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Can't Raise Temperature of Black Oxide tank enough

Help. I have a 250 gallon tank for a black oxide operation. It is gas fired and has caused me no problems.

The problem... the last three days I have not been able to get the temperature of the tank above 270 F. I have added an extraordinary amount of Black Oxide to the tank thinking that my mixture was off. To no avail. The temperature has remained at 270 F. It has not mattered how much salt I have added the temperature will not increase. I have checked the burners and they appear to be functioning well. I have checked the temperature controller and thermocouples and they are also fine.

What next?

Terry Murphy
- Bridgeport, Connecticut, USA

First off, be really, really careful with this problem. Remember that water boils at 212 degrees F, such that a sudden unanticipated mixing of a slug of water with a slug of 270+ degree solution can cause the water to flash to steam and make the tank erupt.

I don't think I would attempt to troubleshoot this problem except wearing head-to-toe protection.

But what I would do is take a small beaker [beakers on eBay or Amazon [affil links] of the solution into the lab and heat it under a hood there. This will tell you whether the problem is that your gas burners are not producing enough heat, or that the boiling point of the solution is 270 degrees so that it is impossible to get it hotter.

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

Terry -

First double check your temp probe using a mercury thermometer. Secondly, adding more salt to the bath makes the boiling point rise even higher, so it will take even longer to get to the operational point of the bath. Another possibility is that you have a sludge build-up on the bottom of the tank that is insulating the bath from the heat source - try scooping the bottom to see if there is an unusual amount of sludge present. Occasional rectifying helps to minimize sludge formation.

Hope this helps.

Dan Brewer
chemical process supplier - Gurnee, Illinois