Next-generation milling and dispersing machines can help ceramic manufacturers significantly improve the efficiency and flexibility of their slip preparation processes

Ceramic bodies generally consist of a mixture of clays and hard materials. In many ceramic manufacturing processes, all of the raw materials are ground together in ball mills to the required fineness. However, hard materials and clays vary in their grindability, dispersability and wear characteristics; as a result, the milling time-and consequently energy consumption-is usually dictated by the most difficult material in the mixture. Clays and other soft materials are often over-processed, and a significant amount of energy is wasted.

A much more efficient way to prepare ceramic bodies is to grind and disperse each raw material separately according to its specific requirements. This method optimizes throughput while minimizing energy consumption and wear. However, achieving cost benefits from this type of preparation requires grinding and dispersing machines that are both highly effective and flexible, so that they can easily be adjusted to meet specific material properties. Although machines capable of separately preparing raw materials have been available for quite some time, such systems often have high operational safety, energy and space requirements that make them impractical for many ceramic manufacturing operations.

A new generation of milling and dispersing machines has been designed to overcome these drawbacks. By using the right equipment, today's ceramic manufacturers can significantly improve the efficiency and flexibility of their slip preparation processes.

Conventional Grinding Limitations

In a ball mill, grinding occurs when a particle is crushed between two pieces of grinding media. Grinding efficiency therefore depends on:

• The weight of the grinding media.
• The amount of media used.
• The number of contacts (between two pieces of grinding media or between the grinding media and mill lining) per unit of time.

These parameters can only be changed to a certain extent. For instance, most ceramic applications use aluminum oxide (Al₂O₃) grinding media because they are cost-effective and offer favorable grinding properties. A higher weight can be obtained by using fewer media with a larger surface area, but fewer contacts will occur between the grinding media and the material to be ground. Another possibility is to use media with a higher density, such as zirconium oxide (ZrO₂) or steel. However, the former is expensive, while the latter often presents contamination issues in ceramic applications.

The amount of grinding media (and therefore the amount of grinding surface) can be increased if smaller media are used, but this results in a lower grinding media weight and less grinding capacity. Finally, the number of contacts between the media and material can be improved by increasing the speed (rpm) of the ball mill, but most ball mills are already operating at their maximum level.

Grinding machines that can overcome these limitations are necessary to achieve a decisive improvement in the grinding process.

Next-Generation Grinding

A new generation of agitated media mills* has been designed to handle both dry and wet grinding in an efficient, continuous operation (see Figure 1). The mill is composed of a rotating cylindrical grinding chamber, one or two agitators (depending on the size of the machine), and a static deflector associated with a feeding tube for the material being ground. The agitator(s) is/are positioned eccentrically with respect to the grinding chamber's axis of rotation and can rotate in the same direction as the grinding chamber or in the opposite direction, depending on the desired grinding effect. The lining used in the grinding chamber and agitator is typically made of steel, polyurethane or Al2O3, depending on the application and desired wear resistance, and the grinding beads can be either steel or ceramic.

In a grinding operation, the grinding chamber is filled to approximately 80% of its capacity with small (3 to 7 mm in diameter) grinding beads. The size of the grinding media depends on the initial particle size of the material to be ground and the required fineness.

The coarse material is continuously fed into the mill through a loss-in-weight batching system (see Figure 2) and is sent through a feeding tube to the bottom of the grinding chamber. The rotating movement of the chamber drags the material to the inside and mixes it with the grinding media. As soon as the material enters the chamber, the high pressure of the load at the bottom of the grinding chamber, which is caused by the weight of the grinding media, develops into a very intense grinding action. The intense movement of the grinding media provides a high dynamic input into the agitator (most of the grinding action takes place in this zone), and the compression of the grinding media in front of the deflector produces a high level of attrition between the grinding media and the material.

The rotation of the grinding chamber ensures that all areas experience an equal amount of grinding action. As a result, the energy supplied to the agitator is always high, intense contact between the grinding media and the agitator is always guaranteed, and material buildup on the wall and bottom of the grinding chamber is prevented.

During the grinding process, the fine material is pulled from the bottom of the grinding chamber to above the grinding media, where it is automatically extracted and sent through an air classifier. Oversized particles are recirculated back into the mill with the coarse grain for additional grinding until the desired fineness is achieved. The grinding media, which is consumed during the grinding process, is continuously replaced through the material feeding tube.

The system's design ensures highly efficient grinding. Figure 3 shows the test results from a feldspar grinding operation. The starting grain size was relatively coarse (approximately 4 mm)-usually a grain size distribution of <2 mm is preferred. (Keep in mind that grinding is the most expensive way to reduce the particle size of a material. Ideally, hard materials should be pre-crushed as finely as possible to maximize material preparation efficiency. Using a finer feeding material will increase throughput and reduce specific energy consumption [kWh/t]).

The operating parameters of the agitated media mill-the grinding throughput, the agitator's rotational speed, the size and volume of the grinding media, and peripheral variables such as the speed of the air classifying wheel and the amount of air circulation in the grinding chamber-can all be changed to fully optimize the grinding process. As a result, each material is ground with a minimum amount of energy because the energy consumption corresponds directly to the grinding properties of the raw material and the required particle size. Additionally, each material can be processed with an optimal throughput and ground to a precise fineness, giving operators full control over the material preparation process.

