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Screening is an essential aspect of processing ceramic materials. An array of commercial applications for ceramics—from building materials to aerospace—is facilitated by the diversity of available ceramic products. The ceramic product materials that support these demanding and differing applications are equally diverse. Ceramic product materials range in size, shape, composition and structure.
Material screening is critical to ensure quality for the input materials and, ultimately, the final product. Often, screeners are simply specified on the principle of size and capacity. However, diverse screener options and features are also available. The necessity and use of these screener features accommodates the diversity in production and maintenance philosophy. Screener specification is optimized through a combination of size and features.
Several of the common and critical variables necessary to appropriately specify a screener include:
• Type of product (wet vs. dry)
• Volume of product (tons/hour, gal/min)
• Particle size distribution (percentage distribution of particle sizes through range of applicable measurement units)
• Cut/separation points
• Moisture content (for dry product)
• Slurry viscosity (for wet product)
• Percent of solids in slurry (for wet product)
• Special considerations (abrasive, toxic, explosive, etc.)
The purpose of the screener is also an important consideration when determining the optimal screener size and features. The process might be a simple scalping application to eliminate upper-range particles and the statistical outliers of product (extreme oversize particles or foreign particles). The screening process of the raw materials might be to ensure specific size qualities of the raw material or to eliminate the fines (small particles detrimental to the downstream process). By ensuring input product quality, the resulting process improvement is often economically justified by correction or failure cost of final product.
The screening of the raw input materials is necessary to ensure process stability during production. In addition, the reduction of raw material product variability upstream inevitably reduces product variability downstream. Improved process performance reduces costly rework, lost product and field issues.
As a general concept of total quality management systems, every dollar spent on prevention will result in a return of $10 or $100. For ceramic screening, it may be necessary to implement a raw material screening process to ensure input raw product quality.
Wet Screening During Ceramic Production
Ceramic process screening is often performed in a slurry or suspension. A round screener is commonly applied due to scalability for low- and mid-range volume slurry screening. The compact dimensions of a round screener are attractive where footprint dimensions are an operational concern. Rectangular screeners are typically applied for larger-volume processes due to increased efficiency at a larger scale.
Beyond Screener Size
Depending on process and operational philosophy, certain available features on screeners can improve screening performance, maintenance performance and efficiency. The consideration and selection of screener features helps manufacturers choose a screener that adequately meets customer requirements beyond size.
Two prominent types of seal designs are available: external and internal. The traditional external-type seal offers a cost-effective design (see Figure 1). It is suitable for most applications and can be considered dust tight.
In this design, the screen frame and seal are not in contact with the product. The external seal maximizes the screen surface area because the seals are external to the screen surface.
The internal “U” seal provides features and benefits that should be considered for special applications with specific requirements (see Figure 2). The internal screen seal is gas-tight over 100 lbs per square foot (5,000 Pascal). This type of seal offers reduced downtime with improved efficiency to exchange seal rings. It does not require additional adhesive to fix the seal.
Types of Clamping Systems
Two prevalent clamping philosophies can be classified as horizontal and vertical. Horizontal clamping uses a ring that is horizontally secured and tightened to maintain seal integrity (see Figure 3). The economical horizontal seal ring entirely encompasses the sealing area, so it provides a complete and equal distribution of applied sealing pressure.
Vertical clamping uses a sealing system in which clamps apply vertical pressure to vertically compact and compress the screener to maintain seal integrity. A vertical clamping system with QSS quick clamps does not require any tools to open the screener. Instead, the vertical clamping system is engaged (or disengaged) via a simple lever action. The vertical clamps remain attached to the machine in order to reduce the likelihood of lost or damaged rings that would otherwise have been removed during maintenance. With QSS quick clamps, the seal integrity can be easily inspected and visually monitored (see Figure 4).
Screen Mesh Construction Type
In addition to considering the mesh size, the construction of the mesh cloth is a factor in the effectiveness of the screener on the process. The mesh cloth is the critical contact point between the screener and process material where the actual separation of the particles occurs.
The construction of the cloth determines the diameter thickness of the wire in the mesh. The percentage of open area is also determined by the method with which the wires are joined to form the mesh. Three common types of mesh construction include tensile bolting cloth, mill grade and woven wire.
Screener Body Construction
It is also necessary to consider any special properties of the material being screened and the impact on both that material and the screening machine. Improperly specified material in the screener construction can introduce contamination into the process. Abrasive material may excessively wear an improperly constructed machine and unnecessarily increase maintenance cost.
Common screener body materials are mild steel, 304 stainless steel or 316L stainless steel. Seal materials can vary based on chemical composition and temperature properties of the material being screened. Typical seal materials include Perbunan® (acrylonitrile-butadiene rubber, NBR), silicon, viton and ethylene propylene diene monomer (EPDM).
It may also be necessary to consider certification against ignition for volatile environments. This may include specific features like grounding, ignition testing and special hazardous environment motors.
Based on maintenance requirements and philosophy, additional screener customization is possible. For example, another feature often requested to assist with maintenance efficiency and safety is a hinge feature for the screener lid (see Figure 5). The processing of diverse products requires the consideration of the purpose and available options to specify a screening solution, vs. simply determining the size of the machine.
To find suppliers of screening equipment and other products, visit www.ceramicindustry.com/suppliers.