There are a couple of possibilities:

Some applications have successfully used a "foam breaker" impeller. The device is essentially an additional impeller, typically large diameter (50% to 70% of the tank diameter), two or four narrow blades, radial flow (straight, vertical blades), positioned to operate at all times above the maximum liquid level. The mechanical effect of an impeller cutting through the foam layer will often break the bubbles causing the foam. Be careful, if the liquid level rises to the foam breaker location, the mixer will overload. Also check for mechanical factors, such as natural frequency problems caused by the additional weight of another impeller.

If your problem develops as a consequence of gas dispersion (unclear from your description), then a different or other additional impeller may help. The cupped-blade disk turbine (CD-6), often called a Smith impeller, when operated backwards may help coalesce gas bubbles in the liquid, thus reducing foaming problems.

As in any case involving additional or alternate impellers, a mechanical review of the installation for motor loads, shaft stress levels and natural frequencies is essential for safety and reliability.