Where the
world gathers for metal finishing
Q&As since 1989


-----

Treating chromic rinse water

 

Dear Sir/s:

We are planning to set-up tank for zinc chromating. We foresee a problem that we will be regularly producing large amounts of rinse water after the chromic(hexa) dip (1000 gal per day). Is there any cost efficient way of recovering the chrome or treating the water prior to discharge? Any process that would turn the water to clear and not yellowish?

Thanks in advance.

Dino Kaw
- Quezon City

---

 

Dino,

I came across a very simple evaporator system which might be of interest but where I personally had absolutely zero input. It has been running for years in a Canadian shop.

It is crude, rude, inexpensive and apparently works OK.

It consists of 5 items. A simple scrubber shell, some packing, a pump, an eliminator grid, a holding tank below and some spray piping.

The scrubber shell is connected horizontally to the push fan and at the other end has the eliminator. At the top is a very simple spray header. The pump is a mild steel unit.

The fan is also a mild steel but of centrifugal design.

The packing is the cheapest one can buy. The only sophisticated thing is the single bank PVC sine curved mist eliminator consisting of vertical blades @ 20 mm centres.

The concept, I believe, is to pump the highly diluted chrome wastes over the packing where it is distributed and, especially in summer, is quickly evaporated. Once the holding tank gets depleted somewhat, this means a far more concentrated chromic solution which then goes back to the chrome tanks.

As said, crude, rude, inexpensive to buy and cheap to run.

I hope this will help you somewhat.

Cheers!

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

---

You might need to contact a wastewater treatment consultant, Dino, as it isn't safe to just start adding chemicals you are not familiar with. But, in brief, the pH is lowered to about 3.5-4.0, sodium bisulfite is added as a reducing agent until the yellow color vanishes and the wastewater turns blue-green, then the pH is raised to precipitate the chromium.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

---

You may find it cheaper and more effective to concentrate the chromate by utilizing a weak base or strong base anion ion exchange resin. The resins are very good at removing chromates, and can be regenerated using NaOH. The small concentrated solution can then be batch treated by reducing to trivalent chrome and precipitation.

Hope this helps.

Juzer Jangbarwala
Hydromatix, Inc. - Santa Fe Springs, California
 

All of the above answers are correct but none ask the question "Why are you generating this quantity of rinse water"? Are you barrel processing or rack processing? Have you investigated more efficient means of rinsing including sprays, blow offs, counterflows, etc. to minimize rinse water generation? Once you have minimized rinse water generation then you can look for a technology or approach to handle the generated waste. In the US 1,000 gal/day ( 8 hours?) is a fairly large volume of water for a chromate rinse in many facilities.

Hope this gets you thinking.

Gene Packman
process supplier - Great Neck, New York