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Re: chromium recovery/ water recycling



Don,

Here are some ideas on chromium recovery/water recycling you may want to consider.  You would have to research each technology and its application for the specific facility and determine the cost effectiveness.
   
For water reduction/reuse:

1.  Install two stage counterflow rinsing--Your description appears to be a single-stage rinse process and one suggestion might be to modify the system to two-stage counterflow rinses.  These rinses theoretically require approximately 97% less water than single-stage and achieve comparable rinse quality. In addition, there is also considerable drag-out lost to single-stage rinsing that typically end up at the WWTP.  Also, consider reducing the flow rates.
2.  Segregate the waste streams so you can reuse the rinse water
3.  Possibly change the tank configurations-- Sometimes tank layout modifications can help you cost effectively utilize chemical recovery technologies by reducing drag-out and rinse water flow rates.
4.  Install conductivity control systems--These systems seem cost effective and they reduce the water use by adding water to rinse tanks only when necessary instead of at a continuous rate.
5.  Use of DI water 

For chemical recovery I would suggest installing some in-line metal recovery process.  Again these recovery processes would have to be analyzed in greater detail with the specific application in mind.  Below are some ideas:  
			Atmospheric evaporation seems to work well with chrome and is, in most cases, the most cost-effective form of recovery. 
	Ion exchange could be used for process bath recovery treatment on the rinsewater from the chrome acid bath process. 
	Reverse osmosis is another idea, but might not be suitable because of the high oxidation potential. 
	Electrowinning can remove chromium, however, in order to operate efficiently, a high concentration of metals is needed.  In your case, the chromium concentration is only 1 ppm in the rinse water.
	Use of an electrolytic purification process to recover chromic acid from rinse water. 
  
Good luck and I hope this helps.
Koren


Koren Carpenter
Environmental Engineer
Pollution Prevention Section
517-241-8177

>>> <Donatus.Wielkens@dep.state.fl.us> 03/27 12:31 PM >>>

Dear P2tech members,

I am looking for options for a facility to recover chromium and recycle rinse
water from their conversion coating process.  Conversion coating is also known
as chrome phosphate etch.  A process that treats the metal surface, here
aluminum extrusions, providing protection and good adhesion for the final 
paint coating. Aluminum is extruded to profiles of up to 40 ft length. 
Workpieces are pretreated, painted, and shipped to customers for final
manufacturing. Production is 18 million pounds/year.

Extrusions are taken through a five-diptank chromium conversion process prior
to an electrostatic applied paint finish. Stage 1) caustic wash, 2) water
rinse, 3) chrome phosphate dip, 4) water rinse, 5) water rinse.  Tanks 1, 3
and
5 are heated. Flow of effluent is constant from tanks 2, 4 and 5 to a water
treatment system.  Chromium concentration effluent tanks 4 and 5 is less than
1.0 (one) mg/liter!  Occasionally, contents of tanks 1 and 3 are partly
discharged into the treatment system. Hexavalent and trivalent chromium in the
water is treated using the standard chromium reduction/chemical precipitation
process.  This process generates 136,000 pounds of partly dewatered  sludge
(filter press) for disposal  and 2.6 million gallons of water for discharge to
sewer.  This is "industry practice", we have been told. Non-chrome
alternatives
are not an option (customer specs).  Also, I think tanks 1 and 2 contain large
quantities of dissolved aluminum which could be considered for recovery by
commercial processes. 

I have searched the archives and the Internet; no success so far. 

Are there feasible, cost-effective options?  Would appreciate your input.

Thanks, Don Wielkens, Donatus.Wielkens@dep.state.fl.us FDEP, P2Program