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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
publicly reply to Merle Mulder

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
publicly reply to Ted Mooney

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

Merle Mulder [returning]
Employee - RSA
publicly reply to Merle Mulder

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 [affil links] 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
publicly reply to Ted Mooney

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