Flexible screw conveyor moves cat litter pellets gently
Screw self-centers within tube, proving clearance and limiting product damage
Cat litter is usually made from sodium bentonite clay, a material that is relatively inexpensive and highly absorbent, but obtained by environmentally controversial strip mining.
A new sodium-bentonite-free cat litter, produced by PURR-fect Solutions, LLC in its Salt Lake City plant, is made from fly ash, a byproduct of coal-fired power plants that is normally dumped into landfills, which is itself another environmental concern.
PURR-fect Solutions is a spinoff from Ceramatec Inc., a research-and-development company that developed the process, in which fly ash — a fine powder consisting mainly of alumina and silica oxides — is formed into pellets by mixing it with an aqueous solution.
The fragile wet pellets are conveyed by a Flexicon flexible screw conveyor to a circular fluid bed dryer, which reduces their moisture content from about 5 wt% to 0.3 wt%. The pellets are then mixed with clumping agents and odor-control ingredients to derive the final product.
The litter’s absorption properties are similar to those of bentonite and it is cost-competitive with other products on the market, says Chett Boxley, general manager of PFS and a former Ceramatec researcher.
Conveying the wet pellets to the dryer is a critical part of the process, he says. “The pellets are somewhat fragile, and when you transport them it’s very important that you preserve the morphology and size distribution of the particles.”
Geometry prevents degradation
PURR-fect Solutions found a solution with the flexible screw conveyor. It consists of a rotating, flexible steel screw in a 10 ft. long by 4 in. OD tube. The screw is rotated by a 2 hp (1.5 kW) electric motor with gear reducer located at the top end of the conveyor above the discharge point, eliminating material contact with seals.
A key feature of the design is that the screw automatically self-centers within the tube, providing clearance between the screw and the tube wall. This provides sufficient space for particles to flow without damage.
Boxley was introduced to flexible screw conveyors by Clark Draney of Sandy, Utah-based Process Control Equipment, a manufacturers’ representative who had worked with Ceramatec. Boxley decided against bucket elevators due to their higher cost and the possibility of fugitive dust.
The cat litter manufacturing process begins with pneumatic conveying of fly ash from a storage silo to a pan pelletizer, where it is mixed with an aqueous solution that contains an activator to promote pellet formation. The resultant pellets contain about 5 wt% moisture, as noted earlier, and range in size from around 100 to 6 mesh (140 to 3,350 µm).
The newly formed pellets drop into a small hopper, pass through a transition adapter and into the inlet of the flexible screw conveyor, which conveys the pellets at a 45 degree incline to the fluid bed dryer.
Screw design is critical
As it happened, PURR-fect Solutions learned by experience about the importance of the appropriate screw geometry. “At first we picked a less-expensive spiral that we thought would work,” says Boxley, “but the pellets rubbed against the wall at the entrance to the conveyor. This smeared the wall and caused the screw to bind.”
Boxley acknowledges that “it was our mistake. When we told Flexicon about it they helped us to do some troubleshooting and got us the right screw.”
PFS has been running the conveyor since the startup of its pilot plant in November 2012 and has had no trouble with it, says Boxley. “All we do is give the motor an occasional squirt of oil,” he says.
The tube interior does not require any cleaning, he adds. It can be evacuated by reversing the screw and removing the pellets from the bottom of the conveyor, but PFS doesn’t have to do that. “At the end of a production run we just throw in a bucket of dry pellets to get the last of the pellets out of the tube,” he says.
At present the start-up company is producing about 650,000 lb/yr of cat litter, which is sold to regional pet stories. The pilot plant is operated for only a few hours per day and production can be easily tripled by going to an eight hour shift.
PFS runs the motor at only about 90 rpm, but it can operate at twice that speed without harming the pellets, says Boxley, noting that the speed is dictated by the capacity of downstream equipment.
Flexicon Corp. designs and manufacturers bulk handling equipment and custom-engineered and integrated plant-wide systems that transport, discharge, fill, weigh, blend, deliver or feed a broad range of powder and bulk solid materials.