New Technology for Efficient Grinding

July 1, 2002

A new mill uses a unique agitator design to optimize circulation while keeping maintenance and clean-up work to a minimum.

A constantly increasing awareness of quality and a general desire to increase efficiency have significantly increased the demands made on grinding and dispersion technology. Existing procedures must be optimized in every aspect, and new product technologies often require new, more efficient manufacturing processes and plants.

To meet these requirements, a new grinding and dispersion concept has been developed.* The new mill uses a unique agitator design to optimize circulation and modify the conventional passing procedure in the mill. Within these new parameters, the mill attains the high throughput necessary for circulation processes while keeping maintenance and clean-up work to a minimum. At the same time, the new mill efficiently converts the applied drive energy into dispersion performance.

Figure 1. The conventional ball mill has the disadvantage of concentrating the mill media near the separation sites. This action creates backmixing, which broadens the size distribution.

Circulation Procedures

In circulation milling, the product is driven in a circle through the mill back into an agitated holding tank. It is then mixed with the undispersed material until the required product quality has been attained.

Circulation milling allows the use of less capital equipment and less cleaning labor when compared with traditional discrete-pass operations. However, the efficiency of circulation milling requires that high throughputs be run so that the greatest number of circulations is possible within a reasonable period of time. Conventional mills are often limited in throughput capacity, since their grinding shells are designed to provide sufficient residence time to effect dispersion on most materials. As a result, hydraulic packing (an accumulation of grinding media near the separating device) often occurs when using a conventional mill in a circulation operation.

The agitator ball mill, which is well known on the market and intended for single-pass operation, has a cylindrical horizontal milling container in which an agitator shaft with several coaxial agitator disks are arranged (see Figure 1). The milling container is filled up to 85% with hard spherical grinding media (0.3-3.0 mm), which are set into motion by the rotation of the agitator discs.

With circulation and one-pass, high-performance processes, the grinding media are dragged along by the strong product current caused by the product feed pump and the stickiness of the product in the direction of the separating device, where the grinding media accumulate. This hydraulic packing results in increases in the milling container pressure, product temperature and mechanical wear in the outlet area. Recirculation bores in the agitator discs provide some relief, but they also heighten backmixing and therefore create a broader product net particle distribution.

Figure 2. The new agitator (a) is the primary component of the new mill (b). The grinding path is shown in c.

To prevent these drawbacks, an agitator element** was developed for use in the new mill that stabilizes the grinding media in the milling container (see Figures 2a and 2b). The new element does this in an axial direction, and its operating principle also prevents backmixing in the milling container, leading to a tighter dwell time distribution and a higher specific energy density.

Theoretical Background

The energy density in the mill is directly proportional to the degree of fineness that can be expected from the product flowing through it. The crushing in agitator ball mills with conventional disc agitators occurs in two zones with high energy density. The first zone almost encompasses the agitator disc, and the second lies at the grinding shell wall. Within the area of the agitator disc, the grinding media are accelerated in the direction of the shell wall and absorb kinetic energy during the acceleration process. This energy can be used to crush the particles as they impact other grinding media or the grinding shell wall.

There is a strong shear rate at the agitator disc. In addition, the grinding media collide with one another due to differential tangential velocity and can release their potential energy into seized particles. During turbulent flow, 90% of the energy input into the grinding chamber is used for dispersion and crushing in these zones, which makes up approximately 10% of the grinding volume.

Calculations also indicate that the grinding media in conventional agitator mills are not circulated all the way into the area adjacent to the agitator shaft. Unground product can be found in these areas, and recirculation often destroys the product’s uniform particle size distribution. This phenomenon is sometimes called bypassing.

The inefficient distribution of energy density and the unfavorable circulation behavior in conventional agitator ball mills necessitates the construction of big mills with many agitator discs and a high filling volume of grinding media to attain the required performance.

Figure 3. Particle size distribution during identical specific energy input.

High Energy Density Zones

In the new agitator, four main stress zones with high energy density can be identified: at the accelerator carrier disc, between the blades, at the grinding cylinder wall and at the steering disc (see Figure 2c).

