"[contains]Potassium Alum"
- Is that a compuound containing aluminium?

Yes it is:

Formula:

KAl(SO4)2 12H2 O



Elements: Potassium, Aluminium, Sulphur, Oxygen, Hydrogen.

So how can theya dvertise their product as Alumium free?

I think it's a case for the Trade Description's Act...