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pH balancing our caustic waste tank

Q. I work for a aluminium extrusion company, we have caustic waste tanks which we are trying to get the pH down to under 12 as per regulations. Can you help us find a solution. The volume of the tank takes 720L of water and 200L of caustic

Merle Mulder
Employee - RSA
October 28, 2010

A. Hi, Merle. I don't think there is any magic electronic way to do this. I think you have no choice but to neutralize with acid -- probably sulfuric or hydrochloric.

Regards,

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

Q. How much hydrochloric acid should we add and what will the reaction be?

Merle Mulder [returning]
Employee - RSA

A. Hi, Merle. If nobody knows how to calculate it, they also may not know how to do the neutralization safely, considering heat of reaction, etc. You may need a permit for the treatment to be legal depending on local laws. The main reaction will be:

NaOH + HCl => NaCl + HOH

There will be other side reactions as the dissolved aluminum is converted to alum salts -- the quantity depending upon how much aluminum is dissolved in the caustic.

You never make an add to a 920 liter volume based on a theoretical calculation though (the calculation is for a reasonableness check as you proceed with titration); you neutralize a small beaker [beakers on eBay or Amazon] quantity and calculate the required additions by scaling up from your test sample. Good luck.
Regards,

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

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