Dewatering Screens and Screws: Dry Idea
The washing and dewatering plants were installed and running by mid 2006. Folmar explains that fines-laden water slurry captured from the wash plant is pumped to the circular thickener. A dry polymer make-down system mixes dry batch polymer flocculants with water and injects the polymer into the slurry upstream of the thickener. The flocculants attach and help the fines to settle more quickly to the bottom of the tank. The clarified water at the top of the thickener tank flows over weirs and uses gravity to run into the fresh water tank, where it is pumped back to the wash plant for reuse. BoDean Co. owner Dean Soiland explains that, while a system of totes can be installed to premix the flocculants, he prefers having his crew mix each 250-gallon batch individually for better control. This is because plate presses typically require significantly reduced polymer usage.
From the bottom of the thickener, the slurry of 50-percent solids pumps to holding tanks adjacent to the filter press. A nuclear densometer calculates mud density as it enters the press.
“The press works over pressure and time,” says Ian Scroggins, wash plant operator for the thickening/filter press system at the Mark West Quarry. Scroggins explains that each press plate is covered by its own filtering cloth. The plates hang vertically in a row. At the beginning of a cycle, pumps fill the spaces between the plates (called chambers) with the thickened slurry mixture. The plates work like a mold as they slowly tighten to 225 pounds per square inch, compacting the fines between them and filtering the water first through the outer layer of mud, through the cloths, and finally through holes in the plates to be captured beneath, where it is sent back to the fresh water system.
At the end of the cycle, when the maximum amount of water has been pressed from the fines, the press opens at one end and the plates move aside one by one at a speed of 30 plates per minute, allowing the caked fines to drop to a stockpile below. The filter cakes weigh 300 pounds each, for a total of 21 tons processed per cycle.
“We pump a slurry that’s 45- to 50-percent mud. If we pumped more often, we would have more water in the slurry and a wetter cake at the end of the process,” Scroggins says. “We can make the cakes as dense as we want.” The caked fines that drop from the filter press are so dry and compact they have a consistency of hard clay. “For the reclamation of water, you will never get anything as dry as these cakes from a (traditional) belt press,” Scroggins adds.
Scroggins says that a full press cycle takes about an hour. Because the thickener makes more mud than Mark West Quarry can process through the filter press in one shift, the press is programmed to run automatically after personnel leave the quarry at the end of the day.
“The press could run 24 hours a day if we wanted,” he says, “but, typically, we set up the thickener to fill the mud tanks to capacity before we leave for the day.” The filter press processes it automatically, running 16 to 18 hours per day and shutting off when the tanks hit a certain level. In the morning, when a shift begins, the fines are all in cakes at the bottom of the press, and there is no mud left in the tanks.
Within two months of the press’ startup, BoDean built a second mud tank to hold more fines slurry for processing through the filter press, according to Folmar. “The initial cost to purchase this press was more than if we had bought a belt press,” he says. “But over time, if you don’t have storage for additional drying of fines or room for settling ponds, it pays for itself. In fact, with this technology, settling ponds may just go the way of the past.” Additionally, BoDean Co. has found a local use for the filter cakes that carries its sustainable practices even further — selling the material for use as pond liners in Napa Valley’s wineries.
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