The mixture of grinding media and product is drawn into the area of the agitator shaft and, because of the agitator construction, is rapidly accelerated to the outside by the multitude of blades in the agitator. An internal circulation is built up at the radial-axial level through the pressure differential between the intake and outlet of the impeller. This force is locally much greater than the axial product current. The external effect of the agitator shaft is created in the new agitator through this forced flow. This effect does not occur with conventional discs. With the new agitator, the energy input into the grinding chamber is used for crushing in zones that take up around 30-40% of the milling volume, as opposed to 10% for a conventional mill.

Figure 4. Comparison of agitator discs.

Figure 3 illustrates the influence of the new agitator on the grinding result compared to a conventional agitator disc. The product used is a 70% calcium carbonate suspension. The figure also shows the particle size distribution using identical specific energy input. The distribution indicates a better dwell time distribution compared to the conventional agitator ball mill (see Figure 4).

As shown in the formula at right, the specific energy can also be understood as a product of the stress intensity and the number of stresses, where Ev is the specific energy, Md is torque, Vs is the volume (in liters), w is the frequency and t is time. The movement of the grinding media causes the crushing. The higher the torque (Md), the higher the forces causing the crushing that is found where higher stress intensity results.

The probability of particles reaching the active crushing space increases with increasing rotational speed (w) and longer crushing duration (t). The result is the stress frequency. In the new agitator, the stress intensity is high based on the greater crushing zones, and the stress frequency is much higher through the forced internal circulation.

Figure 5. Movement and speed relationships of the mixture in the new agitator (r1, r2 - interior and exterior radius of the impeller; u1, u2 - circumferential speeds; w1, w2 - relative velocity of the mixture; v1, v2 - absolute speeds of the mixture).

Flow Relationships

Figure 5 shows the relationship between the movement and speed of the delivery medium on a blade of the new agitator. The mixture of product and grinding media that enters axially in the center is diverted from the turning agitator in a radial direction through the blade channel to the exterior circle. The centrifugal force supplies the mixture with energy on the way through the internal channel.

The speeds u1 and u2 determine the centrifugal force. Because the channel cross-section in the exterior circle is larger than in the internal circle, w1 > w2 applies according to the continuity equation.

The blade forces cause the speed changes that occur in the agitator. The angular momentum increase between the internal and external accelerator perimeter determines the energy transferred to the mixture in the blade channel. To keep pressure loss as low as possible, the mill mixture enters the agitator radically with an absolute speed of v1. The exit angle determines the stress intensity.

Design Solution

In any mill, the geometrical proportions of the grinding chamber and agitator determine the efficiency of the crushing and dispersion, the throughput, wear and heat development. The proportion of the suction, pressure and exit face velocity must be in an optimum relation; otherwise, the pumping effect and therefore internal circulation are disturbed. The position and shape of the blades also play an important role. Blades that are too flat do not grind, while blades that are pitched too sharply cause movement that is far too aggressive, resulting in excess wear and inefficient grinding.

The geometry of the new agitator is the result of many laboratory tests and an extensive cybernetic examination. All of the results and realizations of recent years have been incorporated into the new mill. The high energy density created with the new agitator enables a compact design, and zones with low energy density have been avoided. The use of ceramic and other high-grade materials ensures high cooling performance and low abrasion despite the high energy density. Additionally, the media charge level, which can reach 90% in conventional mills, has been reduced to as low as 55% through the function of the new agitator. Table 1 shows some practical examples of test data.

With the new mill, the stress intensity can be adjusted so that the principle is equally effective in wet milling applications, where the breaking down of agglomerates and aggregates is required more than genuine crushing. Strong hydraulic movement through the new agitator also leads to high shear forces.

While the new mill has demonstrated superior performance in the recirculation process, discrete mill operation is also successful. Benefits of the new mill include increased production capacity with reduced energy, floor space and labor.

For More Information

For more information about the new mill and agitator, contact CB Mills, div. of Chicago Boiler Co., 1300 Northwestern Ave., Gurnee, IL 60031; (800) 522-7343 or (847) 662-4000; fax (847) 662-4003; e-mail sales@cbmills.com; or visit http://www.cbmills.com. For more information about WAB, visit http://www.wab.ch.

*The Efficient Circulation Mill (ECM), developed by WAB and distributed by CB Mills in North America and Latin America.
**The Dyno-Accelerator