Recapturing ultrafines may create a saleable byproduct
The argument for finding an efficient way to recover fines in aggregate processing is undeniable. Any time a producer can find a way to make profitable use of a processing byproduct, it is a win-win situation.
“If you’re washing in your operation, you traditionally must have some sort of pond where you send your wastewater. The wastewater always contains super-fine material that can’t be captured with standard washing equipment.” says Jarrod Rice, aggregates manager for Derrick Equipment Co. He explains that in the settling pond, gravity and time allow the ultra-fine material to settle to the bottom. “If you’re not pulling those fines from your pond and stacking them, then your pond will grow and grow; they require continual management,” he adds.
The typical method for managing a settling pond has been to pull large buckets of the fine material from the bottom of the pond, stack it in a stockpile, and wait for it to dry. “There is a cost associated to the time and equipment dedicated to that process. Some of that cost is associated to equipment not being used on its primary function. But if you can capture even 50 percent of your fines before they enter the pond, versus spending $1 million a year to pull those fines from the pond and dewater them, you’re saving $500,000 a year in unprofitable labor,” Rice says. “By capturing and dewatering fines as part of the process, you have a saleable product.”
Settling ponds aside, ultra-fine material can be captured through mechanical or chemical means. For mechanical separation, the use of hydrocyclones, dewatering screens, and centrifuges allows the producer to speed up the separation and drying process with fines.
According to Dean Fogal, superintendent for Valley Quarries’ Mt. Cydonia #2 plant in Pennsylvania, the maintenance of the settling pond and belt press were two reasons the company chose to use a different solution. The operation installed a HI-G Dryer, combination of hydrocyclones, and a dewatering screen to capture up to 70 percent of the plant’s discharge slurry solids, recovering the +400 mesh materials. “To capture and dry the remaining ultra-fine solids, underflow from our thickener (about 30 percent solids by weight) now flows to a centrifuge, which dewaters them to stackable and conveyable form,” he explains.
Port Colborne Quarries in Ontario, Canada, chose to install mechanical fines recovery equipment in late 2011 because an increase in washed material was causing the settling ponds to fill quickly with ultra-fine material. “We were having to clean our ponds twice a year, and the product we recovered from them was unusable,” says Tim Cassibo, operations manager. The operation installed a system which uses hydrocyclones and a high-G dewatering screen to capture and dewater material as small as 500 mesh to 80 percent solids by weight.
In order to meet stringent specifications, sand and other aggregate materials often must be washed. Washing can be achieved by wet screening plants, log washers, blade mills, coarse and fine material washers, classifying tanks, and dewatering screens, which remove clay, silt, and other deleterious material. Wet classifying also is one step in sizing and separating smaller particles. As a byproduct of washing, wastewater from the process carries ultra-fine material. The question of what to do with this material has plagued aggregate operations for years.
The most traditional and common method for dealing with fines is to send them to a series of settling ponds. In these ponds, as a result of gravity, the fine material eventually settles to the bottom of the pond. Because of the maintenance involved with ponds, and the fact that the aggregates industry has increased its efforts toward better sustainability, many producers have begun to use additional methods – or other methods entirely – to allow for reduction/elimination of ponds, capturing reusable water and finding a use for ultra-fine material.
Capture those fines
Many operations are beginning to employ equipment such as hydrocyclones, dewatering screens, clarifiers, thickeners and flocculants, centrifuges, and presses to capture ultra-fines, as well as reclaim/reuse wash water. In many of these cases, producers have been able to reduce the size and number of settling ponds at their operations – and in some cases, producers have completely eliminated them. At the same time, the ultra-fine material is ultimately captured and can be stockpiled for haulage and/or reuse.
How it’s done
In this scenario, the -200 mesh material and water overflowing from the sand screw enters a cyclone feed pump, which sends it to a series of 4-inch hydrocylones. The water spins much like a tornado, throwing larger material to the sides, where it falls onto a dewatering screen, where the -200 x 500 mesh material is stockpiled as a clean sand product and the smaller material re-enters the cyclone feed pump. The ultra-fine material (-500 mesh) rises up from the center of the hydrocyclone’s spinning water and overflows into a thickener, which consolidates it. Clean water from the thickener is routed back into the wash system; the consolidated underflow then enters a centrifuge, which dewaters the -400 mesh ultra-fines into stackable and conveyable form.
Finding a market
Capturing and dewatering ultra-fines provides not only sound environmental practices, but can lead to reduced costs and greater profitability. From pond, swimming pool, or septic system liners to paint fillers to golf course sub-base to aglime to crop dust to livestock bedding and more, producers are finding a wide variety of uses for their captured and dewatered fines. Producers should take some time to research markets for their ultra-fine material, and train their sales team to sell this valuable material.
Dean Fogal is superintendent for Valley Quarries’ Mt. Cydonia plants. Valley Quarries Inc. is a subsidiary of New Enterprise Stone & Lime Co., Inc., and is based in Chambersburg, Pa.
