigher quality grinding beads do cost more—but they can produce great long-term benefits that easily justify their higher initial cost. Benefits such as increased grinding efficiency are easy to demonstrate. If the mill can produce 10 or 20% more product in the same amount of time, the dollar value can easily be tabulated. The goal is always to get more gallons through a smaller grinding chamber without damaging the mill—and higher-quality beads can help achieve this goal.
Other benefits are harder to quantify. For example, using a higher-quality grinding bead will usually result in a longer cycle of consistent production in the plant. If the mill produces the same desired finished particle size over a longer time cycle, a definite savings will be realized. The curve of quality vs. hours of operation will flatten out. Quality control time cycles can be increased, and samples won’t have to be checked as often. Using high-quality grinding media will enable you to reduce the yo-yo effect in manufacturing—when the process is constantly changing as media and mill parts wear, quality control is a moving target. On the other hand, longer lasting media and mill parts will ensure consistent quality control with much less effort, greatly reducing the stress levels of process engineers and production managers.
Mill maintenance costs will usually be reduced when a higher quality grinding media is used, resulting in a longer mean time between failures (MTBF). For example, if the preventive maintenance cycle can be increased to 2000 hours instead of 1000 hours, this means the mill is down for maintenance half as frequently. This change in MTBF has a definite average cost associated with it—both in downtime (lost production) and maintenance costs for replacement parts and labor.
When viewed in this long-term context, the initial cost of grinding beads should be the least significant factor in your media selection process. In other words, a lower cost per pound of beads can result in a significantly higher cost per pound of finished goods produced in the long run.
Improved ManufacturingImproved grades of ceramic grinding media are available in zirconium silicate, cerium stabilized zirconium oxide and yttrium stabilized zirconium silicate. These high quality ceramics should be evaluated if issues such as high efficiency, ultra-fine particle size, minimum contamination and reduced mill maintenance are critical to your company.
If you are currently using lesser grades of ceramic or glass media, you have a great deal to gain by testing these new offerings. Look for a solid composition, a consistent microstructure, high density, excellent roundness, a smooth surface and a uniform size distribution. Additionally, work with your equipment supplier to find the best product for your equipment and application. The effect of high quality grinding media on your product quality, productivity and mill maintenance costs can be significant.