Sieving is a fundamental process of separating coarse from fine materials using a surface with perforations or mesh. This technique dates back to ancient Egypt, where sieves made from woven grass and reeds were used to size grain. Sieving remains a standard method for separating materials by particle size, such as different-sized grains. Small sieves help with various cooking tasks in kitchens, while industrial sieves operate on a larger scale.

Both kitchen and industrial sieves work by letting finer materials fall through the screen's openings, leaving coarser particles behind. Industrial sieves often use a vibrating motion to help particles grind against each other until they are small enough to pass through the screen's holes. These sieves have many uses, from separating stones from sand for construction materials to filtering solid waste in water treatment processes.

Rotary Sifter or Industrial Sieve? 

Rotary sifters are screening machines sometimes considered industrial sieves, though this is a misnomer. Also referred to as rotary sieves or centrifugal sifters, instead of a vibratory force, they primarily use centrifugal force to classify and regulate materials during processing. Rotary sifters generally have elongated housings supporting a drive motor at one end that directly or indirectly powers the machine, along with a bearing for the central rotor at the other end. Below the machine’s housing, a discharge hopper directs separated materials to downstream processes.

Oversized material and contaminants are directed to a flanged outlet at the end, where the bearing is located and removed from processing. Most rotary sifters have two doors: one for quick checks of the screen without removing it and another for screen maintenance and replacement. The position of the feed inlet and primary outlet can vary by model and manufacturer, and rotary sifters can be made from different types of steel. The screens are also made from various materials, including nylon cloth, perforated plates, stainless steel mesh, and woven wire.

Industrial sieves, also known as vibratory screens, vibratory sieves, vibratory screeners, or flat deck screeners, come in different sizes and shapes. A common design uses two or more circular decks for more precise classification and screening. Vibrations, caused by an offset motor attached directly to the screener, help agitate the material for sieving. Industrial sieves typically have an inlet above the dust cover and an outlet below the screen to align material flow. These sieves are also made from various types of steel, with screens constructed from materials like nylon mesh, perforated plates, steel wire, and wire cloth.

Operational Differences of Rotary Sifters & Industrial Sieves

Unlike typical industrial sieves that use vibration to separate material, rotary sifters instead use centrifugal force, which is why they’re commonly referred to as centrifugal sifters. Screening starts at the feed inlet, where material is pushed into a cylindrical chamber through rotating paddles on a miniature auger. The screens through which material passes are fitted and clamped into a rigid frame closely fitted around the spinning paddles. As material hits these rotating paddles, centrifugal force accelerates finer materials through the screen apertures while coarser material is forced to the end of the chamber, where it discharges into a hopper. As oversized and near-sized particles strike the screen’s surface, they can plug the screen’s apertures, so brushes or other mechanisms are used to keep material from blinding the screen.

In contrast, industrial sieves rely specifically on vibration to agitate and separate materials by particle size. This vibration is provided by an offset motor that’s mounted outside the screening area. Fed from above, material makes contact with a sieving screen, where the vibrations from the motor cause it to be spread across the screen’s surface to instigate particle separation by size. Undersized particles fall through the screen’s apertures, while oversized particles are swept off the sieving screen and into reject ports. Sometimes, a deck underneath the screen will also be fitted with rubber balls or nylon rings designed to impinge on the screen, keeping the sieve clear of any near-size particles.

Benefits of Rotary Sifters & Industrial Sieves

Rotary sifters and industrial sieves both play roles in material handling, each with its own advantages. Rotary sifters effectively manage various friable materials, like flour or sugar clumps. The centrifugal force they generate prevents screen blockages by keeping near-sized particles clear, eliminating the need for a mechanical anti-blinding mechanism. They excel at removing small amounts of off-spec or oversized material that can reduce product quality. Additionally, rotary sifters operate quietly, so nearby workers don't need noise protection.

Industrial sieves handle large volumes of powders and can separate the product into multiple-size fractions. Their inline inlets and outlets, along with compact design, make them easy to integrate into existing processing systems, especially those with limited height. With fewer parts than rotary sifters, industrial sieves are simpler to clean and maintain, and their screens are more accessible to replace. Since the vibrating mechanism is outside the housing, these machines are less prone to damage from foreign objects.

Applications for Rotary Sifters & Industrial Sieves

Rotary sifters and industrial sieves are commonly used in similar industries and applications. Rotary sifters handle bulk materials in agriculture, chemicals, and food processing sectors. They efficiently perform bulk screening or coarse separation, quickly processing large quantities of grains, granules, or powders. In contrast, industrial sieves offer finer separation for industries such as cosmetics, food processing, and pharmaceuticals. Thanks to their precision in ensuring uniform particle sizes, they are particularly useful for batch screening and quality control.

