@Names little
I am not an expert in this field but simple fact that we are selling tantalum to local and overseas customers while others are stockpiling and/or struggling to find a buyer tells me all about quality of our product.
Perhaps you can find answer in A40 report which is quoting that we sold to GAM tantalum concentrate (3.13% TA2O2) as well
high-grade concentrate grading in excess of 30% contained TA2O2 to Stark.
As per your quotes, if your are not the expert and other experts are wrong it's time to move on or just call A40 and asked if they can explain to you.
Please note that A40 is quoting HIGH GRADE and that could be a reason why Stark is one of our customers.