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Another class of fine media mills uses an agitator system in which a number of pins are mounted perpendicular to the agitator shaft axis. These mills also disperse particulates by moving the media/product mixture in a way that creates differential media velocities and shear. As with disc agitation systems, the maximum media velocity—and thus the maximum shear applied to the particulate product—is obtained at the end or periphery of the rotating pin agitator.
While both of these systems have been successfully used to disperse ceramic glazes and other ceramic materials, they require a significant amount of processing time and energy to achieve a complete dispersion. Recently, a new disc system* was developed that overcomes these problems while providing improved dispersion results.
The New Disc DesignThe new disc system is composed of a number of pins or pegs mounted on a urethane-coated disc surface. Unlike prior pin mill technologies, the pins are positioned parallel to the agitator shaft axis and close to the periphery of the rotating disc, resulting in a greatly enhanced level of dispersion capability. The parallel pins are forced through the normal accelerating flow of the media/product mixture, and this forcing action diverts the product flow around the pins.
The new disc configuration creates a disruption in flow across the flat disc surface and results in a pulsating flow pattern both toward and away from opposing disc surfaces. This combined action increases the velocity of the media/product mixture as it flows around the pins beyond the velocity levels normally attained at the disc periphery. The result is an increase in the maximum shear level attainable at a given agitator tip speed compared to conventional disc and perpendicular pin (peg) agitation systems operated at the same tip speed.
The higher media/product shear level obtained with this new pin disc agitator system also results in a significant increase in the rate of product dispersion compared to conventional disc systems. Tests conducted in production operations have shown an increase in dispersion capacities ranging from 150 to 200 percent of those achieved in fine media mills with conventional disc agitator systems operated under identical process conditions (see Figure 1).
The number of discs required to achieve the optimum results varies, depending on the application. In some cases, replacing just one or two standard discs with the new pin discs will provide the required processing improvements, while in other cases all of the standard discs might need to be replaced. Tests using a company’s current operation and processing conditions can be used to determine how many of the new discs will be needed to produce an enhanced dispersion.
Test ResultsTo prove the capabilities of the new disc design, researchers at a ceramic plant processed a 90 percent glass frit/10 percent ceramic powder formulation in water at 50 percent total solids with surfactant in an SM45 Supermill.1 An 80 percent media charge of 0.5 mm YTZ media2 was used to process the formulation. The specific gravity of the slurry was approximately 2.0 (16.7 lbs/gallon), and its viscosity before milling was 100 cps as measured on a Brookfield viscometer. Five of the new pin discs and four standard urethane-coated discs were used in the test.
An identical slurry processed using nine standard discs required 240 minutes of milling to reach D50 (i.e., 50 percent of all particles were smaller than the maximum particle size) of 0.500 microns and D99.99 (i.e., 99.99 percent of all particles were smaller than the maximum particle size) of 1.700 microns. The total energy required to complete the batch was 70 KWh, and the maximum flow rate was 18 liters per minute (285 gallons per hour) before packing began.
The mill with five new pin discs and four standard discs was almost twice as efficient as the equivalent mill using only urethane-coated discs, and it was also more efficient compared to a comparable pin disc configuration. Replacing five of the standard discs with the new pin discs reduced the processing time from 240 minutes to about 125-130 minutes total—including one discrete pass. The total energy was reduced from 70 KWh to 57 KWh—nearly a 20 percent reduction in energy consumption.
The new discs also reduced the D99.99 particle size by 0.200 microns vs. the previous standard—a very important benefit for the ceramic plant. The smaller particle size produced a more uniform finished product with better quality and increased performance in the final application (electronics). An overview of the batch results is presented in Table 1.
“The mill has never run this smooth—with stable readings of pressure, power consumption and product temperature. We believe that the mill was previously operating on the edge of hydraulic media packing. Adding the new AP4 pin discs has eliminated the problem,” the plant operator said.
References:1. The SM45 Supermill is supplied by Premier Mill, A Lightnin Company, Reading, Pa.
2. YTZ grinding media is a trademark of Tosoh Corp., headquartered in Tokyo, Japan.
*The AP series, developed by Premier Mill, A Lightnin Company