By Earl Fox, Facility Manager

Chemical Recovery Series I : Phosphoric Acid

Waste reduction at D-CHN goes beyond recycling scrap aluminum; conserving and reclaiming liquids are also part of the program.  D-CHN uses large volumes of sulfuric acid, phosphoric acid, and sodium hydroxide in its anodizing process.  It did not take management long to realize that a reduction in chemical usage directly translated into big savings in raw material and disposal costs.

How does D-CHN recycle phosphoric acid?  In anodizing, aluminum is immersed in a brightening solution of phosphoric.  Some of the aluminum dissolves into phosphoric solution.  When the concentration of dissolved aluminum builds to a certain level, the solution becomes contaminated and must be replaced.  In D-CHN one single stage hot rinse and two counter-current water tanks connected to pH neutralization unit were used to rinse bright- dipping aluminum part prior to anodizing as shown in diagram below.   In order to neutralize phosphoric rinse water, 1.3 gallon of 50% sodium hydroxide (NaOH) consumes each 1.0 gallon of 85% phosphoric acid.

D-CHN devised a method to rejuvenate the three rinse waters. First, all three rinse waters were made as dead rinse waters.  By doing this, rinse water #1 thru #3 starts to accumulate phosphoric acid content in the rinse.  We learned that about 10 – 12 days take to reach phosphoric acid level at 58 – 60 weight percent at rinse water #1.  Second, D-CHN pumps out the rinse water #1 to vendor who sold to a customer of 60 wt% of phosphoric acid for their sanitized application.  D-CHN has shipped rinse water #1 to the vendor every 10 – 12 days per cycle.    This two-step process recycles 7,200 gallon of phosphoric acid (60%), reduces 3,000 gallon of sodium hydroxide (50%) usage, and saves $8,000 D-CHN each year in purchase of sodium hydroxide chemical.

Sulfuric acid is inexpensive, but neutralization of the acidic waste is quite expensive.  D-CHN is also planning to install diffusion dialysis system to recycle sulfuric acid near future.

About Mike Sung

Chemical Engineer
This entry was posted in Company Activities. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s