Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/756888
Recapturing ultrafi nes may create a saleable byproduct OPERATIONS T he argument for fi nd- ing an effi cient way to recover fi nes in aggregate processing is undeniable. Any time a producer can fi nd a way to make profi table use of a pro- cessing byproduct, it is a win-win situation. "If you're washing in your oper- ation, you traditionally must have some sort of pond where you send your wastewater. The wastewater always contains super-fi ne 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-fi ne material to settle to the bottom. "If you're not pulling those fi nes from your pond and stacking them, then your pond will grow and grow; they require contin- ual management," he adds. The typical method for managing a settling pond has been to pull large buckets of the fi ne 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 fi nes before they enter the pond, versus spending $1 million a year to pull those fi nes from the pond and dewater them, you're sav- ing $500,000 a year in unprofi table labor," Rice says. "By capturing and dewatering fi nes as part of the pro- cess, you have a saleable product." Settling ponds aside, ultra-fi ne material can be captured through mechanical or chemical means. For mechanical separation, the use of hy- drocyclones, dewatering screens, and centrifuges allows the producer to speed up the separation and drying process with fi nes. According to Dean Fogal, super- intendent 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-fi ne solids, underfl ow from our thickener (about 30 percent solids by weight) now fl ows to a centrifuge, which dewaters them to stackable and conveyable form," he explains. Port Colborne Quarries in Ontario, Canada, chose to install mechanical fi nes recovery equipment in late 2011 because an increase in washed ma- terial was causing the settling ponds to fi ll quickly with ultra-fi ne material. "We were having to clean our ponds twice a year, and the product we recovered from them was unusable," says Tim Cassibo, operations manag- er. 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. AGGREGATES MANAGER In this scenario, the -200 mesh material and water overfl which sends it to a series of 4-inch hydrocylones. The water to the sides, where it falls onto a dewatering screen, where sand product and the smaller material re-enters the cyclone up from the center of the hydrocyclone's spinning water Clean water from the thickener is routed back into the wash centrifuge, which dewaters the -400 mesh ultra-fi nes into In order to meet stringent specifi cations, sand and other aggregate materials often must be washed. Washing can be achieved by wet screening plants, log washers, blade mills, coarse and fi ne material washers, classify- ing 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-fi ne material. The question of what to do with this material has plagued aggregate operations for years. Wash up How it's done 1 4 .