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| The 4660 Mixer. |
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| The 4660 Mixer. |
In two distinct applications, a combination of Flygt pumps and mixers is being used to reclaim and ultimately recycle industrial waste material. The first application is for turning a mountain of synthetic gypsum created by a power plant into new wallboard. The second application is for recycling returned concrete material and wastewater. In both cases, the mixers and pumps are working together to create and pump abrasive slurries.
Mixing and pumping gypsum
The Tennessee Valley Authority (TVA) had a waste disposal problem at their 260-MW Cumberland power station. Since 1993, TVA had been dumping limestone (synthetic gypsum) from their flue gas desulfurization (FGD) system onto a vacant part of their property, resulting over the years in a 15 to 20 foot high, 80-acre mountain of gypsum.
To turn the waste disposal cost of getting rid of the gypsum into an economic development opportunity, a three-party venture, involving the power plant, an adjacent gypsum-processing plant, and a neighboring wallboard production facility was created. Synthetic Materials, a firm enlisted by the TVA, built a gypsum recovery plant on TVA's property to dewater the gypsum produced by the FGD process, and sell it to a neighbouring wallboard plant, so turning a profit on a previously unused product.
Slurry rafting
To overcome the occasional shortfalls in raw material when one or both units at the power plant were off line, Synthetic Materials purchased a raft system from ITT Flygt to obtain additional gypsum from the 80-acre stockpile. The raft consists of a Flygt Model 5540 submersible abrasion resistant slurry pump and a Flygt Model 4660 submersible mixer. The 5540 abrasion resistant pump incorporates a 20 HP submersible motor mounted on a wet end, consisting of a split volute with a replaceable high chrome liner insert and a highly efficient, technologically advanced impeller specially designed for slurry pumping.
The 4660 mixer uses a state-of-the-art direct drive motor and high chrome propeller in conjunction with an integral jet ring for increased efficiency. The raft is located in a flooded pit inside the gypsum pile. The concept uses the submersible 4660 mixer to create a gypsum slurry consisting of 20% solids, which is then moved by the submersible 5540 slurry pump to the cyclones for dewatering.
Because of the complex rheology of the gypsum slurry, Flygt was provided with a sample of the gypsum material to test at their mixing laboratory in Trumbull, Connecticut. Flygt scientists were able to offer the proper mixing recommendation based on this testing, and the equipment is said to have performed flawlessly.
The stockpile now accounts for a significant portion of the gypsum plant's input of raw material and has helped Synthetic Materials meet production goals. Gypsum processing has slim profit margins, so it needs reliable, high-input volume to boost the bottom line. The factors have to be such that the gypsum can be processed and delivered at a better cost than the next low cost source. Flygt provided Synthetic Materials with a cost-effective solution they needed to make their process profitable.
Reclaiming unused concrete
Harbor Ready Mix was founded in 1985. Located about 40 miles south of San Francisco, this concrete plant can produce 100 yards of ready mix an hour -- for a total of 55,000 to 85,000 cubic yards a year.
Harbor Ready Mix previously disposed of returned concrete at a nearby concrete reclaim center that converts it into road base. Disposing of the returned mix, ranging from 5 yards up to 30 yards a day, presented a significant cost of doing business. Reducing the ongoing expense was a primary incentive for creating a recycling system at this plant.
Complying with environmental regulations
In addition to reducing disposal expense, a reclamation system would also help Harbor Ready Mix comply with California's increasingly stringent environmental regulations. Some five years ago, the EPA enacted tougher rules that subject California's batch plant operators to a permit process and holds them accountable for all water used in the operations. Complying with the stricter regulations, aggravated by increasingly frequent water shortages, further justified the system. Reclamation required the segregation of the sand, gravel, and greywater from unused concrete. Requirements dictated that all recovered concrete ingredients be recycled, including all greywater, typically the most difficult to contain and reuse.
Contained within a closed-loop system, the greywater would ultimately be reintroduced and mixed with municipal water at a controlled rate to achieve an acceptable specific gravity for subsequent batches of concrete. The rest would be filtered and reused at the head end of the reclaimer for flushing and washing the fleet of returning trucks.
The recycling process begins when drivers dump their returned concrete into a "rapid reclaimer" whose trommel screen separates sand from gravel for immediate transfer back to the plant. The remaining cementious slurry and associated water used to flush and wash the trucks flows by gravity through two 6-inch diameter pipes into the first of three open pits.
Two corrosion-resistant, 15-HP Model 4660 ITT Flygt mixers, commonly used in wastewater treatment plants, are mounted in adjacent corners of Pit #1. Operating at 580 RPM and running on an alternating cycle, the mixers keep the cementious solids in suspension within the 10,000 gallon pit. Some of the slurry is gravity fed through filters into Pit #2 and subsequently Pit #3 where the clarified water is then pumped back to the head end of the reclaim process.
The majority of the stored greywater is drawn from Pit #1 by an 8-HP Model 2102 Flygt submersible pump and fed back to the plant through a 3-inch line. The recycled greywater is then mixed with either municipal or clarified water from Pit #3 at a controlled ration to keep it compatible with the specifications of the current batch of mix being blended at the plant.
In addition to saving disposal charges incurred in the past for returned concrete, the system should help the batch plant comply with increasingly stringent environmental regulations aimed at controlling runoff of tainted stormwater from industrial properties.
The system also eliminates the routine clean out of the square holding ponds common at this and other batch plants. The open pits filled over time with hardened cemetious sludge and had to be scooped out with a front loader every six to eight weeks. The material was too alkaline for use as shoreline fill, so it was either hauled to the reclaim center, carefully designated spot fills or to a landfill.
Harbor Ready Mix expects to save most of the $30,000 to $35,000 a year that was previously paid to dispose of returned concrete. The system enables them to recycle virtually all of the returned mix and the greywater accumulated from flushing the mixers and washing the trucks. They project recovery of their investment within four to five years.
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