Milling Finer Dispersions

A new fine media mill can produce dispersions that are significantly higher in maximum product quality than existing fine media mills while operating under the same conditions of agitator tip speed and media type, size and load.

As the bar for quality continues to be raised for many ceramic products, manufacturers are increasingly faced with the need to develop finer dispersions with narrower particle size distributions. However, many existing fine media mills are already operating close to their limits. In many cases, mill optimization strategies such as increasing agitator tip speeds and using finer grinding media have already been put in place, yet the required product quality still cannot be obtained. Based on historical demands for increased product quality, there is no doubt that further improvements will be required.

Recognizing these needs, researchers recently developed a new fine media mill* to meet this challenge. The mill is equipped with a new agitator design that enables it to produce dispersions that are significantly higher in maximum product quality than existing fine media mills while operating under the same conditions of agitator tip speed and media type, size and load.

The Mathematical Dispersion Model

A detailed mathematical dispersion model, called OPTIMIZE^TM, was used to develop the new mill. This model has been used for some time to predict the effect of changing the operating conditions used for making a dispersion. In many cases, its accuracy also allows the evaluation of new mill and agitator designs without having to build them.

The model requires input of product quality versus average residence time data for a given product batch and the definition of the conditions under which it was made. The basic quality versus residence time equation has been previously published.¹ The OPTIMIZE version of this equation is:

QUALITY = QMAX - (QMAX - QSTART) x EXP ( RATE x RTIME)

where QUALITY is the product quality at residence time (RTIME); QMAX is the maximum product quality at infinite residence time; QSTART is the starting (premix) product quality; RATE is the dispersion rate; and RTIME is the product average residence time in the mill. This relationship is shown in Figure 1.

The finite values of QMAX and QSTART are computed by the curve-fitting segment of the model and are not entered as data. QSTART can be measured when particle size is used as the quality measurement but the clumps of slurry inherent in the premix sample result in large measurement errors. QMAX, by definition, is the maximum product quality obtainable with a specific media mill, operated under a set of defined conditions for an infinite residence time. For this reason, it is best to extrapolate the curve fit to determine the values of QSTART and QMAX.

Note that the product quality versus residence time curve will change with any change in operating conditions. The RATE and QMAX values used in the model are affected by changes in the mill configuration and the mill operating conditions. The value of the OPTIMIZE model is that it can predict the effects of these changes and redefine the product quality versus residence time curve for the revised conditions or a different mill.

The Significance of QMAX

Actual product quality measurements are normally based on physical measurements of dispersion samples, including particle size, transparency, absorption and scattering coefficients. The OPTIMIZE model converts these physical measurements into a product quality index, in which the selected quality standard is given a value of 100%. QMAX is defined as a quality value relative to that of the standard and is given as a percentage of the standard.

As mentioned earlier, one of the characteristics of the dispersion industry is that, for certain end uses, the requirements for product quality usually increase with time. Figure 2 is a typical product quality versus residence time curve, where the original product standard was produced at a residence time of 15.00 minutes and valued at 100%. The maximum quality (QMAX) attainable using the mill operating conditions is 5% higher in measured quality than the standard and, thus, has a product quality of 105% relative to that of the standard. The requirement for an increased dispersion quality results in the selection of a new quality standard, using the same mill operating conditions and, therefore, having the same product quality versus residence time curve but requiring a residence time of 17.88 minutes to produce the new quality standard. This new standard is now assigned the standard value of 100% and the QMAX, which has an unchanged finite value, is only 3% higher than this new standard and, modelwise, has a relative value of 103% of the standard.

Further increases in the required product standard quality, without a change in the mill operating conditions, will narrow the difference between the increased product quality standard and the QMAX, until the product quality standard is virtually equal to the finite QMAX of the quality versus residence time curve. Assuming that the relative QMAX value is only 0.5% higher than the newest standard, the QMAX, which is unchanged in measured quality value, now has a relative value of 100.5% of the newest standard. The newest product quality standard, produced under the same operating conditions as the original standard, now requires a residence time of 28.21 minutes. Further improvements in the standard product quality using the same process operating conditions are impossible, since QMAX represents the maximum quality attainable under these operating conditions.

The obvious answer to increasing the product quality further is to change the process operating conditions to provide a new product quality versus residence time curve having a higher measured QMAX value. However, if the mill operating conditions have already been optimized in producing the original product quality versus residence time curve, further improvement in the product quality is not possible with the existing mill. Therefore, a new mill capable of higher shear and providing a higher QMAX value under the same operating conditions is required to meet future quality requirements.

Designing the New Mill

In the design phase of the new mill, the capabilities of both existing fine media mills and the future product quality requirements to be faced by fine media mills were extensively reviewed. Testing revealed that while some mills were more productive than others on a mill volume basis, the ultimate product quality attainable with all media mills was comparable—i.e., the maximum product quality produced by all tested mills, using the same operating conditions, was essentially the same, although some mills achieved this maximum quality with less residence time than others.

Based on this evaluation, researchers identified and prioritized the design characteristics for the new mill. These included:

1) The mill must produce product dispersions that are significantly higher in maximum product quality than existing fine media mills when operated under the same conditions of agitator tip speed and media type, size and load. This criterion had the highest priority. Researchers were not interested in any mill system that would not show such an improvement.

