- Published: Saturday, 01 August 2009 08:00
- Written by CP Staff
In a state where land, water and material hauling are at a premium and tough environmental regulation is a fact of life, California's Spragues' Ready Mix (SRM) has deployed the first closed-loop recycling equipment combination of its kind: a concrete reclaimer separating fine and coarse aggregate, coupled with a slurry-recycling filter that dewaters cement fines to yield nearly ASTM C 94-grade batch water (suspended solids less than 60ppm).
Launched in 1927, family-owned Spragues' ventured into the ready mixed business in 1955. The founder's great-grandsons own and operate the company; Mike Toland is president and CEO, Steve Toland is vice president and COO. They currently operate two plants in southern California Û Irwindale (headquarters) and Simi Valley. A company that specializes in supplying to smaller to mid-size contractors, Spragues' experiences a fair share of unused concrete returned back to the plant. The Irwindale facility's ready mixed output of up to 45 loads/day or 30 loads on average, for example, yields returns up to 30 yd./day (approximately 6 to 7 percent). Its original approach to aggregate reclaiming and slurry processing (or disposal) was to pour concrete into block forms for resale, air dry returned concrete, and haul away the compounded waste or wash out into weir ponds.
In an effort to be more environmentally sensitive, SRM tried different concrete reclaimers with some success. However, the resulting gray water (cement fines) was just as difficult to manage as the returned concrete itself. Spragues' experimented by introducing gray water back into the batch, but had problems maintaining daily water consistency. The company decided to purchase a Liebherr LRS 708 Concrete Reclaimer and Alar Auto-Vac AV640-C Slurry-Water Recycling Filter combination.
Featuring a 29-yd. hourly capacity, the Liebherr LRS 708 is especially suitable for mid-sized ready mixed plants like Spragues'. Its dual mixer-truck discharges allow two vehicles to empty simultaneously up to a 270-degree angle around the feed hopper. An alert system informs the plant operator and mixer driver about the operational status of the entire reclamation system. The 100 percent hot-dipped galvanized unit is erected at ground level and requires no special ramps or foundations for operation. Liebherr engineers have addressed problems associated with fibers, lightweight aggregate, bearing lubrication and better fines separation.
The Auto-Vac is a self-cleaning, dewatering filter that separates water from cement fines, generating _-micron clean, recyclable water and dry, manageable solids for easy transport or reuse. The AV640-C is carbon steel constructed, pre-piped, pre-wired, and mounted on structural-steel auxilary and filter skids.
Since the equipment was installed earlier this summer, Mike Toland notes, Spragues' plant staff has met an initial objective of reclaiming returned concrete sufficient to reduce the waste stream by 90 percent, and is on track to recycle all inbound material and truck washout by year's end. The Auto-Vac filtered water, which does not influence set times and finishability the way reclaimed gray water had, is recycled into new batches, he adds.
The Liebherr/Alar recycling system is exceeding our exacting needs for concrete and water reclamation. The systems have been easy to install and maintain by our own people, affirms Steve Toland. We look forward to years of doing our part for the environment and saving money in the process.
Full water recycling and zero-discharge are more prevalent in European ready mixed operations. However, EPA's Storm Water Initiative and proposed Clean Water Act expansion could steer U.S. plants to full recycling solutions through stricter enforcement and stiff penalties for noncompliance.
In essence, the drive for the Spragues' project was economical. The producer already had an environmental program in place, but the buildup of cement fines and viscosity in the gray slurry water caused excessive water make-up and storage fees, exorbitant dredging and hauling costs due to wet tonnage, and increased admixture use. Top dollar went to hauling, where diesel truck owners continue to increase rates due to oil, fuel surcharges, insurance, driver wages and other payments. Spragues' needed to reduce the wet tonnage by dewatering the dredged material. Space issues prohibited the use of land to act as a drying bed, so hauling was frequent.
SEPARATE AND DEWATER
The LRS 708 separates rock, sand and other aggregate materials, producing a clean, readily transported or recycled material. The LRS generates 12,000 gallons of gray slurry water/eight-hour shift. The specific gravity can be as high as 1.25 (35 percent solids), but averages about 20 percent solid content by weight. Typically, one yard of returned concrete equals 650 lb. of cement fines.
After the Liebherr system reclaims, the gray slurry-water (containing cement fines) is pumped into an agitated holding tank. The objective is to keep the solids in suspension and not let the active concrete settle or set in the tank. The slurry is pumped from the mixing tank to the Alar Auto-Vac. Prior to reaching the Alar system, a metered amount of polymer is injected into the filter feed line and blended with the slurry through a series of in-line, 90-degree, injection feed pipes. The polymer increases the micron size of the cement fines, allowing for faster filtration, cleaner effluent, and drier solids, as it conserves diatomaceous earth (the consumable filter media used during the Auto-Vac operation).
The polymer-treated slurry water feeds into the Auto-Vac filter system. The suspended solids accumulate on the surface of the filter drum and _-micron clean water is vacuumed through the filter media. A variable-speed knife advances, removing the solids from the drum surface with lathe-like precision, and leaves a clean layer of filter media to capture more fines. The active concrete material will not blind or plug the filter. The filtered water is pumped into a nonpotable clean water tank for reuse in the batch, reclaimer and washout process.
The dewatered, filtered cement fines can be collected for transport off site. The dry cement fines measure an estimated 65 percent dry solid content by weight (the consistency of damp sand). The Alar Auto-Vac AV640 at Spragues' removes an average of 3,000 lb. of cement fines/hour, equal to 4.6 yd./hour. The unit is capable of pulling out 1,200-2,200 lb. of cement fines/seven-hour period.
The Auto-Vac filter is occasionally run when the batch plant is not in operation to clean the tank/weir bottom, yard and runoff water. Both systems can be sold and operated separately.