Since cement fluidizes when full of air, a fully fluidized floor has become a common means of reclaim for bulk-storage facilities. But a new hybrid system of airslides and a reclaim screw is an innovative—and less expensive—option for processing soft powders safely. Dome Technology and Laidig Systems joined forces to develop this Fluidized Screw technology, and its benefits are now being realized at St. Marys Cement domes in Charlevoix, Michigan, and Chicago, Illinois.
The alternatives and their limitations
Dealing with cement can be tricky and dangerous, especially if systems aren’t in place to reduce entry into the storage facility. “The biggest danger associated with other methods of cement reclaim is that sometimes personnel are required to enter the storage dome in the event that a hard-pack area needs to be broken up. In such cases personnel are exposed to the risk of engulfment in the avalanche from collapsing piles,” said Wyn Laidig, president of storage and reclaim-system provider Laidig Systems.
To combat safety concerns, an airslide system has become typical for cement reclaim—it’s a “really common way to handle fine-grained products like cement,” said engineer for Dome Technology Adam Aagard. A fully fluidized floor comprised of troughs side by side and with an acceptable slope throughout the floor is especially common and provides nearly 100 percent cleanout.
But it’s especially expensive too. That’s why Dome Technology and Laidig developed their hybrid system as an innovative—and less expensive—option for processing soft powders.
The Fluidized Screw—safety plus efficiency
At both Charlevoix and Chicago, Laidig’s Fluidized Screw combines a rugged mechanical screw reclaimer on the floor of the storage dome with an efficient air-gravity system embedded also in the same floor.
The floor consists of an aerated center hub and ten airslides embedded into the dome’s sloped floor. The air-distribution system in the dome floor causes the cement to slide; this airflow is made possible by troughs two to three feet wide with special fabric covers that allow air permeation from below and into product.
The airslides are arranged like spokes in a wheel to reclaim most of the product. When the system starts up, various air gravity conveyor spokes are turned on in sequence. In the process pie-shaped piles of material are left behind.
Secondary reclamation takes place as the mechanical screw, located in a “home position” over one of the radial spokes, breaks down remaining piles and hard pack, mobilizing the product so it can also be reclaimed. “At every spoke you’ll be able to draw down cement, but between the spokes there will be peaks (of cement),” Aagard said. “From that point, the screw comes in and sweeps out remaining product.”
A PLC control system activates air-gravity conveyor zones to coincide with the location of the mechanical screw; at shutdown the screw automatically returns to the nearest home position over one of the radial spokes.
The hybrid system provides a major advantage to companies with prolonged storage where hard pack is likely. But a benefit for all customers is that “Laidig’s fully automated controls mean no personnel entry and ‘push-button’ reclamation with only minimal personnel supervision,” Laidig said.
This reclaim model will allow plant managers at Charlevoix to achieve a throughput of 100 metric tons per hour, and at Chicago—where a bigger system was requested for quicker loading—300 metric tons reclaimed in 60 minutes. Ninety-five percent of this reclaim is made possible with airslide technology, and the remainder is delivered via the screw sweep.
“The fluidized screw … will provide a more mechanically reliable solution, as compared to other mechanical reclaim systems. It will also provide for more complete reclaim of stored product compared to other mechanical systems or aerated floors,” said Charlevoix plant manager Randy Pryor.
Editor’s note: This is an excerpt from a Dome Technology feature published in the July 2016 issue of World Cement. For the full text, click here or here.