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Attrition Milling For Wet FGD Limestone

FGD Attrition Milling

Attrition milling is another approach beginning to gain acceptance in wet FGD. Attrition milling is also not new. Various forms of attrition milling have been used across a wide range of industries and applications, such as paints, pigments, pharmaceuticals, ceramics and coal slurries. The attrition mill could be considered on the other end of the loose generic description of stirred media or stirred ball mills. Similarly, some have classified the ball mill as being in the family of attrition mills.

The tailoring of the mill design to the specific application can be critical with attrition mills, as they operate at higher speeds, smaller volumes and higher localized energy concentrations. This mill type was chosen as an example of how mill system advancement, optional mill system arrangements and sparing philosophies can impact overall system design and process effectiveness.

The mill was designed specifically for limestone grinding with a reduction ratio equivalent to that typically applied to the vertical ball mill in FGD service. This is taken to be the comminution of ¼” x 0” mill feed size to a 95% less than 325 mesh product size. Since the design of the attrition mill is suited to obtaining extremely fine grinding, finer product sizes are possible and are evaluated to some extent later.

The original attrition mill was invented by Dr. Andrew Szegvari in the 1920s. The invention is significant in that it marked the beginnings of agitated media milling. As mentioned earlier, the stirred ball mill term is often used as it depicts a grinding mill, which utilizes a stationary vessel and internally agitated balls. The concept is integral to the design of the attrition mill. Power input is used more directly for agitating the media rather than for rotating a large heavy tank and its contents, as in the case with conventional horizontal ball milling.

Optimal fine grinding requires both impact and shearing force. In the mill, the rotation of the horizontal arms directly imparts energy to the grinding media, thus causing the balls to randomly collided with one another. These collisions create the necessary impact forces to break down individual particles in the slurry. In addition to the impact forces of the media, the balls are also spinning in different directions, thereby creating shear forces on the adjacent slurry. The combination of these impact and shearing forces results in efficient size reduction.

During wet milling process of FGD limestone, both limestone and water are continuously fed from the top of the mill through a specially designed feed housing. This funnel shaped housing has a cylindrical extension, which projects into the grinding vessel of the attrition mill. The shaft of the mill extends down through the housing and the cylindrical extension with the media agitator arms being disposed just beneath the extension. The rotating shaft utilizes several radially projecting, angled impeller blades, which are within the cylindrical extension. When the shaft rotates, these blades create a pumping action, which forces the limestone and water into the grinding media bed.

Generally, attrition mills use media ranging in size from 3mm to 10mm. However, in the wet FGD application, the mill design was altered to incorporate 12.7mm through-hardened carbon steel balls. The specialized mill design allows the 6mm (¼”) feed size of the limestone. Attrition arm tip speed is approximately 270 m/min. After the limestone and water slurry has passed through the grinding media bed, the ground limestone slurry overflows and discharges from the top of the mill into a separation tank. A low speed mixer is incorporated into the separation tank, which allows the fines to continuously overflow to the mill product tank while coarse limestone particles settle to the bottom. A mill recirculation pump then carries the coarse limestone particles back to the mill to be re-ground.

The power consumption of these high efficiency attrition mills is relatively low. For example, a mill equipped with a 150hp mill motor produced 9.75 metric tons per hour of 95% minus 325 mesh limestone slurry product when fed with ¼” x 0” stone feed size. This equates to a specific energy based on the mill installed motor power of only 10.4 kWh per short ton of material processed.

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