Ceramic Industry

Shake, Rattle and Roll: Selecting Vibrating Screens for Ceramic Slurry Screening

April 1, 2001
Table 1. Vibrating screen characteristics.
Controlling the particle size of ceramic slurries results in proper bonding and a smooth surface on the finished part. While a variety of methods can be used to achieve an acceptable particle size range, vibrating screens are typically the lowest cost method of continuous particle size separation where an “absolute” cut point is required. Key advantages of vibrating screens are summarized in Table 1.

Figure 1. Wet capacity (gpm) vs. micron separation for a round vibrating screen.

Circular Vibrating Screens

The most popular configuration of vibrating screens for fine screening in the ceramics industry is a circular screen. Circular vibrating screens are generally available from 18 to 72 in. in diameter and have the capability of producing separations from 2 to 625 mesh (11,000 to 25 microns). Typical meshes for ceramic applications are in the 80- to 100-mesh range (180 to 156 microns).

Once the mesh opening (micron rating) has been chosen to meet the particle size objectives, flow capacity is the key machine sizing issue. Figure 1 shows a general guide for wet screening flow capacities in round screens.

Other parameters affecting wet screening capacity include:

  • Slurry viscosity (high viscosity reduces capacity)
  • Percent solids loading in slurry (high solids reduces capacity)
  • Amount of near-sized particles (significant near-size reduces capacity and causes screen blinding)
Note that the flow capacities per square foot of screen can be significantly reduced with high solids ceramic slurries (>35% by weight solids).

Round vibrating screens are easy to install. Because they transmit minimal vibration, they are typically bolted directly to the floor or manufacturing structure. Units can be configured to fit into the tightest floor space requirements, and low profile designs address installations where headroom is an issue.

Optimal screen performance is achieved by the fully adjustable screening action. Weight plates installed on the upper and lower ends of the motion-generator shaft impart vibration around the center of mass. Rotation of the top weight creates vibration in the horizontal plane, which causes material to move across the screen to the periphery. The lower weights act to tilt the machine, causing vibration in the vertical and tangential planes. Users can set the speed and spiral material pattern across the screen to maximize throughput and screening efficiency.

Figure 2. Wet capacity (gpm) vs. micron separation for a rectangular screen (4 x 9-ft screen deck).

Rectangular Vibrating Screens

When extremely large volumes of ceramic slip require processing, a rectangular vibrating screen may be used. Rectangular screens provide the most efficient separation and highest capacities for ceramic manufacturers. Up to five 48-in. round screens can be replaced by a single rectangular unit. And unlike old rectangular screen technology, the newest units provide positive screen sealing systems and pre-tensioned screens. These new technologies yield virtually no grit bypass, thus maintaining the highest slip quality. The capacity of rectangular screens is shown in Figure 2.

Two different types of rectangular machines—elliptical motion (EM) and universal motion (UM)—are commonly used for ceramic slip processing. The elliptical motion machine employs an engineered, unidirectional motion along the full length of the screen deck. Combining the adjustable ellipse profile with strong conveying action facilitates handling of large volumes with enhanced screen life.

With the universal motion design, users can precisely dial in the ideal motion for each application. Round, elliptical or linear motion can be customized with field adjustments to optimize the process. This capability is especially beneficial for changing processes.

Both EM and UM machines allow for field changes of the angle of motion, frequency, amplitude and deck angle to maximize throughput and screening efficiency. As with vibrating round screens, installation requires simply bolting to existing flooring. Due to an isolated mount suspension system, transmitted vibration is virtually eliminated.

The Importance of Screen Design

For any vibrating screener, screen design will be a key factor in unit performance and maintenance. Screen cloth that is highly pre-tensioned across a frame and securely bonded provides the best screening surface with the longest life. Cleaning kits incorporated into the screen design provide dynamic anti-blinding action when needed. Additionally, designs that enable screen changes without tools minimize maintenance costs and downtime.

Laboratory testing on a small separator is the best means to confirm capacity and efficiency due to the complexities involved in each process. Typically a 30-gallon drum of product is shipped to the screen manufacturer’s lab for testing. After a four to eight hour test, capacity on a full-scale machine can be predicted within 15%.

Vibrating screens play an important role in ceramic manufacturing operations. Careful analysis of all factors before ordering will result in smooth field startups and optimal production performance.

For More Information

For more information about round or rectangular screens, contact Sweco at P.O. Box 1509, Florence, KY 41022-1509; (800) 80SWECO or (859) 283-8400; e-mail info@sweco.com; or visit http://www.sweco.com.

Vibrating screens (supplied by Sweco) at Syracuse China help the company maintain product quality.

Avoiding Product Contamination

Ceramic manufacturers like Syracuse China appreciate the quality assurance that vibrating screens provide in their process. “Throughout our manufacturing process, we wage a continuous battle against contamination in our product,” said Joe Benoit, senior ceramic engineer at Syracuse China. “These screens allow us to remove non-desirable material from our clay bodies that could cause a defect in the final part.

“We normally link in-line electro-magnets after the screening step to enhance the removal of the very fine tramp metal that may pass through the screen. The screens and electro magnets work in tandem to keep our losses to less than 1%. Defects would easily be 10-15% higher without this process.”