Q&A: State of the Art in Screening & Separation of Powders & Bulk Solids

Recently, Powder & Bulk Solids presented “State of the Art in Screening & Separation of Powders & Bulk Solids” as part of its DryPro webinar series. The industrial revolution, and more recently computers and controls, have rapidly accelerated the development of screening/separation processes. Powerful and inexpensive new tools and resources are now available to inventors.

Dr. Timothy J. Bowser, P.E., Food Process Engineer at Oklahoma State University Robert M. Kerr Food & Agricultural Products Center, summarized the top new technological inventions and applications in the area of screening and separation, focusing on the food and ag industry. The webinar began with an overview of screening/separation theory and modern technologies being used to improve them.

After the presentation portion of the webinar, a live Q&A took place. Here are the questions and answers:

Q. How do you see the future of new, developed materials vs existing working machines? Do they have to be upgraded or replaced?

Some equipment, especially large-scale equipment, can be upgraded and be very effective. But sometimes economically it’s just better to buy something new. It depends on how close it is to the end of its mechanical lifespan. And of course, the advances we’ve talked about: How much can you incorporate some of these advancements into the existing equipment or do you have to totally redesign it?

Q. What is the most effective separation technology for 3D printing polymer powder?

The most effective technology just depends on what you’re trying to separate. If it’s magnetic particles you’ve seen some really good options in the slides I’ve shown, take a look back at the Eriez slides. There are some great options. You really need to speak with some of the manufacturers of the equipment. If it’s magnetic you’re talking about, some of the companies we’ve talked about here can make some excellent recommendations for what you need. If you need to separate based on particle size, we have also seen some excellent options in this presentation. This webinar didn’t cover other separation processes, like density or color classification, that you may need to look into.

Q. I work for an organic fertilizer production development company, and we are looking to separate almond shells from husks. We currently sift manually and are looking for more effective and automated options, do you have any suggestions for this application? We also grind and feed the shells into our process. Are there technologies that integrate these processes?

There are definitely some technologies that can do several steps at the same time. I think you need to expand your operations beyond screening, I think you’re on the right track. And I’d love to talk to you about this offline if that’s OK. There’s lots of options here and it would take quite a while to get through those. Air separation is one technique that comes to mind. Many companies combine air separation with screening. Uniform feed rates are important for the success of many processes. Many of the screening equipment suppliers incorporate size reduction and feeders into their systems. You can certainly use a feeding-size reduction-screening process to drop shells directly into your system.

Q. What's the benefit of an inclined sieve?

I think one of the main benefits is that it helps to move the product through the sieve by gravity. So, you have a gravity component that moves the product along the direction of the incline. There are disadvantages to everything too. One of the biggest issues is, with any sieve, how the particle is presented to the aperture. And the size of the aperture, blinding, etc. and all that factors into the angle of the incline. And that’s one of the great things about using these numerical models that they’re developing recently to simulate these sieves. You can plug in your sieve design and run it through every conceivable angle of inclination to see how it does. Many manufacturers are doing it these days and are heading in that direction. And it can result in optimization of the sieve that they sell to you and that they design for you.

Q. What advancements have been made in highly abrasive products while screening?

That’s the materials, so coatings and construction materials as they advance are becoming more durable and have longer life. And one of the biggest issues with these more durable, longer-life materials is how do you machine them? How do you manufacture something that is so hard? You can’t drill it, bend it, or cut it. It’s difficult to manage. These new advanced materials make things so much better. You have some metals that are extremely durable, and you can use polymers to help capture them and hold them in place. If you’re able to put those polymers in a space and area where they aren’t subject to wear and they have been combined with the metal that is more wear-tolerant, you will have more durable screening equipment.  There are plenty of other options for wear materials such as composite materials and nano particles.

Q. What do we need to consider for selecting magnetic separators when working with poorly flowing powders?

With poorly flowing powders, we touched on a few items. Some are how they flow across matrices of magnetic bars like we saw in one of the examples. I think one of the really exciting options is the vibrating filter bed like I showed on the Eriez example. There are other options like pneumatic conveying — where the product is conveyed through the separation device using air so it flows better. You have other options like fluidized beds low-friction coatings, anti-static hardware, and anti-blinding devices.

Q. Have you noticed that the longer RO-TAP sieving is run, the smaller the particle size becomes?

I think that’s true in some cases because you have some degradation of the particles themselves so that can happen. With a tough particle, it’s not going to happen as much. Especially with some of the more durable minerals and agricultural materials. For friable particles, the process needs to be adjusted to reduce friction and residence time. One way to do this is to use oversized equipment to reduce the number of collisions between particles.

Q. What is the best way to avoid particle clumping?

There’s a lot of options to reduce agglomeration and one of the ways I mentioned was in the SWECO example that used ultrasonics to break up particles at the sieve. There are options for mixing and blending prior to that as well, different size reduction techniques, conveying, even electrostatic treatments to keep products from clumping. So, there is a wide variety of options and things to look at that must be tailored to your product.

Q. You talked about the reverse sieving method. Does anyone make commercial equipment to implement this method, and would it be economical for my application?

Yes, there are a lot of companies that make it. I think they just need to use their standard equipment and reconfigure it. Is it economical for you? If you have a product that is valuable, not a low-cost commodity, this could be an excellent option. I would definitely take a look at it, and if you don’t have the ability to look at it yourself, I’d find a supplier that has a laboratory where you can run the product and they can set up and test a reverse sieve method for you.

Q. Which separators/sieves designs have minimal dust accumulation and less chances of explosions?

I really prefer the designs that are well enclosed, that have minimal gaskets, and minimal openings. It’s how they’re designed with simple, clean lines. The units can combine aspiration to remove dust particles. So much depends on the particles and how they process through the system. The good manufacturers have some options that are available but the thing to look for is very ergonomic, sanitary, and clean design. These go together. Also, look for minimal retention and good air handling.

View the State of the Art in Screening & Separation webinar on demand here.