Tim Cassibo is operations manager for Port Colborne Quarries Inc. (PCQ) in Port Colborne, Ontario, Canada. PCQ has been a division of Rankin Construction Inc., based in St. Catharines, Ontario, since 2007.
Jarrod Rice is the aggregates manager for Derrick Equipment Co. He has worked for Derrick since 2012, and has held various roles in the company’s technical and commercial departments during that time period. He is a graduate of the University of California, Davis, with a degree in mechanical engineering.
Voices of Experience
Before Rankin Construction, parent company for Port Colborne Quarries (PCQ) in Ontario, Canada, purchased the aggregate operation in 2007, PCQ had been a dry crushing and screening facility. But specification demands for asphalt sand — both internally for Rankin’s asphalt plants, and externally for other customers — led the company to add a wash plant.
“When we put in the wash plant, we knew that two or three years down the road, we’d want a system that could help us recover material that was going into our settling ponds,” says Tim Cassibo, operations manager for PCQ. When it became apparent that the quarry was having to clean its settling pond twice a year, Rankin decided it was time to put a means in place to capture material before it could reach the settling pond.
“We were dedicating workers and equipment and time to cleaning the pond, which took all of them away from production. It was a cost liability for us,” Cassibo says.
PCQ installed a HI-G Dewatering Machine, which combines small hydrocyclones surrounding a radial manifold with a high-G dewatering screen.
With the fines recovery system in place, ultra-fines down to the 400/500 mesh level are captured from the washing plant’s hydrocyclone effluent, and dewatered to a dry and manageable 80 percent solids by weight — saleable for such products as aglime and dense flowable fill. The ROI for PCQ’s fines recovery system was realized in less than two years.
“We’re now effectively recovering 75 percent of our washing operation’s waste material, and we’re converting it into a profitable product that we can sell. In addition, we have been able to extend the life of our settling pond by at least four or five times,” Cassibo says. “We’ve gone from cleaning it twice a year to now cleaning it every three or four years.”
“Several years ago, we were experiencing problems with our fines recovery process because the belt presses we used for dewatering the ultra-fines required a lot of repairs and personnel time,” says Dean Fogal, superintendent for Valley Quarries’ Mt. Cydonia plants in Pennsylvania. “We also were devoting time and equipment to cleaning our settling ponds.”
The solution was the installation of a combination of hydrocyclones paired with dewatering screens, which capture approximately 70 percent of solids from the plants’ discharge slurry and recover material down to 400 mesh.
At the Mt. Cydonia #2 Plant, the remaining 30 percent of the ultra-fine solids (-400 mesh) are sent in a slurry to a radial thickener device, which consolidates them for dewatering. Valley replaced its belt press with a high-capacity centrifuge to dewater the thickener underflow (about 30 percent solids by weight). The centrifuge dewaters the ultra-fines into a stackable, conveyable form that is 72 percent solids by weight. The water from the centrifuge is returned to the thickener feed as a recirculating load.
Fogal says the settling pond has been eliminated, because 100 percent of its solids are dewatered, and all of the process water is recovered. “Our processing plant uses about 2,800 gallons of water per minute — 168,000 gallons per hour,” he notes. “Aside from about 2,500 gallons per hour that goes out with the product, the rest of our water circulates back to the plant.” He explains that the plant’s sand product typically is stacked at 22 percent moisture. The material is allowed to drain off and is sold at 6 to 8 percent moisture. Water that drains from the product is channeled back to the system.
“The entire process is much more environmentally friendly and has resulted in cost savings. Plus, we’re now able to sell our solid ultra-fines for such products as cattle bedding,” Fogal says.
According to Jarrod Rice, aggregates manager for Derrick Equipment Co., mechanical equipment uses the same gravitational principal for separating ultra-fines as a settling pond — requiring a significantly smaller footprint and making the process more efficient. These machines include hydrocyclones, dewatering screens, and centrifuges.
A hydrocyclone, he says, works in the same way as a tornado. As water spins within, heavier solid material is pushed to the outside, where gravity causes it to fall out of the bottom. At the same time, smaller material is pushed upwards in the center of the vortex and flows out of the top as a slurry. “Larger hydrocyclones handle greater volumes, but make a coarser material cut,” Rice explains. “Because we want to capture finer material, we make up volume using a series of smaller hydrocyclones.”
For purposes of capturing material, the heavier (coarser) material in a hydrocyclone falls to a high-G dewatering screen, from which it can be stacked at up to 80 percent solids by weight. The lighter (finer) material slurry flows to either a thickener system or to the settling pond.
“At that point, even if your ultra-fine material flows to a settling pond, you’re way ahead in pond maintenance requirements,” Rice says. “If you process it more, chemical flocculants can be used to cause the solids to consolidate and fall. The underflow then might report to a type of press — or it can be sent to a centrifuge.”
A centrifuge accepts material continuously, rather than in batches. It has an outer bowl that spins at a high rate. The thickener underflow enters as a slurry. Centrifugal energy sends fine solids to the wall and clear water to the center. A conveyor augers the solids out of the discharge end at up to 75 percent solids by weight, from which they can be conveyed and stacked. Water from the centrifuge is sent back to the plant.