2) The mill must be capable of operation in the circulation process mode at a product feed rate of up to 300 times the mill liquid volume per hour or more. This criterion provides a narrow particle size distribution to the dispersion by resulting in a high number of product passages through the mill.

3) The mill should be designed to use the ultra-fine media being introduced to the industry. The criteria for the new mill included a requirement for handling media as fine as 0.1 mm in diameter.

4) The mill should have a higher dispersion rate, as measured by the OPTIMIZE dispersion model, than existing fine media mills at the same operating conditions.

5) The mill should be designed with sufficient power input to operate the agitator system over a tip speed range of 2000 to 3000 FPM and have suitable provision to remove the agitation generated heat.

Based on the above criteria, researchers developed and evaluated a number of agitator designs aimed primarily at meeting the first criterion. One disc design, called the AP disc, showed extreme promise and was developed further.

The use of the AP discs resulted in a significant increase in the maximum product quality obtainable by a media mill when compared with an identical mill operated under the same conditions of agitator speed, media type, size and load. The OPTIMIZE model indicated that the primary AP disc mechanism was the acceleration of the mill media to a velocity far beyond that normally obtained with a disc. Calculations indicated that, with the AP disc, the velocity of the accelerated media was essentially double that obtained with a standard disc of the same dimensions. This resulted primarily in an increase in the maximum level of shear available in the mill, with a consequent improvement in maximum product quality and dispersion rate.

The AP discs also consumed more power than the standard discs, requiring an improvement in the normal cooling capability through design changes. To meet the objective of an increased product flow rate for circulation milling, the shell design was changed relative to previous fine media mills of similar volume to provide more area for product flow and more cooling surface area per unit volume. A newly designed screen module was developed to allow the handling of ultra-fine media to a size as small as 0.1 mm.

Operation of the new mill is enhanced by the use of a computer-controlled startup and operation to provide precise control of process variables due to the high energy capability of the AP discs in a small volume mill. The computer assures that the system will automatically adjust to the operating conditions required for stable processing.

Testing the New Mill

Using the A6P disc, comparison runs were made versus existing standard discs—using the same number of each disc type—under identical conditions of agitator tip speed, media type, size and load. One run focused on a 20% calcium carbonate dispersion. In this example, the mill was operated at an agitator tip speed of 2700 FPM, with an 80% media load of 0.7-1.2 mm ceria stabilized zirconium oxide media.** The circulation process was used with a 119 liter batch size and a circulation rate of 240 GPH for seven hours. The circulation rate was limited by the standard discs, which are more sensitive to hydraulic media packing.

The results of this comparison illustrated the benefits of the AP disc concept. The batch processed using the existing fine media mill standard discs had a measured product quality versus residence time curve fit QMAX median particle size of 0.503 micron on a volume basis. The batch processed using the A6P discs had a measured product quality curve fit QMAX value of only 0.310 microns. This is a significant improvement in the measured QMAX value achievable under the same operating conditions. Use of the OPTIMIZE model allowed the effective agitator tip speed of the A6P disc mill to be defined at over 5000 FPM—i.e., the QMAX obtained with the AP discs, operated at a tip speed of 2700 FPM, were equivalent to that which would be obtained with standard discs operated at a tip speed in excess of 5000 FPM. (Most existing mills are limited by mill and media wear constraints to agitator tip speeds of about 3000 FPM.) The higher maximum shear obtained under these conditions resulted in a significant increase in the maximum product quality achievable in the A6P disc mill.

Predicting Results from the Model

The advantages of using the A6P disc mill over existing mills can be seen in the example given in Figure 3. This figure illustrates a situation in which the existing mill has been optimized for an existing product and, because the required product quality has increased to the point where it almost matches the available QMAX, no further improvement can be anticipated.

This figure compares two otherwise identical mills, one using the existing standard fine media mill discs and the other using the A6P discs and operated under identical conditions. The current disc technology mill will produce the existing product at a residence time of 28.43 minutes, but cannot increase the product quality beyond the current standard. The use of the optimized mill using the new A6P discs will produce the same product quality at a residence time cycle of 10.99 minutes but, more importantly, can produce a product with a quality increase of up to 105% of the current standard in less residence time than the current, lower quality product.

Meeting Increased Quality Standards

Increasingly finer dispersions will continue to be required in many segments of the ceramic industry—and many media mills are not up to the challenge. Tests have shown that operating the new mill with the advanced AP disc technology under the same conditions will continue to significantly exceed the product quality achievable with existing mills. The new mill can be used to produce finer particle size ceramics and generate nanosize particles with narrow distributions. Additionally, the finer particle size attainable with this mill could be used for electronic printing inks to allow even finer printed circuits to be developed—thereby generating new markets in the future.

For More Information

For more information about fine media mills, contact Premier Mill, A Lightnin Company, One Birchmont Dr., Reading, PA 19606-3298; (610) 779-9500; fax (610) 779-9666; e-mail pmsales@lightnin.spx.com; or visit www.premiermill.com.

*The QMAX Supermill™, supplied by Premier Mill, A Lightnin Company, Reading, Pa.
**Zirconox, a registered trademark of Jyoti Ceramic Industries Pvt. Ltd.