At Lone Star Industries Cape Girardeau, Missouri cement plant, reliable coal supply is crucial for keeping the entire clinker production process on-line. If coal supply is interrupted, plant personnel have only 15 minutes to restore feed before the pre-calciner and rotary kiln shut down. Unfortunately, Lone Star Industries was experiencing severe problems with their coal mill feeding system.

Coal is transferred from outdoor stockpiles to a pulverizing roller mill via a belt conveyor. The coal mill operates under a vacuum of 10 to 12 inches of water column, thus an air seal must be maintained at the mill’s feed inlet. For nearly twenty years, a triple-flap gate was used at the feed inlet to provide an air seal. This gate was chosen over a conventional rotary valve airlock because of potential excessive wear, leakage, and buildup. When air leaks into the mill, the pulverized coal is not conveyed away efficiently. Consequently, coal will accumulate in the mill and start clogging the unit and overflow onto the ground.

Since the triple-flap gates were installed, problems were experienced (e.g., coal buildup, gate misalignment, and leakage into the mill), resulting in equipment downtime, increased labor cost, and unscheduled plant shutdowns. The constant problems with the triple-flap gate forced Lone Star to pursue a new feed method to the roller mill. The new feeder was required to be robust, handle abrasive coal, and must run continuously – 24 hours a day, 365 days a year.

Jenike & Johanson recommended a Solids Pump® pressure-sealing unit to feed the coal with minimal gas leakage between the conveyor belt and roller mill which are at different gas pressures. The Solids Pump® can operate with almost any solid that can be conveyed in a screw conveyor, provided that its fines content and/or compressibility are high enough to allow a seal to be formed. It consists of three primary functional components: the inlet/conveying section; the sealing section; and the discharge section. The functional requirements of each section are determined by the flow properties of the bulk solid and the flow rate and gas sealing requirements of the system in which the screw is to be installed.

The inlet section can accept material from an up-stream feeder or conveyor, or in some applications, it can be directly connected to a bin and serve as a feeder. The sealing section is designed so that the bulk solid being conveyed through the screw forms a plug of sufficient length and density to provide the necessary gas seal without high solids pressures. By keeping the solids pressures low, screw torque and wear are minimized. The sealing mechanism does not rely on mechanical tolerances or moving parts.

The discharge section can be designed to break up a cohesive plug and deliver a uniform stream of material to downstream process equipment. Alternatively, with free flowing materials and some co-current applications, the discharge section is designed to provide stability to the plug. A weighted gate can be included at the discharge of the Solids Pump® to help the plug develop during startup and minimize gas leakage when the feeder is empty.

Lone Star installed the 16 inch diameter Solids Pump® during an annual plant shutdown without any problems. Savings in reduced maintenance costs alone paid for the installation of the Solids Pump® within 8 months (return on investment was 115%)! After one year of operation, Lone Star decided to install a second Solids Pump® to supply coal to another mill. Once again, the equipment paid for itself, and Lone Star continues to experience major cost savings.