A three-stage blending process can yield maximum efficiency and prolong wear life.

By Florian Festge

They say good things come in threes. In the case of screening for aggregate operations, profit comes in three distinct phases: layered, basic, and sharp screening. But how good they are — whether they maximize efficiency and improve the potential for success — depends on having the right screen media on the deck for each of the three phases.

A three-phased approach to screening, with the proper media used during each phase, can improve operational efficiency.

A relatively new method for achieving that goal is a blended screen media approach that incorporates the right screen media for each phase and the right mix of media for the process overall. The goal in implementing a blended solution is to find the optimal combination of open area and durability. Hit the sweet spot, and an operator can maximize product quality and profits by increasing efficiency of classification while minimizing downtime and maintenance costs.

Three distinct phases and outcomes

During the first phase of screening, the layered phase, a deep bed containing coarse and fine particles hits the screen at the feed end of the deck. In the basic phase, the particles begin to stratify as fine material settles at the bottom and larger material climbs to the top of the bed. The sharp phase occurs toward the discharge end of the deck. It is during this final phase that the near-size particles move into direct contact with the screen media and have the last opportunity to fall through the openings.

There are typically two scenarios aggregate operations want to avoid when it comes to screening.

The first situation is the result of a deck that completes screening too early. In this scenario, particles travel only about a third of the way down the deck until all the undersize material has passed through the openings. While the main mission — classifying the material — is still achieved, the entire deck isn’t being used to its full capability. The results are diminished profits from premature screen media wear.

Consider this example: An operation produces 400 tons per hour and charges $10 per ton. Due to high abrasion, the media with high open area wears out quickly and needs to be replaced after only two weeks of operation. Replacing the top deck takes two people roughly four hours. At about $4,000 an hour of product not being processed and sold, that screen change-out costs $16,000 in lost revenue alone.

The second scenario involves screening that does not finish completely. In this case, undersized particles travel over the discharge end and contaminate the material. This can double a company’s production cost if the material needs to be re-screened, not to mention the lost efficiency and time. And if the material ends up too contaminated to be sold, profitability is lost altogether. In the worst-case scenario, a company may not recognize it is producing contaminated product, which can lead to costly warranty claims and negative image.

Consider this example: The same operation produces 400 tons per hour, and it costs the company $6 per ton to produce the material. Due to insufficient open area at the discharge end of a vibrating screen, the media does not allow the near-sized material to pass. As a result, the final product is contaminated and must be reworked. This means that the company needs to rework its material for a total cost of $12 per ton. With a sales price of $10 per ton, the company is losing $800 per hour until all of the material is re-processed.

A possible third scenario, optimal screening, is the ultimate goal of every operation. The bulk of undersized material passes through the openings about two-thirds of the way down the screen deck, and the last third of the deck allows all the near-sized particles to find an opening. The final product meets specification and can be sold for top dollar. However, while a company is often satisfied with achieving optimal screening, it seldom recognizes the potential to increase profits even further.

Layered: Typically appears at the feed end and contains a mix of coarse and fine particles, with a deep bed depth. In this phase, media should be impact-resistant and have a high wear life.
Basic: Occurs in the middle of the deck where the particles are stratifying and near-size and oversize particles are at the top of the bed. In this phase, the media should have a balance of open area and wear life.
Sharp: Taking place at the discharge end, near-size and oversize particles are in direct contact with the media. This phase is critical, and screen media should have maximum open area.

Consider this example: The same operation produces 400 tons per hour and charges $10 per ton. Its screening is considered optimal, and it is producing high-quality product. Due to greater impact at the feed end of a vibrating screen, the screen media at that end wear out sooner than the remaining panels and need to be replaced. Replacing the first section takes two people roughly one hour. At about $4,000 an hour of product not being processed and sold, that screen change-out costs $4,000 in lost revenue.

When screening is finished early, it is completed within the first one third of the deck, and undersize particles have passed the media openings. The customer loses out on the maximum potential of the screen media.

In all three scenarios, the key to achieving the most profitable screen surface is finding the right balance between wear life and open area — the amount of open space on a screening surface. Increasing open area normally leads to a decrease in wear life, while reducing open area usually increases the wear life of the screen media.