Pneumatic systems and handling cement with SCMs
Editor’s note: This article ran in its entirety in the Feb. 2017 issue of World Cement.
By Rebecca Long Pyper for Dome Technology
The go-to option for moving cement is a pneumatic system—and for good reasons. Fluidized floors boast complete reclaim of stored material, low system maintenance and elimination of restrictions that would inhibit flow.
“Pneumatic systems are pretty much bulletproof,” said engineer for Dome Technology Adam Aagard. “Vendors will do a lot of the basic conveyance design to ensure that the right pump, compressor and blowers are selected,” and will also help cement companies identify the appropriate size of equipment depending on reclaim needs.
But according to David Bergenstock, FLSmidth sales manager of pneumatic transport systems and products, another important considerations is the type of cement being handled. Cement producers often blend Type I and Type II Portland Cement with supplemental cementitious materials (SCMs) like fly ash, ground blast furnace slag and pozzolans to increase long-term strength or to improve other mechanical or chemical properties. These materials may also improve conveyability and lubricity, while presenting an environmental benefit by consuming “waste” materials from other processes.
But once SCMs are added, “the product becomes a new product; it becomes a blended cement,” Bergenstock said. “No one should assume that just because a material is cementitious, it will act exactly as a cement in a pneumatic conveyance system.”
Some companies with an existing conveying system designed for Portland cement may presume that if supplemental material is added, the product will convey similarly. But instead, conveying may occur at a different capacity—even with adding only a small percentage of SCMs. “It doesn’t take very much for this to change the conveyability of the product,” Bergenstock said.
To ensure the expected results, cement companies should seek out labs that can test the blended product’s particle-size distribution and chemical composition to ensure the reclaim and conveying system will perform as required. For instance, a product with 10 percent of supplemental material will likely convey differently than a product with 30 percent.
“The message is that (blended cement) can always be conveyed, but the system has to be designed specific to the product being handled,” Bergenstock said. FLSmidth operates a lab that offers blended-cement analysis, in addition to general fluidizing and conveying testing.
Regardless of the material being handled, certain qualities are consistent in the design of pneumatic systems. The centerpiece of such a system is the fluidized floor in all its varieties, and “if you don’t want to deal with moving parts, the fluidized floor is your system,” Aagard said.
There is no most-popular type of fluidized floor, but different types operate differently. The simplest, an inverted cone or pyramid that feeds product to the center where it drops to a reclaim tunnel under the floor, is also the least expensive. But with an inverted cone, a reclaim tunnel is necessary. The tunnel will require waterproofing or added dome height and fill to get the tunnel above grade—costs that should be factored in when considering budget.
A dihedral floor will mobilize product to one side of the floor, and a double-dihedral floor feeds cement to two sides. An obvious advantage with these models is the need for a reclaim tunnel is eliminated as product is fed to the side, not the center, “It’s nice to discharge to the side with all the equipment outside,” Aagard said, freeing up more room inside for storing product and reducing safety concerns inherent with reclaim-tunnel maintenance.
One perk to the double-dihedral floor is the ability to run an access tunnel under the peak in the floor. Aeration pipe can be run through this tunnel, drastically reducing penetrations to the exterior of the dome wall where pipe would otherwise run around the perimeter.
Companies with existing but dated fluidized floors might opt to renovate their existing floor, which is possible and usually costs less than constructing a new storage facility, Aagard said.
To read the full text, which includes information about pneumatic conveyance and its energy consumption, visit the World Cement website.