plating, anodizing, & finishing Q&As since 1989
R.O. WATER REQUIREMENT FOR SS ELECTROPOLISHING?
I am involved in setting up a new electropolishing line. The products to be processed are welded and/or formed 316L stainless steel components, ranging in size from 1 ft3 to 60 in. x 30 in. x 24 in. The process steps are 1. Nitric-HF pickle @150deg., 2. ambient rinse, 3. Phosphoric-Sulfuric acid electropolish @125 deg., 4. Dragout rinse @120 deg.,5. ambient rinse, 6. 20% nitric acid dip @120 deg., 7.ambient rinse, 8. 160 deg. rinse. The water source at this location has total hardness of 400-450 ppm. We intend to re-use rinse water, passing it through a deionizer. We have been advised to soften and reverse osmosis filter the water used for rinsing and solution evaporation make-up. Is this necessary, what problems will we have if we use untreated water or just softened water in this process? Thank you.Thomas Kemp
First of two simultaneous responses -- 2006
A lot of people use tap water in electropolishing processes. However, a DI or RO final rinse will eliminate water spots. It is also recommended for solution make-up.
Since you are planning to recycle the rinses with a deionizer, you could use the same unit to produce DI make-up water, but this will increase the frequency of regenerations and the volume of metal-containing waste water that needs to be treated. If this is a problem, then an RO can solve it.
We see this situation in plants that operate with zero discharge of process waste water. In these cases, an RO system for make-up is a good way to reduce the amount of waste that will need to be evaporated and hauled off-site.
consultant - Cleveland Heights, Ohio
Second of two simultaneous responses -- 2006
A correction - toal hardness of the water supply is 224 ppm, TDS is 470 ppm, silca is 9.2 mg./l. Sorry about that.Thomas Kemp
Purer water (RO or DI) rinses better. Tap water forms precipitates such as calcium sulphate & fluoride which also scale RO membranes.
I suggest using 2 rinse tanks at each rinse step and counterflowing the rinses: 7b to 7a to 5b to 5a to 2b to 2a. See Ted's articles in the finishing.com On-line Library:
"Plating Shops for the Nineties and Beyond" and
"20 Ways to Cut Water Usage in Plating Shops."
For the high purity water supply, consider 'double RO' -- replacing the DI unit with a 2nd RO unit.
To reduce the liquid wastewater volume to treat, add a small RO unit to the reject stream of the 1st RO [provided flow is high enough to keep concentrations low enough to avoid clogging membranes].
150 F is hot for a nitric-HF pickle Ø easy to over pickle (etch) & much fuming.
125 F is a bit low for the phosphoric-sulfuric EP solution. Raising it to 140-150 F and adding surfactant will lower viscosity, reducing dragout.
Perhaps add a spray rinse prior to the EP dragout rinse Ø reduces changes of the latter.
If using copper fixturing (best conductivity), include stations for racking after pickling and unracking prior to nitric passivation.
- Goleta, California
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The amount of dissolved solids in your influent water is hardly significant compared to the amount of dragout you will be removing in the de-ionization processes. True, the rinses will last longer, but not much and hardly worth the effort. If you are going to deionize your rinses I hope you have a lot of time and a big wallet for regeneration and replacement membranes.
- Shiner, Texas, USA
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