Fly ash is a fine, powder-like material generated from coal-burning power plants. And despite an increase toward more sustainable energy production, coal-burning power plants are still prevalent, producing fly ash that could serve as a valuable commodity. Thus now is an important time as any to evaluate the way in which we look at this material, and consider it not a waste, but an opportunity for use in a multitude of industrial applications. Through conditioning and agglomeration technologies, fly ash can be transformed from a waste, to a more marketable product, forever giving meaning to the term “waste-to-value.”
Advantages to Fly Ash Processing: Waste to Value
Fly ash agglomeration offers a host of advantages in terms of processing solutions and product value. Depending on the unique sample, fly ash can be used in the following industrial applications:
- Cement- Fly ash that contains high amounts of calcium can be used in cement. Pigment can even be added during processing, to create a premium colored cement product.
- Berms– Fly ash can be used as a fill in berms along highways.
- Fertilizer- There is some potential for fly ash to be utilized in fertilizer products; deposits high in sulfur can be blended with other materials to create fertilizers. While using the material as a fertilizer has been discussed, it is rarely, if at all, done.
Conditioning Fly Ash for Landfill
While many sources of fly ash can lend themselves to more marketable products, not all is usable, and some samples should be landfilled. In these situations, conditioning or agglomerating fly ash prior to landfill is particularly advantageous in its own regard. Because fly ash is a fine powder, it can be very dusty and easily wind-blown, making it difficult to transport, as well as tough to manage at a landfill. Pre-conditioning or de-dusting the material makes it easier to transport and handle, and more stable for landfilling, so it does not end up blowing away into undesired areas. This stabilization may also help prevent toxic metals from leaching out of the fly ash.
Process Overview
The primary equipment used for processing, whether that be for landfill, or for a marketable product, includes pin mixers, paddle mixers, and disc pelletizers, with the equipment selection being dependent upon the desired end-product. If one is seeking fine agglomerates, less than 1 mm or 18 mesh, a pin mixer is the equipment of choice. For larger agglomerates, the paddle mixer, or pin mixer and disc pelletizer combination is the best option.
As mentioned, the end product’s intended use will often dictate which equipment is most suitable for the operation. Paddle mixers, for example, create a wide range of agglomerates, though not as uniform in shape as those produced by other agglomeration equipment. Fly ash processed for landfill would likely not require a uniform end product, and thus the paddle mixer would suffice as the equipment of choice. As a cement additive however, uniform particle size distribution may be very important, and a pin mixer/disc pelletizer combination would be better suited to achieve the desired end product characteristics.
Below is an overview of each piece of agglomeration equipment involved in fly ash processing:
Paddle Mixers
Paddle mixers are composed of two shafts spanning the length of the mixer, each affixed with pitched paddles. The shafts rotate at a constant speed, in opposition to each other. The action of the paddles moves the fly ash from the top of the trough, down each side, and then back up the middle. This creates a kneading and folding over motion, ideal for thorough mixing. Additionally, in a fly ash application, the paddle mixer is fitted with a binder spray system that feeds in binder at a constant, specified rate. This, in combination with the motion inside the paddle mixer, thoroughly mixes and de-dusts the feed with the binder, creating a homogeneous mix.
Pin Mixers
Pin mixers can be used as a stand-alone device, or in combination with a disc pelletizer. Unlike a paddle mixer, pin mixers are comprised of a single shaft, fitted with rods, or “pins.” The shaft rotates at a constant, high speed, causing an intense spinning action inside the mixer. This spinning motion not only eliminates air and reduces water volume between the fly ash particles, but it also begins to form micro-pellets. These micro-pellets can be considered a finished product, or move on to further processing on the disc pelletizer. Again, a binder spray system is added to help de-dust the fly ash and promote the formation of micro-pellets. When used in combination with a disc pelletizer, these micro-pellets serve as an ideal starting point for creating premium fly ash pellets on the disc pelletizer.
Disc Pelletizers
Disc pelletizers, also known as pan granulators, use tumble growth agglomeration, a process where the material gradually rolls and builds against itself in the presence of a surface moisture and possibly a binder, in order to form round pellets. In this application, fly ash would exit the pin mixer and be fed onto the disc pelletizer at a constant rate, along with additional binder to increase tackiness of the material. As the disc rotates, the tacky micro-pellets roll against themselves, picking up additional fines, and essentially “growing” into larger pellets. Once pellets reach the desired size, they exit the disc pelletizer. Disc pelletizers are the equipment of choice when uniform pellets are of utmost importance.
The fly ash processing systems noted above are commonly performed in FEECO’s lab. Because the material composition varies from deposit to deposit and desired end-product use differs as well, our lab technicians may test single pieces of equipment or various combinations of equipment which may be required for processing. Testing in the FEECO Feasibility Lab is always recommended when processing fly ash, to become familiar with the nuances of each specific deposit.
FEECO has worked with fly ash since our founding in 1951. From that day forward, we’ve done everything from feasibility testing in the lab, to manufacturing custom processing systems and equipment. For more information on our experience with the material, or for help with your fly ash by-product, contact us today!