Conventional Blunging Limitations

As mentioned previously, the key to efficient slip preparation is to prepare each raw material separately. This means that the clays should also be prepared and dispersed in separate steps, instead of being ground with the hard material in the ball mill.

Of course, clays containing hard materials should be ground. However, clays with few or no hard materials should instead be dispersed, or blunged, in separate machinery designed for this purpose. Dispersing clays is more cost-effective than grinding, and it allows the size of the grinding chamber (and therefore the overall mill size) to be reduced. It also enables manufacturers to use clays of virtually any density-including those with a very low density. Any impurities can be separated through screening.

Most conventional dispersers and blungers consist of a stationary tank with a dispersing tool mounted on a shaft. The tank is filled with the appropriate amount of water needed to achieve the desired slip density, and then the clay is added. (The clay must be pre-crushed to minimize the amount of time required for adequate dispersion.) The necessary energy for dispersing the clay in the water is introduced through the dispersing tool. Since the specific energy input of conventional systems is relatively low, the dispersion process typically requires three to five hours or more, even when pre-crushed clay is used. As a result, most plants must use a large number of dispersers and a significant amount of floor space to adequately handle their dispersion operations.

Next-Generation Blunging

A next-generation intensive blunger** introduces a considerably higher amount of energy into the slip. As a result, the processing time is shorter, the machines are smaller and the space requirements are lower. Figure 4 compares the new system with conventional blungers.

In the new blunger, the entire blunging tank (also called a pan) rotates. One or several eccentrically positioned rotating dispersing tools are mounted at the top of the machine and reach into the rotating pan (see Figure 5). A combined bottom/wall scraper continuously cleans the pan bottom and cylindrical pan wall, and directs the material to be prepared toward the high-performance dispersing tools. An exit gate specifically designed for liquid media ensures an even discharge flow. (The prepared liquid can also be extracted using a pump, if necessary.)

The dispersion process is carried out in several different steps (see Figure 6). First, a chunk of raw clay (up to 20 cm in diameter, depending on the blunger type) is fed into the blunger, along with 20-50% of the water required to prepare the final slip. This produces a stiff plastic mix. High levels of energy are introduced to this plastic mix through the dispersing tool, and high shearing forces shear down the clay agglomerates. The plastic mix is continuously directed toward the dispersing tool(s) through the rotating dispersing pan and the bottom/wall scraper.

After the clay has disintegrated, the remaining water is added and mixed into the clay to produce a homogeneous slip. Any additives can be introduced in either the initial or second mixing phase.

The finished slip is discharged into an intermediate tank located below the blunger, and is then pumped through a screen (to separate the impurities) and into a storage tank while the next batch is being prepared. The intermediate tank holds approximately 1.5 times the amount of slip as the blunger.

In addition to minimizing the amount of dispersion time required, the new blunger also provides several other benefits. The rotating pan, bottom/wall scraper and the use of a plastic mix prevents material from adhering to the dispersing pan and provides a "self-cleaning" effect. The specific energy requirements are lower compared to dispersing all of the materials in a conventional blunger (see Figure 7). Additionally, the ability to control the viscosity of the slip makes it easier to screen out impurities, therefore allowing the use of less pure-and less expensive-clays. Finally, each slip will be dispersed and stored separately for later processing.

Preparing the Final Slip

To prepare the final slip, the ground and dispersed raw materials are blended in a conventional batch blunger or in the next-generation continuous intensive blunger, along with the amount of water necessary to achieve the desired finished density (see Figure 8). Additives, liquefiers and pigments can also be added during this stage. If different slip qualities or colors are required, the slip composition can be changed quickly and easily, without the extensive cleaning time and product losses often associated with changing a formulation ground in a ball mill.

Increasing Efficiency and Flexibility

Using the next-generation mill and blunger to prepare raw materials in separate steps is a flexible, highly efficient process. With these technologies, ceramic manufacturers can achieve high-quality slips with customized colors and properties while using significantly less energy than conventional ball milling.

For more information about next-generation milling and dispersing technologies, contact Maschinenfabrik Gustav EIRICH, Walldürner Str. 50, D-74736 Hardheim, Germany; (49) 062-83-510; fax (49) 062-83-51325; e-mail eirich@eirich.de ; or visit http://www.eirich.com .

*The MaxxMill‚, supplied by Maschinenfabrik Gustav Eirich GmbH & Co. KG, Hardheim, Germany
**The EIRICH Intensive Blunger, supplied by Maschinenfabrik Gustav Eirich GmbH & Co. KG

SIDEBAR: Energy Savings at a Glance

The table below shows an example of the energy savings that can be achieved with the next-generation milling and dispersing equipment. These data are based on the preparation of a porcellanato tile body with 60% hard material and 40% clay. (Remember that the required energy consumption for both clay and hard material depends on the dispersing and grinding properties of the individual materials.) The data for the calculations were taken from a material similar to that used in the ceramic industry.

In conventional wet grinding processes using a ball mill, approximately 40-60 kWh/t of energy (depending on the raw material properties) are required to produce a slip that is 2.5% > 45 µm. Using the technologies described in this article, a slip with the same properties can be achieved using just 28.6 kWh/t-a 46-70% savings in energy consumption.

These calculations are obviously assumptions, and actual results will vary depending on the materials being processed. However, it is clear that this high-performance equipment presents a significant potential for energy savings.