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Meeting Permit Limits at the Process Machine Removal of Fine Particles from Wastewater Using Plate Centrifuge Technology

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A recycler of strategic metals (nickel and titanium) has historical discharged treated wastewater to the local POTW. They operate under a permit limiting total nickel to of 1.0 ppm, which was problematic for the facility. They self-reported nickel excursions to state authorities, and an NOV was in process. The facility sought assistance to solve the problem before the NOV was received. The facility receives nickel and titanium alloy metal chips and turnings from international clients. The metal is crushed, washed in a strongly alkaline solution, rinsed, and dried in wash/rinse/dry tumbler units. Spent rinse water is discharged onto a vibratory screen which separates large particles and agglomerates for recycling. Rinse water then flows into a twocompartment settling tank, to an equalization tank, and finally to a sand filter. Laboratory analysis on wastewater showed that 94.5% of nickel particulates in a sample from the equalization tank were <1.0 microns, and 93.4% of nickel particulates in a sample from sand filter effluent were <1.0 microns (Table I). It was evident that these fine particulates caused excursions, and that the sand filters were incapable of removing them. The challenge was identifying efficient particle removal technology that was robust for use in the shop, unaffected by oil and grease in waste water, easy to run and maintain, and costeffective over its service life. In addition, a machine-side solution was preferred so that valuable nickel could be separated for recycling before mixture with titanium lowered its value. A bench-top test of flocculation was conducted and physical separation technology and equipment (e.g.cyclonic separation, conventional centrifugation, advanced vibratory screeners, engineered separation tank systems) was evaluated. All were rejected, as testing proved the equipment ineffective in removing the fines, or manufacturers would not guarantee removal. A high-speed disk centrifuge system which was designed and built for metal removal showed promise based on laboratory tests. Testing per ASTM D 2709 (the BS&W method) demonstrated greater than 99% particulate removal efficiency. Therefore was a week-long shop floor trial of the centrifuge, using actual production equipment, was arranged. The trial unit was set up as a machine-side (pre-treatment) device. Rinse water was fed into the centrifuge inlet port from the first chamber of the settling tank. The centrate was automatically discharged to the wastewater treatment system, and separated metal and other solids were automatically discharged into a collection drum. Periodically, water samples were drawn from the inlet and outlet ports on the centrifuge control module. Laboratory analysis on pre-and post-centrifuge wastewater was positive. Nickel removal ranged from 81-96%, and total nickel levels from this pilot test were significantly under permit limits (Table II.). An extended shop trial initiated in October 2006 tested extended use on all rinse units. Performance has exceeded expectations, with permit requirements met on all samples. A side benefit of centrifugation was that total suspended solids, primarily oil and grease, are also efficiently removed (Table II). In conclusion, disk centrifuge technology appears to remedy elevated nickel discharges. It has been shown to work reliably with minimal intervention by the facility's operators, and references from other users of plate centrifuges indicated long service life.
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Keywords: HIGH-SPEED DISK CENTRIFUGE; METAL RECYCLER; NICKEL; PARTICLE REMOVAL; PERMIT LIMITS

Document Type: Research Article

Publication date: 2007-10-01

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