CED System Issues: Recovering paint, Ultrafiltration problems

A. Good day!
Have you solved your issues in the UF membrane elements and bag filters? What were the reasons?

One of the reasons for clogging bag filters and high coagulation in UF membrane elements are particles of coagulated E-paint due to the high pH of the E-paint.
1.First of all, check microbial contamination in the demin-water, anolyte system, E-paint, and the last demin-water rinse tank before E-tank. High levels of microbial contamination can cause an increase the pH of E-paint and its coagulation. If the m/c is high, use the appropriate biocide, disinfection and acid additive.
2.Check and follow E-coating parameters of supplier: solid content, conductivity, p/b ratio, temperature, etc.
3.Also, the transfer of alkaline contaminants from the pretreatment line on the painted parts, conveyor parts might cause pollution and coagulation in the E-paint.

All the best.

Dear Dmitriy_Belanovich,
Very correct answer & I appreciate your exact answer.

A. About your only hope is to fix the cause of the contamination, which is probably poor final rinsing or excessive phosphate coating as Ted alluded to. A DI rinse is almost mandatory ahead of CED baths to avoid problems like this. Once you correct the cause I'm afraid short of dumping the bath you'll just have to let the symptoms dissipate on their own. Phosphate ions in the coating won't generally cause much problem, but of course you'll have to keep an eye on performance.

As long as phosphate levels are very high, I'd try to keep regenerating the membranes as much as necessary. Then replace them when the problem is over, they'll be aged badly by all the extra regeneration. There is no way no get the phosphate ion out without affecting the paint, maybe your paint vendor can recommend a method and then deal with the results by adding only the affected components, but that's a last resort before dumping the tank.

As for the cause, look first at rinsing, then check your phosphate coating to make sure it is the correct coating weight AND is adherent to the surface. Good luck

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A. Dear Mr Narsimhan,
By separating permeate and paint thru UF membrane and using permeate for cascade rinses, you actually form a closed loop which reduces your carry-over losses as well as solvent and additive losses.
If you discard the permeate and use fresh water for rinses, then you will loose the solvent and additives in permeate which you will have to add extra in main tank. Also, permeate removes extra paint more effectively than water since it contains these solvents and additives.
Further, cascading brings the paint back to main tank which is carried over to rinses. This saves your paint losses.
Hope the system is clear to you now.

A. Dear Mr. Narasimhan:

If you are actually running an E-coat system, the answer, as given by Mr. Kulkarni will be right in front of you. Allow me to give the same answer using different words.

You separate permeate from the paint and then mix them together. Paint = Permeate + Concentrated Paint = Paint

If you look at your E-coat system, the permeate goes into the Permeate rinse 2 tank, cascades into Permeate rinse 1 tank and then finally cascades into the E-coat tank.

What goes into Permeate 2 tank is clear permeate. What cascades into E-coat tank is not clear permeate. Instead it is permeate with some solid content. The function of permeate is to recover the paint which goes out of the paint bath along with the component and gets accumulated in the rinse tank.

I hope this explains the E-coat industry first separates the permeate and then allows it back into the E-coat paint.

A. Dear Narasimhan,
It is a long time since I was involved with EC (1979) and now I only supply powder coating. However, the basic principles I do not think have change much. The UF Membrane if I am correct is to remove some of the solubilising agent from the tank, which consists of resin, solvent, solubilising and possibly pigment if it is coloured. The ratios of these are finally balanced and if you are to maintain the tank in good working order, these must be kept within certain limits. The solubilising agent if allowed to increase above a certain limit in comparison with the other ingredients would cause the applied wet coating to be removed (washed off) by the demin water rinse. Your lab tests for solids, MEQ's and ph etc should indicate to the person in charge of the plant what is required to keep the tank in balance -- more paint, water, less/more solubilising agent.

A. The answer, my friend, is swirling in the tank!

If the quality of coating is perfect, the chemistry of the E-coat tank is in perfect health. If the customer has severe coating problems, your UF membrane could be affected. Just walk around the shop floor and see the final coating quality. You will know whether it is safe to put in the new membrane or not.

UF membranes are certainly affected by ionic contamination. But in that case the coating is already being affected and there can also be generation of sludge in the tank.

Check if regular tests are being carried out on the tank.

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Ed. note: How did Bob Dylan miss that line :-)

A. Hi Prasad. I'm neither an e-coat nor a UF expert, but from experience in somewhat analogous cases, it certainly seems that the higher the flow rate the lower the loss of wasted non-deposited paint from the final rinse ... but the more expensive the recovery process. I would also assume that the non-rinsed non-deposited paint could cause various problems like rubbing off, staining, etc., if it's excessive.

Regards,

A. Hi Gerardo. We appended your question to a thread on a related subject which indicates that the answer is clearly "Yes!" for electrocoated paint. We'll see if people have anything to say about autophoretic.

Regards,

A. Hello sunil,
you have to wash ultra filter by help of permeate first, then make a solution of solvent (which you are using in the lacquer)
-- 4 liter and emulsion stabilizer - 1 liter and 15 liter DM water.

Run the ultra filter into this solution up to 1 hour and then cool down than run for next 1 hour.

Slowly your permeate will increase.

Note: always after running ultra filter you have to clean ultra filter by permeate once a week.

A. It's been a while since I worked with E-coat, a coil coating line with e-coat primer. Apparently the only one in the world then or since. The reeds over time get plugged so you need to back flush them, just don't dead head them, an expensive mistake.

"Electrocoating: a Guidebook for Finishers"
by Electrocoat Association

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A. Hello Jayakumar ponniasalan,
the pH of the solution and the solid content will effect on the permeate rate (ultra filter flow rate), so if flow rate decreases then you have to wash the ultra filter by help of emulsion stabilizer and solvent which you are using in CED.

A. The solid content will affect the ultra filter flow rate, and you have to wash the ultra filter once a week at least

A. Hi Jay. Hopefully a reader has a quick rule of thumb for you, but Machu spends a 7-page subchapter on sizing and deploying UF units for various situations. Good luck.

Regards,