Removing excess aluminum from Sulfuric Acid anodizing

Some say control the dissolved aluminum between 5 and 15 gm/L, I say 8 to 12.

1. Resin removal systems are available in the industry. some say they do not work, others say they have held the aluminum under 8 gm/L for several years with proper maintenance of the equipment

2. Aluminum How-To has a work sheet (IRS-like) procedure for calculating how much to dip out and how much to add back. To my regular clients I furnish this on an Excel program whereby you enter alum gm/L as is, alum gm/L desired, free acid as is, free acid desired, tank gallons (that is 5 numbers), then punch one button and the read out displays how much to dip out and how much acid to add back.

3. Electroplating shops use the clean aluminum bearing spent acid for pickling and acid dipping.

Albert

Check out Eco-Tec on the Environmental Equipment page of this website. They make an "Ano-Pur" unit for the purpose you are looking for. They are quite effective.

Adding a compound with Fluoride in it would precipitate out AlF3. Be careful with fluoride compounds though, they're nasty.

There are systems for removing aluminium from anodizing baths based on ion-exchange. But unless you have a very large installation, the cost would be prohibitive. You're left with dumping. Unless you have other significant contaminants, it is best to do a partial dumping rather than a complete one.

You do not quote figures. I wonder if you are keeping the "free" sulfuric acid correctly? Provided that you replace the sulfuric acid then the solubility of aluminium sulphate is very high. It can eventually lead to problems because of high viscosity. Often "drag-out" will keep the aluminium at a satisfactory level.