The authoritative public forum
  for Metal Finishing since 1989
-----

Drag-in tank for Chrome Plating

2004

As per of the set-up of a new chrome-plating shop, I plan to install a 30 gal drag-in tank to cumulatively collect drag-outs. Then, I plan to simply heat-up the chromic acid/water mixture to 80 °C until the natural evaporation makes the concentration right enough for make-up to the chrome tank.

Questions are :-

1) Anything fundamentally wrong with this set-up?
2) Is it safe to heat up the chromic acid drag-outs to 80 °C?
3) Do I need a fume extractor for the drag-in tank?

Thank you.

Syed Salim
Plating Shop - Kuala Lumpur, Selangor, Malaysia

---

First of two simultaneous responses --

The largest problem is you will be putting back all of the contaminants that were dragged out. This is not a problem if you have a porous pot in your chrome tank to remove the unwanted metal ions and if you filter the solution to remove insoluble trash. Heating the tank to 80 °C is way too hot to get satisfactory rinsing, is somewhat a danger from splashes for your employees and is not necessary. Consider running your chrome tank a couple of degrees warmer. Note that this will expand the allowable envelope for bright plating, but will require a higher current density and will also plate slightly slower if the CD is not raised enough. Yes, it will require a hood, but not as much as the plating tank as there will be far less entrained chrome as there is no gas bubbles as in the plating tank. Return of the drag out works fine for some platers and will cause major problems for others. (Not initially, where it will look fantastic)

James Watts
- Navarre, Florida
2004

---

Second of two simultaneous responses -- 2004

Hi Syed,

My background is in plastics (chem plant eqpt) not plating so bear with me.

If you heat the chrome to 80 °C I'd think it would very safe to do so but there are definite problems.

I had a failure mode on two scrubbers (actually they were mist eliminator banks) due to a night shift heating up the diluted chrome tanks ... resolved only when they ceased heating up the tanks.

There is perhaps another option for you to consider. This involves a m.s tank, a m.s. fan, some mist eliminator (vertical PVC blades) along with some mass transfer packing and a m.s. pump.I've never done this ... but it seems logical and effective to me. Obviously you'd need someone to intelligently put all this together.

The process is one of this fan pushing air into this tank. At the opposite end is the aforesaid eliminator bank with the air discharging to atmosphere. In the middle is the packing above which is a distributor (perforated plate 25% open area works fine) into which the diluted chrome goes. The tank has a drain, of course, and the liquid goes to another tank using the aforesaid pump to pump the liquid back up again.

What will happen is that you'll get NATURAL evaporation and hence a concentration increase in the liquid and very little downtime.

I hope that this makes sense to you ... just heating up the liquid will cause pollution problems.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

Interesting design, thanks for the input. I will give it a thought. By the way, do I REALLY need to filter the drag-outs prior to pumping back to the chrome tank? In my case, there is no prior plating process prior to chroming (eg. nickel). So, there is no likelihood of heavy metal contamination in the chrome tank, right? Except maybe organic insolubles that had escaped NaOH dip and rinse. I want to minimise waste-treatment costs but I do not want to contaminate my chrome tank.

Syed Salim
- Malaysia
2004

---

2004

Hi again Syed,

You ask about filtering the dragouts. Ah, I really don't know but would think, based on what you said, that this is not necessary.

I didn't make that perforated plate idea easily understood.

Re the m.s. tank ... the duct from the fan would go, say, 6" below the top. The top has a flange so that a lid, if necessary, can be fitted.

Go 3" down ... make a mini ledge. Instal a loose buy supported perforated plate.(the plastic PVC perforated plates have about 25% to 26% open area). Above which have a pipe with coarse 1/8" dia holes @ 2" or so centres. The length (for the packing) could be 3 feet or so.

Obtain some packing, preferably smallish. Enter the tank via a transition @ max. 400 fpm. But exit into the eliminator can be l,000 fpm.

What happens is that when you pump into that top void area, due to the perforated plate's small open area, you'll get excellent distribution of the liquid across the packing.

Then at the end of the shift or the end of the day, you should find that your diluted chrome is now much stronger and can be pumped back into the chrome tank. This is trial and error, of course.

Does this make sense? I hope so and wish you success.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).