A significant investment is required to operate any stirred milling process. The technology is employed to meet exacting quality demands, primarily relating to achieving a particular particle size distribution in a specific product. However, these demands can also be defined by other criteria, such as color strength, gloss and reactivity.
Evaluation of a particular process can be theoretical, experimental or practical, but usually results in defining the “specific energy,” or the energy required to process a unit volume of the product.
The mill supplies the energy for the process, while the media transfers that energy to the product to affect the grind. Each mill type specifies the minimum bead charge required, typically between 75% and 85% of the mill volume. Filling above this range will result in increased wear and mill temperature; operating below this range will reduce milling efficiency, and product particle size will increase, as shown in Figure 1.
FIGURE 1 | Particle size versus bead size.
Specifying the type of bead to be used is more complex and relates to both the size and density of the media. Higher density results in higher bead impact intensities, which consequently improve the achieved grind. Generally, the highest density media that allows for good mill operation is recommended.
Bead size selection is influenced by various factors, including the target size reduction ratio and the formulation’s rheological properties. For each process, there will be an optimum bead size—1 mm in the example detailed in Figure 2.
FIGURE 2 | Effect of filling ratio.
If beads are too large, energy is wasted on excessively intense impacts; if they are too small, impacts lack the energy needed to break target particles. These factors are defined and controlled at the start of a milling operation to ensure acceptable performance.
Stirred milling provides the intense, aggressive environment necessary to break down target particles. However, this environment also causes wear on the mill and media. Wear on mill liners and discs requires periodic maintenance, typically defined in the machine’s operating manuals.
But what about the media?
Mill Volume Control – Topping Up
There are many types of beads used in stirred mills, with ceramic-based products being the most widely adopted. Ceramic media are inert, nonabrasive, and cost effective. A technologically advanced grade is based on engineered yttria-stabilized zirconia, with a density of 6.0 g/cc. The best options are mono-sized. Wear rates depend on formulation and operating conditions; for this example, a wear rate of 0.02% per hour results in a 10% media loss after 500 hours. Another 500 hours will decrease the mill volume below 70%, reducing efficiency.
To maintain grind parameters, mill volume must be adjusted by topping up the mill to replace the worn bead. Topping up can be regulated through power draw, product quality, operating hours, or visual observation.
The objective is to keep the mill volume within the 75%-85% range, avoiding overfilling while providing practical topping-up intervals. Eventually, the question arises of how long this practice can continue safely. The endpoint is when bead size approaches the separator system size—after about 4,000 hours in the given example with a 1 mm bead and a screen or gap set at 2½ times smaller. Running the mill beyond this point is not recommended, as beads will become trapped, pressure will increase and the separator could be damaged.
A second, equally important consideration is the effect of a charge with a continuously reducing bead size on grind quality.
Media Size Control
In a simplified case, if bead size decreases by 10% every 500 hours and losses are made good with new bead additions at the same interval, bead charge evolution may resemble Table 1.
TABLE 1 | Bead charge evolution.
After 2,000 hours, the average bead size in the mill is approximately 30% smaller than that of the new beads. Topping up has not maintained the optimum bead size, which significantly impacts grind quality and mill efficiency. Beads must provide a minimum stress intensity to break target particles, as illustrated in Figure 3.
FIGURE 3 | Particle size versus stress intensity.
The original process target would be to have bead size in the optimum range at the base of the curve. A 30% deviation from this point would likely push efficiency into the lower range of the curve.
Conclusions
There are limits to maintaining an effective media charge in a stirred mill. One critical point is dictated by the mill itself: media must remain at sizes safely above the separator system size. This is the extreme limit of the process. Other factors, such as maintaining bead charge volume and bead size, should prompt action before the mill is compromised. Topping up maintains bead volume but eventually fails to provide the necessary bead size for efficient grind. At this point, a more significant action, such as replacing the entire charge or screening out ineffective beads, is required.
Determining when to take this action is key to a proficient maintenance program. Monitoring mill output allows for a reactive approach, where action is taken after quality or throughput decline. This method can be costly due to productivity loss before remediation. Bead audits, which involve taking small samples to assess the bead mix, allow proactive decisions about charge replacement. Typically, bead condition is assessed after a set operating time, with future charges reviewed at similar intervals.
A rule of thumb is to use a bead charge evolution table to set a maximum top-up level, depending on product sensitivity. A good starting point is a maximum top-up level of 30%, keeping average bead size above 70% of the original and stress intensity at acceptable levels. In the given example, this equates to an operating time of up to 2,000 hours. After this period, the charge should be replaced. The used charge can be sieved at an appropriate level, and the beads can be used for future topping up.
Note that beads are often sold in size ranges (e.g., 0.8-1.0 mm or 1.0-1.2 mm); this must be considered when creating a maintenance plan. Bead options vary, with each formulation having a specific wear rate. Lower-density beads typically wear faster than the zirconia media in this example, so wear rates must be factored into the maintenance plan.
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