Tantalum isn't in high demand at the moment. Galaxy Resources had an agreement previously with GAM and GAM would have been buying from Pilbara had they been interested. However they apparently signed an agreement with another company which was surprising.
I have never really factored tantalum into the value of this company. It's just a by product that may or may not give us some good return and potentially offset the mines running costs. @shades2 is probably the best to discuss this with as they have done way more research than I have.
For now I would forget about tantalum.
GAM = Global Advanced Metals they are an offshoot of Talison.