Screen Separations Decoded

Many users of screening equipment do not fully understand all of the nuances that fit together in order for their powder or slurry to be separated at a specific size. A majority of screen users do not fully know the cost for their organization by inefficient screening. You do not have to be a screening expert, but your success will increase by looking at several key factors in your effort to separate materials based on size.

Blinding occurs when materials get stuck in the screen openings and block that hole from allowing undersized particles to pass. Preventing blinding is the #1 priority in developing a screening solution.

One of the key elements is the screen surface energy. Some technologies use a finesse approach to screen separations like a gyratory motion. This type of technology is available in 2- or 3-dimensional movement and, depending on the configuration, it can incorporate unique anti-blinding devices to keep the screen openings clear. The most advanced cleaning systems on gyratory motion machines are air knife cleaning which blows a curtain of air from beneath the screen to clean it while processing powders.

The other main type of energy spectrum used is vibrational energy. The traditional vibratory screening machines utilize ball cleaning and ultrasonic energy put into the screen cloth to keep them clear. The high-energy vibratory machines use physics and a non-tensioned working screen cloth to prevent blinding.

Nature dictates whether your material performs best by coaxing it through the screen (gyratory) or pounding it through with energy (vibratory). We rarely see a material perform equally well on both types of energy spectrum.

As the need for finer and more refined powders have become commonplace, it has placed a burden on the separation requirements both on raw materials and finished products. Every successful solution has a combination of technology (type of screener) and technique that provide the success--or failure to achieve success. Developing this solution is a scientific approach that includes extensive testing to verify performance.

Consistency is important. If your feed material characteristics, metering of the feed onto the screen, screen cloth opening, environment, and your unit operating without blinding are all consistent, then you will get consistent output of the fractions. This consistency will yield repeatable results. This is the primary reason why testing a small sample volume and then making a technology decision based on that small test is risky. As the owner of Elcan Industries told me years ago when I was a prospective customer, “Everything looks good on a five-gallon bucket test.” And I had just made the mistake of buying something that worked in the lab but not in our plant. Sometimes, higher volume testing isn’t feasible, but we encourage all prospective customers to test as large a quantity as possible.

Another mistake to avoid is getting caught up in the price of the screening technology or the “we have always made this product this way” perspective. Capable testing will provide information that allows you to make an informed business decision on whether newer technologies provide valuable benefits. Many systems are geared toward low capitalized cost upfront but high operating costs for parts and operational labor. The bigger factor to consider is cost of ownership when calculating efficiency, yield, and operational characteristics. There are many applications where the lower priced/older technology is the best choice for that application. However, if you are working with a product that has value, the more advanced screening technologies that may be priced much higher are often the lowest cost systems when you factor in the benefits. The benefits of better yield and efficiency often extend to other areas of your process. For example, better efficiency can lead to less recycle, which then increases forming volume capability by running less recycle. This benefit optimizes the forming system process without investing directly in that area. Over the life of the equipment, this cost differential will make the initial price of those systems insignificant.

Screening is a science-based approach that when an optimized solution is developed, it removes the mystery of the outcome and increases productivity.

Greg Riter is director, sales & engineering, Elcan Industries (Tuckahoe, NY). For more information, visit elcanindustries.com.