Basicity of Polyaluminum Chloride

In former days, when I did a lot of polymer-focused jar testing, I had three products in my polymer test kit that I never fully understood. The products themselves were straightforward enough as they were PACls or polyaluminum chloride coagulants. Where my understanding stalled out was the additional description of these products in terms of their “basicity.”

These PACls were differentiated as follows:

• PACl – High basicity

• PACl – Medium basicity

• PACl – Low basicity

I never had any idea what was meant by the term basicity and I guess, for the longest time, I was just too lazy to do a little research to find out. Absurd as it may seem, I always had to test all three of these products whenever I though a PAC might work best because I didn’t know the difference between them. I now want to explain basicity and the difference between the three products.

The following excellent description is quoted from the third edition of the Nalco Water Handbook (see page 6.16). This textbook is an outstanding reference and one I highly recommend you get.

Polyaluminum chloride (PACl) is a generic term for a range of products that were initially developed in Japan in the 1970s. PACl minimizes the alkalinity consumption problem, because these materials incorporate some of the required alkalinity to form aluminum hydroxide. These products are essentially partially neutralized aluminum chloride and have the empirical formula:

From the empirical formula, aluminum hydroxide would have “x” equal to 3 with the aluminum +3 valence completely neutralized by hydroxide. Zero hydroxide is simply aluminum chloride . Aluminum hydroxide (three hydroxides and zero chlorides) has a basicity of 100% with basicity defined as:

The basicity of a given PACl product is an averaged parameter with the actual aluminum speciation being a complex mixture that can vary from monomeric aluminum (Al+3) to a range of partially neutralized PACl molecules. Precise aluminum distribution is complex and the subject of much debate that is beyond the scope of this book.