Metal finishing Q&As since 1989

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Treatment of wastes from powder coating pre-treatment line

2005

Hello everyone,

I work in a company of Greek interests, dealing with powder coating applications for the past ten years. We apply powder coatings on metal substrates, mainly aluminum and iron, to be subsequently used primarily in the building industry (doors, windows, etc.).

At the moment, the chemical pre-treatment process of metal substrates to be coated involves the use of dipping tanks that contain acidic solutions, whose main components are: HF, H2SO4, H3PO4, HNO3, Surfactants and, most importantly, Chromic Acid.

Waste treatment is of physical/chemical nature (solid/liquid separation) and involves the use of sedimentation tanks, where the precipitation of sludge takes place after the addition of suitable reagents (HCl, NaOH, Poly-electrolytes, etc.). The whole process operates in the batch mode, since only small volumes need to be treated ¡Ö 40 ¨C 50 m3/week (single shift).

At the moment, conventional waste treatment serves us very well, however, we would like to consider more advanced solutions. Please do note that we currently operate much more on an "experience", rather than "scientific" manner. Considering the nature of our wastes and volumes to be treated, how suitable would extended aeration systems be? Oxidation ditch systems seem to be a very good choice for waste treatment, but we do not know much on its applicability in cases like ours. Is anyone in position of providing some information concerning:

1. Applicability in cases like ours
2. Description of suggested systems (e.g. number of
3. Relevant drawings
4. Sizing of systems / area to be covered
5. Details on system installation
6. Costs

Obviously, in case oxidation ditch systems are not suitable, I would like to hear any suggestions/opinions on more applicable solutions.

Warmest regards,

Georgios Papayannakis
- Athens, Greece

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When you say "aeration", you mean biological treatment. Please note that you would use this kind of purification technique only for BOD/COD removal, mostly for domestic waste water. In your case, the first thing to do is chrome reduction (pH 2, sodium bisulphite) followed by neutralisation till pH 9, preferably with lime and then flocculation and sedimentation. More advanced systems include vacuum evaporation, but need chrome reduction anyway. In order to reduce the amount of waste water a closed loop rinsing circuit over an ion exchanger can be considered.
Sincerely,

Walter Provost
PES bvba - Aalst - Belgium
2006

Use RO to concentrate the waste and then go for treatment. If segregation of rinses is possible, we can try of reusing the concentrate back for rinsing. Otherwise the concentrate can be conventionally treated which will reduce the hydraulic load on the treatment plant. The biggest advantage is that the extracted water can be reused back for your process

Aravind A Narayan
- Coimbatore, Tamilnadu, India
2006