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Solving the aluminum fluid waste problem.

A recent project of the Illinois Waste Management & Research Center, Champaign, IL, illustrates the complex issues in disposal of grinding wastes and how existing technologies can dramatically help. The center's client, a company manufacturing highly polished aluminum disks for the computer industry, was facing punitive surcharges for disposal of large volumes of wastes containing grinding fluids, rinse waters, and alkaline cleaners. A significant problem was the high biochemical oxygen demand (BOD) caused by chemicals in the wastewater. The manufacturer had to reduce pollution to mandated levels or ship the wastes to another acceptable location.

Proposed solutions were to reduce the volume of discharge and/or reduce the BOD content of the wastes. The option of reducing discharge volume carried an additional possibility of reclassification of the company as a small volume user with higher limits for BOD wastes. Lowering the BOD content called for one of several strategies that ranged from alternate manufacturing processes with lower or no BOD wastes to development of recycling and waste treatment facilities. For the latter, end-of-the-pipe treatment appeared too expensive and the biocide in grinding coolant contaminants could upset biological treatment processes. Therefore, attention turned to reducing volume and strength of contaminants in the wastewater.

Knowing that recycling can effectively reduce discharge levels, engineers faced a key problem in removing contaminants selectively from active ingredients in waste cleaners and coolants. For example, improper filtration of grinding coolants could strip active surfactants. Appropriate separation technologies included conventional filtration (bag, cartridge etc), membrane filtration, hydroclones, centrifugation, and flocculation.

Implementation of cascade rinse was recommended after a detailed cost-benefit analysis and establishing criteria for measuring rinse water quality which, in some cases, was as simple as measuring total dissolved solids (TDS) or total organic carbon (TOC).

For alkaline cleaning wastes, filtration and recycling seemed ideal because the water volume had been purposely set high to reduce the concentration of suspended solids in waste streams. In. considering options, membrane filtration proved to be too expensive and presented problems with solution chemistry by filtering out nonionic surfactants. A 5[micro] cartridge filter was a compromise that gave adequate reduction of suspended solids.

Recycling of the grinding fluid was more difficult because grinding was sensitive to size and concentration of fines left in coolant, Membrane filtration seemed an appropriate choice for purifying the fluid. A pilot study was undertaken with a 0.1[micro] ceramic membrane from EnviroPure Solutions, Wheaton, IL, to evaluate productivity, fouling, cleanabiity, and volume of coolant recovery. The two-stage study focused on functioning of the filter and the effects on filtration on coolant chemistry BOD, and strategies to reduce BOD.

The primary factors affecting cost in grinding coolant filtration were the volume of coolant passing through the filter and the tendency for fouling of the filter. Tests to reduce fouling by back-pulsing of coolant proved ineffective. However, reducing the pressure across the filter, which required throttling back the output stream, slowed formation of cakes that foul the filter and reduce capacity. Filters could be successfully regenerated with a thorough flushing of the system followed by treatment with a weak solution of nitric acid. Pilot field tests of a simulated continuous coolant flow recovered nearly 92% of the coolant. Recycled grinding fluid lasted three cycles before requiring addition of a 1% coolant concentrate to correct fluid chemistry. Filtration removed 88% of BOD mass and reduced BOD loading in sewer discharge by 50% due to the smaller total volume of concentrate.

Even with filtration and recycling of liquids, however, the problem with BOD in wastes remained, albeit concentrated. Attempts to isolate the source of the BOD showed that the primary source of BOD was the surfactant in the grinding coolant. Working with the fluid provider, engineers found a functionally equivalent coolant with a chemistry that would reduce BOD levels by 70%.
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Publication:Tooling & Production
Date:Mar 1, 1999
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