A Vertical Grinding Solution
Horizontal bead mills have been used for a number of years in applications ranging from de-agglomerating and dispersing tableware and sanitaryware glazes to grinding alumina, silica, silicon carbide, silicon nitride, zirconia and other technical ceramics. In 1986, a new grinding media separation system* was introduced that provided higher throughputs, lower chamber pressures and reduced process temperatures compared to conventional separators. Over the past 15 years, improvements to this technology have reduced, and, in most applications, eliminated the historical problem of grinding media compression in a horizontal bead mill. The technology successfully enables the horizontal bead mill to be used to reduce the particle sizes of abrasive ceramic materials by drastically reducing the wear of the grinding media and mill components.
Horizontal bead mills incorporating this advanced grinding media separation system have been used to process coatings for catalytic converters; synthetic zeolites and alumina for cracking catalysts used in oil refining; alumina, ceria, silica and other chemical/mechanical polishing (CMP) slurries; aluminum oxide (Al2O3), zirconium oxide (ZrO2) and silicon nitride (Si3N4) for wear parts and other technical applications; electro- and piezo-ceramics (e.g., barium titanate); materials for medical prosthetics; nanoparticles of titania for ultraviolet protection and silica for ink jet paper coatings. They have also been used to autogenously grind silicon carbide (SiC) with SiC granulate; to autogenously grind boron carbide (B4C) with B4C granulate; and to grind zirconium silicate using quartz sand. However, while horizontal mills work very well in many of these applications, some demanding processes exist where the abrasive and corrosive natures of the slurry are not ideally suited for a horizontal mill. For example, milling many abrasive, hard ceramic materials causes wear on the grinding media, the internal or wetted mill components (e.g., the agitator discs), the mill lining and the mechanical seal. Additionally, many of today’s technical ceramics require a particle fineness of less than 1 µm, resulting in high energy input requirements and real grinding or particle size reductions with an energy demand of >500 kWh/tsolid. Materials that require a metal- and contamination-free milling process and low contamination from the grinding beads can also be unsuitable for use in a horizontal mill due to the wear of the mechanical seal. A new mill design is needed to meet the needs of these processes.
Design Criteria
One of the most demanding ceramic grinding processes is the contamination-free grinding of SiC slurries to a submicron particle size. Since SiC is extremely abrasive, conventional types of grinding media such as alumina and zirconia typically lead to high contamination rates and high costs due to the fast wear rate of the media. The traditional method for grinding SiC has been to reduce the particle size using steel media in steel ball mills. However, removal of the metal contaminants from the slurry is a difficult, time-consuming process and is therefore expensive.Engineers at Netzsch Feinmahltechnik GmbH, Selb, Germany, decided that a better solution would be to use SiC grains as the milling media. This approach would eliminate the problem of high media costs because grain is much less expensive compared to formed alumina, zirconia or even SiC beads. The use of SiC grains also leads to contamination-free material because the product to be ground is the same as the media (referred to as “autogenous grinding”).
Tests on a horizontal lab mill showed that the SiC grains worked very well as grinding media, producing median particle sizes in the range of 0.5 to 1 micron. However, the tests also showed that the SiC grains wear rapidly—as much as 0.15 kilograms of media were required per kilogram of product ground—compared to other processes. Rapid wear of the media also affects the energy input to the machine due to the decreasing level of shearing. To maintain an efficient process, grinding media would continuously need to be added to the mill. Although conventional methods are available to manage this process with both vertical and horizontal mills, these options are expensive and can be a source of maintenance problems due to blocking and wear.
Additionally, SiC grains are themselves extremely abrasive. Tests with a horizontal mill showed that the mechanical seal would only last for a few hours in this environment.
Engineers agreed that the new mill design must address the problems of wear of the mechanical seal, simplify the addition of grinding media (SiC grains) and include wear-resistant mill construction materials. Table 1 summarizes the process requirements.
The New Vertical Media Mill
To address these process demands, engineers decided that the ideal solution would be a vertical mill because a mechanical seal would not be required to seal the vessel. The mill would be lined with an elastomer grade of polyurethane (PU) because of its well-known abrasion resistance characteristics.The resulting vertical mill, shown in Figure 1, offers a relatively high energy density compared to ball mills, attrition mills and other classic vertical grinding systems. Additionally, the mill uses the advanced grinding media separation system at both the discharge and feed sides of the mill to centrifuge the media out of the slurry. The separation system is installed at the top of the mill as an integral part of the agitator and incorporates a vibratory feeder that runs off the mill’s kilowatt draw. As the media wears down, the kilowatt draw drops. Using a PLC, the vibratory feeder is automatically turned on when the kilowatt draw reaches a low set point and turned off when the target kilowatt draw is achieved.
Eight rubber-coated centric grinding disks ensure efficient grinding with minimized wear (see Figure 2). Grinding media is automatically added during the grinding process through a system that feeds the media to the “center” of the classifying rotor, ensuring that blockage does not occur. The mill’s 30-kW drive motor with a frequency inverter provides an agitator speed of 196-900 rpm (2.6-11.7 m/s) with high energy density, and product is pumped through hose pumps at rates up to 18 kilograms of slurry per minute.
Advantages of the New System
The primary advantage of the new vertical mill is its simplified operating system. The machine has only a few wear parts, including the agitator discs, chamber lining and separation system components. The media addition system is easily controlled and simple in design, eliminating the possibility of parts wearing. The use of a peristaltic pump to feed the mill reduces maintenance costs because the tubing is inexpensive and easy to replace.Several of the new mills have been installed in ceramic plants and are being used to grind SiC using SiC grain as the grinding media. This experience shows wear life of the agitator parts in excess of 2000 hours, which is remarkable considering the abrasive nature of this application. The process is easily automated with flow, temperature and pressure switches to allow unattended operation.
Grinding SiC in the new mill is typically a batch process, with the slurry circulating in a holding tank, as shown in Figure 3, to ensure product consistency. The batch is run until a predetermined energy input is reached, providing a consistent level of particle size reduction and, therefore, a repeatable process. Monitoring the tank level is not required in this type of operation. However, the machine can also be used for discrete pass processing for applications that do not require extreme particle size reduction.
In addition to SiC grinding, the system will also work well in many ceramic and abrasive slurry applications where grinding media from 600 microns to 3 mm is required and the slurry viscosity is suitable (below 2000 m Pas).
Future Applications
One possible future application for the new vertical mill is for grinding catalyst slurries with high acid content. Currently, horizontal mills or attrition mills with 1.5 to 2 mm zirconia grinding beads are typically used for this process; however, the abrasive, acidic slurries attack the seal rings and minimize the life of the mechanical seal. Since the new vertical machine does not use a mechanical seal, this problem is eliminated. Because this process is basically a de-agglomeration of ceramics to obtain a uniform dispersion or homogenization of the various metal solutions and ceramic components, high flow rates through the mill are required. The new mill can operate at flow rates up to 20 kg/minute.Other potential applications include products with an initial coarse particle size (larger than 500 microns) that require fine grinding, even down to submicron sizes. The coarse material can be fed into the mill from the top, and the classifying rotor keeps the coarse particles in the mill until they are ground to the appropriate size. One example of this application is grinding zircon sand using quartz sand or zircon sand as the media. These products are currently milled using horizontal mills, but wear of the mechanical seal in this process is a maintenance issue.
Successful tests using the mill to grind boron carbide have also been made, and this process could be applied to grinding alumina using corundum sand.
With the introduction of this new vertical mill, ceramic manufacturers can grind even the most abrasive, corrosive, high solids slurries to submicron particle sizes—without experiencing excessive equipment wear.