Known also as rotary sieves, rotary sifters or rotary screeners, centrifugal sifters are a type of rotary sifting machine. These sifters separate and classify dry granulates and powders by particle size, though modified sifting machines are also used for bulk solids in other forms, as well as slurries and liquids. All in all, centrifugal sifters are versatile pieces of equipment that can handle all sorts of bulk materials.

Rotary Sifting Machines & Their Uses

Centrifugal sifter operation is relatively simple. Material is fed into the machine’s drum, which rotates to create centrifugal force. As the drum turns, it separates particles by size, allowing right-sized particles to pass through a screen and on to the next stage of processing. These smaller particles exit at the bottom of the sifting machine, with larger particles directed to a separate hopper from which they can be discarded or returned to the sifter for further processing.

Centrifugal sifter applications include: 

  • Seed crops: Used after grains and other seed crops are ground into a powder.
  • Recycling: Used to classify granules of plastics and other recycled materials, centrifugal sifters help guarantee greater particle uniformity.  
  • Powder coatings: Rotary sifting machines help remove lumps and contaminants from powder coatings, ensuring a smooth finish.
  • Plastics: Before molding or extrusion, centrifugal sifters are often used when processing plastics.
  • Pharmaceuticals: Before being formed into tablets, active and inactive powdered pharmaceutical ingredients are classified and separated within rotary sifting machines before being blended together.
  • Mining: In the initial phases of processing ores, centrifugal sifters separate coarse from fine material.
  • Mineral processing: These sifting machines can screen silica, limestone, gypsum, and other mineral powders, as well as gravel, sand, and other aggregates.
  • Livestock feed: Before packaging, centrifugal sifters separate oversized and broken pellets in livestock feed to ensure uniform size while also evenly introducing powdered additives throughout the feed.
  • Food processing: Centrifugal sifters are used for sifting powdered ingredients like cocoa, spices, and sugar while processing foods. They also remove debris like stones and husks from grains and cereals in the early processing stages.  
  • Flour milling: These sifters classify flour particles and remove contaminants.
  • Cosmetics: Finely powdered ingredients for makeup, skincare and other cosmetics must undergo screening in rotary sifting machines to ensure uniform particle sizes, which in turn promote smoother texture, along with consistency and quality.  
  • Chemical processing: These sifting machines remove impurities from plastic resins, pigments, detergents, catalysts, and other powdered chemicals, while also ensuring they’re properly classified and separated by particle size.

For friable materials like flour or sugar that tend to lump together, the centrifugal force produced by rotary sifting machines keeps screens from becoming blocked. This eliminates the need for mechanical mechanisms to prevent material from clogging or sticking to the screen’s apertures, which reduces throughput and operating efficiency. Centrifugal sifters work well for removing oversized or undersized particles from material during processing. These rotary sifting machines are often preferred for high-traffic areas, as their comparatively quiet operation doesn’t require workers to use noise protection.

Prater Case Studies

Prater Industries manufactures two types of centrifugal sifters, each with a variety of models. Our signature Rota-Sieve® is a rotary sifting machine that can deal with sifting larger throughputs of agglomerated, granulated and powdered bulk materials. Meanwhile, our 700 Series centrifugal sifter can handle both smaller and larger processing applications with a quick-take-apart design for easier maintenance.

These two case studies show the benefits of choosing Prater centrifugal sifters: 

Processing Cocoa with a Centrifugal Sifter

One of the largest cocoa processors in North America came to us looking to serve a new customer that had specific sifting and packaging requirements. While typically their customers accepted 50-pound bags of cocoa, this customer needed it packaged in 2000-pound bulk bags. This required that the company reconfigure the sifting system for each customer, a time-consuming and labor-intensive practice.

Further, due to the fact these larger bags weren’t tailored to the system, a significant amount of cocoa powder was lost. The bagging procedure also created issues regarding the handling and weighing of the bags. Prater engineers reconfigured the system to permanently optimize production, redesigning and installing the system to include an RS-91 centrifugal sifter with a dual-bag filling station, along with automated controls and a heavy-duty PAV rotary airlock valve.

Rotary Sifting Machine for the Powder Coating Industry

A leader in the powder coating industry approached Prater about a new product line for one of their new customers. During the development of their processes for this customer, the company realized that our Rota-Sieve®, which was an integral part of their system, allowed oversized particles into the final product rather than screening them out. This resulted in blemishes to the powder coating when applied, even in cases where the screens were classified down to 200 mesh (74 microns). While they determined neither the process nor the product’s chemistry were to blame, they couldn’t identify the exact problem. Time was a factor in resolving the issue, so they requested a Prater representative come onsite to resolve this challenge.

A representative from Prater came that same night, touring the facility and meeting with all stakeholders. After inspecting the centrifugal sifter, it became clear that certain components no longer fit tightly, which allowed oversized particles to slip between them and into the end product. Our engineering team came up with a solution for the Rota-Sieve®, modifying the sifter by designing a new part to prevent this from occurring. This not only ensured that our customers' new revenue stream continued to flow but also improved the performance of Prater’s signature rotary sifting machine.

To learn more about our centrifugal sifters or other processing equipment, contact the material handling specialists at Prater today.