Evaporation of chrome plating rinse water

A. Atmospheric evaporation is difficult to get to a sludge state because it cakes up on the source of heat. It is a real mess to handle in many cases. It works very well for lowering the amount of liquid in a rinse tank or plating tank so that you can plow back more rinse water. This normally has a very negative effect on the plating tank over a period of time. It should have a hood over it. Many EPA inspectors will require some type of cover such as partial mesh. The wastewater is by definition a haz waste and has to be "covered" except for additions or removals.

A. What I read into your question is that you are concerned if anything hazardous is getting into the air. It is highly unlikely that anything but water vapor is produced, since the pH is essentially neutral, i.e.; there are no acid fumes to worry about. Neutral inorganic substances have a much greater boiling point than water, so much so that one would require intense heat to worry about any hazardous fumes. The only substances which would evaporate at the temperatures you mention would be solvents. You would know if solvent vapors were even possible from looking at your process, but from what you described it sounds unlikely. It sounds like your process is "safe", not counting mechanical items such as tank integrity, electrical connections, heater failsafes for loss of liquid etc., and so on... It has been a controversial subject in the past if evaporation is actually waste treatment and requires a treatment permit, but the last I had heard, was that it was not. However, the sludge is a hazardous waste and as such must be dealt with in terms of disposal.

A. Hi Rob,

I have, perhaps, a partial reply to your question re evaporating (chromic or alkalies, for that matter).

What you must NOT do is to steam heat the tank as you will definitely get emission problems even using a fairly good 'dry chrome' inertial scrubber (e.g.. Houdaille Industries, Oshawa, Ont. 1979). Emission problems solved after telling the night shift not to use steam heat!

You could get a very inexpensive 'concentrator'. This would consist of an el-cheapo steel fan, an el-cheapo steel pump, some packing (e.g.. Tripack PP packing) and some non-plugging jets. At the end of this horizontal package is a single high quality PVC blade bank. In other words this is a crude air scrubber. Hence, you use ordinary air, and there would be no heating costs.

I must admit that I have never made one of these but do know of a Company that has ... with success.

Maybe this will be of some interest to you. I hope so,.

Cheers !

A. Re Mandar Sunthankar's idea of a 'swamp cooler', maybe he could tell people more about it (personally I had never heard of it and don't know how it functions) but it sure sounds inexpensive.

Back in the 70's at a trade seminar I was demonstrating a gut standard PVC twin blade type horizontal mist eliminator. The demo just showed water being sprayed into it quite heavily and nary a single visible droplet even dripping off the 2nd blade bank.

What was interesting is that by the end of the day, the water tank was half empty and dirty. In other words evaporation was occurring and also a 'cleaning' of the clean, breathable air in the exhibition hall.

The other observation was that this was the first time that COLD water was not available for this unit, only hot water and the RETURNED SPRAY WATER FROM THE ELIMINATOR DRAIN WAS C O L D !

You can draw your own conclusions from this!

Cheers!

A. Freeman, the conclusion I would draw is that the trade show floor was air-conditioned to low humidity such that the wet bulb temperature was surprisingly low. It takes about 1000 BTU to evaporate a pound of water, so evaporation has a powerful cooling effect (which is how cooling towers work). But you can't cool the water to lower than the wet bulb temperature.

Unfortunately, James Watts is correct about the attitude of some regulators, sometimes, to the idea of evaporation. Although most think, like Ward Barcafer, that evaporation is an environmental good, I know a shop owner who was criminally charged for evaporating waste (despite the face that sludge dryers are sold at every trade show, and that every wet scrubber evaporates enormous amounts of water from its reservoir of recirculating hazardous waste). The charges were eventually dropped against that shop owner, but this hostile anti-manufacturing atmosphere that has turned the Arsenal of Democracy into an empty shell, is very risky to you personally. There should be false arrest charges filed against some regulators :-)

A. Hi Arthur. That's not quite the right approach. What should be done is reduction of the hexavalent chrome to trivalent, followed by precipitation. After the precipitation and clarification (or filtration), the clarified water may be suitable for discharge.

Have a qualified chemist (which might or might not be you; I don't know you) take a small quantity of the water and try this in the lab: Reduce pH to 4.0, add sufficient sodium metabisulphite to reduce the hexavalent chrome to trivalent as indicated by ORP controller and change of color from light amber to light blue-green. Add an alkaline to raise the pH to about 9.0, and observe the formation of floc and loss of any tint to the water. Add a coagulant like ferrous sulphate ferrous sulphate [affil links], calcium chloride, or alum. Add a polyelectrolyte and filter or allow to settle. Test the filtrate/decantate for chrome. Then you'll know whether my strategy can work.

Regards,