Industries in which both rotary sifters and industrial sieves can be used include:  

  • Agriculture: Both types of machinery can be used to clean granulated fertilizers of dust or larger clumps and remove impurities like chaff, dust, stones, and other contaminants from seeds.
  • Animal feed: Both types of screeners are used to ensure feed pellets are uniform in size. Industrial sieves prevent clumping and other uneven distribution, while rotary sifters tend to be used more to separate out oversized or broken pellets prior to packaging. Both machines can also be used to ensure powdered additives are evenly distributed within feed.
  • Chemical processing: Both types of screening machinery remove contaminants from catalysts, detergents, pigments and other powdered chemicals to ensure properly sized particles.
  • Construction: Screening lime, silica, and other cement ingredients, rotary sifters, and industrial sieves ensure materials used in construction are consistently uniform, including the coarse sand and gravel often used for concrete; industrial sieves are also used for ceramic roofing and floor tiles by making sure the particle size of ceramic ingredients is uniform, which enhances quality and strength.
  • Cosmetics: Both screening machines guarantee smoother textures for finely powdered ingredients used for skincare, makeup, and other cosmetics by separating to make particle sizes more uniform; rotary sifters also sift pigments to ensure the quality and consistency of various cosmetic formulas.
  • Food processing: Rotary sifters and industrial sieves are used to sift flour, spices, sugar, and other powdered ingredients to remove foreign matter and lumps and ensure uniform flavors and textures. However, rotary sifters are also used to remove husks, stones, and other debris from cereals and grains in the earlier stages of processing.
  • Mineral processing: Rotary sifters and industrial sieves are used to screen gypsum, limestone, silica, and other powdered minerals; both machines also aid in classifying aggregates like sand and gravel.  
  • Mining: While rotary sifters tend to be used more in initial stages of ore processing to separate fine from coarse material, industrial sieves separate mined materials like coal, limestone and ores containing metal elements by particle size.
  • Pharmaceutical: Both rotary sifters and industrial sieves help ensure active and inactive pharmaceutical ingredients in powder form meet requirements for particle size so that they can be blended uniformly and molded into tablets; industrial sieves are also used for removing solid particles from liquid medicinal suspensions during production.
  • Plastics: Rotary sifters are used more often for processing plastic pellets before extrusion or molding processes, while industrial sieves tend to be used more for plastic resins in powdered or granular form.
  • Powder coating: Both screening machines keep powder coatings free of contaminants and lumps, ensuring smooth finishes on powder coated surfaces.
  • Recycling: Rotary sifters and industrial sieves are used to classify and ensure the granule size of recycled plastics is uniform, including recycled tires that are then used for playground asphalt, playground surfaces, and new tires; for applications like making construction materials or new glass containers, industrial sieves are also sometimes used for recycling and separating crushed glass according to particle size.

Rotary sifters and industrial sieves work in both pneumatic conveying and gravity-fed systems. They can accommodate various screens, allowing different particle sizes to pass through. Rotary sifters, with their spinning paddles, can break apart very soft clumps, or prills. They are effective for friable materials with low moisture and fat content, making them useful for screening pet food before extrusion.

Industrial sieves are common in the food and beverage industry to filter liquids like cream, milk, and other dairy products, removing impurities. They also eliminate solids such as pulp and seeds in juice-based drinks. In the paint industry, sieves ensure even pigment dispersion for smoother paint coats. These machines filter creams and lotions for cosmetics, removing solid contaminants to make them smoother and more uniform.

Prater’s Innovative Design That Supplants Industrial Sieves 

Prater Industries’ Rota-Sieve® is a type of rotary sifter with a particularly original design. While it behaves like most other rotary sifters, Prater’s design doesn’t require an anti-blinding device. Instead, the Rota-Sieve® creates a natural vibration at the screen that augments material flow when oversized and near-sized particles hit it. Our rotary sifter is designed to handle blends, finished products that require sifting or scalping, or raw materials in bulk for sieving operations. Its uncomplicated yet innovative design makes both standard inspections and maintenance easier, making it ideal for a wide variety of applications.

Prater’s Rota-Sieve® is ideal for materials that include: 

  • Agglomerated materials
  • Bulk powders that flow freely
  • Granulated matter

Whether looking for an industrial sieve or rotary sifter, the Rota-Sieve® can separate material down to 200 mesh (74 microns) without requiring anti-blinding devices. To learn more about our rotary sifter or other equipment, contact the material handling experts at Prater.